*2.2. Characterizations*

The morphologies of the starting powders, composite powders, and sintered composites were observed using field-emission scanning electron microscopy (FE-SEM, JSM 7001F, JEOL, Tokyo, Japan), and energy dispersive spectroscopy (EDS) revealed the chemical compositions of the powders and the composites. X-ray di ffraction (XRD, CN2301, Rigaku, Tokyo, Japan) with a Cu-K α radiation source (λ = 1.5405 Å) using a step size of 0.02◦ (2θ), the scanning rate of 1◦/min from 20 to 100◦ to identify the phase constitutions of the composite powders and sintered composites.

To measure elastic modulus of the composites, SPSed specimens with a 4 mm cube were ground flat and mechanically polished up to 4000 grit. An ultrasonic technique was used to determine elastic modulus of the composites (5055PR Pulse receiver 5055PR and Oscilloscope 9354CM oscilloscope, LeCoroy Co., NY, USA). A frequency of 5 MHz was applied; the longitudinal velocity and transverse velocities *y* within the specimens were determined. Nanoindentation tests were performed on the specimens using a commercial nanohardness tester (Nanoindenter XP, MTS, MN, USA) equipped with a Berkovich indenter and measured more than 10 times per each specimen. In each test, the indenter was driven into the sample surface (loading half-cycle) at a rate of 10 nm/s and the peak load ranges from 0 to 35 mN. The Vickers hardness of the specimens was measured using a micro-Vickers hardness testing machine (Mitutoyo, HM200, Kawasaki, Japan) with an indenter load of 300 N.

#### **3. Results and Discussion**
