CONSIDERATIONS ON THE THEORY OF FRICTION: FRICTION WITHOUT WEAR

Abstract

From studies of the friction between metal surfaces it has been possible to form a new concept of friction in relation to wear. Its basis is the fact that certain intermetallic compounds are unlikely to stick on common metals. Friction can be produced by adding a small amount of metal to the intermetallic compound and it is shown theoretically that the condition of friction without wear is then achieved. The elastic vibrations produced in the metals lead to the necessary sticking followed by shearing to cause friction. Practical results are given where the lives of clutches have been prolonged by factors of up to 100; similar improvements are gained for the brakes of trains and aircraft. New phenomena are explained that limit the life of the friction materials even though wear is negligible.     

PRESSING AND SIZING FACTORS AFFECTING THE PRECISION AND MECHANICAL CHARACTERISTICS OF SINTERED PARTS

Abstract

All stresses, and particularly those due to friction, which occur during pressing and sizing operations have consequences on the dimensions, on internal stresses, and eventually on micro-cracks. This friction can have an influence on the variations of the density from one point to another but, when used in a proper way, it can help the movement of punches and increase homogeneity. Three different types of re-pressing are considered—drawing, sizing, and coining—and examples are given to show their influence on mechanical properties and especially on the fatigue limit.     

SOME FUNDAMENTAL STUDIES IN THE COLD COMPACTION OF PLASTICALLY DEFORMING SOLIDS

Abstract

Studies have been made of the behaviour under compression of materials which exhibit plastic deformation at normal temperatures, as exemplified by indium and lead. In particular, attention has been focused on the effects of die-wall friction and on the influence of the relative volume. The die-wall friction effects produced by a solid plug of the material were compared with those produced by a particulate fill of the same material when compressed under similar conditions, and a correlation of these effects with the shear strength of the material was made. A theoretical equation relating relative volume at either end of the compact is suggested.     

PRELIMINARY OBSERVATIONS ON THE USE OF THE INDUCTION-COUPLED PLASMA TORCH FOR THE PREPARATION OF SPHERICAL POWDER

Abstract

The operating characteristics of an induction-coupled plasma torch are described. Preliminary observations are given on its use for the spheroidization of chromium, alumina, molybdenum, magnesia, tantalum, and tungsten powders.     

THE INFLUENCE OF SILICON AND OTHER ELEMENTS IN CONTROLLING INTERPARTICLE FRICTION AND SINTERING OF BERYLLIUM POWDER

Abstract

The effect of small additions of activating elements such as silicon on the consolidation behaviour of beryllium powder has been investigated. Evidence is given that compacts of activated powder have more uniform high density than those produced from non-activated material. Studies carried out on prepared beryllium discs show that silicon modifies the micro-structure of the surface layer of beryllium oxide and, in consequence, affects its sliding behaviour and bonding characteristics.

From these results a model is proposed to account for the observations made on both sintered and hot-pressed beryllium which leads to the conclusion that, in addition to interparticle bonding, some measure of metal particle rearrangement is necessary for maximum densification. Activating elements may, in modifying the surface characteristics of the individual powder particles, assist in achieving an improved balance between particle sliding on the one hand and interparticle bonding on the other. In taking into account the bulk consolidation characteristics as well as the micromechanics of the process, the model also explains the observed influence of particle-size distribution on porosity in the compact.

The extent to which friction and sliding can influence compaction has been demonstrated by using a system of coloured Plasticine balls to simulate individual powder particles. Analysis of the behaviour of the Plasticine compacts substantiates the proposed model of the hot pressing of beryllium powder.     

THE SINTERING OF PYROPHORIC POWDERS

Abstract

The room-temperature oxidation behaviour of reactive metal and ceramic powders is reviewed and it is shown that many materials under normal circumstances oxidize to give a limiting film thickness of ～50Å. Since the thickness of oxide films has an important bearing on sintering characteristics the control of oxidation may be essential for some materials and desirable for others.

