The value of ceramics

Materials World magazine
10 May 2019

The Ceramic Society talks about the importance of ceramic materials, their numerous beneficial properties and the prospects for future applications.

Ceramic materials are often defined by what they are not. They are inorganic, non-metallic materials – everything that isn’t a polymer or a metal. Following the incorporation of the Institute of Ceramics into the Institute of Materials when it was formed back in 1993, ceramics have been represented by The Ceramics Society, which exists to provide a forum for the exchange of information, knowledge and state-of-the-art industrial practice on all aspects of ceramics in the UK.

Its activities focus on the technical, educational, academic and professional issues relating to any IOM3 member, including those who are not directly involved with ceramics. The society is structured into three committees – Ceramics Science, Cementitious Materials and the International Clay Technology Association (ICTa). The society is also closely linked with the non-IOM3 affiliated, Society of Glass Technology (SGT).

Breaking down ceramics

Ceramics can be broadly divided into the following:

  • Traditional ceramics – often defined as those based on clays, which provide everything from bricks to cups, and tiles to washbasins
  • Refractories – materials that can withstand high temperatures and chemical attack, used to line furnaces for melting metals and glass, or firing other ceramics
  • Cementitious materials – those based on cements and mortars which form the basis of the construction industry, and
  • Advanced ceramics – loosely defined as those displaying very specific engineering properties. These can be broken down into bioceramics for use in healthcare, from artificial bones and joints to dental materials, electroceramics that underpin the electronics industry including communications equipment and display technology, and engineering ceramics for the manufacture of all types of materials, in energy generation and storage and defence, among many others.

Products made from ceramics deliver performance other materials can’t, for example, when others reach their limits, ceramics can survive enormous stresses, extreme temperatures, nuclear radiation or highly aggressive chemicals. They provide reliable solutions in all types of industrial production, provide clean energy and safe transport, and enable people to communicate around the planet. Once you know where to look, it is very difficult to find a technology that does not incorporate a ceramic.

The total global annual ceramic market was estimated to be US$158bln in 2014, with growth anticipated to rise to about US$287bln by 2022. Advanced ceramics are expected to be the fastest growing segment in terms of volume at a compound annual growth rate (CAGR) of over 9.0% from 2015-2022, while the construction industry is expected to be the primary market driver over the period, owing to urbanisation, higher per capita income, population rise and changing economies. The largest geographic market by mass is the Asia Pacific, with its size estimated at over 110 million tonnes in 2014. Within this, China was the largest regional market on account of its booming automobile industry, coupled with high infrastructural investment to meet the housing needs of the country.

Clearly, the range is too broad to review every application in which ceramics are used. Here we discuss three major advanced ceramic sectors – medical devices, electronic ceramics and aerospace ceramic matrix composites – reviewing the types of ceramics used, the properties available, and what the future might hold for these materials.

Medical devices

Hip replacements are now a routine and very successful orthopaedic implant and ceramic hip implants lead the market, with over 50% of the prosthetics based on a ceramic ball and, sometimes, a ceramic cup.

On average, one ceramic hip replacement is implanted every 30 seconds. They are based on alumina, zirconia and zirconia-toughened alumina, which offers the benefit of excellent bio compatibility, low friction/wear, and has the lowest risk of osteolysis, or loosening, while solving issues associated with hypersensitivity to metals. Ceramic implants are also available, or are in development, for dental, knee, shoulder, wrist and cranial surgery.

In scanners, imaging and treatment systems, magnetic resonance imaging (MRI) and computer tomography systems require ceramics to insulate and connect the instrumentation controls and power feedthroughs from the outside world to the sealed cryogenic magnetic field inside the MRI magnet assembly. Ceramics can be joined to metals to produce high reliability hermetic seals that are electrically insulating, can be non-magnetic and capable of operating at the very low temperatures that occur around the super-cooled magnetic coil. The magnetic coil can be made from a superconductive ceramic, yttrium barium copper oxide.

Ceramics are also used in sensing and patient positioning functions, the latter in the form of piezoelectric ceramics such as lead zirconate titanate that is able to interchange electrical and mechanical energy. Alumina is also used for components involved in X-ray generation, to insulate the high voltages and provide the thermal performance, while the X-rays themselves are used for imaging and in treatment systems.

Proton therapy is a new generation system for targeted cancer treatment and uses a mini synchrotron to create a high-energy proton beam to treat tumours. This needs hermetic seals and high voltage insulating components, both of which are provided via the use of ceramics, such as alumina, among other materials. Spectroscopy equipment, including mass spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy, are used for drug development and life sciences research and developments. In these applications, ceramics provide chemically inert, non-magnetic, electrically insulating and hermetic components.

