Study On The Microstructure And Corrosion Resistance Of TiAlN Based Hard Coatings

TC4（Ti-6Al-4V）titanium alloy has been widely used in the field of Marine engineering equipment because of its advantages of high strength,low density and excellent machining and forming properties,which could meet the service requirements of mechanical devices in complex environments.However,the limitation of its corrosion in the environment of hydrochloric acid,sulfuric acid and other reducing acids hindered its further use.In view of this situation,this paper adopted multi-arc ion plating technology to prepare TiAlN based hard coating on the surface of TC4titanium alloy,and studied the microstructure,mechanical properties and the influence of different corrosive media on the corrosion resistance of coating system by doping Si、Cr elements and constructing TiAlN/CrN multi-layer coating structure.The results showed that:（1）The microstructure of TiAlSiN and TiAlCrN The microstructure of TiAlSiN and TiAlCrN coatings showed the following characteristics:the fracture morphology was smooth and compact,without obvious columnar crystal structure,grain size was refined to varying degrees,and the size of surface defects such as large particles and holes was significantly reduced,and the density was improved to varying degrees.In addition,the hardness value decreased a little,and the bonding force increased significantly.The electrochemical corrosion test results showed that compared with TiAlN coating,the self-corrosion current density of TiAlSiN and TiAlCrN coating could be reduced to 1～3 orders of magnitude,and the impedance modulus of TiAlSiN and TiAlCrN coating could be significantly increased.Immersion corrosion test revealed that TiAlSiN coatings showed slight corrosion signs,accompanied by certain corrosion products.However,there was no obvious difference in morphology and weight of TiAlCrN coating before and after corrosion.（2）The fracture of TiAlN/CrN multilayer coating was straight and clear,with periodic layered structure and no obvious columnar crystal.The number of defects such as large particles and holes on the surface was significantly reduced,and the density was significantly increased.In addition,the hardness value also appeared a slight attenuation,the adhesion significantly improved.The electrochemical corrosion test results showed that compared with TiAlN coating,the self-corrosion current density of TiAlN/CrN coating could be reduced to 1-3 orders of magnitude in Cl^-containing solution,and the impedance modulus of TiAlN/CrN coating could be significantly increased.The immersion corrosion test also further demonstrated the good corrosion resistance.However,in 5 wt.%H₂SO₄ solution,the self-corrosion current density of the coating increased from 1.157×10^-6 A·cm^-2 to 3.595×10^-6A·cm^-2,and the polarization resistance decreased from 36 kΩ·cm^-2 to 12kΩ·cm^-2,andtheimpedancemodulusdecreaseed simultaneously.Correspondingly,the change of corrosion weight by immersion was also obvious.The failure might be caused by dislocations and other defects at the interface of the layered structure of the multi-layer coating,which formed an important channel for SO₄^2-corrosion,the different reactivity of elements in the multi-layer coating in different corrosive media,and the complex chemical reaction between the coating and active substances.