D B. Marshall, J.B. Davis/ Current Opinion in Solid State and Materials Science 5(2001)283-28 Al2O3 fiber /AL2O3-LaPO4 matrix plumbite with intrinsically weak cleavage planes, analo- gous to mica. These approaches have not proven as effective as either the porous matrix approach or the weakly bonded interface materials described in the previ ous sections [34]. Nevertheless, some useful progress has been made in demonstrating composite properties, refining coating methods, and in defining the relative merits of porosity being concentrated at an interface or distributed 50英z 瞿 throughout the matrix Porous interphases are most commonly produced by 100 mixing fugitive carbon with the fiber coating material ●1100°c/1h; Room Temp Zirconia. rare-earth aluminates. and zircon have been 1100°c/24h; Room Temp investigated recently [45-48]. Composites have been ■1100c/1h;1025 C Test produced by hot pressing sapphire fibers with porous zirconia coatings in an alumina matrix [45, 46]. Althor the composites had relatively low strengths (-100-130 MPa for 0/90 cross-ply ) they were not degraded after ycling at 1200C (1300 cycles)and aging at tempera- Fig. 2. Effect of notch depth and heat-treatment conditions on net-section rengths measured at room temperature and at 1025C. Monazite/ tures that would severely degrade composites with poly umina matrix, Nextel"610 fibers. Specimen width: w=13 mm. crystalline oxide fibers (100 h at 1400C Several groups have produced composites with a thin continuous gap between the fibers and matrix by using a Several studies have shown that infiltration of Nextel fugitive carbon coating on the fibers [49-52]. Useful fiber tows and fabrics with solution precursors, followed insight into the merit of the approach is provided in Ref. by heat treatment to form monazite, often leads to strength [491, where comparisons with uncoated control specimens degradation [40-**44]. Many combinations of precu indicated that the gap was effective in enabling fiber chemistry, fiber composition, temperature, and time have pullout in composites of Nextel 720 fibers in a dense been assessed, with strength measurements being obtained calcium alumino-silicate(CAS)matrix, whereas the gap both from individual fibers with thin monazite coatings of had no effect composite with a porous matrix of 100 nm thickness(see Section 6 for coating method) mullite-alumina. Although the starting strengths were low [*42-**44 and from infiltrated fiber tows that form in this study (130 MPa), there was no degradation in the mini-composites [40, **41]. Although the detailed mecha- porous matrix composite after 500 h at 1150C and a smal nisms of degradation in most cases are not known, it is loss(-20%)in the CAs matrix composite after aging for clear that the mechanisms involve the precursor chemistry 500 h at 1000C rather than the monazite itself, since degradation does not The use of layered crystal structures presents several occur with coatings formed from aqueous slurries of challenges:(1) to form the desired phase at a rhabdophane particles (hydrated monazite) and in low temperature that the fibers are not degraded during minicomposites formed by infiltration with slurries of processing; (ii) to orient the weak basal planes of the monazite particles. Indeed, in these cases the strengths crystals parallel to the fiber surface; and (iii) to control the after heat treatment are often higher than strengths of roughness of the fracture in the coating to allow con- uncoated control specimens. Strength loss is also avoided strained sliding of the fibers. Progress has been made in in some cases when fiber tows are infiltrated with slurries lowering the formation temperature for refractory hexa- consisting of alumina particles in solution precursors described above, possibly a result of the alumina acting as techniques [53-57. However, although textured coatings internal buffer. Many observations are consistent with a have been grown on single-crystal YAG plates at 1200C, hypothesis that trapped gases remaining from the pre- there is a suggestion that on polycrystalline alumina fibers (Nextel 610) there may not be an adequate driving forc during heat treatment and react with the fiber surface for grain growth and texturing at this temperature(which the limit to which these fibers may be exposed during 5. Layered and porous interphases 6. Fiber coating methods Some of the earliest attempts to produce damage-toler nt oxide composites were based on forming weak fiber Although chemical and physical vapor methods have coatings,either by the introduction of porosity or by use of been used to deposit coatings of monazites and hexalumi- layered crystal structures such as B-alumina/magneto- nates [29], the deposition of stoichiometric multicom286 D.B. Marshall, J.B. Davis / Current Opinion in