employ processes which have a large throwing power. All piping was made of stainless steel and heated to 500 The CVD process is one of such processes. Deposition K. Typical deposition conditions were: deposition tem of SiC or BN coatings on fibers with CVD process has perature 300-500oC, pressure 500 Pa. 4 been proven to be successful. 3 This paper describes The deposition equipment is shown in Fig 3. In this methods for coating of Nextel M 720-mullite/alumina- equipment, two different evaporators were installed, in fibers with a porous ZrO2-layer and with a C/ZrO2- order to produce multi-component coatings if necessary double layer resulting in a fugitive coating, sealed with a The reaction gas was transported into a quartz tube in a porous oxide layer resistance heated furnace. The internal diameter of the quartz tube was 10.5 cm and the aluminosilicate fiber fabrics, cut into rectangles of 50x 50 mm, were installed 2. Experimental in this quartz tube. The fabrics were arranged at a position perpendicular to the gas flow. Maximum 4 2. Materials samples could be placed in the resistance furnace. The deposited quantity was measured by the mass differ Precursors with high vapor pressures at moderate ence before (1) and after the deposition (m2) temperatures are preferred for CVD-coatings. Candi The molar deposition rate was calcu date precursors are chlorides, alkoxides and B-dikato- lated by assuming that ZrO, is deposited: nates. Zirconium acetylacetonate [Zr(acac)4 is a B- dikatonate and has a vapor pressure of approx 700 Pa (1) at 200 C, which is high enough to be used in CVD processes. The CVd processing of Zr(acac)4 to produce ZrO2-coatings on flat substrates was described in the where M is the molar mass of Zro literature. 4-7 These studies reported that some carbon The carbon precoating of the Nextel fibers was carried was entrapped in the coating system, because of the out under a pressure of 12 hPa at a deposition tem incomplete decomposition on the substrate surface. This perature of 950%C for 45 min. The carrier gas was pro characteristic of the precursor may be benefited for the pane and its flow rate was set to 167 sccm synthesis of porous ZrO] -layers, without application of another substance for achievement of porosity. The 2.3. Processing of composite Zr(acac)4 powder was of 98% purity and provided by FLUKA(Germany) The pre-mullite(Siral) powder was calcined at about The woven fiber fabrics(8 harness Atlas) were of 1100C prior to preparation of the slurry. X-ray dif- mullite/alumina [ Nextel(720)] and were provided by 3M fraction data of this calcined powder yielded a weakly (Minnesota, USA). The fabrics had the form shown in crystalline y-Al2O3 phase, accompanied by some Sio Fig. I. Woven fiber tows were consisted of fibers with a rich amorphous phase Formation of mullite under given diameter of 12 um. The chemical composition was 85 hot-pressing conditions occurred after I minute holding wt%Al,O3 and 15 wt%SiO, with a density of 3.4 time at 1250C. Just at the hot-pressing temperature, cm3. Each fiber tow contains approximately 400 single without any holding time, no mullite formation was observed. For a full transformation to mullite, it was The matrix material for the composites was produced necessary to hold for 15 min at 1250C under 20 MPa by hot-pressing pre- mullite powders (Siral, CONDEA, Prolonged holding times yielded no improvement in the Germany). This powder has a purity of 99.99% and is densification and mullitization. In order to avoid fiber amorphous in the as-received form damage, the optimum holding time was limited to 15 min Slurry was prepared by mixing of the calcined pow 2. 2. CVD-process and experimental equipmen ders with binder and disperser in an aqueous media Although, for coating, the fiber fabrics were used, for Decomposition temperature of Zr(acac)4 is rather the preparation of the prepegs, the fiber tows were low, hence evaporation rate becomes unstable, if it is pulled out of the fabric to fabricate ID-unidirectional heated for longer times at temperatures above 160C. fiber composites. As-coated fiber tows were immersed in This behavior requires special evaporation equipment, the slurry and formed to rectangular shaped plates on which is shown schematically in Fig. 2. The powdery plaster of Paris moulds. After drying the prepegs in air, precursor is transported through a rotating plate with the stacked prepegs were uniaxially cold-pressed under holes into the evaporator which is heated to 200C and approximately 2 MPa pressure. Hot-pressing of the there evaporates spontaneously. The carrier gas with ZrO2-coated fiber composites was carried out in air at the precursor then is transported through tubes to the 1250 C for 15 min under 20 MPa uniaxial pressure CI urnace. The precursor concentration in the gas was ZrO2-coated fiber composites were also hot-pressed adjusted to 0. 