R.S. Hay et al. Journal of the European Ceramic Society 20(2000)589-597 of wet tubing in the instrument since background runs The spheroidization of 50 nm thick coatings at consistently found this gas despite attempts to run in a 1200C is another concern(Fig. 7). Previous analysis dry environment. 43 This leaves N2 or N2O as the most and observations of the Rayleigh instability of poly- likely products of high temperature degassing, with crystalline thin films suggests that monazite coatings on H,O, CO, CO2, and undetected trace gases as additional other oxides may be prone to this instability. 44 45 Fc possibilities. If H2O or N2 were the principal high tem- example, the high monazite-alumina interphase bound- perature corrosive species, control experiments in air ary energy causes a large equilibrium contact angle B (72%nitrogen) and with water substituted for coating between monazite and alumina, which in turn allows precursor could be expected to degrade fiber strength in spheroidization of polycrystalline coatings at a smaller a manner similar to the coated fibers. Unfortunately, grain size(D) for a given coating thickness(a). Spher this was not the case. Water"coated"fibers had some oidization is predicted when: 44 of the highest strengths of any control experiment(Fig 10). Mass spectrometry of coating precursor was done 3 sin e in argon, but fiber coating was done in air, so the gases a 2-3cos 0+c0s38 evolved during coating might be more highly oxidized forms of carbon and nitrogen than those found by mass A more complex expression in later work5 yields simi- spectrometry. However, fibers coated in argon with this lar results. The grain size must be less than twice the precursor at 1300 C had 1.24 GPa tensile strength, a coating thickness if the film is to be stable with respect negligible increase over those coated in air to spheroidization at high 8. Coatings discussed in this The partial pressure of the evolved gases is another paper(Figs. 5-7)had unusually large grain sizes in concern. Partial pressures measured by mass spectro- comparison to coatings made from other precursors, so metry are quite low, because they are diluted by the they may be particularly prone to spheroidization argon atmosphere used for measurement. However, the Spheroidization kinetics of these coatings should partial pressure of a gas evolved in a closed coating pore depend on the surface diffusion coefficients of mon- may be much higher than atmospheric and depend on azite 46 However, when coated fibers are incorporated in the bursting strength of the pore and the driving force a dense matrix, the rate determining process must for gas formation. If these gas filled pores are adjacent change to interphase boundary diffusion control, which to the fiber surface, fiber surface flaws can be exposed to is generally slower than surface diffusion. 47 It is also high pressure gas from precursor decomposition. As the necessary to transport either matrix or fiber material to pore volume fraction decreases with coating tempera- accommodate monazite spheroidization, which could Ire(Fig. 5), it becomes more likely that the porosity is further retard spheroidization if diffusive mass transpor closed and that gas decomposition products are sealed in in the fiber or matrix is slow compared to monazite the coating. The large tensile strength drop between the 1000 and 1100 C coated fibers( Fig. 10)could, therefore, be related to densification and coarsening of coating por- 5. Summary and conclusions osity and the consequent transition from an open to hermetic coating. However, if the main cause of strength Lowering the deposition temperature of monazite degradation was environmental effects independent of the from 1300 to 900C for an ethanolic monazite precursor presence of a coating, such as those measured by control causes coatings from this particular pr to hay experiments, then it might also be argued that a hermetic higher pore volume fraction, smaller pore size, and coating should seal the fiber from this environment, and defectively crystallized grains with poorly defined grain the fiber should be stronger. More information is neces- size. The porosity is mostly eliminated and the grains sary to confidently explain these results recrystallize during heat-treatment at 1200.C. Thin As deposited coatings were porous, particularly at coatings spheroidized, which may be a problem with low temperature, but the porosity formed during coatings like monazite that have high substrate inter- deposition at 900 C was not stable at 1200 C. These phase-boundary energy. Spheroidization kinetics may coatings recrystallized to a small grain size at 1200oC. be retarded by incorporation of the coating in a dense Coatings deposited at 1300.C had a large grain size with matrix, and spheroidization can be prevented by stabi somewhat larger intragranular porosity. Heat-treat- lization of a small coating grain size in comparison to ments at 1200C were not done on these 1300C coated coating thick fibers. The large grain size and consequent lack of grain Fibers coated at 900oc did not lose much tensile boundary diffusion pathways might cause this intra- strength, but fibers coated at higher temperatures were granular porosity to be more resistant to ening more severely degraded. Control experiments that than the porosity present at 900C. Further experiments mimicked the deposition process in some cases were are necessary to establish the T-t path dependence of nearly as severely degraded as coated fibers. The high coating microstructure. strengths of fibers coated at low temperatures was notof