Lubricant Particles Research Articles

The influence of TiC, CaF 2 and MnS additives on friction and lubrication of sintered high speed steels at elevated temperature

Friction behaviours of sintered high speed steels containing TiC, CaF 2 and MnS additives and lubrication mechanisms of these additives have been investigated at sliding conditions at 600°C. Results shown that these additives strongly affected friction behaviours of the sintered high speed steels. Ceramic carbide TiC, as a bonding agent and enhancement phase, bonded the solid lubricant CaF 2 and MnS surrounding it and supported these solid lubricant particles so that friction process become stable. Fluoride calcium CaF 2 has a better high temperature lubrication properties than MnS, and that the addition of TiC + CaF 2 in the sintered high speed steels achieved excellent friction performances both a stable and a low friction coefficient value. Analysis results by X-ray indicated that the surface film appearing on worn path consists of some molten metal matters and very fine carbide particles. During sliding, the surface film separated contacts and resulted in a lower friction coefficient. Because of the sintered high speed steels usually to be used to manufacture high temperature components, these results are helpful for their engineering use.

Experimental study of magnetic slurries regarding magnetic properties

The aim of this study was to assess the basic magnetic properties of a mixture composed of a commercial lubricant and soft magnetic iron particles, where the particle content is significantly higher than in conventional magnetic fluids, due to the fact that the particles verge on the sedimentation level. This so called magnetic slurry can, then, be regarded as a highly ductile magnetic conducting material for tentative use in a magnetic circuit that can change shape under operation. The high particle content results in high saturation field densities that extend the application field towards electric power engineering. This paper, therefore, focuses on properties like relative permeability, losses, the Q-factor and the efficiency for the power frequencies 50 and 400 Hz. It shows that the relative permeabilities are low (below 10). The values of the efficiency and the Q-factor decrease considerably above 0.5 T. That limits the application of the studied slurry in electric power engineering. In such applications, the powder properties must be improved.

Liquid phase sintering of metal matrix composites containing solid lubricants

Attempts to improve the wear properties of sintered high speed steels have been made by the simultaneous addition of hard ceramic particles (TiC) or (NbC) alongside particles with solid lubricative properties such as (MnS) or (CaF 2). Preliminary investigations carried out to study any interactions between such particles and a sintered high speed steel matrix indicated that the steel matrix carbides were modified by a chemical reaction which occurred between the added ceramic particle and its high speed steel matrix; the result of which was to give a substantial improvement in hardness. The solid lubricant remained chemically unaltered by the sintering process, and tended to reduce hardness. Both types of particulate addition raised the sintering temperature needed to achieve full density due to their effect on solidus/liquidus temperatures. Studies of the sintering kinetics in such materials confirmed that densification was due to a liquid phase sintering mechanism and that solution/re-precipitation of carbide phases played a major role in the densification process. Coarsening of the solid lubricant particles into large agglomerates occured during sintering by a process of the transportation of smaller particles into pore interstices by the liquid phase.

Reactions and stability of ceramic and solid lubricant particles in high speed steels : J.D. Bolton, A.J. Gant. (University of Bradford, Bradford, UK)

Reactions and stability of ceramic and solid lubricant particles in high speed steels : J.D. Bolton, A.J. Gant. (University of Bradford, Bradford, UK)

Characterization of composite tribological coatings: composition, microstructure and mechanical properties

The present study aims to characterize two kinds of plasma composite coatings (Cr 2O 3·CaF 2 and Cr 2O 3·BaF 2) with very good tribological properties. Chemical and quantitative image analyses show that solid lubricant content (CaF 2 and BaF 2) in the coatings is less than but proportional to the nominal solid lubricant content in mixed powders for spraying. The microstructure of the composite coatings is characteristic of a homogeneous distribution of solid lubricant powders within the Cr 2O 3 matrix: on a sliding surface, the coating's microstructure consists of the spheroidal solid lubricant particles dispersed in a continuous Cr 2O 3 matrix; on cross-section the distinct wavy multilayers (Cr 2O 3 and solid lubricant) have been compacted to build the composite coating. During sliding, the matrix has flowed and the lubricant particles have been removed partially, smashed to very fine fragments and incorporated with the matrix of which one thin soft film has been made. It is the thin soft film that offers the coatings' very good tribological properties. However, the solid lubricant results also in degradation of the mechanical properties. Consequently there is one optimal solid lubricant content (about 20 vol.%) at which the coating has the best tribological and mechanical properties.

