One-Step In Situ Spark Plasma Sintering (SPS) Synthesis of Dense W2B5−x Composites with High Strength and Toughness

A novel method for fabricating W 2 B 5− x -based ceramics was developed using in situ SPS of W and B 4 C mixed powders with various molar ratios (1.6:1, 1.8:1, and 2:1). The results revealed a primary reaction between W and B 4 C yielding W 2 B 5− x and graphite, alongside a minor residual B 4 C phase. All compositions achieved near-full densification, featuring a uniform microstructure composed of coarse B 4 C grains and fine graphite particles dispersed within the boride matrix. Quantitative phase analysis indicated that increasing the W/B 4 C molar ratio led to a progressive increase of the W 2 B 5− x phase and a slight increase of graphite but a marked decrease of the residual B 4 C. Mechanically, Vickers hardness declined from 15.63 GPa  ± 0.40 GPa to 14.28 GPa  ± 0.34 GPa (under a 30 kg load) as the W/B 4 C ratio increased, attributable to the diminished content of high-hardness B 4 C and the grain coarsening of the W 2 B 5− x . The flexural strength exhibited a similar downward trend, with a maximum value of 620.4 MPa ± 45.4 MPa achieved at a W/B 4 C ratio of 1.6. Remarkably, fracture toughness improved from 7.94 MPa·m 1/2  ± 0.04 MPa·m 1/2 to 9.21 MPa·m 1/2  ± 0.08 MPa·m 1/2 , primarily because of graphite-induced toughening mechanisms such as crack deflection, branching, and bridging.