2.1

Study design

Experimental resin-based luting agents were prepared with tuned elastic moduli: low E (2.6 GPa), intermediate E (6.6 GPa), and high E (13.3 GPa). Tuning of the E was accomplished by altering the formulation of the resin phase. The resin-based luting agents were used to coat acid-etched, silanized feldspar ceramic disks. In total, five groups were tested, including the positive control group (untreated ceramic) and negative control group (acid-etched, silanized ceramic not coated with luting agents). Thirty specimens were tested in each group to allow appropriate Weibull analysis. The response variables were biaxial flexural strength ( σ _bf , MPa), characteristic strength ( σ ₀ , MPa), and Weibull modulus ( m ). A 3D finite element analysis (3D-FEA) was used to simulate the biaxial flexural test employed in the study. Scanning electron microscopy (SEM) was used to investigate the morphology of the bonded interfaces. Fractographic analysis of the fractured surfaces after mechanical testing was also carried out using SEM.

2.2

Formulation of experimental resin-based luting agents

The experimental resin-based luting agents were formulated using a combination of the monomers urethane dimethacrylate (UDMA), 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]-propane (BisGMA), triethyleneglycol dimethacrylate (TEGDMA), methyl methacrylate (MMA), and/or bisphenol-A ethoxylated dimethacrylate with 30 ethylene oxide units between the aromatic backbone and the insaturates (BisEMA30), all from Esstech Inc. (Essington, PA, USA). The monomer types and fractions added to each formulation were selected in order to tune the E of the materials, which was accomplished by testing and adjusting many formulations in pilot studies. The final compositions of the organic matrix and E of the three luting agents tested (low E , intermediate E , high E ) are presented in Table 1 . For all luting agents, a 0.4% mass fraction of camphorquinone (Sigma–Aldrich, St. Louis, MO, USA) was used as photosensitizer, and a 0.8% mass fraction of ethyl 4-dimethylamino benzoate (Sigma–Aldrich) was used as coinitiator. The organic matrix was loaded with 67% mass fraction of barium-borosilicate glass particles (2 μm average size) coated with 1% mass fraction of silane coupling agent (V-119-4120; Esstech). The materials were mechanically mixed using a centrifugal mixer (SpeedMixer DAC150; FlackTek, Landrum, SC, USA) to produce homogeneous pastes.

2.3

Preparation of ceramic disks

Feldspar ceramic CAD/CAM blocks (I14 A1C Vitablocs Mark II for Cerec; Vita Zahnfabrik, Bad Säckingen, Germany) were milled under water-cooling to generate a cylindrical shape (12 mm diameter). The cylinders were sectioned using a diamond saw (Isomet 1000; Buehler, Lake Bluff, IL, USA) generating disks of 0.8 ± 0.1 mm in thickness. This thickness is consistent with ceramic laminate veneers restorations under clinical conditions. The disks were wet-polished with 600 and 1200-grit SiC abrasive papers (Norton S.A., São Paulo, SP, Brazil). All disks were observed using ×40 magnification in a stereomicroscope to ensure that they were free from visible cracks or flaws. The final dimensions of each disk were measured using a digital caliper accurate to 0.001 mm (Mitutoyo, Tokyo, Japan). A total of 150 ceramic disks was obtained and randomly allocated into the five groups described before (n = 30): negative control, positive control, low E , intermediate E , and high E .

2.4

Determination of E of the resin-based luting agents

Five disk-shaped specimens of each luting agent were prepared using a cylindrical silicone mold (5 mm inner diameter, 2 mm thickness). The mold was filled with each luting agent, and the top surface was covered with a Mylar strip. A ceramic disk was positioned over the Mylar strip, and light-activation was carried out through the ceramic for 60 s using an LED unit (Radii; SDI, Bayswater, Victoria, Australia) with irradiance of 1200 mW/cm ² , which was monitored throughout the experiment. The specimens were wet-polished using 400, 1200, 1500, 2000, and 2500-grit SiC abrasive papers (Norton S.A.). Final polishing was carried out using felt disks (Buehler) and diamond suspensions with 3, 1, and 0.25 μm particles (Buehler). Five Knoop hardness indentations were made on the top surface of each specimen, under a load of 50 kgf for 15 s, using a microindenter (FM-700; Futuretech, Tokyo, Japan). The decrease in the length of the indentation diagonals caused by the material elastic recovery is related to the hardness and E ratio ( H/E ), which was calculated using Eq. (1) :

