Contact damage of tetrahedral amorphous carbon thin films on silicon substrates
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k Hoffman School of Materials Science and Engineering, University of New South Wales, NSW 2052, Sydney, Australia
Avi Bendavid and Phil J. Martin Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), Materials Science and Engineering, Lindfield, NSW 2070, Australia (Received 10 March 2009; accepted 24 August 2009)
We have investigated the fracture behavior of tetrahedral amorphous carbon films, with thicknesses 0.15 (ultrathin), 0.5 (thin), and 1.2 (thick) microns on silicon substrates. To that end, the systems were progressively loaded using a nanoindenter with a spherical tip, and surface and cross sections were subsequently examined using a focused ion beam miller at different loads. A transition was found as a function of film thickness: for ultrathin and thin films, cracking (radial and lateral) initiated in the silicon substrate and followed a similar path in the films. Thicker films, on the other hand, provided a higher level of protection to the substrate, and cracking (lateral and radial at the interface) was constrained to the film. The damage modes and the transition obtained differ from those that occur in thick coatings. Lateral cracks are highly dangerous, leading to delamination of thick films and to spallation when thinner films are used. The results have implications concerning the mechanical reliability of microelectromechanical systems. I. INTRODUCTION
Amorphous carbon films, also known as diamond-like carbon (DLC), have found extensive use due to their excellent combination of physical and chemical properties, such as very high hardness and elastic modulus, high electrical resistivity, chemical inertness, and low coefficient of friction1–6 DLC films contain a mixture of sp2 (graphitic) and sp3 (diamond) carbon phases. By varying the content of hydrogen impurities, films with different properties can be obtained.1–4 Tetrahedral amorphous carbon (ta-C) films, with a high sp3 fraction, typically above 0.8, and negligible hydrogen content, present the best combination of mechanical properties. Hydrogenated DLC films have been traditionally applied to ductile substrates, for example steel or titanium, in applications such as cutting tools,7 biomedical implants,8 or automotive parts.9 On the other hand, taC films find use in microelectromechanical systems (MEMS), where the substrate material, typically silicon, is brittle.1 Ta-C films applied to silicon substrates provide higher levels of stiffness and load-carrying capacity, a)
Address all correspondence to this author. e-mail: [email protected], [email protected] DOI: 10.1557/JMR.2009.0405
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J. Mater. Res., Vol. 24, No. 11, Nov 2009 Downloaded: 01 Dec 2014
and a significantly lower coefficient of friction, thereby reducing wear by stiction.10,11 However, ta-C films are also brittle, and thus susceptible to other forms of wear, for instance chipping or spallation due to fracture.12,13 Miniaturized components in MEMS are often subjected to concentrated loads and impacts that could cause
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