Micro-scale abrasive wear of silicon nitride, sialon–TiB2 composites

7/27/2019 Micro-scale abrasive wear of silicon nitride, sialonTiB2 composites

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Wear 258 (2005) 942952

Micro-scale abrasive wear of silicon nitride, sialonTiB2compositesand D2 tool steel using a multiple load method

A.H. Jones

Materials Research Institute, Sheffield Hallam University, Howard St., Sheffield S11WB, UK

Received 9 August 2004; received in revised form 9 August 2004; accepted 23 September 2004

Available online 23 November 2004

Abstract

The addition of TiB2 to sialon has previously been shown to lead to improvements in tribological performance when using pin-on-disc

testing. In this study the specific wear rates of Si3N4, sialonTiB2 composites and D2 tool steel have been measured using micro-scale abrasive

testing. Two variations of the micro-scale abrasive wear test have been used, free ball and fixed ball, and the results are compared. A method

has been developed for the accurate measurement of the specific wear rate using multiple loads on the fixed ball apparatus. The multiple load

method eliminates the dependence of the result on the accuracy of the balancing of the load arm. Scanning electron microscopy has been used

to study the active wear mechanisms and the effect of increasing the applied load. The severity of contact model has been evaluated as a

method for predicting the transition from three body rolling wear to two body grooving wear with varying load. Increasing the applied load

led to slurry starvation and ridge formation before the predicted two body grooving wear could occur.

2004 Elsevier B.V. All rights reserved.

Keywords: Micro-scale abrasion; Silicon nitride; Sialon; TiB2; Tool steel; Multiple load method

1. Introduction

Silicon nitride and sialon materials have been shown to

have properties that make them suitable for many tribologi-

cal applications. High hardness, high strength and resistance

to chemical attack or corrosion are all attributes that are desir-

able for good tribological performance under a wide range of

conditions. However, the relative brittleness of such ceramics

and the expense in producing components to net shape has

held back the more wide spread use of ceramics to replace

metallic alloys.

Tailoring and improving the properties of silicon nitride orsialon materials can be achieved by the addition of secondary

phases. In particular, the addition of particulate hard phases

has been used as an approach to produce materials with in-

creased hardness, toughness and tribological performance.

The use of compounds with low resistivity such as TiN or

TiB2can produce composites that are electrically conductive

Tel.: +44 114 225 3894; fax: +44 114 225 3501.E-mail address:[email protected].

[1].Such composite materials combine the properties of ce-

ramics with the advantage of being able to electro-discharge

machine (EDM) the material to net shape without expensive

diamond grinding[2].

In a previous study by the author an optimised synthesis

procedure was developed for the production of sialonTiB2composites. At a levelof 40 vol.%TiB2 the composites exhib-

ited enhanced hardness and toughness and were electrically

conductive [2,3]. They also exhibited a dramatic decrease

in the specific wear rate under like-on-like, dry sliding, pin-

on-disc testing; a decrease far greater than the proportional

increase in hardness or toughness[4]. No decrease in the co-efficient of friction was observed and hence the improvement

was attributed to the tribochemical effect of the presence of

TiB2. An adherent tribofilm was formed which served to pro-

tect the underlying material from further wear.

This result was achieved using high stresses and in unlu-

bricated conditions (load = 5 N, pin end radius3 mm). Suchconditions can occur in applications where lubrication may

fail temporarily and these materials would hence be benefi-

cial in such situations. However, the work described here has

0043-1648/$ see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.wear.2004.09.049


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A.H. Jones / Wear 258 (2005) 942 952 943

Table 1

Sample identification, synthesis conditions (temperature/time/pressure, HP: hot pressed, HIP: hot isostatically pressed), density ( ), percentage of theoretical

density (t.d.), hardness (HV10) and fracture toughness (KIc) as measured by indentation after Anstis[10]

Material Synthesis conditions Density,

(g/cc) (% t.d.)

Hardness HV,

(GPa)

Fracture

toughnessKIc,(MPam1/2)

Si3N4 (TSN) HIP 3.21 (100%) 14.9 4.3

10 vol.% TiB2(STB10) HIP 1650 C/60 min/120 MPa 3.27 (98%) 15.3 35 vol.% TiB2(STB35) HP 1650

C/60 min/30 MPa 3.64 (99%) 16.9 40 vol.% TiB2(STB40) A HP 1650

C/60 min/30 MPa 3.64 (98%) 16.0 4.6B HP 1660 C/60min/50MPa + HIP 1360C/90 min/160 MPa 3.66 (98%) 16.9 5.3C HP 1600 C/60 min/30 MPa 3.66 (98%) 16.7 4.5D HP 1670 C/60min/50MPa + HIP 1360C/90 min/160 MPa 3.68 (99%) 16.6 5.7

D2 steel (D2) Hardened and tempered 8.0

studied these materials under abrasive conditions in order to

ascertain whether a similar improvement in wear resistance

is obtained under wet, low load, abrasive conditions due to

the presence of TiB2.

