A fundamental study of nanocomposite hard coatings formed by ion-assisted deposition

Nanocomposite TiN/Si_3N_4 thin films were deposited by ion-assisted deposition (IAD) in a reactive gas atmosphere, over a wide range of substrate temperatures. Composition was determined by Rutherford backscattering spectrometry; microstructure was investigated by glancing incidence x-ray diffraction and transmission electron microscopy; hardness was determined by nanoindentation. The films consisted of closely packed TiN crystals within an amorphous Si_3N_4 matrix. A maximum hardness of 48 GPa for a mean crystal size of #approx#4nm and a silicon content of <10% was recorded. During IAD the material to be deposited is transported to the substrate by means of sputtering. The sputtered atoms condense upon a substrate and they are bombarded with a second ion beam. This has many effects on the film growth process that leads to improved film quality, adhesion, and the formation of novel metastable phases. The independent control of the ion to atom arrival ratio and the ion-assist energy is a key feature of IAD, which makes it an ideal process for a study of this type. An important part of the study is the use of nanoindentation to measure the hardness of the films; an emerging technique with many problems which had to be overcome in order obtain meaningful results. It involves driving a diamond indenter into the sample and a load/displacement curve is obtained; this can be analysed and related to the hardness of the films. It is proposed that the high hardness is the result of a Young's modulus enhancement mechanism that occurs when the crystal size is less than 10 nm. A factor limiting the hardness of the films could be the existence of small crystalline zones within the amorphous region of the film. The existence of a strong Ti-Si bond at the interface between the crystallites and the amorphous regions could increase the hardness. (author)Available from British Library Document Supply Centre-DSC:DXN027913 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo