Influence of Isothermal CVD and CV Conditions on the Deposition Kinetics and Structure of BN 1191 12 64202 0.30 38 5o8 f=14x10°kos 04s 望 8642 050100150200250300350400 P。 sition from the fiber surfac·nm) 5040 Evolution of the greatest coherent domain and the interlayer Position from the fIber surface (nm) in the bn thickness as a function of the mass flow rate for the and La are shaded gray and white, respec- mples were taken in the middle of the fibrous preforms(the Fig. 8. Evolution of the greatest coherent domain and the interlayer cimen reference distance in the Bn thickness for different processes. Samples were taken in the middle of the tows or of the fibrous preforms(the 6.2 pecimen reference). Le and La data are shaded gray and white, re- spectively(T= 700C, P= 1.33 kPa, NH /BCl3= 1, H/BCl3= 1, La. In any case, the interlayer distances remain relativel andf=7×10°kg/s) small, with average values of -0.35 nm, all along the bn film (Fig. 12). An additional experiment that was conducted at a flow rate of 56 x 10 kg/s led to a bn structure that was similar to that obtained at a flow rate of 14 x 10-6 kg/s the Icvi processes. No"skin effect" was observed in the most At each position of the substrate to be coated, the deposition external portion, in contrast to the CVD coatings. The range of and growth conditions obviously are dependent on the partial the interlayer distances was narrow, and values as low as 0.33 ressures of all the species that are present. Therefore, the nm(which corresponds to the theoretical distance for a hex- structure of the bn coating that has been deposited using the agonal BN crystal)were observed. The protected-ICVI process standard conditions has been investigated for three different roduced the best organization, with lower standard deviation positions in the fibrous preform. For example, Fig. 13 shows of the interlay results that have been obtained with the protected -ICVI pro- The evolution of the structure of the deposits in the middle cess. For this fairly low mass flow rate, the results that have of the NovoltexTM preform has been observed for three total been obtained with classical ICVI are not very different. In any flow rates--7x 10 6, 14 x 10, and 28 x 106 kg/s--for both case, the 6.1(6.3) specimen presents the best organization, as ICVI processes. Figure 9 shows the variation of the greatest indicated by its greater coherent domains. However, all the coherent domain and the interlayer distance from the carbon- variations that can be noted are moderate in comparison to the fiber/BN interface to the external portion of a coating that has variations of the structure that has been produced by using the been deposited via classical ICVI. The two lowest mass flow other experimental parameters. This observation is confirmed rates lead to similar structures; however, the BN organization by the examination of the variations of the interlayer distances seems to be slightly better at a flow rate of 14 x 10"b kg/s. In The interlayer distances remain relatively small and are only contrast, the bn organization is altered for the highest floy slightly dispersed, regardless of the position and the infiltration (i.e, at 28x" kg/s), especially at distances greater than -50 process nm from the fiber surface. Figures 10(a)and(b) respectivel A particular skin effect was observed for the CVD processes illustrate the structures that have been obtained for this flo (Fig 8). Observation of the microtexture with and without the rate at a distance of -50 nm from the fiber and near the fiber/ thermal post-treatment clearly shows that this effect results BN interface. Figure 10(a) is representative of one of the poor- from the post-treatment. For ICVI processes, such an effect est BN lattice organizations. However, this organization never appeared. In addition to the standard treatment, different presents characteristics that are even better than the structure treatment times were tested at 1000C (I and 4 h)and a treat- that is generally observed in the CVD coatings, except in their ment at 1200 C was performed for 2 h. Moreover, other treat- external portion. In contrast, with the protected-ICVI process, ments were conducted at 1.33 kPa in various gaseous atmo- this relatively high flow rate produces the best BN organiza- spheres, which included hydrogen, argon, nitrogen, and tion, as shown in the micrographs in Fig. 11. Figure 12 con- helium. Unfortunately, no improvement of the BN lattice firms the large dimensions of the greatest coherent domain ganization was observed. An alternate CVD process that in- The crystal growth seems to be preferentially columnar, i.e., Le volved several short deposition steps, usthe