SILICON NITRIDE MATERIALS WITH LOW FRICTION COEFFICIENT I. SCHULZ, M. HERRMANN, T. REICH, C. SCHUBERT Fraunhofer-Institut für Keramische Technologien und Sinterwerkstoffe, Winterbergstr. 28, D-01277 Dresden, GERMANY; e-mail: schulz@ikts.fhg.de SUMMARY A new Si3N4 material with nanosized microstructure containing fine equiaxial grains and SiO2 rich grain boundary phase was developed using fine ß - Si3N4 powder. The great potential of this Si3N4 material for bearings without lubricants was shown in rolling wear tests. Wear mechanisms in silicon nitride materials under dry fretting and rolling wear conditions using fine ß - Si3N4 powder and conventional α - Si3N4 powder was investigated. A different wear of grains in dependence on orientation to load was detected. An improvement of wear resistance was reached by refinement of microstructure and increasing of grain boundary strength.

Keywords: dry friction, wear, silicon nitride, microstructure

1 INTRODUCTION Silicon nitride based ceramics are increasingly being used in wear applications due to their high temperature reliability, hardness and corrosion resistance compared with metals. The use of silicon nitride materials in wear applications e. g. ball or sleeve bearings is often limited by the relatively high friction coefficient under unlubricated conditions. Up to now materials were developed with the goal of optimising the mechanical properties. The development of high-performance Si3N4 materials for wear parts in the production of cutting and forming tools, machines and engines was characterised by median grain sizes > 0.3 µm.

In this study possibility was found to reduce the friction coefficient and to increase wear resistance of silicon nitride materials by fabrication of silicon nitride materials with special microstructures. The goal of this paper is also to examine the wear behaviour and wear mechanism in silicon nitride ceramic under fretting and rolling wear condition in correlation to microstructure. 2 EXPERIMENTS The Si3N4 powder used was produced by PCT in Riga, Latvia in a inductive coupled N2-plasma using Si in the presence of 6 wt.-% Y2O3 and 3,2 wt.-% Al2O3. in order to make comparison to a conventional α - Si3N4 powder SN-E10 (UBE) was investigated too (tab. 1). The amount of ß – nuclei in the powder was obtained by calculation of a density of nuclei as Vß/dß

3 with the volume fraction of ß – Si3N4 Vß and the crystallite size of ß – Si3N4 dß by X – ray diffraction (measurement phase content, grain size by Rietfeld method (REFINE ++ system)). The powders were milled with the additives in a planetary mill in cyclohexane, dried, heated out and sintered at 1700oC, 30M Pa. 60 min to 99,9% density in a hot press. For mechanical characterisation 4-point bending strength at ground bares, fracture toughness KIC by DCM method were conducted. The microstructure of the samples was characterised by Field Emissions Microscopy. The wear behaviour at polished specimens from different

materials against a rolling ball from Si3N4 (HIP SN TN03NM, Toshiba) was studied under oscillating sliding conditions in the ball-on-disc configuration at contact stresses of 1 GPa and 3 Hz frequency. The friction coefficient and wear behaviour was measured in a rolling wear test equipment at a couple of rings from same material with outer diameters 70 mm and polished treads for different contact stresses without lubricant. Tests with increasing load were performed at a velocity of 400 rpm, 1% slip over 10000 rotations. After each run the treads were observed to detect cracks. The wear images were analysed by optical microscopy, FESEM and AFM.

powder Production method

ß- content ß-crystallite size

Relative density of nuclei

% nm SN-E10 Diimide1) 1,7 58±10 1

PP1 PCH2) 6,2 20±3 46 PP2 PCH3) 69 61±4 22 1) diimide synthesis 2) plasma chemical synthesised powder as received 3) plasma chemical synthesised powder heat treated

Table 1: Characteristic of used powder 3 RESULTS

3.1 Heat treatment

The plasma chemical powder PP1 contains 30% ß, α and amorphous phase. The powder PP2 was heat treated in presence of the oxinitride phase at a temperature < 1500oC to get a crystalline high ß – containing powder with fine ß nuclei. In the absence of the oxinitride liquid α – Si3N4 [98Som] is formed preferentially and higher temperatures are needed for ß – Si3N4 crystallisation connecting with strong grain growth [00Hir]. During the crystallisation of the plasma chemical produced powder the surface area was reduced by wetting the fine ß – crystallites with oxinitride liquid too. In this way a improvement of processing procedure and of resistance against hydrolysis were obtained. SN-E10 powder

contained also fine ß- nuclei. In table 1 a relative density was used by expressing relative to ß – nuclei density of SNE-10 equal 1. The heat treated plasma chemical powder PP2 exhibited nearly the same crystallite size than SN-E10 however a 22 times higher amount of ß – nuclei was measured. It was not possible to increase the ß – content of SN-E10 by a heat treatment without grain growth [5].

