R.H.J. Hannink

The development of microstructure in silicon nitride-bonded silicon carbide

Abstract The microstructure of a commercial silicon nitride-bonded silicon carbide ceramic composite, formed via the nitridation of silicon powder-SiC preforms, has been characterised using analytical transmission electron microscopy. It comprised coarse particles of SiC dispersed in a matrix which was a mixture of crystalline phases based on Si 2 N 2 O and β-Si 3 N 4 , and an amorphous phase. Qualitative microanalysis of amorphous matrix regions revealed the presence of significant concentrations of aluminium, silicon, calcium and oxygen, while aluminium and oxygen were also readily detectable in both of the surrounding crystalline phases. It is thus suggested that the crystalline phases in the matrix are in fact O- and β-sialons respectively. A mechanism, combining reaction bonding and solution-precipitation in a (Ca,K)-aluminosilicate liquid phase at the reaction temperature, is proposed to describe the development of the matrix microstructure.

RBAO composites containing TiN and TiNTiC

Abstract TiC and TiN were tested as candidate materials for particle reinforcement of reaction bonded aluminium oxide (RBAO). As part of the reaction bonding process Al-containing green bodies have to be heated in air in order to oxidize the Al; therefore the oxidation behaviour of any added particles is of critical importance. Previous studies have shown that neither TiC nor TiN is sufficiently oxidation-resistant to withstand the heating cycle necessary to completely oxidize the Al in the process. Therefore, two alternative approaches were investigated. First, TiC and TiN were retained in incompletely reacted RBAO, resulting in an Al 2 O 3 - TiC TiN composite with alloy and possibly intermetallic phases. These metal phases were obtained from Al and Ti Zr which formed when Al reduced TiO 2 and, to a lesser extent, ZrO 2 . Second, the reaction of AlN and TiO 2 to form Al 2 O 3 and TiN at temperatures above 1350 °C was investigated. The resulting material contained Al 2 O 3 , ZrO 2 , TiN and a nitrogen-containing aluminium titanate.

Induced Plastic Deformation of Zirconia

Indentation techniques have been used to study the low temperature deformation characteristics of zirconia based ceramics. Analysis of Knoop hardness anisotropy showed the low temperature slip systems were {100} , the same as the high temperature slip system. Hardness as a function of temperature, up to 600°C revealed the possibility of a different deformation process being active above 400°C. Deformation about large load indents in Mg-PSZ were studied to examine the extent of cooperative transformation phenomena.

Toughened PSZ Ceramics-Their Role as Advanced Engine Components

Toughened partially stabilized zirconia (PSZ) ceramics possess a number of advantageous properties for advanced engine components, in particular for adiabatic engine systems. Magnesia partially stabilized zirconia (Mg-PSZ), is one of the toughest ceramics yet developed also possessing excellent insulating, high strength, good thermal shock and wear resistance properties. In addition PSZ has a thermal expansion coefficient and elastic modulus similar to iron and steel. Over the last three years, it has been shown as a result of the combination of these thermal and mechanical properties, that Mg-PSZ is suitable for applications as varied as cylinder liners, valve guides, valve seats, piston caps, hot plates and tappet (cam follower) inserts. In non-automotive applications this material can be used in extrusion dies, plastic moulding dies, powder metallurgical dies, dry conveyor bearings and prothesis implants.

FACTORS CONTRIBUTING TO R-CURVE BEHAVIOUR IN ZIRCONIA CERAMICS

Three distinct types of microstructures have been found to develop R-curve behaviour in zirconia ceramics. These include materials where: a transformation process zone forms about the crack tip in partially stabilized zirconia (PSZ) containing metastable tetragonal precipitates, microcracking/crack branching occurs in decomposed monoclinic zirconia, and crack deflection by coarse monoclinic precipitates in overaged PSZ materials. Of these the greatest toughening increment occurs where a transformation process zone forms about the crack tip. This R-curve behaviour is accompanied by “plastic” like behaviour during the initial crack loading of a brittle ceramic. Examples of the R-curves for PSZ materials containing metastable tetragonal precipitates and microcracking in decomposed monoclinic zirconia are presented. Also substantial differences are found between K and J approaches for determining the R-curve.

Some Interfacial Related Properties of Transformation Toughened Ceramics

The role of interfaces in the sintering, heat treatment and resulting mechanical properties of PSZ and some TZP materials are considered. This is first illustrated by the addition of SrO to Mg-PSZ to modify the grain boundary chemistry and resultant behaviour on heat treatment. The growth of t-ZrO2 precipitates in Ca-PSZ is controlled by the Wagner interfacial relationship. Finally, the role of interfaces, particularly grain boundaries on the strength and toughness of PSZ and TZP materials is discussed.

Particle Toughening in Partially Stabilized Zirconia Influence of Thermal History

The thermomechanical properties of partially stabilized zirconia ceramics may be dramatically influenced by thermal treatments. This review describes how a variety of cooling processes and isothermal ageing treatments may be used to control the microstructure and hence make magnesia-partially stabilized zirconia (Mg-PSZ) one of the strongest and toughest sintered ceramics known. By careful selection of thermal treatment cycles the strength and fracture properties of Mg-PSZ may be tailored so as to make the material suitable for a variety of industrial and engineering applications.