Structural ceramic nanocomposites are reviewed with emphasis on the Al 2 O 3 SiC and Si 3 N 4 SiC systems. The incorporation of a nanosized second phase, such as SiC, into a ceramic matrix can lead to an improvement in mechanical properties. It is still unclear, however, whether those improvements can directly be related to an intrinsic ‘nanocompossite effect’ or to other factors. This review is divided into three parts. First, basic processing routes for nanocomposites, namely conventional powder processing, sol-gel processing and polymer pyrolysis are presented in detail. Second, the mechanical properties of different nanocomposites are compared. Finally, models which attempt to explain the improvements in these properties are explored. It will be shown that the strength increase can best be related to a reduction in processing defect size. For applications the most interesting properties of nanocomposites are their wear, creep and high temperature performance.

Structural ceramic nanocomposites

The most commonly used bearing couple in prosthetic hip or knee joint replacements consists of a cobalt–chrome (CoCr) metal alloy articulating against ultrahigh-molecular-weight polyethylene. Ceramics have been used as an alternative to metal-on-polyethylene in joint replacement surgery of arthritic hips and knees since the 1970s. In prosthetic hip and knee bearings, ceramic surfaces offer a major benefit of drastically reduced wear rates and excellent long-term biocompatibility, which can increase the longevity of prosthetic hip and knee joints. This benefit is important clinically because hip and knee replacement has become a very common surgical procedure, particularly in the United States, and because these procedures are being increasingly performed in younger patients who place greater demands on the prosthetic bearings. However, ceramics are brittle and the risk of catastrophic bearing failure in vivo, while rare, is a major concern. Improvements in material quality, manufacturing methods, and implant design have resulted in a drastic reduction of the incidence of such failures, so that modern ceramic bearings are safe and reliable if used with components of proven design and durability. Future material improvements are actively being investigated to reduce the risk of ceramic-bearing failures even further. The purpose of this article is to review the structure, properties, applications, and limitations of the ceramics that have been used in orthopedic bearings, and to describe the new ceramic composite materials and surface treatments that will be available for joint replacement surgery in the near future.

Ceramics for Prosthetic Hip and Knee Joint Replacement

The recognition that bulk nanoceramic materials, having grain sizes typically 100 nm or finer, possess appealing mechanical, physical and tribological properties has generated considerable recent research activity. A major challenge in the research on bulk nanoceramics and nanoceramic composites is concerned with the aspect of processing, in particular restriction of grain growth during sintering. In this regard novel processing techniques have been developed with the aim of fabricating bulk ceramic nanomaterials. The superior properties exhibited by these bulk nanoceramics and ceramic nanocomposites, compared with conventional ceramic materials, have opened up prospects for their use in applications demanding better mechanical properties. The present review highlights outstanding issues related with the processing of nanoceramics and ceramic nanocomposites, discusses the novel processing techniques currently used to develop these materials, critically analyses the property modifications resulting...

Consolidation–microstructure–property relationships in bulk nanoceramics and ceramic nanocomposites: a review

The paper reviews the wear behaviour of oxide ceramics. Wear maps are considered and consequently mild and severe wear are defined. Since the use of ceramics in engineering applications require operation in the mild wear regime, the paper concentrates on mild wear mechanisms, but also considers factors which control the transition to severe wear. Within the mild wear regime, the formation of tribofilms are discussed and the manner in which dislocation activity leads to the wear transition is considered. The wear of so-called ceramic nanocomposites, for which no time dependent wear transition has yet been observed, is considered and the reasons for enhanced performance discussed. The role of transformation toughening in zirconia ceramics is considered in detail, and reasons for the generally poor wear response of these materials defined.

The wear behaviour of oxide ceramics-A Review

A set of dense magnesia-magnesium aluminate spinel composites has been prepared by hot-pressing magnesia powder using 0 to 30 wt.% of spinel powder of mean particle size 3, 11 and 22 μm. Bend stength, modulus, and fracture toughness, have been measured. Strength and modulus decrease with increasing spinel content, and for a given loading, spinel particle size, as a result of microcracking caused by thermal expansion mismatch between the magnesia matrix grains and the spinel particles, the effects of which may be intensified by recrystallization of spinel. Fracture is predominantly transgranular for the pure magnesia, and intergranular for the composite materials. Possible reasons for the differences in behaviour between this system and the SiC–Al 2 O 3 system with a similar thermal expansion mismatch are examined.

Mechanical properties of magnesia-spinel composites

Wet erosive wear rates for alumina-silicon carbide nanocomposites have been measured and compared with those for pure polycrystalline aluminas of similar grain size. In pure materials for mean grain sizes >2 μm the dominant wear mechanism appears to be grain-boundary microfracture, leading to grain pullout; for finer grain sizes the worn surfaces are smooth. Within both grain size ranges significant reductions in wear rate are found for the nanocomposite materials, thus extending the span of established benefits obtained through the incorporation of silicon carbide nanoparticles into ceramic materials.

Effects of silicon carbide nano-phase on the wet erosive wear of polycrystalline alumina

Mechanical Properties of Model Magnesia-Spinel Composite Materials

The wet erosive wear rates of a set of hot-pressed, dense, fine-grained zircon materials of grain size in the range 2 to 6 μm have been measured. There is no correlation between wear rate and hardness or fracture toughness: wear rate instead appears to be strongly linked to grain size and pore fraction. The wear rate-grain size relationship is explicable in terms of a model based on grain-boundary microfracture and crack linking, which allow grain detachment to occur. Wear rates for this set of zircon materials are slightly lower than those measured for a set of pure polycrystalline aluminas with a similar grain size range.

Wet erosive wear behaviour of fine-grain zircon ceramic

Wear resistant ceramics form approximately 25 % of the advanced technical ceramics market, and the most widely used materials on a tonnage basis are the sintered aluminas. An important application is wear resistant tiles for lining plant handling slurries of abrasive grits and powders. The identification of factors leading to the minimisation of erosive wear rate is of significance for the optimisation of materials selection, and in improving plant efficiency by reducing the loss of production time required for component replacement. This contribution describes results obtained with two sets of alumina materials of a range of tailored composition and grain size, which were subjected to a simple wet erosion test using a slurry of alumina grit in water. Wear rates were measured on the basis of weight loss under standardized conditions, and analyzed with respect to materials microstructures and mechanical properties. The best correlations between wear rate and materials' related parameters were obtained with grain size and porosity, and on this basis it is expected that it will be possible to design materials of improved resistance to wear.

R.W. Davidge

Papers

Effects of silicon carbide nano-phase on the wet erosive wear of polycrystalline alumina

Mechanical Properties of Model Magnesia-Spinel Composite Materials

Wet erosive wear behaviour of fine-grain zircon ceramic

The reduction of erosive wear rates of advanced technical ceramics

The Wet Erosive Wear of Polycrystalline Alumina with Silicon Carbide Particulate Inclusions