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CHAPTER 1

INTRODUCTION

1.1 Tribology

Tribology is the science and technology of interacting surfaces in relative motion. It

includes the study and application of the principles of friction, lubrication and wear. The

study of tribology is commonly applied in bearing design but extends into almost all other

aspects of modern technology. The tribological interactions of a solid surface's exposed face

with interfacing materials and the environment may result in loss of material from the

surface. The process leading to loss of material is known as "wear". Major types of wear

include adhesion, abrasion, erosion, and corrosion.

Polymers and their composites form a very important class of tribo engineering

materials and are invariably used in mechanical components such as gears, cams, bearings,

bushes, bearing cages, etc., where wear performance in non lubricated condition is a key

parameter for material selection. Composites are subjected to abrasive wear, friction in many

applications. The wear performance of composites deteriorated due to the inclusion of fillers.

The effects of wear are dimensional changes, leakage, lower efficiency, etc. [1].

The most common fiber-reinforced polymer composites are based on glass fibers,

cloth, mat, or roving embedded in a matrix of an epoxy or polyester resin. Reinforced

thermosetting resins containing boron, polyaramids, and especially carbon fibers conformed

to high levels of strength and stiffness. Carbon fiber composites have a relative stiffness five

times that of steel. Because of these excellent properties, many applications are uniquely

suited for epoxy and polyester composites, such as components in new jet aircraft, parts for

automobiles, boat hulls, rocket motor cases, and chemical reaction vessels.

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Although the most dramatic properties are found with reinforced thermosetting resins

such as epoxy and polyester resins, significant improvements can be obtained with many

reinforced thermoplastic resins as well. Polycarbonates, polyethylene, and polyesters are

among the resins available as glass-reinforced composition. The combination of inexpensive,

one step fabrication by injection molding, with improved properties has made it possible for

reinforced thermoplastics to replace metals in many applications in appliances, instruments,

automobiles, and tools.

Modern composites are usually made of two components, a fiber and matrix. The

fiber is most often glass, but sometimes Kevlar, carbon fiber, or polyethylene. The matrix is

usually a thermoset like an epoxy resin, polydicyclopentadiene, or a polyimide. The fiber is

embedded in the matrix in order to make the matrix stronger. Fiber-reinforced composites

have two things going for them. They are strong and light. They are often stronger than steel,

but weigh much less. This means that composites can be used to make automobiles lighter

and thus much more fuel efficient [2].

While attention of academia and industry on materials properties is largely focused

on mechanics, wear causes losses in industry at least not smaller than fracture caused by

mechanical deformation. We discuss the importance of tribology for polymer-based materials

(PBMs). Traditional tribology developed originally for metals cannot be applied to PBMs for

at least two reasons. First, PBMs are viscoelastic and their properties depend on time-in

contrast to metals and ceramics. Second, external liquid lubricants, which work well for other

classes of materials, are easily absorbed by PBMs; swelling is the result. We and others are

developing tribology of PBMs taking into account among others: viscoelasticity, materials

brittleness defined in 2006 and connections of brittleness to recovery in sliding wear

determination, relation of friction and scratch resistance to surface tension, and effects of

magnetic fields on polymer tribology [3].

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The purpose of the research in tribology is to minimize and remove losses that occur

due to friction and wear at all levels, where rubbing, grinding, polishing, and cleaning of

surfaces take place. Tribological parameters include surface roughness, mechanisms of

adhesion, friction and wear, and physical and chemical interactions of lubricants (if present).

Interacting surfaces must be understood for optimal function and long-term reliability of

components and devices and economic viability. Basic understanding of the nature and

consequences of materials interaction at the atomic and molecular level leads to the rational

design of materials for the specific applications. Micro and nanotribology are new areas of

Tribology when one tries to improve tribological properties by using respectively fillers

(Silicon carbide, Alumina, Silicon dioxide, Graphite, Zirconium oxide and Nano-clay etc.)

with sizes in the μm or nm range. Tribological techniques (equipment and methods) designed

for testing on those small scales represent a growing area. From the viewpoint of materials

users, it is advantageous to replace metal parts in various industries such as manufacturing of

cars, airplanes, bearings, gears, etc. by polymer based materials (PBMs). The advantages

include lower density, less need for maintenance, and also lower cost [4].

1.2 Composite Materials

Rapid advancement in industrial activities, particularly in the last few decades has

resulted in the need for developing new multifunctional materials that possess unique

combination of properties. However, conventional engineering materials are unable to meet

this requirement of such special combination of properties such as high strength coupled with

low density. This paved way for the emergence of new class engineering materials –

composites.

Composites are materials consisting of two or more chemically distinct constituents,

on a macro-scale, having a distinct interface separating them. One or more discontinuous

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phases are, therefore, embedded in a continuous phase to form a composite. The

discontinuous phase is usually harder and stronger than the continuous phase and is called the

reinforcement, whereas, the continuous phase is termed as the matrix. In general, fibers are

the principal load carrying members while the matrix keeps them at the desired location and

orientation, acts as a load transfer medium between them, and protects them from

environmental damages [5].

