Advanced Ceramic Materials I Focus


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). RBSN gives a relatively low-density product compared with HPSN and SSNM nitride types, because of this feature HPSN and SSN materials which are suited to for more demanding applications. 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: Silicon carbide is a highly wear-resistant material with good mechanical properties including high temperature strength and thermal resistance of up to 1650ºC. It has low density, high hardness and wear resistance and excellent chemical resistance. Its characteristics mean the material is ideal for applications such as fixed and moving turbine components, seals, bearings, ball valve parts and semiconductor wafer processing equipment. Other specialist applications include beams and profiled supports, rollers, tubes, batts and plate setters, as well as thermocouple protective sheaths.


component. Therefore, when designing and manufacturing with ceramic to give high performance, reliable parts with high strength it is best to keep the shape simple. For example, avoid sharp edges and corners, and sudden changes in cross section.


Metallising/brazing


For many applications it is often necessary to join ceramic to metal to create the finished part. Ceramic-metal bonding is one of the biggest challenges that have faced manufacturers and users due to the inherent differences in the thermal expansion coefficients of the two types of materials. Various methods are available including mechanical fasteners, friction welding and adhesive bonding but by far the most widely used and effective method for creating a leak-tight, robust joint between ceramic and metal is brazing. This starts with the chemical bonding of a metallisation layer on the ceramic to create a wettable surface upon which braze alloy will flow between the two components during the brazing process. In order to achieve maximum bond


strength, ceramic parts are typically metallized with molybdenum manganese (MoMn) and then plated with nickel (Ni). They are then ready for brazing and are generally assembled with metals that


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 in 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. Active braze alloys provide a single-step approach for joining ceramics to metals by eliminating the need for prior metallization of the ceramic surface, as the active components promote wetting. As it can be done in a single step it replaces metallising, firing and electroplating and offers time and cost savings.


Co-fired assemblies


For specific applications, such as flow meters, a metal feed-through can be produced 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. This needs excellent knowledge and control about the sintering cycle temperature and careful selection of the metal.


have very similar


Shape of the components Having chosen the material, the first step is to consider is the shape of the final engineered component. There are certain shapes that will cause weaknesses in the


expansion properties as ceramic. Morgan Advanced Materials manufactures WESGO braze alloys and supplies high-purity, low vapour pressure alloys, including precious metal filler materials, non-precious alloy filler


Coating and glazing The roughness of the final product depends on the grain size. If the grain size is large, then the product will have a rough finish and, after grinding, cavities could be formed. In order to achieve an excellent surface finish parts can be glazed. This is important if parts are to be exposed to dust or pollution. In this case,


the glossy


surface can be cleaned easily to avoid surface leakage and flashovers.


An advanced solution In summary, many factors determine the 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. As applications make greater demands on one or any combination of these properties, ceramics not only become the materials of choice, but in many cases, the only viable option in terms of materials that can survive in the extreme conditions of the application. Advanced ceramics are meeting the needs for higher performance critical components in a wide variety of applications. Through a detailed understanding of the ceramic-metal bonding techniques such as the metallisation process and the advantages of glazing and coating, designers and manufacturers are becoming able to devise increasingly complex components.


Morgan Advanced Materials | www.morganadvancedmaterials.com


Dieter Steudtner is Sales Manager at Morgan Advanced Materials


www.cieonline.co.uk


Components in Electronics


February 2014 15


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