Microstructural Evolution and Failure Mechanisms of Plasma-Sprayed Pure Si and Si–15 mol% HfO2 Bond Coatings in Dry-Oxygen and Water-Vapor Environments at 1623 K

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Abstract
To address the softening and limited oxidation and water-vapor corrosion resistance of Si bond coats at ultra-high temperatures, Si and Si–15 mol% HfO2 (denoted 15HS) composite bond coats were deposited on pressureless sintered SiC substrates by air plasma spraying (APS). Their phase evolution, cross-sectional microstructure, and failure mechanisms were examined after 20–200 h at 1623 K in dry oxygen and in a 90% H2O–10% O2 environment. In dry oxygen, a SiO2 thermally grown oxide (TGO) layer formed on the Si coating surface, cracked and spalled with time, and the coating thinned progressively. In the 15HS coating, HfO2 reacted in situ with SiO2 to form HfSiO4; the combined volume expansion and spallation resistance of the dense layer caused the coating to thicken rather than thinning, and the TGO remained dense and crack-free. Under water-vapor corrosion, the Si coating was continuously consumed through gas-phase volatilization of SiO2 as Si(OH)4, accompanied by porosity and through-thickness cracks; after 200 h the coating thickness was only ~70% of the initial value. The 15HS coating instead underwent a three-stage evolution—homogeneous oxidation, layered construction, and barrier stabilization—after which a dense double-layer barrier formed, with an outer HfSiO4 layer and an inner SiO2 band, giving a thickness retention of ~90%, well above that of the Si coating. Mechanistic analysis showed that HfO2 particles remained in place while Si4+ diffused outward and converted to HfSiO4 at the HfO2 surface. HfSiO4 protected the coating through two mechanisms: blocking OH−/H2O diffusion pathways and suppressing Si(OH)4 volatilization. These findings inform the compositional optimization and structural design of Si-based bond coats for ultra-high-temperature water-vapor service.
Funding Information
  • National Research and Development Program Project (2023YFB4302400)
  • Tianjin Municipal Education Commission (2025KJ165)

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