p/m sintering by atmospheric pressure microwave plasma

7/27/2019 P/M Sintering by Atmospheric Pressure Microwave Plasma

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P/M Sintering by Atmospheric Pressure Microwave Plasma

*K. Cherian, Ph.D., S. Kumar, Ph.D., D. Kumar Ph.D., M. Dougherty Sr., D. TaschMicrowave Technologies, Dana Corporation, 2910 Waterview Drive, Rochester Hills, MI 48309

ABSTRACT

Microwave plasma at atmospheric pressure has beenused successfully to sinter powder metal samples andcomponents. Under suitable experimental conditions,P/M green samples were initially de-lubed and thensintered in the atmospheric microwave plasma. Someexperimental results are presented which show theresulting sintered material characteristics to becomparable to or better than that achieved byconventional means. Some advantages of theatmospheric plasma processing method overconventional methods are also discussed.

INTRODUCTION

PLASMA PROCESSING A NATIONAL PRIORITY

Plasma processing technologies are of vital importanceto several of the largest manufacturing industries in theworld. A recent National Academy of Sciences report

states:"Plasma processing technologies are of vital importanceto several of the largest manufacturing industries in theworld. Foremost among these industries is theelectronics industry, in which plasma-based processesare indispensable for the manufacture of very large-scale integrated (VLSI) microelectronic circuits (or chips). Plasma processing of materials is also a critical technology in, for example, the aerospace, automobile,steel, biomedical, and toxic waste management industries. Most recently, plasma processing technology has been utilized increasingly in the emergingtechnologies of diamond film and superconducting filmgrowth. Because plasma processing is an integral part of the infrastructure of so many vital American industries, it is important for both the economy and the national security that America maintain a strong leadership role in

this technology." (National Research Council, Plasma Processing ofMaterials: Scientific Opportunities and TechnologicalChallenges (1992), National Academy Press ,Washington, D.C.) [1].

With this background the significance of newdevelopments in plasma materials processing, offeringdistinct advantages over existing technologies, wouldbecome more evident.

PLASMA CLASSIFICATION

Plasma consists of charged, excited and neutraparticles (e:electrons, i:ions and n:neutrals) resultingfrom the partial ionization of atoms or molecules of a gas(and is thus an electrical conductor). Plasma may serveas a chemical reactor; interaction of plasma electronswith the feed gas present produces short-lived activespecies. These can react with a surface, or react witheach other to produce secondary short-lived chemicaprecursors for deposition as thin films. Plasmas couldalter the normal chemical reaction pathways, thusoffering the potential to obtain materials with propertiesand specific phases not easily attainable by other routes

Plasmas may be classified into two:

a) Non-thermal or non-equilibrium plasma: characterizedby electron temperature much higher than the ions oneutral atoms. Te>>Ti~Tn

b) Thermal or equilibrium plasma: characterized byequilibrium or near equality in temperature between thethree components. Te~Ti~Tn

(where Te, Ti & Tn represents temperature of electronsions and neutrals respectively)

The temperatures of plasmas could rise to extremelyhigh levels, and this could therefore serve as anexcellent heat source as well. Sustained plasma with

judicious control of the plasma composition could be aneffective materials synthesis and processing toolMicrowaves have been found to be useful in materialsprocessing – with and without plasma. The mainfeatures of both these microwave-processingapproaches are summarized below.

ATMOSPHERIC PRESSURE NON-PLASMAMICROWAVE MATERIALS PROCESSING

Microwave processing has received significant attentionin recent times as an alternate and better route fomaterials synthesis and processing. Non-plasmamicrowave heating processes have been investigatedwith conventional (magnetron) and millimeter (gyrotron)wavelengths, with some degree of success. Recentreviews by Clark and Sutton [2], Schiffman [3], Katz [4]and Sutton [5] deal with the potential use of microwaveprocessing for a range of materials – from bacon andpotato chips to wood, rubber, ceramics andsemiconductors. Rapid heating rates, reduced

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processing times, energy savings, finer microstructures,novel and improved properties of processed materialsand finer microstructures are some of the intrinsicadvantages of this processing route that makes itattractive.

Conventional microwave processing takes advantage ofthe dielectric or microwave-coupling characteristics ofmaterials. Microwave heating is therefore verydependent on the material being processed, and alsoother factors such as geometry, size, and mass of thesample [6,7]. In microwave heating, heat is generatedinternally within the material, leading to “inside-out”heating, unlike conventional heating where the heat isgenerated externally and transferred by mostly radiativetransfer, leading to “outside-in” heating. Microwaveheating is therefore fundamentally different fromconventional heating, but its direct applicability had beengenerally limited to “microwave absorbers” or incorporating such materials as susceptors.

