TaB2-based ceramics: Microstructure, mechanical properties and oxidation resistance

TaB₂-based ceramics were hot pressed in low vacuum with addition of 5-10 vol% MoSi₂. Temperatures in the range of 1680-1780 °C led to relative density around 90-95%. The hardness was about 18 GPa, the fracture toughness 4.6 MPa m^1/2 and the room temperature flexural strength was around 630 MPa, but abruptly decreased above 1200 °C to 220 MPa. The composite containing 10 vol% of MoSi₂ was tested in a bottom-up furnace in the temperature range 1200-1700 °C for 30 min. The microstructure appeared covered by a SiO₂ layer, whose thickness increased with the temperature, but the bulk remained unaltered up to 1600 °C. At 1700 °C the specimen vaporized. Nanoindentation was employed on the oxidized cross sections in order to detect eventual mechanical properties modification associated to chemical/microstructural change, like formation of Ta-B-O solid solutions.     

Effect of boron addition on microstructure,mechanical properties and oxidation resistance of TaC ceramics

TaC ceramics with 0.03–0.60?wt% of boron additions were prepared by hot pressing at 2100?°C for 1?h under a pressure of 40?MPa. Effects of boron content on densification, phase composition, microstructure, mechanical properties and oxidation resistance of the TaC ceramics were investigated. When the boron content was 0.12?wt% and above, full density was obtained due to reactions between boron and oxygen impurity at presence of TaC. Minor phases of TaB₂ and C were formed in the 0.24 and 0.60?wt% B compositions after gas-out of the oxygen impurity. Microstructure of the TaC ceramics was refined with increasing in boron content. The TaC ceramic with 0.24?wt% of boron showed the best mechanical properties with a Vickers hardness, flexural strength and fracture toughness of 17.7?GPa, 534?MPa and 4.6?MPa?m^1/2, respectively. When more boron was added, interfacial bonding of the TaC grains was strengthened causing a decrease in fracture toughness. Oxidation resistance of the TaC ceramics increased with boron content. Particularly, the 0.60?wt% B composition showed a weight gain of 0.0018?g/cm² after oxidization at 800?°C in air for 3?h.     

Optimized mechanical properties and oxidation resistance of low carbon Al2O3-C refractories through Ti3AlC2 addition

The mechanical properties and oxidation resistance of the Al₂O₃-C refractories are of critical importance for iron and steel making processes. However, the evaporation of antioxidants related phases such as Al(g), Si(g), and SiO(g) would deteriorate these properties, especially during high-temperature treatment/application. Therefore, in the present work, a small amount of Ti₃AlC₂ compared with Al was introduced to overcome these problems. The phase compositions, microstructures, mechanical properties, and oxidation resistance of Ti₃AlC₂ containing refractories were investigated. The partial oxidation of Ti₃AlC₂ led to inherited lamellar structures such as Ti₃Al_1-xC₂, TiC, and granular Al₂TiO₅ phases. The controlled oxidation of Ti₃AlC₂ and its volume expansion contributed to the compact-structure, thereby limiting the escape of Si and SiO vapors at high temperatures. Consequently, the mechanical properties and oxidation resistance of Ti₃AlC₂ containing Al₂O₃-C refractories treated at 1600 ℃ were improved.     

Microstructure and mechanical behaviors of 2D-Cf/ZrB2-SiC composites at elevated temperatures

Microstructure and mechanical behaviors of the 2D-C_f/ZrB₂-SiC composites at RT to 1800 ℃ were studied. It is indicated that the interface structure is critical, which affects the matrix phase composition and cracks deflection path in the intra-bundle area. It subsequently determines the load transmitting, cracks propagation behaviors and the mechanical properties of the composites. Flexural strength of the composites increases slightly at 1000 ℃ by the release of residual stress and self-healing of defects. Partial decomposition and crystallization of polycarbosilane (PCS) derived SiC result in the weakening of the SiC matrix at 1500 ℃. On the other hand, residual carbon reacts with ZrO₂ impurity above 1300 ℃. As a result, the matrix turns to porous structure at 1500 ℃, leading to the formation of short micro cracks. At higher temperature of 1800 ℃, quasi-creep mechanical behavior is presented in the composite.     

