Effect of heat treatment after accelerated aging on phase transformation in 3Y-TZP.

Our purpose was to investigate the effect of heat treatment on the reversibility of the tetragonal to monoclinic transformation in 3Y-TZP, and associated surface roughness. The goals were to determine the onset temperature of the reverse transformation, and characterize surface roughness after accelerated aging, and after aging followed by heat treatment. 3Y-TZP disc-shaped specimens were sintered at temperatures from 1300 to 1550 degrees C. The reversibility of the transformation was investigated by X-ray diffraction (XRD) after accelerated aging, followed by heat treatment at temperatures from 350 degrees C up to 850 degrees C. The surface roughness (R(rms)) was measured by atomic force microscopy after polishing, after accelerated aging for 1 or 10 h, and after aging followed by heat treatment. XRD showed that the fraction of m-phase increased linearly with grain size after aging for 10 h (1.0-29.8%). The transformation was reversed for all groups after heat treatment at 850 degrees C/min., with only trace amounts of m-phase remaining for the group sintered at 1550 degrees C. A significant increase in mean surface roughness was observed after accelerated aging (1.59-7.45 nm), compared to polished groups (0.83-1.0 nm). However, the mean surface roughness after accelerated aging for either 1 or 10 h, followed by heat treatment at 850 degrees C/min. (1.18-2.1 nm), was not significantly different from that of the polished groups. This was attributed to the reverse transformation. XRD revealed that the monoclinic to tetragonal transformation, was complete after heat treatment at 500 degrees C for 1 min, for specimens sintered at 1550 degrees C and aged 10 h.

Effect of coloring with various metal oxides on the microstructure, color, and flexural strength of 3Y-TZP.

The purpose of this study was to investigate the effect of cerium and bismuth coloring salts solutions on the microstructure, color, flexural strength, and aging resistance of tetragonal zirconia for dental applications (3Y-TZP). Cylindrical blanks were sectioned into disks (2-mm thick, 25-mm in diameter) and colored by immersion in cerium acetate (CA), cerium chloride (CC), or bismuth chloride (BC) solutions at 1, 5, or 10 wt %. The density, elastic constants, and biaxial flexural strength were determined after sintering at 1350 degrees C. The crystalline phases were analyzed by X-ray diffraction before and after aging in autoclave for 10 h. The results showed that the mean density of the colored groups was comparable with that of the control group (6.072 +/- 0.008 g/cm(3)). XRD confirmed the presence of tetragonal zirconia with a slight increase in lattice parameters for the colored groups. A perceptible color difference was obtained for all groups (DeltaE* = 2.57 +/- 0.48 to 14.22 +/- 0.98), compared with the control. The mean grain size increased significantly for the groups colored with CC or CA at 10 wt %, compared with the control group (0.318 +/- 0.029 mm). The mean biaxial strength of CA1%, CA5%, and BC1% groups was not significantly different from that of the control group (1087.5 +/- 173.3 MPa). The flexural strength of all other groups decreased linearly with increasing concentration for both cerium salts (860.7 +/- 172 to 274.4 +/- 67.3 MPa). The resistance to low temperature degradation was not affected by the coloring process. Coloring with cerium or bismuth salts produced perceptible color differences even at the lowest concentrations. A decrease in flexural strength at the higher concentrations was attributed to an increase in open porosity.

Effect of crystallization heat treatment on the microstructure and biaxial strength of fluorrichterite glass-ceramics.

The purpose of this study was to evaluate the effect of crystallization heat treatment on the flexural strength of potassium fluorrichterite glass-ceramics for biomedical applications. After melting and casting, discs were sectioned from the glass ingots, randomly divided into 10 groups, and heat treated at various temperatures ranging between 890 degrees C and 925 degrees C for various durations. One group was air-abraded after heat treatment. XRD revealed the presence of fluorrichterite and fluormica for all heat-treated groups. SEM confirmed the presence of both fluorrichterite and fluormica crystals. Crystal size increased with temperature and duration of the heat treatment. The mean flexural strength after air abrasion was not significantly different than that of the other groups except the group heat treated at 925 degrees C. The mean flexural strength was maximum after heat treatment at 900 degrees C for 1 h (497.6 +/- 25.5 MPa) and decreased linearly with the longer durations of heat treatment at 900 degrees C.