Two common micro-scale abrasive tests have been devel-

oped in recent years. Both tests, which are described in moredetail below, use the concept of a ball rotating against the

sample in the presence of an aqueous slurry of abrasive.

The first type of test is commonly known as a free ball

test where the ball is rotated against the sample by means of

friction from a rotating shaft. The clear disadvantages of this

system are: the uncertainty in the number of ball revolutions

due to the possibility of slip between the ball, and shaft and

the load being limited to that obtained by the mass of the

ball on the inclined sample. It has also been shown that non-

spherical wear scars can result as the angle between ball and

sample is adjusted when trying to adjust the load[5].

A more recent improvement of the free ball test is com-monly known as a fixed ball test, where the ball is directly

driven; for example, clamped between two co-axial drive

shafts[6]. This method not only ensures an exact measure

of the sliding distance, but also allows a range of loads to be

applied using a pivoted load beam. One disadvantage of the

test is that the ball rotates in a single orientation which can

lead to the ball developing a flat, worn area as opposed to

the free ball test where the ball generally changes orientation

during the test and produces even wear over the ball surface.

The fixed ball test has shown good reproducibility and is cur-

rently being studied as the basis for a new European standard

for micro-scale abrasive wear[7,8].

A recent study by Adachi and Hutchings [9] has attempted

to determinethe wear mode that canbe expectedundercertain

conditions when using this test. Adachi and Hutchings used

a severity of contact model to relate the relative hardnessof ball (Hb) and sample (Hs) to a quantity called the severity

of contact (S), which is given as

S= WAH

where 1

H= 1

Hb+ 1

Hs

where W is the load (N) and v the volume fraction of the

abrasive in the slurry, A the interaction area and is given

byA = (a2 + 2RD) in which R is the ball radius and D the

abrasive particle diameter. The Hertzian contact area radius,

a, is given by:

a = 3WR4E 1/3

in which

E=

1 2bEb

+

1 2sEs

and is Poissons ratio

Adachi and Hutchings used a range of materials and test con-

ditions to experimentally determine a relationship which pre-

dicted that, for three body rolling wear:

S= WAH

Hs

Hb

Table 2

The test conditions for free ball and fixed ball micro-scale abrasive wear testing

Parameter Free ball Fixed ball

Sliding distance (S) 225 m (no slip assumed) 50 m (627 revolutions)Load (W) 0.10.3 N (sample dependent) 0.15 N

Sliding speed (s) 0.5ms1 0.13ms1Ball 25.4 mm, SA51200 (worn in) hardness (HV) 8.9 GPa 25.4mm, SA51200 (shot blasted) hardness (HV)8.9 GPaLiquid media De-ionised water De-ionised water

Abrasive

Type SiC SiC

Size (D0.5) 5m 6.2m

Conc. 0.75 g cm3 (0.189 vol. fraction) 0.75 g cm3 (0.189 vol. fraction)Rate 1 drop/s 3 drops/s

Temperature/humidity 2225 C/3545% RH 2225 C/3545% RH


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944 A.H. Jones / Wear 258 (2005) 942952

The constants were determined to be = 0.076 and

=0.49.Using the above relationship, the properties of the materi-

als given inTable 1and the test conditions given inTable 2,

it should therefore be possible to predict how the wear mode

will change with variation in test conditions, such as increas-

ing load or decreasing abrasive concentration. In this workonly the effect of increasing load was studied.

2. Experimental details

2.1. Materials

Details of the synthesis of the composites and their prop-

erties have been given elsewhere[3]. A commercial hot iso-

statically pressed (HIP) Si3N4material (Toshiba TSN-3NH)

was used as a baseline material. It was characterised as being

a lowz value sialon with yttria and alumina sintering addi-tives and a small amount of rutile (TiO2) added to givea black

colour. In order to compare the tribological performance of

ceramic materials with wear resistant metallic systems a D2

tool steel was included for comparison. The D2 tool steel had

the nominal composition by weight, 1.5% C, 12% Cr, 0.9%

Mo, 0.7% V and contained chromium carbides in a marten-

sitic matrix.

