Análise e medida de fatores que afetam as curvaturas retardadas em porcelanato

Os revestimentos cerâmicos de porcelanato, tanto os esmaltados como os não esmaltados, apresentam um fenômeno conhecido como “curvaturas retardadas”, que consiste na mudança de curvatura das peças depois da saída do forno. Este fenômeno é mais problemático a medida de aumentam as dimensões das peças. Neste trabalho se quantificou a variação da curvatura ao longo do tempo em peças industriais de porcelanato esmaltado, observando-se que a curvatura parece apresentar uma evolução em um sentido (habitualmente em direção ao sentido côncavo, ou em forma de barco) para, após transcorrido certo tempo, inverter esta tendência. A cinética deste processo foi parametrizada considerando que existem dois mecanismos, simultâneos e antagônicos, com cinéticas diferentes.As análises teóricas apontam que somente existem dois fatores que podem produzir curvaturas retardadas: as tensões residuais e a expansão dos suportes. Em ambos os casos é necessário condições adicionais para que as curvaturas efetivamente se manifestem: Deste modo, por exemplo, a presença de tensões residuais não é sinônimo de curvaturas retardadas, mais sim um mecanismo adicional que permita a liberação progressiva destas tensões; este mecanismo é chamado de fluência. Além disso é necessário que o perfil de tensões residuais não seja simétrico em relação ao plano central da peça. No que diz respeito a expansão dos suportes, também são necessárias condições especiais para que estas possam provocar curvaturas retardadas; em particular, é necessário que estas sejam diferentes nas duas faces da peça (face lisa e tardoz). Uma expansão uniforme provocaria uma leve mudança dimensional, mas não uma curvatura retardada, nem mesmo para peças esmaltadas. A medida dos fatores que influenciam sobre as curvaturas retardadas é complexa pois requer o emprego de técnicas diferentes das habitualmente empregadas para a caracterização de revestimentos cerâmicos. Foram criados, especialmente para este trabalho, procedimentos para a medida dos diferentes fatores que produzem as curvaturas retardadas e se estudou a influencia de algumas variáveis sobre estes fatoresGlazed and unglazed porcelain tiles exhibit a phenomenon known as delayed curvatures, which consists of the change in tile curvature after the tiles leave the kiln. This phenomenon becomes more problematic as tile size increases. In this study, the variation of curvature in time has been quantified in industrial glazed porcelain tiles. It was observed that the curvature usually displays an evolution in one direction (customarily in a concave direction or vessel shape) which, after a certain time has elapsed, reverses this trend. The kinetics of this process has been parameterised, assuming there are two simultaneous, opposing mechanisms, with different kinetics. Theoretical analyses indicate that only two factors can produce delayed curvatures: residual stresses and expansion of the tile body. In both cases, additional circumstances need to occur for delayed curvatures to appear; thus, for example, the presence of residual stresses is not synonymous with delayed curvatures, but an additional mechanism is needed that allows progressive release of these stresses, a mechanism known as creep. In addition, the condition that the stress profile is not symmetrical with respect to the centre plane of the tile also needs to be obeyed. In regard to the expansion of the bodies, special conditions must also occur for these to cause delayed curvatures. In particular, it is necessary for the expansion at the tile fair face and at the rib face to be different. Uniform expansion would cause a slight dimensional change, but not a delayed curvature, not even in the presence of glaze. The measurement of the factors that influence delayed curvatures is complex because different techniques from those typically used in characterising ceramic tiles are required. Procedures have been fine-tuned to measure the different factors that give rise to delayed curvatures, and the influence of certain variables on these factors has been studiedVersió editoria

