Andreas M. Glaeser

Grain Boundary Migration in Ceramics

During ceramic fabrication, densification processes compete with coarsening processes to determine the path of microstructural evolution. Grain growth is a key coarsening process. This paper examines grain boundary migration in ceramics, and discusses the effects of solutes, pores, and liquid phases on grain boundary migration rates. An effort is made to highlight work in the past decade that has contributed to and advanced our understanding of solute drag effects, pore-boundary interactions, and the role of liquid phases in grain growth and microstructural evolution. Anisotropy of the grain boundary mobility, and its role in the development of anisotropic (anisometric) microstructures is discussed as it is a central issue in recent efforts to produce ceramic materials with new combinations of properties and functionality.

New approaches to joining ceramics for high-temperature applications

Abstract Micro-designed multilayer interlayers have been used to join both oxide and non-oxide ceramics. The approach allows the formation of ceramic-ceramic joints with high melting point metals at temperatures that are typically several hundred degrees lower than those required for more conventional joining methods. The new joining approach employs a thin transient liquid phase (TLP) layer to allow joining by a brazing-like process. Several distinct interlayers have been used to join alumina ceramics successfully; work using Ni-based interlayers has demonstrated the potentially beneficial impact of “reactive” metal additions to the TLP. The method has also been applied to the joining of silicon-based ceramics, and has led to the fabrication of joints with reproducibly high strengths.

Ceramic joining III bonding of alumina via Cu/Nb/Cu interlayers

A method of ceramic-ceramic joining that exploits a multilayer interlayer designed to form a thin, potentially transient layer of liquid phase has been used to join alumina to alumina. Microdesigned multilayer Cu/Nb interlayers were used to achieve bonding at 1150 °C. Flexure strengths of as-bonded samples ranged from 119 to 255 MPa, with an average of ≈ 181 MPa. The ability to form ‘strong’ ceramic/metal interfaces is also indicated by instances of ceramic failure. Microstructural and chemical characteristics of fracture surfaces were evaluated using SEM, EDS and microprobe. The impact of post-bonding anneals of 10 h duration at 1000 °C in gettered argon on room-temperature joint strength was assessed. High strengths (198 to 238 MPa) were obtained. The retention of strength following annealing in low oxygen partial pressure argon differs from the behaviour previously observed in Cu/Pt bonded alumina. Effects of the anneal on interfacial microstructure were determined, and an explanation for this difference in behaviour is proposed.

Joining of alumina via copper/niobium/copper interlayers

Alumina has been joined at 1150 degrees C and 1400 degrees C using multilayer copper/niobium/copper interlayers. Four-point bend strengths are sensitive to processing temperature, bonding pressure, and furnace environment (ambient oxygen partial pressure). Under optimum conditions, joints with reproducibly high room temperature strengths (approximately equal 240 plus/minus 20 MPa) can be produced; most failures occur within the ceramic. Joints made with sapphire show that during bonding an initially continuous copper film undergoes a morphological instability, resulting in the formation of isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. For optimized alumina bonds, bend tests at 800 degrees C-1100 degrees C indicate significant strength is retained; even at the highest test temperature, ceramic failure is observed. Post-bonding anneals at 1000 degrees C in vacuum or in gettered argon were used to assess joint stability and to probe the effect of ambient oxygen partial pressure on joint characteristics. Annealing in vacuum for up to 200 h causes no significant decrease in room temperature bend strength or change in fracture path. With increasing anneal time in a lower oxygen partial pressure environment, the fracture strength decreases only slightly, but the fracture path shifts from the ceramic to the interface.

Ceramic joining II partial transient liquid-phase bonding of alumina via Cu/Ni/Cu multilayer interlayers

Multilayer Cu/Ni/Cu interlayers that form a thin layer of a Cu-rich transient liquid phase have been used to join alumina to alumina at 1150 °C. The method and bonding conditions yield an assembly bonded by a Ni-rich (>94 at% Ni) interlayer at a temperature substantially lower than those normally required for solid-state diffusion bonding with pure Ni interlayers. Flexure strengths of as-bonded beams ranged from 61 to 267 MPa with an average of 160 MPa and a standard deviation of ±63 MPa. The highest flexure strengths were observed in samples where failure occurred in the ceramic. Post-bonding anneals of 10 h duration in air and gettered-argon at 1000 °C decreased the average room temperature strength to 138 and 74 MPa, respectively. In as-processed and annealed samples, varying degrees of interfacial spinel formation are indicated. Spinel formation may contribute to the scatter in as-processed samples, and the decrease in strength values resulting from annealing.

