November 2002 Interface Design for Oxidation-Resistant Ceramic Composites 2609 CONTROL Uncoated) BN, Porous Oxide, Monazite 200 um BNS (b) (e) 20D Monazite Fig. 12.(a),(b), and (c) SEM micrographs of fractured surfaces of the control minicomposites at various magnifications showing complete lack of matrix crack deflection at the fiber/matrix interfaces. (d) BN-coated composite shows much better pullout than the controls (e) Composite with porous ZrO -SiO coating performs similar to the composite with BN coating. (f)Composite using monazite as interface coating shows some evidence of crack deflection but The sed on ultimate failure stren fract room-tem sticity exhibited by monazite under pushin tests. All the composites that used fiber conditions p results are promising; however, the inher- strengths significantly higher than the ently low modulus of the glassy matrix and its microcracked control that used uncoated fibers(Fig. 11). Moreover, porous- tructure is likely to result in very low load sharing by the matrix. oxide-coated fibers were similar to BN-coated fibers in strength This raises the question whether the matrix supports sufficient load aphy, and fiber pushin behavior. Monazite-coated fibers to permit a fair evaluation of the interface effectiveness. ielded about the same ultimate strengths as BN-coated fibers, but with much less fiber pullout. Further investigation using fiber ushin tests showed that monazite debonded readily but that (5) Interpretation of fractography frictional resistance was higher than that of bn- or rous- Evaluation of the effectiveness of interface control strategies oxide-coated fibers; this may have been partly related to the depends on qualitative analysis of fractography of composites(Fig
The evaluation was based on ultimate failure strengths, fractography, and fiber pushin tests. All the composites that used fiber coatings had ultimate strengths significantly higher than the control that used uncoated fibers (Fig. 11). Moreover, porousoxide-coated fibers were similar to BN-coated fibers in strength, fractography, and fiber pushin behavior. Monazite-coated fibers yielded about the same ultimate strengths as BN-coated fibers, but with much less fiber pullout. Further investigation using fiber pushin tests showed that monazite debonded readily but that frictional resistance was higher than that of BN- or porousoxide-coated fibers; this may have been partly related to the room-temperature plasticity exhibited by monazite under pushin conditions.27,108 These results are promising; however, the inherently low modulus of the glassy matrix and its microcracked structure is likely to result in very low load sharing by the matrix. This raises the question whether the matrix supports sufficient load to permit a fair evaluation of the interface effectiveness. (5) Interpretation of Fractography Evaluation of the effectiveness of interface control strategies depends on qualitative analysis of fractography of composites (Fig. Fig. 12. (a), (b), and (c) SEM micrographs of fractured surfaces of the control minicomposites at various magnifications showing complete lack of matrix crack deflection at the fiber/matrix interfaces. (d) BN-coated composite shows much better pullout than the controls. (e) Composite with porous ZrO2–SiO2 coating performs similar to the composite with BN coating. (f) Composite using monazite as interface coating shows some evidence of crack deflection but no significant pullout.107 November 2002 Interface Design for Oxidation-Resistant Ceramic Composites 2609
2610 Journal of the American Ceramic Socien-Kerans et aL. Vol. 85. No. 1I 12), which can be easily misinterpreted. For example, a"brushy" temperatures result in a more ordered structure, with decreasing fracture surface with long lengths of many fibers can be the result basal interplanar spacing, which has been reported to correlate of either fiber pullout or matrix disintegration. For all but perhaps with an increase of oxidation resistance. 6, 6, 32 BN coatings that ballistic applications, it seems evident that it is desirable for the are very highly crystallized are reported to have grains that are matrix to retain mechanical integrity and ability to transfer load randomly oriented, which can result in a less-effective crack- between fibers as late into the failure process as possible; there- deflecting layer than the layered basal planes of turbostratic BN. 9 fore, it is important to distinguish between the two cases. The Rebillat et al. 33 have reported an increase in interfacial shear presence of holes from which fibers have pulled out is definitive stress with highly crystalline Bn coatings, but here the basal evidence of pullout, although matching of fracture surfaces is planes are textured similar to that found for turbostratic BN and necessary to establish degree of pullout versus matrix disintegra- also result in greater resistance to oxidation. tion. It is also possible to have pullout but eventual disintegration A common impurity in CVD/CVI-deposited BN coatings is of the matrix at a point sufficiently far in the failure process as to oxygen, which results in a bn with larger basal interp e irrelevant spacing. Bn