Si’an Chen

Preparation and Mechanical Properties of C/SiC Joints with Pins or Bolts

C/SiC substrates and M3 bolts were prepared by precursor infiltration and pyrolysis (PIP) process firstly, then the joints with four joining methods (2 φ3mm soft pins, S-A; 2 M3 bolts, S-B; 2 φ3mm soft pins plus 8 φ1mm soft pins, S-C; 2 M3 bolts plus 8 φ1mm soft pins, S-D) were assembled, finally several PIP cycles were continued to finish densification. The influences of different joining methods and PIP cycles (3 or 6) on the stripping properties of C/SiC joints were investigated, and the fracture modes were analyzed. The stripping loads of the joints adopting 6 PIP cycles are almost two to six times the values adopting 3 PIP cycles, and the corresponding fracture modes are pins or bolts rupture and pull-out, respectively. When adopting 3 PIP cycles, the bearing ability of the joints with bolts (S-B and S-D) is about 51% higher than that of joints with pins (S-A and S-C), but when adopting 6 PIP cycles, the values with pins (S-A and S-C) is about 67% higher than that with bolts (S-B and S-D). Besides, the introduction of φ1mm pins enhances the bearing ability of the joints with 3 and 6 PIP cycles about 45% and 14%, respectively.

Effect of deposition time on microstructures and growth behavior of ZrC coatings prepared by low pressure chemical vapor deposition with the Br2-Zr-C3H6-H2-Ar System

ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition (LPCVD) with the Br2-Zr-C3H6-H2-Ar system. The effects of deposition time on the microstructures and growth behavior of ZrC coatings were investigated. ZrC coating grew in an island-layer mode. The formation of coating was dominated by the nucleation of ZrC in the initial 20 minutes, and the rapid nucleation generated a fine-grained structure of ZrC coating. When the deposition time was over 30 min, the growth of coating was dominated by that of crystals, giving a column-arranged structure. Energy dispersive X-ray spectroscopy showed that the molar ratio of carbon to zirconium was near 1:1 in ZrC coating, and X-ray photoelectron spectroscopy showed that ZrC was the main phase in coatings, accompanied by about 2.5mol% ZrO2 minor phase.

Compression Property of C/SiC and Inconel X-750 Springs from Room Temperature to 1000°C

The compression properties of C/SiC composite and Inconel X-750 helical springs were investigated from room temperature (RT) to 1000°C in air. The density of C/SiC spring is 1.74 g/cm3, only ~1/5 of X-750 value (8.17 g/cm3) and the spring constants of C/SiC and X-750 springs at RT are 3.47 and 5.61 N/mm, respectively. The spring constants of X-750 spring decreased with increase of temperature. X-750 spring could keep excellent property below 600°C, but its spring constant was only 36.7% of RT value at 800°C and permanent deformation appeared. At 1000°C, it could not restore and was destroyed. The spring constants of C/SiC spring at 400°C and 600°C were appreciably higher than the RT value, and then decreased with temperature elevating. Above 800°C, the spring constant decreased with test progressing because of the oxidation of carbon fibers and SiC matrix. But it has a spring constant of 2.40 N/mm (69.2% of the RT value) at 1000°C and can revert to its original dimensions.

Low Temperature Preparation of ZrC Coatings on C/C Composite via Molten Salt Reaction

Zirconium carbide (ZrC) coatings were prepared on C/C composite via molten salt reaction process at relatively low temperatures of 800-1000°C. During the reaction process, potassium fluorozirconate (K2ZrF6) played a role transporting zirconium from the molten salt to the C/C composite surface. Elevating reaction temperature increased the growth rate of coatings, simultaneously leaded to rougher coatings. The coatings growth rate increased with reaction time at first and then decreased gradually. The ZrC coatings prepared at 900°C for 5h was ~2m thickness. At the early stage, the low solubility of zirconium in the molten salt leaded to the low coatings growth rate. Secondly, the growth rate of the ZrC coatings was controlled by the chemical reaction between C/C composites and zirconium once zirconium was saturated in the molten salts. Thirdly, the control step of coatings formation turned into the diffusion of carbon through the formed ZrC coatings and which leaded to a gradual decrease of growth rate.

Preparation of Cf/SiC Composites by Chemical Liquid-Vapor Deposition Process

The conversion of the liquid polycabosilane (LPCS) into silicon carbide was investigated by IR, XRD, which indicated the feasibility of the transition from LPCS to SiC ceramics above 900°C. The FTIR spectra and XRD Pattern of the Cf/SiC composites show that the matrix deposited at 1200°C has silicon carbide structure with the crystallite size of β-SiC phase of about 41 nm, while the SiC phase is amorphous at 900°C. The carbon fiber reinforced silicon carbide composites (Cf/SiC) were hereby prepared at 900°C and 1200°C, through chemical liquid-vapor deposition (CLVD) process using LPCS as precursor. Flexural strength of 224 MPa for Cf/SiC specimen with density of 1.81g·cm-3 was obtained after being prepared at 1200°C for 30 minutes. The load-deflection curve has shown that the fracture behavior of the Cf/SiC composites is a typical non-brittleness. The results indicate that the CLVD process has a great advantage and prospect to prepare Cf/SiC composites in future.

Preparation and Properties of 2D C/SiC Composites by CLVD Processing

Chemical liquid-vapor deposition (CLVD) process is a new style of fast densification, which combines the advantages of PIP process and CVI process. 2D C/SiC composites were prepared at 800~1200°C for 3~4 hours with liquid polycarbosilane and carbon fiber cloth by CLVD process with induction heating, and had the density of 1.7 g/cm3, the flexural strength of 84.6MPa, and the flexural modulus of 20GPa. XRD pattern of the sample proved that the matrix was β-SiC. It was found that SiC deposited mainly around single fiber instead of among fiber bundles and layers.