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Background

Yttria-tetragonal zirconia polycrystalline (Y-TZP) is a non-silicate ceramic of great interest in the field of prosthetic and implant dentistry due to its excellent mechanical properties such as flexural strengths of 900 to 1200 MPa, fracture resistance of more than 2000 N, and fracture toughness of 9 to 10 MPa. Y-TZP is biocompatible and its aesthetic properties are excellent when veneered with porcelain [1, 2]. In spite of the high mechanical properties, Y-TZP has poor bond strength to underlying substrate even when resin-based cements are used due to the lack of glassy matrix and the absence of SiO₂ [3-6]. A strong bond is of importance for the long-term clinical success and becomes even more important when bonded restorations are considered (resin-bonded FDPs, and veneers, for example) [7, 8]. This bond may be improved by surface treatment of Y-TZP [9, 10]. Different surface treatment methods have been proposed to improve the bond strength to resin cement, including chemical and/or mechanical surface treatments, but none of these methods provide long-term stability of the bond [11].

Sandblasting of Y-TZP with 50 μm aluminum oxide (Al2O3) particles results in a rough surface that could enhance bond strength results [12], but it reduces the flexural strength of the bulk of the zirconia in a range of 25-50%, thus compromising the longevity of the prostheses [13, 14]. The decline in flexural strength was explained by the formation of cracks (>4 µm) that are too large to be counteracted by transformation toughening. Transformation toughening is a stress-activated mechanism due to the transformation from tetragonal (t) to monoclinic (m) phase  which results in a 3-4% crystalline volume increase, potentially arresting crack propagation [13, 15]. Tribochemical silica coating includes air abrasion with 30 μm silica-coated alumina particles [11] and has been used not only to roughen zirconia surface without damaging it but also to chemically activate it by adding silica to the surface thus making it more receptive for chemical bonding [16], but a wide range of parameters (including particle size, pressure, working time) end up in contradictory results [17-19].

The combination of both chemical and mechanical methods to alter the morphology and/or chemical composition of the surface tends to produce better and even a more stable bond [20-24]. In this regard, adding the phosphate ester monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP) to bonding agents (primers or adhesive) appears to enhance bond strength to zirconia, because chemical bonds are formed between the MDP acidic groups (phosphoric acid) and the oxide layer of zirconia [25, 26]. A combined MDP/silane adhesive was shown to improve bonding between resin cement and zirconia [16, 27-29], and when combined with a compatible resin cement resulted in the highest initial and the long-term bond strength to zirconia [30].

Recent investigations suggest the use of laser (Light Amplification by Stimulated Emission of Radiation) as a surface modifying treatment. Different types of lasers (such as Er:YAG, Nd:YAG, and CO₂) have been used to increase the surface roughness of zirconia [31]. However, surface destruction due to thermal effects of high power lasers is a reason for concern [32], and results in contradictory bond strength result [32-35]. While some studies concluded that lasers are not effective to improve bond strength and seemed to have a minimal impact on zirconia surface roughening; which might be attributed to less absorption of laser energy by zirconia, others showed that surface preparation with CO₂ and Er:YAG lasers increased the bond strength of resin cement to zirconia and indicate this technique as an alternative for bonding to ZrO₂ surfaces [18, 31, 36]. Considering the recent advancement in laser technology with the development of ultrashort pulsed laser [37], which includes a gentle surface irradiation with variable wavelengths, the application of this laser on the Y-TZP surface warrants investigation, as it could increase the roughness of the Y-TZP surface with no surface destruction and enhance the stability of the bond to the substrate without compromising the mechanical performance of the prosthesis, such as those described above.

Research Questions:

What is the effect of zirconia surface roughness, energy, morphology and chemical composition and the long-term bond strength to resin cement? Will the use of ultrashort pulse laser improve the long-term bond strength of resin cement to zirconia vs. other mechanical and/or chemical treatments? Will the use of ultrashort pulse laser affect the fatigue limit of zirconia vs. other mechanical and/or chemical treatments?

