Application of two-step sintering cycle to 20–30 nm α-Al2O3 compacts to investigate microstructure, hardness, strength, density and corundum formation

Abstract

This paper discusses the hardness variation of 20–30 nm α-Al2O3 sintered specimens by two-step sintering (TSS) technique. This study mainly reveals the microstructure transformation of nano alumina into corundum grains. The uniaxial load of 4 ton was used to produce alumina compacts of 11 mm diameter and 3.5 mm thickness. In this two-step sintering process, high step temperature (T1) varied from 1250 °C to 1600 °C, whereas low step temperature (T2) was between 1100 °C to 1550 °C. To achieve hard nano alumina structure, seven TSS cycles were designed by varying the heating rate (°C/min), high step temperature (T1), high step temperature holding time (t1), low step temperature (T2), and low step temperature holding time (t2). After each TSS cycle, the Vickers hardness (HV), load-bearing capacity (KN), ultimate compressive strength (MPa), yield strength (MPa) and density (g cm−3) measurements of each sintered sample were carried out. The significant improvements observed in hardness and density of sintered nano alumina with an increase in high step temperature (T1) as well as holding time (t1). The microstructure transformation and material elemental analysis were carried out using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) respectively. It is evident from SEM micrographs that the specimen of sintered samples with high step temperature (T1) 1600 °C with 2 h holding time, low step temperature (T2) 1550 °C with 10 h holding time produced corundum grain transformation in the nano alumina matrix.