Densification,microstructure, and mechanical properties of V-substituted HfB2-based ceramics

This study firstly developed Hf_1-xV_xB₂ (x = 0, 0.01, 0.02, 0.05) powders, which were derived from borothermal reduction of HfO₂ and V₂O₅ with boron. The results revealed that significantly refined Hf_1-xV_xB₂ powders (0.51 μm) could be obtained by solid solution of VB₂, and x ≥ 0.05 was a premise. However, as the content of V-substitution for Hf increased, Hf_1-xV_xB₂ ceramics sintered by spark plasma sintering at 2000 °C only displayed a slight densification improvement, which was attributed to the grain coarsening effect induced by the solid solution of VB₂. By incorporating 20 vol% SiC, fully dense Hf_1-xV_xB₂-SiC ceramics were successfully fabricated using the same sintering parameters. Compared with HfB₂-SiC ceramics, Hf_0.95V_0.05B₂-20 vol% SiC ceramics exhibited an elevated and comparable value of Vickers hardness (23.64 GPa), but lower fracture toughness (4.09 MPa m^1/2).     

Enhanced mechanical properties,thermal shock resistance and ablation resistance of Si2BC3N ceramics with nano ZrB2 addition

The mechanical properties, thermal shock resistance, and ablation resistance of nano ZrB₂ modified Si₂BC₃N ceramics were investigated. The results show that ZrB₂ stimulated microstructure evolution obviously. Therefore, the maximum strength and fracture toughness reach 559.6 MPa and 6.77 MPa·m^1/2, which are improved by 61.0% and 29.4%, respectively. Furthermore, the residual strengths of 10 wt% ZrB₂ containing composites tested at 1000 ℃ retain 363.6 MPa, which is much higher than 97.7 MPa of pristine Si₂BC₃N ceramics. Besides, the ablation resistance of ZrB₂ modified Si₂BC₃N ceramics at 3000 ℃ is enhanced remarkably and the linear and mass ablation rates of ZrB₂-10 are only 0.009 mm/s and 1.91 mg/s, respectively. The ablation in the ultra-high temperature zone is totally dominated by the ZrB₂ component, and the thermochemical erosion is determined by the oxidation resistance of ZrB₂ in the thermal affected zone.     

Microstructure,oxidation and thermal shock resistance of graphene reinforced SiBCN ceramics

SiBCN ceramics were prepared using various volumes of graphene platelets (GPLs) as nanofiller. The effects of the nanofiller on microstructure, and oxidation and thermal shock resistance of as-sintered ceramics were investigated. The phase composition and microstructures were very similar for all investigated ceramics consisting primarily of β-SiC, BNC and small amounts of α-SiC with relatively homogeneously distributed 5–10 nm thick GPLs in the matrix. For SiBCN ceramics incorporating graphene as nanofiller, a porous oxide layer forms at 1500 °C and the oxidation behavior shows a linear kinetics by thickness measurement method. Gas evolution during heating lead to a passive oxidation behavior and weight loss. Graphene reinforced SiBCN ceramics exhibit thermal shock resistance superior to monoliths of the same material. The graphene distributed in SiBCN matrix can dissipate the energy of crack growth and acts as a stopper to cracks. The toughening mechanisms offered by graphene, including pull-out and bridging appear to aid in ameliorating thermal shock effects. Furthermore, the existence of a dense oxide surface layer retards oxygen diffusion into the inner matrix and heals surface pores and cracks, which also contributes to thermal shock resistance.     