Research Hypotheses:

1. Ultrashort pulse laser surface treatment (LST) will increase the surface roughness, and surface energy of Y-TZP substrates, similarly to that of other mechanical and/or chemical surface treatments.
2. LST will improve long-term bond strength between Y-TZP and resin cement as opposed to other mechanical and/or chemical surface treatments.
3. Fatigue resistance of Y-TZP will not be affected by LST, as opposed to the mechanical surface treatments.

Objectives:

1. To characterize the effect of LST on Y-TZP surface in terms of surface morphology, chemical composition, and phase transformation, and compare it to the effect of other surface treatments (sandblasting, tribochemical silica coating).
2. To evaluate the effect of ultrashort pulse laser on the bond strength between Y-TZP and resin cement, and compare it to the effect of other surface treatments.
3. To study the effect of ultrashort pulse laser on the fatigue behavior of Y-TZP, and compare it to the effects of other mechanical and/or chemical treatments
4. To characterize the type of flaws (intrinsic vs extrinsic) for the different treatments.

 

Materials & Methods

Fully sintered high translucency Y-TZP (Lava™ Plus, 3M ESPE) was used for the study. This material combines both mechanical and excellent optical properties. Tong H et al. studied the characterization of three commercial Y-TZP ceramics and concluded that there is a delicate balance between mechanical and optical properties of the current commercial Y-TZP ceramics. They also concluded that High-Translucency Y-TZP possessed the best translucency properties and could be the best candidate for monolithic dental restorations [38]. Another study by Alghazzawi and Janowski compared 6 brands of zirconia stated that the composition is similar between all studied zirconia types which include 4 elements (yttrium, zirconium, hafnium, and oxygen), in addition to an equiaxed grains size (< 1 μm) [39]. This is consistent with our preliminary finding, in which we studied this material (Lava™ Plus, 3M ESPE) in comparison to two other commercially available Y-TZP (Zirlux FC and BruxZir®) using scanning electron microscopy (SEM) (JEOL 6610LV, California, USA) and Energy-dispersive x-ray spectroscopy (EDS). The results confirmed a similar composition; mainly Zr~70 wt. %, O~ 22.5 wt. %, and C~7% (Figure 1).  Further characterization need to be done including XRD and SEM after thermal etching to confirm similar crystalline phase, alumina content, and gain size in order to be able to say that Lava™ Plus is a good representative of this group of materials and the results of this study could be applied universally to other materials from this group.

All other materials used in the study are listed in Table 1.

To address the research objectives and test the hypotheses, three projects were conducted:

1. Surface Characterization:

Y-TZP blocks (4×8.8×8.8mm) were prepared, surface finished up to 600 grit, ultrasonically cleaned for 10 min in distilled water, and air-dried [33]. Subsequently, all specimens were randomly distributed into three groups (n=3/group) according to the surface treatment: (1) SIL (tribochemical silica coating); (2) SNB (sandblasting) and (3) CTRL (control group; no surface treatment). Three additional groups will be prepared after getting an access to the laser facility: (4) LSR A (single pulse laser at low energy); (5) LSR B (single pulse laser at high energy); (6) LSR C (several pulses laser).

Surface for SIL, SNB and CTRL were characterized as follows:

1. Surface roughness and chemical composition analysis:

A three-dimensional analysis of the surface topography was done using atomic profilometer (P-16+ Profiler, KLA-Tencor Corporation, Milpitas, CA, USA). Three areas (0.2*0.2 mm each) were measured for each specimen. This part was conducted at Ontario Centre for Characterization of Advanced Materials (OCCAM), Faculty of Applied Science & Engineering / University of Toronto.

For qualitative analysis, selected specimens were examined using SEM at 200X, 500X, and 2000x magnification, at a low vacuum level, operating voltage of 15 kV, and a working distance of 10 mm. EDS were further used to evaluate the availability of chemical components at 500X magnification, at a low vacuum level, 15 kV, and a working distance of 10 mm [40].