Microstructural and crystallographic surface changes after grinding zirconia-based dental ceramics.

The purpose of this study was to evaluate microstructural and crystallographic phase changes after grinding 3Y-TZP dental ceramics. Ceramic blanks were sintered according to manufacturer's recommendations and divided into four groups: (A) as-sintered control, (B) diamond-ground manually under water, (C) ground and polished, and (D) ground and annealed at 1000 degrees C for 1 h. Bulk specimens were analyzed by X-ray diffraction to characterize the crystalline phases. The microstructure was investigated by SEM. XRD analyses showed that the control group and the group that was ground and annealed contained only tetragonal zirconia. However, after grinding or after grinding followed by polishing, rhombohedral zirconia and strained tetragonal zirconia were present, without any detectable amount of monoclinic zirconia. Annealing led to the disappearance of both residual lattice strain and the rhombohedral phase. The microstructure of the ground and polished specimens was characterized by significant residual surface damage associated with grain pullout to a depth of about 20 microm. This type of damage could have an impact on the long-term fatigue behavior of 3Y-TZP.

Effect of niobium content on the microstructure and thermal properties of fluorapatite glass-ceramics.

Niobium oxide has been shown to improve biocompatibility and promote bioactivity. The purpose of this study was to evaluate the effect of niobium oxide additions on the microstructure and thermal properties of fluorapatite glass-ceramics for biomedical applications. Four glass-ceramic compositions with increasing amounts of niobium oxide from 0 to 5 wt % were prepared. The glass compositions were melted at 1,525 degrees C for 3 h, quenched, ground, melted again at 1,525 degrees C for 3 h and furnace cooled. The coefficient of thermal expansion was measured by dilatometry. The crystallization behavior was evaluated by differential thermal analysis. The nature of the crystalline phases was investigated by X-ray diffraction. The microstructure was studied by SEM. In addition, the cytotoxicity of the ceramics was evaluated according to the ASTM standard F895--84. The results from X-ray diffraction analyses showed that fluorapatite was the major crystalline phase in all glass-ceramics. Differential thermal analyses revealed that fluorapatite crystallization occurred between 800 and 934 degrees C depending on the composition. The coefficient of thermal expansion varied from 7.6 to 9.4 x 10(-6)/ degrees C. The microstructure after heat treatment at 975 degrees C for 30 min consisted of submicroscopic fluorapatite crystals (200--300 nm) for all niobium-containing glass-ceramics, whereas the niobium-free glass-ceramic contained needle-shaped fluorapatite crystals, 2 microm in length. None of the glass-ceramics tested exhibited any cytotoxic activity as tested by ASTM standard F895--84.

Effect of heat pressing on the mechanical properties of a mica-based glass-ceramic.

Previous work has shown that heat pressing of mica-based glass-ceramics can lead to crystal alignment along the direction of pressing. The purpose of this study was to evaluate the effect of heat pressing on the fracture toughness of mica-based glass-ceramics. Glass rods (12 x 60 mm) were prepared by melting the glass composition at 1400 degrees C for 2 h. Ingots (12 x 12 mm; n = 5) and discs (12 x 1.5 mm; n = 10) were cut from the rods. The discs were heat treated to simulate heat pressing and served as controls. A machinable mica-based glass- ceramic (Dicor MGC) also served as control. Bar-shaped wax patterns (2 x 4 x 22 mm) were invested and heat pressed at 875 degrees C. The elastic constants were determined with the use of the pulsed ultrasonic velocity method. The fracture toughness was measured by the indentation strength technique. The degree of texture was assessed qualitatively by X-ray diffraction and quantitatively on digital SEM micrographs. The results showed that the mean fracture toughness of the heat-pressed specimens (1.96 +/- 0.19 MPa. m(0.5)) was significantly higher than that of the heat-treated controls (1.51 +/- 0.21 MPa. m(0.5)) or the proprietary mica glass-ceramic (Dicor MGC; 1.66 +/- 0.04 MPa. m(0.5)) (p <.001). It was concluded that heat pressing led to a significant increase in fracture toughness in mica glass-ceramics because of crystal alignment along the direction of pressing.

