Bond strength enhancement of zirconia-porcelain interfaces via Nd:YAG laser surface structuring

Author’s Accepted Manuscript Bond strength enhancement of zirconia-porcelain interfaces via Nd:YAG laser surface structuring Bruno Henriques, Douglas Fabris, Júlio C.M. Souza, Filipe S. Silva, Óscar Carvalho, Márcio C. Fredel, Joana Mesquita-Guimarães www.elsevier.com/locate/jmbbm

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S1751-6161(18)30232-7 https://doi.org/10.1016/j.jmbbm.2018.02.031 JMBBM2708

To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 6 December 2017 Revised date: 22 February 2018 Accepted date: 26 February 2018 Cite this article as: Bruno Henriques, Douglas Fabris, Júlio C.M. Souza, Filipe S. Silva, Óscar Carvalho, Márcio C. Fredel and Joana Mesquita-Guimarães, Bond strength enhancement of zirconia-porcelain interfaces via Nd:YAG laser surface structuring, Journal of the Mechanical Behavior of Biomedical Materials, https://doi.org/10.1016/j.jmbbm.2018.02.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Bond strength enhancement of zirconia-porcelain interfaces via Nd:YAG laser surface structuring Bruno Henriquesa,b,c*,Douglas Fabrisa, Júlio C.M. Souzab,c, Filipe S. Silvab, Óscar Carvalhob, Márcio C. Fredela, Joana Mesquita-Guimarãesb a

Ceramic and Composite Materials Research Group (CERMAT), Federal University of Santa Catarina

(UFSC), Campus Trindade, Florianópolis/SC, Brazil b

CMEMS-UMinho, University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal

c

School of Dentistry (DODT), Postgraduate Program in Dentistry (PPGO), Federal University of Santa Catarina, Campus Trindade, 88040-900, Florianópolis/SC, Brazil *Corresponding author at: Tel.: +351 253 510220; fax: +351 253 516007. [email protected] (B. Henriques)

Abstract Objectives: The aim of this study was to evaluate the effect of laser surface structuring on the bond strength of feldspar-based porcelain to zirconia, as compared to conventional sandblasting treatment. Materials and methods: Thirty cylindrical zirconia substrates, previously sintered, were divided in three groups according to the type of surface conditioning: 1) sandblasting with 50µm Al2O3; 2) laser structuring (Ø25 µm holes); and 3) laser structuring (Ø50 µm holes). Porcelain was injected onto the zirconia substrates. X-ray diffractometry (XRD) was used to evaluate the influence of the laser treatment on zirconia crystallographic phases. Shear bond strength test was performed. Micrographs using SEM were used to evaluate the zirconia surface after each surface treatment and to evaluate the fracture surface after the shear test.

Results: The laser-structured groups presented the highest shear bond strength (65±16 MPa and 65±11 MPa, for the 25 µm and 50 µm holes, respectively). The sandblasting samples presented shear bond strength of 37±16 MPa. XRD analysis showed that there was no phase transformation on the thermally affected surface due to laser action. Microcracks were created at some holes due to the high temperature gradient generated by laser. Significance: Laser structuring significantly increased (up to 75%) the shear bond strength of zirconia to veneering porcelain as compared to conventional sandblasting treatment. Therefore, laser structuring arises as a surface conditioning method for producing stronger and long lasting zirconia-porcelain interfaces.

Keywords: zirconia, porcelain, laser structuring, shear bond strength

1.

Introduction

Unalloyed zirconia can assume three crystallographic forms depending on the temperature. At ambient pressure, from ambient temperature to 1173°C, its form is monoclinic (m). The structure is tetragonal (t) between 1173°C and 2370°C, and cubic (c) above 2370°C up to the melting point. These transformations are reversible. Upon cooling, the transformation from the tetragonal to the monolithic phase causes a substantial increase in volume (around 4-5%), resulting in compressive stresses sufficient to lead to catastrophic failure (Denry and Kelly, 2008). Oxides such as calcia (CaO), magnesia (MgO), ceria (CeO2) and yttria (Y2O3) are used to stabilise the tetragonal phase at room temperature and control the t → m transformation, resulting in a metastable phase that presents good crack propagation resistance and, therefore, high

