1194 Journal of the American Ceramic Society-Leparoux et al. Vol. 82. No. 5 erally believed to lead to poor crystal organization. 34,35 Thus, it CVD deposits. In all cases, any possible influence on the initial is not surprising that all the CVd deposits present a poor or- growth conditions would be hidden by the unfavorable CVD ganization at 700C(Fig. 8). Forced flow toward single fibers, conditions that have been used in this study. However, it seems thin tows, or 2D single cloth(either stationary or moving )car that the structure can be improved by using a high-temperature be used to obtain sufficiently uniform coatings that can be post-treatment. In fact, only poorly organized films exhibit a better organized at higher temperature. 6 The drawbacks of skin effect in the outermost 10-20 nm of the BN coating such a process, in terms of the amount of fibers that could be Although the graphitization of BN from a turbostratic structure coated and processing complexity, are clear only occurs at-2000C, it has been proved that the external With regard to the ICVI process, the supersaturation is al- reorganization was the result of the treatment at 1000oC under ways lower than that with CVD at 700C; however, the gas- vacuum. Lacrambe3 also has shown that moderate tempera- phase chemistry also can be changed by the influence of dif- tures(13000-1700C)lead to better crystallization of BN that ferent parameters. The residence time especially is greater, has been prepared via CVD. However, he attributed this im- because of diffusion inside the preform; the conversion yield provement to the evolution of his samples when previously also is much higher, because of the specific surface of the exposed to a moist environment In our case, the substrates porous preform. To first address the possible influence of the were kept in the reactor under vacuum between the deposition variation of the supersaturation, it is clear that some conditions stage and the thermal post-treatment. However, they do contain lead to high supersaturations and then to organization of oxygen as an impurity, and they exhibit resistance to the ICVI lental conditions that have moisture when they are poorly organized 30 It could then be been studied here. these conditions are classical ICvi with high suggested that the thermal treatment allows some decrease in flow rates and the external and upper positions in the preform he content of some impurities, such as oxygen and hydrogen n contrast, all the deposits that have been elaborated via pro atoms that are incorporated in the depos This phenomenon ts. When they are tected-ICVI exhibit greater coherent domains and smaller in- removed, the BN lattice can rearrange terlayer distances and dispersions, in comparison to similar would be favored by an initially very poor organization, and it tings that have been obtained from the classical ICVi pro- would be limited to the surface of the coatings. The structure of For both ICVI and protected-ICVI processes, an interme- each of the BN sequences that are deposited by the alternate liate flow rate leads to the best organization of the film: in the CVD process seems to be sufficiently organized to prevent case of protected-ICVI, the optimal organization is obtained for such a reorganization. However, additional investigations are a higher mass flow rate than that for classical ICVI(see Figs needed to valid these hypotheses and 12). This observation has been confirmed by the experi- ment that was conducted at a flow rate of 56 x 10 kg/s, which did not drastically debase the organization when protected VI was used, in contrast to the experiment that was con From the results that have been presented in Section Ill and ducted without screens at a flow rate of 28 x 106 kg/s discussed in Section IV, the following conclusions can be All these results show that the BN crystalline organization drawn can be optimized under conditions where the ICVI process D The chemical vapor deposition(CVD) process is gov departs both from a pure mass-transfer control (i.e, from al erned either by a combination of mass transfer with chemica most-total conversion at the gas/solid interface )and obviousl kinetics at low flow rates or by the heterogeneous kinetics from a pure kinetic control (which is obtained with CVd at alone at higher flow rates. In this latter case, the deposition rate h flow rates ). These particular infiltration is dependent only on the lowest partial pressure of the reactants ifficult to discuss, because this result might be attributed to BCIs or NH3), and a very good longitudinal uniformity of the different changes in the deposition mechanism. If the surface deposit thickness is obtained. At a deposition temperature of nechanism of the ICVi process remains unchanged from the the supersaturation is high in any case. For the isother imiting CVD process, the BN organization would be improved mal-isobaric chemical vapor infiltration (ICVI)process, the organization of the ICVI deposits that have been obtained at cause of diffusion of the gaseous phase in the fibrous substrate high flow rates without protection is classical, deterioration of and the great deposition yield(because of its high specific the structure at low flow rates is unusual. The poorest organi- surface area). Therefore, the influence of the concentration of zation of the bn deposits has been obtained at the lowest flow the inlet reactant species on the deposition rate is more com- rates, especially in downstream and inside locations(e.g, with