452 K.K. Chawla Journal of the European Ceramic Sociery 28(2008)447-453 Coatings such as bn alone or sic/bn double coati on effectively for mullite fiber/mullite matrix composites in I um that they impart damage tolerant characteristics to mullite fiber/mullite matrix composites. However, both BN and sic/BN BN coatings are nonoxides, and thus are susceptible to oxidation at high temperatures in air. Of course, SiC is more resistant to oxi dation than BN. What this points to is the need for oxide analogs of BN that would function as interphase materials at high temper atures. Beta-aluminas or micas are possible candidate materials. Layered oxides having B-alumina and magnetoplumbite struc- tures should be explored as possible easy cleavage coatings in mullite-based composites 6. Conclusions Chemical interactions between oxides are often severe such that selection of possible mullite-based composites or oxide fiber/oxide matrix composites, in general, is limited Even in the absence of chemical bonding a strong mechanical bond component may be present. This originates from radial M compressive stress due to thermal expansion mismatch and/or the surface roughness of interface. Coatings such as bn alone or sicbn double coating func Fig 8. Fracture surface of mullite fiber, Nextel 550(F)with double coating of tion effectively for mullite fiber/mullite matrix composites i SiC/BNinamullite matrix(M). The interface between BN and SiC is highlighted that they provide a nonbrittle fracture and increased work of with a dotted line. Note the sliding occurred along the fiber/BN coating. The fracture at room temperature SiC coating remained bonded to the mullite matrix Need oxide analogs of bn that would function as inter phase materials at high temperatures. Beta-aluminas or micas are possible candidate materials. Layered oxides having B- higher work of fracture in the coated fiber composites than in the alumina and magnetoplumbite structures should be explored uncoated fiber composite. The BN/SiC double coating system as possible easy cleavage coatings in mullite-based compos consists of 0.08 um BN and0. 16 um SiC-layers. In this case BN is rather turbostractic, and does not undergo any microstruc- tural change up to 1300C. An interesting high magnification Acknowledgments micrograph of the fracture surface of mullite fiber, Nextel 550 with double coating of SiC/BN in a mullite matrix(M)is The work presented in this paper is summary of work done shown in Fig 8. The interface between BN and SiC is high- over more than a decade and involves contributions from many lighted with a dotted line. Note the sliding occurred along the students, Post-Docs, and colleagues. In particular, I wish to fiber/BN coating. The SiC coating remained bonded to the mul- acknowledge the important part played by Professor Dr. H lite matrix. The outer thin SiC layer of the double coating system Schneider during a period of very fertile collaboration. I wish to improves the oxidation resistance of Bn up to 1200-1300C in thank the following agencies that sponsored parts of this work spite of a partial oxidation of SiC to SiO A word of caution is in order here. The application of a coat- US Office of Naval Research ing on fiber can affect its strength. If the thermal mismatch can US Department of Energy, Office of Transportation Tech- result in a residual tensile strength in the fiber, it will weaker nologies, through the High Temperature Materials Laboratory the fiber. The tensile strength of Nextel 480 fiber increased User Program and HTML Faculty Fellowship Program Oak with increasing BN coating thickness until 0. 2 um For coatings Ridge National Lab greater than 0.2 um in thickness, the fiber strength decreased. German Aerospace Research Establishment(DLR) There are two factors affecting the strength of the coated fiber North Atlantic Treaty Organization(NATO) simultaneously. One is the smooth boron nitride coa heals the surface defects of such fiber and thus, contributes to References rength enhancement. The second one is the volume fraction of the coating material which is a weak material. When coating 1. Chawla, KK, Coffin, C and Xu, Z.R., Int Mater. Rev, 2000, 45, 165 thickness is below 0.2 um, the first factor plays a dominant role 2. Schneider, H. and Komarneni, S,ed, Mullite. Wiley-VCH, Weinheim in the strength enhancement. When coating thickness is above 3. Chawla, K.K., Ceramic Matrix Composites(2nded. ) Kluwer, 0.2 um, the second factor becomes dominant to reduce the fiber 4. Cook, J and Gordon, J E. Proc R Soc. Lond., 1964, A228. strength 5. He, M -Y and Hutchinson, J w, Int J Solids Struct, 1989. 25. 