A. De Noni

Multilayered ceramic composites – a review

Abstract 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.

Wear performance of alumina-based ceramics - a review of the influence of microstructure on erosive wear

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 materials 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.

Superfícies fotocatalíticas de titânia em substratos cerâmicos: Parte I: Síntese, estrutura e fotoatividade

Resumo A fotocatalise e um processo de oxidacao avancada capaz de transformar uma grande variedade de poluentes orgânicos toxicos em substâncias inofensivas em condicoes ambientes. Porem, a utilizacao do dioxido de titânio na fase anatase em produtos cerâmicos e limitada devido a transformacao para fase rutilo em temperaturas superiores a 400 °C. As pesquisas apontam caminhos promissores para inibir a formacao da fase rutilo atraves da introducao de dopantes na estrutura do semicondutor. Neste trabalho, foi realizada uma revisao dos principais aspectos encontrados na literatura especializada sobre fotocatalise com dioxido de titânio, em particular a relacao entre estrutura quimica e atividade fotocatalitica. Palavras-chave: fotocatalise, dioxido de titânio, superficies cerâmicas.

Superfícies fotocatalíticas de titânia em substratos cerâmicos. Parte II: substratos, processos de deposição e tratamento térmico

A fotocatalise com dioxido de titânio tem sido utilizada para uma grande variedade de aplicacoes, que se relacionam em particular com decomposicao de compostos orgânicos toxicos produzindo moleculas simples como o dioxido de carbono e agua, destruicao de poluentes da agua conta-minada, do ar, bacterias e celulas cancerigenas [1, 2]. Em geral, a forma polimorfica anatase e preferida devido a sua alta atividade fotocatalitica. No entanto, essa fase nao e estavel a altas temperaturas e sua aplicacao em produtos cerâmicos e limitada. Varios aspectos estao sendo estudados e a eficiencia fotocatalitica e dependente de parâmetros relacionados com a natureza do substrato, metodo de depo-sicao e tratamento termico.

Estudo de composições cerâmicas à base de alumina e vitrocerâmico do sistema LZSA para obtenção de estruturas multicamadas por tape casting

Ceramic matrix composites have been developed in recent decades as an efficient way of improving the fracture toughness of ceramic materials. Multilayered ceramic composites have been highlighted by their relatively low cost and easy production. These composites are made of stacked layers of two different ceramic materials and present as toughening mechanism the crack path deviation layer by layer in the composite. In this work, compositions based on alumina and LZSA glass-ceramic (Li2O-ZrO2-SiO2-Al2O3) were studied to compose the layers in order to obtain multilayer composite with strong interfaces. Thus, compressive residual stresses in thin LZSA-based layers interspersed with Al2O3-based layers, under low tensile residual stress, were expected to obtain. The compositions were defined by factorial design, generating specimens by pressing that were sintered and characterized by determination of the coefficient of thermal expansion and modulus of elasticity. Three Al2O3-based compositions, in wt% (CA1: 100% Al2O3; CA2: 90% Al2O3 and 10% ZrO2; CA-PC: 90% Al2O3, 5% ZrO2 and 5% SiCwhisker) and three LZSA-based compositions (CF6: 70% LZSA, 10% Al2O3 and 20% SiCwhisker; CF8: 50% LZSA, 10% Al2O3, 20% TiO2 and 20% SiCwhisker; CF-PC: 75% LZSA, 5% Al2O3, 10% TiO2 and 10% SiCwhisker) were selected. Such compositions were designed to generate residual compressive stress in the LZSA-based layers (up to -32.35 MPa) and residual tensile stress on the Al2O3-based layers (at least 0.27 MPa). The tape casting technique was used to produce sheets of the selected compositions, using amounts of binders and plasticizers reported in the literature. The compositions were then prepared by tape casting obtaining homogenous, flexible ceramic sheets showing enough green strength to be handled. The alumina-based sheets thickness ranged between 130 and 210 μm, while LZSA-based sheets varied between 180 and 230 μm.

Use of mechanically-activated kaolin to replace ball clay in engobe for a ceramic tile

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 m2/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.

Deflocculant consumption of clay suspensions as a function of specific surface area and cation exchange capacity

Abstract Ceramic tile production by the wet route requires clay suspensions with a high solid content and low viscosity. In this work the deflocculation of clays in aqueous suspensions was investigated by varying the type of clay and additive. Three kaolinitic and two illitic clays were characterized and dispersed with deflocculants based on lithium, sodium and potassium silicates and polyacrylates. The clays were characterized by chemical and mineralogical analyses, particle size distribution, zeta potential, organic carbon content, cation exchange capacity (CEC) and specific surface area (BET). Deflocculation curves were determined by measuring the viscosity for 50 wt.% clay slips. The results indicate that additive consumption is closely related to CEC and BET, which correspond respectively to the chemical and physical characteristics of the clay mineral’s surface. Moreover, viscosity values at the deflocculation point are closely related to BET.

Wet Processing of Triaxial Ceramics Using a Mixture Design Approach

An optimum amount of deflocculant was determined for three-component suspensions, based on a mixture design approach. Three raw materials were used, characterized as clay, feldspar and quartz. Ten compositions were prepared using the mixture design approach. Deflocculation curves were measured for suspensions with 40 wt.% of solids, adding sodium silicate as dispersant. The optimum deflocculant amount (ODA) was determined for each suspension, corresponding to the lowest value of apparent viscosity considering two nearest experimental points. It was observed that ODA increases linearly with the increase of the clay fraction in the mixture. A response surface and polynomial regression could be used to identify the composition range that satisfies usual conditions in wet ceramics processing.