Wear and Friction Behavior of Ni-SiC Composite Coatings

Thin Films in Tribology / D.D o w s m et al. (Editors) 1993 Elsevier Science Publishers B.V.

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Wear and Friction Behavior of Ni-Sic Composite Coatings E.A. Rosset, S. Mischlcr, D. Landolt Materials Department, Ecole Polytechnique Fkdkrale de Lausanne, 1015 Lausanne, Switzerland

The sliding wear and frictional behavior o f Ni metal matrix composite coatings have been studied by using a reciprocating motion wear test rig. The effect of the heat treatment on structure and wear behavior is investigated. An attempt is made to elucidate the role of the interface between the particles and the matrix. Atomic Force Microscopy ( A M ) and Scanning Electron Microscopy (SEM) have been used to characterize thc worn surfaces. 1. INTRODUCTION Electroless and galvanic deposition techniques are cheap and flexible methods to deposit functional metal films on a wide range of substrates. They offer an easy way to produce metal matrix composite coatings. For example, one can obtain self-lubricating and anti-wear coatings by embedding either solid lubricant particles (i.e. P T E ) or hard particles (ie. carbides or nitrides) in a metallic matrix. This paper presents a study of NIP - SIC composite coatings. I t is well known that their tribological properties can be significantly improved with an appropriate thermal treatment. The influence of the thermal treatment on wear properties has becii widel y studied i n terms of performance improvement but the wear mechanisms are not been yet fully understood. The aim of the present paper is to compare the sliding wear and frictional behaviour of a Nil’Sic composite before and after a thermal treatment. Wear rates and frictional coefficients are determined under dry sliding wear conditions against alumina. The morphology of the worn surfaces and o f the wear debris are observed by Scanning Electron Microscopy and Atomic Force Microscopy in order to understand the w a r mechanisms. Simple fracture test ivere carried out i n

order to observe the dominant mechanical failure mode of the composites. 2. EXPERIMENTAL

2.1 Test Material Sliding wear conditions were established by rubbing an alumina pin against a metal matrix

Figure 1. Wear test rig. composite coating plated on steel. Technical pure alumina rods of 4 mm diameter were supplied by Metoxit AG, Thayngen, Switzerland. SIC particles, 1.8p m mean diameter, sup-

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plied by Lonza - Werke Gmbh (UF-lo), Waldshut, Germany, are embedded by co-deposition in an electroless NIP metal matrix 40 pm thick. The phosphorus content is 7-8 wt%. The volumic ratio of S i c particles is 27%. Thermal treatment consists in heating the coating for 5 hours at 290°C in a nitrogeiiihydrogen atmosphere and increase the hardness to 1200 HV0.05 . For fracture surface analysis the coating was separated from the substrate by dissolution of the steel substrate in hydrochloric acid. Fracture surfaces were produced by bending the isolated coating in air. 2.2 Wear test The wear tests were carried out in the reciprocating pin-on-plate rig depicted schematically in Fig. 1. Friction tests are done in air, 50-60 %RH, 20-23 "C. Sliding wear conditions are established by rubbing the end of a vertically mounted pin, figure 1 (1). against a flat sample (2). The sample holder is located above a load cell (3) allowing for the measurement of the normal force. The frictional force is measured with a piezoelectric force transducer (4). An IR optical sensor (5) measures the horizontal pin displacement and a lasermeter (Keyence LC-2100)(6) measures the vertical pin displacement with a 0.2 pm resolution. Reciprocal pin motion is provided by an electrodynamic vibration exciter (Bruel Kjaer 4809)(7). During the wear test the frictional and the normal forces as well as the horizontal and vertical pin displacements transients were monitored using a Macintosh IIfx computer equipped with a National Instrument general purpose 16 bits I/O board controlled by the LabView 2 data acquisition and control software. Real time data analysis provides the wear rate and coefficient of friction evolution. The oscillating pins were prepared by machining the ends of the alumina rods in the shape of truncated cones (120" included angle). The

diameter of the flat end was 0.5 mm giving an apparent contact area of 0.2 mm2. The pin was oscillating at a frequency of 5 Hz. The stroke length of 5 mm corresponded to an average sliding velocity of 50 mm/s. The applied normal load was of 5 N resulting in a nominal contact pressure of 25 MPa. The samples were tested just after a 5 minutes ultrasonic cleaning in ethanol and were dried with a warm air jet. The initial roughness, Ra, was 1.8pm for the non heat treated (HT) samples and 1.6 p m for the HT samples, respectively, over a cut-off length of 0.8 mm. 2.3 Wear analysis The surface topography was measured by use of a Taylor-Hobson talysurf 10 profilometer and a Park Scientific Instruments SFM-BD2 Atomic Force Microscope (AFM). The morphology was observed with a Cambridge Stereoscan 650 SEM. EDAX analysis was performed using a Cambridge Stereoscan 250 SEM. The structure was analysed with a Siemens D500 Kristalloflex RX diffractometer. 3. RESULTS

3.1 Structure The X-Ray spectrum recorded before heat treatment (Fig. 2a) exhibits well defined peaks corresponding to the a-Sic as well as a broad peak

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Diffraction Angle 20 Figure 2. X-Ray Spectrum of a) Non Hr sample, b) HT sample.

