Synthesis and characterization of Al2O3NbC composite and the effect of Y2O3 addition on its sintering

Alumina composites containing ceramic particles have been intensively studied because they show improved mechanical properties and some advantages over fibres and whiskers based composites mainly for processing. The embedment of NbC particles in an Al2O3 matrix has been investigated in this work. Composite materials were obtained by the conventional ceramic processing with compositions Al2O3-x NbC where x=10, 20, and 40 weight %. Materials were sintered in argon at 1650 deg C/30 min, and at 1750 deg C/15min using a graphite resistance furnace. Three sets of samples were processed: containing NbC powder as received; containing NbC powder milled for 30 h; and containing 3 weight % of Y2O3 added to the Al2O3 matrix. After sintering samples were analysed to evaluate the densification grade. Phases were determined by X-ray diffraction, and samples were analysed by optical microscopy, scanning electron microscopy and transmission electron microscopy to observe the microstructure features. Hardness and fracture toughness at room temperature were also determined by the indentation method, and the sintering kinetic was evaluated by dilatometry. The present work shows that Al2O3-NbC composite can be produced by using pressureless sintering, leading to materials with densities higher than 95% of the theoretical density for all composites containing NbC and sintered at 1750 deg C. The same result can be obtained for composites containing 10 and 20 weight % of NbC, and sintered at temperatures as low as 1650 deg C. Higher concentration of NbC in the Al2O3 matrix promotes an improvement of fracture toughness. Samples with 40-wt % of NbC show KIC equals to 4.5 MPa.m1/2. This value is compared to the ones previously determined for Al2O3 - TiC composites, available commercially and used in ceramic cutting-tools. (author)