toughness. The addition of 2-3mol% of Y2O3 fully stabilise the tetragonal phase, resulting is small 100% metastable tetragonal grains (Y-TZP). Yttria stabilised zirconia polycrystal (Y-TZP) has been widely used in biomedical applications, such as framework in all-ceramic dental restorations, due to its biocompatibility, chemical stability, excellent flexural strength (up to 900-1200 MPa) and high fracture toughness (7-10 MPa/m1/2) (Piconi and Maccauro, 1999) Y-TZP presents better aesthetic compared with the traditional PFM (porcelain-fused-tometal) restorations, and it has been an increasing interest in the development of new techniques to enhance its clinical performance. The strong zirconia core is veneered with a feldspar-porcelain layer, which mimics the colour of natural teeth. Studies have shown that zirconia frameworks present high stability (Sailer et al., 2007; Von Steyern et al., 2005). However, the chipping of the veneering ceramic is the main failure mode of zirconia restorations, with cracks growing parallel to the interface (Pang et al., 2015; Zhang et al., 2013), resulting in high failure rates in long term clinical use (Sax et al., 2011). The failure rate of all-ceramic restorations has been reported as higher than in PFM restorations (Guess et al., 2008). The high fracture rate of all-ceramic restorations is due to the poor bonding between the zirconia framework and the veneering porcelain, which causes the delamination and chipping of the porcelain layer. Several factors can influence the bonding strength between framework and veneer, such as residual stresses due to coefficient of thermal expansion mismatch between layers, poor wetting of the core by the veneering ceramic, microstructural and interceramics defects, and lack of proper framework support (Aboushelib et al., 2006; De Jager et al., 2005; Dündar et al., 2007; Santos et al., 2016). Therefore, the proper control of such factor can increase the bonding strength and decrease the failure rate of all-ceramic restorations.

Several studies on zirconia surface treatment have been carried out to improve between the zirconia framework and the veneering porcelain. Studies have shown that surface treatments such as sandblasting and application of a liner can increase the bonding strength, although such findings have not been supported by all published reports (Aboushelib et al., 2006; Kim et al., 2011; Liu et al., 2013). The introduction of a graded glass-zirconia structure or even a homogeneous intermediate layer with intermediate composition can improve significantly the adhesion between zirconia and porcelain (Liu et al., 2015; Santos et al., 2016). CNC-micromachined zirconia surfaces have also shown increased the bond strength to porcelain (Santos et al., 2016). The use of laser technology on zirconia surface treatment has been of interest for different purposes. Laser has been used to increase the surface roughness of zirconia and increase its shear bond strength when bonded with porcelain (Liu et al., 2013; Ural et al., 2010). Laser also has been used to improve wettability characteristics of zirconia surface, changing the way biological fluids interacts with the material, improving the bone-implant interface (Hao and Lawrence, 2006). Micromachining using laser in zirconia also has been studied (Li et al., 2016). Despite its benefits comparing with others surface treatments, the heat generated by the laser has some undesirable sideeffects on the ceramic surface, such as micro-cracks, spatter and heat-affected zones (Stübinger et al., 2008). Laser could also destabilise the metastable tetragonal phase in the Y-TZP, transforming it to monoclinic. Thus, it is important its correct parameterization to avoid such problems. The aim of this study is investigate the effect of laser surface structuring on the shear bond strength between laser structured zirconia surfaces and the veneering porcelain. Two different structuring parameters were tested and their results were compared with the conventional sandblasting technique, which is one of the most used surface

treatments for zirconia-based dental restorations. The null hypothesis of this study was that laser structuring would not significantly improve the zirconia-porcelain bond strength.

2.

Materials and methods

2.1.

Materials

Zirconia powder stabilized with 3mol% yttria TZ-3YBE (Tosoh, Japan) with high purity (99%) and density of 6.05 g/cm3 was used. This powder is spray dried with a range agglomerate size of 20 to 120µm and an average size of 60 µm. For the veneering layer, feldspar-based porcelain (VITA VM9, Bad Säckingen, Germany) was used.

