Chemical Vapor Infiltration Technology Research Articles

Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology

Spray drying, binder jetting and chemical vapor infiltration (CVI) were used in combination for the first time to fabricate SiC whisker-reinforced SiC ceramic matrix composites (SiCW/SiC). Granulated needle-shaped SiCW was spray dried into SiCW spherical particles to increase flowability and thereby increase printability. Then, binder jetting was employed to print a novel SiCW preform with two-stage pores using the SiCW spherical particles. The subsequent CVI technology produced pure, dense, and continuous SiC matrix with high modulus and strength. Consequently, SiCW/SiC with appropriate mechanical properties was obtained. Finally, the challenges of the novel method and the ways to improve the mechanical properties of SiCW/SiC are discussed.

RVE Model with Porosity for 2D Woven CVI SiCf/SiC Composites

A representative volume element (RVE) model with porosity for 2D woven chemical vapor infiltration (CVI) SiCf/SiC composites is presented, and its mechanical properties are analyzed. Samples are divided after a tensile test, and their cross sections are scanned with a scanning electron microscope. The size of the feature structure of the RVE model is determined based on the measurement statistics of the feature structure parameters. In accordance with CVI technology, the deposition rates of the matrix in each direction along the surface of fiber bundles are assumed to be similar. The porosity structure is formed naturally when the RVE model is established. The RVE model conforms to the real structure and accurately shows the location and geometric shape of internal porosity. The relative error of the tensile modulus value estimated from the RVE model through the asymptotic expansion homogenization method and experimental data is 3.26%. Therefore, the RVE model is accurate and efficient.

Evaluation of opening-hole shapes for rivet connection of a composite plate

The rivet joint is an important mechanical connection for hot structures made of ceramic matrix composites. The development of chemical vapor infiltration technology provides a possible application of novel rivets for these structural components. However, connectors can still be the weakest link for structural integrity due to the severe stress concentration around the rivet joint holes. The geometry parameters of the rivet hole can be key influencing factors to the structural strength. A comparative evaluation is presented to analyze the influence of opening-hole shapes on the structural stress concentration of a composite plate. The opening-hole shapes considered include circle, ellipse, and racetrack. The finite element (FE) models of specimens with the three shapes of rivet holes were constructed. The stress distributions of the specimens under tensile loading were analyzed to compare the stress concentration factor (SCF) of different opening-hole shapes. An optimal shape, i.e. racetrack-shaped, was determined based on the SCF reduction of specimen-level comparison. Then, FE models of a composite plate containing rivet holes were built to verify the outperformance of the racetrack-shaped hole. A stress concentration reduction of the plate with racetrack-shaped holes was observed compared to the one with circular holes.

Effect of chemical vapor infiltration treatment on the wave-absorbing performance of carbon fiber/cement composites

Short carbon fibers were treated at high temperatures around 1100°C through chemical vapor infiltration technology. A thinner layer of pyrocarbon was deposited on the fiber surface. The dispersion of carbon fibers in a cement matrix and the mechanical properties of carbon fiber/cement composites were investigated by scanning electron microscopy (SEM) and other tests. The reflectivity of electromagnetic waves by the composites was measured in the frequency range of 8.0–18 GHz for different carbon fiber contents of 0.2wt%, 0.4wt%, 0.6wt%, and 1.0wt%. The results show that the reflectivity tends to increase with the increase of fiber content above 0.4wt%. The minimum reflectivity is −19.3 dB and the composites exhibit wave-absorbing performances. After pyrocarbon is deposited on the fiber, all the reflectivity data are far greater. They are all above −10 dB and display mainly wave-reflecting performances.

Influence of CVI treatment of carbon fibers on the electromagnetic interference of CFRC composites

Abstract Short carbon fibers were treated at temperatures around 1100 °C in a furnace through chemical vapor infiltration technology. The fiber surface was observed by scanning electron microscopy. The reflectivity of electromagnetic radiation by the composites that were reinforced by surface-treated carbon fibers and by as-received ones was measured in the frequency range of 8.0–18.0 GHz. The reflectivity for different carbon fiber contents of 0.2%, 0.4%, 0.6% and 1.0 wt% was investigated. Results showed that the reflectivity of the composites that were reinforced by untreated carbon fibers tended to increase with the increasing fiber contents. The minimum reflectivity was −19.3 dB, far less than −10 dB, when the fiber content was 0.4% and there were wave-absorbing properties. However, after surface treatment, the minimum reflectivity was −8.1 dB for the same fiber content of 0.4%, indicating significant wave-reflecting properties. The achieved reflectivity values after surface treatment were generally greater than those without treatment.

Densification mechanism of chemical vapor infiltration technology for carbon/carbon composites

Carbon/carbon composites were fabricated using pressure-gradient chemical vapor infiltration(CVI) technology with propane (C 3H 6) as the carbon precursor gas and nitrogen (N 2) as the carrier gas. The chemical process of deposition of pyrolytic carbon was deduced by analyzing the component of molecules in gas phase and observing the microstructure of deposition carbon. The results show that the process of deposition starts from the breakdown of C—C single bond of propene (C 3H 6), and forms two kinds of active groups in the heterogeneous gas phase reaction. Afterwards, these active groups form many stable bigger molecules and deposit on carbon fiber surface. At the same time, hydrogen atoms of the bigger molecules absorbed on carbon fiber surface are eliminated and the solid pyrolytic carbon matrix is formed in the heterogeneous reaction process.

Graphite powder reinforced pyrocarbon, a new kind of C/C composite

Graphite powder reinforced pyrocarbon (G/C) was prepared by thermal gradient chemical vapor infiltration technology. The microstructures and basic properties of the G/C were investigated by polarized light microscopy, SEM, electrical and mechanical tests. It is found that pyrocarbon enwraps the graphite powder particles and fills the pores between them effectively. The G/C is isotropic, has a density of 1.85g/cm3, and has a higher volume resistivity (148.4μΩ·m) than graphite and the other C/C composites. It has excellent mechanical properties with a flexural strength of 50MPa and a compressive strength of 120MPa, twice those of pure graphite. The compressive strength of the G/C is similar to, and the flexural strength is lower than, those of carbon felt/pyrocarbon composites.