Study On Microstructures And Properties Of Composite Coatings Prepared By Plasma Spray Welding

The wear failure is one of the most common failures for the material surface, which can bring about enormous economy lost and great injuries. The surface modification used for preparing composite coatings on material surface is an efficient mean to improve the wear resistance, such as air plasma spraying(APS), high velocity oxy-fuel(HVOF) spraying, laser cladding, plasma spray welding,and so on. The coatings prepared by APS and HVOF spraying have a mechanical bond with the substrate, relatively small in the thickness and contain many pores, so their applications are limited. The expensive equipment and high maintenance costs affect the extensive application of laser cladding. In addition, it is difficult to obtain a perfect surface quality for large area cladding because of the existence of slags and pores. The coatings prepared by plasma spray welding have gained a lot of attentions due to their attractive advantages such as the compactness of coatings, the metallurgical bond in the coating/substrate interface and an obvious increase in coating thickness. The ceramic particles reinforced Ni based composite coatings were designed and prepared on Q235 low carbon steel by plasma spray welding, and microstructures and properties of coatings were investigated in this work. The purpose was to reveal the characteristics of the coatings and to provide some foundations for improving the coating properties.Firstly, effects of plasma spray welding parameters(welding current, welding distance, welding speed) on the surface quality and fusion ratio were studied. The results indicated that the optimized parameters for improving coating quality are I=45A-55 A, S=12mm-13 mm and V=30mm/min-35mm/min. Microstructures and properties of Ni60 A coating were investigated, based on the parameters. The Ni60 A coating consists mainly of γ-Ni, Cr23C6, Cr7C3, Ni3 Si, CrB and Cr5B3 phases. The main phase in the Ni60 A coating is γ-Ni, and Cr23C6 has the second content. The wear resistance of Ni60 A coating has a significant improvement compared with the low carbon steel substrate, and the main wear mechanism is micro-cutting and the fracture of chromium carbide particles.Based on results on Ni60 A coating, the WC particles reinforced Ni-based coatings were prepared by plasma spray welding with mixed powders(Ni60A+NiCr-Cr3C2+WC-12Co). The results indicated that the nucleation rates of chromium carbides(Cr23C6, Cr7C3) increase with the increasing of the mass fraction of NiCr-Cr3C2, which is beneficial to improve the wear resistance of coating. The WC content in the coatings increases with increasing the mass fraction of WC-Co12, but an excessive WC-Co12 mass fraction(20 wt.%) results in forming the partially dissolved WC particles in the coating, affecting the wear resistance of coatings. The WC particles reinforced Ni-based coatings characterized by WC particles embedded in γ-Ni matrix. The reaction of WC is thermodynamically possible to take place. Homogeneous nucleation is the leading form of WC precipitation, and the partially dissolved WC particles can also become the nucleus of WC. The microhardness and abrasive wear resistance of coating are improved due to the existence of hard WC particles. The wear resistance of coating is more than 13 times as that of substrate. The main wear mechanism of WC particles reinforced Ni-based coatings is the micro-cutting and fracture of reinforcement particles.The in-situ TiC particles reinforced Ni-based coatings were prepared by plasma spray welding with mixed powders(Ni60A+NiCr-Cr3C2+Ti). The coatings are characterized by in-situ TiC particles embedded in Ni matrix. It is noted that the interface between TiC particles and Ni matrix is close, clean and free from defects. The in-situ TiC content in the coatings increases with the mass fraction of Ti and NiCr-Cr3C2. When the mass fraction was 8.58 wt.%, the coating surface oxidized badly and the holes were observed obviously. The homogeneous nucleation is the leading form of TiC precipitation, and the Ti-Si-C polycompound particles can also become the nucleus of TiC. The wear resistance of coatings is improved, and the best wear resistance of coating is more than 15 times as that of substrate. This is because the carbides form a wear resistance skeleton which also effectively prevent abrasive particle from making an impression. Ti C particles in the matrix were formed in an in-situ manner, which could improve the bonding strength between the TiC particles and matrix. They firmly embedded in the Ni matrix and hardly fell off during wear tests. The main wear mechanism of in-situ TiC particles reinforced Ni-based coatings is the micro-cutting and scratch.The in-situ NbC particles reinforced Ni-based coatings were prepared by plasma spray welding with mixed powders(Ni60A+NiCr-Cr3C2+Nb). The coatings are characterized by in-situ NbC particles embedded in Ni matrix. The in-situ NbC content in the coatings increases with the mass fraction of Nb and NiCr-Cr3C2, but an excessive Nb mass fraction(13.08 wt.%) results in forming the partially dissolved Nb particles in the coating, affecting the wear resistance of coatings. The homogeneous nucleation is the leading form of NbC precipitation, and the partially dissolved Nb particles can also become the nucleus of NbC. The growth patterns of NbC particles are two-dimensional disc-shaped nucleus growth and spiral growth. The wear resistance of coatings is improved, and the best wear resistance of coatings is more than 14 times as that of substrate. The main wear mechanism of NbC particles reinforced Ni-based coatings is the micro-cutting and the removal of reinforcement particles. The in-situ TiC particles reinforced Ni-based coatings have the strongest wear resistance among the three composite coatings, but the process stability is affected by pure Ti powders.