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. Feature
Developments 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-performance ceramic composites. With recent discoveries in materials and manufacturing concepts, composites with all-oxide constituents have emerged as leading candidates, especially for components requiring a long service life in oxidizing environments. 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 outstanding issues that require resolution for successful implementation. Emphasis is directed toward material systems and microstructural concepts that lead to high toughness and longterm durability. These include: the emergence of La monazite and related compounds as fiber-coating materials, the introduction 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 material 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 insulation. 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 designs. (ii) Although presently of secondary concern, long-standing 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 uncorrelated 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. Additionally, 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 fabrication, 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 strategies for oxide CFCCs. Among commercially available oxide fibers, preference has been given to two specific types: (i) Nextelt 610—a polycrystalline, small-diameter (10 mm) alumina fiber, with high strength to 10001–11001C; and (ii) Nextelt 720—a polycrystalline mullite/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
3310 Journal of the American Ceramic Society--Zok Vol. 89. No. I Coating Debond Matrⅸx crack Pullout Porous Debond crack Matrix crack Debonding/sliding 圈 Interface Fig 1. Microstructural concepts for enabling crack deflection in continuousfib Il. Developments in Fiber vancement in the underpinning science and technology have been made by investigators at the U.S. Air Force Research Lab- Undoubtedly, the most significant development in fiber coatings oratory and at Rockwell Scientific(formerly Rockwell Science has been the discovery that rare-earth phosphates such as La- scientific and engineering challenges have been identified and monazite bond weakly to other oxides. - In addition to forn ing low-toughness interfaces, monazite is non-toxic; insoluble in addressed. The key developments and outstanding issues follow water, acids, and bases does not decompose up to its melting Among the numerous recipes for monazite coatings, the most promising uses rhabdophane (LaPO4. 1/ 2H2O) sols derived point(>2000%C); and is not easily reducible below 1400 C In from La(NO3)3 and H3 PO4.- To mitigate the deleterious ef. addition, it has anomalously low hardness(relative to other re- fects of nitric acid (a by-product of the reaction that forms fractory ceramics), thereby facilitating plastic deformation dur- ing fiber-matrix sliding -.Concurrent with the development monazite), the sols are repeatedly washed in de-ionized water of monazite, other mixed-oxide compounds(niobates, tung- before application. Otherwise, significant reductions in fiber states, and vanadates) have been pursued as candidate coating strength are obtained following coating( Fig. 2) materials,23-27 although none exhibits a spectrum of propertie In one successful implementation, the coating is applied by rival that of monazite cible li- The monazite discovery proved pivotal in the resurgence of quid used to minimize bridging between coated fibers.31.32The xide CFCCs. During the past decade, the most substantive ad- fibers are then passed through an in-line furnace(typically at 900-1200C)and spooled. Both Nextel610 and Nextel720 fibers endure this process with negligible strength loss. In its present form, this coating method is restricted to individual tows. Thus, to make useful shapes, the fibers must be first coated in tow form and subsequently woven into the desired architec- ture. The draw back is that the weaving can damage the coating a consequence of the weak interfacial bond The thermochemical compatibility of monazite with a wide range of oxide fibers has been definitively demonstrated. For virtually all systems of interest(including Nextel610 and 720, sapphire, single-crystal mullite, and AlO3/ZrO, and Al,O3/yt rium aluminum garnet eutectics). interfaces with monazite are sufficiently weak to allow debonding to occur when cracks ap- Nextel 720 Fiber roach from within the monazite(Fig. 3),33 even when the re- sidual radial compressive stresses are large(Fig. 4).However, 1400 the resistance to subsequent sliding appears to be considerably Temperature(°C) higher than that of C-or BN-coated fibers in SiC-based CFCCs Sliding stresses of the former systems are typically in the range Fig. 2. Effects of heat-treatment temperature on the strength of Nex of 130-250 MPa, dependent on the thermal expansion coeffi tel"720 fibers after coating with either washed or unwashed rhabdo- cients of the three constituents as well as the radial misfit strain hane sols(Adapted from Hay and boakye2) roduced by surface roughness when sliding occurs(Fig
