Preparation Of Plasma Spraying/Spray Welding Ni-WC Coatings And Calculations On The Refining Mechanism Of Microstructure

With the rapid development of modern industry,the higher requirements for durability of machinery and equipment have been put forward.It is key to enhance the surface performance of the workpieces.Plasma spraying and plasma spray welding are important strengthening methods used in surface engineering technology,by which,not only the size and shape of workpieces are restored,but also more excellent wear and corrosion resistance may be obtained.In this work,Ni-based WC composite coatings prepared with plasma spraying and plasma spray welding were taken,respectively.The evolution of microstructure and wear resistance of the coatings was studied.The micro-mechanical properties of the main wear-resistant phases WC and W₂C were also investigated.On this basis,the adhesion energy,interfacial energy,interface bonding of LaAlO₃/Ni interface and LaAlO₃/WC interface were calculated by the first-principles method,which analyzed the refinement mechanism of Ni-binder and WC by La inclusion LaAlO₃.Finally,the interface properties of TiC/WC were studied,which gave explanations for the refinement mechanism of WC by TiC.The combination between plasma sprayed coating and matrix is mainly mechanical one,while that between plasma spray welded coating and matrix is mainly metallurgical one.The former's microstructure has a large number of unmelted WC particles,and some other Cr₇C₃,Cr₃C₂ chromium carbides;while the latter only has a small amount of unmelted WC particles,and a large number of W-Cr-Ni-Fe-C five elements secondary phase,some Cr₇C₃,Cr₃C₂ chromium carbides.The wear resistance of the sprayed coating is higher about 1.5 times than that of the spray welded coating.The calculated results indicated that,in various W-C compounds,h-WC,?-W₂C and ?-W₂C are stable,and ?-W₂C is metastable,while ?-W₂C and c-WC are unstable.The chemical bonding of all W-C compounds is a mixture of metallic bond,ionic bond and covalent bond.All elasticity modulus values of h-WC are the largest,which exhibits brittleness.The other five W-C phases exhibit some ductility.The theoretical hardness value of h-WC is the highest,and that of c-WC is the lowest.All W-C phases exhibit a certain elastic anisotropy.For the LaAlO₃/Ni interface,in the most range of ??_Al,the interfacial energy of LaAlO₃/Ni interfaces is smaller than that between primary Ni phase and Ni melt(?_Ni?l?/Ni?s?).Under these conditions,LaAlO₃ can be the heterogeneous nucleus of Ni grains.For the LaAlO₃/WC interface,when ??_Al is low,the interfacial energy of La O3-W-terminated interface is near to zero,which suggests the ultra-high stability of the La O3-W-terminated interface.Compared to other LaAlO₃/WC interfaces,LaAlO₃ is more likely to be the heterogeneous nucleus of WC.For the TiC/WC interface,over the entire range of ??_C,the interfacial energy of Ti-Hole-C interface is the lowest.Especially,when ??_C is low,the interfacial energy of Ti-Hole-C interface is negative,which must be smaller than that between primary WC phase and metal melt.Compared to other interfaces,TiC is more likely to be the heterogeneous nucleus of WC.