Formation of Polycrystalline Boron Carbide B4C with Elevated Fracture Toughness

We have studied the processes of formation of high-density polycrystalline boron carbide B₄C during sintering under high pressure (5.5 GPa) and at high temperature (2200 K). Study of the structure by transmission electron microscopy showed that under the indicated conditions for sintering B₄C powders with original particle size range 2–80 µm, processes of primary recrystallization occur, as a result of which interlayers of nanodispersed grains (10–50 nm) are formed in the near-surface regions of the original coarse grains. The microhardness of such material is 41 GPa, and the fracture toughness is 7.2 MPa · m^1/2.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 90–101, January–February, 2005.     

Investigation of the wetting of diamond and graphite by molten metals and alloys V. Carbide-formation kinetics at the graphite/metallic melt interface

Conclusions An investigation was conducted of the properties of the intermediate layer on the graphite/liquid copper-chromium alloy interface. It has been established that this layer consists of carbides. The kinetics of growth of the carbide layer was studied. It has been found that the rate of increase of the layer thickness as a function of time over the temperature range 1150–1250°C obeys the law h^1.5 = S · t.Translated from Poroshkovaya Metallurgiya, No. 2, pp. 76–79, February, 1968.     

Sintering of boron carbide containing small amounts of free carbon

Conclusions A study was made of the sintering kinetics of grade ch. boron carbide, technical boron carbide cleaned of impurities, and boron carbide synthesized from the elements. An investigation was carried out also into the reactive sintering of a mixture of boron and carbon black. Reactive sintering fails to yield dense parts in boron carbide. The best sinterability is exhibited by a fine technical boron carbide powder cleaned of free carbon and other contaminants. Parts with a porosity of less than 5% can be produced from such a powder by pressing and subsequent sintering. However, it is not the presence of free carbon that controls the sintering behavior of boron carbide, since pure and synthesized boron carbide powders containing no free carbon are characterized by poor sinterability. The high activity of the technical powder is probably linked with the presence of structural defects and stresses generated in the course of its manufacture, during milling and quenching. The annealing spectrum of these defects covers a wide temperature range, and consequently the energy of activation for the densification of boron carbide steadily grows with increase in shrinkage.Translated from Poroshkovaya Metallurgiya, No. 7 (151), pp. 27–31, July, 1975.     

Preparation of porous parts from chromium carbide with the aid of volatile fillers

Summary The influence of chlorides on porosity variation during the sintering of chromium carbide specimens was examined. Chromium chloride must be regarded as the most effective pore-generating addition, the optimum quantities of the additions investigated being 2–4% CrCl₃ · 10H₂O, 6–8% CoCl₂ · 6H₂O, and 1–2% NiC1₂ · 6H₂O.Because of the activating effect of the chlorides, to obtain chromium carbide specimens of equal or even higher strength it is possible to lower their sintering temperature by 200–300°C compared with that required for similar specimens containing no additions. The maximum residual porosity attained is 50–52%.Translated from Metallurgiya, No. 4(52), pp. 34–37, April, 1967.     

Influence of grain size and porosity on the high-temperature friction of titanium carbide

Conclusions It was established that in friction of a TiC-TiC pair in vacuum the coefficient of friction at 250C and the wear rate at 1250C are practically independent of grain size. At higher temperatures these characteristics have an inverse relationship to grain size.It was shown that with an increase in porosity both the wear rate and the coefficient of friction increase. With an increase in temperature the influence of porosity on the wear rate decreases.With variations in porosity in the 1–10% range, in grain size in the 1–50 m range, and in temperature in the 20–1500C range the wear rate changes within limits of 10–45% and the coefficient of friction within limits of 3–35%.Translated from Poroshkovaya Metallurgiya, No. 9(297), pp. 56–61, September, 1987.     

Structure and properties of powders of some composite materials based on wurtzite boron nitride

Conclusions Addition of titanium carbide and zinc or aluminum oxide to a granulating blend of wurtzite boron nitride produces a low-strength sintered mass which disintegrates easily along the BN granule boundaries under mechanical action, ensuring the production of granules of a given grain size.Individual particles of composite material based on wurtzite boron nitride obtained with admixtures of zinc oxide are characterized by good compressive strength.Translated from Poroshkovaya Metallurgiya, No. 6(270), pp. 81–85, June, 1985.     

Cyclic strength of hard metals

Conclusions The fatigue limit of the titanium carbide and tungsten carbide alloys investigated on a basis of 5·10⁸ cycles lies in the range (20–30)·10⁷ Pa, and is thus comparable with the endurance of type ShKh high-carbon (1% C-Mn-Si-Cr) ball-bearing steels. The strength and character of fracture of the hard metals are determined by the properties and structural state of their phase constituents. The highest strength is exhibited by tungsten carbide and titanium carbide alloys with evenly distributed equal-sized carbide grains. The character of fracture of the hard metals varies depending on their method of loading, from brittle in static loading to tough-and-brittle in cyclic loading. On time bases not exceeding 10⁶ cycles titanium carbidehard metals are comparable in fatigue resistance to the standard tungsten-containing hard metals.Translated from Poroshkovaya Metallurgiya, No. 9(273), pp. 67–71, September, 1985.     

