Characterization of a Ceramic-Metal-Ceramic Bond: Chemical Vapor Deposited (CVD) Silicon Carbide Joined by a Silver-Base
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Characterization of a Ceramic-Metal-Ceramic Bond: Chemical Vapor Deposited (CVD) Silicon Carbide Joined by a Silver-Based Active Brazing Alloy (ABA) James V. Marzik1, Toshi Oyama2, Warren J. MoberlyChan3, and William J. Croft3 1 Morgan Advanced Ceramics, Inc, Hudson, NH, U.S.A. 2 Morgan Advanced Ceramics, Inc, Hayward, CA, U.S.A. 3 Harvard University, Cambridge, MA, U.S.A. ABSTRACT Chemical vapor deposited (CVD) silicon carbide (SiC) ceramic material was joined to itself via an air stable, silver-based active brazing alloy (ABA). The microstructure and microchemistry of the interface was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Results were compared to previous studies on the active alloy brazing of sintered silicon carbide using higher copper alloys. INTRODUCTION Active brazing as a joining technology for ceramic materials has been a subject of interest for several years [1-3]. The use of active brazing alloys (ABA) eliminates the need to metallize the ceramic surface prior to brazing. The active metal (titanium in most cases) promotes wetting, and reacts with the ceramic to form an intimate bond at the interface. The reaction between silicon carbide and various titanium-containing alloys has been previously reported [4-11]. In previous work involving ceramic-metal joining [4-6,9-11], the SiC has been produced by pressureless sintering or other traditional ceramic processing techniques involving sintering additives. We report here on the brazing of a high purity SiC produced by chemical vapor deposition (CVD) using an air stable silver-based active brazing alloy. EXPERIMENTAL The silicon carbide used in this study was produced by a commercial chemical vapor deposition process in which methyltrichlorosilane was reductively pyrolyzed at 1300-1400°C and 0.25 atm, in a hot-walled chamber onto graphite substrates [12]. The resulting polycrystalline silicon carbide material was 99.999+% pure and theoretically dense, and can be considered a bulk material with deposit thicknesses in the range of 3-15 mm. The grains have the cubic β-SiC structure with numerous stacking faults parallel to the CVD growth plane. Samples were ground flat (surface finish Ra = 0.2-0.3 µm), and brazed under vacuum using the active brazing alloy Ag-ABA™ (Cu 8 atomic percent, Ti 3.1%, Al 3.7%, balance Ag). Samples were brazed for 10-30 minutes at 900-950ºC. Braze joints were typically 1 cm2. SEM/EPMA analyses were carried out on a digital SEM (Leo 982, Carl Zeiss), and a Noran Vantage energy dispersive x-ray spectroscopy (EDS) system. Cross section TEM was performed to analyze the brazed joints. 3mm disks were ultrasonically cut from thin sections (10 atomic % solubility of Cu in Ag at the eutectic temperature. Thus the suggestion that pure Cu has precipitated out at the Ag grain boundaries and triple junctions seems unlikely for the moderate 5% Cu in the overall braze composition. The TEM
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analyses, both EDS and electron diffraction, ind
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