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Application of zirconia surface coating to improve fracture resistance and stress distribution of zirconia ceramic restorations
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Application of zirconia surface coating to improve fracture resistance and stress distribution of zirconia ceramic restorations Firas Abdulameer Farhana,b, Eshamsul Sulaimanb, Muralithran G. Kuttyb, a b
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Prosthodontic Department, College of Dentistry, Baghdad University, Bab Al-Muadham Campus of the University of Baghdad, 1417 Baghdad, Iraq Restorative Department, Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Zirconia surface coating Zirconia ceramic restoration Fracture resistance Fractographic analysis, Finite element analysis
Zirconia ceramic restoration (ZCR) has a higher fracture incidence rate than metal ceramic restoration. Different surface treatments were used to improve fracture performance of ZCR such as grit blasting (GB) by aluminium oxide powder. This type of surface treatment generate residual stresses on veneering ceramic causing crack initiation and ending with a fracture. In order to overcome the stress generated by GB, zirconia surface coating is used as a surface treatment to improve fracture resistance and to accommodate stresses along the ZCR layers. Fifty zirconia ceramic crowns were fabricated and divided according to the type of surface treatment into three groups; the first group is (ZG), involving 20 cores were coated with a mixture of partially-sintered zirconia powder (PZP) and glaze ceramic powder; the second group is (ZL), including of 20 cores were coated with PZP and liner ceramic paste. The third group is grit blasting (GB), preparing of 10 fully sintered cores at 1350 °C which then abraded by 50 µm aluminium oxide powder. The groups ZG and ZL were further subdivided into ZG26, ZG47, ZL26 and ZL47 based on two PZP sizes (47 and 26 µm). Each treated core was veneered with the veneering ceramic layer. Fracture resistance (FR) was measured by the universal testing machine. Finite element analysis (FEA) was used to simulate the stress distributions on the coated and non-coated zirconia crown models. The ZG47 group had higher FR (647.92 ± 97.33 N) and a significant difference (P < 0.00) compared to GB and other coated groups. The FEA exhibited lower and evenly distributed stresses of the zirconia glaze model than the zirconia liner and the non-coated models. The ZG47 coating considered as an alternative method to GB treatment which increases the FR which significantly improved the clinical performance of the ZCR.
1. Introduction Clinical follow-up studies of zirconia-ceramic restoration (ZCR) showed that it was an appropriate substitute to the metal-ceramic restorations because of its excellent aesthetics, high strength, and superior biocompatibility [1,2]. Furthermore, the introduction of computer aided design/computer aided manufacturing (CAD/CAM) technologies in dentistry had facilitated the design and fabrication of ZCRs with minimal flaws and short time [3]. In general, the flexural strength of the zirconia core fabricated by CAD/CAM is higher than core produced by the conventional method such slip casting [4] ZCR with veneering layer considered as a favoured choice in restorative dentistry than monolithic zirconia restoration. Because it has appearance matching the colour shade of the natural tooth while monolithic restoration lacks this property and it is too tough, which may wear the opposing tooth [5]. Unfortunately, the ZCR had met with some technical complication
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which was related to the weak bonding between zirconia and veneering ceramic at the interface area as a result of insufficient mechanical interlocking which was enhanced by providing surface roughness [6]. Furthermore, the zirconia structural matrix lacks glassy content to provide a chemical bond with the veneering ceramic material. Therefore, this will increase the fracture incidence rate of ZCRs when compare with metal-ceramic restorations and recognised as the main cause of clinical failure [7]. In order to improve bonding properties of zirconia substrate, surface treatment was suggested, such as grit blasting (GB) by aluminium oxide (Al2O3) powder which is used to offer microroughness and enhancing micromechanical interlocking between zirconia and ceramic materials [8]. Nevertheless, this type of treatment may cause a phase transition of zirconia atomic matrix from tetragonal (t) to monoclinic (m). This transition accompanied with structural expansion which generates stresses at zirconia/ceramic interface causing microcracks and fracture susceptibility [9,10].
Corresponding author. E-mail addresses: fi[email protected] (F.A. Farhan), [email protected] (E. Sulaiman), [email protected] (M.G. Kutty).
https://doi.org/10.1016/j.ceramint.2018.08.246 Received 14 August 2018; Received in revised form 20 August 2018; Accepted 21 August 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Farhan, F.A., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.08.246
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Consequently, previous studies [11,12], proposed to use a thin film of zirconia coatings applied by airbrush spraying technique over the zirconia substrate surface to create surface roughness without phase transition. The coating composed of a micron-sized of zirconia powder with two particle sizes (26 and 47 µm) which were mixed separately with a glaze ceramic powder and liner ceramic paste. The finding of these studies showed a higher surface roughness and shear bond strength than GB treatment. According to this background, the current study aimed to evaluate the zirconia spray coating on fracture resistance to improve the clinical performance of ZCR. Moreover, the distribution of stresses on the layers of zirconia ceramic crown was examined under a loading condition similar to fracture resistance test, using three-dimensional finite element analysis (FEA). 2. Materials and methods 2.1. Preparation of zirconia coating powders Yttria-stabilized zirconia powder (SIGMA-ALDRICH Co., USA) was compressed by a hydraulic press to produce compacted tablets which were sintered at 1100 °C for 2 h to produce partially-sintered zirconia tablets. The tablets were grounded by a ball milling machine (XQM, Hunan, China) to produce a partially-sintered zirconia powder (PZP) with harder and denser properties. Two sets of powder were separated using a sieve shaker machine equipped with 50, 45, 40, 35, 30, and 25 µm meshes. According to the preliminary experiments, two PZP were collected between meshes 25–30 µm and 45–50 µm respectively. The average particle sizes were 26.0 ± 0.3 µm and 47.0 ± 0.5 µm, which agreed with the previous study achieved by the authors [12].
Fig. 2. Grit blasting procedure for all core surfaces.
