Quantitative analysis of scratch-induced microabrasion on silica glass

We employ instrumented nanoindentation for obtaining quantitative information on the onset of scratch-induced microabrasion on silica glass. For this, in situ evaluation of lateral force and friction coefficient is compared to post mortem optical inspection, following edge-forward scratching with a Berkovich indenter at velocities of 10-500 μm/s under continuously increasing normal load of up to 300 mN. In the two approaches, the onset of microabrasion is identified from the occurrence of pop-ins in the load-displacement curve and phenomenologically determined from the scratch pattern, respectively. Obtained data are analyzed in terms of a Weibull distribution, assuming that microabrasion sets-on as a result of acting stress as well as surface state. Aside of the occurrence of occasional outliers at low load (probably induced through individual surface defects), data indicate two underlying probability functions, i.e., the probability for the propagating scratch to hit a surface flaw and the probability that such an event causes an observable micro-crack. Dominance of the former leads to an exponential function with Weibull modulus ~ 1, reflecting a purely random distribution with load-independent probability of failure. This is observed in particular at high scratching velocity after passing a certain normal load. For the latter, the Weibull modulus increases with increasing scratching velocity, that is, from ~ 1.6 to 4.4, at intermediate load. Here, low Weibull modulus at low load is attributed to the increasing time of local strain, which leads to a reduction of the load-dependence of micro-cracking relative to a faster-moving scratch. In the present case, the critical lateral load for microabrasion of silica (50th percentile) is around 30-40 mN. Within the employed experimental conditions, this value is practically independent of scratching velocity.