Microscratch Properties of Polycarbonate by Spherical Indenter

The effects of normal load, scratch speed and the number of scratches on the scratch performance of polycarbonate were studied by using 100Cr6 spherical indenter. The results showed that in the single scratch, the friction coefficient and the sum of adhesion and viscoelastic friction coefficients increased nonlinearly; penetration depth and ploughing friction coefficient increased linearly with the increase of normal load. In the 15th scratch, the friction coefficient, penetration depth and ploughing friction coefficient increased linearly, the sum of adhesion and viscoelastic friction coefficients decreased linearly with the increase of normal load. In the single scratch and the 15th scratch, the friction coefficient, the sum of adhesion and viscoelastic friction coefficient increased first and then decreased with the penetration depth, and ploughing friction coefficient decreased linearly as the scratch speed increases. In addition, with the normal load increasing or scratch velocity decreasing, the residual depth increased and the elastic recovery rate decreased. The Knoop hardness decreased with the increase of normal load, which indicated the indentation size effect in polycarbonate. For multi-pass unidirectional sliding wear test, with the increase of the number of scratches, the scratch width increased linearly, and the scratch hardness decreased linearly. Moreover, the friction coefficient, penetration depth and residual depth increased with a power law dependence on the number of scratches. In the process of multiple scratches, a hardening layer around the scratch groove formed, and the hardening layer after each scratch became more compacted. As a result, the growth rates of friction coefficient, penetration depth and residual depth gradually decreased and reached stable levels after a certain number of scratches. The numbers of stable scratches for penetration depth and residual depths decreased with the increase of normal load. In the 15th scratch, under low normal loads and scratch speeds, the strain rate decreased with the increase of normal load or the decrease of scratch speed, which led to the deterioration of resistance to plastic deformation and the linear decrease of scratch hardness. Polycarbonate exhibited stress whitening, ironing and regular zig-zag type scratch tracks. The scratch morphology was severely damaged with both residual scratch width and geometric scratch width increasing linearly with the increase of normal load. The residual scratch hardness decreased with a power law dependence on the normal load, while the geometric scratch hardness increased due to work hardening and the increase of strain as the normal load increased. The damage extent of scratch surface decreased with the increase of scratch speed. The residual scratch width and geometric scratch width decreased linearly with the increase of scratch speed. In addition, the residual scratch hardness and geometric scratch hardness increased with the increase of scratch speed, and finally tended to be stable. The derivative of scratch hardness with respect to the square of strain rate was related to the scratch speed: as the scratch speed increased, the slope of the curve gradually decreased. As the scratch speed was large enough, it tended to be zero. Under low normal loads or high scratch speeds, the material was compressed and sunk in around the borderlines of the scratch groove, giving rise to the geometric scratch width greater than the residual scratch width. Nevertheless, for high normal loads or low scratch speeds, a small amount of material piled up on both sides of the scratch groove, resulting in a smaller geometric scratch width than the residual width.