Mechanical properties of multilayer pvd Ti/TiB 2 coatings

Abstract

To obtain from a tribological perspective acceptable PVD coatings of covalently bonded materials, such as borides and oxides, it is essential to control and often reduce the very high compressive stresses which develop during coating growth, since such stresses not only reduce coating adhesion but also limit film thickness. A possible way to obtain lower stress levels could be to utilise multilayer coatings, that is to combine the stressed material with a more ductile material in a layered structure. In the present study a highly stressed PVD TiB₂ material was layered with ductile Ti. The multilayer period, i.e. the combined thickness of one Ti lamella and one TiB₂ lamella, and the lamellar thickness ratio of the two phases were varied in order to evaluate their influences on some important mechanical properties and on the abrasive wear resistance of the Ti/TiB₂ coatings. The coatings were deposited using magnetron sputtering of TiB₂ and electron beam gun evaporation of Ti. To evaluate the coating phase composition X-ray diffraction was used. Mechanical and tribological properties were examined by nanoindentation (hardness and modulus), beam deflection (residual stress), scratch testing (cohesion and adhesion), and dimple grinding (abrasive wear resistance). Hardness, Young's modulus, compressive residual stress, and abrasive wear resistance were found to increase with the fraction of TiB₂ in the coatings, and an inverse relation was found for the critical normal load obtained in scratch tests, i.e. cohesion/adhesion decreased with increasing TiB₂ fraction. The investigation showed that a hybrid PVD technique combining electron beam evaporation of Ti and magnetron sputtering of TiB₂ can be used to produce coatings with increased fracture toughness, decreased residual stress, and relatively high hardness as compared to homogeneous PVD TiB₂. It was also found that the yield/fracture strength of Ti was insufficient, i.e. it could not accommodate the level of stress generated in the TiB₂ phase. Therefore, by choosing another material with higher yield/fracture strength than Ti it is believed that the properties of the multilayer coating could be improved even further.