Il. Denry et al., EFFECT OF ION-EXCHANGE ON THE MICROSTRUCTURE, STRENGTH, AND THERMAL-EXPANSION BEHAVIOR OF A LEUCITE-REINFORCED PORCELAIN, Journal of dental research, 77(4), 1998, pp. 583-588
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.