3.2 Microstructure development At low sintering temperature and short sintering time it was possible to generate materials with fine equiaxial grains using fine ß - Si3N4 powders. These new nanosized materials were characterised by a special microstructure consisting of fine ß - Si3N4 grains with diameters d50 < 0,1µm. The manufacture of these very fine-grained microstructures was especially successful if powder with a high amount of fine ß - Si3N4 nuclei as PP2 was used. In addition the known tendency of refinement of microstructure and decreasing of aspect ratio of grains with decreasing of Y2O3/Al2O3 ratio [99Rie] was used to modify materials from plasma powder and SN-E10 (tab. 2).

With increasing Al2O3/Y2O3 ratio a decreasing of aspect ratio, bending strength and fracture toughness was obtained for each powder. In difference to materials made from plasma powders materials based on SN-E10 powder were found to be generally coarser (Fig. 1, 2) and some silicon oxinitride inclusions appeared at a ratio Y2O3/Al2O3 = 0,2 using SN-E10. With high Al2O3 content the viscosity of liquid phase degreased. For that reason the materials from PP2 with Y2O3/Al2O3 – ratio of 0,7 and 0,4 could be hot pressed even at 1650oC.

2222µµµµmmmma)

2222µµµµmmmmb)

Figure 1: Microstructure of a material with a

Y2O3/Al2O3 ratio of 1,9(a) and 0,4 (b) using a heat treated plasma powder

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2222µµµµmmmmb)

Figure 2: Microstructure of a material with a

Y2O3/Al2O3 ratio of 1,9(a) and 02 (b) using a fine grained α - Si3N4 powder SN-E10

powder Y2O3/

Al2O3 Grains with A1)

> 5,5

Grain thickness,

σ25oC KIC

(DCM)

d50 d90 % µm µm MPa MPa

m1/2 SN-E10 1,9 19 0,12 0,28 968 4,5 SN-E10 1,1 15 0,15 0,35 791 4,2 SN-E10 0,2 7 0,12 0,28 634 3,4 PP2 1,9 14 0,09 0,17 675 4,2 PP2 0,7 14 0,09 0,17 491 3,3 PP2 0,4 8 0,08 0,12 363 2,5

1) amount of grains with a aspect ratio A >5,5 Table 2: material properties

3.3 Wear behaviour The investigation conducted showed that changes in microstructure resulted in a different wear behaviour of the materials. Materials produced from plasma powder with homogenous fine microstructures showed friction coefficients < 0,1 and no wear under sliding contact without lubrication a contact stress of 1 Gpa and low velocity. A major difference was obtained by investigating of materials produced from plasma powder with large ß – Si3N4 grains and Si2N2O inclusions which were sintered at temperatures higher than 1700oC. The friction coefficients of these materials were found to be > 0,4 and a high wear under dry fretting wear conditions was measured. Under dry fretting wear materials from SN-E10 showed also friction coefficients < 0,1 if they have a lower aspect ratio, a relative fine microstructure

due to a high Al2O3 content and a low hot press temperature of 1700oC. The friction coefficient was found always proportional to the wear rate as demonstrated for materials from plasma powder and SN-E10 (fig. 3).

PP2

SNE-10

Figure 3: Correlation between wear and friction coefficient in materials from heat treated plasma

powder PP2 and SN-E10 measured under dry fretting wear at 1 GPa Hertz pressing and 3 Hz frequency

Only the most homogeneous and finest material with a Y2O3/Al2O3 ratio of 0,7 from PP2 showed friction coefficients < 0,1 until Hertz pressing of 1,8 GPa over 10000 rotations at 80 times higher velocity after 100 times higher distance in rolling wear tests with 1% slip (fig. 4). Cracks were not detected at the friction trace by optical microscopy. All other materials listed in table 2 produced from PP2 and SN-E10 powders exhibited friction coefficients between 0,4 and 0,5. Cracks over the friction trace were detected (fig. 5) beginning at Hertz pressing 1,0 GPa.

An explanation of the different wear behaviour was found by studying wear tracks at relative low contact stresses by FESEM and AFM.

