SiC/RBSN

Because RBSN employs gaseous reactants, the SiC/RBSN material tends to have higher levels of frequently interconnected, residual porosity than the SiC/Si3N4 composite fabricated by pressure assisted sintering methods. Interconnected residual porosity remains an important issue for two reasons oxidation and thermal conductivity. Internal oxidation can lead to internal stresses which may cause premature matrix cracking and fiber delamination. Thus, to avoid internal oxidation protective coatings may be necessary for these materials. 

The steps involved in the fabrication of SiC/RBSN composites are shown in Fig. 1. The details of the composite fabrication procedure were described in Reference 6. The starting materials for composite fabrication were SiC fiber mats and silicon powder cloth. The SiC fiber mats were prepared by winding the SiC fibers with desired spacing on a cylindrical dmm. The fiber spacing used depended on the desired fiber volume fraction in the composite. The fiber mats were coated with a fugitive polymer binder such as polymethylmethacralate (PMMA) to maintain the fiber spacing. 

FIGURE 2. Room temperature tensile stress-strain curves for 1-D and 2-D SiC/RBSN composites containing -24 vol% fibers, and unreinforced RBSN [8, 9]... 

FIGURE 3. Variation of room temperature tensile strength with tested volume for 1-D SiC/RBSN containing -24 vol% fibers and hot-pressed silicon nitride [8, 15]. 

The variation oftensile strength with temperature in airfor a 1-D SiC/RBSN composite is shown in Fig. 4 [16]. The elastic modulus and matrix cracking strength decreases slowly with increase in temperature, but the ultimate tensile strength remains relatively constant from 25 to 600°C. Beyond this temperature it decreases due to oxidation of the carbon coating on the SiC fibers and creep effects. 

II. 1.2b. 1.Thermal Expansion Thermal expansion of unidirectional SiC/RBSN composite is mainly a function of constituents volume fractions and measurement direction relative to the fiber, and is not affected by constituents porosity. Measurement of linear thermal expansion with temperature in nitrogen for the 1-D SiC/RBSN composites parallel and perpendicular to the fibers indicates a small amount of anisotropy (Fig. 5). This is attributed to small difference in thermal expansion coefficients of SiC fibers (4.2 x 10 ) and RBSN matrix (3.8 x 10 ) as well as anisotropic thermal expansion of carbon coating on SiC fibers. In the fiber direction, linear thermal expansion is controlled by the SiC fiber, and in the direction perpendicular to the fiber, it is controlled by the RBSN matrix. 

FIGURE 4. Variation of mechanical properties with temperature for 1-D SiC/RBSN composites containing 24 vol% fibers in tested air [16]. 

FIGURE 5. Linear thermal expansion curve during heating in nitrogen for 1-D SiC/RBSN composites containing -24 vol% fibers measured parallel and perpendicular to the fibers [17]. 

FIGURE 6. Thermal expansion and contraction curves for a 1-D SiC/RBSN composite containing 24 vol% fibers measured transverse to the fibers in (a) nitrogen, and (b) oxygen showing influence of internal oxidation [17]. 

Internal oxidation of SiC/RBSN is severe between 800 to 1100°C, and the depth of the oxidation damage zone is directly related to the pore size the smaller the pore size, the lower is the oxidation induced damage. In addition, internal oxidation is also found to generate tensile residual stresses which affect strength properties of SiC/RBSN. 

Thermal cycling of SiC/RBSN in nitrogen had no effect on mechanical properties, but in oxygen environment, composite properties degraded due to internal oxidation (discussed in Section 1.2d) and swelling as shown in Table 11. 

TABLE n. Room temperature tensile properties of unidirectional SiC/RBSN composites... 

The coefficient of linear thermal expansion, specific heat, thermal diffusivity, thermal conductivity for 1-D and 2-D SiC/RBSN at four temperatures in nitrogen measured parallel and perpendicular to the fibers are summarizedin Tables III and IV. In general, through-the thickness thermal conductivity value at room temperature for SiC/RBSN composites is low when compared with a value 7 W/m-k for the unreinforced RBSN or with a value of 30 W/m-k for the sintered silicon nitrides [13]. Both weak bonding between the SiC... 

TABLE III. Thermal property data for 1 -D SiC/RBSN composites in nitrogen [ 18,19]. 

FIGURE 8. Room temperature 4-point flexural strengths for 1-D SiC/RBSN composites containing 30 vol% fibers and monolithic RBSN (NC350) after quenching [20]. 

The thermal shock resistance ofunidirectionally reinforced SiC/RBSN composites was evaluated using the water quench method. Both room temperature flexural (Fig. 8) and tensile properties (Fig. 9) of 1-D SiC/RBSN composites were measured before and after quenching and compared with the flexural properties of quenched unreinforced RBSN under similar conditions. 

Creep resistance is of primary concern in rotating components of a turbine engine. High creep rates can lead to both excessive deformation and uncontrolled stresses. Creep resistance of fiber-reinforced ceramic matrix composites depend on relative creep rates of, stress-relaxation in, and load transfer between constituents. The tensile creep behavior of SiC/RBSN composites containing 24 vol% SiC monofilaments was studied in nitrogen at 1300 C at stress levels ranging from 90 to 150 MPa. Under the creep stress conditions the steady state creep rate ranged from 1.2 x 10 to 5.1 x 10 At stress levels below... 

TABLE V. Physical property data for Hi-Nicalon SiC/RBSN composites [11]. 

Room temperature physical property data for as-processed 1-D and 2-D SiC/RBSN tow composites are shown in Table V. The composites contained 24 vol% SiC fibers and 36 vol% porosity. 

FIGURE 13. Cross sections of a CVD SiC/glass former coated SiC/RBSN monofilament composite after 10 h burner rig testing in air at 1600°C showing stability of carbon core and coating. 

Oxidative stability of surface coated SiC/RBSN monofilament composites in burner rig testing prompted interest in utilization of this composite for uncooled components for small engine applications. Turbine vanes were machined from blanks of 1-D and 2-D SiC/RBSN composites, and surface coated with a layer of CVD SiC and glass former. Both uncoated and coated vanes were engine tested in at 1315°C for 10 h. The uncoated vanes showed severe damage, but the surface coated vanes survived engine tests with minimal damage [28]. 

Various studies indicate that strong, tough, creep and impact resistant SiC/RBSN and SiC/Si3N4 composites can be fabricated. The highlights of these studies are the following ... 

SiC/RBSN composites containing monofilaments are limited to simple shapes. However, SiC/RBSN shape capability can be improved by using textile processes (weaving, braiding) to form multi-fiber bundles or tows of ceramic fibers into 2-D and 3-D fiber architectures. 

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