Properties and structure of niobium carbide heating elements

Conclusions In operation at 1900°C in a carbon-free atmosphere at a residual pressure of 10^–2-2· 10^–3 Pa a heating element of composition NbC + Nb₂C + Nb experiences a steady loss of carbon at a rate of 0.5–1 m/h from its surface, with the formation of a metallic phase. Heating elements of this composition may be recommended for service in vacuum resistance furnaces at temperatures of up to 1900°C, where they can be expected to have a useful life of the order of 1000 h. Niobium monocarbide heating elements operating at 2100–2500°C in a carbon-containing atmosphere under a pressure of 10^–2-2·10^–3 Pa experience no change in phase composition or shape and only minor changes in structure, and possess good thermal fatigue resistance. They can thus be employed in vacuum electric furnaces under these conditions. Long experience (more than two years) with the operation of a laboratory vacuum electric furnace with a niobium carbide heating element in the temperature range 1000–1500°C has demonstrated that the use of niobium carbide heating elements widens scope for new electro-thermal equipment.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 92–97, March, 1982.     

Some mechanical properties of silicon carbide fibers

Conclusions Electroerosion of silicon carbide fibers in contaminated mercury electrical contacts decreases their strength by between one-half and two-thirds. During prolonged holding under load at room temperature the creep of silicon carbide fibers is close to zero, while at 1173°K l/l 10^–7 l/day. X-ray structural analysis of silicon carbide fibers annealed for (1.8–10.8)·10³ sec in a hydrogen atmosphere at 1273–1773°K and for (3.6–18)·10³ sec in a nitrogen atmosphere at 1773–1973°K revealed no recrystallization in (4–5)·10^–7-m-thick layers of polycrystalline SiC deposited from a gaseous phase. A correlation was found between the strength of silicon carbide fibers and the number of flashes forming during their fracture.Translated from Poroghkovaya Metallurgiya, No. 1(253), pp. 55–59, January, 1984.     

Behavior of ultrafine titanium carbide powders during annealing and sintering

Conclusions An investigation was carried out into the effect of hydrogen annealing temperature on the composition and particle size of a titanium carbide powder produced by plasma-chemical synthesis (S = 8.2 m₂/g, 20.1% total C, 2.1% free C). It was found that heating substantially increased the specific surface of the powder and, according to electron microscopical examinations, slightly decreased its mean particle size. The amount of free carbon began to increase in the range 1400–1483°K, and at the same time the particle size of the powder grew. The greatest-increase in relative density was exhibited by specimens pressed in the pressure range 58.8–392 MPa, using polyvinyl alcohol as plasticizer. The highest density (porosity 5.5%) was attained in specimens pressed under a pressure of 58.8 MPa and sintered isothermally in a vacuum at 2090°K.Translated from Poroshkovaya Metallurgiya, No. 11(275), pp. 45–49, November, 1985.     

Structure and properties of sintered nickel-molybdenum strip

Conclusions The powder rolling method enables NMoZh-30-5 type alloys to be produced whose mechanical properties are comparable to those of cast and rolled alloys. Uniformity of microstructure and properties in finished strips is achieved by sintering rolled bars for a long time (not less than 10–18 h). By employing cold plastic working with reductions of 20–70% and varying the grain size in the range 0.01–0.03 mm, it it possible to regulate the mechanical properties of such an alloy.Translated from Poroshkovaya Metallurgiya, No. 1(217), pp. 35–41, January, 1981.     

Formation of highly-porous ceramic based on silicon carbide

Data for the effect of charge composition and characteristics of the starting silicon carbide powder and poreforming agent on the formation of highly porous SiC-base ceramics are studied and summarized. It is shown that porous (up to 70%) polycrystal SiC materials obtained on the basis of finely crystalline (3–5 µm) silicon carbide with introduction of ammonium bicarbonate (grain size 0.1–0.2 mm) as a pore-forming agent into the initial charge have the best structural parameters with sufficient mechanical strength and high gas permeability.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 5–6, pp. 48–54, May–June, 1994.     

