First things first, metal oxides, of which alumina and zirconia are two, are not metals. For example, rust – iron oxide – is not the same as iron metal. Secondly, it’s worth noting the naming of these materials, a formula whereby the suffix ‘-ium’ is replaced by ‘-a’. Therefore, aluminium oxide is often called alumina and sits firmly within the ceramics family of materials.
Ceramics have undergone a massive evolutionary change over the last 40 years in relation to their performance and applications. The associations that ceramics have with craft, brittle china and terracotta still exist, but the new breed of technically advanced ceramics infuses new life and new associations into this primordial material.
Distinguished by hardness – ceramic knives don’t lose their sharp edge as easily as metal – inertness and stiffness, ceramics are moving into the territory of metals. Such is the allure of advanced ceramics that designers have, since the commercialization of advanced ceramics, been looking at ways to introduce them into applications that range from cars to mobile phones. The biggest obstacles to this being production costs and the tolerancing of small components, which need to fit with a plastic injection moulding. Alumina and zirconia are two of the most widely used advanced ceramics and are helping to morph the definition of ceramics into a material of advanced technology and luxury when applied to consumer-facing design.
Image: Vertu’s Constellation mobile phone with monocrystalline aluminium oxide screen
•Versatile processing
•Excellent hardness
•Good corrosion resistance
•Good stiffness
•Good thermal stability
•Excellent electrical insulation
•Higher resistance to wear than zirconia
•Brittle
•Expensive, but less than zirconia
•Melting point: 2072ºC (3700ºF)
Sources
Alumina is one of the most widely available advanced ceramics and is obtainable in a range of purities to suit different applications. Australia has by far the largest reserves of bauxite, the principal ore of aluminium oxide.
Cost
£3.20 ($5) per kg.
Sustainability issues
As an inert material, ceramics do not degrade. In order to produce ceramics an irreversible chemical reaction needs to take place, therefore, unlike thermoplastics, which can be reheated and reused, ceramics are not recyclable in the sense that they cannot be remoulded. However, they can be crushed and used as fillers and gravel for various industrial applications.
Production
Alumina can be extruded, dry – powder – wet compression moulded and sintered from powder. It can be machined, including diamond grinding, in its fired form. When formed and sold as a flat sheet it can also be laser cut. Due to this, variations on production of alumina are suited to both one-off and high-run batch or mass production. It can be readily joined to metals using metallizing and brazing techniques.
Typical applications
Alumina is one of the most popular advanced ceramic materials. Used as an electrical insulator on spark plugs, its good wear resistance makes it ideal for parts, such as dies, bearings and, of course, kitchen knives. Its wide availability and its range of properties also make it suitable for electronic substrates and for making ceramic fibres and papers. It is also used for bulletproof vests and replacement balland-socket hip joints, where its wear-resistance means that small particles that are usually produced by metal replacement joints and which cause resistance are not created. It is also used in high-end mobile phones from Vertu and also for watches from Rado.
Derivatives
Zirconia toughened alumina (ZTA) is used in mechanical applications. It is considerably higher in strength and toughness than alumina. This is as a result of the stress-induced transformation achieved by incorporating fine zirconia particles uniformly throughout the alumina. Typical zirconia content is between 10 and 20%. As a result, ZTA is more expensive than alumina but offers increased component life and performance.
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–Strong and tough –Versatile processing –Extremely hard –Good wear resistance –Widely available |
–Brittle –High cost –Not recyclable |