Communications of the American Ceramic Society a Ym 20 FIBRE ZRO 3. Bilayer PVD coated fiber heat-treated in air at 1400oC for Fig. 4. Bilayer PVD coated fiber heat-treated in argon at 1200C for 3 h. Note the presence of only one hexaaluminate reaction interphase reduction of the Ce4++, Ce3+ a phase separation of the mobile cation, Ce+, from the destabilized zirconia into the the tetragonal()Ce-stabilized to monoclinic zirco- outer alumina phase. Alternatively if the hexaaluminate wa a (on cooling below Tt-m)+ as predicted by the phase diagra m destabilization alumina then the continual depletion of the p phase would is related to the corresponding increase in ic adii of the control the reaction This would also account for the lack of cerium cation Ce3+(1. 07 A)compared to Ce++(0.94 A)leading evidence for the pyrochlore phase during routine examination o a decrease in solubility of CeO, in the tetragonal Zro2. This in the sEM. There is presently insufficient data, however, to results in the destabilization of the t phase in reducing envi- accurately describe the reaction kinetics. Further detailed ronments. Zhu et al.26 also noted that no such phase separation analysis of the evolution of the interface using higher resolu transformation occurred in air or in reducing atmospheres tion microscopy (tEm)is clearly required to be able to resolve ow 1000C. Although the cerium was reduced under these these claims conditions, it was additionally noted by these authors that The observation of the morphologically distinct basal planes hase separation/transformation only occurred at a critical tem of the hexaaluminate allel to the Zro/alo it perature of over 1200C when Zr++ was also reduced to Zr+. terface implied in Figs. 4 and 2 is encouraging in view of the The reducing atmosphere was hydrogen, which is severe known debonding requirements in CMCs. This special orien- enough to reduce the zirconium from its pentavalent to its tation exhibited by the MP layers most likely arises from the trivalent state. The reduction of the zirconia may not, however growth anisotropy of the be a prerequisite for the destabilization phenomenon where hexaaluminates will grow with their basal planes parallel to the there is a sink for the reduced Ce3+ -the alumina shell and/or reaction direction because the rapid transport paths along the fiber in the present case. In other studies concerning the effects basal planes are aligned with the transport direction, Alterna- of various atmospheres on the phase relations in the system tively, it is commonly the free surface energy that determines Zr-Ce-0, 28,29 the Ce4+ in solid solution with the Zro2 is re the growth habit of"single crystal"thin films. In the case of duced to Ce3+ at increased temperatures in reducing environ- pure alumina, the free surface energy of the fully relaxed basal Xrd data from such small specimens and the pyrochlore phase related structures such as B/MP-dunee mip mplicaaincrys. ments(CO, H, NH,,), in vacuums(10- to 10-2 Pa), and in inert planes(0001)is significantly lower than most other gen partial pressures. In this study it was not possible to gate Atmospheres(Ar, He)as well as in atmospheres with low on allographic directions. 30 This result has clear and, possibly, was not observed using EDX analysis in the SEM(but see Of the samples investigated, none showed full conversion of t possible to determine if the outer alumina shell to hexaaluminate. It is anticipated that ffusion of the free Ce+ ion or the reaction of alumina with the formation of the hexaaluminate layer is controlled by the the compound Zr2 Ce20, forms the hexaaluminate observed at diffusion of oxygen to the surface of the coated fiber along an activity gradient which decreases as the surface is approached There are two competing reaction fronts during heat treat In porous materials, such as these, argon/oxygen diffusion may ment, one at the fiber surface and one at the Pvd alumina be channeled along pores and through the columnar structure of shell/PVD zirconia interface. In all the specimens examined, he Pvd coatings, thus increasing the yield of reduced ce+at ome evidence of hexaaluminate formation was detected. Fi- the outermost interface resulting in a tendency for this layer to bers heat-treated at 1200 C exhibited a single hexaaluminate grow preferentially( Fig. 5). It is thus probable that tailoring of reaction layer situated between the two PVd layers( Fig 4).At the hexaaluminate interface may be achieved by variation of this stage no hexaaluminate was observed at the zirconia/fiber deposition parameters, i.e., controlling coating porosity and interface. It was also observed that very little grain growth had structure. The schematic in Fig. 5 represents the gradient in the taken place in the outer layer but the zirconia layer is signifi- postulated valence state of the Ce ion which is initially in state cantly more granular. EDX analysis yielded significant levels Ce and in solid solution with the zirconia In reducing envi- f Ce(1.5 at. %)retained within the zirconia at 1200C. At ronments the valency changes to its Ce+ state(and at suffi 1400 C hexaaluminate was readily observed at both interfaces ciently high temperatures, >1200.C, Zr+-Zr+ also)which Fig. 2). The evolution of the hexaaluminate layers(Figs. 2 and s no longer able to stabilize the tetragonal form of the zirconia. 