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Available online at www.sciencedirect.com

Wear 265 (2008) 375–386

Tribology of titanium boride-coated titanium balls againstalumina ceramic: Wear, friction, and micromechanisms

Curtis Lee a, Anthony Sanders b, Nishant Tikekar a, K.S. Ravi Chandran a,∗a Department of Metallurgical Engineering, The University of Utah, 135 South, 1460 East Rm. 412, Salt Lake City, UT 84112, United States

b Ortho Development Corporation, Draper, UT 84020, United States

Received 3 April 2007; received in revised form 22 October 2007; accepted 20 November 2007Available online 16 January 2008

bstract

The tribological performance of titanium alloy (Ti–6Al–4V) balls coated with a dual boride layer comprised of titanium diboride (TiB2) anditanium boride (TiB) whiskers mated against alumina ceramic disks has been determined using lubricated ball-on-disk wear testing. Measurementsf coefficient of friction values and volumetric wear were made and electron microscopic investigation of wear spots and tracks was performed.he wear rate of the boride-coated titanium alloy balls was 40 times less than that of 97% dense alumina balls. Measurements of wear track width

nd depth corroborated this result. The superior wear resistance is attributed to the hardness and the unique structure of the dual (TiB2 + TiB)hisker layer and the consequent smoothness of the wear surface created during the wear process. The material removal mechanism is abrasive inature in the boride-coated balls compared to grain fracture and pullout in alumina.

2007 Elsevier B.V. All rights reserved.

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eywords: Borided Ti–6Al–4V; Tribology; Ball-on-disk wear testing; Alumina

. Introduction

The higher specific strength and the superior corrosion resis-ance of titanium and its alloys, relative to other competing

etals, make them desirable in many high performance appli-ations. However, its tendency to gall and seize due to low workardening behavior is a barrier to its use in applications whereontact stresses and relative sliding between surfaces are high1]. Several surface modification techniques, including chemicalapor deposition (CVD) [2], physical vapor deposition (PVD)3], laser surface treatment [4], thermal oxidation [5], ion nitrid-ng [6], and solid-state diffusion [7,8] have been developedo create hard and wear-resistant layers comprised of one or

ore hard compounds of titanium. While the common objec-ive behind these techniques has been either the creation of hardurface layers of nitrides, carbides or borides or subsurface lay-

rs rich in N, C and O, there are some inherent disadvantages.or example, the thicknesses of layers that can be made byVD and PVD techniques are limited thicknesses of 0.5–10 �m

∗ Corresponding author. Tel.: +1 801 581 7197; fax: +1 801 581 4937.E-mail address: ravi.chandran@utah.edu (K.S.R. Chandran).

Ta(l(ta

043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2007.11.011

nium boride coating; Titanium alloy

9], and result in coating only line-of-sight surfaces. The maxi-um hardness levels achievable by the impregnation of O and N

toms by thermal oxidation and ion nitriding are often limited toround 12 GPa (HV) [10]. Methods based on laser surface treat-ent suffer from process complexity and cost, surface oxidation

uring melting, and the general restriction to planar surfaces4].

Surface hardening of titanium by the solid-state diffusion ofis advantageous because the process itself is simple and inex-

ensive, does not need complicated equipment, and becauseomplex geometries can be coated. In this process, B frompowder mixture surrounding the specimen is diffused into

he titanium surface at high temperatures. The diffusion leadso a dual layer coating consisting of a continuous monolithiciB2 outer layer and an inner layer consisting of discrete TiBhiskers, generally penetrating normal to the surface (Fig. 1(a)).he dual layer architecture provides a gradual transition fromhard ceramic outer layer (hardness of TiB2 layer is ∼30 GPa

HV) [11]), through the next layer (hardness of TiB whisker

ayer is ∼20 GPa (HV) [12]), to the ductile metal substratehardness of titanium is ∼4 GPa [13]). This dual layer struc-ure holds tremendous potential for sustained wear resistancend low friction under contact conditions, for the layers are

376 C. Lee et al. / Wear 265 (2008) 375–386

F r and(

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2

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ig. 1. Microstructure of (a) dual layer boride coating consisting of a TiB2 layeb) alumina disk, and (c) alumina ball.

ot discretely structured as in the case of CVD and PVDoatings. Additionally, the hardness levels are much higherhan those achievable by thermal oxidation and ion nitrid-ng.

