Abstract
Background and aims: The objective of this study was to investigate the interaction of infrared laser light on Computer Aided Design and Computer Aided Manufacturing (CAD/CAM) ceramic surfaces.
Material and Methods: Sixty CAD/CAM ceramic discs were prepared and divided into two different groups: lithiumdisilicate ceramic (IPSe.maxCADs) and Zirconia ceramic (IPSe.maxZirCADs). The laser irradiation was performed on graphite and non-graphite surfaces with a Carbon Dioxide laser at 5W and 10W power in continuous mode (CW mode) and with Neodymium Yttrium Aluminum Perovskite (Nd:YAP) laser at 10W. Surface textures and compositions were examined using Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS). Thermal elevation was measured by thermocouple during laser irradiation.
Results: The SEM observation showed a rough surface plus cracks and fissures on CO2 10W samples and melting areas in Nd:YAP samples; moreover, with CO2 5W smooth and shallow surfaces were observed. EDS analysis revealed that laser irradiation does not result in modifications of the chemical composition even if minor changes in the atomic mass percentage of the components were registered. Thermocouple showed several thermal changes during laser irradiation.
Conclusion: CO2 and Nd:YAP lasers modify CAD/CAM ceramic surface without chemical composition modifications.
Keywords: CAD/CAM Ceramics, Laser, SEM, EDS, Thermocouple
Introduction
The demand of ceramic restorations has become increasingly popular in dentistry and the continuous need of precision is still a challenge to be achieved. With the improving the Computer Aided Design and the Computer Aided Manufacturing (CAD/CAM) ceramic technology, one of the most important challenges is to integrate it with the other new technologies.
The internal surface of the ceramic restoration must be prepared with the aim to increase the micromechanical retention of the cement. Surface treatment of CAD/CAM ceramic increases the surface in contact with the tooth structure creating micro-porosities and enhancing the potential for mechanical retention of the cement 1, 2).
Different surface treatment methods have been proposed to provide roughness and promote micro-mechanical retention 3, 4).
The identification of non-destructive methods to treat inert ceramics and modify their mechanical and chemical properties may result a good help to produce an activated surface.
Surface treatment (laser irradiation) has been involved in the industrial field using high power lasers and represents a controllable and flexible technique aiming to modify the surface properties of different materials 5, 6). Laser processing parameters during such surface modifications are expected to influence the surface microstructure 7) and the hypothesis developed was that Carbon dioxide (CO2, 10,600nm) and Neodymium Yttrium Aluminum Perovskite (Nd: YAP, 1340nm) lasers could modify the CAD/CAM surface ceramic specimens (E.maxCAD, Emax ZirCAD). These modifications could be linked to laser-target tissue interaction and also thermal elevation.
Aim of the study
The aim of this study was to characterize ceramic specimens (Emax CAD, Emax ZirCAD) irradiated with a CO2 (10,600nm) or Nd:YAP lasers (1340nm).
The characterization of the surfaces before and after laser irradiation by using Scanning Electron Microscopy associated with Energy Dispersive Spectroscopy (EDS) allowed to observe the morphological changes and chemical modifications.
Moreover, the effect of thermal treatment during laser irradiation was evaluated for both ceramics (Emax CAD, Emax ZirCAD) by using thermocouple device.
Materials and Methods
A). Tested ceramics
1) Lithiumdisilicate (EmaxCAD)
This material is composed of a lithium silicate with micron-size lithiumdisilicate crystals in between, which are submicron lithium orthophosphate crystals creating a highly filled glass matrix. Lithiumdisilicate is modified by increasing the crystal content to approximately 70% and by refining the crystal size, thus improving the flexural strength to approximately 360 MPa 8–12).
2) Zirconia (Emax ZirCAD)
This material is partially stabilized by the addition of small amounts of other metal oxides for reaching a flexural strength ranging between 900MPa and 1100 MPa 13).
The physical and chemical properties of tested ceramics are detailed in Table 1.
