Mechanical Technologies-Heat Treatment

Investigation of Surface Layer Produced by Direct Current

and Active Screen Plasma Nitriding

Presented by Nora Berrah

MSc Mechanical Engineer

Neptun Code: ICAM08

Supervisor: Prof. Dr. Miklós Tisza, full professor

Consultant: Andrea Szilágyiné Biró, assistant lecturer

2016/2017

University Of Miskolc

This Thesis is Dedicated to my two beloved and deceased uncles

Mohamed Larbi Berrah and Nasseur Oussedik.

I miss you.

Acknowledgements

There are a number of people without whom this thesis might not have been written, and to whom

I am greatly indebted.

First and foremost I would like to thank Allah. In the process of putting this book together I

realized how true this gift of writing is for me. You give me the power and the strength to believe in

my passion and pursue my dreams. I could never have done this without the faith I have in you, the

Almighty.

I would like to express my deepest sense of Gratitude (with a capital and bold g) to my supervisor,

Madam Szilágyiné Bíró Andrea, who offered her continuous advice and encouragement throughout

the course of this thesis. I thank her for the systematic guidance and great effort she put into training

me in the scientific field. Like a charm!

I would like to thank Madam Németh Alexandra Kitti and Madam Nagy Nóra for their help on the

pin-on-disc and surface roughness test and results, without them, I would not be able to finish this

work.

I would like to express my very sincere gratitude to Prof. Tisza Miklós and Dr. Gáspár Marcell

Gyula from the University of Miskolc and Prof. Youb Khaled BENKAHLA from the University of

Science and Technology Houari Boumediene for the support to make this thesis possible.

To my teachers from the University of Miskolc, especially To Professor Habil György Szeidl, and

from The University of Science and Technology Houari Boumediene. I learned a lot of things from

you, so thank you for all what you have done for me.

To Fatima Zohra, my beautiful Twin, I will not be who I am without you, you were always here

for me, you are my sister, my best friend and Ii can’t imagine my life without you.

To Chahinez, you are like a sister, you were always here for me, In the good and bad moments of

my life and Ii will never forget that.

To Feriel, The most crazy and beautiful person that I have ever met, you took a big place in my

heart and I will never forget the beautiful moments that we spent together during this last 5 years.

To Katia (ma katchouuu), thank you for all the chatting that we had, all the time that you spent to

make me in a better mood.

To Neila, you were supposed to be just a friend from the university but during these last 3 years

you became more.

To Chouaib, Adel, Riadh, Ramzi and Walid, my crazy Algerian friends of Miskolc, thanks guys

for the time we all spent together, the many lengthy discussions and dinners we had. This time was

thoroughly enjoyed and I am looking forward to more good times.

To my friends of Miskolc, this experience I had with these amazing people, especially Zsolt

Csikja (Zsolti), Raghawendra and Fruzina (wech labass), I spent very good times with you guys and I

discovered a lot of beautiful things in your company.

I would like also to thanks the Kaloun Family: tonton Ali, Marguerite, Nordin (nounou) and Said.

You welcomed me as if I were a member of your family, thanks to your kindness Ii felt less

homesick.

Most importantly to my family, my uncles, aunts and cousins (Berrah and Oussedik), especially

To my beloved parents Abdelkrim (papou) and Farida (moumi), my brothers and sisters, Youcef,

Meryem, Abdelhadi and fatima Zohra, my brother in law Said and to my beautiful little niece

Mayssa. I love you so much. Without you I definitely would not have written this. You were always

there even when I could not see through the woods, keeping things in perspective for me and always

staying positive. Thank you is not enough thanks for all that you guys did for me. The support came

in many different ways and I will never forget what you have all done. Now it is my turn!

3

Content

Introduction ....................................................................................................................................... 4

1. Heat treatment Process ............................................................................................................ 5

1.1. Definition of heat treatment .................................................................................................... 5

1.1. Types of the bulk heat treatment ............................................................................................. 6 1.1.1. Annealing ......................................................................................................................... 7 1.1.2. Normalizing: .................................................................................................................... 7 1.1.3. Tempering (high temperature) ......................................................................................... 8 1.1.4. Hardening:........................................................................................................................ 8

1.1.5. Case Hardening .............................................................................................................. 10

2. Nitriding ................................................................................................................................ 12

2.1. Phases and microstructure ..................................................................................................... 12

2.2. Microstructure ....................................................................................................................... 13 2.2.1. Compound zone ............................................................................................................. 14

2.2.2. Diffusion zone ................................................................................................................ 15

2.3. Processes ............................................................................................................................... 15

The three main methods used are: ............................................................................................... 15 2.3.1. Gas Nitriding .................................................................................................................. 15 2.3.2. Salt Bath Nitriding ......................................................................................................... 16

2.4. History and Process ............................................................................................................... 17

2.5. Plasma nitriding technology and equipment ......................................................................... 19 2.5.1. Plasma nitriding technique ............................................................................................. 20

2.6. Plasma nitriding of a steel ..................................................................................................... 21

2.7. Difference between Plasma nitriding and gas nitriding ........................................................ 22

3. Direct Current and Active Screen Plasma Nitriding. ............................................................ 23

3.1. The Active Screen Plasma Nitriding Technology ................................................................. 24 3.1.1. Principle of the Active Screen Plasma Nitriding Technology ....................................... 26

3.2. The Difference Between Active Screen and Direct Current Plasma Nitriding Technology . 28

4. Experiments and Results ....................................................................................................... 30

4.1. Description of the experiments ............................................................................................. 30 4.1.1. Base material, sample preparation, heat treatment parameters ...................................... 30 4.1.2. Testing methods ............................................................................................................. 31

4.2. Results ................................................................................................................................... 35 4.2.1. Direct current plasma nitriding results ........................................................................... 35 4.2.2. Active screen plasma nitriding result ............................................................................. 42

4.2.3. Comparison of the ASPN and DCPN process results .................................................... 53

4.3. Result Discussion .................................................................................................................. 55

5. Summary ............................................................................................................................... 56

6. References ............................................................................................................................. 57

4

Introduction

Heat treatment is a process used to modify the microstructure of metals and alloys, like steel and

aluminium to achieve properties that are beneficial for the lifetime of a component. These properties

are for example the increasing of the surface hardness, temperature resistance, ductility and strength.

Nitriding process, was first developed in 1905-1914, is defined as one type of heat treatment

process and is described by the introduction and the diffusion of atomic nitrogen (N) into the surface

of a component to make it more case hardened.

It is one of the most competent methods of surface hardening processes in case of iron base

materials during fifty years and plays an important role in many industrial applications, like the

manufacture of aircraft, bearings, and automotive components.

The nitriding process is a simple heat treatment cycle compared to the other case hardening

techniques, since no quenching is required after nitriding, cracking or distortion is unlikely, and

components can be machine-finished before treatment, but on the other hand because of the slowing

of the process the main disadvantages are economic.

During the nitriding process, crystal structure of the ferrite is not modified or developed into the

face centered cubic (fcc) lattice of austenite. This means that the steel in the ferritic (with cementite)

phase during the entire process.

Plasma nitriding (Ion nitriding) process is one of surface hardening nitriding process types; on

which the reactivity of the nitriding media is due to the gas ionized state and not to the temperature.

In this process, high electric fields are used to generate ionized molecules of the gas around the

surface to be nitrided. This highly active gas with ionized molecules is called plasma. This process is

a smart choice if our purpose is to get parts which have both nitrided and soft area.

The direct current plasma nitriding (DCPN) is one of the most important technologies today

because the process control is simple; the energy and gas consumption is low. It has high

development potential nowadays.

The active screen plasma nitriding (ASPN) technology is also a very important technology used in

nowadays: this method avoids a lot of obstacles and issues. The principle of this method is that the

metal screen acts as a radiation heater and there is no direct glow discharge at the component, no

overheating is happening and the effect caused by the electrical field during the DCPN are

eliminated. This technology is more used and developed to improve the reliability.

