Effect of Pre-Oxidation on High-Temperature Oxidation Behavior of Al-Si Coating on Nickel-Based Superalloy

Abstract

Under the condition of long-time high-temperature oxidation, oxidation in Al-Si coating will lead to degradation of the coating. To solve this problem, the Al-Si coating was treated before the oxidation experiment at different pre-oxidation temperatures and times. The structure, morphology and element distribution of the oxide film were characterized by XRD, SEM and EPMA, and the oxidation kinetics curves were drawn. The results show that the oxidation resistance of the pre-oxidized samples is improved in the process of constant temperature oxidation at 1000 °C. When the pre-oxidation temperature is set at 950 °C, the mass gain of the samples is the lowest, and the accelerated decomposition of Cr2O3 caused by high pre-oxidation temperature is avoided. After oxidation for 300 h, the oxide film on the surface is still continuous and dense without peeling. The samples pre-oxidized at 950 °C for 7 h show the best oxidation resistance in a 1000 °C constant temperature oxidation process, more α-Al2O3 oxides are generated on the coating surface and the element diffusion between the matrix and the coating caused by overly long oxidation time is avoided.

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

“…Reactions (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.…”

“…The thickness of SiO 2 -based oxide scale (layer I in Figure 15 ) reduced to ~6.8 μm, while the Mo 5 Si 3 diffusion layer (layer IV in Figure 15 ) thickened to ~113.6 μm. Compared to the MoSi 2 -based coating without the MoB diffusion barrier in our previous works [ 18 , 19 ], it is worth mentioning in this work that even after 10 h of oxidation, the MoSi 2 layer was not completely converted to Mo 5 Si 3 , which could still provide the reservoir of Si for the formation of the compact SiO 2 -based glass. It is inferred that the MoB diffusion barrier played an important role in restraining the depletion of MoSi 2 by hindering the Si diffusion toward the substrate, which proved to be beneficial to the long life of the coating.…”

“…Some flaky particles were inlaying in the clusters, and were supposed to be Al 2 O 3 according to element mapping in Figure 6 h,i, which was originated from the pack mixture during pack cementation. Al 2 O 3 has been proved to be beneficial to the high-temperature oxidation resistance of the MoSi 2 coating in our previous work [ 18 , 19 ]. Nevertheless, Al 2 O 3 was not detected by XRD in Figure 5 due to low content.…”

“…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 SiO 2 -based oxides scale (containing e.g., SiO 2 , ZrO 2 , B 2 O 3 ) 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 ].…”