Oxidation resistance of ZrB2-based monoliths using polymer-derived Si(Zr,B)CN as sintering aid

The focus of the present work is the investigation of the influence of polymer-derived ceramics, used as sintering aids for preparing ZrB₂-based monoliths, on their high-temperature oxidation behavior. For the preparation of the monoliths, ZrB₂ powder was coated with polymer-derived SiCN, SiZrCN, or SiZrBCN and subsequently densified via hot-pressing at temperatures as low as 1800°C. To investigate the oxidation kinetics, thermogravimetric analysis (TGA) was performed at 1300°C in synthetic air with exposure times of 50 and 100 h. A detailed study of the materials oxide scale and subsurface microstructure was conducted using optical microscopy, electron probe microanalysis, scanning electron microscopy, and X-ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations using the CALPHAD method, which led to a better understanding of the oxidation mechanism. In comparison to the literature data of ZrB₂–SiC, the results show improved oxidation resistance for all three investigated materials. The formation of gaseous species during oxidation, in particular CO, CO₂, B₂O₃, and SiO, within the oxide scale of the monoliths was rationalized via CALPHAD calculations and used to explain the oxidation behavior and kinetics and also the formation of bubbles in the subsurface region of the oxidized specimens.     

Production of porous SiC by liquid phase sintering using graphite as sacrificial phase: Influence of SiO2 and graphite on the sintering mechanisms

Porous silicon carbide (SiC) is a promising ceramic for high-temperature applications due to its unique combination of properties. In the present work, a fabrication route for porous SiC is presented using graphite spherical powder as sacrificial phase to introduce porosity. By varying the initial amount of sacrificial phase, high-performance SiC materials with porosities in the range 30–50% were manufactured and characterized in terms of microstructure, density, thermal conductivity and flexural strength. The materials were fabricated by liquid phase sintering in presence of 2.5 wt.% Al₂O₃ and Y₂O₃ as sintering additives. The results indicate that the SiO₂ present in the starting SiC powders interacts with the sintering additives forming liquid phases that promote densification and weight loss. Besides, an Al-Si liquid phase is formed at higher sintering temperatures, whose contribution to densification is inhibited in presence of graphite due to the formation of Al-rich carbides.     

Influence of additives content on the high temperature oxidation of silicon nitride based composites

A life prediction tool for mechanical and electrical applications of electroconductive structural ceramics is essential in order to know the limit for engineering uses. The aim of this work was to study the influence of additives content on the oxidation behaviour, in pure oxygen between 900 and 1400 °C, of two fully dense Si₃N₄–35 vol.% TiN composites. For this purpose, a hot pressed material (HP), containing 3.7 wt.% of Y₂O₃ and Al₂O₃ as sintering aids, was compared to an hipped material (HIP), containing 0.4 wt.% of the same additives. Up to a temperature T<∼1200 °C, where the oxidation of the composite is mainly governed by the preferential oxidation of TiN, the two materials exhibit paralinear kinetics with very close oxidation resistance. Contrarily, the hipped material shows a better oxidation resistance at T>∼1200 °C, when the oxidation of the Si₃N₄ matrix takes place. The formation of a compact silica sub-scale acts as an efficient diffusion barrier leading to asymptotic kinetics, with final weight gains exhibiting a negative temperature dependence. In the case of the HP material, i.e. in presence of a higher content of additives, a deterioration of the protective nature of the scale is provoked by the increased mobility of the impurity cations (Y³⁺, Al³⁺) linked to a decrease of the viscosity of the secondary glassy phase. The kinetics have a paralinear shape up to 1400 °C, with final weight gains increasing as a function of temperature. Therefore, this study confirms the deleterious influence on oxidation resistance of additives used for a better sintering of powders and the beneficial effect of hot isostatic pressing for which lower amounts of aids are necessary in comparison with hot pressing.     

