Micromechanics modeling of creep fracture of zirconium diboride–silicon carbide composites at 1400–1700 °C

The creep deformation of the ultra-high temperature ceramic composite ZrB₂–20%SiC at temperatures from 1400 to 1700 °C was studied by a micromechanical mode in which the real microstructure was adopted in finite element simulations. Based on the experiment results of the change of activation energy with respect to the temperature, a mechanism shift from diffusional creep-control for temperatures below 1500 °C to grain boundary sliding-control for temperatures above 1500 °C was concluded from simulations. Also, the simulation results revealed the accommodation of grain rotation and grain boundary sliding by grain boundary cavitation for creep at temperatures above 1500 °C which was in agreement with experimental observations.     

Solid-state pressureless sintering of silicon carbide below 2000 °C

To date, solid-state pressureless sintering of silicon carbide powder requires sintering aids and high sintering temperature (>2100 °C) in order to achieve high sintered density (>95% T.D.). Two-step sintering (TSS) method can allow to set sintering temperature lower than that conventionally required. So, pressureless two-step sintering process was successfully applied for solid-state sintering (boron carbide and carbon as sintering additives) of commercial SiC powder at 1980 °C. Microstructure and mechanical properties of TSS-SiC were evaluated and compared to those obtained with the conventional sintering (SSiC) process performed at 2130 °C. TSS-SiC showed finer microstructure and higher flexural strength than SSiC with very similar density (98.4% T.D. for TSS-SiC and 98.6% T.D. for SSiC).     

Dense tungsten carbide bulks by self-densification reaction sintering of nanodiamond with tungsten at 1700 °C

Tungsten carbide (WC) bulks from the sintering of WC powders possess excellent physical and chemical properties, widely used in high-precision optical molds, seals, nozzles and many other scenarios. To produce dense WC bulks, however, needs a very high sintering temperature (>1900 °C) and super fine WC powders due to the poor sinterability. The high sintering temperature required puts a huge strain on processing equipment, and the preparation of super fine WC powders involves many complex steps. In this work, dense WC bulks are prepared by spark plasmas sintering via self-densification reaction of nanodiamond with tungsten at 1700 °C. The use of nanodiamond enables dense WC bulks of a density of 98.1% and Vickers hardness of 21.8 GPa for a volume expansion associated with the change of diamond to graphite greatly reduces the porosity of as-prepared WC bulks, demonstrating a novel mechanism of self-densification reaction sintering. Besides, reaction between tungsten and diamond produces far less volume shrinkage than other allotropes of carbon, partly contributing to the densification of the as-prepared WC bulks. Our creative strategy has significant advantages over prevailing methods in preparing high-density WC bulks, facilitating the application of WC bulk materials in industrial production and providing a new insight into the preparation of dense bulks of other metal carbides.     

Densification and strengthening mechanism in spark plasma sintering of B4C–VB2 ceramics via carbide boronizing

To reduce the negative effects of the long-time and B₂O₃ phase on the traditional sintering process for B₄C-based composite ceramics, nearly fully dense B₄C–VB₂ composite ceramics were prepared by reactive spark plasma sintering (SPS) technology at 2000 °C with B and V₈C₇ powders as raw materials in this paper. The effects of the degassing time during SPS on the microstructure and the mechanical properties of the final products were investigated in detail. The results revealed that the proper degassing time was beneficial for the vent of B₂O₃ during the sintering process, which refined the grain size, promoted densification and improved the mechanical properties of the composite ceramic. However, the redundant degassing time increased the holding time at high temperature, resulting in abnormal grain growth and mechanical performance deterioration. In the present work, the optimal degassing time was 6 min, and the final product prepared under the above conditions exhibited excellent comprehensive performance with a relative density of 99.2%, Vickers hardness of 31.2 GPa, bending strength of 654 MPa and fracture toughness of 5.7 MPa m^1/2. In addition, the strengthening and toughening mechanisms of the products were mainly attributed to the residual thermal stresses and bridging structure caused by the fine B₄C and VB₂ grains distributed uniformly.     

