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ELSEVIER Wear 181-183 (1995) 571-580

Tribological behavior of silicon nitrideunlubricated sliding between 22 C

materials underand 1000 C

WEAR

A. Skopp, M. Woydt, K.-H. HabigFederal I nstitute for Matetial s Research and Testing @AM), Unter den Eichen (97, 12200 Berl in, Gemany

Received 29 April 1994; accepted 20 October 1994

AbstractFriction and wear of 13 different materials with silicon nitride (S& NJ matrix were investigated under u nlubricated conditions.

Tests were performed in laboratory air at sliding velocities between 0.03 and 5 m s--r and at temperatures in the range of22 to 100 0 C. These materials were tested in the same test rig by means of the pin-on-disc configuration with self-matedcouples to evalu ate tribolog ical influences of compo sition, structure and other properties, which were varied to reach frictioncoefficients below 0.2 and wear coefficients below 1 X 10e6 mm3 N-r m-r.

For mono lithic silicon nitride materials, high friction coefficients between 0.4 and 0.9 and wear coefficients between 5 X 10e6and 1 x lo-* mm3 N- m-r were measured. By adding boron nitride, the friction coefficient could be reduced to 0.1 at roomtemperature. Simultaneously, after running-in, the wear rate was also reduced by a factor of five in comparison to monolithicsilicon nitride. Through the addition of titanium nitride, the wear was reduced by a factor of five at room temperature andeven by two orders of magnitude at a high temperature of 800 C.

All materials and worn surfaces as well as wear debris were extensively characterized by means of SEM, plus energy-dispersive X-ray , X-ray diffraction, small-spot electron spectroscopy for chemical analysis, electron-probe microan alysis, Aug erelectron spectroscopy , Fouri er transform infrared spectroscopy and transmission electron microscopy before and after thetribological tests. Combined with theoretical estimations on mechanical and thermal stresses, which are based on calculationsof hot-spot (flash) and bulk temperatures, wear processes and wear models are proposed.

The monolithic silicon nitrides investigated reveal a mechanically dominated wear mechanism with fatigue between thegrains and the amorphous grain boundaries. Tribochemical reaction layers (SiO,, SiO,N,, Si3N,.Y,03) did not tend to decreasefriction and/or wear in a wide range of test conditions. Only at higher temperatures above 40 0 C and higher sliding velocities(above 0.3 m SK at 1000 C and above 1 m s-r at 400 C) was a more pronounced wear reduction caused by the tribochemicalformation of protective layers and their plastic deformation.

The addition of titanium nitride (TiN) or boron nitride (BN ) led to microstructures which were mainly free from glassygrain-boundary phases. More important, these silicon nitride composites are able to form relatively soft lubricious oxides likeTiOz_, and H,BO, or an intrinsically solid lubricant like BN.H,O on a hard substrate, so that then a tribochemical wearmechanism dominates.

Considering tribological and surface analysis results as well as theoretical calculations, wear maps are given for the variousmonolithic silicon nitride m aterials and for the composites Si,N,-TiN and S&N,-BN. They give indications for the furtherdevelopment of ceramic materials.Keywords: Silicon nitride; Wear maps; High temperature; Tribology; T-maps; Ceramics composites

1. Introduction

For moderately stressed silicon nitride slidin g pairs(low mean contact pressure and/or low velocities) inhumid atmospheres up to temperatures of about 800C (in humidity-saturated gases), low friction and wearcoefficients have been measured [l-8]. In dry and normalatmospheres and for more highly stressed sliding couples

0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights resewedSSDI 0043-1648(94)07082-2

of silicon nitride materials, only high friction coefficientsand high wear coefficients un der unlub ricated conditionshave been pub lished [1,2,6-121. To compare the tri-bological results of ceramics in a wide range of testingconditions, wear maps have been proposed by differentauthors [12-181. However, nearly all of the numerousexisting silicon materials have been tribologically char-acterized in different test rigs under differing testing

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conditions [l-8,16-17]. This hinders the comparison ofthe tribological behavior and the evaluation of influencesfrom fabrication, composition and structure.

