Kinetics and High-Temperature Oxidation Mechanism of Ceramic Materials in the ZrB<Subscript>2</Subscript>–SiC–MoSi<Subscript>2</Subscript> System

This study is devoted to the fabrication of the ZrB₂–SiC–(MoSi₂) compact ceramics according to hybrid technology (self-propagating high-temperature synthesis (SHS) + hot pressing), as well as to investigating its phase composition, structure, and high-temperature oxidation kinetics. Reaction mixtures are prepared according to the following scheme: mechanical activation (MA) of Si + C powders; wet admixing of Zr, B, and Si + C MA-mixture powders; and drying mixtures in a drying oven. The ZrB₂–SiC SHS composite powder is formed in a reactor in a combustion mode by elemental synthesis. Compact samples with a homogeneous structure and low residual porosity not exceeding 1.3% are formed by hot pressing the SHS powder. Two compositions are selected for testing, notably, the first one calculated for the formation of ZrB₂ + 25% SiC; the second composition is similar to the first one, but with the addition of 5% of the MoSi₂ commercial powder. The microstructure of the samples is presented by dispersed dark gray rounded SiC grains distributed among light faceted ZrB₂ grains. The sample with the MoSi₂ additive has a more finely dispersed structure. The high-temperature oxidation of the samples at 1200°C results in the formation of SiO₂?ZrB₂–(B₂O₃) complex oxide films on their surface with a thickness on the order of 20–30 μm, which serve as an efficient diffusion barrier and lower the oxidation rate. Their structure also contains ZrSiO₄ complex oxide after prolonged holding (longer than 10 h). In addition, an insignificant weight loss of the samples is observed after 10 h testing, which is caused by the volatilization of gaseous oxidation products (B₂O₂, CO/CO₂, MoO₃). The sample with the MoSi₂ additive shows better resistance to oxidation.     

Synthesis of TiC–TiB–TiB2 composite powders by solid reaction of powders

In the present work, TiC–TiB–TiB₂ diffusion-layer-coated B₄C composite powders were synthesised via a powder immersion method using Ti and B₄C powders as reactants. The phase compositions and microstructure of the treated powders were characterised by employing X-ray diffraction and scanning electron microscopy. No significant reaction between B₄C and Ti could be detected at 800°C. After treatment at 900°C, the products generated were composed of TiC and TiB. After treatment at 1000°C, the products generated were primarily composed of TiC and TiB, with a small amount of TiB₂. The composition and proportions of the produced phases varied with process temperatures and the composition of the initial powders used. Powder mixtures with a Ti/B₄C molar ratio of 3.5:1 and treated at 1000°C for 14?h were more suitable for synthesis of TiC–TiB–TiB₂-coated B₄C composite powders.     

Investigation of Microstructure,Mechanical and Wear Behaviour of B<Subscript>4</Subscript>C Particulate Reinforced Magnesium Matrix Composites by Powder Metallurgy

In the present study, magnesium and magnesium matrix composites reinforced with 10, 20 and 30 wt% B₄C particulates were fabricated by powder metallurgy using hot pressing technique. The microstructure, mechanical properties and wear behaviour of the samples were investigated. Microstructure characterization showed generally uniform distribution of B₄C particulates. XRD investigations revealed the presence of Mg, B₄C and MgO phases. The mechanical properties of the investigated samples were determined by hardness and compression tests. Hardness and compressive yield strength significantly increased with increasing B₄C content. The reciprocating wear tests was applied under loads of 5, 10 and 20 N. Wear volume losses decreased with increasing B₄C content. Abrasive and oxidative wear mechanisms were observed.     

Wrought Fe-Cr-Al alloys

Conclusions It is recommended that semifinished blanks from Fe-Cr-Al alloys (Fechrals) with or without disperse strengthening-phase additions should be manufactured from alloy powders (rather than from mechanical mixtures); the blanks should be additionally consolidated by hot isostatic pressing, and the resultant rods subjected to hot plastic working by extrusion. Hot isostatic pressing increases the processing ductility of Fe-Cr-Al alloys, reduces the size of their ferrite grain, and improves their mechanical properties. Free forging of blanks produced by hot isostatic pressing does not ensure good quality of rods: Their mechanical properties show scatter due to large grain size variations and the presence of microcracks. The mechanical and physical properties of Fe-Cr-Al alloys produced by powder metallurgy techniques are virtually identical with those of cast alloys of the same compositions. At the same time, P/M alloys are less prone to grain growth at t>900°C. Additional strengthening with disperse refractory oxides (2 wt. % TiO₂ or ZrO₂) increases about 1.5–2 times the strength of Fe-10A1-25Cr alloys at room and high temperatures right up to 1200°C.Translated from Poroshkovaya Metallurgiya, No. 5(221), pp. 43–53, May, 1981.     

