Isabel K. Lloyd

Influence of microstructure and chemistry on the fracture toughness of dental ceramics

OBJECTIVES the primary aim of this research was to measure fracture toughness for several groups of dental ceramics, and determine how this property is affected by chemistry and microstructure. METHODS Fracture toughness (KIc) values were obtained using Single Edge Precracked Beam (SEPB) and Single Edge V-Notch Beam (SEVNB) methods. Dynamic Youngs modulus, which often scales with strength and has been used in explaining the microstructure/toughness relationship on a theoretical basis, was also obtained for the three groups of materials comprising this study. The first group, consisting of micaceous glass ceramics, included model materials that varied systematically in microstructure but not in chemistry. The second group, the feldspathic porcelains, varied significantly in microstructure, but little in chemistry. The ceramics comprising the third group were significantly different in both chemistry and microstructure. RESULTS Upper toughness limits for the micaceous glass-ceramics and feldspathic porcelains were significantly raised compared to the base glasses, but remained under 2 MPa m(1/2). The highest toughnesses were associated with high percent crystallinity, large grains and high aspect ratios. The third group KIc values were 2.8 MPa m(1/2) for a lithium disilicate glass-ceramic, 3.1 MPa m(1/2) for a glass-infused alumina, and 4.9 MPa m(1/2) for zirconia. SIGNIFICANCE the correlations between microstructural characteristics and measured properties supports theoretical predictions in the literature. From a practical standpoint, microstructural effects were found to be important, but only within a limited range; the chemistry apparently defined a band of achievable property values. This suggests very large increases in fracture toughness are unlikely to be attained by changes in microstructure alone. A functional relationship determined for the micaceous glass-ceramics enables quantitative predictions of fracture toughness based on the microstructure.

Materials Design of Ceramic-based Layer Structures for Crowns

Radial cracking has been identified as the primary mode of failure in all-ceramic crowns. This study investigates the hypothesis that critical loads for radial cracking in crown-like layers vary explicitly as the square of ceramic layer thickness. Experimental data from tests with spherical indenters on model flat laminates of selected dental ceramics bonded to clear polycarbonate bases (simulating crown/dentin structures) are presented. Damage initiation events are video-recorded in situ during applied loading, and critical loads are measured. The results demonstrate an increase in the resistance to radial cracking for zirconia relative to alumina and for alumina relative to porcelain. The study provides simple a priori predictions of failure in prospective ceramic/substrate bilayers and ranks ceramic materials for best clinical performance.

“EDGE TOUGHNESS” AND MATERIAL PROPERTIES RELATED TO THE MACHINING OF DENTAL CERAMICS

Abstract Seven dental ceramic compositions, including zirconia, glass-infused alumina, polycrystalline alumina, porcelain and three different machinable glass ceramics (MGCs) were tested for edge flaking. The chosen load range encompassed the forces typically associated with machining operations for these materials. The results were examined with regard to material properties and the applicability of a parameter defined as “edge toughness” to machin-ability. It was determined that a non-linear low-load chipping relationship may be more relevant to machinability than the “edge toughness” parameter calculated from the high-load linear region.

Microwave sintering of ZnO at ultra high heating rates

In this paper, a unique processing approach for producing a tailored, externally controlled microstructure in zinc oxide using very high heating rates (to 4900 °C/min) in a microwave environment is discussed. Detailed data on the densification, grain growth, and grain size uniformity as a function of heating rate are presented. With increasing heating rate, the grain size decreased while grain size uniformity increased. At extremely high heating rates, high density can be achieved with almost complete suppression of grain growth. Ultrarapid microwave heating of ZnO also enhanced densification rates by up to 4 orders of magnitude compared to slow microwave heating. The results indicate that the densification mechanisms are different for slow and rapid heating rates. Since the mechanical, thermal, dielectric, and optical properties of ceramics depend on microstructure, ultrarapid heating may lead to advanced ceramics with tailored microstructure and enhanced properties.

Structural investigation of chalcogenide and chalcohalide glasses using Raman spectroscopy

Abstract HV- and HH-polarized Raman spectra of the chalcogenide glasses Ge 30 As 10 Se (60− x ) Te x (25⩽ x ⩽35) and the chalcohalide glasses Ge 30 As 10 Se 30 Te (30− y ) I y (0⩽ y ⩽30), Ge 30 As 10 Se 35 Te (25− z ) I z (0⩽ z ⩽20) were investigated. For the chalcogenide glasses, the main structural units include [AsSe 3− x Te x ] mixed pyramidal units, [GeSe 4− x Te x ] mixed tetrahedral units, and Ge–Te–Ge chains. The substitution of iodine for tellurium in the chalcohalide glasses results in the formation of Ge–I, As–I, and Se–I bonds which break up the three-dimensional network. The main structural units for the chalcohalide glasses are [AsSe x Te y I z ] mixed pyramidal units where x + y + z =3, [GeSe x Te y I z ] mixed tetrahedral units where x + y + z =4, and GeSe 3/2 I mixed tetrahedral units. The symmetry properties of these structural units has been determined through the dependence of the depolarization ratio on the frequency shift.