Under severely stressed unlubricated sliding condi-tions, three silicon materials have been tested by Iwasaand Toibana [9] and by Childs and M imaroglu [lo].They were tested against themselves in the same testrig to investigate mainly the effects of composition andprocessing route. In a small range of testing con ditions,the results of Iwasa and Toibana [9] indicate that thecomposition has an influence on the wear. The lowestwear was measured for S&N, with 5 wt.% La,OJ5 wt.%Y,O,, increasing for Si,N, with 5 wt.% YZ045 wt.%A&O ,, and was highest for S&N, with 5 wt.% MgO .The tests by Childs and Mimarog lu [lo] indicate alower wear coefficient for the rotating specimen, whensilicon nitride w ith a lower glass-ph ase content (additionof 2% MgO instead 4% MgO) was used.

For eight silicon nitride materials with different com-positions (varied additives and additive contents be-tween 4 and 20 wt.%), different production routes an ddifferent microstructures, tribological tests were per-formed under unlubricated and highly stressed slidingconditions [11,12]. All materials were investigated inone test rig with the pin-on-disk configuration. Am bientatmosphere was air with relative hum idities between20% and 50% (at room temperature). For slidingvelocities between 0.03 and 5 m s-l and temperaturesbetween 22 and 1000 C, the influences of compositionand structure on the tribological behavior were small.For these materials the friction coefficients were high,in the region between 0.4 and 0.9, and the wearcoefficients w ere higher than 5~10-~ mm 3 N- m-l,increasing with ambient temperature to values up tolo- mm3 N-l m-l. The interrelationships of me-chanical and chemical aspects were extensively inves-tigated by various methods of surface analysis, and bytheoretical estimations [11,12,19,20].

To overcome the unsatisfactory tribological s lidingbehavior of conventional silicon nitride materials, sec-ondary phases (which should have self-lubricious ca-pabilities with low friction and high wear resistance)were added . S econdary phases should also give a higherresistance against mechanical wear and, if possible,better mechanical properties.Ti-based phases like TiN, with the ability to formTiOa_x films [21-231, and BN, which can build upintrinsic self-lubricating films (which are expected toshow a beneficial tribological behavior [24,25]), werechosen as proper candidates to be added to S&N ,.Recent results with TiO,_, phases (e.g. Mag ntli phasesof Ti,O,_, with n =4-10) show that the films are notstable against changes of temperature and atmosphericpressure [23] and that the measured low friction valuesare not reproducible. Tests have shown that the tri-bological behavior of hexagonal BN and that of materials

containing BN depends on the ambient atmosphere[24,25]. Low friction and low wear have been measuredin hum id air. Gardos [25] has stated that low frictionis due to the unique role of water vapor in the frictionof lamellar compou nds. Erdemir et al. [26,27] haveascribed very low friction coefficients to the formationof self-lubricating boric acid films on boron-containingsurfaces.The use of these p hases in composites, which couldhave some stabilizing effect on the desired phases, hasbeen shown only in a few publications [28,29]. Recently,experimental results with S&N ,-TiN under dry oscil-lating sliding conditions were published by Imada etal. [28] and s how that the wear coefficient of Si,N,was distinctly reduced by the addition of TiN. Thewear then decreased with increasing humidity and de-creasing load . Tribological tests with Si3N,-BN (2 to30 wt.% BN ) sliding pairs have been performed byIwasa and K akiuchi [29] and reveal a drop in the wearcoefficient by an order of magnitud e to low6 mm 3 N-m -l, when more than 20 wt.% BN was added to Si,N+Simultaneously, a friction-reducing effect was not ob-served, with friction coefficients above 0.5.

Under severe (highly stressed) dry sliding conditions,friction and/or wear of silicon nitride-based materialshas been reduced over a wide range of speeds andtemperatures by the addition of titanium nitride (TiN)or hexagonal boron nitride (BN) [30,31]. The surfacesand reaction layers have been investigated by variousmethods of surface analysis [1 2,19,20,30]. On the basisof some of these results, as well as on the calculationof contact tem peratures and thermal stresses [12], thiswork reveals in a comprehensive form the mechan ismsof wear for self-mated Si,N, m aterials, giving wearmap s and indications for the development of siliconnitride materials with an improved tribological b ehavior.

