Microstructural evolution and ablation behaviors of NbSi2/SiO2–Nb2O5/X (X=MoSi2, MoSi2-Yb2O3, MoSi2-Yb2O3–ZrC) multilayer coatings on Nb alloy in different ablation environments

Abstract

To improve the high-temperature ablation resistance of Nb alloys, multiphase-multilayer ceramic coatings were prepared using a novel strategy combining halide activated pack cementation (HAPC) and liquid plasma-assisted particle deposition and sintering (LPDS) methods. The MoSi2, Yb2O3, and ZrC particles were deposited individually or co-deposited on the NbSi2 bottom layer to obtain NbSi2/SiO2–Nb2O5/MoSi2 (Mo), NbSi2/SiO2–Nb2O5/MoSi2-Yb2O3–SiO2 (MoYb), NbSi2/SiO2–Nb2O5/MoSi2-Yb2O3–ZrO2–ZrC (MoYbZr) multilayer ceramic coatings, respectively. Results showed that all three coatings remained intact after propane ablation (1500 °C), exhibiting excellent ablation resistance. For the harsher oxyacetylene ablation environments with higher temperatures (1800 °C), Mo coating suffered from long cracks, bubbling, and localized spalling and MoYb coating sprouted massive cracks without spalling. The oxide scale integrity of the MoYbZr coating was well maintained without cracks, exhibiting excellent resistance to oxyacetylene ablation attributed to the incorporation of Yb2O3 and ZrC. The mass and linear ablation rates of MoYbZr coating were only 0.137 mg/s and 0.196 μm/s after oxyacetylene ablation, which were 25.14 % and 49.61 % lower than that of MoYb coating, and 88.01 % and 75.37 % lower than that of the Mo coating, respectively. The Yb2O3 strengthened the Si–O bonding in the SiO2 network and increased the viscosity of the SiO2 scale, thus enhancing the self-healing ability of the oxide scale and slowing down the evaporation of SiO2. The oxidation products (ZrO2 with SiO2) from MoSi2 and ZrC further reacted to produce ZrSiO4, which can inhibit microcrack creation and extension. Micropores in the Zr–Si–O oxide scale mitigated thermal stresses, inhibited crack initiation and extension. The Yb2O3, Yb2SiO5, and ZrSiO4 embedded in the SiO2–ZrO2 scale acted as pinning effects, enhancing the high-temperature structural stability, thereby improving the ablation resistance of the MoYbZr coatings.