Hot pressed (HP) and HP and HIP sialons with TiB2 ad-

ditions of 0, 10, 35 and 40 vol.% were tested. Four samples

containing 40 vol.% TiB2 were used, each being produced

under slightly different conditions summarised inTable 1.

All sialon materials had1 wt.% yttriaaluminasilica glass(40 wt.%:25 wt.%:35 wt.%, respectively) added as a sintering

aid. The microstructure consisted of mixed /sialon matrix

with grain size


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A.H. Jones / Wear 258 (2005) 942 952 945

Fig. 2. Configuration of the free ball and fixed ball micro-scale abrasive

wear test (adapted from Gee et al. [7]).

not only by the mass of the ball but by the friction coefficient,

sample tilt and lateral position of the axis to the ball centre.

Due to the difference in the height and diameter of each sam-

ple the actual sample tilt and test position varied from crater

to crater as the apparatus was adjusted to accommodate the

sample. As such the load applied was sample dependent andvaried between0.03 and 0.18 N, as measured by a load cellmounted behind the sample while the test was running.

The slurry consisted of a fine, angular SiC powder with

a nominal mean size (D0.5) of 5 m provided via the CSM

Instrument Company for use with their equipment. The con-

centration was 0.75 g/ml in de-ionised water (volume fraction

of abrasive = 0.189). Theslurry was continually agitated in its

dispensing container by using a magnetic stirrer. It was found

that sufficient mixing was only achieved when a vortex was

formed in the slurry by the stirrer. Lack of a vortex resulted

in a varying concentration being delivered to the sample and

frequent blockage of the delivery tube. The vortex mixing did

not occur when containers with non-flat bottoms were used

or when low speeds were used for stirring. This observation,

while apparently obvious and of importance to the test, has

not been mentioned in previous work.

The distance travelled was calculated from the number of

rotations of the drive shaft and the diameter of ball (25.4 mm)

with the assumption of no slip between the ball and shaft. A

worn in ball was always used as the surface finish of new

balls has been shown to affect results [11] (the ball undergoes

negligent wear in such tests). The test conditions are given in

Table 2.All tests used the same nominal distance, however,

since there was variation in the actual number of revolutions

completed in a fixed time from test to test, the actual distance

andload for each test was recorded andused in the calculation

of the specific wear rate,free(for the free ball test) for each

wear scar on a sample.

In free ball testing each samplewas testedfive times (n = 5)

at the same load and distance to givea measure of the repeata-

bility and to ensure that any inhomogeneity in the materialsdid not affect the results. The worn volume (V) was calcu-

lated, assuming a spherical cap geometry to the wear scar,

from:

V= b4

64R, b R

wherebis the wear scar diameter andRthe radius of the ball

(r= 12.7 mm). The specific wear rate,free(mm3 N1 m1),

was calculated from the known load (W) and sliding distance

(S) via the following relationship:

=V

SW

whereVis in mm3,Sis in m andWis in N. The standard de-

viation,n 1, and the standard error,, ( = n1/

n)

were also calculated for each material.

2.3. Fixed ball testing

Fixed ball testing was carried out using a Phoenix Tribol-

ogy TE66 tester (Phoenix Tribology Ltd., Woodham House,

Whitway, Newbury, RG209LF, UK) withthe ballclamped be-

tween two co-axial drive shafts and the load applied by a piv-

oted load arm. The configuration of the test is shown in Fig. 2and details of the tests conditions are given in Table 2.The

balls were supplied by Phoenix Tribology in a pre-roughened

condition achieved by shot blasting using alumina shot in a

proprietary process. The ball was rotated between each test

to a new orientation to avoid the development of a flattened

wear track on the ball.

An angular SiC abrasive was used (supplied by Anglo

Abrasives, Sheffield, UK) and was found, upon measurement

using a Malvern particle size analyser, to have a mean size

(D0.5) of 6.2m. Theabrasive was kept agitated as described

above.

For the majority of samples a series of tests were carried

out, each at a fixed sliding distance but with different loads

in the range 0.15 N. By using this multiple load method

the specific wear rate,fixed, was determined from a plot of

wear volume per unit sliding distance (mm3 m1) againstload (N). The gradient of a linear regression analysis of these

points gavefixed (mm3 N1 m1). Only wear scars that ex-

hibited three body rolling wear were used in the linear re-

gression analysis. Care was taken to examine the wear scars,

especially those for higher loads (>1 N), for any evidence of

groove or ridge formation; signs of slurry starvation during

the test. Wear scars with such features were not including in

generating the linear regression fit.


