FRACTURE-TOUGHNESS, DIAMETRICAL STRENGTH, AND FRACTOGRAPHY OF AMALGAM AND OF AMALGAM TO AMALGAM BONDS

Chevron-notch fracture toughness, diametrical tensile strength and fractography were evaluated for bulk amalgams and for bonds formed between new and 1-day-old amalgams of the same type. Three types of bonded specimens were prepared: 1) by mechanically roughening the 1-day-old amalgam with 600-grit paper; 2) using a new mercury-rich amalgam; and 3) using a bonding resin, either 4-META or a phosphate ester monomer. Similar values in bond properties were obtained with all bonding techniques for two commercial dispersed-phase bonded amalgams, one of which contained palladium; however, bulk fracture toughness of the palladium-containing amalgam was significantly less than for the palladium-free amalgam. This result reveals that the bonding of amalgam to amalgam, at least for these two amalgams, is a surface-related phenomenon, and thus, the traditional reporting of bonding properties as a percentage of bulk properties loses meaning. Short-rod geometry was more representative of the interfacial bond properties since these samples fractured within the interfacial bonds, while diametrical strength samples often fractured slightly away from the interface. The use of bonding resins did not improve bond fracture toughness for either amalgam, while the diametrical strength improved for one of the amalgams. The use of mercury-rich amalgam significantly improved the fracture toughness over all other techniques for one amalgam, while proving to be similar to a 600-grit preparation for the second amalgam. Fractographic analysis revealed the propagation of the fracture along the bonded interface to be through the copper-tin phase, either surrounding the dispersed phase or distributed in the matrix, and through the silver-mercury phase, often by intergranular fracture. Remains of resin were detected, having been broken loose by fracture or still adhering in isolated regions. Distinctions in the fractography between the bulk amalgams may explain the significant differences detected in their fracture toughnesses.