Carbon & Graphite Brazing

Carbon is considered to be the sixth most abundant element by weight in the earth's crust which together with the unique diversity of organic compounds and unusual polymer-forming ability at the temperatures transforms the element into a chemical basis of all known life. While graphite is one of carbon's allotrope. It is an electrical conductor & holds the distinction of being the most stable form of solid carbon.

Preference over Structural Ceramics
The structural ceramics of carbon, graphite & ceramic-matrix composites are similar to the carbon composites though carbon based materials differ from ceramics in many ways which are :

• The carbon based materials oxidize at temperatures ranging from 400 to 650°C.
• The materials have high melting points & low coefficient of thermal expansion.
• These materials have significant amounts of open porosity.
• They have higher thermal conductivity.
• They are less sensitive to thermal shock.
• Are quiet creep resistant.
• They vary vastly in the degree of crystallinity

Production of Materials
There are several production procedures through which carbon & graphites are manufactured & the process yields materials consisting an innumerable amount of crystalline perfection & properties. Polycrystalline graphites are manufactured from cokes derived from the residue after the volatile materials are separated from the petroleum.

Carbon-carbon composites have carbon or graphite fibers which are either woven into two-dimensional or three-dimensional bodies of the desired shape which is there in a carbon or graphite matrix. The production of carbon-carbon composite bodies from either processes, such as liquid impregnation or chemical vapor infiltration, are marked with high tensile strength & low coefficients of thermal expansion which helps it in resisting thermal shock or stresses.

Brazing Characteristics
The varied characteristics of brazing related to carbon & graphite are mentioned in this section.

• Wettability
  Carbon & graphites have wetting nature & are strongly influenced by impurities consisting of oxygen, water either absorbed on the surface or in the bulk material.
  
  
• Thermal Expansion
  The effect of coefficient of thermal expansion while brazing carbon & graphite is given ample consideration. In case of graphite brazing, carbon's or graphite's type & grade needs to be ascertained which helps in establishing the characteristics of a particular material.

Filler Metals
The common brazing filler metals cant easily be made wet with graphite. There is little or no wetting action when they ball up at the joint. Therefore, atleast two techniques are applied which helps in overcoming the deficiency & these are :

• Either a more readily wettable layer coating is done on graphite.
• There can be usage of brazing filler metals consisting of strong carbide-forming elements.

Graphite brazing is possible even when coated with either a metallic or intermetallic layer which makes the brazing process quiet easy & successful when used with a conventional filler metal.

Applicable Heating Methods
The brazing operations of carbon & graphite should be executed in environments which is devoid of oxygen, therefore, the carbon based material starts oxidizing at approx 400°C. Mostly, carbon based components make use of furnace brazing but at times, induction heating is as well used. The process is accomplished by varied ways :

• Either in a vacuum
• By use of high purity inert gas protection

Applications
Carbon & graphite have abundant specialized applications such as :

• Used as electrodes in metallurgical applications & moderator materials in nuclear applications
• Used in rocket nozzles
• Guide vanes
• Nose cones
• Electric motor brushes & switches
• Bushings & bearings
• High temperature heat exchangers
• Plumbing
• Heart valves
• Air frame composites
• High performance brake linings
• Posts for synthetic teeth