MODULE 2

WEAR

INTRODUCTION

Need for Maintenance:

1. Deterioration

2. Breakage /Failure

3. Obsolescence

Reasons 1 & 2 can be attributed to friction and wear between contacting surfaces

WEAR: The removal of material from or the impairment of a solid surface resulting from friction or impact

Wear - occurs when two mating parts are in relative motion

Generally considered to be undesirable.

There are instances where wear is useful - polishing, grinding and other surface finishing operations, lead wearing in a pencil to make writing possible etc.

Wear results in material removal from the surfaces in contact

It can never be avoided, but can only be controlled by the proper lubrication and use of wear resistant material

SURFACE TOPOGRAPHY

All solid surfaces are uneven.

Surfaces composed of peaks and valleys

called ASPERITIES

When su

When 2 nominally plane and parallel surfaces are brought into contact, contact initially occurs at only a few points.

When normal load is increased, the surfaces move closer together and a larger number of asperities come into contact

True/Actual contact area < Apparent/Nominal contact area (geometrical area measured)

When relative motion (sliding) takes place between surface, these asperities come into contact and tries to resist sliding, causing friction and wear

FRICTION

One of the first people to investigate friction

was Leonardo da Vinci

Friction is the force resisting the relative

motion of solid surfaces, fluid layers, and

material elements sliding against each other

Types of friction:

1. Dry friction

static friction ("stiction") between non-moving surfaces

kinetic friction between moving surfaces

2. Fluid friction

3. Lubricated friction etc.

LAWS OF SLIDING FRICTION

empirical relations

Three Laws of Friction

First two laws - Amontons Laws

First Law: The frictional force (Ff) is proportional to the

normal load (N)

is independent of normal load N

Mathematically,

Where,

Ff frictional force

N total normal reaction/load at contact interface

- coefficient of friction

Value of varies from 0.001 (lightly

loaded rolling bearing) to greater than 10

(clean metals sliding against themselves in

vacuum)

Most common materials, ranges from

0.1 to 1.

NOTE: Polymers do not usually obey first

law.

Second Law

Frictional force is independent of the apparent area of contact

Experiment Normal load, held constant, apparent area of contact increased

is independent of apparent are of contact

NOTE: Second law not obeyed by POLYMERS

Third Law

Found by Coulomb

Friction is independent of sliding velocity

Friction Force to initiate sliding more than that necessary to maintain it.

Hence,

s (coefficient of static friction) > d (coefficient of dynamic friction)

d is nearly independent of sliding velocity

At very high speeds (tens or hundreds of m/s), d falls with increasing velocity

Coefficient of Friction

Independent of:

Normal Force

Apparent Area of contact

Nearly independent of sliding velocity

Depends solely on the materials of the

surfaces in contact.

WEAR

Classifying wear: 1. Based on the conditions in which the wear

occurs: 1. Presence of lubricant - Lubricated /Dry(Un-

lubricated) wear

2. Presence of abrasive particle: Abrasive Wear/ Sliding wear

2. Lim and Ashby wear classification (into 4 basic mechanisms)

Seizure Melt Oxidation Plasticity

SLIDING WEAR

Also known as adhesive wear

Terms associated with sliding wear:

Scuffing: (UK) localized surface damage due to lubrication breakdown

at high sliding speeds.

Scoring: (US) synonymous with scuffing

Galling: Severe from of scuffing

Gross surface damage

Damage resulting from un-lubricated sliding at low speeds

TESTING METHODS Used to:

Study wear mechanisms

To extract useful design data (wear rates, etc.)

TRIBOMETERS or TRIBOTESTERS instruments used for wear testing.

Several different geometrical arrangements are employed:

Ring on ring (line or face contact as in Fig. A & B)

Pin on disk (flat face or on the rim as in Fig. C & D) Most commonly used

Block on ring (Fig. E)

Pin on flat (Fig. F)

Arrangements classified into two categories:

Symmetrical Wear rates of two surfaces of same material should be

same

Not used often

E.g.: Ring on ring (Fig. A & B)

Asymmetrical Most commonly used

E.g. Pin of disk

Contact may be:

Conformal (extended nominal contact area)

Counter-formal/concentrated (point or line contact)

Of the mating pair/contact pair,

the pin/block is treated as the SPECIMEN (component for which wear rate is measured)

the disk/flat/ring is treated as the COUNTERFACE

FOUR BALL TESTER:

Method for evaluating lubricant performance

Lower 3 balls rotated together in a carrier and move relative to upper ball

Upper ball is held stationary and pressed downward under a fixed normal load

Balls made of std. rolling bearing steel

SPECIMENS AND TESTING

Specimen Dimensions:

Few mm to tens of mm

Asymmetric test specimen dimensions usually less than 25mm, while counter-face

larger.

