J Vicens et al. Aerospace Science and Technology 7(2003)135-146 BN coating was close to stoichiometric BN with an excess of studied by Sun et al. [42]. Below 1408K, the constant carbon In BN(C)/LAS composites, Si and Al diffuse from creep rates were extremely low(10-9.s-)and at 1473K the matrix into the coating introducing the crystallization constant creep rates were an order magnitude higher. The of BN, and bore diffuses into the matrix in opposite di- 0/90 fibre-reinforced composites exhibited long creep- rection impeding the glass-ceramic conversion. The resid- strain recovery. From the microstructure investigations, it ual boron-bearing glassy regions degrade the high tempera- was concluded that the dual sic/bn coating provides an ture strength. In order to prevent interdiffusion phenomena, effective barrier to reaction and diffusion. Moreover the a Sic diffusion barrier(100-250 nm thick) was deposited n coating allows debonding to occur with an extensive CVD on the BN(C)coating. These studies performed on Si ibre pull-out of the fibres. Tensile fatigue experiments were Nicalon/LAS and Sic Nicalon/BMAS composites fabricated also conducted. The composites survived 10 cycles without with such BN/SiC dual interphase showed that such com- fracture up to 1473 K under a maximum stress slightly posites have significant potential properties for applications higher than the proportional limit stress of the matrix to-1473 K. They could withstand stress levels higher than More recently Widjaja et al. [49] found a creep-induced the proportional limit at high temperatures for long period residual stress strengthening mechanism in SiC Nicalon- This short review demonstrates the interest of the dual fibre/BMAS composites After creep experiment at 1373K, interphase concept: one layer(BN) coating as an oxidation the BMAs matrix can be put in compression by the elastic resistant and mechanical fuse and the other(SiC) as an recovery of the fibres if the load is removed at room oxidation resistant diffusion barrier. One problem which temperature. This increases the stress at which matrix ne Bn layer which is attribute g of the SiC layer over cracking begins. The state of residual stresses was supported ed to thermally induced by X-ray diffraction results. It was shown also that the stresses [40] creep-load transfer process did not embrittle the fibre/matrix interface because fibre pull-out behavior was maintained 5.2. Mechanical behaviors and microstructure of sic/BM One problem which demands further studies is the debond dual-coated Nicalon/BMAS composites ing of the SiC overlayer from the BN layer during matrix infiltration [40]. Different coatings were also investigated Sic/Bn dual-coated Nicalon-fibre-reinforced BMAS ma- thinner SiC/BN coatings, Si3N4/BN coatings and BN coat trix composites(0/90 cross plies) were studied by Sun et ings alone for example[43]- Si3N4 is not suitable because it al.[41]. The mechanical properties of the composites were dissolves in the BMAs matrix during composite fabrication evaluated by three point bending and tensile testing at both It was concluded that further improvements in the perfor- room temperature and high temperatures(up to 1573 K)in mance of Sic/Bn coating may be expected through an in- r The composite strength was excellent up to 1473 K, the crease of the SiC coating with a bn thickness of 350 nm ultimate strength at 1473 K and the elastic modulus were The introduction of roughness with Bn underlayers may found respectively to be 565 MPa and 69 GPa. Mechanical also increase adhesion of the Sic overlayer properties decrease significantly at 1573 K because of ma- trix softening. A degradation of the mechanical properties occurs after annealing in air for 500 h at 1473 K 6. Conclusion ush-out experiments showed a strongly fibre/matrix interface with a debonding energy (J/m)equal to 8. J/m Considerable interest has been placed upon and a frictional sliding stress t 139 MPa in the ceramized opment of glass and glass ceramic matrix compe state. This is much higher than values observed in classical intermediate-temperature applications(1473 K). This ex glass ceramic composites [31] plains the numerous varieties of glass-ceramic composites Interfaces were studied by TEM and observations were reinforced by SiC fibres which have been fabricated. De- correlated with mechanical properties. It was shown that a spite these high number of different types of composites, nanoscale silica/carbon sublayer was formed at the BN/Sic this class of composites displays very typical fibre-matrix Nicalon fibre interface during long-term exposure to oxy- interphase characteristics. a quite good description of the gen at high temperature. This sublayer appearing between complex nature of the fibre-matrix interfacial zone is now 1373 K and 1473 K was supposed to be responsible for the available. This has been possible using a combination of decrease in the fibre/matrix bonding strength at high tem- a high number of very specific analytical techniques up perature. But good interfacial properties are maintained at the nanometer scale on thin foils of composites or on ex 1373 K for long-term exposures tracted fibres from the matrix. Using these complementary techniques and by studying composites with different ma- 5.3. Bending creep behaviourof sic/BN coated trices, a mechanism for explaining the developed interfaces fibre/BMAS composites and the kinetic of formation has been proposed Among the different glass-ceramic matrix composites The flexural creep and fatigue behavior of the the sic/bn dual coated Sic Nicalon fibre reinforced BMas composites and the associated microstructure changes matrix composites emerges as very good candidates for low-144 J. Vicens et al. / Aerospace Science and Technology 