M. Rühle

The thermodynamics of spinel interphase formation at diffusion-bonded Ni/Al2O3 interfaces

Abstract Interface microstructural development during solid state diffusion bonding of Ni to single-crystal α-Al2O3 has been studied by electron microscopy. Nickel aluminate spinel (NiAl2O4) interphase layers ∼ 1 μm thick formed under high vacuum bonding conditions. Very high vacuum (VHV) annealing caused the spinel to disappear, indicating that its stability depends critically upon the oxygen activity. High vacuum diffusion bonding utilizing initially oxygen-free Ni and oxygen-containing Ni established that spinel formation requires a threshold oxygen activity, and furthermore, that the source of the required oxygen can be oxygen initially dissolved in the Ni. Thermodynamic calculations confirm that the threshold oxygen level necessary to stabilize the spinel increases from 0.006 at.% (60 at.ppm) at 1273 K to 0.025 at.% (250 at. ppm) at 1663 K. Further analysis indicates the spinel exhibits a maximum thickness determined by the difference between the initial and threshold oxygen concentrations and the Ni thickness. Considering the solubility limit of oxygen in solid Ni, the spinel thickness is limited to ∼0.005 times the Ni thickness. The reaction is explored further in the context of diffusion path concepts with a calculated Niue5f8Alue5f8O phase diagram.

Structure and chemistry of metal/ceramic interfaces☆

Abstract The present state of knowledge is reviewed concerning the structure and chemistry of metal/ceramic interfaces. Experimental observations are described for several model systems and open problems concerning different aspects of structure and properties of heterophase boundaries are discussed.

EFFECTS OF PLASTICITY ON THE CRACK PROPAGATION RESISTANCE OF A METAL/CERAMIC INTERFACE

Fracture experiments have been conducted on a gold/sapphire interface. The interface is found to fail by interface separation in a nominally “brittle” manner with a critical strain energy release rate, Gc ≈ 50 Jm−2, substantially larger than the work of adhesion, Wad ≈ 0.5 Jm−2. Evidence of plastic deformation on the gold fracture surface, such as blunting steps and slip steps, suggest that plastic dissipation is the primary contribution to the measured Gc. Calculations suggest that the majority effect occurs in the plastic zone through the crack wake. The interface is also found to be susceptible to slow crack growth.

On the toughening of ceramics by whiskers

The toughening of ceramics by whiskers is now an established phenomenon. However, the prevalent toughening mechanism have not been rigorously identified, and microstructures that afford optimum toughness cannot yet be specified. Various toughening mechanisms have been proposed. These include deflection, microcracking and bridging. The dominant micro-structural characteristics differ substantially for each of these mechanisms. Consequently, it is important to establish the prevalent mechanisms operative in specific material systems. The purpose of the present study is to investigate toughening in a SiC whisker toughened Al/sub 2/O/sub 3/, with the objective of establishing experimental procedures that allow identification of the principal toughening mechanism.

Microcrack and transformation toughening of zirconia-containing alumina

Abstract Experimental studies on the microcrack and transformation toughening of zirconia (ZrO 2 )-containing alumina are presented. The investigated ceramics contain about the same volume fractions of tetragonal and monoclinic ZrO 2 particles respectively. The microcrack parameters (microcrack length, microcrack density and microcrack opening) and the transformation behavior are determined by transmission electron microscopy studies. The microstructural results are used to explain the mechanical properties of the toughened material.

Structures of Nb/Al 2 O 3 Interfaces Produced by Different Experimental Routes

Nb/Al 2 O 3 interfaces were produced by (i) diffusion bonding of single crystalline Nb and Al 2 O 3 at 1973 K, (ii) internal oxidation of a Nb-3at.% Al alloy at 1773 K, and (iii) molecular beam epitaxy (MBE) growth of 500 nm thick Nb overlayers on sapphire substrates at 1123 K. Cross-sectional specimens were prepared and studied by conventional (CTEM) and high resolution transmission electron microscopy (HREM). The orientation relationships between Nb and Al 2 O 3 were identified by diffraction studies. HREM investigations revealed the structures of the different interfaces including the presence of misfit dislocations at or near the interface. The results for the different interfaces are compared.

