Influence of Deposition Parameters on the Composition and Structure of Reactively Sputtered Nanocomposite TaC/a-C:H Thin
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Jane Y. Howe and James Bentley Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Gary L. Doll The Timken Company, Canton, Ohio 44706
Jeffrey T. Glass Duke University, Durham, North Carolina 27708 (Received 30 March 2005; accepted 28 June 2005)
The properties of nanocomposite tantalum carbide/amorphous hydrocarbon (TaC/a-C:H) thin films depend closely on reactive magnetron sputtering deposition process conditions. The chemical composition and structure trends for TaC/a-C:H films were obtained as a function of three deposition parameters: acetylene flow rate, applied direct current (dc) bias voltage, and substrate carousel rotation rate. Films were deposited according to a 23 factorial experimental design to enable multiple linear regression modeling of property trends. The Ta/C atomic ratio, hydrogen content, total film thickness, TaC crystallite size, and Raman spectra were statistically dependent on acetylene flow rate, applied dc bias voltage, or both. Transmission electron microscopy revealed a nanometer-scale lamellar film structure, the periodicity of which was affected mostly by substrate carousel rotation rate. The empirical property trends were interpreted with respect to hypothesized growth mechanisms that incorporate elements of physical vapor deposition and plasma-enhanced chemical vapor deposition.
I. INTRODUCTION
Nanocomposite metal carbide/amorphous hydrocarbon (MC/a-C:H) thin films deposited by reactive sputtering of metal or metal carbide targets are being used increasingly as tribological coatings on precision steel components.1–3 This class of materials emerged in the 1980s4–9 as a surface coating that imparted low friction and high wear resistance to surfaces. Techniques such as x-ray diffraction and x-ray photoelectron spectroscopy were originally utilized to infer the nanocomposite structure of these materials. 9–13 Transmission electron microscopy results further confirmed the nanocomposite structure in the 1990s.14–18 The nanocomposite film structure offers the surface free energy and elasticity of a polymer (a-C:H matrix) with the hardness and wear resistance of a carbide ceramic (MC nano-crystals). This combination of properties makes MC/a-C:H films a key surface engineering option. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0324 J. Mater. Res., Vol. 20, No. 9, Sep 2005
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Many MC/a-C:H composition and structure studies as referenced throughout this manuscript have emphasized the impact of hydrocarbon gas flow rate and applied bias voltage deposition parameters on film properties. However, the studies relied mostly on observations from onefactor-at-a-time experiments. Only one other published study to the authors’ knowledge utilized statistical multifactor designed experiments to study this material system. Villiger et al.19 studied the effects of the C2H2 flow rate (via an optical emission monitor setting), applied bias voltage, Ar flow rate, and N2 flow rate on the pr
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