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Objective The aim of the present study was to assess the effect of water storage on the quasi-static properties and cyclic fatigue behavior of four contemporary CAD/CAM resin composite materials. Methods The CAD/CAM resin composites Grandio Blocs, LavaTM Ultimate, CerasmartTM and Brilliant Crios, as well as the direct resin composite Grandio SO, were evaluated. Rectangular plates were cut from the blocks or fabricated using a silicon mold to obtain specimens for fracture toughness (KIc, n = 10), biaxial strength (σ0, n = 30) and cyclic fatigue testing (n = 30). Half of the specimens was stored for 24 h in dry conditions and the other half was aged for 60 days in distilled water at 37 °C. KIc was determined using the Compact-Tension (C(T)) method and σ0 and cyclic fatigue were tested using the Ball-on-Three-Balls assembly. Additional disc-shaped specimens (n = 5) were produced to obtain water sorption curves of the materials. Weibull statistics and two-way ANOVA with Tukey’s post-hoc test were used for data assessment. Results The highest water sorption was observed for LavaTM Ultimate (42.6 μg/mm3), whereas Grandio SO displayed the lowest uptake (14 μg/mm3). A statistically significant drop in KIc and σ0 was measured for all materials after water storage, except for the σ0 of CerasmartTM. Water ageing had a dissimilar effect on the cyclic fatigue behavior, increasing the slow crack growth susceptibility of LavaTM Ultimate, but decreasing it for CerasmartTM and Brilliant Crios. Significance Contemporary CAD/CAM resin composites are susceptible to water driven degradative processes, although differences in filler content and resin matrix constitution play an important role in how it impacts their mechanical properties.

Here we use the 3-point bending with eccentric notch test (3-PBEN) to investigate the fracture behavior of a pressable and a CAD/CAM lithium disilicate (LS2) glass-ceramics under combined mode-I and mode-II loading. The effect of the bulk texturization in the beams of the pressable LS2 is made visible through the fracture trajectory following the most energetically favorable path dictated by the crystallite alignment. The CAD/CAM LS2 shows an isotropic fracture mode but increasing fracture energy with mode-II contribution, similar to the pressable variant.

Objective Strength is one of the preferred parameters used in dentistry for determining clinical indication of dental restoratives. However, small dimensions of CAD/CAM blocks limit reliable measurements with standardized uniaxial bending tests. The objective of this study was to introduce the ball-on-three-ball (B3B) biaxial strength test for dental for small CAD/CAM block in the context of the size effect on strength predicted by the Weibull theory. Methods Eight representative chairside CAD/CAM materials ranging from polycrystalline zirconia (e.max ZirCAD, Ivoclar-Vivadent), reinforced glasses (Vitablocs Mark II, VITA; Empress CAD, Ivoclar-Vivadent) and glass-ceramics (e.max CAD, Ivoclar-Vivadent; Suprinity, VITA; Celtra Duo, Dentsply) to hybrid materials (Enamic, VITA; Lava Ultimate, 3M ESPE) have been selected. Specimens were prepared with highly polished surfaces in rectangular plate (12 × 12 × 1.2 mm3) or round disc (O = 12 mm, thickness = 1.2 mm) geometries. Specimens were tested using the B3B assembly and the biaxial strength was determined using calculations derived from finite element analyses of the respective stress fields. Size effects on strength were determined based on results from 4-point-bending specimens. Results A good agreement was found between the biaxial strength results for the different geometries (plates vs. discs) using the B3B test. Strength values ranged from 110.9 MPa (Vitablocs Mark II) to 1303.21 MPa (e.max ZirCAD). The strength dependency on specimen size was demonstrated through the calculated effective volume/surface. Significance The B3B test has shown to be a reliable and simple method for determining the biaxial strength restorative materials supplied as small CAD/CAM blocks. A flexible solution was made available for the B3B test in the rectangular plate geometry.

