Study On The Enhancement Of Bongding Strength For High Velocity Oxy-Fuel Sprayed Cermet Coating

High velocity oxy-fuel (HVOF) sprayed coating is characterized as dense and high bonding strength. Owing to the importance of adhesion and cohesion on the application of the coating,it is valuable to study the enhancement of the coating for the improvement of the coating performance and widening of the coating application.Basing on the comparing of the results on the roughness obtained by two different characterization methods,the influence of surface roughness on the surface bonding strength was investigated.Amechanism was proposed for the effect of surface roughness S on the bonding strength,by which the bonding strength can be enhanced through enhancement of mechanical interlock.For Cr₃C₂-NiCr coating,composite coating was deposited by adding some Ni-based alloy with lower melting point to Cr₃C₂-NiCr, and the cohesion was enhanced by heat treatment in which Ni-based alloy melted. The enhancement of the cohesion was approved by examination on the coating microstructure and erosion test.The influence of grit size and gas pressure on the surface roughness was investigated. Results showed that Ra increased with the increase in gas pressure and decreased with the decrease in grit size.The surface roughness S was characterized by both conventional roughness tester and laser microscopy.It is approved that the results obtained by laser microscopy was much better than that obtained by conventional surface roughness tester.S decreased with decrease in grit size.For the mild steel in surface roughness Ra of less than 2μm,the WC-Co coating deposited with powder in size of 19-25μm presented bonding strength of 60⁸0MPa,and that for 30-50μm presented bonding strength of 40⁵0MPa.The bonding strength of the coating deposited with coarse powder can not be improved by increasing the particle velocity through increasing fuel gas flow.Results showed that the particle velocity of the fine powder and coarse powder was 437(±89)m/s and 580(±143)m/s,respectively. Mechanism on the influence of S on the bonding strength was proposed. When Ra is relatively small,high bonding strength can be formed by mechanical interlock if S is comparable to the spray powder size.The mechanism was approved by the fact that the coating bonding strength on the hard stainless steel with smaller Ra reached 60¹00MPa.The bonding strength of Cr₃C₂-NiCr coating was decreased by adding Ni60 powder in the Cr₃C₂-NiCr spray powder.The bonding strength decreased with the increase of the content of Ni60 powder.In the composite coating of Ni60 and Cr₃C₂-NiCr,the chemical bonding was enhanced by the heat treatment at 1080℃owing to the fusion of Ni60.The enhancement of the chemical bonding was approved by the fact that the erosion performance at 90°of the coating was improved by heat treatment.