Analytical transmission electron microscopy with high spatial resolution — possibilities and limitations

SummaryDuring the past three decades, there has been a proliferation of new physical techniques for elemental analysis within a transmission electron microscope (TEM). Emphasis in analytical transmission electron microscopy (AEM) is put on the analysis of extremely small specimen volumes with high sensitivity. These physical techniques involve irradiation of the specimen with the electrons of the electron microscope in order to (i) produce a particular excitation of the elements in the specimen which can be detected by the emission of particles or of electromagnetic radiation or (ii) obtain quantitative information about the specimen by scattering or absorption of the incident electrons. In AEM, usually characteristic X-rays and electron energy losses are analyzed leading to the chemical composition of a small specimen volume. Simultaneously, information on structure and morphology of the specimen can be obtained by conventional TEM.The spatial resolution at which an electron microscope operates ranges from 0.5 nm to 10 nm and this resolution is determined by (i) the actual probe size in the AEM, (ii) the spreading of the beam within a sample, (iii) the size of the interaction region that gives rise to the detected signal and (iv) the signal-to-noise ratio of the signal.The possibilities and limitations of the techniques are elaborated for different examples which concentrate on the evaluation of structure and chemistry of interfaces in metallic and ceramic specimens.

Diffraction effects along the normal to a grain boundary

Abstract 1. 1. The intensity calculated by Vaudin et al. is the excess intensity caused by the distortions in the interface region. This was first stated in VBS. The lack of an explicit identification in the earlier papers by Vaudin et al. was one of the major causes of the ensuing disagreements. 2. 2. The intesities for grain boundaries calculated by Vaudin et al., both with and without segregation, are the same as the excess intensities calculated by Vitek and Ruhle for the same grain boundary models. 3. 3. VBS have shown mathematically that for bicrystals of realistic size, the square of the averaged amplitude is the excess intensity caused by the distortions at the interface. The criticisms by VR of the amplitude-averaging technique should be ignored. 4. 4. The excess intensity is easier to measure experimentally, and to interpret because the shape of the diffracting bicrystal affects both the shape and intensity of the calculated total intensity streak profile. 5. 5. The quantification of the electron diffraction patterns by Vaudin et al. was premature. The problems associated with quantification are discussed at length in VLSS and VR. 6. 6. VLSS suggested that the long streaks observed in diffraction patterns from NiO bicrystals were due to large expansions at the interface. VR pointed out that the total rigid body translations associated with these expansions were unrealistically large. However, the alternate explanation by VR in terms of faceting is not consistent either with the fine details of the diffraction patterns or the diffraction effects that are expected from a faceted grain boundary.

Ferroelastic Domain Switching In Tetragonal Zirconia

Ferroelastic domain switching is one of the possible toughening mechanisms in ceramic materials. Microstructural evidence of domain reorientation (switching) in polydomain tetragonal zirconia single crystals is observed upon the application of a unidirectional compressive stress. Dark field imaging of the three {112} tetragonal twin variants in a {111} zone indicates that two sets of twin variants grow at the expense of the third set upon application of uniaxial compression. The diminishing variant is the one with its c axis parallel to the compression axis. Indentation experiments on uniaxially compressed samples show an anisotropy in crack length. Crack propogates more easily along the loading direction. A construction for the orientation relationship of domains and their twin boundaries is presented.

Microanalytical Tem Studies of Ceramic Materials

Frequently, the processing of dense, strong, and tough ceramics requires the addition of sintering aids. The materials are therefore composed of various components. For the understanding of the properties of the ceramics it is essential that the microstructure and the distribution of the different components within the material are known. The possibilities and limitations of microanalytical TEM studies will be described for different structural ceramics, e.g. ZrO 2 , NiO, SiC, and SiAlON.