II. Developments in Fiber Coatings (1) Monazite Undoubtedly, the most significant development in fiber coatings has been the discovery that rare-earth phosphates such as Lamonazite bond weakly to other oxides.20–22 In addition to forming low-toughness interfaces, monazite is non-toxic; insoluble in water, acids, and bases; does not decompose up to its melting point (420001C); and is not easily reducible below 14001C. In addition, it has anomalously low hardness (relative to other refractory ceramics), thereby facilitating plastic deformation during fiber–matrix sliding.23,24 Concurrent with the development of monazite, other mixed-oxide compounds (niobates, tungstates, and vanadates) have been pursued as candidate coating materials,25–27 although none exhibits a spectrum of properties to rival that of monazite. The monazite discovery proved pivotal in the resurgence of oxide CFCCs. During the past decade, the most substantive advancements in the underpinning science and technology have been made by investigators at the U.S. Air Force Research Laboratory and at Rockwell Scientific (formerly Rockwell Science Center, where the monazite discovery was made). Numerous scientific and engineering challenges have been identified and addressed. The key developments and outstanding issues follow. Among the numerous recipes for monazite coatings, the most promising uses rhabdophane (LaPO4 1/2H2O) sols derived from La(NO3)3 and H3PO4. 28–30 To mitigate the deleterious effects of nitric acid (a by-product of the reaction that forms monazite), the sols are repeatedly washed in de-ionized water before application. Otherwise, significant reductions in fiber strength are obtained following coating (Fig. 2). In one successful implementation, the coating is applied by passing continuous tows through the sol, with an immiscible liquid used to minimize bridging between coated fibers.31,32 The fibers are then passed through an in-line furnace (typically at 9001–12001C) and spooled. Both Nextelt 610 and Nextelt 720 fibers endure this process with negligible strength loss.30 In its present form, this coating method is restricted to individual tows. Thus, to make useful shapes, the fibers must be first coated in tow form and subsequently woven into the desired architecture. The drawback is that the weaving can damage the coating: a consequence of the weak interfacial bond. The thermochemical compatibility of monazite with a wide range of oxide fibers has been definitively demonstrated. For virtually all systems of interest (including Nextelt 610 and 720, sapphire, single-crystal mullite, and Al2O3/ZrO2 and Al2O3/yttrium aluminum garnet eutectics), interfaces with monazite are sufficiently weak to allow debonding to occur when cracks approach from within the monazite (Fig. 3),33 even when the residual radial compressive stresses are large (Fig. 4).24 However, the resistance to subsequent sliding appears to be considerably higher than that of C- or BN-coated fibers in SiC-based CFCCs. Sliding stresses of the former systems are typically in the range of 130–250 MPa, dependent on the thermal expansion coeffi- cients of the three constituents as well as the radial misfit strain produced by surface roughness when sliding occurs (Fig. 5). Fig. 1. Microstructural concepts for enabling crack deflection in continuous-fiber ceramic composites. Fig. 2. Effects of heat-treatment temperature on the strength of Nextelt 720 fibers after coating with either washed or unwashed rhabdophane sols. (Adapted from Hay and Boakye29). 3310 Journal of the American Ceramic Society—Zok Vol. 89, No. 11