To understand the effect of surface cleaning after surface treatment, two samples received SIL and SNB then subjected to ultrasonic cleaning for 5 min in distilled water using an ultrasonic cleaner (FS5 Fisher Scientific, Sheboygan, Wisconsin, United States) and further analyzed using SEM and EDS to compare them to the non-cleaned groups.

This part was conducted at the Earth Sciences Department / University of Toronto.

Data Analyses: The average roughness values (Sa) were measured using compatible software (Apex Software). Sa values were analyzed by one-way ANOVA and Tukey’s test (α = .05).

2.  X-ray Diffraction Analysis (XRD):

A diffraction analysis was conducted using a Philips XRD system, including a PW 1830 HT generator, to determine the effects of  treatments on the crystalline phases present in the surface of the samples [41].

This part was conducted at the Earth Sciences Department / University of Toronto.

Data Analyses: A quantitative value of phase composition of specimens (the relative monoclinic phase) were obtained using Reference Intensity Ratio (RIR) analysis (X’Pert Quantify, PANalytical) after matching the sample patterns with a reference pattern (ICDD pdf4+) [42]. The relative monoclinic phase values were analyzed by one-way ANOVA and Tukey’s test (α = .05).  The relationship between the roughness and relative monoclinic phase was assessed by Pearson’s correlation analysis.

3. Contact Angle:

The reactive level of the surface (wettability) were determined using contact angle measurements [43]. A goniometer (NRL C.A. goniometer, Ramé-Hart, Inc., Mountain Lakes, NJ) was used for this purpose; where a microsyringe was used to place a 20 mL droplet of distilled/deionized water on the sample surfaces. For each droplet, the contact angle on either side was measured using a microscope at 300X and the average was reported as a single measurement. Five measures were acquired per sample. This part was conducted at Prof. Santerre’s Lab, IBBME / University of Toronto.

Data Analyses: The values were analyzed by one-way ANOVA and Tukey’s test (α = 0.05).

2. Zirconia-composite Bond Strength:

Zirconia-composite bond strength was assessed using microtensile bond strength. Y-TZP blocks (4×8.8×8.8mm) were prepared as described in the first project and then divided into three groups (n=5/group) according to the surface treatment: (1) SIL; (2) SNB; (3) CTRL. Three additional groups will be prepared and tested after getting an access to the laser facility: (4) LSR A; (5) LSR B; (6) LSR C. Composite resin blocks (4×8.8×8.8mm) were prepared and cemented to the prepared Y-TZP slices using a bisphenol A-glycidyl-methacrylate (Bis-GMA) based adhesive resin cement combined with a silane and MDP-containing adhesive (Scotchbond™ Universal, 3M ESPE) according to the manufacturer recommendations. Cementation was done using a special jig in which a constant load of 6 N was applied until the cement is completely cured (~5 min) [44]. After cementation and storage in distilled water at 37°C for 24 hours, each specimen was transversely cut to produce 1×1 mm² beams using a water-cooled sectioning saw (IsoMet™ 1000 Precision Sectioning Saw, BUEHLER, Lake Bluff, Illinois, USA). These beams were incubated in distilled water, and microtensile tested (3 beams per specimen) after 48 hours of storage at a crosshead speed of 0.5mm/min. using a microtensile tester (Microtensile tester, BISCO Inc., IL, USA). Further testing will be performed after aging of specimens for 2, 4 and 6 months. Selected samples will be examined using SEM at 100X to determine the failure mode (Adhesive, cohesive or mixed), and further examined using EDS for SIL group.

Data Analyses: The test load (N) was recorded and calculations made using the following equation:

S = L / A; where S is the bond strength (MPa), L = test load (N), A = adhesive area (mm²)

The current were analyzed by one-way ANOVA and Tukey’s test (α = 0.05). Later on, when the longer incubation period will be completed, a two-way ANOVA will be considered, with the independent factors being mechanical surface treatment and aging period. Tukey’s test (α ═ 0.05) will be used for comparison among groups.

