Graphite bonding and carbon bonding is another aspect of ceramic to metal bonding in that carbon with its allotropes, has a range of crystalline structure that can resemble ceramic. Most engineered ceramic are covalently bonded and as such are not as easy to react with as other metals. For example, graphite is ‘partially covalently bonded” while carbon in its diamond form, is 100% covalently bonded and this “ceramic like”… in between carbon can take many forms, including but not limited to…
- Solid Graphite: pressed, filled and sintered powders
- Graphite Foam: liquid carbonaceous precursors foamed and graphitized.
- Pyrolytic Graphite (TPG): vapor deposited epitaxially grown carbon layers
Soldering and brazing are processes that can metallurgically bond carbon materials to metals. Brazing (> 450˚C) is a bonding method that uses molten active braze filler metals with Si, Fe, Ti, Hf or Zr additions that can react to form compounds with carbon in the interfaces between carbon and metal. Brazing is normally done at temperatures above 800˚C, depending on the selected braze filler. Most brazing is done vacuum or other protective atmosphere so not to oxidize or otherwise decompose the carbon materials. One of the limits in brazing of carbon materials is the type of intermetallic compounds (IC’s ) that form, typically carbides, which may make the joints brittle or porous.
For soldering, S-Bond Technologies has developed active soldering (< 450˚C) methods for graphite bonding that starts with S-Bond metallization treatment followed by active soldering with it active solders to most metals, ceramics, and glass. Such active soldering processes eliminates the need for chemical flux which can be absorbed and thus contaminate the graphite components. The reactions with active solders’ reactive elements, generate a reacted “solderable” surface to which metals, glass, ceramics and other crystalline materials can be bonded. Reliable joints have been made between graphite and carbon based materials with all metals including steel, stainless steels, titanium, nickel alloys, copper and aluminum alloys…
Active solder (S-Bond) joints:
- Are ductile, based on Sn-Ag or Sn-In alloys
- Exceeds the strength of carbon and graphite
- Are thermally conductive, with S-Bond alloys having k = 50 W/(m-K)
- Are metallic and this electrically conductive with a metallurgical bon
S-Bond alloys has been shown to wet and adhere to a wide variety of the graphite and carbon, including:
- All Grades of Graphite
- Metal infiltrated graphite
- Graphite and Carbon Foams
- Pyrolytic Graphite
- High conductivity graphite fibers
Graphite and carbon based materials are normally used in thermal management, electrical connections, sensors or sliding seals and are used for its, high electrical and thermal conductivity, and/or low friction properties. They find application in electrical sensors, power leads and feed throughs, and motor brushes. S-Bond joining creates a metallurgical bonded solder connection, creating much higher conductivity and lower profile electrical connections, especially useful for small sensors and brushes.
More recently, Graphite Foams offer revolutionary advancements in thermal management. Graphite foams, graphite fibers and pyrolytic graphite can all be joined using S-Bond processes. Examples of applications and components are shown below. S-Bond joints have been proven to be thermally conductive and enhance the performance of graphite based thermal management devices, especially those made from graphitic foams.
S-Bond Technologies has developed extensive experience in active brazing and active S-Bond solder bonding of graphite, carbon and carbide to metals. Contact Us to evaluate our joining solutions for your graphite bonding and carbon joining applications.