Evaluating the fracture properties and fatigue wear of tetrahedral amorphous carbon films on silicon by nano-impact testing

A repetitive contact technique, nano-impact testing, has been used to investigate the fracture properties of tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The impact test has shown clear differences in the resistance to impact wear of ta-C films with their thickness, for film thicknesses between 5 and 80 nm. The resistance to impact-induced fracture decreases as the film thickness increases. This may be due to the maximum shear stress being closer to the film-substrate interface, or reflect reduced toughness in the thicker films that are less able to deform as the substrate deforms plastically during the repetitive contact test. The mechanism of impact-induced failure on these thin films is (i) the initial impact stage where plastic deformation causes cracks to nucleate sub-surface, (ii) fatigue-further nucleation and growth of sub-surface cracks (with little or no change in probe depth) and (iii) crack coalescence and film fracture leading to a rapid change in probe depth as the film fails. The influence of impact load on fracture probability has been investigated. Fracture probability increases sharply as the impact load is increased from 100 to 300 μN. The greater load provides the stresses necessary to nucleate and propagate the sub-surface cracks; at low load the driving force for the cracks to coalesce and spall the coating is much reduced, so failure is less likely to occur within the test duration.