Preparation And Related Basic Research Of C₃N₄/TiN Composite Coatings Used In The Last-stage Steam Turbine Blade For The Water Impact Erosion-resistance

Middle-frequency (MF) magnetron sputtering and cathodic arc assisted MF-magnetron sputtering were used to deposit C₃N₄ and C₃N₄/TiN composite coatings onto the Si wafer, stainless steel, high speed steel and cemented carbide. The structure, mechanical performance, tribological properties and liquid-impact erosion (LIE) resistance of the C₃N₄/TiN composite coatings were studied systematically. Related basic research results were applied on the last-stage turbine blade and the main results are as follows:(1) MF-magnetron sputtering technique was used to deposit C₃N₄ coatings onto Si wafer. XRD results showed that there existedα,βphases of carbon nitride and theβphase is the main phase. XPS and FTIR analysis revealed that the ratio of C and N is between 1.06-1.24. C atom was present in the form of sp and sp hybridization; the main bonding mode is C-N.(2) A home-made cathodic arc-assisited medium frequency magnetron sputtering system was used to deposit C₃N₄/TiN nano-composite coatings onto stainless steel and high speed steel (HSS) substrates. The hardness of the composite coating is high than 40 GPa and it belongs to the superhard coatings. The analysis of AFM and SEM showed that the as-deposited C₃N₄/TiN coatings exhibited smooth surface and excellent adhesion.(3) The hardness and adhesion of the coatings is different when varied the substrate material, the possible reason is analyzed according to the testing results of hardness, thermal shock, tribology and wear performance. There is no obvious change on the properties of the coatings deposited onto the 2Cr13 stainless steel when the shocking temperature is lower than 400℃and the oxide is present when the temperature is higher than 550℃. The tribological results showed that both the friction coefficient and the wear of the mated ball increase with increasing the load. The C₃N₄/TiN composite coatings exhibited excellent tribological performance.(4) The high-pressure water jet equipment was establishedand based on the research of liquid impact erosion and the system performance is tested by using 12Cr steel, high Cr martersitic white irons and Stellite 6B. The results showed that this system could be used to simulate the liquid impact erosion easily and with many advantages such as simple in structure, convenient in operation and easy in evaluation.(5) The carbon nitride composite coating exhibits excellent LIE performance according to the erosion testing by using stainless steel, high Cr martersitic white irons and Stellite 6B. The LIE lifetime of carbon nitride coating is 50 times compared with stainless steel and three times with Stellite 6B.(6) It can be seen from the failure morphology of C₃N₄/TiN coatings deposited on the 2Cr13 substrate that the crack derives from the area where the streess is concentrated on the surface of coatings and spread with the increase of the impact number. As a result the fracture of the coatings occure from the surface to the sub-surface step-by-step and finally the whole coatings were damaged. The failure pattern of the coatings is typically brittle fracture.(7) The application of the coated 2Cr13 blade in a 50 MW steam turbine showed that the C₃N₄/TiN composite coating is an excellent water impact erosion resistant material.