The purpose of my project work is to lead to a better understanding of the plasma nitriding

process and the effect of this process on a surface layer produced by a direct current and active

screen plasma nitriding. It may then be possible to apply the scientific insight so gained to the

optimization of nitriding practice and the development of nitridable steels.

5

1. Heat treatment Process

1.1. Definition of heat treatment

Heat treatment is the controlled heating, holding at temperature and cooling of metals to modify

their physical, mechanical properties and sometimes-chemical composition. Heat treatment is also

done due to manufacturing processes that either heat or cool the metal such as welding or forming.

Thus it is a very useful manufacturing process that can not only help other manufacturing process,

but can also improve product performance by increasing strength or other desirable properties

[3][17].

Figure 1. Heat Treating Thermal Cycles [25]

Heat Treatment

It is a process in which the structure of metals or metallic alloys is modified by a heating-holding-

cooling cycle (Figure 1) in order to achieve the desired properties (high or low hardness, strength,

toughness, ductility…)

6

Techniques

Heat treatment is performed using different techniques, which depend on the desired shape and

the required role that we want give to the final component that we will get. It could be annealing,

tempering, hardening, carburizing, nitriding, boriding, normalizing or stress relieving.

Objectives of heat treatment (heat treatment processes)

The major objectives can be:

to harden a steel and increase it strength and wear resistance (by hardening)

to make the steel more ductile, malleable and softness ductility and softness (by tempering or

annealing)

to obtain a steel which is more tough (by quenching and tempering)

to refine the grain size of the steel (recrystallization annealing, full annealing, normalizing)

to reduce the internal stresses (induced by differential deformation e.g. by cold working)

to improve machinability (full annealing and normalizing)

to improve cutting properties of tool steels (hardening and tempering)

to improve surface properties (surface hardening, corrosion resistance-stabilizing treatment

and high temperature resistance-precipitation hardening, surface treatment)

to improve electrical and magnetic properties (hardening, phase transformation

recrystallization, tempering, age hardening)

In heat treatment, the component is heated to the prescribed temperature. The rate of heating to

this temperature is important. In case of materials with residual stresses, which produced by cold

work, the parts should be heated more slowly than usual to avoid distortion.

After heating and holding on temperature, the material is cooled at a specific rate, which will

result the desired properties (like annealing, normalizing, hardening...)

1.1. Types of the bulk heat treatment

Bulk heat treatment can be defined as the modifying of bulk properties of a material by

application of heating and cooling cycles, in an appropriate atmosphere.

Mechanical properties:

7

Figure 2. Fe-𝐹𝑒3C Phase diagram Indicating Heat treating temperature and the processes that

correspond to each range of temperature [16]

We can say that during the annealing process, the steel is heated 30 to 50 °C above 𝐴3

temperature in case of hypo-eutectoid steels and 30 to 50 °C above 𝐴1 temperature in case of hyper-

eutectoid steels.

We can also see that we have a process called spheroidizing; this process means that high carbon

steels may be annealed just below the lower critical temperature to improve machinability [15]

1.1.1. Annealing

Annealing is known as the opposite of hardening, you anneal metals to decrease the hardness and

strength, relieve the internal stresses and soften them. Increase their ductility and malleability and in

each case refine their grain structure.

1.1.2. Normalizing:

Normalizing is a heat treatment process that established a more uniform carbide size and

distribution make grain size refinement, which help the following heat treatment operations and

produce a final product. Normalizing usually consist of austenizing hypo-eutectoid steels at a range

temperature of 30-80 °C above the 𝐴3 transformation temperature followed by slow cooling (Figure

2).For hyper-eutectoid steels, the austenitizing temperature is between 30-80°C above 𝐴1.

We can see in the figure given below the structure difference of the spacing of the cementite

plates in the pearlite between annealing and normalizing processes (Figure 3).

8

Normalizing of steel is always considered both from a thermal and microstructural viewpoint.

From a thermal standpoint, normalizing process consists of austenitizing followed by a relatively

slow cooling. [12].

Figure 3 Difference in pearlitic structure due to annealing and normalizing au-dessous[12]

Ferrite is very soft compared to cementite, which is very hard. Because of the closing of the

cementite plates together in case of normalized medium pearlite, they tend to stiffen the ferrite so it

will not yield as easily, which make the hardness increasing. If the hardness of the annealed coarse

pearlite is about 180 HB, then the hardness of the normalized medium pearlite will be of about

223 HB [12]

1.1.3. Tempering (high temperature)

Tempering is a process of heat treating, which help to increase the toughness of iron-based alloys.

This process is generally used after the quenching process to reduce some of the excess hardness and

consist on heating the metal to a certain temperature below the critical one for a certain period,

allowing it to cool in still air.

The exact temperature that we get determines the amount of hardness that removed and depends

on the composition of the alloy and on the desired properties (hardness, strength) of the finished

product.

1.1.4. Hardening:

Hardening is a metallurgical and metalworking process which is defined as the capacity of a steel to

transform partially or completely from austenite to some percentage of martensite at a given depth when

cooled under some given conditions. This cooling operation occurs from a temperature above the

transformation points 𝐴1(in case of plain-carbon hypereutectoid steel) or the transformation point 𝐴3

9

(in case of hypo-eutectoid steels at a certain speed on which the surface of throughout, there is a

significant increase in hardness, usually through the formation of martensite.

The purpose of this process is to get the highest possible hardness of the steel. This possible

maximum hardness theoretically depends just on the carbon content of the steel and practically its

hardenability, where the dimensions of the workpiece and conditions during heat treatment also play

role.

Hardening process has two main methods:

Through Hardening ("Quenching & Tempering")

Surface Hardening

A. Through Hardening

This process is described of general hardening process (for example quenching and low

temperature tempering) which is used to provide uniformity and strength of hardness for the entire

part from the surface to the core and so wear resistance of metals and alloys. This is realized by

heating the entire part to an appropriate temperature for the desired period, which is needed to

perform to structural changes.

For the quenching parts have to be tempered at low temperature (160-180°C) to the required

hardness, which creates a tempered martensitic microstructure. The through hardening is used on

steels which has carbon content between 0.25-0.70weight% C. During this process, the parts are

cooled rapidly by immersing into water, oil or another suitable liquid to transform the material fully

hardened structure. Oil is a very common quenching medium for this process.

In certain cases, such as lower carbon steel, we use water as a quenching medium, since it

provides the highest cooling rate and thus higher hardness. When it is properly quenched and

tempered, the result will be a martensitic microstructure.

The final stage of the through hardening process is the tempering of quenched steel, which is used

to decrease the undesirable residual stress, and it causes spontaneous cracking. This tempering

decreases the hardness by the low temperature reheating of a part.

B. Surface Hardening

Surface Hardening is a process by which a hard, wear resistant surface is provided for the steel,

with ductile but tough core.

Surface hardening techniques can be classified into two important categories:

Processes that do not modify the surface chemical composition (flame hardening).

Processes that change the surface chemical composition (case hardening or thermochemical

processes).

10

I will emphasize processes that change the surface chemical composition (case hardening) in this

project.

The main difference between these methods is that the through hardening process is used on

medium and high carbon steels and Case hardening is used on low (below 0.25 C %) carbon steels

1.1.5. Case Hardening

This process is considered as a heat treatment process that produces a surface which is wear

resistance, while maintaining toughness and strength of the core.

Case hardening methods include:

Carburising

Nitriding

Carbo-Nitriding

Boronizing

Cyaniding

Carburising

It increases strength and wear resistance by the diffusion of carbon into the surface of the steel.

This treatment is applied to low carbon steels (with carbon content between 0.005 to 0.3%) after

machining as well as high alloy steel (bearings, gears and other components).

The depth of the diffusion (penetration) of carbon is in function of the time, the temperature and

the composition of the carburizing agent[4].

Figure 4 Typical continuous carburising furnace [30]

Figure 4 shows the zones of a typical continuous carburizing furnace. The workpieces are pre-

heated and then kept for a certain time at an elevated temperature (generally between 820 and 940°C)

in the austenitic region of the specific alloy.

