Strong Velocity-Weakening of Nanograins at High Slip-Rates

It has been observed that slip localization zones in some experimental and natural faults consist of crystalline or amorphous nanograins of different minerals. Prolonged grinding of silicate rocks (e.g., quartz rock and granite) is known to produce amorphous silica nanograins and mechanical properties of the material (especially under wet condition) have been attributed to a mechanism of fault weakening. Also, recent high- velocity friction tests on carbonate rocks showed that faults can be weakened by thermal decomposition of calcite into nanograins of lime and carbon dioxide and the lubrication effect of the nanograins would be critical for the fault weakening. However, mechanical behavior(s) and friction mechanism(s) of fault slip zones with nanograins, especially at high slip-rates, are still poorly understood, despite their potential importance to the understanding of seismic faulting. In this contribution, we show you our experimental results indicating velocity-weakening of nanograins (probably caused by still unknown mechanical behaviors of nanograins) rather than by temperature-related weakening behavior. In our high-velocity friction tests on Carrara marble at seismic slip-rates, we have tried to "cool" the simulated fault with liquid nitrogen and compressed air during frictional sliding, and found, in the simulated fault coated with nanopowders of lime (CaO) formed by thermal decomposition, no correlation between friction and temperature measured with thermocouples (i.e., no temperature-related weakening behavior), although strong "velocity-weakening" behavior appeared. The observation was confirmed by another experiment: from (1) the calculated "maximum" sliding surface temperature [Carslaw and Jaeger, 1959] using the mechanical data, with an assumption of strong slip localization into a very thin layer, and (2) the measured temperature with thermocouples at a place just below the sliding surface and close to the periphery of the specimen, it was found that, at the "constant" velocity of 1.3 m/s, friction at ~420°C was very low (~0.15 in terms of friction coefficient) and was almost the same as that at 1260°C. On the other hand, interestingly, we could observe friction at the deceleration stage (when sliding velocity decreases gradually from 1.3 m/s to zero) was recovered notably to a higher value with decreasing velocity (thus velocity-weakening), although temperature change was < 100°C. Also, this weakening may not be explained by flash heating weakening, because of extremely small contact sizes of nanograins. From this previously unexpected behavior, now we are open to a possibility that friction mechanism(s) in nanograins may be different from those in ordinary grains (> μm) and we wonder what kind of friction mechanism operates in the nanograins. To get an insight into the questions, additional friction experiments on nanograins of different minerals are being conducted, and the experimental results as well as TEM observation results will be presented at the meeting.