Effect of zinc on strength and fatigue resistance of amalgam

Objectives. This study was conducted to determine the effect of zinc in amalgam on the static mechanical properties and resistance to fatigue-crack propagation of amalgams. Methods. Fatigue, creep, compressive and flexure tests were performed on high-Cu Dispersalloy (Johnson and Johnson Dental Products Co., East Windsor, NJ, USA) and low-Cu Velvalloy (S.S. White Dental Products Int., Philadelphia, PA, USA) in both Zn-containing and Zn-free formulations. Linear Elastic Fracture Mechanics principles were used to characterize the fatigue behavior (crack lengths were monitored). Results. The incorporation of Zn into these amalgams significantly improved their fatigue and creep resistance, while the effect of Zn on the static compressive and flexure strengths was not consistent. Zn significantly increased (p < 0.05) the resistance to fatigue crack propagation during Stage II crack growth for both amalgams, and increased the variations in crack velocity for a given stress intensity difference, without visibly altering the path or nature of the fatigue cracks. Possible influences on fatigue behavior were the mixed microstructure (particles and matrix), the nature of the crack tip, and creep. Significance. The superior resistance to tensile fatigue crack propagation of amalgams containing small amounts of Zn (not, vert, similar1 wt%) in vitro compared with amalgams with no Zn correlated with the superior resistance to marginal breakdown in vivo of Zn-containing amalgams. The range of stress intensities over which stable cracks could propagate was small, while the large variations observed in fatigue crack growth rates for individual materials invalidate predicting a unique fatigue life from the empirical equations obtained.