2. Experimental details2.1. M a t er i a l s

Eight commercially available silicon nitride materialswere tribologically characterized: SSi,N, (ND2 00),HIPRBSi3N_, (411), HIPS&N, (FX-950), G PSSi3N,(SN73), GPSSi,N_, (ESK), GPSRBSi,N, (ESK),HPSi,N, (ESK) and HIPS&N, (ESK), where S=sin-tered, HP = hot-pressed, HIP = hot-isostatically-pressed,RB = reaction-bonded, GPS = gas-pressure-sintered.Beside the production process, the amoun t, distributionand type of additives as well as the structure of thesepolycrystalline materials were varied over a broad range.Material characterization - chemical composition,structure and physical properties - is given elsewhere[11,12,19,20].

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In addition, S&N, with 30 wt.% TiN and Si,N4-BNwith 5, 10 and 20 wt.% BN were tribologically char-acterized. Analyses with TEM show that, in contrastto the polycrystalline silicon nitride m aterials, thesematerials are mainly free of glass phases.

volumetric wear coefficient is the sum of the volumetricwear coefficients of the stationary and the rotatingspecimens.

3. Results and discussion2.2. Tes t r i g and exper im en ta l p rocedu reAll materials were investigated in the same high-

temperature tribometer in a pin-on-disk configuration[32]. The schematic drawing and the arrangement ofthe specimens are shown in Fig. 1. The stationaryspecimen with a curved surface is pressed against theplane surface of a rotating disc (rotating specimen).With increasing wear, the contact pressure decreasesaccording to the test geometry, with a defined pointcontact at the beginning , so that statements concerningthe influence of the mean co ntact pressure are alsopossible. During the tests, up to a sliding distance ofabout 1000 m, sliding velocity (0.03, 0.1, 0.3, 1 and 3m s-l), normal lo ad (10 N) and temperature (22, 400,800 and 1000 C) were kept constant. At room tem-perature the tribological tests were performed in lab-oratory air with relative hum idities between 20 and50% . Friction force, total linear wear and furnacetemperature were continuously mea sured.

The following definitions are used:Friction coefficient f= friction force FJnorm al loadVolumetric wear coefficient k = wear volume W V /

(normal load F,Xsliding distance s)The differential volumetric wear coefficient k (= W V /

(FN As ) ) was calculated at a sliding distance of 1000m, when after running-in the wear rate was stationary.The wear volume w as calculated from the wear scardimension s and profilometric measurem ents. In theliterature, the wear factor is often u sed (here synon -ymous with the volumetric wear coefficient). The total

3.1. Expe r imen ta l r esu l t sFigs. 2-4 summarize the results of previous work

[11,12]. At higher temp eratures of 400, 800 and 100 0C (Fig. 2 shows exam ples for 800 C) for all conventionalsilicon nitride materials tested, the total volumetricwear coefficient drops with increasing sliding velocityby up to two orders of magnitu de, whereas at 22 Cthe total volumetric wear coefficient increases at slidingvelocities above 0.5 m s- by a factor of five to ten[11,12]. At sliding velocities between 1 m s- and 3m s-l, only for GPSSi,N , (SN73 ) with additions ofCeO, and ZrO, were a lower wear rate of 5 X10p6

Solid state frlctlonSIZN~ I SIGNS

lo+FN = IO N; rel humldlty at 22C. 20 - 40 %

2 10-6L ,,I. I (..I ,.,J10. lo- 100 10%

Sliding velocity v [m/s] --)Fig. 2. Total volumetric wear coefficient for unlubricated self-matedsliding pairs of conventional polycrystalline Si3N4 materials as afunction of velocity at 22 C and 800 C.

