June 14, 2016
To reliably service harsh environments, functional assemblies often depend upon a set of properties selected from more than one material. Excluding fasteners, assemblies fashioned of dissimilar materials are typically formed utilizing high temperature processes such as diffusion bonding, brazing and welding.
The challenge of designing such products are multifaceted and require consideration not only of the effects of the harsh environment to which they will be exposed in service, but also consideration of the effects of extreme joining temperature used to form them. Those bonding temperatures may exceed the melting point of some or all of the constituents. These material properties include but are not limited to mechanical, chemical, physical, electrical, optical and thermal properties of the constituent materials. These must be considered independently as well as in context with an assembly.
The chemical potential that drives joining is directly related to the surface free energy of each mating component. This measure of a surface’s readiness to bond is controlled by its cleanliness, finish, morphology, chemical or physical treatments to enhance wetting; and, the nature of the joining atmosphere: e.g. vacuum, inert or reducing.
Both direct and indirect methods are used to introduce heat to the joining process. Conductive or convective heating are examples of direct heat; whereas radiative or inductive heating are both indirect means. Thermal conductivity, heat transfer, thermography and thermal expansion characteristics of all system materials are sensitive to the chosen method of heating. Additionally, most high temperature joining processes exclude air. The oxidizing presence of moisture or oxygen found in air, even at very low residual levels, can impede the bonding of dissimilar materials. System design and utilization of protective atmospheres must take into account the heat transfer as it is affected by the optimum atmosphere.
Quite frequently, metals must be joined to insulators or optical elements in order to perform a function such as the transmission of electricity, light or fluid, into or out of a harsh process or environment. There is no irony in that dissimilar materials have dissimilar material properties. This fact gives rise to the greatest challenges in materials joining: How to transition a joint between a ductile metal and a brittle dielectric insulator – from metallic bonding to covalent or ionic bonding. The interface created by bonding insulator-to-metal is graded, i.e. insulator-to-semiconductor-to-metal using carefully controlled high temperature processes. These processes often employ transition materials and filler metals such as braze alloys and welding rod.
After the environmental conditions have been considered, and a list of candidate materials have been collected which will stand up to the harsh application, there still remains the task of addressing seal integrity, process stressors and the compatibility of select materials for joining at elevated or reduced temperature. Here arise questions of gap control, residual mechanical sealing stress and chemical compatibility. Chief among these concerns is the matching of thermal coefficients of expansion for dissimilar materials. The residual stress which arises from dissimilar materials returning to room temperature post-joining at elevated temperature can be large enough to break one or more of the components or interfaces.
Insulator Seal, for over 30 years, has created a catalog of nearly 20,000 solutions for dissimilar material joints used in harsh environments. And this set of solutions continues to grow as the ISI technical staff, with a collective 100 years of design experience, addresses new designs every day. In fact, over 25% of all active projects are entirely new. A full two-thirds of all products are not “standard” but have been designed or co-developed by Insulator Seal in recent years. These applications range from temperatures as low as -269 oC to as high as 1,000 oC; from pressures as low as 10-15 atm to greater than 103 atm; from air-conditioned laboratories to down-hole petroleum drilling operations; from two sub-sea miles to the depths of Martian space.
A representative list of materials with which Insulator Seal has experience is provided below. Technology is constantly advancing; and, meeting environmental challenges can be critical to the success of your design. If materials of interest are not listed here, please contact Insulator Seal directly to discuss your application.
Dielectric and Crystalline Materials Commonly Sealed by Insulator Seal:
- Sapphire (crystalline Al2O3)
- Alumina (85-99.9% Al2O3)
- Yttria Stabilized Zirconia (YSZ – Y2O-ZrO2)
- ZTA (ZrO2 – Al2O3)
- Quartz (crystalline SiO2)
- Fused Silica (amorphous SiO2)
- Aluminum Nitride (AlN)
- Silicon Nitride (Si3N4)
- Zinc Selenide (ZnSe)
- Zinc Sulfide (ZnS)
- Cleartran™ (Multi-Spectral ZnS)
- Calcium Fluoride (CaF2)
- Magnesium Fluoride (MgF2)
- Glass (Schott™ BK-7)