Synthesis and strengthening mechanism for (Ti,Nb)3SiC2/Al2O3 ceramics: A combined experimental and first-principles investigation

(Ti,Nb)3SiC2/60 vol%-Al2O3 composite ceramic samples with different Ti/Nb atomic ratios were prepared by hot-pressing sintering using Ti/Si/Al/TiC/NbC/Al2O3 powders as starting materials. X-ray diffraction analysis showed that the solid solubility of Nb is approximately 7.5 at%. Excessive NbC powder addition can lead to the generation of an impurity phase. Scanning transmission electron microscopy and X-ray photoelectron spectroscopy images reveal that most of the Nb-doped atoms aggregate at 2a-sites, as this configuration exhibits the lowest system energy. The optimized hardness, flexural strength and fracture toughness of (Ti,Nb)3SiC2/Al2O3 were determined to be 16.0 GPa, 672 MPa and 6.9 MPa m1/2, which corresponds to an increase of 15.9%, 18.7% and 25.4% respectively, compared with Ti3SiC2/Al2O3 composite ceramics. By means of first-principle calculations, the strengthening mechanism is derived from both intragranular improvement in (Ti,Nb)3SiC2 solid solutions and intergranular enhancement of (Ti,Nb)3SiC2/Al2O3 grains interfaces.