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at 45". The presence of this peak indicates an amorphous structure of the Ni-P matrix. After heat treatment several X-Ray lines (Fig. 2b) appear superimposed to the broad peak corresponding to the amorphous state. This indicates that a partial recristallisation of the matrix has occurred. The lines observed in Fig. 2b correspond to the compounds a-Sic; Ni,P and Ni,Si. It is known that, by thermal activation, amorphous NIP undergoes an eutectic reaction leading to the formation of two phases: pure Nickel and Ni,P. For a phosphorous content of 7.5 % the ratio of the volume fractions of the two phases is 5050 [ 11. The Ni,Si phase forms by diffusion of Si from the S i c particles in the Ni matrix. Moos and co-workers [2] observed a complete Sic dissociation in a NIP composite with S% S i c after a heat treatment of 2 hours at 600°C.

3.2 Wear tests The evolution of the coefficient of friction (p) and of the vertical pin displacement with the number of cycles is plotted in Fig. 3. The value of the coefficient of friction scatters considerably between different tests on the same sample and assume the value of 0.3 f 0.1 independently on the heat treatment. The wear coefficients were determined by profilometry after the wear test and correspond to 3.8 and 8.8 lo* mm3/Nm for samples with and without heat treatment respectively. The deviation of the wear coefficients was 20%. The wear track was generally flat independent of heat treatment. Numerous pits were observes on the non HT samples, whilst only few were present on the HT sample. The morphology observed in figure 4 top exhibits geometrical features. Cracks, probably due to fatigue, were present at the ends of the pits in the sliding direction. The surface morphology of the pits shown in figure 4 bottom corresponds to the original sur-

face. These pits resulted from the valleys which have not interacted mechanically with the pin yet and were due to the original sample roughness. Very few cracks, similar to the non HT samples, were observed. 05

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Figure 3. Coefficient of friction and vertical pin displacement evolution, without HT (top), with HT (bottom). 'The AFM images recorded on the flat parts of the wear track surface (Fig. 5) shows the presence of grooves on both the matrix and the Sic particles. The observed grooves indicate abrasion by sub-micron ploughing. The S i c particles protrude slightly (about 20 nm) from the matrix. The AFM image shown in figure 6 has been recorded in a pit of the non HT sample and indicates a faceted surface morphology. Wear debris were found aggregating at the end of the wear tracks on both samples. Significant differences in debris size and size distribu-

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Figure 5. Imbedded palticles in the backs. AFM topogmphy of 11011HT sample (top) aid HT sample (bottom).

Figure 4. Wear track surfaces, Noii HT sample (Top), HT sample (Bottom).

Figure6. A I M typal topography of a pit in a non HT sample.

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tion are found by scanning electron microscopy (Fig. 7). On HI' samples wear debris exhibit a very fine (submicron) and uniform size distribution. The size of wear debris generated on non HT samples ranges from 5 p m to 0.2 pm for most of the debris, the larger debris exhibiting a faceted geometry. Using E D A S analysis several large debris were identified as being Sic particles. Considerable metal transfer on the alumina pin was found during sliding on the non H?' samples. In this case the pin flat end was almost completely covered with a dark film. EDAX analysis showed that this film was composed of Ni, P and Si. The irregular morphology of this adhesion film is clearly illustrated by the SEM image (Fig. 8 top). 'The flat end of the pin rubbed against the HT sample was only locally covered with a dark films. EDAX analysis showed that the dark stains observed on the SEM images (Fig. 8 bottom) contain Ni, P and Si whilst the rest of the surface exhibits only an EDAX signal corresponding to the A1 from the Alumina pin.

3.3 Fracture surfaces SEM images of fracture surfaces are shown in Fig. 9. The morphology of these surfaces depends on thermal treatment of the sample: the no11 HT sample exhibits a faceted fracture surface with a facet size in the range 1 to 5 pm. Several Sic particles were identified on the non FIT fracture surface (dark features in the centre of Fig. 9 top). The shape and the average size of the facets observed in Fig. 9 top correspond very well with the geometry and dimensions of the SIC particles. This indicates that in the non HT sample the fracture propagates preferentially at the interface between particle and matrix. The fracture surface features observed on the H T sample are much finer than the average size of the Sic particles. This indicates that the fracture path goes, at least partially, through the Sic particle.