2.2.Specimens preparation The zirconia substrates were prepared via press and sintering (N=30). The samples were pressed in a mold of 10 mm diameter and the applied pressure was 100 MPa. Afterwards the samples were sintered at 1500 ºC for 2h with a heating rate of 5 ºC/min. The final dimensions of the samples, after sintering, were 8 mm diameter and 5 mm height. The samples were divided in three different groups according to the type of surface treatment (n=10): the first one is the control group (here after referred as CG), whose samples undergone a conventional sandblasting with 50 µm Al2O3 particles. The other two groups of zirconia substrates were laser structured using a Nd:YAG laser (OEM Plus, Italy) with the following characteristics: 6W of power, 1064 nm of wavelength, pulse duration of 36 ns, frequency of 20kHz and maximum pulse energy of

0.3 mJ. Equally spaced microholes with Ø25 µm (hereafter referred as L25) and Ø50 µm (hereafter referred as L50) were laser machined on the zirconia surface resulting in different hole density: 300 holes/mm2 and 100 holes/mm2 for L25 and L50, respectively. After laser structuring, the laser irradiated zirconia surface got a dark color as a small layer of materials loses oxygen due to high temperatures and fast cooling rates. Because this change in color is aesthetically undesired, a thermal treatment at 900°C during 30 minutes was applied to the laser treated zirconia substrates for oxygen recovering and restoring the white color. The veneering porcelain was applied over the zirconia substrates by the injection technique, using a Press furnace (Programmat EP 5010, Ivoclar Vivadent), as depicted in Figure 1. Porcelain ingots, compatible to the investment ring dimensions, were produced by press and sintering the porcelain powders. The firing cycle used in this study followed the manufacturer instructions. The firing temperature was 1000°C during 15 minute of holding time. The heating rate used was 50°C/min after reaching the initial temperature of 700°C. Pressing time was 5 minutes at 1000°C. The investment ring was removed from the furnace immediately after the end of the pressing program and placed on a grid to cool down to room temperature.

2.3. Analysis of zirconia surface and zirconia-porcelain interface X-ray diffractometry examinations were carried out in the laser ablated zirconia substrates to evaluate the influence of laser surface conditioning on the zirconia crystallographic phases. The crystalline phases analysis was carried out in an X-ray diffractometer (Philips X’Pert, PANalytical, Netherlands) before and after the thermal treatment of the laser structured zirconia substrates at 900 ºC. The radiation source was Kα line of copper (λ=0.15141 nm), with 40 kV and 30 mA. The scan was done

continuously in 2-theta setting from 0 to 80º with step size of 0.02º and a step time of 2 s. For the identification of the crystalline phases the software (X’pert high score plus, PANalytical, Netherlands) was used. The zirconia substrates were analysed both prior and after the porcelain injection by scanning electron microscopy (HITACHI TM3030, Krefeld, Germany) in order to evaluate the size and distribution of holes and the infiltration of porcelain in the holes. Samples were gold sputtered for SEM inspection. After the shear test, the samples were randomly selected and their fracture surface were analysed using the SEM to determine the failure mode for each surface treatment. Cross sectioned samples were also analysed. For that, samples were embedded in a self-curing resin, wet-ground using grit SiC sand-papers down to 4000 Mesh and polished with diamond paste (1 μm).

2.2.

Shear bond strength test

The shear bond strength tests were carried out at room temperature and performed in a universal testing machine (Instron 8874, MA, USA), with a load cell of 25 kN capacity and under a crosshead speed of 0.5 mm/s. Tests were performed in a custom-made stainless steel apparatus similar to that described by (Henriques et al., 2011) The apparatus consisted in two sliding parts A and B, each one with a hole perfectly aligned to the other. After aligning the holes, the specimens were there inserted and loaded in the interface until fracture. The shear bond strength (MPa) was calculated dividing the highest recorded fracture force (N) by the cross sectional area of the bonded porcelain (mm2).

