Oxidation Behavior of (Mo,Hf)Si2-Al2O3 Coating on Mo-Based Alloy at Elevated Temperature

Abstract

To improve the oxidation resistance of Mo-based alloys, a novel (Mo,Hf)Si2-Al2O3 composite coating was fabricated on a Mo-based alloy by the method of slurry sintering. The isothermal oxidation behavior of the coating was evaluated at 1400 °C. The microstructure evolution and phase composition of the coating before and after oxidation exposure were characterized. The anti-oxidant mechanism for the good performance of the composite coating during high-temperature oxidation was discussed. The coating had a double-layer structure consisting of a MoSi2 inner layer and a (Mo,Hf)Si2-Al2O3 outer composite layer. The composite coating could offer more than 40 h of oxidation-resistant protection at 1400 °C for the Mo-based alloy, and the final weight gain rate was only 6.03 mg/cm2 after oxidation. A SiO2-based oxide scale embedded with Al2O3, HfO2, mullite, and HfSiO4 was formed on the surface of the composite coating during oxidation. The composite oxide scale exhibited high thermal stability, low oxygen permeability, and enhanced thermal mismatch between oxide and coating layers, thus improving the oxidation resistance of the coating.

Fabrication and Oxidation Resistance of a Novel MoSi2-ZrB2-Based Coating on Mo-Based Alloy

“…Some flaky particles were inlaying in the clusters, and were supposed to be Al2O3 according to element mapping in Figure 6h,i, which was originated from the pack mixture during pack cementation. Al2O3 has been proved to be beneficial to the high-temperature oxidation resistance of the MoSi2 coating in our previous work [18,19]. Nevertheless, Al2O3 was not detected by XRD in Figure 5 due to low content.…”

“…It also can the composite coating was retained intact and the SiO2-based glassy oxide sc protective since no through-thickness crack was formed in the oxide scale, or and the oxide scale was well adhered to the coating. 16)-( 20) are possible to conduct when the MoSi 2 -ZrB 2 coating samples were exposed to an oxidizing environment at 1600 • C. As mentioned in previous work [18,19], the standard Gibbs free energy (based on one mol oxygen) of Reaction ( 16) is more negative than that of Reaction (17). Thus, the oxidation of MoSi 2 to form Mo 5 Si 3 and SiO 2 by Reaction ( 16) was dominant.…”

“…SiO2-based oxide scale (layer I in Figure 15) reduced to ~6.8 μm, while the Mo5S layer (layer IV in Figure 15) thickened to ~113.6 μm. Compared to the MoSi2-ba without the MoB diffusion barrier in our previous works [18,19], it is worth in this work that even after 10 h of oxidation, the MoSi2 layer was not com verted to Mo5Si3, which could still provide the reservoir of Si for the formation pact SiO2-based glass. It is inferred that the MoB diffusion barrier played an role in restraining the depletion of MoSi2 by hindering the Si diffusion towa strate, which proved to be beneficial to the long life of the coating.…”

“…The oxidation parabolic constant K P , which corresponds to the slope of the curve, was approximately 1.2811 mg 2 •cm −4 •h −1 . It is reported in previous works [18,19] that the weight gain of the coating is likely to relate to the formation of an oxide scale on the coating surface during oxidation. Chemical reactions of the coating with oxygen were conducted to form an SiO2-based oxides scale (containing e.g., SiO2, ZrO2, B2O3) when exposed to high-temperature oxidizing environments.…”

“…To improve the thermal stability of the SiO 2 oxide scale on the MoSi 2 coating, one of the research hotspots is to develop MoSi 2 -based composite coatings doped with alloying elements (e.g., B [10,11], Al [12,13], Ti [14], Zr [15], and Hf [13]) or/and oxides (e.g., Al 2 O 3 [16][17][18][19][20], ZrO 2 [18,21], HfO 2 [18,22]), carbides (e.g., ZrC [23]), and borides (e.g., ZrB 2 [24,25]), which have been confirmed to be effective in improving the performance and longevity of the MoSi 2 coating at elevated temperatures. The B-doped MoSi 2 coating has been proven to perform excellent oxidation resistance benefiting from the formation of a protective borosilicate scale on the coating [10,11].…”