CVD-Coating of Fabric Sheets in Combination with the LSI-Process

Abstract

Due to short processing times and fairly low manufacturing costs the Liquid Silicon Infiltration Process (LSI) developed at the DLR in Stuttgart is a well established process to produce ceramic matrix composites (CMC) without any fiber coating [1]. The key step is the liquid silicon infiltration of porous carbon/carbon composites (C/C) whose distinct microstructure is formed in the preceding pyrolysis step of carbon fiber reinforced plastics (CFRP). These were produced by resin transfer molding (RTM) in a first step. This process leads to C/C-SiC composites being characterized by high mass-specific properties in combination with an extreme thermal shock stability. By applying special process pa-rameters the microstructure as well as the physical properties can be tailor-designed to match spe-cific requirements. However, compared to CMCs reinforced with coated fibers [2-5], C/C-SiC has a low damage tolerance and tensile strength since the fiber matrix bonding (FMB) is too high. An improvement can be achieved by a fiber pretreatment at about 600 °C prior to composite manufacturing [6]. However, the superior properties of composites manufactured with coated fibers cannot be achieved. Unfortunately, single fiber coating increases CMC costs considerably [7]. In order to overcome this problem, woven C-fabrics have been coated with pyrolytic carbon (pyC) and then submitted to the LSI-process yielding dense C/C-SiC. In a first step, C-fabric sheets (30 mm in width and 300 mm in length) were coated with pyC in both a CVD hot wall and alternatively in a CVD cold wall reactor under static condi-tions at about 900 °C. Toluene was used as a precursor whereas argon served as a carrier gas. Deposition times ranging from 10 to 20 min resulted in an increase in weight of 50 to 250 mg corre-sponding to a mean layer thickness of 39 to 189 nm on each single fiber. The pyC-coated C-fabrics exhibited increased stiffness with increased coating thickness as well as reference samples were submitted to the conventional LSI-process. Besides SEM-investigations on the pyC-coating, this paper reports on the physical and the mechanical properties of the composites manufactured via LSI-processing in all stages – CFRP, C/C and especially C/C-SiC.