The primary functions of the matrix are to transfer stresses between the reinforcing

fibers/particles and to protect them from mechanical and/or environmental damage whereas

the presence of fibers/particles in a composite improves its mechanical properties such as

strength, stiffness etc. The objective is to take advantage of the superior properties of both

materials without compromising on the weakness of either. The reinforcements impart their

special mechanical and physical properties to enhance the matrix properties. A synergism

produces material properties unavailable from the individual constituent materials, while the

wide variety of matrix and strengthening materials allows the designer of the product or

structure to choose an optimum combination.

Composites are used in aircraft, helicopters, space-craft, satellites, ships, submarines,

automobiles, chemical processing equipment, sporting goods and civil infrastructure, and

there is the potential for common use in medical prosthesis and microelectronic devices.

Composites have emerged as important materials because of their light-weight, high specific

strength and stiffness, excellent fatigue resistance and outstanding corrosion resistance

compared to most common metallic alloys such as steel and aluminium. Other advantages of

composites include the ability to fabricate, directional mechanical properties, low thermal

expansion coefficients and high dimensional stability. It is the combination of outstanding

physical, thermal and mechanical properties that makes composites attractive to use in place

of metals in many applications, particularly when weight-saving is critical [6].

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1.2.1 Classification of Composite Materials

Generally, composites are described through either of the constituents, the matrix or

the reinforcement. Based on the matrix material, composites are classified as:

i. Metal Matrix Composites

ii. Ceramic Matrix Composites

iii. Polymer Matrix Composites and

iv. Hybrid Composites

1.2.1.1 Metal matrix composites (MMCs)

In MMCs, ceramics or metals in the form of fibers, whiskers or particles are used as

reinforcements in the metal matrix. Most commonly used matrices are aluminum,

magnesium, copper, titanium and zinc and their alloys. The most commonly used

reinforcements are silicon carbide, alumina, boron, graphite and fly ash. In comparison with

most polymer matrix composites, MMCs have certain superior mechanical properties, namely

higher transverse strength and stiffness, greater shear and compressive strengths and better

high temperature capabilities. Metal matrix composites are used for light weight as well as

for high temperature applications.

1.2.1.2 Ceramic matrix composites (CMCs)

Ceramic composites are being used in recent years. These composites are mainly used

for high temperature applications and in electronic industries. In CMCs, the matrix materials

are ceramics and reinforcements are either metals or ceramics. Ceramic matrix composite

development has lagged behind the other composites for two main reasons. First reason is

that most of the processing routes for CMCs involve high temperatures and can only be

employed with high temperature reinforcements. The second reason that has hindered the

progress of CMCs is concerned with high temperatures usually employed in production. The

differences in the coefficients of thermal expansion between the matrix and reinforcement

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lead to thermal stresses on cooling. The best example for the ceramic matrix composite

components is concrete. Metal and Ceramic matrix composites find relatively few

applications compared to Polymer matrix composites because of the involved processing

methods and high cost.

1.2.1.3 Polymer matrix composites (PMCs)

Synthetic polymers are attractive engineering materials because they offer good

strength to weight ratio as components often requiring little post- casting surface treatment,

prior to use.

Figure 1.1 Classification of polymer composites based on reinforcements [5].

Fiber reinforced composites

Single layered

composites

Multi layered

composites

Laminates

Continuous fiber

reinforced composites

Composites

Particulate reinforced composite

Random

orientation

Hybrids

Discontinuous fiber

reinforced composites

Preferred

orientation

Unidirectional

reinforcement

Bidirectional

reinforcement

Random

orientation

Preferred

orientation

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The mechanical properties of common polymers when compared to metals are,

however, not very good and are inadequate for many structural applications. In particular,

their strength and stiffness are low compared with metals and ceramics. This means that there

is a considerable benefit to be gained by reinforcing polymers with fibers or fillers.

Polymer matrix composites have particularly attracted a wider usage and lot of

interest because of their relative ease of processing, low density, desirable electrical and

thermal properties and excellent chemical and corrosion resistance. These find wide

applications ranging from specialized functions in aerospace, automotive, electronics

engineering to day-to-day consumer industries like construction and transport. Classification

of polymer based composites on the nature of reinforcement is simplified and presented in

Figure 1.1 [5].

1.3 Polymers

Polymer matrix composites, as the name indicates, consist of an engineering polymer

as the matrix material. The term 'engineering polymer' is often replaced by 'engineering

plastics'. The term engineering polymer is defined as a synthetic polymer resin-based material

that has a load bearing abilities and high performance characteristics, which permit it to be

used in the same manner as metals or ceramics [13]. Other properties of engineering

polymers include mouldability and a good balance of mechanical properties.

Engineering polymers are of the thermoplastic and thermosetting type. A

thermoplastic is one, which dissolves in selected solvents and which may be heated and

cooled, reversibly, without decomposition. Few of the well-known thermoplastics used in

composites are polyamide (PA), polycarbonate (PC), acetals, polyethylene (PE),

polyetheretherketone (PEEK), poly vinyl chloride (PVC) and polystyrene (PS).

