of the ame The hot-pressing environment utilized above is typically re ducing but from experimental considerations it was also de of In reducing of oxygen), and at significantly lower temperatures, the phase assemblage is unstable and compounds such as Ce+AlO3 and I. Experimental Detai Ce2O3. 1lAl2O3(MP/B alumina) can form. Under reducing circumstances Ce4+ reduces to Ce+ by the reaction 2Ce02- the binary zirconia- kon sapphire fibers(Saphikon Inc, Milford, NH) by rf physical ceria phase diagram.2 Figures 2 and 3 show the difference in apor deposition(PVD)magnetron sputtering. Two PVD coat- constitut on for the bilayer coated fibers which have been sub- gs were deposited. The first layer in contact with the fiber jected to heat treatments at 1400C for 3 h in an argon atmo ere and 6 h in an air atmosphere, respectively. the grain additional set of coated fibers were supplied with only one reaction layers are clearly present in the fiber heat-treated in PVD ZrO /10% CeO2 coating so that the interfaces could be argon, one at the fiber surface and the other at the original studied as a function of single and combined layer chemistries interface een the two PVD layers. EDX analysis confirms Two or three short lengths of fiber from both series of coated fiber were heat-treated at either 1200 or 1400c for times B2 in Fig. 2, with compositions(charge balance uncorrected) varying between 10 min and 6 h in order to study the change CeAl o. 019.7 and CeAlz 1 O1s. 9, respectively, approximating in interfacial characteristics, viz., microstructure and composi Ce -/-MP alumina 3 TI tion with increasing time. In each instance the fibers were zirconia layer, labeled"ZrO2, "is depleted in cerium. No such heated(6C/min) from room temperature and cooled under phase instability was observed in coated fibers heat-treated in controlled conditions, using identical gas flow rates and alu- air under similar conditions(Fig. 3). This is consistent with mina crucibles. Fibers were sectioned and carbon coated prior some earlier work concerning Ce-stabilized zirconia. No to examination in the scanning electron microscope(SEM) other physical changes can be detected in the air-treated sample (EOL JSM-6100, Tokyo, Japan). All semiquantitative analysis and importantly no B/MP-alumina formation was observed as performed with the ultrathin window in position in the ighlighting the importance in composite fabrication X-ray detector(LINK/ISIS energy dispersive X-ray microanal ysis(EDX) system with 30 take-off detector) Divalent or other impurities were not found in any of the speci- mens during routine EDX analysis, suggesting that earlier re- shell will simulate the matrix in a composite and also form orted findings 6. 17, 18, 20, 24(discussed previously) could not ac- protective sheath for the"active"Ce-doped zirconia layer be count for the formation of the hexaaluminates or for the neath. The outer layer is very similar in composition to the destabilization of the Ce-(ss-Zro2 phase.(Identical ex matrix material but the residual stresses are anticipated to be ments were performed using single-coated fibers with no outer lifferent because of differing thermal histories. Both types of alumina shell. /MP-alumina was observed at the fiber/coatin fiber were heat-treated in air in addition to argon to give interface for all times at 1400oC but only at times exceeding 1 indication of the importance of the atmospheric conditions h in the 1200C experiments. The removal of the alumina shell does not appear to have inhibited the formation of hexaalumi nate Il. Results and discussion Work by heussner and Claussen and Zhu et al Figure l shows a fracture end of an as-received fiber. Both material of similar composition to the cerium-doped oatings on the fiber surface have a columnar grain structure used in this study, showed that in atmospheres of and EDX analysis correlated well with that expected from the original sputter tions. The coatings appear to be con tinuous with the columns of the outer alumina coating match ng those from the inner zirconia layer. 'Typical SEM EDX spot sizes were ar dimension to the crys In the ternary system CeOyZrO2/Al,O3 the binary phase resulting in only approximate analytical composit FIBR ZRO CE-ZRO2 ALUMINA Fig. 1. Bilayer PVD coated Saphikon fiber in theas-received Fig. 2. Bilayer PVD coated fiber heat-treated in argon at 1400oC for 6 h in argon1874 Communications of the American Ceramic Society Vol. 80, No. 7 The hot-pressing environment utilized above is typically re￾ducing but from experimental considerations it was also de￾cided to perform a series of experiments in an inert (argon) atmosphere. 