Journal of Friction and Wear最新文献

Research on microstructure changes and structural defects (using the example of dislocation density) of samples made of high-speed steel R6M5 and irradiated with a single high-energy laser pulse as well as the effect of these changes on the abrasive wear resistance of the studied material is presented in the article. It was found that the proposed laser treatment significantly affects the microstructure of the irradiated samples. This is expressed in the almost complete disappearance of the banded distribution of carbides in the samples, which indicates a partial redistribution and dissolution of “heavy” carbides of the Me₆C type containing tungsten and molybdenum in the material matrix (martensite). In this case, the configuration of heavy Me6C carbides is located between the formulas Fe₃(W,Mo)₃C–Fe₄(W,Mo)₂C. Also, exposure to a high-energy laser pulse leads to an increase in the quantity and size of secondary carbides in the alloy matrix, significantly enriched in tungsten and molybdenum. In addition, there is a change in the type of vanadium carbides from Me₂C to VC, accompanied by a significant decrease in the amount of molybdenum and tungsten in it. Using the Thixomet image analysis program, it was determined that the number of grains of heavy Me₆C carbides in terms of volume decreased by 1.30–1.58 times, depending on the distance from the point of interest to the irradiation place of the sample. At the same time, the average size and direction (anisotropy) of the material grains has not changed. The results of X-ray phase analysis showed that after treatment, the number of main phases of the samples remained practically unchanged in distribution, but there was an increase in the responses intensity. Based on the above, the change in the structure defectiveness was determined using the example of dislocation density. The results of abrasive wear tests showed that there is an increase in the wear resistance of irradiated samples by 1.58–2.48 times, depending on the distance from the point of interest to the irradiation place of the sample. In this case, the value of the greatest wear resistance (2.48 times) is achieved with the greatest microstructure changes and structure defectiveness, which corresponds to the distance from the point of interest to the irradiation place of the sample equal to 20 mm. The obtained results allow us to recommend the use of the microstructural analysis method for assigning optimal modes of laser hardening of materials.

The paper presents the results of a study of the chemical and phase composition, structure, physicomechanical, and tribological characteristics of two types of vacuum ion-plasma antifriction carbon coatings alloyed with molybdenum and tungsten, respectively. The first of these types of coatings has a highly oriented linear-chain structure; the second type is an amorphous hydrogenated carbon. The tribotechnical characteristics of the studied coatings were evaluated on a four-ball machine with a modernized friction unit and were carried out both under dry friction and under friction in the boundary lubrication mode in inactive (polyalphaolefin oil PAO-4), surface-active (PAO-4 + 1% by weight of oleic acid) and chemically active (the same oil + 2% by weight of DF-11 additive) lubricating environments. It was found that the coating with both “monocrystal” carbon and diamond-like carbon, both alloyed and unalloyed, significantly reduced friction losses and wear of the steel samples to which the coatings were applied, and alloying of these coatings reduced friction almost as much as the studied lubricants. Thus, under dry friction of a ball made of ShKh-15 steel on the cylindrical surfaces of rollers made of the same steel, the friction coefficient obtained was ~0.8; when a ball rubs against rollers coated with a molybdenum-doped “monocrystalline carbon” coating, the dry friction coefficient is ~0.7; additionally, in an inactive oil environment, this friction pair provides a friction coefficient of 0.12–0.14; and in a chemically active sulfur-containing oil environment, the friction coefficient decreases to 0.05. For a friction pair of steel –molybdenum-doped diamond-like coating, with boundary lubrication by a surface-active composition, the minimum friction coefficient is 0.09. Taking into account the results of the proposed study will allow optimizing the process of creating highly effective lubricants for heavily loaded friction units.