4)in terms of thickness, extent, and composition was indicative The destabilization and phase partitioning that occurs can pro- a diffusion-rate-controlled reaction where the reaction prod mote the formation of these in situ reacted hexaaluminate lay uct, in this case hexaaluminate, inhibits the further diffusion of ers. Alternatively, the reduced valent Ce3+ ions, no longer ableJuly 1997 Communications of the American Ceramic Society 1875 Fig. 3. Bilayer PVD coated fiber heat-treated in air at 1400°C for 3 h. reduction of the Ce4+ + Ce3+ resulted in a phase separation of the tetragonal (t) Ce-stabilized zirconia into monoclinic zirco￾nia (on cooling below T,-,,J + pyrochlore (P) phase Zr,Ce207 as predicted by the phase diagram.25 The t + m destabilization is related to the corresponding increase in ionic radii of the cerium cation Ce3+ (1.07 A) compared to Ce4+ (0.94 A) leading to a decrease in solubility of CeO, in the tetragonal Zro,. This results in the destabilization of the t phase in reducing envi￾ronments. Zhu er a1.26 also noted that no such phase separation and transformation occurred in air or in reducing atmospheres below 1OOO"C. Although the cerium was reduced under these conditions, it was additionally noted by these authors that phase separatiodtransformation only occurred at a critical tem￾perature of over 1200°C when u" was also reduced to Zs+. The reducing atmosphere was hydrogen, which is severe enough to reduce the zirconium from its pentavalent to its trivalent state. The reduction of the zirconia may not, however, be a prerequisite for the destabilization phenomenon where there is a sink for the reduced Ce3+-the alumina shell andor fiber in the present case. In other studies concerning the effects of various atmospheres on the phase relations in the system Zr-Ce-0,28*29 the Ce& in solid solution with the Zr02 is re￾duced to Ce3+ at increased temperatures in reducing environ￾ments (CO, H, NH,), in vacuums (lo-' to lo-, Pa), and in inert atmospheres (Ar, He) as well as in atmospheres with low oxy￾gen partial pressures. In this study it was not possible to gather XRD data from such small specimens and the pyrochlore phase was not observed using EDX analysis in the SEM (but see previous footnote). It was thus not possible to determine if diffusion of the free Ce3+ ion or the reaction of alumina with the compound Zr2Ce,07 forms the hexaaluminate observed at the interfaces. There are two competing reaction fronts during heat treat￾ment, one at the fiber surface and one at the PVD alumina shell/PVD zirconia interface. In all the specimens examined, some evidence of hexaaluminate formation was detected. Fi￾bers heat-treated at 1200°C exhibited a single hexaaluminate reaction layer situated between the two PVD layers (Fig. 4). At this stage no hexaaluminate was observed at the zirconidfiber interface. It was also observed that very little grain growth had taken place in the outer layer but the zirconia layer is signifi￾cantly more granular. EDX analysis yielded significant levels of Ce (-1.5 at.%) retained within the zirconia at 1200°C. At 1400°C hexaaluminate was readily observed at both interfaces (Fig. 2). The evolution of the hexaaluminate layers (Figs. 2 and 4) in terms of thickness, extent, and composition was indicative of a diffusion-rate-controlled reaction where the reaction prod￾uct, in this case hexaaluminate, inhibits the further diffusion of Fig. 4. Bilayer PVD coated fiber heat-treated in argon at 1200°C for 3 h. Note the presence of only one hexaaluminate reaction interphase. the mobile cation, Ce3+, from the destabilized zirconia into the outer alumina phase. Alternatively if the hexaaluminate was formed via a reaction of the P phase-Zr,Ce,O,-with the alumina then the continual depletion of the P phase would control the reaction. This would also account for the lack of evidence for the pyrochlore phase during routine examination in the SEM. There is presently insufficient data, however, to accurately describe the reaction kinetics. Further detailed analysis of the evolution of the interface using higher resolu￾tion microscopy (TEM) is clearly required to be able to resolve these claims. The observation of the morphologically distinct basal planes of the hexaaluminate growing parallel to the Zr02/A1203 in￾terface implied in Figs. 4 and 2 is encouraging in view of the known debonding requirements in CMCs. This special orien￾tation exhibited by the MP layers most likely arises from the growth anisotropy of these phases, discussed earlier. Often hexaaluminates will grow with their basal planes parallel to the reaction direction because the rapid transport paths along the basal planes are aligned with the transport direction. Alterna￾tively, it is commonly the free surface energy that determines the growth habit of "single crystal" thin films. In the case of pure alumina, the free surface energy of the fully relaxed basal planes (OOO1) is significantly lower than most other main crys￾tallographic directions.30 This result has clear implications on the preferred growth orientations of alumina and, possibly, related structures such as P/MP-alumina. Of the samples investigated, none showed full conversion of the outer alumina shell to hexaaluminate. It is anticipated that the formation of the hexaaluminate layer is controlled by the diffusion of oxygen to the surface of the coated fiber along an activity gradient which decreases as the surface is approached. In porous materials, such as these, argodoxygen diffusion may be channeled along pores and through the columnar structure of the PVD coatings, thus increasing the yield of reduced Ce3+ at the outermost interface resulting in a tendency for this layer to grow preferentially (Fig. 5). It is thus probable that tailoring of the hexaaluminate interface may be achieved by variation of deposition parameters, i.e., controlling coating porosity and structure. The schematic in Fig. 5 represents the gradient in the postulated valence state of the Ce ion which is initially in state Ce4+ and in solid solution with the zirconia. In reducing envi￾ronments the valency changes to its Ce3+ state (and at suffi￾ciently high temperatures, >1200"C, Uk + W+ also) which is no longer able to stabilize the tetragonal form of the zirconia. The destabilization and phase partitioning that occurs can pro￾mote the formation of these in situ reacted hexaaluminate lay￾ers. Alternatively, the reduced valent Ce3+ ions, no longer able