Abstract
Objectives
(1) To determine the surface roughness and material loss of a Y-TZP ceramic before and after tribochemical grit blasting and (2) to characterize the changes in elemental surface composition and the phase transformations after tribochemical treatment.
Methods
Machined bar shaped specimens (Zeno, Wieland) were subdivided into three groups. After grit blasting for 10, 20 and 30 s respectively, half of the specimens of each group were ultrasonically cleaned in ethanol for 10 min. The other half was rinsed with a water spray. Surface roughness was measured using an electro-mechanical profilometer. The elemental composition of the samples was obtained by energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) was used for phase transformations determination.
Results
The median Ra increased significantly from 0.24 to 0.32–0.38 μm after grit blasting. Augmentations were also noted for R max , R v and R p . The highest roughness parameters were, obtained for water sprayed specimens and samples abraded for 30 s. Loss of material ranged between 1 and 3 μm for 30 s grit blasting. Tetragonal and cubic phases were identified in ‘as machined’ specimens. Grit blasting resulted in domain switching and lattice deformations. The elemental composition comprised Si and Al. The duration of grit blasting did not significantly, influence the atomic percentages of Si or Al. Significantly lower values for both Si and Al were noted, after ultrasonic cleaning.
Significance
Grit blasting with CoJet™ Sand resulted in an increase of surface roughness, a removal of maximum 3 μm of material and coated the surface with submicron silica and alumina particles.
1
Introduction
Ongoing developments in high strength ceramics have led dental clinicians to make use of yttria-stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP) in structural applications. Current Y-TZP ceramics consist of 0.2–0.5 μm diameter equiaxed grains of ZrO 2 and 1.75–3.5 mol% (3.5–8.7 wt%) Y 2 O 3 . Depending on composition, firing time and temperature, Y-TZP ceramics may contain mixtures of tetragonal and cubic phases or of two tetragonal ( t and t ′) phases . The enhanced mechanical properties of Y-TZP , that is, a flexural strength of 800–1100 MPa and fracture toughness in the 6 and <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='8MPam’>8MPam−−√8MPam
8 MPa m
range are due to tetragonal to monoclinic phase transformations and their associated mechanisms of volume increase and development of compressive strain fields that oppose crack propagation . The phase transformation may be triggered by externally applied stresses , airborne particles abrasion , or hydrothermal aging .
Due to their superior mechanical properties, zirconia-based fixed dental prostheses can be cemented using conventional techniques. Indeed, most manufacturers of adhesive resin cements do not recommend any particular surface treatments of the Y-TZP ceramic prior to cementation. Nevertheless, efforts have been made to modify the surface properties of the zirconia ceramic with the objective of improving adhesion using surface pretreatments such as silanization , alumina grit blasting and a laboratory tribochemical silicoating system Rocatec™ . Recent in vitro studies showed that the bond strength of MDP and resin based cements to a Y-TZP ceramic increased significantly after tribochemical silica coating using the chairside CoJet™ Sand system (30 μm alumina silica-coated particles) .
The CoJet™ Sand manufacturer (3 M ESPE, Seefeld, Germany) recommends to remove any residual blast-coating agent with a stream of oil-free air, and most clinicians will do so using either gentle air or air–water sprays to rid the surface of loose particles. One group ultrasonically cleaned their Y-TZP surfaces in 96% isopropanol after silica coating without detrimentally effecting adhesion .
The particle sizes for grit blasting of zirconia range between 25 and 110 μm. It was observed that blasting with 110 μm alumina particles significantly increased the zirconia’s static flexural strength but detrimentally affected its overall reliability due to damages such as lateral crack formation and diffuse surface flaws . When polished Y-TZP surfaces were grit blasted with 50 or 120 μm particles, a reduction of static flexural strength and reliability was observed after large grit treatment . Conversely, the surface damage using 50 μm alumina particles on Y-TZP polished surfaces resulted in a substantial loss of strength under high frequency loading due to coalescing microcroacks .