ICVI processes. No “skin effect” was observed in the most￾external portion, in contrast to the CVD coatings. The range of the interlayer distances was narrow, and values as low as 0.33 nm (which corresponds to the theoretical distance for a hex￾agonal BN crystal) were observed. The protected-ICVI process produced the best organization, with lower standard deviations of the interlayer spacing. The evolution of the structure of the deposits in the middle of the Novoltex™ preform has been observed for three total flow rates—7 × 10−6 , 14 × 10−6 , and 28 × 10−6 kg/s—for both ICVI processes. Figure 9 shows the variation of the greatest coherent domain and the interlayer distance from the carbon￾fiber/BN interface to the external portion of a coating that has been deposited via classical ICVI. The two lowest mass flow rates lead to similar structures; however, the BN organization seems to be slightly better at a flow rate of 14 × 10−6 kg/s. In contrast, the BN organization is altered for the highest flow (i.e., at 28 × 10−6 kg/s), especially at distances greater than ∼50 nm from the fiber surface. Figures 10(a) and (b) respectively illustrate the structures that have been obtained for this flow rate at a distance of ∼50 nm from the fiber and near the fiber/ BN interface. Figure 10(a) is representative of one of the poor￾est BN lattice organizations. However, this organization presents characteristics that are even better than the structure that is generally observed in the CVD coatings, except in their external portion. In contrast, with the protected-ICVI process, this relatively high flow rate produces the best BN organiza￾tion, as shown in the micrographs in Fig. 11. Figure 12 con￾firms the large dimensions of the greatest coherent domain. The crystal growth seems to be preferentially columnar, i.e., Lc > La. In any case, the interlayer distances remain relatively small, with average values of ∼0.35 nm, all along the BN film (Fig. 12). An additional experiment that was conducted at a flow rate of 56 × 10−6 kg/s led to a BN structure that was similar to that obtained at a flow rate of 14 × 10−6 kg/s. At each position of the substrate to be coated, the deposition and growth conditions obviously are dependent on the partial pressures of all the species that are present. Therefore, the structure of the BN coating that has been deposited using the standard conditions has been investigated for three different positions in the fibrous preform. For example, Fig. 13 shows results that have been obtained with the protected-ICVI pro￾cess. For this fairly low mass flow rate, the results that have been obtained with classical ICVI are not very different. In any case, the 6.1 (6.3) specimen presents the best organization, as indicated by its greater coherent domains. However, all the variations that can be noted are moderate in comparison to the variations of the structure that has been produced by using the other experimental parameters. This observation is confirmed by the examination of the variations of the interlayer distances. The interlayer distances remain relatively small and are only slightly dispersed, regardless of the position and the infiltration process. A particular skin effect was observed for the CVD processes (Fig. 8). Observation of the microtexture with and without the thermal post-treatment clearly shows that this effect results from the post-treatment. For ICVI processes, such an effect never appeared. In addition to the standard treatment, different treatment times were tested at 1000°C (1 and 4 h) and a treat￾ment at 1200°C was performed for 2 h. Moreover, other treat￾ments were conducted at 1.33 kPa in various gaseous atmo￾spheres, which included hydrogen, argon, nitrogen, and helium. Unfortunately, no improvement of the BN lattice or￾ganization was observed. An alternate CVD process that in￾volved several short deposition steps, using standard condi￾Fig. 8. Evolution of the greatest coherent domain and the interlayer distance in the BN thickness for different processes. Samples were taken in the middle of the tows or of the fibrous preforms (the 6.2 specimen reference). Lc and La data are shaded gray and white, re￾spectively (T 4 700°C, P 4 1.33 kPa, NH3/BCl3 4 1, H2/BCl3 4 1, and f 4 7 × 10−6 kg/s). Fig. 9. Evolution of the greatest coherent domain and the interlayer distance in the BN thickness, as a function of the mass flow rate for the classical ICVI process. Lc and La are shaded gray and white, respec￾tively. Samples were taken in the middle of the fibrous preforms (the 6.2 specimen reference). May 1999 Influence of Isothermal CVD and CVI Conditions on the Deposition Kinetics and Structure of BN 1191