Mechanical characteristics of electrotechnical materials of the system Si3N4-SiC

Conclusions With selected optimal technology of producing materials Si₃N₄,-SiC, their mechanical characteristics may change within fairly broad limits, and they are determined primarily by the composition of the initial charge. Material with optimal composition has a bending strength of 500 MPa and a critical stress intensity factor 6.8 MN/m^3/2.To obtain ceramics with high values of _b and K_1c, it is expedient to use finely disperse highly active silicon carbide (10–30 volume%), and also oxide-free-activators for hot pressing.Increasing the grain size of the conducting phase SiC to 120 m and the amount of activating additive leads to reduced _b of the materials, however, the overall level of strength remains fairly high (>200 MPa).Translated from Poroshkovaya Metallurgiya, No. 1(313), pp. 57–61, January, 1989.     

The effect of production conditions on the structure formation and on the electrical conductivity of hotpressed materials in the system nitride-silicon carbide

Conclusions n materials of the system Si₃N₄-SiC are hot-pressed, there occur processes of carbidization of Si₃N₄ and of reduction of the surface film of SiO₂ accompanied by weight loss and increased concentration of SiC. The degree of such physicochemical transformations is well correlated with the magnitude of the specific weight losses. The electrical conductivity of the investigated materials is basically determined by the content of the conducting phase SiC in the initial charge; however, when the concentration of the introduced silicon carbide is low, then the amount of secondary SiC forming in hot pressing plays a considerable role.Translated from Poroshkovaya Metallurgiya, No. 2(290), pp. 51–54, February, 1987.     

Structure and Certain Properties of Hot-Pressed Boron Carbide-Based Ceramic with Calcium-Silicon Additive

The influence which the composition of powder mixtures, the treatment conditions which the mixtures are subjected to, and the conditions under which the hot-pressed composite materials B₄C – (5-10 mass%) calcium-silicon are fabricated exert on the structure, nature of failure, and mechanical properties of these materials is investigated. Optimum properties are possessed by material containing 10 mass% of addition. It is shown that the structure, morphology, and dispersivity, as well as the nature of the distribution of the components that are added to the composite material (secondary phase) vary as the temperature of hot pressing changes. Maximal mechanical characteristics of the composite material (σ_bend = 560 MPa, K _1c = 4.7 MPa·m^1/2, HV = 37 GPa) are attained at hot-pressing temperatures in the range 2000-2100°C.     

Synthesis and properties of ceramics in the SiC — B4C — MeB2 system

The effect of impurities and additives of titanium and zirconium borides on the structure and mechanical properties of SiC — B₄C ceramics over a broad temperature range has been investigated. The ceramics was fabricated by hot pressing without a protective medium. Introduction of borides is accompanied by improvement in all the studied mechanical properties at room temperature, and the nature of hardening of the ceramics is practically independent of the type of SiC powders used. At high temperatures, the mechanical behavior of the ceramics is determined by the impurity composition: the ceramics obtained using abrasive powders loses strength beginning at 600°C, while using powders with decreased impurity content makes it possible to preserve the strength of the material up to a temperature of 1400°C. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 29–42, May–June, 2000.     

Effect of distribution of B4C on the mechanical behaviour of Al-6061/B4C composite

In this work, the effect of mixing parameters on the distribution of B₄C in 6061-Al alloy and its correlation with mechanical behaviour was studied. 6061-Al alloy powder was mixed with 10 mass-% B₄C powder in a ball mill and powder rotator mixer by varying mixing time from 1 to 5?h. Mixing was performed in both wet and dry conditions in a ball mill while only dry condition was used in the powder rotator mixer. The green compacts were sintered at 630°C. The quadrat method was used to quantify the distribution of B₄C particles in the microstructures of sintered Al/B₄C composite. The results showed that the distribution was improved with mixing time but the density, hardness and compression strength of Al/B₄C composites were reduced with time during ball milling. On the other hand, the distribution of reinforcement, density, hardness and compressive strength of Al/B₄C composites was improved with mixing time in the powder rotator mixer.     