MICROWAVES PROCESSING OF METAL PARTS

Metals generally do not absorb or couple withmicrowaves. One method of applying microwaveprocessing to “non microwave absorbers” such asmetals has been through the use of suscepters, in bulkor coating form, to absorb microwave energy and radiateheat, in the close vicinity of the part to be processed.Conventional microwave techniques have beeninvestigated for processes such as sintering and joiningof metal parts. Since the energy transfer occurs throughmicrowave-material coupling, the dielectric properties ofthe material components being processed decide theprocessing set-up configurations. For poor couplers,secondary heaters are often needed to raise the

temperature of the samples to the level from where theywill begin coupling better with the radiation. In the caseof powder metal samples, it has been found thatmicrowave coupling may occur, depending on theparticle size. Further, gyrotron radiation (higherfrequency microwaves) offers the possibility of very highrates of localized heating for materials which will couplewell with the millimeter wavelengths [8], but the set upsand maintenance costs can be expensive.

Non-plasma atmospheric pressure microwave materialsprocessing approach therefore could yield certainbenefits, though with limitations where solid metal parts

processing are involved. Processing with microwaveplasmas could therefore enable faster processing, andatmospheric pressure plasmas may lead to even moreefficient processing. The major roadblock in this regardhad been the difficulties to initiate and sustainmicrowave plasmas in a specific processing volumes atatmospheric pressure; plasma jets and small volumeplasmas would not suffice for larger area and volumeparts processing [9,10].

CONVENTIONAL MICROWAVE PLASMAPROCESSING AND CONSTRAINTS

Various techniques have been developed for plasmainitiation and sustainment; these involve microwaves, r-or dc as excitation sources, coupled with lowepressure/vacuum chambers, electrodes or arcs and jetswith associated power sources and circuitry [6]. In thecase of microwave plasmas, low-pressure environments

make it easier to initiate and sustain the plasma withreasonable power input, but this brings in severaconstraints when viewed from an industrial applicationsperspective. These include:

i) Need for expensive vacuum pumps and systems.

ii) Processing volume constraints, depending on thevacuum chamber size and pump efficiency.

iii) Not suited for continuous processing, only batchprocessing is feasible; this, together with processingvolume limitations, restricts throughput.

iv) Pump down and vacuum break periods add to overalprocessing time per batch and processing economics.

v) Precursor densities low at low pressures, leading tolow synthesis/deposition rates.

Therefore, the low-pressure microwave plasmaprocessing applications have generally been limited toselect “high value” added uses, e.g., steps required formanufacture of semiconductor devices, magnetic mediaor deposition of specialized films for specific high techapplications.

The fact that vacuum conditions are necessary makeslow pressure plasma impractical to use in industriesrequiring high rates of throughput, e.g., for commerciaautomotive components fabrication, textile industry, etcAn atmospheric plasma treatment process, on the othehand, should be well suited for continuous processingbut its technological development has been limited bythe inability to ignite and sustain plasma at atmosphericpressure using reasonable levels of microwave powerTherefore, alternate methods of generating atmosphericplasmas have been investigated at several laboratoriesworldwide, and these include dielectric barrier dischargecorona discharge, plasma torch, atmospheric pressure

plasma jet configurations etc. All of these have certainoperating constraints especially where volumetricheating to high temperatures is required.

THE DANA BREAKTHROUGH: THEATMOPLASTM (ATMOSPHERIC PRESSUREMICROWAVE PLASMA) PROCESS

With the above background, the significance of the Danabreakthrough in microwave plasma processing becomesevident. Dana scientists have discovered a method toinitiate and sustain microwave plasma at atmospheric



pressure. 2.45 GHz or 915 MHz microwaves may beused and the power requirements to initiate the plasmaby this route are relatively low, about 1 kW or less.Under specific conditions 90-95% of the microwaveenergy may be coupled with the plasma to enable fastheating of metal samples to very high temperatures. Thisenables controllable atmospheric plasma processing ofmaterials without the limitations, constraints orrestrictions of the current plasma and microwaveprocessing techniques outlined earlier. This process isnow known as the AtmoPlasTM process. The sustainedplasma thus obtained may be utilized as an efficientheating source, as a chemical reactor, or both. In theAtmoPlas

TMprocess, the microwave energy is absorbed

by the plasma which then transfers it as heat to the bodybeing processed; dielectric characteristics of the body isthereby irrelevant, and use of secondary heaters is notnecessary.

