The main goal of this study is to fabricate Mo-Si-B based claddings containing Mo 5 SiB 2 on nickel substrate using tungsten inert gas surfacing process in order to improve oxidation resistance of Ni. In this regard, the cladding procedure was performed in different manners using elemental powders, elemental powders+molybdenum interlayer and alloyed powders+ molybdenum interlayer. The results showed that in the absence of molybdenum interlayer, the chemical composition of cladding was strongly changed due to dilution with substrate. The use of two molybdenum interlayers reduced dilution significantly. The phase evolution during cladding procedure is significantly dependent on conditions of feedstock powders. In the case of unmilled powders, the desirable Mo 5 SiB 2 phase was not formed in the clad worthwhile Mo 2 Ni 3 Si and Mo 5 SiB 2 phases were formed after using alloyed powders. To produce alloyed Mo-Si-B powders, the mechanical alloying and subsequent heat treating process were performed. After investigation on Mo-Si-B system, two kinds of powder mixtures including Mo-12.5Si-25B (P1) and Mo-25Si-10B (P2) (at.%) were selected as reference powder mixtures .In addition, four other powder mixtures were considered by decreasing molybdenum content of reference powder mixtures in a Si/B constant ratio. It was indicated that phase evolutions during mechanical alloying and subsequent heat treatment of P1 powders depends on milling conditions, milling time and also time of heat treatment. Finally, 20h milling and subsequent annealing for 5h at 1100°C led to formation of various Mo-Si-B alloyed powders containing Mo 5 SiB 2 phase. Phase types of these powders were dependent on composition of powder mixtures. Weld cladding of alloyed powders led to formation of a multi-phase claddings consisting of Mo 5 SiB 2 +Mo 2 Ni 3 Si matrix phase. It was also indicated that microstructure and phase composition of claddings depends on phase structure of feedstock powders. The results of oxidation tests at 1000°C showed that the presence of Mo 5 SiB 2 improves the oxidation resistance of claddings, while its effect depends on microstructure and composition of claddings. Although the cladding produced using P1 based powders contained Mo 5 SiB 2 , but, the multiphase structure of claddings including nonresistant phases such as Mo 2 B resulted in spallation of oxide films comprised of complex compounds. In contrast, the presence of continuous (Mo,Ni) 5 (Si,B) 3 phase in microstructure of claddings produced by P2 based powders led to formation of SiO 2 protective layer on the surface during oxidation procedure. Here, these claddings exhibited excellent oxidation resistance at high temperatures. Keywords : Weld-Cladding, Mo-Si-B, Mo 5 SiB 2 , Mechanical alloying, Microstructure, Oxidation.
