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Silicon nitride/silicon nitride joints with refractory metal (W and Mo) interlayers were vacuum brazed using an active braze, Cu-ABA (Cu–3Si–2Al–2.25Ti, wt%), and two interlayer arrangements in a double-lap offset configuration: Si 3 N 4 /Cu-ABA/W/Cu-ABA/Mo/Cu-ABA/Si 3 N 4 and Si 3 N 4 /Cu-ABA /Si 3 N 4 . Titanium segregated at the Si 3 N 4 /Cu-ABA and Mo/Cu-ABA interfaces, but not at the W/Cu-ABA interface. The room-temperature compression-shear strength values of Si 3 N 4 /Cu-ABA/Si 3 N 4 and Si 3 N 4 /Cu-ABA/W/Cu-ABA/Mo/Cu-ABA/Si 3 N 4 joints were 118±24 MPa and 22±5 MPa, respectively. Elevated-temperature compression tests showed that Si 3 N 4 /Cu-ABA/Si 3 N 4 joints had strength of 31±6 MPa at 1023 K and 17±3 MPa at 1073 K. Likewise, Si 3 N 4 /Cu-ABA/W/Cu-ABA/Mo/Cu-ABA/Si 3 N 4 joints had strength of 19±4 MPa at 1023 K and 13±3 MPa at 1073 K. Knoop microhardness profiles revealed hardness gradients across the joints. The effect of joint microstructure and test configuration on the mechanical behavior is discussed. [ABSTRACT FROM AUTHOR]

Two types of silicon carbide (SiC) substrates, CVD-SiC and SA-TyrannohexTM (SA-THX), were diffusion bonded using three different metallic interlayers: Ti (10 and 20 m) coating on SiC deposited using physical vapor deposition (PVD), Ti foils (10 and 20 μm), and a Mo-B foil (25 μm). The microstructures of the resulting interfacial reaction zone were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in samples prepared using focused ion beam (FIB) technique. The effect of interlayer material and thickness, and the direction of the SiC fibers in SA-THX, on the interfacial microstructure and phase composition was studied. Sound diffusion bonds developed in most samples except a few that showed microcracking. In SA-THX/PVD-Ti samples, chemical reactions occurred more rapidly with thin (10 μm) interlayers and SA-THX fibers bonded parallel to the interlayer. Microcracks developed in samples with large amounts of an intermediate phase and small amounts of stable phases. This microcracking occurred due presumably to a large coefficient of thermal expansion (CTE) mismatch and CTE anisotropy. The SA-THX samples with the Mo-B interlayer formed crack-free bonds even in the presence of reaction phases with anisotropic CTEs. This was attributed to a lower CTE mismatch of chemical phases with the SA-THX substrate in Mo-B interlayer system than in samples with Ti interlayers. [ABSTRACT FROM AUTHOR]

Abstract: Copper-base active metal interlayers were used to bond in situ reinforced silicon nitride (Honeywell AS800) at 1317K for 5 and 30min in vacuum. The joints were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron back scattered diffraction (EBSD), and transmission electron microscopy (TEM). A Ti-rich interaction zone (∼3.0–3.5μm thick) formed at the Si3N4/braze interface. This reaction layer grew toward the inner part of the joint with a featureless microstructure, creating a strong bond. Regions of a Ti-rich phase were frequently found next to the reaction layer but surrounded by the Cu alloy. Extensive Ti and Si enrichments were noted at the interface but there was no evidence of interfacial segregation of Y, La, and Sr (from Y2O3, La2O3 and SrO, added as sintering aids). The reaction layer thickness and composition did not change when brazing time increased from 5min to 30min suggesting rapid growth kinetics in the early stages of reaction. The joints were crack-free and showed features associated with plastic deformation, which indicated that the metal interlayer accommodated strain associated with CTE mismatch. The inner part of the joint consisted of highly textured large grains of the braze alloy. [Copyright &y& Elsevier]

Abstract: Si3N4 (SN-281)/Cu-clad-Mo joints, brazed using a soft (YS: 341MPa) and ductile (43% elongation) Pd–Co braze were sound and exhibited an interaction zone comprised of Pd66Mo14Cu10Co6Si3 and Pd74Mo11Co8Cu6Si. Similar joints made using a less ductile and stronger Pd–Ni braze led to cracking from large CTE mismatch-induced strain energy (∼64mJ to 348mJ). [Copyright &y& Elsevier]

