Advanced ceramics such as alumina, aluminum nitride, zirconia, silicon carbide, silicon nitride and titania-based materials, each with their own specific characteristics, offer a high-performance, economic alternative to conventional materials such as glass, metals and plastics.
The demand posed by new and changing applications is to improve operation at a reduced cost. New materials are being continuously engineered and adapted in order to address the needs of specific and often unique applications.
Joining ceramics to metals creates its own engineering challenges that require specialist expertise. Morgan Advanced Materials is a global leader in the metallisation and joining of ceramics. Its application engineers have worked with customers all over the world to provide high-integrity solutions for components of all sizes, shapes and specifications.
Desired Properties for the Application
Physical properties such as hardness, strength, wear resistance, corrosion resistance and thermal stability are considered while choosing a material. Each of these can be optimised depending on the choice of material.
Designers need to consider the various ceramic materials available to them. Several ceramic materials are favoured and have a proven track record for their mechanical, electrical, thermal and/or chemical properties. Examples are detailed below:
Alumina is a versatile material that offers a combination of good mechanical and electrical properties. It is suited for a variety of applications, which include X-ray tubes, electron tubes, laser devices, aerospace devices, high vacuum applications, flow meters, pressure sensors and wear components.
It has good stiffness and strength, good resistance to wear and high hardness. Alumina is offered in many grades ranging from 60% to 99.9% with additives designed to enhance properties such as wear resistance or dielectric strength. It can be formed using several ceramic processing methods and can be processed machined or net-shaped to produce a variety of sizes and shapes.
Furthermore it can be readily joined to other ceramics or metals using specially developed metallising and brazing techniques.
Aluminum Nitride (AlN)
Aluminum nitride (AlN) exhibits very good thermal conductivity. Other properties include excellent thermal shock resistance and corrosion resistance. Based on these properties AlN is used in power electronics, aeronautical systems, railways, opto-electronics, semiconductor processing, microwave and military applications. Typical applications include heaters, windows, IC-packages and heat sinks.
Zirconia offers corrosion and chemical resistance at high temperatures up to 2400°C – well above the melting point of Alumina. Magnesia Partially Stabilised Zirconia (Mg-PSZ) and Yttria Tetragonal Poly-crystal Zirconia (Y-TZP) are suited to engineering or structural applications where exceptional mechanical strength and properties such as hardness, wear and corrosion resistance are required.
The high temperature capability of zirconia products has resulted in the development of Fully Stabilised Zirconia (FSZ) grades for crucibles, nozzles and other components for molten metal handling applications.
Silicon nitride has very good high-temperature strength, creep and oxidation resistance, while its low thermal expansion coefficient provides good thermal shock resistance when compared with most other ceramic materials.
It has high fracture toughness, hardness, chemical and wear resistance, and is manufactured in three main product types: reaction bonded silicon nitride (RBSN), hot pressed silicon nitride (HPSN) and sintered silicon nitride (SSN).
Typical applications include: bearing ball and roller elements, cutting tools, valves, turbocharger rotors for engines, glow plugs, non-ferrous molten metal handling, thermocouple sheaths, welding jigs and fixtures and welding nozzles.
Silicon carbide is highly wear-resistant with good mechanical properties includinghigh temperature strength and thermal resistance of up to 1650°C. It has low density, high hardness and wear resistance and excellent chemical resistance. The applications of SiC are fixed and moving turbine components, seals, bearings, ball valve parts and semiconductor wafer processing equipment. A significant area of use is in specialist thermal processing applications, including beams and profiled supports, rollers, tubes, batts and plate setters, as well as thermocouple protective sheaths.
Shape of the Components
Once the material has been decided, the shape is the next consideration. There are certain shapes that will cause weaknesses in the component. When working with ceramic, simple shapes are consistently seen to provide the strongest result.
In some applications there is a requirement to join ceramic to metal to create the finished part. Various methods are available including mechanical fasteners, friction welding and adhesive bonding, but by far the most widely used and effective method for creating leak-tight, robust joint between ceramic and metal is brazing. This begins with the chemical bonding of a metallisation layer on the ceramic to create a wettable surface upon which the braze alloy will flow between the two components during the brazing process.
Morgan Advanced Materials manufactures WESGO® braze alloys and supplies high-purity, low vapour pressure alloys, including precious metal filler materials, non-precious alloy filler materials and active braze alloys. Precious brazing filler metals are derived from gold, silver, platinum and palladium--based materials and exceed the most stringent requirements imposed by the power tube, aerospace, semiconductor, medical, electronic and vacuum industries which they serve. Non-precious alloy filler materials are ideal for applications including tooling for mining and heavy industry equipment. They are suitable for brazing applications between 500ºC and 1200ºC.
A metal feedthrough can be produced for specific applications, such as flow meters, by placing a wire in the ceramic in the pre-sintered (green) stage. As the ceramic shrinks during the sintering process, it compresses on the metal and forms a gas-tight seal.
Coating and glazing
The roughness of a final product is based on the grain size. If the grain size is large, then the product will have a rough finish which can lead to cavities being formed after grinding. In order to achieve an excellent surface finish, parts can be glazed.
To summarise, many factors determine the optimum material from which components are manufactured. It is most important to consider the application and the performance requirements based on thermal, mechanical, electrical and chemical properties. Ceramic materials’ hardness, physical stability, extreme heat resistance, chemical inertness, biocompatibility, superior electrical properties and, not least, their suitability for use in mass produced products, make them one of the most versatile groups of materials in the world.
This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.
For more information on this source please visit Morgan Advanced Materials.