Abstract
The initiation of fracture and onset of chipping at plastic-elastic indentations and scratches on near (111), (111) and (001) silicon surfaces has been studied by optical and high resolution scanning electron microscopy. It is shown that a pattern of surface cracks are initiated at a critical width of impressions or scratches. It is proposed that the observed size effect is governed by the strain energy criterion put forward by Puttick et al (1979) according to which the critical size of indentations in a highly brittle material should be alpha x (EGamma)/(y2). The critical size parameter is evaluated for near (111) silicon; good agreement is found between the theory and observations. Indentations and scratches on near (111) silicon appear to produce cleavage on planes close to {110} rather than {111} planes, together with a preference for crack propagation in the surface layer in the directions [112], [121] and [211]rather than the reverse directions. This pattern of fracture exerts a marked influence on the anisotropy of abrasion of these surfaces. Similar behaviour is found on (111) silicon. Indentations on (001) silicon also appear to initiate surface fracture on or close to {110} rather than {111} planes, with no sign of asymmetry in the crack pattern. The mechanics of fracture at such plastic-elastic indentations is discussed and it is proposed that {110} cleavage is initiated by a dislocation reaction similar to that associated with indentation fracture in ionic crystals. The coefficient of friction ? (the ratio of the applied normal load to the tangential force) has been measured as a function of load (1 to 30g) on near (111) and (001) surfaces using Vickers diamond indenters having sharp and blunt tips. It is found that at light loads no penetration of the slider occurs; u is low and the resultant damage is associated with the nucleation of a high density of dislocations in a thin surface layer only. At higher loads considerable penetration of the slider occurs; u rises to a higher value due to the ploughing. No anisotropy can be detected using the blunt indenter. A decrease in u at cracking is observed in [112] and [110] on (111) and (001) surfaces respectively using the sharp indenter.