Progress in pressureless sintering of boron carbide ceramics – a review

ABSTRACT

Boron carbide (B₄C) ceramics has many outstanding performance, such as extremely high hardness, low density, high melting point, high elastic modulus, high thermoelectromotive force, high chemical resistance, high neutron absorption cross section, high impact and excellent wear resistance. Therefore, B₄C ceramics can be used in various industrial applications, such as lightweight ceramic armour, high temperature thermocouples, neutron absorber, reactor control rods in nuclear power engineering, polishing media for hard materials, abrasive media for lapping and grinding, and wear resistant components (blasting nozzles, die tips and grinding wheels). Pressureless sintering is the method with industrialised application value for B₄C ceramics, however, it is impossible to sinter pure B₄C ceramics to high densities without additives by pressureless sintering. So sintering additives must be used to promote the densification of B₄C ceramics. The different sintering additives used to promote the densification of boron carbide will be described in this review, including carbon additives, metallic additives, oxide additives, non-oxide additives, combined additives and rare earth oxide additives. Finally, the recent research trends for sintering methods and sintering additives of B₄C ceramics will also be proposed.     

Oxidation of Composite Materials Based on Aluminum Nitride

A method for studying the heat resistance of composite aluminum nitride-based ceramic materials in air at 1073 – 1273 K is developed that allows the change in mass to be measured with an accuracy of 0.15 – 0.17 mg. The interaction between AlN-based composite materials and a phosphate binder (H₃PO₄) is studied and compared with hot-pressed specimens. A mechanism for the effect of the binder on the kinetics of oxidation is proposed. The relatively low activation energies (152 and 205 kJ/mole) suggest that the oxidation process is mainly determined by the diffusion of aluminum ions through the -Al₂O₃ film.     

Effects of different types of sintering additives and post-heat treatment (PHT) on the mechanical properties of SHS-fabricated Si3N4 ceramics

Porous silicon nitride (Si₃N₄) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La₂O₃, Sm₂O₃, Y₂O₃, and Lu₂O₃) on the phase compositions, development of Si₃N₄ grains and flexural strength (especially high-temperature flexural strength) were researched. Si₃N₄ ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si₃N₄ ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C.     

Effect of CaF2 on sintering behavior and thermal shock resistance of Y2O3 materials

Y₂O₃ materials have become a popular candidate for preparing refractory crucibles for ultra-pure high-temperature alloy melting in recent years. However, its difficulty in sintering and poor thermal shock resistance limited its industrial application. The effect of CaF₂ on the densification microstructure, mechanical properties, and thermal shock resistance of Y₂O₃ materials was investigated in this paper. The main purpose of this study was to optimize the amount of CaF₂ added in the preparation of Y₂O₃ materials to improve its thermal shock resistance and get better mechanical properties. The mechanism of the densification process of CaF₂-doped Y₂O₃ materials was analyzed by phase analysis and microstructure. The results showed that successive doping of large Ca²⁺ ions caused more lattice distortion in the Y₂O₃ materials, and the diffusion rate of Y³⁺ was increased, thus enhanced grain boundary diffusion and promoted sintering densification in the Y₂O₃ materials. Meanwhile, the addition of CaF₂ also significantly reduced the apparent porosity and enhanced the mechanical properties of the materials. The improvement of these properties was attributed to the increased relative density of CaF₂-doped Y₂O₃ materials and the high sintering activity of CaF₂. In addition, crack deflections effectively improved the thermal shock resistance of the materials. The residual flexural strength ratio of Y₂O₃ materials doped with 1 wt % CaF₂ was increased by 21.2% after thermal shock test compared with undoped specimens.     

Oxidation behaviour and microstructure of a dense MoSi2 ceramic coating on Ta substrate prepared using a novel two-step process

Tantalum (Ta) alloys are important ultra-high-temperature structural materials owing to their excellent high-temperature mechanical properties and processability. However, they exhibit poor high-temperature oxidation resistance. In this study, a dense MoSi₂ ceramic coating was prepared on a Ta substrate using an innovative multi-arc ion plating process and halide activated pack cementation in order to improve its ultra-high-temperature oxidation resistance. This ceramic coating exhibited a low roughness space arithmetic (287.1 ± 26.3 nm) and a dense structure. The relationship between the thickness of the coating and the duration of pack-cementation at 1250 ℃ was parabolic. The coating had a service life of more than 12 h at 1750 ℃, and showed excellent high-temperature oxidation resistance because of the uniform and dense structure of the coating and the rapid formation of a dense SiO₂ layer with low O₂ permeability during high-temperature oxidation.     