Table 1: Composition and physical properties of tested ceramics.
| Lithium Disilicate | Zirconia | |
|---|---|---|
| Composition (wt %) | SiO2 57.0–80.0 | ZrO2 87.0–95.0 |
| Li2O 11.0–19.0 | Y2O3 4.0–6.0 | |
| ZrO2 0.0–8.0 | HfO2 1.0–5.0 | |
| MgO 0.0–5.0 | Al2O3 0–1 | |
| K2O 0.0–13.0 | ||
| P2O5 0.0–11.0 | ||
| ZnO 0.0–8.0 | ||
| Al2O3 0.0–5.0 | ||
| Coloring oxides | ||
| 0.0–8.0 | ||
| Physical properties | Flexural strength (biaxial) 360 ± 60 Mpa | Flexural strength (biaxial) 900 x 50 Mpa |
| Chemical solubility 40 ± 10 µg/cm2 | Chemical solubility <10 µg/cm2 | |
| Coefficient of thermal expansion (100–400 °C) 10.15 ± 0.4x10−6 K1 | Coefficient of thermal expansion (100–400 °C) 10.75 ± 0.25 10−6 K1 |
B). Lasers used
1) CO2 laser
A CO2 laser (Dream Pulse Lasers, Daeshin Enterprise Corp., Korea, 10,600nm wavelength) was used with two different output power values (5W and 10W) corresponding to two different power densities (1592.3 W/cm2, 3184.7 W/cm2). Irradiation was conducted in non-contact and continuous mode (working distance 2 mm, spot size of the aiming beam 0.1mm).
2) Nd:YAP laser
Nd:YAP laser (Nd:YAP Lokki, Lobel Medical, France, wavelength 1340nm), previously proposed for surface modification with the aim to form a glazed surface layer on the ceramics 14), was used. A 320 µm diameter fibber and output power of 10W corresponding to a power density of 14185 W/cm2 were used.
Due to the high absorption level of this wavelength in black chromophores, the ceramic surfaces were stained via a black pencil. Irradiation parameters are summarized in Table 2.
Table 2: Irradiation parameters.
| Laser | CO2 | Nd: YAP | |
|---|---|---|---|
| Output Power | 5W | 10W | 10W |
| Operating Mode | C.W | C.W | Pulsed |
| Surface Area | 0.0314 cm2 | 0.0314 cm2 | 0.0008 cm2 |
| Density | 1592.3 W/cm2 | 3184.7 W/cm2 | 14185 W/cm2 |
Thirty ceramic discs of lithiumdisilicate (IPSe.max CADs, Ivoclar Vivadent), and thirty discs of Zirconia ceramic (IPSe.max ZirCADs, Ivoclar Vivadent) were sliced until to reach a thickness of 3mm with a low speed saw (ISOME™, Buehler, ITW Company, USA) by using a diamond blade diamond wafers blades Arbor size 0.5 (12.7mm); ceramic samples were then roughed by diamond dental bur (Grain size 100µm) and randomly distributed into six groups for both types of CAD/CAM ceramic:
Control group (Group A, n=6), Ceramic discs irradiated with CO2 laser 5W (Group B, n=6), Ceramic discs irradiated with CO2 laser 10W (Group C, n=6), Graphite ceramic discs irradiated with Nd:YAP laser 10W (Group D, n=6), Non graphite ceramic discs irradiated with Nd:YAP laser 10W (Group E, n=6).
C). Scanning Electron Microscopy (SEM)
The ceramic samples surface morphology characterization was conducted using Scanning Electron Microscopy (JSM-5310LV, Jeol Ltd., Japan) after surface metallization (Ion sputter Jeol JFC 1100E).
D). Energy Dispersive Spectroscopy (EDS)
Energy Dispersive Spectroscopy was conducted to identify the chemical composition of the samples. During EDS, the samples were exposed to an electron beam inside a Scanning Electron Microscope (SEM) (JSM-35CF, Jeol Ltd., Japan). These electrons collided with the electrons within the sample, causing some of them to be knocked out of their orbits. The vacated positions were filled by higher energy electrons emitted by x-rays during the process. By analysing the emitted x-rays, the elemental composition of the sample could be determined. Samples were analysed in low tension and low vacuum without metallization.