SIGNIFICANCE OF THE BREAKTHROUGH

The novelties and potential advantages of theAtmoPlas

TMover conventional plasma as well as non-

plasma microwave processing technologies include thefollowing:

System advantages:

i) No need for expensive vacuum chamber andassociated pumping systems.

ii) No processing volume constraints related to thechamber size and pump efficiency.

iii) Suited for batch and also continuous processingadopting suitable conveyer system; this would remove

processing volume limitations and the resulting restrictedthroughput.

iv) No pump down and vacuum break periods to add tooverall processing time and processing economics.

v) No electrodes and associated circuitry needed as insome other atmospheric pressure plasma systems.

Advantages as a chemical reactor:

vi) Since the plasma is at or close to atmosphericpressure, etching and reaction rates can be higher

compared to that achieved by low-pressure systems.

vii) In an atmospheric pressure plasma, precursordensities and hence deposition rates can be significantlyhigher than with low-pressure deposition processes.

Advantages as an effective heating source:

viii) Uniform volumetric heating of samples possible, forvarious industrial processes such as sintering, joining,etc

ix) Not restricted to microwave susceptible / dielectricmaterials only as in conventional microwave processing.

x) Secondary heaters not needed for poor microwavecoupler materials processing.

xi) Not restricted to nano-size grained metal greenbodies for effective sintering, as in conventionamicrowave processing.

xii) Costlier single mode microwave set-ups with limitedE-max and H-max node points not necessary.

xiii) Plasma immersion and selective heating of requiredregion only is possible by suitable design of the cavity –thus protecting components sections not amenable toheat treatment.

xiv) Very rapid thermal build up possible withconventional microwave frequencies such as .915 and2.45 GHz – almost comparable to that obtained by the83 GHz gyrotron processing system, at a fraction of theestablishment and operating costs.

APPLICATION OF ATMOPLASTM PROCESS TOMETAL PROCESSING: P/M SINTERING.

The AtmoPlasTM

process offers a broad range oopportunities in materials processing for industriaapplications; these include heat treatment, joiningsintering, carburization, etc., of metal components. Thefollowing sections deal with some preliminary resultsfrom application of the AtmoPlas

TMtechnology to powde

metal sintering.

The AtmoPlasTM

laboratory set-up.

Fig. 1: Schematic of the AtmoPlasTM

laboratory set-up

Figure 1 shows the AtmoPlasTM

set-up used for powdemetal sintering in the laboratory. This is capable ofdelivering up to 6 kW continuous 2.45 GHz microwave



power. The outer cylindrical chamber is water cooledand is made of aluminum. Under the chamber, themagnetron and its power supply are mounted as shown,and microwave energy is fed into the chamber through awave guide with a three stub tuner to match theimpedence of magnetron and the plasma load. Anisolator serves to protect the magnetron from reflectedmicrowave power by dumping it into a water load. Amode stirrer, operating in the aluminum chamber, helpsmake the microwave field uniform in the chambervolume. On one side of the chamber is a sliding door, towhich a horizontal aluminum platform is fixed. The partsto be sintered are placed in a ceramic cavity, as shownin the figure. Stainless steel tubes provide gas inlets andoutlets into this cavity. Plasma is initiated and sustainedin the cavity volume, and the part temperature ismonitored by a dual wavelength pyrometer. Varioussafety interlocks automatically shut down the system ifmicrowave leakage is detected during operation.

Atmospheric Pressure Microwave Plasma Sintering ofP/M Cam Lobes

Sintering trials were carried out on green powder metalcam lobes. The metal powder used for making the greenbodies was a copper-phosphorus steel alloy, similar toMPIF FC-0205. Each of the cam lobe green sampleshad dimensions of about 31 mm at the longest region,25 mm at the broadest region and 9 mm height, withaverage weight 28 gms.

De-lubing and sintering

Both de-lubing and sintering steps were carried out inthe same AtmoPlas

TMprocess cavity as consecutive

operations. However, the plasma modes employed for

each of these steps were different; filamentary plasmawith high Ar flow was found to be effective for the de-lubing step, whereas steady uniform plasma, with theplasma chemistry adjusted if necessary to preventdecarburization effects, was employed successfully forsintering. De-lubing and sintering trials were carried outinitially with one green sample, and the process wassubsequently upgraded for simultaneous multiple partprocessing.