Wear in Zn-Al-Si alloys

The zinc-aluminum based casting alloy ZA-27 is useful in bearing applications. Its tribological properties may be improved by replacing its small copper content with silicon. These are further improved if the silicon particle size is controlled by suitable additions of strontium. This effect is related to the observation that wear properties of the alloy are governed by the development of a thin surface film of the Al-rich α phase. The integrity of this surface film during wear is controlled by the interplay between the lubricant film thickness and particle size. Wear performance is optimum when the silicon particles are larger in diameter than the lubricant film thickness, but small enough that they may embed in the bearing surface and be covered by the surface film. It is suspected that a similar conclusion may hold for particles such as grit from a dirty operating environment.

Secondary ion mass spectrometry (SIMS) evaluation of magnesium stearate distribution and its effects on the physico-technical properties of sodium chloride tablets

Lubricant film formation on sodium chloride particles by two commercial (A and B), and one high-purity sample of magnesium stearate (C) prepared in an acidic medium, has been studied. After mixing with 0.1 or 0.5 wt.% concentration of lubricant, the distribution on individual host particles was found to be non-uniform when assessed using imaging SIMS. For all samples, areas of high as well as low lubricant concentration were observed. Static SIMS spectra for sodium chloride and sodium chloride-magnesium stearate mixes have also been obtained. Positive and negativeion ratios were compared to interpret the spectra and identify the mechanism for lubricant film formation. The adsorption of lubricant molecules has been considered to be one of the most important mechanisms for excipient surface coverage. The observed Na +/Mg + ratio of 521.11 for the uncoated excipient was reduced to 0.94, 1.25 and 13.36 after 2 min. mixing with 0.5 wt.% concentration of A, B and C respectively. Since adherence of lubricant particles to host particle surfaces was not found after this mixing period, these reductions indicated the formation of molecular layers on the sodium chloride particles. Also, as the ratio after mixing for C was much higher than for the other samples, a relatively incomplete molecular coverage was envisaged. The distribution of the lubricant on the excipient surface has been discussed with reference to its influence on lubrication and the implication on the properties of the finished product. The static SIMS spectra have also been compared to reflect the fundamental crystalline structures of these lubricants.

Sintered 6061 aluminium alloy—solid lubricant particle composites: sliding wear and mechanisms of lubrication

Aluminium alloy (6061) matrix composites containing up to 14 vol.% natural graphite particle dispersions were prepared by a powder metallurgy route. Dry sliding wear rates of composites were evaluated against a rotating EN25 steel disc using a pin-on-disc apparatus. Worn surfaces and debris were examined in the scanning electron microscope. Model experiments were conducted on the etched surfaces of a 6061 aluminium alloy-14 vol.% graphite composite with graphite particle protrusions. Wear rates of composites increased linearly with the graphite volume fraction. Worn surfaces did not reveal the presence of a graphite film and the majority of debris were flaky in nature. This anomalous behaviour with graphite was attributed to the increased porosity (interconnected and interfacial) in the composites. Wear rates of composites in the etched condition showed a substantial decrease over that of the matrix alloy. Worn surfaces of composites in this case showed patches of graphite layers adhering to them.

Method of extending useful life of instrument strings

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Hydrodynamic behaviour of two-phase (liquid-solid) lubricants

An analytical hydrodynamic model was developed to predict the behaviour of a simple two-phase (liquid-solid) lubricant. An experimental study was also carried out with such lubricants using conventional full-film hydrodynamic bearings. Both the analytical and the experimental results showed that the presence of suspended solid particles in a newtonian lubricant enhanced the load-carrying capacity and reduced friction.

Dynamics of Solid Lubrication as Observed by Optical Microscopy

A bench metallograph was converted into a 'micro contact imager' by the addition of a tribometer employing a steel ball in sliding contact with a glass disk. The sliding contact was viewed in real time by means of projection microscope optics. The dynamics of abrasive particles and of solid lubricant particles within the contact were observed in detail. The contact was characterized by a constantly changing pattern of elastic strain with the passage of surface discontinuities and solid particles. Abrasive particles fragmented upon entering the contact, embedded in one surface and scratched the other; in contrast, the solid lubricant particles flowed plastically into thin films. The rheological behavior of the lubricating solids gave every appearance of a paste-like consistency within the Hertzian contact.