Table 1 Surface treatments and experimental groups used in the study. Group
N
Type of surface treatment
ZG26
10
ZG47
10
ZL26
10
ZL47
10
GB (control)
10
Coated with a mixture of zirconia powder with a particle size 26 µm and glaze porcelain Coated with a mixture of zirconia powder with a particle size 47 µm and glaze porcelain Coated with a mixture of zirconia powder with a particle size 26 µm and liner ceramic Coated with a mixture of zirconia powder with a particle size 47 µm and liner ceramic Grit Blasting by 50 µm aluminium oxide powder
2.2. Tooth and master die preparation An artificial maxillary first premolar was positioned in a full mouth dental model (Nissin dental model, Kyoto, Japan) and prepared according to the clinical instructions proposed for ZCR. The prepared tooth was duplicated with a vinyl polysiloxane impression (Reprosil, DENTSPLY, USA) to create a mould that filled with molten wax to produce a wax pattern which invested and then casted with Co/Cr alloy (Remanium, Dentaurum, Germany) to fabricate five metal master dies (Fig. 1A). Each metal die was embedded in epoxy resin (Ultrathin resin, PACE Technologies, USA) by positioning 2 mm apical to the finishing line using cubic silicone mould 30 × 30 × 30 mm length, width and height respectively (Fig. 1B). An impression was made of each metal ceramic restoration metal die and poured with the type IV dental die stone material (Elite Rock, Zermack, Italy) to produce stone die (Fig. 1C).
2.3. Preparation of CAD/CAM cores Fifty zirconia cores were designed and fabricated by Cercon CAD/ CAM technology (Cercon-Smart Ceramic System, DeguDent GmbH, Germany). The stone dies were scanned by a laser scanner to generate computer data and defined the prepared tooth parameters including cementing gap, finishing line and correction of the surface line angles.
Fig. 1. (A) Wax pattern converted to the metal die. (B) Master metal die placed in an epoxy resin. (C) Stone die. 2
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Fig. 5. Fracture resistance test. Table 2 The material properties used in the FEA.
Fig. 3. Coating procedure for unsintered zirconia core.
The final step is designing the core data over the prepared tooth which includes configuring the individual anatomic shape and wall skeleton thickness. The core data was transferred to the milling machine which has a special metal holder (milling frame) to mount unsintered zirconia block (Cercon Base 47, DeguDent GmbH, Germany) near the rotary instrument to carve out and produce the final shape of zirconia core.
Material
Young's modulus (GPa)
Poisson's ratio
Reference
Veneer ceramic Zirconia core Dentin Enamel Resin cement Coating layer ZG Coating layer ZL
69 210 18.6 84.1 16 9.75 5.64
0.28 0.22 0.30 0.30 0.24 0.3 0.3
[18] [19] [20] [18] [20] ROM ROM
ROM: Rule of mixture. GPa: Gigapascal.
50 µm aluminium oxide (Al2O3) powder at 4 bar air pressure for 10 s with 10 mm distance of blasting nozzle to the core (Fig. 2). The classification of groups is summarised in Table 1. The coating procedure was achieved by mixing equal weight ratios of PZP and glaze powder or liner paste using a magnetic stirrer at 500 rpm for 15 min to produce a homogeneous slurry. The unsintered cores were coated with an airbrush mini spray gun (model 130-dual action airbrush kit, Taiwan) fixed vertically to the zirconia core. The coating parameters were designated according to the pilot experiments and agree with a previous study using the same parameters that produce a uniform thickness and efficient roughness [12]. These parameters include; air pressure at 2.5 bars, total spray time of 2 s, and a
2.4. Surface treatments and study grouping Unsintered zirconia cores were divided according to the type of surface treatments into three groups; the first group is (ZG), consisting of 20 unsintered zirconia core coated with a mixture of PZP and glaze powder. The second group is (ZL), containing 20 unsintered zirconia core coated with a mixture of PZP and liner paste. The ZG and ZL groups were subdivided according to the selected particle sizes of PZP (26 and 47 µm) into ZG26, ZG47, ZL26 and ZL47, for each one 10 cores. The third group is grit blasting (GB) as a control group, including 10 fully sintered zirconia cores at 1350 °C and subsequently blasted with
Fig. 4. (A) Silicone mould with a treated zirconia core used as a veneering build up reference. (B) Final zirconia ceramic crown cemented on a metal die. 3
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Fig. 6. Treated core with different surface treatments. (ZG26 and ZG47) cores coated with zirconia glaze, (ZL26 and ZL47) cores coated with zirconia liner and (GB) core treated with grit blasting.
Fig. 7. Cross-sectional SEM image of the coated zirconia ceramic crown with ZG47.
Germany) was mixed with the appropriate amount of a special ceramic liquid to produce a ceramic slurry which was added and condense to the silicone mould after lubricating with an isolating agent to prevent sticking of the ceramic slurry to the silicone mould to produce a final crown shape (Fig. 4B). The crown was sintered in a ceramic furnace (Programat EP 5000, Ivoclar Vivadent, Schaan/Liechtenstein) as instructed by the manufacturer.
distance of 13 cm (Fig. 3). After coating the cores were sintered at 1350 °C. The surface morphology of the cores with different surface treatments and the cross-sectional view were evaluated by scanning electron microscope (SEM). 2.5. Veneering ceramic application All treated zirconia cores were veneered with the ceramic layer to produce the final shape of ZCR using a separable silicone mould (Fig. 4A). Ceramic powder (Cercon Ceram Kiss, DeguDent GmbH, 4
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2.7. Fractographic examination
Table 3 Descriptive analysis and One-way ANOVA for fracture strength of crowns with different surface treatments. (GB) cores treated with grit blasting, (ZG26 and ZG47) cores coated with zirconia glaze mixture, (ZL26 and ZL47) cores coated with zirconia liner mixture. Group
N
Mean in Newton
SD
F
Sig.