In the AtmoPlasTM

process, delubing could be done in~10 – 15 min at average temperatures of ~700C in thefilamentary plasma mode. In the conventional process

being compared to, the delubing stage took 45 min. Afterdelubing, the plasma was transitioned to the steadyplasma mode with the proper plasma chemistry and tothe required sintering temperature. Sinteringtemperature comparable to that used in conventionalsintering process (~1150C) was tried first, followed byhigher temperatures. The following sinteringtemperatures were investigated: ~1150C, ~1250C &~1350C. The sintering period was kept constant in theseruns, at 20 min. Figures 2 - 4 show how the parts wouldappear at various stages of the processing run: multipleparts in the cavity prior to plasma processing, plasmatransition from the de-lubing stage to the sintering stage

(filamentary to steady plasma) and the uniformly heatedparts immediately after the plasma is extinguished oncompletion of a run.

Fig. 2: Multiple cam lobes in the plasma cavity prior toplasma processing.

Fig. 3: Plasma transitioning from the filamentary tosteady modes.

Fig. 4: Heated parts immediately after plasma switch-off

Analysis

Representative samples from these runs were selectedfor analysis at Dana‟s Advanced Technology ResourceGroup‟s A2LA Accredited Advanced Materials Lab. Inaddition to metallographic analysis, direct Rockwell Banalysis were performed in accordance with ASTM E 1803, average of 5 measurements, and density estimations



were done per ATRG Material Science DepartmentWork Instruction WI-0092 (rev.5).

RESULTS AND DISCUSSION

The AtmoPlasTM sintered samples were analysed forhardness, density, decarburization effects if any, andmicrostructure. The results from the conventional andAtmoPlasTM processed samples are shown in Tables 1 –3. When compared against properties of conventionallysintered (~1150C) cam lobes the analysis of cam lobessintered at ~1150C, ~1250C & ~1350C by theAtmoPlas

TMprocess for a similar period (20 min),

showed significantly interesting results.

While the conventionally processed sample showed anaverage hardness of 89 HRB, the AtmoPlas

TM

processed samples showed apparently better results.With higher temperatures of sintering for the sameduration, the hardness values apparently showsignificant increase, as shown in Table1.

Table 1: Hardness: Conventional & AtmoPlasTM

processed

Sample Hardness (HRB)

Conventional sintered, ~1150C, 20 min 89

AtmoPlasTM

sintered, ~1150C, 20 min 93.04

AtmoPlasTM sintered, ~1250C, 20 min 93.72

AtmoPlasTM

sintered, ~1350C, 20 min 105.96

The post processed density values too appear toincrease steadily with the sintering temperatures, asshown in Table 2. Checking for possible decarburizationeffects, neither of the samples depicted in the tables andprocessed at increasingly higher temperatures in theAtmoPlas

TMprocess showed any decarburization

effects.

Table 2: Density: Conventional & AtmoPlasTM

processed

Sample Decarburization Density(g/cc)

Conventional sintered,~1150C, 20 min

None 6.58

AtmoPlasTM sintered,~1150C, 20 min

None 6.42

AtmoPlasTM sintered,~1250C, 20 min

None 6.57

AtmoPlasTM

sintered,~1350C, 20 min

None 7.00

Metallographical studies apparently reveal the effects ofhigher sintering temperatures on the microstructureThere appears to be clear evidence for reduction inporosity with higher temperatures of sintering. Thisseems to fit in with the density values mentioned aboveThe general microstructural observation is that ofnetwork of grain boundary ferrite surrounding grains opearlite In the samples sintered at higher temperatures(~1250C & 1350C), network of proeutectoid ferrite withspherodized carbides around pearlite were alsoobserved. Such aspects of the microstructure can be beimproved upon, with further experimentation involvingchanges in processing parameters. Overall, themicrostructural observations appear to support thehardness and density measurement results mentionedearlier.

Table 3: Microstructure: Conventional & AtmoPlasTM

processed P/M samples.

Sample Microstructure

Conventional sintered,

~1150C, 20 min

Unetched

AtmoPlasTM


Network of grain

boundary ferrite (lightregions) surroundinggrains of pearlite (darkregions), the blackregions are porosityvoids.