plex. The deposition rate is lower than that observed in the high HCI concentration, which could hamper the BN organi- lower rocess, and, accordingly, the supersaturation is much a high deposition yield). This result might be attributed to a CVD (2) The structure of BN coatings that have been deposited for the heterogeneous reaction on the bulk substrates. On the fr other hand, the residence time of the gaseous species is osition conditions, including the processing type(CVD or hanged through the infiltration itself, the variation of the total ICVI, protected or not by diffusion screens ). The present CVD flow rate, and the use of diffusion screens. The homogeneous conditions lead to poor organization of the BN deposits with a formation of intermediate precursors, which leads to a different large interlayer spacing, in accordance to the high supersatu- surface mechanism that favors or hinders the organization of ration. The ICVI process allows deposition of a fairly well- the BN deposit, cannot be excluded either. Evolution of the gas organized BN coating on all fibers of a fibrous preform under hase with time already has been observed in the deposition of conditions where the supersaturation is intermediate. Poorer pyrocarbon via pulsed CvD, 7 which is a process that allows organization that is obtained at lower supersaturation could be the residence time to be monitored by using the pulse duration due to the high yield of deposition, which leads to important The lattice organization and orientation is improved by inter- HCI formation and very different deposition conditions at the mediate"maturation times"of the gaseous phase, because par- ticular intermediates could form. Therefore other thoroug diate species that are produced by some homogeneous reac- studies would be necessary to understand how the bn or tions cannot be excluded under conditions where the residence nization is improved. The influence of the amount of HCl time also is ve or the formation of intermediate species could especially be necessary to vary different. However, additional studies validate these hypothese examined (3) Good thickness uniformities can be obtained via ICVI The above-mentioned discussion about the influence of the despite the influence of mass transfer. In regard to that point, deposition conditions on the BN structure shows that the nature the gaseous product HCl has no influence on the deposition of the fiber obviously cannot influence the structure of the rate. The use of diffusion screens and the adjustment of theerally believed to lead to poor crystal organization.34,35 Thus, it is not surprising that all the CVD deposits present a poor or￾ganization at 700°C (Fig. 8). Forced flow toward single fibers, thin tows, or 2D single cloth (either stationary or moving) can be used to obtain sufficiently uniform coatings that can be better organized at higher temperature.36 The drawbacks of such a process, in terms of the amount of fibers that could be coated and processing complexity, are clear. With regard to the ICVI process, the supersaturation is al￾ways lower than that with CVD at 700°C; however, the gas￾phase chemistry also can be changed by the influence of dif￾ferent parameters. The residence time especially is greater, because of diffusion inside the preform; the conversion yield also is much higher, because of the specific surface of the porous preform. To first address the possible influence of the variation of the supersaturation, it is clear that some conditions lead to high supersaturations and then to poor organization of the ICVI deposits (for the experimental conditions that have been studied here, these conditions are classical ICVI with high flow rates and the external and upper positions in the preform). In contrast, all the deposits that have been elaborated via pro￾tected-ICVI exhibit greater coherent domains and smaller in￾terlayer distances and dispersions, in comparison to similar coatings that have been obtained from the classical ICVI pro￾cess. For both ICVI and protected-ICVI processes, an interme￾diate flow rate leads to the best organization of the film: in the case of protected-ICVI, the optimal organization is obtained for a higher mass flow rate than that for classical ICVI (see Figs. 9 and 12). This observation has been confirmed by the experi￾ment that was conducted at a flow rate of 56 × 10−6 kg/s, which did not drastically debase the organization when protected￾ICVI was used, in contrast to the experiment that was con￾ducted without screens at a flow rate of 28 × 10−6 kg/s. All these results show that the BN crystalline organization can be optimized under conditions where the ICVI process departs both from a pure mass-transfer control (i.e., from al￾most-total conversion at the gas/solid interface) and obviously from a pure kinetic control (which is obtained with CVD at high flow rates). These particular infiltration conditions are difficult to discuss, because this result might be attributed to different changes in the deposition mechanism. If the surface mechanism of the ICVI process remains unchanged from the limiting CVD process, the BN organization would be improved normally first at lower supersaturations. Although the poor organization of the ICVI deposits that have been obtained at high flow rates without protection is classical, deterioration of the structure at low flow rates is unusual. The poorest organi￾zation of the BN deposits has been obtained at the lowest flow rates, especially in downstream