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946 A.H. Jones / Wear 258 (2005) 942952

The quality of the fit of the linear regression was measured

by the quantity R2, where R2 = 1 is a perfect linear fit to the

data. In all results presented here R2> 0.999, showing an ex-

cellent linear fit to the data. Errors for fixedwere determined

from the standard error of the gradient.

Generally, results from loads in the range 0.251 N were

used to calculate fixed from the multiple load method de-scribed above. Results from the tests carried out using three

indents at a single load of 0.25 N were used as a compari-

son with those obtained from the multiple load method. The

higher applied loads (e.g. 1.55 N) were used to study the

effect of load on the transition between two body grooving

wear and three body rolling wear.

Any non-zero value for the x-axis intercept of the linear

fit was regarded as primarily due to an offset in the applied

load caused by the act of balancing the load arm. Balanc-

ing is carried out to compensate for the weight of the sample

and is performed manually, is operator dependent and subjec-

tive. It is recognised that other effects may contribute to the

x-axis intercept, such as the uncertainty in the gradient, butthe primary cause apparent to this author is the uncertainty

in the balance. Hence, the multiple load method of deter-

miningfixedshould be independent of the uncertainty in the

applied load caused by balancing and is hence more accurate

than determination offixedfrom a single load measurement.

This was especially important for low applied loads (


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A.H. Jones / Wear 258 (2005) 942 952 947

Fig. 3. SEM images of a free ball test wear scar on Si3N4 (TSN): (a) low

magnification showing appearance of wear scar and (b) high magnification

showing detail of worn surface.

from between the ball and sample. This phenomenon has not

been described elsewhere in the literature on this subject and

indicates both the need to further understand the exact pro-

cesses that occurs during such tests, and that the free ball test

is somewhat unreliable and much more complicated that its

simple nature suggests.

3.3. Fixed ball test results

In the fixed ball tests the value offixed was determined

from the multiple load method using the gradient of a plot of

worn volume per unit sliding distance versus applied load, as

described above. The plots in Figs. 57 show examples of the

data for all loads for samples D2, TSN and STB35, respec-

tively. The filled data points indicate those which were used

Fig. 4. High magnification images of sialon40 vol.% TiB2 (STB40A): (a)

polished unworn surface and (b) centre of the wear scar after the free ball

test, with no evidence of abrasive wear on the surface.

to obtain fixed. The unfilled data points were not included

in the linear regression analysis due to the appearance of the

wear scars; indicating some form of wear mechanism other

than three body rolling wear.

Theresults for fixed ball specific wear rate aresummarised

inTable 4which lists the specific wear rates, fixed, the load

offset taken from thex-axis intercept, and the calculated load

for transition from three to two body wear. A plot of the

results for free ball and the fixed ball test (both single load

and multiple load methods) is shown inFig. 8.

The fixedball results show significantdifferences between

themultiple load methodand the singleload method. It canbe

argued that due to the multiple load methods independence

of any offset in the applied load (due to balancing), that the

multiple load results can be regarded as more accurate. The

load offset (or error) is small in all cases except for the D2

sample. The offsets are different because each time a new


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948 A.H. Jones / Wear 258 (2005) 942952

Table 4

The results obtained from single load and multiple load methods using the fixed ball test

Ranking Sample Specific wear ratefixed(103), (mm3 N1 m1) Load error (N) Transition load (N)Single load method (0.25 N) Multiple load method

Best D2 1.5 1.03 0.04 0.11 3.6

TSN 1.3 1.30 0.05 +0.003 3.5STB10 1.8 STB40A 2.5 2.14 0.08 0.05 3.5STB35 2.5 2.42 0.04 0.03 3.5

Worst STB40C 2.2

Load error is the off-set in the applied load due to balancing the load arm. The transition load is the applied load for transition from three body rolling to two

body grooving calculated after the method of Adachi and Hutchings[8].

Fig. 5. Results from the fixed ball micro-scale abrasive wear test on D2 tool

steel. Filled data points are those exhibiting three body rolling wear only

and were used to obtainfixedvia a linear fit. Unfilled points exhibited some

degree of grooving or ridging in the wear scars. The vertical dotted line (- - -

- -) represents the calculated boundary between three body rolling wear and

two body grooving wear, after Adachi and Hutchings.

sample is tested the load arm has to be re-balanced to account

for the different masses of the samples.