7 (2003) 135–146 BN coating was close to stoichiometric BN with an excess of carbon. In BN(C)/LAS composites, Si and Al diffuse from the matrix into the coating introducing the crystallization of BN, and bore diffuses into the matrix in opposite di￾rection impeding the glass–ceramic conversion. The resid￾ual boron-bearing glassy regions degrade the high tempera￾ture strength. In order to prevent interdiffusion phenomena, a SiC diffusion barrier (100–250 nm thick) was deposited by CVD on the BN(C) coating. These studies performed on SiC Nicalon/LAS and SiC Nicalon/BMAS composites fabricated with such BN/SiC dual interphase showed that such com￾posites have significant potential properties for applications to ∼1473 K. They could withstand stress levels higher than the proportional limit at high temperatures for long period. This short review demonstrates the interest of the dual interphase concept: one layer (BN) coating as an oxidation resistant and mechanical fuse and the other (SiC) as an oxidation resistant diffusion barrier. One problem which needs more study is the debonding of the SiC layer over the BN layer which is attributed to thermally induced stresses [40]. 5.2. Mechanical behaviors and microstructure of SiC/BN dual-coated Nicalon/BMAS composites SiC/BN dual-coated Nicalon-fibre-reinforced BMAS ma￾trix composites (0/90◦ cross plies) were studied by Sun et al. [41]. The mechanical properties of the composites were evaluated by three point bending and tensile testing at both room temperature and high temperatures (up to 1573 K) in air. The composite strength was excellent up to 1473 K, the ultimate strength at 1473 K and the elastic modulus were found respectively to be 565 MPa and 69 GPa. Mechanical properties decrease significantly at 1573 K because of ma￾trix softening. A degradation of the mechanical properties occurs after annealing in air for 500 h at 1473 K. Push-out experiments showed a strongly fibre/matrix interface with a debonding energy (J/m2) equal to 8.65 J/m2 and a frictional sliding stress τ ∼ 139 MPa in the ceramized state. This is much higher than values observed in classical glass ceramic composites [31]. Interfaces were studied by TEM and observations were correlated with mechanical properties. It was shown that a nanoscale silica/carbon sublayer was formed at the BN/SiC Nicalon fibre interface during long-term exposure to oxy￾gen at high temperature. This sublayer appearing between 1373 K and 1473 K was supposed to be responsible for the decrease in the fibre/matrix bonding strength at high tem￾perature. But good interfacial properties are maintained at 1373 K for long-term exposures. 5.3. Bending creep behaviour of SiC/BN coated fibre/BMAS composites The flexural creep and fatigue behavior of the same composites and the associated microstructure changes were studied by Sun et al. [42]. Below 1408 K, the constant creep rates were extremely low (∼10−9·s−1) and at 1473 K constant creep rates were an order magnitude higher. The 0/90◦ fibre-reinforced composites exhibited long creep￾strain recovery. From the microstructure investigations, it was concluded that the dual SiC/BN coating provides an effective barrier to reaction and diffusion. Moreover the BN coating allows debonding to occur with an extensive fibre pull-out of the fibres. Tensile fatigue experiments were also conducted. The composites survived 105 cycles without fracture up to 1473 K under a maximum stress slightly higher than the proportional limit stress of the matrix. More recently Widjaja et al. [49] found a creep-induced residual stress strengthening mechanism in SiC Nicalon- fibre/BMAS composites. After creep experiment at 1373 K, the BMAS matrix can be put in compression by the elastic recovery of the fibres if the load is removed at room temperature. This increases the stress at which matrix cracking begins. The state of residual stresses was supported by X-ray diffraction results. It was shown also that the creep-load transfer process did not embrittle the fibre/matrix interface because fibre pull-out behavior was maintained. One problem which demands further studies is the debond￾ing of the SiC overlayer from the BN layer during matrix infiltration [40]. Different coatings were also investigated: thinner SiC/BN coatings, Si3N4/BN coatings and BN coat￾ings alone for example [43]. Si3N4 is not suitable because it dissolves in the BMAS matrix during composite fabrication. It was concluded that further improvements in the perfor￾mance of SiC/BN coating may be expected through an in￾crease of the SiC coating with a BN thickness of 350 nm. The introduction of roughness with BN underlayers may also increase adhesion of the SiC overlayer. 6. Conclusion Considerable interest has been placed upon the devel￾opment of glass and glass ceramic matrix composites for intermediate-temperature applications (∼1473 K). This ex￾plains the numerous varieties of glass–ceramic composites reinforced by SiC fibres which have been fabricated. De￾spite these high number of different types of composites, this class of composites displays very typical fibre-matrix interphase characteristics. A quite good description of the complex nature of the fibre-matrix interfacial zone is now available. This has been possible using a combination of a high number of very specific analytical techniques up to the nanometer scale on thin foils of composites or on ex￾tracted fibres from the matrix. Using these complementary techniques and by studying composites with different ma￾trices, a mechanism for explaining the developed interfaces and the kinetic of formation has been proposed. Among the different glass–ceramic matrix composites, the SiC/BN dual coated SiC Nicalon fibre reinforced BMAS matrix composites emerges as very good candidates for low-