SOlId state rlctlonSi3N, / Si,N,F,- 10 N :T - 22C

lo+1 6 . .I10-z 10-I 10o 10Slld;ng vel oci ty v Ims) -iii 2;

Fig. 3. Critical frictional power density for the transition betweena high wear rate and a lower wear rate of Si,N4 composites forunlubricated sliding.ig. 1. High-temperature tribometer and test configuration.

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0.03 01 03 1 3 ms 0.03 0.1 0.3 1 3msShdlng vel oaty v

Fig. 4. Differential volumetric wear coefficient of the stationaryspecimen for the composites Si3N4-TiN and S&N,-BN in comparisonto conventionally S&N., materials at 22 C and 800 C.

mm 3 N-l m-l and a lower friction coefficient of 0.2measured. It should be noted that un der all test con-ditions the wear volume of the rotating specimen washigher than that of the stationary specimen [11,12].The total volumetric wear coefficients were above lop5mm 3 N- m-l and increased with increasing temper-ature.

The wear rate of silicon-nitride-based compositesdecreases with increasing sliding distance, wh ereas thewear rate of polycrystalline silicon nitrides is nearlyindependent of sliding distance (as has been sho wnearlier [11,12]). From the linear wear vs. sliding distancecurves, a critical w ear volume (at the end of running-in) can be determined for the composites. From thisvalue, the critical mean contact pressure as well as thecritical frictional power density can be calculated [12].This critical frictional power den sity at the transitionbetween high and low wear rates is shown in Fig. 3as a function of sliding velocity for composites withBN or TiN.

After a sliding distance of 1000 m the differentialvolumetric wear coefficients of conventional Si,N, ma-terials and those of the composites S&N ,-TiN andSi3N,-BN at 22 C and 80 0 C are compared in Fig.4. The wear rate of silicon nitrides is distinctly reduced(up to orders of magnitud e) when secondary phasesare added to the silicon nitride matrix. Th is was observedover a wide range of sliding velocities at lower and athigher temperatures.

The surfaces and the wear debris were investigatedby means of scanning electron microscopy (SEM ) plusenergy-dispersive X-ray (EDX ), sm all spot X-ray dif-fraction (XRD ), Fourier transform infrared micro spec-troscopy (micro FT-IR), electron probe micro analysis

(EPM A), Auger electron spectroscopy @ES ), small-spot electron spectroscopy for chemical analysis (ESCA /XPS) and transmissio n electron microscopy (TEM ) withconvergent beam electron diffraction (CBED ) and en-ergy-dispersive spectroscopy (EDS). The results arepublished elsewhere [11,12,19,20].

The wear maps presented later are based on theresults show n in the figures and on the results fromwear surface analysis, as well as from hot-spot tem-perature calculations.3.2. Ca lcu la t i on o f t empera tu res on s l i d i ng su r f aces

The temperature calculations of Ashby and co-work-ers [33,34] and Kuhhnann-Wilsdorf [35,36] were usedin this work to estimate the bulk a nd the flashtemperatures (hot spot at the real asperity contacts)at the interface of rubbing surfaces. They are basedon theoretical models developed earlier by Blok [37],Archard [38] and Jaeger [39,40].

The surface temperatures in the tribocontact can beestimated from the frictional power density, givenas the product of sliding speed, load per mean contactarea and friction coefficient. Fu rthermore, the totalsurface temperature at the microcontacts is the sumof the ambient temperature, the bulk temperature andthe flash temperature. The calculations consider thetemperature dependence of thermomechan ical prop-erties of Si,N,. Influences due to wear particles andtribochemical reaction layers were neglected in themodels used.