Solid State and Materials Science 5 (2001) 283–289 plumbites with intrinsically weak cleavage planes, analo￾gous to mica. These approaches have not proven as effective as either the porous matrix approach or the weakly bonded interface materials described in the previ￾ous sections [34]. Nevertheless, some useful progress has been made in demonstrating composite properties, refining coating methods, and in defining the relative merits of porosity being concentrated at an interface or distributed throughout the matrix. Porous interphases are most commonly produced by mixing fugitive carbon with the fiber coating material. Zirconia, rare-earth aluminates, and zircon have been investigated recently [*45–48]. Composites have been produced by hot pressing sapphire fibers with porous zirconia coatings in an alumina matrix [*45,46]. Although the composites had relatively low strengths (|100–130 MPa for 0/90 cross-ply), they were not degraded after cycling at 12008C (.1300 cycles) and aging at tempera￾Fig. 2. Effect of notch depth and heat-treatment conditions on net-section tures that would severely degrade composites with poly- strengths measured at room temperature and at 10258C. Monazite/ crystalline oxide fibers (100 h at 14008C). alumina matrix, NextelE 610 fibers. Specimen width: w513 mm. Several groups have produced composites with a thin continuous gap between the fibers and matrix by using a Several studies have shown that infiltration of Nextel fugitive carbon coating on the fibers [*49–52]. Useful fiber tows and fabrics with solution precursors, followed insight into the merit of the approach is provided in Ref. by heat treatment to form monazite, often leads to strength [*49], where comparisons with uncoated control specimens degradation [40–**44]. Many combinations of precursor indicated that the gap was effective in enabling fiber chemistry, fiber composition, temperature, and time have pullout in composites of Nextel 720 fibers in a dense been assessed, with strength measurements being obtained calcium alumino-silicate (CAS) matrix, whereas the gap both from individual fibers with thin monazite coatings of had no effect in a composite with a porous matrix of |100 nm thickness (see Section 6 for coating method) mullite-alumina. Although the starting strengths were low [**42–**44] and from infiltrated fiber tows that form in this study (|130 MPa), there was no degradation in the mini-composites [40,**41]. Although the detailed mecha- porous matrix composite after 500 h at 11508C and a small nisms of degradation in most cases are not known, it is loss (|20%) in the CAS matrix composite after aging for clear that the mechanisms involve the precursor chemistry 500 h at 10008C. rather than the monazite itself, since degradation does not The use of layered crystal structures presents several occur with coatings formed from aqueous slurries of challenges: (i) to form the desired phase at a sufficiently rhabdophane particles (hydrated monazite) and in low temperature that the fibers are not degraded during minicomposites formed by infiltration with slurries of processing; (ii) to orient the weak basal planes of the monazite particles. Indeed, in these cases the strengths crystals parallel to the fiber surface; and (iii) to control the after heat treatment are often higher than strengths of roughness of the fracture in the coating to allow con￾uncoated control specimens. Strength loss is also avoided strained sliding of the fibers. Progress has been made in in some cases when fiber tows are infiltrated with slurries lowering the formation temperature for refractory hexa￾consisting of alumina particles in solution precursors as luminates as low as 10008C using sol–gel and doping described above, possibly a result of the alumina acting as techniques [*53–57]. However, although textured coatings an internal buffer. Many observations are consistent with a have been grown on single-crystal YAG plates at 12008C, hypothesis that trapped gases remaining from the pre- there is a suggestion that on polycrystalline alumina fibers cursors can build up high pressures in dense coatings (Nextel 610) there may not be an adequate driving force during heat treatment and react with the fiber surface for grain growth and texturing at this temperature (which is [**44]. the limit to which these fibers may be exposed during processing) [*53]. 5. Layered and porous interphases 6. Fiber coating methods Some of the earliest attempts to produce damage-toler￾ant oxide composites were based on forming weak fiber Although chemical and physical vapor methods have coatings, either by the introduction of porosity or by use of been used to deposit coatings of monazites and hexalumi￾layered crystal structures such as b-alumina/magneto- nates [29], the deposition of stoichiometric multicom-