E). Thermocouple
A temperature-measuring device (thermocouple) (USB TC-08,*AS299/111*, Pico Technology, UK) was connected to the ceramic discs and computer by two dissimilar conductors displaying the rise of temperature on the surface of the ceramic discs during laser irradiation.
Results
A). Scanning Electron Microscopy (SEM)
The Scanning Electron Microscopy observation of the two sample groups (EmaxCAD and Emax ZirCAD) metallized and irradiated surface showed two types of surface: smooth and rough surfaces. They were evidenced some melting and burning areas due to the cumulative effect of the laser irradiation.
The presence of some cracks with variable intensities was to put in relationship to the thermal elevation by laser irradiation. With magnification x200, SEM observations they were evidenced:
1) Lithiumdisilicate (EmaxCAD) Figure 1: A, B, C, D and E
Group A (Control): rough surface with shallow depression.
Group B (CO2 5W): Two types of surface; smooth and shallow depressions.
Group C (CO2 10W): Presence of cracks and multiple holes of different size over a rough surface.
Group D (Nd:YAP Graphite): Rough surface with the presence of melting areas with irregular holes.
Group E (Nd:YAP): Melting areas with some burned areas appeared on rough surface.
Figure 1:
SEM of EmaxCAD
2) Zirconia (E.max ZirCAD) Figure 2 : A, B, C, D and E
Group A (Control): Rough surface showed shallow depressions.
Group B (CO2 5W): Smooth and shallow depressions were present on the surface.
Group C (CO2 10W): Presence of cracks and multiple holes with dissimilar size over an unsmooth surface.
Group D (Nd:YAP Graphite): Presence of melting areas with irregular holes.
Group E (Nd:YAP): Melting areas with some burned zones on the surface.
Figure 2:
SEM of Emax ZirCAD
B). Energy Dispersive Spectroscopy (EDS)
The EDS probe indicates the concentration of the chemical elements of the area spot where it is positioned. Results showed slight differences in the chemical composition between control group and randomly selected irradiated ceramics.
1) Lithiumdisilicate (EmaxCAD):
The identified elements of non-irradiated EmaxCAD are very similar to those described by the manufacturer. Carbon represents 2.41%, Oxygen 53% and Silicon dioxide 31%.
In comparison with control group it was evidenced: CO2 5W: a slight increase of Sodium (1.24%) and Silicon dioxide (34.25%) was identified, while with CO2 10W no differences were evidenced with the exception of a slight enhancement of Silicon dioxide (34.08%). Nd:YAP graphite: results showed a Carbon decrease (0.64%) and an Oxygen increase (58.34%), while with Nd:YAP without graphite, no carbon was detected with increase of Silicon dioxide (35.46%). A descriptive analysis is reported on the Tables 3, 4, 5.
Table 3: EDS of EmaxCAD Group Control.
| Unit | % Mass | % Atom |
|---|---|---|
| C K | 2.41 | 4.13 |
| O K | 52.90 | 67.89 |
| Na K | 0.36 | 0.32 |
| Al K | 0.83 | 0.63 |
| Si K | 30.97 | 22.64 |
| K K | 5.15 | 2.71 |
| Zn L | 0.36 | 0.11 |
| Zr L | 7.02 | 1.58 |
| Total | 100 |
Table 4: EDS of EmaxCAD CO2 Laser.
| CO2 5W | CO2 10W | ||||
|---|---|---|---|---|---|
| Unit | %Mass | %Atom | Unit | %Mass | %Atom |
| C K | 2.17 | 3.58 | C K | 2.35 | 3.88 |
| O K | 53.72 | 66.40 | O K | 53.56 | 66.27 |
| Na K | 1.24 | 1.07 | Na K | 0.52 | 0.45 |
| Al K | 0.99 | 0.72 | Al K | 0.85 | 0.62 |
| Si K | 34.25 | 24.11 | Si K | 34.08 | 24.02 |
| P K | 1.97 | 1.25 | P K | 2.86 | 1.83 |
| K K | 5.66 | 2.86 | K K | 5.78 | 2.93 |
| Total | 100 | Total | 100 |
Table 5: EDS of EmaxCAD Nd:YAP Laser.