High-temperature reaction between silicon carbide and nitrogen under pressure

The effects of temperature and nitrogen pressure are studied on the SiC Si₃N₄ transformation of silicon carbide powders of various phase compositions, specific surface areas, and contents of mixtures. It is shown that the degree of transformation increases with nitrogen pressure up to 10 MPa and that, in all temperature and pressure ranges of nitrogen, it is higher for bulk free powder than the preliminarily compacted material. In 30–60 min, a complete transformation of SiC into Si₃N₄ occurs under 10 MPa nitrogen pressure and at 1650–1750°C temperature.Institute of Inorganic Chemistry, Slovakian Academy of Sciences. Institute of Superhard Materials, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, No. 4(364), pp. 1–6, April, 1993.     

Formation of a TiB2-reinforced copper-based composite by mechanical alloying and hot pressing

The microstructural changes during mechanical alloysing and subsequent hot pressing of Cu, Ti, and B powder mixtures were studied using scanning electron microscopy, X-ray diffraction, differential thermal analysis, and electron microprobe analysis. In particular, the changes in the Cu grain size, lattice parameter, and lattice strain of the powder mixtures and the formation of new phases (TiCu₄ during mechanical alloying and TiB₂ during hot pressing) were investigated. The mechanism of the in-situ formation of TiB₂ particles in the resultant copper composite was also studied.     

Processing,Microstructure, and Mechanical Properties of B4C ― TiB2 Particulate Sintered Composites. Part I. Pressureless Sintering and Microstructure Evolution

Pressureless sintering of boron carbide ceramics containing 0-25 vol. % TiB₂ phase, produced via an in-situ chemical reaction between B₄C, TiO₂, and elemental carbon, was studied in the isothermal and constant-heating-rate regimes. The presence of TiB₂ results in a decrease in activation energy for sintering from 717 kJ/mol at 0 vol. % TiB₂ to 266 kJ/mol at 25 vol. % TiB₂. Ceramic bodies of B₄C TiB₂ particulate composites with relative densities of up to 99% were sintered without pressure at temperatures of 2050-2100°C. Grain boundary diffusion is the primary mechanism of TiB₂ particle coarsening. TiB₂ particle size is bimodal depending on whether the particle is confined within a B₄C grain or located on the grain boundary. Densification behavior of the B₄C TiB₂ system is identical at different heating rates in the temperature range of 1800-2150°C.     

Boriding of titanium OT4 from powder saturating media

The possibility of application of boriding media based on boron carbide—which additionally contain chromium, titanium, and silicon—for the diffusion hardening of titanium alloys is considered. Boriding in amorphous boron is performed for comparison. The microstructure, elemental composition, and phase composition of diffusion coatings on the OT4 titanium alloy formed by saturation in powder media are investigated. Hardening boride layers are formed on the titanium alloy form saturating media based on amorphous boron and multicomponent mixtures based on boron carbide. In all cases, the phase composition of the coating corresponds to phases TiB, Ti₂B₅, and Fe₂Ti. It is revealed that coatings from 30 to 150 μm thick are formed in conditions of the solid-phase saturation of titanium from powder mixtures due to the diffusion. Temperature-temporal conditions of formation of boride layers on OT4 titanium from powder saturating media are investigated and optimal modes for the formation of operable boride coatings are established. The optimal temperature range for processes of chemical-thermal boriding of titanium (900–1150°C) and saturation time (from 2.5 to 5 h) are determined. The maximal thickness of the operable boride coating on the OT4 titanium alloy is established, being from 180 μm in the case of saturation from B_amorph and up to 240 μm for the 50% B₄C + 20% SiC + 25% CrB₂ + 5% NaCl mixture at 950°C and saturation time of 4 h. Herewith, it should be noted that it was considered that the largest coating thickness is that retaining on the hardened sample surface.     

Synthesis of Al2O3/TiCN-0.2%Y2O3 Composite by Hot Pressing

Al2O3/TiCN composites were synthesized by hot pressing.The influences of components and HP temperature on mechanical properties,such as bending strength,breaking tenacity and Vickers hardness were investigated.The results showed that the mechanical properties of Al2O3/TiCN composite increased with temperature when hot pressing temperature is below 1650 ℃.The mechanical properties reached their maximums when the composites were sintered at 1650 ℃ for 30 min under hot pressing pressure of 35 MPa,the value of bending strength,breaking tenacity and Vickers hardness was 1015 MPa,6.89 MPa·m1/2,and 20.82 MPa,respectively.When hot pressing temperature was above 1650 ℃,density decreased because of decomposition with increased temperature,and mechanical properties dropped because of rapid growth of grains in size at high temperature.Microstructure analysis showed that the addition of Y2O3 led to the formation of YAG phase so as to inhibit the growth of crystals.This helped to improve breaking tenacity of the composites.TiCN particles with diameters of 1 μm dispersed at Al2O3 grain boundaries,inhibited grain growth and enhanced mechanical properties of the composites.SEM study of the propagation of indentation cracks showed that the bridge linking behavior between matrix and strengthening phase might lead to the formation of the coexisted field of crack deflection,branching and bridge linking.The mechanism of this phenomenon was that the addition of Y2O3 improved the dispersion of TiCN particles so as to enhance the tenacity of the composites.The breaking tenacity was changed from 5.94 to 6.89 MPa·m1/2.     