The effect of annealing at 1400 °C on the structural evolution of porous C-rich silicon (boron)oxycarbide glass

A precursor SiBOC glass was annealed at 1400 °C for 1, 3, 5 and 10 h and then it was HF etched in order to dissolve the SiO₂/B₂O₃ phase and to obtain a porous C-rich oxycarbide glass. The porous material was studied by N₂ absorption. The pore diameter of the porous C-rich SiBOC glass ranges between 2 and 5 nm and continuously increases with increasing annealing time. The pore volume also increases with the annealing time up to ≈1.0 cm³/g which is close to the pore volume estimated from the chemical composition (1.04 cm³/g) assuming complete dissolution of the silica-based phase.     

Improved ablation resistance of C–C composites using zirconium diboride and boron carbide

Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon–carbon (C–C) composites at high temperature (1500 °C). Our approach combines using a precursor to ZrB₂ and processing them with B₄C particles as filler material within the C–C composite. An oxyacetylene torch test facility was used to determine ablation rates for carbon black, B₄C, and ZrB₂–B₄C filled C–C composites from 800 to 1500 °C. Ablation rates decreased by 30% when C–C composites were filled with a combination of ZrB₂–B₄C particles over carbon black and B₄C filled C–C composites. We also investigated using a sol–gel precursor method as an alternative processing route to incorporate ZrB₂ particles within C–C composites. We successfully converted ZrB₂ particles within C–C composites at relatively low temperatures (1200 °C). Our ablation results suggest that a combination of ZrB₂–B₄C particles is effective in inhibiting the oxidation of C–C composites at temperatures greater than 1500 °C.     

Calcium–magnesium–aluminosilicate corrosion behaviors of rare-earth disilicates at 1400 °C

Environmental barrier coatings (EBCs) are used to prevent oxidation of underlying ceramic matrix composite (CMC) structural components in gas turbines. When the siliceous minerals deposit on the surface of EBCs, a glassy melt of calcium–magnesium–aluminosilicate (CMAS) will be formed, leading to the EBCs degradation. In this study, seven rare-earth disilicates (RE₂Si₂O₇, RE = Yb, Lu, La, Gd, Eu, Sc, and Y) were fabricated to analyze their CMAS corrosion behaviors. The results indicated that the RE₂Si₂O₇ could react with the CMAS in the temperature range of 1250–1350 °C. Reaction zones formed at the interfaces. For the Yb₂Si₂O₇, Lu₂Si₂O₇, La₂Si₂O₇, Eu₂Si₂O₇ and Gd₂Si₂O₇, the reaction zones dissolved into the molten CMAS and separated from the RE₂Si₂O₇. As for the Sc₂Si₂O₇ and Y₂Si₂O₇, the reaction zones could stay at the interface. They could effectively block the molten CMAS to penetrate into the RE₂Si₂O₇ and protect them from CMAS corrosion.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

Development of multi-phase B–Si–C ceramic composite by reaction sintering

A dense ceramic composite in the system B–C–Si has been synthesized by the reaction sintering technique based on infiltration of silicon melt at 1550 °C under vacuum into a porous compact made of boron carbide and petroleum coke powder. The final material is around 99% dense and microstructurally contains B₄C, SiC and Si as the major phases. The B₄C-phase reacted at its interface with Si-phase, which is explained in terms of dissolution of Si in the carbide phase.     

Fracture toughness enhancement of silicon carbide composites with hydrophilic-modified Tyranno® Si–Al–C fibre addition

The surface properties of Tyranno® Si–Al–C (SA) fibre (mean length: 0.7?mm) were modified from hydrophobic to hydrophilic nature by acid treatment of carbon-coated SA fibre, in order to enhance dispersibility or fracture toughness within silicon carbide (SiC) matrix. Two types of carbons, i.e. amorphous carbon and graphite carbon, were used to coat the surface of SA fibres (SA(AC) and SA(GC) fibres, respectively) by the pyrolysis of polyvinyl alcohol at 500°C for 30?min and 1800°C for 30?min, respectively. Then, carboxyl groups were formed on the SA(AC) and SA(GC) fibre surfaces through acid treatment (concentrated H₂SO₄ and HNO₃) at room temperature. Owing to the acid treatment, SA fibre could be homogeneously mixed with SiC particles in polar solvent (acetone). The fracture toughness of SiC composites containing 40 mass% of acid-treated SA(GC) fibre (PVA/SA ratio?=?2.5) hot-pressed at 1800°C for 60?min attained 8.3?MPa?m^1/2.     