During the thermal cycle, the components are in an enriched carbon atmosphere such that nascent

atoms of carbon can diffuse into the surface layer of the component.

11

Carbo-Nitriding

This process is known as a case hardening process which is defined by the diffusion of both

carbon and nitrogen into the base metal in the temperature range 700-900°C.

In carbonitriding, atoms of carbon and nitrogen diffuse interstitially into the metal, creating

barriers to slip, increasing the hardness and the young modulus near the surface.

It is a modified form of carburising process, and it’s used for plain carbon or low alloy steel.

There is also a salt path process which is defined as the nitrocarburising (cyaniding) technique.

This process also involves the diffusion of both carbon and nitrogen into the surface layers of the

steel. In most common way cyaniding, the steel is heated in a liquid bath of cyanide – carbonate –

chloride salts and then quenched in brine, water or oil [5].

Boriding

Boriding is a surface hardening process by which boron is introduced to a metal by diffusion. The

resulted surface contains metal borides, nickel boride, cobalt borides...etc. As pure materials, these

components have high hardness and wear resistance.

Their favorable properties are manifested even when they are a small fraction of the bulk solid

[1].

Boronized parts has high wear resistance, often two to five times higher than components treated

with conventional heat treatments such as hardening, carburizing, nitriding, nitrocarburizing or

induction hardening [24].

Most borided steel surfaces will have iron boride layer hardness ranging from 1200-1600 HV.

Nickel-based superalloys such as Inconel and hastalloys will typically have nickel boride layer

hardness of 1700-2300 HV [7].

Figure 5 Example of deep case boriding (DHB-DC) [31]

12

2. Nitriding

Nitriding is a special thermochemical case hardening process which means diffusion of nitrogen

into the surface of a metal to increase wear resistance, surface hardness and fatigue life by

dissolution of the nitrogen and forming hard nitride precipitations. The diffusion temperature is a

relatively low one (generally below 590°C) so there is little or no distortion as a result of the process

and hardening occurs without quenching.

During the process the core properties are not (or just minimal) affected by the nitriding process

in case of that the final tempering temperature (before nitriding) for the product was higher that the

nitriding process temperature.

As a result, a high strength product is produced with extremely good wear resistance with little or

no dimensional change. Quenching is not required for the production of a hard case [5][8].

2.1. Phases and microstructure

The nitriding process, which is a ferritic thermochemical method, is defined by the introduction of

nitrogen into the surface adjacent of a component by a diffusion process (this technique is based on

the solubility of nitrogen in iron), generally at a temperature between 500°C to 580°C.

In order to understand the principles of Nitriding process, we should understand first the iron-

nitrogen equilibrium diagram (Figure 6).

Figure 6 Iron-Nitrogen equilibrium diagram [19]

13

When treated in a nitrogen containing medium at a given temperature, nitrogen will diffuse and

dissolve in iron.

When the nitrogen content is higher than 0.1%, γ’-nitride 𝐹𝑒4𝑁 is formed; but if the concentration

of the nitrogen exceed 6%, the γ’-nitride 𝐹𝑒4𝑁 starts to increase to nitride (𝐹𝑒2−3𝑁). Below

500°C,

ζ –nitride would begin to form: the nitrogen content in this phase is approximately 11% and its

chemical formula is 𝐹𝑒2𝑁 – during the nitriding process, we will not produce so high nitrogen

content.

The higher the nitrogen content, the more potential for Ɛ phase to form. As the temperature is

further increased to γ’ phase temperature at 490°C, the limit of solubility begins to decrease at a

temperature of approximately 680°C. The equilibrium diagram shows that control of nitrogen

diffusion is critical to process success [27].

Figure 7 shows the connection between nitrogen content and the microstructure changes in a pure

iron during nitriding [18].

Figure 7 Relation between nitrogen content (weight %) and the microstructure changes [32]

2.2. Microstructure

The microstructure of nitrided iron is shown in Figure 8 and Figure 9. We can say that the

compound layers is composed of sub layers of γ’-nitride 𝐹𝑒4𝑁 phase and Ɛ nitride (𝐹𝑒2−3𝑁) phase.

The phase Ɛ is close to the surface and the γ’ phase is near to the diffusion zone (diffusion layer).

14

Figure 8 Schematic description of the microstructure of nitrided iron [20]

Figure 9 Schematic of typical nitrided case structure [20]

2.2.1. Compound zone

The compound zone (white layer) is a hard layer that cannot diffuse into the steel and remains on

the surface. After the process the structure of the compound zone, consists of Ɛ nitride (𝐹𝑒2−3𝑁)

phase and γ’-nitride 𝐹𝑒4𝑁 phase.

The outer part of the white layer can be porous: the formation of the porosity is the result of the

decomposition of thermodynamically unstable nitrides into iron and nitrogen gas at discontinuities

[26].

The presence of the pores in the compound layer has contradictory effects: because it results in

poor adherence and low surface hardness, but - in each case - pores will form reservoirs to hold

surface lubricant, which is needed, is some cases. Therefore, an optimum density of porosity in the

compound layer would help to increase the wear resistance.

15

The thickness of compound zone is approximately between 10 µm to 14 µm of which the first 2

µm to 4 µm may contains fine pores as described above.

2.2.2. Diffusion zone

There are two mechanisms which determine the diffusion zone hardness. The first one is

hardening due to interstitial solution of nitrogen in steel microstructure. Even a small amount of

nitrogen increases the hardness of steel drastically.

The second mechanism is the precipitation hardening caused by the formation of iron nitrides or

alloying elements. (For alloyed steels, this hardening mechanism is predominant)

The properties of the nitrided steels are the following:

High resistance to abrasion wear

High surface hardness

Lower friction coefficient

Improved corrosion resistance to about 500°C

Increased fatigue strength

2.3. Processes

The three main methods used are:

Gas Nitriding

Salt Bath Nitriding

DC Plasma Nitriding (DCPN)

2.3.1. Gas Nitriding

In this case hardening process, the nitrogen is introduced into the surface of a solid ferrous alloy

by holding the metal at a suitable temperature (below 𝐴1, for ferritic steels) in contact with a

nitrogenous gas, usually ammonia (495-565 °C).

The nitrogen diffuses into the steel and hydrogen is exhausted. A typical nitriding set up is

illustrated in the Figure 10.

16

Figure 10 Schematic figure of a gas nitriding system[41]

2.3.2. Salt Bath Nitriding

This thermochemical process is called salt bath nitriding because the parts that have to be nitrided

are immersed into a salt bath containing molten salt combinations. The salt mixtures originally had

60-70% by weight NaCN and 30-40% KCN. Salt bah nitriding is a process during which nitrogen

and carbon are diffused simultaneously into the surface of the material.

During the salt bath nitriding, process the nitriding medium is a nitrogen containing salt like

cyanide salt. The advantages of this process is that by controlling the composition of the salt, we can

control the chemical reactivity of the bath, which will decrease the process temperature, and the

lowering of the process temperature is really important since it means that the distortions of the

treated parts will be less and gives the ability for processing a broader band of steels grades without

attenuate their mechanical properties. Salt bath nitriding process is faster process that gas nitriding

due to a better heat and nitrogen transfer because of the properties and high reactivity of the bath. To

resume, it could be said that 10 hours of gas nitriding correspond to 4 hours of processing in a salt

bath Plasma Nitriding [26].

Figure 11 Description of the process flow of salt bath nitriding process [33]

17

2.4. History and Process

The developing of plasma nitriding process began in the 1920’s as an alternative to conventional

gas nitriding. After World War II the plasma nitriding process received widespread acceptance in

Germany, Russia, China and Japan. The process was not introduced into the United States until after

1950 and has only been used as a production process for the last 20-25 years [8].

During plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to

the gas ionized state. In this technique intense electric fields are used to generate ionized molecules

of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called

plasma, which gave the name to the technique.