Any formation of thin SiO2 – containing layers reducing friction was not detected at the friction trace of materials with friction coefficients < 0,1 despite intensive search by FESEM and AFM.

material 1 (1,2GPa)

material 1 (1GPa)

material 2 (1,2GPa)material 2 (1GPa)

material 2 (1,8GPa)

Figure 4: Friction coefficient measured by dry rolling wear of material 1 using a commercial fine-grained α-

Si3N4 powder SN-E10 and material 2 using a heat-treated plasma powder PP2 at different Hertz pressing

(slip = 1%, velocity = 0.8 m/s)

Figure 5: Wear track of material 1 using a commercial

fine-grained α - Si3N4 powder SN-E10 after rolling wear at Hertz pressing of 1,0 GPa (slip = 1%,

velocity = 0.8 m/s)

Materials with a medium wear state and friction coefficients between 0,2 and 0,4 were characterised by breaking out the grain boundary phase (fig. 6). This resulted in the removal of grains when the contact stress or time was high enough. This wear state was characterised by heavy wear and a friction coefficient > 0,4.

Mainly rolling friction was found in rolling contact with a small rate of sliding friction in difference to fretting wear. But in evaluation of our tests the same wear mechanism was detected. At the beginning smoothing of friction trace took place. The wear started with a disruption of grain boundary phase too. With increasing contact stress or time intergranular cracks were formed across the trace until eruption of single grains proceeded.

A possible mechanism of wear in silicon nitride ceramic was indicated in a micrograph of the friction trace made by AFM, which shows a material from PP2 in the moment of transfer to heavy wear. At this characteristic load a characteristic surface profile was formed which pointed out the selective wear of grains in dependence to the direction of the ß – Si3N4 crystals (fig. 7).

1111µµµµmmmm

Figure 6: Wear track of material 1 Plasma powder after fretting wear (friction coefficient = 0,25)

The picture shows that grains with c-crystal axis vertical to the surface were abraded less than grains which are orientated parallel or vertical to the friction trace.

0nm

2500nm

1250nm

0nm 1250nm 2500nm

0 n m0 n m0 n m0 n m 2 5 0 0 n m2 5 0 0 n m2 5 0 0 n m2 5 0 0 n m

3 0 n m3 0 n m3 0 n m3 0 n m

Figure 7: Surface profile of material from PP2 powder

after fretting wear (friction coefficient = 0,18) From this model it can be concluded that materials with the same grain size showed a reduced wear with increasing grain boundary strength. From the literature [1, 2] is known that materials with high Al2O3 content and SiO2 rich grain boundaries possess a high grain boundary strength. These materials exhibited lowest wear in our tests. 4 CONCLUSIONS Si3N4 materials with homogeneous microstructure containing fine equiaxial grains with diameters d50 < 0,1µm were produced from fine ß - Si3N4 powder.

Fine-grained ß - Si3N4 powder could be generated by crystallisation of plasma chemical powder in presence of oxinitride liquid.

In dry fretting wear tests materials with homogeneous microstructure containing fine equiaxial grains using heat treated plasma chemical powder or SN-E10 showed friction coefficients < 0,1 and no wear.

Only homogenous material containing finest equiaxial grains using plasma chemical powder and a grain boundary phase with high Al2O3 content exhibited friction coefficient < 0,1 under higher contact stress in rolling wear tests until high loads.

The same wear mechanisms could be detected for dry fretting and rolling wear in silicon nitride. With increasing contact stress or time the wear started with smoothing of friction trace, continued with intergranular crack until eruption of single grains proceeded.

Based on the results of the tests it was concluded that improved wear resistance can be reached by the refinement of microstructure and an increase of grain boundary strength by to SiO2 enrichment. 5 REFERENCES [1] Peterson, I. M., Tien, T_Y; Effect of the Grain Boundary Thermal Expansion Coefficient on the Fracture Toughness in Silicon Nitride, J. Am. Soc., 78 (9) 2345-5 (1995) [2] Sun, E. Y., Becher, P. F., Plucknett, K. P., Hsueh, C.-H., Alexander, K. B., Waters, S. B.; Microstructural Design of Silicon Nitride with Improved Fracture Toughness, J. Am. Soc., 81 (11) 2831-40 (1998) [3] Somiya, S., Mitomo, M., Yoshimura; M.; Silicon Nitride – 1, Elsevier, London New York, (1998), 83-102 [4] Riedel, G., Bestge, H., Herrmann, M.; Correlation between Structure and Properties of Si3N4 Materials with Y2O3 / Al2O3 Sintering Additives, CFI/DKG 76 (1999) [10] No. 1, 24-27 [5] Herrmann, M., Klemm, H., Schubert, C.; Handbook of Ceramic Hard Materials, Weinheim: Wiley-VCH 2 (2000) 749-801 [6] Hirata, T., Akiyama, K., Morimoto, T.; Synthesis of ß - Si3N4 particles from α - Si3N4 particles, J. of the European Ceramic Society 20 (2000) 1191-1195