A study was undertaken to evaluate the effect of artificial aging through steam and thermal treatment as influencing the shear bond strength between three different commercially available zirconia core materials, namely, Upcera, Ziecon, and Cercon, layered with VITA VM9 veneering ceramic using Universal Testing Machine. The mode of failure between zirconia and ceramic was further analyzed as adhesive, cohesive, or mixed using stereomicroscope. X-ray diffraction and SEM (scanning electron microscope) analysis were done to estimate the phase transformation (m-phase fraction) and surface grain size of zirconia particles, respectively. The purpose of this study was to simulate the clinical environment by artificial aging through steam and thermal treatment so as the clinical function and nature of the bond between zirconia and veneering material as in a clinical trial of 15 years could be evaluated.
The most recent introduction to the dental ceramics family is zirconia, which in its pure form is a polymorphic material. Fixed partial denture and crowns are available options for replacing lost tooth structure. Fabrication of fixed prosthesis involves layering of metallic substructure or all ceramic core structure with an aesthetic ceramic veneering material. The underlying substructure provides the required strength and the overlying ceramic gives the required esthetics [
Yttrium oxide partially stabilized tetragonal zirconia polycrystal (Y-TZP) was introduced in dentistry as a core material in the 1990s. The name of zirconium is said to be derived from the Persian word “Zar”-“gun” meaning golden in color [
Many studies on the use of zirconia have been conducted, but some features still remain unclear. There is a lack of information on how temperature change and water treatments affect the material properties and clinical durability of veneered zirconia cores. Unlike other prostheses in the body, oral prosthesis is constantly subjected to moisture with fluctuating pH and temperature. Recent studies have shown an indication that water molecules have an influence on the bond between veneered and veneering material [
The substructure (zirconia core) of the restoration is covered by veneering ceramic. Studies have shown the bond between the zirconium core and the veneering ceramic to be influenced by water molecules which eventually affects the clinical durability of the restoration in function [
Aging of zirconia may have detrimental effects on its bonding with veneering ceramics; mechanical stresses and wetness exposure accelerate this process [
Three types of zirconia were selected for this study: Cercon (DeguDent, Hanau, Germany), Ziecon (Jyoti Ceramic Industries Pvt., Ltd., Nashik, India), and Upcera (Shenzhen Upcera Co., Ltd., China) (Table
Zirconia systems evaluated in this study.
Name | Composition | Manufacturer |
---|---|---|
( |
ZrO2 (>93%), Y2O3 (4.95–5.35%), HfO2 (0.8%), Al2O3 (0.15–0.35%), trace elements (SiO2, NaO2, Fe2O3) | Cercon DeguDent, Germany |
( |
ZrO2 (94–96%), Y2O3 (4.05–6.0%), HfO2 (1-2%), Al2O3 (0–0.1%), trace elements (SiO2, NaO2, Fe2O3) | Jyoti Ceramics, Nashik, India |
( |
ZrO2 + HfO2 + Y2O3 (99.6 wt%), Al2O3 (<0.5 wt%), SiO2, NaO2, Fe2O3 (<0.2%) | Upcera, Shenzhen, China |
Firing schedules of zirconia (as provided by the manufacturer).
Core material | Manufacturer | Sintering temperature | Sintering cycle time |
---|---|---|---|
Cercon | Cercon DeguDent, Germany | 1,350°C | 8 hrs |
Ziecon | Jyoti Ceramics, Nashik, India | 1500°C | 8 hrs |
Upcera | Upcera, Shenzhen, China | 1,450°C–1500°C | 8 hrs |
Though thermocycling has been advocated in the literature as a conventional aging test, autoclaving induced low-temperature degradation is an established method for accelerated aging of Y-TZP materials. Autoclaving at 134°C for 5 hours is the standard aging protocol according to ISO 13356 valid for Y-TZP implants for surgery. 20 samples from each group were autoclaved at 134°C for 5 hrs to simulate oral conditions for 15 years [
The prepared samples were steam cleaned and taken up for ceramic layering application. Ninety samples of different zirconia core material were layered using VITA VM9 (VITA Zahnfabrik, Bad Säckingen, Germany) veneering ceramic material (Table
Properties of veneering material as provided by the manufacturer.
Veneering material | Manufacturer | Flexural strength (MPa) | CTE |
---|---|---|---|
VM9 | VITA Zahnfabrik, Bad Säckingen, Germany | 96 | 8.8–9.2 |
Firing schedules of the veneering ceramic according to manufacturer.
