30 N P Bansal Journal of the European Ceramic Society 29(2009)525-535 Table I Mechanical properties of unidirectional Hi-Nicalon/BN/SiC/BSAS composite annealed at various temperatures for 100 h in air; Vt=0.32(#Hi-NIC-BSAS-6-24-97) Annealing temperature(C) Density(g/cm) Weight change after annealing Ec(GPa) MPa) (%) Ou(MPa) 0.091 50 14 171 69 3.04 0.092 19 14 Deformed 43% a Measured at room temperature in three-point flexo dation of silicon carbide fibers, resulting in low-melting glassy the top debonded layer where damaged broken pieces of the phase which migrates to the sample surface. The bubble for- fibers are also present. producing amorphous silica and volatile CO and CO? gased ation may be related to the oxidation of Hi-Nicalon fibe 3.5. X-ray diffraction analys Stability of BN in moisture and oxygen containing atmo- spheres is an intrinsic problem in the long-term use of this KRD pattern recorded from surface of the as-fabricated material as fiber-matrix interface. 4 At 700C, the sensitivity CMC panel is given in Fig. 7. It shows the presence of only to moisture is controlled by the crystalline structure. Also, for- monoclinic celsian along with a trace amount of hexacelsian. mation of boric acid is minimal below 800C. However, B2O3 Additional diffraction peaks at d values of 0.504 nm(20=17.5) reacts readily with water to form HBO2. Therefore, in the pres- and 0.312 nm(20=28.5%)were detected in samples annealed at ence of moisture, B2O3 will undergo significant weight loss. The 900, 1000, and 1100C. The peak at d=0.312 nm was much product of hydrolysis is predominantly metaboric acid(HBO2) stronger than the one at d=0.504 nm in the 900C annealed with traces of orthoboric acid(H3 BO3). In dry air, BN shows specimen where as the reverse was true for the Cmc annealed at minimal oxidation up to 800C. At higher temperatures, B2O3 1000 and 1100C. The phase corresponding to these peaks could glass is formed on its surface. B2O3 has a low vapor pres- not be identified. a white shiny and glassy layer was formed on sure(<2 x 10-3 Torr)and volatilizes slowly at temperatures less the surface of the sample annealed at 1200C. The surface layer than 1100C. Most of the Bn coatings on fibers are deposited was found to contain monoclinic celsian and an unidentified at relatively low temperatures(1000C) and generally are amorphous phase from XRD analysis contaminated with carbo n and oxvgen impurities. Also, these BN coatings consist of randomly oriented microcrystalline or 3.6. Mechanical properties turbostratic grains and lack well ordered microstructures. The stability of BN towards moisture and its resistance towards air Apparent stress-strain curves recorded in three-point flex oxidation depend on the degree of its crystallinity. + BN with ure for the unidirectional BSAS matrix composite reinforced large d(002) spacing is much more reactive towards moisture with BN/SiC-coated Hi-Nicalon fibers before and after thermal than those close to the hexagonal BN structure probably because aging in air at various temperatures to 1100C, are presented of its less densely packed basal planes implying weaker atomic in Fig. 8. In earlier studies,20 hot pressed BSAS monolithic bonding. The higher the value of d(002) interlayer spacing, material showed flexural strength of 130 MPa, elastic modulus the less crystalline the material. BN with an interlayer spacing of 96 GPa, and failed in a brittle mode, as expected. In contrast, of d(002)=0.335 nm(3.35 A), which is close to the theoreti- the composites show initial linear elastic behavior followed by cal value of 0.333 nm, showed significantly improved stability an extended region of load carrying capability beyond the ini- towards moisture and air tial deviation from linearity. This indicates load transfer to the The matrix layers on the surface of CMC specimens fibers beyond the proportional limit indicating graceful failure annealed at 1100 and 1200C appear to have cracked and and true composite behavior. Various room temperature mechan delaminated, respectively as seen in the SEM micrographs ical properties of the composites, before and after thermal aging (Fig. 5)taken from the polished cross-sections. A large dif- in air at various temperatures, are summarized in Table 1. The ference in the coefficients of thermal expansion (CTE) of measured elastic modulus of the CMC is in good agreement Hi-Nicalon fiber(3.5 x 10-6oC-)and the oxide matrix with a value of 150 GPa, calculated from the rule-of-mixtures (5. x 10-6oC-)may be responsible for the observed (Ec=VmEm +ViEf where V is the volume fraction and the sub- cracking and delamination. This would provide an easy path scripts c, m, and f refer to the composite, matrix, and fiber, for the ingression of oxygen to the fiber bundles and accelerate respectively) using Em=96 GPa- and Er= 270 GPa. ,There the degradation of fibers from oxidation. No such cracking or is no effect of thermal annealing in arup to 1"C on the values delamination was observed in samples annealed at lower tem- of elastic modulus, yield stress, yield strain, and ultimate stress peratures. Higher magnification SEM micrographs(Fig. 6)from of the composites Mechanical behavior of the CMC annealed the 1200C annealed specimens show the presence of gas bub- at 1200C could not be recorded as this specimen had badly bles on its surface. Severe damage is also observed underneath deformed530 N.P. Bansal / Journal of the European Ceramic Society 29 (2009) 525–535 Table 1 Mechanical propertiesa of unidirectional Hi-Nicalon/BN/SiC/BSAS