When choosing a material for an application, designers and manufacturers need to consider a wide range of factors. Here Dieter Steudtner, sales manager at Morgan Technical Ceramics, WESGO, provides an insight into advanced ceramic materials, their properties, and potential applications
Today’s advanced ceramics offer powerful physical, chemical and electrical properties that make them highly resistant to melting, bending, stretching, corrosion, wear, high voltages and currents. The result is development opportunities for manufacturers in a wide range of industries such as aerospace, defence, automotive, medical, electronics, telecommunications, scientific equipment and semiconductor processing.
Advanced ceramics such as alumina, zirconia, silicon carbide, silicon nitride and titania based materials provide a cost-effective, high performance alternative to traditional materials such as metals, plastics and glass.
Demands, however, are posed by new and changing applications, and the challenge is to improve operational capability while reducing cost. New materials are constantly being developed and engineered to address this need.
When choosing a material, designers need to consider factors including the desired properties. In particular when thinking about the shape, design and application of the part there are a lot of things to be considered in the early stages of development. Fully understanding the materials, however, can ensure well informed choices are made.
The joining of ceramics to metals also creates its own engineering challenges that require specialised expertise. Furthermore, physical properties such as strength, hardness, wear-resistance, corrosion-resistance and thermal stability need to be considered. Designers look for those materials which give the best combination of characteristics for an excellent performance.
Materials
Having identified the material properties needed for individual applications, designers can then consider the various ceramic materials available.
Alumina is a versatile material that offers a combination of good mechanical and electrical properties. It is suitable for a wide range of applications including seal rings, laser tubes, electrical insulators, thread guides, grinding media and wear components. It also has good strength and stiffness, good hardness and resistance to wear.
This material is available in many grades ranging from 60% to >99.9% purity with additives designed to enhance properties such as 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 it can be readily joined to metals or other ceramics using specially developed metalising and brazing techniques.
Zirconia offers chemical and corrosion resistance at high temperatures up to 2400°C – well above the melting point of Alumina. In its pure form, crystal structure changes limit use in mechanical/temperature applications, but stabilised zirconia with calcium, magnesium or yttrium oxide additives, can produce materials with very high strength, hardness and, in particular, toughness.
The material has low thermal conductivity (20% that of Alumina) and is an ionic conductor above 600°C which benefits applications such as fuel cells where ionic movement within a solid material is required. This has lead to applications in oxygen sensors and high temperature fuel cells.
Typical applications include: precision ball valves (balls and seats), high density grinding media, thread guides, cutting blades, medical prostheses, pump seals, valves and impellors, radio frequency heating susceptors and metrology components.
Silicon Nitride has good high temperature strength, creep resistance and oxidation resistance. In addition, its low thermal expansion coefficient gives good thermal shock resistance, compared to most ceramic materials. It has a high fracture toughness, high hardness, chemical and wear resistance.
Three main types are produced: Reaction Bonded Silicon Nitride (RBSN), Hot Pressed Silicon Nitride (HPSN), and Sintered Silicon Nitride (SSN). RBSN gives a relatively low-density product compared with Hot Pressed and Sintered Silicon Nitride. HPSN and SSN offer better physical properties suitable for more demanding applications. Typical applications include: bearing balls and rollers, cutting tools, valves, turbocharger rotors for engines, turbine blades, glow plugs, molten metal handling, thermocouple sheaths, welding jigs and fixtures and welding nozzles.
Silicon Carbide is a highly wear resistant material with good mechanical properties including high temperature strength and thermal resistance of up to 1650°C. It has a low density, high hardness and wear resistance and excellent chemical resistance. This material is ideal for applications such as fixed and moving turbine components, seals, bearings, ball valve parts and semiconductor wafer processing equipment.
Component shape
Having chosen the material, the first step is to consider the shape of the final engineered component – certain ones will cause weaknesses in the component. When designing and manufacturing the ceramic to give high performance, reliable parts with high strength it is therefore best to keep the shape simple, avoiding, for example, sharp edges and corners, sudden changes in cross section and oval parts.
For many applications it is often necessary to join ceramic to metal to create the finished product. Ceramic-metal bonding is one of the biggest challenges due to the inherent differences in the thermal expansion coefficients of the materials. While various 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 by brazing. This starts with applying a chemically bonded layer of metal on the ceramic to create a wettable surface on which braze alloy will flow.
In order to achieve a maximum of adhesive strength, ceramic parts are typically metalised with molybdenum manganese (MoMn) and then nickel (Ni) plated. The parts are then ready for brazing with metals that exhibit similar expansion properties as ceramic, such as Nickel-Cobalt-Iron alloys.
Morgan Technical Ceramics manufacturers and supplies high-purity, low vapour pressure brazing alloys including: Precious Brazing Filler metals derived from gold, silver, platinum and palladium based materials that exceed the most stringent requirements imposed by 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 500°C and 1200°C.
Active braze alloys provide a single-step approach for joining ceramics to metals by eliminating the need for prior metallisation of the ceramic surface, as the active components promote wetting. As it can be done in a single step it replaces metalising, firing and electroplating and offers time and cost savings.
Connection
Another way of connecting ceramic and metal is to sinter in wires, for example, in feedthroughs for flow measurement components. In this method, a wire is placed in the ceramic in the raw ‘green’ stage and is sintered together with the ceramic in a kiln. During the sintering process, the ceramic shrinks and clamps around the wire, forming a strong bond that is gas tight.
This process has a two distinct advantages compared to alternative methods. It is more cost effective as there is only one process to bond the metal to ceramic; and secondly, it is the only possible technology for very small wires.
Coating and glazing
Another consideration in engineering the product is coating and glazing. Coating further optimises the technical properties of the material including its mechanical strength and chemical resistance. It improves durability and resistance to wear for components and glazing seals the material which can also help with improving surface porosity.
The roughness of the final product depends on the grain size – if 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 are glazed. This is important if parts are exposed to dust or pollution – a glossy surface can be cleaned easily to avoid surface leakage.
Morgan Technical Ceramics is a specialist in high quality Diamond-Like-Carbon (DLC) coatings, CVD diamond coatings, DiamondShield plastic coatings and wear resistant glass.
Considering the demands
Many factors determine the material from which components are manufactured. It is therefore important to consider both the application and performance requirements based on thermal, mechanical, electrical and chemical properties.
As applications make greater demands on any one or combination of these properties, ceramics not only become the material of choice but, in many cases, the only viable option in terms of materials that can survive in the extreme conditions required by the application.
As a result, advanced ceramics are meeting the needs for higher performance critical components in a wide variety of applications, in demanding sectors like healthcare, energy and transportation. Through a detailed understanding of the ceramic-metal bonding techniques such as the metallization process and the advantages of glazing and coating, designers and manufacturers are better placed to develop new components.
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