1

Introduction

Tribochemical coating of metal substrates by air-blasting silica-coated alumina particles has been shown to be an effective treatment to promote bonding between metal and composite resins in repair processes of fractured metal-ceramic restorations . This method was found superior to other silica-coating modalities, such as silica-lasing and proved effective for bonding to Ni Cr as well as noble alloys , although for the latter inconsistent results have been reported .

Following grit-blasting, a dry field and extended silane reaction periods (waiting up to 5 min) have been proposed for efficient surface priming . The reaction period is critical because it contributes to solvent evaporation, thin silane film formation to enhance bonding with the substrate rather than intermolecular bonding , and removal of the condensation reaction by-products . Nevertheless, reaction periods of 1–7 min following silanization, failed to significantly differentiate bond strength with resin composites. It has been postulated that the determinant factor for increased interfacial strength is the thickness of the siloxane layer formed, which depends on the silane solution concentration, rather than the reaction period .

All the powders used for intraoral tribochemical coating are based on 30 μm silica-coated alumina particles. Recently, silane encapsulation technology was introduced, for simultaneous tribochemical coating and silanization. This technology includes porous silica microcapsules of ∼30 μm wall thickness, filled with γ-methacryloxypropyl trimethoxysilane (MPTS), and sealed by superficial MPTS condensates . This single-step treatment provides several advantages over the conventional two-step procedures, including reduced chairside time, better reproducibility, no drying period and therefore, a more compatible surface to adhesion of resin composites.

The aim of the present study was to investigate the effects of a tribochemical bonding system based on encapsulated silane on the surface properties of a Co-Cr alloy, in comparison with conventional tribochemical systems. The null hypothesis was that there are no differences in the surface properties of the alloy among the various treatments.

2

Materials and methods

The materials used in the study are listed in Table 1 .

Disk-shape specimens (Ø = 10 mm, h = 2 mm, n = 20) were prepared from cast Co-Cr alloy, metallographically ground-polished (SiC papers, 320–1000 grit size) in a grinding-polishing machine (Ecomet III, Buehler, Lake Bluff, IL, USA) and ultrasonicated (5 min, distilled H ₂ O). The specimens were then randomly assigned into four groups (n = 5 per group) of tribochemically-coating treatments CoJet (CJ), SilJet (SJ), SilJet Plus (SP) and a control of untreated polished alloy (CR). The central part of CJ, SJ and SP specimens was grit-blasted with the corresponding powder employing an intraoral sandblaster (Microetcher IIA, Danville Materials) operated at 2.3 bar air pressure (0.47 L/s flow rate), from 5 mm distance at 90 ° angle for 5 s. All the specimens were dried with a stream of dry, oil-free air for 20 s and then examined by optical interferometric profilometry (OIP), reflection FTIR microscopy (RFTIRM), and scanning electron microscopy/energy dispersive X-ray microanalysis (SEM/EDX).

For OIP analysis an optical profiler (Wyko NT1100, Veeco, Tuscon, AZ, USA) was used on control and grit-blasted surfaces. Measurements were performed under the following conditions: Mirau lens, 40× magnification, vertical scanning mode at 10 mm length, 113.3 × 148.5 μm ² analysis area, tilt correction with 0.1 nm (z-axis) and 0.2 mm (x-axis and y-axis) resolution. The surface roughness parameters measured were (i) Si O C the amplitude parameters Sa (the arithmetic average of the absolute values of the surface height deviations measured from the best fitting plane) (ii) Sz (the 10 point height over the surface, representing the average difference between the 5 highest peaks and 5 lowest valleys), (iii) the hybrid parameter Sdr (the developed area due to the surface texture vs an ideal plane area ratio) and (iv) the functional parameters Sci (the core fluid retention index, describing the volume that a surface would support from 5% to 80% of the bearing ratio in relation to the standard deviation of the height distribution-Sq). Three measurements were performed on each specimen (CR, CJ, SJ, SP groups, n = 5 per group) and averaged as a specimen representative value.

