RBSN matrix

Thermal conductivity is also a function of matrix porosity. As the porosity increases, thermal conductivity decreases. Therefore, internal porosity may significantly reduce the thermal conductivity ofSiC/RBSN composites. The reduced modulus that intrinsically accompanies uniformly distributed porosity in RBSN matrix facilitates load transfer to reinforcements, and also lowers thermal stresses in properly designed composites. 

Where E is the elastic modulus, V is volume fraction of the constituent, and the subscripts f and m refer to the fiber and matrix, respectively. The elastic modulus of the RBSN matrix decreases with increasing porosity. To estimate the elastic modulus of RBSN with known porosity, the following equation may be used [12],... 

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 12. Room temperature tensile-stress-stram curves for 1-D and 2-D Hi-Nicalon SiC/RBSN composites containing - 24 vol% fibers, and unreinforced RBSN matrix [11]. 

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 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. 

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... 

R.T. Bhatt and JT). Kiser, Matrix Density Effects on Mechanical Properties of SiC/RBSN Composites, NASA-TM 103733, 1990. 

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