3. Fatigue Resistance:

Fatigue testing is a reliable method to predict the longevity of indirect materials as a function of propagation of surface defects [45, 46]. Fatigue testing was performed on bars (3×5×30mm). Specimens were divided into groups (n=5/group) according to the treatment: (1) SIL; (2) SNB; (3) CTRL. Each bar received the treatment in the middle of the tensile side on a surface of 5mm×6mm. Three additional groups will be prepared and tested later: (4) LSR A; (5) LSR B; (6) LSR C.

Prior to fatigue testing, the monotonic failure stresses were determined from 3 specimens/group. The maximum stress level (100% load) was set 20% below the average monotonic stresses recorded at failure. Cyclic fatigue (sinusoidal loading/unloading at 15 Hz between 10% and 100% load) was applied in 3-point-bending configuration with 20 mm span distance [47], in a water tank, using a universal testing machine (Instron, model 8501, Corp, Canton, Mass.) at a crosshead speed of 0.5 mm/min.

Data Analyses: Analysis of the results still needs to be completed. S–N diagrams (stress vs. number of cycles to failure) will be generated. Kaplan–Meier survival analysis will be performed to determine the failure stress at the median percentile survival level for 1 million cycles [47]. The statistical analyses will use the log-rank test for comparison between the groups. Selected samples will be examined under a stereomicroscope at 57× magnification and then investigated under SEM to determine the origin of the failure. Fractographic analysis will be performed in accordance with the NIST Recommended practice guide for fractography of glasses and ceramics [48].

Results and Discussion

1. Surface Characterization:

1. Surface roughness and chemical composition analyses:

Sa was significantly higher for both SNB (p<0.001) and SIL (p=0.01) compared to CTRL. SNB was significantly higher than SIL (p=0.02) (Table 2, Table 3, Figure 2). This indicates increased surface roughness after mechanical surface treatments, which would have the potential to increase the micromechanical interlocking of the resin cement to the zirconia surface, positively affecting bond strength. The effect of surface roughness on fatigue resistance is currently under investigation.

SEM observations were consistent with the profilometer findings (Figure 3). SEM for SIL and SNB- treated samples revealed a roughened irregular surface. This would promote resin cement interlocking, but would also result in cracks that could weaken the material. More regular surface resulted from SIL treatment.

EDS confirmed the presence of silica particles on SIL group surfaces, but not on the other groups (Figure 4). This would increase the surface energy, promote a chemical bond with the silane coupling agent (available in the adhesive applied later), and explain the higher bond strength results. The distribution of silica/alumina/zirconia particles was further investigated by EDS mapping. The results show that the distribution areas for Si and Al were identical and they appeared to be pointed in shape, reflecting the points of impact with the CoJet sands. The distribution was not uniform all over the surface reflecting a limitation from the nature of the manual procedure (Figure5).

Ultrasonic cleaning after treating samples with SIL resulted in decrease of the silica content from ~2 wt. % for non-cleaned samples to ~1 wt. % after cleaning (Figure 6). This means the reduction of the silica particles available for bonding with silane and the reduction in the bond strength accordingly. These results are consistent with previous findings which suggested that ultrasonic cleaning of silica-coated zirconia should be avoided [24, 49]. For SNB group, Al particles are still seen on the surface after ultrasonic cleaning, but the bond strength was decreased according to Nishigawa et al., who concluded that ultrasonic cleaning after sandblasting should also be avoided [50].

2. X-ray Diffraction Analysis (XRD):

X-ray diffraction patterns of Y-TZP surfaces after different surface treatments were obtained and matched with a reference pattern (ICDD pdf4+) to determine the crystalline phase (Figure 7).  Reference Intensity Ratio (RIR) analysis for data showed that CTRL group has only a tetragonal crystalline phase on the surface means no cracks formed. Surface treatments with SIL and SNB increase the relative monoclinic phase significantly (p<0.001). This indicates a surface destruction and crack formation which was counteracted by the transformation toughening. SNB resulted in highest percentage resulted from deeper cracks. No significant difference between SIL and SNB (Table 4, Table 5, Figure 8). A strong positive correlation (r = 0.719) between the surface roughness and relative monoclinic phase % was observed (p<0.001) (Figure 9, Figure 10).