coatings with greater dissolved oxygen on Nicalon Another important distinction should be made between little and fibers in a Cvi SiC matrix have resulted in composites with lower no fiber pullout. For no pullout, the interface toughness is too high stress rupture lives in air than CMCs with low-oxygen-containing to deflect cracks. For little pullout, the interface deflects cracks, BN-fiber coatings. After initial formation of a thin Sio, layer but the debond length is very short, which implies a different the fiber surface, enhanced oxidation of the bn occurs because of problem, such as excessive interfacial friction. In such a case, internal oxidation, followed by borosilicate glass formation. On modification to decrease, for example, compressive residual stress the other hand, with low-oxygen-containing BN-fiber coatings nd debonding roughness friction may improve behavior. little oxidation takes place intemally at 950C for times of up to 400 h in dry air. If the amount of oxygen is restricted (low oxygen partial pressures), SiO, forms before either B,O3 or a borosilicate IV. Approaches to Oxidation-Resistant Interface Control glass at the fiber/coating interface. Although BN is stable within a crack-free composite, under loads that exceed the matrix The focus of this work is on approaches other than carbon and cracking stress, oxygen is not restricted from reaching internal BN. Nevertheless, most of the effort toward improved oxidation resistance has been devoted to bn coatings (in SiC-based com- nterfaces. At high temperatures where borosilicate formation posites); hence, that topic is briefly reviewed in the first part of this occurs rapidly, cracks are also rapidly sealed to minimize further section. Several approaches to inherently oxidation-resistant inter- oxygen ss. A larger problem exists at intermediate tempera- tures of 600%-900oC, where oxidation of bn can be rapid, but the face control have been pursued. Some of the approaches and a few formation of the borosilicate sealant is slow. 17, 50, 118 At lo of the compositions are intended to be compatible with SiC-based fibers but most of the work has been done on oxide fibers. The emperatures, BN can oxidize to volatile B, O,, rather than limitations and strengths of each concept are discussed, and stable liquid or solid oxidation products. If this occurs, the coating suggestions for future work are made is lost, resulting in possible fiber degradation and loss of load transfer between fiber and matrix. If a borosilicate glass does form ( BN Coatings because of the strong bond formed between fiber anda ks is lost in place of the bn coating, the ability to deflect crac In an attempt to increase the oxidation resistance of interfaces in The presence of water accelerates BN oxidation via the evolution SiC-fiber-reinforced composites, a-BN Died as a of the volatile species HBO,, H, BO,, and H, B, 0 16, 116, 132, 137, 140 logical alternative to carbon-fiber coatings 7-10, 14, C-BN In low-moisture environments(20 ppm H,O)BN is first oxidized has a hexagonal, layered crystal structure, similar to that of carbon, to B2O3, which then forms a borosilicate glass with the oxidation with the same lubricious nature. a-Bn commonly exhibits product of the SiC fiber Volatile HBO3 then forms via the reaction urbostratic defects, i.e., rotational stacking disorder of the basal of residual H,O with the borosilicate glass At high-moisture levels anes. Higher synthesis temperatures typically lead to a more >10% H,O), BN reacts directly with H,O to form the volatile ordered structure. BN has been shown to be thermodynamical molecules HBO2, H3 BO3, and H3 B3O6 directly because of their stable with SiC at the high temperatures of interest for Sic-based high thermodynamic stability. This volatilization leads to recession com Although bn is more oxidation resistant than of the fiber coating. Two methods have been explored to improve carbon, it produces a volatile oxide at temperatures <1000oC the durability of bn in H,O-containing atmospheres. The first The rate of oxidation is highly dependent on structure and impu method is to increase the order of bn by synthesizing and In this section, we summarize research that has focused on improving depositing at higher temperatures analogous to that used to the oxidation resistance and environmental stability of BN-fiber improve oxidation resistance in dry air. 4, 6, 119, 137, 140 The second coatings primarily for SiC-fiber-reinforced composites method is to dope the bn with silicon, which offers much greater Turbostratic BN-fiber coatings are usually applied using CVd stability in moist air by accelerating glass formation rather than ethods. -10, 10, 2, 7-22 although some coatings have been B,O, vaporization. 20, 26, 37 However, improved stability has applied via dip coating in liquid precursors. 25-12 Most CVD/CVI been limited to atmospheres with relatively low moisture contents work recently has been done using the lio ip cursors, and limited (20% H,O) while little improvement has been observed in atmospheres containing 90%H,O. Stability at higher pressures a sole-source precursor to BN. The formation of in situ BN e.g. aircraft ines operate at >30 atm (3 MPa)) is also after reaction under an NH, or a nitrogen atmosphere, respectively. have higher interfacial shear stresses. 