The effect of the substrate on the mechanical properties of TiN coatings

Abstract

TiN coatings are commonly used in industry to impart improved friction and wear performance. It is widely recognised that the substrate plays an important role in determining the mechanical properties and wear resistance of such coatings. TiN coatings are usually applied to hard tool steels where there is substantial load support from the substrate. However, there are many applications where it may be desirable to apply the coatings to substrates with lower hardness or stiffness values than the conventional, hard tool-steel substrates that are commonly used. Coatings can still be effectively used in these applications. However, it is then critical to understand the transitions between where the deformation is contained solely in the coating and where it is a combination of coating/substrate properties that are important in determining the overall mechanical response of the system. This paper therefore reports on a systematic investigation of the effect of the substrate on the mechanical response using a range of mechanical testing techniques. A TiN coating was deposited using ion-assisted PVD onto a number of substrates with differing combinations of modulus and hardness [i.e. a range of Young's modulus (E) to yield stress (Y) E/Y ratios]. The mechanical properties of these coatings have been investigated using nanoindentation, microindentation and scratch testing, and the deformation was observed using scanning electron microscopy. In the microindentation tests, nested cracks were observed around the indentations. In the nanoindentation tests, the indentation response was found to be plasticity-dominated, with little evidence of cracking. The scratch tests showed that the scratch response was controlled by plastic deformation in the substrate, and that the friction coefficient increased as the depth of penetration into the sample increased. For the coatings here, it was observed that the indentation depth/coating thickness ratio required for the deformation to be contained within the coating was less than the usual t/10 ratio.