Optimization of CVD SiC process and preparation of high-purity coatings based on a thermodynamic-fluid dynamics coupled model

Abstract

The mechanism of SiC preparation via chemical vapor deposition (CVD) of the CH3SiCl3(MTS)-H2 system remains unclear. This article integrates thermodynamic calculations, fluid dynamics simulations, and experimental validations to enable a synergistic analysis from thermodynamic equilibrium predictions to fluid dynamics-based dynamic modeling. The results systematically reveal the effects of process parameters on the SiC deposition procedure. It was found that the silicon-rich phenomenon observed at low temperatures is related to the low reactivity of CH4 and the preferential adsorption of chlorosilanes. With increasing deposition temperature, the concentration of silicon-containing molecular species such as SiCl2 rises, while unsaturated hydrocarbons like C2H2 become the dominant carbon sources at high temperature, ultimately producing nearly stoichiometric SiC coatings at 1400 ℃. Notably, thermodynamic calculation results alone exhibited deviations from experimental results, whereas coupling with fluid dynamics simulations, consistency was improved significantly. This research method not only compensates limitations inherent in thermodynamic calculations but also provides reliable theoretical basis and technical support for precise control of CVD parameters and optimization of SiC chemical composition.

1. Introduction

Silicon carbide (SiC) has exceptional wear resistance, high-temperature stability, irradiation resistance, chemical corrosion resistance, and thermal shock tolerance [1−5], making it irreplaceable in aerospace thermal protection, nuclear reactor structural materials, photovoltaic device packaging, and semiconductor devices [6−10]. Polycrystalline SiC prepared via chemical vapor infiltration (CVI) and chemical vapor deposition (CVD) has become a research focus in ceramic matrix composite fabrication and functional coating design due to its excellent geometric adaptability, cost-effectiveness, and high purity [11−17].

Common precursors for CVD SiC preparation fall into two categories: C-H-Si and C-Cl-H-Si systems. The C-Cl-H-Si system is advantageous for preparing stoichiometric SiC because the presence of Cl suppresses homogeneous deposition of Si atoms caused by locally excessive precursor concentration. Methyltrichlorosilane (CH3SiCl3, MTS), a typical precursor of this system, is favored for large-scale industrial SiC production due to its high safety, wide process window, and economic benefits [14−17].

Previous studies have shown that CVI/CVD process parameters, such as temperature, pressure, and H2/MTS ratio, significantly affect the composition, phase content, microstructure, and morphological characteristics of SiC [14, 18−26]. Deviations from the stoichiometric ratio (e.g., excess silicon or carbon) generally deteriorate the physicochemical properties of SiC [14, 20, 21, 27]. Thermodynamic calculations for the MTS-H2 system indicate that [28−32], at equilibrium, SiC and graphite are the primary solid phases, with graphite co-deposition occurring at either excessively low (800−1000 ℃) or high (1400−1600 ℃) deposition temperatures [29, 30]. SiC and Si co-deposition is almost negligible, occurring only when H2/MTS>10^4 [29, 30, 33]. However, experimental observations often deviate from thermodynamic predictions: lower deposition temperatures, higher pressures, and higher H2/MTS ratios tend to promote Si and SiC co-deposition [16, 17, 20, 22, 23, 34−39], whereas higher temperatures, lower pressures, and lower H2/MTS ratios favor graphite and SiC co-deposition [14, 19, 20, 25, 40−42]. This discrepancy is commonly attributed to the omission of kinetic effects in thermodynamic calculations [33, 43]. Under moderate temperature or short gas residence time conditions, gas-phase equilibrium may not be reached within the reactor timescale, making it essential to incorporate kinetic factors to evaluate the evolution of gas-phase species inside the reaction chamber.