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 ion-exchange and firing, leucite transformed into either tetragonal rubidium leucite or cubic cesium leucite. The mean coefficient of thermal contraction (550 to 50 degrees 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 ion-exchange, which also modifies the microstructure, crack deflection patterns, and flexural strength of the material.

Wear performance of bovine tooth enamel against translucent tetragonal zirconia polycrystals after different surface treatments

“…Unlike translucent TZP, the microstructure of POR is not homogenous but is composed of glassy matrix and crystalline phases, such as leucite 22,23) . This nonhomogeneity of the material may provoke wear, especially at the beginning of wear simulation.…”

Insights on Ceramics as Dental Materials. Part I: Ceramic Material Types in Dentistry

“…This reduces the chance of cracking within the ceramic since the ceramic must overcome the compressive stresses [12]. Alternatively, smaller diameter sodium ions can be replaced with larger diameter potassium ions on the feldspar ceramic to put the ceramic under compression in order to minimize crack formation between the ceramic and the metal [28]. Reported improvements of 50% increased flexure strength and 100% improved fracture toughness are found with feldspar ceramics [29,30].…”

Toughening of CAD/CAM all-ceramic crowns by staining slurry

“…This suggests that the toughness increases as the potassium ion concentration is increased in the staining slurry through ion exchange by which sodium ions in the IPS glass are replaced by alkaline ions such as potassium that have a larger radius; this difference in ionic radius is thought to generate compressive stress at the surface 8) . Since the radius of a potassium ion (133 pm) is slightly greater than that (126 pm) of a silver ion, K + ions will generate greater compressive stress on the surface of an IPS block than Ag + ions when both ions have the same ion exchange efficiency with Na + ions (i.e., K + for Na + and Ag + for Na + ) 30) .…”

“…One technique for realizing this involves rapid cooling, which introduces a residual surface compressive stress due to differential contraction between the treated surface and the subsurface region 6) . Ion exchange strengthening is a chemical process in which large alkaline ions such as potassium are substituted into porcelain surface sites that are occupied by smaller ions such as sodium 7,8) . Another technique is dispersion reinforcement by ceramic particles such as alumina 9) and leucite 8,10) , which can hinder crack propagation in porcelain.…”