Spark plasma sintering of UHTC powders obtained by self-propagating high-temperature synthesis

Fully dense ZrB₂–SiC and HfB₂–SiC ultra-high-temperature ceramics (UHTCs) composites are fabricated by first synthesizing via self-propagating high-temperature synthesis (SHS) the composite powders from B₄C, Si, and Zr or Hf reactants, and subsequently consolidating the product by spark plasma sintering (SPS) without the addition of any sintering aid. It was found that the SHS technique leads to the complete conversion of reactants to the desired products and the SPS allows for the full consolidation (>99.5% relative density) under the optimal operating conditions of 1800 °C/20 min/20 MPa and 1800 °C/30 min/20 MPa, for the cases of ZrB₂–SiC and HfB₂–SiC, respectively. Based on the results reported in this work, it can be stated that the combination of SHS and SPS methods represents a particularly rapid and convenient preparation route (lower sintering temperature and processing time) for UHTCs as compared to the techniques available in the literature for the fabrication of analogous products.     

Combustion synthesis of ZrB2-SiC composite powders ignited in air

ZrB₂-SiC composite powders have been synthesized by combustion synthesis in air, using a mixture of Zr, B₄C and Si as raw materials. It was found that the air atmosphere has played an important role in the ignition process of the SHS reaction. Three other kinds of ZrB₂-SiC-ZrC composite powders with different ZrC content were also synthesized, and the ignition time has been measured for better understanding the ignition mechanism. As a result, the composite powders with particle size smaller than 1 µm and oxygen content as low as 0.4 wt.% were obtained.     

Quantitative evaluation of the oxidation behavior of ZrB2-15 vol.%SiC at a low oxygen partial pressure

Oxidation tests of ZrB₂–SiC composite were carried out at 1373–1923 K under a low oxygen partial pressure of 57 Pa. By making composite with SiC, ZrB₂ shows good oxidation resistance. The ZrB₂-15 vol.%SiC composite shows better oxidation resistance at higher temperatures than ZrB₂-30 vol.%SiC with respect to mass decrease. The SiC depleting is the main cause of this mass decrease and is quite significant under the low oxygen partial pressure. The SiC depleting seems to start to occur at around 1673 K or higher. The mass changes of ZrB₂-15 vol.%SiC are quantitatively discussed by introducing an empirical equation. A first attempt of the evaluation of passive/active transition has been conducted using the obtained values.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Effect of graphite flake on the mechanical properties of hot pressed ZrB2-SiC ceramics

A ZrB₂ ceramic containing 20 vol.% SiC and 10 vol.% graphite flake (ZrB₂-SiC-G) was fabricated by hot pressing. It was shown that the fracture toughness was improved due to the introduction of graphite flake, whereas the flexure strength and hardness decreased slightly. The fracture toughness of ZrB₂-SiC-G composite was 6.1 ± 0.3 MPa·m^1/2, which was much higher than that of monolithic ZrB₂, ZrB₂-SiC composite and similar ZrB₂-SiC-C composite. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. The results here pointed to a potential method for improving fracture toughness of ZrB₂-based ceramics.     

Characterization of hot-pressed short ZrO2 fiber toughened ZrB2-based ultra-high temperature ceramics

Two different ZrB₂-based ultra-high temperature ceramics were produced by hot pressing: ZrB₂ + 20 vol.% SiC particle + 15 vol.% ZrO₂ fiber and ZrB₂ + 20 vol.% SiC whisker + 15 vol.% ZrO₂ fiber. The microstructures were analyzed by using transmission electron microscopy and high-resolution transmission electron microscopy. It was shown that a clean interface without any impurities was identified in ZrB₂-based hybrid ceramics with SiC whiskers and ZrO₂ fibers, which would significantly improve the toughening mechanism. The results of high-resolution transmission electron microscopy showed that stacking faults in SiC whiskers resulted from an insertion of a (111) layer, which would be one of the main reasons for material anisotropy. However, the interface between the SiC particle and ZrO₂ fiber was found to be ambiguous in ZrB₂-based hybrid ceramics with SiC particles and ZrO₂ fibers due to the slight reaction. The orientation relationship between t-ZrO₂ and m-ZrO₂ phases obeyed the classical correspondence: (100)_m//{100}_t and [001]_m//〈001〉_t, which further verified the feasibility of phase transformation toughening mechanism.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Microstructure,hardness and indentation toughness of high-temperature C40 Mo(Si,Al)2/SiC composites prepared by SPS of MA powders

The C40 Mo(Si_0.75Al_0.25)₂ and Mo(Si_0.75Al_0.25)₂/SiC materials containing micro-, nano-scale structure and Mo/Mo₅Si₃ phases have been prepared by spark plasma sintering (SPS) of mechanically alloyed (MA) powders. Sintered composites have hardness around 14 GPa. The 1.84 MPa m^1/2 toughness of C40 Mo(Si,Al)₂ can be 30% improved by addition of 20 vol.% SiC.     

