In-situ Fabrication of (Ti, Mo)Si₂/MoSi₂ Composite Coating to Protect Mo Substrate Against Cyclic Oxidation at 1200 C

(Ti, Mo)Si-2/MoSi2 composite coatings were prepared on Mo substrate by the continuous deposition pack cementation method. The X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and thermodynamic calculation were used to characterize the composite coatings and to analyze the formation mechanism. The results show that the co -deposition process cannot achieve the titanium deposition effectively. The titanium-modified MoSi2 coatings can be prepared by a two-step deposition process of titanizing and siliconizing. The coatings contain three layers: the outer layer is (Ti, Mo)Si-2 ternary compound layer; the second layer is MoSi2 layer; the layer between the MoSi2 and Mo substrate is the Mo5Si3 transition layer. The siliconizing temperature shows negligible effect on coating structure. The growth rate of titanium-modified MoSi2 coating is slightly lower than that of single MoSi2 coating. The growth of (Ti, Mo)Si-2/MoSi2 composite coating is dominated by the inward diffusion of Ti and Si. Ti is concentrated on the outer layer of the coating. Si diffuses through the (Ti, Mo)Si-2 compound layer and interacts with the substrate to form the MoSi2 layer and Mo5Si3 transition layer. In the titanizing process, the free state AIF(3) is introduced by the reaction among pack mixtures. In the subsequent siliconizing process, a trace amount of Al in free state is precipitated in the form of Al3Mo phases in the (Ti, Mo)Si-2 layer near the upper interface of MoSi2 layer. During the cyclic oxidation tests at 1200 degrees C, the (Ti, Mo)Si-2/MoSi2 composite coatings do not lose mass obviously after exposure in oxidation atmosphere for 180 h. A dense composite oxide layer consisting of SiO2 and TiO2 can be formed by the oxidation of (Ti, Mo)Si-2 phase. This composite oxide layer can fill the surface cracks of the coating and continuously block the oxygen diffusion, so the oxidation resistance of (Ti, Mo)Si-2/MoSi2 composite coating in the periodic oxidation environment is far superior to that of the single MoSi2 coating.