Veneering ceramic | Temperature (°C) | Time (min) | Heating rate (°C/min) | Firing temperature (°C) | Holding time (min) |
---|---|---|---|---|---|
Liner | |||||
VITA VM9 | 500 | 6 | 55 | 930 | 1 |
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Dentin | |||||
VITA VM9 | 500 | 6 | 55 | 910 | 1 |
A metal jig was used for holding the samples to determine the shear bond strength. The samples were held in such a manner that the junction of zirconia substructure/veneering ceramic interface was faced towards the chisel load applicator. Universal Testing Machine (Model 3345, Instron Corp., Norwood, MA, USA) with a 10 kN load cell and crosshead speed of 0.5 mm/min was used. A chisel load applicator was used to direct a parallel shearing force to the substructure/veneer ceramic interface. The fracture load was obtained with the help of the graph on the digital monitor attached to the machine. The drop in the graph determined the point of debonding (fracture load) of each sample. Fracture load (kg) was converted to the shear bond strength (MPa) by use of the cross-sectional area of disc. The following formula was used to calculate the shear bond strength (MPa):
After shear bond strength analysis, all the debonded samples were analysed under a stereomicroscope (20x) to evaluate the nature of bond failure. Each specimen was placed on carbon coated flat platform in such a manner that the surface between zirconia substructure and veneering ceramic faces the pointer of stereomicroscope, so as to identify whether there was cohesive failure, adhesive failure, or a combination of both, that is, mixed failure.
The crystalline phases on the surfaces of the specimens were analysed by a Philips X’Pert 1 X-ray diffractometer. Scans were performed in the
After phase analysis, to observe the surface microstructure for the surface particle size evaluation, specimens were sectioned and polished with a diamond wheel and grinding machine and coated with gold using a sputter-coating technique. The specimens were observed by field emission scanning electron microscopy. Electron imaging was performed at an accelerating voltage of 15 kV to investigate surface geometry with Zeiss EVO 40 scanning electron microscope at the magnification of 30,000x. 10 areas were randomly selected and analysed for each specimen (Figures
(a) Microstructural analysis of grain size of zirconia specimens without autoclave treatment (without artificial aging) under scanning electron microscope at the magnification of 30,000x. (b) Microstructural analysis of grain size of zirconia specimens with autoclave treatment (with artificial aging) for 5 hours under scanning electron microscope at the magnification of 30,000x.
The data was entered into MS Excel spreadsheet and analysed, using SPSS version 11 statistical software. Descriptive statistics were calculated for each variable for the three groups. Techniques applied were Student’s
Table
Comparison of shear bond strength (MPa) in all the groups showing mean, standard deviation, and significance for the samples within subgroups.
Upcera | Ziecon | Cercon |
|
|
---|---|---|---|---|
Comparison in between subgroups without artificial aging | ||||
Mean ( |
24.43 ± 7.13 | 24.70 ± 6.76 | 27.9 ± 6.54 | 0.66 |
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Comparison in between subgroups with artificial aging | ||||
Mean ( |
24.0 ± 7.53 | 23.9 ± 8.03 | 24.20 ± 8.87 | 0.99 |
Table
Type of bond failures after debonding of the samples among all the three groups as examined under stereomicroscope.
Groups | Subgroups |
Number of samples showing cohesive failure | Number of samples showing combined failure |
---|---|---|---|
Upcera | Control ( |
9 ( |
1 ( |
Test ( |
14 ( |
6 ( | |
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Ziecon | Control ( |
8 ( |
2 ( |
Test ( |
15 ( |
5 ( | |
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|||
Cercon | Control ( |
8 ( |
2 ( |
Test ( |
14 ( |
6 ( |
Table
The comparison between the mean of phase transformation (m-phase fraction) Vol% values and their respective standard deviation of all the three groups, that is, Upcera, Ziecon, and Cercon, within each subgroup.
