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.

The mechanical properties and microstructural evolution of metal-ceramic bonds produced using a transient liquid phase (TLP) are described. Alumina (Al2O3) was joined at 500 degrees C, 600 degrees C, and 700 degrees C using a multilayer In/Cusil-ABA (R) (commercial copper-silver eutectic brazing alloy)/In interlayer. The introduction of thin In cladding layers allows the system to bond at much lower temperatures than those typically used for brazing with Cusil-ABA (R), thereby protecting temperature-sensitive components. After chemical homogenization, the interlayers retain an operating temperature range similar to that of the brazed joints. TLP bonds made at 500 degrees C, 600 degrees C, and 700 degrees C with holding times ranging from as low as 1.5 h to 24 h had average fracture strengths above 220 MPa. The effects of bonding temperature and time on fracture strength are described. Preliminary analysis of the interlayers shows that the Ag-In or Cu-In intermetallic phases do not form. Considerations unique to systems with two-phase core layers are discussed. Experiments using single-crystal sapphire indicate rapid formation of a reaction layer at 700 degrees C, suggesting the possibility of making strong bonds using lower temperatures and/or shorter processing times.

Partial-transient-liquid-phase (PTLP) bonding of advanced ceramics employs an A/B/A sandwich-style interlayer that is designed such that the outer cladding, A, forms a transient-liquid phase that disappears at the bonding temperature due to diffusion of A into the core layer, B. The resultant bonds can have re-melt temperatures that are significantly higher than the bonding temperature. The success of PTLP bonding relies on the proper selection of the interlayer components: the transient liquid must be able to flow into and fill strength-limiting interfacial flaws, the adhesion between the interlayer and the bulk ceramic must be sufficiently high to prevent interfacial failure, the formation of strength-reducing brittle phases at the interface should be minimized, and the residual stresses due to coefficient of thermal expansion (CTE) mismatch should be minimized. The composition of the transient liquid predominately determines the interfacial characteristics of the bond, while the core composition determines the residual stresses in the assembly. In recent work, Al2O3 bonded using Ni/Nb/Ni interlayers produced joints that were of such high strength that all bonded samples failed exclusively in the ceramic and not at the joint during 4-point bend testing.

The wetting characteristics of the Ni-Nb transient-liquid and the CTE of Nb are favorable for the fabrication of strong PTLP-bonded Al2O3. However, for other ceramic systems, using a binary interlayer system such as Ni-Nb may not be desirable. When using binary interlayers it is not possible to control the composition of the transient liquid and the core independently. In order to expand PTLP bonding to other advanced ceramics, this study examined a new interlayer design that employs a surface-modified core, such as Mo-surface-modified Nb, instead of a homogeneous core, such as pure Nb. A surface-modified core is a core layer with an intentionally inhomogeneous composition in order to better control the composition of the transient-liquid and the core layer independently. It was found that Al2O3 PTLP bonded using a Mo-surface-modified V core and a Ni cladding had fracture strengths of 302±29 MPa. This is comparable to those using Ni/Nb/Ni interlayers, 341±28 MPa. In both assemblies, all of the samples failed in the ceramic bulk. The insights gained from these experiments were used to develop interlayer design guidelines for the PTLP bonding of other advanced ceramics.

A numerical model of the interaction between liquids and rough solids is presented in this work. Partial-Transient-Liquid Phase (PTLP) bonding has proven an effective method of joining ceramic materials. The joining process is facilitated through the development of a thin-liquid layer between ceramic and metallic solids. Successful joining requires a liquid that will spread to fill interfacial voids, which can act as critical flaws that decrease the strength of the joined assembly. A full understanding of this method requires a model of the liquid behavior between dissimilar, rough solids. This work discusses the effect of wetting angle and surface roughness on the behavior of the liquid layer.

Numerical simulations are presented that test possible liquid configurations on rough surfaces, determining the preferred geometry based upon the total interfacial energy. Use of computational methods allows independent control of surface roughness, liquid volume, and interfacial energies. In this way, the effect of the liquid contact angle and the amplitude and wavelength of surface-roughness features on the liquid behavior is examined. Emphasis is placed upon surface-roughness parameters and the correlation with preferred liquid configurations. Models are presented and discussed for liquids in contact with one rough surface and for a liquid entrapped between two dissimilar surfaces. Comparison of the one-surface simulation to previous studies of liquid/roughness behavior is provided.

In liquid-based joining methods, the ability of the liquid layer to fill interfacial voids is strongly affected by surface roughness. Numerical simulations of a liquid flowing to fill an inter-solid gap are presented. The importance of dissimilar surface roughness, dissimilar contact angles, and inter-solid distance is discussed. It is found that increasing surface roughness can act to aid liquid-based joining in certain systems.

Transient-liquid-phase (TLP) bonding was used to join high-strength, high-purity Al2O3 ceramic. This method uses a multilayer interlayer (B/A/B sandwich structure), which forms thin transient-liquid layers between the Al2O3 and the refractory core layer (A), then isothermally solidifies through a diffusive mechanism. The presence of thin liquid layers allow interfacial gaps and voids to be filled, while allowing bonding times comparable to those used for conventional brazing. It was shown that TLP bonding produces high-strength joints with re-melt temperatures that are significantly higher than the bonding temperatures used. This study explores the interrelationships between the processing conditions, fracture strengths of the joints, wetting behavior of the TLP, and the diffusion/isothermal solidification kinetics. In particular, when Ni/Nb/Ni interlayers were utilized, four-point bend tests revealed that the fracture strengths of the joints matched those of the monolithic Al2O3, even after a 5-min holding time at 1400°C, the bonding temperature. The resulting interlayer consists of >99% Nb, which has a melting point >2000°C. Sessile-drop wetting experiments revealed that the Ni-Nb liquid alloy formed during bonding exhibits relatively low contact angles (≈90°) on Al2O3, which enables the formation of nearly flaw-free interfaces. An analytical diffusion calculation model was also used to describe the isothermal-solidification and homogenization kinetics, and will be used in future studies to aid new interlayer designs.

Heterogeneous materials may become structurally unstable under an applied stress. In this thesis, the effects of two dissimilar stresses are studied. First, the methods of statistical mechanics are used to analyze the effects of mechanical stress on the strength of heterogeneous materials. A phenomenological multi-scale model is presented that analyzes inelastic deformation in a model natural composite, nacre. A kinetic Monte Carlo technique is developed to study the mechanical response of the biopolymer. The results of this model are used to generate a cellular automata model of the composite material. Under certain conditions, the sizes of plastic events in this model follow an apparent power-law distribution. The dynamics are found to be similar to earthquakes, where the slip sizes exhibit a scale-free distribution.

Second, a continuum theory is generated to understand how a model microstructure responds to thermal stresses. At elevated temperatures, a structure may spontaneously change shape in order to minimize its overall surface energy. To this end, the stability of a hollow-core dislocation to pearling and coarsening is considered using a linear stability analysis. There is a competition between elastic energy and the anisotropic surface energy. It is shown that sufficiently small hollow-core dislocations are stable with respect to both forms of structural instability, suggesting a route to stabilize nanometer-scale wires.