Jason R. Heffelfinger

Steps and the structure of the (0001) α-alumina surface

Abstract The initial stages of facet formation on the (0001) α-alumina surface are explored through annealing vicinal single crystals for different lengths of time and characterizing the surface with atomic force microscopy. Faceting of the (0001) surface begins with the formation of 0.2 nm or ∼ c 6 high steps ( c = 1.3 nm). These steps show a preference for existing as pairs; the pairs then bunch together and form facets which are typically multiples of c in height. In this fashion, single facets and faceted domains initially exist together on the same surface. A faceted terrace-and-step morphology develops as the facet domains coalesce with longer annealing times.

Mechanisms of surface faceting and coarsening

Abstract The progression of surface faceting on ceramic substrates is explored using the {1010} and (0001) surfaces of Al2O3 (α-Al2O3, corundum structure) as model systems. Atomic-force microscopy and electron microscopy techniques are used to monitor the progression of faceting as a function of annealing time. Whether the surface facets into a hill-and-valley structure, such as the {1010} surface, or into a terrace-and-step structure, such as the (0001) surface, faceting starts with the nucleation and growth of individual facets. The growth of individual facets is found to promote the nucleation of adjacent facets, and thus leads to the formation of facet domains. As domains that are out of phase with each other coalesce regions with a high density of facet junctions are formed. Facet junctions, points where two facets merge to form one, are found to have an important role in facet coarsening. The mechanism for the coarsening of the completely faceted surface involves the motion and elimination of facet junctions. Both the facet wavelength and density of facet junctions are monitored as a function of annealing time for the {1010} and (0001) Al2O3 surfaces.

On the faceting of ceramic surfaces

Abstract A model is proposed by which ceramic surfaces change from flat vicinal surfaces into those of a faceted hill-and-valley structure. This model is based on scanning electron microscopy, transmission electron microscopy and atomic-force microscopy observations of facet formation on annealed ( 10 1 0 ) α-alumina surfaces. The progression of surface faceting starts with the nucleation and growth of an individual facet. The growth of this facet creates local surface disturbances that promote the nucleation of adjacent facets; thus, domains of faceted surface form on the otherwise smooth surface. Through time, these domains coalesce and the facets coarsen into the final hill-and-valley morphology.

CHARACTERIZATION AND MICROANALYSIS OF INTERFACIAL REACTIONS IN METAL-MATRIX COMPOSITE SYSTEMS

Understanding the solid‐state reactions involved in metal/ceramic systems is important when combining the two types of materials into a composite. In this investigation, the solid‐state reaction between Al2O3 (alumina) and a β‐Ti alloy has been characterized by transmission electron microscopy (TEM), scanning electron microscopy, parallel‐acquisition electron energy‐loss spectroscopy and X‐ray energy‐dispersive spectroscopy. Two different systems were used to investigate this reaction. The first system utilizes a controlled reaction geometry and involved diffusion bonding single‐crystal α‐alumina and a β‐Ti alloy. Here, three interfacial regions were found to form: a region of intermetallics (Ti3Al and TiAl) located near the alumina interface, an α‐Ti region, and a β‐Ti region (rich in Mo, the β‐phase stabilzer). Analysis of cross‐section TEM samples of this reaction revealed the presence of both Ti3Al and TiAl at the alumina interface. Orientation relationships between the intermetallics and the alumina are discussed. In the second, system, interfacial reactions inside metal–matrix composites that contain alumina and a β‐Ti alloy were investigated. Here, different coatings used in the MMCs are investigated for their ability to prevent the reaction between the matrix and fibres. Reaction products inside the MMCs are compared with those found in the model reaction geometry.

The effect of surface structure on the growth of ceramic thin films

Thin-films of yttria-stabilized zirconia (YSZ) and Y O (cubic structure) have 2 3 been grown on annealed substrates of (0001) and {1010} Al O (alpha-Al O, 2 3 2 3 corundum structure) by pulsed-laser deposition. These model systems demonstrate the effect that surface structure, such as surface steps and facets, can have on ceramic thin-film growth. The near-(0001) Al O surface facets into a 2 3 terrace-and-step morphology where the terraces correspond to the (0001) surface. YSZ grew on, or near, surface steps on the nominally (0001) surface such that it was preferentially aligned with the (110) YSZ plane parallel to the (1120) Al O 2 3 plane. On the large (0001) terraces, the grains grew so that the (110) YSZ plane was aligned parallel to the (1010) Al O plane. The {1010} Al O surface facets 2 3 2 3 into a hill-and-valley structure that corresponds to the {1012} and \[1011} planes. Growth of Y O onto this surface again produced two different orientations. For 2 3 growth which occurred on the {1011} facet, ...

Microscopy of interfaces in model oxide composites

Solid‐state reactions are known to occur in composite materials during fabrication, processing or service. In the present study, a model approach for studying such reactions is illustrated. The goal of this approach is to understand the effects of, for example, temperature, time and other driving forces on the earliest stages of such reactions. Three different materials systems were used in order to investigate some of the fundamental processes occurring. Two of the systems involve spinel formation while the other is more complicated, since three different compounds can form between the end‐members. For all of these systems a thin‐film reaction geometry was utilized. High‐quality thin films of the various oxides were deposited on bulk substrates by pulsed‐laser deposition.

Interfacial reactions in metal-matrix composites

The interfacial reaction between Al{sub 2}O{sub 3} (alumina) and a {beta}-Ti alloy has been characterized by transmission electron microscopy, scanning electron microscopy, and X-ray energy-dispersive spectroscopy. Diffusion bonding single-crystal alumina and a {beta}-Ti alloy was found to produce three interfacial regions: a region of intermetallics (Ti{sub 3}Al and TiAl) located near the alumina interface, an {alpha}-Ti region, and a {beta}-Ti region (rich in Mo, the {beta}-phase stabilizer). Of the intermetallics to form, Ti{sub 3}Al was found to form first and have an aligned, planar interface with the alumina. TiAl formed second and was found to separate grains of Ti{sub 3}Al and the alumina. Reaction products observed in the diffusion-bonded alumina/{beta}-Ti couples are compared with those observed in metal-matrix composites (MMCs), where a {beta}-Ti alloy matrix is reinforced with alumina fibers. Different coatings used in MMCs are investigated for their ability to prevent the reaction between the matrix and fibers.