Influence of water vapour on high-temperature oxidation of Al2O3-MgO-doped hot-pressed silicon nitride

Silicon nitride is particularly sensitive to high-temperature oxidation. The intensity of oxidation is influenced by the chemical composition of the amorphous phases present at the grain boundaries and consequently by the sintering additives responsible for their formation. The presence of water vapour increases Si₃N₄ oxidation also in intermediate temperature conditions. In this study the influence of water vapour pressure at high temperature (1200°C) on the corrosion of hot-pressed silicon nitride (HPSN) doped with Al₂O₃-MgO was evaluated. The water vapour has a great influence on the devitrification of the amorphous oxide upper layer, due to the formation of crystalline oxides (primarily cristobalite and tridymite). This process increases the oxidation rate, consequently increasing the porosity of the exposed surface. The microstructural evolution of HPSN in the presence of water vapour at 1200°C was analysed by SEM and XRD.     

Influence of refractory oxide additives on the stability of alumium titanate

Conclusions Batch additives incorporated in the form of various oxides as a means of improving the physicochemical properties of aluminum titanate can be divided into two groups. The first consists of additives which prevent the decomposition of the titanate at temperatures below 1200°C, and stabilize its high-temperature modification. In terms of the degree of activity these oxides are arranged in the series: MgO, TiO₂, SiO₂, Cr₂O₃, La₂O₃. It is possible to assume that their influence is due mainly to the formation of limited solid solutions, and also the development during sintering of impurity grain boundary phases. The second group of oxides (SnO₂, NiO, excess Al₂O₃, etc.) contributes to the sintering of aluminum titanate by forming secondary crystalline phase on the grain boundaries of the ceramic. In this connection in obtaining sintered ceramic materials based on aluminum titanate it is preferable to consider additives of the first group or their combinations with additives of the second group.Translated from Ogneupory, No. 4, pp. 29–31, April, 1987.     

Densification behaviors and high-temperature characteristics of Si3N4 sintered bodies using Al2O3–Yb2O3 additives

The densification behaviors (include α–β transformation) and high-temperature characteristics (especially oxidation resistance and high-temperature strength properties) of Si₃N₄ sintered bodies using Al₂O₃–Yb₂O₃ based sintering additive are investigated.Densification and α–β transformation behaviors were investigated by varying the compositions of Al₂O₃–Yb₂O₃ additives. In terms of the influence of the Y₂O₃/Al₂O₃ ratio on densification behavior, a greater Yb₂O₃/Al₂O₃ ratio tends to inhibit densification. The α–β transformation tended to be delayed in sintered bodies with a small additive amount of 3.4 mass%. Compared with the transformation behaviors of the sintered bodies using Al₂O₃–Y₂O₃ additives, those using Al₂O₃–Yb₂O₃ additives exhibited a narrower temperature zone for α–β transformation, which attributed to the finer structure for the sintered body using Al₂O₃–Yb₂O₃ additives. This is affected by the difference in solubility of Si₃N₄ in the two kinds of glass phase.High room temperature strength of 900–1000 MPa was obtained for sintered bodies with a 10.0 mass% addition of additives, and this is considered to be due to the finer micro-structure. Precipitation of a Yb₄Si₂N₂O₇ phase at the grain boundary glass phase, as induced by crystallization processing, enables the improvement of 1300 °C strength to about 650–720 MPa. Crystallization processing resulted in a 30% reduction in the amount of weight change during oxidation (from 3.42 to 2.46 mg/cm²), demonstrating the effectiveness in improving oxidation resistance.     