Results from experiments with infrared thermography[41] for sapphire/Si,N , sliding pairs were introducedin the calculations to tune the T-MAP S 4.0 diagram[33,34] and for comparison, as has been described inRef. [12 ]. It should b e said that hardness and thermalproperties of sapphire and Si,N, are similar, and thatthe tribological test conditions are comparable withthose in the experiments of Griffioen et al. [41]. There-fore, the IR measurem ents give an indication of thesurface temperatures in Si,NdSi,N , sliding contacts andof the numb er of asperities under the conditions ofthis work. Griffioen et al. [41] measured, for 8.9 N and1.53 m s-l, hot-spot temperatures up to 1900 C. Theaverage numb er of hot spots was 7 f3. The thermo -graphically m easured flash temperatures show a widespread, probably because the asperity size lies in therange of 1 to 30 pm. Simultaneously, the flash tem-perature increases with the asperity size (under theassump tion that the total area of the microcontacts isconstant). Therefore, the temperature map has to betuned due to experimental results. W ith the propertiesadditionally given in the T-MA PS diagram , good agree-ment of measured and calculated temperatures hasbeen obtained . Only at higher slidin g speeds w ith highflash temperatures do the differences increase.

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Table 1Calculation of the flash temperature for one microcontact as function of sliding velocity for temperature-dependent and temperature-independentproperties: M odel Kuhlmann-Wilsdorf [35,36] cf= 0.5, N ,.,x= 1, T,=22 C ). Calcula tion program, Bartelt [42 ]. Calculatio ns for constant physicalproperties (AT,), temperature-dependent hardness (AT,) and temperature-dependent properties (ATJ2,(m/s)

SSi3N4/SSi3N4 (ND 200)AT, AT2(C) (C)

AT ,(C)

HIP-Si~N,/HIP-S&N4 (FX-950) S&N,-TiN/Si,N,-TIN (ESK)AT , AT, AT, AT, AT2 AT3(C) (C) (C) (C) (C) (C)

0.03 41 36 31 56 56 59 25 -0. 1 13 7 11 9 131 182 17 6 211 810. 3 34 5 34 2 453 600 52 2 732 2371.0 1007 85 2 1114 1420 96 1 1055 7131.5 1392 1030 1303 1992 p 1350 1480 998 _3. 0 2396 x1 1320 2590 n 3261 a 1764 1945 a 1750 - _

a T,,,, higher than sublimation temperature at 1 atm.

Lines of constant bulk and flash temperatures cal-culated using the software T-MA PS 4.0 of Ashby an dco-workers [33,34] are show n in Fig. 5 as a functionof mean contact pressure and sliding velocity. Theyreveal the strong influence of sliding velocity on theflash temperature, which exceeds the oxidation tem -perature of S&N , at nearly 1 m s-l.

Ishigaki et al. [6] have found an increasing wear rateabove 0.2 m s- and a phase transition from a-Si,N4to P-Si3N, at a sliding velocity of 1.5 m s-l. In air thisphase transition occurs n ormally at 1550 C, so thatthis is a further experimental indication for tribologicallyinduced high temperatures.

According to the model of Kuhlm ann-W ilsdorf[35,36], the flash temperatures can be calculated as afunction of the numb er and size of microcontacts. Basedupon this model, a computer program [42] was usedto calculate the bulk and the flash temperatures. First,the calculations were performed with constant physical

Fig. 5. Temperature map for unlubricated self-mated sliding pairsof SSiSNJSSiSN, (ND200); for comparison, experimentally detectedtemperature data and temperatures calculated with the model ofKuhlmann-Wilsdorf [35,36] are additionally given.

properties (at ambient temperature), then with a tem-perature-dependent hardness and , finally, with tem-perature-dependent properties (hardness, thermal con-ductivity, specific heat, thermal diffusivity, Youngsmodu lus) [12]. The temperature dependence of thesevalues wa s mainly available up to a temperature of 800 C, some up to 1900 C. Table 1 shows the calculatedflash temperatures under the condition of only onemicrocontact for constant p roperties and temperature-dependent properties (giving quite different flash tem-peratures, especially at higher slid ing velocities).

