Effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of randomly packed hollow sphere (RHS) cell wall

Cell wall of stainless steel randomly packed hollow sphere structures (RHS) were densified by spark plasma sintering (SPS) in the temperature range 1000-1200 °C. The porosity of RHS cell wall before and after SPS are inspected by quantitative image analysis of scanning electron microscope (SEM) micrographs. The micro-indentation technique examines the micro-hardness and Young's modulus of the cell wall. Results show that the micro-hardness and Young's modulus of the cell wall increases with the increase of SPS process temperature. An empirical relation is proposed to relate the cell wall Young's modulus with the porosity as well as the pore morphology and microscopic cracking in the RHS cell wall. The results suggests that porosity, pore morphology and micro-cracking alone might not fully be responsible for the moderate increase of the cell wall Young's modulus after SPS, and submicroscopic cracking in the RHS cell wall might pose as one of the factors that contribute the degradation of Young's modulus of highly densified cell wall. The successful densification is attributed to the unique sintering feature of the SPS. With the uneven Joule heating at porous sites and the spark plasma at the mass gap, pores are eliminated while the overall RHS structure is retained. The massive electric current (∼ 1000 A) enhances the densification process, resulting in a very short processing time and relatively low processing temperature.