The objective of this study was to investigate the tribolog-cal characteristics of the dual layer boride coating against aard ceramic. The wear resistance of this dual layer coating isxpected to be favorable due to several factors: (i) TiB2 hasvery high hardness and is a wear-resistant ceramic, (ii) TiBhisker tensile strength has been estimated to be as high as500 MPa [14] and (iii) the abrasive wear rate of Ti + 40% TiB2intered composite was found to be about 500× lower thanhe base Ti–6Al–4V alloy [15]. The present study employedalls and disks of alumina (Al2O3) ceramic for comparativevaluation of the Ti–6Al–4V alloy balls coated with the dualoride layers. The structure of the TiB2 + TiB whisker dual

ayer coating, surface roughness outcomes of coated/polishedalls, friction coefficient, wear performance, and the mecha-isms of wear have been determined and are discussed in thisork.

SF

i

able 1roperty data for alumina balls and disks

rticle Density (g/cm3) Purity Hardness

l2O3 ball 3.87 99.5% 1700 HV

l2O3 disk 3.92 99.8% 83 RC@45 N

TiB whisker layer, as observed in Ti–6Al–4V disk sample and SEM images of

. Materials and experiments

.1. Materials and ball preparation

Grade 200 Ti–6Al–4V alloy balls, Ø6.35 mm (Bal-Tec, Losngeles, CA) were chosen for this study. Surface roughnesseasurements of these balls by contact profilometer yielded

verage values: Ra and Rz of 0.05 �m and 0.70 �m, respectively.rade 25 alumina (Al2O3) balls, Ø6.35 mm (Hoover, Sault Ste.arie, MI) were used for comparison. Surface roughness lev-

ls of these balls were: Ra and Rz of 0.04 �m and 0.6 �m,espectively. For all the ball-on-disk wear tests, the counter-ace material was a Ø76.2 mm alumina disk (CoorsTek, Golden,O). The disks were lapped to a flatness of 2.5 �m and Ra of.5 �m. The material property data for the alumina balls andisks supplied by the manufacturer are summarized in Table 1.

EM images of the unworn disk and ball surfaces appear inig. 1(b) and (c).

The dual layer boride-coated balls were prepared by coat-ng procedures discussed in previous work [7,8]. Briefly, this

Flexural strength (MPa) Compressive strength (MPa)

324 2,068372 2,068

C. Lee et al. / Wear 265 (2008) 375–386 377

Table 2Diameter, sphericity, and roughness of Ti–6Al–4V balls before and after boriding and after polishing

Ball condition Diameter (mm) Sphericity (�m) average (S.D.) Ra (�m) average (S.D.)

As-received 6.359144 0.152 (0.051) 0.050 (0.0053)Borided 6.374562 19.80 (11.9) 0.555 (0.1960)Borided + polished (average) 6.355537 9.25 (5.21) 0.114 (0.0290)BB

itamtTbt

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2

wicwtTsiAmtac

sm0lpd

nta

μ

wtt(bistUtt

2

tof3piw

h

V

where h is the spot depth, r is the ball radius, d is the spot diam-eter, and V is the volume of material lost by wear. The secondtechnique used a Zygo New View 5032 scanning white lightinterferometer (SWLI) (Zygo Corporation, Middlefield, CT),

Table 3Wear testing parameters

orided + polished (best areas) –orided + polished (dull areas) –

nvolved surrounding the Ti–6Al–4V balls in a powder mix-ure composed of a B source, an activator, and a de-oxidant ingraphite crucible. The powder mixture was prepared by ballilling for 16 h. The balls were situated in the crucibles such

hat each ball was surrounded by at least 2.54 cm of powder.he boriding treatment was done at 1050 ◦C for 24 h followedy slow cooling to room temperature. One treated ball was sec-ioned for metallographic examination.

After boriding, the balls were polished by a commercial ballearing manufacturer (ITI, Dexter, MI). Six balls were usedn determining the average diameter, surface roughness, andphericity prior to and after polishing; the results are given inable 2. The polished surfaces had some roughness variationue to differing degrees of engagement with the ball polishingounterfaces, caused by the asphericity of the as-borided balls.onsequently, specific locations that provided the best finishnd the thickest coating were identified for wear testing. Fur-her, Vickers micro-indentation hardness measurements (LECO

-400, St. Joseph, MI, USA), made on balls secured in a collet,ere supplemental in identifying the spots of coating suitable

or wear testing. The hardness measurements revealed that theardest coating locations corresponded to spots on the ball thatad a relatively smooth surface texture.

