As the cost of producing SiC ceramic and as the technology to “cast” complex parts with SiSiC composite ceramics matures, there is increasing applications for bonding SiC based ceramics to metals. Applications are expanding in electronics (LED’s and semiconductors), optical mirrors, energy production and aerospace.
The challenge in many of the applications seen above is to interface and be bonded to metals for electrical connections, water channeling, enclosure seals, feedthroughs and support.
As in many other ceramics there are two challenges; 1) Wetting and adhering to the SiC / SiSiC surfaces and 2) accommodating the CTE mismatch during joining and in many applications, in service.
Joining processed to bond to SiC is similar to other oxide ceramics such as alumina and zirconia, except in joining to SiC / SiSiC, there is a stronger chemical potential driving the bonding at temperature over 800˚C. The constituent elements of SiC ceramics, carbon and silicon, can be reduced creating in the joints with metal, free silicon or carbon, that in turn can react with the metals in either the brazing filler being used or in the base metals being joined to. The reaction of metals (Ti, Ni, Fe, Cr, etc.) form carbides and/or silicides that are the “binding” reaction phase, however, if not controlled in thickness can embrittle any joint. Thus, in joining processes, the metal: carbon-silicon phases need to be controlled. Time, temperature and barrier coatings are the ways deleterious interactions can be minimized.
As in other ceramic joining, brazing or soldering can be used to join SiC to ceramics. In both cases of brazing or soldering there are two methods… One is first depositing a metallization that adheres to the SiC then brazing or soldering directly to the metallization. The other process is active brazing directly with braze filler that contain Ti and or other reactive elements (Hf of Zr) and in a brazing process, when these active fillers melt, the SiC can be joined directly to the metal. In active brazing caution must be taken to minimize the temperature (preferably below 850˚C) and keep brazing times as short as possible to minimize over reaction in the metal-SiC braze interface.
Soldering of SiC-metal joints can eliminate the over reactions that brazing can initiate, provided service temperatures are below 200˚C and joint stresses do not exceed 5,000 psi (34.5 MPa). Recently, active soldering of SiC has recently emerged as a viable process [see S-Bond Technical Blog on SiC: Metal bonding]. In this process, a chemical bond to the SiC / SiSiC is created in an S-Bond Technologies, proprietary process which first coats the SiC surfaces to be bonded with an “active solder paste”. Then the pastes are reacted at temperatures over 860˚C in a vacuum furnace to create a S-Bond metallization layer on the area to where the bond to the metal is to be made. In a secondary operation at 250˚C, an active solder filler is directly melted onto the metal and onto the pre- S-Bond metallized SiC surface and while still molten, pressing and sliding the two active solder tinned surfaces together with sufficient alloy to create the joint. The result is a highly hermetic joint with excellent bond strength.
S-Bond Technologies is using its active solder processes to as a solution to a number of applications where SiC and SiSiC needs to be joined to metal for supports, enclosures, water fittings and electrical connections. For more information see www.s-bond.com