Ternary phase induced microstructure regulation and strengthening in (W, Ti)C-Co cermet materials containing hBN

Abstract

The application of (W, Ti)C-Co cermet materials in harsh environments is constrained, necessitating further performance enhancement. In this study, (W, Ti)C-25 wt% Co samples with 0, 0.5, 1.0, 1.5, and 2 wt% hBN were fabricated via vacuum liquid-phase sintering. Phase evolution, grain boundary and phase interface characteristics and their correlation with properties were comprehensively analyzed via XRD, SEM, EDS, and EBSD. Results show that hBN addition induces the formation of a ternary W₂Co₂₁B₆ phase. This new phase consumes W from (W, Ti)C and Co from the binder, which induces rightward shifts in (111) diffraction peaks of both phases. Microstructural analysis reveals transition of hard phase morphology toward equiaxed morphologies with reduced grain sizes, indicating grain growth inhibition effect of hBN. Crucially, hBN modifies grain boundary characteristics. With 1 wt% hBN addition, ultra-low-energy Σ3 boundaries partially convert to medium-energy Σ9 boundaries while preserving overall low-Σ (3 ≤ Σ ≤ 29) boundary density, enabling simultaneous enhancement of hardness (4.5 %) and fracture toughness (14.68 %). Larger addition of hBN promotes detrimental transformation of low-Σ boundaries to high-energy random configurations, causing precipitous toughness decline (24.6 %) despite maximum hardness gain (8.91 %). Furthermore, hBN reduces friction coefficients effectively with abrasive wear as the main failure mechanism, though wear rate first increases then decreases due to the changes in the generation of W₂Co₂₁B₆. Overall, hBN optimizes properties via phase evolution, grain boundary and phase interface modification.