Influence of experimental factors and film thickness on the measured critical load in the nanoscratch test

A model system showing clear film failure in nanoscratch tests, Si-containing a-C:H film on glass, was selected to investigate the influence of various experimental parameters on the measured critical load in the nanoscratch test. No dependence on critical load was found for this system on either (1) scratch speed, (2) loading rate, or (3) increase in load per unit scratch distance (dL/dx) when dL/dx is much less than 1 N/mm, whilst critical loads only slightly increase at higher dL/dx (1-5 N/mm). This suggests that tests under widely different loading conditions can be compared directly in contrast to what has been reported for conventional macro-scale scratch testing where dL/dx is set to 10 N/mm and results are strongly dependent on both loading rate and scan speed. It has also been found that the film thickness has a marked influence on the critical load in the nanoscratch test. The role of the ratio (hardness/modulus) on the linear increase in critical load with film thickness is discussed.