Superplasticity and machinability in a four-phase ceramic

Fine grained four-phase ceramic materials were fabricated to have a combination of high temperature superplasticity and room temperature machinability. The composite ceramics were made of 25 vol.% 3Y-TZP/8YSZ–25 vol.% Al2O3–25 vol.% MgAl2O4–25 vol.% LaPO4, using LaPO4 powders fabricated in-house. X-ray diffraction and scanning electron microscopy revealed that the grain size for the best mixed samples was in the range of 600 nm, tetragonal ZrO2 transformed into monoclinic, and reactions in the sintered samples produced a new phase, magnetoplumbite (LaMgAl11O19) with lath-like grains. The formation of magnetoplumbite was facilitated by the presence of yttria and by a liquid phase generated at elevated temperatures. These four-phase ceramics had a maximum hardness of 12 GPa and fracture toughness was no more than 3 MPa√m. Deformation rates at 1400 °C under 40 MPa stress were in the superplastic range of 10−3 s−1 for most compositions. These four-phase ceramics were machinable as demonstrated using conventional tungsten drill bits.