Showing papers on "Sintering published in 1968"

Sintering of ZnO: I, Densification and Grain Growth

Abstract: The densification rate and grain growth rate of pure MgO powder compacts are measured between 1450 and 1650° C in air. Densification rate in a semilog plot appears to be linear up to about 94% of the theoretical, followed by marked nonlinearity. The time dependence of grain growth is 1/2 at the beginning and then decreases considerably with further sintering. Application of lattice diffusion model to the densification and grain growth data gives calculated diffusion coefficients in fair agreement with the directly measured diffusion coefficients for magnesium; they are also in fair agreement with the literature value of the diffusion coefficients calculated from various other kinetic processes in MgO.

Microstructural Development During the Liquid-Phase Sintering of Two-Phase Alloys, with Special Reference to the NbC/Co System

Abstract: An investigation was made of the grain growth and other microstructural changes occurring during the liquid-phase sintering of NbC alloys with ∼20 wt % cobalt. The effects of sintering time, sintering temperature, and small alloying additions were studied. It was found that the grain growth of NbC in liquid cobalt, at 1420° C, can be described by the equation: $$\bar d^3 - \bar d_0 ^3 = {\text{K}}t$$ where $$\bar d$$ is the mean linear intercept of the grains after time t, and $$\bar d_0$$ the initial mean intercept, K being a temperature-dependent constant with an “activation energy” of 95±15 kcal/mole. This equation suggests that grain growth occurs by a solution/ precipitation process controlled by diffusion in the liquid phase. Small alloying additions of WC, TiC or NbB2 inhibit the growth and/or alter the growth process, as well as affecting such properties as the shape and contiguity of the carbide grains. The relative significance of grain coalescence to grain growth in a liquid phase is discussed. By examining theoretically the effect of anisotropy of interface energy on the cube ⇌ sphere grain-shape change, it has been possible to explain the observed sensitivity of grain shape towards sintering conditions.

Process for producing sintered diamond compact and products

Abstract: PROCESS INCLUDING PRELIMINARY CLEANSING AND ESSENTIAL PRECONDITIONING TREATMENT OF FINELY DIVIDED DIAMOND PARTICLES FOLLOWED BY COMPACTION OF THE PRECONDITIONED PARTICLES AT HIGH TEMPERATURES AND PRESSURES IN THE DIAMOND STABLE REGION TO PRODUCE DENSE SELF-BONDED SINTERED DIAMOND COMPACT. INCORPORATION OF FORON, SILICON OR BERYLLIUM AS SINTERING AID AGENTS WITH THE PRECONDITIONED PARTICLES STILL FURTHER PROMOTES SINTERING AND BONDING OF THE COMPACT.

Shape Sensitivity of Initial Sintering Equations

Abstract: Initial sintering equations are shown to depend as much on possible particle shape variations in powder systems as on the mechanism of material transport. Also, competing mechanisms, such as surface diffusion, may alter the generally assumed relations between neck growth and shrinkage and so may affect the sintering kinetics. When shrinkage occurs by volume diffusion, both effects may be overcome by measuring the size of the interparticle boundary as well as the shrinkage during sintering. When grain boundary diffusion is the mechanism of shrinkage, both the cross-sectional area and the radius of curvature of the neck must be measured during sintering. The usefulness of simultaneous measurements of neck growth and shrinkage is demonstrated with literature data for copper.

Densification processes in the tungsten carbide-cobalt system*

Abstract: Densification of the tungsten carbide-cobalt system has been investigated by determining the effect of the principal sintering variables-composition, temperature and time of sintering, particle size, ball-milling-and by studying the processes that occur. Considerable shrinkage takes place during heating, before the eutectic temperature is attained. A 9% cobalt alloy sintered entirely in the solid state to give comparable density and mechanical properties to those attained by liquid-phase sintering, but the sintering time was increased by a factor of 10. Densification proceeds from nuclei created by ball-milling, which packs the porous cobalt agglomerates with tungsten-carbide particles; if the cobalt particles are only mixed with the tungsten carbide, then on sintering they flow out into the matrix leaving behind voids that do not fill. Densification is characterizedby two features: first, tungsten-carbide particles cement together with cobalt between grains to form clusters and filaments; secondl...

