Fracture toughness ( K IC ) of a group of CAD/CAM nonmetallic blocks was measured.
Two were glass-ceramic-based, 2 were resin-composite-based, and one feldspathic porcelain, testing both glass-ceramic materials before and after firing.
An alternative loading approach was followed, using a modified test arrangement that was quite easy to follow, with a simplified specimen preparation.
ANOVA revealed significantly different mean K IC among materials, with the fired specimens revealing significantly highest values.
This in-vitro study determined plane-strain fracture toughness ( K IC ) of five different chairside CAD/CAM materials used for crown fabrication, following alternative innovative loading approach of compact tension test specimens.
Rectangular-shaped specimens were cut from CAD/CAM blocks ( n = 10): Vita Mark II (Vident) (VMII); Lava-Ultimate (3M/ESPE) (LU); Vita Enamic (Vident) (VE); IPS e.max CAD (Ivoclar Vivadent); crystallized and un-crystallized (E-max and E-max-U, respectively); and Celtra Duo (Dentsply) fired and unfired (CD and CD-U, respectively).
Specimens were notched with thin diamond disk prior to testing. Instead of applying tensile loading through drilled holes, a specially-made wedge-shaped steel loading-bar was used to apply compressive load at the notch area in Instron universal testing machine. The bar engaged the top ¼ of the notch before compressive load was applied at a cross-head speed of 0.5 mm/min. Fracture load was recorded and K IC calculated. Data was statistically-analyzed with one-way ANOVA at 95% confidence level and Tukey’s tests.
Means and SDs of K IC in MPa m 1/2 for VMII, LU, VE, E-max, E-max-U, CD and CD-U were: 0.73 (0.13), 0.85 (0.21), 1.02 (0.19), 1.88 (0.62), 0.81 (0.25), 2.65 (0.32) and 1.01 (0.15), respectively. ANOVA revealed significant difference among the groups ( p < 0.001). CD and E-max had significantly highest mean K IC values.
Mean K IC values of the tested materials varied considerably, however, none of them reached mean K IC of dentin (3.08 MPa m 1/2 ) previously reported. For E-max and CD, specimens firing significantly increased mean K IC . The modified test arrangement was found to be easy to follow and simplified specimen preparation process.
Ceramics are inorganic products with nonmetallic characteristics. These compounds are fired at higher temperatures to attain desirable properties . Dental ceramics have excellent esthetic properties and are highly biocompatible . New handling and processing technologies have led to a wider range of available modern ceramic materials for CAD/CAM machining , with a more easy fabrication procedure .
Poor longevity of earlier ceramic materials due to increased fracture rates was a main complication of these materials . Ceramics are considered to be brittle in nature having increased susceptibility to fracture under tension. This brittleness results in the development of cracks with subsequent crack propagation and finally catastrophic failure . Moreover, ceramic restorations present in the oral cavity are subject to thermal, chemical and mechanical influences, which concentrate stresses on minute surface areas. These concentrated stresses cause strain(s) to develop .
Dental restorations must be mechanically stable and durable during function , in order to resist deleterious effects of the harsh oral environment. A significant factor affecting strength and mechanical behavior of ceramic materials is the distribution of existing flaws . During CAD-CAM milling procedure, machining and grinding of the blocks creates surface damage in the form of micro-cracks and flaws which may propagate slowly causing failure of the restoration. To counteract this side effect of grinding, polishing or glazing of the outer surfaces of the restoration is routinely performed .
Fracture toughness ( K IC ) is an intrinsic property of a material that relates to its resistance to crack propagation which finally causes its failure . This property is concerned with the critical stress intensity, K , at the crack tip . The critical stress intensity depends on the tri-axial strain conditions and crack instability that occurs under plane strain conditions at a minimum K to be referred to as fracture toughness ( K IC ) .
A restorative material with high fracture toughness ( K IC ) shows better fracture resistance and longevity in service as compared to materials with lower fracture toughness ( K IC ) . For brittle materials, fracture toughness ( K IC ) is one of the most significant mechanical properties that is independent of specimen shape, flaw size and stress concentration .
