Key Engineering Materials Vol. 333

Abstract: It has been demonstrated, through theory and experiments, that compressive layers arrest large surface and internal cracks to produce a stress below which the material will not fail. This enables the materials to have a Threshold Strength. The stress intensity function, K, was derived for a crack sandwiched between two compressive layers. This function suggests that the threshold strength is proportional to the magnitude of the residual, compressive stress, the thickness of the compressive region, and inversely proportional to the distance between the compressive regions. All of these factors have been experimentally examined for laminar composites containing thin, compressive layers. Cracks that propagate straight though the layer obey the K function used to model this behavior. Crack bifurcation, which occurs at high compressive stresses, produces a larger threshold strength than predicted. Crack bifurcation is not fully understood. During the initial studies, differential thermal contraction during cooling from the densification temperature was used to develop the compressive stresses. A molar volume change to induce the compressive stress was also used to develop the compressive stresses. In one case, it was shown that the compressive stresses could arise when the compressive layer contained a material that underwent a structural phase transformation during cooling. In another, ion exchanged glass plates that are subsequently bonded together also produce a threshold strength. Factors that affect the threshold strength are reviewed.

Abstract: Ceramic laminates can be designed to combine high strength with flaw tolerance. In this paper, the designing approach based on the mechanical response of residual stresses free biological layered structures is revised. The main design tools are analysed and different ceramic-ceramic systems combining stiffness, high strength and flaw tolerance with thermo-mechanical stability are described. Two main approaches have been used depending on the relative toughness of the layers and the interfaces between them. Laminates constituted by layers separated by weak interfaces, to originate crack deflection and delamination along the interface, show high thermal shock resistance but limited resistance to shear stresses and, thus, to wear. Laminates with strong interfaces that combine stiff and high strength external layers with flaw tolerant internal ones are appropriate for wear applications. In this group of materials, the combination of layers with the same phase composition and different microstructures avoids residual stresses due to thermal expansion mismatch, but the attainment of such microstructural differences implies the co-sintering of layers with large differences in the green state. The generation “in situ” during sintering of the desired microstructural differences represents an interesting alternative in terms of processing for this group of materials.

Abstract: In this paper a method to produce laminated ceramic composites containing residual stresses is described. The method consist in superimposing thin layers obtained by tape casting, their worm-pressing and sintering. Detailed information on the process and on the slurry compositions are reported. The reasons why laminated structure can exhibit improved performances are also illustrated. The model on which a multilayer composite, containing residual stresses, can be designed is briefly illustrated. The relationship among the physical, chemical and microstructural properties of the different layers, necessary to stimulate the residual stresses outlined.

Abstract: Multilayered materials and coating are complex structures proposed among others to face the structural requirements of ceramics. The development of reinforcement mechanism by laminated structures can be due to deflection criteria or to the presence of residual stresses and requires of tailored laminates. These designs are characterized by the phases, thickness and distribution of the layers as well as the joining strength between them. In this sense water based colloidal processing techniques are used to fabricate layered structures by consolidating the layers from fluid dispersions of the powders in water. In these processing methods the phases presented in the final laminate are mainly given by the composition of the starting slurries while the changes in thickness and sharpness of the layers are controlled by acting on the processing parameters. The achievement of stable slurries is a shared step for all the colloidal processing techniques. In the water based slurries the stability will be dominated by the polar media, the surface behavior of the particles and the presence of dispersant additives to increase the repulsion between particles. The stable slurry ensures an effective milling and dispersion of the phases as well as high solid loadings, if required. Further processes associated to shaping and consolidation of the layers requires the incorporation of additives and-or water removal. The shaping methods based on aqueous slurries can be classified taking into account the process of solid-water separation. For each of those shaping methods, the nature and amount of the additives is different in order to get the optimum rheological behavior and green strength after drying. Depending on the thickness of layers and coatings as well as the shape and dimensions of the samples, the shaping method can be selected alone and combined with others.

Abstract: Electrophoresis is the effect that when an electric field is applied to a suspension of a powder in a liquid, the powder particles move under influence of this field. Frequently the powder particles also deposit at one of the electrodes. The form of the electrode determines the form of the deposit, hence shaping is possible. The current insights into the science and technology of electrophoretic deposition (EPD) will be summarized. EPD is well suited for shaping layered microstructures (laminates), by simply changing repeatedly between two or more suspensions during deposition. Tubular laminates consisting of silicon carbide layers and crack deflecting graphite interlayers have been produced. These tubes demonstrate an enhanced fracture energy and a gradual mode of failure. Another area of advanced ceramics where the use of EPD makes sense are functionally graded materials (FGM) in which one tries to combine in one component high hardness and high toughness. EPD allows the formation of FGM by depositing from a powder suspension to which a second suspension is continuously added during the process. An example will be shown of a graded WC-Co hardmetal.

Abstract: Dense, uniform and crack-free mullite (3Al2O3·2SiO2) coatings were deposited on Sibased substrates by chemical vapor deposition using the AlCl3–SiCl4–H2–CO2 system. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The growth rate and crystal structure of mullite were dependent upon temperature, pressure, reactant concentration, and reactant ratios. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied.

Abstract: A review is given of "misted" CSD deposition. This technique uses stoichiometrically correct sol-gel solutions but is not a spin-on process. Instead a monodisperse mist of droplets as large as 3 microns in diameter or as small as 0.3 microns is deposited on a substrate. This technique has the great advantage over sol-gel spin-on processing in that it is suitable for nonplanar structures, including nanotubes and nano-wires. One could coat a variety of objects with this technique, including anything from non-planar flash-goggles to a parabolic mirror or focal-plane array of pyroelectric detectors. Yet it is much simpler and less expensive than conventional chemical vapour deposition (CVD). We illustrate its use with functionally graded layers on platinised silicon wafers, on nanotubes of piezoelectrics, and most recently [Pollard, Gregg, et al.] on 100 Gbit/cm2 arrays of Pt nanowires on Si substrates (the latter are 30-nm diameter, spaced 50 nm apart, embedded in porous alumina and capped with lead zirconate titanate capacitors).

Abstract: The progress of research efforts on the mechanical properties of ceramic laminates is reviewed. Laminates with weak interface are described with respect to their failure mode and the criterions to achieve graceful failure. For laminates with strong interfaces basic principles concerning the residual stresses and their influence on crack propagation are introduced. The implications for strength, indentation strength, strength distributions, macroscopic R-curves and threshold strength are discussed.