Study Of Solid Particles Erosion Resistance Of The Thermal Sprayed Cr₃C₂-NiCr Coatings

The solid particle erosion phenomenon is serious in the flow passage of steam turbine and it become aggravated with the increasing unit efficiency. In order to protect the flow passage, the thermal spray as an effective measure is widely used. In this paper, the Cr3C2-NiCr thermal sprayed coating was researched, which is frequently used to defend the passage components. How the preparation technology and particle size influenced on the microstructure and performance of the coating was also studied. Eventually, the relationship among the microstructure, performance and the erosion resistance could be found by the investigation of solid particles erosion mechanism of different coatings, which would provide a theory basis in the further preparation of erosion resistant coatings.The Cr3C2-NiCr cermet coatings were fabricated by detonation spray (DS) and high velocity oxyen-fuel (HVOF) spray technologies, and three different kinds of spray powders. Then detonation spray Diamalloy3007 coating (D1), high velocity oxyen-fuel spray Diamalloy3007 coating (H1), high velocity oxyen-fuel spray homemade sub-micron coating (S2) and high velocity oxyen-fuel spray H.C.Starck coating (N3) were prepared by Sulzer-Metco Diamalloy3007 powder, homemade sub-micron powder and H.C.Starck powder, respectively. And then, the microstructure and micro-defects of the four as-sprayed coatings were observed and analyzed by optical microscopy (OM), scanning electron microscopy (SEM/BSE) and X-ray diffraction. The porosity, apparent carbide content, dissolved carbon content, hardness and fracture toughness had been studied through series of experiments as well as the relationship between the microstructure and performance. The Erosion Wear Mechanism of D1 and H1 coatings were investigated by the solid particle erosion test on the condition of simulating the actual working environment of the steam turbine blades. For D1 and H1 coatings, the test temperature is 600℃and 620℃, the impact speed is 210 and 420 meters per second, the imapct angle is 18 and 90 degrees, respectively. As a result, the factors that affected the properties of the coatings could be obtained.The structure and performance results show that the detonation spray (DS) is superior to high velocity oxyen-fuel spray technologies in preparing Cr3C2-NiCr coatings The D1 and N3 coating performance are similar, which are more preceding than H1 and S2 coatings. The coatings hardness and fracture toughness have relation to the microstructure, and three conclusions have been obtained. The hardness and fracture toughness decrease as the porosity increased. Then along with the increase in apparent carbide content, the hardness increases while fracture toughness decrease. The hardness and fracture toughness decrease with the increasing dissolved carbon content. The solid particle erosion test reveals that the coatings mass loss rate would like to increase as the impact angle or the impact speed improved, while it decreases with the increasing test temperature. Both the D1 and H1 coating display the wear characteristic of brittle materials.The erosion mechanism analysis indicates that the Microcutting and Microploughing are the main erosion mechanism at 18℃, with the pull-out of carbide and small block exfoliation. Furthermore, the block exfoliation is more severe at the speed of 420 meters per second at the same impact angle. The erosion mechanism at 90 degrees includes block exfoliation, the plastic deformation of binder phase and the extrusion of the carbides, meanwhile, the brittle fracture character has also been observed with a speed of 420 meters per second. The test result demonstrates that the block exfoliation is the main factor to increase the mass loss rate of coatings.In conclusion, the microstructure characters of superior wear resistant coatings were assessed:while the coatings defects and the apparent carbide content are same, the smaller the carbides size, the better the wear resistance. Under the same coatings defects, the higher apparent carbide content and the smaller carbide size, the better wear resistance. The wear resistance decreases with the increasing dissolved carbide content. The wear resistant of coating reduces as the defects increased in the case of same uniform apparent carbides content, dissolved carbides content and carbides size. In addition, the wear resistant have relevant to the cohesive strength of coating, the higher the cohesive strength of the coating, the superior wear resistance it will attain.