Characterisation of High Strength TiN/TiB _x Multilayered Ceramic Coatings

Abstract

The microstructure of ceramic multilayered coatings deposited using TiN and TiB 2 targets has been investigated using HRTEM, EELS mapping and XRD. The coatings predominantly consist of nanocrystalline layers of alternating TiN and TiB, the latter being deposited nonstoichiometrically from the TiB 2 target as TiB.B27. Deposition of regions which are highly deficient in boron results in the coherent growth of large columnar TiN grains through many multilayer periods. IntroductionImprovements to the lifetimes of metal cutting tools have long been achieved by applying hard coatings. Current physical vapour deposition techniques provide the means of fabricating various configurations such as single-layer coatings, multiphase composite coatings, and complex configurations of two or more materials in multilayered or phase-gradient coatings [1].As outlined by Quinto [2], the major properties required by a hard coating are good adhesion, high microhardness and good chemical stability (especially with respect to the workpiece) at the working temperature. The choice of a TiN/TiB 2 multilayered ceramic system was made with these parameters in mind. The suitability of TiN as a hard coating is demonstrated by its widespread use in prolonging the lives of high speed metal cutting tools in industry [3]. TiB 2 is also an extremely hard and chemically stable ceramic, finding many applications as an inert dispersion hardener, particularly in structural intermetallic alloys.An especially good adhesion of TiN to TiB 2 has been predicted by Holleck [4] who noticed similar Ti lattice spacings on the TiN (111) and the TiB 2 (0001) planes suggesting a possible coherency between the layers. TiB 2 is observed to grow epitaxially on bcc metals with the relationship (10-10) TiB2 //(001) bcc and <0001> TiB2 //<010> bcc , where atomic matching occurs at the (10-10) TiB2 // (001) bcc Ti interface plane [5]. We can therefore predict a similar TiN -TiB 2 epitaxial growth relationship of (10-10) TiB2 // (001) TiN and <0001> TiB2 // <110> TiN , with low misfit matching on the (10-10) TiB2 // (001) TiN Ti planes.The use of multilayered systems has been shown to be of profound benefit to the hardness of coatings due to the addition of the layer interfaces and periodic changes in elastic modulus which are thought to hinder the movements of defects [6,7]. Many multilayered systems use layers of similar crystal structure in an attempt to guarantee coherence between the alternating materials; for instance the nitride films [8] which use transition metal nitrides with the NaCl Interface Controlled Materials. Edited by M. Rühle and H. Gleiter