Interesting aspects of indentation and scratch methods for characterization of thermally-sprayed coatings

In order to improve the knowledge on the use and significance of instrumented indentation and scratch testing on thermally-sprayed materials, a wide range of tests was performed on thermally-sprayed ceramic, cermet and metal coatings. A scale-dependent behavior of hardness was observed as a function of indentation depth for all coatings: at low penetration depths, the hardness value depends on the intralamellar material properties, whereas at larger depths it reflects the long-range cohesive strength of the coating. In all cases, hardness becomes independent of the indentation depth above a threshold value of ~ 2000 nm. The elastic modulus is also scale-dependent, but it never stabilizes to a depth-independent value, probably on account of crack opening/closing mechanisms. Scratch test on the cross-section has been deeply investigated and identified as a comparative method to quantify the cohesion of the coatings.