| Nd:YAP Graphite | Nd:YAP | ||||
|---|---|---|---|---|---|
| Unit | %Mass | %Atom | Unit | %Mass | %Atom |
| C K | 0.64 | 1.04 | O K | 53.01 | 67.48 |
| O K | 58.34 | 71.40 | Na K | 0.71 | 0.63 |
| Na K | 0.71 | 0.61 | Al K | 1.00 | 0.75 |
| Al K | 0.87 | 0.63 | Si K | 35.46 | 25.71 |
| Si K | 32.07 | 22.36 | P K | 2.26 | 1.48 |
| P K | 2.04 | 1.29 | K K | 7.56 | 3.94 |
| K K | 5.33 | 2.67 | |||
| Total | 100 | Total | 100 | Total | |
2) Zirconia (Emax Zircad):
The identified elements of non-irradiated Emax Zircad are very similar to those described by the manufacturer. Carbon represents (11.84%) Oxygen (24.11%) and Zirconium (62.67%).
In comparison with the control group, result showed: CO2 5W: slight increase of Zirconium (67.12%) and decrease of Carbon (7.16%); with CO2 10W Carbon decreased (5.65%). Nd:YAP Graphite: results showed decrease of Carbon (6.03%) and slight increase of Oxygen (27.41%), while with Nd:YAP without graphite, no carbon was detected with increase of Zirconium (71.32%). A descriptive analysis is reported on the Tables 6, 7, 8.
Table 6: EDS of Emax ZirCAD Group Control.
| Unit | % Mass | % Atom |
|---|---|---|
| C K | 11.84 | 30.93 |
| O K | 24.11 | 47.28 |
| Zr L | 62.67 | 21.55 |
| Hf M | 1.38 | 0.24 |
| Total | 100 |
Table 7: EDS of Emax ZirCAD CO2 Laser.
| CO2 5W | CO2 10W | ||||
|---|---|---|---|---|---|
| Unit | %Mass | %Atom | Unit | %Mass | %Atom |
| C K | 7.16 | 20.27 | C K | 5.65 | 16.17 |
| O K | 25.72 | 54.70 | O K | 27.28 | 58.57 |
| Zr L | 67.12 | 25.03 | Zr L | 67.07 | 25.26 |
| Total | 100 | Total | 100 |
Table 8: EDS of Emax ZirCAD Nd:YAP Laser.
| Nd:YAP Graphite | Nd:YAP | ||||
|---|---|---|---|---|---|
| Unit | %Mass | %Atom | Unit | %Mass | %Atom |
| C K | 6.03 | 17.06 | O K | 28.68 | 69.63 |
| O K | 27.41 | 58.17 | Zr L | 71.32 | 30.37 |
| Zr L | 66.55 | 24.77 | |||
| Total | 100 | Total | 100 | ||
C). Temperature measurement
Starting with ambient room temperature (27°C), it was observed that there was a thermal elevation during laser irradiation on tested ceramics.
1) Lithiumdisilicate (EmaxCAD)
CO2 5W: the temperature reached 59.17°C with an average temperature elevation of (32.17°C).
CO2 10W: the temperature reached 73.28°C with an average temperature elevation of (46.28°C).
Nd:YAP Graphite: the temperature reached 101.06°C with an average temperature elevation of (74.06°C).
Nd:YAP: the temperature reached 114.52°C with an average temperature elevation of (87.52°C).
A descriptive analysis is reported on the Figure 3: A, B, C, D.
Figure 3:
Thermal mesuarement of EmaxCAD (A) CO2 5W (B) CO2 10W (C) Nd:YAP Graphite (D) Nd:YAP
2) Zirconia (Emax Zircad)
CO2 5W: the temperature reached 59.44°C with an average temperature elevation of (32.44°C).
CO2 10W: the temperature reached 93.10°C with an average temperature elevation of (66.10°C).