Non-additive sintering of Si2N2O ceramic with enhanced high-temperature strength,oxidation resistance,and dielectric properties

The high-temperature mechanical and dielectric properties of Si₂N₂O ceramics are often limited by the introduction of a sintering aid. Herein, dense Si₂N₂O was prepared at 1700 °C by hot-pressing oxidized amorphous Si₃N₄ powder without sintering additives. A homogeneous network with short-range order and a SiN₃O structure was formed in the oxidized amorphous Si₃N₄ powder during the hot-pressing process. Si₂N₂O crystals preferentially nucleated at positions within the SiN₃O structure and grew into rod-like and plate-like grains. Fully dense ceramics with mainly crystalline Si₂N₂O and some residual amorphous phases were obtained. The as-prepared Si₂N₂O possessed a good flexural strength of 311 ± 14.9 MPa at 1400 °C, oxidation resistance at 1500 °C, and a low dielectric loss tangent of less than 5 × 10⁻³ at 1000 °C.     

The effect of different sintering additives on the electrical and oxidation properties of Si3N4–MoSi2 composites

The fabrication and properties of electrically conductive Si₃N₄–MoSi₂ composites using two different sintering additive systems were investigated (i) Y₂O₃–Al₂O₃ and (ii) Lu₂O₃. It was found that the sintering atmosphere used (N₂ or Ar) had a critical influence on the final phase composition because MoSi₂ reacted with N₂ atmosphere during sintering resulting in the formation of Mo₅Si₃. The electrical conductivity of the composites exhibited typical percolation type behaviour and the percolation concentrations depended on the type of sintering additive and atmosphere used. Metallic-like conduction was the dominant conduction mechanism in the composites with MoSi₂ content over the percolation concentrations due to the formation of a three-dimensional percolation network of the conductive MoSi₂ phase. The effect of the sintering additives on the electrical and oxidation properties of the composites at elevated temperatures was investigated. Parabolic oxidation kinetics was observed in the composites with both types of additives. However, the Lu₂O₃-doped composites had superior oxidation resistance compared to the composites containing Y₂O₃–Al₂O₃. It is attributed to the higher eutectic temperature and crystallisation of the grain boundary phase and the oxidation layer in the Lu₂O₃-doped composites.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Liquid phase sintered silicon carbide (LPS-SiC) ceramics having remarkably high oxidation resistance in wet air

Numerous studies on the oxidation characteristics of common silicon carbide ceramics have revealed very good oxidation resistance in dry atmospheres, provided that the oxygen partial pressure is sufficiently high. On the contrary, as other SiO₂-passivated ceramics, SiC shows poor oxidation resistance in moist atmospheres. In this contribution, the oxidation behaviour at 1400 °C of fully dense SiC samples with Lu₂O₃–AlN and Lu₂O₃–Ho₂O₃ sintering additives is investigated. Similar to results recently reported for a novel Si₃N₄ material, it is demonstrated that Lu₂O₃ yields greatly improved oxidation behaviour as compared to other liquid phase sintered SiC materials. In particular, the Lu₂O₃–Ho₂O₃ additive leads to passive oxidation under a 0.01 MPa partial pressure of water.     

Effect of additives on the thermal conductivity of zirconium diboride based composites – A review

Re-entry space vehicle necessities sharp leading edges for better aerodynamic performance and, hence, require advanced thermal protection materials with improved safety for crew members. Material possessing high thermal conductivity and oxidation resistance are desirable at nose cap and wings leading edge of spacecraft. Consequently, the thermal shock resistance improves due to reduced thermal gradient and stresses. ZrB₂ has drawn strong impetus for futuristic space vehicles as thermal protection materials under extreme thermal environments. This study reviews the effect of the incorporation of non-carbonaceous and carbon additives on the thermal conductivity of ZrB₂ ceramics and based composites. Several factors such as the purity of starting powder, initial particle size, amount of sintering aids, processing route, porosity, the grain size of ZrB₂ matrix, distribution of secondary phases in the matrix and sinter density of the final composite, controls the overall thermal conductivity of ZrB₂ based composites.     

