We get many calls about hermetically sealing sapphire (single crystal aluminum oxide) to metal flanges and this post addresses some of the applicable commercial methods used. Sapphire to metal seals many times are used in sensor housing like those seen in Figure 1. In this application these titanium housing are actively soldered (S-Bond) to the edge of the sapphire window.
Sapphire joining historically has been done by both brazing (filler melt above 450 C) and soldering (fillers melt below 450C)… [see “Soldering vs Brazing article by R Smith]. In both cases of brazing or soldering there are two methods… One is first depositing a metallization that adheres to the sapphire then brazing or soldering directly to the metallization. Metallizations include the Mo-Mn process or vapor deposition of thin titanium, followed by thin gold or nickel layers. 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 sapphire can joined directly to the metal; however the high temperature brazing temperatures (> 800˚C) limits the metals and/or the size of sapphire that can be brazed due to high stresses that can fracture the sapphire window upon cooling from the brazing temperatures.
To counter the high CTE difference stresses: active soldering of sapphire windows has recently emerged as a viable process. In active soldering, there are two options… for lower stressed joints, an “adhesive bond” can suffice and this is done by directly melting S-Bond 220 solder fillers at 250C onto the metal and onto the ceramic surfaces and activating the alloy wetting by rubbing the active solder into the two surfaces… then while still molten, pressing and sliding the two active solder tinned surfaces together with sufficient alloy to seal the joint.
If the higher joint strengths are needed or thermal cycling will be significant (creates thermally induced stresses at the interface) then a chemical bond on the ceramic may be needed. To prepare the ceramic S-Bond Technology uses a proprietary process which first coats the sapphire with an “active solder paste” then at temperatures over 860˚C in a vacuum furnace … the pastes react with the sapphire and to create a 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 directly melted onto the metal and onto the pre- S-Bond metallized sapphire surface, then while still molten, pressing and sliding the two active solder tinned surfaces together with sufficient alloy to seal the joint. The result is a highly hermetic joint with excellent bond strength.