Effect of Deep Cryogenic Pre-treatment for Substrates on Mechanical Property of DLC Films

Abstract

2µm thick DLC multilayer films were deposited on W9 high-speed steel with deep cryogenic pre-treatment (DCT) for 30 hours in liquid nitrogen by unbalanced magnetron sputtering technique, which were compared with the DLC films without DCT. The results show that compound systems with DCT exhibit higher film-substrate bonding strength and superior friction wear property, which are closely related to the DCT effect that martensitic transformation and decomposition, grain refinement and carbides precipitation. Thus the hardness, strength and residual stress of substrate are improved remarkably, and deformation compatibility between film and substrate under external load become more coordinating. Meanwhile, surface morphology of substrate is effectively adjusted by phase transformation, as a result, element diffusion and mechanical interlocking effect of film-substrate interface are optimized with added grain boundary and fine uniform organization. IntroductionDiamond-like Carbon (DLC) films, a kind of fascinating materials, having some features of both graphite and diamond, such as high hardness, good wear resistance, low friction coefficient, chemical inertness and biocompatibility, which have been extensively used in many fields due to its excellent properties [1][2][3][4]. However, high residual compressive stress and weak adhesion of DLC films to substrate has restricted their working life and applications [5][6]. To reduce the residual stress and to enhance the adhesion and mechanical properties of DLC films, a lot of techniques have been studied by researchers [7][8][9][10][11], such as changing sedimentary means, doping third elements (N, Cr, Ti, W, etc), incorporating metal transition layer. Nonetheless, previous studies have their own limitations because these ways focus primarily on changing the properties of films.Deep cryogenic treatment is a special kind of heat treatment technology, and has been efficiently performed on metal materials since 1940. Samples are cooled down in liquid nitrogen for some periods, then gradually warmed up to room temperature, which is used regularly after quenching and before tempering. A lot of studies suggest that DCT allows the occurrence of phase transition and microstructure refinement, it can significantly improve some properties of metal substrate [12][13][14]. From previous studies, we believe that the hardness, strength and surface state of metallic substrates are equally important for bonding strength and properties of films [15][16]. In this paper, deep cryogenic treatment was used to improve the properties of metallic substrates. we mainly investigated the mechanical property of DLC films from film-substrate bonding strength and friction performance. Moreover, the effect of DCT on microstructure and surface state of substrates were significantly discussed as well as the optimization mechanism of film-substrate interface.