3. Contact Angle:

Contact angle values were significantly reduced for SIL and SNB compared to CTRL (p<0.001). SIL was significantly lower than SNB (p<0.001). Although SNB resulted in a rougher surface and expected to have a lower contact angle, it seems that silica particles found on SIL surfaces play a role decreasing the contact angle even more. The lower the contact angle the better wettability of the adhesive and cement later, which resulted in a higher bond (Table 6, Table 7, Figure 11).

2. Zirconia-composite Bond Strength:

The results showed that mechanical and/or chemical treatment (SIL and SNB) significantly increased microtensile bond strength compared to the CTRL. SIL results were significantly higher than both SNB and CTRL (p<0.001). This is consistent with the surface characterization findings and could be explained by the increased surface roughness and the presence of silica to chemically bond to the silane in the adhesive.  (Table 8, Table 9, Figure 12).

Conclusion

Within the limitation of this study, it is clear that the SIL resulted in a better bond strength on the short term. Although SNB resulted in a higher surface roughness, SIL combines the roughness with the addition of silica; resulted in a lower contact angle and made the surface ready for a chemical bond with silane available in the adhesive. Further investigation is needed to assess this bond after long term storage. The changes in crystalline phase (t-m) after surface treatments (SNB and SIL) indicates cracks formation and warrants further investigation for any possible effect or changes on the material fatigue resistance.

Significance

This study is investigating the effect of ultrashort laser treatment on characterization, immediate and long-term bond strength between resin cement and zirconia and the fatigue behavior of zirconia.

Increasing the roughness of the Y-TZP intaglio surface is the only alternative to enhance the stability of the bond by generating micromechanical interlocking between the substrate and the low viscosity resin cement. There is a possibility that laser ablation may do so without compromising the mechanical performance of the prosthesis. The results of this study have great potential to be used as preliminary data for a future project aiming at optimizing the parameters of laser for application in different ceramic materials.

Progress since last committee meeting

• Coursework: Registration in Advances in Dental Materials course (DEN1070H) – starts in January

• Experimental work: All the experiments explained in Materials & Methods section was completed, including Surface Characterization (Surface roughness analysis, X-ray Diffraction Analysis (XRD), Surface Energy), Zirconia-composite Bond Strength,andFatigue Resistance for SIL, SNB, and CTRL.

• Publication: An abstract was submitted towards an oral presentation for the coming IADR conference, San Francisco March 2017. Title: “Bonding between Zirconia and Resin Cement – Effect of Surface Treatment”

• Grants / Funds applications:

1. An application was submitted to the Alpha Omega Foundation of Canada Research.
2. An application was submitted to the Dental Research Institute

 

Future directions:

• Continue bond strength testing for SIL, SNB and CTRL after 2, 4 and 6 months storage.
• Fatigue Analysis of the results for SIL, SNB and CTRL needed to be completed.
• Sample preparation and testing for LSR A, LSR B, and LSR C groups, following the same methodology.
• detailed future plan (Gantt-Chart) future plan described in Table 10.

Appendices:

• Tables

1. Table 1: Materials used in the above-mentioned projects.

Material	Composition	Lot number	Expiry date	Manufacturer
Lava™ Plus	Tetragonal polycrystalline zirconia partially stabilized with 3mol-% Yttria	520217	Sep-16	3M ESPE
CoJet™ System Sand	Silicatized sand (particle size 30 µm)	620599	Feb-19	3M ESPE
Blasting Compound	Aluminum Oxide (particle size 50 µm)	1629071	NA	Ivoclar Vivadent Inc.
Filtek ™ Z250	-TEGDMA (triethylene glycol dimethacrylate)  -UDMA (urethane dimethacrylate) -Bis-EMA (Bisphenol A polyetheyleneglycol diether dimethacrylate)	N736805	Sep-18	3M ESPE
RelyX™ Ultimate	Methacrylate monomers, Filers, Initiator, Dark cure activator for Scotchbond Universal	618736	Jul-17	3M ESPE
Scotchbond™ Universal	MDP phosphate monomer, Silane, Vitrebond copolymer	613345	Dec-17	3M ESPE

2. Table 2: ANOVA table for surface roughness (Sa) values, where surface treatments (SIL, SNB, and CTRL) are the independent factor.

