[SSS15-P05] Frictional strength of agate at intermediate slip rates in air and argon atmospheres
Keywords:frictional strength at intermediate slip rates, agate, in air, in argon atmosphere
At V = 1 cm/s, steady-state friction coefficients μss at TBG = RT in air and argon are ≈0.65 without noticeable difference, suggesting that moisture-adsorption by wear materials does not affect μ. Thermography shows that the slip surface temperature (TSS) ranges from 50°C to 70°C. In contrast, μss at TBG = 100°C in air is ≈0.4, suggesting decreasing μss due to increasing TSS. XRD and FTIR analyses show that wear materials after all these experiments are largely amorphous, and contain similar amounts of adsorbed water. This implies that silica gel is formed even soon after the experiments at dry conditions.
At V = 10 cm/s, μss at TBG = RT in air and argon as well as μss at TBG = 100°C in air are ≈0.3 without noticeable difference. Thermography shows that TSS reaches 170°C when μ reaches the maximum followed by subsequent significant weakening. TSS tends to synchronize with μ, suggesting a feedback so that increasing μ causes increasing TSS followed by decreasing μ and then decreasing TSS. Thus μ is likely controlled by TSS at this V.
At V = 1 mm/s, μss at TBG = RT in air is ≈0.8, which is significantly higher than μss (≈0.65) at V = 1 cm/s. Thermography shows that TSS is ≈40°C, which is lower than TSS (50−70°C) at V = 1 cm/s. The difference in μss between V = 1 mm/s and V = 1 cm/s is attributable to different amounts of water adsorbed by amorphous wear materials at different TSS. Adsorption of water by amorphous wear materials is known to increase µ.
Thus our results show that agate exhibits significant weakening only when TSS is high, but not due to the presence of silica gel. Additional friction experiment on silica gel gouge at V = 1 mm/s and TBG = RT in air also reveals that its μss is ≈0.6, similar to μss of common rocks and minerals, casting doubt on the lubrication effect of silica gel. The reason for the significant weakening of quartz rocks at V ranging from 1 mm/s to 1 cm/s during which TSS does not exceed 100°C, however, remains unknown.