There is a lot of interest today in joining engineered-ceramic materials (such as alumina) to themselves or to metals. Active metal brazing (AMB) has shown itself to be an interesting method to accomplish this instead of the standard moly-manganese process of ceramic surface preparation prior to brazing (a process that has been around for many years).
I’ve written blog articles about this topic back in September 2009 and again this year in January. Part 2 addresses additional questions raised by the January blog.
1. Is any special preparation required for the ceramic?
2. What gap clearances are needed for AMB?
3. Which brazing filler metals (BFM) can be used?
4. What temperatures are required?
For any brazing process to work effectively (including AMB), the BFM must be able to form a strong, permanent bond with the base materials (alumina, metals, etc.) being joined. So, surface-prep and gap-clearance are going to be important. Let’s look at those two items:
1. Surface Preparation: Preparation of the ceramic and metal surfaces to be joined should be treated in the same manner as any other surface to which someone wants to braze. It must be clean (free of any surface contaminants such as oils, grease or dirt). Additionally, the oxides on the surface must be kept to a minimum.
That will be easier to achieve with metals than with alumina, since alumina ceramics consist of oxide particles sintered together. So, we will start by getting the surface of the alumina very clean, removing any oils/greases, etc. via a degreaser matching the type of oil being removed (mineral-based oils use standard solvent-based degreasers, whereas water-based oils will require a water-based degreaser to remove them effectively). Then when the surface has been thoroughly cleaned, do a final rinse cleaning with acetone (rather than water or alcohol) so that no moisture components are adherent to the surface of the alumina. Then handle the parts with gloved hands.
2. Gap clearances: Treat this ceramic-to-metal braze just as you would any other brazement. Namely, be sure the gap – at brazing temp – is 0.000-0.003 inches (0.000-0.075 mm) if possible. Knowing that most metals expand much more than the alumina, the expansion-rate differences must be considered in the design of the parts and the brazing sequence.
Consequently, it is typical that a low-expansion metal, such as Kovar, is first brazed to the ceramic surface (since the expansion rate of Kovar is somewhat close to that of the alumina) and then this assembly is next brazed to whatever metal then needs to be joined to it. Thus, you would essentially be brazing the newer metal to the Kovar substrate (brazing metal-to-metal).
It is very important to always try to calculate the change in dimensions that will occur when brazing one metal to another so that you have the desired brazing gap at process temperature.
3. Brazing Filler Metal (BFM): When brazing metal to ceramic (such as the Kovar to the alumina), special “active” brazing alloys (ABAs) should be used, since they typically contain about 3-5% of titanium added to their chemistry in order to allow them to effectively react with, and bond to, the alumina substrate (this was discussed in more detail in my January blog).
The choice of BFM to join the secondary metal component to the Kovar substrate will depend on the end-use service conditions to be encountered. Many of the standard BFMs may be considered.
4. Brazing temperature: The brazing temperature should typically be about 100°F (50°C) above the liquidus temperature for the particular BFM selected. The time at temperature may vary considerably from one application to another and cannot be specified as “one for all.” What is important, however, is the heating and cooling ramp-rates, since you are dealing with a fragile ceramic-to-metal bonded joint in one case with a secondary low-expansion metal to high-expansion metal in the other. Therefore, use the slowest heating/cooling rates that you can for the given situation to minimize joint stresses and prevent cracking of any of the brazed-joint interfaces.
Additionally, as mentioned in the previous blog, since you are working with base materials that are extremely sensitive to oxidation (titanium, ceramics, etc.), the brazing should typically be performed in a clean, low leak-rate vacuum furnace.