Effect Of C And Si-doping On Properties Of WBC(Si) Hard Coatings And Its Mechanism

AlB₂-type WB₂ is a highly promising hard self-lubricating coating.Its hardness is35⁴0GPa and the wear rate is about 10^-7mm³/mN,which has been a competitive candidate as a tool coating for nonferrous metals and their alloys processing in the aviation and aerospace field.However,the key problem for the AlB₂-type WB₂ coatings is their relative low toughness and high compressive stress.The multicomponent coating constructed by C and Si doping is expected to further improve the comprehensive performance of the WB₂ coatings.In this paper,WBC and WBCSi coatings were prepared on YG8 cemented carbide and Si wafers by direct current（reactive）magnetron sputtering.The effects of C and Si doping on the microstructure,residual stress,mechanical and tribological properties of AlB₂-type WB₂-based WBC（Si）coatings were investigated systematically.The WBC coatings were deposited by reactive magnetron sputtering using a WB₂ target with a constant working pressure（C₂H₂+Ar）at 0.7 Pa.By increasing the C₂H₂ partial pressure P_C2H2（0⁰.035Pa）,the phase composition of the WBC coating changed from WB₂/a-WC（a represents amorphous structure）to a-WB₂/a-WC and then to a-WB₂/a-WC/a-C in turn.The preferred orientation of the coatings changed from（101）to（001）to amorphous structure,and the cross-sectional morphology gradually evolved from a columnar structure to an amorphous shape.The residual stress of the coatings changed from tensile stress to compressive stress due to C doping,and the compressive stress gradually increased with the increase of P_C2H2,which was up to 1.5GPa.When P_C2H2=0.015Pa,the WBC coatings with a small amount of a-C had higher hardness and toughness,which led to the lowest wear rate about 9×10^-8mm³/mN.WBCSi coatings with different Si contents（0¹3.2at.%）were prepared by changing the Si target power with P_C2H2=0.015Pa at a constant working pressure 0.7Pa.By increasing the Si content,all the deposited WBCSi coatings were amorphous,and its degree of short-range order was reduced,the cross-section morphology exhibited the amorphous structure;both the reduced short-range order of the coatings and the formation of a-WSi₂ and a-SiC greatly released the residual compressive stress of the WBCSi coatings.At the same time,the residual compressive stress of the coatings increased with the increase of the film thickness within a certain range;The hardness and the elastic modulus of all the amorphous WBCSi coatings presented the similar values.The lowest wear rate about 1.4×10^-7mm³/mN was obtained for the WBCSi coatings with 10 at.%Si.WBCSi coatings with different Si contents（0⁸.94at.%）were synthesised by DC magnetron sputtering technology using a WB₂/C target by changing the Si target power.As the Si content increased,the crystallinity of the coating was increased,the columnar structural unit was refined and the grain size gradually decreased.The WB₂/a-WSi₂ or（and）WB₂/a-WSi₂/a-SiC nanocomposite were formed,which significantly reduced the residual stress of the WBCSi coatings.In addition,the hardness of the WBC coating increased from28.6GPa to 39GPa,and both the toughness and wear resistance of the coatings were also improved.The WBCSi coatings with 8.94at.%Si showed the lowest wear rate about2.0×10^-7mm³/mN.