Plasma nitriding, which is an extension of the conventional nitriding process, is an efficient

method to increase hardness and wear resistance of metals and alloys by using a gas that when it is

exposed to an electrical potential is ionized and glows. The basic technological advantage of this

method is the low temperature at which the process is conducted, resulting in very small dimensional

deformations and distortions.

Ion/plasma Nitriding of components is characterized by their active participation in glow

discharge since they serve as cathodes. To understand this characterization, it is necessary to refers to

the Paschen curve which compares input voltage with the current density on the steel part surface.

(see Figure 12)

Figure 12 Paschen curve showing the relationship between voltage and current and the various glow

discharge characteristics.[37]

18

The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is

needed (as is the case of gas nitriding with ammonia). There are hot plasmas nitriding typified by

plasma jets used for metal cutting, welding, cladding or spraying. There are also cold plasmas

nitriding, usually generated inside vacuum chambers, at low-pressure regimes.

Usually steels are beneficially treated with plasma nitriding. This process permits the control of

the nitrided microstructure: allowing nitriding with or without compound layer formation. Not only

the performance of metal parts is enhanced, but working lifetime also increased, and so the strain

limit and the fatigue strength of the metals. For instance, mechanical properties of austenitic stainless

steel like resistance to wear can be significantly increased and the surface hardness of tool steels can

be doubled.

Figure 13 General treatment cycle of plasma nitriding [28]

Table 1 Processing parameters of the general treatment cycle of plasma nitriding

Phase Description

1-Pumping The vessel is purged and then filled up with gas at a chosen

pressure

2-Heating Divided into several phases, depending on the temperature

and physical or chemical reaction between the furnace

atmosphere and the part.

2a-Pre-oxidation (usually not done) A thin oxide film is produced in an atmosphere of water

vapor to accelerate nitro-cementation (i.e.to accelerate the

nitriding process)

3-Sputtering Positive ions impacting on the surface heats the job to

slightly below nitriding temperature and ejects surface

atoms

4-Nitriding Nitrogen ions absorbed into the surface to form finely

dispersed nitrides

5-Oxidation (Upon customer request) A thin oxide film is produced in an atmosphere of water

vapor to prevent corrosion of part after treatment

6-cooling Done as fast as possible without causing part distortion

19

2.5. Plasma nitriding technology and equipment

Plasma technology is defined as a physical principle: solids becomes liquids, liquids becomes

gaseous by supplying energy. If more energy is applied to a gas, it is ionized and goes into the fourth

state of matter (the energy-rich plasma state), which was discovered by Irvng Langmuir in 1928.

Plasma, since it occurs by adding energy is defined as a gas which contains charged and neutral

species, which includes some electrons, positive or negative ions, atoms, and molecules. There is not

however a distinct phase changes in going from a neutral gas to plasma.

The degree of ionization, which is an important parameter of a plasma - is the fraction of the

original neutral species (atoms or/and molecules), which have become ionized. When it is low it

means that the plasma is partially ionized (the neutral particles play a big role) and when it is high,

the plasma is fully ionized (the degree of ionization approaches unity, and the neutral particles play

little or no role). This ionization requires certain energy to form plasma.

The plasmas are normally used for a wide variety of processes like thermochemical diffusion

processes, thin film deposition, etching, cleaning, cutting, welding and materials processing initiated

and sustained by electric feels, which are produced by direct current (dc) or alternating current (ac)

power supplies. It is a very good way to combine non-thermal energy field materials processing.

They are also referred to as electric discharge, gaseous discharge or glow discharges.

Figure 14 Plasma sphere [29]

The general frequencies of the alternating current are 100 kHz (at the low end of the spectrum),

13.56 MHz (in the radio frequency portion of the spectrum) and 2.45 GHz (in the microwave

region).

There are different technologies that can be used today for a plasma nitriding process, but we will

mainly focus on glow discharge plasmas, which are more widely used in industry.

20

Plasma techniques are usually used under different values of vacuum; there are researchers who

work on plasmas at 1 atmosphere pressure applied to materials processing techniques. With this

investigation, more industrial applications can be built.

2.5.1. Plasma nitriding technique

The plasma nitriding process (called also direct current plasma nitriding) is defined as the

formation of plasma under vacuum conditions.

To ensure this process the following equipment’s are needed (Figure 15):

A vacuum vessel with a pumping system to get a gas pressure of about 10-1000 Pa

(approximately 1-10 Torr)

A gas supply for nitrogen-containing gases

An electric power supply to produce a glow discharge

An electric power supply for separate heating

A process control unit.

The parts, which have to be treated, are electrically insulated from the surrounding chamber

(vacuum chamber) and connected to the direct current power supply. The workload form the cathode

(-) within the anodic chamber (+).

The electrically positives ions in the gas are accelerated to the negative workload. From this

bombardment, the parts are heated up and the ions are able to form nitrides at the surface. The

specific voltage-to-current characteristic has to be considered for the handling of the plasma process

[13].

Figure 15 Plasma nitriding equipment draft [13]

21

2.6. Plasma nitriding of a steel

Plasma nitriding is a process, which has many advantages but contains also limitations which are

mainly based on the effects resulting from the formation of an electrical field like edge (corner)

effect, arcing, hollow cathode, sputtering and non-homogenous temperature of the workload.

Figure 16 Schematic of the edge effect [10]

The hollow cathode effect appears when parts are placed very close together or they contains deep

holes of small diameters, the overall discharge current can increase so much so that local melting

takes place.

In case of the arcing, a very high local temperature is produced which results in localized melting

or/and sputtering of material from this point on the surface.

The most important one is the edge (corner) effect; it is a problem that can occur in any type of

heat treatment process. It happens when nitrogen penetrates into the surface from all angles of the

corner and therefore the edges which consist of nitrogen supersaturated structure and nitride

networks becomes very brittle and can easily break down. The edge problem is very common in all

nitriding processes with high nitrogen potentials [10][11]. In case of plasma nitriding, the glow

discharge enhances this effect by the plasma field.

To overcome these common problems, many efforts have been made in the past few years. One of

the recent steps was the development of the active screen plasma nitriding technology.

22

Advantages of Plasma Nitriding

Parts which are plasma nitrided are usually ready to use after the process without any extra

machining.

Plasma nitriding convey a nitriding depth upper to 0.6 mm, far deeper than any other process,

which helps to eliminate the need for a secondary treatment.

This process can be more controlled than the other.

Plasma nitriding increase the stress limits of treated metals.

Plasma nitriding can be performed at lower temperature than other nitriding processes [9].

Disadvantages of Plasma Nitriding

High capital cost is including due to the limitation of ion nitriding.

The process need for precision fixing with electrical connections.

Plasma nitriding process has a long time processing compared to other short cycle

nitrocarburizing processes.

2.7. Difference between Plasma nitriding and gas nitriding

Plasma nitriding imparts a hard wear resistant surface without brittleness, galling, or spalling.

This eliminates costly cleaning or grinding to remove the brittle white layer associated with gas and

salt nitriding. Plasma nitriding has a higher surface hardness and maintain the material’s core

properties, which is due to the lower processing temperatures associate dwith palsma nitriding. A

uniform glow discharge, which envelops the whole surface, achieve a consistent hardness and case

depth.

With the gas nitriding, the process is reliant on the decomposition of ammonia. The

decomposition relies on the surface of the steel acting as a catalyst during the reactionary period of

the process cycle. As a result of the decomposition, and because of fixed gas, the surface metallurgy

is fixed. This means the surface will consist of the compound layer, which is a mixture of two phase

and the diffusion zone, and under that, is the core material. The compound layer can be reduced by a

two-stage process involving higher process temperature. The danger of this process is the risk of

cracking particularly on sharp corners. The ion nitride process on the other hand uses the same

elemental gases of nitrogen and hydrogen, but no catalyst. The process relies to control the surface

metallurgy (depending on the steel analysis).therefore the surface metallurgy is variable. More

materials can be selected for plasma nitriding, including cast iron, mild steel, mold, tool, high speed

and stainless steels.[9].

