Effects of rolling and hot pressing on mechanical properties of boron carbide-based ceramics

A study of hot pressed B₄C-based laminates, after rolling and without rolling, has been performed to elucidate the existence of fracture resistance/crack length anisotropy induced by this processing technique. While the crack lengths/fracture resistance was affected significantly by the presence of the residual stresses in B₄C/B₄C–ZrB₂ laminates, no differences in Vickers crack lengths were observed in B₄C/B₄C laminates prepared by rolling and hot pressing, as compared to the crack lengths seen in pure B₄C ceramics prepared by hot pressing without rolling. X-ray diffraction analysis confirmed that no texture has been formed during the rolling and hot pressing of B₄C ceramics.     

Fracture toughness assessment of silicon carbide-based ceramics and particulate-reinforced composites

The fracture toughness of sintered silicon carbide (α-SiC) and silicon carbide reinforced with particulate titanium diboride (TiB₂/SiC) has been evaluated using specimens in bending containing chevron notches and through-thickness precracks at ambient and elevated temperatures in air and in vacuum. Fracture toughness values measured from through-thickness precracked test-pieces are lower at all test temperatures. The particulate reinforcement has been shown to toughen the matrix significantly at room temperature only. At the test temperature of 1200°C the difference in toughness between the two materials is reduced and increasing the temperature to 1600°C further reduces this difference, to the extent that the two materials have values of fracture toughness which are indistinguishable. This provides strong evidence that the dominant toughening mechanism in the composite is the effect of thermal residual stresses which are relieved as the temperature is increased. Fractographic observations suggest that the bonding between the SiC and TiB₂ particulate is relatively weak because interfacial decohesion of particles is observed at all test temperatures. Nevertheless, surface roughness measurements indicate that there may also be a contribution to the toughness from increased crack deflection in the composite material at room temperature only.     

Oxidation resistance of silicon nitride ceramics with various additives

Five silicon nitride ceramics with various additives were evaluated for their oxidation resistance at 1300 ° C in flowing dry and wet air. In a dry atmosphere, the oxidation of all types of Si₃N₄ except one proceeded in two stages with different reaction rates nearly following the parabolic law. In a wet atmosphere, all types showed a linear relationship between weight gain by oxidation and water varpour content. The water vapour in the atmosphere slightly accelerated the oxidation. The influence of oxidation on room-temperature strength was complex, but there was no observed distinguishable difference of the effect on the flexural strength between dry and wet oxidation.     

The effect of Fe addition on processing and mechanical properties of reaction infiltrated boron carbide-based composites

Dense metal-ceramic composites based on boron carbide were fabricated using boron carbide and Fe powders as starting materials. The addition of 3.5–5.5 vol% of Fe leads to enhanced sintering due to the formation of a liquid phase at high temperature. Preforms, with about 20 vol% porosity were obtained by sintering at 2,050 °C even from an initial boron carbide powder with very low sinterability. Successful infiltration of the preforms was carried out under vacuum (10⁻⁴ torr) at 1,480 °C. The infiltrated composite consists of four phases: B₁₂(C, Si, B)₃, SiC, FeSi₂ and residual Si. The decrease of residual Si is due to formation of the FeSi₂ phase and leads to improved mechanical properties of the composites. The hardness value, the Young modulus and the bending strength of the composites fabricated form a powder mixture containing 3.5 vol% Fe are 2,400 HV, 410 GPa and 390 MPa, while these values for the composites prepared form iron free B₄C powder are 1,900 HV, 320 GPa and 300 MPa, respectively. The specific density of the composite was about 2.75 g/cm³. The experimental results regarding the sintering behavior and chemical interaction between B₄C and Fe are well accounted for by a thermodynamic analysis of the Fe–B–C system.

