Density Functional Theory Modeling of Heterogeneous Reactions of Hydrocarbon Intermediates on Silicon Carbide: Surrogate Kinetic Model Development for Chemical Vapor Infiltration

Compared to traditional materials, those with enhanced strength, toughness, and high-temperature resistance are preferred in advanced engineering industries such as aerospace, automotive, nuclear, and semiconductor [1]. Ceramic Matrix Composites (CMC) are used in these industries because of the advantageous characteristics of ceramics and composites [2]. Their excellent material properties have made CMC one of the most commonly used materials in high-tech applications [3]. CMC offers better strength and durability, resistance to high temperature and pressure, corrosion and oxidation resistance, lower weight, and improved fuel efficiency compared to conventional materials [4]. Their manufacturing is based on the principle that combining two or more materials can enhance properties. CMC can effectively overcome the shortcomings of conventional materials in harsh environments, making them essential in high-performance applications [5]. Their ability to withstand high temperatures makes them suitable for use in gas turbines and jet engines, thereby enhancing performance and fuel economy [6]. Improved fracture toughness contributes to reliability and safety by absorbing the energy released during a crack [7]. Their lightweight nature is advantageous in aerospace and automotive sectors, promoting speed and energy savings [8]. Moreover, due to their resistance to corrosion and oxidation, CMCs require less maintenance and demonstrate durability in extreme environments. Tailoring the fiber and matrix combinations enables the use of CMC requirements in nuclear reactors and hypersonic vehicles. Their resistance to fatigue and creep also makes them suitable for applications involving cyclic loading, ensuring long-term reliability [9]. All these properties highlight the importance of CMC in developing energy-efficient, high-reliability solutions for advanced industries. SiC is preferred for CMC due to its outstanding mechanical and thermal properties, electrochemical stability, and unique crystal structure [10]. Due to its excellent mechanical, chemical, and electrical properties, SiC is well-suited for harsh environments. Its resistance to deformation under mechanical loading is reflected in its high elastic modulus (approximately 410 GPa) [11], making it stiffer than most structural ceramics and metals. This characteristic is crucial in applications such as space-borne components and turbine blades, where dimensional stability under extreme stress is required. SiC also forms a hard, adherent silicon dioxide (SiO2) layer when exposed to oxygen at elevated temperatures, making it highly resistant to oxidation and corrosion [12]. 

This passivizing oxide layer prevents further degradation even in aggressive environments such as corrosive gases or oxidizing agents at high temperatures. Another notable property of SiC is its wide band gap, which ranges from 2.3 to 3.3 eV, depending on the polytype (e.g., 3C, 4H, or 6H-SiC) [13,14]. This wide band gap allows SiC-based devices to operate under high voltages, frequencies, and temperatures where conventional dielectric semiconductors like silicon typically fail due to thermal breakdown, excessive leakage currents, or insufficient carrier mobility [15]. This is attributed to the high intrinsic electrical resistivity, thermal conductivity, and breakdown electric field of SiC [16]. Furthermore, SiC is highly suitable for structural applications in chemically and thermally extreme environments due to its low density, chemical inertness, and thermal stress resistance. Its high bond energy ( 4.6 eV) contributes to its mechanical and thermal stability [17]. Even above 1200°C, SiC maintains outstanding creep resistance [18], fracture toughness (approximately 4–5 MPa·√m) [19], hardness (2500–2800 HV) [20], and flexural strength (400–600 MPa) [21]. With a low thermal expansion coefficient ( 4.0 × 10−6/K) and high thermal conductivity (120–270 W/m·K), it offers exceptional thermal shock resistance [22].

Among fabrication techniques, the Chemical Vapor Infiltration (CVI) process is highly effective for producing SiC-based Composites of Materials (CMC). Introduced by Bickerdike in 1962, CVI enhances the density of porous structures while preserving fiber integrity and ensuring control over SiC deposition and material characteristics [23]. CVI operates at 600°C–1600°C and avoids undesirable reactions in high-temperature or liquid phase techniques. It enables uniform infiltration into complex porous preforms without clogging or cracking. Partial densification enables fracture resistance through fiber pull-out [24]. As a result, CVI remains a preferred method for manufacturing high-purity SiC CMC with complex geometries. To deposit SiC during CVI, selecting an appropriate gaseous precursor is critical. Methyltrichlorosilane (CH3SiCl3, or MTS) is the most commonly used precursor due to its molecular composition, which provides both Si and C for near-stoichiometric SiC deposition [25]. MTS decomposes in the reactor at temperatures ranging from 600°C to 1100°C, producing silicon carbide (SiC) and byproducts including chlorosilanes, hydrocarbons, and hydrogen chloride (HCl) [26]. Alternative precursors include silane (SiH4), chlorosilanes (SiCl4, SiHCl3), and organosilicons such as tetramethylsilane. Each precursor presents trade-offs in terms of deposition rate, safety, stoichiometric control, and cost [27]. MTS offers reasonable control and efficiency with fewer safety concerns than silane, which remains the precursor of choice. However, ongoing research explores improved formulations to enhance performance and environmental compatibility [28]. This study investigates the surface chemistry of MTS decomposition for SiC formation using quantum mechanical principles. A key objective is to determine the reaction pathway and rate-limiting steps of the decomposition process. Modeling these reactions is essential for process optimization, reducing experimental costs, and improving material quality [29]. Despite prior research, gas-surface interactions during SiC deposition remain poorly understood due to the complexity of physicochemical phenomena across different time and length scales [30,31]. Existing models often oversimplify reaction pathways and ignore interactions between hydrocarbon byproducts and SiC [32–36]. The formation and behavior of intermediates CH4, CH3, SiCl2, and HCl are not explained by most of the simple models of MTS decomposition, which consider the reaction as a simple reaction and are concerned only with the direct formation of Si-C bonds. However, the rapid dynamic interactions of these byproducts with the SiC surface result in secondary reactions, site competition, surface poisoning, and possible etching, which have a controlling influence on the deposition process [37–39]. These models lose basic phenomena like adsorption/desorption behavior, the influence of surface coverage and hydrogen availability, the contribution of transition states and energy barriers, and the kinetic competition between deposition and etching by overlooking such complicated interactions. Hence, such simplicity results in faulty predictions of growth rates and quality of the SiC film. More sophisticated methods, like surrogate models or chemical pathway analysis based on DFT, are needed to simulate the actual surface-sensitive chemistry of SiC CVI [40–43]. Carbon deposition is particularly challenging due to slow reaction rates, and accurate modeling of Si–C bond formation is crucial [44,45]. MTS first decomposes into Si- and C-containing species [46, 47], which undergo surface reactions to form SiC [48]. The unimolecular decomposition of MTS, with an activation energy of 340 kJ/mol, is a significant rate-limiting step [49,50]. This study employs quantum chemical modeling within a multiscale framework to assess carbon deposition during MTS decomposition. We use first-principles Density Functional Theory (DFT) to calculate activation energy, identify transition states, and map the Potential Energy Surface (PES). The approach includes vibrational frequency analysis, zero-point energy (ZPE) correction, and evaluation of the Gibbs free energy. These data are used to fit an Arrhenius model, allowing predictions of growth behavior under various CVI conditions. The methodology bridges atomic-level interactions with macroscopic process control, offering a bottom-up approach to multiscale modeling.

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