GB ZG26 ZG47 ZL26 ZL47
10 10 10 10 10
418.24 541.49 647.92 466.74 472.95
116.80 114.82 97.33 139.59 124.59
5.552
P < 0.001*
Three fracture modes were defined including; an adhesive mode takes place at the zirconia core/veneer interface where the zirconia core was mostly exposed, a cohesive mode observed in the veneering layer bulk and the mixed mode representing both adhesive and cohesive modes [13,14]. All surfaces of the fractured crowns were visually detected to separate fracture modes based on the fractographic classification mentioned before. In order to validate the visual results, the remaining veneered areas of differentiated crowns were examined under a stereomicroscope (SZ X7, Olympus) at 0.8 X magnification to confirm the classification of the fracture modes. The SEM analysis was used to examine the fracture manner of tested sample.
* Significant difference between groups at P < 0.05. Table 4 Multiple comparisons by Tukey HSD test of the fracture strength values between the zirconia ceramic crowns with different surface treatments. (GB) cores treated with grit blasting, (ZG26 and ZG47) cores coated with zirconia glaze mixture, (ZL26 and ZL47) cores coated. (I) Group
(J) Group
Mean Difference (I-J)
Sig.
GB
ZG26 ZG47* ZL26 ZL47 ZG47 ZL26 ZL47 ZL26* ZL47* ZL47
− 123.252 − 229.678 − 48.499 − 54.708 − 106.425 74.752 68.54 181.178 174.969 − 6.208
0.161 0.001 0.892 0.843 0.286 0.631 0.703 0.012 0.017 1.000
ZG26
ZG47 ZL26
2.8. Finite element analysis (FEA) An extracted human maxillary premolar tooth collected from oral and maxillofacial department clinics of University of Malaya (Ethics No.: DF RD1201/1007(P), Dental Committee, Faculty of Dentistry, University of Malaya) was scanned by 3D dental laser scanner system (Maestro 3D Dental Scanner-MDS400, Italy) to produce a group of Stereolithography (STL) files. Three FEA models were constructed using the SolidWorks CAD system (Dassault Systemes Corp) and divided into; zirconia glaze coated model (ZG), zirconia liner coated model (ZL) and non-coated model (NC). The Young's modulus (E) and Poisson's ratio (v) values of each model component were obtained from the literature (Table 2) while the coating layers were estimated by applying the “rule of mixture” [15]. All solids components were assumed homogeneous, linear, elastic, and isotropic through the entire distortion and the meshes were constructed by tetrahedral elements of 0.5 mm in size. The number of nodes and elements were determined by convergence analysis to obtain accurate data in the shortest processing time. A static load of 200 N simulating masticatory force was applied on an area of 2.5 mm2 on the palatal cusp of the crown model at an oblique angle of 45° to the long axis of the tooth to simulate a masticatory force [16,17].
* Significant difference between groups at P < 0.05.
2.6. Fracture resistance test (FR) All tested crowns for five groups were cemented on their respective metal die by conditioning the inner surface of the crown with hydrofluoric (HF) acid. RelyX U200 self-adhesive resin cement (RelyX U200, 3M ESPE, Neuss-Germany) was applied to the inner surface of the crown to cement on its individual metal master die. The cemented crown was placed in a specially designed stainless steel holder for testing by universal testing machine (Shimadzu AGS-X series, Tokyo, Japan) which positioned crown obliquely at 45° to the long axis of stainless steel knife edge rod that mounted on the vertical arm of the machine (Fig. 5) with crosshead speed of 0.5 mm min−1 until fracture occurred. The final crown was checked with their specific metal die.
2.9. Statistical analysis All the obtained results from the fracture resistance test were analysed with One-way analysis of variance (ANOVA) and post hoc multiple comparisons by Tukey HSD test between groups at a significance level of P < 0.05 using Statistical Package for Social Sciences (SPSS 22, SPSS Inc.).
Fig. 8. Stereomicroscope images failure modes of tested zirconia ceramic crowns. 5
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Fig. 9. SEM images of mixed failure. The black arrow indicated the point of load, white arrows indicated arrested fracture line.
3. Results
3.4. Stress distribution by FEA
3.1. Surface morphology of the treated cores
The stress distribution patterns of the crown models showed propagation and concentrated of the maximum tensile stress around the location of loading point on the occlusal surface and then directed toward the veneer/core interface area to terminate at the cervical area and inner side of the core. The ZG model showed less stress (580 MPa) than the ZL model (650 MPa) and the NC model (832 MPa). In addition, the ZG model showed less stress concentration along the interface area than the other models (ZL and NC) (Fig. 10A). On the other hand, the stresses scattering on the prepared tooth model was higher in the NC model than ZG and ZL models (Fig. 10B).
The SEM analyses of treated zirconia cores (Fig. 6) showed different surface morphologies. The ZG26 core showed less prominence roughness than the ZG47 core which showed a prominent surface roughness with deep undercuts. The core coated by ZL26 and ZL47 showed shallow prominence roughness and undercuts compared to the ZG47. The GB cores showed the lowest surface roughness. A cross-sectional view of a coated zirconia crown showed a uniform mechanical interlocking between the veneering ceramic layer and treated zirconia core (Fig. 7).
4. Discussion 3.2. Fracture resistance assessment
The low bonding strength at zirconia/veneering interface area of ZCR and residual stresses generated from phase changes in the zirconia structural matrix are the most crucial factors that play an important role in the clinical performance of ZCR [21–23]. The surface treatment is an effective way to improve the bonding properties of the zirconia substrate to the other ceramic material by providing surface roughness to increase mechanical interlocking between zirconia and veneering layer [8,24]. In particular, GB is the common surface treatment used to roughen zirconia surface which provides reasonable bond strength to the veneering ceramic layer [25]. According to several studies, GB generates changing in the crystallographic phases of zirconia from (t) to (m) which accompanied with volume changes at the zirconia surface and creates stress on the veneer layer, which negatively affects the adhesion between veneering ceramic and zirconia substrate [10,26,27]. The present study used PZP as a surface treatment to avoid phase transition because the coating procedure was accomplished with the unsintered zirconia substrate which then fully sintered. The surface properties (including roughness, coating thickness and phase transition) and shear bond strength were evaluated by the previous study by Farhan et al., 2018 [12]. The findings of the study concluded that the zirconia coating with coarse particle size produced high surface roughness, uniform thickness and higher shear bond strength. The selection of two sizes of PZP (26 and 47.0 µm) was confirmed according to the pilot study after measuring the surface roughness and coating thickness. An even coating thickness is very important when applied to the ZCR which limited by the thickness of restoration layers and natural tooth reduction which covered by restoration.