Research and development of refractory oxidation resistant diborides

Abstract: : The oxidation, mechanical and physical properties of zirconium diboride and hafnium diboride and composites prepared from these diborides with appropriate additives, have been determined as a function of composition, microstructure and test temperature. The composites were designed to enhance oxidation resistance, strength and thermal stress resistance without sacrificing high temperature stability; the principal additives were silicon carbide or graphite. Several hundred diboride billets, in sizes from two inches diameter to six inches square, were fabricated by conventional hot pressing. A unique role for ceramic additives has evolved in enhancing the fabricability of diboride materials and producing fine grained crack free billets. All power materials and hot pressed microstructures subjected to properties evaluations have undergone extensive characterization through qualitative and quantitative chemical analyses, phase analyses and grain size and density measurements. An exploratory fabrication effort was initiated to develop alternate means to hot pressing for producing dense diboride materials; hot forging, plasma spraying and sintering are being studied. In addition, the hot pressing of diboride compositions containing additives such as SiC whiskers, carbon filaments or tungsten filaments is being studied.

High temperature gas isostatic pressing of crystalline bodies having impermeable surfaces

Abstract: CRYSTALLINE METAL AND CERAMIC BODIES THAT HAVE PREVIOUSLY BEEN SINTERED TO THE EXTENT THAT THE ONLY POROSITY REMAINING IN THE BODY IS CLOSED POROSITY ARE ISOSTATICALLY HOT PRESSED TO A STILL HIGHER DENSITY BY SURROUNDING THE BODY WHICH IS MAINTAINED AT SINTERING TEMPERATUR WITH A GAS AT A PRESSURE OF AT LEAST 10,000 PSI THE GAS MUST BE INERT TO THE BODY BEING PRESSD AND MUST NOT PERMEATE SAID BODY DURING THE HOT ISOSTATIC PRESSING STEP WHILE PRIOR ISOSTATIC HOT PRESSING PROCESSED HAVING REQUIRED THE USE OF AN IMPERMEABLE CONTAINER OR AN ADDITIONAL COATING LAYER FOR THE WORK PIECE NEITHER ARE REQUIRED IN THE PRESENT PROCESS

Initial Sintering of MnXO-Al2O3

Abstract: Effects of manganese oxide on the initial sintering kinetics of compacts of 5-μ diameter alumina were studied by isothermal shrinkage measurements from 1450° to 1650°C. The observed rates were characterized by assuming a volume-diffusion mechanism. Variations in sintering rate with both oxygen partial pressure in the furnace and impurity concentration, as well as the observed activation energy, indicated oxygen-ion diffusion was the rate-limiting step during densification.

Electrical sintering under liquid pressure

Abstract: A method of electrical sintering of a mass particles of conductive or nonconductive powder in which the powder is compacted between the electrode by means of liquid pressure applied through a flexible membrane The fluid pressure may be supplied within the body or from the outside and may follow the shrinkage of the mass which can be spark sintered or fused by resistance heating An elongated element can bridge the electrodes for retention in the mass to form a reinforcing member or can serve as an auxiliary heating element The body thereby sintered can be surrounded by a lightly compacted, porous layer of a heat resistant material (eg, graphite) to form a force-transmitting medium between the membrane in the sinterable mass

Cutting of deposit forming steel and cutting tools for such steels

Abstract: Efficient cutting of deposit forming steels, such as described in French Pat. No. 1,387,441, with tungsten carbide cutting insert having a high tungsten carbide content over an extended period of time is difficult. Efficient cutting of such deposit forming steels over a wide speed range from about 50 to about 350 m/min is made possible by embodying in the cutting edge surface stratum of the cutting tool at least 15 percent of one or more of the carbides or borides of Ti, Ta, Zr, Nb and V, or at least 50 percent aluminumoxide in the tool surface stratum may be embodied either by diffusion, or by affixing such surface layer to a known tungsten carbide insert, or by sintering stratified compacts of powder particle mixtures cohering a thick powder mixture layer containing a large proportion of tungsten carbide is covered along one or on its opposite layer surfaces with a thin powder mixture strata, each of which contains at least 15 percent of one or more of the carbides or borides of Ti, Ta, Zr, Nb and V.

Role of Crystallinity in the Sintering of Urania Powders

Abstract: The crystallinity in UO2 powders prepared by the ADU route at different calcination temperatures (600°-1600°C) and its influence on their sintering behaviour (sintering temperature 1000°-1600°C) in hydrogen were studied by X-ray difiraction techniques. Powder patterns were made for the loose powders and back reflection for the sintered pellets. It was noticed that higher the calcination temperature, the greater was the degree of crystallinity developed in the powders and poorer was their sinterability. The crystallinity of the sintered pellets was mainly dependent on the sintering temperature and crystallinity of the original powder and is not directly related to the sintered density. By sintering pellets of poorly crystalline powders at low temperatures (about 1200°C) sintered pellets of near theoretical density but with poorly developed crystallinity could be obtained.