.2. Wear testing

Wear testing was performed on a custom-made ball-on-diskear-testing machine designed to meet the ASTM G99 spec-

fication. Basically, it consisted of a rotary stage in which theounterface alumina disk was secured, and above it, a collet intohich the test ball was mounted. The rotary stage was designed

o function inside a reservoir containing the lubricating liquid.he collet had a rear screw that was used to tightly grab andecure the ball along its equator, preventing ball movement dur-ng testing. The collet was attached to a force–torque sensor (ATIutomation, Apex, NC, USA; max. normal force: 200 ± 0.6 N,ax. shear: 65 ± 0.2 N) for measurement of the normal and

angential forces on the ball during wear. The collet–sensorssembly was attached to the bottom of a loading stage thatarried weights for normal loading.

Wear tests were run under a load of 5.0 kg (49 N) and at apeed of 0.12 m s−1. The disk and the ball were submerged in aineral oil (PTI Process Chemicals, McHenry, IL) with an s.g. of

.8395 at 25 ◦C, and a viscosity of 106 cP at 40 ◦C. The test trackength was approximately 1.0 km. (Summary of test conditionsrovided in Table 3.) A National Instruments DAQPad-6020Eata acquisition system was used to continuously monitor the

LTSL

– 0.024 (0.0089)– 0.125 (0.0521)

ormal and tangential loads on the ball. The coefficient of fric-ion (COF) was calculated and smoothed using a central movingverage technique:

j =(

1/n∑j+(n/2)−1

i=j−(n/2) Fi

1/n∑j+(n/2)−1

i=j−(n/2) Ni

)(1)

here μj is the average COF at a discreet distance (dj), n ishe number of data points in the average, and Fi and Ni arehe measured horizontal and normal forces at a given distancedi), respectively. A moving average is used to report the COFecause the sensitivity of the sensor to both the normal and hor-zontal forces resulted in large scatter in the data. In part, thecatter was also exacerbated due to stick-slip friction betweenhe ball and the disk, especially in the uncoated Ti–6Al–4V ball.sing a moving average reduced scatter and produced a rela-

ively smooth line that could be analyzed and used to comparehe average COF of different materials.

.3. Determination of volumetric wear

Volumetric wear on the balls was determined by twoechniques. The first technique followed the ASTM G99-04 rec-mmendations, assuming that the wear spot is an ideally flateature. The wear spot was imaged using a Nikon Nexiv VMR020 optical coordinate measuring machine (CMM) at an appro-riate magnification. The wear spot edge was identified at 10◦ncrements, and a best-fit circle was fit to 36 edge points. Theear volume was then calculated using the following equations:

= r −√

r2 −(

d2

4

)(2)

=(

πh

6

)(3d2

4+ h2

)(3)

oad 5 kgemperature Room temperatureliding distance 1000 mubrication Light mineral oil

3 r 265 (2008) 375–386

wtwtftfcdw

tIwuifrtatSbtdmm

Fig. 2. (a) Raw scan of a wear spot on a ball and (b) after subtraction of a best-fit

Fo

78 C. Lee et al. / Wea

hich used a direct method to measure volumetric wear. Inhis technique, the wear spot, including some adjacent regions,as scanned and used to create a 3D image (Fig. 2(a)). From

he scanned data, the instrument calculated a best-fit sphereor the unworn regions and then subtracted that sphere fromhe entire data set. In the resulting data set, the unworn sur-ace appeared flat and the worn surface appeared as a sphericalrater (Fig. 2(b)). The instrument’s software then analyticallyetermined the volume of the crater, which was the volumetricear.There were small discrepancies between the results of the

wo wear measurement techniques due to problems in practice.n some instances, there was difficulty in measuring the aluminaear spots with the SWLI technique because the slope of thenworn surface around the spot deflected light away from thenstrument’s objective. As a result, only partial data was obtainedrom the unworn surface. In the borided balls, two measurementsesulted in significant discrepancies (47% and 29%), but three ofhe five measurements had discrepancies less than 3%. The aver-ge discrepancy for the alumina balls was 17%. In cases wherehere were discrepancies, CMM data was relied upon more thanWLI because the wear spots appeared flat and circular, andecause there is precedence for using the wear spot diameter

o calculate volumetric wear in alumina balls [16]. Also, theifficulty in obtaining data from the unworn regions of the alu-ina balls with SWLI makes the volume calculations using thatethod suspect for error.