Objective Development of residual stresses is a potential source of premature fractures in glassy materials, being of special interest in novel lithium silicate glass-ceramics that require a crystallization firing to achieve their final mechanical properties. The aim of this work was to assess the influence of various firing tray systems and the application of different cooling protocols on the development of residual stresses in Suprinity PC crowns. Their effect on the in vitro lifetime of the restorations was also studied. Methods Thirty crowns were milled out of Suprinity PC blocks and crystallized using one of five different commercial firing tray systems (n = 6). Samples in each group were cooled following a fast (FC = 5.5 °C/s), a slow (SC = 0.4 °C/s) or the manufacturer’s reference cooling (REF ). Obtained crowns were sagittally or transversally sectioned and the magnitude and distribution of residual stresses was determined using the light birefringence method. Extra crowns of three of the subgroups (n = 8) were produced and submitted to chewing simulation for 106 cycles or until fracture ensued. Results Average residual stresses ranged between 0 and 1.5 MPa (peaks of 5 MPa). Highest stress magnitudes were observed at the support areas of groups using firing pins, leading to thermal cracks in FC samples and premature failures in the REF subgroup. The use of fibrous pads and firing pastes limited the development of residual stresses, whereas application of SC regimes extended the lifetime of the restorations. Significance Development of residual stresses during crystallization firing in lithium silicate glass-ceramics results critical for their mechanical performance and should be therefore avoided by ensuring a homogenous cooling of the structures.

Objectives The present study evaluated the effect of the coefficient of thermal expansion (CTE) mismatch and the cooling protocol on the distribution of residual stresses and crack propagation in veneered zirconia bilayers. Methods Ceramic discs with two different CTEs (Vita VM9 and Lava Ceram) were fired onto zirconia plates and cooled following a slow (0.5 °C/s) or a fast (45 °C/s) cooling protocol. The residual stress distribution throughout the veneer thickness was assessed by means of depth-wise Vickers indentation after sequentially sectioning the bilayers parallel compared to normal to the interface. A mathematical solution for the residual stress distribution was used as reference. Additionally, Hertzian cone crack propagation in the veneers was induced by cyclic contact loading and measured at different number of cycles to estimate the crack growth rate. Results The higher CTE mismatch of the VM9 group generated an important stress gradient with high compressive residual stresses near the interface, hindering the crack propagation. The low CTE mismatch group (Lava Ceram) developed only a slight stress gradient and higher cone crack growth rates. No differences were observed between the two cooling protocols applied regarding stress magnitude and crack propagation behavior. Significance The CTE mismatch has a predominant role in the generation of residual stress gradients within the veneer, which directly influences contact-induced crack propagation. Based on the results, the cooling protocol had no significant effect on the residual stress distribution in zirconia-veneer bilayers.

Objectives Chemical and mechanical degradation play a key role on the lifetime of dental restorative materials. Therefore, prediction of their long-term performance in the oral environment should base on fatigue, rather than inert strength data, as commonly observed in the dental material’s field. The objective of the present study was to provide mechanistic fatigue parameters of current dental CAD/CAM materials under cyclic biaxial flexure and assess their suitability in predicting clinical fracture behaviors. Methods Eight CAD/CAM materials, including polycrystalline zirconia (IPS e.max ZirCAD), reinforced glasses (Vitablocs Mark II, IPS Empress CAD), glass-ceramics (IPS e.max CAD, Suprinity PC, Celtra Duo), as well as hybrid materials (Enamic, Lava Ultimate) were evaluated. Rectangular plates (12 × 12 × 1.2 mm3) with highly polished surfaces were prepared and tested in biaxial cyclic fatigue in water until fracture using the Ball-on-Three-Balls (B3B) test. Cyclic fatigue parameters n and A* were obtained from the lifetime data for each material and further used to build SPT diagrams. The latter were used to compare in-vitro with in-vivo fracture distributions for IPS e.max CAD and IPS Empress CAD. Results Susceptibility to subcritical crack growth under cyclic loading was observed for all materials, being more severe (n ≤ 20) in lithium-based glass-ceramics and Vitablocs Mark II. Strength degradations of 40% up to 60% were predicted after only 1 year of service. Threshold stress intensity factors (Kth) representing the onset of subcritical crack growth (SCG), were estimated to lie in the range of 0.37–0.44 of KIc for the lithium-based glass-ceramics and Vitablocs Mark II and between 0.51–0.59 of KIc for the other materials. Failure distributions associated with mechanistic estimations of strength degradation in-vitro showed to be useful in interpreting failure behavior in-vivo. The parameter Kth stood out as a better predictor of clinical performance in detriment to the SCG n parameter. Significance Fatigue parameters obtained from cyclic loading experiments are more reliable predictors of the mechanical performance of contemporary dental CAD/CAM restoratives than quasi-static mechanical properties.