42 The Nh, treatment of the Nextel 3 12 fiber did not form a discrete Other modifications to the BN-coating system have been made BN layer, but only enriched the amorphous surface layer in boro to either improve oxidation resistance or prevent reactions between d nitrogen, while depleting it of oxygen, to a depth of <5 the bn coating and matrix. among the earliest modifications was nm.In the case of the Sic fibers, the hexagonal-BN grains grew the incorporation of a Sic layer over the Bn coating, whose with their basal planes normal to the fiber surface. Although these mary purpose was to BN-coated Sic fibers displayed the same high tensile strengths coating to the matrix and te diffusion of any matrix species (3 MPa)as the original unreacted fibers, they have not yet been into the bn coating. 43-145 This combination pro- tested in a composite vides stability up to 1 100C during fast fracture and creep, with The oxidation resistance of turbostratic BN is highly dependent oxidation of fibers and bn coatings occurring only in the near- of crystallinity, crystallographic orientation urface regions. However, under cyclic loading and when the purities, all which are affected by the synthesis and/or matrix-cracking stress is exceeded, oxidation of the fibers and the ion conditions. In general, higher synthesis and deposition BN coating can occur in the interior of the composite as well
12), which can be easily misinterpreted. For example, a “brushy” fracture surface with long lengths of many fibers can be the result of either fiber pullout or matrix disintegration. For all but perhaps ballistic applications, it seems evident that it is desirable for the matrix to retain mechanical integrity and ability to transfer load between fibers as late into the failure process as possible; therefore, it is important to distinguish between the two cases. The presence of holes from which fibers have pulled out is definitive evidence of pullout, although matching of fracture surfaces is necessary to establish degree of pullout versus matrix disintegration. It is also possible to have pullout but eventual disintegration of the matrix at a point sufficiently far in the failure process as to be irrelevant. Another important distinction should be made between little and no fiber pullout. For no pullout, the interface toughness is too high to deflect cracks. For little pullout, the interface deflects cracks, but the debond length is very short, which implies a different problem, such as excessive interfacial friction. In such a case, modification to decrease, for example, compressive residual stress and debonding roughness friction may improve behavior. IV. Approaches to Oxidation-Resistant Interface Control The focus of this work is on approaches other than carbon and BN. Nevertheless, most of the effort toward improved oxidation resistance has been devoted to BN coatings (in SiC-based composites); hence, that topic is briefly reviewed in the first part of this section. Several approaches to inherently oxidation-resistant interface control have been pursued. Some of the approaches and a few of the compositions are intended to be compatible with SiC-based fibers, but most of the work has been done on oxide fibers. The limitations and strengths of each concept are discussed, and suggestions for future work are made. (1) BN Coatings In an attempt to increase the oxidation resistance of interfaces in SiC-fiber-reinforced composites, -BN has been studied as a logical alternative to carbon-fiber coatings.7–10,14,109–112 -BN has a hexagonal, layered crystal structure, similar to that of carbon, with the same lubricious nature.113 -BN commonly exhibits turbostratic defects, i.e., rotational stacking disorder of the basal planes. Higher synthesis temperatures typically lead to a more ordered structure.114 BN has been shown to be thermodynamically stable with SiC at the high temperatures of interest for SiC-based composites.115 Although BN is more oxidation resistant than carbon, it produces a volatile oxide at temperatures 1000°C.116 The rate of oxidation is highly dependent on structure and impurities. In this section, we summarize research that has focused on improving the oxidation resistance and environmental stability of BN-fiber coatings primarily for SiC-fiber-reinforced composites. Turbostratic BN-fiber coatings are usually applied using CVD methods,8–10,110,112,117–122 although some coatings have been applied via dip coating in liquid precursors.123–125 Most CVD/CVI processes use BCl3 or BF3 and NH3 as precursors, and limited work recently has been done using the liquid borazine (B3N3H6) as a sole-source precursor to BN.126 The formation of in situ BN coatings also has been attempted on Nextel 312TM aluminum borosilicate fibers127–129 and on boron-doped SiC fibers130,131 after reaction under an NH3 or a nitrogen atmosphere, respectively. The NH3 treatment of the Nextel 312 fiber did not form a discrete BN layer, but only enriched the amorphous surface layer in boron and nitrogen, while depleting it of oxygen, to a depth of 50 nm.129 