Ceramics International

Available online 7 February 2026
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Ceramics International

Multilayer SiC/Y2O3 coatings deposited on the inner wall of a long graphite tube by atmospheric long laminar plasma spraying

https://doi.org/10.1016/j.ceramint.2026.02.027Get rights and content

Abstract

To meet the high-temperature protection requirements of slender graphite guide tubes used in uranium alloy melting, a SiC–Y2O3 composite ceramic coating was developed via Atmospheric Long Laminar Plasma Spraying (ALPS) combined with reaction sintering. The coating was applied to the inner surface of graphite guide tube with an inner diameter of 57 mm and a length of 200 mm. A Si coating was deposited onto the inner wall by ALPS and then transformed, through a two-step vacuum heat treatment at1400-1500 °C, into a C/SiC interfacial structure consisting of a dense outer SiC layer (∼10 μm) and a subsurface region containing dispersed SiC. Subsequently, a Y2O3 coating was deposited on the SiC bond coating by ALPS under oblique-angle inner-wall spraying. This structure is primarily attributed to a strong shadowing effect under oblique-angle deposition: because the ALPS jet impinges on the inner wall at a very small angle (about 15°), surface features generated during spraying – including previously deposited coating ridges, secondarily splashed particles and dust inclusions accumulated in the confined space – act as obstacles that locally block the incoming particle flux and promote directional columnar growth with inter-column gaps. After a single thermal-shock cycle at 1500 °C for 60 s, the SiC/Y2O3 coating remained macroscopically intact without spallation, and the hardness of the Y2O3 coating increased from 4.85 GPa (as-sprayed) to 8.94 GPa. These results demonstrate that ALPS is a promising technique for producing robust ceramic coatings on complex internal graphite surfaces and that the SiC/Y2O3 multilayer design provides an effective protective coating system for the severe thermal and chemical environment encountered in uranium-alloy processing.

Introduction

Uranium and its alloys exhibit excellent nuclear properties and are widely used in advanced nuclear systems [1,2]. Due to their high melting points and chemical reactivity, graphite crucibles and guide tubes are commonly employed in their melting processes, owing to graphite's exceptional thermal stability, thermal shock resistance, and machinability [3,4]. However, at elevated temperatures, graphite reacts with molten uranium, forming carbides and causing metallic contamination [[5], [6], [7]]. Therefore, effective protection of graphite inner surfaces against molten uranium and uranium alloys is of great importance.
To mitigate this, applying ceramic barrier coatings on graphite surfaces has become essential [[8], [9], [10]]. Among potential candidates, yttrium oxide (Y2O3) is favored for its high melting point (∼2410 °C), chemical inertness, and compatibility with uranium alloys [[11], [12], [13]]. Atmospheric Plasma Spraying (APS) has been used to deposit Y2O3 coating, effectively reducing carbon diffusion and improving structural integrity under thermal cycling [14,15]. However, due to the coefficient of thermal expansion (CTE) mismatch between graphite (∼4 × 10−6 K−1) and Y2O3 (∼8 × 10−6 K−1), coatings often suffer from cracking or delamination [16,17]. To alleviate these issues, an intermediate silicon carbide (SiC, ∼6 × 10−6 K−1) coating is often introduced, typically by chemical vapor deposition and pack cementation techniques, to improve thermal compatibility and to suppress carbon migration from the graphite substrate into the ceramic top coating [[18], [19], [20]].
In many practical applications, the graphite components used for uranium-alloy melting are deep cavities or slender tubes, where the inner surface is the critical area requiring protection. For such geometries, conventional internal diameter plasma spraying (ID-PS) becomes difficult or even infeasible: when the inner diameter is small, the internal spray gun cannot penetrate deeply enough, or the stand-off distance becomes too short for the ceramic particles to fully melt, leading to poor coating quality and non-uniform thickness [21]. Gas-phase deposition techniques such as CVD can provide dense SiC coating, but they are costly, time-consuming and difficult to apply to long tubes with large required coating thicknesses. As a result, Y2O3 coating on graphite inner walls are still often prepared by brush-coating followed by sintering, which usually yields limited thickness, weak adhesion and inadequate thermal-shock resistance [10,22]. This situation highlights the need for new coating technologies capable of depositing high-quality Y2O3 ceramic coating on confined graphite inner surfaces.
Atmospheric Long Laminar Plasma Spraying (ALPS) is a plasma spraying technique specifically developed to generate a long, stable laminar plasma jet in ambient atmosphere, as schematically illustrated in Fig. 1. Compared with conventional atmospheric plasma spraying, ALPS is characterized by reduced turbulence intensity, suppressed air entrainment, and a significantly extended high-temperature core along the axial direction. Owing to these characteristics, ALPS enables more uniform temperature and velocity distributions over long spray distances, which is particularly advantageous for thermal spraying in confined or internal geometries. As a result, particles can be effectively heated and transported over extended stand-off distances while maintaining stable deposition conditions, making ALPS well suited for inner-wall coating applications in long and narrow cavities [[23], [24], [25]].
In our previous work and related studies, Atmospheric Long Laminar Plasma Spraying has been successfully used to deposit ceramic coatings on the inner walls of cylindrical metallic substrates, such as YSZ-based thermal barrier coating, where an inclined columnar microstructure and good thermal-cycling performance were observed [27]. These results indicate that a long laminar plasma jet combined with oblique-angle deposition in confined geometries can produce columnar coatings with enhanced thermal-shock resistance. Therefore, in this work we employ ALPS, combined with vacuum reaction sintering, to form a SiC/Y2O3 composite coating on the inner walls of slender graphite guide tubes, in order to overcome the above limitations.
In this study, a high-temperature SiC/Y2O3 composite coating was fabricated on the inner wall of a graphite guide tube with an inner diameter of 57 mm and a length of 200 mm by combining ALPS with vacuum reaction sintering. A Si coating was first deposited by ALPS and then converted into a C/SiC interfacial structure consisting of a dense outer SiC layer and a subsurface region containing dispersed SiC, which serves as a bond coating and diffusion barrier. A Y2O3 coating was subsequently deposited on this SiC bond coating by ALPS under oblique-angle inner-wall spraying, leading to an inclined columnar microstructure characteristic of the long laminar jet in the confined space. The microstructure, mechanical properties and thermal-shock behaviour of the coatings were systematically investigated, with emphasis on clarifying the formation mechanism of the inclined columnar Y2O3 coating and the protective role of the SiC/Y2O3 multilayer design for graphite guide tubes in uranium-alloy melting.