ovember 2006 Oxide Fiber Composites 3311 (a) 1 20 um um Fig 3. Fracture surfaces of an alumina/alumina continuous-fiber ceramic composite after 5 h of exposure at 1200.C: (a)uncoated fibers, (b)monazite- coated fibers. ( Courtesy Kristin Keller, AFRL. Reprinted with permission) Although the low hardness of monazite(5GPa2) facilitates plastic option. It can be deposited readily onto tows or woven fabric ccommodation of the misfit, the coating is less effective than C or by chemical vapor deposition or through pyrolysis of organic BN in mitigating these stresses In the latter, the low radial stiffness precursors and is readily oxidized at moderately high tempera- of the coatings allows for elastic accommodation of the misfit with tures. 16, 17 Although straightforward in principle, the approach nly moderate radial pressure and hence low sliding stress has two potential drawbacks: () matrix sintering treatments Three outstanding issues remain. (i Presently, there is no es- must be performed in an inert(non-oxidizing)environment, and tablished method for coating woven fiber cloths or preforms (i once the coating is oxidized, the fibers are unprotected from (distinct from tows). Such capability would circumvent the prob- the surrounding matrix and may be susceptible to bonding lems of weaving coated tows. (ii) The sliding stress of monazite ontact points coated fibers in dense ceramic matrices is considerably higher Preliminary feasibility studies have yielded encouraging re- than that in C- and BN-coated CFCCs, by as much as an order sults. When carbon-coated Nextel"720 fibers were embedded in of magnitude If excessively high, this may compromise compos- a dense calcium aluminosilicate matrix and the carbon subse ite toughness. Relative to SiC-SiC composites, more attention must be directed to thermal expansion mismatch and surfac quently oxidized, significant enhancements in fiber pullout were roughness effects in the oxide systems (ini) Although the issue of ture exposure was also improved. A more recent investigation fiber strength retention has been addressed, an assessment of the has also shown the benefits of combining fugitive coatings with efficacy of monazite coating on Nextel" 720 fibers(the highest orous matrices. For this purpose, composites were fabricated by infiltration of a mullite-20% alumina slurry into a carbon- demonstrated. It will likely require the use of a mullite-based coated Nextel" 720 preform, repeated impregnation and pyr- matrix, to minimize residual stress and allow fiber sliding subse- olysis of an alumina precursor, followed by oxidation of the uent to debonding, while retaining chemical compatibility with carbon. Preliminary results are presented in Fig. 6. With the the fibers. The large difference in thermal expansion coefficients fugitive coating, the composite exhibits significantly greater pull- of alumina (8x 10 K )and 720 fibers(6x 10 K)pr out as well as higher notched strength and fracture energy. The ludes the use of alumina-rich compositions as matrix choices mprovements are attributable to the combined effects of matrix porosity and the interfacial gap formed following carbon re- (2) Fugitive Coatings moval. The long-term stability and role of gap thickness in such Application of fugitive coatings to oxide CFCCs has received ystems is the focus of ongoing investigation. surprisingly little attention. Carbon appears to be the be 400 Al2O3 matrix LaPO4 coating LaPo Al2O3/ZrO2 Mullite Sapphire YAG/Al2O Sapphire Fig 4. (a) Microstructure and (b) fiber pullout in a dense LaPOa matrix 1200-1000-800-600-400-2000 reinforced with large-diameter sapphire fibers. Courtesy of Janet Radial misfit stress(MPa) Davis, Rockwell Scientific. Reprinted from J. Eur. Ceram. Soc., 19 J B. Davis, D B. Marshall, and P.E. D. Morgan,""Oxide Composites Fig. 5. Effects of radial misfit stress(from both thermal expansion mis- of AlO3 and LaPO4. pp. 2421-2426, 1999, with permission from atch and microstructural roughness) on the sliding stress of several Elsevier) monazite-coated fibers( Adapted from Davis et al. -
Although the low hardness of monazite (5 GPa20) facilitates plastic accommodation of the misfit, the coating is less effective than C or BN in mitigating these stresses. In the latter, the low radial stiffness of the coatings allows for elastic accommodation of the misfit with only moderate radial pressure and hence low sliding stress. Three outstanding issues remain. (i) Presently, there is no established method for coating woven fiber cloths or preforms (distinct from tows). Such capability would circumvent the problems of weaving coated tows. (ii) The sliding stress of monazitecoated fibers in dense ceramic matrices is considerably higher than that in C- and BN-coated CFCCs, by as much as an order of magnitude. If excessively high, this may compromise composite toughness. Relative to SiC–SiC composites, more attention must be directed to thermal expansion mismatch and surface roughness effects in the oxide systems. (iii) Although the issue of fiber strength retention has been addressed, an assessment of the efficacy of monazite coating on Nextelt 720 fibers (the highest temperature commercially available oxide fiber) has yet to be demonstrated. It will likely require the use of a mullite-based matrix, to minimize residual stress and allow fiber sliding