sphere, as determined from the unworn regions in (a).

ig. 3. (a) Microstructure of the coating on the ball after boriding treatment, and (b and c) after ball polishing. (b) Represents an area of maximal coating thicknessn the ball and (c) corresponds to an area where minimal coating thickness remained.

r 265 (2008) 375–386 379

3

3

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uwPraapow

3

mhnC0Cf

Fig. 4. (a) COF of uncoated Ti–6Al–4V ball against alumina and (b) COFof boride-coated Ti–6Al–4V and alumina balls, both against alumina. Testswd

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TW

Sn

766666666

C. Lee et al. / Wea

. Results

.1. Microstructure of dual layer boride coating

After boriding, the Ti–6Al–4V balls were examined macro-copically and microscopically and no uncoated areas wereound. Thus, the process was effective in uniformly treatinghe entire outer surface. The coating thickness of a sectionedall was measured using an optical microscope. The monolithicuter TiB2 layer was measured as 5 ± 1 �m and the total coatinghickness (consisting of both layers) was found to be 17 ± 9 �m.he coating appeared to be fully dense.

Microstructures of the coating and the substrate are shownn Fig. 3(a)–(c). The boriding process resulted in a dual layeriB2 + TiB whisker coating on the ball surface and a lamellar+ � microstructure in the interior. The interior microstructure

s typical in heat treatment conditions involving slow coolingrom above the �-transus temperature of the Ti–6Al–4V alloy.he outer layer of TiB2 and the substrate are separated by the

ntermediate TiB whisker layer; the TiB whiskers are visible asray, while the TiB2 layer appears essentially white.

The surface finish produced by ball polishing was not entirelyniform. Macroscopically, the surface appeared patchy withell-polished regions interspersed with relatively dull regions.ost-polish metallographic examination of cross-sectioned ballsevealed regions which varied in coating thickness (Fig. 3(b)nd (c)). The duller surface regions coincided with areas wheresignificant part of the coating was apparently removed duringolishing (Fig. 3(c)). Such regions often appeared to lack theuter TiB2 layer; however, they typically retained a layer of TiBhiskers penetrating into the substrate.

.2. Coefficient of friction

The COF of metals and alloys are typically higher than hardaterials like ceramics. Uncoated titanium and its alloys exhibit

igh friction even under lubricated conditions, due to the ductileature and low work hardening behavior of titanium [13]. The

OF in a test run on an uncoated Ti–6Al–4V ball ranged from.3 to 0.4 in the present test conditions as shown in Fig. 4(a). TheOF values (taken as the RMS value for the entire test record)

or the five tests of coated balls (along with the other tests) are

tro

able 4ear and COF for the materials tested

pecimename

Material Distance(km)

Wear spotØ (mm)

Wear vol

CMM

0306 Ti–6Al–4V 0.399a 4.56 8.350929 TiB2 + TiB 1.021 0.478 0.00080926 TiB2 + TiB 1.027 0.716 0.00411004 TiB2 + TiB 1.021 0.738 0.00461018 TiB2 + TiB 1.021 0.556 0.00151020 TiB2 + TiB 1.012 0.722 0.00421106 Alumina 1.012 1.552 0.09141103 Alumina 1.012 1.644 0.11570922 Alumina 1.212 1.729 0.1417

a Test was terminated after 400 m because nearly half of the ball was worn away.

ere run using light mineral oil as a lubricant and under a 5 kg load. (Note theifference in scales between the two graphs.)

resented in Table 4. In Fig. 4(b), representative COF data fromtest of a borided Ti–6Al–4V ball mated to an alumina disk is

ompared with COF data from a test of an alumina ball mated ton alumina disk. The boride-coated Ti–6Al–4V balls performeds well or slightly better than the alumina balls under the presentest conditions. The RMS COF ranged between 0.11 and 0.12or the tests run on borided Ti–6Al–4V balls and was 0.12 forhe alumina balls.

