Mechanical Behaviors And Deformation Mechanisms Of Arc Ion Plated Nitride Coatings

Transition metallic nitrides,as a material family owning high hardness,have been widely used as wear protection coatings.But their brittle nature limits their application under extreme loading conditions and extension towards multiple working situations.Understanding the underlining deformation mechanisms is the precondition to optimize their brittleness behaviors.In this work,Cr-Al-N and Ti-Mo-N were selected as two representing systems with high and low brittleness respectively.They were prepared using arc ion plating technique,and process,composition,structure and mechanical behaviors have been systematically studied.The emphasis has been put on analysis of cross-scale mechanical behaviors and revealing grain boundary(GB)plasticity that governs global mechanical properties.Owing to their highly brittle nature,the CrAlN coatings show typical brittle behaviors during deformation,that plastic flow can hardly be found,and grain boundary sliding leads to cracking with large width and smooth cracking path.Although stress-induced twinning and fcc-hcp transition were found,they have little impacts on the total brittle behaviors.As a result,CrAlN coatings usually show negative SRS(strain rate sensitivity),m values.Lowering the N content can bring about metallic behavior of the coatings leading to positive m value.But it also causes dramatic drop in hardness and wear resistance,destroying the load-bearing capacity of the coatings.By increasing the bias voltages,one can create higher compressive stress in the coatings that impedes columnar grain sliding,thus enhancing hardness and wear resistance of the coatings.But meanwhile,such method induces more lattice defects,rising the risks for the trans-grain cracking,limiting the positive impacts of the compressive stress.In comparison,CrAlN coatings deposited in conditions with high-ionization rate and temperature possess columnar crystalline structures with very low defect density and high compressive stress.Such structure has very low risk for trans-grain cracking and columnar grain sliding is hindered,endowing the coatings with super-hardness and load-bearing capacity,and positive m value,which is unusual in brittle system,is formed as a result.Mo solution were predicted,via first-principle studies,to increase valence electron density of the lattice,causing hardening-toughening effect.Accordingly,TiMoN coatings should exhibit totally different mechanical behaviors compared to CrAlN.However,insufficient experimental focus has been put on the TiMoN systems.The previous result of systematic investigation on Ti Mo₈N coatings of our group show that N₂ pressure is the dominant process parameter.The present work prepared Ti Mo₈N coatings with various N₂ pressure,and systematically studied the correlation between process,structure and mechanical properties.Emphasis was put on revealing the deformation mechanism of Ti Mo₈N&Ti Mo₈N coatings.The results show that addition of Mo promote the formaiton of bct phase,so that Ti Mo₂₇N coatings deposite in 0.15-2.0 Pa all demonstrate fcc+bct dual phase structure.Mo reduction was caused by angle loss due to different atom weight.Driven by angle loss and thermodynamic,Ti and Mo elements formed layered segregation at nanoscale.Mo addition leads to strong solid solution hardening.The hardness of as-deposited Ti Mo₂₇N coatings range from 28.1-35.4 GPa,much higher than 23 GPa of Ti N counterpart,but no better than that of Ti Mo₈N coatings because Mo segregation reduce the actual solid solution effect.Due to the existence of bct phase,the load-bearing capacity of Ti Mo₂₇N coatings is weaken-the highest crack generating load is only 200 g.But due to the stronger solid lubricating effect,friction coefficient and wear rate are both significantly lower.When strained,full and partial dislocations in Ti Mo₈N and Ti Mo₂₇N coatings are activated,enabling the motions of sub-GBs,GB curvature and deformation twinning mechanism.Additionally,strain-induced amorphorization was also unveiled in Ti Mo₂₇N coatings.The synergy of these plastic mechanisms are responsible for the high toughness of TiMoN coating,as predicted by the former theoretical study.Ti Mo₂₇CN coatings were prepared with various C₂H₂ flow rate to investigate the influence of C content on structure and mechanical properties.C addition improves deposition rate and promote Ti-N and Mo-C segregation.Therefore,with rising C content,layered segregation of Ti and Mo grows more phenomenal.Grain size is also reduced,accomanied by transition from columnar crystalline structure to amorphous+equiaxed nanocrystals composite structure,and disappearance of bct phase and(111)orientation of fcc phase.Hardness,toughness,load-bearing capacity and wear resistance first increase then decrease as C content rises.Highest hardness reach 37.6 GPa when C content is 19.56at%and the lowest is 30.9 GPa at C content of 34.95at%,lower than that of Ti Mo₂₇N coatings.An unique structure,characterized by highly parellel columnar GBs,3D GB and stable GB cracks was formed,when C content is 19.54at%.As a result,multiple plastic mechanisms including low inherent brittleness,3D GB strengthening,pattern effect of highly parallel GBs and strain-induced amorphization,take place when strained,resulting in substantial plasticity at GBs,so that crack tips are blunted,and the coating show high toughness and positive m value.When C content is 34.95at%,the thickness of amorphous phase exceeds the critical dimension of main shear band,causing softening-brittlement effect,leading to a negative m value and decreased hardness and toughness.