The Interface Design And Properties Of Diamond-Like Carbon Films With High Adhesion Strength Deposited On Aluminum Alloy

Due to the excellent properties of low density,high strength-to-weight ratio,good machinability and decoration,aluminum alloys are widely used in aviation and aerospace industries,automotive,ships and other fields.With the rapid development of modern manufacturing and the continuous expansion of new fields,the use of aluminum alloys is facing higher requirements.Surface modification is one of the most effective ways to improve the low hardness,insufficient wear resistance and corrosion resistance of aluminum alloys.As an environmental friendly surface modified material,diamond-like carbon?DLC?film has the advantages of high hardness,low friction and good chemical inertness,which is expected to be an ideal coating for aluminum alloy protection.To solve the key problem of low adhesion caused by the large differences in physical properties of the aluminum alloy/DLC system,the thesis aims to design the the strong interface structures between the DLC film and aluminum alloy substrates using linear ion beam combined with magnetron sputtering technology.Based on the element selection and interface failure analysis of typical interlayers,the effects of interlayer with different thicknesses and structures on the combining performance are systematically studied.Several optimized DLC coating with strong bonding interface were obtained,and the tribological properties are evaluated.The main work and conclusions are as follows:Firstly,the typical interlayer elements Si and Ti are selected,then Si/DLC and Ti/DLC films with Ti and Si interlayer are prepared on the aluminum alloys,respectively.A comparative study of the different interlayers on the microstructure,mechanical properties,bonding strength and failure mechanism of the films are investigated.The results showed that the introduction of Si and Ti interlayers make no contribution to the morphology and structure of DLC films.Both interlayers have effects of reducing the internal stress of the film and the mismatch of the mechanical properties between the film and substrate.In particular,the Si/DLC film exhibits high toughness and good resistance to crack initiation and expansion when resisting scratch damage.Compared with the pure DLC film,the adhesion strength of Si/DLC film is increased from 8.4 N to 10.7 N.The main reason for the adhesive failure is that cracks initiate at the weak interface between the Si interlayer and the substrate.Then these cracks propagate near the interface and cause the interfacial fracture;However,the Ti interlayer in the Ti/DLC film has insufficient ability to coordinate the plastic deformation between the film and the substrate.At the same time,the weak interfacial adhesion between the Ti interlayer and the top layer DLC causes crack initiation and upward propagation,which causes the premature brittle flaking of the DLC film and reduces the adhesion strength to 7.1 N.Secondly,in order to further improve the adhesive properties,a-C:H:Si:O films with different doping content of silicon and oxygen are prepared by linear ion beam to build the interlayer structure with gradient change of hardness and modulus..The results show that the introduction of Si and O elements mostly forms O-Si-C bonded Si Ox Cy phase with C element,and a small amount of Si Ox phase was formed through Si-O bonding,and the percentage of sp² increased.As the Si and O content increases,the I_D/I_G value and the G peak fluctuate slightly.The obtained hardness and modulus of the three doped films are 155.8/20.7 GPa,134.2/17.3 GPa,115.0/14.8 GPa respectively,realizing the gradient change of hardness and modulus.Based on this,the Ti/a-C:H:Si:O/a-C:H multi-layer film system composed of a Ti sublayer,a interlayer with a silicon-oxygen gradient doped layer,and the DLC surface layer is designed,which combined with the optimization of the film thickness of each layer to achieve the best adhesion strength?16.5 N?.Analyzing the adhesion mechanism,the thicker Ti sublayer can significantly reduce the internal stress of the film and coordinate the plastic deformation between the film and the substrate.The modulus gradient interlayer a-C:H:Si:O has enhanced interfacial toughness and reduced mechanical properties between the film and the substrate.Therefore,it has excellent resistance to crack initiation,propagation and plastic deformation,and its failure behavior shows the characteristics of a tough film.Finally,the selected single/multi-layer transition layer DLC film system is used to study the friction behavior in atmospheric environment.The results show that compared with the DLC film without the interlayer,the load-carrying capacity of Si/DLC film is improved from 5 N to 15 N and the wear rate is reduced by 1 time,and the friction life is increased by 6 times at a load of 5 N;The Ti/a-C:H:Si:O/a-C:H multi-layer film system can further increase the load-bearing capacity of aluminum alloy/DLC film to more than 20 N,the friction coefficient and wear rate are 0.146 and 6.5×10^-16mm³/N?m when the load is 20 N.The excellent friction performance of these films is mainly due to the formation of amorphous carbon transfer film on the surface of the friction pair and the improvement of toughness.