Testing Standards:

Sphere on disk (DIN 50324)

Pin on Disk (ASTM G 99)

Block on Ring (ASTM G 77)

Quantification of Wear:

Several parameters are used to

quantify wear. The most important are:

1. Wear volume (V)

2. Wear height (h)

3. Wear rate (w) and

4. Wear coefficient (KA)

Also, is measured along with wear.

Wear volume and wear height are the parameters most commonly used by engineers and designers

They are not very useful for characterizing the process of wear in general terms, because they are heavily dependent on variables that may change significantly from problem to problem

Wear rate is defined as the volume removed per sliding distance

w= V/s

TESTING PARAMETERS 1. Loads:

Range: fractions of N to several kN

2. Nominal area of contact Varies for counter-formal contact

3. Sliding Speed: Range: fractions of mm/s to hundreds of m/s Affects rate of frictional heat dissipation, thus contact

interface temperature.

4. Duration of test

5. Atmospheric Conditions Water vapour, oxygen etc.

6. Presence of lubricant

1 & 2 combined and rep as Nominal Pressure (Load/Nominal Area)

THEORY OF SLIDING WEAR:

ARCHARD WEAR EQUATION

Developed by Holm and Archard

Highlights the main variables which

influence sliding wear

Also, gives a method to describe severity

of wear by means of WEAR

COEFFICIENT (K)

Developed mainly for metals

Derivation

Assumptions:

True contact area will be sum of individual contact areas.

This area is proportional to the normal load.

Under most conditions, the local deformation of asperities will be plastic.

Assume a single asperity contact (circular in plan/top view)

Radius a

Maximum contact in stage (c)

The fraction of the normal load supported by it = dW

dW = Pa2

Where,

P yield pressure of plastically deforming asperity (close to indentation hardness, H)

As sliding continues, it leads to formation and destruction of asperity contact junctions.

Wear is associated with removal of materials from the asperities

Volume of material removed depends on the size of asperity junction from which it originated.

Assumed that,

Volume of material removed from1 asperity junction (dV) proportional to cube of contact dimension (a3)

Volume can be taken to be a that of hemisphere of radius a

i.e., dV = (2 a3)/3

NOTE: All asperities will not give rise to wear particles. (assume a proportion does so)[ kappa]

Hence, Avg. vol. of material (dQ) worn away per unit sliding distance (for single asperity contact, through a distance of 2a)

dQ = dV/2a

i.e., dQ = ( a2)/3

Overall Wear Rate (Q) is the sum of the

contributions over the whole real area of

contact:

Q = dQ = /3 a2..(1)

Total Normal Load (W)

W = dW = P a2 (2)

From (2),

a2 = W/P..(3)

(3) in (1):

Q = kW/3P

Hence, the Archard Wear equation:

Q= KW/H Where,

Q = Overall Wear rate

K = Wear coefficient/coefficient of wear = k/3

H = Hardness of the softer material (=P)

K wear coefficient: Dimensionless Always less than unity Higher value indicates increased wear severity

In engineering applications,

(K/H) = k (dimensional wear coefficient)

is more useful.

k = Q/W volume of material removed by wear (mm3) per unit

sliding distance (m), per unit normal load on the contact (N)

Unit mm3 (Nm)-1

Helps in comparing wear rates of different class of materials. E.g.: Metals and elastomers

NOTE: It is assumed in Archard wear

equation that:

Q depends only on

normal load (W) &

Hardness or Yield Strength of softer surface (H)

According to eqn, if K is a constant for a

given sliding system:

1. Volume of material lost by wear

proportional to distance slid. (i.e., Q is a

constant)

2. If W is varied, Q should vary in proportion

Statement 1: Found to be experimentally true.

Transient behaviour is sometimes noted at the start, where wear during the initial running in period may

be higher/lower that steady state wear rate (where

equilibrium surface conditions have been established)

Statement 2: Strict proportionality not found b/w Q and normal

load (W)

Over limited ranges, Q varies directly with W.

Abrupt transitions from low to high Qs and vice versa is observed.

This case can be understood better by considering the example of sliding contact of leaded Brass

against Hard Stellite ring.

At Low loads,

Q increases with W

K 2 x 10 -6

At loads of 5 to 10 N,

Sharp increase in wear rate (100x)

Transition point

Follows archard eqn

At even high W,

Still follows archard eqn

K 10 -4

Regime of wear at low loads, below transition MILD WEAR

Regime above transition SEVERE WEAR

MILD WEAR: Fine wear debris Predominantly oxide Worn surface relatively smooth Sliding surfaces separated by oxide films with occasional

direct metallic contact

Low (0.15)

SEVERE WEAR: Larger particles Metallic debris Roughened surface Metal to metal contact High (0.25- 0.3)

Transition from mild to severe wear:

Due to change in nature of sliding contact

Extensive metal to metal contact occurs

Transition from severe to mild wear:

Occurs when two competing processes balance, i.e.,

1. Rate of exposure of fresh metal surface by severe

wear

2. Rate of oxidation of the surfaces by the

surrounding atmosphere.