Synthesis of highly Li-ion conductive garnet-type solid ceramic electrolytes by solution-process-derived sintering additives

The sintering and processing of garnet-type solid ceramic electrolytes (e.g., Li₇La₃Zr₂O₁₂ (LLZ)) are challenging because the material composition and microstructure at high temperatures must be carefully controlled to obtain the stabilization of highly conductive cubic phase and dense ceramic. Liquid-phase sintering using sintering aids is typically used for densifying ceramic materials, as it is a faster and/or lower-temperature process. In this study, we used solution-process-derived sintering additives to sinter garnet-type solid electrolytes highly effective in terms of relative density and properties at 1000 °C (10 h). The liquid phase formation during the sintering was rationalized to establish the optimal sintering conditions. The use of 1.2-vol% 75Li₂O∙25B₂O₃ and 1.5-vol% Al₂O₃ as sintering additives was highly effective in densifying a Ta-doped LLZ, achieving a high ionic conductivity of 0.8 mS cm⁻¹ (25 °C) with low activation energy (9 kJ mol⁻¹) and almost negligible contribution of the grain boundary resistance (10 %).     

New strategy for impedance measurement of surface oxidation characteristic in ceramic fiber by controlling aspect ratio

Silicon carbide fibers (SiC_f) are promising materials for high-temperature applications, because of their excellent thermal and mechanical properties. Due to the inevitable oxidation of SiC_f used at high-temperature, the surface and interfacial properties of SiC_f can be changed. Impedance analysis is necessary to confirm the useful information for various properties. However, since single SiC_f has a long length and a very small cross-section area, its capacitance is so small that impedance analysis is almost impossible. In this study, an in-situ analysis tool that can estimate the impedance of SiC_f to confirm the resistance elements is proposed using bundled fiber specimens. It is confirmed that it is possible to measure the impedance of the SiC_f under high-temperature and oxidation atmospheres by controlling the aspect ratio of the specimen. This tool provides a new pathway that can be applied in various systems by investigating the electrical properties according to the oxidation behavior.     

Effect of additives introduced by ball milling on sintering behavior and mechanical properties of hot-pressed B4C ceramics

The ball-milling process is a usual route employed to break up agglomerates in the powder or mix the powder with additives when preparing ceramics. Although it is well known that a powder can be “contaminated” by the wear particles from the milling balls, the study dealing with how the preparation and properties of a hard material would be affected by additives just introduced by scrape of milling balls has been scarce. In the present work, sintering behavior and mechanical properties of hot-pressed B₄C with additives derived from milling balls were investigated. Polyoxymethylene, ZrO₂, Al₂O₃ and Si₃N₄ were selected as different ball materials. The results show that the sinterability of B₄C could be significantly enhanced because of the incorporation of one of these additives, i.e. ZrO₂, Al₂O₃ and Si₃N₄ (approximately 3–6 vol%). As a result of improvement in density, excellent mechanical properties of B₄C ceramics were obtained. Among them a flexural strength of B₄C added by ZrO₂ reached 630 MPa.     

Effects of Gd2O3 and Yb2O3 on the microstructure and performances of O'-Sialon/Si3N4 ceramics for concentrated solar power

O'-Sialon/Si₃N₄ composite ceramics for solar absorber were prepared in situ through pressureless sintering from Si₃N₄ and low pure Al₂O₃ with different rare-earth oxides (i.e., Yb₂O₃ and Gd₂O₃). This study investigates the effects of Yb₂O₃ and Gd₂O₃ on phase composition, microstructure, densification, oxidation resistance and solar absorptance. The results revealed that Yb₂O₃ and Gd₂O₃, which were applied as flux materials, significantly reduced the sintering temperature and improved the densification, oxidation resistance of the composite ceramics. Moreover, the introduction of the two sintering aids promoted the formation of O'-Sialon through the liquid-phase sintering mechanism. The samples doped with Gd₂O₃ exhibited more O'-Sialon content, lower porosity, and better oxidation resistance compared with those doped with Yb₂O₃. Especially, sample A2 (6 wt% Gd₂O₃ additional) sintered at 1600 °C exhibited the best comprehensive properties for 10.10% water absorption, 23.29% porosity, 105.57 MPa bending strength, 75.16% solar absorption. Dense oxide layers were generated on sample surfaces after oxidation at 1300 °C, which protected the ceramics samples from further oxidation. However, the two additives had characteristic reflection peaks in the ultraviolet–visible region (300–400 nm) and were also blamed for high reflectivity in the near–infrared region, which resulted in the decrease in absorption.