Grit blasting a CAD–CAM machined surface with alumina particle sizes of 25, 50 and 110 μm, did not lead to differences in biaxial strength nor to a decrease in reliability when the samples were stored in water . Further, when ‘as machined’ Y-TZP CAD–CAM surfaces were grit blasted with 50 μm alumina particles, a significant improvement in strength and reliability was registered . It is reasoned that CAD–CAM ceramic frameworks harbour surface flaws induced during fabrication and finishing procedures but that grit blasting smoothens machining defects and creates a compressive layer due to phase transformations.
Besides, the adhesion to resin based cements is enhanced by grit blasting. Considering the improved adhesion when using the CoJet™ Sand, several questions remain open; in particular, concerning the effects of a “soft” blasting of the zirconia frame (30 μm particles, 2.5 bars).
In this regard, the present project is part of a research line aimed at establishing the relationships between the surface textures generated by clinically relevant preparation methods and the fatigue resistance of zirconia ceramics. More specifically, the objectives of the present study were to
- (1)
Determine the surface roughness of a Y-TZP ceramic after machining using CAD–CAM techniques before and after tribochemical grit blasting (CoJet™ Sand).
- (2)
Determine the loss of material after tribochemical treatment.
- (3)
Characterize the changes in surface elemental composition using EDS (energy dispersive X-ray spectroscopy) after tribochemical treatment using different cleaning methods.
- (4)
Characterize the phases and phase transformations before and after tribochemical treatment using XRD (X-ray diffraction).
2
Materials and methods
2.1
Specimens
18 bar shaped zirconia specimens (Zeno ® Zr, Wieland Dental, Pforzheim, Germany), 40 mm × 3 mm × 5 mm in size were used as machined by the manufacturer. Prior to surface preparation, they were subjected to four firing cycles (930, 900, 890, 880 °C) to duplicate the effect of veneering ceramic application. To determine the effect of grit blasting and treatment duration, the samples were divided into three groups of n = 6. They were grit blasted in their midportion with 30 μm silica-coated alumina particles (CoJet™ Sand, 3M-ESPE, Seefeld, Germany) on a surface of ca. 6 mm × 5 mm for 10, 20 and 30 s respectively. Grit blasting was performed using an air abrasion gun (RondoFlex 2013, KaVo, Biberach, Germany) at 2.5 bars (manufacturer’s instructions), with the tip inclined approximately 45° and at a distance of 7 mm. Half of the specimens of each group was then ultrasonically cleaned in pure ethanol for 10 min using a sonic device (Micro 10+, Unident, Switzerland) set a low frequency range (28–34 kHz). The conditioned surface was always facing up. The other half of the specimens was rinsed with a gentle air–water spray for 10 s with tap water and dried at room temperature ( Fig. 1 ).
2.2
Surface roughness and loss of material
Surface roughness was measured using an electro-mechanical profilometer (M1, Mahr, Göttingen, Germany) equipped with a drive unit (PGK, Mahr) and a measuring sensor (MFW-250, Mahr) with a 40 nm z -resolution. The specimens were mounted on an XY cross-table. Before and after grit blasting, areas of 12 mm × 3 mm, 5.6 mm × 1.75 mm and 0.25 mm × 0.25 mm were scanned using a 2 μm radius diamond stylus at a speed of 0.5 mm/s and an applied force of 0.5 mN. Frames of 21, 51, and 501 line profiles with y -axis steps of 140, 60, and 0.5 μm were obtained. The vertical loss after abrasion was computed after superposing the profiles of the grit blasted and the untreated areas lateral to the grit blasted zones. Three tracings 5 mm in length and separated by a total distance of 140 μm were evaluated. The largest interdistance of each pair of tracings served as basis for comparisons. A 3D imaging software optimized for freeform designs (Imageware, Siemens PLM, Camberley, UK) was used for this purpose. Helpful functions such as non-homogenous scaling (to magnify the z -coordinate), best fit registration (virtual 3D adjustment to the unaffected lateral areas), point to curve fitting and point to curve difference plot have been used.