In the case of the SiC fibers, the hexagonal-BN grains grew with their basal planes normal to the fiber surface. Although these BN-coated SiC fibers displayed the same high tensile strengths (3 MPa) as the original unreacted fibers, they have not yet been tested in a composite.131 The oxidation resistance of turbostratic BN is highly dependent on structure, degree of crystallinity, crystallographic orientation, and impurities, all which are affected by the synthesis and/or deposition conditions. In general, higher synthesis and deposition temperatures result in a more ordered structure, with decreasing basal interplanar spacing, which has been reported to correlate with an increase of oxidation resistance.16,116,132 BN coatings that are very highly crystallized are reported to have grains that are randomly oriented, which can result in a less-effective crackdeflecting layer than the layered basal planes of turbostratic BN.119 Rebillat et al.133 have reported an increase in interfacial shear stress with highly crystalline BN coatings, but here the basal planes are textured similar to that found for turbostratic BN and also result in greater resistance to oxidation. A common impurity in CVD/CVI-deposited BN coatings is oxygen, which results in a BN with larger basal interplanar spacing. BN coatings with greater dissolved oxygen on Nicalon fibers in a CVI SiC matrix have resulted in composites with lower stress rupture lives in air than CMCs with low-oxygen-containing BN-fiber coatings.17 After initial formation of a thin SiO2 layer on the fiber surface, enhanced oxidation of the BN occurs because of internal oxidation, followed by borosilicate glass formation. On the other hand, with low-oxygen-containing BN-fiber coatings, little oxidation takes place internally at 950°C for times of up to 400 h in dry air. If the amount of oxygen is restricted (low oxygen partial pressures), SiO2 forms before either B2O3 or a borosilicate glass at the fiber/coating interface.134 Although BN is stable within a crack-free composite, under loads that exceed the matrixcracking stress, oxygen is not restricted from reaching internal interfaces. At high temperatures where borosilicate formation occurs rapidly, cracks are also rapidly sealed to minimize further oxygen ingress. A larger problem exists at intermediate temperatures of 600°–900°C, where oxidation of BN can be rapid, but the formation of the borosilicate sealant is slow.17,50,118,135–139 At low temperatures, BN can oxidize to volatile B2O3, rather than forming stable liquid or solid oxidation products. If this occurs, the coating is lost, resulting in possible fiber degradation and loss of load transfer between fiber and matrix. If a borosilicate glass does form in place of the BN coating, the ability to deflect cracks is lost because of the strong bond formed between fiber and matrix. The presence of water accelerates BN oxidation via the evolution of the volatile species HBO2, H3BO3, and H3B3O6. 16,116,132,137,140 In low-moisture environments (20 ppm H2O) BN is first oxidized to B2O3, which then forms a borosilicate glass with the oxidation product of the SiC fiber. Volatile HBO3 then forms via the reaction of residual H2O with the borosilicate glass. At high-moisture levels (10% H2O), BN reacts directly with H2O to form the volatile molecules HBO2, H3BO3, and H3B3O6 directly because of their high thermodynamic stability. This volatilization leads to recession of the fiber coating. Two methods have been explored to improve the durability of BN in H2O-containing atmospheres. The first method is to increase the order of BN by synthesizing and depositing at higher temperatures analogous to that used to improve oxidation resistance in dry air.14,16,119,137,140 The second method is to dope the BN with silicon, which offers much greater stability in moist air by accelerating glass formation rather than B2O3 vaporization.120,126,137 However, improved stability has been limited to atmospheres with relatively low moisture contents (20% H2O), while little improvement has been observed in atmospheres containing 90% H2O.141 Stability at higher pressures (e.g., aircraft engines operate at 30 atm (3 MPa)) is also unknown. Although improvements in oxidation resistance have been demonstrated, silicon-doped BN also results in interfaces that have higher interfacial shear stresses.142 Other modifications to the BN-coating system have been made to either improve oxidation resistance or prevent reactions between the BN coating and matrix. Among the earliest modifications was the incorporation of a SiC layer over the BN coating, whose primary purpose was to prevent diffusion of boron from the coating to the matrix and to prevent diffusion of any matrix species into the BN coating.14,118,135,141,143–145 This combination provides stability up to 1100°C during fast fracture and creep, with oxidation of fibers and BN coatings occurring only in the nearsurface regions. However, under cyclic loading and when the matrix-cracking stress is exceeded, oxidation of the fibers and the BN coating can occur in the interior of the composite as well.144 2610 Journal of the American Ceramic Society—Kerans et al. Vol. 85, No. 11