A novel cBN–hardened B12(C,Si,B)3–SiC composite was successfully fabricated by reactive spark plasma sintering (SPS), and its unlubricated sliding wear performance was evaluated. Powder mixtures with varying B4C:Si:cBN ratios (vol%) were consolidated under different SPS conditions, and the resulting materials were characterised microstructurally and mechanically to identify suitable processing conditions for obtaining a dense (>98%) fine-grained (<1 μm) cBN–hardened (∼33.4 GPa) B12(C,Si,B)3–SiC composite—namely, 72B4C:18Si:10cBN (vol%) SPSed at 1500 °C, with a heating rate of 200 °C/min, a dwell time of 15 min, and an applied pressure of 75 MPa. This novel composite was then tested tribologically against diamond at a 40 N load under unlubricated sliding conditions, demonstrating a threefold improvement in wear resistance (∼(2.7 ± 0.3)·107 (N·m)/mm3) relative to that of its cBN–free reference counterpart—with very mild abrasion and no evidence of plastic grooving or grain pull-out. This enhanced performance results from the cBN third phase increasing the overall hardness of the composite and promoting the formation of a more protective and lubricating oxide tribolayer. Incorporating cBN into ceramic microstructures could thus be an effective strategy to enhance hardness and tribological performance, provided that sintering conditions are sufficiently mild to prevent its transformation into hBN.
High-entropy A6B2O17 (A = Hf, Zr; B = Ta, Nb)/8YSZ double-layer ceramic coatings with three compositions are fabricated by atmospheric plasma spraying (APS). Thermal cycling tests are performed at 1150 °C with a 5 min heating hold followed by 5 min air cooling, while isothermal oxidation tests are performed at 1150 °C. Among the three coatings, H3Z1T2N1 coating exhibits the longest thermal cycling lifetime (1823 ± 153 cycles), followed by the H1Z1T2N1 coating (1123 ± 136 cycles) and H1Z3T2N1 coating (826 ± 114 cycles). After 180 h oxidation, the thermally grown oxide (TGO) thicknesses are 5 μm, 5.6 μm, and 6.6 μm, respectively. The H3Z1T2N1 coating possesses the lowest oxidation rate constant (kp = 0.1263 μm2·h−1). This superior performance is attributed to dense vertical cracks induced by higher thermal expansion, which relieve thermal stress, suppress oxygen ingress, and delay interfacial failure.
In this study, a compositional optimization strategy was formulated within the 85BaTiO3-15Na0.5Bi0.5TiO3 (85BT-15NBT) system to attain X9R-type stability while maintaining a comparatively high dielectric response. Nb/Y co-doping was introduced to adjust the phase structure and dielectric behavior of the parent 85BT-15NBT. Rietveld refinement revealed a structural evolution from a tetragonal P4mm symmetry to pseudocubic Pm3ˉm symmetry. X9R requirement, with a room-temperature permittivity of 1496 and a dielectric loss of 1.82%, was achieved. CaZrO3 modification increased the relative permittivity to 1679 along with a dielectric loss of 1.50% without compromising the temperature stability. High-temperature impedance and Arrhenius analyses revealed improved high-temperature insulation and CZ-induced changes in the local defect and chemical environment. These findings illustrate that compositional optimization is effective for designing lead-free X9R dielectric ceramics for high-temperature MLCC applications.
