3310 Journal of the American Ceramic Society--Zok Vol. 89. No. I Coating Debond Matrix crack Pullout Porous matrix Debond crack latrix crack Debonding/sliding 圈 Interface gap 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 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- 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 trium aluminum garnet eutectics), interfaces with monazite are sufficiently weak to allow debonding to occur when cracks ap- Nextel 720 Fibe 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(FigII. Developments in Fiber Coatings (1) Monazite Undoubtedly, the most significant development in fiber coatings has been the discovery that rare-earth phosphates such as La￾monazite bond weakly to other oxides.20–22 In addition to form￾ing 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 re￾fractory ceramics), thereby facilitating plastic deformation dur￾ing fiber–matrix sliding.23,24 Concurrent with the development of monazite, other mixed-oxide compounds (niobates, tung￾states, 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 ad￾vancements in the underpinning science and technology have been made by investigators at the U.S. Air Force Research Lab￾oratory 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 ef￾fects 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 li￾quid 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 architec￾ture. 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/yt￾trium aluminum garnet eutectics), interfaces with monazite are sufficiently weak to allow debonding to occur when cracks ap￾proach from within the monazite (Fig. 3),33 even when the re￾sidual 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 Nex￾telt 720 fibers after coating with either washed or unwashed rhabdop￾hane sols. (Adapted from Hay and Boakye29). 3310 Journal of the American Ceramic Society—Zok Vol. 89, No. 11