Particle Tribology

Progress in wear prevention can be made only when a better understanding of the mechanisms by which wear occurs and of the controlling factors has been achieved. Much of the research on wear has been directed towards the study of surfaces in relative motion and the changes brought about by their interaction and the lubricant or environment present. Little attention has been paid to the particles generated by the interaction of surfaces in relative motion and thus valuable information has been neglected. Each particle present in a lubricant carries with it the history of the conditions under which it was formed, and thus careful examination of particles can yield specific information concerning the condition of the moving surfaces from which they were produced. Techniques of particle examination are reviewed and the various theories of wear are considered in the light of information obtained by the examination of lubricant contaminant particles. The development of a powerful new technique of machinery condition monitoring by particle tribology to allow a prognostic approach to failure prevention and the safe switch from expensive periodic maintenance to failure prevention maintenance is outlined with examples from the work of NEL. The role of particle tribo-analysis in the development of machinery to ensure a maintenance and failure free life of sufficient duration to adequately cover capital expenditure is discussed.

Theoretical Effects of Solid Particles in Hydrostatic Bearing Lubricant

Theoretical Effects of Solid Particles in Hydrostatic Bearing Lubricant

Anisotropy of sintered antifriction materials containing a solid lubricant

1. It is shown that when a sintered antifriction material with a solid lubricant having a laminar structure, such as a silver/molybdenum disulfide composite, is produced by pressing in a die set, it exhibits anisotropy of mechanical and frictional characteristics. 2. In the manufacture of dry friction components from such materials, it is necessary to allow for the possibility of preferential orientation of the solid lubricant particles and to choose accordingly the method of producing such parts and the design of the friction unit.

MECHANICAL AND TRIBOTECHNICAL PROPERTIES OF MULTICOMPONENT SOLID LUBRICANT COMPOSITES BASED ON ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE

Multicomponent composites with ultra-high molecular weight polyethylene (UHMWPE) matrix reinforced by short carbon fibers (CF) and filled with solid lubricant particles of finely dispersed polytetrafluoroethylene (PTFE) have been studied. It is shown that simultaneous loading of two kinds of microfillers (enforcing and solid lubricant) ensures increasing both mechanical properties (elastic modulus, yield point, shore D hardness) and wear resistance of three-component UHMWPE composites at variation of triboloading conditions. It is shown that at moderate sliding velocity (V = 0.3 m/s) and load (P = 60 N) the rational composition providing maximum wear resistance under dry sliding friction is “UHMWPE + 5 wt. % fluorolite + 5 wt. % CF” (wear resistance is doubled). The latter results from the pattern of the formed permolecular structure and friction surface material response onto tribotechnical loading (due to formation of transfer film).Under severe tribotesting conditions (P = 140 N × V = 0.5 m/s) the two-fold increase in wear resistance demonstrates the composite “UHMWPE + 5 wt. % fluorolite + 10 wt. % CF”. This effect is mostly governed by enforcing action of short carbon fibers. The mechanism of this improvement might be explained in the following way. Friction heating induced increase of the temperature gives rise to local melting and surface layer plasticization. Presence of enforcing fibers ensures better protection of the friction surface from combined action of compressive and shear forces transferred from rotating steel counterface. Friction coefficient, topography of wear track surfaces and wear mechanisms of multicomponent UHMWPE composites are discussed taking into account the data on permolecular structure formation and the temperature in the tribocontact zone.

Dry-Film Lubricants from Molybdenum Disulfide Bonded with Microfibrous Boehmite

Binders that have been tried for powdered, lamellar, dry lubricants have shortcomings such as hindering the realignment of the lubricant particles or encapsulating the lubricants, making a wearing-in process necessary. A microfibrous form of colloidal alumina (boehmite) is shown to act as a superior nonencapsulating binder for molybdenum disulfide in the formation of ductile, water-resistant dry lubricant films. Such films exhibit lower coefficients of friction than have been reported for molybdenum disulfide or graphite films bonded with any other material. These new lubricating coatings have good load-carrying ability and durability at any temperature below the thermal decomposition temperature of molybdenum disulfide (700 F). Optimum performance of these boehmite-bonded films is obtained when: (a) the weight ratio of fibrillar boehmite to MoS2 is near 0.20; (b) the film is 0.2 to 0.5 mil thick; (c) the substrate is hard and highly polished; and (d) the film is applied as an alkaline dispersion (pH = 10...