Elastic constants, Vickers hardness, and fracture toughness of fluorrichterite-based glass-ceramics.

OBJECTIVES: To determine the elastic constants, Vickers hardness, and indentation fracture toughness of fluorrichterite glass-ceramics in the system SiO2-MgO-CaO-Na2O-K2O-F. METHODS: Five glass compositions were prepared with increasing sodium content. The compositions were melted at 1400 degrees C for 2 h and cast into 60 x 12 mm rods. Discs (1.5 x 12 mm) were cut from the glass rods, nucleated in the temperature range 600-650 degrees C for 1 h and crystallized at 900 degrees C for 0.5 h. The density of the glass-ceramics was measured by Archimedes' method. The elastic constants were determined by standard ultrasonic velocity technique. The fracture toughness was evaluated by the indentation technique. The indentation crack patterns and microstructure were investigated by scanning electron microscopy. The crystalline phases were identified by X-ray diffraction. A commercially available lithium disilicate glass-ceramic served as control. RESULTS: The fluorrichterite glass-ceramics containing 3.8 and 5.6 wt% sodium had the lowest mean fracture toughness. The glass ceramic containing 1.9 wt% sodium and the control material had the highest mean fracture toughness. The values obtained for these two materials (2.26 +/- 0.15 MPa m0.5 and 2.29 +/- 0.31 MPa m0.5, respectively), were not significantly different (p = 0.997). There was no linear relationship between the amount of sodium present in the glass-ceramic composition and the mean fracture toughness. The fluorrichterite glass ceramics exhibited a dual microstructure consisting of both fluorrichterite and mica crystals. SIGNIFICANCE: Higher fracture toughness appeared to be associated with a higher density of fluorrichterite crystals.

Effect of post-processing heat treatment on the fracture strength of a heat-pressed dental ceramic.

The purpose of this study was to evaluate the effect of post-processing heat treatment on the fracture strength of a heat-pressed lithium disilicate dental ceramic (OPC 3G). Bar-shaped wax patterns (2 x 4 x 22 mm; n = 60) were invested and heat pressed in an automated pressing machine according to manufacturer's recommendations. The bars were polished to 1200 grit and annealed at 600 degrees C for 1 h. Two groups were left untreated as controls, one of which was Vickers indented in oil prior to testing in three-point bending. Two groups were indented and then heat treated at either 800 or 825 degrees C for 30 min. The corresponding control groups were heat treated, then indented prior to testing. Optical micrographs of the indentations were taken before and after heat treatment under differential interference contrast. The fracture toughness and/or fracture strength of the specimens was determined by the indentation-strength technique in three-point bending. Indentation-generated cracks were not discernible by optical microscopy after heat treatment at either 800 or 825 degrees C for 30 min, even under 1000 x magnification. ANOVA and Tukey's test showed that heat treatment in air at 825 degrees C for 30 min significantly increased the fracture strength of a heat-pressable lithium disilicate ceramic. This result was attributed to healing of the flaws present in the material.

Effect of processing variables on texture development in a mica-based glass-ceramic.

