Sintering and tribomechanical properties of gel-combustion-derived nano-alumina and its composites with carbon nanotubes

Fully pure nano-α-alumina (Al₂O₃) was prepared following gel-combustion method. Near theoretically dense monolithic Al₂O₃ and its composites reinforced with multiwalled carbon nanotubes (MWCNTs) were prepared using spark plasma sintering (SPS) at 1500 °C under 40 MPa within 10 min. The shrinkage curves were guided in sequence by the crystallization of the amorphous mass followed by a solid-state sintering. The differential nature of electrical conductivity of both composite phases resulted in enhanced densification through localized joule heating. Formation of ~ 1-μm-sized equiaxed matrix grains with uniform distribution of structurally survived CNTs in it was observed in the sintered composites. Within the investigated loading span, the highest Vickers hardness (H_V) values were obtained only at 0.5 wt% MWCNT loading in matrix Al₂O₃. Improvements in H_V values for the composites at 0.2 and 2 kgf indentation loads were found to be ~ 18 and ~ 12%, respectively, in comparison with those obtained for pure matrix phase. Quantitative indentation size effect analyzed through standard mathematical models indicated the role of matrix grain refinement and proper matrix–filler load sharing in changing the true hardness. On the contrary, increased CNT concentration leaded to increased sensitivity toward size effect due to the extreme flexible nature of the filler. Unlubricated linear scratch experiments revealed ~ 30–45% lower specific wear rate (W_R) values of the composite specimens compared to SPS-processed monolithic Al₂O₃. Microstructure and scar profile observations were utilized to describe such enhanced wear resistance of present composites.