EFFECT OF ION-EXCHANGE ON THE MICROSTRUCTURE, STRENGTH, AND THERMAL-EXPANSION BEHAVIOR OF A LEUCITE-REINFORCED PORCELAIN

Leucite (KAlSi2O6) is used as a reinforcing agent in some porcelains f or all-ceramic restorations; however, it increases their coefficients of thermal expansion, imposing constraints on the processing of the ma terial. The potassium ions in leucite are exchangeable for rubidium or cesium ions, leading to rubidium leucite or cesium leucite (pollucite ). Both rubidium leucite and pollucite exhibit a lower coefficient of thermal expansion and inversion temperature than leucite. The purpose of this study was to evaluate the effects of rubidium and cesium leuci tes on thermal expansion, microstructure, crack deflection patterns, a nd flexural strength of a leucite-reinforced porcelain. A dental porce lain powder was mixed with rubidium or cesium nitrate and heat-treated . porcelain bars (n = 3) and discs (n = 15) were made with the exchang ed powders. X-ray diffraction analyses were performed before and after bars were fired. Controls were made of untreated Optec HSP porcelain powder, formed into bars and disks, and baked following manufacturer's recommendations. The density of all specimens was determined by Archi medes' method. The thermal expansion behavior of the materials was mea sured by dilatometry. The microstructure and Vickers indentation crack patterns were investigated by scanning electron microscopy. X-ray dif fraction showed that after ion-exchange and firing, leucite transforme d into either tetragonal rubidium leucite or cubic cesium leucite. The mean coefficient of thermal contraction (550 to 50 degrees C) was sig nificantly (p < 0.003) greater for the control material, followed by t he rubidium-exchanged material, and lowest for the cesium-exchanged ma terial. Crack pattern analyses revealed that the cesium-exchanged mate rial exhibited a significantly lower number of crack deflections compa red with those in the two other materials (p < 0.001). The microstruct ure of the two exchanged porcelain materials was dense, with well-disp ersed small crystals as well as larger rubidium or cesium leucite crys tals. The mean flexural strength of the rubidium-exchanged material wa s significantly higher than those of the other materials, which were n ot significantly different. It was concluded that the thermal expansio n of leucite-reinforced porcelain can be lowered by ion-exchange, whic h also modifies the microstructure, crack deflection patterns, and fle xural strength of the material.