Mechanical properties and deformation mechanisms of B4C–TiB2 eutectic composites

Samples of B₄C–TiB₂ eutectic are laser processed to produce composites with varying microstructural scales. The eutectic materials exhibit both load dependent and load independent hardness regimes with a transition occurring between 4 and 5 N indentation load. The load-independent hardness of eutectics with a microstructural scale smaller than 1 μm is about 31 GPa, and the indentation fracture toughness (5–10 N indenter load) of the eutectics is 2.47–4.76 MPa m^1/2. Indentation-induced cracks are deflected by TiB₂ lamellae, and indentation-induced spallation is reduced in the B₄C–TiB₂ eutectic compared to monolithic B₄C. Indentation-induced amorphization in monolithic B₄C and the B₄C phase of the eutectic is detected using Raman spectroscopy. Sub-surface damage is observed using TEM, including microcracking and amorphization damage in B₄C and B₄C–TiB₂ eutectics. Dislocations are observed in the TiB₂ phase of eutectics with an interlamellar spacing of 1.9 μm.     

Oxidation protection of B4C modified HfB2-SiC coating for C/C composites at 1073–1473 K

B₄C modified HfB₂-SiC coating for C/C substrate was designed to expand the application of HfB₂-SiC based coating in low-medium temperature environment. The oxidation protection behavior of HfB₂-SiC based ceramic coatings with and without B₄C at 1073, 1273 and 1473 K was tested and analyzed. The experimental results reveal that the oxidative damage of HfB₂-SiC coated C/C reduces by over 20% after introducing B₄C, which may be due to the protection of borosilicate glass with more suitable viscosity during oxidation. Meanwhile, B₄C can improve the oxidation protection ability of HfB₂-SiC coating best at 1473 K. And the introduction of B₄C can reduce the mass loss of HfB₂-SiC coated C/C sample by 77.6% after oxidation for 58 h at 1473 K. The fluidity of glass film becoming better with temperature-rising, and the fluid borosilicate glass layer makes the coated samples have the best anti-oxidation properties at 1473 K among these three temperatures.     

Oxidation behavior of high-entropy carbide (Hf0.2Ta0.2Zr0.2Ti0.2Nb0.2)C at 1400–1600 °C

Oxidation behavior of high-entropy carbide (Hf_0.2Ta_0.2Zr_0.2Ti_0.2Nb_0.2)C (HTZTNC) was investigated over temperature range of 1400–1600 °C. Results showed improved oxidation resistance of high-entropy carbide compared with individual carbide ceramics. In oxide layer, Ta₂O₅ and Nb₂O₅ were found to be dominant phases at 1400 °C, whereas ZrTiO₄ and HfTiO₄ were main phases obtained at 1500 and 1600 °C. Moreover, these complex dense oxide layer structures on the surface of HTZTNC at high temperature led to excellent oxidation resistance. The observation of Ti-depleted layer at 1500 and 1600 °C after 20 min of oxidation indicated that oxidation mechanism involved outward diffusion of titanium oxide, which was further confirmed by reoxidation experiments. In sum, these findings are promising for future development of high-entropy ultrahigh temperature ceramics with good oxidation resistance.     

Flexural strength and fracture behavior of ZrB2–SiC ultra-high temperature ceramic composites at 1800 °C

ZrB₂–15 vol.%SiC and ZrB₂–30 vol.%SiC composites with smaller starting particle sizes in which the particle sizes of ZrB₂ and SiC are 2 μm and 0.5 μm, respectively, demonstrated marked plasticity and significant reduction in the flexural strength at 1800 °C. The flexural strengths of these two composites are 112 ± 12 MPa and 48 ± 10 MPa, respectively, and their corresponding strength retentions are 13% and 7%, respectively. Large ZrB₂ grains were commonly observed in the samples containing 15 vol.%SiC, which are always the sites for the crack initiation. Cavities were found in the samples containing 30 vol.%SiC and the grain boundaries are the main sites for the crack and cavity nucleation. To improve ultra-high temperature strength, larger starting particle sizes (ZrB₂ and SiC are 5 μm and 2 μm, respectively) were used for the preparation of ZrB₂–15 vol.%SiC. This sample fractured in an elastic manner up to 1800 °C and showed a very high strength with a value of 217 ± 16 MPa.     

