A contribution to understanding the results of instrumented indentation on thermal spray coatings - Case study on Al2O3 and stainless steel

This paper is intended to show the benefits of instrumented indentation for advanced testing of highly heterogeneous materials such as thermal spray coatings. These materials possess a particular microstructure composed of splats, unmelted particles, cracks and voids. Due to this structure, their mechanical properties are known to be load dependent. This paper presents a thorough study of this subject on two representative types of thermal spray coatings: Al2O3 ceramics and 316 L stainless steel, both sprayed by water stabilized plasma system. The instrumented indentation, offering a broad range of loading forces, was chosen for characterization of these coatings at various scales: from the smallest structural units up to the coating as a whole. The dependence of mechanical properties on the indentation load was reflected in gradual decrease of hardness and Young's modulus with increasing indentation load (scale effect) and it was observed on all tested coatings. The scale effect of a thermal spray coating can be divided into three stages: low loads, intermediate loads and high loads. These three stages correspond to measurement of local properties, scale effect and global coating properties. Besides, also other important aspects of instrumented indentation of thermal spray coatings, such as sudden drops in penetration depth and failure mechanism, were identified. Since the utilized materials represented a typical microstructure of thermal spray metals and ceramics, it is expected that the conclusions can also be applied to other types of thermal spray coatings with similar structure.