The mean FR values of the tested crowns in Newton (N) revealed that the coated groups have greater values than the GB group (418 N ± 116.80) as shown in Table 3. Moreover, the ZG47 group provides a higher value (647.92 N ± 97.33) than other tested groups. One-way ANOVA showed a significant difference between all tested groups at (P < 0.05). Moreover, the ZG47 group showed a significant difference with other tested groups except for ZG26 group (Table 4).
3.3. Fractographic analysis of tested crowns The fracture modes observed under the stereomicroscope classified to adhesive, cohesive and mixed modes as typically as shown in Fig. 8. The cohesive mode presents only in the ZG47 group (20%) and a dominance mode was mixed which is the most failure mode of coated groups (70%), while an adhesive mode was higher in the GB crowns with (70%). SEM images of the tested crowns with mixed fracture mode showed the site of the fracture initiation was the point of contact with the loading device. The high stress under the loading point probably lead to the crack initiation and propagating toward the cervical area. Hertzian cone crack and arrested fracture lines were detected along the bulk of veneer layer in cohesive and mixed modes, which could be referred to the enhanced bonding between veneering ceramic to the zirconia core and utilised to determine the direction of the fracture path (Fig. 9). 6
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Fig. 10. (A) Von Mises stresses on zirconia ceramic models. (ZG) crown model coated with zirconia glaze, (ZL) crown model coated with zirconia liner and (NC) crown model without coating. (B) Stress distribution on zirconia ceramic crown models.
studies, the liner ceramic paste was reduced the surface roughness by filling a portion of the pores and undercuts of longitudinal projection produced by PZP [12,28,29]. The fractographic study of the tested crowns showed cohesive mode only in the ZG47 group by recognising the fracture area at the bulk of the veneering ceramic layer, which indicated a higher fracture resistance value. In contrast, the adhesive mode was recognised as a high incidence mode in the GB group which related to lesser fracture resistance value of tested crown (Fig. 8). SEM analysis showed the direction of fracture initiation at the loading point and then propagated toward the bulk of veneering layer forming the Hertzian cone crack that
The surface morphology of the coated cores showed a higher surface roughness than GB groups (Fig. 6) which resulted from the longitudinal projections formed by the coating mixture. Moreover, the ZG47 group showed a highest FR value (647.92 N) compared to the other coated groups while GB showed the lowest value (Table 3). This due to the improvement of mechanical interlocking at the zirconia core/veneer ceramic interface which produced by the coarse size of zirconia powder (Fig. 7). Furthermore, the ZG47 coating composed from a combination of the two powders (PZP and glaze ceramic powder) which produced more longitudinal projection. The ZL coating composed from powder and paste (PZP and liner ceramic paste) and according to the previous
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represents the expanding path of a fracture in a cone shape which depends on the degree of bonding between the veneering ceramic and the zirconia core (Fig. 9) [30]. Consequently, in order to settle the influence of zirconia coatings on the stress distribution through the ZCR layers, FEA was used to explain the fracture initiation and propagation. The FEA showed a higher stresses level at the area underneath the loading force and the maximum stress was extended from the loading site on the occlusal surface and decreased toward the inner parts of the crown model. The ZG model showed less stress (580 MPa) than the ZL model (650 MPa) and NC model (832 MPa) (Fig. 10A) which support the high fracture resistance result of the ZG47 group. The stress at the interface area evenly distributed in the model ZG while for other models showed an unequally and intense stresses (Fig. 10B). This unequal stresses may facilitate crack initiation at the core/veneer interface area during mastication and lead to chip off or fracture of the veneering ceramic layer. Furthermore, accumulation of stress at the inner side of the core in ZL and NC models caused stress on the prepared tooth compared with ZG model. For the ZG model, the stress value ranged from 200 to 260 MPa while values increased in ZL model from 466 to 641 MPa and for the NC, the level was higher than in the ZG and ZL model with stress above 790 MPa. According to these results, the location of the high stress at the zirconia core could affect the prepared tooth supported crown and may cause further trauma to the restored tooth. The limitations of the present study include the effects of the fatigue failure under cyclic loading to simulate the clinical performance of the ZCRs were not evaluated. Therefore, further studies are needed.
