Effect Of Cryogenic Treatment On Mechanical Properties Of DLC / Alloy Steel Composite System And Its Mechanism

Diamond like carbon (DLC) is a kind of amorphous carbon film formed by hybridization of sp3 and sp2 bonds. It is widely used as a novel hard film material in the fields of mechanical, electronic, optical, thermal, acoustic, medical and so on due to the excellent performances. But some defects of poor film-substrate cohesion and fracture toughness caused by overlarge residual stress of DLC film have limited its application in some big load conditions. In order to further improve the usability of DLC films, the mechanical behavior of DLC film is now improved popularly through doping the third element, introducing a stress buffer layer or by post-annealing etc. However, the various types of film modification process have their intrinsic limitations, which is difficult to obtain the material of DLC film having high hardness, high toughness, good adhesion and wear alike good comprehensive mechanical performance.A lot of studies suggest that cryogenic treatment (CT) can significantly improve the mechanical properties of metal substrate materials such as hardness and wear resistance. In particular, cryogenic treatment allows the occurrence of martensitic phase transition, micro structure refinement and fine carbide precipitation in high-speed steel alike alloy steel materials, but literatures on CT of integrated film-substrate system using this kind of alloy steel as substrate had rarely been reported. As such, It is much worthwhile to make clear how CT will influence bonding strength of DLC film-substrate composite system and whether the mechanism applied to high-speed steel or other alloy steel is still equally applicable to the film-substrate system.In this paper, the 2μm thick DLC films were deposited by unbalanced magnetron sputtering technique on W9 high-speed steel and 304 stainless steel alike different alloy steel substrates respectively followed by two kinds of cryogenic treatment process route. According to the different rules and features of effect on mechanical properties of the DLC/alloy steel composite system by cryogenic treatment, the function and mechanism of effect on film-substrate bonding strength of DLC film/high-speed steel as well as friction and wear property of DLC film/304 stainless steel by cryogenic treatment were respectively discussed in the paper.The results are as follows:(1) The nano-hardness of DLC film on W9 high-speed steel substrate increases by about 20% after 30h cryogenic treatment, which is because that cryogenic treatment can produce the effect of miniaturization and densification on the organizational structure of DLC film. Meanwhile, the films shedding of Rockwell indentation edge of DLC film/high-speed steel is significantly reduced by cryogenic treatment followed by the enhancement of film-substrate bonding strength, the DLC film/alloy steel substrate composite system’s capability of resistance to deformation pushed by large load is enhanced by cryogenic treatment. (2) The adhesion force of DLC film on W9 high-speed steel substrate increases approximately from 1.1N to 1.5N after 30h cryogenic treatment, and the structure of substrate is refined and carbon content in substrate surface of film-substrate interface significantly increases. This is because that cryogenic treatment allows the occurrence of martensitic transformation and decomposition increasing substrate grain boundary which facilitate the interface diffusion of carbon from DLC to the substrate. The element C diffusing to grain boundaries of the substrate surface is proposed to form new carbides which enhance the pinning effect of DLC film on the substrate to improve film-substrate force. (3) The cryogenic treatment, annealing treatment and composite treatment of cryogenic+annealing all have improved the friction and wear property of DLC film/304 stainless steel in different degrees and reduced the friction coefficient and wear volume of DLC film. In particular, the single cryogenic treatment performs best. It is because that the three kinds of treatment all can improve the fracture toughness of DLC film and reduce surface roughness of DLC film, but only single cryogenic treatment doesn’t reduce but increases film’s hardness and makes DLC film get the best lubricity and highest hardness-to-elasticity ratio, which other methods cann’t reach to. (4) The cryogenic treatment, annealing treatment and composite treatment of cryogenic+annealing all can adjust the residual stress of film/substrate composite system, but annealing process often sacrifices the hardness of DLC film as the price. But compared to single cryogenic treatment, by making the fracture toughness of DLC film better,the annealing treatment and composite treatment of cryogenic+annealing appropriately offset the negative impact of largely reducing DLC film’s hardness on DLC film’s abrasion resistance, but inferior in terms of the improvement of DLC film’s lubrication friction characteristics.