and inside locations (e.g., with a high deposition yield). This result might be attributed to a high HCl concentration, which could hamper the BN organi￾zation during growth, even if HCl does not act as an inhibitor for the heterogeneous reaction on the bulk substrates. On the other hand, the residence time of the gaseous species is changed through the infiltration itself, the variation of the total flow rate, and the use of diffusion screens. The homogeneous formation of intermediate precursors, which leads to a different surface mechanism that favors or hinders the organization of the BN deposit, cannot be excluded either. Evolution of the gas phase with time already has been observed in the deposition of pyrocarbon via pulsed CVD,37 which is a process that allows the residence time to be monitored by using the pulse duration. The lattice organization and orientation is improved by inter￾mediate “maturation times” of the gaseous phase, because par￾ticular intermediates could form. Therefore, other thorough studies would be necessary to understand how the BN orga￾nization is improved. The influence of the amount of HCl or the formation of intermediate species could especially be examined. The above-mentioned discussion about the influence of the deposition conditions on the BN structure shows that the nature of the fiber obviously cannot influence the structure of the CVD deposits. In all cases, any possible influence on the initial growth conditions would be hidden by the unfavorable CVD conditions that have been used in this study. However, it seems that the structure can be improved by using a high-temperature post-treatment. In fact, only poorly organized films exhibit a skin effect in the outermost 10–20 nm of the BN coating. Although the graphitization of BN from a turbostratic structure only occurs at ∼2000°C, it has been proved that the external reorganization was the result of the treatment at 1000°C under vacuum. Lacrambe38 also has shown that moderate tempera￾tures (1300°–1700°C) lead to better crystallization of BN that has been prepared via CVD. However, he attributed this im￾provement to the evolution of his samples when previously exposed to a moist environment. In our case, the substrates were kept in the reactor under vacuum between the deposition stage and the thermal post-treatment. However, they do contain oxygen as an impurity,39 and they exhibit poor resistance to moisture when they are poorly organized.30 It could then be suggested that the thermal treatment allows some decrease in the content of some impurities, such as oxygen and hydrogen atoms that are incorporated in the deposits. When they are removed, the BN lattice can rearrange. This phenomenon would be favored by an initially very poor organization, and it would be limited to the surface of the coatings. The structure of each of the BN sequences that are deposited by the alternate CVD process seems to be sufficiently organized to prevent such a reorganization. However, additional investigations are needed to valid these hypotheses. V. Conclusions From the results that have been presented in Section III and discussed in Section IV, the following conclusions can be drawn: (1) The chemical vapor deposition (CVD) process is gov￾erned either by a combination of mass transfer with chemical kinetics at low flow rates or by the heterogeneous kinetics alone at higher flow rates. In this latter case, the deposition rate is dependent only on the lowest partial pressure of the reactants (BCl3 or NH3), and a very good longitudinal uniformity of the deposit thickness is obtained. At a deposition temperature of 700°C, the supersaturation is high in any case. For the isother￾mal–isobaric chemical vapor infiltration (ICVI) process, the limiting contribution of mass transfer is always observed, be￾cause of diffusion of the gaseous phase in the fibrous substrate and the great deposition yield (because of its high specific surface area). Therefore, the influence of the concentration of the inlet reactant species on the deposition rate is more com￾plex. The deposition rate is lower than that observed in the CVD process, and, accordingly, the supersaturation is much lower. (2) The structure of BN coatings that have been deposited from BCl3–NH3–H2 mixtures is highly dependent on the de￾position conditions, including the processing type (CVD or ICVI, protected or not by diffusion screens). The present CVD conditions lead to poor organization of the BN deposits with a large interlayer spacing, in accordance to the high supersatu￾ration. The ICVI process allows deposition of a fairly well￾organized BN coating on all fibers of a fibrous preform under conditions where the supersaturation is intermediate. Poorer organization that is obtained at lower supersaturation could be due to the high yield of deposition, which leads to important HCl formation and very different deposition conditions at the gas/solid interface. The influence of the formation of interme￾diate species that are produced by some homogeneous reac￾tions cannot be excluded under conditions where the residence time also is very different. However, additional studies are necessary to validate these hypotheses. (3) Good thickness uniformities can be obtained via ICVI, despite the influence of mass transfer. In regard to that point, the gaseous product HCl has no influence on the deposition rate. The use of diffusion screens and the adjustment of the 1194 Journal of the American Ceramic Society—Leparoux et al. Vol. 82, No. 5