A Novel PVD Coating For Cutting Titanium Alloys:Desgin And Its Performance

It’s an emergency task to research and develop a novel coated carbide tool,which can improve the production efficiency and reduce the cost for machining titanium alloys.First,the abrasion behavior of various commercially used coated carbide against 304 steel and TC4 alloy was studied.It proved that Cr-based coating is suitable as the protection film for cutting TC4 A alloy.Then,a new CrN/CrSiCN multilayer composite coating was deposited on to YG8 carbide.The micro-structure and structure of the coating were characterized by Scanning electron microscopy(SEM),hardness tester,ball cratering tester,XPS,X-ray diffraction,surface roughness tester and scratch tester.The sliding wear of CrSiCN against TC4 alloy was investigated systemically using SFT-2M ball-on-disk instrument.The results shows that:(1)The sliding wear resistance of a monolithic layer Cr30Al70 N coating was superior than those other two coatings with Ti based top surface layer,e.g.,Ti33Al67 N layer and Ti50Al50 N layer.The affinity with counter surface of 304 steel and TC4 alloy ranked downward as the following order: Cr30Al70N/Ti33Al67 N,Cr30Al70N/Ti50Al50 N,and monolithic layer Cr30Al70 N coating.The wear mechanisms of these three hard coating dominantly include smearing,adhesion and oxidization.The main wear mechanism of these coatings is local tearing,which results in coating spalling.The nature of the locally tearing is that the cemented carbide is in direct contact with the TC4 alloy or 304 steel substrate to form high adhesive strength junction.Therefore,TC4 alloy should be selected to choose Cr-based coating.Therefore,TC4 alloy should be selected to choose Cr-based coating.(2)After DOE tests,the optical parameters of CrSiCN coating deposition are work vacuum pressure 0.3Pa,bias voltage 50 V,Cr-Si alloy cathode target current 60 A.When the C/N ratio was 2:1 in a CrSiCN coating,it achieved the best comprehensive mechanical performance,surface hardness 3049 HV0.01,and the adhesion strength 94 N.The CrSiCN coating with C/N ratio 1:1 exhibited the worst mechanical properties,which had surface hardness 2411HV0.01 and adhesion strength 11 N.(3)With the C content of a coating increasing,the coating gradually transformed from a crystalline state to an amorphous state.Si element in the CrSiCN coating mainly existed in the form of a-Si3N4/a-SiC,a-SiCxNy amorphous phase.In addition to solid solution in the CrN phase,there was a large number of C being amorphous state in the film.(4)The sliding wear resistance of a CrSiCN coating against TC4 alloy performed well.One reason was that the existence of amorphous C in the coating promoted formation of alubricating transfer layer between the sliding counters and retarded locally spalling of a coating.Another reason was that the transition CrN layer widened and appeared melting into the substrate and top coating layer,in that the adhesion strength between the coating and the substrate improved.(5)The dominant wear mechanisms of CrSiCN coating against TC4 alloy were also smearing,adhesion and oxidization.Locally tearing of a coating was the main wear failure mode,which was resulted from the directly contact between the TC4 alloy and the carbide substrate.So the adhesion strength of a coating played significance.Comparing the wear resistance of CrSi CN coating with monolithic layer CrAlN coating,the former had adhesion strength 94 N,and the latter 60 N.Although the friction coefficient of the former coating is only about 67% of the the latter,the spalling area of the former is less than 1/6 of the CrAl N coating.(6)In the high-speed milling TC4 titanium alloy,CrSiCN composite coating cutter life is21.5 min as the reference flank wear is standard.It has a 144% increase in tool life compared to the carbide knife,which is 50% higher than the GT-1 composite coating cutter;CrSiCN composite coating cutter can greatly inhibit the formation of titanium bond and chip formation during processing and reduce the friction and wear of chips and milling cutter,improve the ability of milling cutter,reduce the temperature rise of milling area and attached.