Preparation And Properties Of Superhard And Anti-corrosion CrB₂ Coatings

Due to the excellent properties such as high hardness,high strength,low friction coefficient,wear resistance and corrosion resistance,hard coatings have been widely used in various surface protection fields,such as cutting tools,machined dies and precision parts.With the development of modern manufacturing industry and the expandment of new fields,many mechanical parts and components are facing worse service conditions.In order to further improve the efficiency of equipment and extend its service life,more and more attention has been paid to superhard coatings in recent years.Traditional superhard coatings often need preparation conditions of high temperature and high pressure.Transitional metal boride coatings have strong valence bonds and abundant electronic structures,they are expected to make superhard coatings more widely used.As Cr is an excellent corrosion resistant element,CrB₂ coating is expected to meet the requirements of marine equipment for good mechanical properties and excellent electrochemical properties.In this paper,CrB₂ coatings were prepared by DC magnetron sputtering at 300oC,and then prepared under different target magnetic fields and substrate bias.The main research results and conclusions are as follows:?1?CrB₂ coatings had strong?001?preferred orientation at 300oC,it had compact nanocolumnar microstructure.The mechanical and electrochemical properties of the coatings were tested by nano indentation,electrochemical impedance spectroscopy and electrochemical polarization curves.The hardness of the coatings was up to 45.7±1.6 GPa which reaches the standard of superhard coating.The loading-unloading curve was smooth and there was no pop-in phenomenon,which further proves that the coating was compact and has few defects.The corrosion current density of the coating decreased by about two orders of magnitude relative to the substrate,and the polarization resistance increased by about two orders of magnitude.They proved that the coating had excellent corrosion resistance.The obvious passivation phenomenon was due to the chemical reaction of CrB₂ coating in the corrosive medium and the formation of Cr₂O₃.?2?By setting a non-uniform gradient magnetic field,CrB₂ coatings were prepared under different magnetic field intensities,and the effects of target magnetic field intensity on the microstructure,structure and properties of the coatings were studied.With the increase of magnetic field strength from 40 mT to 100 mT,XRD showed that the?001?preferred orientation of the coating increased gradually,the grain size increased gradually,and the thickness of the coating increased from 1.78?m to 2.75?m.When the target strength was 80 mT,the hardness of the coating reached the maximum value 47.75±2.49 GPa,and the corrosion resistance of the coating reached the best.The corrosion current density of the coating was 3.6×10^-9A·cm^-2,When the magnetic field intensity further increased to 100 mT,the hardness and electrochemical performance of the coating decreased gradually.?3?The effects of bias voltage on the microstructure and properties of CrB₂coatings were studied by using different substrate bias under the optimum magnetic field.The results showed that the mechanical and electrochemical properties of the coatings reached the best when the bias voltage was-50 V.The hardness of the coating was 49.78±1.44 GPa,corrosion current density was 2.17×10^-9 A·cm^-2,because at this configuration,the coating had strong?001?orientation,small grain size,low porosity and good adhesion.When the bias pressure was further increased,the porosity of the coating increased,the bonding force decreased,and the grain growed up,which resulted in the decrease of the mechanical and electrochemical properties of CrB₂ coating.