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Unlike, thermoplastics, thermosetting polymers do not dissolve in solvent and do not

get softened on heating. Thermosetting resins are usually low viscosity liquids or low

molecular weight solids that are formulated with suitable additives known as cross-linking

agents to induce curing and with filler or fibrous reinforcements to enhance properties as well

as thermal and dimensional stability. Thermosetting polymers become permanently hard

when heat is applied and do not soften upon subsequent heating. During the initial heating,

covalent cross-links are formed between adjacent molecular chains. These bonds anchor the

chains to resist motions and the cross linking is generally extensive. Hence, thermoset

polymers are harder, stronger, and more brittle than thermoplastics and have better

dimensional stability.

The thermosets include vinyl esters, polyesters, phenolic and epoxy resins. Some of

the thermoplastics and thermosets with their characteristics and applications are listed in

Table 1.1 [14]. Limited water resistance, working temperature range and shrinkage associated

with polyesters and phenolic resins have made the epoxies to stand out, despite being a little

on the higher side when viewed from cost considerations. Epoxies offer excellent water

resistance, higher working temperatures and very low shrinkage coupled with easy

processability. Finally a third category of polymers may be mentioned here, which display

rubber like elasticity called 'elastomers'. Natural rubber (NR), acrylonytrile butadiene rubber

(ABR) and styrene butadiene rubber (SBR) are to be mentioned in this class. These are used

in specific applications where large deformations are required or where toughening with

fillers in some of the polymers are called for. Phenolic resins (like the popular one and

commercially known as Bakelite) find wider applications, especially in non-engineering day-

to-day applications involving the plastic material. These can be compounded with a large

number of fillers or resins to form blends for improved properties [15].

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1.3.1 Thermoplastics

Unlike thermoset resins, thermoplastic are not cross-linked; rather they derive their

strength and stiffness from the monomer units, which are of very high molecular weight. The

thermoplastics are anisotropic in nature and tend to melt. Composites with thermoplastics are

generally processed by injection molding and extrusion process. Polypropylene (PP),

polycarbonates (PC) and polyamides (PA) come under this type.

Polypropylene is a thermoplastic polymer that was developed in the middle of the 20th

century. Over the years, polypropylene has been used in a number of applications, most

notably as fiber for carpeting and upholstery for furniture and car seats. One of the main

drawbacks to polypropylene is the product has a resistance to the addition of paint or ink once

the cooling process has completed.

Nylon has many advantages like ability to be very lustrous, semilustrous or dull. It has

very good durability, high elongation and excellent abrasion resistance. But it has the

disadvantages like high moisture pick-up with related dimensional instability, high shrinkage

in molded sections, attacked by oxidizing agents, strong acids and bases.

1.3.2 Thermoset Resins

These are generally high-density liquid polymers, which are converted into hard

brittle solids by the process of curing. On curing, these materials form a covalently-bonded

three-dimensional network as shown in Figure 1.2. The mechanical properties of the

composite depend upon this networking. Thermoset resins are isotropic in nature, the most

important property of this category being the response to heat. It does not melt on heating

once curing is over. However, a loss in stiffness at the heat distortion temperature is noted.

Compared to polyesters, epoxies have better toughness and environmental resistance, low

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moisture absorption and much less shrinkage during curing. These details have been

compiled by Seymour [13].

Table 1.1 Characteristics and uses of a few polymeric materials.

Material Type Major Characteristics Typical applications

Thermosetting polymers

Epoxies

Rigid, clear, tough, chemical resistant,

excellent adhesion properties, high

resistance to cracking, low curing

shrinkage

Adhesives, coatings, embedding,

potting, electrical components,

pump components, cardiac

pacemakers

Polyesters

(unsaturated)

Rigid, clear/opaque, tough, chemical

resistant, fire resistant, high strength, low

creep, good electrical properties and low

temperature impact resistance, low cost

Boat hulls, building panels, car

bodies, lorry cabs, tanks and

ducting, compressor housings,

embedding and coatings

Vinyl esters Rigid, translucent, good corrosion

resistance, low viscosity

Chemical tanks, ducts, piping,

process equipment (partially in

corrosive environments)

Thermoplastic polymers

Nylons

Rigid, translucent, tough, hard wearing,

fatigue and creep resistant, resistant to

fuels, oils, fats and most solvents

Gear wheels, bushings, zips,

pressure tubing, synthetic

fibers, bearings carburettor

parts

Polyethylene

(low density)

Flexible, translucent/waxy, durable,

weatherproof, good low temperature

toughness (-60°C), easy to process, low

cost, excellent chemical resistance

Squeeze bottles, toys, utility

kitchen ware, high frequency

insulation, garment bags,

chemical tank linings

Polyurethane

(thermoplastic)

Flexible, clear, elastic, wear resistant,

impermeable

Soles and heels for sport shoes,

football boots, hammer heads,

seals

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Chain

Cross-link

Figure 1.2 Network polymer (Thermoset).

In general, curing is carried out at a single or multiple higher temperatures. However,

the curing can be achieved at room temperature. After the optimum cross-linking is over, the

composite is further cured at a relatively higher temperature for a shorter time period, known

as post curing.