Production of inorganic fibrous materials

Abstract: 1,224,435. Inorganic fibres. CELANESE CORP. 27 Feb., 1969 [28 Feb., 1968], No. 10576/69. Headings C1A and C1N. An inorganic filamentary material is produced by forming an acidic spinning solution of an organic filament-forming polymer containing an inorganic compound dissolved or suspended therein, spinning the solution, initially oxidizing the precursor filamentary material by heating it in a free oxygen-containing atmosphere to obtain an infusible charred product retaining a filamentary configuration and incapable of sustaining combustion when exposed to a match flame and, finally, sintering the charred product at 1200‹ to 2800‹ C. The polymer may be a polyamide, preferably polyhexamethylene terephthalamide. The inorganic compound is preferably titanium dioxide, boric oxide or aluminium oxide. The product fibre may be a carbide, the sintering being carried out in a nitrogen or argon atmosphere, or an oxide, the sintering being carried out in a free oxygen-containing atmosphere. Examples describe the preparation of filamentary titanium dioxide, aluminium oxide, titanium carbide, aluminium carbide and boron carbide.

Heat treated ferromagnetic particles

Abstract: FERROMAGNETIC ALLOY PARTICLES OF IRON, NICKEL OR COBALT, AND OPTIONALLY CHROMIUM SUPERSATURATED WITH BORON, NITROGEN OR PHOSPHORUS WHICH ARE PREPARED BY REDUCTION OF A SOLUTION CONTAINING SALTS OF THE APPROPRIATE METALS, ARE CONVERTED TO POLYPHASE PARTICLES OF SUBSTANTIALLY UNCHANGED SIZE HAVING IMPROVED MAGNETIC PROPERTIES BY HEATING BELOW THEIR SINTERING TEMPERATURES. THE FERROMAGNETIC PARTICLES ARE USEFUL FOR MAKING MAGNETS AND FOR MAKING MAGNETIC RECORDING MEMBERS.

Fast-Neutron Effects on the Carbides of Titanium, Zirconium, Tantalum, Niobium, and Tungsten

Abstract: Solid cylindrical specimens (½- × ½-in.) of the monocarbides of Ti, Zr, Ta, Nb, and W, made by 1) hot pressing, 2) slip casting and sintering, and 3) explosion-pressing and sintering, were irradiat...

Exothermic effects during sintering of a mixture of nickel and aluminum powders. II

Abstract: When a mixture of nickel and aluminum powders is sintered there is an exothermic effect which involves a rise of several hundred degrees in the temperature of the compact in a few seconds. The magnitude and nature of the exothermic effect depend on the aluminum content of the mixture, the degree of dispersion of the powders, and the initial porosity of the specimens. The temperature at which the exothermic effect begins moves towards lower temperatures with increase in the degree of dispersion, with increase in the concentration of aluminum in the mixture, and with decrease in the porosity. The magnitude of the heat effect increases with increase in the aluminum content. It is shown that the exothermic effect in the sintering process arises mainly from decomposition of the liquid phase and crystallization of chemical compounds with large heats of formation. A definite role is played by the metallothermic reduction of nickel oxide by aluminum present in the liquid phase and by the formation of intermetallic compounds at the boundaries of dissimilar particles of powder during sintering in the solid phase.

Effects of Dopents on Sintering of Zinc Oxide and Nickel Oxide

Abstract: The influence of dopents (Li2O, Na2O, Al2O3 and In2O3) on the sintering of ZnO and NiO has been studied in the temperature range 750° to 1200°C in air. The dopents, Li2O and Na2O, were found to promote the sintering of ZnO and the other dopents, Al2O3 and In2O3 to inhibit. The result is explained by the diffusional transfer of interstitial zinc ions.While for the sintering of NiO, the Li2O and Na2O were found to promote the sintering, the Al2O3 and In2O3 to inhibit. This result shows evidently that material transport in NiO does not depend on the concentration of nickel ion vacancies, but on the concentration of electron holes, trivalent nickel ions Ni3+, which have smaller ionic radii than bivalent nickel ions.

The sintering of nickel/aluminium spheres to nickel plates

Abstract: Spheres of nickel, and of nickel containing 0.5,1.0, 2.0, and 10.0 wt % aluminium, all of approximately 100μm diameter, were sintered under vacuum to nickel plates for various times within the temperature range 1000 to 1300° C. Neck growth was determined as a function of time. Values for the exponent of time in the neck growth equation,x/a = Atn of 0.19, 0.18, 0.15, and 0.16 were found for the above compositions, respectively. An activation energy of 33 kcal/mole was found for sintering nickel spheres to nickel plates. An activation energy of 69 kcal/mole was found for sintering Ni/1.0% Al spheres to nickel plates.

