Microstructure and mechanical properties of Mo–Si–C and Zr–Si–C thin films: Compositional routes for film densification and hardness enhancement

Abstract

Transition metal carbides exhibit high hardness, thermal stability and excellent wear resistance making them attractive for applications such as tool and die coatings. However, when deposited in thin film form using physical vapor deposition methods the mechanical properties can degrade due to poor film density. In this study we examine the addition of SiC to two transition metal carbides, Mo2C and ZrC, as a possible route for film densification and hardness enhancement. Mo–Si–C and Zr–Si–C films were deposited with a range of Si concentrations using rf magnetron co-sputtering. Films were characterized using X-ray photoelectron spectroscopy, X-ray diffraction, electron microscopy and nano-indentation and micro-indentation hardness testing. For Mo–Si–C the Si additions caused significant alterations to the microstructure, affecting the grain size, the amount of amorphous phases, and crystal structure. The hardness of the Mo2C films containing Si first increased slightly with Si content, and declined at higher Si concentrations, but was never above that expected for bulk Mo2C. For Zr–Si–C, two sets of experiments were conducted: one with a large substrate-to-source distance and low rate of deposition, giving compositionally uniform films, and the second with a very short distance giving a compositionally graded film. For the first set, ZrC films without Si exhibited high porosity and low hardness, but with 31.5% SiC, the films were at the bulk levels for ZrC and the porosity was significantly reduced. This hardness improvement was explained in terms of a continuous re-nucleation process and resultant nanocrystalline grain structure. For the second set of films a hardness enhancement above rule of mixtures was observed, with a maximum of 4500 HK10 for films containing 9.3% SiC. This was explained in terms of an implantation of Si atoms.