1188 Journal of the American Ceramic Society-Leparoiex et al. Vol. 82. No 5 tion conditions. 30 To gain some insight into the correlations The thickness of these fibrous substrates was always very low between the deposition conditions and the BN structure, fir 0. 4 mm); therefore, the deposition conditions that have beer deposition conditions where the CVd process is purely con oIled by the surface kinetics have been determined. 30,t These or similar to(for tows) those from the CVD process. Different onditions allow investigation of the influence of the different fibers have been used to observe the influence of the nature of let gaseous species on the surface kinetics. Then, how the the substrate: these fiber types include T300(ex-PAN, average deposition conditions are influenced by mass transfer and de diameter of 8 um, Torayca, produced by Soficar, Abidos, art a fur France) and P55(pitch-derived, average diameter of 10 um ion of either the process type (CVD or ICVI), processing Thornel, produced by Amoco Polymers, Chicago, IL) carbon parameters, inlet gas composi iber, as well as NicalonTM NLM 202 fiber(ex-PCS, average given in this study. The influence of diffusion cages that are diameter of 15 um) positioned around the fibrous preform also is reported. Finally, The infiltration process(ICvI) was investigated by us the bn structure is studied using transmission electron micros- three-dimensional (3D)woven architecture(NovoltexTM, Soci- copy(TEM), under these various CVD and ICVI condition ete Europeenne de Propulsion(SEP), Saint-Medard-en-Jalles and the variations of the uniformity of the deposit are reported. France)that was composed of carbon fibers that were 9 um in diameter(ex-PAN, treated by SEP), this NovoltexTM material Il. Experimental Procedure had an average pore diameter of 45 um and a specific surface area of -0.25 m/g. The fibrous preform (150 mm x 18 mm x BN was deposited by using different isothermal CVD pro- Suring 15 mm x 18 mm x 3.3 mm. Each element was [el (I Materials and Deposition Procedure 10 mm) was cut into thirty intermediate samples, each me cesses at a temperature(T)of 700oC, under a reduced pressure enced by using the nomenclature"m n, "where m=1-10 for (P)of 1.3 kPa, from BCI-NH -H, gas mixtures of different the longitudinal position and n -3 for the transverse pos inlet composition. The deposition apparatus and procedure tion(see Fig. 1 ). Thus, different parameters have been intro- have been described in detail previously 30-32 Considering the duced to characterize the uniformity of the deposit. For the poor stability of Bn that was obtai the deposits were always thermally treated at 1000 C at the end thickness uniformity, which is calculated as the ratio between of the deposition stage, directly in the reactor, for 2 h unde he lowest and the highest relative mass increases along the vacuum(residual pressure of I Pa preform(m = 1-10). The variable IU represents the infiltration bn deposition was performed simultaneously on bulk sub- uniformity, which is defined as the average of ten ratios be- strates and a fibrous preform, to investigate both CVD and tween the internal elemental relative mass gain(n= 2)and the ICVI, respectively(Fig. 1). One bulk substrate was composed of ten graphite rings that were mounted on a holder. The n= 3). The total uniformity (TU) is defined as TU =LU X deposition process also was applied to one-dimensional (ID) IU. For the bulk substrates, the variable U represents the lon- fiber substrates, which consisted of either small tows or mon gitudinal uniformity, which is calculated as the ratio between filaments that were mounted on a carbon fixture. and to two- he relative weight increase at the bottom and top of the ten dimensional(2D) single-plane-weave fabrics In any case, the graphite rings after smoothing the curve that reports the relative ain surface of the substrate was parallel to the gaseous flow. weight gain of the ten substrates In some cases, diffusion screens were placed in fron fiber substrates(they were not used to protect the ngs). These screens created a pure diffusive volume the substrates, which were separated from the convective floy In case of the Cvd process on monofilaments or small tor the screens were made of carbon sheets with many holes bored into them The screens were mounted on both sides of the fiber carbon-based cement. In of Icvi the diffusion screens were composed of graphite plates that com- etely surrounded the preform(Fig. 2). The gaseous phase could only diffuse through oblong holes that had been bored into the plates that were positioned in front of the main surface substrate of the substrate. The axis of these holes was perpendicular to the external convective-flow direction and their diameter was four times smaller than the thickness of the plate. In the fol- lowing text, the terms"protected-CVD"and"protected-ICVI are used to respectively denote CVD and ICVI processes that were performed with protection screens ( Structural Characterization The BN-coated fibers were characterized via TEM. The analyses were performed at CNEA(Universite d'Orleans)on thin foils that had been prepared in a conventional manner, by inclusion in a methylmethacrylate resin or in Araldit (Cib Specialty Chemicals Holding, Basel, Switzerland) and cutting normal to the fiber axis via ultramicrotomy. Transverse varia- tions of the BN structure were obtained by scanning different zones of the coating, from the interface at the fiber to the external portion of the Bn deposit. Generally, three sections were observed and one representative BN coating was choser to be studied on each n. The dimensions of the greatest coherent domain and the interlayer distance, which is normally 0.333 nm for hexagonal BN. were measured on the TEm im Fig. 1. Schematic of the bulk substrates and the fibrous preform with ages. In the reported results, La represents the dimension of the their holders in the deposition reactor. in-plane extension and Le represents the extension of the co-tion conditions.30 To gain some insight into the correlations between the deposition conditions and the BN structure, first, deposition conditions where the CVD process is purely con￾trolled by the surface kinetics