Sputter Deposited ZrC and NbC Thin Films – Studies on Microstructure, Texture and Hardness
Transition metal carbides have great industrial importance with a wide area of applications. Unlike many ceramic materials which can be produced from raw materials found in nature, the refractory carbides generally do not exist in the natural state. Synthesis of these carbides is costly and exacting. Sputtered coatings of the refractory metal carbides are of great interest for applications where hard wear-resistant materials are desired. Understanding how the experimental conditions affect the microstructure and properties in reactive sputtering deposition process is still an area of intense research activity. Reactively sputtered zirconium carbide thin films were grown on (100) silicon substrate and the influence of substrate temperature on the properties of the films were investigated. The substrate temperature was varied from ambient to 500°C and partial pressures of the sputter gas and reactive gas (argon and methane) were optimised to obtain crystalline films. Structural characteristics showed that the films exhibit nanocomposite structure consisting of ZrC nanocrystallites embedded in amorphous carbon typically at lower growth temperature (TS < 300°C), and at higher growth temperatures film were highly textured. In addition, Films deposited at 325 °C showed a distinct increase in FWHM which had considerable effect on the mechanical properties of the film. Maximum hardness of 24.8 GPa was seen at 325ºC. The changes in atomic bonding structures, their relative fractions with respect to substrate temperature were discussed. We also report superhard nanocrystalline nanocomposite NbC thin film deposited on Si (100) under 500˚C growth temperature via reactive magnetron sputtering. The pronounced nano hardness and modulus value of 42 GPa and 267 GPa at 40/60 C/Nb ratio were found to be strongly dependent on the grain size and higher percentage of carbide content. HRTEM studies further confirm the formation of nanocomposite structure with nanocrystalline grains embedded in amorphous matrix. The influence of vapour incidence angle (α= 0˚ to 75˚) on optimized ZrC and NbC thin films were investigated by depositing films in Oblique angle deposition geometry (OAD). The anisotropic growth rate of crystallographic planes and the mechanism of development of micro structural features in OAD of carbide films have been investigated. XRD and pole figure measurements indicated that the films grown at higher growth temperatures (800°C) exhibited higher degree of preferred orientation coupled with larger crystallite size whereas the films deposited at room temperature displayed random polycrystalline nature. The strong increase in porosity with increase in deposition angle with distinctly separated nanometer sized columns resulted in lowering of hardness and reduced modulus value. The film with zero incidence angle exhibited a maximum hardness and reduced modulus of 28 GPa and 223 GPa respectively. On the other hand, NbC films deposited with OAD, remained to be polycrystalline in nature with less intense peaks and also exhibited loss of preferential orientation indicating lower crystal quality with increase in vapor deposition angle. It is apparent that variation in crystallographic texture coupled with sculptured nanostructures are solely material dependent properties. Nano metric modulated ZrC/NbC superlattice multilayer structure performance has been evaluated for structural stability and hardness enhancement. Multilayers present superlattice effect in XRD patterns, which are attributed to the precise periodical stacking of crystalline monolayers also confirmed by cross section FESEM. X-ray photoelectron spectroscopy depth profile analysis was performed to get information on chemical composition of modulated layers and also to get an insight on the interface region. Hardness and modulus value of 43.2 GPa and 272 GPa was observed which is higher than individual monolayers response to mechanical loading. The enhanced hardness is possibly due to the inhibition of dislocation motion along the interface and also due to strain effects at the interface.