The outcome of the temperature calculations withthe model of Kuhlm ann-W ilsdorf [35,36] and that ofthe temperature map s constructed according to the T-MA PS 4.0 program of Ashby and co-workers [33,34]are close, if in both models only the temperaturedependence of the hardness (which decreases betw een22 C and 1000 C by nearly a factor of two) is considered.Wh en additionally the temperature dependence of thethermal conductivity (which decreases between 22 and1000 C, also by nearly a factor of two) is used, thenthe calculated temperatures above 0.3 m s- are higherthan those computed with the program of Ashby et al.In this model the temperature dependence of thethermal properties, like that of the thermal conductivity,is not considered.

The thermal stresses calculated on the basis of cor-responding flash temperatures [12] are show n as afunction of sliding velocity in Fig. 6. Above slidingvelocities of 0.5 m s-, the thermal stresses in thevicinity of the microcontact can exceed the bendingstrength of S&N ,. The simulation of thermal stresseswith the laser pulse method [43,44] on surfaces revealed,that - above critical values of the laser power d ensityin the rang e of 0.8 to 2 kJ slR cm- [43] - it comesto spontaneou s thermal crack ing and decomposition.

A comparison of these critical valu es with tribo-logically induced frictional p ower densities (definedas the frictional power (NR =fiFN) divided by the contactarea A and a factor of two for the distribution of the

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0pmqasaBap%m_w1a~a(

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-WAA6~

I3NN (OONS/NS LOOaSpO0(EJMV

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(a)

lb1Fig. 8. Wear models: (a) mechanically dominated wear mechanismwith intergranular fracture and delamination for Si3N4/Si3N4; (b)tribochemically dominated wear mechanism with the formation ofstable triboreaction layers or films on Si3N4-TiN and S&N,-BN.

the wear volume at the rotating disk is between fivefoldand about two orders of magnitu de higher than thewear volume of the stationary specimen [11,12]. Ahomogeneous amorphous CVD S&N, coating (withoutglassy grain boundaries) on S&N , reduced the wearrate of the rotating disk by a factor of five for slidingspeeds below 0.3 m s- at room temperature. At highervelocities thermal cracking, caused by high thermalstresses and the low thermal conductivity of this coating,probably occurs. Then the wear rate was as high as,or higher than, for uncoated polycrystalline Si,N,. Tri-bochemically formed phases such as SiOz, SiOH andSiO,N,,, which were also determined in the wear scaras well as in/on w ear particles with various su rfaceanalysis techniques [11,12,20], could not reduce thewear rate sufficiently. After removing loosely adheringwear particles and loose layers in an ultrasonic bath,investigations with SEM indicate also microfracture onthe surface below these layers. Recently performedTEM analysis of cross-sections reveals also intercrys-talline microfracture below the worn surface [20].

It should be considered that wear debris influencesthe tribological behavior, especially for the Si,N&N ,pairs. The wear debris often agglom erates and attachesto one or both of the sliding surfaces and forms m oreor less dense layers. These layers were plastically de-formed, broken away and sometimes transferred to thecounterbody [11,12]. At higher temperatures thickerlayers, built up by multiple passes, w ere found on thestationary specimen, protecting it against wear. Re-moving these adhering particles during the tribologicaltests led to a drop of the friction coefficient to 0.2-0.3for a short time.3.3.2. Si,N, composites with TiN or with BN

TEM investigations of the structure of these materialsdid not show any amorphous grain-boundary phase.Additionally, in Si,N,-TiN the residual stress es can bereduced by plastic deformation of TiN grains, whichis indicated by dislocations. Therefore, both aspectsreduce thermomechanical wear, and tribochemicallyformed reaction layers like Ti02_, and H3B0 ,, wh ich

Sohd state fnctionQN4 I &N4, FM = IO N

600 -mlcrocracktng \- delamlnatlon \

surfaceatigue- mlcrocracking- delaminatton- tnbooxidatlonplastIcallydeformedsurfaces

b03 0.1 0.3 1Slldlng veloctty v [m/s] -----&

3 t;