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Oxidation resistance of thin boron carbo-nitride films on Ge(100) and Ge nanowires

Chemical vapor deposition of thin (< 10 nm) films of amorphous boron carbo-nitride (BC_0.7N_0.08, or BCN) on Ge(100) and Ge nanowire (GeNW) surfaces was studied to determine the ability of BCN to prevent oxidation of Ge. X-ray photoelectron spectroscopy was used to track Ge oxidation of BCN-covered Ge(100) upon exposure to ambient, 50 °C deionized water, and a 250 °C atomic layer deposition HfO₂ process. BCN overlayers incorporate O immediately upon ambient or water exposure, but it is limited to 15% O uptake. If the BCN layer is continuous, the underlying Ge(100) surface is not oxidized despite the incorporation of O into BCN. The minimum continuous BCN film thickness that prevents Ge(100) oxidation is ~ 4 nm. Thinner films (≤ 3.2 nm) permitted Ge(100) oxidation in each of the oxidizing environments studied. GeNWs with a 5.7 nm BCN coating were resistant to oxidation for at least 5 months of ambient exposure. High resolution transmission electron microscopy images of HfO₂/BCN/Ge(100) cross-sections and BCN-coated GeNWs reveal clean, abrupt BCN-Ge(100) interfaces.     

Oxidation mechanisms in Si-Al-O-N ceramics

Oxidation mechanisms in single- and two-phase Si-Al-O-N ceramics have been studied using scanning and transmission electron microscopy together with energy-dispersive X-ray microanalysis. Silicate layers formed on single-phase (β′) ceramics are non-crystalline, with viscosity and resulting oxidation kinetics controlled by outward diffusion of grainboundary segregated impurities. Aluminium substitution inβ′ is important in compensating for the viscosity reduction imposed by the divalent ion impurities and inhibiting crystallization. Crystallization, induced only on slow furnace cooling, produces mullite and cristobalite phases. Two-phase (β′and matrix) ceramics exhibit comparatively poor oxidation kinetics with formation of a porous crystalline silicate layer due to the continued availability of a high concentration of metallic ions in the matrix phase.     

Oxidation kinetics of hexagonal boron nitride powder

The isothermal oxidation of hexagonal boron nitride powders was carried out at 900–1050°C in dry oxygen and air. The oxidation kinetics were found to obey linear rate law and were described by the surface chemical reaction-controlled shrinking cylindrical model. The apparent activation energies were found to be 298 and 330 kJ mol^–1in dry oxygen and air, respectively. The oxygen partial pressure dependence of the oxidation rate constant at 1000 °C is well represented by a Langmuir-type equation. Microscopic examination of the oxidized sample after removal of the oxide scale with water suggested that the rate of attack by oxygen was determined by an anisotropy due to the crystallographic direction, similar to oxidation in graphite. The volatilization of B₂O₃ was observed only in dry oxygen and obeyed a linear rate law, and was found not to affect the oxidation reaction.     

Oxidation resistance of carbon-ceramics composite materials sintered from ground powder mixtures of raw coke and ceramics

Carbon-SiC-B₄C composite materials were prepared from ground powder mixtures of petroleum raw coke, SiC and B₄C by powder sintering, without the use of any special binder and hot-pressing process. Dense composites with a fine microtexture were obtained. Oxidation tests were carried out on the composites at temperatures from 1000 to 1300° C under an air flow. The oxidation resistance depended strongly on the SiC/(SiC + B₄C) ratio and total contents of SiC and B₄C in the composites, which determined the compositions of B₂O₃ and SiO₂ in the protective film formed at the surface of the composite block during oxidation. In optimum ratios, from 63 to 87%, the composites showed such a high oxidation resistance that they were comparable with Si₃ N₄ at 1200° C.     

Oxidation bonding of porous silicon carbide ceramics

A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al₂O₃ and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al₂O₃ content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al₂O₃, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.     

TiC-TiB2复相陶瓷的氧化行为研究

对自蔓延高温合成法制备的TiC-TiB2复相陶瓷的氧化行为及机理进行了研究.结果表明:在低于1000℃范围内,随氧化温度和时间的增加,氧化增重符合抛物线规律;在600℃时,TiB2首先被氧化成TiO2,在1000℃时TiB2、TiC均被氧化成TiO2,表面形成致密的TiO2氧化膜,阻止内部继续发生氧化,这表明该材料具有较好的高温抗氧化能力.     