subsequent to debonding, while retaining chemical compatibility with the fibers. The large difference in thermal expansion coefficients of alumina (B8 106 K1 ) and 720 fibers (6 106 K1 ) precludes the use of alumina-rich compositions as matrix choices. (2) Fugitive Coatings Application of fugitive coatings to oxide CFCCs has received surprisingly little attention. Carbon appears to be the best option. It can be deposited readily onto tows or woven fabric by chemical vapor deposition or through pyrolysis of organic precursors and is readily oxidized at moderately high temperatures.16,17 Although straightforward in principle, the approach has two potential drawbacks: (i) matrix sintering treatments must be performed in an inert (non-oxidizing) environment, and (ii) once the coating is oxidized, the fibers are unprotected from the surrounding matrix and may be susceptible to bonding at contact points. Preliminary feasibility studies have yielded encouraging results. When carbon-coated Nextelt 720 fibers were embedded in a dense calcium aluminosilicate matrix and the carbon subsequently oxidized, significant enhancements in fiber pullout were obtained.17 The retention in properties following high-temperature exposure was also improved. A more recent investigation has also shown the benefits of combining fugitive coatings with porous matrices.34 For this purpose, composites were fabricated by infiltration of a mullite–20% alumina slurry into a carboncoated Nextelt 720 preform, repeated impregnation and pyrolysis of an alumina precursor, followed by oxidation of the carbon.34 Preliminary results are presented in Fig. 6. With the fugitive coating, the composite exhibits significantly greater pullout as well as higher notched strength and fracture energy. The improvements are attributable to the combined effects of matrix porosity and the interfacial gap formed following carbon removal. The long-term stability and role of gap thickness in such systems is the focus of ongoing investigation. Fig. 3. Fracture surfaces of an alumina/alumina continuous-fiber ceramic composite after 5 h of exposure at 12001C: (a) uncoated fibers, (b) monazitecoated fibers.33 (Courtesy Kristin Keller, AFRL. Reprinted with permission). Fig. 4. (a) Microstructure and (b) fiber pullout in a dense LaPO4 matrix reinforced with large-diameter sapphire fibers.35 (Courtesy of Janet Davis, Rockwell Scientific. Reprinted from J. Eur. Ceram. Soc., 19, J.B. Davis, D.B. Marshall, and P.E.D. Morgan, ‘‘Oxide Composites of Al2O3 and LaPO4,’’ pp. 2421–2426, 1999, with permission from Elsevier). Fig. 5. Effects of radial misfit stress (from both thermal expansion mismatch and microstructural roughness) on the sliding stress of several monazite-coated fibers. (Adapted from Davis et al. 24). November 2006 Oxide Fiber Composites 3311
3312 Journal of the American Ceramic Society--Zok Fugitive C el61 No coat Al2O3-LaPOA 3 mm 2 Fig. 7. (a) Microstructure and (b) fiber pullout in a p aPO4/ ockwell Scientific. Reprinted from J. Eur. Ceram. 02 0.6 .0 19,JB.Davis, D.B. Marshall, and P.E. D Morgan, "Oxide Compos A Displacement(mm) Elsevier) some sense)all three principal toughening schemes: porous matri- Notched tensile behavior of a porous mullite-alumina matrix d monazite coati ngs. The initial pro- ed by Nextel 720 fibers, showing the effects cessing steps are identical to those used to produce porous-matrix ( Courtesy J H. Weaver) FCCs with an interfacial gap(described above). Following oxi dation of the carbon, a monazite precursor is repeatedly impre (3) Hybrid Concepts nated and pyrolyzed, thereby filling the interface gaps for One approach that obviates the problems associated with weav occupied by carbon as well as between the matrix particles ing of coated tows and simplifies processing involves a hybrid- (Fig. 8). If successful, this approach could provide an effective ization of the coated-fiber and the porous-matrix schemes Here, route to fabricating coated fiber composites with virtually any woven uncoated preforms are infiltrated with a monazite pre- architecture and configuration. A critical assessment of perform- cursor solution containing fine alumina particles. Following pyrolysis, a layer of monazite is formed on the fibers as well as composite exhibits extremely large pullout lengths(> 100R, (1) Microstructural Concll e between the alumina particles. The resulting matrix consists of a porous two-phase mixture of LaPOa and Al2O3( Fig. 7(a). The II. Matrix-Enabled damage tolerance When introduced in the mid-1990s, the porous-matrix co Ivity in typ cept was motivated principally by two factors: (i) the lack of a performance characteristics appear to be a consequence