1.3.3 Epoxies

Epoxies have found a special place in the family comprising thermoset engineering

polymers because of their excellent mechanical properties with chemical and corrosion

resistance as evident from the listings made in the Table 1.1. Moulded or cast epoxies have

excellent dimensional stability and low shrinkage. Hence, these are used as dies for stamping

metal sheets and as models for production articles. Another attribute, which has placed

epoxies above others, is the easy processability with the addition of a curing agent and with

or without application of heat [16]. Epoxy prepolymers contain epoxide end groups (Figure

1.3a) and pendant hydroxyl groups as the repeating units in the chain. Hence, these can be

cured or hardened by addition of polyamines, which react with epoxide groups at room

temperature, or by the addition of cyclic anhydrides, which react with hydroxyl pendant

groups at elevated temperatures.

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Most of the epoxies are prepared by the reaction of epichlorohydrin and bisphenol-A.

Owing to the presence of pendant hydroxyl groups, epoxies bond well to the other materials.

A common version of epoxy based groups used in engineering applications is the diglycidyl

ether of bisphenol-A (DGEBA) that contains two epoxide groups one at each end of the

molecule as shown in the chemical representation cited in Figure 1.3b [15].

O

C C

Figure 1.3 (a) Epoxide group

CH3

CH3

O O

CH3

CH3

O OCH CH2

OH

CH2 CH2 CHCH2

O

CH2 CHCH2

O

Figure1.3 (b) Structure of DGEBA [15]

Epoxy prepolymers are hardened or ‘cured’ to form a rigid shape by the addition of a

curing agent, as mentioned above, with or without application of heat. The curing or

hardening reaction involves addition of these in small amounts, which initiates transformation

of liquid resin to solid state. The hydrogen atoms of the curing agents react with epoxide

groups at room temperature or with pendant hydroxyl groups at elevated temperatures to

form a network of cross links. The density or the extent of these cross-linking determines

many of the physical and mechanical properties of the polymer. The cross linking is also

responsible for higher glass transition temperature (Tg) of epoxies, which determines the

operable range without losing the stiffness properties.

Many polyamines can be used to cure liquid epoxy prepolymer. To mention a few,

diethylene triamine (DETA) and triethylene tetramine (TETA) are used frequently for curing

at room temperature. Since, these reactants are toxic and skin irritants, adequate ventilation

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must be provided and the skin of the applicator must be protected from contact with the

curing agents to prevent dermatitis.

1.4 Reinforcements

As emphasized earlier, reinforcements are responsible for providing superior levels of

strength and stiffness to the composites. In a continuous fiber-reinforced composite, the fibers

provide virtually all the strength and stiffness. Even in particulate-reinforced composites,

significant improvements were obtained. They can be broadly divided into two types viz.,

fillers and fibers, depending on their shape and nature.

1.4.1 Fibers

Fibers are important variety of reinforcing agents. They have good strength and

directional property. Carbon, glass and Kevlar are the most important fibrous materials used

for making composites [17]. Among these, the glass fibers are quite popular and are available

in different types, depending on their composition like E-glass, C-glass, and S-glass. Fibers

are also used as unidirectional woven clothes.

1.4.2 Fillers

Fillers are materials often added to polymers to improve tensile and compressive

strength, tribological characteristics (including abrasion) and dimensional and thermal

stability.

1.4.2.1 Microfillers

A wide range of microfillers are used starting from metallic powders to elastomeric

fillers. Oxides such as SiO2, ZrO2, Al2O3, TiO2, CuO, CuS, CuF2, PbS, CaS and boron nitride

are some of the commonly used metallic fillers. Among other inorganic fillers, metallic

powders, copper and mild steel have been used [18]. Other than particulate form, few other

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fillers like mica have been used which appear as flakes [19]. Organic fillers, which have

caught the attention of composite manufacturers, are mainly elastomers.

1.4.2.2 Silicon carbide

Silicon Carbide (SiC) is the only chemical compound of carbon and silicon. It was

originally produced by a high temperature electro-chemical reaction of sand and carbon.

Silicon carbide is an excellent abrasive and has been produced and made into grinding wheels

and other abrasive products for over one hundred years. Today the material has been

developed into a high quality technical grade ceramic with very good mechanical properties.

It is used in abrasives, refractories, ceramics, and numerous high-performance applications.

The material can also be made an electrical conductor and has applications in resistance

heating, flame igniters and electronic components. Structural and wear applications are

constantly developing.

Key Properties

Low density

High strength

Low thermal expansion

High thermal conductivity

High hardness

High elastic modulus

Excellent thermal shock resistance.

1.4.2.3 Silicon dioxide

The chemical compound silicon dioxide (SiO2), also known as silica (from the Latin

silex), is an oxide of silicon with the chemical formula SiO2. It has been known for its

hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well

as in the cell walls of diatoms.

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Silica is manufactured in several forms including fused quartz, crystal, fumed silica

(or pyrogenic silica, trademarked Aerosil or Cab-O-Sil), colloidal silica, silica gel, and

aerogel.