Oxidation of reduced Y-doped semiconducting barium titanate ceramics

Oxidation-induced microstructural changes in reduced yttrium-doped barium titanate (Ba_{1 – x}Y_xTi_{1 – x}⁴⁺Ti_x³⁺ O₃) are studied using samples sintered in a reducing atmosphere ( = 10^–4Pa) and then oxidized in air at 1150 and 1350°C. The results indicate that oxidation leads to precipitation of Ba₆Ti₁₇O₄₀ and, at relatively high doping levels, Y₂Ti₂O₇. These phases increase the electrical resistance of the outer layer of the grains in the ceramics.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 93–100.Original Russian Text Copyright © 2005 by Vyunov, Kovalenko, Belous, Belyakov.     

Oxidation and corrosion of ceramics and ceramic matrix composites

Ceramics and ceramic matrix composites are candidates for numerous applications in high temperature environments with corrosive gases and deposits. High temperature oxidation and corrosion issues must be considered for these materials. The most important oxidation and corrosion studies have recently focused on four major areas. These are: (I) oxidation of precursor-based ceramics, (II) studies of the interphase material in ceramic matrix composites, (III) water vapor interactions with ceramics, particularly in combustion environments, (IV) development of refractory oxide coatings for silicon-based ceramics. This paper explores the most current work in each of these areas.     

Hardness and cracking resistance of structural ceramics

Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 27, No. 3, pp. 12–18, May–June, 1991.     

High-temperature deformation of fully-dense fine-grained boron carbide ceramics: Experimental facts and modeling

Boron carbide ceramics are the hardest material in Nature after diamond and the cubic phase of boron nitride. Due to this fact, their room-temperature fracture properties are the object of intense research. Paradoxically, high-temperature deformation is essentially unknown, because very high temperatures and stresses are necessarily required and high-quality specimens have not been available until recently. In this paper, the high-temperature compressive creep of fine-grained boron carbide polycrystals is reported. The breakdown of the classical power-law for high-temperature plasticity in ceramics is found. An analytical model is proposed. The model assumes that deformation is produced by dislocation glide. However, since the formation of twins is energetically favorable in this material and they act as strong barriers for dislocation glide, their motion turns to become progressively more difficult as elongation proceeds. The combination of increasing twin barriers and dislocations in mutual interaction is proposed to be the mechanism for high-temperature plasticity in this material. The model is validated with the experimental results. Final elongation of boron carbide specimens is reported to be over 100%, although this material cannot be described as a superplastic ceramic.     

High resistance boron treated steels for railway applications

Abstract

The present study analyses different boron contents (between 10 and 160 ppm) on the structure of a 0·2C–2Mn–1Si (wt-%) steel deformed at a starting temperature of 1050°C in a T. J. Pigott laboratory rolling mill. The steel was made in a laboratory open induction furnace using high purity raw materials and cast into metallic moulds. This experimental steel has proved to have tribological properties, under dry rolling/sliding contact, as good as those for the 0·8% pearlitic steels used in railway applications. Before thermomechanical processing, the steel ingots (70 × 70 × 70 mm) were homogenised at 1100°C for 1·5 h. The thermomechanical treatment was carried out by a reversed multipass process to reach a level of deformation of 60%. Plastic deformation was finished at ～920°C for all the rolled steels and the plates (70 × 150 × 20 mm) were then water quenched and/or air cooled to room temperature. Results show more bainitic structures as boron content increases in the air cooled steel after hot rolling. For the quenched steels, the structure becomes more martensitic as boron content increases. The best combination of mechanical properties was obtained for the air cooled 76 ppm boron containing steel, which had a lower bainitic structure. This steel had the yield strength of 750 MPa, 15% elongation and the hardness of 40 HRC. Materials characterisation was carried out by optical and transmission electron microscopy (TEM). Results are discussed in terms of the boron segregation towards grain boundaries, the effect of boron on the steel hardenability, as well as on the boron carbonitrides (CNB) precipitation.