Research & Technical Programmes

Densification mechanism of nano alumina by spark plasma sintering
Dibyendu Chakravarty, Tata Narasinga Rao and G. Sundararajan

The mechanisms involved in the densification of nanostructured powders during spark plasma sintering (SPS) are being investigated with nano alumina. Preliminary work shows a significant role of the applied stress on grain growth and density attained by SPS. Sintering under higher stress of 30 MPa shows better density (>99%) and smaller grains as compared to that with lower stress (5 MPa). Slower heating rate leads to better densification with limited grain growth. TEM with nano MgO additions reveal the presence of Mg-rich phases at triple points; dislocations and subgrains are also observed in the microstructure. Higher hardness and fracture toughness have been achieved compared to conventionally prepared compacts.

Al2O3: 1400°C/5MPa/10mins hold Al2O3:1400°C/30MPa/10mins hold

Mg-rich nano phase and pores at triple point Dislocations and subgrains in SPS alumina

Nanostructured alumina based ceramic cutting tools
Dibyendu Chakravarty and Joydip Joardar

Al2O3-based nano/ultrafine-composites containing ZrO2 and TiC are being developed by spark plasma sintering for cutting tool inserts. It has been observed that TiC resides at the grain boundaries of Al2O3 and retards grain boundary migration leading to finer grains with increased strength and hardness while ZrO2 improves toughness of the alumina matrix by stress induced tetragonal to monoclinic martensitic phase transformation.

High Strength porous alumina by spark plasma sintering
Dibyendu Chakravarty and Tata Narasinga Rao

Porous alumina was fabricated by Spark Plasma Sintering (SPS) with Al (OH) 3 as the starting powder. The decomposition of Al (OH) 3 produced a series of transitional alumina phases and finally the stable α-alumina phase was obtained. SPS parameters were optimized to yield samples having a continuous network of nanopores with pore size 5-50 Nm and bending strength>100 MPa, which could be used as devices for medical applications. The pore size and distribution became wider with increased sintering temperature and pressure. Phase transformation, pore morphology, pore size distribution, strength and microstructure evolution under SPS conditions are also evaluated.