11. Experimental Details The oxide coatings were deposited onto single-crystal Saphi￾kon sapphire fibers (Saphikon Inc., Milford, NH) by rf physical vapor deposition (PVD) magnetron sputtering. Two PVD coat￾ings were deposited. The first layer in contact with the fiber was Ce (12%) doped zirconia and was -1.3 pm thick and the outer layer was alumina which was -3.5 km thick (Fig. 1). An additional set of coated fibers were supplied with only one PVD ZrO,/lO% CeO, coating so that the interfaces could be studied as a function of single and combined layer chemistries. Two or three short lengths of fiber from both series of coated fiber were heat-treated at either 1200” or 1400°C for times varying between 10 min and 6 h in order to study the change in interfacial characteristics, viz., microstructure and composi￾tion with increasing time. In each instance the fibers were heated (6”C/min) from room temperature and cooled under controlled conditions, using identical gas flow rates and alu￾mina crucibles. Fibers were sectioned and carbon coated prior to examination in the scanning electron microscope (SEM) (JEOL JSM-6100, Tokyo, Japan). All semiquantitative analysis was performed with the ultrathin window in position in the X-ray detector (LINKASIS energy dispersive X-ray microanal￾ysis (EDX) system with 30” take-off detector). Bilayer coated fibers are used such that the outer alumina shell will simulate the matrix in a composite and also form a protective sheath for the “active” Ce-doped zirconia layer be￾neath. The outer layer is very similar in composition to the matrix material but the residual stresses are anticipated to be different because of differing thermal histories. Both types of fiber were heat-treated in air in addition to argon to give an indication of the importance of the atmospheric conditions. 111. Results and Discussion Figure 1 shows a fracture end of an as-received fiber. Both coatings on the fiber surface have a columnar grain structure and EDX analysis correlated well with that expected from the original sputtered compositions. The coatings appear to be con￾tinuous with the columns of the outer alumina coating match￾ing those from the inner zirconia layer. In the ternary system CeO,/ZrO,/Al,O, the binary phase assemblage based on the ceridzirconia solid solution (ss) and alumina are reported stable at temperatures below 1600°C in air. In reducing environments, however (lower partial pressures of oxygen), and at significantly lower temperatures, the phase assemblage is unstable and compounds such as Ce3+A10, and Ce,O, 1 lAl,O, (MP/P alumina) can form. Under reducing circumstances Ce”+ reduces to Ce3+ by the reaction 2Ce0, + Ce203 + $,, which has implications for the binary zirconia￾ceria phase diagram.,’ Figures 2 and 3 show the difference in constitution for the bilayer coated fibers which have been sub￾jected to heat treatments at 1400°C for 3 h in an argon atmo￾sphere and 6 h in an air atmosphere, respectively. The grain structures of both fibers are observed to have coarsened but mo reaction layers are clearly present in the fiber heat-treated in argon, one at the fiber surface and the other at the original interface between the two PVD layers. EDX analysis confirms the presence of two in situ reacted interphases, labeled pl and p2 in Fig. 2, with compositions* (charge balance uncorrected) CeAl,,,,O,,., and CeAl,,,O,,,,, respectively, approximating the structural formula Ce,~,.Al12~y0,,~z-MP a1~mina.l~ The zirconia layer, labeled “ZrO,,” is depleted in cerium. No such phase instability was observed in coated fibers heat-treated in air under similar conditions (Fig. 3). This is consistent with some earlier work2, concerning Ce-stabilized zirconia. No other physical changes can be detected in the air-treated sample and importantly no P/MP-alumina formation was observed highlighting the importance in composite fabrication of using reducing or inert atmospheres to form the p/MP interphase. Divalent or other impurities were not found in any of the speci￾mens during routine EDX analysis, suggesting that earlier re￾ported finding^^*'^*'^,^^,*^ (discussed previously) could not ac￾count for the formation of the hexaaluminates or for the destabilization of the Ce-(ss)-ZrO, phase. (Identical experi￾ments were performed using single-coated fibers with no outer alumina shell. PW-alumina was observed at the fibedcoating interface for all times at 1400°C but only at times exceeding 1 h in the 1200°C experiments. The removal of the alumina shell does not appear to have inhibited the formation of hexaalumi￾nate.) Work by Heussner and Cla~ssen*~ and Zhu et aL2, using material of similar composition to the cerium-doped zirconia used in this study, showed that in atmospheres of low Po,, Typical SEM EDX spot sizes were of similar dimension to the crystallite sizes. However, beam broadening would sample a certain proportion of neighboring phases resulting in only approximate analytical compositions. Fig. 1. Bilayer PVD coated Saphikon fiber in the “as-received” state. 6 h in argon. Fig. 2. Bilayer PVD coated fiber heat-treated in argon at 1400°C for