Numerical modeling of SiC by low-pressure chemical vapor deposition from methyltrichlorosilane

The development of functional relationships between the observed deposition rate and the experimental conditions is an important step toward understanding and optimizing low-pressure chemical vapor deposition(LPCVD)or low-pressure chemical vapor infiltration(LPCVI).In the field of ceramic matrix composites(CMCs),methyltrichlorosilane(CH3 SiCl3,MTS)is the most widely used source gas system for SiC,because stoichiometric SiC deposit can be facilitated at 900°C–1300°C.However,the reliability and accuracy of existing numerical models for these processing conditions are rarely reported.In this study,a comprehensive transport model was coupled with gas-phase and surface kinetics.The resulting gas-phase kinetics was confirmed via the measured concentration of gaseous species.The relationship between deposition rate and 24 gaseous species has been effectively evaluated by combining the special superiority of the novel extreme machine learning method and the conventional sticking coefficient method.Surface kinetics were then proposed and shown to reproduce the experimental results.The proposed simulation strategy can be used for different material systems.

ZrC-ZrB_2-SiC ceramic nanocomposites derived from a novel single-source precursor with high ceramic yield

For the first time, ZrC-ZrB_2-SiC ceramic nanocomposites were successfully prepared by a single-source-precursor route, with allylhydridopolycarbosilane(AHPCS),triethylamine borane(TEAB),and bis(cyclopentadienyl) zirconium dichloride(Cp_2 ZrCl_2) as starting materials. The polymer-to-ceramic transformation and thermal behavior of obtained single-source precursor were characterized by means of Fourier transform infrared spectroscopy(FT-IR) and thermal gravimetric analysis(TGA). The results show that the precursor possesses a high ceramic yield about 85% at 1000 ℃.The phase composition and microstructure of formed ZrC-ZrB_2-SiC ceramics were investigated by means of X-ray diffraction(XRD) and high resolution transmission electron microscopy(HRTEM).Meanwhile, the weight loss and chemical composition of the resultant ZrC-ZrB_2-SiC nanocomposites were investigated after annealing at high temperature up to 1800 ℃. High temperature behavior with respect to decomposition as well as crystallization shows a promising high temperature stability of the formed ZrC-ZrB_2-SiC nanocomposites.

Ablation characteristics of mosaic structure ZrC-SiC coatings on low-density, porous C/C composites

Mosaic structure ZrC-SiC coatings were fabricated on low-density, porous C/C composites via thermal evaporation and an in-situ method. ZrC was packed in a typical lamellar mode, and the mosaic structure was formed by the deposition of Zr and Si atoms on the shallow surface of the porous C/C composites.Ablation analysis showed that the defects in the coatings originate from the boundary between the ZrC and holes created by the consumption of SiC at 2500℃. After ablation for 200 s at 3000℃, a dense ZrO2 layer formed on the coating surface, and the defects were sealed owing to the continuous supply of ablative components. The mass and line ablation rates of the Zr C-SiC coatings were-0.46 ± 0.15 mg cm^-2·s^-1 and-1.00± 0.04 μm s^-1, respectively.

Damage behavior of atomic oxygen on CVD SiC coating-modified carbon/carbon composite in low earth orbit environment

In this work, samples of carbon/carbon(C/C) and chemical vapor deposited(CVD) SiC-coated C/C samples were investigated to understand the AO damage mechanism in low Earth orbit(LEO) environment. The ground-based simulated atomic oxygen(AO) generator was employed. Results indicate that the CVD SiC coating exhibited improved radiation resistance properties against AO radiation as evidenced by a 16%better strength retention ratio, 60% less mass ablation, and increased strength stability. The magnitude of these influences affected the surface morphology, as observed by scanning electron microscopy(SEM)and surface resistance meter test results. The variations in the surface constituents were confirmed by X-ray diffraction(XRD) and X-ray photoelectron spectroscopy(XPS) results. The main products left on surface after AO exposure are SiO2 and SiCxOyfilm. Additionally, Si atoms are found to be the preferential reacting element in the SiC coating, and this process is accompanied by graphite precipitation, grain growth, and crack necking. Also, the damage mechanism of the AO-exposed SiC coating was revealed and is discussed.

Preparation of ZrC-SiC composite coatings by chemical vapor deposition and study of co-deposition mechanism

In this work, the Zr C-SiC composite coatings were co-deposited by chemical vapor deposition(CVD)using ZrCl4, MTS, CH4 and H2 as raw materials. The morphologies, compositions and phases of the composite coatings were characterized by scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy(EDS) and X-ray diffraction(XRD). The results indicated that the morphologies, compositions and phases of the composite coatings were related to the deposition temperature, the flow rate of the carrier H2 gas, and the ratio of C/Zr. Moreover, the co-deposition mechanism of the composite coatings was also studied. It was found that different deposition temperatures resulted in different deposition mechanisms. At temperatures in the range of 1150–1250℃, the Zr C-SiC co-deposition was controlled by the surface kinetic process. At temperatures in the range of 1250–1400℃, the Zr C-SiC co-deposition was controlled by the mass transport process.