594 R.S. Hay et al./ Journal of the European Ceramic Society 20(2000)589-597 Table I Tensile strength measurements Deposition Environment Heat-treatment Heat-treatment Tensile Weibull temperature temperature time modulus (°C) (° (h) (GPa) 123 1000 Morante 6.85 Monazite Argon Monazite Air 0123 nnnn aaumuu rrrr Water/20% HNO, 111 Control Water/20% HNO/l-octanol 5.11 likely environmental assisted surface crack growth. Pre- heat-treatment atmosphere, and one from the monazite vious work showed that coating thickness did not affect precursor. It is not clear whether these effects are inde- tensile strength, and that different monazite precursors pendent or synergistic If degradation is predominantly used for fiber coating under identical conditions had environmental, hermetic matrices that seal fibers from much different tensile strengths, which was consistent the environment could preserve fiber strength in a with environmental effects specific to each precursor. CMC. For heat-treated fibers, atmospheric moisture or Flaws in weakly bonded fiber coatings should not func- silica from MoSiz heating elements are possible corro tion as fiber flaws, 9 which is consistent with lack of a sive chemical species coating thickness effect and the known debonding At 1200 C segregation of lanthanum from monazite properties of monazite. to interfaces in the fiber(Fig 8)may also cause strength Observations made here and elsewhere suggest that degradation. Lanthanum doped alumina has facetted there are separate effects on fiber strength, one from the and elongated grains. 3 Lanthanum doping also retards sintering and creep in alumina.39,40 Effects on strength have, to our knowl- 14 12 alumina basal planes causes easier intergranular fracture 3 along those planes analogous to cleavage in p-alumina and magnetoplumbite, 41.42 then significant weakening can be expected. We note that X-ray diffraction suggests our precursor was lanthanum rich(although TEM did not corroborate this). Therefore lanthanum activity may have been buffered to a relatively high value by la3 PO7 LaPO4, instead of a lower value fixed by LaPO4 LaP3Oo More measurements of lanthanum segregation as a function of temperature, time, and lanthanum Logit(s))at 1200 C activity at the two different buffers needs to be measured and a more definitive correlation with fiber tensile strength should be made Identification of the chemical species causing strength 100011 1200 l300 degradation remains problematic. Mass spectrometry sts that only N, or CO, H,O, and N,O or CO gases were evolved by the precursor above 700C(Fig. Fig 10. Tensile strength of coated fibers and control experiments vs coating temperature or heat-treatment time at 1200.C. Log(o) scale on 3). The black sample and argon atmosphere suggest that x-axis refers to solid line connecting squares. Coating temperature limited oxidation of carbon and partial reduction of the le on x-axis refers to dashed line connecting circles. Numbers for nitrates from the lanthanum precursor N2O control experiments are given in Table I more likely. The measured H20 could also be an artifactlikely environmental assisted surface crack growth. Pre￾vious work showed that coating thickness did not a€ect tensile strength, and that di€erent monazite precursors used for ®ber coating under identical conditions had much di€erent tensile strengths, which was consistent with environmental e€ects speci®c to each precursor.6 Flaws in weakly bonded ®ber coatings should not func￾tion as ®ber ¯aws,19 which is consistent with lack of a coating thickness e€ect and the known debonding properties of monazite. Observations made here and elsewhere6 suggest that there are separate e€ects on ®ber strength, one from the heat-treatment atmosphere, and one from the monazite precursor. It is not clear whether these e€ects are inde￾pendent or synergistic. If degradation is predominantly environmental, hermetic matrices that seal ®bers from the environment could preserve ®ber strength in a CMC. For heat-treated ®bers, atmospheric moisture or silica from MoSi2 heating elements are possible corro￾sive chemical species. At 1200C segregation of lanthanum from monazite to interfaces in the ®ber (Fig. 8) may also cause strength degradation. Lanthanum doped alumina has facetted and elongated grains.39 Lanthanum doping also retards sintering and creep in alumina.39,40 E€ects on strength have, to our knowl￾edge, not been reported. If lanthanum segregation to alumina basal planes causes easier intergranular fracture along those planes analogous to cleavage in b-alumina and magnetoplumbites,41,42 then signi®cant weakening can be expected. We note that X-ray di€raction suggests our precursor was lanthanum rich (although TEM did not corroborate this). Therefore lanthanum activity may have been bu€ered to a relatively high value by La3PO7/ LaPO4, instead of a lower value ®xed by LaPO4/ LaP3O9. More measurements of lanthanum segregation as a function of temperature, time, and lanthanum activity at the two di€erent bu€ers needs to be measured, and a more de®nitive correlation with ®ber tensile strength should be made. Identi®cation of the chemical species causing strength degradation remains problematic. Mass spectrometry suggests that only N2 or CO, H2O, and N2O or CO2 gases were evolved by the precursor above 700C (Fig. 3). The black sample and argon atmosphere suggest that limited oxidation of carbon and partial reduction of the nitrates from the lanthanum precursor to N2 and N2O is more likely. The measured H2O could also be an artifact Table 1 Tensile strength measurements No. Deposition temperature ( C) Coating Environment Heat-treatment temperature ( C) Heat-treatment time (h) Tensile strength (GPa) Weibull modulus 1 900 Monazite Air 1.95 6.85 2 1000 Monazite Air 1.78 5.66 3 1100 Monazite Air 1.32 4.95 4 1200 Monazite Air 1.39 4.09 5 1300 Monazite Air 1.21 5.09 5a 1300 Monazite Argon 1.24 5.07 6 900 Monazite Air 1200 0.02 1.14 3.77 7 900 Monazite Air 1200 2 1.21 4.00 8 900 Monazite Air 1200 100 0.83 3.43 9 Control Air 1200 100 1.04 4.97 10 1200 Control Air 1.50 4.79 11 1300 Control Air 1.51 4.40 12 1300 Control Water 1.77 4.23 13 1300 Control Water/20% HNO3 1.59 4.08 14 1300 Control Water/20% HNO3/l-octanol 1.85 5.11 Fig. 10. Tensile strength of coated ®bers and control experiments vs. coating temperature or heat-treatment time at 1200C. Log(t) scale on x-axis refers to solid line connecting squares. Coating temperature scale on x-axis refers to dashed line connecting circles. Numbers for control experiments are given in Table 1. 594 R.S. Hay et al. / Journal of the European Ceramic Society 20 (2000) 589±597