	Sum of Squares	df	Mean Square	F	Sig.
Between Groups	.520	2	.260	18.320	.000
Within Groups	.341	24	.014
Total	.861	26

 

 

3. Table 3: Mean surface roughness ( Sa ) values and standard deviation with subset tables (Tukey’s test) for Y-TZP samples after receiving different surface treatments (SIL, SNB, and CTRL)

Treatment*	Mean	Std. Deviation
CTRL	.216 a	.048
SNB	.556 b	.191
SIL	.392 c	.061

Treatment	Subset*
	1	2	3
CTRL	.216
SIL		.392
SNB			.556

* Different letters indicate significant difference

according to Tukey’s test (α = 0.05)

4. Table 4: ANOVA table for relative monoclinic phase (%), where surface treatments (SIL, SNB, and CTRL) are the independent factor.

	Sum of Squares	df	Mean Square	F	Sig.
Between Groups	1547.556	2	773.778	224.645	.000
Within Groups	20.667	6	3.444
Total	1568.222	8

5. Table 5: Relative monoclinic phase (%) and standard deviation with subset tables (Tukey’s test) of Y-TZP samples after receiving different surface treatments (SIL, SNB, and CTRL)

Treatment*	Mean	Std. Deviation
CTRL	.00 a	.000
SNB	29.33 b	2.52
SIL	26.00 b	2.00

Treatment	Subset*
	1	2
CTRL	.00
SIL		26
SNB		29.33

* Different letters indicate significant difference

according to Tukey’s test (α = 0.05)

6. Table 6: ANOVA table for Contact angle measurements (°), with surface treatments (SIL, SNB, and CTRL) are the independent factor.

	Sum of Squares	df	Mean Square	F	Sig.
Between Groups	15446.711	2	7723.356	356.306	.000
Within Groups	910.400	42	21.676
Total	16357.111	44

7. Table 7: Contact angle measurements (°) and standard deviation with subset tables (Tukey’s test) of Y-TZP samples after receiving different surface treatments (SIL, SNB, and CTRL)

Treatment	Mean	Std. Deviation
CTRL	50.97 a	6.30
SNB	20.63 b	4.45
SIL	6.57 c	2.37

Treatment	Subset*
	1	2	3
CTRL	50.97
SNB		20.63
SIL			6.57

* Different letters indicate significant difference

according to Tukey’s test (α = 0.05)

8. Table 8: ANOVA table for Zirconia-composite Bond Strength (MPa) values, with surface treatments (SIL, SNB, and CTRL) are the independent factor.

	Sum of Squares	df	Mean Square	F	Sig.
Between Groups	168.533	2	84.267	72.723	.000
Within Groups	48.667	42	1.159
Total	217.200	44

9. Table 9: Zirconia-composite Bond Strength (MPa) values and standard deviation with subset tables (Tukey’s test) after receiving different surface treatments (SIL, SNB, and CTRL) and cemented using resin cement.

Treatment*	Mean	Std. Deviation
CTRL	5.30 a	.7
SNB	6.40 b	.7
SIL	9.9 c	1.5

Treatment	Subset*
	1	2	3
CTRL	5.3
SNB		6.40
SIL			9.9

* Different letters indicate significant difference

according to Tukey’s test (α = 0.05)

10. Table 10 : Gantt-Chart

• Figures:

1. Figure 1:

  

Figure 1: EDS evaluation for different Y-TZP samples’ shows similar chemical components of the surface, mainly Zr~70 wt. %, O~ 22.5 wt. %, and C~7 wt. %.