1 mol% at a gas flow of 200 sccm argon. under the same conditions, however in flowing argonemploy processes which have a large throwing power. The CVD process is one of such processes. Deposition of SiC or BN coatings on ®bers with CVD process has been proven to be successful.3 This paper describes methods for coating of NextelTM 720-mullite/alumina- ®bers with a porous ZrO2-layer and with a C/ZrO2- double layer resulting in a fugitive coating, sealed with a porous oxide layer. 2. Experimental 2.1. Materials Precursors with high vapor pressures at moderate temperatures are preferred for CVD-coatings. Candi￾date precursors are chlorides, alkoxides and b-dikato￾nates. Zirconium acetylacetonate [Zr(acac)4] is a b￾dikatonate and has a vapor pressure of approx. 700 Pa at 200C, which is high enough to be used in CVD￾processes. The CVD processing of Zr(acac)4 to produce ZrO2-coatings on ¯at substrates was described in the literature.4±7 These studies reported that some carbon was entrapped in the coating system, because of the incomplete decomposition on the substrate surface. This characteristic of the precursor may be bene®ted for the synthesis of porous ZrO2-layers, without application of another substance for achievement of porosity. The Zr(acac)4 powder was of 98% purity and provided by FLUKA (Germany). The woven ®ber fabrics (8 harness Atlas) were of mullite/alumina [Nextel(720)] and were provided by 3M (Minnesota, USA). The fabrics had the form shown in Fig. 1. Woven ®ber tows were consisted of ®bers with a diameter of 12 mm. The chemical composition was 85 wt% Al2O3 and 15 wt% SiO2 with a density of 3.4 g/ cm3 . Each ®ber tow contains approximately 400 single ®bers. The matrix material for the composites was produced by hot-pressing pre-mullite powders (Siral, CONDEA, Germany). This powder has a purity of 99.99% and is amorphous in the as-received form. 2.2. CVD-process and experimental equipment Decomposition temperature of Zr(acac)4 is rather low, hence evaporation rate becomes unstable, if it is heated for longer times at temperatures above 160C.4 This behavior requires special evaporation equipment, which is shown schematically in Fig. 2. The powdery precursor is transported through a rotating plate with holes into the evaporator which is heated to 200C and there evaporates spontaneously. The carrier gas with the precursor then is transported through tubes to the furnace. The precursor concentration in the gas was adjusted to 0.1 mol% at a gas ¯ow of 200 sccm argon. All piping was made of stainless steel and heated to 500 K. Typical deposition conditions were: deposition tem￾perature 300±500C, pressure 500 Pa.4 The deposition equipment is shown in Fig. 3. In this equipment, two di€erent evaporators were installed, in order to produce multi-component coatings if necessary. The reaction gas was transported into a quartz tube in a resistance heated furnace. The internal diameter of the quartz tube was 10.5 cm and the aluminosilicate ®ber fabrics, cut into rectangles of 5050 mm2 , were installed in this quartz tube. The fabrics were arranged at a position perpendicular to the gas ¯ow. Maximum 4 samples could be placed in the resistance furnace. The deposited quantity was measured by the mass di€er￾ence before (m1) and after the deposition (m2)
m ˆ m2 ÿ m1. The molar deposition rate was calcu￾lated by assuming that ZrO2 is deposited: n : m ˆ
m AM …1† where M is the molar mass of ZrO2. The carbon precoating of the Nextel ®bers was carried out under a pressure of 12 hPa at a deposition tem￾perature of 950C for 45 min. The carrier gas was pro￾pane and its ¯ow rate was set to 167 sccm. 2.3. Processing of composites The pre-mullite (Siral) powder was calcined at about 1100C prior to preparation of the slurry. X-ray dif￾fraction data of this calcined powder yielded a weakly crystalline g-Al2O3 phase, accompanied by some SiO2- rich amorphous phase. Formation of mullite under given hot-pressing conditions occurred after 1 minute holding time at 1250C. Just at the hot-pressing temperature, without any holding time, no mullite formation was observed. For a full transformation to mullite, it was necessary to hold for 15 min at 1250C under 20 MPa. Prolonged holding times yielded no improvement in the densi®cation and mullitization. In order to avoid ®ber damage, the optimum holding time was limited to 15 min. Slurry was prepared by mixing of the calcined pow￾ders with binder and disperser in an aqueous media. Although, for coating, the ®ber fabrics were used, for the preparation of the prepegs, the ®ber tows were pulled out of the fabric to fabricate 1D-unidirectional ®ber composites. As-coated ®ber tows were immersed in the slurry and formed to rectangular shaped plates on plaster of Paris moulds. After drying the prepegs in air, the stacked prepegs were uniaxially cold-pressed under approximately 2 MPa pressure. Hot-pressing of the ZrO2-coated ®ber composites was carried out in air at 1250C for 15 min under 20 MPa uniaxial pressure. C/ ZrO2-coated ®ber composites were also hot-pressed under the same conditions, however in ¯owing argon 538 K. Nubian et al. / Journal of the European Ceramic Society 20 (2000) 537±544