Efecto de la nitruración en la degradación hidrotérmica de circona tetragonal policristalina estabilizada con Y₂O₃

Abstract

Se ha estudiado el efecto de la nitruración a 1650 ºC durante 2 horas en la degradación hidrotérmica de una cerámica policristalina (Y-TZP) estabilizada con 2.5% molar de itria. Se ha observado que la microestructura superficial de la cerámica nitrurada es resistente a la transformación de fase t→m en presencia de agua a 100 ºC durante el período de tiempo investigado (850 horas), mientras que el material de partida se transforma en la superficie, pero no sufre ninguna reducción significativa en la resistencia. Sin embargo, en las muestras nitruradas se observa una disminución de la resistencia a flexión biaxial con el tiempo de ataque hidrotérmico, la cual se atribuye a la presencia de una cantidad importante de fase monoclínica en el interior de la muestra. Es decir, los granos situados en esta zona son más transformables en las probetas nitruradas, sin que el agua desempeñe aparentemente ningún papel en la degradación de la resistencia de estas muestras. El comportamiento observado frente a la degradación hidrotérmica en las condiciones estudiadas se atribuye al cambio en la estructura cristalina de la capa superficial y a la transformabilidad de la fase tetragonal a fase monoclínica en el interior de las muestras. Palabras clave: Y-TZP, nitruración, transformación de fase, degradación hidrotérmica Effect of nitridation on hydrothermal aging of Y-TZPThe effect of nitridation at 1650 ºC during 2 hours on hydrothermal aging of Y-TZP stabilized with 2.5 % moles of yttria has been studied. It has been shown that the superficial layer of nitrided samples was resistant to t→m phase transformation in water at 100 ºC during the investigated time period (850 hours). Conversely, the non-nitrided material transformed at the surface but no significant reduction in biaxial strength was observed. However, the biaxial strength of nitrided samples diminishes with hydrothermal aging time, which is thought to be due to the presence of an important amount of monoclinic phase inside the sample which is due to the high transformability of tetragonal grains. It appears that water does not play any important role in degradation of nitrided samples. The observations has been associated to changes in crystalline structure of a surface layer and to the high transformability of the tetragonal phase to monoclinic one inside the nitrided samples.Keywords: Y-TZP, nitridation, phase transformation, hydrothermal aging. INTRODUCCIONLas cerámicas con base circona presentan unos valores relativamente elevados de tenacidad de fractura (4-10 MPam 1/2 ) y de resistencia mecánica (>1 GPa), lo cual hace posible su utilización en aplicaciones estructurales. Sin embargo, un factor que limita sus aplicaciones es la fácil activación de la transformación de la fase tetragonal a la monoclínica (t→m) cuando se ponen en contacto con agua u otros agentes químicos (por ejemplo, H 2 SO 4 ) durante largos períodos de tiempo. La transformación t→m es autocatalítica y se inicia en la superficie del material avanzando lentamente hacia el interior; después de larg...

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