Unidirectional ceramic matrix composites

As noted earlier, CVl is nsed primarily to form ceramic-fiber-reinforced ceramic matrix composites. The most common of these combinations is SiC fiber/SiC matrix composites. One commercially available product has a two-dimensional 0/90 layup of plain weave fabric and fiber volume fraction of about 40%. This same composite can be fabricated with unidirectional fibers and with 45° architectures. The most commonly used SiC fiber for the preforms is Nicalon , the mechanical properties for which were provided earlier in Section 5.4.2.7. A number of other carbide and nitride fibers are also available, including Si3N4, BN, and TiC. Preform geometries can be tailored to the application in order to maximize strength and toughness in the direction of maximnm stresses. The reactions used to form the matrix are similar to those used in CVD processes (cf. Section 7.2.4) and those described previously in Eq. (3.105). 

Kastritseas, C., Smith, P.A., Yeomans, J.A. (2004c), The onset of thermal shock damage in unidirectional fibre-reinforced ceramic matrix composites , Proceedings of the 5th International Conference in High Temperature Ceramic Matrix Composites (HTCMC-5), Seattle, WA, 235-240. 

Pryce, A.W., Smith, P.A. (1992), Behaviour of unidirectional and crossply ceramic matrix composites under quasi-static tensite loading , Journal of Materials Science, 27(10), 2695-2704. 

Figure 6.1 shows a typical stress-strain curve for a unidirectional SiCf/CAS composite loaded in uniaxial tension parallel to the fibers. The features of this curve are represenative of many ceramic matrix composites. In order to distinguish between the various damage states that a composite undergoes, it is convenient to divide the stress-strain curve into several sequential parts. 

O. Unal and N. P. Bansal, Temperature Dependency of Strength of a Unidirectional SiC Fiber-Reinforced (Ba, Sr)Al2Si208 Celsian Composite, in Advances in Ceramic Matrix Composites TV (J. P. Singh and N. P. Bansal, Eds.), Am. Ceram. Soc., Westerville, OH Ceram. Trans., 96, 135-147 (1999). 

The slice compression test has been applied to polymer-matrix composites even though it was developed to probe the interface in ceramic matrix composites [16]. A thin slice sample of unidirectional composite is produced with the cut surface perpendicular to the fiber axis. The surfaces are cut and polished to be parallel to each other and perpendicular to the fibers. The thin slice is loaded in compression in the fiber axis direction with two plates. One of the plates is made of a very hard material such as silicon nitride and the other of a soft material, e.g. pure aluminum which can deform as the fibers are compressed into it. The thickness of the slice must be controlled to allow the fibers to debond without failing in compression as well as allowing them to slide inside through the matrix. The depth of the fiber indentation into the plate can be related to the interfacial shear strength [17]. 

Peters PWM, Ludenbach G, Pleger R, Weiss R, Influence of matrix and interface on the mechanical properties of unidirectional carbon/carbon composites, J Eur Ceramic Soc, 13(6), 561-569, 1994. 

Unal and N. P. Bansal, "In-Plane and Interlaminar Shear Strength of a Unidirectional Hi-Nicalon Fiber-Reinforced Celsian Matrix Composite," Ceram. Int., 28[5] 527-40 (2002). 

Another application of dental composites is orthodontic archwires. One example is a unidirectional pultruded S2-glass-reinforced dimethacrylate thermoset resin. Depending on the yam of glass fiber used, the fiber volume fraction varied from 32 to 74 percent The strength and modulus were comparable with those of titanium wires. Orthodontic brackets were also made from composites with a polyethylene matrix reinforced with ceramic hydroxyapatite particles, resulting in isotropic properties and good adhesion to enamel. 

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