Abstract: Basic friction coefficients are important parameters affecting the shear strength of structural surface in rock masses. The basic friction coefficient determined by conventional laboratory test methods is influenced by mineral composition, temperature, and other factors. To systematically reveal the basic frictional properties of structural surfaces, the macro and micro friction coefficients of sandstone were investigated separately, and their relationship was established. First, mineral components and mechanical parameters of the sandstone were determined by X-ray diffraction and nanoindentation tests. Second, the friction coefficient in the macro scale was examined by tilt test and direct shear test, and the direct shear test was carried out on flat structural surface specimens dimensioning 10 cm×10 cm×5 cm subjected to constant normal stresses of 1, 2, 3, 8, and 12 MPa. The results show that the friction coefficient decreases in the logarithmic form as the normal stress rises, whereas it increases in the logarithmic form as the shear rate grows. Then, the friction coefficient in the micro scale was examined using nano-scratch test, and the results show that the friction coefficients of feldspar minerals first decrease and then remain unchanged with increasing load under low load conditions, and the friction coefficients of quartz minerals first decrease and then increase with increasing load. In addition, the friction coefficients of both minerals show an increasing trend with increasing shear rate under low load conditions, and remain stable under high load conditions. Finally, based on the macro-micro friction coefficient test results, the rate- and state-dependent friction (RSF) law was used to establish the linear regression equation linking the basic friction coefficient of red sandstone and the mineral friction coefficient, and the reliability of this empirical relationship was verified by the direct shear test with errors ranging from 0.17% to 0.91%. This study provides a new idea for the determination of the basic friction coefficient.

Key words: friction coefficient, microscale, direct shear test, nano-scratch, RSF law