For RFTIRM analysis, an FTIR microscope (AutoImage, Perkin-Elmer, Beaconsfield, Bacon, UK) was used attached to an FTIR spectrometer (Spectrum GX, Perkin-Elmer) operated under the following conditions: Liquid N ₂ -cooled mercury-cadmium telluride (MCT) detector, 4000–650 cm ⁻¹ wavenumber range, 4 cm ⁻¹ resolution, 400 × 300 μm ² aperture and 100 scans co-addition per site. Spectra acquisition and interpretation were performed by AutoImage 5.0/Spectrum 5.0.1 software (Perkin-Elmer). All spectra were subjected to Kramers-Kroning and baseline corrections. Measurements were performed at two randomly selected regions on each specimen (n = 2 per group) after sandblasting and silane application (CR, CJ, SJ groups) plus 1 h storage at 37 ° C (all groups). For silane treatments, S-Bond was applied on the grit-blasted regions using a microfiber brush, left undisturbed for 60 s and then air-dried for 10 s. The 1250–900 cm ⁻¹ wavenumber range was defined as the region of interest, since the major Si O C and Si O Si vibrations appear at this region . The absorbance spectra recorded, were further subjected to Gaussian peak fitting at standard width/variable shape mode and 3% zero baseline. Peak fitting analysis was performed by PeakFit v4.12 software (Seasolve, Framingham, MA, USA) and goodness of the fit was assessed by r ² and sum of squares due to error (SSE) parametric models.

For SEM/EDX analysis the specimens (CR, CJ, SJ, SP groups, n = 3 per group) were examined in a SEM (Quanta 200, FEI, Hilsboro, OR, USA) coupled to an energy dispersive X-ray spectrometer (Sapphire CDU, EDAX Int, Mahwah, NJ, USA) employing a liquid N ₂ -cooled Si (Li) detector with a super ultra-thin Be window (SUTW+, EDAX Int). Secondary (SE) and atomic number contrast backscattered electron images (BE) of selective regions were acquired in the SEM at 15 kV accelerating voltage and 90 μA beam current in high vacuum mode (5 × 10 ⁻⁶ Pa) under 1000× magnification. The specimens were further subjected to elemental analysis by the EDX system. Spectra were collected from each region of interest in area scan mode (15 kV accelerating voltage, 110 μA beam current), 130 × 120 μm ² analysis area at 1000× magnification, 34% detector dead time and 131.4 eV resolution. EDX spectra were subjected to C and ZAF (atomic number, absorption, fluorescence) corrections. Qualitative and quantitative elemental analysis was performed in non-standard mode utilizing a software (Genesis v 5.2, EDAX Int). Elemental mappings of Al and Si were recorded under 5000× and 1000× magnifications and the area coverage by Al + Si was calculated at regions with maximum Al and Si distributions (5000×) and at randomly chosen regions (1000×). The same conditions were used for the analysis of the powders, at 500 × magnification, which were pressed against an aluminum stab covered with self-adhesive graphite tape.

A second series of 15 specimens per group were prepared as above. Five specimens per group were used to evaluate the wettability of the silane treated surfaces with distilled water and the rest for assessment of the shear bond strength with a resin composite. The wettability study was performed by the sessile drop method. Briefly, a 5 μL triple-distilled water drop was applied by a micro syringe on SP and silane-treated CR, CJ, SJ specimen surfaces placed horizontally on an adjustable (x,y, z-tilt) micrometric table. After 10 s, the meniscus of the droplet formed was photographed with a digital camera under ambient conditions (23 ° C, 60% RH, 1:1 magnification) by focusing at the highest meridian plane of the drop section that intersects the equilibrium three-phase line . The contact angle (θ) was measured trigonometrically (θ = 2 arctn 2 h/b, where h the height and b the base length of the meniscus). This method is considered suitable for small drops . One photograph was taken per specimen, since top-view images showed that the droplet was symmetric about a central vertical axis.

For the shear bond strength study (SBS), the SP and silane-treated CR, CJ and SJ specimen surfaces were covered with a 100 μm-thick masking tape, leaving free circular areas (Ø = 2 mm). All treated surfaces were covered with a thin layer (∼0.1 mm thickness) of a light-cured opaquer (Accolade Opaquer TC), which was irradiated for 20 s. Then, cylindrical molds (Ø = 2.4 mm, h = 2 mm) were placed over the treated surfaces, filled with a light-cured flowable composite (StarFlow, A2 shade), and irradiated for 30 s. Irradiation was performed with a LED curing unit (Bluephase 16i, Ivoclar Vivadent, Schaan, Liechtenstein) operated in standard mode (1600 mW/cm ² irradiance with turbo tip). All specimens were stored in distilled water (5 days/37 ° C), then subjected to thermal-cycling in water (3000×, 5/55 ° C, 2 cycles/min, 10 s dwell time, 5 s transfer time) and debonded under shear loading (notched-edge blade method) in a universal testing machine (Tensometer 10, Monsanto, Swindon, UK) operated at 1.0 mm/min crosshead speed. Debonded metal surfaces were subjected to failure mode analysis under a stereomicroscope (M80, Leica Microsystems, Wetlzar, Germany) at 10× magnification. Failure mode was classified as follows: Type I (adhesive at the alloy-composite interface), Type II (cohesive within the composite) and Type III (mixed, type I and II).

