To investigate the influence of surface damage on the fatigue behavior of zirconia under two different loading conditions.
Materials and methods
One hundred twenty zirconia bar-shaped received either airborne particle abrasion using 50 μm or 120 μm alumina particles while polished specimens served as control. The specimens were subjected to two fatigue regimes: dynamic fatigue (1,000,000 cycles, 1 hz and 0.5 s contact time) or static fatigue (a constant load applied for 5000 s) under water using the staircase application of the load. The flexure strength after fatigue (dynamic fatigue strength) was compared to the initial flexure strength of the tested specimens ( α = 0.05). The critical crack shape and size of fractured specimens was examined using scanning electron microscopy.
Compared to the initial flexure strength of the tested specimens, dynamic fatigue strength was 86.3% for the polished specimens, 73.4% for 50 μm particle abrasion, and 42.3% for 120 μm particle abrasion while the static fatigue strength was 85.9%, 78.5%, and 51.5% respectively. Significant statistical differences ( F = 223.679, P < 0.001) were found between different surface treatments but not between dynamic and static fatigue strengths for the same type of surface treatment.
The dynamic and static fatigue strengths of zirconia are significantly influenced by type of surface damage.
Within the limitations of this study, surface damage have great influence on fatigue behavior of zirconia.
During function, dental restorations are subjected to different types of stresses that change in magnitude and direction during each masticatory cycle . Nowadays, the tendency to use metal free restorations became the driving force to shift to the more esthetic and more biocompatible all-ceramic restorations. Nevertheless the brittleness of these materials stood as a barrier against their wide-spread use at least in the posterior region of the mouth . The introduction of zirconia to the dental field opened the design and application limits of all-ceramic restorations as today long span and complex zirconia restorations are manufactured using CAD/CAM systems with high accuracy and success rate.
Pure zirconia undergoes a crystal phase transformation during cooling from tetragonal to monoclinic phase starting at about 1170 °C, which is accompanied by a volume increase of approximately 4.5% . However, addition of dopants such as CaO, MgO, Y 2 O 3 , and CeO 2 stabilizes the tetragonal phase at room temperature. The stabilized tetragonal phase could revert to the more stable monoclinic phase when subjected to stress which generates compressive stresses at the tip of propagating crack contributing to superior mechanical properties and higher fracture toughness. Nevertheless, zirconia ceramics are still brittle materials and are very sensitive to surface defects and structural flaws .
According to literature, the fracture strength of brittle ceramics is strongly related to the propagating crack size . For the same material, a large structural defect would result in lower failure stress and vice versa. This relationship is clearly illustrated using the following formula:
σ = K Y a
where σ is the failure stress, K the fracture toughness of the material, Y the geometric constant, and ( a ) is the depth the critical crack. There is also a strong correlation between the initial crack size a i , subcritical crack growth (SCG), and the lifetime (cyclic fatigue number N f ) of brittle ceramics . The closer the initial crack size (a i ) is to the critical size ( a c ) the more rapidly the crack grows to a fatal stage. This relationship is illustrated using the following formula:
N f = 2 Δ σ − n A Y n n − 2 C i 2 − n 2 − C c 2 − n 2