Silica is used primarily in the production of glass for windows, drinking glasses,

beverage bottles, and many other uses. The majority of optical fibers for telecommunications

are also made from silica. It is a primary raw material for many white ware ceramics such as

earthenware, stoneware, porcelain, as well as industrial Portland cement.

Silica is a common additive in the production of foods, where it is used primarily as a

flow agent in powdered foods, or to absorb water in hygroscopic applications. It is the

primary component of diatomaceous earth which has many uses ranging from filtration to

insect control. It is also the primary component of rice husk ash which is used, for example,

in filtration and cement manufacturing.

Key Properties

Near zero thermal expansion

Exceptionally good thermal shock resistance

Very good chemical inertness

Can be lapped and polished to fine finishes

Low dielectric constant and Low dielectric loss

1.4.2.4 Alumina

Alumina is available from Ceramaret with a purity of upto 99.98%. It is a

polycrystalline material with a grain size of 1 to 10 microns, an average density of 3.90 g/cm3

and a hardness about 2000 VHN. Alumina filler is an inorganic material that has the potential

to be used as filler in various polymer matrices. Aluminium oxide (Al2O3) commonly referred

to as alumina, can exist in several crystalline phases which all revert to the most stable

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hexagonal alpha phase at elevated temperatures. This is the phase of particular interest for

structural applications. Alumina is the most cost effective and widely used material in the

family of engineering ceramics. The raw material from which, we can have good mechanical

properties. Some of the properties are listed below.

Key properties

Good wear resistance.

Good dielectric resistance.

Good thermal conductivity.

High strength and stiffness.

Resistance to abrasion.

High hardness.

Resistant to thermal shock.

1.5 Advantages of Fiber/Filler Reinforced Polymer Composites

The major advantages of PMCs compared to un-reinforced materials are as follows:

Higher strength-to-density ratios

Higher stiffness-to-density ratios

Ability to tailor properties to meet wide-ranging performance specifications

Cost effective manufacturing processes

Moulding to close dimensional tolerances, with their retention under in-service

conditions

Good impact, compression, fatigue and electrical properties

Lower coefficients of thermal expansion

Improved abrasion and wear resistance

Excellent chemical and corrosion resistance

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Excellent fire resistance

Improved damping capabilities

Polymers and their composites find extensive usage in many engineering applications

as an alternative product to metal based ones. As the present study deals with the tribological

behaviour of polymer based composites, a brief introduction about the basic concepts of

tribology is presented below.

1.6 Basic Concepts of Tribology

1.6.1 Wear

Wear is described as the progressive loss of material from the operating surface due to

the relative motion between that surface and the contacting surface known often by the term

counter surface [7]. Wear of metal occurs by the plastic deformation of the surface and by

detachment of particles, which form wear debris. In metals, this process may occur by contact

with other metals, non-metallic solids, flowing liquids or solid particles or liquid droplets

entrained in the flow of gases. Till date, much of the knowledge on tribological behaviour of

composite materials is empirical, and limited predictive capability exists. Nevertheless,

attempts have been made to generalize the tribological behaviour of composite materials and

to understand the contribution of interdisciplinary sciences to tribological behaviour [20]. The

wear process may be generally classified into adhesive, abrasive, erosive, impact, corrosive,

fretting and so on. Of these, adhesive and abrasive wear phenomena are generally

encountered in engineering applications. In contrast to metals and ceramics, polymers exhibit

lower coefficients of friction, with values typically between 0.1 and 0.5, whether self-mated

or sliding against other materials. They are therefore often used without lubrication in

tribological applications, usually sliding against harder counterfaces. Their strengths are also

much lower and it is therefore reasonable to consider metallic or ceramic counterfaces when

sliding against polymers to act as rigid bodies. Nearly all the deformation due to contact or

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sliding takes place within the polymer, and the surface finish of the hard counterface has a

strong influence on the mechanism of the resulting wear. As the emphasis in the present work

is laid on abrasive wear phenomena of polymer composites, the adhesive and abrasive wear

aspects is described briefly in the section to follow.

1.6.1.1 Adhesive wear

Adhesive wear is defined as the process occurring due to sliding or rolling contact

between two solid surfaces leading to material transfer between the two surfaces or loss from

either surface. Wear may result from adhesion between the polymer and counterface which is

smooth and involve deformation only in the surface layers of the polymer. On the other hand,

if the counterface is rough, then its asperities will cause deformation in the polymer to a

significant depth; wear then results either from abrasion associated with plastic deformation

of the polymer, or from fatigue growth in the deformed region. When two surfaces slide on

one another, their topographic features allow only the contact of asperity peaks as shown in

Figure 1.4 [21]. These contact points or 'junctures' represent the real area of contact. The

wear due to the contact of two surfaces follow an equation by Archard on an asperity contact

model:

(1.1)

Where:

V - is the wear volume,

S - is the sliding distance,

L - is the normal load,

H - is the indentation hardness value of softer of wear pair and

k - is a constant which effectively is required to make the formula fit really within an order of

magnitude.

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This equation represents a steady state wear. However, for all practical purposes,

three regions of wear can be identified as shown in Figure 1.5 [22].