where a is the minor and b the major diagonal of the Knoop indentation in the fully loaded state, given by a constant 0.140647, a ′ and b ′ are the altered dimensions when fully recovered, and α ₁ = 0.45 is a proportionality constant. The five readings made in each specimen were averaged.

2.5

Resin-coating of ceramic disks

One of the ceramic disk surfaces was etched with 10% hydrofluoric acid for 90 s (Dentsply Caulk, Milford, DE, USA), washed for 60 s, and dried with water- and oil-free compressed air for 30 s. Two layers of a silane coupling agent (Dentsply) were applied, and after 60 s, they were dried with compressed air for 30 s. A standard volume of resin-based luting agent was applied to the center of the disk, and a Mylar strip was lightly pressed to extrude the luting agent. The specimen was placed on top of a highly polished disk (14 mm diameter, 2 mm thickness) of a resin composite (Filtek Z350 XT; 3M ESPE, St. Paul, MN, USA, shade A2D) used to emulate dentin. The flexural modulus of this resin composite reported by its manufacturer is 11.4 GPa. The disk was centrally orientated on a leveled loading platform, and its top ceramic surface was loaded with a 5 N load for 60 s. Loading was applied using a 10-mm-diameter ball indenter with an intermediary 2-mm-thick rubber layer to reduce contact-induced damage . Excess luting agent was removed using a microbrush, and light-curing was performed through the ceramic for 60 s. The thickness of the resin-based luting agent layer was measured afterwards. Resin-ceramic specimens with luting agent thickness outside the range between 100 μm and 150 μm were discarded and replaced by new specimens. The specimens were dry-stored at 37 °C for 24 h in lightproof containers.

2.6

Biaxial flexural test

The σ _bf of the specimens was measured on a mechanical testing machine (DL500; EMIC, São José dos Pinhais, PR, Brazil) using a ball-on-ring setup. The specimens were placed on a 10-mm-diameter knife-edged support and centrally loaded with a spherical indenter (4 mm diameter) at a crosshead speed of 1 mm/min. A thin section of rubber dam sheet was placed between the support and disk to accommodate slight distortions in specimen geometry. The σ _bf (MPa) of the uncoated ceramic disks (monolayer configuration) was calculated using Eq. (2) :

where P is the fracture load (N), ν the Poisson’s ratio (0.25) of ceramic , t the disk thickness (mm), a the radius of the knife-edged support (mm), R the radius of the disk-shaped specimen (mm), and b the radius of the loading contact area at the center of the specimen (mm), determined using Eq. (3) :

The σ _bf of the ceramic specimens coated by the resin-based luting agents (bilayer configuration) was calculated according to the analytical solutions described and tested before . First, the E of the ceramic ( E*1
E 1 *
) and resin-based luting agents ( E*2
E 2 *
) were calculated as a function of the Poisson’s ratio of the ceramic and luting agent, according to Eq. (4) :

where E ₁ is the E of ceramic , E ₂ the measured E of the resin-based luting agents, and ν ₁ and ν ₂ the Poisson’s ratios of the ceramic (0.25) and resin-based luting agents (0.27) . The neutral plane ( tn ) of the coated ceramic specimens was calculated as a function of the ceramic and resin-based luting agent thicknesses ( t ₁ and t ₂ ) and the calculated E*1
E 1 *
and E*2
E 2 *
, using Eq. (5) :

The σ _bf of the coated specimens was calculated at z-axial positions at the center of the disks, where the ceramic surface at the bonded interface is located (position z = 0) and the resin-based luting agent surface above the ring of the ball-on-ring setup is located (position z = −t ₂ ), according to Eqs. (6) – (8) :

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