Schematic of sliding tribometer: (a) -metal button, (b) copper slider, (c) diamond coated tracks, (d) copper vee-track base, (e) natural diamond counterfaces, (f) thermometer, (g) solenoid, (h) heater, (i) pivot, and (j) counterweight.
Optical micrograph of the diamond tips, which slide across the diamond film substrates. The tips are 90° cones, which initially end in a sharp point with a radius, as shown on the left. After an experimental run, the tips wear into a truncated cone with a typical radius of . Most of this wear occurs in the initial run-in period. The tip shown on the right was cycled thousands of times to make the worn region more obvious.
The normal force on the block from the track has contributions from (a) the vee-track shape and (b) the angle the track is tilted for the block to slide. This contributes factors of and , respectively, giving a total normal force of .
Friction coefficient as a function of cycle number for various types of diamond film substrates in air at room temperature. The plot shows that DLC undergoes the largest and fastest run-in process, while MCD undergoes the smallest and slowest change.
Schematic of cryostat: (a) vacuum feedthrough, (b) vacuum can, (c) 40 K shield, (d) 4 K shield, (e) spotlight, (f) sliding tribometer, and (g) high speed camera.
Friction coefficient as a function of relative cycle number at room temperature, showing the effects of pressure. The points to the left of the dotted line are obtained in atmospheric air after the run-in process observed in Fig. 4. The points to the right of the dotted line (UHV, ) are shifted to begin at cycle 100. Only MCD (lowest hydrogen content) is affected by vacuum.
Friction coefficient of MCD as a function of relative cycle number. The sample was worn in a room temperature in air. No further sliding cycles were performed until the sample reached 7 K under cryogenic vacuum. At low temperature, less than 10 sliding cycles were required to reach a steady state high friction state.
Friction coefficient for several types of diamond substrate as a function of temperature. The friction is similar for both cooling and heating runs for all materials. The friction for MCD is high and for DLC low, and both have only a weak temperature dependence, with a slight tendency toward higher friction at low temperature. The friction in UNCD makes a reversible transition between high and low friction states in the temperature range of 120–220 K. All measurements under cryogenic vacuum.
Friction coefficient of UNCD1 plotted vs relative cycle number at constant temperatures near the beginning temperature of the observed friction rise regime as seen in Fig. 8. Friction was measured at 295 K, then at lower temperature (228 or 206 K), then again at 295 K. The arrows indicate time progression, but are not linear fits to the data. Data were taken after temperature cycling (Fig. 8) but before breaking cryogenic vacuum. All measurements under cryogenic vacuum.
Diamond film characteristics. Roughness values, measured before friction tests, have no effect on friction after initial run-in. Surface hydrogen content is for the top 500 nm of the film. All films are deposited on silicon wafers and are thick except for MCD, which is thick.
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