The oxidation behaviour of uranium carbide and uranium nitride in various concentrations of oxygen and water vapour in argon is described and its effects on the sintering behaviour and composition changes that take place during sintering are considered.

Suitable handling conditions for these materials are provided by glove-boxes filled with argon; there is a brief discussion of the factors that might apply if they were used in other areas of powder metallurgy.

The application of high-purity handling atmospheres in the preparation, handling, and sintering of tungsten powders has given pellets that had densities of 95% theoretical and oxygen contents <10 ppm after sintering in hydrogen at 1550°C.     

DEFORMATION AND DENSIFICATION OF POROUS PREFORMS IN HOT FORGING

Abstract

Observations are reported on plane-strain upsetting, between unlubricated flat punches, of powder preforms of varying density at 1160°C (1435 K). Macroscopic deformation behaviour is characterized by the pressure/strain relationship and by the ratio of lateral to vertical flow. Increased preform porosity affects lateral flow similarly to increased friction in forging of dense material.

Densification is studied as a function of strain and pressure. Lower preform density requires not only larger strains but also higher pressures for a given final density.

The density distribution in typical forgings is charted by means of hardness measurements. Zones of incomplete densification are revealed where local pressure was reduced by lack of constraint, or where strain was impeded by friction effects.     

THE EFFECT OF THE TEST FLUID AND TEST METHOD IN THE MEASUREMENT OF THE PERMEABILITY OF POROUS METAL BEARINGS

Abstract

Previous work on the determination of the fluid permeability of porous materials is reviewed, and methods for the measurement of the gas and liquid permeability coefficients of porous metal bearings are described. It is shown that the liquid permeability in porous metal is not independent of time and pressure as is the gas permeability. By the use of relatively simple experimental methods, the influence of surface-active additives in the mineral oil on the liquid permeability can be studied. By a complete understanding of the interactions between the lubricating oil and the metal surfaces of the porosity, an improvement in the performance prediction of porous metal bearings is to be expected.     

THE DEVELOPMENT OF VACUUM-SINTERED STEELS

Abstract

The work described is concerned with the development of vacuum-sintered ferrous materials that can be used for high-strength structural parts. The reasons for choosing this production route are given, and the development of the materials is traced from the simple iron–carbon alloys to the more complex iron–carbon–nickel–chromium alloys. The metallography of the various series of alloys is included and the mechanical properties are fully reported. Tensile strengths > 70 tons in ^?2 were obtained after heat-treatment. An indication of the dimensional changes that took place during sintering is also given.     

THE DENSITY OF PACKING OF TWO-COMPONENT POWDER MIXTURES

Abstract

The problem of the bulk density attained by a mixture of two powders, of uniform particle size, is considered. It is shown that there are three limiting cases for which the bulk density may easily be calculated and that, from these cases, the bulk density corresponding to a wide range of conditions may be deduced.

It is suggested that a clear distinction must be maintained between a perfect ordered mix, designated a “hyperperfect” mix, and a perfect randomized mix, the randomized mix being the only type attainable by use of a practical mixing machine. The perfection of the mixing of the two component powders is an important factor in the problem and, for mixes as obtained from practical mixing machines, the bulk density is considerably lower than would be the case with the theoretically perfect randomized mix.

The assumption upon which most of the work on the density of mixtures is based, namely that of a “hyperperfect mix”, is inappropriate to industrial mixing processes, so that such treatments are invalid.     

THE ROLE OF ARC-PLASMA IN METALLURGY

Abstract

Arc-plasma is introduced by a comparison with conventional heat sources. The mechanism and requirements of arc-plasma devices and equipment are then considered. An arc-plasma materials-deposition unit for automatic operation in an atmosphere of controlled composition and pressure is described. Photographs of objects plasma-spray-formed in tungsten are shown, together with microstructures of plasma-sprayed refractory deposits.

Metallurgical applications of arc-plasma technology are considered under four headings:

Plasma as a user: the requirements of powders for arc-plasma purposes are discussed.

