Evaluation of magnetron-sputtered TiB2 intended for tribological applications

To date, the coating industry has dramatically changed the possibilities for effective machining of many different materials. There are, however, some materials that still are problematic to machine. An example of such a material is particle-reinforced aluminium. For this material an effective tool material must possess high hardness to minimise abrasive wear. In addition, it must have a low adhesion to aluminium in order to prevent built-up edges. In respect of this TiB2 is a very interesting material, because it exhibits a very high hardness and is inert to aluminium (liquid). The latter indicates a very low adhesion of aluminium on TiB2. In this work, TiB2 coatings were deposited by physical vapour deposition (PVD) on cemented carbide substrates using direct-current (DC) magnetron sputtering. The aim of the work was to evaluate how various deposition parameters, as well as the choice of working gas, influence the formation and tribological properties of TiB2 coatings. All coatings were evaluated with respect to their phase composition, chemical composition, hardness, elasticity, residual stress, cohesion, adhesion and abrasive wear resistance. All the coatings displayed X-ray diffraction patterns of the hexagonal TiB2 phase with a preferred (0001) orientation. Besides being very hard, all coatings were found to exhibit a high compressive residual stress level that was found to increase with negative substrate bias. The coating adhesion was found to be relatively good, whereas the coating cohesion was fairly low due to the high compressive stress in the coatings. Not surprisingly, all coatings displayed an extremely high abrasive wear resistance. It can be concluded that a PVD TiB2 coating with good adhesion and overall good mechanical and tribological properties can be obtained, provided no negative substrate bias is used. A further improvement, in respect of lowering the residual stress, can be achieved by using xenon instead of argon as working gas. (C) 2000 Elsevier Science S.A. All rights reserved.