Another important observation is that,in comparisonto the

free ball test results, the ranking of the materials using fixed

ball testing is reversed. In the fixed ball tests the D2 tool steel

Fig. 6. The results for silicon nitride (TSN). Filled points represent wear

scars with three body rolling wear that were used to calculate fixed. Unfilled

points represent wear scars where ridging was seen to occur.

Fig. 7. Results from a sialon35 vol.%TiB2material. Filled points represent

those with only three body rolling wear characterused to obtain fixedfrom a

linear regression analysis. Non-filled points exhibited wear scars with some

element of grooving or ridging.

exhibits the lowest wear rate and the sialon40 vol.% TiB2materials exhibited the highest wear rates, the opposite resultto that from the free ball test.

3.4. SEM of fixed ball test wear scars

Scanning electron microscopy of thewear scars from fixed

ball testing on D2 tool steel (Fig. 9a) revealed a pattern of

Fig. 8. Graphical representation of the results from free ball and fixed ball

testing. Results are shown for both a single load and multiple loads for fixed

ball tests.


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A.H. Jones / Wear 258 (2005) 942 952 949

Fig. 9. SEMimages of thesurfaceof worn D2 tool steel:(a) high magnifica-

tion of thewornsurface, (b)the edge of thewear scar showing wear resistant

carbides and (c) EDX spectrum of a region in (a) showing high levels of Si

indicating embedded SiC in the worn surface.

multiple, small, sharp indents caused by the hard, angular

abrasive particles rolling through the interface. At the edge

of the wear scar it was observed that the hard carbides were

more resistant to the rolling abrasive (Fig. 9b) while the softer

martensitic matrix was more severely indented by the abra-

sive particles. An EDX spectrum of the surface of the wear

scar (Fig. 9c) revealed a high level of Si and some C. This was

attributed to the presence of small particles of SiC embedded

in the surface.

The Si3N4wear scar appeared substantially different from

the metallic sample (Fig. 10a). The morphology of the worn

surface was rounded with no clear evidence of indentation

as is seen in metallic materials but also no clear evidence

Fig. 10. SEM images of the worn surface of the Si3N4 (TSN) sample: (a)

high magnification of the worn surface and (b) the edge of the wear scar

showing boundary between worn and unworn material.

of grain fracture or pull-out. It is possible that the freshlyexposed Si3N4surface reacted with the water present in the

slurry forming a thin oxide or hydroxide layer on the wear

surface. Such a reaction may explain the rounded and soft

appearance of the worn surface. At the edge of the wear scar

(Fig. 10b) there was no clear evidence for the mechanism

of wear, possibly due to the fine scale of the microstructure

and lack of contrast between phases. In the case of the D2

sample the size of the phases and their composition provided

a sharp contrast in back scattered electron mode, which made

observation of the wear mechanism far easier.

In Fig. 11, a sialon40 vol.% material (STB40A), the mor-

phology of the wear scar was similar to that of TSN but with

the TiB2 particlesclearlyvisible in thestructure. There was no

clear evidence for TiB2or sialon grain pull-out. EDX spectra

from unworn and worn surfaces are shown inFig. 12.There

was a small increase in the carbon and content of the surface

suggesting some entrapment of SiC particles. The increased

oxygen content suggests that oxidation and/or hydroxylation

of the surface may have occurred. The mechanism of wear

was thus assumed to be similar to that for TSN but the role

of TiB2was not clear from these observations.

There is significant residual stress in the microstructure

due to the difference in the coefficient of thermal expansion

() between Si3N4 and TiB2 (Si3N4= 3.03.5

106 K1,


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950 A.H. Jones / Wear 258 (2005) 942952

Fig. 11. SEM images of the worn surface of sialon40vol.% TiB2(STB40A): (a) high magnification of the worn surface and (b) the edge of

the wear scar showing boundary between unworn (top) and worn (bottom)

material.

Fig. 12. EDX spectra from (a) the unworn and (b) the worn surface of a

sialon40vol.% TiB2 composite. The worn surface shows an increase in

oxygen (from reaction with water) and carbon (from embedded SiC).

Fig. 13. Ridges observed on wear scar on D2 tool steel with load = 2.5 N.(a) Low magnification of the wear scar and (b) the ridges showing the three

body rolling character of their surface.

TiB2= 68 106 K1). It is possible that the residualstress, locked into the structure upon cooling from the sinter-

ing temperature, plays a part in the higher wear rate of TiB2containing materials by exacerbating grain removal during

the abrasion process.

