Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014)
    Thin Films Monday Sessions
       Session TF-MoM

Paper TF-MoM3
Deposition and Tribomechanical Properties of Hf-B-C Thin Films

Monday, December 8, 2014, 9:20 am, Room Makai

Session: Self-Organized and Nanostructured Thin Films
Presenter: Elham Mohimi, University of Illinois at Urbana-Champaign, USA
Authors: E. Mohimi, University of Illinois at Urbana-Champaign, USA
T. Ozkan, University of Illinois at Urbana-Champaign, USA
S. Babar, University of Illinois at Urbana-Champaign, USA
P.J. Sempsrott, University of Illinois at Urbana-Champaign, USA
A.A. Polycarpou, Texas A&M University
G.S. Girolami, University of Illinois at Urbana-Champaign, USA
J.R. Abelson, University of Illinois at Urbana-Champaign, USA
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The tribomechanical properties of thin film coatings can be enhanced by alloying to afford a multicomponent nanostructure. Our group previously reported the conformal growth and favorable mechanical properties of HfB2 and Hf-B-N hard coatings by chemical vapor deposition (CVD) at low substrate temperature. Here, we alloy C into HfB2 in order to reduce the friction coefficient and enhance the hardness. A useful analogue is C-alloyed TiB2, which exhibits super hardness and good thermal stability. However, there have been no previous studies of Hf-B-C alloys.

Hf-B-C nanocomposite coatings are deposited by CVD onto Si or stainless steel discs using the precursor hafnium borohydride, Hf(BH4)4, with a co-flow of dimethylbutene (DMB), (CH3)3CCH=CH2, as the carbon source. Depositions are performed in a high vacuum chamber with base pressure of 10-8 Torr, hafnium borohydride pressure 0.1-0.5 mTorr and DMB pressure 0.05-0.4 mTorr at substrate temperatures of 250-600°C. DMB also acts as growth inhibitor – it reduces the film growth rate by a factor of 2-6 compared to growth using the precursor alone, an effect which enhances conformality. For higher temperature growth, DMB increases the film density and decreases the surface roughness. XPS analysis indicates a mixture of HfB2, HfCx and B4C phases, however, this is uncertain due to the small shifts between different bonding states.

As-deposited films are XRD amorphous with hardness values of 9-26 Gpa and reduced modulus of 99-208 Gpa. Upon annealing at 700°C for 3 hours under inert gas atmosphere, the films transform to a partially nanocrystalline structure, which increases the hardness to 17-34 Gpa and the elastic modulus to 158-248 Gpa. The tribological properties of Hf-B-C films are superior to those of HfB2 films. This is attributed to graphitic attachment of carbon atoms on contacting surfaces as evidenced by EDS analysis of the wear scar surface. Summarizing, this system affords conformal growth at low growth temperatures, suitable for the fabrication of complex structures such as MEMS.