23

3. Direct Current and Active Screen Plasma Nitriding.

Nitriding, in addition to carburizing and coatings, has developed into a process that will be

applied for a long time in the future.

The most important nitriding technologies of today are:

Salt bath Nitriding

Gas Nitriding

Direct Current Plasma Nitriding (DCPN)

Active screen plasma nitriding

Plasma nitriding or direct current plasma nitriding (DCPN) are processes which offers many

advantages like reducing energy consumption and the complete removal of any environmental

hazard. This process is defined as the treatment of the components, which are subjected to a high

cathodic potential, from which the plasma forms directly on the component surface to heat the

components and to provide the active nitriding species. The active screen plasma nitriding is a

technique used to treat low alloy steels, stainless steels, tool steels and other steels to achieve

identical nitriding effects as the direct current plasma nitriding with eliminating of disadvantages of

direct current. Cathodic cage plasma nitriding is defined as one type of the active screen plasma

nitriding, which is able to avoid the problems that a component can have like edge effects or hollow

cathode effect.

Advantages of Conventional Direct Current Plasma Nitriding

There are many advantages for the DC plasma nitriding but the more important are the following:

The process control is simple

The energy consumption is low

High development potential

Limitattions of Conventional Direct Current Plasma Nitriding

High equipment investment costs

It requires a higher maintenance

Trained staff required

The applications are limited

24

Figure 17 Gear wheels [35]

The direct current plasma nitriding technology is every time and constantly using due to improved

safety and environmental impact. For example, plasma nitriding as it works with a safe nitrogen gas

eliminates the hazardous gas byproducts that could be produced which cannot be allowed to escape

to the work area instead of the gas nitriding process.

3.1. The Active Screen Plasma Nitriding Technology

The active screen plasma nitriding is a technology which can avoids all the problems and issues

that the direct current plasma nitriding is facing. The active screen acts as a radiation heater, so there

is not so high voltage at the components, overheating does not happen and any effect due to electrical

field is eliminated.

All these parameters (screen hole size, the gas mixture and the treatment time…etc ) ensure

proper temperature control and uniformity; allow treat parts with different dimensions at the same

time, which increases the process to efficiency. Even the complex geometry can be treated uniformly

and efficiently which is not possible with the DCPN. Applying active screen plasma nitriding

technology, non-conductive materials can be nitrided such as rubber or polymers.

25

Figure 18 Direct current plasma nitriding[35]

The active screen plasma nitriding plays two main roles:

The first one is that the active screen, on which plasma is applied, radiates heat to the work

load. The latter is brought to the aim temperature under vacuum.

The second role that the aspn plays is that a high energized species which are directed to the

workload by gas flow and BIAS are generated from this process,

The principles of DCPN and ASPN furnace are shown in the next pictures[15].

Figure 19 Active screen plasma nitriding [35]

26

3.1.1. Principle of the Active Screen Plasma Nitriding Technology

In such a novel nitriding process, the active screen plasma nitriding is composed of a workload

which is surrounded by a large metal screen (mesh), on which a high voltage cathodic potential is

applied (Figure 19).

The components are in a floating potential or subjected to a relative lower – so called – BIAS

voltage, so the workload is heated rather by radiation from the screen. The temperature is more

uniform and stable, as a result defects which appears due to direct exposure of the workload to highly

negative potential are eliminated or reduced.

The screen has two main roles during this process:

To heat the components to the nitriding temperature by radiation from it (the screen).

Providing active nitriding species to the component surface, these actives species are

principally excited neutral atoms and molecules.

The holder of the workload is electrically insulated to a low BIAS to ease the flow of actives

species to the components. In this way, sputtering takes place on the mesh instead of the surface.

Since plasma is not formed on the component surface, many of problems associated with the direct

current plasma nitriding disappear. Finally, we can say that after an active screen plasma nitriding

process, the surface has better quality than after a conventional direct current plasma nitriding: the

local temperature problems are reduced, which produce more uniform hardening effects and

properties.

Some fields of application for the active plasma nitriding:

Hydraulics

Automotive

Aerospace

Figure 20 Piston rings[35]

27

Cathodic Cage Plasma Nitriding

This technique presents more advantages compared to the conventional method such as the

production of layers with uniform thickness but the most important advantage of this process is the

complete elimination of the edging effect, which is due to the production of the plasma on the

cathodic cage and not directly on the samples. (Figure 21)

During this process, the samples are totally involved by a metallic cage, where a high cathodic

potential is applied. The heat is radiated to the workload, being the cathode of the circuit; it is

subjected to severe sputtering that increases its temperatures. A ceramic disc which is in the cathodic

cage is placed on the samples holder in order to electrically isolate the samples from the cathode.

Besides, the temperature inside the nitriding chamber is uniform, and the thermal gradient is very

small. Thus, heat is transmitted to the workload by radiation and to a lesser extent by convection.

However, little information has been reported regarding the effect of the thickness and hole-size of

the cage on the nitriding properties.

Figure 21 Schematic presentation of the sample position inside the cathodic cage [36]

The economical and environmental advantages of the plasma nitriding.

Economical advantages:

The active screen plasma nitriding and direct current plasma nitriding are considered as a good

process because of their economical and environmental advantages.

The reasons that make the ASPN and DCPN a good process choice are:

28

it needs only 60 to 200l/h gas mix

No ammonia is needed

Finally, this process accepts steam-cleaned parts and allows mixed loads and can works fully

automatically.

Environmental advantages

The active screen plasma nitriding is considered as a friendly environmental process because it

doesn’t harm and this is due to the no need of effluent burning systems, and the using of only 10%

gas as compared to gas nitro-carburizing equipment.

As another good point of using this technique, the actives screen plasma nitriding uses pure gases

like nitrogen, hydrogen, or oxygen. For the unused gases, the vacuum pumping system of the aspn

released them to the atmosphere.

Figure 22 Piles of flanges [35]

3.2. The Difference Between Active Screen and Direct Current Plasma Nitriding

Technology

The most important difference between the conventional direct current plasma nitriding and the

active screen plasma nitriding is that the cathodic potential is applied on a metal screen, which

surrounds the working table, and the components to be treated are placed in a floating potential.

Under this electrical condition, the plasma can be formed on the surface of the metal instead of

the sample surface. In each cases layer produced by ASPN has a more depth compound layer than in

case of DCPN [34].

As a consequence, the active screen plasma nitriding could overcome the limitations of the

conventional direct current plasma nitriding techniques.

29

Figure 23 Parts during direct current plasma

nitriding [34]

Figure 24 Active screen during plasma nitriding

[34]

30

4. Experiments and Results

4.1. Description of the experiments

4.1.1. Base material, sample preparation, heat treatment parameters

During the experiments, the specimen that I used was 51CrV4 called the Chrome Vanadium Steel.

Its Chemical composition is defined on the table below:

Table 2.Chemical Composition of the 51CrV4, weight%

C Si M P (max) S (max) Cr V

0.47-0.55 0.40 0.70-1.10 0.025 0.025 0.90-1.20 0.10-0.25

± 0.02 ± 0.03 ± 0.05 + 0.005 + 0.005 ± 0.05 ± 0.02

The 51CrV4 can be used for the production of many materials like disc springs.

During the nitriding

the gas used was decomposed ammonia,

the plasma nitriding process temperature was 520°C,

the pressure was 2.0 mbar,

the treatment took 8 hours,

the main voltage was around 600V,

the BIAS voltage were,

o 100%=205 V

o 50%=105 V.

In case of the active screen plasma nitriding process, I used two others specimens which have the

same properties than the direct current plasma nitrided specimen to be able to compare these two

specimens result after applying an ASPN treatment with the one on which we applied a dcpn

process, the voltage that I used was 250 V with two different BIAS the first one was 50% and the

second one was 100 % BIAS.

