The Effect Of Cryogenic Treatment On The Mechanical Properties Of Amorphous Carbon Film/high Speed Steel And Its Mechanism

Amorphous carbon films is mainly composed by amorphous and microcrystalline structure of sp~2,sp~3 hybrid carbon.According to different internal structure of films,which including diamond-like carbon film(DLC)and graphite-like carbon film(GLC).As a representative of ideal anti-wear 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.However,high residual internal stress and weak adhesion of film-substrate has restricted their working applications and life.Nonetheless,previous performance optimization studies have their own limitations,it’s difficult to obtain the amorphous carbon films that with good bonding strength and excellent mechanical properties.Cryogenic treatment is a relatively mature technology of material processing,a lot of studies suggest that cryogenic treatment can make material occurrs plastic deformation,refined,phase change,etc.It has been widely applied in crystals and amorphous materials such as metal,ceramic and plastic to improve their comprehensive mechanical properties.However,the research that cryogenic treatment is applied to amorphous carbon films is rarely reported.In this paper,2 μm thick DLC multilayer films and 4 μm thick GLC multilayer films were deposited on W9 high-speed steel by unbalanced magnetron sputtering ion deposition technique.The two groups of film/substrate composite systems were cryogenic treated by liquid nitrogen.At the same time,in view of the DLC film/high speed steel,cryogenic pre-treatment was used for substrate as a contrast experiment.The research focus on changes of film/substrate self-organization and interfacial microstructure after cryogenic treated,then discusses the effect and mechanism of cryogenic treatment on mechanical properties such as film-substrate bonding strength,frictional properties of DLC and GLC amorphous carbon films.The results are as follows:DLC films/high-speed steel composite systems:(1)Cryogenic treatment can promote transformation,decomposition of martensitic and precipitation of fine carbide in W9 high-speed steel,its texture become fine homogenization and grain boundary is increased.After cryogenic treatment,residual compressive stress of substrate increased from-369.20 MPa to-446.02 MPa and hardness increased from 851.9HV to 917.2HV.(2)Although cryogenic treatment had no significant effect on hybrid bond of Celement in DLC films,film-substrate bonding strength of DLC films/high-speed steel is improved by cryogenic pre-treatment and cryogenic treatment.The removal of thin film plaque under load is relieved and crack density is reduced.This is beacuse that the increase of grain boundary and fine homogenization of substrates surface after cryogenic treatment are favorable for short-range diffusion of interface elements and mechanical interlocking of film-substrate.Similarly,cryogenic treatment improves membrane-based coordination performance and optimizes internal structure of DLC films.(3)The average coefficient of friction of untreated,cryogenic pre-treated and cryogenic treated samples respectively are 0.148,0.130,0.122.The friction coefficient is reduced after cryogenic treatment and cryogenic pretreatment,furrow wear and wear amount are reduced,and wear resistance is enhanced.It is mainly because the effect of cryogenic treatment that optimization of membrane-based interface,increase of hardness,strength and residual compressive stress inhibits generation and expansion of cracks during friction and wear.GLC films/high-speed steel composite systems:(1)The columnar crystal structure of Cr transition layer in GLC films/high-speed steel compound systems is broken after cryogenic treatment,and fibrous structure is eliminated.Meanwhile,Cr nanocrystalline in Cr/C gradient layer is refined,and the average grain size is reduced from 20 nm to 5nm,organization become more dense and homogeneous,thus achieve effect of fine-grain strengthening.(2)The failure mode of untreated GLC film/high-speed steel compound system is massive layer peeled and failure of films is seriously.After cryogenic treatment,the failure probability of film itself and interface is reduced,and membrane peeling eased,crack density and length are smaller,finally to improve film-substrate bonding strength.(3)After cryogenic treatment,friction and wear properties of each layer in GLC films are improved,thus film failure is slowed down.it’s because that the internal structure of films is fine,homogeneous and interface fusion is better,which will reduce stress concentration and improve bearing capacity of GLC films,thus inhibits the fracture and interfacial failure during friction and wear.