Roughness parameters were calculated using a Gaussian profile filter (ISO 11562) with the cut-off wavelength set at λ c = 0.8 mm and the evaluation length at l n = 5.6 mm. The surface roughness parameters used in this study were the average surface roughness ( R a ), maximum roughness depth ( R max ), the distance between the highest peak and the reference line ( R p ) and the distance between the deepest valley and the reference line ( R v ). Finally, 3D surface topographies were generated from the 0.25 mm × 0.25 mm area frames.
2.3
Elemental composition
The grit-basted surfaces were analyzed for elemental composition using EDS (energy dispersive X-ray spectroscopy). The data were obtained using a SEM (Leo VP 1455VP, Carl Zeiss, Oberkochen, Germany) fitted with an energy dispersive X-ray spectrometer (INCA, Oxford Instruments, Abingdon, UK). The primary electron energy was varied from 5 to 20 keV. The other testing parameters were set to WD: 15 mm, process time: 5 s, live time: 60 s, dead time: 30–40%. Three different areas were selected for each sample. Each area measured 80 μm × 40 μm and was scanned five times.
2.4
Phase composition
XRD (X-ray diffraction) was employed to quantify the monoclinic and tetragonal phases before and after grit blasting. XRD was performed using a diffractometer (D5005, Bruker AXS, Karlsruhe, Germany) with Ni filtered Cu Kα 1 radiation (1.5418 Å) at 40 kV and 35 mA. Diffraction data were collected from 26 to 65° 2 , with a step size of 0.01° and a counting time of 2 s per step. A quantitative value of phase compositions of the ‘as machined’ Y-TZP ceramic was obtained by applying Rietveld refinement analysis (Topas, Bruker AXS).
2
Materials and methods
2.1
Specimens
18 bar shaped zirconia specimens (Zeno ® Zr, Wieland Dental, Pforzheim, Germany), 40 mm × 3 mm × 5 mm in size were used as machined by the manufacturer. Prior to surface preparation, they were subjected to four firing cycles (930, 900, 890, 880 °C) to duplicate the effect of veneering ceramic application. To determine the effect of grit blasting and treatment duration, the samples were divided into three groups of n = 6. They were grit blasted in their midportion with 30 μm silica-coated alumina particles (CoJet™ Sand, 3M-ESPE, Seefeld, Germany) on a surface of ca. 6 mm × 5 mm for 10, 20 and 30 s respectively. Grit blasting was performed using an air abrasion gun (RondoFlex 2013, KaVo, Biberach, Germany) at 2.5 bars (manufacturer’s instructions), with the tip inclined approximately 45° and at a distance of 7 mm. Half of the specimens of each group was then ultrasonically cleaned in pure ethanol for 10 min using a sonic device (Micro 10+, Unident, Switzerland) set a low frequency range (28–34 kHz). The conditioned surface was always facing up. The other half of the specimens was rinsed with a gentle air–water spray for 10 s with tap water and dried at room temperature ( Fig. 1 ).
2.2
Surface roughness and loss of material
Surface roughness was measured using an electro-mechanical profilometer (M1, Mahr, Göttingen, Germany) equipped with a drive unit (PGK, Mahr) and a measuring sensor (MFW-250, Mahr) with a 40 nm z -resolution. The specimens were mounted on an XY cross-table. Before and after grit blasting, areas of 12 mm × 3 mm, 5.6 mm × 1.75 mm and 0.25 mm × 0.25 mm were scanned using a 2 μm radius diamond stylus at a speed of 0.5 mm/s and an applied force of 0.5 mN. Frames of 21, 51, and 501 line profiles with y -axis steps of 140, 60, and 0.5 μm were obtained. The vertical loss after abrasion was computed after superposing the profiles of the grit blasted and the untreated areas lateral to the grit blasted zones. Three tracings 5 mm in length and separated by a total distance of 140 μm were evaluated. The largest interdistance of each pair of tracings served as basis for comparisons. A 3D imaging software optimized for freeform designs (Imageware, Siemens PLM, Camberley, UK) was used for this purpose. Helpful functions such as non-homogenous scaling (to magnify the z -coordinate), best fit registration (virtual 3D adjustment to the unaffected lateral areas), point to curve fitting and point to curve difference plot have been used.
