Interfacial reaction kinetics and microstructural evolution of C/SiC composites to metal joints

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2021-9-09
Saltık, Simge
In the present study, joining of two dissimilar materials namely C/SiC composites and Ti6Al4V alloys by brazing using Ag-Cu based brazing filler alloys have been investigated. The effect of active Ti element content, additive particle size/amount in the filler alloy together with the reinforcement structure, material properties of the C/SiC composite on the interface evolution mechanism and resulting mechanical performance of the brazed joints have been examined. Additionally, apart from the brazing studies, the relationship between brazing performance and wetting behavior have been correlated by modeled wetting experiments conducted under actually applied brazing conditions. According to the measured contact angle values obtained from the wetting experiments, it can be stated that Ticusil filler alloy with the highest amount of Ti considerably improves the wetting behavior of both monolithic SiC ceramic and C/SiC ceramic composite material with contact angles of 10° and 15°, respectively. 915 °C and 15 min were determined to be the optimized brazing parameters for this brazing filler alloy to ensure almost complete wetting and a uniform reaction layer for effective adhesion of the C/SiC composite surface. Resulting joints with ⁓1 µm thick reaction layer, which were obtained using Ticusil brazing under this condition had the highest shear strength of ⁓33 MPa. Varying amounts of SiC particles of two different sizes (nano and micro scale) were incorporated into the Ticusil brazing filler alloy to control and optimize its wetting characteristics on ceramic surfaces. In this regard, 2 wt% nano level SiC and 1 wt% micro level SiC particle addition led to the highest improvement in the shear strength of the joints by 35% and 8%, respectively, compared to their no additive containing versions. Although the recorded contact angle values were nearly same (42° for 2 wt% nano level and 37° for 1 wt% micron level), higher increment in the mechanical performance was achieved by the addition of nano level SiC particles due to more homogeneous reinforcement effect of the nanoparticles. Results indicated that the optimum contact angle for the highest joint performance is ⁓40° for both nano and micron level additive containing Ticusil filler alloy. Composites reinforced by two different types of carbon fiber structures, namely 3D and 2D, had similar wettability characteristics, and hence showed similar reaction layer and interlayer morphologies. In addition to this, carbon nanotube (CNT) impregnation to the matrix of the 2D C/C-SiC composites improved the wettability of its surface by the brazing filler alloy. On the other hand, despite better wetting 2D C/C-SiC composites with or without CNT impregnation showed inferior mechanical characteristics which can be attributed to the level of residual stresses caused by material property variances as well as imperfections at the joint interlayer.

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Citation Formats
S. Saltık, “Interfacial reaction kinetics and microstructural evolution of C/SiC composites to metal joints,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.