R. Stevens

Hafnia and hafnia-toughened ceramics

Hafnia (HfO2) and hafnium-based materials are traditionally regarded as technologically important materials in the nuclear industry, a consequence of their exceptionally high neutron absorption coefficient. Following the discovery of transformation toughening in the mid 1970s, a considerable research effort has been devoted to zirconia (ZrO2)-toughened ceramics (ZTCs). They are considered to be potentially useful materials for structural applications at low and intermediate temperatures (T<1000 °C). Their unsuitability for high-temperature structural applications (T>1000 °C) is related to the low temperature of the tetragonal to monoclinic transformation in ZrO2. On the basis that HfO2 exhibits a similar crystal structure and in particular that its tetragonal to monoclinic transformation temperature (∼1700 °C) is approximately 700 °C higher than that for ZrO2, it has been suggested that high-temperature transformation toughening could be possible in HfO2-toughened ceramics (HTCs). Although the concepts behind this suggestion are universally appreciated, only a limited success has been made of the fabrication and the microstructural and mechanical property evaluation of these materials. The fracture toughness values obtained so far in HfO2 toughened ceramics are, in fact, considerably lower than those obtained in their ZrO2 counterparts. A great deal of further research work is therefore required in order to understand fully and to exploit toughened ceramics in the HfO2-based and HfO2-containing systems. This review covers the science and technology of HfO2 and HfO2-toughened ceramics in terms of processing, phase transformation, microstructure, and mechanical properties.

Zirconia-toughened alumina (ZTA) ceramics

In the previous decade, a considerable amount of work has been done on the alumina-zirconia ceramic composite system with a particular emphasis on improving the mechanical properties utilizing the recognized toughening mechanisms. Zirconia-toughened aluminas (ZTA) can be regarded as a new generation of toughened ceramics; for example, a toughness of >12 MPa m0.5 has been obtained, compared with 3 MPa m0.5 for commercial alumina ceramics. The fracture strength of ZTA is also greatly in excess of that for alumina. The mechanical properties of ZTA are critically dependent on their microstructures, which can be designed in terms of specific applications and controlled by means of powder preparation and densification processes. This review also includes details of the possible future development of ZTA; these are expected to involve the development and measurement of the mechanical properties for high-temperature engineering applications.

Microstructure of zirconia-yttria plasma-sprayed thermal barrier coatings

The objective of this paper is to report on the characterization of the highly complex microstructure of zirconia coatings, which arise as a result of the plasma-spraying process. The fine structure has been observed to change through the thickness of the coating, behaviour which has been related to the cooling rate and crystallization of the deposited material. Microstructural features such as an amorphous bond coat/ceramic interfacial film and a grain-boundary glassy phase, which are believed to have a significant effect upon coating properties such as adhesion and compliance, have been shown to be present.

Phase composition and properties of plasma-sprayed zirconia thermal barrier coatings

The use of X-ray diffraction combined with TEM analysis has been used to study the crystalline structure and change in phase composition of zirconia coatings containing 6–12 wt% Y2O3. The optimum composition for maximum durability, observed for coatings within this composition range, is believed to be related to the microstructure developed on rapid cooling and to the volume fractions of t′, c and m phases formed during the evolution of the coating. The amount of these phases present in commercial thermal barrier coatings has been determined using X-ray diffraction and the mechanisms of toughening deduced from TEM examination of the sections of the coatings. The results obtained are discussed in relation to the degree of toughness and hence the thermal shock resistance which is a major factor in determining service life.

Structure, properties and production of ?-alumina

The crystal structures of the β-alumina compositions have been described and used to explain the fast ion transport for which these materials are renowned. Measured values of both the single crystal and polycrystalline ionic conductivity show a wide variation; this is explained in terms of the range of chemical compositions of the β-alumina system and also the variety of measuring techniques used. Dopants or impurity ions can have a significant effect on the physical properties of the β-aluminas. The ionic conductivity, the stability of the material and the densification during sintering have been considered in relation to the nature and level of a range of dopants described in the literature. The optimization of the ionic and mechanical properties has been achieved by development of the fabrication techniques and it is this which accounts for much of the present research. Thus the many different methods of producing both single and polycrystalline material have been described, including the range of sintering routes currently available. The advantages and disadvantages of each production route in terms of the resulting properties have also been discussed.

