What is common to a knife used by a Japanese sushi master, a necklace of American diamonds, and a turbine blade of a jet engine? All these articles make use of zirconia ceramics! Among the advanced ceramic materials, zirconia ceramics perhaps have the widest range of applications. This is because of the impressive properties of zirconia ceramics, which in turn, are a result of the wide range of microstructures that can be obtained in these materials. The properties of interest to an engineer include melting temperatures in excess of 2000C, resistance to chemical attack, resistance to fracture and wear, resistance to heat conduction, and the ability to conduct electricity through the migration of oxygen ions. Biocompatibility is the other attractive feature of zirconia ceramics.
Pure zirconia (ZrO2) is not used in any practical application. Other oxides such as yttria (Y2O3), calcia (CaO) and magnesia (MgO) are common additives in the range of 3-28 wt%. These additives are called ‘stabilisers’ for the following reasons: At room temperature, pure ZrO2 has a monoclinic (m) crystal structure, which transforms to tetragonal at ~1170C. The tetragonal (t) phase itself transforms to a cubic fluorite phase © at 2370C which remains stable up to the melting point of 2680C. The volume of monoclinic zirconia is up to 9% higher than tetragonal zirconia. If a component is made of pure zirconia, it would be sintered at temperatures in excess of 1200C, which is usual for ceramic components. During cooling from the firing temperature, the t to m transformation would occur, accompanied by sudden volume expansion which would lead to build up of stress and shattering of the component. In the presence of a stabiliser oxide, instead of monoclinic, the tetragonal or the cubic phase remains stable down to room temperature, depending on the amount of stabiliser added. This helps in avoiding the catastrophic volume change accompanying the t to m transformation. Moreover, one can adjust the stabiliser content and the sintering temperature to stabilise a mixture of tetragonal and cubic phases down to room temperature. These various forms of stabilised zirconia have different properties and applications, as described below.
Zirconia is used as special refractories in steel-making. The submerged entry nozzle for continuous casting is made of zirconia. Magnesia-zirconia is used for making stir plug/blocks and well blocks in ladles for VOD (vacuum oxygen decarbonisation). Other small but critical components in the continuous casting process are made of zirconia. The high melting temperature, chemical inertness and wear resistance of the zirconia refractories have created a niche for these materials in steelmaking technology.
It is the celebrated transformation toughening that has enabled ceramic engineers to come up with incredible products like knives, scissors and even sparkless hammers of zirconia ceramics. Currently, yttria is the most common stabiliser used. About 5wt% yttria addition yields a fully tetragonal material. This is called tetragonal zirconia polycrystal (TZP). This material exhibits the phenomenon of transformation toughening. The tetragonal phase is retained at room temperature in a metastable manner, meaning it is prevented from becoming monoclinic due to the inability of the atoms to move rapidly. However, if a large stress is applied, the atoms can jump in a coordinated move to generate the monoclinic phase. This stress-induced transformation is martensitic in nature, similar to what happens in certain types of steel. The most important consequence of this stress-induced transformation is that the resistance of the material to sudden fracture increases. Thus, the fracture toughness of TZP can be as high as ~15 MPa m-1/2, compared to ~1 MPa m-1/2 for the common soda-lime glass.
Transformation toughening can also be obtained by using higher yttria contents of about 6-12 wt%. With suitable sintering schedule, fine dispersion of tetragonal particles in cubic matrix can be obtained. This partially stabilised zirconia (PSZ) also has high fracture toughness, though not the same as TZP. The fracture toughness of non-zirconia ceramics can also be enhanced by dispersing tetragonal particles in them. Zirconia toughened alumina (ZTA) is a prime example of such ceramics. This material has the great combination of hardness and fracture toughness, which is very handy in making high performance cutting tools with long lifetime. Commercial applications of transformation toughened zirconia materials include extrusion dies, thrust bearings, valve seats, valve guides for turbocharger rotors, scissors used in paper industries, roller guides, bearings in seal-less pumps, magnetic drives, seal faces and ball milling media. Transformation toughened zirconia ceramics are also used in prosthetics, such as femoral ball heads in artificial hip joints and dental restoratives. These go a long way in alleviating the suffering of many individuals.
Moving on to another vital aspect of the versatile zirconia ceramics, it is a fact that all modern jet engines are built using a layer of zirconia on all critical components in the hot sections. These layers, called thermal barrier coatings (TBC’s) have very low thermal conductivity and therefore protect the metallic components against excessive heat of the combustion gases in the jet engines. But what is the need for these protective coatings? To get to the answer, consider the energy resources for the aviation sector. Not only do we live in an energy starved world, but also in an era of great environmental concerns. While surface transport may move to new energy resources, such as electricity from solar or wind power technology, aviation will depend on fossil fuel for the foreseeable future. This is because the new energy resources do not offer the high power to weight ratio which is crucial for any aerospace vehicle. Hence, the aircraft engineers are looking at enhancing the fuel efficiency of modern jet engines like never before. It is well known that fuel efficiency increases with increasing operating temperature. Nickel base superalloys used for constructing the critical components of a jet engine are already very close to the highest possible operating temperatures. Zirconia based thermal barrier coatings offer the opportunity to further increase the gas turbine engine’s operating temperature while still keeping the superalloys in their comfort zone.