A Theory of Liquid-Solid Hydrodynamic Film Lubrication

This investigation was undertaken for tile purpose of extending the design theories for hydrodynamic bearings to include the effects of solid particles in a liquid base lubricant. A set of nonlinear, coupled partial differential equations is developed to include the effects of the solid particles. Solutions of the mathematical model by numerical analysis are compared to the results obtained in actual bearing tests with a universal bearing test machine. Increased friction from the solids is shown to be limited to a certain range of operation such that at Sommerfeld numbers above or below this range there is only a slight increase in the friction above that obtained with the liquid alone. Good agreement between the theoretical solutions and experimental values was obtained by using experimentally determined particle shear strengths. Contributed by the ASLE Committee on Lubricant Fundamentals and presented at the Annual Meeting of American Society of Lubrication Engineers, held in Pittsburgh, Pennsylvania, M...

SOME OBSERVATIONS ON THE EFFECT OF LUBRICATION ON THE CRUSHING STRENGTH OF TABLETS.

Abstract The effect of mean compaction pressure, base particle size, lubricant particle size, and lubricant concentration, on crushing strength of tablets has been determined for crystalline aspirin, hexamine, sucrose and sodium chloride, and simple granulations of sucrose and of hexamine. Magnesium stearate powder was used as a lubricant. The lubricant decreased the strength of all tablets, the reduction being greatest for the crystalline materials. Variation of lubricant particle size had no effect on strength, and the behaviour of the granulations was not predictable from the characteristics of the crystalline materials. The results are compared with those of other workers.

A study of some factors affecting rolling contact fatigue life : Thomas L. Carter. NASA Technical Report R-60, 1960. ii, 48 pp., tabs., diagrs., photos. GPO price, $ 0.50. (Supersedes NACA TN 3925; 3030; 3933; 4161; 4163; 4216; and NACA RM E57K12)

A series of investigations using the fatigue spin rig to study the effect of several factors contributing to rolling contact fatigue life is summarized. Ball specimens of 1/2 and 9/16 inch diameter were tested at maximum theoretical Hertz compressive stresses in the range of 600,000 to 750,000 psi. Life was found to vary inversely with the tenth power of stress. In forging fiber studies, a greater concentration of failures and poorer life were observed where the greatest angle of intersection between the fiber flow lines and the surface occurred. This effect was independent of alloy composition. Higher lubricant viscosity was found to increase fatigue life, Lubricants having the same viscosity but of different base stock produced wide differences in life that correlated with the pressure viscosity coefficient of the lubricant. Higher temperature produced lower fatigue life. Dry powder lubricants produced poor fatigue life at 450 F; failure appearance indicated that the lubricant particles probably acted as minute stress raisers. In metallographic studies, nonmetallic inclusions were found to have a deleterious effect on fatigue life the inclusion size, location, composition, and condition of the matrix being contributing factors; failures were by shear cracking in the subsurface zone of maximum shear stress and eventual propagation into a shallow surface spall. Vacuum melting improved fatigue life, although a general correlation between cleanliness and fatigue life was not found. Life results for ten different bearing materials are presented.

Effect of Fiber Orientation, Temperature and Dry Powder Lubricants on Rolling Contact Fatigue

The rolling contact fatigue spin rig was used to study the interrelation of fiber orientation and fatigue. In both balls and specially oriented races, a concentration of fatigue failures was observed in regions with the highest angle of intersection of fiber flow lines with the surface. In one race specimen fewer failures per unit area were obtained with increasing distance from the billet axis so that variations in cleanliness in the race may have been important. Further tests are necessary to determine the actual importance of cleanliness. In races, increase in fiber orientation angle also resulted in poorer fatigue life. The fatigue life of M-1 tool steel balls decreased with increasing temperature when using a synthetic lubricant, and was low with dry powder lubricants at 450°F, possibly due to the lubricant particles acting as mechanical stress raisers within the contact zone. Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Los Angeles, Calif., Oc...