Misinterpretation of friction as fracture in shallow scratch tests with classical and universal size effect laws

Abstract

Microscratch responses of 14 bulk metallic glasses were investigated by Rockwell C diamond indenter under progressive normal load. Effects of mechanical properties on microscratch variables are quantified for bulk metallic glasses. Penetration depth increases linearly with normal force, and a power-law function can be used to express the variation of residual depth with normal force under small loads. Residual depth fluctuates under large loads; and elastic recovery rate, scratch friction coefficient, lateral hardness, and scratch hardness all tend to be stable under large loads. The asymptotic scratch friction coefficient decreases linearly with the increase in scratch hardness. The asymptotic elastic recovery rate is found to be proportional to the ratio of elastic modulus over Knoop hardness, and scaling relationships among hardness values and yield strength are found. Different scratch-based methodologies of assessing fracture toughness are compared and discussed. The successful characterization of fracture toughness by scratch-based methods requires sufficiently large loads, and different methodologies have different scopes of application.