Determining the Limit of Hardness in Ternary Carbide thin Films
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DETERMINING THE LIMIT OF HARDNESS IN TERNARY CARBIDE THIN FILMS James E. Krzanowski, Jose L. Endrino, and Sirma H. Koutzaki, Mechanical Engineering Dept., University of New Hampshire, Durham, NH 03824 ABSTRACT In this study we examine the structure and mechanical properties of ternary carbide films, with the objective of understanding the role that residual stress, density, grain size, texture and crystallinity play in determining film hardness. The main variables we used to alter film structure are the nominal film compositions and substrate bias voltage. The systems examined here are Ti-Hf-C and Hf-Si-C, and are compared to previous work on Ti-Si-C. The Hf-Si-C films show hardness levels above that of HfC alone. Residual stress measurements show a compressive stress to be present, with the exception of the –25V sample where the stress was tensile. However, compositional analysis of the biased samples by XPS reveals that Si content in the films is reduced by the application of substrate bias. Consistent with this, the films with higher bias have larger grain sizes and better crystallinity. For the Ti-Hf-C films, the results for hardness vs. bias are consistent with known stress and densification effects of the bias, and values are consistent with rule-of-mixtures expectations. INTRODUCTION Recent research reports on the mechanical properties thin films with nano-scale heterophase structures have claimed dramatic property enhancements, including hardness levels exceeding that of diamond and a high fracture toughness [1,2]. Studies conducted on CVD metal/silicon nitride thin films [1,3-5] revealed microstructures consisting of a nanocrystalline (2-4 nm) metal nitride phase (such as TiN) embedded in an amorphous silicon nitride phase. It was proposed that structure resulted in super hard films due to the nanoparticle strengthening of TiN and the crack propagation resistance provided by to the silicon nitride interlayer. Studies of nano-composite films consisting of TiC and amorphous carbon [2] also reported high hardness and toughness and attributed it to a multiphase nano-scale microstructure. The effect of film microstructure on the mechanical properties of TiN thin films was previously studied by Bull and Rickerby [6]. Their study showed a relatively low hardness (10 GPa) in TiN films deposited by sputter ion plating without bias, but by increasing the substrate bias they were able to obtain films with hardness levels comparable to bulk TiN. The residual (compressive) stress was also found to increase with bias. They explained their results in terms of a modified structural zone model proposed by Messier et al. [7]. In the model, the substrate temperature for transition from zone 1 to zone T is reduced with increasing bias due to bombardment induced mobility of the adatoms. While zone 1 can have considerable inter-columnar porosity, zone T was defined as consisting of poorly defined but densely packed fibrous grains. It was
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suggested that while the hardness of the grains themselves did not change, elimina
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