2.3.

Statistical analysis

The results were analyzed using one-way ANOVA followed by Tukey HSD multiple comparison test. The Shapiro-Wilk test was first applied to test the assumption of normality. Differences between surface treatment and presence of interlayer in terms of shear bond strength were tested using t-test. P values lower than 0.05 were considered statistically significant.

3. Results 3.1. Morphology and microstructure characterization Figure 2 shows the zirconia surface after the laser structuring with 25 µm (a) and 50 µm (b) diameter holes. It is visible a high quality on the laser patterning. The hollows are reproducible and present always the same distance. As can be seen in detail, some holes presented small cracks resulted of residual stresses due to the laser heating. In order to investigate if the Nd:YAG laser patterning has destabilized the tetragonal phase of the zirconia substrate, X-ray diffraction was performed. In Figure 3 is shown the diffractograms of the zirconia surfaces after laser structuring and after the heat treatment. Analyzing the diffractograms it can be seen that the laser structuring did not promote any destabilization of the tetragonal phase. Both, before and after the heat treatment, the diffractograms are virtually identical, i.e., their crystallographic phases are the same. The peaks show that only the tetragonal phase is present on zirconia.

On Figure 4, it is visible that the porcelain has been adequately injected on the sandblasted (a) and on the laser structured surface (b and c). It can be seen the surface

roughness profile of the sandblasted sample and good adhesion between the two materials. Comparing the laser structured samples, it can be seen that L25 samples (Fig. 3(b)) have holes with approximately 25 µm of diameter and 50 µm deep, while the L50 samples presents potholes with approximately 50 µm of diameter and 100 µm deep. Besides that, the analysis revealed that L25 samples have potholes closer to each other, while L50 specimens presented deeper and more separated potholes. It can be seen the microcracks on the detail created by the laser surface treatment on both samples. Residual pores can be noted at the porcelain region. 3.2. Shear bond strength analysis The shear bond strength recorded for the different interface designs are shown in Figure 5. The sandblasted samples displayed the lowest resistance to shear stress (37±16 MPa). The laser patterning technique showed a significant increase in the bond strength of ~75% relative to the sandblasted samples. The resistance recorded for both L25 and L50 specimens were virtually identical (65±16 MPa and 65±11 MPa, respectively), although the latter showed less deviation in the recorded bond strengths.

The failure type of the specimens can be classified as cohesive, adhesive or mixed. In this study, only mixed failure was registered. The sandblasted sample (Figure 6(a)) showed porcelain remnants on zirconia asperities, besides the region where porcelain cohesive failure has occurred. The laser structured samples (Figures 6(b) and 6(c)) presented a region with porcelain cohesive failure and the region with adhesive failure, where the porcelain stayed retained at the holes. The L50 samples (Figure 6(c))

presented more retained porcelain due to its bigger holes e smaller untreated zirconia surface.

3.3. Discussion This study evaluated the influence of laser patterning on zirconia surface on the shear bond strength between veneering feldspathic porcelain and zirconia. Laser patterning was compared with the conventional sandblasting technique. Crystallographic changes due to laser heating also have been investigated. Sandblasting is still the most used zirconia surface treatment to increase the adhesion with feldspathic porcelain. However, its benefits are controversial. Although some studies show that sandblasting can increase the shear bond strength of zirconia-based restorations (Liu et al., 2013; Nakamura et al., 2009), others say that sandblasting does not influence the bonding between zirconia and porcelain (Fischer et al., 2008). It can even decrease long-term reliability of restorations due to flaws induced by the sandblasting. Thus, alternative surface treatments are being studied to increase the shear bond strength of restorations and, therefore, their life-time. Zirconia surface treatment with laser patterning resulted in a significant increase (>75%) in shear bond strength compared with the traditional treatment (sandblasting). There was virtually no difference in the shear bond strength for the different sizes of holes used on this study. The holes create a mechanical interlocking effect that increases the adhesion between the porcelain and zirconia. This fact can be proved by Figures 5(b) and 5(c), which show remaining porcelain at the holes after the shear test. A study

