BAI Xiaoyun ¹, WANG Xu ¹, FU Lu ¹, CAI Yuelei ¹, LIN Hui ¹, ZHOU Zhiqiang ¹, GUO Litong ²

(1. Zhejiang Harog Technology Co., Ltd., Huzhou 313117, Zhejiang, China; 2. School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China)

Extended abstract:

[Background and purposes] Although CVD for SiC using methyltrichlorosilane (MTS) is a mature technique, most prior studies have focused on the effects of isolated parameters or single performance metrics. However, a systematic optimization of multiple interdependent process variables, such as temperature, pressure and H₂/MTS ratio, with the explicit aim of synergistically enhancing both oxidation and thermal shock resistance remains scarce. Furthermore, the mechanistic linkage between optimized process windows, resultant microstructures and ultimate coating performance is often inadequately elucidated, limiting the development of robust coatings for industrial applications. A meticulously designed experimental matrix was designed to unravel the synergistic effects of temperature, pressure and H₂/MTS ratio. Our goal was not merely to deposit SiC, but to engineer coatings with optimal microstructure with both high-temperature stability (oxidation) and mechanical resilience (thermal shock). Through in-depth characterization of growth kinetics, crystallography and surface chemistry, the process windows were correlated with specific growth regimes (reaction-controlled vs. mass-transport-controlled). Crucially, a unique optimal parameter combination was identified, which yields coatings with unprecedented optimal performance. The validated protocol provides a significant advancement over conventional CVD recipes, offering a clear pathway for manufacturing next-generation durable SiC-coated graphite material (graphite/SiC) with extended service life and reliability.

[Methods] SiC coatings were deposited on cleaned graphite substrates in a hot‑wall vertical CVD reactor (Model: SGL-1700L, Shanghai Jvjing Precision Instrument Manufacturing Co., Ltd.) equipped with mass flow controllers (MKS Instruments) for precise gas delivery. A gas mixture of Ar (99.999%) at 800 sccm and H₂ (99.999%) at 600 sccm was introduced into the chamber. Liquid methyltrichlorosilane (MTS, 99.99%) served as the source of Si and C. The MTS was heated to 55 ℃ and delivered into the chamber via continuous bubbling, using H₂ at 150 sccm as the carrier gas. To systematically identify the optimal coating performance, three key variables over defined ranges were studied, including deposition temperature (1150, 1200, 1250 ℃), total system pressure (1, 2, 5, 10 kPa) and H₂/MTS molar ratio (5, 10, 15).

[Results] We present a comprehensive and innovative optimization of the CVD process using methyltrichlorosilane (MTS) on graphite substrates. The synergistic effects of deposition temperature (1150–1250 ℃), total system pressure (1–10 kPa) and H₂/MTS molar ratio (5–15) on microstructure, growth kinetics and service-performance metrics (oxidation and thermal shock resistance) are systematically and quantitatively examined. For the first time, a unique optimal parameter set (1200 ℃, 10 kPa, H₂/MTS=5) was validated, which yields dense (111)-oriented β-SiC coatings with near-stoichiometric composition. Coatings produced under these conditions exhibit exceptional performance, including a remarkably low mass loss of only 1.46 % after 3 h at 1000 ℃ in static air and outstanding thermal shock resistance withstanding up to 58 severe cycles (900 ℃→room-temperature water quenching) before failure. This work not only provides deep insight into the growth mechanism under varied CVD regimes, but also offers a "predictable and scalable process-design strategy", representing a substantial advancement for protecting graphite components in demanding semiconductor- processing and aerospace environments.

[Conclusions] This study is aimed to present a significant and innovative advancement in the CVD of SiC protective coatings for graphite through systematic multivariate optimization. The work is to not only examine the influence of individual process parameters, but also emphasize the synergistic effects of microstructure, crystal structure and elemental analysis on enhancing the oxidation resistance and thermal shock resistance of graphite/SiC. Our key contributions are as follows.(1) Identification of a unique optimal parameter set: a graphite/SiC composite with excellent overall performance can be obtained under the conditions of deposition temperature of 1200 ℃, a system total pressure of 10 kPa and H₂/MTS ratio of 5.(2) Establishment of microstructure–performance correlations: the coating produced under these optimized conditions exhibits a dense crack-free β-SiC microstructure with a preferred (111) orientation, which is identified as the key reason for its outstanding functional performance.(3) Demonstration of record‑high combined performance: the graphite/SiC prepared with the optimized parameters shows a mass loss rate of only 1.46% (equivalent to 2.34 mg·cm⁻²) after 3 h exposure to an oxygen-containing atmosphere at 1000 ℃, while withstanding 58 thermal shock cycles from 900 ℃ to room-temperature water quenching. This performance surpasses most of the results reported in the open literature.(4) Provision of a scalable process framework: based on an in-depth understanding of the growth mechanism transition and the validated optimal process parameters, a reliable and scalable CVD technical route has been provided for the industrial production of high-performance SiC coatings.

Key words: CVD; Graphite/SiC; oxidation resistance; thermal shock resistance; process optimization; synergistic enhancement effect