Statistical analyses of the roughness parameters, surface wettability by water, extent of Al + Si surface coverage and SBS were performed by one-way ANOVA plus Holm-Sidak multiple comparison tests and by Chi-square test for failure mode analysis (α = 0.05).

2

Materials and methods

The materials used in the study are listed in Table 1 .

Disk-shape specimens (Ø = 10 mm, h = 2 mm, n = 20) were prepared from cast Co-Cr alloy, metallographically ground-polished (SiC papers, 320–1000 grit size) in a grinding-polishing machine (Ecomet III, Buehler, Lake Bluff, IL, USA) and ultrasonicated (5 min, distilled H ₂ O). The specimens were then randomly assigned into four groups (n = 5 per group) of tribochemically-coating treatments CoJet (CJ), SilJet (SJ), SilJet Plus (SP) and a control of untreated polished alloy (CR). The central part of CJ, SJ and SP specimens was grit-blasted with the corresponding powder employing an intraoral sandblaster (Microetcher IIA, Danville Materials) operated at 2.3 bar air pressure (0.47 L/s flow rate), from 5 mm distance at 90 ° angle for 5 s. All the specimens were dried with a stream of dry, oil-free air for 20 s and then examined by optical interferometric profilometry (OIP), reflection FTIR microscopy (RFTIRM), and scanning electron microscopy/energy dispersive X-ray microanalysis (SEM/EDX).

For OIP analysis an optical profiler (Wyko NT1100, Veeco, Tuscon, AZ, USA) was used on control and grit-blasted surfaces. Measurements were performed under the following conditions: Mirau lens, 40× magnification, vertical scanning mode at 10 mm length, 113.3 × 148.5 μm ² analysis area, tilt correction with 0.1 nm (z-axis) and 0.2 mm (x-axis and y-axis) resolution. The surface roughness parameters measured were (i) Si O C the amplitude parameters Sa (the arithmetic average of the absolute values of the surface height deviations measured from the best fitting plane) (ii) Sz (the 10 point height over the surface, representing the average difference between the 5 highest peaks and 5 lowest valleys), (iii) the hybrid parameter Sdr (the developed area due to the surface texture vs an ideal plane area ratio) and (iv) the functional parameters Sci (the core fluid retention index, describing the volume that a surface would support from 5% to 80% of the bearing ratio in relation to the standard deviation of the height distribution-Sq). Three measurements were performed on each specimen (CR, CJ, SJ, SP groups, n = 5 per group) and averaged as a specimen representative value.

For RFTIRM analysis, an FTIR microscope (AutoImage, Perkin-Elmer, Beaconsfield, Bacon, UK) was used attached to an FTIR spectrometer (Spectrum GX, Perkin-Elmer) operated under the following conditions: Liquid N ₂ -cooled mercury-cadmium telluride (MCT) detector, 4000–650 cm ⁻¹ wavenumber range, 4 cm ⁻¹ resolution, 400 × 300 μm ² aperture and 100 scans co-addition per site. Spectra acquisition and interpretation were performed by AutoImage 5.0/Spectrum 5.0.1 software (Perkin-Elmer). All spectra were subjected to Kramers-Kroning and baseline corrections. Measurements were performed at two randomly selected regions on each specimen (n = 2 per group) after sandblasting and silane application (CR, CJ, SJ groups) plus 1 h storage at 37 ° C (all groups). For silane treatments, S-Bond was applied on the grit-blasted regions using a microfiber brush, left undisturbed for 60 s and then air-dried for 10 s. The 1250–900 cm ⁻¹ wavenumber range was defined as the region of interest, since the major Si O C and Si O Si vibrations appear at this region . The absorbance spectra recorded, were further subjected to Gaussian peak fitting at standard width/variable shape mode and 3% zero baseline. Peak fitting analysis was performed by PeakFit v4.12 software (Seasolve, Framingham, MA, USA) and goodness of the fit was assessed by r ² and sum of squares due to error (SSE) parametric models.