Numerous techniques have been commonly used for testing fracture toughness ( K IC ). These include: the indentation strength (IS), indentation fracture (IF), the single-edge-notched beam (SENB), single-edge pre-cracked beam (SEPB), compact tension (CT), chevron notched short rod/chevron notched short bar (CNSR/CNSB) and the double torsion double cantilever beam (DCB) .
The single-edge-notched beam (SENB) and compact tension (CT) test geometries are suggested for dental materials, because both tests require a smaller specimen size to fulfill plane strain conditions as compared to configurations of specimens of other tests .
Traditionally, the compact tension (CT) test was used for determining fracture toughness ( K IC ) of resilient materials, such as dentin and resin composites . It has not as yet been used for testing brittle dental ceramic materials.
This study aimed to apply the compact tension (CT) test design in a modified form to measure fracture toughness ( K IC ) of a group of restorative CAD/CAM materials including ceramics and nanoceramic resin composites.
Materials and methods
Machinable CAD/CAM blocks representative of 5 different material types were cut into rectangular-shaped specimens ( n = 10 specimens/material) being 4 mm thick. The specimens conformed to the dimensions prescribed for the compact tension test, ASTM E-399 , however, no holes were drilled. The materials comprised a lithium disilicate glass ceramic, IPS e.max CAD (Ivoclar Vivadent) tested before and after crystallization; a fully-crystallized zirconium-reinforced lithium silicate glass ceramic, Celtra Duo (Dentsply), also tested before and after firing; a feldspathic porcelain, Vita Mark II (Vident); a nanoceramic resin composite, Lava Ultimate (3M/ESPE); and a hybrid ceramic/composite material, Vita Enamic (Vident) ( Table 1 ).
|Material||Type of material||Lot/batch no.||Manufacturer|
|Celtra Duo (CD)||Zirconia-reinforced lithium silicate||18017606||Dentsply|
|IPS e.max CAD (E-max)||Lithium disilicate glass ceramic||N44759||Ivoclar Vivadent|
|Lava Ultimate (LU)||Nanoceramic resin composite||N316515||3M/ESPE|
|Vita Enamic (VE)||Hybrid ceramic or polymer-infiltrated ceramic network||43441||Vita|
|Vita Mark II (VMII)||Feldspathic porcelain||6500||Vita|
Rectangular cuts of the different materials were sliced from CAD-CAM blocks using a water-cooled low-speed diamond saw (Buehler Ltd., Lake Bluff, IL, USA). The width and height of the specimens varied according to block size, 15 mm × 12 mm (E-Max and E-Max-U), 14 mm × 12 mm (VE, CD and CD-U), and 12 mm × 10 mm (VMII and LU). Specimen thickness was maintained at 4 ± 0.5 mm. Three to 4 specimens were cut from each block. The specimens were notched with a 0.15 mm double-sided diamond disk (Hyperflex double-sided, Brassler, USA). Sides of that notch were laterally widened through applying the disk at a slight angle. Effective notch length ( a ) related to the specimen width ( W ) according to the requirement 0.45 ≤ a / W ≤ 0.55. A digital caliber (Mitutoyo Corporation, Kanagawa, Japan) was used for measuring specimen dimensions with an accuracy of ± 0.01 mm ( Figs. 1 and 2 ). Individual precise dimensions of each specimen were used in the equation to calculate its K IC .
Instead of applying tensile loading through drilled holes, a specially-made wedge-shaped steel loading bar with a 1.8 mm thick tip was used to apply compressive load at the notch area engaging the top ¼ of the notch. An Instron universal testing machine (Model 4301, Instron Corp., Canton, MA, USA) was used at cross-head speed 0.5 mm/min ( Fig. 3 ).
Maximum load at fracture was recorded for each specimen and K IC was calculated according to the formula:
K IC = P × Y 2 B × W 1 / 2
where P = maximum load required for fracture in MPa, Y 2 = a tabulated function of a / W , B = thickness, and W = net width in mm.
Statistical analysis was subsequently performed using one-way ANOVA at a 95% confidence interval and Tukey’s multiple comparison tests (IBM SPSS Statistics 20).