After the processes, the cross section of specimens were grinded to create a flat surface, polished

to make the surface smooth and shiny, and etched for examination of the microstructure using a

microscope., The grinding papers that I used were: P180, P320, P 800, P1000 and finally P2000 and

in case of the polishing operation I used aluminium oxide suspension. Pin-on-disc and surface

roughness test were performed to determine the friction coefficient as a function of the wear path and

surface height as a function of distance across the surface.

31

I measured the hardness of the specimen on the cross section as a function of the distance from

the surface; using the Mitutoyo microvickers hardness tester (Figure 30) and I took some

microscopic pictures of the specimens with the Axio Observer D1m (Zeiss) inverted microscope.

(Figure 31)

4.1.2. Testing methods

The Pin-on-Disc testing

Pin-on-disc testing (Tribometer) is a method to characterize and determine the wearing

parameters of a specimen, like the coefficient of friction (which is determined by the ration of the

frictional force to the loading force on the pin) and rate of wear between two materials. This method

consists of a stationary pin holder in contact with a rotating disc (see Figure 25). The pin can have

different shape, but the spherical ones are usually used to simply the contact geometry.

In case of my study, my purpose was to determine the coefficient of friction, which is

characterized by the resistance of the relative movement between two surfaces in contact and is

defined as:

µ=FT/FN

Where:

FT is the tangential force

FN is the normal force.

Figure 25 (a) Schematic figure of typical pin-on-disc test; (b) Pin-on-disc testing machine [20]

32

The surface roughness testing method

Surface roughness test is a measuring method to charachterize the texture of a manufactured

surface. Although there are many definitions of surface roughness, all of them are based on a

statistical representation of the high frequency surface deviations (peaks and values) from the local

mean surface heigh.

During my test, I used the surface roughness tester (Figure 26) to determine the cross-sectional

area of the wear track (Figure 27 Wear track profile made by surface roughness .

Figure 26 Surface roughness equipment; AltiSurf520

Figure 27 Wear track profile made by surface roughness test

33

The grinding method

This method is defined as a finishing abrasive machining process used to produce very fine

finishes and very accurate dimensions but it has many other applications area of manufacturing and

tool making [38].

Before the microVickers hardness measuremnts and microscopic examinations it was necessary to

grind the cutted cross-section of the spceimens.

Figure 28 Grinding equipment

The polishing method

It is also considered as a finishing process for smoothing a workpiece surface using an abrasive

and a work wheel or a leather strop. It is often used to enhance the appearance of an item, prevent

contamination of instruments, remove oxidation, creat a reflective surface or prevent corrosion in

pipes.In metallurgy and metallography, polishing is used to create a flat and defect-free surface for

examination of a metal microstructure under a microscope.[39]

Figure 29 Polishing equipment [40]

34

The Mitutoyo micro-Vickers hardness tester

Digital micro hardness tester with XY stage and digital ocular for direct reading of hardness

indentations with no requirement for tables.

Features

Digital measurement ocular

Hardness results displayed

Load range 10grams-100grams

Digital micrometer stage

Full programmable set up

Specification

Test load: 10,25,50,100,200,300,500,1000 g

Load control: Automatic (Loading, duration and unloading)

Load duration: 5 to 90 seconds in 1-seconds increments

Accuracy: ±1%

Resolution :0.01 µm

Max.Specimen height: 90mm

Max specimen depth: 100mm

Power supply: 240 V 60 Hz single phase

Figure 30 The Mitutoyo micro-Vickers hardness tester

35

The Axio Observer D1m (Zeiss) inverted microscope

The AxioObserver is a research-grade metallography designed for examination of mounted

samples. Contrasting methods for reflected light include brightfield, darkfield, circular DIC,

polarization and fluorescence.

Features

Light manager, which stores illumination settings of each objective and contrast technique,

recalled automatically as each objective is positioned

Low-position fine and coarse focus knobs

Full integration with axioVision provides automatic scaling when changing objectives to

ensure accurate measurements (D1m and Z1m only)

Innovative contrast techniques: advanced darkfield, C-DIC

12.5-1500X optical magnification range

6-position objective turret offers 1.25 to 150X objective magnification without exchange

Figure 31 The Axion Observer D1m (Zeiss) inverted microscope

4.2. Results

4.2.1. Direct current plasma nitriding results

After making the measurements, I was able to show my result in a graph. (Figure 32) on which I

could determine the case depth hardness and the base material hardness which are 400 HV with a

distance of 0.5 mm from the surface for the first one and 350 HV for the second one (generally case

depth hardness = base material hardness +50 HV).

36

Figure 32 Layer hardness, DCPN process

On the microscopic pictures, we can see that the cross-section of the specimen, after nitriding. It is

composed of a white layer and a diffusion layer, the thickness of the white layer is approximatively

4 µm (Figure 33).

Figure 33 Microstructure of the surface layer, DCPN (N=1000x; etching: Nital)

Pin on dic testing results

The Figure 34 and Figure 35 show the results of a pin on disc testing after respectively 60 and 120

minutes for a direct current plasma nitrided specimen.

The parameters that we used for the pin on disc testing of the Figure 34 (for a time duration of 60

minutes) are:

The diameter of the pin: 3 mm

The velocity: 160 m/s

0

100

200

300

400

500

600

700

800

0 0,2 0,4 0,6 0,8 1 1,2

Har

dn

ess,

HV

0,1

Distance from the surface (mm)

37

The force applied :60 N

The temperature: room temperature

The duration :60 min

Figure 34 Coefficient of friction as a result of the pin-on-disc test, DCPN, wearing time: 60 min

The parameters that we used for the pin on disc testing of the Figure 35 (for a time duration of

120 minutes) are:

The diameter of the pin: 3mm

The velocity: 160 m/s

The force applied :60 N

The temperature: room temperature

The duration :120 min

Figure 35 Coefficient of friction as a result of the pin-on-disc test, DCPN, wearint time: 120 min

0

0,2

0,4

0,6

0,8

0 50 100 150 200

CO

F, μ

Path, m

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 100 200 300 400

CO

F, μ

path, m

38

Figure 36 Surface of the workpiece after DCPN process

Figure 37 Surface of the workpiece after a pin on disc testing (N=8x)

a) (b)

Figure 38 (a) twear=120 min (b) twear=60 min (N=50x)

Figure 37 and Figure 38 show us microscopic pictures about the surface of the direct current

plasma nitrided workpiece after a pin on disc testing at two different duration (60 min and 120 min).

39

The red weared powder means that the wear is low; we can see also that the surface of the

specimen is not so homogeneous. It contains a high quantity of particles.

After getting the results, I could make two conclusions from:

When we reach the white layer, the coefficient of friction is low.

When we reach the diffusion zone, the friction is the higher (approximately 0.43 )

In case of the 60 min experiment, the progressivity is more homogeneous than the 120 min

experiment and the coefficient of friction attend it maximum when the path is 180 m. For the 120

minutes experiment, the progressivity is inhomogeneous, the coefficient increase and decrease

suddenly when it reach 100 m. And the coefficient of friction stay stable (0.43 from 180 m to 350 m)

which is not really realistic because normally the coefficient of friction can’t be more than 0.1 but

unfortunately I didn’

Finally, we can say that there is no big difference between the two measurements, the graphs are

both increasing.

Surface roughness results

Figure 39 Horizontal cross section of the wear track, DCPN, twear=60 min

40

Figure 40 Vertical cross section of the wear track, DCPN, twear=60 min

The average area of the hole:

ADCPN 60min = 1

4(10815 + 11625 + 13177 + 11487) =11 776

41

Figure 41 Horizontal cross section of the wear track, DCPN, twear= 120 min

Figure 42 Vertical cross section of the wear track, DCPN, twear=120 min

42

The average area of the hole:

ADCPN 120min = 1

4(14046 + 10121 + 13764 + 10011) =11 986

The area of the hole is correlated to the wear resistance. When the wearing time is higher the area

is higher.

4.2.2. Active screen plasma nitriding result

After measuring 3 times in different places on the surface layer (and for different distance from

the surface) the hardness of the specimen on which we applied an active screen plasma nitriding

process with 100% BIAS and 50 % BIAS, I was able to resume the results on the graphs below

(Figure 43).

