BRAZING & SOLDERING TODAY assemble several identical stacks of the kW range using this glass-ceramic composite sealant. The tensile strength of Crofer 22APU alloy butt joints joined with BaO-CaOSiO2 YSZ composite glass-ceramic material was about 3 N/mm2 as joined, about 4 N/mm2 after aging at 800°C for 500 h, and 30 thermal cycles RT-750°C. Another promising composition was the same glass-ceramic reinforced with 20 wt-% of silver particles, and it exhibited tensile strength above 5 N/mm2, while aging decreased the strength of such joints. New Nickel-Tantalum Brazing Alloy for Joining Ceramic Joints Stable above 1000°C Ceramic materials are widely used for high-temperature applications, but it is still a challenge to produce ceramicceramic or ceramic-metal joints stable at 58 FEBRUARY 2014 temperatures above 1000°C using metallic brazes. A potential Ni-38 at.-% Ta brazing filler metal for high-temperature applications (>1000°C) was developed and evaluated at the Fraunhofer Institute for Ceramic Technologies and Systems, Dresden, Germany. This Ta-Ni eutectic alloy was used for brazing alumina and SiC samples (Ref. 5). The melting temperature of the above composition is 1395°C; therefore, brazing in a vacuum furnace was executed at 1410°–1600°C. TiH2 powder in the amount of 10–20 wt-% was added to promote wetting of the braze on ceramic materials. After dissolution of titanium in the Ta-Ni alloy during brazing, the solidus temperature of formed Ta-Ni-Ti went down to 1230°C. A thin brittle intermetallic layer at the interface with cracks was found. However, bending strength of brazed joints at 1400°C was satisfactory — 50–70 MPa for alumina brazed joints and 200 MPa for silicon carbide brazed joints. Their strength increased by increasing brazing temperature to 1480°C. Bio-Compatible, Titanium-Cobalt- Based Active Brazing Coating of Medical Implants Owing to their outstanding mechanical properties, wear and corrosion resistance, as well as inert behavior, highperformance alumina and zirconia ceramics are currently in extensive use as medical implants. However, these inert ceramics do not form a chemical bond to the bone tissue, so their use is still limited. A promising approach was evaluated in RWTH Aachen University, Surface Engineering Institute, Germany, to functionalize ceramic surfaces via a coating using high-vacuum active brazing and producing biocompatible surfaces on an alumina substrate (Ref. 6). TECHNOLOGY NEWS For info, go to www.aws.org/ad-index For info, go to www.aws.org/ad-index
Welding Journal | February 2014
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