November 2002 Interface Design for Oxidation-Resistant Ceramic Compe CONTROL ( Uncoated) BN. Porous Oxide Monazite 2001u Monazite 501um 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 no significant pullout, u7 The evaluation was based on ultimate failure phy, and fiber pushin tests. All the composi ontrol that used uncoated fibers(Fig. 11). Moreover, xide- coated fibers were similar to BN-coated fibers in strength likely to result in very low load the question whether the matrix fractography, and fiber pushin behavior. Monazite-coated fibers to permit a fair evaluation of the interface 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 (5) Interpretation of Fractography frictional resistance was higher than that of BN- or porous oxide-coated fibers: this may have been partly related to the
Journal of the American Ceramic Sociery-Kerans er Vol, 85. No. II 2), which can be casily misinterpreted. For example 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 correlat ith an increase of oxidation resistance. D, 6.13 BN coatings that allistic 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 of the matrix at a point sufficiently far in the failure process as oxygen, which results in a Bn with larger basal pacing.bN coatings with greater dissolved oxygen 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 on 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 odification to decrease, for example, compressive residual stress the other hand, with low-oxygen-containing BN-fiber coatings and debonding roughness friction may improve behavior 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 artial pressures), SiO, forms before either B,O, or a borosilicate IV. Approaches to Oxidation-Resistant Interface Control glass at the fiber/coating interface. Although BN is stable within The focus of this work is on approaches other than carbon and a crack-free composite, under loads that exceed the matrix BN. Nevertheless, most of the effort toward improved oxidation cracking stress, oxygen is not restricted from reaching internal interfaces, At high temperatures where borosilicate formati resistance has been devoted to BN coatings (in Sic-based com- occurs raDd.A larger problem exists at intermediate tempera- cracks are also rapidly sealed to minimize further face control have been pursued. Some of the approaches and a few tures of 600-900C, where oxidation of BN can be rapid, but the fibers, but most of the work has been done on oxide fibers. The temperatures, BN can oxidize to volatile B.O, rather than forming limitations and strengths of each concept are discussed, and is lost, resulting in possible fiber degradation and loss of load suggestions for future work are made. transfer between fiber and matrix. If a borosilicate glass does form in place of the BN coating, the ability to deflect cracks is lost (1 BN Coatings because of the strong bond formed between fiber and matrix. In an attempt to increase the oxidation resistance of interfaces in SiC-fiber-reinforced composites, c-Bn has udied as a of the volatile species HBO2. H3 BO, and H, B 06. 16. 1 16.132.137100 logical alternative to carbon-fiber coatings, 7-10, 14, I0-112 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 B.O,, which then forms a borosilicate glass with the oxidation with the same lubricious nature. 3 a-BN commonly exhibits product of the SiC fiber Volatile HBO, then forms via the reaction turbostratic defects, i. e, rotational stacking disorder of the basal of residual H,O with the borosilicate glass. At high-moisture levels planes. 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 HBO,, H, BO,, and H, B, O directly because of their stable with SiC at the high temperatures of interest for SiC-based high thermodynamic stability. This volatilization leads to recessior composites. Although BN is more oxidation resistant the f the fiber coating. Two methods have been explored to improve carbon, it produces a volatile oxide at temperatures <1000C. the durability of BN in H,O-containing atmospheres. The first The rate of oxidation is highly dependent on structure and impurities method is to increase the order of bn by synthesizing and In this section, we summarize research that has focused on improving siting at higher temperatures an s to that used to the oxidation resistance and environmental stability of BN-fiber improve oxidation resistance in dry air. 4. 16, I 19. 137. 40The 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 applied via dip coating in liquid precursors. 23-125 gs have been B, 0, vaporization. 20, 126, 1.37 However, improved stability has been limited to atmospheres with relatively low moisture contents processes use BCl, or BF, and NH, as precursors, and limited (20% H,O), while little improvement has been observed in work recently has been done using the liquid borazine(B,N3H6)as atmospheres containing 90%H, 0. 4 Stability at higher pressures a sole-source precursor to BN. The formation of in situ BN (e. g, aircraft engines operate at >30 atm (3 MPa))is also coatings also has been attempted on Nextel 312 aluminum unknown. Although improvements in oxidation resistance have after reaction under an NH, or a nitrogen atmosphere, respectively ave higher interfacial shear stresses. 4 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 boron to either improve oxidation resistance or prevent reactions between m.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 primary purpose was to prevent diffusion of boron from the BN-coated SiC fibers displayed the same high tensile strengths coating to the matrix and to prevent diffusion of any matrix species (3 MPa)as the original unreacted fibers, they have not yet been into the BN coating. 14. 1 18, 135, 41 143-145 This combination pro- tested in a composite vides stability up to 1 100"C during fast fracture and creep, with The oxidation resistance of turbostratic BN is highly dependent dation of fibers and BN coatings occurring only in the near on structure, degree of crystallinity, crystallographic orientation, surface regions. However, under cyclic loading and when the and impurities, all which are affected by the synthesis and/or matrix-cracking stress is exceeded, oxidation of the fibers and the deposition conditions. In general, higher synthesis and deposition BN coating can occur in the interior of the composite as well