Effect of the dispersion of nanometric silica particles on the thermal stability of a nanostructured iron based powder

An iron based powder has been mechanically ground with nanometric silica particles in order to increase the thermal stability of the nanostructure. The powder has been annealed at different temperatures between 200 and 750 °C. Microhardness tests revealed that without the addition of SiO2, the hardness is constant up to 400 °C, whereas the dispersion of SiO2 keeps a constant hardness value up to 600 °C. A similar trend has been obtained for the microstructural parameters (crystalline domain size and dislocation density) obtained by a line profile analysis of the XRD spectra; the powders lose the nanostructure after annealing at 600 °C for the material without SiO2 and above 750 °C with SiO2 addition. Thermal analysis has been used to calculate the activation energy of the grain growth process and to extrapolate the temperature at which the powder could be sintered, without losing nanostructure, by spark plasma sintering (SPS). Preliminary attempts of sintering by SPS seem to confirm the indication obtained by the present work.