Abstract

Hexagonal boron nitride (h-BN) is a very versatile material that can be used in a number of applications due to its unique combination of properties. This paper reviews typical h-BN qualities and their applications. The use of h-BN as a composite material with zirconium oxide for side dams in thin-strip casting is looked at in particular detail. Recent results for corrosion, wear and high-temperature compressive stress of MYCROSINT^® SO are presented here for the first time.

Abstract

ZrB₂–SiCw composites were prepared through hot-pressing at a low temperature of 1800 °C, and Al₂O₃ plus Y₂O₃ were added as sintering aids. Analysis revealed that additives may react with impurities (i.e. surface oxygen impurities and residual metallic impurities) to form a transient liquid phase, thus promote the sintering and densification of ZrB₂–SiCw composites. The content of additives was found to have a significant influence on the sinterability, microstructure and mechanical properties of ZrB₂–SiCw composites. ZrB₂–SiCw composite prepared with a small amount of additives (3 vol.%) provided the optimal combination of microstructure (relative density of 98.3%) and excellent properties, including flexural strength of 783 MPa and fracture toughness of 6.7 MPa m^1/2. With further addition of additives, SiC whiskers were inclined to gather together and be enveloped by excessive liquids to form core-rim-like structures, which lead to little decrease in mechanical properties.

Abstract

ZrB₂–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB₂–SiC–BN (40 vol%ZrB₂–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m^1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB₂–SiC (80 vol%ZrB₂–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB₂–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB₂–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB₂–20 vol%SiC and 43.1 vol%ZrB₂–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm², respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB₂–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB₂–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB₂–SiC–BN.

Systematic studies were carried out on the sintering of ZrB₂–SiC ceramics without any external pressure being applied. The densities, mechanical properties and microstructures with different contents of B₄C were investigated. The results showed that B₄C additions were beneficial not only for the sintering process but also for the oxidation resistance. The sintered ZrB₂–SiC ceramics with 0.4 wt.% B₄C addition had an average strength of 382 ± 24 MPa and a modulus of 405 ± 34 GPa. The hardness and toughness were 13.4 ± 0.4 GPa and 3.7 ± 0.2 MPa m^1/2, respectively. The effect of B₄C addition on the oxidation resistance of ZrB₂–SiC was studied by observing the mass changes and the residual strengths of samples after oxidized treatments.

Adding a small amount of vanadium carbide as sintering aids, nearly fully dense ZrB₂–SiC composites were obtained by pressureless sintering methods at 2000–2100 °C. Thermodynamic calculations and experimental results proved that VC is better at removing the surface oxide impurities of ZrB₂ than WC, and played a very important role during the densification process. The mechanical properties of the sintered ceramics were also reported.

A pressureless densification mechanism and microstructure development in ZrB₂–20 vol.% SiC ceramics with boron additive were investigated. Results showed that the boron addition removed oxygen impurity and improved the densification process. By adding 1 wt.% boron, fully dense ZrB₂–SiC ceramics with a fine-grained microstructure was obtained. However, a higher boron addition would lead to fast grain growth of both SiC and ZrB₂ grains and a deteriorated densification behavior by forming a liquid phase. The mechanical properties of the sintered ceramics with 1 wt.% boron addition were also evaluated.

Abstract

In this paper, ZrO₂ removing reactions of Groups IV–VI transition metal carbides (MCs, M = Hf, Nb, Ta, W, Ti and V) in ZrB₂ based ultra-high temperature ceramics (UHTCs) are investigated. Distinct roles of various MCs were observed during this process. According to thermodynamic analysis and experiment verifications, the sequence for oxide removing ability of different MCs is WC > VC > NbC > TaC > HfC > TiC. The importance of this study concerns the establishment of a map of reactivity of the transition metal carbides against ZrO₂, in order to choose the proper additive for the densification of ZrB₂-composites.Utilizing right reactions and controlling proper sintering atmospheres, high density ZrB₂–SiC ceramic could be eventually obtained by pressureless sintering at temperatures (around 2000–2200 °C).

A pressureless sintering process with two pre-sintering heat treatments was developed for the densification of HfB₂. The use of two isothermal holdings, at 1650 and 2000 °C, prior to the final sintering temperature (2200 °C) proved to be more effective at removing oxygen impurities and rearranging the initial powder compact microstructure. The mechanical properties of HfB₂ with 2 wt.%B₄C, including hardness (19.5 GPa), E modulus (529 GPa) and strength (469 MPa), were comparable to hot-pressed samples or higher than the values using other sintering aids.

