Micro-impact testing: a new technique for investigating fracture toughness

Abstract

A novel micro-impact technique has been developed to simulate the repetitive contact conditions which many thin films and coatings experience in service. In this paper we describe how the technique has been used to investigate the impact resistance of ceramic coatings of almost identical microhardness. There were very clear differences in impact behavior between the coatings, which correlated with measurements of their fracture toughness determined from crack inspection. Coatings with low fracture toughness values exhibited high wear rates and a marked susceptibility to brittle fracture. The high surface roughness of some of the ceramic samples precluded the determination of critical loads by scratch testing. In contrast, the micro-impact technique is less affected by sample roughness than scratch testing, and therefore, is able to provide useful data on a wider range of materials. The micro-impact technique offers an important new method for the determination of the fracture toughness of thin films where methods involving the direct visualization of cracks are impossible.