Ceramic Brazing

Ceramics include a vast array of materials which actually are refractory, inorganic, and nonmetallic materials. These materials are backed up with properties making them useful in many varied product areas. Ceramics are comparatively harder than steel & are heat & corrosion resistant. As per its characteristic, ceramics are categorized into two classes which are :

• Traditional Ceramics
• Advanced Ceramics

The materials are produced from powders easily formed to take a desired shape. The ultimate densified part is achieved only after they are heated at an extremely high temperature with or without the usage of external pressure's application. Lot of care is taken while manufacturing structural ceramics since that ensures the controlling of chemical composition helping it to achieve high densities.

Characteristics
Ceramics have many features which makes them quiet a powerful material used in many vital applications. These are :

• High strength
• Excellent wear resistance
• High hardness
• Excellent corrosion & oxidization resistance
• Low thermal expansion
• High electrical resistivity

Materials
Ceramic materials include wide array of chemical properties. Together they help in forming a strong ceramic material.

• Aluminum Oxide
• Zirconium Oxide
• Silicon Carbide
• Silicon Nitride
• Sialons

Ceramic Brazing
Ceramics are easily brazed using either direct methods like diffusion bonding or indirect methods like brazing. The methods help tremendously in bonding ceramics to any metal involving varied bonding techniques.

• Brazing Techniques
  It is difficult to wet liquid metals in case of structural ceramics which are considered to be the most stable compounds. Since, wetting is important to produce usable braze joints, lot of effort is needed to produce wetting which in turn can wet the ceramic surfaces using the braze filler metals. This is achieved through two procedures which are :
  • Promotion of wetting to the ceramic surfaces before brazing applying coatings.
  • Activation of wetting by using alloying braze filler metals with elements.
  
  
  
• Commercial Active Filler Metals
  The quantity of commercially available active braze filler metals are less in numbers, especially in the case of dealing with high temperature applications. The ones available are in combination of Ag (Silver), Cu (Copper) & Ti (Titanium) & these are :
  • Ag-27Cu-4.5Ti
  • Ag-3sCu-2Ti
  • Ag-27.5Cu-2.0Ti
  • Ag-23.5Cu-14.5In-1.25Ti
  • Ag-1Ti

Processes & Equipment
Ceramics comparatively take longer duration to get heated at a definite temperature which is difficult to be heated evenly by any localized heating techniques. And this happens with ceramics marked with oxides as A!O3 & ZrO2 with low thermal conductivities. Even in case of ceramics being electrical insulators at low temperatures, ceramics are induction heated either using a metal or graphic susceptor.

Vacuum or controlled atmosphere brazing in inert gas is an effective process considered vital for brazing ceramics. Lot of care is taken when heating ceramics as its quite slow while brazing which in turn helps the balanced maintenance of the materials at the same temperature. An easy way to produce an effective joint is to equilibrate the temperature of assemblies exactly below the solidus temperature of the filler metal. This is followed by a slow controlled heating which further helps in creating the ultimate brazing temperature.

Applications
Structural ceramics are widely used as :

• Cutting tools
• Bearings
• Machine tool parts
• Dies
• Pump seals
• High temperature heat exchangers
• Internal combustion & turbine engine parts