Accomodating Thermal Expansion Mismatch

Metal fillers (solders and brazes) can join a wide variety of materials, including aluminum, copper, stainless steel, refractory metals with aluminum oxide, aluminum nitride, silicon carbide and other oxide, nitrides and carbides… However, the success of bonding ceramic to metals depends on the exact ceramic and metal materials and the joined assembly size and geometry. This discussion is based on the coefficient of thermal expansion (CTE) mismatch of the materials being joined. Materials expand at different rates depending on the composition (atomic elements), structure (atomic arrangement) and thermal properties. A material’s volume will change equationbased on the relationship and when derived to any linear dimension, the relationship of the increase of length per unit length per °C (or °F) is established that leads to the linear expansion relation.

chartA table of common metals, ceramics and glass is seen below showing that materials vary widely. Many errors or “miscalculations” occur from aluminum soldering to any other metal or ceramics. With a linear CTE of 23 x 10-6 / °C, aluminum is one of the most expanding metals when heated. Alternatively, SiC, quartz and tungsten have close zero or not much expansion at all when heated. When bonding with metal fillers (brazes and solders) the relative differences in CTE will, after bonding, lead to differences in strain that must be accommodates by distortion (bending) or stress, as seen in the sketch below. Although the bending may be exaggerated, the result is ceramics can easily be cracked or joints be fractured if joint designs are managed properly.

The most common design error made in joining is to solder or braze bond large metal components to ceramics with relatively large bond areas. For example if WC cemented carbide is brazed to stainless steel, their largely different CTE, when cooling from the brazing temperatures (many times over 850C), the cemented carbide component cracks or the metal deforms and may actually shear the braze joint. Therefore it is incumbent on the component designer to account for CTE mismatch to be compatible both with bond processing and in service.

So with CTE being a major concern when ceramic:metal bonding, the design consideration are:

  1. CTE of the ceramic and metal… selecting lower CTE metals can be one solution.
  2. Size and geometry of joint… smaller joints and joints that are symmetric can offset strains.
  3. Bonding temperature… when bonding at lower temperatures, accumulated stresses are lower.

When making ceramic:metal bonds, outside of adhesive or glass bonding, brazing and soldering are the most common bonding methods to create connections or hermetic seals. Braze filler metals melt above 450C where soldering filler metals melt below 450C. From a CTE perspective solder joints would create less stressed joints when ceramic:metal bonding. But many times an assembly is subjected to service temperatures above the remelt temperature of solders or the solders may not be compatible with the service atmosphere so braze filler metals bonding above 850C are used.

material_tableLower filler metals such as S-Bond active solders, can bond aluminum to ceramic using low temperature. In solder joining, unlike brazing (over 450C) the component parts require heating to ~ 250 °C. Looking at the CTE values in the table below, a 12” plate of Aluminum will grow by almost 0.060” while the 12” plate of aluminum oxide will only grow by about 1/10 that amount with a CTE of 4.4 ppm for Alumina vs. the 23 ppm for Aluminum. Thus the alumina plate only grows at 250°C by about 0.010”… so upon cooling, the aluminum will try to return to length, by 0.060” where the alumina will only return, upon being bonded at 250°C by 0.010” setting up a strain difference to bending of the assemble plates, as seen in the figures above.

So when ceramic:metal bonding CTE mismatch must be taken into their assembly designs by some of the following techniques.

  1. Using better matched CTE materials (e.g. ceramic to Kovar®).
  2. Using multi-layers to over a distance accommodate CTE.
  3. Bond smaller areas/components or make a mosaic breaking the larger CTE materials into smaller pieces.
  4. Stiffening a design to resist bowing (may still fracture joint).
  5. Use lower temperature joining processes, such as exothermic materials that only heat the joint areas, a recent commercially developed nanofoil has been developed and can reheat and solder joints via a patented NanoBond® process.
  6. Use design analytical modelling to better understand CTE mismatch and how it may be mitigated by design and/or process or filler metal material selections

We look forward to your discussion and contribution to this forum thread. If you have questions regarding ceramic:metal bonding, Contact Us.

 

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