Huiming Cheng

The properties of carbon fibre/SiC composites fabricated through impregnation and pyrolysis of polycarbosilane

Unidirectional carbon fibre reinforced SiC composites were prepared from four types of carbon fibres, PAN-based HSCF, pitch-based HMCF, CF50 and CF70, through nine cycles or twelve cycles of impregnation of polycarbosilane and subsequent pyrolysis at 1200°C. The polycarbosilane-derived matrix was found to be β-SiC with a crystallite size of 1.95 nm. The mechanical properties of the composites were evaluated by four-point bending tests. The fracture behavior of each composite was investigated based on load-displacement curves and scanning electron microscope (SEM) observation of fracture surfaces of the specimens after tests. It was found that CF50/SiC and CF70/SiC exhibited high strength and non-brittle fracture mode with multiple matrix cracking and extensive fibre pullout, whereas HSCF/SiC and HMCF/SiC exhibited low strength and brittle fracture mode with almost no fibre pullout. The differences in the fracture modes of these carbon fibre/SiC composites were thought to be due to differences in interfacial bonding between carbon fibres and matrix. Values of flexural strengths of CF70/SiC and CF50/SiC were 967 MPa and 624 MPa, respectively, which were approximately 75% and 38% of the predicted values. The relatively lower strength of CF50/SiC, compared with CF70/SiC, was mainly attributed to the shear failure of CF50/SiC during bending tests.

Characteristics of several carbon fibrereinforced aluminium composites prepared by a hybridization method

The properties and microstructures of several high-strength and high-modulus carbon fibrereinforced aluminium or aluminium alloy matrix composites (abbreviated as HSCF/Al and HMCF/Al, respectively, for the two types of fibre) have been characterized. The composites evaluated were fabricated by pressure casting based on a hybridization method. It was found that the strength degradation of high-modulus carbon fibres after infiltration of aluminium matrices was not marked and depended upon the type of aluminium matrix. However, the strength of high-strength carbon fibres was greatly degraded by aluminium infiltration and the degradation seemed to be independent of the type of aluminium matrix. The longitudinal tensile strength (LTS) of CF/Al composites was very different between HMCF/Al and HSCF/Al composites. The HMCF/Al composites had LTS values above 800 MPa, but the HSCF/Al composites had only about 400 MPa. In contrast, the transverse tensile strength of the HSCF/Al composites, above 60 MPa, was much higher than that of the HMCF/Al composites, about 16 MPa. Chemical reactions were evident to the interface of high-strength carbon fibres and aluminium matrices. There was no evidence of chemical products arising between high-modulus carbon fibres and Al-Si alloy and 6061 alloy matrices. However, it was considered that some interfacial reactions took place in pure aluminium matrix composites. Fracture morphology observation indicated that the good LTS of CF/Al composites corresponded to an intermediate fibre pull-out, whereas a planar fracture pattern related to a very poor LTS and fibre strength transfer. The results obtained suggested that interfacial bonding between carbon fibres and aluminium matrices had an important bearing on the mechanical properties of CF/Al composites. An intermediate interfacial bonding is expected to achieve good longitudinal and transverse tensile strengths of CF/Al composites.

Effect of silicon additions on characteristics of carbon fiber reinforced aluminum composites during thermal exposure

The effects of Si additions on the behavior of high modulus carbon fiber reinforced aluminum matrix (CF/Al) composites during thermal exposure at 773 K for different times have been investigated. The composites were fabricated via hybridization with a small volume fraction of SIC particles using a pressure-casting process, The change of longitudinal tensile strength, the strength degradation of carbon fibers, and the microstructural observations on the interfaces of CF/pure Al composites and CF/Al-Si composites after thermal exposure undoubtedly indicate that the alloying element Si in an aluminum matrix can effectively prohibit the interfacial reactions at the fiber/aluminum interface and has positive effects on the characteristics of CF/Al composites.

TANSO no. 176 — AbstractEffects of grinding and addition of MoSi2 or LaB6 on structural change of petroleum coke and electrical resistivity of the sintered compacts

Petroleum coke with and without addition of 5 mass % LaB6 or MoSi2 was ground by alumina mortar for up to 100 h and then heat-treated at 1500 °C and 2500 °C for 1 h. The ground powders and the heat-treated powders were investigated by microscopy and X-ray analysis to clarify a change of morphology and structure of the powder. Electrical resistivity of sintered compact prepared from the ground powder was examined. Grinding treatment made the particle of the coke powder smaller and roundish up to 30 h independently of the additive. Dense sintered compacts were obtained from the ground powder, especially from the powder containing LaB6, at 2500 °C. The ground powder, whose structure was amorphous, had a tendency not to graphitize easily. By 1500 °C heat treatment, the additive had no effect on the structural change of the powder. On the other hand, addition of LaB6 caused an increase of interlayer spacing and a suppression of lowering in crystallite size by 2500 °C heat treatment. Addition of LaB6 or MoSi2 lowered electrical resistivity of the sintered compact obtained at 2500 °C.

EFFECT OF BORON–DOPING ON OXIDATION OF CARBON FIBER

Influence of boron–doping by different methods was investigated on X–ray parameter and oxidation resistance of carbon fibers with different microstructure. With diffusion of boron into carbon fiber, d(002) spacing decreased and crystallite size, Lc(002), increased. However, the change of the X–ray parameter was a little different among carbon fibers with different microstructure. Many small grains of precipitates formed on fiber surface by reaction of carbon and boron in the cases of using B2O3 and B4C powder as boron–doping. With formation of the precipitates, degradation in strength was observed. The degradation was considered to be due to formation of defects around the precipitates. Weight–loss curve with oxidation temperature shifted about 50–150° to higher side than those that of non–doped fibers in all cases. However, formation of pits by oxidation was observed for boron–doped carbon fiber by B2O3 and B4C powder method. While, formation of precipitates and defects were few on the surface for the fibers boron–doped by PVD method and it was suggested that homogeneous boron–doping is preferable to improve oxidation resistance of carbon fiber.