Effect of Ion Exchange on the Microstructure, Strength, and Thermal Expansion Behavior of a Leucite-reinforced Porcelain

Abstract

Leucite (KAlSi2O6) is used as a reinforcing agent in some porcelains for all-ceramic restorations; however, it increases their coefficients of thermal expansion, imposing constraints on the processing of the material. 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 leucites on thermal expansion, microstructure, crack deflection patterns, and flexural strength of a leucite-reinforced porcelain. A dental porcelain powder was mixed with rubidium or cesium nitrate and heat-treated. Porcelain bars (n = 3) and discs (n = 15) were made with the exchanged 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 Archimedes' method. The thermal expansion behavior of the materials was measured by dilatometry. The microstructure and Vickers indentation crack patterns were investigated by scanning electron microscopy. X-ray diffraction showed that after ionexchange and firing, leucite transformed into either tetragonal rubidium leucite or cubic cesium leucite. The mean coefficient of thermal contraction (550 to 50°C) was significantly (p < 0.003) greater for the control material, followed by the rubidium-exchanged material, and lowest for the cesium-exchanged material. Crack pattern analyses revealed that the cesium-exchanged material exhibited a significantly lower number of crack deflections compared with those in the two other materials (p < 0.001). The microstructure of the two exchanged porcelain materials was dense, with well-dispersed small crystals as well as larger rubidium or cesium leucite crystals. The mean flexural strength of the rubidium-exchanged material was significantly higher than those of the other materials, which were not significantly different. It was concluded that the thermal expansion of leucite-reinforced porcelain can be lowered by ionexchange, which also modifies the microstructure, crack deflection patterns, and flexural strength of the material.