The use of advanced ceramics as restorative dental materials is strongly increasing, owing to the introduction of Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) milling techniques which allow the fabrication of large and complex restorations with very high-dimensional accuracy [
Among the ceramic materials for dental applications, the zirconia-based ones are very widespread, because of their transformation toughening capabilities [
The aim of the present study is to analyze the mechanical behavior of commercially available Y-TZP ceramics for dental applications and to estimate the effects of different processing conditions, which usually occur during production by CAD/CAM techniques. In addition to this, eight commercially available ceramic veneers, to be used for the production of all-ceramic restorations in combination with Y-TZP structures, were analyzed by using three-point bending tests on two-layered specimens. Finally, a systematic comparative analysis of the eight selected ceramic veneers was carried out.
The mechanical strength of a commercial yttria-stabilized zirconia, to be used as core material for the production of crowns and bridges in combination with CAD/CAM techniques, was analyzed by three-point bending tests on standard specimens. Furthermore, the mechanical behavior of eight different types of veneering ceramics was analyzed by flexural tests of two-layered zirconia-veneer specimens and microhardness measurements.
The bending tests of zirconia-based core material were carried out following standard ISO procedures and recommendations (ISO 6872). Beam specimens, with length
The three-point bending tests were done under displacement control (cross-head feed rate equal to 1 mm/min) by using a universal testing machine (Instron 8500), with a 5 kN load cell, controlled by a TestStar II (MTS) controller.
In order to analyze the effect of surface finishing and coloring process on the flexural strength, four different specimen types were produced and tested, for example, Type A, Type B, Type C, and Type D (Table
Nomenclature of zirconia core specimens.
Specimen type | Description |
---|---|
Type A | Sintered |
Type B | Colored and sintered |
Type C | Sintered and polished |
Type D | Colored, sintered, and polished |
Type A specimens were tested when sintered.
Type B specimens were colored before sintering, by using commercial dyes.
Type C specimens were polished after sintering; in particular, surface polishing was done by using water-cooled carborundum disks, with progressively finer alumina grits, ranging between 400, 800, and 1200, respectively.
Type D specimens were colored, sintered, and polished. As the surface condition can have a great influence on the mechanical behavior of the core material, the surface roughness of each specimen type was measured by using a contact measuring system (MarSurf III, Mahr). The surface roughness was measured along the longitudinal and transversal direction of the specimens. Five measurements for each direction were carried out, with a traveling distance of 2 mm.
Flexural tests of two-layered core/veneer specimens, for each veneering ceramic analyzed, were carried out by using the same equipment and testing parameters described in the previous section. The specimens, with length
The specimens were analyzed by three-point bending tests, with a test span equal to 15 mm, and the load was applied on the veneer surface; that is, the veneer layer is subjected to compressive stress. In order to carry out a comparative analysis between the eight selected veneering ceramics, the total strain energy per unit volume was calculated.
In fact, this energetic parameter allows a comparative study between the different types of core/veneer specimens, as it describes the overall mechanical behavior of the two-layered system. The total strain energy per unit volume was calculated from the experimentally measured load-deflection curves, by considering the volume of the specimen between the test spans.
The results, concerning the experimental tests carried out on zirconia core specimens and on two-layered core/veneer systems, are described in this section. In particular, the results of bending tests performed on zirconia specimens are firstly discussed; then, a comparative analysis of the eight selected ceramic veneers is given.
Twenty specimens for each type listed in Table
Flexural strength and surface roughness for the four types of zirconia specimens.
Specimen type | Surface condition, Ra ( |
Flexural Strength (MPa) | Weibull parameters (MPa) | ||
---|---|---|---|---|---|
Average | SD | Modulus | Characteristic strength | ||
Type A | 1.75 ± 0.47 | 688 | 100 | 8 | 729 |
Type B | 1.27 ± 0.36 | 733 | 109 | 7 | 779 |
Type C | 0.13 ± 0.03 | 982 | 75 | 15 | 1005 |
Type D | 0.12 ± 0.03 | 991 | 46 | 22 | 1007 |
Table
Ten two-layered specimens for each veneering ceramic were analyzed by three-point bending tests. Three different failure mechanisms have been observed as shown in Figure Simultaneous failure of zirconia and veneer (Type F). Complete interfacial debonding of the veneer (Type D). Serrated fracture of the ceramic veneer (Type S), that is, the crack spreads in the veneer, approaches the zirconia-veneer interface, and then kinks again into the veneer.
Failure mechanisms of the two-layered zirconia-veneer specimens together with schematic depictions of the force-deflection curves and optical observations of the fracture surfaces.
In particular, Figure
Failure mechanism of two-layered zirconia-veneer specimens.
Veneer type | Failure mechanism* |
---|---|
Lava Ceram |
|
Ceramco PFZ |
|
Vita VM9 |
|
Triceram |
|
Zirox |
|
GC Initial ZR |
|
IPS e.max |
|
Sakura Interaction |
|
On the basis of the previous considerations, a first qualitative comparison of the eight selected ceramic veneers was made. In Table
Table
In Figure
Strain energy per unit volume at failure and veneer damage for the two-layered specimens.
Figure
The zirconia-based ceramic materials are the most promising for dental application, because of their transformation toughening capabilities [
In the present study, the authors analyzed the mechanical behavior of a commercial yttria-stabilized zirconia for dental application (Kavo Everest Bio ZS Blank), as well as its interaction with eight commercial veneering ceramics.
The effects of surface roughness as well as of the coloring process on the mechanical strength of the stabilized zirconia, analyzed by standard three-point bending tests, were firstly examined. The results indicate that surface roughness plays a critical role in the mechanical strength of zirconia structures, because a strong increase in the average flexural strength, from about 700 MPa to 1000 MPa, is observed after a mechanical polishing treatment of the test specimens. The results also show that the flexural strength of the polished specimens is characterized by smaller standard deviations (and higher
Furthermore, the results show that the coloring process, carried out by using a commercial coloring liquid (
It is worth noting that the effects of surface roughness, at the core/veneer interface, on the adhesion mechanisms were not analyzed in this investigation; however, even if it is expected to play a significant role in failure mechanisms of all-ceramic restorations, the surface morphology of the zirconia dental frames, obtained from CAD-CAM techniques, is not modified prior to ceramic veneering, due to both economic and technological issues. Therefore, this work was aimed at the identification of the best combinations of core/veneer, based on a reference surface condition, with roughness values (
On the contrary, the gingival surfaces of dental bridges are normally not veneered and, consequently, the roughness of the zirconia frame plays a very important role in the mechanical strength, because they are subjected to tensile stresses during chewing; therefore, high care should be devoted to both milling and/or possible postmilling laboratory procedures of zirconia frames in order to avoid the formation of rough surfaces, especially in the connector area, where geometric discontinuities and sources of stress concentration are present.
The results of this research indicate that close attention should be paid to the preparation procedure of zirconia-based crowns and bridges by CAD/CAM process, with the aim of obtaining smooth surfaces, because a strong effect of surface roughness on the mechanical strength was observed. Furthermore, no significant effects of the coloring process on the mechanical behavior were measured. Finally, comparative studies of several commercial ceramic veneers, to be used for the realization of all-ceramic systems, show different mechanical behavior and failure modes between the selected veneers. Further studies should be carried out to measure the wear properties of the ceramic veneers, as well as the interaction with natural enamel.
This study was supported by Tecnologica Srl (Crotone, Italy). The authors wish to thank Mr. Roberto Marrelli and his technical team (Odontoiatric Center Calabrodental, Crotone, Italy) for the valuable support in specimen preparation.