Carbon matrix composite densification

In the fabrication process of three dimensional carbon fiber reinforced SiC matrix composite, Suzuki and Nakano [207] applied PCVI as the final densification process for the specimen, which was made by the joint process of slurry infiltration and organosilicon polymer pyrolysis. The open porosity and bulk density of the specimen changed from 5.3% and 2.63 g-cm (relative density of 94%) to 3.5% and 2.67 g cm (relative density of 95%) by the apphcation of PCVI (1173 1223K, total 90,000 pulses). The flexural strength of the specimen increased over 20% (mean value =153 MPa, maximum value = 174 MPa). 

A CO2 laser-roller densification process is used to fabricate a unidirectional carbon-fiber-aluminum-matrix composite. Although good densification is obtained, the mechanical properties of the composites are low, probably due to fiber degradation. 

There are two basic types of processes used to make CAMCs. The first is chemical vapor infiltration (CVI). CVI is a process in which gaseous chemicals are reacted or decomposed, depositing a solid material on a fibrous preform. In the case of CAMCs, hydrocarbon gases like methane and propane are broken down, and the material deposited is the carbon matrix. The second class of processes involves infiltration of a preform with polymers or pitches, which are then converted to carbon by pyrolysis (heating in an inert atmo-sphere). After pyrolysis, the composite is heated to high temperatures to graphitize the matrix. To minimize porosity, the process is repeated untU a satisfactory density is achieved. This is called densification. Common matrix precursors are phenolic and furan resins, and pitches derived from coal tar and petroleum. 

Ceramic Matrix Composites (CMC) performed by a hybrid process is described in this paper. This process is based on (i) the chemical vapor deposition of carbon interphase on the fiber surface, (ii) the introduction of mineral powders inside the multidirectional continuous fiber preform and (Hi) the densification of the matrix by Spark Plasma Sintering (SPS). To prevent carbon fibers and interphase from oxidation in service, a self-healing matrix made of silicon nitride and titanium diboride was processed. A thermal treatment of 3 minutes at 1500 C allows to fully consolidate by SPS the composite without fiber degradation. The ceramic matrix composites obtained have an ultimate bending stress at room temperature around 300 MPa and show a self-healing behaviour in oxidizing conditions. 

Figure 31. Weight-loss of unidirectional carbon/carbon composites by isothermal oxidation in air, as affected by Zn2 207 inhibitor or by SiC coating (32,49) The composites were fabricated with 50 vol.-% high-modulus Modmor I fibers, coal-tar pitch as matrix precursor, four densification cycles, and final heat treatment to 1400°C.

After densification of the preform, the matrix pores become blocked, preventing any more carbon precursor from penetrating the fiber array and it is believed that at this stage that the composite can be cooled quickly, so that the matrix cracks due to a thermal mismatch, allowing the densification process to be continued by filling the cracks so formed. This process can be repeated some 4-6 times. 

Chemical vapor deposition (CVD) involves heating a fiber preform in a gaseous environment to deposit the matrix, present in the gaseous phase, on to the fiber. The term chemical vapor infiltration (CVI) is used to describe CVD densification occurring within the fiber preform as distinct from a simple surface deposition technique. Kohno describes the carbon infiltration of carbon-carbon composites [65]. 

Aveston eta/. [50] suggested that this is not necessarily always the case, and that the overall orientation efficiency would also depend on the response of the matrix to the local flexural stresses. If the matrix is sufficiently weak, it will crumble, and the flexural stresses will be effectively relaxed. They thought this to be the case in the carbon fibre reinforced cement tested in their work. Stucke and M umdar [52] applied this mechanism to account for the embrittlement of glass fibre reinforced cement and suggested that the densification of the ageing matrix around the fibres leads to a build-up of flexural stresses in the fibres in the cracked zone, which in turn results in premature failure. In the younger composite, the matrix interface is more porous and weaker, and crumbles before any significant flexural stress can develop in the fibres. 

Figure 8. Backscattered SEM morphologies of the polished cross-sections of the composite showing well densification inside and between fiber bundles (a) low and (b) high magnifications (inset in b detail of the distribution of carbon fiber, pyrolytic carbon, and matrix)...

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