wet tubing in the instrument, since background runs consistently found this gas despite attempts to run in a dry environment.43 This leaves N2 or N2O as the most likely products of high temperature degassing, with H2O, CO, CO2, and undetected trace gases as additional possibilities. If H2O or N2 were the principal high tem￾perature corrosive species, control experiments in air (72% nitrogen) and with water substituted for coating precursor could be expected to degrade ®ber strength in a manner similar to the coated ®bers. Unfortunately, this was not the case. Water ``coated'' ®bers had some of the highest strengths of any control experiment (Fig. 10). Mass spectrometry of coating precursor was done in argon, but ®ber coating was done in air, so the gases evolved during coating might be more highly oxidized forms of carbon and nitrogen than those found by mass spectrometry. However, ®bers coated in argon with this precursor at 1300C had 1.24 GPa tensile strength,6 a negligible increase over those coated in air. The partial pressure of the evolved gases is another concern. Partial pressures measured by mass spectro￾metry are quite low, because they are diluted by the argon atmosphere used for measurement. However, the partial pressure of a gas evolved in a closed coating pore may be much higher than atmospheric and depend on the bursting strength of the pore and the driving force for gas formation. If these gas ®lled pores are adjacent to the ®ber surface, ®ber surface ¯aws can be exposed to high pressure gas from precursor decomposition. As the pore volume fraction decreases with coating tempera￾ture (Fig. 5), it becomes more likely that the porosity is closed and that gas decomposition products are sealed in the coating. The large tensile strength drop between the 1000 and 1100C coated ®bers (Fig. 10) could, therefore, be related to densi®cation and coarsening of coating por￾osity and the consequent transition from an open to a hermetic coating. However, if the main cause of strength degradation was environmental e€ects independent of the presence of a coating, such as those measured by control experiments, then it might also be argued that a hermetic coating should seal the ®ber from this environment, and the ®ber should be stronger. More information is neces￾sary to con®dently explain these results. As deposited coatings were porous, particularly at low temperature, but the porosity formed during deposition at 900C was not stable at 1200C. These coatings recrystallized to a small grain size at 1200C. Coatings deposited at 1300C had a large grain size with somewhat larger intragranular porosity. Heat-treat￾ments at 1200C were not done on these 1300C coated ®bers. The large grain size and consequent lack of grain boundary di€usion pathways might cause this intra￾granular porosity to be more resistant to coarsening than the porosity present at 900C. Further experiments are necessary to establish the T±t path dependence of coating microstructure. The spheroidization of 50 nm thick coatings at 1200C is another concern (Fig. 7). Previous analysis and observations of the Rayleigh instability of poly￾crystalline thin ®lms suggests that monazite coatings on other oxides may be prone to this instability.44,45 For example, the high monazite±alumina interphase bound￾ary energy causes a large equilibrium contact angle  between monazite and alumina, which in turn allows spheroidization of polycrystalline coatings at a smaller grain size (D) for a given coating thickness (a). Spher￾oidization is predicted when:44 D a  5 3 sin3  2 ÿ 3 cos  ‡ cos3  …1† A more complex expression in later work45 yields simi￾lar results. The grain size must be less than twice the coating thickness if the ®lm is to be stable with respect to spheroidization at high . Coatings discussed in this paper (Figs. 5±7) had unusually large grain sizes in comparison to coatings made from other precursors,6 so they may be particularly prone to spheroidization. Spheroidization kinetics of these coatings should depend on the surface di€usion coecients of mon￾azite.46 However, when coated ®bers are incorporated in a dense matrix, the rate determining process must change to interphase boundary di€usion control, which is generally slower than surface di€usion.47 It is also necessary to transport either matrix or ®ber material to accommodate monazite spheroidization, which could further retard spheroidization if di€usive mass transport in the ®ber or matrix is slow compared to monazite. 5. Summary and conclusions Lowering the deposition temperature of monazite from 1300 to 900C for an ethanolic monazite precursor causes coatings from this particular precursor to have a higher pore volume fraction, smaller pore size, and defectively crystallized grains with poorly de®ned grain size. The porosity is mostly eliminated and the grains recrystallize during heat-treatment at 1200C. Thin coatings spheroidized, which may be a problem with coatings like monazite that have high substrate inter￾phase-boundary energy. Spheroidization kinetics may be retarded by incorporation of the coating in a dense matrix, and spheroidization can be prevented by stabi￾lization of a small coating grain size in comparison to coating thickness. Fibers coated at 900C did not lose much tensile strength, but ®bers coated at higher temperatures were more severely degraded. Control experiments that mimicked the deposition process in some cases were nearly as severely degraded as coated ®bers. The high strengths of ®bers coated at low temperatures was not R.S. Hay et al. / Journal of the European Ceramic Society 20 (2000) 589±597 595