SiC/RBSN composites

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]... 

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]. 

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]. 

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]. 

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. 

Internal pores in SiC/RBSN composites are unavoidable and reduce their oxidation resistance and thermal conductivity. Functionally graded oxidation resistant surface coatings appear to avoid internal oxidation problems for unstressed conditions. 

R.T. Bhatt, Mechanical Properties of SiC/RBSN Composites , in Tailoring Multiphase and Composite Ceramics, (1986), p. 675 edited by R.E. Tressler, G.L. Messing, C.G. Pantano and R.E. Newnham, Plenum Press, New York. 

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

R.T. Bhatt, Tensile Properties and Microstmctural Characterization of Hi-Nicalon SiC/RBSN Composites, Ceramics International, 26, pp. 535—539, 2000. 

R.T. Bhatt, Tension, Compression, and Bend Properties for SiC/RBSN Composites, Proc. of the 8th ICCM Conference Vol. m, 23-A, 1991. 

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