Ceramic materials are of technical and commercial interest due to their chemical, mechanical and thermal performance, leading ceramics to meet many engineering requirements. Alumina (aluminum oxide) is one of the primary representatives of this class of materials because of its high fracture toughness, hardness and density, which enable its use in the production of highly critical parts. One such application involves protection against abrasion and erosion wear. The wear properties of a ceramic can be improved not only by controlling its material characteristics but also by controlling the fabrication process, which defines the material's microstructure. Many studies of the effects of the microstructure on these properties have been published. The objective of this study was to review the effects of the microstructure on the erosive wear resistance of alumina-based ceramics. Four factors that control the erosive wear of alumina were identified: (i) the effects of dopants on the diffusivity of the grain boundaries, (ii) the fabrication route, (iii) the sintering mechanisms and (iv) the alumina grain size. The published experimental results related to these topics are described and provide a clear understanding of the erosive wear of alumina.

With the aim of improving the toughness of ceramic materials, laminated composites have been successfully developed since Clegg et al. (1990) inserted weak interfaces using very thin graphite layers between silicon carbide sheets and obtained a composite that exhibited non-catastrophic fracture characteristics. The weak interface must allow the crack to deviate either by deflection or delamination; in other words, the interface must exhibit a fracture resistance that is lower than that of the matrix layer. In parallel, ceramic laminated composites with strong interfaces were developed in which the residual tensile and compressive stresses appeared in alternate layers during cooling after sintering. These composites are prepared by stacking ceramic sheets produced by lamination or tape casting or by the sequential formation of layers by slip casting, centrifugation or electrophoretic deposition. The techniques may be combined to obtain a composite with the most adequate configuration. This work presents a review about the obtainment of multilayered ceramic composites as a toughening mechanism of ceramic plates.

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Fracture toughness enhancement of ceramic materials through multilayered ceramic composites has been developed since 1990. Toughening mechanisms are based mainly on delamination, deflection, bifurcation or crack arrest effect. Delamination and crack deflection occur by means of weak interfaces. Bifurcation (and deflection as well) and crack arrest effects are result of residual stresses arising from the thermal expansion coefficient mismatch or phase transformation on alternating layers. The main manufacturing methods of these composites are slip casting of two ceramic materials, and stacking and pressing of ceramic tapes obtained by tape casting or rolling technics, followed by suitable sintering process. This review aims to present general aspects of research performed around the theme so far. It is verified that occurs the enhancement of ceramic toughness and reliability with this technic, so it is possible to enlarge its range of application in engineering.

Ceramic engobe is an intermediate layer between the substrate and the glaze of a ceramic tile. It is basically composed by plastic material, clays, and non-plastic material, feldspar and frit. Light-colored clays with good plasticity and low-fire temperature are used in ceramic engobe formulations, typically ball clays. However, these clays contain different accessory minerals, which can adversely affect the opacity and the whiteness. The use of washed kaolin, with a lower content of accessory minerals, tends to lead to higher opacity and whiteness. In addition, its mechanical activation can increase the plasticity, allowing its use as a replacement for ball clay. The aim of this study was to investigate the effect of the use of mechanically-activated kaolin to replace ball clay in an engobe employed in the production of ceramic tiles. Samples of kaolin were activated by high energy mill (1 and 4 h, at 500 rpm) and characterized by surface area and particle size measurements, crystallography, infrared spectroscopy and differential scanning calorimetry. Cylindrical specimens of four ceramic engobe formulations were submitted to splitting tensile strength and bulk density tests. The specimens were fired in a laboratory kiln and characterized through the determination of water absorption, bulk density, relative density and by reflectance spectrophotometry. Crystallographic analysis with Rietveld refinement and microstructural analysis by scanning electron microscopy were also carried out. After the high energy milling, the kaolin had less crystallinity and the specific surface area increased from 4.6 to 46.1 m 2 /g. The use of mechanically-activated kaolin as a replacement for ball clay in a ceramic engobe increased the mechanical strength, crystalline phase content and whiteness.

Ceramic materials present interesting properties and generate products that meet engineering requirements referring to this class for several applications. Aluminum oxide is a very good example of this class of materials, being applied widely in high technology areas especially where the abrasive and erosive wear are strongly demanded. Many factors interfere on the wear of alumina: material composition, characteristics of the used oxides, and the manufacturing process that defines the final microstructure. It is know that the grain size of the alumina has a strong influence on the wear rate and is the focus of several studies. The grain growth during sintering as well as the diffusion process can be controlled by using dopants in the alumina composition. Thus the aim of this paper is to present a review about the dopants interference on the grain size of alumina, which in turn influences the wear behavior of the alumina. The results described allow broad understanding on the subject

INTRODUÇÃOMateriais cerâmicos encontram diversas aplicações por suas propriedades de alta rigidez, resistência à compressão, resistência ao desgaste, resistência química e habilidade de suportar elevada temperatura. No entanto, sua inerente