Chemical Vapour Infiltration (cvi) of Silicon Carbide Fibre Preforms

An alternative fabrication technique is proposed for ceramic matrix / ceramic fibre composites involving filling a fibre fabric preform with a SiSi N slurry and subsequently reaction bonding. For the first processing step in this process, chemical vapour infiltration (CVI) was evaluated as a means of both binding together and protecting the fibres. Sic (Nicalon) fibre p re fom consisting of through-stitched 2D fabrics were idiltrated. The prefom thickness was more than 4 mm and a 3 pm TiCEiN layer was subsequently deposited on all the fibres, even in the centre of the preform after less than 8 h. The purpose of the CVI layer was firstly to bind the fibres together, but also to protect the fibre/matrix interfacial carbon film from reaction with the subsequently infiltrated matrix. The remaining porosity of the preform was filled with a SiJSi N -powder slurry which was nitrided to form a reaction bonded Sig N 4 (RBSN) matnx. The advantage of this combined CVIKBSN route is a substantla1 cut in processing time compared to a process where most of the porosity is filled using CVI. 1 .Introduction Ceramic composite materials are becoming more and more interesting for aerospace applications where low density, good oxidation and corrosion resistance and high strength at temperatures above 1000°C are required (1-4). The general principle of long fibre reinforcement of ceramics is not so much to reinforce, i.e. to strengthen or stiffen (which is the case for polymer or metal matrix composites), but rather to overcome poor reliab'ity and brittleness of ceramic materials by creating a "graceful" fracture due to energy absorbing, crack growth controlling mechanisms (5). These mechanisms can only operate if the fibrelmatrix bond strength is sufficiently low in relation to fibre strength to prevent cracks from propagating through the fibres and to result in a fracture behaviour where the fibres remain undamaged in spite of multiple matrix cracking. The currently most usual method of obtaining a weak fibrelmatrix interface is to deposit a thin coating of a low shear strength material such as boron nitride or graphite on the fibres. Unfortunately both BN and C are readily oxidised at temperatures well below 1000°C as well as being fairly reactive towards many ceramic matrix materials at the processing temperature. A class of ceramic composites manufactured by the chemical vapour infiltration (CVI) of three-dimensionally woven fibre preforms (see Figure 1.) have been developed, mainly in France (6,7). Sic (Nicalon) fibres in a CVI-Sic matrix are currently being evaluated as a potential aeroengine material. In this material the pure SIC matrix protects the fibrelmatrix interface from oxidation during use. To build up the entire matrix by CVI is a tedious process in which Sic is deposited at a low deposition rate inside the pore structure of a Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991260 C2-492 JOURNAL DE PHYSIQUE 1V fibre preform. When the surface pores become closed, the partly infiltrated preform is taken out of the reactor and diamond ground in order to open up the surface. This process requires processing times of the order of weeks to months and is consequently fairly costly. The resulting mechanical properties at high temperatures and in oxidking environments of the composites are, however, excellent. The aim of the present work has been to develop a production method for ceramic composites, avoiding time consuming CVI and using fairly low cost matrix materials, while stiU obtaining adequate high temperature properties. The general approach was to use CVI only for the first matrix infiltration, in order to obtain a protective ceramic coating on the thin carbon layer already present on the fibre surface. The remaining porosity in the preform was fdled using a previously developed method (8-11) involving Si(s)/Si N (s) slurry intihation and reaction with N2(g) (nitridation) to form a reaction bonded Sx3 N 4 (RBSN) matrix. Figure 1. Some examples of 3-dimensionally woven fibre preforms. TiCmiN was chosen as fibre mating on the basis of thermodynamic computer-aided calculations (12) which showed that silicon forms Si N in N rather than reacting with TiN (see Table I.), and that T ic is inert towards carbon and SIC. Furthermore, to use a Ti-based rather than a Si-based coating makes it more straightforward to use a standard CVD equipment as used by the cemented carbide industry. Table 1. Calcgated equilibrium compositions in the TiISi-system at P = 1 atm (12). 1. NITROGEN ATMOSPHERE Reactants (moles): 1 T i ( s ) , 3 Si(s) and 10 N (g). at 1400°C = > 1 Si N (s) 1 TiN(s) 8 N (g) 2. ARGON ATMOSPERE Reactants (moles): 1 T i ( s ) , 3 Si(s) and 1 Ar(g) Several investigations have shown that it is possible to deposit various ceramic coatings on ceramic monofilaments, ceramic fibre tow, or single fabrics in a laboratory scale reactor (13,14). Little has however been reported on process parameters for full scale CVI of larger components. In the present investigation the possibility of performing chemical vapour infiltration using a production scale reactor was therefore evaluated. 2.Experimental Orthogonal eight harness satin weaves of Sic fibres (Nicalon NL 607, C-coated grade, Nippon Carbon Ltd, Japan, fibre mean diameter 15 pm) were stitched together using more flexible lower diameter (10 pm) Si-Ti-C-0 fibres (Tyranno, UBE Industries, Japan). In this way two-directional through-stitched stacked fabric preforms of more than 4 mm thickness were produced. The priciple of the fibre architecture is close to the 2.5D preform shown in Figure 1. The Sic fibres were supplied by the manufacturer with a thin carbon surface coating to ensure a low fibrelmatrix bond strength in ceramic systems. The CVI was performed at T ion AB in a production scale CVD reactor (250 Tandem, Berna Ltd, Schweiz) with a cylindrical shape (2251750 mm inner diameterheight). Tic1 $, H and CH at 80 mbar, 1040°C for 2.25 h were used for the Tic layer and Tic1 4 , H 2 , N and HCI at 400 mbar, 950°C for 2.75 h yielded the T i . The SilSi N slurry infiltration and subsequent nitridation methods have been reported elsewhere (8-11). 3.Results and Discussion As can be seen in Figure 2 it was possible to infiltrate the fibre preform to its centre and deposit a TiCfliN layer on the Sic fibres. The process conditions used were not very different from the conditions used for CVD coating of non-porous material. These conditions could be used since only a fairly thin ceramic layer was desired. If a high proportion of the porosity is to be tilled using CVI more care must be taken in optimisiig the deposition rate so as to obtain a higher deposition rate at the centre of the fibre preform than at the surface (l5,16). The 3 +m ceramic layer provided rigidity to the flexible preform so that no support was needed during the subsequent slurry infiltration. Figure 3 shows the fully infiltrated SiC/(TiCfl'ii-CVI)/RBSN composite material after nitriding. As predicted the TiCfl'ii-layer has not reacted, neither with the Sic fibres during CVI nor with the silicon during the reaction bonding step. Figure 2. SEM micograph of TiCfliN-infiltrated Sic-fibre preform. From the centre of a 4 mm thick piece. (Back-scattered electron image. Fibre diameter 15 pm) C2-494 JOURNAL DE PHYSIQUE IV