Synthesis and spark plasma sintering of Si3N4–ZrN self-healing composites

A Si₃N₄–ZrN wear-resistant self-healing composite material was developed. Si₃N₄–ZrN composite ultrafine powders were synthesized at a temperature of 1200 °С via solid-state reactions without milling and densified by spark plasma sintering at 1650 °C to a relative density of 97 ± 0.5%. Balls 13.494 mm in diameter for ball bearings manufactured by spark plasma sintering had a fine-grained structure with a grain size of 200–500 nm, Vickers hardness of 22.5 ± 1.8 GPa, and indentation fracture toughness of 6.2 ± 0.4 MPa. The tribological properties of the composite were investigated under static and dynamic loading. The self-healing capability of the Si₃N₄–ZrN composite was evaluated in the temperature range 500–550 °С. High-temperature three-point bending tests of notched specimens showed a bending strength of 383 ± 21 MPa at room temperature and 413 ± 30 MPa at 500 °С, which confirmed the self-healing of the composite.     

Densification and ablation behavior of ZrB2 ceramic with SiC and/or Fe additives fabricated at 1600 and 1800 °C

The effects of Fe and SiC additions on the densification, microstructure, and ablation properties of ZrB₂-based ceramics were investigated in this study. The sample powders were conventionally mixed by cemented carbide ball then sintered by spark plasma sintering. The ablation rates and behavior of the ceramics were investigated under an oxyacetylene torch environment at about 3000 °C. A sample with high relative density (96.3%), high flexural strength (415.6 MPa), and low linear ablation rate (−0.4 µm/s) was obtained via SPS at 1600 °C. Adding 4 vol% Fe was more beneficial to the density of ZrB₂ sintered at 1600 °C as compared to ZrB₂ sintered at 1800 °C. The ablation behavior and rates were similar among samples sintered at 1600 °C and 1800 °C.     

Solubility limit of Si in YAG at 1700 °C in vacuum

Measurement of the solubility limit of Si in yttrium aluminum garnet (YAG-Y₃Al₅O₁₂) is crucial for understanding the mechanisms by which Si influences grain boundary mobility, and the mechanisms by which grain boundaries migrate. In the present work, the solubility limit of Si in YAG at 1700 °C in vacuum (5 × 10⁻⁶ Torr), which are the most common sintering temperature and environment for YAG, was measured for the first time. Measurements were conducted by wavelength dispersive spectroscopy (WDS), using polished YAG specimens with 3700 ppm Si (0.8 wt% SiO₂). Si content to ensure saturation with Si. The accuracy of the WDS result was confirmed by using a series of doped specimens and by comparing to inductively coupled plasma mass spectrometry ICP-MS results. The results indicate that the solubility limit of Si in YAG at 1700 °C (5 × 10⁻⁶ Torr), is 980 ± 60 ppm. The measured Si solubility was found to significantly depend on the cooling rate, where for furnace cooled specimens the measured Si solubility was 650 ± 60 ppm. A second phase in triple junction was repeatedly observed when higher content of Si was used, confirming this work results.     

Flexural creep of zirconium diboride–silicon carbide up to 2200 °C in minutes with non-contact electromagnetic testing

Flexural creep of ZrB₂–30 vol% SiC ultra high temperature ceramic composite was studied at 1700–2200 °C and 20–50 MPa using the novel method of electromagnetic Lorentz force loading of electrically heated specimens. Experiments were conducted in air and in non-oxidizing atmospheres. The apparent activation energy for creep was 344 ± 35 kJ/mol for non-oxidizing conditions. The stress exponent was 1.4 ± 0.4. The creep rate was slightly higher in air due to a decrease in the size of the load bearing substrate because of oxidation. There was no evidence of electric field effects. Creep experiments could be performed up to 2200 °C very quickly, with experiments conducted in a few minutes.     

Thermal stability of MoSi2-TiB2-CrB2 composites at 900–1400°C

Slip casting procedure with subsequent firing in air is used to synthesize composites (100 ? x)MoSi₂?x MB₂, where M = Ti, Cr, (Ti, Cr) and x = 10, 20, and 30 mol % at 900–1400°C. The MoSi₂-CrB₂ and MoSi₂-(Ti, Cr)B₂ composites containing 20 and 30 mol % of borides are found to possess the highest protective properties. The optimum temperature range for sintering is 1100–1400°C.