Scratch hardness and chipping of dental ceramics under different environments

OBJECTIVE The goal of this program was to identify promising environments that could efficiently minimize machining-induced damage of dental materials. METHODS Single point abrasion (SPA) scratch testing was used on five materials to determine the scratch hardness and amount of edge chipping as functions of chemical environment, including air, water, saline and glycerol solutions. Limited testing was also done under additional environments expected to promote chemomachining effects via crack growth promotion or debris removal. A conical diamond indenter and a conventional tungsten carbide machining tool were used in the scratch tests. One-way ANOVA analysis was used to determine statistical differences among the variables. RESULTS There was a consistent trend across materials that the water and saline yielded the lowest values of scratch hardness, air the next lowest, and the tests performed in glycerol yielded the highest hardness values. The measured hardness values using the conical diamond tool in the glycerol environments were about twice the hardness values measured under water and saline solutions. Environmental effects on chipping were minimal, but a linear relationship between load and per cent chipping was determined for the WC tool within the 10-50 N test range. The choice of scratch tool strongly affected scratch hardness and chipping tendency. SIGNIFICANCE The chemical environment had an effect on machining characteristics, but the effects were more dependent on tool interactions rather than material specific properties. As a result, it may not be possible to utilize a particular single environment to substantially improve the damage response of dental materials to machining operations. Improvements in damage resistance can be environmentally obtained, but only for shallow cuts (finishing operations).

Microwave sintering of high-density, high thermal conductivity AlN

Microwave energy was used to sinter high thermal conductivity AlN ceramics (160–225 W/mK). The effects of sintering time, temperature, and amount of additive on phase composition, phase distribution, densification behavior, grain growth, and thermal conductivity were studied. The thermal conductivity of AlN was greatly improved by the addition of Y 2 O 3 , extended sintering time, and higher sintering temperatures. Thermal conductivity development in Y 2 O 3 -doped AlN showed two distinctive time regimes: (i) densification, where full densification, secondary phase formation, concentration and segregation, and rapid purification of AlN grains occur, accompanied by a large increase in thermal conductivity; (ii) postdensification, where grain growth and secondary phase sublimation/evaporation occur, yielding a further increase in thermal conductivity. Our results indicate that microwave sintering is a promising approach for synthesis of high thermal conductivity AlN ceramics.

Microwave sintering and properties of AlN/TiB 2 composites

The effect of TiB 2 on the densification behavior and properties of microwave-sintered AlN/TiB 2 ceramic was investigated. The densification of the composite was significantly retarded in nitrogen atmosphere due to strong nitridation of TiB 2 compared to sintering in argon atmosphere. The densities of the AlN/TiB 2 composites containing different amounts of TiB 2 all reached 99% of the theoretical density during 2 h of sintering at 1850 and 1900 °C. Microstructure analysis revealed that the TiB 2 particles were dispersed in the AlN matrix while AlN grains retained its contiguity. This microstructure led to a composite with superior properties; thermal conductivity as high as 149 W/(m K) was achieved. The microwave sintered composites are harder and tougher than pure AlN. Microwave-sintered AlN/TiB 2 composite is a promising material for structural applications in which high thermal conductivity and controlled dielectric loss are important.

Joining Dental Ceramic Layers With Glass

OBJECTIVE Test the hypothesis that glass-bonding of free-form veneer and core ceramic layers can produce robust interfaces, chemically durable and esthetic in appearance and, above all, resistant to delamination. METHODS Layers of independently produced porcelains (NobelRondo™ Press porcelain, Nobel BioCare AB and Sagkura Interaction porcelain, Elephant Dental) and matching alumina or zirconia core ceramics (Procera alumina, Nobel BioCare AB, BioZyram yttria stabilized tetragonal zirconia polycrystal, Cyrtina Dental) were joined with designed glasses, tailored to match thermal expansion coefficients of the components and free of toxic elements. Scanning electron microprobe analysis was used to characterize the chemistry of the joined interfaces, specifically to confirm interdiffusion of ions. Vickers indentations were used to drive controlled corner cracks into the glass interlayers to evaluate the toughness of the interfaces. RESULTS The glass-bonded interfaces were found to have robust integrity relative to interfaces fused without glass, or those fused with a resin-based adhesive. SIGNIFICANCE The structural integrity of the interfaces between porcelain veneers and alumina or zirconia cores is a critical factor in the longevity of all-ceramic dental crowns and fixed dental prostheses.

The temperature dependence of ferroelectric imprint

Abstract Unidirectional voltage pulse stressing can induce a significant amount of asymmetry in the retention characteristics in certain LSCO/PLZT/LSCO thin films. A large asymmetry was developed within 1000 s of unidirectional pulsing with a 100-Hz (50% duty cycle) square wave at 125°C while no significant retention asymmetry was developed at 25°C in the same time frame. The change in respective switched and non-switched polarizations after voltage pulse stressing follow an Arrhenius behavior. The thermal activation energies (Ea) derived from the Arrhenius plots are Ea = 0.21 eV for the change in switched polarization and an Ea = 0.56 eV for the change in non-switched polarization.