Experimental analysis of the machinability in the thermally assisted milling process of zirconia ceramics

• Thermally assisted milling of Y-TZP which is directly heated by a heater was proposed.
• A series of straight milling experiments in which the workpiece is heated to 450 °C at maximum was conducted.
• The cutting resistance and the tool wear amount were reduced significantly by the proposed method. On the other hand, the surface roughness of the machined grooves was not worsen.
• Fracture on the side walls of the generated grooves was enhanced in the thermally assisted machining, which attributes to the decrease of fracture toughness at elevated temperature.
• Effectiveness of the thermally assisted milling of Y-TZP was confirmed in a way that it reduces cutting resistance and tool wear by utilizing enhanced fracture at elevated temperature while maintaining surface roughness.

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is a promising material for dental restoratives. The fabrication method of such dental restoratives is usually as follows: (1) hot pressing of zirconia powder (2) machining of pre-sintered zirconia (3) sintering. However, the current method has disadvantages such as low geometrical accuracy and long process time. It was adopted since the fully sintered Y-TZP is extremely difficult to machine. If a suitable milling process for fully sintered Y-TZP can be established, it will enable an accurate, efficient, and cost-effective process. The milling process is important for three-dimensional free-form fabrication, which is required for manufacturing dental restoratives. In this study, we propose a thermally assisted milling process in which a fully sintered Y-TZP workpiece is directly heated to several hundred degrees by a heater to soften it and enhance its machinability. In a practical situation, the machinability must be predicted in advance to determine the proper machining conditions, including the workpiece temperature. In order to make such predictions, the machining phenomena occurring during thermally assisted machining must be understood. In addition, the quantitative relationships between the workpiece temperature, cutting conditions, and the resulting machinability must be characterized. In the present study, we conducted a series of straight-milling experiments to observe the machining phenomena and obtain the quantitative relationships between the machining conditions and the machinability of Y-TZP using the proposed thermally assisted milling process. A series of experiments were performed using the central composite design method. The results of the experiments revealed that raising the workpiece temperature significantly reduces cutting resistance and the tool wear. Although a greater amount of fracturing occurred when the workpiece was heated, the resulting roughness of the bottom surface of the grooves did not increase. Quantitative relationships between the machining conditions, including the workpiece temperature, and the machinability were established using a curve-fitting method.