It is clear that the dual layer boride coating on titanium ledo a decrease in the COF along with an increase in the hardness,elative to the uncoated titanium alloy. This is consistent withther surface-hardened titanium specimens where a significant

ume (mm3) Wear rate (mm3/(N m)) COF (RMS)

SWLI CMM SWLI

– 4.27 × 10−4 – 0.350.0015 1.60 × 10−8 3.00 × 10−8 0.110.0041 8.14 × 10−8 8.14 × 10−8 0.110.0045 9.19 × 10−8 8.99 × 10−8 0.110.0021 3.00 × 10−8 4.19 × 10−8 0.100.0042 8.46 × 10−8 8.46 × 10−8 0.110.1103 1.84 × 10−6 2.22 × 10−6 0.120.0898 2.33 × 10−6 1.81 × 10−6 0.120.1332 2.38 × 10−6 2.24 × 10−6 0.12

380 C. Lee et al. / Wear 265

Tabl

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ecim

enav

erag

esof

wea

ran

dC

OF

Mat

eria

lD

ista

nce

(km

)W

ear

spot

,Ø

(mm

)H

eigh

t(�

m)

Wea

rvo

lum

e(m

m3)

Wea

rra

te(m

m3/(

Nm

))C

OF

(RM

S)

CM

MSW

LI

CM

MSW

LI

TiB

2+

TiB

1.02

0(0

.006

)0.

642

(0.1

18)

16.7

(5.7

)0.

0030

(0.0

018)

0.00

33(0

.001

4)6.

08×

10−8

(3.5

1×

10−8

)6.

55×

10−8

(1.7

5×

10−8

)0.

1095

(0.0

05)

Alu

min

a1.

078

(0.1

15)

1.64

2(0

.089

)10

8.2

(11.

9)0.

1163

(0.0

251)

0.11

11(0

.021

7)2.

19×

10−6

(2.9

9×

10−6

)2.

09×

10−6

(2.4

4×

10−6

)0.

120

(0.0

02)

Stan

dard

devi

atio

nsap

pear

inpa

rent

hese

s.

r[mlums

3

cvbnrova

wdlmbrdacisiabicbphoasvTfUmia

4

4c

p

(2008) 375–386

eduction in COF was seen [17,18]. For example, Itoh et al.17] reported a COF of 0.05–0.2 for N+ implanted Ti–6Al–4Vated against itself, using a load of 460 mN and immersed in

ubricating oil. Met et al. [18] reported a COF of 0.03–0.08sing a smooth fine-grained diamond coating on Ti–6Al–4Vated against itself under a 13 N load and lubricated with ringers

olution.

.3. Volumetric wear of balls

Table 4 provides a compilation of volumetric wear and spe-ific wear rates observed in individual balls. There is someariability in the wear of coated Ti–6Al–4V balls. This maye due to the variability in the coating thickness, and is probablyot linked to the variations in surface roughness. The volumet-ic wear data of all of the TiB2 + TiB coated balls is at least anrder of magnitude less than the alumina balls. The worst COFalue for the coated balls is less than the best COF value for thelumina balls. Table 5 provides average wear and COF results.

Optical micrographs of the smallest and largest borided ballear spots are presented in Figs. 5 and 6, respectively. Theseemonstrate the nature of wear progression through the dualayer boride coating. Fig. 5(a) and (b) illustrates the generalorphology of the wear spot and the surface topography of the

all. In Fig. 5(b), a dual contrast can be seen, indicating theemnant TiB2 layer (white areas in optical micrographs and thearker phase in the SEM images) and the TiB whisker layer (grayreas). This outcome can be imagined as the result of a planarross-section through the ball, where the plane transects the coat-ng while grazing the substrate. In Fig. 6(a) and (b), showing thepecimen with the largest wear spot, the TiB2 layer is clearly vis-ble, but the TiB layer is much less evident. Both are visible onlylong the edge (Fig. 6(b)). This is indicative of wear progressioneyond the TiB whisker layer, such that the imagined intersect-ng plane now transects the substrate in large part. It is likely thatlose to the end of the test some of the load was actually borny the TiB2 layer (the elastic modulus of TiB2 is 560 GPa, com-ared to that of Ti–6Al–4V, which is 110 GPa), thus explainingow the ball was able to resist wear despite the inner regionsf the wear spot seeming to consist almost entirely of the Ti-lloy matrix. It is also possible that these regions also containedome TiB whiskers that contributed to the wear resistance, iniew of the presence of some long, sporadic TiB whiskers in theiB layer of the coating (Fig. 3(a)). SEM images of a wear spotrom a representative alumina ball are presented in Fig. 7(a)–(d).nlike the worn surfaces of borided Ti–6Al–4V balls, the alu-ina balls exhibited grain fracture during the wear process. This

s indicated by the relatively rough topography of the wear spotnd the associated pits corresponding to the grain-pull-outs.