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Section snippets

ALPS processing

Atmospheric Long Laminar Plasma Spraying (ALPS) was employed as the heat source for coating deposition. In this process, a direct-current non-transferred arc plasma torch was operated with Ar and N2 as the plasma-forming gases to generate a long, straight and quasi-laminar plasma jet under atmospheric conditions. As shown in Fig. 2a–c, the visible jet extends several hundred millimetres from the torch exit and remains nearly parallel without obvious divergence or turbulence, providing an

Microstructure of the reaction-sintered SiC bond coating

The surface and cross-sectional morphologies of the SiC bond coating, formed via vacuum reaction sintering of the plasma-sprayed Si layer, are shown in Fig. 6. The surface (Fig. 6a–d,g) exhibits a relatively dense and rough texture, with an average roughness of 6.31 ± 0.15 μm. XRD analysis confirms that the sprayed Si has been completely transformed into 3C-SiC, with no residual free silicon detected, indicating a full reaction with the graphite substrate.
Cross-sectional SEM images (Fig. 6b, c,

Conclusions

In this work, a multilayer SiC/Y2O3 coating system was successfully fabricated on the inner wall of a long, narrow graphite tube using atmospheric long laminar plasma spraying (ALPS). The results demonstrate that ALPS provides an effective approach for depositing functional ceramic coatings inside confined internal structures that are inaccessible to conventional internal-diameter thermal spraying techniques.
Stable Y2O3 deposition was achieved at extended spray distances of 150–300 mm, forming

CRediT authorship contribution statement

Yi-Fan Lei: Writing – original draft, Software, Conceptualization. Li-Hua Deng: Resources, Investigation. Dong Chen: Writing – review & editing, Resources, Project administration, Funding acquisition. Xin-Jian Zhang: Resources, Funding acquisition. Jie Chen: Writing – review & editing, Supervision, Resources, Funding acquisition. Sen-Hui Liu: Writing – review & editing, Supervision, Resources, Project administration, Data curation, Conceptualization. Chang-Jiu Li: Resources, Investigation,

Declaration of competing interest

The authors declare that there is no conflict of interest regarding the publication of this paper. The manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere.

Acknowledgements

This work was supported by National key research and development program of China (No. 2021YFB4001400), Key research and development plan of Shaanxi Province (S2023-YF-LLRH-QCYK-0201), Xi'an Advanced Functional Coating Technology International Science and Technology Cooperation Base. National Natural Science Foundation of China (U2430212).

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