of ()the suitable suite of coatings for oxide fibers, and ( ii) the expectation monazite coating enabling crack deflection, and (i) the low of reduced manufacturing costs resulting from the absence of matrix stiffness reducing the radial constraints on the fiber oatings. Although the concept has proven to be an effective hence reducing the sliding resistance. Contrary to other reports lternative to fiber coatings for enabling damage tolerance, it of fiber strength degradation following exposure to acidic pre- as several inherent limitations: (i)CFCCs with two-dimension cursors, the reported combination appears to be innocuous. It al (2D) fiber architectures exhibit low thermal conductivit has been suggested that the alumina buffers the solution, strength, and fracture resistance in the through-thickness direc- making the fibers less susceptible to reaction with the precur ion;(ii)regardless of fiber architecture, these composites are non-hermetic;(iii) they are expected to have l compressive strengths than the dense matrix counterparts, because of the re- In addition to the combined porous-matrix/coated fiber scheme. a second hybrid duced constraint on fiber microbuckling: and(iv) they are more Mullite Monazite lextel 720 fibe n 10 Fig 8. Scanning electron micrographs of an oxide continuous fiber ceramic composite(using backscatter electron imaging). Monazite is present within the interface gap produced by arbon as well as between matrix particles. Monazite precursor provided by Janet Davis, Rockwell
(3) Hybrid Concepts One approach that obviates the problems associated with weaving of coated tows and simplifies processing involves a hybridization of the coated-fiber and the porous-matrix schemes. Here, woven uncoated preforms are infiltrated with a monazite precursor solution containing fine alumina particles.35 Following pyrolysis, a layer of monazite is formed on the fibers as well as between the alumina particles. The resulting matrix consists of a porous two-phase mixture of LaPO4 and Al2O3 (Fig. 7(a)). The composite exhibits extremely large pullout lengths ( 100R, with R being the fiber radius; Fig. 7(b)) and virtually no detectable notch sensitivity in typical specimen configurations. These performance characteristics appear to be a consequence of (i) the monazite coating enabling crack deflection, and (ii) the low matrix stiffness reducing the radial constraints on the fiber, hence reducing the sliding resistance. (Contrary to other reports of fiber strength degradation following exposure to acidic precursors, the reported combination appears to be innocuous. It has been suggested that the alumina buffers the solution, making the fibers less susceptible to reaction with the precursors and the decomposition products formed during precursor pyrolysis).29,35 In addition to the combined porous-matrix/coated fiber scheme, a second hybrid approach has emerged, using (in some sense) all three principal toughening schemes: porous matrices, fugitive coatings, and monazite coatings.34 The initial processing steps are identical to those used to produce porous-matrix CFCCs with an interfacial gap (described above). Following oxidation of the carbon, a monazite precursor is repeatedly impregnated and pyrolyzed, thereby filling the interface gaps formerly occupied by carbon as well as between the matrix particles (Fig. 8). If successful, this approach could provide an effective route to fabricating coated fiber composites with virtually any architecture and configuration. A critical assessment of performance and durability of this class of composite has yet to be made. III. Matrix-Enabled Damage Tolerance (1) Microstructural Concept When introduced in the mid-1990s,5–8 the porous-matrix concept was motivated principally by two factors: (i) the lack of a suitable suite of coatings for oxide fibers, and (ii) the expectation of reduced manufacturing costs resulting from the absence of coatings. Although the concept has proven to be an effective alternative to fiber coatings for enabling damage tolerance, it has several inherent limitations: (i) CFCCs with two-dimensional (2D) fiber architectures exhibit low thermal conductivity, strength, and fracture resistance in the through-thickness direction; (ii) regardless of fiber architecture, these composites are non-hermetic; (iii) they are expected to have lower compressive strengths than the dense matrix counterparts, because of the reduced constraint on fiber microbuckling; and (iv) they are more susceptible to wear.36 Fig. 6. Notched tensile behavior of a porous mullite–alumina matrix reinforced by Nextelt 720 fibers, showing the effects of a fugitive carbon coating. (Courtesy J. H. Weaver). Fig. 7. (a) Microstructure and (b) fiber pullout in a porous LaPO4/ Al2O3 matrix reinforced with Nextelt 610 alumina fibers. (Courtesy of Janet Davis, Rockwell Scientific. Reprinted from J. Eur. Ceram. Soc., 19, J.B. Davis, D.B. Marshall, and P.E.D. Morgan, ‘‘Oxide Composites of Al2O3 and LaPO4,’’ pp. 2421–2426, 1999, with permission from Elsevier). Fig. 8. Scanning electron micrographs of an oxide continuous-fiber ceramic composite (using backscatter electron imaging). Monazite is present within the interface gap produced by removal of the fugitive carbon as well as between matrix particles. (Monazite precursor provided by Janet Davis, Rockwell Scientific). 