1053-452 K.K. Chawla / Journal of the European Ceramic Society 28 (2008) 447–453 Fig. 8. Fracture surface of mullite fiber, Nextel 550 (F) with double coating of SiC/BN in a mullite matrix (M). The interface between BN and SiC is highlighted with a dotted line. Note the sliding occurred along the fiber/BN coating. The SiC coating remained bonded to the mullite matrix. higher work of fracture in the coated fiber composites than in the uncoated fiber composite. The BN/SiC double coating system consists of 0.08m BN and 0.16m SiC-layers. In this case BN is rather turbostractic,11 and does not undergo any microstruc￾tural change up to 1300 ◦C. An interesting high magnification micrograph of the fracture surface of mullite fiber, Nextel 550 (F) with double coating of SiC/BN in a mullite matrix (M) is shown in Fig. 8. The interface between BN and SiC is high￾lighted with a dotted line. Note the sliding occurred along the fiber/BN coating. The SiC coating remained bonded to the mul￾lite matrix. The outer thin SiC layer of the double coating system improves the oxidation resistance of BN up to 1200–1300 ◦C in spite of a partial oxidation of SiC to SiO2. A word of caution is in order here. The application of a coat￾ing on fiber can affect its strength. If the thermal mismatch can result in a residual tensile strength in the fiber, it will weaken the fiber. The tensile strength of Nextel 480 fiber increased with increasing BN coating thickness until 0.2m. For coatings greater than 0.2m in thickness, the fiber strength decreased. There are two factors affecting the strength of the coated fiber simultaneously. One is the smooth boron nitride coating which heals the surface defects of such fiber, and thus, contributes to strength enhancement. The second one is the volume fraction of the coating material which is a weak material. When coating thickness is below 0.2 m, the first factor plays a dominant role in the strength enhancement. When coating thickness is above 0.2m, the second factor becomes dominant to reduce the fiber strength. Coatings such as BN alone or SiC/BN double coating func￾tion effectively for mullite fiber/mullite matrix composites in that they impart damage tolerant characteristics to mullite fiber/mullite matrix composites. However, both BN and SiC/BN coatings are nonoxides, and thus are susceptible to oxidation at high temperatures in air. Of course, SiC is more resistant to oxi￾dation than BN. What this points to is the need for oxide analogs of BN that would function as interphase materials at high temper￾atures. Beta-aluminas or micas are possible candidate materials. Layered oxides having -alumina and magnetoplumbite struc￾tures should be explored as possible easy cleavage coatings in mullite-based composites. 6. Conclusions • Chemical interactions between oxides are often severe such that selection of possible mullite-based composites or oxide fiber/oxide matrix composites, in general, is limited. • Even in the absence of chemical bonding, a strong mechanical bond component may be present. This originates from radial compressive stress due to thermal expansion mismatch and/or the surface roughness of interface. • Coatings such as BN alone or SiC/BN double coating func￾tion effectively for mullite fiber/mullite matrix composites in that they provide a nonbrittle fracture and increased work of fracture at room temperature. • Need oxide analogs of BN that would function as inter￾phase materials at high temperatures. Beta-aluminas or micas are possible candidate materials. Layered oxides having - alumina and magnetoplumbite structures should be explored as possible easy cleavage coatings in mullite-based compos￾ites. Acknowledgments The work presented in this paper is summary of work done over more than a decade and involves contributions from many students, Post-Docs, and colleagues. In particular, I wish to acknowledge the important part played by Professor Dr. H. Schneider during a period of very fertile collaboration. I wish to thank the following agencies that sponsored parts of this work: • US Office of Naval Research. • US Department of Energy, Office of Transportation Tech￾nologies, through the High Temperature Materials Laboratory User Program and HTML Faculty Fellowship Program Oak Ridge National Lab. • German Aerospace Research Establishment (DLR) • North Atlantic Treaty Organization (NATO). References 1. Chawla, K. K., Coffin, C. and Xu, Z. R., Int. Mater. Rev., 2000, 45, 165. 2. Schneider, H. and Komarneni, S., ed., Mullite. Wiley-VCH, Weinheim, 2005. 3. Chawla, K. K.,Ceramic Matrix Composites(2nd ed.). Kluwer, Boston, 1998. 4. Cook, J. and Gordon, J. E., Proc. R. Soc. Lond., 1964, A228, 508. 5. He, M.-Y. and Hutchinson, J. W., Int. J. Solids Struct., 1989, 25, 1053–1067.