Microstructure and mechanical properties of continuous carbon fiber-reinforced ZrB2-based composites via combined electrophoretic deposition and sintering

A process combining electrophoretic deposition (EPD) with hot pressing (HP) was developed to fabricate continuous carbon fiber-reinforced ZrB₂-based composites (C_f/ZrB₂-based composites). ZrB₂-based ultra-high temperature ceramic (UHTC) particles were uniformly pre-coated on continuous carbon fibers via EPD. Then, the UHTC-coated carbon fibers were stacked and hot pressed to prepare the C_f/ZrB₂-based composites. Microstructure observations revealed that almost no micro-pores were found in the inter-bundle and intra-bundle regions of fibers after HP. The flexural strength, fracture toughness and the work of fracture of the C_f/ZrB₂-based composite were measured as 199 ± 26 MPa, 6.71 ± 1.29 MPa·m^1/2, and 754 ± 58 J/m², respectively. Based on the observations of non-brittle fracture behavior, fractured morphology and crack propagation, the enhanced fracture properties were mainly attributed to the multiple toughening mechanisms, such as fiber pull-out, fiber bridging, crack deflection and branching along the interfaces.     

Ultra-high temperature ceramics based on ZrB2 obtained by pressureless sintering with addition of Cr3C2, Mo2C,and WC

ZrB₂-MeC and ZrB₂-19 vol% SiC-Me_xC_y where Me=Cr, Mo, W were obtained by pressureless sintering. The capability to promote densification of ZrB₂ and ZrB₂-SiC matrices is the highest for WC and lowest for Cr₃C₂. The interaction between the components results in the formation of new phases, such as MeB (MoB, CrB, WB), a solid solution based on ZrC, and a solid solution based on ZrB₂. The addition of Cr₃C₂ decreases the mechanical properties. On the other hand, the addition of Mo₂C or WC to ZrB₂-19 vol% SiC composite ceramics leads increased mechanical properties. Long-term oxidation of ceramics at 1500 °C for 50 h showed that, in binary ZrB₂-Me_xC_y, a protective oxide scale does not form on the surface thus leading to the destruction of the composite. On the contrary, triple composites showed high oxidation resistance, due to the formation of dense oxide scale on the surface, with ZrB₂-SiC-Mo₂C displaying the best performance.     

The effect of oxidation on the mechanical properties and dielectric properties of porous Si3N4 ceramics

The effect of oxidation temperature and time on the microstructures, phase compositions, mechanical properties, and dielectric properties of porous Si₃N₄ ceramics was investigated in the temperature range from 900 °C to 1300 °C for 1 h, 5 h, and 24 h. The weight gain measured either at lower temperature (900 °C) for long time (24 h) or at higher temperature (1300 °C) for 1 h demonstrated that the porous Si₃N₄ ceramics were easily oxidized under the current test conditions. Results showed that the amount of open pores, flexural strength, compressive strength, and dielectric constant all decreased with the increase of oxidation temperature independent upon the oxidation time. The oxidation product SiO₂ was low-temperature quartz in mild condition (low temperature, short time) and cristobalite in severe condition (high temperature, long time). The existence of cracks on the oxide scale was due to the phase transformation of SiO₂ and thermal expansion coefficient mismatch between SiO₂ and Si₃N₄.     

Improvement of sinterability and mechanical properties of ZrB2 ceramics by the modified borothermal reduction methods

Starting from ZrO₂ and boron (molar ratio: 1:4), four ZrB₂ powders were synthesized by borothermal reduction method, three of which were designed to introduce minor modifications by combining solid solution with Ti and/or water-washing. The sinterability, microstructures, mechanical properties and thermal conductivity were investigated. In comparison with the conventional borothermal reduction, the modified methods offered significant improvement in terms of densification of ZrB₂ ceramics, particularly the mixture that included water-washing. Owing to the refined particle size and boron residues, ZrB₂ ceramics from the modified borothermal reduction which included water-washing demonstrated nearly full densification, Vickers hardness of 14.0 GPa and thermal conductivity of 82.5 W/mK after spark plasma sintering at 2000 °C for 10 min. It was revealed that the properties of ZrB₂ ceramics could be enhanced utilizing the proposed minor modification, starting from the same raw materials and adopting the same sintering conditions.     

