Preparation And Wear Properties Of TiB₂-NiTi Coating Deposited By Low-Temperature High Velocity Oxygen Fuel Spraying

Titanium diboride?TiB₂?metal-based ceramic coating with excellent wear resistance has been widely used in engineering application,which were usually prepared by the technology of high velocity oxygen fuel spraying.However,the powders would be seriously oxidized by the high flame current temperature,which affected the quality and performance of coating.In this work,the sturcure of the high velocity oxygen-fuel spraying gun has been modified to reduce the gas temperature.TiB₂-NiTi composite powders have been prepared by the agglomerated and sintered method,and TiB₂-NiTi cermet coating with low oxygen content has been deposited successfully.The microstructure,composition and morphology of self-granulated powders and coatings were characterized by XRD,SEM,EDS,et.al.The cavitation test and abrasive wear test of the coating were completed in the simulated turbine application environment.The results show that:In the agglomerated and sintered self-granulation,the optimum fabricating parameters for the centrifugal atomization spray drying powders in the first step has been obtained by adding PAA of 0.4 wt.%,PVA of 1.6 wt.%,solid content of 50 wt.%,spray drying in-temperature of 180?,out-temperature of 90?,rotational speed of the atomizer of 20000 r/min.The optimum parameters of spraying granulated powders in the second step are at 1350?for 1 h.The obtained powders with the phase composition of TiB₂,NiTi,TiO,exhibit an apparent density of 1.56 g/cm³,fluidity of 10.1 s/50 g and a median diameter of 40?m.The optimization parameters of the coating deposition by the modified low-temperature high velocity oxygen-fuel spraying gun were:oxygen flow rate of 1500SCFH,kerosene flow rate of 5.5 gal/h,cooling nitrogen flow rate of 1000 L/min,spraying distance of 200 mm and powder feeding rate 30 g/min.The TiB₂-NiTi coating deposited by the modified low-temperature high velocity oxygen-fuel spraying gun possess a typical layered dense structure with microhardness of 919.9±52.8(Hv_0.2),the fracture toughness of 2.4±0.3 MPa·m^1/2 and the porosity of 4.16%.With cooling nitrogen flow rate from500L/min to 1500 L/min,the oxygen content of coating decreases from 20.06 wt.%to15.68 wt.%.The cavitation test indicates that the cavitation morphology shows cavitation pits with different shapes.During the cavitation process of 2 h,the maximum depth of cavitation pit reaches 101.16?m at 75 min.When the cavitation testers'amplitude is 60?m,the weight loss of the coating in deionized water and NaCl solution is 13.46 mg and15.53 mg,respectively.The TiB₂-NiTi coating showed excellent cavitation resistance with the weight loss was 5.54 mg at an amplitude 40?m in deionized water.The abrasive wear test shows that furrows and exfoliation pits are the typical morphology of abrasive wear.When the load is 40 N,70 N and 100 N,the relative wear resistance of the composite coating is 7.6,7.4,and 6.9,respectively.The relative wear resistance of the coating is up to 8.5,when the abrasive grain size is 100 mesh.The relative wear resistance of the coating is reduced to 6.9,when the TiB₂ content is 40 wt.%.