Si3N4-TIN I S&N,-TiN; FN = 10 N

tribooxldatlon Ik t, ?? l O~"mm/ Nm f > 0.5400 -?32%

surface fatigueE 200 - -microcrackingF L,-5 1O%m/N.f,O5

"" OW 1 3Shding velocity v [m/s] -

%N4-BN I SI,N.,-BN, F~ = IO N

surfaceatigue- microcracking- delamlnatlon

0.0 01 0,3 1 3Slidmg velocity v [m/s] p

Fig. 9. Dominating wear mechanisms with friction coefficients anddifferential volumetric wear coefficients for self-mated pairs of S&N,,S&N,-TiN and S&N,-BN as functions of sliding velocity and ambienttemperature (schematic wear map).were determined with various surface analysis methods[12,20], become predominan t. This is shown in Figs.8 and 9. Wh ereas the layers on S&N ,-TiN reduce thewear for all testing con ditions, the layers on Si3N4 -BNfail at temperatures above 150 C owing to the thermaldecomposition of H,BO, at this temperature [26,27,46].3.4. Wear maps

Wear m aps with the dominating wear mechanisms,and the corresponding friction coefficients and wear

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coefficients as a function of temperature and slidingvelocity are shown schematically in Fig. 9 for S&N, aswell as for the composites S&N,-TiN and S&N,-BN.

Mech anical w ear mechan isms (abrasive wear, grain-boundary fatigue, delamination) with intercrystallinecracking and, secondarily, wear particles determine thewear rate of the eight various conventional siliconnitrides. For higher sliding velocities above 0.5 m s-l,high surface temperatures with thermal stresse s andthermal cracking can increase the wear rate at roomtemperature, with the exception of GPSSi,N , (SN73)(Fig. 2) [12]. At higher ambient temperatures the wearrate drop s by an order of magnitu de above a criticalsliding velocity; the critical sliding speed decreases withincreasing ambient temperature [11,12] and is nearlyindependent of the type of silicon nitride used in thematerial p airing [12]. Distinct plastically deformed sur-face regions and tribochemically formed reaction layerswere found in this low-wear region.

Probably the plastic deformation of the tribochem-ically formed layer and/or of a layer of compacted weardebris and the plastic deformation of the surface regionsreduce the mechanical stresses at the contact and leadto the decreasing wear rate. (The thermom echanicalwear model of Ting and Winer [45] considers thesuperposition of thermal and isothermal stresses andthe temperature-dependent material properties, andthey discuss plastic deformation on the surface as awear mechan ism. A wear criteria equ ation, which isgoverned by three non-dime nsional parameters, is usedto predict the transition between mild and severe wearfor hertzian contact. The critical temperature for thetransition between mild and severe thermom echanicalwear was also calculated.) In any case, the wear ratefor the applied test conditions was above 10e5 mm3N-l m-l, and the friction coefficient was in the rangeof 0.4 to 0.9, so that con ventional silicon nitride materialsare not appropriate for most applications under u n-lubricated conditions.

Tribochemically dominated w ear mechan isms deter-mine the tribological behavior of the silicon-nitride-based com posites with additions of TiN or BN. Stablesurface layers of Ti02_, (mainly TiO, in the rutilestructure) on Si,N,-TiN and thin films of H,BO, orBN. (H,O) on Si,N,-BN were determined [12]. Si-multaneously, according to the nearly glass-free grainbound aries, the wear resistance of these compositesagainst mechanically caused grainboundary fatigue ormechanical wear is increased.

For the stability of tribochemical films, the contactpressure is important. There exist critical mean contactpressures and critical frictional power densities. B elowthese values [12,30] the wear rate drops by orders ofmagnitud e. The critical mean contact pressures andthe critical frictional power densities for these com-posites are dependent on the sliding velocity. At room

temperature the critical mean contact pressures lie inthe range betw een 3 and 1 4 M Pa. The critical frictionalpower densities increase with sliding velocity a nd arein the range between 0.3 and 4 W mm- for Si,N,-TiNand between 0.01 and 10 W mmw 2 for Si,N,-BN.

4. ConclusionsConsidering published results of tribological tests and

surface a nalysis on various polycrystalline silicon nitridematerials and on the silicon-nitride-based materialsS&N,-TiN and S&N,-BN [11,12] and the results of thiswork, especially the new calculations on friction-inducedsurface temperature increase, wear mechan isms wereelucidated, wear models deduced and schematic wearmaps constructed.