Toughening mechanisms in duplex alumina-zirconia ceramics

A series of Al2O3-ZrO2 ceramics has been fabricated using both conventional sintering and a hot-pressing route, which results in various microstructures including (i) Al2O3 with well-dispersed ZrO2 single crystals; (ii) Al2O3 With TZP (tetragonal zirconia polycrystals) agglomerates (20 to 50μm); and (iii) Al2O3-ZrO2 duplex structures, in which both well-dispersed ZrO2 single crystals and TZP agglomerates are dispersed. The fracture strength of the composites has been measured by means of three-point bending and the fracture toughness by means of the micro-indentation technique. The microstructural characterization was carried out using scanning and transmission electron microscopy, and phase analysis of the zirconia by means of X-ray diffraction. The high toughness values of ≈ 12 M Pa m1/2 measured for the duplex structure have been correlated with the toughening mechanisms operative and the fracture strength with the matrix grain size and with larger defects present in the structure. A combined toughening process is proposed to account for the improved properties, including transformation toughening, microcrack toughening and crack deflection, which are discussed in context with the property measurements and the microstructural observations.

Effects of TiO2 addition on the sintering of ZrO2·TiO2 compositions and on the retention of the tetragonal phase of zirconia at room temperature

The production of tetragonal zirconia polycrystalline (TZP) ceramics and the identification of factors controlling retention of the tetragonal phase in the ZrO2·TiO2 system have been investigated. In this binary system, it was not possible to retain tetragonal zirconia polycrystals at room temperature for a range of compositions sintered above 1200 °C. A decrease in the martensitic transformation temperature of zirconia with titania addition was observed, but the effect was insufficient to retain the tetragonal phase at room temperature. In solid solution, the TiO2 additions act to suppress ZrO2 densification, this leading to grain growth when attempts are made to attain higher densities. The use of fine powders, fast firing or sintering in reducing conditions altered densification but was not able to generate a final grain size sufficiently small to avoid spontaneous tetragonal→monoclinic transformation on cooling. Based on the results obtained for ZrO2·MOx systems, the main factors involved in the retention of tetragonal zirconia at room temperature are discussed in an attempt to incorporate thermodynamical and the stress field effects.

Fabrication and microstructure-mechanical property relationships in Ce-TZPs

Ceria-stabilized tetragonal zirconia polycrystals(Ce-TZPs) have been fabricated via conventional sintering of commercially available electrofused and electrorefined CeO2-doped ZrO2 powder at 1550°C for various periods from 0.5–30 h. The resultant grain sizes of the sintered materials were in the range 2–15 μm. The sintering of such electrorefined powder appears to occur by a liquid state sintering process, evinced in terms of the grain-size dependence on sintering time at 1550°C and by direct TEM observation. The mechanical properties of the sintered materials have been characterized, including single-edge notch bend fracture toughness and three-point bend fracture strength. The grain-size dependence of these properties in the CeO2-stabilized tetragonal polycrystals is very much different from that in Y2O3 stabilized tetragonal zirconia polycrystals (Y-TZPs). The transformation plasticity, which is represented by the yield stress behaviour and the total strain to fracture, plays an important role in the microstructure-property interrelationship in the Ce-TZPs.

The effects of notch width on the SENB toughness for oxide ceramics

Abstract The fracture toughness of a range of oxide ceramics, namely aluminas with various grain sizes, Y2O3-stabilized tetragonal zirconia polycrystals (Y-TZP), MgO-partially stabilized zirconias (Mg-PSZ) and zirconia-toughened ceramics (ZTC), has been measured by the single edge notch beam (SENB) technique using machined notches of varying widths from 150 to 1800 μm. It was found that the response of each material was critically dependent on its microstructure. Experimental results show that where ‘dynamic’ toughening mechanisms occur, such as transformation toughening, SENB toughness depends strongly on the notch width. For example, the SENB toughness of Y2O3-stabilized tetragonal zirconia polycrystals and MgO-partially stabilized zirconia ceramics, which are typical of transformation toughened ceramics, increases significantly with increasing notch width when the notch width is less than 1 mm. A further increase in the notch width results in only a slight increase in the SENB toughness. Zirconia-toughened alumina (ZTA) ceramics show similar behaviour, although the increase in SENB toughness is smaller than that for Y-TZP ceramics. The notch width dependence of SENB toughness for the transformation toughened ceramics is related to the transformation zone at the notch tip, induced by diamond blade machining. In contrast, the SENB toughness of alumina ceramics shows very little response to the variation in notch width. A discussion on the relationships between SENB toughness values measured and the operative toughening mechanisms is presented.

Effect of zirconia additions on the reaction sintering of aluminium titanate

Yttria-stabilized zirconia was added to a chemically prepared mixture of alumina and titania. The effect of the zirconia on the microstructure and resultant properties was studied following reaction sintering to form aluminium titanate. An increase in mechanical strength was observed with little effect on the excellent thermal properties of the aluminium titanate. This was attributed to generation of extra microcracks by the transformation of the zirconia phase and the unusual microstructure produced by the presence of zirconia.