ournal . Am. Ceram Soc., 80 [7 1873-76(1997) In Situ Reacted Rare-Earth Hexaaluminate Interphases Markys G Cain, Rebecca L. Cain, t and Michael H. Lewis entre for Advanced Materials, University of Warwick, Coventry CV4 7AL, United Kingdom John Gent Rolls royce plc., Derby DE2 8BJ, United Kingdom A novel in situ reaction between a ceria-doped zirconia is taken up is dependent on the charge and radius of the cation interphase coating on Saphikon fibers and an outer alu- within the interspinal planes. A large number of rare-earth mina coating has resulted in the formation of oriented alkali, and alkaline-earth cations can stabilize the hexaalumi hexaaluminate platelets which can act as a low fracture nate structure in either its MP or B-alumina form. 3. 4 It is nergy interface barrier for crack deflection in oxide-oxide expected that the B-alumina and MP layered materials both ceramic- matrix composites( CMCs). The reaction proceeds only in reducing environments where the reduction of the of B-alumina, values of the fracture energy anisotropy, GG 3+ vale basal destabilization phenomenon consistent with previously re- enable their basal planes to deflect cracks since this fract ported findings. The diffusion of the cerium from the zir. energy ratio is <0. 25. The anisotropy in the growth kinetics conia into solid solution with the alumina can stabilize the of the hexaaluminates imparts a platelike or acicular morphol layered hexaaluminate structure. Preferred orientational ogy. This microstructural characteristic has been used as a growth of the hexaaluminate parallel to the coating inter- toughening agent in composites of alumina, zirconia, and alu- face was observed which is the required orientation for mina/zirconia containing the B/mP phase, processed in either a chanced debonding at the fiber/matrix interface in long dispersed form or formed in situ 16-21 The success of a refrac- fiber-reinforced CMCs tory layered-type hexaaluminate interphase would depend very much on the relative orientation of the B-alumina or MP basal planes with respect to the debonding interface- that is, the fiber surface in fiber-reinforced CMCs. Significant research has been conducted in this area and it remains uncertain wheth Lites(CMCs), the goal is to produce a material that will with- optimized such that the majority of the layered interphase pos sesses the required orientation. 6. 8, 22 This paper reports on a reason, much effort is devoted to oxide-oxide Cmc syste ly apyrores olenngle-crystal alumina fibers. An earlier study se- novel in situ formed hexaaluminate interphase material which imates the desired basal orientation with respect to the oxidation resistant 1-9 The success of an interface material for incorporation within oxide-oxide Cmcs depends largely upon lected rare-earth (rather than alkali)stabilized hexaaluminates tures. The former requirement can be attained through eith methods based on liquid precursors and magnetron sputter maintaining a low debond energy between the interface ing.8.9 Although matrix crack deflection was demonstrated by material and the fiber or matrix or through the materials microscopic observation of layer- plane separation in lanta own intrinsic, and necessarily low, fracture energy Materials num hexaaluminate crystals, there was limited preference for that possess a low intrinsic fracture enex lp (ucture alu- case saphikon monofilaments), resulting in high average C-axis orientation normal to the surface of the fibers (in this high-temperature stability include the rare-eart minates,8,9, which have a layered or sheetlike structure con- debond energies.23 There was also concern about filament taining weak basal planes which have been shown to readily cleave, 12 Hexaaluminates based on either the magnetoplum- strength degradation due to intrusion of the hexaaluminate bite or B-alumina structure have been proposed as suitable platelets into the filament surface during crystallization. An alternative approach which uses the fiber precoating as a interphase candidate materials II for use at temperatures urrently set by the silicate-reinforced Sic source of rare-earth stabilizing cation is described here. A for- tuitous in situ interface reaction has been observed when Ce. fiber composites. The magnetoplumbite (MP) and B-alumina doped zirconia/a alumina bilayer coated sapphire fibers are tructures are very similar, each having spinel blocks com- incorporated within an alumina matrix forming a hexaalumi sed of Ar+ and O?- ions with the same structure as spinel i3 nate interface. 8 The in situ formation of B or MP aluminas he differences lie in the contents and arrangement of the within oxide CMCs has been previously reported and shown to stabilizing cations between the spinel layers. The structure that occur in the presence of certain impurities (Mn, Mg, Sr ted the morph nd exact crystal structure of the hexaaluminate and, in tur the fracture toughness of the composite Preliminary investi D J. Green-contributing editor gations using composites containing Ce-doped zirconia/a alumina bilayer coated sapphire fibers were fabricated via the hot-pressing route to achieve acceptable densities. The process- ing atmosphere was considered of crucial importance in cata lyzing the in situ reaction. This paper reports on the evolution of the in situ reacted hexaaluminate interface in bilayer coated Saphikon sapphire fibers as a function of time and temperatIn Situ Reacted Rare-Earth Hexaaluminate lnterphases Markys G. Cain,* Rebecca L. Cain,' and Michael H. Lewis Centre for Advanced Materials, University of Warwick, Coventry CV4 7AL, United Kingdom John Gent Rolls Royce plc., Derby DE2 8BJ, United Kingdom A novel in sihc reaction between a ceria-doped zirconia interphase coating on Saphikon fibers and an outer alu￾mina coating has resulted in the formation of oriented hexaaluminate platelets which can act as a low