For SEM/EDX analysis the specimens (CR, CJ, SJ, SP groups, n = 3 per group) were examined in a SEM (Quanta 200, FEI, Hilsboro, OR, USA) coupled to an energy dispersive X-ray spectrometer (Sapphire CDU, EDAX Int, Mahwah, NJ, USA) employing a liquid N ₂ -cooled Si (Li) detector with a super ultra-thin Be window (SUTW+, EDAX Int). Secondary (SE) and atomic number contrast backscattered electron images (BE) of selective regions were acquired in the SEM at 15 kV accelerating voltage and 90 μA beam current in high vacuum mode (5 × 10 ⁻⁶ Pa) under 1000× magnification. The specimens were further subjected to elemental analysis by the EDX system. Spectra were collected from each region of interest in area scan mode (15 kV accelerating voltage, 110 μA beam current), 130 × 120 μm ² analysis area at 1000× magnification, 34% detector dead time and 131.4 eV resolution. EDX spectra were subjected to C and ZAF (atomic number, absorption, fluorescence) corrections. Qualitative and quantitative elemental analysis was performed in non-standard mode utilizing a software (Genesis v 5.2, EDAX Int). Elemental mappings of Al and Si were recorded under 5000× and 1000× magnifications and the area coverage by Al + Si was calculated at regions with maximum Al and Si distributions (5000×) and at randomly chosen regions (1000×). The same conditions were used for the analysis of the powders, at 500 × magnification, which were pressed against an aluminum stab covered with self-adhesive graphite tape.

A second series of 15 specimens per group were prepared as above. Five specimens per group were used to evaluate the wettability of the silane treated surfaces with distilled water and the rest for assessment of the shear bond strength with a resin composite. The wettability study was performed by the sessile drop method. Briefly, a 5 μL triple-distilled water drop was applied by a micro syringe on SP and silane-treated CR, CJ, SJ specimen surfaces placed horizontally on an adjustable (x,y, z-tilt) micrometric table. After 10 s, the meniscus of the droplet formed was photographed with a digital camera under ambient conditions (23 ° C, 60% RH, 1:1 magnification) by focusing at the highest meridian plane of the drop section that intersects the equilibrium three-phase line . The contact angle (θ) was measured trigonometrically (θ = 2 arctn 2 h/b, where h the height and b the base length of the meniscus). This method is considered suitable for small drops . One photograph was taken per specimen, since top-view images showed that the droplet was symmetric about a central vertical axis.

For the shear bond strength study (SBS), the SP and silane-treated CR, CJ and SJ specimen surfaces were covered with a 100 μm-thick masking tape, leaving free circular areas (Ø = 2 mm). All treated surfaces were covered with a thin layer (∼0.1 mm thickness) of a light-cured opaquer (Accolade Opaquer TC), which was irradiated for 20 s. Then, cylindrical molds (Ø = 2.4 mm, h = 2 mm) were placed over the treated surfaces, filled with a light-cured flowable composite (StarFlow, A2 shade), and irradiated for 30 s. Irradiation was performed with a LED curing unit (Bluephase 16i, Ivoclar Vivadent, Schaan, Liechtenstein) operated in standard mode (1600 mW/cm ² irradiance with turbo tip). All specimens were stored in distilled water (5 days/37 ° C), then subjected to thermal-cycling in water (3000×, 5/55 ° C, 2 cycles/min, 10 s dwell time, 5 s transfer time) and debonded under shear loading (notched-edge blade method) in a universal testing machine (Tensometer 10, Monsanto, Swindon, UK) operated at 1.0 mm/min crosshead speed. Debonded metal surfaces were subjected to failure mode analysis under a stereomicroscope (M80, Leica Microsystems, Wetlzar, Germany) at 10× magnification. Failure mode was classified as follows: Type I (adhesive at the alloy-composite interface), Type II (cohesive within the composite) and Type III (mixed, type I and II).

Statistical analyses of the roughness parameters, surface wettability by water, extent of Al + Si surface coverage and SBS were performed by one-way ANOVA plus Holm-Sidak multiple comparison tests and by Chi-square test for failure mode analysis (α = 0.05).