Deformation by scratches and frictional properties of MgO crystals

Abstract

The subsurface structure of MgO crystals frictionally damaged on their (001) faces by scratching with a diamond stylus in the 〈100〉 direction was investigated using a scanning electron microscope in both the cathodoluminescence and the secondary electron modes. The friction is independent of the load below a critical value, and above this value it depends on the load W according to μ ∝ Wn where μ is the coefficient of friction. The subsurface beneath the sliding contact consists of four characteristic material zones: zone I is severely deformed with a high defect density which causes quenching of the luminescence; zone II is a plastically deformed intermediate zone showing enhanced luminescence; zone III is a slipped zone which on etching shows a dislocation pit pattern; zone IV consists of non-deformed crystalline material. The normalized depth da, where d is the depth of each deformed zone and a is the track half-width, is independent of the load. The frictional behaviour was examined by using empirical relations for the damage to estimate the plastic work done during the scratching process.