10:45 AM - 12:15 PM
[SCG50-P08] Evaluating the Effect of Humidity on Microdeformation in Single-Crystal Quartz Using Nanoindentation
The frictional force is proportional to the real contact area of micro projections (termed asperities) on friction surfaces, according to the adhesion theory of friction. In friction experiments on rocks under constant normal stress, static friction increases in proportion to the logarithm of load holding time (here referred to as logarithmic healing) (Deiterich, 1972). The increase of the real contact area in proportion to the logarithm of load holding time is also known. As the said logarithmic healing has been reported to be humidity-dependent in several experiments, it was assumed that the rate of increase of real contact area would also be humidity-dependent. However, it has recently been suggested that the rate of increase in real contact area may not be affected by humidity, based on experimental results using the nano-indentation method with quartz (Thom et al., 2018). On the other hand, Onoe and Tsutsumi (2020, JpGU) showed that the humidity dependence of logarithmic healing in single-crystal quartz is most pronounced below 20% humidity, upon conducting friction experiments under various humidity conditions. Therefore, we further verified the humidity dependence of the rate of increase of real contact area in the low-humidity range.
In this study, we investigated the effect of humidity on the rate of increase of microdeformation area, especially in the low-humidity range, by conducting nanoindentation tests under low humidity conditions (5-20% RH, room temperature) on the synthetic quartz crystal which is identical to the one used by Onoe et al. and by calculating the time increase of real contact area due to load holding. A conventional nanoindentation tester (DUH-211S, Shimadzu) was used for the tests, and the humidity in the acrylic chamber covering the sample section was controlled using a dry air generator and a humidity control device. Indentation tests with different load holding times were made at each humidity condition and the indentation hardness was calculated from the load-depth data recorded in each test. The real contact area was attempted to be determined from the obtained indentation hardness. However, the contact area values obtained by this method varied widely and significant results could not be achieved. Meanwhile, the depth evolution data during load holding strongly suggested that the time increase of real contact area was independent of humidity for 0-30% RH humidity condition, a result consistent with previous studies. The mechanism of the humidity dependence of logarithmic healing independent of the increase in real contact area needs to be clarified in the future.
As for the cause of the contact area variation, it was suspected that the composite modulus used to calculate the indentation hardness may have been affected by thermal drift, as the strain rate in load-holding experiments on quartz is lower, especially for longer periods. Therefore, based on the method of Liu et al. (2014), we first obtained the composite modulus at high-speed strain by load-unload experiments without load holding and then tried to estimate the contact area by applying the obtained composite modulus to the long-time load-holding experiments. The results showed no improvement in the data. The depth transition data showed that the behaviour of the load-unload tests was not consistent despite the same experimental conditions, and some of the depths even decreased from a certain point during load holding. Follow-up tests have shown that in many cases, the depths show a significantly lower depth increase or even a decrease when the experiments are made in succession. Whether these behaviours are due to some characteristic of the tester or to the homogeneity of the physical properties of the quartz requires further investigation.
In this study, we investigated the effect of humidity on the rate of increase of microdeformation area, especially in the low-humidity range, by conducting nanoindentation tests under low humidity conditions (5-20% RH, room temperature) on the synthetic quartz crystal which is identical to the one used by Onoe et al. and by calculating the time increase of real contact area due to load holding. A conventional nanoindentation tester (DUH-211S, Shimadzu) was used for the tests, and the humidity in the acrylic chamber covering the sample section was controlled using a dry air generator and a humidity control device. Indentation tests with different load holding times were made at each humidity condition and the indentation hardness was calculated from the load-depth data recorded in each test. The real contact area was attempted to be determined from the obtained indentation hardness. However, the contact area values obtained by this method varied widely and significant results could not be achieved. Meanwhile, the depth evolution data during load holding strongly suggested that the time increase of real contact area was independent of humidity for 0-30% RH humidity condition, a result consistent with previous studies. The mechanism of the humidity dependence of logarithmic healing independent of the increase in real contact area needs to be clarified in the future.
As for the cause of the contact area variation, it was suspected that the composite modulus used to calculate the indentation hardness may have been affected by thermal drift, as the strain rate in load-holding experiments on quartz is lower, especially for longer periods. Therefore, based on the method of Liu et al. (2014), we first obtained the composite modulus at high-speed strain by load-unload experiments without load holding and then tried to estimate the contact area by applying the obtained composite modulus to the long-time load-holding experiments. The results showed no improvement in the data. The depth transition data showed that the behaviour of the load-unload tests was not consistent despite the same experimental conditions, and some of the depths even decreased from a certain point during load holding. Follow-up tests have shown that in many cases, the depths show a significantly lower depth increase or even a decrease when the experiments are made in succession. Whether these behaviours are due to some characteristic of the tester or to the homogeneity of the physical properties of the quartz requires further investigation.