Porcelain-Fused-to-Metal (PFM) Dental Materials Processed Using Microwave Energy

Objectives: The most commonly used biomaterials for the fabrication of dental crowns, inlays and onlays include dental ceramics, metal alloys and composites (porcelain-fused-to-metal dental materials). Traditionally, dental restorative materials (e.g., dental crowns) have been processed by radiant heating in conventional dental furnaces. Microwaves have been used for processing various industrial materials (i.e., ceramics, metals and composites) in the past. However, the use of microwaves for processing biomaterials has only been shown recently by few authors. We investigated the feasibility of using microwave energy to sinter commonly used porcelain-fused-to-metal dental materials. Our goal was to show the feasibility of using alternate processing techniques that can reduce the sintering time, while maintaining and/or improving the physical properties (mechanical and microstructural) of the final biomaterial(s).

Methods: Four groups of dental alloys (Ni-Cr, Co-Cr, Au-Ag-Pd, and Ag-Pd) with matching low-fusing porcelains were sintered in a research microwave (operating frequency of 2.45 GHz and maximum power output of 2 kW) and tested for their mechanical properties with a three-point bend test (ISO 9693); the results were compared to similar specimens sintered in a conventional dental furnace.

Results: The average bond strength values (ISO 9693) from all four groups of specimens sintered in the conventional furnace and microwave furnace specimens were 58.5±16.6 MPa and 72.3±20.2 MPa respectively. The porcelain fusion temperatures (evident from XRD analysis) for microwave sintered specimens were lower by approximately 200 K when compared to conventionally sintered equivalents. The microwave sintered specimens showed increased diffusion of chromium metal species from the metal substrate along the interface into the overlying porcelains.

Conclusions: Our results indicate that microwave sintering can be used as an effective alternative to conventional sintering, while saving time and energy as well as improving the mechanical and microstructural properties of the biomaterial(s).