J.Am. Ceran.Soe,89p3309-3324(2006 DO:10.l111551-2916.2006.01342x c The American Ceramic Society urna Developments in Oxide Fiber Composites frank zoot Materials Department, University of California, Santa Barbara, California 93106 Prospects for revolutionary design of future power generation Notwithstanding this progress, the long-term durability of systems are contingent on the development of durable high-per- SiC-Sic composites continues to be plagued by two persistent formance ceramic composites. With recent discoveries in mate problems. ()In combustion environments that contain rials and manufacturing concepts, composites with all-oxide water vapor, recession occurs by volatilization of the silica constituents have emerged as leading candidates, especially for scale. Environmental barrier coatings must then be used components requiring a long service life in oxidizing environ- in order to achieve minimum durability goals in practical de- ments. Their insertion into engineering systems is imminent. The signs. (ii) Although presently of secondary concern, long-stand intent of this article is to present a synopsis of the current ing problems with oxidation embrittlement at intermediate understanding of oxide composites as well as to identify out- temperatures remain unresolved. These deficiencies have ues that require resolution for successful implemen- purred interest and developments in all-oxide cFCCs tation. Emphasis is directed toward material systems and Indeed, oxide systems have emerged as leading contenders for microstructural concepts that lead to high toughness and long applications requiring long service lives(>10 h) in oxidizin term durability. These include: the emergence of La monazite environments and related compounds as fiber-coating materials, the introduc High toughness in CFCCs is achieved by one of three tion of the porous-matrix concept as an alternative to fiber microstructural design paths(Fig. 1). All seek to promote un- coatings, and novel strategies for enabling damage toleranc orrelated fiber failure resulting in high fiber bundle strength while retaining long-term morphological stability. Additionall and energy dissipation during subsequent pullout The most materials and mechanics models that provide insights into ma- common approach uses a fiber coating that either forms a terial design, morphology evolution, and composite properties weak interface with the fibers or has an inherently low fracture toughness (Fig. 1(b)). It has been utilized extensively in SiC/SiC, C/SiC, and C/C fiber composites, principally through C and bn coatings Similar mechanisms can be enabled through the use of fine-scale matrix porosity, obviating the need for a fiber coating (Fig. I(c). To ensure durability, the matrix must be phase compatible with the fibers, because demand for high-temperature thermostructural materials of their intimate contact in the absence of a coating. Addition- ntinues to grow, fueled principally by power generation ystems for aircraft engines, land-based turbines, rockets, and ally, the pore structure must be retained at the targeted use ly, hypersonic missiles and fight vehicles. Typi temperature. The third approach uses fugitive coatings most omponents include combustors, nozzles, and thermal insula ones that are volatilized by oxidation after composite fabrica- tion. With their high melting point, strength, and toughness, tion, leaving a narrow gap at the fiber-matrix boundary continuous-fiber ceramic composites( CFCCs)offer the greatest (Fig. I(d)). The present article highlights the most signifi- ant developments in the implementation of these design strat of these system egies for oxide CFCCs Among commercially available oxide fibers, preference has Over the past 2 decades, the vast majority of CFCC research been given to two specific types: () NextelmM610-a polycrys has focused on Sic-SiC systems. The supporting manufacturing technology has reached a high level of sophistication and talline, small-diameter(10 um) alumina fiber, with high strength maturity. Large components are routinely manufactured to 1000%.C; and (ii) Nextel"720-a polycrystalline mul and have been tested in turbine engines and burner rigs. and moderately elevated temperatures(relative to 610), but with superior creep resistance and microstructural stability at high temperatures, to about 1200oC. 9 Although most activities in D. Green-contributing editor high-performance oxide CFCCs have focused on these, some concept demonstrations have used large diameter(>100 um) sapphire and eutectic alloys. The latter are not amenable to pt No. 22223. Received August 8, 2006: approved September 7. 