Abstract

A pressureless sintering process, using a small amount of boron carbide (≤2 wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100 °C to 2350 °C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300 °C) to liquid phase sintering (LPS, above 2300 °C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B₄C on the microstructure development and mechanical properties of the sintered ceramics were investigated.

ZrB₂–20 vol.% MoSi₂ composites with elongated ZrB₂ grains were prepared by reactive hot pressing using Zr, Si, B and Mo as raw materials. The ductile behavior of MoSi₂ at high temperature helped the rod-like ZrB₂ grains align under the applied pressure during sintering, resulting in a partly textured composite. The anisotropic growth of ZrB₂ grains improved the toughness of the composite remarkably.

Abstract

The reactions of aluminum borates (9Al₂O₃ · 2B₂O₃ and 2Al₂O₃ · B₂O₃) with aluminum nitride (AlN) have been used as a new chemical route to synthesize alumina-boron nitride (Al₂O₃–BN) composites. Reaction mechanisms were investigated by TG-DTA and static reaction process. The reactions started at around 1200 °C and completed at around 1500 °C. Soaking at temperatures higher than 1800 °C resulted in the reverse reaction that caused great weight loss. Hot pressing promoted the reactions due to the improved diffusion process. The in situ formed BN phase was in agglomerate shape located at the pockets of Al₂O₃ matrix particles and this distribution was suggested to be beneficial to the strength of materials with weak phase dispersoids. The fracture surface analysis demonstrated that the main fracture mode was transgranular, indicating the existence of a strong Al₂O₃ network in the in situ synthesized composites. The prepared composites exhibited high strength, low Young’s modulus and high strain tolerance.

Abstract

The reaction mechanism and microstructure development of strain tolerant in situ SiC–BN composites fabricated from the in situ reaction of Si₃N₄, B₄C and C were investigated. This exothermic reaction took place at about 1400°C in an argon atmosphere according to the results of X-ray diffraction analysis and differential thermal analysis. The reaction finished after hot pressing at 1700°C for 60 min, and densification occurred mainly in the temperature range of 1700°C to 2000°C. In spite of poor sinterability of BN, composites with rather high density were obtained. Chemical composition analysis of the composites obtained showed that there was no obvious change in the composition after hot pressing. The in situ formed SiC was of the β-type with a quasi-spherical shape, whereas the in situ BN was graphitic hexagonal with a flake shape, and was located at the grain boundaries of SiC. The composite obtained showed a very fine and homogeneous microstructure. The bending strength of the composite was high, while the elastic modulus decreased substantially.

Highly c-axis oriented ZrB₂-based ultrahigh-temperature ceramics (UHTCs) were prepared via strong magnetic field alignment during slip casting, followed by spark plasma sintering, using ZrB₂ and β-SiC as starting materials. Results showed that hardness was superior on the surface perpendicular to the grain alignment. Oxidation resistance was obviously better on the surface of (001) direction. The anisotropic properties of the textured materials would provide an opportunity to the material design and selection of UHTCs.

Based on recent preliminary results of textured ZrB₂-based ceramics, c-axis oriented HfB₂-based ultrahigh-temperature ceramics with a Lotgering orientation factor as high as 0.91 were prepared by slip casting in a strong magnetic field alignment, followed by spark plasma sintering. The textured sample displays significantly anisotropic properties. Compared with textured ZrB₂-based ceramics, the HfB₂-based material shows much better oxidation resistance.

High-hardness B₄C was prepared by sintering samples textured by a strong magnetic field alignment technique (12 T). The c-axis was highly oriented perpendicular to the applied magnetic field. The hardnesses measured on the surface perpendicular and parallel to c-axis were 38.86 ± 2.13 and 31.31 ± 0.79 GPa, respectively. The high values of hardness and elastic modulus were in agreement with those reported for B_4.38C monocrystal.

Textured and platelet-reinforced ZrB₂-based ultra-high-temperature ceramics (UHTCs) are prepared via reactive hot pressing and successive hot forging. The textured materials display significantly anisotropic mechanical properties, and the flexural strength has been improved by 52% owing to the orderly arrangement of platelet ZrB₂ grains. The texturing of ZrB₂-based platelet composites should open up an effective approach to optimizing the properties of ZrB₂-based UHTCs.

Textured hexagonal boron nitride ceramics were prepared by hot pressing, using boron nitride powders as starting materials. The results showed that a high sintering temperature and moderate external pressure helped to form a textured microstructure when using boron nitride powder with a low degree of order and a broad particle size distribution. The flake-like boron nitride grains tended to orient along the hot pressing direction via pressure-induced preferential grain growth. The obtained textured boron nitride ceramics demonstrated strong anisotropic mechanical and thermal properties.