A first systematic study on the conventional wire electrical discharge machining (Wire-EDM) of a Cf-ZrB2-SiC composite fabricated through slurry infiltration followed by hot pressing. The machining performance was evaluated using equi-energy (E) and anti-electrolysis equi-energy (Ae 2) pulse strategies with plain and coated wires under both roughing and finishing modes. The cutting rate was found to be approximately similar for both plain and coated wires; however, the coated wire produced a narrower kerf in roughing conditions and a wider kerf in finishing compared to the plain wire. Machining with the Ae 2 pulse resulted in a higher material removal rate (MRR) and lower surface roughness, with the most pronounced observed when using the coated wire. Wire-EDM produced various damage features in the UHTCMC, such as shiny recast layers with pores, cracks, burning, and splitting of carbon fiber, pitting, fiber-matrix debonding, and so forth. The wire surface topography differed markedly between plain and coated wires; however, a systematic axial wear band was observed in both cases.
Ultra-low-temperature co-fired ceramic (ULTCC) technology is attractive for miniaturized radio frequency front-end modules. Li2Co(WO4)2 ceramics are promising candidates owing to their relatively low sintering temperature and favorable dielectric properties. In this study, high-purity Li2Co(WO4)2 ceramics were synthesized via a conventional solid-state reaction route using Li2CO3, CoO, and WO3 as raw materials. The thermal behavior of the precursor powders was evaluated by thermogravimetry-differential scanning calorimetry and derivative thermogravimetry, while phase formation, microstructure, densification, and microwave dielectric properties were investigated for ceramics sintered at 625 degrees C-700 degrees C. Thermal analysis indicated that calcination at 600 degrees C largely suppressed the major mass-loss processes, while X-ray diffraction confirmed the formation of phase-pure Li2Co(WO4)2 ceramics after sintering. The sample sintered at 675 degrees C achieved the highest bulk density of 6.467 g & centerdot;cm- 3, corresponding to a relative density of approximately 93.55% and an estimated total porosity of approximately 6.45%. Correspondingly, the best overall microwave dielectric properties were obtained at 675 degrees C: epsilon r = 12.488, Q & times; f = 10,979.1 GHz, and tau f = -71.95 ppm/degrees C. Comparison with literature-reported values suggests that Q & times; f is particularly sensitive to processing conditions and microstructural defects. These findings provide guidance for controlling densification and dielectric loss in tungstate-based ULTCC ceramics.
Advancing ultra‐high‐temperature materials requires superior oxidation resistance for hypersonic flight, atmospheric reentry, and propulsion systems, yet their service temperatures remain limited to below 3000°C. Herein, we put forward a component modulation strategy for the fabrication of SiC‐enhanced high‐entropy ceramics, with SHC‐3 demonstrating the lowest mass ablation rate (1.58 mg·s −1 ) and linear ablation rate (0.67 µm·s −1 ) while exhibiting outstanding ablation resistance under various extreme high‐temperature conditions. The incorporation of SiC particles significantly enhances the densification of SHC‐3, increasing its hardness to 33.1 GPa, while the observed crack deflection induced by the SiC particles is considered a key mechanism for strengthening and toughening the material. The superior ablation resistance of SHC‐3 is attributed to the synergistic effects of SiC‐assisted densification and crack deflection, the formation of a SiO 2 ‐rich protective phase under ultra‐high‐temperature ablation conditions, and the lattice‐distortion‐related diffusion retardation of the high‐entropy carbide matrix, which collectively suppress oxygen penetration and slow material degradation. This study demonstrates significant progress in the application of high‐entropy ceramics at extreme temperatures up to 3000°C and lays the foundation for designing advanced materials capable of operating at even higher temperatures.
Polymer‐derived ceramic (PDC) coatings on carbon fibers (CFs) protect against high‐temperature oxidation; however, thermomechanical stress and cracking during precursor pyrolysis can expose fiber surfaces, creating pathways for oxygen ingress. In this study, a boron‐ and hafnium‐modified polysilazane precursor is used to prepare PDC coatings on CF single plies via a modified polymer infiltration and pyrolysis (PIP) process. Rather than studying high‐temperature stability limits of SiHfBCN‐coated CF composites, we track macroscale mass changes and localized crack evolution. Up to three PIP cycles for fabrication and an oxidation temperature of 550°C were used as a model system to investigate defects in PDC coating, oxygen ingress, and CF degradation. The relationship among coating layers, crack width, crack spacing, ceramic mass loading, and fiber mass retention is investigated using electron microscopy. After oxidation at 550°C for over an hour, SiHfBCN‐coated samples exhibited the lowest average crack width of 5.6 ± 1.5 µm and the highest crack spacing of 38.5 ± 10.9 µm compared to SiBCN and SiCN coatings. At 36% ceramic mass loading, SiHfBCN retained 25.7% of the fiber mass, compared with 12% for SiCN, 9.2% for SiBCN, and 2.9% for uncoated CFs. These findings provide mechanistic insights into coating defects, microcrack evolution, and CF degradation in PDC‐CF composites.
ABSTRACT Yttria‐stabilized zirconia ceramic (3YSZ) was densified to only 60% relative density (RD) by cold‐sintering process (500 MPa pressure, 180°C temperature) but yielded majority of the pores in nano‐scale range. In contrast, 3YSZ conventionally sintered at low temperature of 1200°C yields a high RD of ∼97% with large macro pores ∼2 µm size. To explore further, cold sintering was done as a pretreatment followed by conventional low‐temperature sintering at 1200°C. This method yielded exceptional properties with a majority of tetragonal phases and a complete densification with a remarkably minimal pore size of ∼0.01 µm having Vickers hardness of 12.6 ± 0.3 GPa and flexural strength of 450 ± 40 MPa. The ceramic specimens obtained by combined cold and conventional sintering (CS) were thoroughly characterized using mercury porosimetry, x‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Raman, confocal Raman, impedance spectroscopy, and AFM. The evolution of pore size during the combined cold and CS stages closely mirrors the behavior seen in the well‐known two‐step sintering process. The combined cold and CS process provides an energy‐efficient two‐step sintering route for densification of micron‐sized 3YSZ ceramics, which are important in ceramic industry from a sustainability point of view.