High‐Temperature Creep of Polycrystalline Magnesia: II, Effects of Additives *

Abstract: The creep of magnesia doped with 0.035 to 2.26 cation % of nine other oxides and three binary mixtures thereof and of three seawater products (about 96, 98, and 99.5y0 MgO) was evaluated in transverse bending at 1200° to 1500°C, with strain rates of about 10−2%/hr, and average grain sizes of 5 to 50p. The results obtained were compared with those for pure magnesia. Most additives accentuated the plastic (diffusion-controlled) nature of the creep process presumably by pinning dislocations and/or slowing grain growth. In most cases the rate-determining diffusing species seemed to be the cation, Mg, but in two cases it was suspected that oxygen boundary diffusion was controlling. Porosities above ˜10% appear to increase the temperature dependence of creep, probably by introducing boundary sliding. The agreement of the creep data with those of other diffusion-controlled processes (electrical conductivity, sintering, and grain growth) is demonstrated.

Lattice strain in tungsten carbide

Abstract: X-ray-diffraction measurements have shown that ball-milling of tungsten carbide powder gives rise to considerable line broadening. Analysis of this broadening by integral-breadth and Fourier methods indicates that the main cause of the broadening is lattice microstrains. X-ray studies have been made on tungsten carbide powders, sintered blocks, and tool tips with cobalt binding, to determine how this strain varies at different stages of treatment of the material. It was found that, after sintering, the tungsten carbide was strain-free, but diamond polishing and mechanical work induced strain in the surface layers.

Sintered hard metal

Abstract: A hard metal article with a tough body and a wear resistant surface is formed by sintering together a compact of two different powders, one powder mixture comprising at least 80%WC, a binder such as Fe, Co or Ni and optionally a minor proportion of a carbide of Ti, Zr, Hf, V, Nb, Ta, and the powder mixture forming the surface layer comprising a major proportion of a carbide of Ti, Zr, Hf, V, Nb, Ta and a minor proportion of Fe, Co or Ni, the sintering being such as to allow the Fe, Co or Ni of the surface layer to migrate into the body. In an example a compact formed of a first layer of 94%WC-6%Co and a second layer of 82% TaC -9%WC - 9%Co is sintered under vacuum at 1440 DEG C. for 1 hour.

Properties of electrolytic and reduced titanium powders and sinterability of porous compacts from such powders

Abstract: As a result of the better sinterability of reduced powders, the mechanical properties of compacts from these powders are higher than those of compacts from electrolytic powder. As a technique for the production of porous titanium, sintering in thoroughly-dried argon is simpler than vacuum sintering and enables porous parts to be produced with mechanical properties fulfilling the requirements of sintered-filter operation.

Preparation of moulded and sintered aluminum nitride

Abstract: A METHOD OF MANUFACTURING A MOULDED AND SINTERED MASS OF ALUMINUM NITRIDE COMPRISING MOULDING AN ADMIXTURE OF ALUMINUM NITRIDE POWDER, ALUMINUM OXIDE POWDER AND METALLIC ALUMINUM POWDER, NITRIDING THE MOULDED MASS IN NITROGEN OR AMMONIA GAS AT A TEMPERATURE BELOW THE MELTING POINT OF METALLIC ALUMINUM, AND SINTERING THE MASS IN NITROGEN OR INERT GAS AT TEMPERATURES RANGING FROM 1,600 TO 2,200*C.

Sintered stabilized zirconia containing discontinuous phase of cobalt-containing oxide

Abstract: The resistance of a zirconia mass or layer to gas penetration is maximized by sintering the zirconia body with a cobalt-containing sintering agent present therein as a separate phase. In the case of stabilized zirconia the cobalt-containing sintering agent may be introduced either during or after stabilization.

Sintering and recrystallization of ZrC-ZrB2 compacts

Abstract: 1. By using finely-divided starting materials, it is possible substantially to improve the sinterability of ZrC, ZrB2, and cermets based on these compounds. The addition of ZrB2 to ZrC or of ZrC to ZrB2 has a very beneficial effect on sinterability. By sintering ZrC and ZrB2 powders with a specific surface area of 4–5 m2/g at 2300–2400°K it is possible to obtain specimens with a total porosity of not more than 2–4%. 2. To describe the process of collective recrystallization over a wide range of temperatures and holding times, Eq. (4) should be used instead of Eq. (1), which is normally employed for this purpose. 3. Grain growth is very intense in ZrC, slightly less intense in ZrB 2, and much less pronounced in ZrC-ZrB 2 cermets. Both the grain size and energy of activation of the cermets diminish with increasing amount of the second component.

Electrical conductivity and sintering in iron oxides at high temperatures

Abstract: Two raw materials were chiefly used, namely laboratory ferric oxide and a high grade hematite. All experiments were carried out in an oxygen atmosphere, in the range 600 to 1300° C.