Abstract: BACKGROUND: The effect of heating under different reduced air pressures on the internal oxidation and metal-ceramic bond strength of palladium-silver alloys requires further research, and controversy exists with respect to the bonding mechanism between palladium-silver alloys and porcelain. 
OBJECTIVE: To assess the effect of heating under different reduced atmospheric pressure on the metal-ceramic bond strength of palladium-silver alloys.  
METHODS: Palladium-silver metallic specimens (25 mm× 3 mm× 0.5 mm) were prepared using a cast method. Specimens were randomly assigned to seven groups, and subjected to heat treatments under six different reduced air pressures of 0.001 8, 0.002 3, 0.003 6, 0.004 6, 0.005 4, and 0.007 1 MPa and under normal atmospheric pressure (0.1 MPa). Scanning electron microscopy and energy-dispersive spectroscopy were used to study the specimens’ microstructure and elemental composition. After treatment, alloys in the seven groups were subjected to a simulated opaque porcelain sintering procedure to analyze the alloying element composition. The palladium-silver alloy-ceramic specimens were prepared and the microscopic morphology of the specimens was observed with scanning electron microscope. The metal-ceramic bond strength was evaluated using a three-point bending test. 
RESULTS AND CONCLUSION: (1) Scanning electron microscope displayed that nodules-like protrusions appeared on the surface of the heat-treated alloy. As the oxygen partial pressure increased, the number and volume of nodules on the surface of the alloy increased rapidly. When the oxygen partial pressure increased to 0.007 1 MPa, the nodules on the surface of the alloy were fused and connected into a cord or mesh, and finally almost covered the surface of the alloy, which was similar to the coverage of nodules on the surface of the normal pressure heat treatment alloy. Compared with that before heat treatment, the oxide content on the alloy surface increased significantly after heat treatment. As the oxygen partial pressure increased, the oxide content on the alloy surface showed a decreasing trend. (2) Scanning electron microscopy showed that the microstructure of the metal-porcelain interface was similar in each group. The porcelain was tightly bonded to the alloy. The cord-like structure was visible under the surface of the alloy. As the oxygen partial pressure increased, the cord-like structure increased, and the depth of its penetration into the metal increased. After the simulated opaque porcelain was sintered, the oxide content on the alloy surface continued to decrease. (3) The group with heating under 0.001 8, 0.002 3, and 0.003 6 MPa had significantly higher bond strength than those of heating under 0.004 6, 0.005 4, 0.007 1 MPa, and 0.1 MPa groups (P < 0.05). No statistical differences were observed in the bond strengths between the other groups (P > 0.05). (4) The results show that heating palladium-silver alloys under a lower reduced air pressure (≤ 0.003 6 MPa) effectively improved its bonding strength with porcelain. The internal oxidation and the content of external oxides on the metal surface of palladium-silver alloys were closely related with air pressure in the heat treatment.

Key words: materials, Palladium-silver alloy, metal-ceramic bond strength, three-point bending test, oxygen partial pressure, heat treatment, scanning electron microscopy, internal oxidation