Modelización mecánica del enfriamiento rápido en sistemas tipo gres porcelánico

This paper analyses the effect of cooling on mechanical behaviour, in particular, on the residual stresses that appear when materials of the porcelain tile type are involved. However, these compositions have a very complex microstructure, in which there are several crystalline phases and the glassy phase is not homogeneous. In this study a simpler composition was therefore formulated, using sodium feldspar as starting material to which quartz with different particle sizes was added. A viscoelastic model was used to estimate the residual stresses that develop during cooling. The parameters of the model were obtained either from the literature or were determined in laboratory tests. An assembly was designed that allowed non-contact measurement of the temperature at the top and bottom surfaces of the test pieces during cooling. The test pieces were subjected to different types of cooling and their residual stresses were then determined by the strain relaxation slotting metho

Fracture toughness and temperature dependence of Young's modulus of a sintered albite glass

Albite glass is an important component in many ceramic compositions, often used as liquid phase during the sintering process. Nevertheless, in spite of its almost ubiquous presence in the final microstructure of these compositions, some properties such as Young's modulus or fracture toughness have not been extensively studied in literature. This paper presents an experimental study on a sintered albite glass obtained from sodium feldspar powder. The microstructure of the resultant amorphous solid was analyzed via scanning electron microscopy and X-ray diffraction. Fracture toughness was determined at room temperature from the critical stress intensity factor (KIC) with a three point bending single-edge notched test (SENB). Young's modulus and coefficient of thermal expansion (CTE) were measured as a function of the temperature. A value of CTE of 6.6 × 10− 6 °C− 1 within the temperature range 300–500 °C was found. Young's modulus values of the albite glass were within the range from 63.6 to 65.2 MPa. SENB tests revealed a fracture toughness of KIC = 0.78 ± 0.06 MPa m1/2, which is of similar value to those found for borosilicate and soda-lime glasses

Relationship between Young's modulus and temperature in porcelain tiles

A focused research was conducted on samples prepared from an industrial porcelain tile composition containing quartz, used to produce ceramic floor tiles, with the aim of evaluating the variation of fired specimens’ Young's modulus with temperature. These samples were fired in controlled laboratory conditions so that specimens with pre-existing cracks were obtained and subject to non-destructive in situ thermo-mechanical measurements (impulse excitation technique) in the 22–700 °C temperature range during heating and cooling processes in order to find evidences to explain the hysteresis phenomenon in the Young's modulus versus temperature curve. The observed irreversible Young's modulus may be directly related to the pre-existent cracks that on heating and cooling are closed and opened up respectively, changing thus the Young's modulus which is well characterized by a hysteresis cycle

Kinetic study of the thermal decomposition process of calcite particles in air and CO2 atmosphere

The thermal decomposition process of calcite particles (0.45–3.60 mm average diameter), made up of porous agglomerates of very small CaCO3 microcrystals, was studied in the 975–1216 K temperature range. The experiments were carried out under isothermal conditions in air atmosphere, in CO2 atmosphere, as well as in a gas stream comprising different concentrations of air and CO2. An equation is proposed that relates the calcite conversion degree to both reaction time and operating conditions. The equation satisfactorily fits to the experimental results obtained in the entire tested range of particle sizes and temperatures in all the studied carbon dioxide concentrations

Wear behaviour of WC plasma sprayed coatings wi th micro and nanostructured powders