Figure 43 Layer hardness, ASPN process, 100% BIAS

Figure 44 Layer hardness, ASPN, 50% BIAS

0

100

200

300

400

500

600

700

0 0,2 0,4 0,6 0,8 1 1,2

Har

dn

ess,

HV

0,1

Distance from the surface (mm)

0

100

200

300

400

500

600

700

800

0 0,2 0,4 0,6

Har

dn

ess

HV

0,1

(HV

)

Distance from the surface (mm)

43

On this graph, we can determine the base material and case depth hardness of the specimen,

which are similar to the direct current plasma nitriding process: 350 HV for the base material and

400 HV for the case depth.

Figure 45 The microstucture of the surface, ASPN, 100 % BIAS (N=1000x; etching: Nital)

Figure 46 The microstucture of the surface, ASPN, 50 % BIAS (N=1000x; etching: Nital)

The microstructure of the specimen after an active screen plasma nitriding with a 100 % BIAS

(Figure 45) and a 50% BIAS (Figure 46) show us the same surface layer composition, which is a

white layer and a diffusion layer for the both. So we can say that there is no big difference at that

point between the ASPN 50 % and 100% with the DCPN process.

44

Pin-on-disc result

The Figure 47 and Figure 48 show the results of a pin on disc testing after respectively 60 and

120 min for a 100 % BIAS active screen plasma nitrided specimen.

Figure 47 Coefficient of friction as a result of the pin-on-disc test, ASPN 100% BIAS, wearing time:

60 min

Figure 48 Coefficient of friction as a result of the pin-on-disc test,

ASPN 100% BIAS, wearing time: 120 min

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 20 40 60 80 100 120 140 160 180

CO

F, μ

path, m

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 50 100 150 200 250 300 350

CO

F, μ

path, m

45

Figure 49 Coefficient of friction as a result of the pin-on-disc test, ASPN 50% BIAS, wearing time:

60 min

Figure 50 Coefficient of friction as a result of the pin-on-disc test, ASPN 50% BIAS, wearing time:

120 min

These two graphes show us that the coefficient of friction is not constant during the wear track. In

case of the 60 min pin on disc test, we can see that the coefficient of friction is increasing until it

reach the diffusion zone and attend it maximum (the coefficient of friction = 0.58) and stay constant.

But in case of the 120 min pin on disc test ,the result is totally inhomogeneous, the coefficient of

friction is increasing and decreasing along the whole path and doesn’t reach a constant value, and

this could be because of the particles which are generated and appear on the surface during the

testing method.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 20 40 60 80 100 120 140 160 180

CO

F, μ

path, m

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 20 40 60 80 100

CO

F, μ

path, m

46

a) (b)

Figure 51 Surface of the workpiece after an active screen plasma nitriding process with (a) 50%

BIAS and (b) 100% BIAS

Figure 51 shows the picture of the workpiece surface after an active screen plasma nitriding

process for the both case (100% and 50% BIAS), as we can see, the surface is nicer and more

homogeneous than the direct current plasma nitrided workpiece surface (Figure 36). The active

screen plasma nitriding compare to the DCPN process is able to avoid the edge effect.

Figure 52 Surface of the workpiece after a pin on disc testing (N=8x)

47

a) (b)

Figure 53 (a) P1=60 min (b) P2=120 min (N=50x)

The Figure 52 and Figure 53 show the microscopic pictures of a 100% BIAS active screen plasma

nitrided workpiece after 60 and 120 min of a pin on disc test.

Surface roughness results

Figure 54 Horizontal cross section of the wear track, ASPN 100% BIAS, twear=60 min

48

Figure 55 Vertical cross section of the wear track, ASPN 100% BIAS, twear=60 min

The average area of the hole:

A100% BIAS ASPN 60min = 1

4(4773 + 3395 + 3179 + 5158) =4126

49

Figure 56 Horizontal cross section of the wear track, ASPN 100% BIAS, twear=120 min

Figure 57 Vertical cross section of the wear track, ASPN 100% BIAS, twear=120 min

50

The average area of the hole

A100%BIAS 120 min = 1

4(2799 + 4623 + 3215 + 5871) =4127

Figure 58 Horizontal cross section of the wear track, ASPN 50% BIAS, twear=60 min

51

Figure 59 Vertical cross section of the wear track, ASPN 50% BIAS, twear=60 min

The average area of the hole:

A50% BIAS ASPN 60min = 1

4(2966 + 3504 + 3943 + 2600) =3253

52

Figure 60 Horizontal cross section of the wear track, ASPN 50% BIAS, twear=120 min

Figure 61 Vertical cross section of the wear track, ASPN 50% BIAS, twear=120 min

53

The average area of the hole:

A50% BIAS ASPN 120min = 1

4(11175 + 11495 + 12665 + 10444) =11 445

4.2.3. Comparison of the ASPN and DCPN process results

After getting the results of the actice screen plasma nitriding process and the direct current plasma

nitriding process, I have to make a comparison between them with a graph (Figure 62) on which we

can see the hardness measuremnts results of three different experiments (ASPN 50%, ASPN 100%,

DCPN). The only difference that I could make from it is that the hardness of the direct current

plasma nitriding is higher than the hardness of the 100% and 50% BIAS active screen plasma

nitriding. It is because of the difference in the voltage apllied on the specimens: when we applied

DCPN process, the voltage on the specimens was higher; so the driving force and energy for the

nitrogen ion also. Thanks to the higher voltage, the nitrogen content of the surface higher; so the

hqrdnes.

The hardness of the base material is approximately the same for the three cases, 350 HV for the

base material, so the limit value to evaluate the case depth is the same: 400 HV. But we reached the

limit at different distances: for the 100% and 50% BIAS ASPN process I reached the case depth at a

distance of 0.3 mm from the surface but in case of the DCPN process, I reached the case depth at a

distance of 0.5 mm from the surface (see Figure 63).

Figure 62 Layer hardness for ASPN processes (50%, and 100% BIAS) and DCPN process

0

100

200

300

400

500

600

700

800

0 0,2 0,4 0,6 0,8 1

Har

dn

ess

0,1

(H

V)

Distance from the surface (mm)

DCPN

ASPN 50 % Bias

ASPN 100% Bias

54

Figure 63 Case depth for the different processes

When I see the surface uniformity of workpiece, after the application of the three processes, I can

say that the active screen plasma nitriding process gives a better result since it eliminates the most

important problem that the DCPN process: is not needed to face which is the edge effect.

Based on microscopic investigation of the surface layers, the microstructure is similar for the

three processes (a compound layer and a diffusion layer); even the thickness of the white layer is

approximately the same.

After the pin on disc testing method, the results that I got shows that the surface of the ASPN

workpiece is more uniform than the surface of the DCPN workpiece.

Finally in case of the surface roughness results after (60 and 120 min), I could deduce that the

area of the hole after a direct current plasma nitriding process in much higher than an active screen

plasma nitriding (see Figure 64). However, the duration didn’t increased the area of the hole for the

DCPN and ASPN 100% , which means that we reached the constant value for the layer after

approximately 60 min of the process. However, in case the ASPN 50% , the average area of the hole

after 60 min increased 3 times after 120 min of the process which means that in that case the value of

the layer needed more time to be reach.

0,320,28

0,5

ASPN 50% ASPN 100% DCPN

Cas

e d

epth

(mm

)

process

55

Figure 64 The average Area of the hole for the different processes

4.3. Result Discussion

The active screen plasma nitriding is a technique used to achieve identical nitriding effects as

the direct current plasma nitriding with eliminating the disadvantages of direct current plasma

nitriding process.

The active screen plasma nitriding is a technology which can avoids most of the problems

and issues that the direct current plasma nitriding is facing.

In practice, the active screen technique has proved to be less sensitive to grease and rust on

the treated parts.