3312 Journal of the American Ceramic Society—Zok Vol. 89, No. 11
ovember 2006 Oxide Fiber Composites 3313 b) 100m (c) Fig 9. Damage and fracture mechanisms in a porousmatrix oxide Fig. 11. Minimal fiber pullout on the fracture surface of a mullite. ntinuous-fiber ceramic composite(a) Crack deflection and interface ased porous matrix continuous-fiber ceramic composite strengthened as infiltrated with an excessive amount of a precursor-derived alumina.(Courtes with epoxy while under load and then sectioned and polished. (b, c) M.A. Mattoni Uncorrelated fiber failure and pullout Material consists of Nextel720 fibers in an eight-harness satin weave and a mullite-alumina matrix. matrix, coated-fiber systems. That is, matrix cracks deflect along the fiber-matrix interface and fibers subsequently fail in an un- To ensure a morphologically stable pore structure, the matri- correlated manner, leading to pullout( Fig 9). Additionally, the ces typically consist of two dissimilar phases, distinguished by degree of notch sensitivity, characterized by open hole tension their sintering kinetics. The major phase is present as a contigu- tests, is comparable in the two classes of materials(Fig. 10). In ous 3D particle network. In turn, the network is bonded by a contrast, when the matrix is sintered or densified excessively, ei- less refractory ceramic or glass binder, in the form of either ther through processing or subsequent elevated temperature(in smaller sinterable particles or the product of precursor pyrolysis. service)exposure, embrittlement ensues. This is manifested in Particles in the main network dictate the long-term stability of planar fracture surfaces with minimal fiber pullout and signi the matrix against sintering, whereas particle junctions formed antly reduced toughness(Fig. Il) y the binder control the mechanical integrity of the matrix Additionally, the junctions at the fiber surface control the inter facial toughness (2) Debonding Mechanics When properly implemented, porous-matrix CFCCs exhibit fracture characteristics similar to those of conventional dense- twofold. Firstly, the bond between the matrix and the fibers is inherently weak. That is, the interface toughness, Ti. can be no greater than that of the matrix itself; for typical porosity levels 30 Notch Insensitive: ON/oo=1 nitude lower than that of the fibers, Tf, thereby ensuring a low. o6061A toughness interface. Secondly, because energy release rates scale ith elastic moduli. the red △1018stee leads to a reduction in the driving force for matrix cracks Oxide CFCCs To achieve high toughness in CFCCs, matrix cracks must deflect into the fiber/matrix interface rather than penetrate into 后08 Mullite- alumina he fibers. (A second condition-that interface sliding occur with SIC CFCCs only moderate resistance-must also be satisfied. ) The condi tions that satisfy this requirement are plotted in Fig. 12(a) Deflection is predicted when the toughness ratio, Ti/Tf is less than the energy release rate ratio, Ga/Gp, associated with defec- Glass PMCs tion and penetration. The latter is a function of the elastic mis- atch parameter 804 S Glass/Polyester △=(Er-Em) ch Sensitive: ON/oo=/ko=0 (Er +Em) 902 where E is the plane strain modulus, and the subscripts f and m denote fiber and matrix, respectively. For porous-matrix sys- tems, A takes on high values(>0.5); hence, the allowable tough ness ratio is also higl Hole Diameter, 2a(mm) Because of similarities in the matrix and fiber constituents in oxide CFCCs of present interest, the nature of bonding at the Fig 10. Open-hole tensile strength of metals, oxide, and Sic uIs-fiber ceramic composites(CFCCs), and polymer matrix composite fiber-matrix interface is similar to that between particles in the go is the unnotched tensile strength and ko is the elastic stress concen- natrix. Consequently, their toughnesses are expected to move in tration factor. All composites have two-dimensional fiber architectures tandem: that is, Ti=ol m where o is a non-dimensional par (either laminated or woven)and loads are applied parallel to one of the meter. As the packing density of matrix particles at the fiber fiber he normalized hole diameter is a/w=0.2 for all cases except surface is lower than that in the bulk, i<Im and hence o<I the oxide CFCC, wherein afw=1/3 For conservative design, o is taken to be 1