El objetivo de este trabajo es estudiar el comportamiento a desgaste de distintos recubrimientos de WC sobre acero inoxidable mediante proyección por plasma atmosférico. Se proyectan dos tipos de polvos comerciales con base cobalto (12 %) y níquel (10 %), con el objeto de analizar la influencia de la matriz e incluso del espesor del recubrimiento. Los recubrimientos obtenidos se caracterizan microestructuralmente por microscopía electrónica de barrido y difracción de rayos X. La caracterización tribológica de los recubrimientos se realiza mediante ensayos pin-on-disc contra bolas cerámicas de alúmina (2.400 HV) y nitruro de silicio (1.600 HV), sin lubricación, obteniéndose las tasas de desgaste y la evolución del coeficiente de rozamiento. Las pistas de desgaste y los residuos generados se caracterizaron mediante Microscopía Electrónica de Barrido, MEB. Los resultados muestran diferentes mecanismos de desgaste en cada par de material ensayado en función de la matriz y de la naturaleza de la bola empleada en el ensayo, encontrándose mayores tasas de desgaste con bola de nitruro de silicio. En una segunda fase, se obtienen recubrimientos, con dos polvos base cobalto cuyos tamaños de partícula son micrométrico y nanométrico, respectivamente, con el objetivo de estudiar la influencia del tamaño de partícula inicial sobre las propiedades a desgaste del recubrimiento. Los resultados obtenidos ponen de manifiesto una mayor resistencia al desgaste de los recubrimientos obtenidos con polvos nanoestructurados, bajo condiciones de carga elevadas, mientras que para cargas bajas ambos recubrimientos presentan un comportamiento similar a pesar de las diferencias microestructurales apreciadasThe aim of the present work is the study of wear behaviour of different WC coatings deposited on stainless steel substrate by means of atmospheric plasma spraying (APS). Two types of WC commercial powders, with different metal binder (12% Cobalt and 10% Nickel) have been deposited in order to analyse the influence of the metal matrix and thickness of the coating in tribological properties.The microstructure of the depositions was characterized using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). On the other hand, tribology characterization of the coatings was made by pin-on-disk wear tests against alumina (2400HV) and silicon nitride (1600HV) ceramic balls, without lubrication. Wear rates and friction coefficient evolution have been calculated. Finally, wear tracks and wear debris have been analysed with the help of SEM.The results of each pair of tested materials show different mechanisms of wear related to the nature of the ball that has been used, obtaining higher wear rates with silicone nitride ball..In a second phase of the study, in order to examine the influence of the initial particle size on the wear properties of the coatings, cobalt based coatings have been analysed with different initial particle size (micrometric and nanometric particles).Results show that nanostructured coatings have higher wear resistance than microstrutured ones for high loads. However for low loads, behaviour is similar in spite of the microstructural differences appreciate

Deposition of Al2O3-TiO2 Nanostructured Powders by Atmospheric Plasma Spraying

Al2O3-13%TiO2 coatings were deposited on stainless steel substrates from conventional and nanostructured powders using atmospheric plasma spraying (APS). A complete characterization of the feedstock confirmed its nanostructured nature. Coating microstructures and phase compositions were characterized using SEM, TEM, and XRD techniques. The microstructure comprised two clearly differentiated regions. One region, completely fused, consisted mainly of nanometer-sized grains of c-Al2O3 with dissolved Ti+4. The other region, partly fused, retained the microstructure of the starting powder and was principally made up of submicrometer-sized grains of a-Al2O3, as confirmed by TEM. Coating microhardness as well as tribological behavior were determined. Vickers microhardness values of conventional coatings were in average slightly lower than the values for nanostructured coating. The wear resistance of conventional coatings was shown to be lower than that of nanostructured coatings as a consequence of Ti segregation. A correlation between the final properties, the coating microstructure, and the feedstock characteristics is give

Simulación de la deformación de baldosas cerámicas durante la cocción

Se ha desarrollado un modelo que permite calcular los perfiles de temperatura en el interior de las baldosas cerámicas durante su cocción; así como estimar la evolución de la curvatura de las piezas durante el calentamiento y cocción.Los resultados de las simulaciones se han contrastado con medidas experimentales obtenidas mediante un horno prototipo de laboratorio, que permite conocer las diferencias de temperatura entre las superficies superior e inferior de las probetas, así como su curvatura

Simulación de la deformación de baldosas cerámicas durante la cocción

Se ha desarrollado un modelo que permite calcular los perfiles de temperatura en el interior de las baldosas cerámicas durante su cocción; así como estimar la evolución de la curvatura de las piezas durante el calentamiento y cocción.Los resultados de las simulaciones se han contrastado con medidas experimentales obtenidas mediante un horno prototipo de laboratorio, que permite conocer las diferencias de temperatura entre las superficies superior e inferior de las probetas, así como su curvatura