Paper TF-FrM5
Conformal CVD Growth of HfB_xC_y and HfB_xAl_y Hard Coatings with Low Coefficient of Friction and High Oxidation Resistance

Friday, November 11, 2016, 9:40 am, Room 105A

Conformal hard coatings with low coefficient of friction, high oxidation resistance and chemical stability are desired for applications such as components with convoluted structures, cutting tools that sustain high temperature, thermal protection systems for extreme environments, and machines with relative motion of parts. We previously reported the conformal growth and excellent mechanical properties of HfB₂ and HfB_xN_y hard coatings by chemical vapor deposition (CVD) below 300°C using the high vapor pressure precursor hafnium borohydride, Hf(BH₄)₄. Here we report a further improvement in the properties of HfB₂ films by alloying with C to reduce the coefficient of sliding friction, and with Al to impart high temperature oxidation resistance.

All Depositions are performed in a high vacuum chamber using 0.1-0.5 mTorr of the hafnium borohydride precursor. Carbon-alloyed HfB_xC_y films are grown using a co-flow of 0.1-0.4 mTorr dimethylbutene (DMB) as the C source at substrate temperatures of 250-600°C. The resulting films contain 5-33 at. % C. DMB also acts as growth inhibitor that reduces the growth rate by a factor of 2-6 compared to growth using the precursor alone; this affords almost perfect conformality, e.g., a step coverage > 90% in a trench of aspect ratio 30:1. The nanoindentation hardness varies from 21 to 9 GPa for films with 5-21 at. % C grown at 300°C, and from 23-25 GPa for films with 28-35 at. % C grown at 600°C. The coefficient of sliding friction is remarkably low, 0.05-0.08 for films with the highest and lowest carbon content, respectively. In addition, the elastic response is more compliant, which is expected to improve the tribological wear performance.

Al-alloyed HfB_xAl_y films are grown using a co-flow of 0.05-0.20 mTorr trimethyl amine alane (TMAA) as aluminum source at a temperature of 250-300°C. In the absence of alloying, HfB₂ films exposed to an oxygen ambient at 800°C will oxidize deeply because HfO₂ does not provide a protective layer and B₂O₃ evaporates rapidly. In sharp contrast, films containing 1-20 at. % Al form a protective aluminum oxide surface layer. Compositional depth profiles confirm the absence of oxidation below this surface layer. In addition these films are morphologically stable: whereas HfB₂ crystallizes, densifies and forms a network of cracks at temperatures above ~ 600°C, the HfB_xAl_y films do not crystallize or crack upon annealing to 800°C in inert or oxidizing atmospheres. Future work, including the co-alloying with C and Al, will explore property optimization in which both low friction and oxidation resistance are desired.