Composition And Structure Design Of Fuctional Composite TiAlN-based Nano-coatings And Subsequent Titanium Cutting Performance

Titanium alloy has been widely used in many industrial fields,including the aeronautic,automotive and biological industries for its high-strength lightweight and good corrosion resistance.However,titanium is hard to machine due to its low thermal conductivity,high chemical affinity,and low elastic modulus.These characteristics lead to high cutting temperature,short life,and high vibration of cutting tools.Coating on tools has provided an effective way to overcome these problems.It is well known that metal nitride hard coatings can improve the lifetime and cutting performance of coated tools.However,there are still many problems such as poor wear resistance,easy oxidation,serious sticking and so on with commercial tool coatings.These problems become more severe during high speed machining titanium alloy.The development of high speed machining technology brings a series of new problems for tool protective coatings,such as higher cutting temperature,more serious tool vibration,adhesive and oxidation wear.Thus,there are needs to further increase the mechanical properties and oxidation resistance of hard coating.Composition,structure design and functional composite are common methods for the development of new coatings.In this paper,magnetron sputtering is used to prepare TiAlN-base nano-coatings.The effect of different doping element,gradient and multilayer structure,functional coating on the properties and titanium cutting performance of the coating was studied.Furthermore,the relationship between microstructure and cutting performance of TiAlN-based nano-coatings was also revealed.The research content and achievements are given in details as follows:The effects of Si,Ta and Cr element on the microstructure,mechanical properties and oxidation resistance of the TiAlN-based nano-coatings were investigated by composition design method.The results show that the coating structure changes from columnar growth to nanocrystal growth by doping Si element.The coating surface roughness and hardness were also improved.However,too high Si content will cause deterioration of the coating hardness and adhesion strength.Adding Ta,Cr element can improve the oxidation resistance of the coating,while improving the mechanical properties of the coating.The addition of Ta into TiAIN coating can retard the formation of ?-TiO2,which is very important for the oxidation resistance.However,a higher Ta content is not conducive to retard the formation of ?-TiO2 phase.The addition of Cr into TiAIN coating can retard the formation of hcp-AIN,while forming a Cr2O3 diffusion barrier on the coating surface.The titanium cutting experiment indicates that the flank wear of TiAlCrN coated tool decreases by about 50%than that of the uncoated carbide tool.The flank wear of TiAlTaN coated tool decreases by about 20%of that of the uncoated carbide tool.However,an increased flank wear of TiAISiN coated tool is observed for its large brittle.The effects of gradient and multilayer structure on the adhesion strength and fracture toughness of the TiAlN-based nano-coatings coating were investigated by structure design method.It is found that the gradient design can improve the quality of the coating surface,reduce internal coating stress and obtain better comprehensive mechanical properties.In TiAISiN coating,for example,gradient TiAISiN coating can maintain more than 70%of the original TiAlSiN coating hardness,but the adhesion force is increased by 300%to more than 100 N.Multilayer structure design can suppress crack propagation within the coating to improve the coating fracture toughness.A maximum KIC of 3.6 MPa m1/2 is obtained in a TiAlN/CrN multilayer coating with a 25 nm bilayer period.The titanium cutting results show that the flank wear of gradient TiAlSiN and TiAlTaN coated tools decrease by 46%and 32%,respectively,compared with the non-gradient coated tools.The flank wear of TiAlN/CrN multilayer coated tool with period thickness of 25 nm is about 33%lower than that of single TiAlN coated tool.The TiAlTaN coating was modified by TaO functional coating.The mechanism of TaO functional coating in reducing adhesive wear was explored.The attained results reveal that the coatings' friction coefficient and surface roughness decreased with the increasing TaO thickness from 0 to 150 nm.The adhesive bonds between coating and titanium also reduced with increasing TaO thickness.This is because of positive interaction energy between the TaO coating and pure Ti.Tool life of the TiAlTaN/TaO coated tool with TaO thickness of 150 nm is increased more than triple the TiAlTaN single coated tool.At a cutting length of 547 m,the flank wear of G-TiAlTaN/TaO coated tool prepared in this work is about 20%lower than that of the Sandvik GC1105 coated tool.