Structure Formation,Electrophysical and Mechanical Properties of an Electrically Conducting Ceramic Composite Based on Silicon and Boron Carbides

How the structure of electrically conducting ceramic composites in the SiC B₄C system is formed has been studied. The mechanical and electrophysical properties have been investigated as a function of the ratio of the components and the sintering conditions.     

Compactability of powdered copper-lanthanum hexaboride composites under hydrostatic treatment

Conclusions The compactability of powder composites copper-lanthanum hexaboride depends largely on the content of additive. Under hydrostatic pressing of powder mixtures the hard inclusions of LaB₆ hinder plastic deformation of copper particles, and thereby they hinder densification of the composite. In sintering of compacts the additive LaB₆ hinders the increase of volume of the specimens; on the other hand, an increase of content of LaB₆ hinders shrinkage of the composite material. In hydroextrusion of sintered compacts, inclusions of lanthanum hexaboride increase the deformation resistance of the material, and that increases the extrusion pressure of the composite, causing the most intense healing of pores.Almost pore-free powder materials of the system Cu-LaB₆ were obtained by hydrostatic pressing at pressures of 800–1000 MPa, sintering in hydrogen at 950°C for 2 h and subsequent hydroextrusion with reduction =80–90%.Translated from Poroshkovaya Metallurgiya, No. 5(353), pp. 18–22, May, 1992.     

Investigation of the Structure and Properties of Materials in the SiC ― TiB2 System

The structure and properties of sintered and hot-pressed materials of the SiC TiB₂ system were investigated. The optimal conditions for obtaining dense ceramics with minimum grain size of the phase components was determined. It was established that composites containing from 25 to 50 vol.@percnt; TiB₂ have a bend strength equal to 450 MPa at 20°C; this increases to 500 MPa at 1600°C for ceramics with a pseudoeutectic structure. The material is highly heat resistant in the temperature range 900-1200°C.     

Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si₃N₄−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m²/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si₃N₄, β-Si₃N₄, β-SiC, and X-ray amorphous Si₃N₄. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Salaspils. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 7–12, January–February, 1999.     

Effects of wet and dry milling conditions on properties of mechanically alloyed and sintered W–C and W–B4C–C composites

Abstract

Tungsten based W–1C and W–2B₄C–1C (wt-%) powders synthesised by mechanical alloying (MA) for milling durations of 10, 20 and 30 h, in wet (ethanol) and dry conditions, were characterised. X-ray fluorescence spectroscopy investigations revealed Co contamination which increased with increasing milling time during wet milling. X-ray diffraction investigations revealed the presence of W and WC phases in all powders, Co₃C intermetallic in the wet milled W–1C powders and W₂B intermetallic phase in both wet and dry milled W–2B₄C–1C powders. As blended and MA processed powders were consolidated into green compacts by uniaxial cold pressing at 500 MPa and solid phase sintered at 1680°C under hydrogen and argon atmospheres for 1 h. X-ray diffraction investigations revealed the presence of W₂C intermetallic phase in sintered composites produced from both wet and dry milled W–1C powders and the W₂B intermetallic phase in sintered material from the wet milled W–2B₄C–1C powder. Sintered composites from wet milled powders showed relative densities >91%, with the maximum density of 99·5% measured for the sintered 30 h wet milled W–2B₄C–1C composites. Microhardness values for the wet milled W–1C and W–2B₄C–1C composites were 2–2·5 times higher than those for dry milled composite powders. A maximum hardness value of 23·7±2·1 GPa was measured for the sintered W–2B₄C–1C composite wet milled for 20 h.     

Recrystallization of silicon nitride powders during hot pressing

Conclusions The hot pressing of comminuted silicon nitride powders enables a uniform fine-grained structure of one and the same mean grain size to be obtained irrespective of the type of starting Si₃N₄, powder. At milling times of more than 100 h no significant decrease in particle size is observed. The recrystallization of milled silicon nitride powders during hot pressing takes place chiefly in the fine fractions appearing during milling. The degree of recrystallization attained is higher with ultrafine active silicon nitride powders (PCS) than with comminuted powders.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 43–47, October, 1982.