Development of brazing process in ceramic matrix composites for in-space applications

Abstract

The rapid development of high-tech in aerospace from the past to the present has led to the investigation of new materials which can be tolerant of extremely harsh working environments. Studies carried out to meet the demands of the aerospace industry have accelerated the development of novel materials such as superalloys, ceramics, and composites. Composite materials are formed by a combination of a matrix and a reinforcing material. These materials, each of which having different properties, provide enhanced properties unobtainable in either single material. Ceramic matrix composites (CMCs), a subgroup of composite materials, are designed to overcome some disadvantages of monolithic ceramics such as intrinsic brittleness and lack of reliability. The CMCs considered in this study consist of carbon fibers embedded in a SiC matrix. Owing to the excellent combination of high hardness, lightweight, wear resistance, heat and thermal shock resistance properties, carbon fiber reinforced SiC (C/C-SiC) ceramic matrix composites are ideal candidates for being used at high temperatures in aerospace applications such as hypersonic craft thermal structures, propulsion chambers and nozzle extensions for advanced rockets. Although there is extensive knowledge about the manufacturing processes of CMCs, their use in aerospace industry is still quite restricted due to the difficulty in integrating them into the main metal structure. Most structural designs in the aerospace industry require joining CMCs to metals. Thus, the joining of CMC materials with metals is one of the most important cutting-edge research topics that need to be improved upon. The brazing process which is used for the joining of materials using filler metals with a solidification temperature of less than the base material and a melting temperature above 450 °C is the most frequently preferred joining method for CMCs. The scope of this thesis mainly consists of two studies. As the output of the first study, C/C-SiC ceramic matrix composite plates were manufactured by liquid silicon infiltration (LSI) process. Firstly, carbon fiber reinforced plastic (CFRP) preforms were produced by infiltrating the liquid polymer precursor into the fiber preform and then curing the fibers inside this phenolic resin matrix via out of autoclave (OOA) technique. CFRP preforms were obtained using two different fiber types. The first fiber type used was randomly oriented fiber reinforcements, the so-called polyacrylonitrile (PAN)-based short fibers, representing cut fibers with typical lengths of 3 mm. The second fiber type used was pitch-based short fibers with lengths of 12 mm. Then, density and porosity tests of the produced CFRP samples were carried out and the process was tried to be optimized by making trials in the furnace cycle, the filling methods of the process, and the excess resin ratio according to these test results. By comparing the quality of the samples obtained from the trial productions, the final process providing the best sample specs was chosen as the final process. By following these process steps, CFRP preforms made of pitch-based and PAN-based short fibers were made ready for the siliconization process. In the second step of the C/C-SiC plate manufacturing process, two steps were taken to form the SiC matrix using the LSI process. First, a porous C/C preform was created through the pyrolysis step using a carbon precursor polymer, which is converted into an intermediate carbon matrix. Then, molten silicon was infiltrated into the C/C preform, resulting in the formation of a SiC matrix through the chemical reaction of silicon and carbon. The density and porosity of the resulting C/C-SiC plates were then tested. In addition, the microstructure of the C/C-SiC composites was examined using Scanning Electron Microscope (SEM) tests and the silicon uptake was evaluated using Radiography tests. As mentioned above, another main study subject of this thesis is to provide a chemical and physical bond between CMC-metal materials by using filler metals containing Cr active element. This study indicated the selection criteria of the best compatible filler metal, the examination of effects of capillarity attraction and wetting on brazing, the determination of furnace cycle and atmosphere, and the examination of joint design variables to maintain proper sealing. The joint microstructure and possible reaction products at the interface were inspected by SEM, Energy Dispersive Spectrometry (EDS) and X-Ray Diffraction (XRD) characterization tests. According to the results obtained from this study, although BCo-1 filler metal exhibited a low wettability on C/C-SiC composites, it exhibited active filler alloy characteristics thanks to the Cr elements in its content. In this way, CMC materials can be joined to TZM refractory metals with filler metals containing Cr active elements without metallization. Banded structures consisting of dissolved graphite dispersed in the brittle silicide matrix at the joint interface reduced the joint strength and caused the joint to crack. The extended holding time at the brazing temperature increased the wettability of the CMC material by the filler metal, but caused the formation of more complex silicides, resulting in unreliable joints. Thanks to the longer carbon fiber length used in the pitch-based CMCs compared to the PAN-based CMCs, the former was more successful in withstanding the compressive stresses caused by the CTE mismatch between metal and CMC. Finally, the higher thermal conductivity of pitch-based CMCs compared to PAN-based CMCs contributed to the improved flow characteristics and melting of the filler metal.