Normal Load

Junctures : Real area ofcontact where plasticdeformation may occur

Gross contact area

Figure 1.4 The real contact area (junctures) and apparent (gross) contact area of two

surfaces [21].

REGION I

REGION II

REGION III

Sliding distance

Volu

me

loss

in w

ear

REGION IRUNNING INWEAR

REGION IIMILD WEAR

REGION IIISEVERE WEAR

Figure 1.5 Variation of sliding wear volume with sliding distance [22].

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Region I represent faster wear during the running in period, region II a slower and

steady state wear and the region III represents the terminal conditions. Under high load

conditions, both Region II and III loose their distinct identity. On the other hand, Region II is

prolonged in lubricated systems. The wear in different regions is influenced by various

factors such as load, speed, oxidation, shape and size of the debris, onset of fatigue and micro

cracks etc.

The wear process has been explained in the literature from the point of view of

surface and subsurface damage [23] known as delamination theory (Figure 1.6). This

delamination approach involves the following steps.

i. The deformation pattern in the form of dislocations and vacancies appear due to sliding

action at the surface and subsurface.

ii. The formation of voids at the subsurface layers occurs due to the continued plastic

deformation. They increase further in the presence of inclusions and large precipitate

particles at the surface.

iii. The voids coalesce either due to the growth or by shearing action of the surrounding

material around hard particles due to the formation of cracks parallel to the wearing

surface.

iv. In continuation of the process, the crack after reaching a critical length due to shearing

action yields sheet like wear particles or debris.

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PLASTIC DEFORMATION

SUB-SURFACECRACK NUCLEATION

WEAR SHEETFORMATION

Figure 1.6 Schematic representations of the various stages involved in the formation of

delamination wear sheets [23].

. In the present investigation, as emphasis is laid on the abrasive wear behaviour of

glass epoxy composites, these aspects are covered in detail in the following sections.

1.6.1.2 Abrasive wear

Abrasive wear is defined as the wear due to hard protuberances forced against and

moving along a solid surface. Mechanisms of abrasive wear can involve both plastic flow and

brittle fracture. It is reported [24] in the literature that the factors responsible for abrasive

wear are hardness, shape and size of the abrading material. Abrasive wear is generally

classified into the following two types [1, 24] .

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1) Two-body abrasion where a hard rough body plough into a softer body and

2) Three-body abrasion where a third body (usually hard granular matter) placed between

the sliding surfaces gets crushed and cut grooves.

The two-body and three-body abrasive wear is shown in Figure 1.7 (a-b). The two-

body wear is generally a low stress type of wear with particles being transported across the

surface with little breakdown in particle size of the abrasive. In three-body wear, due to the

high stress, the particles are deliberately reduced in size. According to Rabinowicz [25] the

abrasion model is as shown in Figure 1.8. The wear volume is expressed as follows:

(1.2)

Where:

V - is the wear volume in mm3, L - is the normal load in Newtons, D - is the abrading

distance in m, H - is the hardness and 2 - is the abrasive cone angle.

Two-body abrasion

Three-body abrasion

Figure 1.7 (a) Two-body and (b) Three-body wear.

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Volume of softermaterial removed

W

2

Adhesive cone

r

2° 2

Figure 1.8 Abrasive wear model from Rabinowicz [25].

A more general relationship suggested for the abrasive wear per unit sliding distance

is:

(1.3)

Where:

K1 - is the probability term as in adhesive wear,

K2 - is the mean proportion of the groove volume removed when wear debris formed and

K3 - is a function of the shape of the particle.

1.7 Factors Affecting Wear

Wear resistance does not form a part of the basic material properties such as thermal

conductivity, melting point or density. The wear phenomenon is affected by various factors

including processing parameters. Some of the key factors influencing the wear rate [26] are

given below.

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i. Design criteria - Transmission of load, type of motion, degree of lubrication,

temperature and environmental factors.

ii. Operating conditions - Speed, contact area, contact pressure and surface condition.

iii. Abrasive characteristics - Hardness, shape, size and their distribution.

iv. Material properties - Composition, hardness, microstructure, work hardening ability

and resistance to corrosion.

Wear is one of the most commonly encountered industrial and domestic problems

leading to replacement or repair of engineering components. Several applications of the FRP

composites require low friction and better wear performance, for example, gears, seals,

bushes, bearings, chute liners and components used in earth moving and agricultural

machineries.

The principal tribological parameters that control the friction and wear performance

of FRP composites can be classified into two categories:

1) Extrinsic to the material undergoing surface interaction i.e., the load normal to the tribo-

contact, the sliding velocity, the sliding distance (transient and steady state period), the fiber

orientation, the environment and temperature, the surface finish and the counterpart.

2) Intrinsic to the material undergoing surface interactions i.e., the reinforcement or filler

type, reinforcement or filler size and its distribution, the reinforcement or filler shape, the

matrix microstructure and finally the reinforcement or filler volume fraction.

For tribologically loaded components, the coefficient of friction, the mechanical load

carrying capacity and the wear rate of the materials determine their acceptability for

industrial applications. Polymer based composite materials are the ones employed in such

applications owing to their ever increasing demand in terms of stability at higher loads,

temperatures, better lubrication and wear properties.

