A new method based on work-displacement curve to assess the toughness of coated system

Abstract

Traditional indentation models relating the toughness to applied load and the length of well-developed radial cracks were initially developed for monolithic bulk brittle materials. Their application can also be extended to coated systems if sufficiently thick, however, the use of such models to thin coatings will be influenced by other factors such as substrate deformation and it is difficult to use traditional approaches to assess sub-threshold cracking. The method of extrapolating the load-displacement curve has been widely used to assess the toughness of a coating when a step in the load-displacement curve is observed which is clearly associated with chipping. This approach completely ignores the change of other mechanical behaviour associated with the fracture event and it cannot be applied to displacement control experiments. The plot of the irreversible work vs. load for a range of applied loads can be used in this case but is time consuming. Also this method does not work well for sub-threshold cracking. Therefore, it is necessary to find a method to solve such problems if the fracture properties of very thin coatings (≤1μm) are to be assessed. In this paper, a method which is based on extrapolating the total work vs. displacement curve at a load where cracking occurs is presented to estimate the fracture toughness of the coated system.