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5. Conclusion The coated crown with coarse particle zirconia glaze mixture (ZG47) showed a highest FR value compare with other coated groups and GB group. The distribution of stress on the ZG crown model was lesser when compared with the other models. Furthermore, the stresses at the interface area were uniformly in ZG model while in the models (ZL and NC) were concentrated in different sites which may lead to fracture initiation. Consequently, it can be concluded that the zirconia coating with ZG47 is useful for decreasing the fracture susceptibility of veneering ceramic and increasing the success rate of ZCR than other types of surface treatment. Acknowledgements This project was sponsored by a postgraduate research grant from the University of Malaya, Malaysia IPPP (grant number PG330-2016A) and University Malaya research fund assistance (BKP) grant number BKS011-2017. References [1] J. Pelaez, P.G. Cogolludo, B. Serrano, L. Lozano, F. José, M.J. Suárez, A four-year prospective clinical evaluation of zirconia and metal-ceramic posterior fixed dental prostheses, Int. J. Prosthodont. 25 (2012) 451–458. [2] G. Aktas, E. Sahin, P. Vallittu, M. Özcan, L. Lassila, Effect of colouring green stage zirconia on the adhesion of veneering ceramics with different thermal expansion coefficients, Int. J. Oral Sci. 5 (2013) 236–241. [3] R.W.K. Li, T.W. Chow, J.P. Matinlinna, Ceramic dental biomaterials and CAD/CAM technology: state of the art, J. Prosthodont. Res. 58 (2014) 208–216.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Application of zirconia surface coating to improve fracture resistance and stress distribution of zirconia ceramic restorations Firas Abdulameer Farhana,b, Eshamsul Sulaimanb, Muralithran G. Kuttyb, a b
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Prosthodontic Department, College of Dentistry, Baghdad University, Bab Al-Muadham Campus of the University of Baghdad, 1417 Baghdad, Iraq Restorative Department, Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Zirconia surface coating Zirconia ceramic restoration Fracture resistance Fractographic analysis, Finite element analysis
Zirconia ceramic restoration (ZCR) has a higher fracture incidence rate than metal ceramic restoration. Different surface treatments were used to improve fracture performance of ZCR such as grit blasting (GB) by aluminium oxide powder. This type of surface treatment generate residual stresses on veneering ceramic causing crack initiation and ending with a fracture. In order to overcome the stress generated by GB, zirconia surface coating is used as a surface treatment to improve fracture resistance and to accommodate stresses along the ZCR layers. Fifty zirconia ceramic crowns were fabricated and divided according to the type of surface treatment into three groups; the first group is (ZG), involving 20 cores were coated with a mixture of partially-sintered zirconia powder (PZP) and glaze ceramic powder; the second group is (ZL), including of 20 cores were coated with PZP and liner ceramic paste. The third group is grit blasting (GB), preparing of 10 fully sintered cores at 1350 °C which then abraded by 50 µm aluminium oxide powder. The groups ZG and ZL were further subdivided into ZG26, ZG47, ZL26 and ZL47 based on two PZP sizes (47 and 26 µm). Each treated core was veneered with the veneering ceramic layer. Fracture resistance (FR) was measured by the universal testing machine. Finite element analysis (FEA) was used to simulate the stress distributions on the coated and non-coated zirconia crown models. The ZG47 group had higher FR (647.92 ± 97.33 N) and a significant difference (P < 0.00) compared to GB and other coated groups. The FEA exhibited lower and evenly distributed stresses of the zirconia glaze model than the zirconia liner and the non-coated models. The ZG47 coating considered as an alternative method to GB treatment which increases the FR which significantly improved the clinical performance of the ZCR.
1. Introduction Clinical follow-up studies of zirconia-ceramic restoration (ZCR) showed that it was an appropriate substitute to the metal-ceramic restorations because of its excellent aesthetics, high strength, and superior biocompatibility [1,2]. Furthermore, the introduction of computer aided design/computer aided manufacturing (CAD/CAM) technologies in dentistry had facilitated the design and fabrication of ZCRs with minimal flaws and short time [3]. In general, the flexural strength of the zirconia core fabricated by CAD/CAM is higher than core produced by the conventional method such slip casting [4] ZCR with veneering layer considered as a favoured choice in restorative dentistry than monolithic zirconia restoration. Because it has appearance matching the colour shade of the natural tooth while monolithic restoration lacks this property and it is too tough, which may wear the opposing tooth [5]. Unfortunately, the ZCR had met with some technical complication
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which was related to the weak bonding between zirconia and veneering ceramic at the interface area as a result of insufficient mechanical interlocking which was enhanced by providing surface roughness [6]. Furthermore, the zirconia structural matrix lacks glassy content to provide a chemical bond with the veneering ceramic material. Therefore, this will increase the fracture incidence rate of ZCRs when compare with metal-ceramic restorations and recognised as the main cause of clinical failure [7]. In order to improve bonding properties of zirconia substrate, surface treatment was suggested, such as grit blasting (GB) by aluminium oxide (Al2O3) powder which is used to offer microroughness and enhancing micromechanical interlocking between zirconia and ceramic materials [8]. Nevertheless, this type of treatment may cause a phase transition of zirconia atomic matrix from tetragonal (t) to monoclinic (m). This transition accompanied with structural expansion which generates stresses at zirconia/ceramic interface causing microcracks and fracture susceptibility [9,10].
Corresponding author. E-mail addresses: fi[email protected] (F.A. Farhan), [email protected] (E. Sulaiman), [email protected] (M.G. Kutty).
https://doi.org/10.1016/j.ceramint.2018.08.246 Received 14 August 2018; Received in revised form 20 August 2018; Accepted 21 August 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Farhan, F.A., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.08.246
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Consequently, previous studies [11,12], proposed to use a thin film of zirconia coatings applied by airbrush spraying technique over the zirconia substrate surface to create surface roughness without phase transition. The coating composed of a micron-sized of zirconia powder with two particle sizes (26 and 47 µm) which were mixed separately with a glaze ceramic powder and liner ceramic paste. The finding of these studies showed a higher surface roughness and shear bond strength than GB treatment. According to this background, the current study aimed to evaluate the zirconia spray coating on fracture resistance to improve the clinical performance of ZCR. Moreover, the distribution of stresses on the layers of zirconia ceramic crown was examined under a loading condition similar to fracture resistance test, using three-dimensional finite element analysis (FEA). 2. Materials and methods 2.1. Preparation of zirconia coating powders Yttria-stabilized zirconia powder (SIGMA-ALDRICH Co., USA) was compressed by a hydraulic press to produce compacted tablets which were sintered at 1100 °C for 2 h to produce partially-sintered zirconia tablets. The tablets were grounded by a ball milling machine (XQM, Hunan, China) to produce a partially-sintered zirconia powder (PZP) with harder and denser properties. Two sets of powder were separated using a sieve shaker machine equipped with 50, 45, 40, 35, 30, and 25 µm meshes. According to the preliminary experiments, two PZP were collected between meshes 25–30 µm and 45–50 µm respectively. The average particle sizes were 26.0 ± 0.3 µm and 47.0 ± 0.5 µm, which agreed with the previous study achieved by the authors [12].
Fig. 2. Grit blasting procedure for all core surfaces.