Roughness parameters were calculated using a Gaussian profile filter (ISO 11562) with the cut-off wavelength set at λ c = 0.8 mm and the evaluation length at l n = 5.6 mm. The surface roughness parameters used in this study were the average surface roughness ( R a ), maximum roughness depth ( R max ), the distance between the highest peak and the reference line ( R p ) and the distance between the deepest valley and the reference line ( R v ). Finally, 3D surface topographies were generated from the 0.25 mm × 0.25 mm area frames.
2.3
Elemental composition
The grit-basted surfaces were analyzed for elemental composition using EDS (energy dispersive X-ray spectroscopy). The data were obtained using a SEM (Leo VP 1455VP, Carl Zeiss, Oberkochen, Germany) fitted with an energy dispersive X-ray spectrometer (INCA, Oxford Instruments, Abingdon, UK). The primary electron energy was varied from 5 to 20 keV. The other testing parameters were set to WD: 15 mm, process time: 5 s, live time: 60 s, dead time: 30–40%. Three different areas were selected for each sample. Each area measured 80 μm × 40 μm and was scanned five times.
2.4
Phase composition
XRD (X-ray diffraction) was employed to quantify the monoclinic and tetragonal phases before and after grit blasting. XRD was performed using a diffractometer (D5005, Bruker AXS, Karlsruhe, Germany) with Ni filtered Cu Kα 1 radiation (1.5418 Å) at 40 kV and 35 mA. Diffraction data were collected from 26 to 65° 2 , with a step size of 0.01° and a counting time of 2 s per step. A quantitative value of phase compositions of the ‘as machined’ Y-TZP ceramic was obtained by applying Rietveld refinement analysis (Topas, Bruker AXS).
3
Results
3.1
Surface roughness and loss of material
An example of surface topography of a 0.25 mm × 0.25 mm area before and after 30 s grit blasting and ultrasonic cleaning is shown in Fig. 2 (for color images see the electronic version). Abrasion erased the helical surface topography resulting from machining but notably increased surface roughness.
Roughness parameters R a , R max , R v , and R p before (i.e. ‘as machined’) and after grit blasting for 10, 20 and 30 s with water spray or ultrasonic cleaning, are tabulated in Table 1A . A Shapiro–Wilk test indicated that the values were not normally distributed. Hence Kruskal–Wallis H and Mann–Whitney U tests ( p < 0.05) were applied to assess the effects of surface treatment ( Table 1B ). Grit blasting with CoJet™ Sand resulted in a significant increase of the median R a values from 0.24 (‘as machined’) to 0.32–0.38 (‘surface treated’). The other surface texture parameters behaved similarly with a significant increase of R max from 2.36 to 2.7–3.3, R v from 0.87 to 1.20–1.39 and R p from 0.98 to 1.11–1.39.
| Abrasion time | Cleaning method | R a (μm) | R max (μm) | R v (μm) | R p (μm) |
|---|---|---|---|---|---|
| As machined | 0.24 [0.15–0.44] | 2.36 [1.39–5.02] | 0.87 [0.51–1.68] | 0.98 [0.55–2.15] | |
| 10 s | Water spray | 0.33 [0.25–0.55] | 3.0 [2.1–5.6] | 1.22 [0.98–2.37] | 1.21 [0.81–1.68] |
| 20 s | Water spray | 0.32 [0.27–0.37] | 2.8 [2.3–4.2] | 1.20 [0.89–1.53] | 1.20 [0.96–1.63] |
| 30 s | Water spray | 0.38 [0.22–0.57] | 3.1 [1.6–5.5] | 1.33 [0.78–2.22] | 1.27 [0.73–1.90] |
| 10 s | Ultrasound | 0.32 [0.19–0.38] | 2.7 [1.8–4.6] | 1.20 [0.69–1.62] | 1.11 [0.71–1.55] |
| 20 s | Ultrasound | 0.32 [0.21–0.37] | 2.7 [1.6–3.4] | 1.20 [0.75–1.50] | 1.15 [0.74–1.49] |
| 30 s | Ultrasound | 0.38 [0.21–0.50] | 3.3 [1.7–4.7] | 1.39 [0.74–1.95] | 1.39 [0.74–1.95] |
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