have been determined.30,31 These conditions allow investigation of the influence of the different inlet gaseous species on the surface kinetics. Then, how the deposition conditions are influenced by mass transfer and de￾part from this limiting case of pure kinetic control, as a func￾tion of either the process type (CVD or ICVI), processing parameters, inlet gas composition, and/or total gas flow rate, is given in this study. The influence of diffusion cages that are positioned around the fibrous preform also is reported. Finally, the BN structure is studied, using transmission electron micros￾copy (TEM), under these various CVD and ICVI conditions, and the variations of the uniformity of the deposit are reported. II. Experimental Procedure (1) Materials and Deposition Procedure BN was deposited by using different isothermal CVD pro￾cesses at a temperature (T) of 700°C, under a reduced pressure (P) of 1.3 kPa, from BCl3–NH3–H2 gas mixtures of different inlet composition. The deposition apparatus and procedure have been described in detail previously.30–32 Considering the poor stability of BN that was obtained at the given temperature, the deposits were always thermally treated at 1000°C at the end of the deposition stage, directly in the reactor, for 2 h under vacuum (residual pressure of 1 Pa). BN deposition was performed simultaneously on bulk sub￾strates and a fibrous preform, to investigate both CVD and ICVI, respectively (Fig. 1). One bulk substrate was composed of ten graphite rings that were mounted on a holder. The deposition process also was applied to one-dimensional (1D) fiber substrates, which consisted of either small tows or mono￾filaments that were mounted on a carbon fixture, and to two￾dimensional (2D) single-plane-weave fabrics. In any case, the main surface of the substrate was parallel to the gaseous flow. The thickness of these fibrous substrates was always very low (<0.4 mm); therefore, the deposition conditions that have been considered in this paper were the same as (for monofilaments) or similar to (for tows) those from the CVD process. Different fibers have been used to observe the influence of the nature of the substrate: these fiber types include T300 (ex-PAN, average diameter of 8 mm; Torayca, produced by Soficar, Abidos, France) and P55 (pitch-derived, average diameter of 10 mm; Thornel, produced by Amoco Polymers, Chicago, IL) carbon fiber, as well as Nicalon™ NLM 202 fiber (ex-PCS, average diameter of 15 mm). The infiltration process (ICVI) was investigated by using a three-dimensional (3D) woven architecture (Novoltex™, Soci￾e´te´ Europe´enne de Propulsion (SEP), Saint-Medard-en-Jalles, France) that was composed of carbon fibers that were 9 mm in diameter (ex-PAN, treated by SEP); this Novoltex™ material had an average pore diameter of 45 mm and a specific surface area of ∼0.25 m2 /g. The fibrous preform (150 mm × 18 mm × 10 mm) was cut into thirty intermediate samples, each mea￾suring 15 mm × 18 mm × 3.3 mm. Each element was refer￾enced by using the nomenclature “m.n,” where m 4 1–10 for the longitudinal position and n 4 1–3 for the transverse posi￾tion (see Fig. 1). Thus, different parameters have been intro￾duced to characterize the uniformity of the deposit. For the fibrous preforms, the variable LU represents the longitudinal thickness uniformity, which is calculated as the ratio between the lowest and the highest relative mass increases along the preform (m 4 1–10). The variable IU represents the infiltration uniformity, which is defined as the average of ten ratios be￾tween the internal elemental relative mass gain (n 4 2) and the average external mass (calculated from values for n 4 1 and n 4 3). The total uniformity (TU) is defined as TU 4 LU × IU. For the bulk substrates, the variable U represents the lon￾gitudinal uniformity, which is calculated as the ratio between the relative weight increase at the bottom and top of the ten graphite rings after smoothing the curve that reports the relative weight gain of the ten substrates. In some cases, diffusion screens were placed in front of the fiber substrates (they were not used to protect the graphite rings). These screens created a pure diffusive volume around the substrates, which were separated from the convective flow. In case of the CVD process on monofilaments or small tows, the screens were made of carbon sheets with many holes bored into them. The screens were mounted on both sides of the fiber fixture, using a carbon-based cement. In case of ICVI, the diffusion screens were composed of graphite plates that com￾pletely surrounded the preform (Fig. 2). The gaseous phase could only diffuse through oblong holes that had been bored into the plates that were positioned in front of the main surface of the substrate. The axis of these holes was perpendicular to the external convective-flow direction, and their diameter was four times smaller than the thickness of the plate. In the fol￾lowing text, the terms “protected-CVD” and “protected-ICVI” are used to respectively denote CVD and ICVI processes that were performed with protection screens. (2) Structural Characterization The BN-coated fibers were characterized via TEM. The analyses were performed at CNEA (Universite´ d’Orle´ans) on thin foils that had been prepared in a conventional manner, by inclusion in a methylmethacrylate resin or in Aralditt (Ciba Specialty Chemicals Holding, Basel, Switzerland) and cutting normal to the fiber axis via ultramicrotomy. Transverse varia￾tions of the BN structure were obtained by scanning different zones of the coating, from the interface at the fiber to the external portion of the BN deposit. Generally, three sections were observed and one representative BN coating was chosen to be studied on each specimen. The dimensions of the greatest coherent domain and the interlayer distance, which is normally 0.333 nm for hexagonal BN, were measured on the TEM im￾ages. In the reported results, La represents the dimension of the in-plane extension and Lc represents the extension of the co￾Fig. 1. Schematic of the bulk substrates and the fibrous preform with their holders in the deposition reactor. 1188 Journal of the American Ceramic Society—Leparoux et al. Vol. 82, No. 5