. Materials and methods
2.1. Sample preparation
Zinc oxide (ZnO) powder of purity better than 99.99% was procured from M/S Alfa Aesar, Germany. This powder was further ground in a planetary ball mill (Fritsch Pulverissette, Germany) using agate pots and balls for 30 h. The ball to charge ratio was kept at 16:1 as decided by our previous experience [25]. Particle size of the starting powder was determined by Transmission Electron Microscope (TEM, Philips Quanta 400) image analysis. A highly diluted suspension of the powder sample in isopropyl alcohol was taken on carbon grit for TEM observations. Powder sample was uni-axially pressed into cylindrical discs of ~10 mm diameter and ~2 to 3 mm thickness by applying 500 MPa pressure in a hydraulic press. Discs were subsequently heated in a box type furnace (Carbolite, UK) at 1300 °C for different holding durations (1, 3, 6 and 10 h). The bulk density of the sintered samples was measured using a “programmable density determination kit” attached to digital balance (Mettler Toledo D205, Mettler Toledo International Inc., Greifensee, Switzerland) using deionized water as a liquid medium. The sintered discs were polished before testing under mechanical load for micro hardness and fracture toughness study.