Showing papers by "Srinivasa R. Bakshi published in 2011"

Spark plasma sintered tantalum carbide: Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties

Abstract: TaC and TaC–1 wt.% B4C powders were consolidated using spark plasma sintering (SPS) at 1850 °C and varying pressure of 100, 255 and 363 MPa. The effect of pressure on the densification and grain size is evaluated. The role of nano-sized B4C as sintering aid and grain growth inhibitor is studied by means of XRD, SEM and high resolution TEM. Fully dense TaC samples were produced at a pressure of 255 MPa and higher at 1850 °C. The increasing pressure also resulted in an increase in TaC grain size. Addition of B4C leads to an increase in the density of 100 MPa sample from 89% to 97%. B4C nano-powder resists grain growth even at high pressure of 363 MPa. The formation of TaB2/Carbon at TaC grain boundaries helps in pinning the grain boundary and inhibiting grain growth. The effect of B4C addition on hardness and elastic modulus measured by nanoindentation and the indentation fracture toughness has been studied. Relative fracture toughness increased by up to 93% on B4C addition.

Spark plasma sintered tantalum carbide-carbon nanotube composite: Effect of pressure, carbon nanotube length and dispersion technique on microstructure and mechanical properties

Abstract: TaC–4 wt.% CNT composites were synthesized using spark plasma sintering. Two kinds of CNTs, having long (10–20 μm) and short (1–3 μm) length, were dispersed by wet chemistry and spray drying techniques respectively. Spark plasma sintering was carried out at 1850 °C at pressures of 100, 255 and 363 MPa. Addition of CNTs leads to an increase in the density of 100 MPa sample from 89% to 95%. Short CNTs are more effective in increasing the density of the composites whereas long CNTs are more effective grain growth inhibitors. The longer CNTs are more effective in increasing the fracture toughness and an increase up to 60% was observed for 363 MPa sample. Hardness and elastic modulus are found to increase by 22% and 18% respectively for 100 MPa samples by addition of long CNTs. Raman spectroscopy, SEM and TEM images indicated that the CNTs were getting transformed into flaky graphitic structures at pressure higher than 100 MPa.

A comparison of mechanical and wear properties of plasma sprayed carbon nanotube reinforced aluminum composites at nano and macro scale

Abstract: The macro- and nano-scale mechanical and wear properties of carbon nanotube (CNT) reinforced Al–Si composite coatings prepared by plasma spraying have been compared in this paper. The composite coatings show a two phase microstructure; one phase being the Al–Si matrix with well dispersed CNTs and the other being the CNT clusters. Nanoindentation testing on the matrix portion with dispersed CNTs indicated an increase in the elastic modulus by 19% and 39% and an increase in the yield strength by 17.5% and 27% by the addition of 5 wt.% and 10 wt.% CNTs respectively. Macro-scale compression tests indicated no improvement in the elastic modulus but an increases in the compressive yield strength by 27% and 77% respectively, by addition of 5 wt.% and 10 wt.% CNTs. Nanoscratch testing carried out on the Al–Si matrix with dispersed CNTs indicated a decrease in scratch volume by 34% and 71% by addition of 5 wt.% and 10 wt.% CNTs respectively. Macro-scale wear tests indicated a decrease in the wear volume by 68% in case of 5 wt.% CNT coatings but an increase in the wear volume by 15% for the 10 wt.% CNT coating. The differences in mechanical and wear properties at nano and macro scales are explained in terms of the bimodal CNT dispersion (well dispersed and clusters) in Al–Si matrix, CNT cluster size and fraction and carbide formation.

In-situ synthesis of TiC/SiC/Ti3SiC2 composite coatings by spark plasma sintering

Abstract: Composite coatings consisting of TiC/SiC/Ti3SiC2 phases have been synthesized using spark plasma sintering (SPS) of Ti/SiC/C powder mixture on titanium substrate. The processing of composite coatings was achieved at 1300 °C and 1400 °C with a uniaxial pressure of 90 MPa and holding time of 8 min. Identification and quantification of the phases have been performed using x-ray diffraction (XRD). X-ray elemental maps obtained using EDS have been used to analyze the evolution of various phases and their distribution in the microstructure. A tentative mechanism for phase evolution based on exothermic reactions between various constituent powders has been proposed. Microhardness of the spark plasma sintered coatings was found to be ~ 3 times that of the Ti substrate resulting in ~ 100 times lower wear weight loss in ball-on-disc wear tests.