composite annealed at various temperatures for 100 h in air; Vf = 0.32 (#Hi-NIC-BSAS-6-24-97) Annealing temperature (◦C) Density (g/cm3) Weight change after annealing Ec (GPa) σy (MPa) εy (%) σu (MPa) – 3.09 ± 0.03 – 137 122 0.091 759 550 3.12 None 145 155 0.108 853 800 3.06 None 150 138 0.096 814 900 3.16 None 151 171 0.114 769 1000 3.04 None 146 134 0.092 819 1100 2.90 None 142 143 0.102 736 1200 Deformed +0.43% a Measured at room temperature in three-point flexure. dation of silicon carbide fibers, resulting in low-melting glassy phase which migrates to the sample surface. The bubble for￾mation may be related to the oxidation of Hi-Nicalon fibers producing amorphous silica and volatile CO and CO2 gases. Stability of BN in moisture and oxygen containing atmo￾spheres is an intrinsic problem in the long-term use of this material as fiber–matrix interface.24 At 700 ◦C, the sensitivity to moisture is controlled by the crystalline structure. Also, for￾mation of boric acid is minimal below 800 ◦C. However, B2O3 reacts readily with water to form HBO2. Therefore, in the pres￾ence of moisture, B2O3 will undergo significant weight loss. The product of hydrolysis is predominantly metaboric acid (HBO2) with traces of orthoboric acid (H3BO3). In dry air, BN shows minimal oxidation up to 800 ◦C. At higher temperatures, B2O3 glass is formed on its surface. B2O3 has a low vapor pres￾sure (<2 × 10−3 Torr) and volatilizes slowly at temperatures less than 1100 ◦C. Most of the BN coatings on fibers are deposited at relatively low temperatures (∼1000 ◦C) and generally are contaminated with carbon and oxygen impurities. Also, these BN coatings consist of randomly oriented microcrystalline or turbostratic grains and lack well ordered microstructures. The stability of BN towards moisture and its resistance towards air oxidation depend on the degree of its crystallinity.24 BN with large d (0 0 2) spacing is much more reactive towards moisture than those close to the hexagonal BN structure probably because of its less densely packed basal planes implying weaker atomic bonding. The higher the value of d (0 0 2) interlayer spacing, the less crystalline the material. BN with an interlayer spacing of d (0 0 2) = 0.335 nm (3.35 Å), which is close to the theoreti￾cal value of 0.333 nm, showed significantly improved stability towards moisture and air. The matrix layers on the surface of CMC specimens annealed at 1100 and 1200 ◦C appear to have cracked and delaminated, respectively as seen in the SEM micrographs (Fig. 5) taken from the polished cross-sections. A large dif￾ference in the coefficients of thermal expansion (CTE) of Hi-Nicalon fiber (∼3.5 × 10−6 ◦C−1) and the oxide matrix20 (∼5.28 × 10−6 ◦C−1) may be responsible for the observed cracking and delamination. This would provide an easy path for the ingression of oxygen to the fiber bundles and accelerate the degradation of fibers from oxidation. No such cracking or delamination was observed in samples annealed at lower tem￾peratures. Higher magnification SEM micrographs (Fig. 6) from the 1200 ◦C annealed specimens show the presence of gas bub￾bles on its surface. Severe damage is also observed underneath the top debonded layer where damaged broken pieces of the fibers are also present. 3.5. X-ray diffraction analysis XRD pattern recorded from surface of the as-fabricated CMC panel is given in Fig. 7. It shows the presence of only monoclinic celsian along with a trace amount of hexacelsian. Additional diffraction peaks at d values of 0.504 nm (2θ = 17.5◦) and 0.312 nm (2θ = 28.5◦) were detected in samples annealed at 900, 1000, and 1100 ◦C. The peak at d = 0.312 nm was much stronger than the one at d = 0.504 nm in the 900 ◦C annealed specimen where as the reverse was true for the CMC annealed at 1000 and 1100 ◦C. The phase corresponding to these peaks could not be identified. A white shiny and glassy layer was formed on the surface of the sample annealed at 1200 ◦C. The surface layer was found to contain monoclinic celsian and an unidentified amorphous phase from XRD analysis. 3.6. Mechanical properties Apparent stress–strain curves recorded in three-point flex￾ure for the unidirectional BSAS matrix composite reinforced with BN/SiC-coated Hi-Nicalon fibers, before and after thermal aging in air at various temperatures to 1100 ◦C, are presented in Fig. 8. In earlier studies11,20 hot pressed BSAS monolithic material showed flexural strength of 130 MPa, elastic modulus of 96 GPa, and failed in a brittle mode, as expected. In contrast, the composites show initial linear elastic behavior followed by an extended region of load carrying capability beyond the ini￾tial deviation from linearity. This indicates load transfer to the fibers beyond the proportional limit indicating graceful failure and true composite behavior. Various room temperature mechan￾ical properties of the composites, before and after thermal aging in air at various temperatures, are summarized in Table 1. The measured elastic modulus of the CMC is in good agreement with a value of 150 GPa, calculated from the rule-of-mixtures (Ec = VmEm + VfEf where V is the volume fraction and the sub￾scripts c, m, and f refer to the composite, matrix, and fiber, respectively) using Em = 96 GPa20 and Ef = 270 GPa.18,19 There is no effect of thermal annealing in air up to 1100 ◦C on the values of elastic modulus, yield stress, yield strain, and ultimate stress of the composites. Mechanical behavior of the CMC annealed at 1200 ◦C could not be recorded as this specimen had badly deformed