Research On Wear Resistance Of Cr₃C₂-NiCr Coating Reinforced By Nano-TiC

Remanufacturing is one of the important tasks of "Made in China 2025",especially for high-end products with high added value,professional repair or upgrade is carried out to make their quality characteristics not lower than the level of new products.The objects of remanufacturing are usually worn parts after long-term service,which mainly include friction wear and erosion wear.For products in high temperature service,oxidation wear is also included in the wear process.Therefore,it is of great significance to study the excellent anti-wear repair coating and preparation process,and analyze its wear failure mechanism to improve the service performance and life of parts.Thermal spraying technology is recognized as a mainstream remanufacturing and surface strengthening technology in the industry.The Cr₃C₂-NiCr coating prepared by thermal spraying technology is widely used to improve the wear resistance and erosion resistance of parts.In order to optimize the thermal spraying process for preparing the Cr₃C₂-NiCr coating,Cr₃C₂-NiCr cermet coatings were deposited by shroud plasma spraying（SPS）and high velocity oxygen-fuel（HVOF）spraying in this study.The erosion test of both coatings was carried out with simulating the environment of multi-angle high-speed erosion of small particles of turbine blades.Then,the phase composition,microstructure,microhardness and erosion wear behavior of both coatings were analyzed.The results showed that the HVOF coating was denser with fewer defects than the SPS coating.The microhardness of the HVOF coating was about 9%higher than that of the SPS coating,and the HVOF coating had better erosion resistance.The erosion rate of both coatings increased rapidly first and then decreased slowly with the increase of impact angle.It was noted that the erosion rate of the SPS coating and HVOF coating reached the maximum at the impact angle of 60° and 75°,respectively.For both coatings,at low impact angles,the erosion wear was dominated by micro-cutting,while it was gradually transformed to fatigue fracture spalling as the increasing impact angle.Specifically,large cracks in the carbide-rich region and extensive fatigue fracture spalling of the SPS coating can be observed at 60° impact angle,while the HVOF coating exhibited micro-cracks and the most severe sporadic spalling of chromium carbide particles at 75°impact angle.With the continuous improvement of equipment performance requirements,the traditional Cr₃C₂-NiCr cermet wear-resistant coating cannot meet the increasingly severe service environment of components.Therefore,the wear resistance of the Cr₃C₂-NiCr coating needs to be improved urgently.In this work,a method to improve the wear resistance of Cr₃C₂-NiCr coatings by adding nano-TiC was investigated.TiC-Cr₃C₂-NiCr powders with different nano-TiC ratios were prepared by the preferred mechanical ball milling,and TiCCr₃C₂-NiCr coatings were prepared by HVOF.The effects of microstructure and phase of the modified coatings on the tribological properties were systematically investigated.The results showed that the nano-TiC diffusely distributed in the binder of Cr₃C₂-NiCr coating which plays an important role in releasing stress,filling cracks and improving wear resistance.At room temperature,the expansion of the spalling of Cr₃C₂ particles was inhibited by the synergistic effect of nano-TiC "pinning",nanoscale dislocations and optimized phase composition in the coating,which improved the wear resistance of the coating.At 800℃,nano-TiC accelerated the formation of oxide film,improved the toughness of oxide film,and promoted the recrystallization of nanoscale Cr₃C₂ in the bonder,which improved the high temperature wear resistance of the coating.The erosion resistance of Cr₃C₂-NiCr coating and modified coating at room temperature and high temperature was compared and studied.At room temperature,the erosion rate of the four coatings first increases rapidly and then decreases slowly with the increase of impact angle.The erosion rate of the first three reached the maximum near the 75 ° impact angle,while the erosion rate of 12%TiC-Cr₃C₂-NiCr coating reached the maximum near the 60°impact angle.Adding 12%TiC coating showed better erosion resistance.The addition of nano TiC strengthened the bonding phase,inhibited the crack initiation and propagation,and improved the fracture toughness,which was conducive to the improvement of coating properties.Under the high-temperature erosion test,the oxide film on the surface of Cr₃C₂-NiCr coating was broken and peeled off by impact,and cracks were left in and around the peeling area,resulting in the intensification of oxidation reaction in the coating to form brittle Cr₂O₃.However,the nano-TiO₂ generated by oxidation was dispersed in the brittle Cr₂O₃,which played the role of sealing holes,releasing stress,reducing cracks,improving the density and fracture toughness of the oxide film,and further improving the oxidation-erosion resistance of the coating.