Synthesis and mechanical properties of Ti3AlC2 by spark plasma sintering

In this paper, spark plasma sintering (SPS), after hot isostatically pressing (HIP) method was reported as a new approach to prepare bulk polycrystalline samples of Ti₃AlC₂. The ternary carbide was fabricated by spark plasma sintering (SPS) at a pressure of 22 MPa and temperature of 1250°C. The raw materials, elemental powders of Ti, Al and activated carbon, were pretreated in the following different ways prior to SPS: one way was to obtain porous Ti₃AlC₂ by self-propagating high-temperature synthesis (SHS) from mixture of Ti, Al and C, and then densify the product by SPS; the second way was to synthesize Al₄C₃ from Al and C firstly, and then mix powders of Ti and C with synthesized Al₄C₃ to fabricate bulk Ti₃AlC₂ by SPS. Obtained polycrystalline Ti₃AlC₂ ceramics had excellent mechanical properties: density was 4.24 ± 0.02 g/cm³, flexural strength was 552 ± 30 MPa and fracture toughness (K _IC) was 9.1 ± 0.3 MPa · m^1/2. It could be concluded that SPS method was a useful method to synthesize bulk Ti₃AlC₂ with excellent properties in a very short time and easily sintering process. The optimal conditions to synthesize Ti₃AlC₂ were also discussed.     

Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

In situ synthesis and properties of Zr2Al3C4/ZrB2 composites

The Zr₂Al₃C₄/ZrB₂ composites are in situ synthesized by spark plasma sintering using Zr, Al, graphite, and B₄C powders as the initial materials. The introduction of ZrB₂ can not only evidently hinder the coarsening of Zr₂Al₃C₄ grains, but also benefit the densification and improve the hardness and Young’s modulus of the Zr₂Al₃C₄/ZrB₂ composites. When the ZrB₂ content is 20 vol.%, the composite shows an optimum fracture toughness value of 4.37 MPa m^1/2, about 20% higher than that of the monolithic Zr₂Al₃C₄. The unique mechanical properties can be mainly ascribed to the contribution of ZrB₂ as the reinforcing phase hindering the crack propagating. Compared with Zr₂Al₃C₄, the Zr₂Al₃C₄/20 vol.%ZrB₂ composite also exhibits a relatively higher thermal conductivity and better oxidation resistance.     

Effect of a weak fiber interface coating in ZrB2 reinforced with long SiC fibers

This study reports the microstructural analysis and mechanical properties of a ZrB₂ ceramic containing long BN-coated Hi-Nicalon SiC fibers. A composite was produced and thoroughly characterized by transmission electron microscopy to study the interfaces at the nanoscale level. Full densification was accomplished by hot pressing at 1450 °C. The fiber in the sintered material retained its pristine aspect, confirming that the coating was effective in preventing degradation due to interactions with the matrix. Pull-out was observed on fractured surfaces, but toughness values were about 4.5 MPa√m, which was comparable to those of ZrB₂ materials with SiC additions in the form of particles or short fibers. However, the composites exhibited a controlled fracture behavior, as confirmed by a notably higher work of fracture, 140 J/m², compared with 20–30 J/m² of unreinforced ZrB₂ or ZrB₂ containing chopped fibers.     

The thermal shock resistance of the ZrB2–SiC–ZrC ceramic

In the present work, the thermal shock resistance of the ZrB₂–SiC–ZrC ceramic was estimated by the water quenching method and the flexural strength of the quenched specimen was measured. The measured critical temperature difference of the ZrB₂–SiC–ZrC ceramic was significantly greater than that of the ZrB₂–15 vol.% SiC ceramic. The improvement in thermal shock resistance was attributed to its higher fracture toughness (6.7 MPa m^1/2) and lower flexural strength (526 MPa) relative to the ZrB₂–15 vol.% SiC ceramic (4.1 MPa m^1/2 and 795 MPa) based on Griffith fracture criterion. Furthermore, the temperature and thermal stress distributions in the specimen during instantaneous water quenching were simulated by Finite element analysis.     

Effect of BN grain size on microstructure and mechanical properties of the ZrB2–SiC–BN composites

ZrB₂–20 vol.%SiC composites containing 10 vol.% h-BN particles (ZSB) with average grain sizes ranging from 1 μm to 10 μm were hot-pressed. The fracture toughness of the ZSB composites was higher than reported results of monolithic ZrB₂ (2.3–3.5 MPa m^1/2) and SiC particle reinforced ZrB₂ composites (4.0–4.5 MPa m^1/2). The improvement in the fracture toughness of the ZSB composites was due to the high aspect ratio of h-BN and weaker interface bonding, which could enhance crack deflection and stress relaxation near the crack-tip. Compared with the flexural strength of the ZrB₂–SiC composites, the reduction in the flexural strength of the ZSB composites was attributed to the weaker interface bonding and the lower relative density. Furthermore, improvement in toughness and the reduction in the strength were valuable to improve the thermal shock resistance of the ZSB composites. The ΔT_c of ZSB5 material is 400 °C which is higher than ZrB₂–20%SiC and ZrB₂–15%SiC–5%AlN.     

Preparation and properties of 2D C/ZrB2-SiC ultra high temperature ceramic composites

Two-dimensional C/ZrB₂-SiC composites were fabricated by chemical vapor infiltration (CVI) process combined with slurry paste (SP) method. ZrB₂ was introduced in the matrix by stacking the pasted carbon cloth with ZrB₂-polycarbosilane slurry. After heat-treated at 900 °C, the stacked carbon cloth preform was infiltrated SiC by CVI process to obtain 2D C/ZrB₂-SiC composites. Mechanical properties such as flexural strength and interlaminar shear strength were investigated. The ablation tests were carried out on an oxyacetylene torch flame. The small linear erosion rates indicate that the composites have good ablation resistance properties. These results demonstrate that CVI combined with SP method is a useful way to fabricate 2D C/ZrB₂-SiC composites.     

Preparation and characterization of ZrB<Subscript>2</Subscript>-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Preparation and mechanical properties of ZrB2-based ceramics using MoSi2 as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.