in the literature shows that CNC-drilled holes present similar effect; however its results are not as good as laser patterning (Santos et al., 2016). Other methods can be used combined with laser patterning. B. Henriques et al. (Henriques et al., 2012, 2011), showed that bond strength can increase up to ~140% using an intermediate composite layer in metal/ceramic restorations between the metallic substrate and the veneering porcelain. Metallic particles of the composite interlayer are bonded to the metallic substrate during the porcelain sintering, promoting additional mechanical interlocking between the substrate and the interlayer. Combining the effect of the mechanical interlocking by machined holes and the presence of a composite interlayer can produce an optimized design that can further improve the shear bond strength of dental restorations (Santos et al., 2016). Creating a functionally graded interlayer between the porcelain and substrate can also strengthen the core-veneer bond in dental restorations (Liu et al., 2015). The existence of defects at the zirconia surface produced by roughening treatments (sandblasting, grinding and laser structuring) is generally regarded for a decrease in the flexural strength of zirconia parts, especially under fatigue conditions (Fonseca et al., 2014; Guess et al., 2010; Hjerppe et al., 2016; Zhang et al., 2006, 2004). The laser structuring used in the present study produced surface defects (holes with 25 µm and 50 µm of diameter having 50 µm and 100 µm depth, respectively), which may significantly reduce the fracture strength of the zirconia structures. However, two factors that may partially mitigate the loss in strength must be considered: the infiltration of the holes by the veneering porcelain, sealing the small cracks; and whether these holes are localized in surfaces under mostly compressive stresses, who tend to close cracks. Nevertheless, further investigation should be carried out to find out what is the actual influence of laser structuring on the flexural strength of

bilayered porcelain/zirconia dental ceramics, in particular as regard to the fatigue tolerance properties and aging resistance. Laser ablation creates a great temperature gradient at the samples, resulting in high residual stresses. Microcracks were created due to this fact at some holes. The high heat generated by the Nd:YAG laser beam facilitates localized melting, but also lead to material cracking (Stübinger et al., 2008). A high-quality machining without thermally induced cracks can be achieved a femtosecond laser source, however its removal rate is too low for practical applications (Li et al., 2016). The laser structuring/machining of green ceramics (unfired) has been explored aiming at avoiding the formation and propagation of micro-cracks during laser processing of sintered ceramics, boosted by their inherent hardness and brittleness, as well as their poor thermal shock resistance (Liu et al., 2017; Nowak et al., 2006; Samant and Dahotre, 2010). Further studies should be carried out to evaluate the influence of these cracks on long-term use. The evaluation of the bond strength behavior after thermo- and/or mechanical cycling tests should also be considered in order to better estimate the performance of these joints under aging conditions similar to those found in the intra-oral environment (Fabris et al., 2017; Gale and Darvell, 1999; Morresi et al., 2014). The absence of such aging tests arises as a limitation of the present study that should be considered when analyzing the results here reported and obtained under static testing conditions. XRD analysis showed that laser patterning did not induce any crystallographic change in the zirconia structure. This result agrees with another study which used laser irradiation to change the zirconia surface and increase its roughness (Liu et al., 2013). Despite the fact that the sandblasted specimens were not analysed by XRD, studies show that sandblasting can induce a t→m transformation, leading to the formation of a compressive layer on zirconia surface, which can increase its flexural strength

(Guazzato et al., 2005; Karakoca and Yilmaz, 2009). Nevertheless, the existence of such layer might impair the long-term reliability and stability of zirconia-based restorations (Liu et al., 2013). The flaws induced by sandblasting can cause strength reduction in long-term use, outweighing any strengthening effect from phase transformation or compressive stresses from this surface treatment (Zhang et al., 2004). Moreover, the different factors acting during the veneering process (e.g. presence of water, effect of temperature and diffusion phenomena, and the development of tensile stress in the framework during cooling) may also impart modifications to zirconia surface that can ultimately lead to phase destabilization with consequences on the stress state of the zirconia surface and in the veneering porcelain (Mainjot et al., 2013). 4. Conclusions The following conclusions can be drawn from this study: 

Zirconia surfaces were successfully structured by laser ablation. Micro-holes with good geometrical quality and equally spaced were this produced. The holes were adequately injected and filled by the dental porcelain.