Wear of Si,NdSi,N , sliding pairs is determined bymechanically, thermomechanically and/or tribochemi-tally dominated wear mechan isms, depending on com-position (addition of secondary phases), structure (grain-boundary properties, amoun t and distribution) and testconditions.

Mon olithic silicon nitride materials reveal high frictionand wear for relatively highly stressed sliding com-ponents between 22 and 10 00 C. Their tribologicalbehavior is primarily determined by mechanical andthermomechanical wear mechanisms. Mechanisms withsecondary effects, su ch as the formation of wear particlelayers, plastic deformation and tribo-oxidation, couldnot reduce the friction and wear significantly. Therefore,utilization of this material under severe dry slidingconditions seems to be very limited.

Addition s of TiN or BN to the S&N , matrix canreduce the wear and/or the friction significantly. Besidesthe optimized mechanical properties of Si,N, compositeswithout any glass phase, the distinct improvement ofthe tribological behavior was due to the (tribochemical)formation of stable triboreaction layers.

The following points for the development of materialswith silicon nitride matrix with improved tribologicalsliding behavior can be deduced from earlier tribologicaltests, surface ana lysis and new theoretical calculations:

(1) To increase the wear resistance of conventionalSi3N4 materials (against mech anical and thermome-chanical wear),??the content of glass phase must be avoided and/or

reduced (if the glass phase is necessary for low-costsintering, a recrystallisation of the secondary phaseand/or an accommo dation of the thermomech anicalproperties of the Si,N, crystal and the phases betweenthem should be beneficial), and

??the thermal ditfusivity and thermal conductivity (re-duce thermal stresses; hinder thermal cracking) mustbe increased.

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(2) To reduce w ear and/or friction, a secondary phasemust be added to the silicon nitride matrix, which??forms stable, wear-resistant and lubricious o xides or

intrinsic solid films, or??is plastically deformable (like TiN particles) to reduce

internal stresses in the material.The comb ination of points (1) and (2) led to silicon

nitride-based materials with friction coefficients of 0.1and differential wear coefficients of about 1 X lob7 mm3N-l m-l, which h ave been measured under severelystressed sliding conditions (high velocities, high loads,high temperatures). The better understanding of thetribological effects from the addition of phases like BNand TiN to silicon-nitride-based materials and fromtheir combination is an important basis for improvingthe tribological performance of these materials. Theaim is to get mechanically and thermally resistantmaterials. Additionally, third phases should build uplayers reducing friction and wear in a wide range ofstressing conditions.

Acknowledgments

This work was partially supported with financialcontributions by. a project of BMF T (German Ministryof Research and Technology) (project no. 03TO O13A )and in the framework of a BRITE/EURAM project(contract no. RllB-183 ; proposal no. P-2148 ). Thissupport is gratefully acknow ledged.

The materials GPSS i,N, (EKasinS), HPS i,N,(EKasinD), GPSRBSi,N,, HIPSi,N, and Si,N, with 30wt.% TiN were provided by Elektroschmelzwerk Kemp-ten GmbH, Germany; SSi,N, (ND200) by FeldmtihleAG, Germany; HIPRB Si,N, (411) by HoechstCeramTec AG, Germany; GPSSi3N4 (SN73) by NGKInsulators Ltd., Japan ; HIPSi,N, (FX-950 ) by TungalloylToshiba, Japan ; and three Si,N,-BN (HIP) materialsby HTM AG, Switzerland.

Dr. Bartelt (BAM -5.23) is gratefully acknowledg edfor the development of a program to calculate hot-spot temperatures according to the model of Kuhlm ann-Wilsdorf. Dr. Klaffke and Dr. Wasche are gratefullyacknowledg ed for helpful discussions.

We thank our colleagues at BAM for their supportand the intensive cooperation. Mrs. Binkow ski is grate-fully acknowledg ed for metallography, photography andfigure layout.

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