Plasma as a producer: extractive metallurgy, production of ultrafine–and spherical–particles, powders of intermetallic compounds and alloys.

Plasma as a fabricator: consideration of spraying, spray-welding, spray-forming, cutting, and arc-plasma furnaces, including King’s rotating-plasma furnace.

Plasma as a tester: pla,sma for the analyst, application to thermal endurance, ablation, and thermal-shock testing.     

THE EFFECT OF COPPER ON THE PROPERTIES OF LUBRICANTS

Abstract

The effect of copper on the properties of mineral oil, polyglycol, aliphatic esters, and silicone fluids has been examined in relation to the case of self-lubricating bearings. The various oxidation mechanisms involved are considered, together with specific examples of the part which additives may play in the reduction of deposit formation, inhibition of oxidation, and prevention of copper corrosion.     

PRODUCTION OF TUNGSTEN ALLOY PENETRATION RADIATION SHIELDS

Abstract

Manufacturing procedures are Manufacturing procedures are given for a series of high-density sintered tungsten-nickel-copper and tungsten-nickel-iron alloys covering a density range of ～17 g/cm³ to > 18·5 g/cm³.

Physical and mechanical property data for these alloys are given and the effect of sintering variables on these properties is discussed.

Reference is made to equipment and techniques employed in the manufacture of radiation shields to meet the most stringent demands brought about over the years by the increased strength of radiation sources and the widening scope for the application of artificial radioactive elements.     

THE EFFECT OF DISPERSOID PROPERTIES ON THE DISPERSION-STRENGTHENING OF CARBONYL IRON

Abstract

The dispersion-strengthening of carbonyl iron by alumina and zirconia has been found to be sensitive to particle morphology. This has been studied by optical and electron microscopy in samples representing each stage of various production sequences.

In the case of alloys that had been sintered at 1350°C and then forged and cold rolled, alumina had a greater room-temperature-strengthening effect than did zirconia; whereas for alloys sintered at the same temperature but tested in the extruded condition there was no such difference. Furthermore, sintering at 1200°C followed by forging and cold rolling produced a similar degree of dispersion-strengthening for each of the two dispersoids. These observations are interpreted in terms of the variation of particle morphology with production history.

Tensile properties were also determined in the range 20–600°C for alloys that had been sintered at 1350°C and extruded. The dispersion-strengthening effect decreased with increasing temperature in a manner that is consistent with the experimentally determined recrystallization temperature of these alloys.     

EFFECTS OF PROCESSING AND POWDER CHARACTERISTICS ON THE WEAR AND FRICTION PROPERTIES OF SOME NICKEL-BASE MATERIALS

Abstract

Pure nickel and nickel-based binary and ternary materials containing tungsten carbide and graphite dispersions have been studied. Three types of carbide powder and six types of graphite, different in size, shape, and structure (one in each group being nickel-coated) were studied to varying degrees. The smallest carbide powder (0·35 μm) possessed the best wear-resistance and the lowest coefficient of friction. The larger and coated types of graphite are somewhat superior; however, considerable amounts (～ 40 vol.-%) are needed to improve wear-resistance substantially and reduce the coefficient. Use of the coated type of carbide leads to rather high coefficients (0·58-0·76) under dry testing conditions. Several effects of the presence of a small amount of oxygen in the sintering atmosphere are discussed; a most interesting result is the marked improvement in the wear-resistance of pure nickel treated to produce a dispersion of nickel oxide. With some powders blending can lead to reduced wear-resistance, while with others increasing the blending time has no effect. Several of the findings confirm the interpretation of the results of the previous study regarding the dependence of wear on the ratio of the volume fraction of tungsten carbide to that of graphite, based on microstructural considerations. Wear-resistance cannot be correlated with high hardness.     