. Discussion

.1. Ball polishing, sphericity and uniformity of boride

oating

The overall surface roughness Ra of the borided balls afterolishing, ∼0.10 �m, meets the specification for an AFBMA

C. Lee et al. / Wear 265 (2008) 375–386 381

Fig. 5. Micrographs of the smallest wear spot of the borided Ti–6Al–4V (#060929) taken with optical microscope (a and b) and SEM (d and e). Note that the TiB2

phase appears white in optical micrographs and as dark phase in SEM micrographs. The TiB appears as light gray in optical micrographs and as dark gray in SEMmicrographs.

Fig. 6. Micrographs of the largest wear spot of the borided Ti–6Al–4V (#061004) taken with optical microscope (a and b) and SEM (c). Note the changes in thecontrast of the TiB2/TiB phases upon going from optical to SEM micrography.

382 C. Lee et al. / Wear 265 (2008) 375–386

n alum

Gpiteacwcirawtlstirmawpaatot

4

tbsnbtmbA3rThlolomz1s

Fig. 7. SEM images of a representative wear spot on a

rade 100 ball. Nonetheless, the surface texture was unex-ectedly non-uniform, and this seems to have arisen from thencreased asphericity of the balls after boriding. Sphericity ishe radially measured envelope between the highest and low-st points on the sphere; thus, it measures spherical error. Theverage sphericity after boriding (∼20 �m) increased signifi-antly from that before boriding (0.15 �m). Also, the sphericityas greater than the thickness of the monolithic portion of the

oating. The ball polishing process works by progressively wear-ng the highest points more than the lowest, with the eventualesult (ideally) of removing all spherical error. As such, somereas (high spots) of the surfaces were polished more (Fig. 3(c)),hile other areas (low spots) were polished less (Fig. 3(b)). In

he high spots, polishing apparently went through the TiB2 outerayer and ended within the TiB whisker zone. These areas macro-copically appeared duller due to polishing differences betweenhe whiskers and the Ti-alloy substrate. In other areas, the pol-shing affected the outer TiB2 layer and finished leaving a highlyeflective and polished surface. The Ra in such areas, selectivelyeasured as 0.02 �m, was smoother than the overall Ra of the

lumina balls (0.04 �m). The hardness results were consistentith the surface finish variation. Hardness indentations on well-olished areas were smaller, indicating the presence of TiB2nd a sublayer of TiB whiskers. Larger indents were obtained in

reas lacking the TiB2 layer. The spherical error resulting fromhe boriding process and its effects in polishing were causesf variation in surface roughness, hardness, and the coatinghickness.

tata

ina ball (#061103) taken at different magnifications.

.2. Wear

The wear spot diameter, depth, and volume were substan-ially less with the borided balls (Table 4) relative to aluminaalls. Although on average, the alumina balls were run for alightly longer distance, the difference was not statistically sig-ificant. The wear difference between the borided Ti–6Al–4Valls and alumina balls, to some extent, appears to be due tohe intrinsic differences in hardness. The hardness of the alu-

ina ball was 17 GPa (HV). The intrinsic hardness of TiB2 haseen reported as 25 GPa (HV) for a polycrystalline material [11].

TiB2 + TiB coating has been reported to have a hardness of2–44 GPa (KHN) [19]. PVD deposited TiB2 coatings have beeneported to have hardness levels in the range of 52 ± 8 GPa [20].iB2 coatings made by fused salt electrolysis were reported toave hardness in the range 22–52 GPa (HV) [21]. In the authors’aboratory, the diffusion based TiB2 + TiB coatings, producedn flat Ti-alloy specimens, have typically exhibited hardnessevels of 25–33 GPa (HV), which is consistent with the rangef hardness values reported by others. Further, flat specimens,echanically polished through the TiB2 layer to the transition

one between TiB2 and TiB, have shown hardness values of8–23 GPa (HV). Fig. 8 illustrates the large variation in theizes of Knoop hardness indentations from the outer TiB2 layer,

hrough the TiB whisker layer, and into the substrate; there isbout a factor of three difference between the indentation size inhe TiB2 layer and that in the substrate. This should translate intobout a factor of 10 difference in hardness values. From the cor-