2006. weaving and remain too expensive to find widespread use in the Force Office of Scientific Research (award number foreseeable future. Brief references to fiber types are included in to whom correspondence should be addressed. e-mail: zok(a enginecring. this article. However, the status of oxide fibers is beyond the scope of this paper. FeatureDevelopments in Oxide Fiber Composites Frank W. Zokw Materials Department, University of California, Santa Barbara, California 93106 Prospects for revolutionary design of future power generation systems are contingent on the development of durable high-per￾formance ceramic composites. With recent discoveries in mate￾rials and manufacturing concepts, composites with all-oxide constituents have emerged as leading candidates, especially for components requiring a long service life in oxidizing environ￾ments. Their insertion into engineering systems is imminent. The intent of this article is to present a synopsis of the current understanding of oxide composites as well as to identify out￾standing issues that require resolution for successful implemen￾tation. Emphasis is directed toward material systems and microstructural concepts that lead to high toughness and long￾term durability. These include: the emergence of La monazite and related compounds as fiber-coating materials, the introduc￾tion of the porous-matrix concept as an alternative to fiber coatings, and novel strategies for enabling damage tolerance while retaining long-term morphological stability. Additionally, materials and mechanics models that provide insights into ma￾terial design, morphology evolution, and composite properties are reviewed. I. Introduction THE demand for high-temperature thermostructural materials continues to grow, fueled principally by power generation systems for aircraft engines, land-based turbines, rockets, and, most recently, hypersonic missiles and flight vehicles. Typical components include combustors, nozzles, and thermal insula￾tion. With their high melting point, strength, and toughness, continuous-fiber ceramic composites (CFCCs) offer the greatest potential for enabling elevations in the operating temperatures of these systems. Over the past 2 decades, the vast majority of CFCC research has focused on SiC–SiC systems. The supporting manufacturing technology has reached a high level of sophistication and maturity. Large components are routinely manufactured and have been tested in turbine engines and burner rigs. Notwithstanding this progress, the long-term durability of SiC–SiC composites continues to be plagued by two persistent problems. (i) In combustion environments that contain water vapor, recession occurs by volatilization of the silica scale.1–3 Environmental barrier coatings must then be used in order to achieve minimum durability goals in practical de￾signs. (ii) Although presently of secondary concern, long-stand￾ing problems with oxidation embrittlement at intermediate temperatures remain unresolved. These deficiencies have spurred interest and developments in all-oxide CFCCs. Indeed, oxide systems have emerged as leading contenders for applications requiring long service lives (4104 h) in oxidizing environments. High toughness in CFCCs is achieved by one of three microstructural design paths (Fig. 1). All seek to promote un￾correlated fiber failure, resulting in high fiber bundle strength and energy dissipation during subsequent pullout. The most common approach uses a fiber coating that either forms a weak interface with the fibers or has an inherently low fracture toughness (Fig. 1(b)). It has been utilized extensively in SiC/SiC, C/SiC, and C/C fiber composites, principally through C and BN coatings.4 Similar mechanisms can be enabled through the use of fine-scale matrix porosity, obviating the need for a fiber coating (Fig. 1(c)).5–15 To ensure durability, the matrix must be phase compatible with the fibers, because of their intimate contact in the absence of a coating. Addition￾ally, the pore structure must be retained at the targeted use temperature. The third approach uses fugitive coatings: ones that are volatilized by oxidation after composite fabrica￾tion, leaving a narrow gap at the fiber–matrix boundary (Fig. 1(d)).16–18 The present article highlights the most signifi- cant developments in the implementation of these design strat￾egies for oxide CFCCs. Among commercially available oxide fibers, preference has been given to two specific types: (i) Nextelt 610—a polycrys￾talline, small-diameter (10 mm) alumina fiber, with high strength to 10001–11001C; and (ii) Nextelt 720—a polycrystalline mul￾lite/alumina fiber with a somewhat lower strength at ambient and moderately elevated temperatures (relative to 610), but with superior creep resistance and microstructural stability at high temperatures, to about 12001C.19 Although most activities in high-performance oxide CFCCs have focused on these, some concept demonstrations have used large diameter (4100 mm) sapphire and eutectic alloys. The latter are not amenable to weaving and remain too expensive to find widespread use in the foreseeable future. Brief references to fiber types are included in this article. However, the status of oxide fibers is beyond the scope of this paper. Feature D. Green—contributing editor This work was supported by the Air Force Office of Scientific Research (award number F49550-05-1-0134), monitored by Dr. B. L. Lee. w Author to whom correspondence should be addressed. e-mail: zok@engineering. ucsb.edu Manuscript No. 22223. Received August 8, 2006; approved September 7, 2006. Journal J. Am. Ceram. Soc., 89 [11] 3309–3324 (2006) DOI: 10.1111/j.1551-2916.2006.01342.x r 2006 The American Ceramic Society