Method to improve the oxidation resistance of ZrB2-based ceramics for reusable space systems

UHTCs are the most suitable materials for space and hypersonic, however in service they lose their properties owing to oxidation damages. Tailoring composition and microstructure on a multiscale level may maintain structural stability in the sub-layers. Sintering of ZrB₂-MoSi₂ ceramics at 1900–2150?°C results in microstructures characterized by partial or complete (Zr,Mo)B₂ solid solutions. This has notable impacts on the performance of the composites subjected to cyclic oxidation at 1650?°C. Coupled XRD, SEM and TEM analyses pointed out the formation of a unique interpenetrating microstructure, where ZrO₂ micro-grains encase stable nano-sized MoB particles. This architecture manifested its suitability during repeated oxidation limiting the effect of oxygen attack to some microns thickness, due to diffused Mo which prevented turbulence and bursting phenomena on the outer glass.This study elucidates the processing conditions that lead to the development of prominent materials for application in new generation thermal protection systems for reusable space components.     

Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping

The oxidative degradation of ZrB₂ ceramics is the main challenge for its extensive application under high temperature condition. Here, we report an effective method for co-doping suitable compounds into ZrB₂ in order to significantly improve its anti-oxidation performance. The incorporation of SiC and WC into ZrB₂ matrix is achieved using spark plasma sintering (SPS) at 1800?°C. The oxidation behavior of ZrB₂-based ceramics is investigated in the temperature range of 1000?°C–1600?°C. The oxidation resistance of single SiC-doped ZrB₂ ceramics is improved due to the formation of silica layer on the surface of the ceramics. As for the WC-doped ZrB₂, a dense ZrO₂ layer is formed which enhances the oxidation resistance. Notably, the SiC and WC co-doped ZrB₂ ceramics with relative density of almost 100% exhibit the lowest oxidation weight gain in the process of oxidation treatment. Consequently, the co-doped ZrB₂ ceramics have the highest oxidation resistance among all the samples.     

Effect of heating-cooling rates on microstructure evolution of ZrB2-based coatings during oxidation

In this work, the influence of heating-cooling rates on the microstructure of ultra-high temperature ceramics (UHTCs) during oxidation was firstly reported. Four different kinds of processes (low-heating and low-cooling rates, low-heating and high-cooling rates, high-heating and low-cooling rates, high-heating and high-cooling rates) combined with static oxidation at 1500 °C were applied on ZrB₂-SiC-WB composite coatings. The results showed that the oxide layers of the samples presented different microstructure characters under different heating-cooling modes. A relatively thick liquid layer with obvious bubble phenomenon was generated under the high-heating rate, while a thin liquid layer without bubbles was formed under the low-heating rate. Certain amount of sheet B₂O₃ crystals with large sizes were formed under the high-cooling rate, while did not for the low-cooling rate. The formation mechanisms of these unique microstructures were analyzed and the influence of heating and cooling rates was explained.     

Sintering behavior and mechanical properties of Ta4HfC5-based composites with ZrB2 additive

Ta₄HfC₅ powder was synthesized using TaCl₅, HfCl₄ and phenolic resin as raw materials. Then, Ta₄HfC₅–10 vol% MoSi₂ ceramics and Ta₄HfC₅–10 vol% MoSi₂ with different proportions of ZrB₂ (10 – 30 vol%) ceramics were sintered by spark plasma sintering. Zr atoms substituted Ta and Hf atoms in Ta₄HfC₅ during the sintering process at 2000 °C. The sintering behavior and microstructure evolution upon the ceramics are discussed. The mechanical properties of the composites were improved compared to the pure Ta₄HfC₅ ceramics. The hardness of Ta₄HfC₅–MoSi₂ with 30 vol% ZrB₂ increased from around 10 GPa to almost 13 GPa, the flexural strength increased from around 245–435 MPa, and the fracture toughness increased from 2.56 ± 0.12 MPa?m^1/2 to 4.46 ± 0.20 MPa?m^1/2.     