The purpose of this study was primarily to determine the feasibility of heat-pressing a mica-based glass-ceramic with the use of commercially available dental equipment, and secondly to evaluate the effect of various processing variables on the degree of crystal alignment in the pressed glass-ceramic. The hypotheses were that the mica-based glass-ceramic could be successfully heat-pressed and that some degree of crystal orientation would be obtained, depending on the processing variables. The glass composition was melted at 1400 degrees C for 2 h and cast into 12 x 60-mm rods. Ingots (10 x 12 mm) were cut from the glass rods, heat treated, and heat-pressed under various conditions. The microstructure of the pressed specimens was investigated by SEM and compared to that of heat-treated controls. The length of Vickers-induced cracks was measured on heat-pressed specimens and heat-treated controls. The results showed that mica-based glass-ceramics could be successfully pressed with the use of commercially available dental equipment. The resulting degree of crystal alignment (texture) along the direction of pressing varied between 35 +/- 6 and 79 +/- 6. There was a linear relationship between the degree of texture and the apparent aspect ratio of the mica platelets. A significant decrease in the length of the Vickers-induced cracks in the direction perpendicular to pressing was observed, associated with an increase in length in the direction of pressing.

Effect of additives on the microstructure and thermal properties of a mica-based glass-ceramic.

Previous work has shown that lithium mica glass-ceramics were excellent potential candidates as dental ceramics. The purpose of this study was to evaluate the effect of various additives on the microstructure and thermal properties of a mica glass-ceramic in the system Li(2)O-K(2)O-SiO(2)-MgO-F. Five glass compositions were prepared: a base composition and four compositions with various additives. The compositions were melted at 1400 C for 2 h and cast into 50 x 8 mm ingots. Differential thermal analyses were performed on the glasses up to 1400 C. Bars (4 x 8 x 25 mm) were cut from the ingots and heat treated at various temperatures. The crystalline phases were analyzed by x-ray diffraction. The microstructure was investigated by scanning electron microscopy. The percent crystallinity, crystal density, and average particle size were calculated from stereology measurements. X-ray diffraction revealed that the major crystalline phase was taeniolite for all glass-ceramics. Differential thermal analyses showed that the crystallization exotherm occurred in the temperature range 600--700 C. The addition of calcium fluoride was most efficient in promoting the growth of mica crystals. Larger mica plate diameter could be beneficial to the crack-propagation resistance of the glass-ceramic. The addition of aluminum phosphate promoted nucleation rather than crystal growth and led to a significantly lower glass transition temperature and crystallization exotherm temperature.

Effect of sodium content on the crystallization behavior of fluoramphibole glass-ceramics.

The purpose of this study was to evaluate the effect of sodium content on the crystallization behavior of glass-ceramics in the system SiO(2)-MgO-CaO-Na(2)O-K(2)O-F. Five glass compositions were prepared with increasing sodium content. The compositions were melted at 1400 degrees C for 2 h and cast into 60 x 12-mm ingots. Differential thermal analyses were performed on the glasses up to 1400 degrees C. Discs (1.5 x 12 mm) were cut from the ingots, nucleated in the temperature range 600-650 degrees C for 1-2 h, and crystallized in the temperature range 900-1000 degrees C for 0.5-4 h. The crystalline phases were analyzed by x-ray diffraction. The microstructure was investigated by scanning electron microscopy. Differential thermal analyses showed that the crystallization exotherm occurred in the temperature range 600-750 degrees C. There was a linear relationship between the amount of sodium and the transformation temperatures. X-ray diffraction revealed the presence of mica and diopside as major crystalline phases in the sodium-free composition. Mica, diopside, and fluorrichterite were present in all other glass-ceramics. The microstructure of the sodium-free glass-ceramic was characterized by the presence of hexagonal mica crystals and prismatic diopside crystals. All other compositions exhibited needle-shaped fluorrichterite crystals (2-5 microm in length) in addition to mica and diopside crystals.

Effect of water exposure on the fracture toughness and flexure strength of a dental glass.