Table 1 Surface treatments and experimental groups used in the study. Group
N
Type of surface treatment
ZG26
10
ZG47
10
ZL26
10
ZL47
10
GB (control)
10
Coated with a mixture of zirconia powder with a particle size 26 µm and glaze porcelain Coated with a mixture of zirconia powder with a particle size 47 µm and glaze porcelain Coated with a mixture of zirconia powder with a particle size 26 µm and liner ceramic Coated with a mixture of zirconia powder with a particle size 47 µm and liner ceramic Grit Blasting by 50 µm aluminium oxide powder
2.2. Tooth and master die preparation An artificial maxillary first premolar was positioned in a full mouth dental model (Nissin dental model, Kyoto, Japan) and prepared according to the clinical instructions proposed for ZCR. The prepared tooth was duplicated with a vinyl polysiloxane impression (Reprosil, DENTSPLY, USA) to create a mould that filled with molten wax to produce a wax pattern which invested and then casted with Co/Cr alloy (Remanium, Dentaurum, Germany) to fabricate five metal master dies (Fig. 1A). Each metal die was embedded in epoxy resin (Ultrathin resin, PACE Technologies, USA) by positioning 2 mm apical to the finishing line using cubic silicone mould 30 × 30 × 30 mm length, width and height respectively (Fig. 1B). An impression was made of each metal ceramic restoration metal die and poured with the type IV dental die stone material (Elite Rock, Zermack, Italy) to produce stone die (Fig. 1C).
2.3. Preparation of CAD/CAM cores Fifty zirconia cores were designed and fabricated by Cercon CAD/ CAM technology (Cercon-Smart Ceramic System, DeguDent GmbH, Germany). The stone dies were scanned by a laser scanner to generate computer data and defined the prepared tooth parameters including cementing gap, finishing line and correction of the surface line angles.
Fig. 1. (A) Wax pattern converted to the metal die. (B) Master metal die placed in an epoxy resin. (C) Stone die. 2
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Fig. 5. Fracture resistance test. Table 2 The material properties used in the FEA.
Fig. 3. Coating procedure for unsintered zirconia core.
The final step is designing the core data over the prepared tooth which includes configuring the individual anatomic shape and wall skeleton thickness. The core data was transferred to the milling machine which has a special metal holder (milling frame) to mount unsintered zirconia block (Cercon Base 47, DeguDent GmbH, Germany) near the rotary instrument to carve out and produce the final shape of zirconia core.
Material
Young's modulus (GPa)
Poisson's ratio
Reference
Veneer ceramic Zirconia core Dentin Enamel Resin cement Coating layer ZG Coating layer ZL
69 210 18.6 84.1 16 9.75 5.64
0.28 0.22 0.30 0.30 0.24 0.3 0.3
[18] [19] [20] [18] [20] ROM ROM
ROM: Rule of mixture. GPa: Gigapascal.
50 µm aluminium oxide (Al2O3) powder at 4 bar air pressure for 10 s with 10 mm distance of blasting nozzle to the core (Fig. 2). The classification of groups is summarised in Table 1. The coating procedure was achieved by mixing equal weight ratios of PZP and glaze powder or liner paste using a magnetic stirrer at 500 rpm for 15 min to produce a homogeneous slurry. The unsintered cores were coated with an airbrush mini spray gun (model 130-dual action airbrush kit, Taiwan) fixed vertically to the zirconia core. The coating parameters were designated according to the pilot experiments and agree with a previous study using the same parameters that produce a uniform thickness and efficient roughness [12]. These parameters include; air pressure at 2.5 bars, total spray time of 2 s, and a
2.4. Surface treatments and study grouping Unsintered zirconia cores were divided according to the type of surface treatments into three groups; the first group is (ZG), consisting of 20 unsintered zirconia core coated with a mixture of PZP and glaze powder. The second group is (ZL), containing 20 unsintered zirconia core coated with a mixture of PZP and liner paste. The ZG and ZL groups were subdivided according to the selected particle sizes of PZP (26 and 47 µm) into ZG26, ZG47, ZL26 and ZL47, for each one 10 cores. The third group is grit blasting (GB) as a control group, including 10 fully sintered zirconia cores at 1350 °C and subsequently blasted with
Fig. 4. (A) Silicone mould with a treated zirconia core used as a veneering build up reference. (B) Final zirconia ceramic crown cemented on a metal die. 3
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Fig. 6. Treated core with different surface treatments. (ZG26 and ZG47) cores coated with zirconia glaze, (ZL26 and ZL47) cores coated with zirconia liner and (GB) core treated with grit blasting.
Fig. 7. Cross-sectional SEM image of the coated zirconia ceramic crown with ZG47.
Germany) was mixed with the appropriate amount of a special ceramic liquid to produce a ceramic slurry which was added and condense to the silicone mould after lubricating with an isolating agent to prevent sticking of the ceramic slurry to the silicone mould to produce a final crown shape (Fig. 4B). The crown was sintered in a ceramic furnace (Programat EP 5000, Ivoclar Vivadent, Schaan/Liechtenstein) as instructed by the manufacturer.
distance of 13 cm (Fig. 3). After coating the cores were sintered at 1350 °C. The surface morphology of the cores with different surface treatments and the cross-sectional view were evaluated by scanning electron microscope (SEM). 2.5. Veneering ceramic application All treated zirconia cores were veneered with the ceramic layer to produce the final shape of ZCR using a separable silicone mould (Fig. 4A). Ceramic powder (Cercon Ceram Kiss, DeguDent GmbH, 4
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2.7. Fractographic examination
Table 3 Descriptive analysis and One-way ANOVA for fracture strength of crowns with different surface treatments. (GB) cores treated with grit blasting, (ZG26 and ZG47) cores coated with zirconia glaze mixture, (ZL26 and ZL47) cores coated with zirconia liner mixture. Group
N
Mean in Newton
SD
F
Sig.