The laser structuring of the zirconia surfaces enhanced the shear bond strength between zirconia and injected feldspathic porcelain up to 75%, as compared to conventional sandblasting surface conditioning.



Although there was no phase transformation due to the laser heating, substantial residual stresses were generated, resulting in cracks at some holes.

Acknowledgements This study was supported by FCT-Portugal (UID/EEA/04436/2013, NORTE-01-0145FEDER-000018 - HAMaBICo), CNPq-Brazil (PVE/CAPES/CNPq/407035/2013-3).

The authors thank to Dr. Adomiro Pacher Filho, owner of the ATM Prótese Dental at Florianópolis/SC, Brazil for the technical support given in the porcelain injection process.

References Aboushelib, M.N., Kleverlaan, C.J., Feilzer, A.J., 2006. Microtensile bond strength of different components of core veneered all-ceramic restorations. Part II: Zirconia veneering ceramics. Dent. Mater. 22, 857–863. doi:10.1016/j.dental.2005.11.014 De Jager, N., Pallav, P., Feilzer, A.J., 2005. The influence of design parameters on the FEA-determined stress distribution in CAD–CAM produced all-ceramic dental crowns. Dent. Mater. 21, 242–251. doi:10.1016/j.dental.2004.03.013 Denry, I., Kelly, J.R., 2008. State of the art of zirconia for dental applications. Dent. Mater. 24, 299–307. doi:10.1016/j.dental.2007.05.007 Dündar, M., Ozcan, M., Gökçe, B., Cömlekoğlu, E., Leite, F., Valandro, L.F., 2007. Comparison of two bond strength testing methodologies for bilayered all-ceramics. Dent. Mater. 23, 630–6. doi:10.1016/j.dental.2006.05.004 Fabris, D., Souza, J.C.M., Silva, F.S., Fredel, M., Gasik, M., Henriques, B., 2017. Influence of specimens’ geometry and materials on the thermal stresses in dental restorative materials during thermal cycling. J. Dent. doi:10.1016/j.jdent.2017.08.017 Fischer, J., Grohmann, P., Stawarczyk, B., 2008. Effect of zirconia surface treatments on the shear strength of zirconia/veneering ceramic composites. Dent. Mater. J. 27, 448–454. doi:10.4012/dmj.27.448

Fonseca, R.G., Abi-Rached, F.D.O., da Silva, F.S.C.P., Henriques, B.A.P. De, Pinelli, L.A.P., 2014. Effect of surface and heat treatments on the biaxial flexural strength and phase transformation of a Y-TZP ceramic. J. Adhes. Dent. 16, 451–8. doi:10.3290/j.jad.a32663 Gale, M.S., Darvell, B.W., 1999. Thermal cycling procedures for laboratory testing of dental restorations. J. Dent. doi:10.1016/S0300-5712(98)00037-2 Guazzato, M., Quach, L., Albakry, M., Swain, M. V., 2005. Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic. J. Dent. 33, 9– 18. doi:10.1016/j.jdent.2004.07.001 Guess, P.C., Kuliš, A., Witkowski, S., Wolkewitz, M., Zhang, Y., Strub, J.R., 2008. Shear bond strengths between different zirconia cores and veneering ceramics and their susceptibility to thermocycling. Dent. Mater. 24, 1556–1567. doi:10.1016/j.dental.2008.03.028 Guess, P.C., Zhang, Y., Kim, J.W., Rekow, E.D., Thompson, V.P., 2010. Damage and reliability of Y-TZP after cementation surface treatment. J. Dent. Res. 89, 592– 596. doi:10.1177/0022034510363253 Hao, L., Lawrence, J., 2006. Effects of Nd:YAG laser treatment on the wettability characteristics of a zirconia-based bioceramic. Opt. Lasers Eng. 44, 803–814. doi:10.1016/j.optlaseng.2005.08.001 Henriques, B., Gasik, M., Soares, D., Silva, F.S., 2012. Experimental evaluation of the bond strength between a CoCrMo dental alloy and porcelain through a composite metal-ceramic graded transition interlayer. J. Mech. Behav. Biomed. Mater. 13, 206–14. doi:10.1016/j.jmbbm.2012.04.019 Henriques, B., Soares, D., Silva, F.S., 2011. Optimization of bond strength between gold alloy and porcelain through a composite interlayer obtained by powder