C. Lee et al. / Wear 265

Fc

ratohtbb

(laiiasactTc1nns[lthbs

4

nbrsor

cacsmlTiTrciwrwtsb

ietSdimTtws

tttesttTuwpaT

4

mdeaZ

ig. 8. Knoop microhardness indentations across the cross-section of the borideoating on Ti–6Al–4V alloy.

elation between the measurements of flat specimen hardnesss well as metallographic examination of post-polish coatinghickness on the borided balls, it is estimated that the hardnessf the borided balls, in locations identified for testing, wouldave been in the range of 20–25 GPa (HV). Consequently, onhe basis of hardness alone, a significant reduction in wear coulde anticipated for the borided balls compared to the aluminaalls.

In these tests, the specific wear rate of the TiB coated balls6.55 × 10−8 mm3/(N m)) was about two orders of magnitudeess than that of the alumina balls (2.09 × 10−6 mm3/(N m)),s shown in Table 5. Within limitations, the specific wear rates useful for comparing these results against results reportedn the literature. Saikko and Karenen examined the wear of

commercial medical implant alumina and reported a lowerpecific wear rate of 1.6 × 10−8 mm3/(N m) [16]. This may bettributable to the higher density (3.98 g/cm3) and the signifi-antly lower roughness of the disk (0.03–0.06 �m Ra). Althoughhere are few directly comparable wear tests of TiB and/oriB2, Pfohl et al. measured the wear of a ∼1 �m CVD TiB2oating against alumina and reported a specific wear rate of.5 × 10−8 mm3/(N m) [22]; however, their test conditions wereot reported. For comparison of other coating systems on tita-ium, Met et al. reported wear results of a 2 �m thick and verymooth (15–35 nm Ra) PVD diamond coating on Ti–6Al–4V23]. The wear rate, in the range of 1.30 × 10−10 mm3/(N m), isower than the TiB2 results reported here. Clearly, further worko improve the surface finish of the boride coatings reportedere are necessary to reduce the specific wear of the boridedalls and to better compare the results against other reportedystems.

.3. Wear mechanisms

Besides differences in hardness, differences in wear mecha-isms affect wear rates [24]. The wear surface of the aluminaalls appears to be relatively rough in comparison to the sur-

ounding, unworn areas. The SEM images of the alumina wearpot clearly show that grains have been fractured and pulledut of the alumina ball (Fig. 7). Wear by grain fracture and grainemoval has previously been reported in sliding wear of alumina

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ouples [25]. In contrast, the wear spots on the borided ballsppear smooth, particularly in the boride layer regions. This islearly observed on the smallest wear spot (Fig. 5). In this wearpot, the TiB2 layer is visible as the white material in the opticalicrographs (it appears as the dark phase in the SEM images),

ocated principally along the periphery. The beginning of theiB zone is visible as the light gray material located principally

n the interior of the spot. Darker gray material, representing theiB whisker + Ti-alloy zone, is scattered throughout the inte-ior. It is apparent that the TiB2 layer is worn through in theentral area, while it is retained at the periphery, as anticipatedn view of the ball’s curvature. Neither boride layer (TiB2 or TiBhisker layer) exhibited any sign of fracture or grain-fracture-

elated wear mechanism. With the exception of some parallelear striations, the boride surfaces appeared smooth. The fact

hat the TiB2 does not appear uniformly around the entirepot circumference may be attributable to asphericity of theall.

The largest wear spot on a borided ball exhibited a sim-lar smoothness of the worn layers. Inward from the spot’sdge, the boride layers are identifiable, first as the white TiB2,hen by the light gray TiB (Fig. 6—colors are reversed forEM image). The TiB2 layer is quite smooth, without any evi-ence of fracture or pull-out related wear processes. Furthernward, within the darker gray interior of Fig. 6(b), there are

any small white specs, which appear to be cross-sections ofiB whiskers. The increased wear rate of this specimen rela-

ive to the one having the smallest wear spot correlates wellith the reduced proportion of boride material in the wear

pot.It can be inferred that the variation in wear rates between

he borided balls is linked to the varying degrees of exposure ofhe TiB whisker + Ti-alloy zone (e.g. Fig. 6) in different balls tohe wear process. While the exposed inner region is undoubt-dly softer than the boride annulus, the TiB whisker contenthould significantly reduce its wear rate compared to that ofhe untreated Ti-alloy. This is supported by previous observa-ions of about 3× reductions in wear rate of a Ti-alloy + 20%iB2 composite (the composite actually contained TiB partic-lates) compared to that of the Ti-alloy base [15]. Eventually,ith increasing linear wear, the whisker layer would be sur-assed, the area fraction of the boride layers would decrease,nd the wear rate should become essentially close to that of ai-alloy ball.