Objective A deeper understanding of the mechanical behavior of dental restorative materials requires an insight into the materials elastic constants and microstructure. Here we aim to use complementary methodologies to thoroughly characterize chairside CAD/CAM materials and discuss the benefits and limitations of different analytical strategies. Methods Eight commercial CAM/CAM materials, ranging from polycrystalline zirconia (e.max ZirCAD, Ivoclar-Vivadent), reinforced glasses (Vitablocs Mark II, VITA; Empress CAD, Ivoclar-Vivadent) and glass-ceramics (e.max CAD, Ivoclar-Vivadent; Suprinity, VITA; Celtra Duo, Dentsply) to hybrid materials (Enamic, VITA; Lava Ultimate, 3M ESPE) have been selected. Elastic constants were evaluated using three methods: Resonant Ultrasound Spectroscopy (RUS), Resonant Beam Technique (RBT) and Ultrasonic Pulse-Echo (PE). The microstructures were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), Raman Spectroscopy and X-ray Diffraction (XRD). Results Young’s modulus (E), Shear modulus (G), Bulk modulus (B) and Poisson’s ratio ( ν ) were obtained for each material. E and ν reached values ranging from 10.9 (Lava Ultimate) to 201.4 (e.max ZirCAD) and 0.173 (Empress CAD) to 0.47 (Lava Ultimate), respectively. RUS showed to be the most complex and reliable method, while the PE method the easiest to perform but most unreliable. All dynamic methods have shown limitations in measuring the elastic constants of materials showing high damping behavior (hybrid materials). SEM images, Raman spectra and XRD patterns were made available for each material, showing to be complementary tools in the characterization of their crystal phases. Significance Here different methodologies are compared for the measurement of elastic constants and microstructural characterization of CAD/CAM restorative materials. The elastic properties and crystal phases of eight materials are herein fully characterized.

The aim of this study was to compare the effect of different mechanical surface treatments and chemical bonding protocols on the tensile bond strength (TBS) of aged composite. Bar specimens were produced using a nanohybrid resin composite and aged in distilled water for 30 days. Different surface treatments (diamond bur, phosphoric acid, silane, and sandblasting with Al₂O₃ or CoJet Sand), as well as bonding protocols (Primer/Adhesive) were used prior to application of the repair composite. TBS of the specimens was measured and the results were analyzed using analysis of variance (ANOVA) and the Student-Newman-Keuls test (α = 0.05). Mechanically treated surfaces were characterized under SEM and by profilometry. The effect of water aging on the degree of conversion was measured by means of FTIR-ATR spectroscopy. An important increase in the degree of conversion was observed after aging. No significant differences in TBS were observed among the mechanical surface treatments, despite variations in surface roughness profiles. Phosphoric acid etching significantly improved repair bond strength values. The cohesive TBS of the material was only reached using resin bonding agents. Application of an intermediate bonding system plays a key role in achieving reliable repair bond strengths, whereas the kind of mechanical surface treatment appears to play a secondary role.

Objectives This study was intended to separate residual stresses arising from the mismatch in coefficients of thermal expansion between glass and zirconia (ZrO2) from those stresses arising solely from the cooling process. Slow crack growth experimentes were undertaken to demonstrate how cracks grow in different residual stress fields. Methods Aluminosilicate glass discs were sintered onto ZrO2 to form glass–ZrO2 bilayers. Glass discs were allowed to bond to the ZrO2 substrate during sintering or prevented from bonding by means of coating the ZrO2 with a thin boron nitrade coating. Residual stress gradients on “bonded” and “unbonded” bilayers were assessed using birefringence measurements. Unbonded glass discs were further tested under biaxial flexure in dynamic fatigue conditions in order to evaluate the effect of residual stress on the slow crack growth behavior. Results When fast-ccoling was induced, residual tensile stresses on the glass increased significantly on the side toward the ZrO2 substrate. By allowing the bond between glass and ZrO2, those tensile stresses observed in unbonded specimens are overwhelmed by the contraction mismatch stresses between the ZrO2 substrate and the glassy overlayer. Specimens containing residual tensile stresses on the bending surface showed a time-dependent strength increase in relation to stress-free annealed samples in the dynamic biaxial bending test, with this effect being dependent on the magnitude of the residual tensile stress. The phenomenon observed is explained here on the basis of the water toughening effect, in which water diffuses into the glass promoting local swelling. An additional residual tensile stress at the crack tip adds an applied-stress-independent (Kres) term to the total tip stress intensity factor (Ktip), increasing the stress-enhanced diffusion and the shielding of the crack tip through swelling of the crack faces. Significance Residual stresses in the glass influence the crack growth behavior of veneered-ZrO2 bilayered dental prostheses. The role of water in crack growth might be of higher complexity when residual stresses are present in the glass layer.