The future of medical technology includes concepts such as smart medical implants that respond appropriately to the body’s needs, wearable devices that interface through a smartphone, earlier and more consistent diagnoses, affordable home-based sensors, improved imaging systems, patient-specific treatments, improved drug development efficiency, artificial intelligence and machine learning, virtual reality and 3D imaging, together with better understanding of biological processes at the atomic and sub-cellular levels. All these rely on the development of enhanced advanced ceramics or will feed into their development. As medical technology becomes more sophisticated, the key component properties required become more demanding – higher voltages, greater precision in components and parts, more compact electronics and the development of materials that integrate multiple functions in the same ceramic material.

Electronic ceramics

More than 65% of the £42bln advanced ceramics global market is based on electronic ceramics, which is predicted to more than double to over £100bln by 2024. Functional electronic ceramics such as dielectrics, ferroelectrics, piezoelectrics, pyroelectrics and thermoelectrics, are used, in bulk or thin-film form, for a wide range of applications, including capacitors varistors (variable resistors – used to protect systems from voltage surges ranging up to lightning strikes), high temperature and harsh environment sensors, transducers, pacemakers, digi boxes, space heaters and high-frequency communication devices.

High-frequency dielectric ceramics are used in mobile communications, satellite television broadcasts, radar, GPS, WiFi, etc. and their material properties underpin the device performance. The key enabler for this technology is the development of materials with higher dielectric permittivity, higher quality factor and lower dielectric loss and, for some applications, the ability to withstand high temperatures and harsh environments. Thus, advanced ceramics fit the bill perfectly. However, there is a theoretical limit to the inherent properties that can be achieved, therefore researchers are now looking at developing artificially designed material structures and architectures – metamaterials – to cater for the ever-demanding needs of the beyond 5G communications sector.

The conventional ceramic fabrication technologies of die pressing, tape casting, slip casting, injection moulding etc. offer insufficient design freedom and often lead to high levels of machining – generating both expense and waste – with high labour and tooling costs. This means that additive manufacturing, often referred to as 3D printing, is increasingly being researched with a view to producing the required complex shaped, smart ceramics with tailored properties. This trend is gaining traction in other ceramic sectors too.

Aerospace materials

Aerospace companies have been developing ceramic matrix composite (CMC) technology for both air and land-based power generation gas turbine applications since the 1980s. CMCs are seen as a key enabler to improve the specific fuel consumption of aero-engines. Research is being targeted towards two principal application sets – silicon carbide (SiC)based materials for core turbomachinery aimed at reducing component cooling flow requirements, and oxide-based materials for weight reduction. The demonstration of technology maturity for introduction into aero-engines is, necessarily, very rigorous and the reliability and life requirements for CMCs are particularly challenging for the SiC-based high-temperature ceramics. These CMCs are expected to operate in a hostile environment combining high temperature, mechanical/thermal stresses, oxidising conditions, a high-velocity gas stream and combustion products, such as water vapour and sulphur oxide (SOx).

Industrial effort has tended to focus on design, material and manufacturing capability development, while more academic-based research underpins development of a mechanistic understanding of damage progression. The latter includes, for example, specialised testing, characterisation of reaction products, degradation monitoring techniques, digital image correlation, tomography and acoustic emission, to inform the development of lifing methods to underpin component life. This arduous requirement set is driving parallel activities to develop prime reliant environmental barrier coatings to protect gas path surfaces.

For CMCs aimed at weight reduction, typically oxide/oxide CMCs, the challenges are different. Cost reduction, maximising weight reduction via analysis while accounting for manufacturing defects, and validating a lifing approach for complex geometries are key. In both CMC types, effectively linking coupon (small sample) testing to sub-element, sub-component and full component testing is challenging and there is inevitably much reliance on very expensive engine testing. However, this step is justified by the huge disruptive opportunity these materials bring.

More recently, in the last few years an entirely new class of CMC has been developed – the ultra-high temperature CMC. These are materials designed to operate successfully at temperatures well over 2,000°C – in some cases up to 3,000°C. Applications range from thermoablation-resistant rocket nozzles through to thermal protection systems for hypersonic flight, typically defined as flight speeds of greater than Mach 5. Materials are typically based on carbon fibres for the structural performance, protected by ‘exotic’ ceramics such as zirconium diboride – typically offering protection to around 2,500°C – and hafnium diboride – 1.7 times denser and roughly 10 times more expensive, but offering 3,000°C capability.

Ceramic materials are opening up new, innovative opportunities for developing and enhancing many advanced technologies across sectors as diverse as construction, energy, healthcare, defence, transport and manufacturing. While ceramics are often not as cheap as many metals and polymers, they increasingly offer solutions to technical problems that competitor materials can’t match and recent developments mean it is now much easier to use multiscale modelling, characterisation and testing to design the composition, structure and manufacturing process to yield new materials that provide exactly the desired properties. For ceramics, the future is exciting.