Effect of ZrB2 content on the densification,microstructure, and mechanical properties of ZrC-SiC ceramics

ZrC ceramics containing 30 vol% SiC-ZrB₂ were produced by high-energy ball milling and reactive hot pressing. The effects of ZrB₂ content on the densification, microstructure, and mechanical properties of ceramics were investigated. Fully dense ceramics were achieved as ZrB₂ content increased to 10 and 15 vol%. The addition of ZrB₂ suppressed grain growth and promoted dispersion of the SiC particles, resulting in fine and homogeneous microstructures. Vickers hardness increased from 23.0 ± 0.5 GPa to 23.9 ± 0.5 GPa and Young’s modulus increased from 430 ± 3 GPa to 455 ± 3 GPa as ZrB₂ content increased from 0 to 15 vol%. The increases were attributed to a combination of the higher relative density of ceramics with higher ZrB₂ content and the higher Young’s modulus and hardness of ZrB₂ compared to ZrC. Indentation fracture toughness increased from 2.6 ± 0.2 MPa⋅m^1/2 to 3.3 ± 0.1 MPa⋅m^1/2 as ZrB₂ content increased from 0 to 15 vol% due to the increase in crack deflection by the uniformly dispersed SiC particles. Compared to binary ZrC-SiC ceramics, ternary ZrC-SiC-ZrB₂ ceramics with finer microstructure and higher relative densities were achieved by the addition of ZrB₂ particles.     

Influence of ZrC addition on the microstructure,mechanical properties and oxidation resistance of Ti(C,N)-based cermets

In this study, Ti(C,N)-WC-NbC-ZrC-Co-Ni cermets were prepared by sintering-hip at 1450?°C. The effect of ZrC addition on the microstructure, mechanical properties, oxidation resistance and wear resistance of Ti(C,N)-WC-NbC-Co-Ni cermets were explored in detail. The results show that ZrC addition plays the role of inhibitor in the dissolution–reprecipitation process, which can increase the wear-resistant carbide phases and inhibit the precipitation of brittle (Ti,W,Nb)(C,N) rim phase. Therefore, the core-rim structures are refined and the Nb content in binder increases, which enhance mechanical properties and oxidation resistance of cermets. With the increasing ZrC content, the oxidation resistance of cermets can be improved constantly, while the transverse rupture strength, fracture toughness and wear resistance of these cermets increase first and then decrease. The cermet with 1?wt% ZrC exhibits the transverse rupture strength of 2549?MPa and highest fracture toughness of 13.0?MPa?m^1/2. The oxidation weight gain of cermets containing 5?wt% ZrC after holding 100?h at 750?°C in air is 2.8?×?10^?6 g?mm^?2, which is only 22% of that in the cermets without ZrC addition.     

Enhanced mechanical properties of Si3N4 ceramics with ZrB2-B binary additives prepared at low temperature

Phase composition, microstructures, and mechanical properties of silicon nitride (Si₃N₄) ceramics were investigated with ZrB₂ and B additives. Results showed that the addition of ZrB₂ and/or B in 2.5 and 5 vol.% promoted the phase transformation of α- to β-Si₃N₄ phase and the formation of bimodal microstructure after hot-pressing at 1500 °C. With the introduction of 2.5 vol.% (ZrB₂-B) binary additives, fracture toughness and strength of Si₃N₄ ceramics increased significantly from 5.2 MPa m^1/2 and 384 MPa to 7.2 MPa m^1/2 and 675 MPa, respectively. However, the hardness of ceramics decreased slightly from 23.5 GPa to 21.3 GPa, which was still higher than typical values reported on Si₃N₄ ceramics (15˜17 GPa).     

O＇－Sialon－ZrO_2复合材料的显微结构与力学性能的研究

研究了Ｏ＇－Ｓｉａｌｏｎ－ＺｒＯ２复合材料的显微结构与力学性能的关系。结果表明，Ｏ＇－Ｓｉａｌｏｎ形成连续网络编织状结构。ＺｒＯ２加入量较少时充当填充结构骨架的作用；ＺｒＯ２加入量增多时（至４０％），会有更多的ＺｒＯ２形成聚集体。随着ＺｒＯ２引入量的增加，材料的常温抗折强度提高，但高温抗折强度下降。Ｏ＇－Ｓｉａｌｏｎ的编织状结构可能阻碍晶界滑移。这种复合材料的高温抗折强度在１４００℃为１１２～１７３ＭＰａ。