AN INVESTIGATION INTO THE RELATIVE EFFICIENCY OF DIFFERENT SAMPLING PROCEDURES

Abstract

Several procedures for the sampling of powders before particle-size analysis have been described, but little information is available as to the merits of the various techniques. In the present work the relative efficiencies of scoop sampling, cone and quartering, the chute riffler, and the spinning riffler have been investigated. Of these, scoop sampling is especially prone to error, since the whole of the powder does not pass through the sampling device and the previous history of the powder is important. The special problems of scoop sampling have been studied in some detail; it is concluded that training of the operator could improve sampling efficiency, but that, in general, this sampling technique should be avoided. The efficiency of the spinning-riffler technique proved far superior to any other, provided that the time for one revolution of the ring of containers was only a small fraction of the total time required for the powder to flow through the sampling device.     

THE COMPACTION OF METAL POWDERS BY ROLLING. I.—THE PROPERTIES OF STRIP ROLLED FROM COPPER POWDERS

Abstract

The investigation described is an extension of earlier work (“Symposium on Powder Metallurgy 1954”, p. 131. 1956: London (Iron and Steel Institute), and Sheet Metal Ind., 1955, 32, 589) which described the effect of rolling pressure and sintering conditions on the mechanical properties of strip rolled from copper powders. The directional variation of U.T.S. and of elongation of sintered strip are shown to be the same as those of solid copper with a similar microstructure, at least for material with up to 16% porosity. The shape of powder particles and the particle-size distribution have a marked effect on the strength of sintered strip by virtue of their effect on the shape and size of the pores in the sintered material. Measurements of electrical resistance reveal a linear relationship between conductivity and porosity over wide ranges of porosity in both “green” and sintered strip. The conductivity increases rapidly during the first few minutes of sintering at 1000° C. Measurements of the resistance in the rolling direction and in the transverse direction, which are independent of local variations in density, have been made on green sheet; the resistance in the rolling direction is the higher. A correlation between this result and the mode of particle deformation is proposed, and is elaborated in Part II of this paper.     

WEAR AND FRICTION STUDIES OF NICKEL–TUNGSTEN CARBIDE–GRAPHITE COMPOSITES

Abstract

Wear and friction studies have been carried out on pressed and sintered composites of 75–95% theoretical density. Carbonyl nickel and tungsten carbide, both with a particle size of 5 μm, and natural crystalline flake graphite with an approximate size of 1·1 μm were used. The wear specimens were run against a rotating (100 rev/min) steel cylinder. The wear-resistance correlates well with the ratio of the volume fraction of tungsten carbide to that of graphite (WC/graphite); there is a minimum in weight loss at a ratio near unity. This effect is explained in terms of powder coating with graphite during blending, which affects grain-boundary formation and grain growth during sintering; supporting metallographic evidence is given. The wear-debris particle size produced is related to wear; the wear rate increases with increasing particle size. Residual porosity in the materials reduces wear-resistance. The wear rate does not correlate with hardness or coefficient of friction; the results of wear tests in oil or water are very similar to those for tests run in air.     

THE MEASUREMENT AND FORM OF POROSITY IN THE LOOSE SINTERING OF COPPER COMPACTS

Abstract

Changes in surface area of specimens of loose-sintered –300-mesh spherical copper powder, measured by the BET gas-adsorption technique, are given for sintering temperatures of 700,800,900, and 1000°C under furnace atmospheres of hydrogen and argon, for times of 0–24 h. Porosity determinations, using a xylene-impregnation technique, show that the porosity is composed entirely of interconnected pores at 700, 800, and 900°C, connected porosity occurring only after 14 h sintering at 1000°C. Determinations of pore-size distribution are also given, measured by a technique based on a “capillary rise of a liquid in a porous material”. Results indicate that for specimens sintered under a hydrogen atmosphere, an overall increase in pore size occurs, whereas for specimens sintered at 800 and 900°C under an argon atmosphere the size of the majority of the pores remains constant, whilst a small percentage of extremely large pores is developed. Permeability coefficients calculated from surface-area and pore-size distribution data are compared with the experimental values.