GB ZG26 ZG47 ZL26 ZL47
10 10 10 10 10
418.24 541.49 647.92 466.74 472.95
116.80 114.82 97.33 139.59 124.59
5.552
P < 0.001*
Three fracture modes were defined including; an adhesive mode takes place at the zirconia core/veneer interface where the zirconia core was mostly exposed, a cohesive mode observed in the veneering layer bulk and the mixed mode representing both adhesive and cohesive modes [13,14]. All surfaces of the fractured crowns were visually detected to separate fracture modes based on the fractographic classification mentioned before. In order to validate the visual results, the remaining veneered areas of differentiated crowns were examined under a stereomicroscope (SZ X7, Olympus) at 0.8 X magnification to confirm the classification of the fracture modes. The SEM analysis was used to examine the fracture manner of tested sample.
* Significant difference between groups at P < 0.05. Table 4 Multiple comparisons by Tukey HSD test of the fracture strength values between the zirconia ceramic crowns with different surface treatments. (GB) cores treated with grit blasting, (ZG26 and ZG47) cores coated with zirconia glaze mixture, (ZL26 and ZL47) cores coated. (I) Group
(J) Group
Mean Difference (I-J)
Sig.
GB
ZG26 ZG47* ZL26 ZL47 ZG47 ZL26 ZL47 ZL26* ZL47* ZL47
− 123.252 − 229.678 − 48.499 − 54.708 − 106.425 74.752 68.54 181.178 174.969 − 6.208
0.161 0.001 0.892 0.843 0.286 0.631 0.703 0.012 0.017 1.000
ZG26
ZG47 ZL26
2.8. Finite element analysis (FEA) An extracted human maxillary premolar tooth collected from oral and maxillofacial department clinics of University of Malaya (Ethics No.: DF RD1201/1007(P), Dental Committee, Faculty of Dentistry, University of Malaya) was scanned by 3D dental laser scanner system (Maestro 3D Dental Scanner-MDS400, Italy) to produce a group of Stereolithography (STL) files. Three FEA models were constructed using the SolidWorks CAD system (Dassault Systemes Corp) and divided into; zirconia glaze coated model (ZG), zirconia liner coated model (ZL) and non-coated model (NC). The Young's modulus (E) and Poisson's ratio (v) values of each model component were obtained from the literature (Table 2) while the coating layers were estimated by applying the “rule of mixture” [15]. All solids components were assumed homogeneous, linear, elastic, and isotropic through the entire distortion and the meshes were constructed by tetrahedral elements of 0.5 mm in size. The number of nodes and elements were determined by convergence analysis to obtain accurate data in the shortest processing time. A static load of 200 N simulating masticatory force was applied on an area of 2.5 mm2 on the palatal cusp of the crown model at an oblique angle of 45° to the long axis of the tooth to simulate a masticatory force [16,17].
* Significant difference between groups at P < 0.05.
2.6. Fracture resistance test (FR) All tested crowns for five groups were cemented on their respective metal die by conditioning the inner surface of the crown with hydrofluoric (HF) acid. RelyX U200 self-adhesive resin cement (RelyX U200, 3M ESPE, Neuss-Germany) was applied to the inner surface of the crown to cement on its individual metal master die. The cemented crown was placed in a specially designed stainless steel holder for testing by universal testing machine (Shimadzu AGS-X series, Tokyo, Japan) which positioned crown obliquely at 45° to the long axis of stainless steel knife edge rod that mounted on the vertical arm of the machine (Fig. 5) with crosshead speed of 0.5 mm min−1 until fracture occurred. The final crown was checked with their specific metal die.
2.9. Statistical analysis All the obtained results from the fracture resistance test were analysed with One-way analysis of variance (ANOVA) and post hoc multiple comparisons by Tukey HSD test between groups at a significance level of P < 0.05 using Statistical Package for Social Sciences (SPSS 22, SPSS Inc.).
Fig. 8. Stereomicroscope images failure modes of tested zirconia ceramic crowns. 5
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Fig. 9. SEM images of mixed failure. The black arrow indicated the point of load, white arrows indicated arrested fracture line.
3. Results
3.4. Stress distribution by FEA
3.1. Surface morphology of the treated cores
The stress distribution patterns of the crown models showed propagation and concentrated of the maximum tensile stress around the location of loading point on the occlusal surface and then directed toward the veneer/core interface area to terminate at the cervical area and inner side of the core. The ZG model showed less stress (580 MPa) than the ZL model (650 MPa) and the NC model (832 MPa). In addition, the ZG model showed less stress concentration along the interface area than the other models (ZL and NC) (Fig. 10A). On the other hand, the stresses scattering on the prepared tooth model was higher in the NC model than ZG and ZL models (Fig. 10B).
The SEM analyses of treated zirconia cores (Fig. 6) showed different surface morphologies. The ZG26 core showed less prominence roughness than the ZG47 core which showed a prominent surface roughness with deep undercuts. The core coated by ZL26 and ZL47 showed shallow prominence roughness and undercuts compared to the ZG47. The GB cores showed the lowest surface roughness. A cross-sectional view of a coated zirconia crown showed a uniform mechanical interlocking between the veneering ceramic layer and treated zirconia core (Fig. 7).
4. Discussion 3.2. Fracture resistance assessment
The low bonding strength at zirconia/veneering interface area of ZCR and residual stresses generated from phase changes in the zirconia structural matrix are the most crucial factors that play an important role in the clinical performance of ZCR [21–23]. The surface treatment is an effective way to improve the bonding properties of the zirconia substrate to the other ceramic material by providing surface roughness to increase mechanical interlocking between zirconia and veneering layer [8,24]. In particular, GB is the common surface treatment used to roughen zirconia surface which provides reasonable bond strength to the veneering ceramic layer [25]. According to several studies, GB generates changing in the crystallographic phases of zirconia from (t) to (m) which accompanied with volume changes at the zirconia surface and creates stress on the veneer layer, which negatively affects the adhesion between veneering ceramic and zirconia substrate [10,26,27]. The present study used PZP as a surface treatment to avoid phase transition because the coating procedure was accomplished with the unsintered zirconia substrate which then fully sintered. The surface properties (including roughness, coating thickness and phase transition) and shear bond strength were evaluated by the previous study by Farhan et al., 2018 [12]. The findings of the study concluded that the zirconia coating with coarse particle size produced high surface roughness, uniform thickness and higher shear bond strength. The selection of two sizes of PZP (26 and 47.0 µm) was confirmed according to the pilot study after measuring the surface roughness and coating thickness. An even coating thickness is very important when applied to the ZCR which limited by the thickness of restoration layers and natural tooth reduction which covered by restoration.