OBJECTIVES: The low fusing dental glass (Duceram LFC) has been advertised as presenting a superior chemical resistance and augmented strength after 16h exposure to water or 4% acetic acid. The purpose of this study was to evaluate the effect of prolonged exposure to water on two mechanical properties (fracture toughness and flexure strength) of LFC. METHODS: Disks and bars were mirror polished and annealed prior to aging in: (1) air (control), (2) water for 24h at 80 degrees C and (3) water for 8 weeks at 80 degrees C. Fracture toughness (K(Ic)) was determined by indentation fracture (IF) and indentation strength (IS) using a 19.6N Vickers indentation load. Flexure strength values were obtained from three-point bending at 0.1mm/min. Statistical analysis was performed using the Weibull distribution, Tukey and Bartlett tests (P<0.05). RESULTS: Both techniques (IS and IF) showed a significant improvement in the K of Duceram LFC after 8 weeks in water (0.88 and 1.14MPa m(0.5)) as opposed to the 24-h values both in water and air (0.77-0.78MPa m(0.5)). However, for flexure strength the Weibull characteristic (S(0)) and the m parameter did not change significantly with water storage (S(0)=90-100MPa, Weibull m =7-8). SIGNIFICANCE: The increase in toughness of Duceram LFC after aging in water is an interesting and favorable observation for a restorative material exposed to the oral environment. Nevertheless, in comparison with other contemporary ceramics, the toughness of this LFC remains in the range of soda-lime-glass or classic feldspar porcelains.

Phase transformations in a leucite-reinforced pressable dental ceramic.

The objectives of this study were to determine the conditions for the formation of sanidine in a pressable dental ceramic (OPC; Jeneric/Pentron). Ceramic discs (16 mm in diameter, 1.3 mm thick; n = 60) were pressed according to the manufacturer's recommendations. One group was left as pressed as the control; the other groups were further heat-treated at temperatures ranging from 700 degrees C to 1100 degrees C for times ranging from 10 min to 24 h. X-ray diffraction was performed on powdered specimens. The microstructure was investigated by scanning electron microscopy (SEM). X-ray diffraction showed that sanidine was the only crystalline phase present after heat treatment at 900 degrees C for 24 h, whereas a mix of tetragonal leucite and sanidine was found in the specimens heat-treated at 900 degrees C for 12 h and 800 degrees C for 24 h. Tetragonal leucite was the only phase present in the control specimens after heat treatment at 950 degrees C and higher, and in the specimens heat-treated up to 1 h at 850 degrees C or 900 degrees C. Tetragonal and cubic leucite were found in the specimens treated at 750 degrees C or 800 degrees C for 30, 60, and 180 min and 700 degrees C for 24 h. SEM confirmed the presence of sanidine in the specimens heat-treated at 900 degrees C for 24 h. All specimens treated for up to 20 min at temperatures ranging from 700 degrees C to 950 degrees CC contained only tetragonal leucite. The clinical significance of this study is that the formation of sanidine is unlikely to occur at the temperature and time ranges needed for the staining or veneering of this leucite-reinforced pressable dental ceramic.

Effect of magnesium content on the microstructure and crystalline phases of fluoramphibole glass-ceramics.

The purpose of this study was to evaluate the effect of magnesium content on the microstructure and crystalline phases of glass-ceramics in the system SiO(2)-MgO-CaO-Na(2)O-K(2)O-F. Four glass compositions were prepared with magnesium content varying from 12-18 wt%. The compositions were melted at 1400 degrees C for 2 h and cast into 30 x 8 mm ingots. Differential thermal analyses were performed on the powdered glasses at a heating rate of 20 degrees C/minute up to 1400 degrees C. Bars (4 x 8 x 25 mm) were cut from the ingots with a low-speed diamond saw, nucleated in the temperature range 600-650 degrees C for 1-2 h and crystallized in the temperature range 900-1000 degrees C for 4-6 h. The crystalline phases were analyzed by X-ray diffraction. The microstructure was investigated by scanning electron microscopy. The Vickers hardness was determined after indentation under a 9.81 N load. Differential thermal analyses revealed that crystallization occurred in the temperature range 700-800 degrees C. X-ray diffraction showed the presence of fluorrichterite as major crystalline phase regardless of the magnesium content in the parent glass or heat treatment temperature. The microstructure consisted of interlocked acicular crystals (5-10 micrometers). The highest magnesium content led to the coexistence of both a mica phase and fluorrichterite. This microstructure promoted crack deflections and arrest.

