Study On Strengthening And Toughening Mechanism And High Temperature Thermal Stability Of CrWN-PtIr Coatings

Glass moulding technology is considered to be an effective method for manufacturing high quality optical glass components due to its high production efficiency and low cost.In the process of die forming,the die needs to bear periodic load and thermal cycle,resulting in the reduction of die life.In order to protect the high precision surface of the mold,the protective coating is prepared on the mold to prolong the service life of the mold.At present,the die coating is divided into three types:carbon coating,nitride coating and noble metal coating.Nitride coating is widely used as a protective coating because of its low cost and good thermal stability.Noble metal coating has been studied by many scholars in glass molding system because of its excellent oxidation resistance.This thesis combines the characteristics of nitride coating and noble metal coating,using noble metal doped nitride coating,design and preparation of mold protection coating and its performance is studied.Cr WN coatings and Cr WN-Pt Ir coatings were prepared by plasma-enhanced magnetron sputtering technology.The effects of noble metal doping on the microstructure,mechanical properties,tribological properties and thermal stability of the coatings were investigated by microstructure characterization,mechanical properties and tribological properties testing and heat treatment experiments.The test results of the prepared coatings showed that all the three coatings showed concentric cubic structure of Cr N and W₂N.The results of XPS show that the Cr WN-Pt Ir coating has a typical composite structure of nanocrystal Cr N and W₂N phases with Pt and Ir.The cross sections of the three coatings all showed columnar crystal structure.The Cr WN-Pt Ir coatings showed excellent surface and cross section morphology due to the doping of precious metals refining the grain size of the coatings.This conclusion was also proved in the roughness test.Transmission electron microscopy（TEM）was used to analyze the cross section of the coating.The growth mode of Cr WN-Pt Ir coating was layered alternating growth,and its interface was clear,dark alternating and hierarchical.The coating also has good hardness and toughness,which can be attributed to the dissolution of precious metal atoms in the lattice forming a solid solution hardening.At the same time,the doping of precious metals refines the grain size,which is beneficial to improve the hardness according to the hall-petch relationship.It is found that the three coatings have good bonding force with the substrate by scratch test.The tribological properties of the coating were characterized by friction and wear experiments.In order to explore the thermal stability of the three coatings,the three coatings were placed in a quartz tube furnace for thermal annealing experiment at 650℃for 4hours.XRD test shows that the phase decomposition of Cr WN coating is inhibited by noble metal doping,which indicates that the noble metal doping makes the coating have higher thermal stability.By observing the surface morphology after heat treatment,it is found that the surface quality of the coating is improved by Pt Ir doping.EDS test shows that slight oxidation reaction occurs on the surface of the three coatings.Although heat treatment is carried out under vacuum,a small amount of oxygen element still exists in the atmosphere and reacts with the coating.These trace oxygen elements have no obvious effect on the surface morphology of the coating.According to the mechanical property test,the aging hardening phenomenon occurred in the coating after heat treatment,and the hardness was improved.The doping of precious metal Pt Ir improved the mechanical properties of Cr WN coating.