Wolfgang Braue

Transmission electron microscopy of microstructures in ceramic materials

Abstract Based on selected examples from the area of Si3N4 ceramics, the value of utilizing transmission electron microscopy (TEM) as a technique to study ceramic microstructures as well as a characterization tool for the development of new materials is demonstrated. In the field of ‘new ceramics’, one Si3N4-based composite is discussed, which was processed via pyrolysis of liquid precursors (polysilazanes). Moreover, it is shown that TEM in general can helpfully accompany ceramic processing techniques. This applies to the characterization of ceramic starting powders as well as to the study of densified materials. The investigation of Si3N4 powders, in particular the influence of the addition of sintering aids via organometallic precursors, which leads to a homogeneous distribution of sintering additives in the powder compact, in contrast to the use of metal oxide powders, is shown. The variation of micro-structures during the densification process of liquid assisted sintering is also demonstrated. The most common application of the TEM technique is to characterize dense ceramic components in the as-processed (as-sintered) state. Post-sintering heat treatment can initiate secondary phase crystallization. However, the very important aspect of microstructure integrity at elevated temperatures, e.g. the stability of microstructures under severe service conditions, is also addressed. Emphasis is placed on the fact that ceramic microstructures, which are typically thought to be rather stable, can undergo serious microstructural changes when temperature and stress is applied simultaneously, which strongly limits potential applications of these materials.

A New Concept for Thermal Protection of All-Mullite Composites in Combustion Chambers

Abstract A new thermal protection concept for all-mullite composite shingles based on a thermally-sprayed mullite layer is described. Because of the insufficient thermal long-term stability of the Nextel™ 720 fibers in the 1273 plus regime, Nextel™ 720 fiber-based ceramic composites are protected by a flame-sprayed mullite coating in order to prevent the composite from thermal degradation in service. The protection layer is deposited on the front side of the ceramic shingle facing the hot gas stream. Front and back sides of the shingles are cooled through film and convection cooling, respectively. Reducing both the composite material and the protection layer to a single phase (mullite) system is a simple, but highly efficient approach to keep thermal and elastic misfit strains at the interface at reasonably low levels. Due to the porous grain texture intrinsic to thermally-sprayed materials, thermal conductivity of the protection layer is low, yielding a considerable thermal insulation effect depending on the layer thickness and the particular heat flow scenario of the combustion chamber. The microstructure/property relationship of the thermal protection layer and its interaction with the underlying composite are discussed focussing on the constraints of real combustion chamber operation conditions.

Effective properties of composites utilising fibres with a piezoelectric coating

Composites with piezoelectric fibres are promising new materials, combining the beneficial properties of very different constituents. Recently hybrid fibres with an inactive core and a piezoelectric coating have been developed. For conventional two-phase systems the correlation between component properties and effective composite behaviour is well approximated using effective field or self-consistent models. Since the latter approaches are commonly based on solutions for homogeneous inclusions, they cannot be directly employed for heterogeneous particles as coated fibres. Different methods are employed to estimate the relevant effective electromechanical parameters of composites continuously reinforced with coated piezoelectric fibres: (a) a unit cell finite element model, (b) an effective field approach built on a rigorous solution for a coated fibre in an infinite matrix and (c) a simple homogeneous field approximation. The results of the approaches are compared and discussed.

Nucleation and Growth of Oxide Constituents on NiCoCrAlY Bond Coats During the Different Stages of EB-PVD TBC Deposition and Upon Thermal Loading

Progression of TGO scale in a NiCoCrAlY/EB-PVD TBC system is monitored via analytical SEM and TEM from the first vacuum anneal of the NiCoCrAlY bond coat through the different processing stages of the EB-PVD Y-PSZ coating (preheating; initial and advanced stages of TBC deposition) and upon cyclic fatigue at T >1100°C. During the early stages segregation of oxide constituents to the TGO scale is dominated by yttria giving rise to a transient “off-plane” mixed zone (Al2O3, ZrO2) microstructure textured perpendicular to the TGO/TBC interface. Subsequently Y-aluminates and spinel are formed. Local phase incompatibility of coexisting oxides is considered critical for the formation of failure-relevant clustered pores, as verified from the bond coat/TGO interface and the TGO mixed zone adjacent to the TBC top layer.

