High thickness Ti/TiN multilayer thin coatings for wear resistant applications

Hard coatings like titanium nitride (TiN) normally contain a high degree of internal stress (usually compressive in-plane parallel with thesurface) owing to growth defects developed during the deposition process and thermal mismatch effects after final cooling; it is, therefore, difficultto produce single-layer TiN coatings thicker than 6–7 μm, without adhesion problems. In the present study, thick coatings (i.e. >10 μm) have beenachieved by alternate multilayering of TiN with Ti interlayers, leading to a tougher and less-stressed film. However, having a constant distributionof titanium interlayer thickness is not necessarily the best solution to achieve maximum performance in terms of wear resistance and hardness. Theresidual stress distribution along the thickness is unlikely to be constant with the inner layers being more stressed due to a greater amount ofthermal differential strain. Following this guideline, a series of numerical simulations was performed in order to calculate the residual stressthrough thickness distribution due to the deposition process. Three sets of multilayered Ti/TiN coatings having both constant and variable Tiinterlayer thickness were modelled and deposited, using a reactive arc PVD process. Mechanical and tribological properties were characterizedusing static and depth sensing Vickers micro-hardness indentation tests, rotating wheel (dimpling grinder) abrasive wear tests and Rockwell Cadhesion tests. Coating interface characterizations were made by SEM-EDS. Results showed that adhesion can be significantly improved byadopting a titanium through thickness quantity increasing towards the interface: an optimized distribution allows also higher hardness and wearresistance to be obtained, as it requires a lower total amount of titanium to obtain good adhesion properties.