Brian Derby

Neutron reflection studies of titanium segregation to metal–ceramic interfaces

Abstract Titanium is an extremely reactive element which is known to promote the wetting of ceramics by liquid metals. The wetting process is believed to be controlled by the segregation of a very thin layer of Ti to the metal/ceramic interface where it undergoes a localised redox reaction with the ceramic. This hypothesis is consistent with cross-sectional microanalysis studies of metal–ceramic interfaces where thin layers of substoichiometric titanium oxides have been identified. The study of titanium segregation is ideally suited to neutron reflection because of the negative scattering length of the dominant isotope. In this study we have characterised the chemical composition of three interfaces between metals and a sapphire (Al2O3) single crystal using specular neutron reflection. The interfaces are: Sn/sapphire, Sn/sapphire containing a thin (≈20xa0nm) Ti interlayer introduced by physical vapour deposition (PVD), and an interface between sapphire and a commercial Ag-Cu-Ti alloy used to braze ceramics. We have evaluated the neutron reflection data collected from the CRISP reflectometer at the ISIS facility using a multilayer model of the interface. The results indicate that the 20xa0nm evaporated layer cannot be distinguished from interface roughness while the interface between sapphire and the commercial braze alloy contains a thin (≈70 nm) titanium suboxide layer.

Acoustic Emissions During Indentation Tests

When a hard tip indents a sample, cracking, delamination and plasticity can all produce acoustic signals in the form of elastic waves. This paper describes how these acoustic emissions (AE) can be monitored using piezoelectric elements and a load-controlled indentation instrument. Signals emitted from thin films and bulk materials are shown and they are correlated with distinct jumps in displacement of the indenter tip. The benefits and difficulties of different methods of monitoring the emissions are also considered.

High temperature neutron reflection spectroscopy of liquid metal/ceramic interfaces

Abstract A high temperature facility to study liquid metal–ceramic interfaces via neutron reflection spectroscopy is described. Preliminary results from a study of sapphire/Sn and sapphire/Sn–3xa0wt%xa0Ti samples up to 1000°C are presented. Results show that the interface generally consisted of a patchy mixed metal/metal oxide surface rather than discreet layers. A study of Ti segregation proved inconclusive although further investigation is required. Overall the technique is suitable for the characterisation of metal–ceramic interfaces at high temperature.

High-temperature neutron reflectometry of liquid metal-ceramic interfaces

Neutron reflectometry has been used to study liquid metal-ceramic interfaces in situ. A high-temperature facility designed and commissioned for use on the SURF reflectometer at the ISIS beamline, Rutherford Appleton Laboratory, is described. Results from a study of two Sn-Ti alloys on sapphire up to 1173 K are presented and the interfacial composition established by interpretation of the recorded reflectivity profiles. Overall the results are in good agreement with previously published work inferred from the analysis of solidified structures and the neutron reflection technique, established as a unique method for the high temperature analysis of buried solid/liquid interfaces.

INTERFACIAL SHEAR STRENGTH TESTING OF CERAMIC MATRIX COMPOSITES

The interfacial frictional shear strengths of five different ceramic matrix composites have been measured using the fibre push down and push through methods and the compression technique. The values determined using the slice compression method were found to be significantly lower than those obtained with fibre pushing methods. A significant fibre poisson ratio effect was observed in three of the five composite systems examined.

CHARACTERIZATION OF ALUMINA/SILICON CARBIDE CERAMIC NANOCOMPOSITES

Ceramics of an alumina matrix containing SiC reinforcements of approximately 100-300 nm diameter have been reported to have considerably higher strengths than monolithic polycrystalline aluminas. Niihara1 has reported strength increases from 320 MPa to 1050 MPa and an increase in fracture toughness from 3.1 MPa√m to 4.7 MPa√m. Although strength and possibly toughness increases over those of similar grain size alumina have been reported by a number of workers2,3,4, maximum strengths of only about 800 MPa have been achieved. In all cases the increase in strength is accompanied by a change in fracture mode from inter granular failure in the monolithic alumina to transgranular failure in the composite.

Microanalysis of Buried Metal/Ceramic Interfaces Using Neutron Reflection

A better understanding of the interfacial phenomena between liquid metals and solid ceramics is very important in developing many technologies of materials processing. The interfacial energy, a key physical factor of the interface, is controlled by local chemistry. The segregation of small quantities of alloying elements to interfaces can have a dramatic influence on the energy as quantified by the Gibbs1 adsorption isotherm. Interfaces between solid ceramics and liquid metals tend to be of high energy and are normally sufficiently high so as to prevent wetting. It is well known that the addition of small quantities of an active element, notably Ti, to many liquid metals substantially reduces the contact angle on ceramic surfaces. This is accompanied by the segregation of Ti to the metal/ceramic interface and the formation of an intermediate phase believed to be TiO.1 The presence of TiO has been confirmed by X-ray diffraction of the exposed interface2 after solidification. It has not been possible to directly characterise the composition of the metal/ceramic interface during wetting experiments because of the presence of the substrate and the wetting droplet which bury the interface. In this article we present results using the technique of specular neutron reflection to characterise such buried interfaces. The results show the capability of neutron reflection for detecting the presence of a thin interfacial layer at a solid ceramic/solid metal interface and the segregation of Ti at a solid ceramic/liquid metal interface.

The Influence of Surface Roughness on Interface Formation in Metal/Ceramic Diffusion Bonds

Many people have identified the problem of joining ceramic parts to ceramic or metal components as one of the steps to be overcome if such materials are to be used in the future. The ceramics of major interest; SiC, Si3N4 and Zr02 appear to be joined best using metal interlayers and the solid phase diffusion bonding technique. Here we consider the formation of the A1/A12O3 interface as a model for other systems.