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Design lubrication

Define application

Define properties required

Mechanical design

Materials selection

Engineer substrate /engineer surface

Figure 1.9 Sequence of steps in designing a tribological component involving surface

engineering [27].

The choice of materials to be selected from the stand point of strength and tribological

considerations is very important. Figure 1.9 [27] outlines the steps involved in designing a

tribological system, although it must be appreciated that for the most effective design some of

the steps will be iterative.

1.8 Basics of Machining

1.8.1 Machining

Conventional machining, one of the most important material removal methods, is a

collection of material-working processes in which power-driven machine tools, such as lathe,

milling machine, and drilling machine are used with a sharp cutting tool to mechanically cut

the material to achieve the desired geometry. Machining is a part of the manufacturing of

almost all metal products. It is not uncommon for other materials to be machined.

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1.8.2 Machining Operations

The three principal machining processes are classified as turning, drilling and milling.

Other operations falling into miscellaneous categories include shaping, planing, boring,

broaching and sawing. Turning operations are operations that rotate the work piece as the

primary method of moving metal against the cutting tool. Lathes are the principal machine

tool used in turning. Milling operations are operations in which the cutting tool rotates to

bring cutting edges to bear against the work piece. Milling machines are the principal

machine tool used in milling. Drilling operations are operations in which holes are produced

or refined by bringing a rotating cutter with cutting edges at the lower extremity into contact

with the work piece. Drilling operations are done primarily in drill presses but not

uncommonly on lathes or mills. Miscellaneous operations are operations that strictly speaking

may not be machining operations in that they may not be chip producing operations but these

operations are performed at a typical machine tool.

For example, a work piece may be required to have a specific outside diameter. A

lathe is a machine tool that can be used to create that diameter by rotating a metal work piece,

so that a cutting tool can cut metal away, creating a smooth, round surface matching the

required diameter and surface finish. A drill can be used to remove metal in the shape of a

cylindrical hole. Many of these same techniques are used in woodworking. More recently,

advanced machining techniques include electrical discharge machining (EDM), electro-

chemical erosion, laser or water jet cutting to shape metal work pieces.

1.8.3 Machining of Composite Materials

Why machining? Machining involves the removal of any extra or unwanted material.

Some of the most common machining processes are drilling, turning and milling. Earlier

composites were machined like metals. But poor surface finish and faster tool wear led to the

further study of composite machining. Unlike metals, composites need separate tools and

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working conditions. Although tools used for machining of metals can still be used for

composites, care must be taken to maintain optimum levels of feed rate, thrust force, and

other factors. Metal tools tend to wear out faster when used for machining of non-metals. One

of the main advantages of composites has been the fact that an entire part can be

manufactured. This minimizes the machining of composites. However with part integration,

sometimes composites need to be joined to form a larger part, which means that a certain

amount of machining needs to be done for composites too.

1.8.4 Drilling

Drilling is an operation of producing the circular hole in metal or composite materials

by means of a revolving tool called drill. The holes created are used primarily for fastening

one component to another, for passing coolants, and for wiring purposes. Drilling has been

widely used to make holes in metals, but due to its availability, it is now being used to make

holes in composite materials. Drilling is widely used because it is a more cost-effective

process than laser beam cutting and because there are not many other processes that produce

a deep circular hole. Drilling is often used in the machining of composites, because of readily

available machinery and because it is simply more cost effective than the more advanced

method of laser beam cutting. Although composites are not metals, industries previously cut

them like metals. This resulted in tool wear, and poor surface finish. Many researchers then

studied the reasons for this. Although similar to metal drilling, composite drilling requires

special drill bits, which are usually coated with tungsten carbide or titanium nitride. Some of

the major factors that determine tool wear are feed rate, geometry of the drill bit, and many

other factors. One of the most popular drill bits has been the twist drill, widely used in the

drilling of metals but lately also used in drilling composites. The most common defect

observed in a hole drilled in composite material is delamination is usually known as the peel

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up of the material. At the exit point, delamination occurs when the drill bit tries to push

through the material.

1.8.4.1 Cutting forces during drilling

Thrust force during drilling can be defined as the force acting along the axis of the

drill during the cutting process.

Mz

Fz

Fz

Fz

Mz

Fz =Thrust force, N

= Torque, N-m

Figure 1.10 Schematic diagram showing the forces acting on the drill.

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The various cutting forces during drilling process are shown in Figure 1.10 Cutting

forces help to monitor the tool wear, since the forces increase with tool wear. Thrust force is

also used to monitor tool wear and, in turn, monitor the tool life. Tool failure can occur if tool

wear is not monitored. Other than being an important factor in the monitoring of tool wear,

thrust force is considered to be the major contributor of delamination during drilling.

Vibratory drilling has been known as one of the methods to reduce thrust force during drilling

of composites. If the thrust force is known, then the machining efficiency can be increased

and higher quantities can be machined. Cutting forces act on the drill as it penetrates through

the work piece removing material and thus generating power. During drilling some

unbalanced radial cutting forces act on the tool which is due to the asymmetric sharpening.

