Preparation And Properties Of MoSi₂-based High Temperature Ceramics Coating

Under high temperature conditions, Graphite materials have good physical andmechanical properties, however, above450℃while under oxidizing conditions it couldlead to severe oxidative so it affects its function. The present work is about a methodusing TIG cladding to improve the oxidation resistance of graphite with substratesurface MoSi2-based ceramic coating layer.The experiment with Mo powder, Si powder, Al powder and AlN powder as rawmaterials, using the method of argon arc cladding to make three coating systems: theMoSi2-Al-SiC, the MoSi2-AlN-SiC and the MoSi2-Al-AlN-SiC, the best coating ratio isn(Mo):n(Si):n(Al):n(AlN)=1:2.2:1.0:0.8, the corresponding optimal parameters are:cladding current:125A, cladding speed:7.0m/h, the cladding voltage range20.9-21.1V,an argon flow of5.0L/min. Oxyacetylene ablation rate rise about1/16compared withgraphite substrate (2.24mg/mm2�'s). The experiment results show that in box-typeresistance under furnace oxidation1100℃and1300℃for10h gain the minimumweight, the results were2.8×10-4g/cm2and12.5×10-4g/cm2, the coating has excellentantioxidant properties.MoSi2ceramic coatings produced by argon arc, the result is that the coating andthe substrate has no clear boundaries, because of the presence of Si in the coating layer,under the effect of an argon arc heat, in the reaction layer and the interface layergenerated a graphite substrate with the formation of a metallurgical bond SiC. Al alloyelements added MoSi2coating eliminates the microcracks which produced during thepreparations, as the Al content increases, the moldability of the coating deteriorates;AlN ceramic reinforcements added to the coating makes better formability.Al replace Si in MoSi2to change the lattice, making metallic bond strong andcovalent bond weak. Plastic toughness of material can be improved, and Mo(Si, Al)2with better oxidation resistance is formed. With increasing of Al content, coatingmicrohardness increased after the first decrease; AlN has a high thermal conductivityand high temperature strength, high temperature of sublimation and decomposition ofAl and nitrogen. mostly like chemical compatibility of MoSi2, and chemicallycompatible with MoSi2matrix. It is a solid solution of the reaction product having anhigh temperature strength at high temperature and SiO2, improving the mechanicalproperties of high temperature MoSi2matrix, changing in the content of AlN hardnessof the coating microstructure which is not affected.Sample polished cross sections are analysed by scanning electron microscope andenergy dispersive microanalysis (BSE-EDS), combined with XRD analysis showed that the cladding layer to generate phase containing mainly for sheet MoSi2and Mo(Si, Al)2mixture, mingled with the rest of the material response. After the oxidation test thecoating formed on the surface of dense, continuous, and oxides of Al and Si solidsolution of AlN protective film impede further oxidation of the coating. Coating theinside of MoSi2, Al and Si constantly being replaced after sufficient oxidation time, acompleted reaction Mo(Si, Al)2phase.