Upcera | Ziecon | Cercon |
|
|
---|---|---|---|---|
Comparison in between subgroups without artificial aging | ||||
Mean (%) ± SD | 6.08 ± 1.16 | 6.49 ± 0.77 | 12.6 ± 1.15 | 0.00 |
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Comparison in between subgroups with artificial aging | ||||
Mean (%) ± SD | 11.4 ± 1.9 | 11.0 ± 0.18 | 14.3 ± 0.70 | 0.04 |
Table
Comparison of surface grain size (
Upcera | Ziecon | Cercon |
|
|
---|---|---|---|---|
Comparison in between subgroups without artificial aging | ||||
Mean ( |
0.43 ± 0.039 | 0.41 ± 0.064 | 0.39 ± 0.060 | 0.532 |
|
||||
Comparison in between subgroups with artificial aging | ||||
Mean ( |
0.42 ± 0.034 | 0.40 ± 0.05 | 0.40 ± 0.032 | 0.781 |
Based on the results obtained, the proposed hypothesis was accepted. There is no significant difference in the mean values of shear bond strength among all samples for both aging and nonaging groups. Hence the proposed hypothesis was accepted and probably multicentric trials could be taken to get the explicable results. Zirconia is classified as a high strength ceramic material; it was introduced for use in dentistry as a biomaterial with unsurpassed mechanical properties. Its clinical application expanded from single crowns, short-span fixed partial denture, and multiunit full-arch zirconia frameworks to implant abutments and complex implant superstructures [
The results showed that there is no significant difference in the mean values of shear bond strength among all samples (Table
Another factor that may affect the shear bond strength is the multiple firing of veneer ceramic over zirconia core; due to multiple firing there is relaxation of residual stresses and phase changes from tetragonal to monoclinic transformation [
Zirconia veneered samples of all groups (aged as well as nonaged) after SBS test showed combined and cohesive (within veneer) bond failure as seen under stereomicroscope, with the predominance of cohesive failure in the veneer layer with no adhesive failure (Table
In the present study different analytical methods are applied, namely, XRD and SEM, to analyse the aging characteristics of the zirconia ceramics.
ISO Standards allow a maximum of 25% of m-phase to be present [
The present study on simulated aging has shown that LTD is observed to be insignificant over a simulated period of 15 years of clinical usage.
The literature is well supplemented with research on phase transformation and its effect on physical properties such as flexural strength and fracture toughness. However, very few studies have evaluated the quantitative phase transformation within zirconia frameworks due to LTD and the correlation of phase transformation with the bond strength between zirconia substructure and veneering ceramic.
In a study Xiao et al. observed the change in the monoclinic phase and compared the low-temperature resistance aging performances of the three clinical frequently used zirconia core materials Lava Frame, Cercon Smart, and Upcera before and after aging. XRD analysis showed that the m-phase contents of the three zirconia materials increased by prolonging the aging time, where in Upcera zirconia was the most sensitive to the aging treatment [
The differences in LTD observed in the three different brands of yttria-stabilized zirconia ceramics may be attributed to differences in grain sizes, distributions of the grains, and additives (e.g., binder for the pressing step) [
Control of grain size in zirconia ceramics is important to maximize the mechanical properties and minimize the possibility of LTD. Grain size may also affect the bond strength; a SEM analysis among zirconia groups of Upcera, Ziecon, and Cercon revealed that, after postaging treatment, the grain sizes in all the three zirconia ceramics were found to be insignificantly the same as that of samples without aging (Table
The strength and other physical properties of zirconia are related to the quality of the zirconia block, that is, its composition, method of compaction and converting into a blank, and homogeneity within the blank. Sintering mechanism may have a determinant influence on the phase transformation, grain size, and the physical and mechanical properties. A wide scope of research is open to investigate further the challenges in improving the properties of zirconia restorations. The study is in vitro, where clinical situations have been simulated. Therefore, the results of this study may not directly transpolate into serviceability of the prosthesis in clinical setting. However, the results do indicate the acceptable values of bond strength upon aging of all commercially available zirconia used in the study as compared to samples without any aging procedure.
From the present study it can be concluded that 15 yrs of clinical usage as induced by artificial aging has no effect on the bond strength of zirconia and the veneering ceramic. The effect of stress generated by clinical usage such as masticatory forces, surface cracks, premature contact, or any other mechanism leads to stress formation within prosthesis which could not be simulated in the study samples. This concentration of stress with the passage of time (aging) could affect the performance of zirconia based restorations clinically.
Within the limitations of the study, following conclusions were drawn: Bond strength between zirconia and veneering ceramic in all samples was comparatively same (24.43 MPa to 27.9 MPa) and aging of 15 years had no effect on bond strength (23.9 MPa to 24.20 MPa). The zirconia veneer interface came out to be stronger than the strength of the veneering ceramic; on average 70% of the samples have shown combined failure; hence to improve the longevity of zirconia restorations the strength of veneering ceramic has to be improved. Bonding of zirconia veneered restoration is not influenced if the phase transformation (% of monoclinic content) is up to 14%. ISO Standards allow a maximum of 25 wt% of m-phase to be present, post accelerated aging test conducted at 134°C with 2-bar pressure for 5 hrs for any commercial zirconia ceramics to be used in dentistry. Surface grain size
The paper is approved tacitly or explicitly by the responsible authorities where the work was carried out.
The authors declare that they have no competing interests.