The Microstructure Regulation,Mechanical Behavior,Strengthening And Toughening Mechanism Of Laminate Heterostructured Magnesium Alloy

Along with the development of industrial technology and the expansion of application fields,the further requests for the wear resistance film at complex working environments are put forward.The wear-resistant films should possess high hardness and toughness,while the high hardness commonly accompanied by the descended toughness.The design and preparation of hard yet tough wear-resistant film is a challenging work.Transition metal nitrides exhibit excellent comprehensive performances,such as high melting point,high hardness and sound wear resistance,well chemical stability,sound corrosion resistance,and the certain conductivity,suggesting the great application potential in hard and wear resistant films at the surface of industrial parts and tool.Tantalum nitride（Ta N）possesses excellent physical and chemical properties,suggesting the potential in surface protective film at under various extreme environments.The preparation of hard yet tough multi-element transition metal nitride films is a great significance for the development of wear resistant film materials.In this paper,by alloying metal elements and regulating the ratio of nitrogen atoms to transition metals（stoichiometry）,the multi-element transition metal nitride films based on tantalum nitride with high hardness and toughness were designed and prepared.The structure,mechanical and tribological properties and the toughening mechanism were researched.Hard and tough with sound wear resistance TaAgN、Ta_1-xMo_xN、Ta Mo N_x、NbTaMo WN_x films were prepared by magnetron reactive sputtering.The microstructure of films was characterized by X-ray diffraction（XRD）,high resolution transmission electron microscopy（HRTEM）and scanning electron microscopy（SEM）.The hardness,fracture toughness were measured by indentation tester and scratch tester.In addition,a ball-on-disk tribometer was used to obtain the wear rate and friction coefficient;Raman spectroscopy and X-ray photoelectron spectroscopy（XPS）were used to analyze the film structure and chemical bond.The prepared Ta N_x films（x=0.95～1.21）maintain a B1 Na Cl structure by changing stoichiometry（N/Ta atomic ratio,x）.The residual stress of films increases with raising nitrogen partial pressure.Raman spectroscopy and XPS results indicate that the Ta_N/N_Tadefects were formed in the sub/over-stoichiometric Ta N_x films.The increased stoichiometric x leads to the decrease of hardness and fracture toughness of Ta N_x films,and increasing the wear rate.The sub-stoichiometric Ta N_0.95 processes higher hardness,toughness and lower wear rate compare to theδ-Ta N_x film with x=0.99～1.21.TaAgN films were prepared by cooperating Ag in Ta N.HRTEM and SEAD results indicate that few solute Ag atoms（1.2 at.%）dissolve into Ta N sublattice,resulting in the grain refinement.The film maintains B1-Na Cl structure.Incorporating only a few solute Ag atoms（1.2 at.%）achieve significant improvement in hardness（～36.1 GPa）,fracture toughness(K_IC=1.51 MPa*m^1/2)and wear resistance(wear rate～1.9×10^-6 mm³/Nm).The main reason for the enhanced mechanical properties by solute Ag is due to the stronger hybridization of p（N）-d-e_g（Me）orbitals and the emergence of Ag d-t_2g stateshe,evidenced by XPS valence band spectrum,improving the strength and metallic characteristics.The hardness,toughness and wear resistance of films decreased with further increasing Ag contents.Ta_1-xMo_xN films were constructed by alloying Mo into Ta N.The films exhibit B1 Na Cl structure in the composition range of x=0.13～0.86.The mechanical and tribological properties of Ta N films are improved by alloying Mo,while the Ta_1-xMo_xN films with balanced Mo and Ta contents（x=0.57）processes a higher hardness（～39.5 GPa）and fracture toughness(K_IC=2.78 MPa*m^1/2),and lower wear rate(～1.9×10^-7 mm³/Nm).The improvement of hardness and toughness are mainly due to the solid solution strengthening and the highly populated shear sensitive d-t_2g metallic states.A decrease in wear rate is due to the high hardness and toughness.The sub-stoichiometric Ta Mo N_x films（x=N/Ta+Mo）were prepared with controlling nitrogen content.The solid solution films maintains B1-Na Cl in the range of x=0.42～0.73.The Raman spectroscopy and XPS results confirmed that the sub-stoichiometric Ta Mo N_xfilms processes TM_N（TM=Ta/Mo）defect.The film with x=0.59 exhibits higher hardness（～36.4 GPa）,fracture toughness(K_IC=1.67 MPa*m^1/2)and better wear resistance(wear rate:～6.5×10^-7 mm³/Nm).The XPS results revealed that the valence band structure of films can be optimized by adjusting the stoichiometry,while the strong p（N）-d-e_g（Me）and d-t_2g（Me）states leads to the high metallic and strength characteristics for the chemical bonds of films,improving the hardness and toughness.The NbTaMo WN_x films with multi components were constructed.The stoichiometric NbTaMo WN_x films exhibit B1-Na Cl structure in the range of x=0.16～0.72.The residual stress of films increases with increasing the stoichiometry x（x=N/TM）.The TM_N defects are formed in the sub-stoichiometric NbTaMo WN_x films evidenced by the Raman spectrum and XPS.The NbTaMo WN_x film with x=0.48 exhibits a higher hardness（30.8 GPa）,fracture toughness(K_IC=1.85 MPa*m^1/2)and wear resistance(wear rate:～5.0×10^-7 mm³/Nm).The enhanced mechanical properties of films are mainly attributed to to the solution strengthening from the multi components,and the high metallic and strength characteristics by regulating the stoichiometry.