Reaction sintering (RS), liquid phase sintering (LPS), PIP, melt infiltration (MI), and their hybrid processes are alternative options. PIP requires development of a near-stoichiometric polymer precursor. The other methods have issues in phase and uniformity control.

The NITE process is based on LPS,5,7,30 which has been improved owing to the progress in reinfor­cing fibers and availability of fine nano-SiC powders. A slurry of p-SiC nanopowders and additives is infil­trated into SiC fabrics and dried for making prepreg sheets. After the layup of the sheets, hot pressing is applied to make NITE-SiC/SiC. Small amounts of sintering aids (Al2O3, Y2O3, SiO2), high temperatures (1750-1800 °C), and pressures ranging from 15 to 20 MPa are required for matrix densification. The NITE process was claimed to present great advan­tages such as flexibility in the shape and size of the components.7 The successful development of NITE is due to appropriate fiber protection and the emer­gence of advanced SiC fibers such as Tyranno-SA3.

2.12.2 Properties of CVI SiC/SiC

Table 1 is a complete list of the mechanical and thermophysical properties of first generation 2D CVI SiC/SiC composites reinforced with SiC Nica — lon fibers of first generation.2, 1 An average strain-to — failure of 0.3% and a tensile strength of 200 MPa have been reported. Higher strengths and strains — to-failure appear in Tables 2 and 3, which give the available properties measured on other generations of SiC/SiC composites reinforced with advanced Hi-Nicalon or Hi-Nicalon type S fibers.3,32,33 The behavior of stronger Nicalon-reinforced SiC/SiC is discussed in a subsequent section. It can be noted that the strain-to-failure can reach 1%, and the ten­sile strength can exceed 300 MPa. As discussed in a subsequent section, a high strain-to-failure can be obtained when the performances of the reinforcing tows and the load transfers during loading have not been impaired as a result of the processing condi­tions. Ideally, the strain-to-failure should coincide with that of reinforcing tows, that is, about 0.8%.

Source: Bouillon, E.; Habarou, G.; Spriet, P.; et al. Characterization and nozzle test experience of a self sealing ceramic matrix composite for gas turbine applications. In Proceedings ofIGTI/ASME TURBO EXPO Land, Sea and Air 2002, Amsterdam, The Netherlands, June 3-6, 2002; Power Systems Composites Datasheet.

The strain-to-failure is an interesting characteristic for CMCs for several reasons. First of all, it is not sensitive to scale effects, so that it may be regarded as an intrinsic property and so various CMCs can be compared easily. Then, it reflects the degree of damage tolerance, whereas the strength reflects the load-carrying capacity. These characteristics need to be differentiated, as most components are usually subjected to strain-controlled loading conditions.

A fracture toughness of 30MPaVm was measured using conventional techniques designed for mono­lithic materials. It can be regarded as a high value when compared to monolithic SiC. However, it is

Source: Morscher, G.; Pujar, V. Int. J. Appl. Ceram. Technol. 2009, 6, 151-163.

worth pointing out that it represents the fracture toughness of an equivalent homogeneous material. As discussed in a subsequent section, critical stress intensity factor (KIC) is not an intrinsic property, and it is not an appropriate concept for long fiber- reinforced composites. Furthermore, besides the resistance to crack propagation, damage tolerance is an important property for CMCs. It cannot be char­acterized by fracture toughness. This situation is new when compared to homogeneous materials. Anyway, the fracture toughness KIc may be regarded as an index to compare materials. It cannot be used for design purposes for the aforementioned reasons.

Table 1 shows that CVI SiC/SiC retains its prop­erties at high temperatures. These properties can be enhanced by using advanced fibers. Durability will be addressed in a subsequent section.

Properties vary according to factors, including preform architecture, fiber type, matrix properties, fiber-matrix bond strength, loading conditions, etc. For instance, high tensile strengths (up to 400 MPa) were obtained with Hi-Nicalon™ SiC fibers,34 or with Nicalon fibers and rather strong interfaces.35 Further details on microstructure versus properties are discussed in subsequent sections. The mechanical behavior of 2D CVI SiC/SiC composites exhibits features that are related to composite microstructure. Thus, it deserves special attention because it differs significantly from that of the more conventional homogeneous materials. A clear understanding will be beneficial to a sound use of CVI SiC/SiC.

Tables 3 and 4 show that the ultimate strength and Young’s modulus tend to decrease under off-axis tensile conditions.36 It is worth pointing out that the strain-to-failure is an invariant. It is interesting to note that in 2D CVI SiC/SiC, the directions of the princi­pal stresses coincide with those of the fiber tows.