metallurgy. Mater. Sci. Eng. A 528, 1415–1420. doi:10.1016/j.msea.2010.10.054 Hjerppe, J., Närhi, T.O., Vallittu, P.K., Lassila, L.V.J., 2016. Surface roughness and the flexural and bend strength of zirconia after different surface treatments. J. Prosthet. Dent. 116, 577–583. doi:10.1016/j.prosdent.2016.02.018 Karakoca, S., Yilmaz, H., 2009. Influence of surface treatments on surface roughness, phase transformation, and biaxial flexural strength of Y-TZP ceramics. J. Biomed. Mater. Res. - Part B Appl. Biomater. 91, 930–937. doi:10.1002/jbm.b.31477 Kim, H.-J., Lim, H.-P., Park, Y.-J., Vang, M.-S., 2011. Effect of zirconia surface treatments on the shear bond strength of veneering ceramic. J. Prosthet. Dent. 105, 315–22. doi:10.1016/S0022-3913(11)60060-7 Li, J., Ji, L., Hu, Y., Bao, Y., 2016. Precise micromachining of yttria-tetragonal zirconia polycrystal ceramic using 532 nm nanosecond laser. Ceram. Int. 42, 4377–4385. doi:10.1016/j.ceramint.2015.11.118 Liu, D., Matinlinna, J.P., Tsoi, J.K.-H., Pow, E.H.N., Miyazaki, T., Shibata, Y., Kan, C.-W., 2013. A new modified laser pretreatment for porcelain zirconia bonding. Dent. Mater. 29, 559–565. doi:10.1016/j.dental.2013.03.002 Liu, R., Sun, T., Zhang, Y., Zhang, Y., Jiang, D., Shao, L., 2015. The effect of graded glass-zirconia structure on the bond between core and veneer in layered zirconia restorations. J. Mech. Behav. Biomed. Mater. 46, 197–204. doi:10.1016/j.jmbbm.2015.02.017 Liu, Y., Liu, L., Deng, J., Meng, R., Zou, X., Wu, F., 2017. Fabrication of micro-scale textured grooves on green ZrO 2 ceramics by pulsed laser ablation. Ceram. Int. 43, 6519–6531. doi:10.1016/j.ceramint.2017.02.074 Mainjot, A.K., Douillard, T., Gremillard, L., Sadoun, M.J., Chevalier, J., 2013. 3DCharacterization of the veneer-zirconia interface using FIB nano-tomography, in:

Dental Materials. pp. 157–165. doi:10.1016/j.dental.2012.11.010 Morresi, A.L., D’Amario, M., Capogreco, M., Gatto, R., Marzo, G., D’Arcangelo, C., Monaco, A., 2014. Thermal cycling for restorative materials: Does a standardized protocol exist in laboratory testing? A literature review. J. Mech. Behav. Biomed. Mater. doi:10.1016/j.jmbbm.2013.09.013 Nakamura, T., Wakabayashi, K., Zaima, C., Nishida, H., Kinuta, S., Yatani, H., 2009. Tensile bond strength between tooth-colored porcelain and sandblasted zirconia framework. J. Prosthodont. Res. 53, 116–119. doi:10.1016/j.jpor.2009.02.007 Nowak, K.M., Baker, H.J., Hall, D.R., 2006. Cold processing of green state LTCC with a CO2 laser. Appl. Phys. A 84, 267–270. doi:10.1007/s00339-006-3612-2 Pang, Z., Chughtai, A., Sailer, I., Zhang, Y., 2015. A fractographic study of clinically retrieved zirconia–ceramic and metal–ceramic fixed dental prostheses. Dent. Mater. 31, 1198–1206. doi:10.1016/j.dental.2015.07.003 Piconi, C., Maccauro, G., 1999. Zirconia as a ceramic biomaterial. Biomaterials. doi:10.1016/S0142-9612(98)00010-6 Sailer, I., Fehér, A., Filser, F., Gauckler, L.J., Lüthy, H., Hämmerle, C.H.F., 2007. Fiveyear clinical results of zirconia frameworks for posterior fixed partial dentures. Int. J. Prosthodont. 20, 383–8. Samant, A.N., Dahotre, N.B., 2010. Three-dimensional laser machining of structural ceramics. J. Manuf. Process. 12, 1–7. doi:10.1016/j.jmapro.2010.01.001 Santos, R.L.P., Silva, F.S., Nascimento, R.M., Souza, J.C.M., Motta, F. V., Carvalho, O., Henriques, B., 2016. Shear bond strength of veneering porcelain to zirconia: Effect of surface treatment by CNC-milling and composite layer deposition on zirconia. J. Mech. Behav. Biomed. Mater. 60, 547–556. doi:10.1016/j.jmbbm.2016.03.015

Sax, C., Hammerle, C.H., Sailer, I., 2011. 10-Year Clinical Outcomes of Fixed Dental Prostheses With Zirconia Frameworks. Int J Comput Dent 14, 183–202. Stübinger, S., Homann, F., Etter, C., Miskiewicz, M., Wieland, M., Sader, R., 2008. Effect of Er:YAG, CO2 and diode laser irradiation on surface properties of zirconia endosseous dental implants. Lasers Surg. Med. 40, 223–228. doi:10.1002/lsm.20614 Ural, Ç., Külünk, T., Külünk, Ş., Kurt, M., 2010. The Effect of Laser Treatment on Bonding Between Zirconia Ceramic Surface and Resin Cement. Acta Odontol. Scand. 68, 354–359. doi:10.3109/00016357.2010.514720 Von Steyern, P.V., Carlson, P., Nilner, K., 2005. All-ceramic fixed partial dentures designed according to the DC-ZirkonR technique. A 2-year clinical study. J. Oral Rehabil. 32, 180–187. doi:10.1111/j.1365-2842.2004.01437.x Zhang, Y., Lawn, B.R., Malament, K.A., Van Thompson, P., Rekow, E.D., 2006. Damage accumulation and fatigue life of particle-abraded ceramics. Int. J. Prosthodont. 19, 442–448. Zhang, Y., Lawn, B.R., Rekow, E.D., Thompson, V.P., 2004. Effect of sandblasting on the long-term performance of dental ceramics. J. Biomed. Mater. Res. - Part B Appl. Biomater. 71, 381–386. doi:10.1002/jbm.b.30097 Zhang, Z., Guazzato, M., Sornsuwan, T., Scherrer, S.S., Rungsiyakull, C., Li, W., Swain, M. V., Li, Q., 2013. Thermally induced fracture for core-veneered dental ceramic structures. Acta Biomater. 9, 8394–8402. doi:10.1016/j.actbio.2013.05.009

Figure 1 – Porcelain injection process used in the production of the zirconia-porcelain cylindrical specimens.

Figure 2 – Zirconia laser structured surfaces of L25 and L50 specimens, with holes with 25 µm (a) and 50 µm (b), respectively.

Figure 3 – Diffractograms of the zirconia surfaces after laser structuring process and after the thermal treatment (tt).

Figure 4 – Sandblasted (a) and laser structured samples with 25 µm (b) and 50 µm (c) holes after the porcelain injection.

Figure 5 – Shear bond strength of the sandblasted and laser structures samples with 25 µm (L25) and 50 µm holes (L50). Asterisks indicate statistically significant differences (p<0.05).

Figure 6 – Fracture surface after shear test of the sandblasted (a) and laser structured L25 and L50 samples, with 25 µm (b) and 50 µm holes (c) respectively.