Effect of heat treatment on microcrack healing behavior of a machinable dental ceramic.

The purpose of this study was to evaluate the effect of annealing in air on the crack healing behavior of a machinable dental ceramic (Vita Mark II). The glass transition temperature and the softening point were determined by dilatometry. Polished Vita Mark II blocks (n = 12) were indented with a Vickers indenter under a 9.8 N load. Optical micrographs were taken immediately after indentation, and the crack lengths were measured. The specimens were annealed at 900 degrees C for either 30 min, 1 h, or 2 h. One group was indented and heat treated at 400 degrees C for 2 h as control. The ratio of crack length after annealing to crack length before annealing treatment was calculated for each indentation. Scanning electron microscopy was performed before and after annealing at 900 degrees C for 1 h. Scanning electron microscopy revealed shortening and blunting of the cracks after annealing. Annealing in air at 900 degrees C for either 30 min, 1 or 2 h significantly reduced the mean crack length of an indented machinable dental ceramic compared to the control group. However, the mean biaxial flexural strength was not significantly affected by an annealing treatment.

Crystallization kinetics of a low-expansion feldspar glass for dental applications.

Previous studies have shown that the crystallization of glasses in the system K2O-Al2O3-SiO2, in the primary field of leucite, is not possible without modification of the composition. Leucite (KAlSi2O6) is used as a reinforcing phase in some compositions for all-ceramic dental restorations. However, because of their higher coefficient of thermal expansion, these materials cannot be veneered with conventional metal-ceramic porcelains. The purposes of this study were to investigate the crystallization behavior of a glass in the system K2O-Al2O3-SiO2 and to evaluate the effect of heat treatment on the crystal size, percent crystallinity, and coefficient of thermal expansion of the material. Ion-exchanged glass powder was prepared by mixing the glass with rubidium nitrate and heat-treating at 450 degrees C for 4, 8, 24, or 48 h. Bars were made from these powders and baked under vacuum at 1038 degrees C for 2 min. The bars made from the powder ion-exchanged for 48 h were further heat-treated for 4 h at either 800 degrees C, 850 degrees C, 900 degrees C, 950 degrees C, or 1038 degrees C. X-ray diffraction analyses showed cubic leucite was the only crystalline phase in the specimens made from the powders ion-exchanged for 4, 8, 24, or 48 h and baked at 1038 degrees C for 2 min. Further heat treatment for 4 h at either 800 degrees C, 850 degrees C, 900 degrees C, 950 degrees C, or 1038 degrees C promoted the growth of cubic leucite. In addition, a second phase identified as tetragonal rubidium-leucite was present in the specimens heat-treated for 4 h at 1038 degrees C. SEM observations showed that all specimens made from the powders ion-exchanged for 48 h exhibited small spherical crystals dispersed in a glassy matrix. The percent crystallinity ranged from 18.9% to 42.9% and the average particle size was between 0.64 to 1.18 microns. The coefficients of thermal expansion ranged between 8.076 and 8.788 x 10(-6)/degree C.

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 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.

Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic.

