Whisker-Reinforced Ceramic Matrix Composites

Abstract

Whisker-reinforced ceramic matrix composites have recently received a great deal of attention for applications as high temperature structural materials in, for example, advanced heat engines and high temperature energy conversion systems. For applications requiring mechanical reliability, the improvements that can be realized in fracture strength and fracture toughness are of great interest. Of particular importance for optimizing the mechanical reliability of these composites is the effect of the whisker/matrix interfacial characteristics on the strengthening and toughening mechanisms. Whisker reinforcements are primarily utilized to prevent catastrophic brittle failure by providing processes that dissipate energy during crack propagation. The degree of energy dissipation depends on the nature of the whisker/matrix interface, which can be controlled largely by the matrix chemistry, the whisker surface chemistry, and the processing parameters.It is generally believed that a strong interfacial bond results in a composite exhibiting brittle behavior. These composites usually have good fracture strengths but low fracture toughnesses. If the interfacial bond is weak, the composite will not fail in a catastrophic manner due to the activation of various energy dissipation processes. These latter composites tend to have high fracture toughnesses and low fracture strengths. Generally, the interface should be strong enough to transfer the load from the matrix to the whiskers, but weak enough to fail preferentially prior to failure. Thus, local damage occurs without catastrophic failure. It is therefore necessary to control the interfacial chemistry and bonding in order to optimize the overall mechanical performance of the composites.

Influence of SiC whisker morphology and nature of SiC/Al2O3 interface on thermomechanical properties of SiC reinforced Al2O3 composites

“…Becher and Wei, 13 Wei and Becher, 14 Becher et al, 15 and Homeny et al 16 have achieved fracture toughness values approaching 10 MPa m 0.5 and flexural strength values approaching 800 MPa. Furthermore, Homeny and Vaughn 17 have demonstrated that the fracture toughness could vary with whisker type from 4 to 9 MPa m 0.5 when utilising whiskers that were similar in all aspects, except for surface chemistry. They have associated the high fracture toughness with the presence of carbon and silicon oxycarbide phases on the whiskers surface.…”

Silicon Carbide Platelet/Silicon Carbide Composites

“…As a result, the weakened matrixreinforcement interface, which increases the yt1 term, alternatively promotes crack deflection around the perimeter of the platelets. This is evident in the increased crack branching around 9 platelets and larger pullout lengths in the alumina-coated SiCP -SiC composites, in contrast to composites containing uncoated platelets where crack paths resemble those in monolithic SiC with little evidence of crack deflection.…”

“…Reinforcement with SiC or C continuous fibers could possibly yield the highest high fracture resistance, but the use of such fibers is hindered by significant processing difficulties. Investigations of SiC particulate reinforcements in a number of ceramic matrix composites have shown that these can be effective in improving fracture toughness (6)(7)(8)(9)(10). While the results obtained with SiC whiskers in oxide matrix composites have been promising, potential health hazards in processing encourage directing attention to other forms of particulate reinforcement, in particular singlecrystal platelets (11)(12)(13)(14).…”

Characterization of interfaces in C fiber-reinforced laminated C–SiC matrix composites

“…The compohigh temperatures then becomes a concern [6][7][8][9][10][11]. Also, site systems that have been studied include whisker-rethe nature and extent of fiber surface roughness has been inforced glasses [1], whisker-reinforced ceramics [2][3][4][5], found to contribute to improved toughness [12][13][14], but, and, more notably, SiC fiber-reinforced ceramic matrices generally, an optimum interfacial coating thickness is [6][7][8][9]. The choice of fiber coating and matrix-reinbelieved to be more beneficial to toughness improvements forcement pairs is largely dictated by chemical comby enabling debonding within the coating [10].…”