Mechanical property development in reactively sputtered tantalum carbide/amorphous hydrocarbon thin films
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Gary L. Doll The Timken Company, Canton, Ohio 44706
Jeffrey T. Glass Duke University, Durham, North Carolina 27708 (Received 21 December 2005; accepted 9 March 2006)
Hardness, elastic modulus, and stress directly influence the ability of tantalum carbide/amorphous hydrocarbon (TaC/a-C:H) thin films to enhance the wear-resistance of steel tribological component surfaces. Designed factorial experiments enabled an evaluation of the effects of acetylene flow rate (QC2H2), direct current bias voltage level (Vb), and substrate rotation rate (Rot) during deposition on the mechanical properties of reactively sputtered TaC/a-C:H films. Significant relationships were found between compressive stress level and Vb, whereas hardness and elastic modulus were dependent primarily on Vb and secondarily on QC2H2 within the studied parameter space. It is proposed that effects of ion bombardment on the a-C:H phase during growth are responsible for property dependencies on Vb. Decreases in hardness and elastic modulus with increasing QC2H2 are attributed to increased hydrogen concentration and a concomitant decreased volume fraction of TaC crystallites in the films.
I. INTRODUCTION
Nanocomposite metal carbide/amorphous hydrocarbon thin films (MC/a-C:H) have been used increasingly on precision steel components1 to reduce friction and make surfaces resistant to adhesive and abrasive wear. One such application is to delay scuffing at the rib-roller end sliding contact in tapered roller bearings when the lubricant supply is interrupted.2 The relationship between MC/a-C:H film hardness and wear resistance can be described by the Archard equation3 V=
kLx , H
(1)
where V is the volume of material removed, L is the applied load, x is the sliding distance, H is the hardness of the worn surface, and k is a nondimensional wear constant. The Archard equation has been applied to estimate the scuffing protection achievable with MC/a-C:H thin film coatings at the rib-roller end sliding contact.2 A a)
Address correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0174 1500
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 6, Jun 2006 Downloaded: 27 Feb 2015
key insight of this model is that wear resistance is proportional to the hardness of the wear surface. The Archard equation applies to sliding wear surfaces in general, but it is only meaningful for thin films that have adequate adhesion to the substrate. A wear-resistant film is not useful if it is removed from the substrate by delamination. Two mechanical properties can affect the adhesion of thin films in service: residual stress level and elastic modulus. High stress levels can degrade film adhesion to the substrate. In addition, films must have a comparable or lower elastic modulus than the substrate to accommodate surface deformation without fracture. Thus, hardness, elastic modulus, and film stress directly influence the ability of MC/a-C:H films to enhance the wear-resistance of steel mechanical component surfaces. An understanding of the rel
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