Etched: 2% Nital

AtmoPlasTM


Network of grainboundary ferrite (lightregions) surroundinggrains of pearlite (dark

regions), the blackregions are porosity voids

Etched: 2% Nital



AtmoPlasTM


Network of grainboundary ferrite (lightregions) surroundinggrains of pearlite (darkregions), the blackregions are porosity voids

Etched: 2% Nital

The AtmoPlasTM

route offers processing advantages aswell. The process for cam lobe sintering involvedessentially 3 stages: de-lubing, sintering and cool-down.All these steps were performed as consecutiveoperations in the same cavity. It was found that whileconventional processing has a de-lubing stage of 45minutes duration, the AtmoPlasTM route could yieldcomparable de-lubing results in about 15 minutes. Thisreduction of the de-lubing time has led to overallreduction of the process time and yielded sintering

results comparable to the longer conventional process.The sintering and cool down periods in the runsmentioned here were deliberately kept comparable withthe conventional process, to be able to compare theresults effectively. There may be the possibility toimprove the processing times for these stages as well,with the AtmoPlas

TMroute.

Also, conventional sintering set-ups have limitations onthe highest processing temperatures possible due to thelimitations of the sintering furnace components such asthe conveyer belts, which also have to be subject to theextended high temperatures conditions in a bulk volumeheating configuration. This can often lead to the sagging

of the belts and their reduced lifetimes. In addition to thecost of their replacements, equipment down time toreplace such defective belts, all lead to increasingprocessing costs. In the AtmoPlas

TMprocess, however,

since the plasma and thereby the heat, is confined to avolume encompassing the parts and not the wholeprocessing chamber volume, the belt does notexperience the very high temperatures; thereby theirperformance and lifetimes would not be affected. Highertemperature sintering would therefore be possibleeconomically.

CONCLUSION

Several constraints of Low Pressure Microwave Plasmaprocessing may be overcome with the new AtmosphericPressure Microwave Plasma Process, AtmoPlas

TM,. This

novel process enables the initiation and sustaining ofvolumetric microwave plasmas at atmospheric pressurein suitable cavities, which may be used for variousmaterials processing applications. Such plasmas may beused as a heater or a chemical reactor, or in acombination mode.

The utility of the atmospheric pressure plasma in aheater mode was successfully investigated. P/Msintering was done through the AtmoPlas

TMroute a

temperatures similar to conventional processingComparable or better results were obtained togethewith reduction in overall processing time.

Possible advantages of this P/M sintering route oveconventional routes were also successfully investigatedP/M sintering was done through the AtmoPlas

TMroute a

higher temperatures that are not very practical in theconventional processing route due to equipmencomponent constraints. Analysis of samples thussintered at higher temperatures but for the same periodas in the conventional route, showed different propertiesDensity and hardness values showed significanincreases, with apparent reduction in porosityAtmoPlas

TMprocessing is thus capable of offering an

alternate P/M sintering route with distinct advantagesover conventional methods.

REFERENCES

1. National Research Council, Plasma Processing oMaterials: Scientific Opportunities and TechnologicaChallenges (1992), National Academy Press Washington, D.C.)

2. D.E. Clark and W.H. Sutton, Annu. Rev. Mater. Sci.1996, 26, 299-331

3. R.F. Schiffman, Ceram. Trans., 1995, 59, 7-17

4. J.D. Katz, Annu. Rev. Mater. Sci., 1992, 22, 153-170

5. W.H. Sutton, Mater. Res. Soc. Symp. Proc., 1992269, 3-19

6. R. Roy, D.K. Agarwal, J.P. Cheng and M. MathisCeram. Trans., 1997, 80, 3-26

7. T. Gerdes and M. Willert-Porada, Mat. Res. SocSymp. Proc., 1996, 347, 531-537

8. K. Cherian, A. Fliflet, S. Ganguly and R. Roy, “NoveApproaches to Materials Synthesis and Processing withMicrowaves”, Proceedings of the Ted White Festschrif

Symposium, Chemical Engineering DepartmentUniversity of Queensland, Australia (December 2001)

9. H. Schlemm and D. Roth, “Radio-frequencyatmospheric pressure plasmas – a new basic approachfor plasma processing,” GALVANOTECHNIK, SpeciaEdition, Issue 6, Vol. 92 (2001).

10. See, for example, papers presented in the “PlasmaGeneration, New Plasma and Ion Sources” and“Atmospheric Pressure Processes” Sessions at the NinthInternational conference on Plasma Surface

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Engineering, Garmisch-Partenkirchen, Germany,September 13-17, 2004.

CONTACT

Dr. Kuruvilla Cherian „s major interests have been insuper hard materials synthesis and development of

novel materials processing technologies involving lasers,microwaves and plasmas. He can be contacted [email protected]

p/m sintering by atmospheric pressure microwave plasma

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