Thermal diffusion o versinte ring of ShKh powder

Conclusions Experiments have shown that to oversinter powders in air and obtain a layer with high mechanical properties and porosity it is necessary to (a) add to the mixture elements which activate adhesion and oversintering, and (b) to heat quickly and reduce the time of holding at temperature.For oversintering ShKh alloy steel powder it is satisfactory to add boron and silicon to the mixture and high frequency induction heat the layer for 80–100 sec at 1250°C. This produces high adhesion of the layer to the base and complete sintering.To produce a porosity of 40–50% the layer is placed on the surface of the parts by spreading a pasty mixture. Less porosity may be obtained by pressing the mixture with a pressure of 2–4 tons/cm². In oversintering ShKh powder hydrolyzed ethyl silicate may be used as a plasticizer.Translated from Poroshkovaya Metallurgiya, No. 5(65), pp. 107–110, May, 1968.     

Effect of structural factors on the mechanical properties of superhard materials based on boron nitride

Conclusions With increasing grain size a transition takes place from intercrystallite to dual-type (inter- and transcrystallite) fracture. The two-phase Geksanit-R is characterised by the smallest grain size, a uniform grain size, and the lowest inclusion and pore contents. It is to these characteristics that the material owes its high hardness and excellent fracture toughness (K_c50–55 kgf/mm^3/2); its effective energy of fracture, y (6.2–7.5)·10⁴ ergs/cm⁴, is an order greater than its true surface enrgy o. The hardness of Geksanit-R sinterings is virtually independent of the ratio between the relative amounts of BN_w and BN_s. The presence in Geksanit-R specimens of 2–7% of graphitelike boron nitride embrittles the material, without substantially altering its hardness. The other BN_s-base superhard materials (including Elber-R, Belbor, and PTNB-IB-1) are less hard than Geksanit-R, which is attributable to their larger grain size and the presence of foreign substances and phases, and exhibit a marked tendency toward brittle rupture.Translated from Poroshkovaya Metallurgiya, No. 10(202), pp. 61–69, October, 1979.     

Electrophysical properties of a porous silicon carbide ceramic

The electrical resistance and thermal electromotive force have been measured for porous silicon carbide resistance materials. It has been established that the range of controlled variation in the electrical resistance is 5·10^–2 to 1·10⁴ · cm, and that the composition of the conducting SiC phase and the concentration of added dielectric constituents have a decisive influence on the electrical characteristics examined.Material Science Problems Institute, Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 4(364), pp. 85–89, April, 1993.     

High-speed shaping of a clad graphite powder

Conclusions High-speed shaping with single-ended load application enables parts of improved quality to be obtained from clad graphite granules. Within the working speed range investigated (from 0.3·10^–3 to 25 m/sec) the uniformity of density distribution over the compact height increases with increasing speed. Increasing the speed of loading in the high-speed working of clad graphite granules to more than 15 m/sec is not recommended because at these speeds the improvement in the uniformity of density distribution over the compact height is accompanied by the layering of the material.The work was carried out under the supervision of Professor V. G. Kononenko.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 21–25, March, 1982.     

WC-Co hard alloys as dispersign-strengthened materials

Conclusions The variations of the yield strength and hardness of WC-Co alloys with composition and carbide grain size are in accord with the Orowan and Ansell-Lenel theories of the dispersion strengthening of alloys. The interparticle spacings determining the yield strength and hardness of WC-Co alloys would be expected to be of the order of 10^–5–10^–6 cm. This finding is confirmed by electron-microscopic observations.Translated from Poroshkovaya Metallurgiya, No. 3 (123), pp. 32–38, March, 1973.     

Fatigue and Compression Characteristics of Al 6061–B 4 c Composite Materials

Aluminum matrix composites reinforced with boron carbide are a kind of materials that are widely used because of high strength, low density, and improved tribological properties. In this study, mechanical properties of Al 6061–B4C composites reinforced with B4C of three different particle sizes were investigated. In the Al 6061–B4C composite materials, produced by the powder metallurgy methods (extrusion of billets obtained by sintering at temperature of 550°C under pressure of 450 MPa), the change of mechanical properties such as hardness, compressive strength, and fatigue life, related to B4C particle size and the applied heat treatment mode (aging at 180°C for 5 h), were investigated. The hardness of the materials is increased with B4C grain size and the heat treatment. After the heat treatment, the fatigue life of Al 6061–B4C (3 μm) material increases slightly, while that of the composite materials decreases with larger size of B4C reinforcement. The fatigue life of the composite materials reinforced with a larger grain size B4C is reduced by heat treatment. While the compression test data of untreated composite materials were similar to each other, the heat treatment increased these values in all samples. The highest increase in the compression strength was observed in the composite reinforced with 17 μm sized B4C. The addition of graphite reduces the deformation ability of the composites.     

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.     

Temperature dependence of the grain size of NbC0.80

Summary A study was made of the effects of temperature in the range 1600–3300°K on the grain size of the nonstoichiometric niobium carbide NbC_0.80. The energy of activation of the grain growth process was found to be 65.5 kcal/mole; this value leads to the conclusion that the grain growth process is controlled by boundary and surface diffusions.Translated from Poroshkovaya Metallurgiya, No. 9(45), pp. 50–54, September, 1966.