To ensure a morphologically stable pore structure, the matrices typically consist of two dissimilar phases, distinguished by their sintering kinetics. The major phase is present as a contiguous 3D particle network. In turn, the network is bonded by a less refractory ceramic or glass binder, in the form of either smaller sinterable particles or the product of precursor pyrolysis. Particles in the main network dictate the long-term stability of the matrix against sintering, whereas particle junctions formed by the binder control the mechanical integrity of the matrix. Additionally, the junctions at the fiber surface control the interfacial toughness. When properly implemented, porous-matrix CFCCs exhibit fracture characteristics similar to those of conventional densematrix, coated-fiber systems. That is, matrix cracks deflect along the fiber–matrix interface and fibers subsequently fail in an uncorrelated manner, leading to pullout (Fig. 9). Additionally, the degree of notch sensitivity, characterized by open hole tension tests, is comparable in the two classes of materials (Fig. 10).9 In contrast, when the matrix is sintered or densified excessively, either through processing or subsequent elevated temperature (inservice) exposure, embrittlement ensues. This is manifested in planar fracture surfaces with minimal fiber pullout and signifi- cantly reduced toughness (Fig. 11). (2) Debonding Mechanics The role of matrix porosity in enabling damage tolerance is twofold. Firstly, the bond between the matrix and the fibers is inherently weak. That is, the interface toughness, Gi, can be no greater than that of the matrix itself; for typical porosity levels (B30%), the matrix toughness, Gm, is about an order of magnitude lower than that of the fibers, Gf, thereby ensuring a lowtoughness interface. Secondly, because energy release rates scale with elastic moduli, the reduction in modulus due to porosity leads to a reduction in the driving force for matrix cracks. To achieve high toughness in CFCCs, matrix cracks must deflect into the fiber/matrix interface rather than penetrate into the fibers. (A second condition—that interface sliding occur with only moderate resistance—must also be satisfied.) The conditions that satisfy this requirement are plotted in Fig. 12(a).37 Deflection is predicted when the toughness ratio, Gi/Gf, is less than the energy release rate ratio, Gd/Gp, associated with deflection and penetration. The latter is a function of the elastic mismatch parameter, D Ef E ð Þ m Ef þ E ð Þ m (1) where E is the plane strain modulus, and the subscripts f and m denote fiber and matrix, respectively. For porous-matrix systems, D takes on high values (40.5); hence, the allowable toughness ratio is also high. Because of similarities in the matrix and fiber constituents in oxide CFCCs of present interest, the nature of bonding at the fiber–matrix interface is similar to that between particles in the matrix. Consequently, their toughnesses are expected to move in tandem: that is, Gi 5 oGm where o is a non-dimensional parameter. As the packing density of matrix particles at the fiber surface is lower than that in the bulk,38 GioGm and hence oo1. For conservative design, o is taken to be 1. Fig. 9. Damage and fracture mechanisms in a porous-matrix oxide continuous-fiber ceramic composite. (a) Crack deflection and interface debonding in a notched bend specimen. The specimen was infiltrated with epoxy while under load and then sectioned and polished. (b, c) Uncorrelated fiber failure and pullout. Material consists of Nextelt 720 fibers in an eight-harness satin weave and a mullite–alumina matrix. Fig. 10. Open-hole tensile strength of metals, oxide, and SiC continuous-fiber ceramic composites (CFCCs), and polymer matrix composites. so is the unnotched tensile strength and ks is the elastic stress concentration factor. All composites have two-dimensional fiber architectures (either laminated or woven) and loads are applied parallel to one of the fiber axes. The normalized hole diameter is a/w 5 0.2 for all cases except the oxide CFCC, wherein a/w 5 1/3. Fig. 11. Minimal fiber pullout on the fracture surface of a mullitebased porous matrix continuous-fiber ceramic composite strengthened with an excessive amount of a precursor-derived alumina. (Courtesy M. A. Mattoni). November 2006 Oxide Fiber Composites 3313