After an active screen plasma nitriding process, the surface has better quality than after a

conventional direct current plasma nitriding: the local temperature problems are reduced,

which produce more uniform hardening effects and properties.

In order to achieve a desirable metallurgical response, masterials for the active screen and the

amount of BIAS applied to the component have to be considered in applications of active

screen plasma process.

4126 4127

11985,5 11776

3253

11445

0

2000

4000

6000

8000

10000

12000

14000

ASPN 100%BIAS 120 min

ASPN 100%BIAS 60 min

DCPN 120 min DCPN 60 min ASPN 50% BIAS60 min

ASPN 50% BIAS120 min

Aver

age

aera

of

the

hole

(µ

m2)

Process

56

5. Summary

The plasma nitriding has several advantages. It has the ability to automate the system, which gives

good reproducibility of results. It improve control of case depth.

Active screen plasma nitriding technology (ASPN) is a new industrial solution that enjoys all the

advantages of traditional plasma nitriding but does not have its inconveniences.

Therefore using the plasma that surrounds the parts to be treated, the workload attains the required

nitriding temperature. The advantage of ASPN in this regard is that the workload is not subject to

sputtering of material and arcing damage during this stage of the treatment where high power is

applied to the parts.

During the experiments made for my specimens ( 100% BIAS and 50% BIAS active screen

plasma nitriding and Direct current plasma nitriding ) and after the different wearing testing

(polishing, grinding, pin-on disc testing, surface roughness ) which were applied to making them

more easy to evaluate and analyze, I could get results and make a comparison between these

processes.

When I saw the surfaces of my workpieces after these processes, I was able to compare and find

the difference between them, which is the edge effect that appear on my workpiece after a DCPN

process and that doesn’t exist after both ASPN processes.

The polishing and grinding helped me to get a better surface for the hardness measurements, so I

was able to resume these results in graphs and deduce that after an ASPN process the specimen has a

lower hardness than after a DCPN process. However, with the ASPN process I reached the case

depth more rapidly than for the DCPN process.

As a final conclusion of my work, I can say that the active screen plasma nitriding technology is

an important technology, which should be more widely used and applied than the DCPN because it

avoids a lot of obstacles and issues that the direct current plasma nitriding process is facing.

57

6. References

[1] Nayar,Alok. Properties Of Steels, In The Steel Handbook. New Delhi: TaTa McGraw-Hill

Education,2009,479

[2] Yang.Mei. Introduction To The Gas Nitriding. In Nitriding-Fundamentals,modeling and process

optimization ( Doctoral Thesis). Worcester Polytechnic Institute, April.2012.

[3] ASM handbook, Volume 4A,Introduction to Surface Hardening of Steels in Steel Heat Treating

Fundamentals and Processes, J.Dosset and G.E.Totten, editors

[4] City steel Heat Treating,Inc Website, Surface Treatments.

[5] ASM handbook, volume 5, Surface Engineering Of Iron and Steels. In Surface Engineering.ASM

International Handbook Committee,1994.

[6] Haruman.Esa.Characterisation of Compound Layers produced by plasma nitrocarburising.

University Of Birmingham,1992.

[7] Thirunavukarasu.H 13MY12 PSG Tech Kovai. Annealing Normalizing, Quenching, Martensitic

Transformation.

[8] NorthEast Coating Technologies. Inc (Techniques Surfaces).Plasma Process. In Quality Surface

Treatment Solutions.

[9] NorthEast Coating Technologies. Inc (Techniques Surfaces).Plasma Vs.Gas. In Quality Surface

Treatment Solutions.

[10] Andrea.Szilágyiné. Biró, Trends Of Nitriding Processes, In Production Processes and

Systems, vol.6.(2013) No.1.pp.58-63

[11] ASM International, « Fundamentals Of Nitriding and Nitrocarburizing ».In Metals

Handbook,2013

[12] K.P.Shah, Heat Treatment Of Steels. In The Handbook On Mechanical Maintenance.

[13] Reinar Grun and Hans-Joachim Gunther, Plasma nitriding in industry-problems, new

solutions and limits. In Materials Science and Engineering.A140 (1991). pp.435-441

[14] Bernal.Andres, Investigation On Nitriding With Emphasis In Plasma Nitriding

Process,Current Technology And Equipment. (Review Article). Materials Processing. Royal

Institute Of Technology-KTH. Stockholm.Sweden.

[15] ALD Vacuum Technologies website. Active Screen Plasma Nitriding. In Technologien.

[16] Ihom, A.P.1 Abella, S.A.2, and Nor Iv,. J.3 1National Metallurgical Development Centre, Jos,

PMB 2116 Jos, 2National Metallurgical Institute Onitsha, Anambra State, 3College of Education,

Katsina-Ala, Benue State. Active Screen Plasma Nitriding: An environmentally Friendly Process

(A review paper). Continental J. Engineering Sciences 5 (2): 67 - 75, 2010.

[17] Gallo.Santiago.Corujeira, Active screen plasma technology.In Active Screen Plasm Surface

Engineering Of Austenitic Stainless Steel For Enchanced Tribological And Corrosion

Properties.(Doctoral thesis) University of Birmingham. 2009. pp.41-43

[18] F. Hoffmann, P. Mayr, F. Preisser and P. Minarski, Nitriding/nitrocarburising at high

pressure,In Heat Treatment of Metals, 1992.3, p80-85.

[19] Lucefin Group, Technical Card of 51CrV4. 2006

[20] Francis Kennedy,Yuan Lu, Lan Baker, Contact temperatures and their influences on wear

during pin-on-disk tribotesting. In Tribology International (Published Paper).February 2015.

[21] Spectrographic siteweb, In Refurbished by application, recent sales, Mitutoyo MVK-H1,

Tong park Industrial Estate

[22] https://www.thebalance.com/what-is-the-metallurgical-term-known-as-tempering-2340024

(26/11/2016)

58

[23] http://www.technologystudent.com/equip1/heat1.htm (26/11/2016)

[24] https://en.wikipedia.org/wiki/Boriding (26/11/2016)

[25] http://www.mdpi.com/1424-8220/14/10/19493/htm (27/11/2016)

[26] http://surface-heat.com/metallurgical-articles/technical-information-arcor-nitrocarborizing/

(27/11/2016)

[27] http://www.ionheat.com/sites/default/files/archivos/investigation_in_plasma_nitriding_proce

ss_0.pdf (27/11/2016)

[28] http://www.tppinfo.com/heat_treatment/plasma_nitriding_coating/plasma_nitriding.html

(27/11/2016)

[29] http://topsy.one/hashtag.php?q=%23plasma (27/11/2016)

[30] https://matenggroup.wordpress.com/2014/12/05/carburising-2/ (27/11/2016)

[31] http://www.ibccoatings.com/boriding-boronizing-dhb (27/11/2016)

[32] http://emrtk.uni-

miskolc.hu/projektek/adveng/home/kurzus/korsz_anyagtech/1_konzultacio_elemei/nitriding_intro

duction.htm (27/11/2016)

[33] http://www.houstonunlimitedinc.com/nitriding.aspx (27/11/2016)

[34] C.X. Li, Tom Bell, and H. Dong.: Active Screen Plasma Nitriding of Austenitic Stainless

Steel, http://www.tandfonline.com/doi/pdf/10.1179/026708402225006240

[35] http://web.ald-vt.de/cms/vakuum-technologie/technologien/vacuum-heat-treatment/aspn-

active-screen-plasma-nitriding/ (27/11/2016)

[36] http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392013000400010

(27/11/2016)

[37] http://blog.sina.com.cn/s/blog_649ce7730102vw4e.html (29/11/2016)

[38] https://en.wikipedia.org/wiki/Grinding_(abrasive_cutting)#Surface_grinding (03/12/2016)

[39] https://en.wikipedia.org/wiki/Polishing_(metalworking) (03/12/2016)

[40] http://www.workmate.com.au/product/polishing-compounds (03/12/2016)

[41] http://saroptstroy.ru/gas-nitriding/ (16/12/2016)