Figure 7. Wear debris. Non HT sample (Top), HI' sample (Bottom).

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Figure 8. Pin surfaces after sliding, on lion HT sample (Top), on HT sample (Bottom).

Figure 9. Fracture surface on non HT sample (Top) and on HT sample (Bottom).

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4. DISCUSSION The present results show that the heat treatment leads to a reduction of the wear coefficient of a factor of 2. The dominant wear mechanisms are also affected by the heat treatment : surface fatigue pits are observed only in non HT samples. On the same samples the extent of metal adhesion to the pin is more impoitant. Wear debris found on HT samples are finer and exhibit a more homogeneous size distribution. Abrasion (Fig. 5 ) occurs on both samples.

The vertical pin displacement scattering (Fig. 3) relates directly to the size of the particles observed (Fig. 7) and thus to the heat treatment. The 2 p m elevation of the pin on the noii HT sample (Fig. 3, top) corresponds to the formation of the transfer coating observed on the pin (Fig. 8, top). Such an elevation is scarcely observable for the HT sample (Fig. 3, bottom) and correlate with the rather clean surface of the pin (Fig. 8, bottom). The observed differences in wear behaviour are a consequence of the structure modification induced by the heat treatment. Before heat treatment the composite is composed of Sic particles dispersed in an amorphous Ni-P matrix. The S-Ray analysis shows that during heating phase transitions and chemical reactions occur which lead to the formation of Ni,P and Ni,Si. According to the literature formation of nickel phosphide takes place in the Ni-P matrix [I]. The formation of Ni,Si is controlled by the diffusion of Si out from the S i c particles into the matrix and is therefore expected to take place at the interface between matrix and particle. Additional compounds formed at the interface in composite materials influence the strength of matrix-particle cohesion and thus the mechanical properties of the material [3]. If the cohesion is too weak no load transfer between matrix and

reinforcing particle occurs. I n this case no improvement of the material properties results from the combination matrix-particle and cracks will propagate at the interface. A stronger interaction leads to a significant load transfer from the matrix to the reinforcing particle and thus cracks can be deviated or stopped. The fracture surfaces (Fig. 9) indicate that the formation of nickel silicide improves the cohesion between Sic particles and the matrix. h o r to the heat treatment the composite cracks along the particleslmatrix interfaces whilst after heat treatment the cohesion is much stronger and leads to a crack propagation through the S i c particles. The capability of the interface to transfer load may play an important role in wear. The S i c particles protrude from the matrix in the wear track and therefore carry most of the applied load during sliding. On non HT samples the load is not distributed to the surrounding matrix because of the weak particle-matrix cohesion. Highly concentrated stresses arise at the sharp edges of the S i c particles. This leads to microcracking with subsequent crack propagation at the interface caused by repeated alternating loading of the surface (surface fatigue). As consequence flaking of material occurs and pits are formed. On HT samples the sliding forces applied on a Sic particle are distributed by the interface to the surrounding matrix so that no stress concentration appears. No flaking of material is then expected. The predictions of this simple model agree well with the observed presence of pits only on the non HT sample. The faceted surface morphologies in the pits and the non HT fracture surface correspond to the model. Considering only the increase in the hardness of the matrix could lead to the opposite prediction. Because of the weak cohesion S i c particles are likely to be teared from the non HT composite during sliding. This can explain the presence of large S i c particles among

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the wear debris on the non I-IT sample Adhesion depends o n physical aiid chemical properties of the materials in contact, the mode and value of loading aiid on the state of the surfaces (roughness, contamination). N o major difference i n roughness, surface chemistry o r i n chemical properties of the material is to be expected between H?’ and non HT samples. Instead an increase of a factor of 2 i n hardness of the NiP phase was observed by Kloos [3] after a similar heat treatment. According to the adhesive law of wear an increase in hardness of the Ni-P matris leads to a reduced material loss.

5. CONCLUSIONS ‘The microstructure and the u.ear behaviour of the studied Ni-1’ i Sic composite coating are modified by the applied heat treatment. A mechanistic relationship between microstructure and wear process has been proposed. The role played by the matrix-SiC interface in the wear mechanism has been shown. REFERENCES [1]J. Osowiecki, W. Form, .I. Forchelet MCmoires el Etudes Scientifiques Revue de h46tallurgie 1 I , 581 (1985) [2]K.H. Kloos, G. Wagner, E. Rrosxit Z. Werkstofftcch 9,305 (1978) [3] R.W. Davidge Interfaces i n Materials, Proceedings of the Colloqium, Brussels, 9 December 1988