Phase formation and microstructure in sputter-deposited Ti-Mo-C and Ti-W-C thin films
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I. INTRODUCTION
METAL carbides are known to have extremely high hardness levels, making them attractive candidates for wearresistant coatings. Transition metal carbides that have been investigated as potential hard coating materials include carbides of titanium, tungsten, and zirconium.[1–6] Titanium carbide and tungsten carbide have been extensively studied to investigate both their intrinsic properties[7–13] as well as methods to deposit them as coatings.[14–17] TiC crystallizes into the rock-salt (B1) structure over a wide but substoichiometric composition range. The hardness of TiC at room temperature is 28 to 30 GPa, the highest of the transitionmetal carbides,[18] although at elevated temperatures, tungsten carbide maintains a higher hardness.[18,19] Nonetheless, the hardness is substantially lower than that of diamond, which is the hardest material found in nature, with a hardness of 80 to 100 GPa. Improving the hardness, and possibly wear resistance, of hard carbide coatings could potentially be achieved through compositional and microstructural modifications. Traditional metallurgical strengthening mechanisms such as grain refining and precipitation hardening can provide possible routes to improving the mechanical properties of these materials. In fact, recent claims by Veprek et al.[20,21] to have obtained nitride-based films with hardness levels higher than that of diamond are based on Hall–Petch strengthening along with a grain-boundary phase to inhibit grain-boundary sliding or microcracking. The application of these principles to carbide-based coatings, which are intrinsically harder than nitrides, therefore, has the potential to produce ultrahard coatings. SIRMA H. KOUTZAKI, Graduate Research Assistant, and JAMES E. KRZANOWSKI, Associate Professor, are with the Mechanical Engineering Department, University of New Hampshire, Durham, NH 03824. Contact e-mail: [email protected] JOSE J. NAINAPARAMPIL, Visiting Scientist, is with the Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted July 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Multiphase carbide coatings can be obtained by either multilayer deposition of the constituent carbides, or by natural phase separation during deposition. For the latter case, coating compositions will need to be based on transitionmetal carbide mixtures that have limited miscibility. Most carbides that are based on the group-IVB transition metals are highly or completely miscible with those in the same group or those in group VB.[22] Examples include TiC-NbC and ZrC-TaC; these carbides all have the rock-salt structure. More structural variety is found in the group-VIB carbides of Cr, Mo, and W. Combining a group-IVB carbide, such as TiC, with a group-VIB carbide, such as Mo2C, is more likely to produce heterogeneous multiphase carbide coatings. We have chosen to investigate this system (TiC-Mo2C) as well as the TiC-WC system. While these are essentially ternary compounds, we can also approach these as pseudobinary system
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