2. Figure 2:

* Different letters indicate statistically significant difference according to Tukey’s test (α = 0.05)

Figure 2: Bar graph illustrating mean surface roughness values. The lowest roughness was presented by the control group, followed by SIL-treated samples. The highest roughness was presented by SNB-treated samples.

3. Figure 3:

                    

I. SEM of CTRL samples revealed a relatively smooth surface lacking any irregularities compared to the later groups. Shallow straight-line grooves appeared on high magnification are resultant from polishing with silicon carbide abrasive paper (600 grit).

                        

II. SEM of SNB samples revealed a roughened and irregular surface. The bright spots (star) are the zirconia particles, the dark spots (arrows) are reflecting the points of impact with sands resulted in a depression and crack formation. Higher alumina content in those dark spots was confirmed using EDS.

                       

III. SEM of SIL samples revealed a roughened surface but more regular than SNB. The bright spots (star) are the zirconia particles; the dark spots (arrows) are reflecting the points of impact with silica-coated alumina sands resulted in a depression and crack formation. Higher alumina and silica content in those dark spots was confirmed using EDS. The cracks formed were smaller in size and number compared to SNB- treated samples.

4. Figure 4:

Z

Figure 4: EDS analysis shows the presence of Si on the surface of SIL-treated samples. Al particles are                              remnant after blasting.

5. Figure 5:

 

 

Figure 5: EDS mapping for SIL-treated samples. SEM (A) revealed a roughened irregular surface after SIL application. B and C show Alumina and Silica distribution respectively, Lower brightness indicates lower Si/Al contents. It is clear that the distribution areas for Si and Al were identical but not uniform, and appeared to be consistent with the dark spots (arrows) on the SEM; reflecting the cracks/ depressions resulted from the impact of the CoJet sands. Zirconia particles (D) are distributed all over the surface and not covered completely by silica.

6. Figure 6:

  

Figure 6: EDS analysis of SIL-treated samples before (A) and after (B) ultrasonic cleaning with distilled water. The spectrum of the SIL-treated group (A) clearly revealed the existence of silica on the surface. From the spectra of the SIL-treated group with ultrasonic cleaning for 5 minutes (B), a decrease in silica content was clearly observed (arrow) when compared with the spectrum of silica coating group.

7. Figure 7:

Figure 5: X-ray diffraction patterns of Y-TZP surfaces after different surface treatments quantified by RIR analysis the X’Pert Quantify software. The top graph and middle graph show that both tetragonal and monoclinic peaks are present in SNB group and SIL group respectively. The bottom graph shows that only tetragonal peaks are observed in CTRL group.

8. Figure 8:

* Different letters indicate statistically significant difference according to Tukey’s test (α = 0.05)

Figure 6: Bar graph illustrating mean monoclinic phase content. No detectable trace of monoclinic phase was observed for the control group. No significant difference was observed between the other two groups.

 

9. Figure 9:

Figure 9:  A scatterplot graph showing the correlation between monoclinic phase % and surface roughness. There is a linear relationship with a strong positive correlation (p<.001).

10. Figure 10:

 

Figure 10:  Figures 2 and 8 beside each other to understand the relationship easier.

11. Figure 11:

* Different letters indicate statistically significant difference according to Tukey’s test (α = 0.05)

Figure 7: Bar graph illustrating the mean contact angle (°). The highest value measured was for the control group. The lowest value measured for SIL-treated samples. Surface treatment (SIL, and SNB) decreased the contact angle significantly compared to the control group.

12. Figure 12:

* Different letters indicate statistically significant difference according to Tukey’s test (α = 0.05)

Figure 8: Bar graph illustrating the mean zirconia-composite bond strength values (MPa) after 48 hours storage. The lowest value recorded was for the control group. SIL – treated surfaces resulted in values significantly higher than the other groups.

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