In-plane microstructure of plasma-sprayed MgAl spinel and 21-mullite based protective coatings: An electron microscopy study

Abstract Planar specimens prepared from thin plasmasprayed Mg-Al spinel and 2 1 -mullite layers are investigated via conventional and analytical transmission electron microscopy in order to evaluate the constraints of rapid solidification on the in-plane microstructure at different scales of resolution. Despite intrinsic differences in the nature and density of structural defects between the two materials studied, their gross-scale microstructures share a characteristic sequence starting from (i) large, spherical core grains, followed by (ii) a radial chillzone exhibiting columnar to dendritic grain morphologies, which eventually leads to (iii) an impingement zone formed by the intersection of different chillzones. Relief of thermomechanical strain from the core grains and chill zones gives rise to an equiaxed subgrain architecture and intensive microcracking in the impingement zone. Moreover, the impingement zone represents the region of highest and most varied lattice defect density.

A comparative high-resolution study of interface chemistry in silicon-nitride-based ceramic matrix composites reinforced with silicon carbide whiskers

Abstract Silicon nitride/silicon carbide(w) ceramic matrix composites synthesized from common starting materials, except for the whiskers themselves, under the same processing conditions have been investigated. High-resolution electron microscopy and high-spatial-resolution electron energy loss nanospectroscopy were used to characterize the whisker/matrix interfaces in these composites both structurally and chemically. The presence of a discontinuous oxygen-rich amorphous layer at the two kinds of whisker/matrix interfaces examined in this paper appeared to be a general phenomenon. The wide difference (a factor of 10) between the structural image and chemical widths (oxygen distributions) of the whisker/matrix interfaces was attributed to oxygen solution into the silicon nitride matrix. The oxygen sources were the sintering aids and surface impurities.

Microstructure and Densification of Sintered (B+C)-Doped β-Silicon Carbide

The role of boron and carbon during densification of sintered β-SiC was investigated through the combined approach of in-situ dilatometry and CTEM/AEM inspection of TEM-foils referring to well-defined densification events. Preliminary data obtained indicate that in the early stages of densification, boron is not enriched in the continuous carbon-rich surface layer covering the β-SiC powder particles nor does it segregate to internal interfaces in high quantities. Small boron quantities are dissolved in the SiC grains- Simultaneously with the β- to α- phase transformation, decomposition of foam-like B 4 C aggregates releases small B 4 C particles, which are bound intragranularly to α-SiC.

Alpha-Si3N4 Precipitation in C/C-SiC Composites From Inherent Fiber Impurity Sources.

The formation of crystalline {alpha}-Si{sub 3}N{sub 4} filaments grown inside the pore channels of HT-carbon fibers from a 2D C/C-SiC composite is investigated by transmission electron microscopy. Precipitation of {alpha}-Si{sub 3}N{sub 4} is promoted by a low carbonization heat treatment of the C/C material prior to liquid silicon infiltration and results from the interaction of a highly reactive nitrogen-rich vapor phase released from the fiber and silicon vapor diffusing ahead of the SiC reaction front into the porous microtexture of the fiber.

A Microstructural Study of Reaction-Bonded Silicon Carbide

Interfaces in Reaction Bonded Silicon Carbide (RBSC) have been characterized by Analytical and High Resolution Electron Microscopy. Both Si/SiC and SiC/SiC interfaces were free of any oxygen impurity segregation, but contained metallic impurity precipitates. Oxygen was detected in the second phase particles in the SiC grains. A model is presented to explain the evolution of these second phase particles in the SiC grains.