Nd:YAP: Graphite: the temperature reached 101.27°C with an average temperature elevation of (74.27°C).
Nd:YAP: the temperature reached 136.16°C with an average temperature elevation of (109.16°C).
A descriptive analysis is reported on the Figure 4: A, B, C, D.
Figure 4:
Thermal mesuarement of Emax ZirCAD (A) CO2 5W (B) CO2 10W (C) Nd:YAP Graphite (D) Nd:YAP
Discussion
Dental ceramics are required to have long-term durability and high physical properties in the oral cavity, but these ceramics exhibit inherent flaws on their surface. All systems proposed for ceramic surface conditioning demonstrated several limits. To overcome these limits, laser was proposed with the objective to improve mechanical properties of CAD/CAM ceramics.
Energy Dispersive Spectroscopy instanced that the laser treatment does not modify the chemical composition leading to the formation of a new chemical compound but it showed a change in the atomic mass percentage of the components. In EmaxCAD ceramics, Silicon dioxide mass percentage increased in the ceramics CO2 5W and 10 W irradiated.
In Emax Zircad ceramics, all samples showed a significant decrease of carbon mass percentage and a significant increase of Zirconium mass percentage.
SEM observations showed that CO2 laser with 5W could roughen ceramic surfaces of EmaxCAD and EmaxZircad; furthermore, at the structure of the ceramic samples irradiated with CO2 10W laser observation, formation of micro cracks and melting textures related to the thermal effect of the laser irradiation was seen.
In contrast, the structure of the ceramics irradiated with Nd:YAP demonstrated multiple holes, micro-cracks and melting grains. This is likely due to the deposition of high quantities of radiation energy in a well-defined portion of the ceramic surface over a short period of time and it leads to a very high energy density accumulation: so, the radiation energy is thermalized and the temperature heats a thin superficial layer 15–17).
Micro-cracks formation on ceramics after CO2 and Nd:YAP laser irradiations could be explained due to the high thermal effects of laser irradiation. This may cause extreme physical stress in the re-hardening ceramic surface 18,19).
During experimental procedure, temperature was measured and results showed that temperature increased more in Emax Zircad than EmaxCAD. This is probably due to the high absorption of laser irradiation in Emax Zircad. Nd:YAP laser in pulsed mode showed more temperature elevation than CO2 laser in continuous mode and not surprisingly temperature increased when output power increased. The comparison between Nd:YAP Graphite group or without graphite for both type of ceramics (EmaxCAD and Emax Zircad) showed that graphite could decrease thermal elevation with the process of vaporization when ceramic surfaces were black coloured.
The sintering temperature of EmaxCAD is 840 °C (1544 °F) and Emax Zircad is 1500 °C (2732 °F). Average temperature increase during laser irradiation of Emax Zircad with Nd:YAP laser (14185W/cm2) on non-black stained surface reached 87°C that represents 15% of the temperature reached during the sintering process and approximately 10% with EmaxCAD. Moreover, average temperature increase with CO2 irradiation 5W (1592.3 W/cm2) was identical for both EmaxCAD and Emax Zircad and represents approximately 5% of the temperature during sintering process.
If we consider that irradiation with CO2 5W is able to create smooth and shallow depressions for both types of ceramics without cracks formation and multiple holes -as observed with CO2 10W- and low temperature increase, it seems acceptable to use this wavelength and this power density for future investigations.
Conclusion
Within the limits of the present study, the most significant finding of this experimental study is that Carbon Dioxide and Neodymium Yttrium Aluminum Perovskite lasers may create surface modifications of the CAD/CAM ceramic indicating the possibility of improving mechanical bonding properties without changing its chemical compositions.
On the basis of the experimental observations (SEM, EDS and thermocouple), the proper treatment seems to be made by CO2 5 W.
Further studies should be performed to study the mechanical properties of irradiated ceramic surface (Micro-hardness, roughness) and the adhesion characteristics after ceramic sealing (Wettability, shear bond strength and micro leakage).
Acknowledgements
Authors thank Mines ParisTech and, in particular, Suzanne Jiacomet for the technical assistance during the EDS observations.
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