This work deals with the prediction of thrust force at which delamination will occur during

drilling of composites.

Torque is the measure of how much a force is acting on the material. The torque

required to operate a drill depends on various factors. Results can be obtained by considering

the drill diameter and the feed and the material being drilled. Torque in general is said as the

cross product of the force acting perpendicular to the direction and the distance from the

point of its application. The present study deals with the performance evaluation of carbide

and HSS tools by monitoring process parameters like thrust force, torque at various cutting

conditions and optimize the drilling parameters in terms of delamination factor, surface finish

etc.

1.8.4.2 Drilling quality

Drilling quality is characterized by the extent of delamination damage, surface

roughness, hole edge quality, and roundness. Delamination is an intrinsic problem in drilling

layered materials because the drill feed motion and the resulting thrust force acting normal to

the stacking plane tend to separate the plies along the weak epoxy layer in between. In

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addition, different drill point geometries behave differently in terms of delamination

response. Delamination leaves cracks between the plies in the drilled component, which may

result in degradation of its mechanical performance. Hole surface roughness is measured on

the walls of the hole in the direction of the feed. It is influenced by the fiber orientation

around the periphery of the hole. The chip formation mechanism resulting surface roughness

is critically dependent on fiber orientation. The drilled edge quality, roundness, and

dimensional accuracy are influenced by delamination, tool wear, and cutting temperatures.

Distortions to the hole may occur due to the different thermal expansion coefficients along

and transverse to the fiber directions and between the polymer matrix and the reinforcement

fibers. This may lead to residual stresses and dimensional variations in the hole diameter. The

studies have shown that proper selection of the drilling parameters and practices is a good

strategy for reducing or eliminating some of the problems prone to drilling. The following

sections are devoted to the details of the drilling quality and the influencing factors.

1.8.4.3 Delamination

Among the many undesirable features produced by drilling, inter laminar

delamination is considered to be the most important one.

(a) Push-out (b) Peel-up

Figure 1.11 Schematic diagram of push-out delamination at exit and peel-up

delamination at entry.

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Delamination is a major concern in drilling FRPs because it may severely affect the

structural integrity and long-term reliability of the machined component. Drilling-induced

delamination occurs at the entry and exit planes of the work piece as illustrated schematically

in Figure 1.11 These are called push-out and peel-up delamination. Two different

mechanisms are responsible for delamination on each side of the laminate.

1.8.4.4 Surface roughness

In a machining process, a specific surface geometry is produced as a result of the

prescribed machine tool kinematics. This surface geometry is called an ideal or theoretical

surface geometry, which follows a repeated pattern. In real life, however, the actual machined

surface deviates from the ideal surface because of the occurrence of tool wear, machine

vibrations, material inhomogeneity, and other factors not related to machine tool kinematics.

The actual machined surface may not have a regular geometry.

y

Mean line

Sampling length, L

maxy

ymin

Waviness

Ideal surface(Lay)

Roughness

R

t

R

p

v

30,59

Rt

Figure 1.12 Schematic representation of a machined surface.

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These effects result in what is called natural surface finish. Figure 1.12 shows the

different definitions used to describe machined surface geometric characteristics. The surface

profile is typically described by its lay, waviness, and roughness. Lay is the macroscopic

contour of the surface and describes the direction of the predominant surface pattern. The

term lay is mostly used to describe flat surfaces and shape is used for contoured surfaces.

Errors in lay and shape result from misalignment of machine components and from

distortions resulting from clamping forces. Waviness is the recurrent deviations from an ideal

surface that are relatively of large magnitude (>0.1mm). These deviations result from

deflections in the machine tool and cutting tool, from errors in the tool geometry and from

machine vibrations. Roughness is the finely spaced irregularities or irregular deviations

characterized by short wavelength as shown in Figure 1.12. Roughness is affected by tool

shape and feed (ideal surface finish) as well as by machining conditions (natural surface

finish).

Surface roughness is most often used to characterize machined surfaces. It is

commonly quantified by statistical parameters such as the arithmetic mean value Ra,

maximum peak to valley height Rt, maximum peak to mean height Rp, mean to valley height

Rv, and ten point average height Rz. The machined surface profile is most commonly

measured by a stylus surface profilometer.

1.9 Chapter Schemata

This thesis, organized on the basis of experimental work contains six chapters.

Chapter 1 gives a brief introduction to polymer matrix composites, classification and wear.

Chapter 2 highlights the literature review on mechanical, tribological properties and drilling

of polymer based composites. This chapter also includes the objectives of the present

investigation.

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The fabrication process, experimental methodology, materials and equipment used for

the experimentation and their procedure is explained in chapter 3. Chapter 4 covers the

experimental investigation on physical and mechanical properties and the effect of various

tribo-parameters on two-body and three-body abrasive wear of fillers (silicon dioxide and

alumina) filled and glass fabric reinforced epoxy hybrid composites. Chapter 5 covers the

detailed experimental investigations of drilling of particulate (silicon dioxide and alumina)

filled and glass fabric reinforced epoxy hybrid composites. The conclusions and suggestion

for future work are given in chapter 6.