An investigation into the effect of substrate on the load-bearing capacity of thin hard coatings

TiN and diamond-like carbon (DLC) coatings were deposited on Ti substrates with and without processing by high-pressure torsion (HPT). The HPT processing refined the grain size of titanium from the coarse-grained (CG) size of ~8.6 µm to the ultra-fine grained (UFG) size of ~130 nm and increased the hardness from ~1.83 to ~3.05 GPa. Scratch test results revealed that all the thin hard coatings had a higher critical load when deposited on the harder UFG Ti substrate compared to those deposited on the softer CG Ti. A three-dimensional finite element model (3D FEM) revealed that the improved load-carrying property of the thin hard coatings on the harder titanium substrates was related to the higher stresses generated within the substrate and its deformation behaviour which reduced the strain at the coating/substrate interface. A model based on the hardness of a two-layered composite was used to explain this effect, and it is shown that the model is reasonably successful in predicting the critical load of a wide range of coating-substrate systems. The analytical model and the results of the 3D FEM modelling emphasize the contribution of the substrate to the load-bearing capacity of thin coatings.