Surface conditions are of interest in all-ceramic restorations since they can control both bonding and strength. Tensile testing methods are commonly used to evaluate surface conditions of ceramics. This work evaluated tensile properties of a feldspathic ceramic as-finished, sandblasted and etched under three stressing conditions: (1) biaxial flexure; (2) monotonic mastication loading, dry; and, (3) cyclic mastication loading, wet.
Materials and methods
Feldspathic CAD/CAM blocks were sliced into Tabs 1 mm thick, n = 135 specimens were divided into 3 groups assigned to as-finished (600 grit SiC; control), sandblasted, and etched. Of the 45 specimens per group, 35 specimens were used for bonded tests and 10 specimens for biaxial flexure testing. Pin-on-three ball biaxial testing was performed per ISO 6872. 35 specimens were bonded to dentin-analog bases and loaded to radial crack pop-in beneath a 3 mm diameter piston. 20 specimens were tested dry with failure determined by acoustic emission methods. 15 specimens, bonded to bases having micro-channels for water transport, were cyclically loaded beneath the 3 mm piston under water at 15 Hz for 500,000 cycles.
Biaxial flexure distinguished among all three surface conditions ( p < 0.05, ANOVA). Monotonic testing could not distinguish among groups. Cyclic testing could not distinguish between sandblasted and etched groups but both were weaker than as-finished.
Mastication loading of bonded specimens creates a different stress state than simple flexure due to contributions of the cement–ceramic interface. Water adds a damage accumulation effect. Tensile stress conditions need to be chosen with the desired outcomes considered.
Use of all-ceramic restorations is increasing due to demands for esthetics and biocompatibility, the advent of automated fabrication systems, new materials having improved durability and handling, and the growing body of positive clinical survival data for systems still on the market. Single-layer (monolithic) ceramics are today commonly used as veneers, inlays, onlays and anterior single-unit crowns. While clinical survival data is quite favorable, brittle fracture is still a common reason for failure, so there has been attention on the strength of esthetic ceramics for the long-term survival rate.
Clinically-determined and laboratory-determined factors often discussed as controlling “use” strength include material characteristics , lab processing , surface treatment , cement type and characteristics of the oral environment. Because cracks initiate from surface flaws during clinical failure , methods of surface treatment have received much attention .
Evidence for improved clinical (and laboratory) performance with bonding has been growing for decades. Usually ceramics need surface treatment to increase the bonding strength, and etching or sandblasting are the most common treatment methods . Although etching is well known for increasing the bonding strength by developing an evenly roughened surface, it depends on the type of ceramic and shows sensitivity to etchant concentration and etch time . On the other hand, sandblasting is easy to use but the volume loss can be highly variable according to the blasting time and residual damage can be sever . While many studies have focused on the effect of etching and sandblasting on the bonding strength, very little attention has yet been given to the fracture resistance of bonded feldspathic porcelain under mastication-derived crack development following surface treatment. For bonded and fully supported ceramics, contrary to flexure tests , effects such as flaw “healing” or bridging by resin and the influence of dentin elastic properties can be also expected. Therefore the results of mastication load tests can be more clinically relevant than tensile tests in bending for effects related to ceramic–cement interactions.
Different tensile test methods can create different stress states and be expected to manifest different responses due to interfacial interactions (ceramic–cement) and mechanisms of damage accumulation. Pure bending tests such as 3-point and 4-point bending and biaxial flexure differ mainly in the surface area at risk, and strengths measured by these techniques can be normalized by the simple Weibull scaling relationship as in Eq. (1) below for converting 4-point results to 3-point:
σ 3 -point σ 4 -point = a 4 -point a 3 -point 1 / m