Since oxide formation plays crucial role in mild wear, any factor affecting it will affect transition from severe to mild wear.

E.g.: Temperature

Temperature

Most direct factor

Temp at sliding interface depends upon: Ambient temp

Frictional power dissipation (Ff x sliding speed) (depends upon) Sliding Speed

Load

UNLUBRICATED WEAR OF

METALS As sliding conditions vary the

mechanism of wear changes

One mechanism cannot be attributed to

wear over a wide range of conditions

Main factors controlling wear mechanism:

Mechanical Stresses

Temperature

Oxidations phenomenon

MECHAINCAL STRESSES

Two types: Normal Stresses Shear stress

Normal stresses under conditions of plastic contact, it will be close to indentation hardness of softer body

But, if surfaces are smooth, closely conforming or lightly loaded, asperity contact will be elastic wear will be very slow (by elastic process like high cycle fatigue)

Shear Stress: Magnitude and position

depends upon .

For < approx. 0.3,

Max shear stress and associated flow beneath surface, and plastic strain (shear)

accumulation is low.

E.g.: Lubricated sliding systems and one having protective oxide layer.

For > approx. 0.3,

Max shear stress on the surface.

Plastic strain (shear) accumulation high.

Plasticity dominated wear mechanisms

prevail:

High Loads

Low sliding speeds

Lead to severe wear

Temperature & Oxidation

Phenomenon Metals form oxide films in air

Rate of film growth depends on temp of

interface (which may be substantially

higher than ambient)

Estimation and measurement of interface

temp difficult

All three factors are inter-related and are

influenced by LOAD and SLIDING

SPEED/VELOCITY.

Fig. below shows the effects of normal

load and sliding velocity on the extend of

mechanical damage and interface

temperature

Increased Normal Load high stresses more severe mechanical damage

Load and sliding velocity determines interface temp (control the power dissipated at the interface)

Low sliding velocity, heat generated will be rapidly conducted away , interface temp will remain low; and the sliding process will be, ISOTHERMAL within a limit.

High sliding velocity, only limited heat conduction can occur, high interface temp. and the limiting condition would be ADIABATIC

High interface temperature high

chemical reactivity of surfaces causing

rapid growth of oxide films in air.

Also - High interface temperature

reduced mechanical strength of asperities

and near surface metal.

May cause melting in extreme cases.

WEAR REGIME MAPS

Also called wear mode map

Dimensionless variables are employed:

Normalized contact pressure (normal load/nominal contact area*indentation hardness of softer material)

Normalized velocity (sliding velocity/ velocity of heat flow)

Sliding velocity given in upper abscissa/ x-axis

E.g. shown for steel (pin on disc contact in air)

Regions in the diagram developed from:

Empirical data from experiments

Simple analytical models for wear.

8 regimes can be identified

REGIME 1(SEIZURE)

Very high contact pressures

Gross seizure of surfaces

Growth in asperity junctions

Real area of contact apparent area of

contact

REGIME II (SEVERE WEAR)

High load

Relatively low sliding speed

Thin oxide layer penetrated by asperity

contact

High surface traction

Metallic wear debris is formed

Severe wear

REGIME III (MILD WEAR)

Low loads

Oxide layer in not penetrated

Low rate of mild wear due to removal of

particles of oxide layer

NOTE:

Regime II and III- thermal effect negligible.

Regime IV and V - thermal effect - important

REGIME IV (SEVERE WEAR)

High Load and speed

High frictional power dissipation

Thermal conduction ineffective

Melting occurs

Wear is rapid and severe

Metal removed as molten droplets

USED IN FRICTION CUTTING

REGIME V (MILD)

Interface temp is high , but below melting

point.

Rapid surface oxidation

Extreme type of oxidation wear takes

place

Thick oxide layer forms and deforms

plastically

Wear debris consists of oxide

REGIME VI, VII & VIII

Occurs over a narrow band of sliding

velocity

Represents transitional behaviour b/w low

speed isothermal and high speed adiabatic

conditions

Regime VI: Thermal effects begin to play role No significant rise of the interface temp. Hot-spots at asperity contacts occur

(reaching flash temperature), leading to patchy oxide growth

Mild wear occurs Wear debris composed mainly of oxides,

removed by spalling

Regime VII: At higher loads metallic asperity contact

occurs despite enhanced oxidation at hot spots

Severe Wear Metallic Debris

Regime VIII:

Higher flash temp that in regime VI

Leads to formation of martensite

Interface temperature above 9100C, so that allotropic transformation can take place

High strength of martensite provide local mechanical support to surface oxide film

Mild wear proceeds with removal of oxide

Constant sliding speed

MECHANISMS OF SLIDING

WEAR

Wear mechanisms operating in the

different wear regimes are either:

1. PLASTICITY DOMINATED WEAR

2. OXIDATIONAL WEAR