OBJECTIVES: Various methods aimed at determining the fracture toughness of ceramics in mode I (KIc) have been described in the literature. The accuracy, scatter and the interexaminer reproducibility of KIc depend strongly on the procedural approach, the test parameters used and the conditioning of the specimens. The purpose of the present study was to compare fracture toughness values obtained using two indentation methods as well as a newly established fracture mechanics test. METHODS: The following methods for KIc determination were applied: (1) indentation fracture (IF), (2) indentation strength (IS) and (3) the single-edge-V-notched-beam test (SEVNB). The materials tested were a low-fusing dental glass (Duceram LFC) and a feldspar-based porcelain (IPS classic). Data were compared by ANOVA and Tukey's multiple comparison test (p < or = 0.05). RESULTS: For both materials, KIc coefficients of variation ranged between 10 and 14% for IF and 7 and 10% for IS. The IS technique demonstrated a load dependency for the IPS porcelain which was not observed when using the IF method. The SEVNB test provided consistent results with coefficients of variation between 1 and 3%. SEVNB toughness values for the IPS porcelain were in agreement with the IS technique. However, halfpenny shaped cracks were observed at the tip of the notch of all LFC specimens thus leading to underestimated KIc values. SIGNIFICANCE: The overall aim of this type of study is to select testing procedures that are as expedient and reliable as possible. This study has shown that all three methods agreed within 10%. However none of the procedures proved absolutely straightforward. Decision on which method to use should be based on a sound understanding of the conceptual limitations and technical difficulties inherent to each technique.

Surface layer characterization after dual ion exchange of a leucite-reinforced dental porcelain.

The dual ion-exchange technique has been shown to improve the strength of feldspathic dental porcelains by first replacing constituent alkali ions with smaller ions above the strain point, and then exchanging the smaller ions for larger ions at temperatures below the strain point. The strength increase is directly related to the thickness of the surface-exchanged layer. This study evaluated the thickness of the exchanged layer after dual ion exchange of a leucite-reinforced dental porcelain. Optec HSP porcelain disks were fabricated, submitted to a dual ion-exchange treatment, and indented at various loads before determining the biaxial flexure strength. The mean flexural strength for the ion-exchanged groups was significantly higher than for control groups, except when the depth of the median crack exceeded 138 microns. Wavelength Dispersive Spectrometry analyses on cross sections showed that the mean potassium amount in the glassy matrix was significantly lower (P < .001) for the treated specimens than for the controls. The mean thickness of the exchanged layer after dual ion exchange was estimated at 140 microns.

Effect of cubic leucite stabilization on the flexural strength of feldspathic dental porcelain.

Previous studies (Mackert and Evans, 1993) have shown that, when feldspathic dental porcelain is cooled, leucite undergoes a transformation from cubic to tetragonal, associated with a 1.2% volume contraction. This contraction leads to the formation of microcracks in and around the crystals and the development of tangential compressive stresses around the crystals. Our aim was to stabilize increasing amounts of the cubic form of leucite in a leucitereinforced dental porcelain, evaluate its effect on the flexural strength, and characterize its microstructure. The hypothesis was that in the absence of crystallographic transformation, the contraction of the leucite crystals would be lower, thereby limiting the formation of microcracks and minimizing the development of tangential compressive stresses around the leucite particles. We prepared 8 porcelain compositions by mixing increasing amounts of either leucite (KAlSi2O6) or pollucite (CsAlSi2O6) with Optec HSP porcelain (Jeneric/Pentron Inc., Wallingford, CT). Porcelain disks were made from each composition (n = 10 per group). X-ray diffraction analyses showed that the amount of stabilized leucite increased with the amount of pollucite added. The microstructure of the specimens containing tetragonal leucite was characterized by twinned leucite crystals, whereas no twinning was observed in the specimens containing cubic leucite. The evaluation of crack deflection showed that significantly less deflection occurred in the specimens containing cubic leucite. The mean biaxial flexural strength for the group corresponding to 22.2 wt% added pollucite, fired at 1038 degrees C, was significantly lower than that for the control group. The group corresponding to 22.2 wt% added leucite fired at 1150 degrees C exhibited a mean biaxial flexural strength significantly higher than that of all other groups that were not significantly different from the control group. Overall, the stabilization of cubic leucite reduced the flexural strength and the number of crack deflections in leucite-reinforced porcelain. Apparently, the development of tangential compressive stresses around the leucite crystals when cooled is responsible for a significant amount of strengthening of feldspathic dental porcelain.