Dissimilar Substrate Bonding with Adhesives
Q: "We have an application that requires a hermetic seal between dissimilar materials. The bonds must be able to withstand the conditions of autoclaving and sustain immersion in a fluid for approximately 30 days. One bond is between ceramic and silicon, and the other is ceramic and SS.
Please advise on materials and other recommendations for surface prep, bond line, etc."
A: There are a few materials that may be candidates for evaluation to bond dissimilar materials. To withstand one autoclaving cycle, followed by immersion in a fluid (I am going to assume a water/aqueous liquid), and give good adhesion to ceramic and silicon, and ceramic and stainless steel, I would recommend either an epoxy or an acrylic-based adhesive. Two-part epoxies will generally withstand these conditions, come in a wide variety of hardness', and give good adhesion. One-part thermal-cure acrylates will survive the autoclaving (1 cycle) with a protected bond line. A protected bond line can be best described as large mating surface areas between the two substrates, with only the edge of the adhesive being exposed to the steam or fluid. I would recommend a bond line thickness of 0.002-0.004 inches for this type of application. If it's too thin you might have voids. If it's too thick you might have too much surface area of the adhesive being exposed to these conditions. In this case, smaller bond line thicknesses are better. Products with a viscosity of 200-1000 cP would be ideal for this bond line thickness. Another adhesive option to improve efficiency in your manufacturing is to look at products classified as Multi-Cure®. Products of these types cure in different ways, including the ability to cure with heat or light. The ability to cure with light would allow these parts to be assembled and tacked in place in seconds and then exposed to heat to cure the remaining shadow areas.
To verify one point: We always double-check if the substrate is silicon or silicone. While made up of the same elements, silicone is a flexible, rubbery material. Silicon, as in silicon wafers, are generally metallic, hard, rock-like surfaces. We work with both materials, but there has been enough confusion over the years that we like to double-check. That little "e" at the end can make a big difference in selecting the proper adhesive. If you are looking at bonding ceramic to silicone, then I would recommend a silicone one- or two-part adhesive.
Regarding surface preparation: A rough surface will (generally) give better adhesion than an electropolished surface. A rough-surface topography often has microscopic mountains, valleys, and pores that the adhesive can fill, which provides additional surface area, as well as a mechanical interlock. A smooth-surface topography only gives one value of the surface area and no mechanical interlock. If the surfaces can be roughened by abrasion, shot preening, scoring, or a chemical primer - these methods will improve the overall bond strength. Making sure that the bonding surface is free of contaminants, oils, release agents, cutting lubricants, or even finger oils can help yield a repeatable bond strength.