Powder blends of Al2O3 and a Cu-Sn-Pb bronze (0-20% bronze) were cold compacted and air-sintered at 1473-1773 K. The Al2O3-bronze composites exhibited networks of metal microchannels, some porosity, and diffusion of Pb in Al2O3 and of Al and O in bronze. The hardness of un-doped Al2O3 increased linearly with temperature, from 455 Knoop microhardness (HK) at 1473 K to 2010 HK at 1773 K, but the hardness of the Al2O3/bronze composite exhibited a sluggish, nonlinear dependence on temperature, with a peak hardness of 1150 HK at 1673 K. Air sintering may be viable to synthesize Al2O3/bronze composites with low bronze contents [ABSTRACT FROM AUTHOR]

Hot-pressed ZrB2–SiC and composites with carbon (ZSC) or SCS-9a SiC fibers (ZSS) were joined using Pd-base brazes, Palco (65% Pd–35% Co) and Palni (60% Pd–40% Ni). Joint integrity was exhibited by the defect-free ZS/Palco/ZS joints with large interaction zones. The ZSS/Palco and ZSC/Palco joints revealed substantial chemical interaction and interfacial cracking due to residual stresses. The joints with Palni exhibited either poor wetting/bonding or cracking. The ZS/Palco joints displayed high Knoop hardness within the ZS region and lower hardness within the Palco interlayer. [Copyright &y& Elsevier]

Abstract: Carbon–carbon, carbon–silicon carbide, and silicon carbide–silicon carbide composites were vacuum brazed to Ti and Hastealloy X using Ni-base metallic glass braze foils (MBF-20 and MBF-30). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) of the joints showed that compositional changes due to substrate dissolution led to secondary-phase precipitation which aided interfacial bonding although inter-laminar shear failure occurred within some composites. Residual thermal stresses in the joint led to hardness gradients; however, stress accommodation by the brazes prevented interfacial cracking. The peak Knoop microhardness in the joints was as high as 1165–1294KHN. [Copyright &y& Elsevier]

Abstract: Advanced carbon–carbon composites were joined to copper-clad molybdenum (Cu/Mo) using four active metal brazes containing Ti (Cu–ABA, Cusin-1 ABA, Ticuni, and Ticusil) for potential use in thermal management applications. The brazed joints were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and Knoop microhardness measurements across the joint region. Metallurgically sound Coints, devoid of interfacial cracks formed in all cases. The joint interfaces were preferentially enriched in Ti, with Cu–ABA joints exhibiting the largest interfacial Ti concentrations. The microhardness measurements revealed hardness gradients across the joint region, with a peak hardness of 300–350KHN in Cusin-1 ABA and Ticusil joints and 200–250KHN in Cu–ABA and Ticuni joints, respectively. [Copyright &y& Elsevier]

Abstract: High conductivity graphite foams with low (125 kg m−3), medium (260–270 kg m−3), and high (650 and 797 kg m−3) densities were integrated with copper-clad-molybdenum using an Ag–Cu–Ti braze alloy (63Ag–35.3Cu-1.75Ti) as powder and foil. The effects of foam density (125–650 kg m−3), cladding thickness (0.1–0.3 mm), and Cu–Mo–Cu ratio (in %: 13/74/13 and 20/60/20), on microstructure and composition were evaluated using optical microscopy and scanning electron microscopy coupled with energy dispersive spectroscopy. Low-density foams showed more extensive braze penetration than high-density foams. The carbon/braze interfaces were well-bonded and enriched in Ti. The joint microstructure and penetration depth were independent of clad layer thickness and Cu/Mo/Cu ratio. Differences in elemental distribution were noted between joints made using foil and powders; with foil, the Ti and Cu concentrations were inversely related (high Ti concentrations overlapped areas of low Cu concentrations and vice versa) whereas with powders, high Cu and Ti concentrations coincided. Although a thick Cu-cladding increases the coefficient of thermal expansion (CTE) of the Cu-clad-Mo substrate and the CTE mismatch relative to foam, this effect is offset by better stress accommodation with a thick clad layer. Thermal calculations verified that thermal resistance decreased with increasing thickness of clad layer in joints. [Copyright &y& Elsevier]

Abstract: Silicon nitride/silicon nitride joints were vacuum brazed at 1317K for 5min and 30min using ductile Cu-base active metal interlayers. The joints were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron back scattered diffraction (EBSD), and transmission electron microscopy (TEM). An inhomogeneous Ti-rich reaction layer (∼2–3μm thick) formed in 5min at the Si3N4/braze interface. The inhomogeneity disappeared after brazing for 30min and was replaced with a compact and featureless reaction zone. TEM studies revealed fine grains in the reaction layer, and larger grains in the inner part of the joint interfaces. The joints were crack-free and presented features associated with plastic deformation, which indicated accommodation of strain associated with CTE mismatch. Electron Backscatter diffraction (EBSD) revealed a highly textured braze alloy interlayer and its crystallographic orientation was determined. The formation of additional phases at the joint interface during brazing is discussed. [Copyright &y& Elsevier]