The mean FR values of the tested crowns in Newton (N) revealed that the coated groups have greater values than the GB group (418 N ± 116.80) as shown in Table 3. Moreover, the ZG47 group provides a higher value (647.92 N ± 97.33) than other tested groups. One-way ANOVA showed a significant difference between all tested groups at (P < 0.05). Moreover, the ZG47 group showed a significant difference with other tested groups except for ZG26 group (Table 4).
3.3. Fractographic analysis of tested crowns The fracture modes observed under the stereomicroscope classified to adhesive, cohesive and mixed modes as typically as shown in Fig. 8. The cohesive mode presents only in the ZG47 group (20%) and a dominance mode was mixed which is the most failure mode of coated groups (70%), while an adhesive mode was higher in the GB crowns with (70%). SEM images of the tested crowns with mixed fracture mode showed the site of the fracture initiation was the point of contact with the loading device. The high stress under the loading point probably lead to the crack initiation and propagating toward the cervical area. Hertzian cone crack and arrested fracture lines were detected along the bulk of veneer layer in cohesive and mixed modes, which could be referred to the enhanced bonding between veneering ceramic to the zirconia core and utilised to determine the direction of the fracture path (Fig. 9). 6
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Fig. 10. (A) Von Mises stresses on zirconia ceramic models. (ZG) crown model coated with zirconia glaze, (ZL) crown model coated with zirconia liner and (NC) crown model without coating. (B) Stress distribution on zirconia ceramic crown models.
studies, the liner ceramic paste was reduced the surface roughness by filling a portion of the pores and undercuts of longitudinal projection produced by PZP [12,28,29]. The fractographic study of the tested crowns showed cohesive mode only in the ZG47 group by recognising the fracture area at the bulk of the veneering ceramic layer, which indicated a higher fracture resistance value. In contrast, the adhesive mode was recognised as a high incidence mode in the GB group which related to lesser fracture resistance value of tested crown (Fig. 8). SEM analysis showed the direction of fracture initiation at the loading point and then propagated toward the bulk of veneering layer forming the Hertzian cone crack that
The surface morphology of the coated cores showed a higher surface roughness than GB groups (Fig. 6) which resulted from the longitudinal projections formed by the coating mixture. Moreover, the ZG47 group showed a highest FR value (647.92 N) compared to the other coated groups while GB showed the lowest value (Table 3). This due to the improvement of mechanical interlocking at the zirconia core/veneer ceramic interface which produced by the coarse size of zirconia powder (Fig. 7). Furthermore, the ZG47 coating composed from a combination of the two powders (PZP and glaze ceramic powder) which produced more longitudinal projection. The ZL coating composed from powder and paste (PZP and liner ceramic paste) and according to the previous
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represents the expanding path of a fracture in a cone shape which depends on the degree of bonding between the veneering ceramic and the zirconia core (Fig. 9) [30]. Consequently, in order to settle the influence of zirconia coatings on the stress distribution through the ZCR layers, FEA was used to explain the fracture initiation and propagation. The FEA showed a higher stresses level at the area underneath the loading force and the maximum stress was extended from the loading site on the occlusal surface and decreased toward the inner parts of the crown model. The ZG model showed less stress (580 MPa) than the ZL model (650 MPa) and NC model (832 MPa) (Fig. 10A) which support the high fracture resistance result of the ZG47 group. The stress at the interface area evenly distributed in the model ZG while for other models showed an unequally and intense stresses (Fig. 10B). This unequal stresses may facilitate crack initiation at the core/veneer interface area during mastication and lead to chip off or fracture of the veneering ceramic layer. Furthermore, accumulation of stress at the inner side of the core in ZL and NC models caused stress on the prepared tooth compared with ZG model. For the ZG model, the stress value ranged from 200 to 260 MPa while values increased in ZL model from 466 to 641 MPa and for the NC, the level was higher than in the ZG and ZL model with stress above 790 MPa. According to these results, the location of the high stress at the zirconia core could affect the prepared tooth supported crown and may cause further trauma to the restored tooth. The limitations of the present study include the effects of the fatigue failure under cyclic loading to simulate the clinical performance of the ZCRs were not evaluated. Therefore, further studies are needed.
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5. Conclusion The coated crown with coarse particle zirconia glaze mixture (ZG47) showed a highest FR value compare with other coated groups and GB group. The distribution of stress on the ZG crown model was lesser when compared with the other models. Furthermore, the stresses at the interface area were uniformly in ZG model while in the models (ZL and NC) were concentrated in different sites which may lead to fracture initiation. Consequently, it can be concluded that the zirconia coating with ZG47 is useful for decreasing the fracture susceptibility of veneering ceramic and increasing the success rate of ZCR than other types of surface treatment. Acknowledgements This project was sponsored by a postgraduate research grant from the University of Malaya, Malaysia IPPP (grant number PG330-2016A) and University Malaya research fund assistance (BKP) grant number BKS011-2017. References [1] J. Pelaez, P.G. Cogolludo, B. Serrano, L. Lozano, F. José, M.J. Suárez, A four-year prospective clinical evaluation of zirconia and metal-ceramic posterior fixed dental prostheses, Int. J. Prosthodont. 25 (2012) 451–458. [2] G. Aktas, E. Sahin, P. Vallittu, M. Özcan, L. Lassila, Effect of colouring green stage zirconia on the adhesion of veneering ceramics with different thermal expansion coefficients, Int. J. Oral Sci. 5 (2013) 236–241. [3] R.W.K. Li, T.W. Chow, J.P. Matinlinna, Ceramic dental biomaterials and CAD/CAM technology: state of the art, J. Prosthodont. Res. 58 (2014) 208–216.
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