Ceramic Materials in Design
Designers and manufacturers can benefit from an in-depth review of advanced ceramic material properties.
Today’s advanced ceramics offer strong physical, chemical and electrical properties that make them highly resistant to melting, bending, stretching, corrosion, wear, high voltages, and currents. This has opened up development opportunities for manufacturers in a wide range of industries—from transportation and healthcare to electronics, scientific equipment and semiconductor processing.
Advanced ceramics such as alumina, aluminum nitride, zirconia, silicon carbide, silicon nitride and titania-based materials can provide cost-effective, high-performance alternatives to traditional materials such as metals, plastics and glass. The demand posed by new and changing applications is to improve operation at a reduced cost. New materials are constantly being developed and engineered to address the needs of individual applications.
When choosing a material, designers must consider many factors, including the desired properties for the application and the available materials. Designers can make well-informed choices by fully understanding these materials. In particular, the shape, design, and application of the part should be considered in the early stages of the engineering process. Ceramic part shapes are often designed to be analogous to metal part shapes. However, this can have a negative impact on the cost and properties of the parts. The joining of ceramics to metals also creates its own engineering challenges that require specialist expertise.
Physical properties such as strength, hardness, wear resistance, corrosion resistance and thermal stability also need to be considered when choosing a material. Each of these characteristics is determined by specific materials. Designers look for materials that give the best combination of characteristics for an excellent performance.
Having identified the material properties needed for individual applications, designers should then consider the various ceramic materials available to them. A number of available ceramic materials are optimized for their mechanical, electrical, thermal and/or
Alumina (Al2O3) is a versatile material that offers a combination of good mechanical and electrical properties. It is suitable for a wide range of applications, including laser devices, X-ray tubes, electron tubes, aerospace devices, high-vacuum applications, flow meters, pressure sensors, and wear components. It has a good strength and stiffness, as well as good hardness and resistance to wear.
Alumina is available in many grades, ranging from 60-99.9%, with additives designed to enhance properties such as wear resistance or dielectric strength. It can be formed using a variety of ceramic processing methods and can be processed net-shaped or machined to produce a variety of sizes and shapes. In addition, alumina can be readily joined to metals or other ceramics using specially developed metallizing and brazing techniques.
Aluminum nitride (AlN) has excellent thermal conductivity, 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 (ZrO2) offers chemical and corrosion resistance at high temperatures up to 2,400°C—well above the melting point of alumina. In its pure form, crystal structure changes limit zirconia’s use in mechanical/temperature applications, but zirconia stabilized with calcium, magnesium or yttrium oxide additives can produce materials with very high strength, hardness and toughness.
Magnesia-partially stabilized zirconia (Mg-PSZ) and yttria tetragonal poly-crystal zirconia (Y-TZP) belong to the group of tough, high-strength zirconias that harness the “transformation toughening” phenomenon unique to this material. They are suited to engineering or structural applications where exceptional mechanical strength and properties such as hardness, wear and corrosion resistance are required. 3Y-TZP is a similar product used primarily for medical implant applications.
Meanwhile, the high-temperature capability of zirconia products has led to the development of fully stabilized zirconia (FSZ) grades for crucibles, nozzles and other components for molten metal applications.
Silicon nitride (Si3N4) has excellent 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).
RBSN gives a relatively low-
density product compared with HPSN and SSNM nitride types. As a result, HPSN and SSN materials are suited 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 (SiC) is a highly wear-resistant material with good mechanical properties, including high-temperature strength and thermal resistance of up to 1,650ºC. It has low density, high hardness and wear resistance, and excellent chemical resistance.
These characteristics mean that silicon carbide 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.
After choosing the material, the next step to consider is the shape of the final engineered component. Certain shapes will cause weaknesses in the component. When designing and manufacturing with ceramics to give high-performance, high-strength, reliable parts, it is best to keep the shape simple. For example, it is advisable to avoid sharp edges and corners, as well as sudden cross-section changes.
For many applications, it is often necessary to join ceramics to metals in order to create the finished part. Ceramic-metal bonding is one of the biggest challenges faced by manufacturers and end users due to the inherent differences in the thermal expansion coefficients of the two types of materials.
Various bonding methods are available, including mechanical fasteners, friction welding and adhesive bonding. The most widely used and effective method for creating a leak-tight, robust joint between ceramic and metal is brazing. Brazing starts with the chemical bonding of a metallization 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 have similar expansion properties to the ceramic.
Precious brazing filler metals are derived from gold, silver, platinum and palladium-based materials; they 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 500-1,200ºC. 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. Because brazing can be done in a single step, it replaces metallizing, firing and electroplating, and offers time and cost savings.
For specific applications, such as flow meters, a metal feedthrough can be produced by placing a wire in the ceramic while it is 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 process requires excellent knowledge about (and control over) the sintering cycle temperature, as well as careful selection of the metal.
Coating and Glazing
The roughness of the final product depends on the grain size. If the grain size is large, the product will have a rough finish and cavities could form after grinding. In order to achieve an excellent surface finish, parts can be glazed. A glaze is important if parts will be exposed to dust or pollution. In these cases, the glossy surface can be cleaned easily to avoid surface leakage and flashovers.
An Advanced Solution
Many factors determine the material from which components are manufactured. It is 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 the extreme conditions of the application. Advanced ceramics are meeting the needs for higher-performance critical components in a variety of applications. Through a detailed understanding of available and effective ceramic-metal bonding techniques, such as the metallization process and the advantages of glazing and coating, designers and manufacturers are becoming aware of the viability of devising increasingly complex components.
For more information, visit www.morganadvancedmaterials.com.