A fact which further highlights the importance of these zirconia ceramic coatings is that gas turbines are used for generating electricity in thermal power plants as well! The requirement for enhancing the efficiency of gas turbines becomes all the more urgent. Therefore, TBC technology has great strategic and socio-economic importance.
The components of jet engines coated with zirconia are the combustor liners, shrouds and the high pressure turbine which bears the brunt of the combustion gases. The thickness of the coatings ranges from about 0.15 to 0.25 mm, with a temperature differential of ~100C across the thickness. Thus, the state-of-the art high pressure turbine blades, made of nickel base superalloys in single-crystal form, are kept at a temperature of ~1050C, although the hot gases are at more than 1200C. All modern jets use internally circulated air to cool the superalloy components. However, TBC’s further enhance the thermal protection, leading to better engine performance. The TBC’s are deposited using plasma spray technique or electron beam physical vapour deposition. Typically, 6-8 wt% of yttria is added to stabilise the tetragonal (t’) zirconia phase. These are the materials of choice because they have very low thermal conductivity (1-2 Wm-1K-1). Moreover, the thermal expansion behaviour is close to that of the superalloys. No transformation toughening can be obtained in TBC’s since it is restricted to low temperatures of a few hundred degrees Celsius. Another important factor is that the t’ phase can destabilise over prolonged thermal exposure. Research efforts are currently directed at improving the reliability, high temperature stability, and thermal conductivity of TBC’s. Oxides of lanthanide series metals (rare-earths) are used as stabilisers for achieving these goals.
At this stage, it may look like zirconia ceramics are all about muscle power and withstanding thermo-mechanical distress. This is far from the truth. Zirconia ceramics possess the amazing ability to conduct electricity through the migration of oxygen ions (O2-). This property is seen at temperatures above 600C. It allows us to build the most efficient power generation device, known as solid oxide fuel cell (SOFC). The importance of efficient power generation was highlighted above. Solid oxide fuel cells convert the chemical energy of the reaction between a fuel and oxygen directly into electricity. The most common fuel used is hydrogen, although SOFC’s can also use certain hydrocarbon fuels. Given the high cost of generating pure hydrogen, and the need to use fossil fuels in the most efficient manner, the fuel flexibility of SOFC’s is one of the best assets of these devices. At the operating temperature of 800 to 1000C, steam is the major by-product of SOFC’s, which can be used for power generation as well. Therefore, a combined cycle power generation plant based on SOFC’s is expected to have almost 70% efficiency!
Zirconia ceramics are used for fabricating the three essential components of an SOFC: electrolyte, anode and cathode. The electrolyte consists of a thin impervious membrane of less than 0.1 mm thickness. The anode consists of a mixture of zirconia and nickel oxide, whereas the cathode consists of zirconia and a special compound called LSM (strointium doped lanthanum manganite).
The zirconia in SOFC’s contains even higher amounts of stabilisers than TBC’s and occurs in the cubic crystal structure. This is because the ionic conductivity is the highest for about 15 wt% of yttria. Similar to TBC’s, there are efforts to use lanthanide series stabilisers to further enhance the properties.
The story of zirconia ceramics and the environment does not end here. The ionic conductivity of zirconia is useful in making the so-called lambda sensors in modern automobiles. This sensor monitors the composition of the exhaust gases of a car engine. Based on the input from the lambda sensor, the computer of the car controls fuel injection into the engine to maximise efficiency. Along with catalytic converters, the O2 sensors have been a boon to the automotive industry which faces increasingly tough emission control regulations and fuel efficiency standards.
Zirconia technology is in the early stages in India. Most of the products made by Indian industry are non-critical in nature, for example small crucibles. Even the raw materials required are almost always imported. Great opportunity exists for developing better processing techniques which can yield more reliable coatings, perhaps at lower costs. Any import substitution in this area will be a significant contribution to indigenisation efforts. In fact, India needs to be more ambitious in this area and become the main supplier of value added zirconia ceramic products to the world. This is because one of the world’s large mineral deposits of zirconium is in India!
Zirconium exists in nature mainly as zircon (ZrSiO4) and some times as the mineral baddeleyite (m-ZrO2). Zircon is found as sandy deposits called zircon sand. The world’s large deposits are found in Brazil, Australia, South Africa, India, Sri Lanka and the USA. India’s zircon deposits are found in the state of Kerala. Baddeleyite is found in Brazil and Russia. The extraction of zirconia from zircon sands is carried out using the following process. Mineral beneficiation is carried out to separate and remove undesirable materials and impurities. For zircon it is mainly silica that is removed, and for baddeleyite, iron and titanium oxides. There are a few routes for extracting zirconia from zircon. These include chlorination, alkali oxide decomposition, lime fusion, and plasma dissociation.
Zirconia has emerged as strategically one of the most important ceramic materials. Researchers in India’s academic institutes, government and corporate laboratories are working on not only indigenising the critical technologies, but also on developing new compositions and processes so that India can emerge as a strong player in the world. The Indian industry has a great opportunity to develop a niche market for zirconia-based products.
Finally, the most glittering application of zirconia ceramics is in the form of gems called American diamond. It consists of cubic zirconia crystals grown from the liquid state and then cut into the gems. Also known as CZ, these sparkling gems rival real diamonds in their visual effect. The author hopes that the next time you buy CZ jewellery, you will be happy not only because of its sparkle but also because of the impact of various zirconia ceramics on our lives!