.4. Nature of wear tracks on disk

The geometry and the depth of the wear tracks and theicrostructure-scale damage on the alumina disk were depen-

ent on the ball material. Fig. 9(a)–(d) illustrates the scanninglectron microscopic analysis of a wear track made on anlumina disk by an alumina ball along with the details ofygo profilometer measurements of wear track geometry.

ig. 10(a)–(d) illustrates similar data for a wear track made byborided Ti–6Al–4V ball.

The wear track created by the alumina ball is deeper andider (Fig. 9(a) and (b)) compared to that made by a borided

384 C. Lee et al. / Wear 265 (2008) 375–386

Fig. 9. The nature of damage in the wear track of alumina disk created by alumina ball: (a) track width, (b) Zygo profilometric topography, (c) track depth profile atone location, and (d) SEM micrograph of worn region inside the track.

Fig. 10. The nature of damage in the wear track of alumina disk created by borided Ti–6Al–4V ball: (a) track width, (b) Zygo profilometric topography, (c) depthprofile at one location, and (d) SEM micrograph of worn region inside the track.

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all (Fig. 10(a) and (b)). The alumina ball wore into the aluminaisk creating a wear track of about 1.13 mm wide and 1.75 �meep (Fig. 9(c)). In contrast, even though the track made byhe borided ball could be seen macroscopically as a faint, grayine, the profilometry data showed that the borided ball barelycratched the surface of the disk (Fig. 10(c)). Further, the micro-copic damage from the alumina ball on the alumina disk isuch worse than the damage created by the borided ball. Inig. 9(d), the alumina appears to be damaged with the grainracture features resembling those of a tensile fracture surface.n comparison, the microstructure of the wear track made by theorided ball (Fig. 10(d)) is relatively smooth and appeared sim-lar to that of the unworn surface (Fig. 1(b)). It is therefore clearhat the relatively reduced wear in the alumina disk + boridedi–6Al–4V ball combination is attributable to an abrasive wearechanism that produces a smooth surface both on the borided

all and on the disk. In contrast, the alumina disk + alumina ballombination seemed to result in a grain-fracture-and-removalear mechanism, ultimately producing a much greater specificear rate.

. Conclusions

. The volumetric wear rate of titanium alloy balls coated witha dual layer boride (TiB2 + TiB whisker) mated against alu-mina was 40 times less than that of alumina balls matedagainst alumina. This was found to be the case even ininstances where a substantial amount of substrate Ti-alloywas exposed during the wear process.

. The width and depth of the wear track on the aluminadisk by the alumina ball was much larger than that madeby the borided ball. The borided ball barely scratchedthe disk surface and showed much superior wear perfor-mance.

. The boride layers wore by a different mechanism thanthe alumina. The alumina wear surfaces exhibited grain-fracture-related wear mechanisms, producing a roughsurface, whereas the boride material wore by an abra-sive process, resulting in a relatively smooth wearsurface.

. The COF of the boride-coated Ti–6Al–4V balls wassmaller than that of alumina balls by a narrow margin.This was observed in spite of the fact that the over-all initial surface roughness of the alumina balls wasbetter.

. The boriding process produced surface roughening andgeometric form changes that influenced the sphericity ofthe balls. The sphericity changes, although slight, affectedthe polishing process and resulted in a variable sur-face texture, hardness, coating thickness, and ultimately,wear.

cknowledgements

The authors gratefully acknowledge the State of Utah Officef Economic Development and Ortho Development for finan-ial support of this work. We are indebted to Dave Edwards

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nd Loveridge Machine Co. for manufacturing the wear test-ng machine, Jack Clark and the Zygo Corporation for Zygoew View 5032 software and training, and Loren Reith foris assistance in the Surface Laboratory at the University oftah.

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