Joining of Ceramics and Ceramic Matrix Composites (CMC) for Aerospace and Energy Applications

SiC-based ceramics and composites (SiC, C/SiC & SiC/SiC) are more and more extensively used as advanced materials for aerospace and energy applications. Existing applications are expanding continuously and require advanced materials, design and joining technologies. The objective of this thesis was to join SiC-based ceramic matrix composites (CMC), ceramics (SiC, Mullite, Alumina) and SiC-based ceramic foams for aerospace and energy applications. The research was aimed to develop strong, oxidation resistant and high temperature stable joints. A novel joining technique defined “RM-Wrap” (RM=Mo, Nb, Ta, W Refractory Metals) has been developed within this thesis. The developed technique is a novel brazing technology named RM-Wrap after the metal used as a wrap to contain one or more silicon foils (e.g. Mo-Wrap when a Mo wrap is used to contain a Si foil). It is a pressure-less joining technology performed at 1450 oC, under an inert environment (Argon flow). Joining materials are in-situ formed composites made of refractory metals silicides (MoSi2, NbSi2, TaSi2 and WSi2) embedded in a silicon matrix. RM-Wrap is a highly tailorable joining technique: the quantity of each phase can be modified and more than one refractory metals can be used together. RM-Wrap has been very effective in joining both coated and uncoated CMC, porous and non-porous materials: ceramics (oxide and non-oxide), CMC (SiC-based) and highly porous substrates (SiC foams) having porosity higher than 80% have been soundly joined. vii The joint morphology (interphase and interface) and elemental composition of the joining material was investigated in detail using FESEM and EDS which showed uniform, continuous and crack free joints. XRD investigation confirms the formation of metal silicides. Oxidation resistance of joints was carried out at 1100 oC for 30 minutes (for CMC joints) and 6 hours (for monolithic ceramic joints) in the air; prior and post oxidation examination of joint morphology showed no morphological change and joints remained firmly joined. Sandwich structures have been developed by Mo-wrap joining two C/SiC as “skins” to the “core” SiC foam. Sandwich structures were tested for thermal shock resistance from RT to 1100 oC in the air for 2 minutes. Three cycles on a single sandwich structure were performed, which remained joined and the joining material composition unchanged. Joints were mechanically tested in three different modes (i) compression, (ii) tensile and (iii) torsion. Joint strength was higher than the interlaminar shear strength of composites as the fracture was always observed in composites. In case of monolithic ceramic (SiC) a mixed failure (cohesive and adhesive) was found, which suggest that the joint strength is comparable to ceramic one. Micro- and nanoindentation tests were also carried out on joining materials.