STRUCTURE AND PROPERTIES OF TI-B-N COATINGS

Crystal structure studies on Ti-B-N coatings of various composition show a good agreement with the phase diagram predictions of Novotny and coworkers (1961). Simple considerations lead us to believe that the performance of such coatings can be optimized if they do not contain a soft phase such as Ti or h-BN and if the composition is located in a zone where the phases TiB2 and TiN coexist in nearly equal parts. The microstructure plays an important role, in particular if the grain size of the coatings is in the nanometre range for Ti-B-N coatings. In this case an inverse Hall-Fetch effect leading to a softening of the coating might be observed. Several preparation techniques for the synthesis of suitably composed coatings are presented and discussed. These are, in particular: (a) Ti+ implantation into hexagonal BN layers; (b) co-sputter deposition from Ti and BN targets; (c) sputtering from a heterogeneously composed Ti-BN target; and (d) Ti/BN multilayer coating interdiffusion. In spite of ultrahigh hardness and good adhesion to metallic substrates, most tribological tests revealed slightly worse performance than TiN and Ti(Al,N) coatings.