Annealing study of palladium-silver dental alloys: Vickers hardness measurements and SEM microstructural observations

Three Pd-Ag dental alloys for metal-ceramic restorations, W-1 (Ivoclar Vivadent), Rx 91 (Pentron) and Super Star (Heraeus Kulzer), were subjected to isothermal annealing for 0.5 hr periods in a nitrogen atmosphere at temperatures from approximately 400 degrees to 950 degrees C. The annealing behavior was investigated by Vickers hardness measurements (1 kg load) and SEM microstructural observations. The highest Vickers hardness occurred at approximately 700 degrees C for W-1 and 650 degrees C for Rx 91. For Super Star, there were two peaks in hardness at approximately 500 degrees and 650 degrees C. Additional use of light indenting loads (25 g for W-1; 10 g for Rx 91 and Super Star) revealed that hardness variations during annealing for W-1 and Rx 91 were related to the palladium solid solution matrix phase. For Super Star, the lower-temperature peak was controlled by multi-phase regions and the higher-temperature peak by the matrix phase. While microstructural changes due to annealing were evident with the SEM for Rx 91 and Super Star, no correlation was possible for W-1 because of its finer-scale microstructure. Although commercial Pd-Ag alloys have a relatively narrow composition range, their microstructures and annealing behavior can vary because of differences in proportions of secondary elements utilized for porcelain adherence and grain refinement elements, as well as other proprietary strategies employed by the manufacturers.