The effect of thermal and plastic mismatch on stress distribution in diamond like carbon film under different interlayer/substrate system

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The poor adhesion for diamond or diamond like carbon (DLC) on steel substrates is attributed to carbon solution and high stress level resulted from the large difference in coefficients of thermal expansion between the film and the substrate. The adhesion is improved by the addition of a metal interlayer with carbide formation and acted as the diffusion barrier. However, the addition of the interlayer might cause larger stress along the interface which will impair the adhesion. Moreover, for DLC deposited on nonferrous substrate, the choice of the interlayer might coupled with the substrate effect. Previous studies have shown that Young's modulus (elastic) mismatch and yield stress (plastic) mismatch between film and substrate has effect on mechanical properties and crack growth stress. The difference in coefficient of thermal expansion (CTE) between interlayer and substrate is the source of thermal stress. These mismatches and the associated stress distribution might provide another perspective about interlayer other than carbide formation. A finite element program which simulate the thermal cooling and indentation process has been utilized to study the stress distribution caused by these mismatches. The former is simulated as deposition process while the latter is related to wear-protection operation. The results show both interlayer Cr and Ti has effects on magnitude and distribution on thermal stress and indentation stress. Such effect is also substrate dependent. To minimize the thermal stress, the thermal coefficient of expansion of the interlayer should lie between the film and the substrate. This provide a guideline for the choice of the interlayer for DLC deposited on different substrates.