At higher loads, usually above 1 N, the wear scars of-

ten appeared to contain grooves, as shown inFig. 13. How-

ever, on examination using SEM, it was observed that these

grooves were in fact ridgesand notthe type of grooves formed

by two body grooving wear, i.e. those formed by plastic

ploughing by abrasive particles. The surfaces of the well

spaced ridges were themselves covered in the characteris-

tic multiple indents of three body rolling wear as shown in

Fig. 13b.

In the case of STB40A a large central ridge was formed

in the wear scar with multiple smaller ridges either side as

shown inFig. 14.This was characteristic of slurry starvation

caused by the high pressures induced by high applied loads

[11].This central ridge was seen to have damaged the ball,

forming a matching valley on the ball (Fig. 15a).


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A.H. Jones / Wear 258 (2005) 942 952 951

Fig. 14. The central ridge observed on STB40A. (a) Low magnification of

the wear scar and (b) the central ridge with parallel ridges with three body

rolling wear character.

A possible explanation is that at the beginning of the test

the small contact area leads to high pressures which excludes

the abrasive slurry and leads to multiple ridge forming by

the direct contact of ball on disc[12].As the test progresses

the contact area increases, the pressure thus decreases and

the slurry is then able to enter the interface. The now ridged

surface of the ball imposes a ridge like morphology on the

wear scar. Some evidence for this explanation exists in ridges

observed by SEM on a ball after an aborted test (only

1020

revolutions) as shown inFig. 15b.The above effect also appeared to result in a reduced spe-

cific wear rate, as demonstrated by the unfilled data points

inFigs. 57. However, this effect is most likely due to the

non-ideal contact geometry imposed by the high loads and

the ridging process.

3.5. Wear transitions

The load at which the transition from three body rolling to

two body grooving should have occurred was calculated for

all materials using their known properties and the test con-

Fig. 15. SEM observation of the ball revealing: (a) a gouged out central

ridge corresponding to the central ridge observed on the sample and (b)

ridges forming after a small number of revolutions at high loads.

ditions after the method proposed by Adachi and Hutchings.

The predicted transition loads are shown in Table 4. However,

such a transition was not observed despite increasing the load

to the maximum possible on the equipment (5 N). Increasing

the load appeared to lead to slurry starvation and ridge forma-

tion before two body grooving wear could occur. This leads

to the conclusion that the severity of contact model can only

be used to describe wear transitions within a limited set of

test conditions where slurry starvation is avoided. In this case

the load would be limited to


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952 A.H. Jones / Wear 258 (2005) 942952

livered to the ball. Flat bottomed containers are necessary

to form efficient vortex mixing.

Fixed ball testing using the multiple load method has beenshown to be a more accurate method than measurements

made at a single load. This method eliminates uncertainty

in the applied load caused by inaccuracy in the balancing

of the load arm. On equipment where a load cell is fittedto measure the load applied to the sample, this approach is

not required.

The free ball and fixed ball techniques can yield signif-icantly different results. Free ball testing demonstrated a

decrease in wear in the presence of TiB2 while fixed ball

testing showed an increase in wear. The later should be

regarded as the more reliable test of abrasive wear resis-

tance.

The wear mechanisms for tool steel and ceramic have beenshown to be quite different. D2 tool steel underwent pref-

erential wear of the softer martensitic matrix via repeated

indentation by the SiC particles as they rolled through the

ball-sample interface. The harder carbides present in themicrostructure resisted indentation and are responsible for

this materials superior wear resistance compared to stan-

dard steels.

Si3N4 containing materials showed no evidence of mul-tiple indentation, and there was no evidence of pull-out

of the Si3N4 or TiB2 grains. The surface appearance and

EDX spectra suggested that a reaction had taken place be-

tween the worn ceramic surface and water. This formed a

soft hydroxide or oxide layer, which was then worn away.

The role of water in the abrasive wear of Si 3N4 materials

is thus worthy of further study.

The determination of wear mode by the examination ofwear scars required close and careful inspection via SEM.

Using optical microscopy alone,it was possible to interpret

the ridges caused by the high loads as two body grooving.

However, SEM determined these features to be ridges with

three body rolling wear morphology.

Increasing the load did not induce the onset of two bodygrooving as predicted by the severity of contact model.

This model is therefore limited in its application and can

only be used to predict wear transitions where the experi-

mental conditions do not lead to slurry starvation and sub-

sequent ridgeformation.In this instance inducing twobody

grooving wear would require loads

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Micro-scale abrasive wear of silicon nitride, sialon–TiB2 composites

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