Ceramic joining IV. Effects of processing conditions on the properties of alumina joined via Cu/Nb/Cu interlayers

Multilayer copper/niobium/copper interlayers consisting of 3 μm thick cladding layers of copper on a 125 μm thick niobium core layer were used to join aluminum oxides at 1150°C or 1400°C, or both. Three microstructurally distinct aluminum oxides were joined—a 25 μm grain size 99.5% pure alumina, a submicron grain size 99.9% pure alumina, and single crystal sapphire. Two-phase interlayer microstructures containing both copper-rich and niobium-rich phases developed during bonding. In some cases, the initially continuous copper film evolved via Rayleigh instabilities into an array of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing conditions (temperature and applied load) and the alumina microstructure (grain size) impacted the extent of film breakup, the morphologies of the copper-rich and niobium-rich phases, the interlayer/alumina interfacial microstructure, and thereby the strength characteristics. Joints possessing a large copper/alumina interfacial area fraction were comparatively weak. Increases in bonding pressure and especially bonding temperature yielded interfaces with higher fractional niobium/alumina contact area. For joined polycrystals, such microstructures resulted in higher and more consistent room temperature fracture strengths. Joined 99.9% alumina polycrystals retained strengths >200 MPa to 1200°C. Relationships between processing conditions, interlayer and ceramic microstructure, and joint strength are discussed.

Low temperature routes to joining ceramics for high-temperature applications

The ability or inability to join ceramics will expand or contract the potential use of ceramics in a wide range of applications. There is a need for fundamental research on the properties of joints between ceramics formed by any method, and a parallel need for methods of ceramic joining that are compatible with mass production of joined assemblies and that will yield reliable and strong joints. The need for reliable, mass production oriented methods of joining addressing high-temperature high-stress applications is particularly urgent. The authors efforts have focused on exploring and development new approaches to joining that combine the processing advantages of liquid-state bonding methods (brazing) with the ability to use refractory metal interlayers normally associated with solid-state bonding methods (diffusion bonding). The approaches under investigation also seek to combine a low processing temperature with the potential for subsequent use at temperatures that approach or exceed the joining temperature.

Model studies of Rayleigh instabilities via microdesigned interfaces

The energetic and kinetic properties of surfaces play a critical role in defining the microstructural changes that occur during sintering and high-temperature use of ceramics. Characterization of surface diffusion in ceramics is particularly difficult, and significant variations in reported values of surface diffusivities arise even in well-studied systems. Effects of impurities, surface energy anisotropy, and the onset of surface attachment limited kinetics (SALK) are believed to contribute to this variability. An overview of the use of Rayleigh instabilities as a means of characterizing surface diffusivities is presented. The development of models of morphological evolution that account for effects of surface energy anisotropy is reviewed, and the potential interplay between impurities and surface energy anisotropy is addressed. The status of experimental studies of Rayleigh instabilities in sapphire utilizing lithographically introduced pore channels of controlled geometry and crystallography is summarized. Results of model studies indicate that impurities can significantly influence both the spatial and temporal characteristics of Rayleigh instabilities; this is attributed at least in part to impurity effects on the surface energy anisotropy. Related model experiments indicate that the onset of SALK may also contribute significantly to apparent variations in surface diffusion coefficients.

Surface and interface properties of alumina via model studies of microdesigned interfaces

The ability to produce controlled-geometry, controlled-crystallography internal voids in ceramics has made possible several new model experiments for studying the high-temperature properties of surfaces and interfaces in ceramics. Recent advances have enabled the production of more complex microdesigned internal defect structures, and have exploited new means of examining them, thus, broadening the range of problems that can be addressed. A particular topic of concern is the effect of surface energy anisotropy on both the driving force for and the mechanism of shape changes. This paper reviews and previews recent research focussing on improving our understanding of surface diffusion in ceramics. Rayleigh instabilities provide one means of examining morphological evolution. The modelling of Rayleigh instabilities in materials with surface energy anisotropy is reviewed, and the results of experiments utilizing microdesigned pore arrays in sapphire are summarized. In a material with anisotropic surface energy and a facetted Wulff shape, the driving force for shape changes hinges on both the absolute and relative surface energies. Microdesigned pore structures have been used to determine the stable surfaces in both undoped and doped sapphire and to provide the relative values of the energies of these stable surfaces. Nonequilibrium shape, controlled-crystallography cavities have been introduced into undoped sapphire, and the effect of crystallographic orientation on their morphological evolution has been studied. Comparisons of the results with predictions of models of surface-diffusion-controlled evolution indicate that surface-attachment-limited kinetics (SALK) play an important role.

Morphological evolution of pre-perturbed pore channels in sapphire

Abstract Internal pore channels having sinusoidal perturbations of controlled amplitude α and wavelength λ oriented along the [1 1 00] and [11 2 0] directions were produced in undoped sapphire. The morphological evolution of these features at elevated temperature was monitored. Three regimes of behavior, dependent upon the perturbation wavelength, were identified. An orientation dependent minimum wavelength for perturbation growth was identified. In some instances, the behavior was not one of amplitude decay, but instead stabilization by facetting. For these cases, the minimum wavelength is referred to as λfacet. In other cases, after sufficiently long anneals, perturbation decay did occur, allowing an experimental determination of λmin. At intermediate wavelengths, channels broke up into isolated pores having spacings that were equal to the imposed wavelength, at rates that depended upon wavelength. The wavelength of maximum evolution rate is termed λmax, and is of the order of 2·λ facet or 2 · λmin. At long wavelengths, perturbations of relatively shorter wavelength developed naturally, albeit slowly, and resulted in the ultimate formation of isolated pores with spacings less than those imposed. Results are compared with those obtained previously when pore channels having no large amplitude perturbations were allowed to evolve at elevated temperatures.