Research On The Thermal Properties And Corrosion Resistance Of Vacuum Sintering Nickel-based--Cermet Composite Coating With Rare Earth

The Ni-based-WC with rare earth composite coating was obtained by the technique of vacuum sintering on the hypoeutectoid steel (Fe-0.45%C). In virtue of SEM, X-ray diffraction and EDX, the microstructure and phase-structure of coating were investigated. This dissertation also studies that the active mechanism of rare earth element La and WC on microstructure and properties of vacuum sintering coating in the way of examining and evaluating in hardness, thermal wear resistance, thermal fatigue resistance and corrosion resistance of coating. The results show that with the appropriate addition of WC and rare earth element La, the microstructure, phases and properties of vacuum sintering Ni60A alloy coating change remarkably. Such change can be described as the followings. Firstly the microstructure of Ni-based alloy coating is composed of Ni-based solid solution and a lot of needle and massive phases. The metallurgical bonding between the matrix and coating has been realized. Secondly the microstructure of composite coating changes when WC is added in. The arborescent crystals of composite coating change to be fine and there are some massive WC phases appearing. Thirdly the composite coating adding rare earth element La makes needle phases reduced or vanished, microstructure changes to be smaller and uniform, and precipitate new phase, such as Cr₅B₃. Fourthly adding rare earth element La improves the state of bonding between the matrix and coating. The micro-hardness of transition area between the matrix and coating changes more mildly and the peak value of hardness appears in the coating side 0.40mm to interface. Fifthly the thermal wear resistance, thermal fatigue resistance and corrosion resistance of coating are enhanced obviously. Sixthly rare earth element La added in coating can raise the corrosion potential, reduce the corrosion current of the composite coating and enlarge the radius of capacitive arc obviously and increase the resistance of coating on the EIS. The corrosion resistance of coating is better in NaCl and NaOH solution but bad in acid. Influence of WC on corrosion resistance is very complex and need to be analyzed according to definite corrosion solution.