fracture energy interface barrier for crack deflection in oxide-oxide ceramic-matrix composites (CMCs). The reaction proceeds only in reducing environments where the reduction of the cerium and zirconium ions to their 3+ valent state causes a destabilization phenomenon consistent with previously re￾ported findings. The diffusion of the cerium from the zir￾coda into solid solution with the alumina can stabilize the layered hexaaluminate structure. Preferred orientational growth of the hexaaluminate parallel to the coating inter￾face was observed which is the required orientation for enhanced debonding at the fibedmatrix interface in long￾fiber-reinforced CMCs. I. Introduction N THE next generation of advanced ceramic-matrix compos- I ites (CMCs), the goal is to produce a material that will with￾stand high temperatures in an oxidizing environment. For this reason, much effort is devoted to oxide-oxide CMC systems￾that is, oxide fibers and oxide matrices, which are inherently oxidation resistant.'-9 The success of an interface material for incorporation within oxide-oxide CMCs depends largely upon the interface material possessing the required debonding char￾acteristics'O and suitable chemical stability at service tempera￾tures. The former requirement can be attained through either maintaining a low debond energy between the interface material and the fiber or matrix or through the materials own intrinsic, and necessarily low, fracture energy. Materials that possess a low intrinsic fracture energy coupled with high-temperature stability include the rare-earth (RE) hexaalu￾minate~~.~.~." which have a layered or sheetlike structure con￾taining weak basal planes which have been shown to readily cleave6J2 Hexaaluminates based on either the magnetoplum￾bite or p-alumina structure have been proposed as suitable interphase candidate materials6.' I for use at temperatures higher than those currently set by the silicate-reinforced Sic fiber composites. The magnetoplumbite (MP) and p-alumina structures are very similar, each having spinel blocks com￾posed of A13+ and 02- ions with the same structure as pin el.'^ The differences lie in the contents and arrangement of the stabilizing cations between the spinel layers. The structure that D. J. Green-contributing editor yanuscript No. 191731. Received June 19. 1996 approved April 22, 1997. Member, American Ceramic Society. 'Now at Wmick Manufacturing Group, University of Warwick, Coventry, United Kingdom. is taken up is dependent on the charge and radius of the cation within the interspinel ~1anes.l~ A large number of rare-earth, alkali, and alkaline-earth cations can stabilize the hexaalumi￾nate structure in either its MP or p-alumina f~rm.'~.'~ It is expected that the p-alumina and MP layered materials both possess large fracture energy anisotropies. For single crystals of p-alumina, values of the fracture energy anisotropy, GJG - 0.01 l2 (Gb normal and Gp parallel to the basal plane) wouli enable their basal planes to deflect cracks since this fracture energy ratio is ~0.25.'~ The anisotropy in the growth kinetics of the hexaaluminates imparts a platelike or acicular morphol￾ogy. This microstructural characteristic has been used as a toughening agent in composites of alumina, zirconia, and alu￾mindzirconia containing the p/MP phase, processed in either a dispersed form or formed in situ.'6-21 The success of a refrac￾tory layered-type hexaaluminate interphase would depend very much on the relative orientation of the p-alumina or MP basal planes with respect to the debonding interface-that is, the fiber surface in fiber-reinforced CMCs. Significant research has been conducted in this area and it remains uncertain wheth￾er the processing and fiber coating route can be satisfactorily optimized such that the majority of the layered interphase pos￾sesses the required orientation.6*8*22 This paper reports on a novel in situ formed hexaaluminate interphase material which approximates the desired basal orientation with respect to the surfaces of single-crystal alumina fibers. An earlier study se￾lected rare-earth (rather than alkali) stabilized hexaaluminates as a potential interphase, on the basis of high-temperature sta￾bility, compatibility with oxides, and the identity of deposition methods based on liquid precursors and magnetron sputter￾ing.8.9 Although matrix crack deflection was demonstrated by microscopic observation of layer-plane separation in lantha￾num hexaaluminate crystals, there was limited preference for C-axis orientation normal to the surface of the fibers (in this case Saphikon monofilaments), resulting in high average debond energies.23 There was also concern about filament strength degradation due to intrusion of the hexaaluminate platelets into the filament surface during crystallization. An alternative approach which uses the fiber precoating as a source of rare-earth stabilizing cation is described here. A for￾tuitous in situ interface reaction has been observed when Ce￾doped zirconidcx-alumina bilayer coated sapphire fibers are incorporated within an alumina matrix forming a hexaalumi￾nate interface.8 The in situ formation of p or MP aluminas within oxide CMCs has been previously reported and shown to occur in the presence of certain impurities (Mn, Mg, Sr etc.)~J7.18*20.24 which also directly affected the morphology and exact crystal structure of the hexaaluminate and, in turn, the fracture toughness of the composite. Preliminary investi￾gations8 using composites containing Ce-doped zirconidcx￾alumina bilayer coated sapphire fibers were fabricated via the hot-pressing route to achieve acceptable densities. The process￾ing atmosphere was considered of crucial importance in cata￾lyzing the in situ reaction. This paper reports on the evolution of the in situ reacted hexaaluminate interface in bilayer coated Saphikon sapphire fibers as a function of time and temperature. 1873