The article is dedicated to the calculation of the acting forces and amplitude–frequency characteristics of the “rotor–stator” system in a state of frictional contact. Unlike known approaches, the normal pressure force between the rotor and stator is defined not only by the elasticity and damping of their materials at the contact point and/or the stator supports but also by the bending complex stiffness of the entire rotor, which can be conceptually treated as having an additional bearing with a clearance. This quasi-static reaction of the rotor to contact with the stator, initially characterized by Coulomb friction during slipping, is extended to dynamic contact with rolling friction of the rotor over the stator, which depends on the normal pressure force between them. In the proposed mathematical model and the practical example considered, the rolling equations are formulated and solved in matrix form concerning the precession speed and rotor displacements, based on system (interconnected with the stator) elastic-inertial characteristics, as well as coefficients of slipping and rolling friction, and internal and external losses. The calculations established finite ranges of possible angular velocities for uninterrupted rolling of the rotor over the stator and demonstrated the possibility of eliminating it by adjusting just two parameters of the system: the damping coefficient of the rotor and the modulus of elasticity of the stator’s contact surface.

Microstructure, phase composition, as well as mechanical and tribological properties have been studied for composites based on a BrAMts9-2 bronze matrix reinforced with tungsten particles. Samples containing 0, 5, 10, and 15 vol % of W particles have been obtained by means of friction stir processing (FSP). It is shown that resulting from a multipass FSP of tungsten-containing samples, a composite structure is formed based on a multiphase bronze matrix consisting of α-Cu solid solution, intermetallic β′-Cu₃Al and γ₂-Al₄Cu₉ as well as uniformly distributed reinforcing tungsten particles. In the course of tribological testing, all the samples demonstrate similar values of friction coefficient ranging within 0.33–0.39. The wear rate of composites reinforced with the use of W are significantly lower than those of samples without W. The minimum wear rate values have been obtained for BrAMts9-2/5% of W samples. The tribo-oxidation effect in tungsten-containing samples is accompanied by the formation of the κ-phase of Al₂O₃ owing to the oxidation of the metastable γ-Al₄Cu₉ phase. The obtained composites can be use to fabricate moving parts of machines and devices, high-voltage electrical contacts, heat-removal elements, as well as for many other practical applications.

The influence of the type of motion friction (sliding/rolling) on the behavior of antiwear additives of lithium grease has been investigated. The tests of plastic lubricant on wear under conditions of sliding friction on a four-ball friction machine (FBFM-1) and as a part of a rolling bearing have been carried out. The multidirectional influence of additives on tribological properties of plastic lubricants is shown. Introduction of detonation nanodiamonds into lithium plastic lubricants (PLs) in concentration of 0.3% by mass in conditions of rolling friction leads to decrease of friction coefficient in relation to basic PLs by 2 times and increase of relative life. In conditions of sliding friction the antiwear properties are improved by 1.7–1.9 times. Addition of tricresyl ether of orthophosphoric acid (TKF) to lithium PLs in rolling friction conditions leads to increase of friction coefficient by 3.65 times and decrease of relative life, and in sliding friction conditions increases antiwear properties by 1.9 times. The results of research allow us to pick the optimum composition of additives depending on working conditions of a friction unit.

The article contains the results of examination of cast-iron pipes of water economizers of water-heating boilers, which were in operation in the period of 1986–1991 and which have no external visual defects. We have determined a dependence of wear-out in the form of graphitic corrosion of the surface layer of inner walls of economizer pipes. Researches have been conducted with the use of an JSM-6390LV scanning electron microscope equipped with an attachment for conduct of energy-dispersive analysis, and with the use of a DuraScan 20 microhardness tester. There are images of areas given, where microhardness testing was performed. The areas were exposed to graphitic corrosion. The dimensions of the layer completely transformed as a result of graphitization and the transition zone were determined. The microhardness of the layer completely transformed as a result of graphitization and the transition zone was measured. It was found that the microhardness of these zones is significantly lower than the microhardness of gray cast iron. We have also presented the results of energy-dispersive analysis and specific diagnostic features identifying the graphitization process. In the article we propose a method of assessment of supporting capacity of cast-iron pipes of water economizers based on the rate of their graphitization. A mathematical model takes into account all main properties, features, physical and geometric characteristic of the real structure, which has an analytical model represented by a beam rigidly restrained on both heads, and which is bent by well-distributed dead weight load arising from the pipe’s own weight and the weight of the water flowing through it. The calculated residual wall thickness of 4.53 mm after wear of its internal cylindric surface is comparable with the known empirical data of statical analysis of emergency situations at economizers. The data given in the article could be used by specialists for expert examination of cast-iron pipes of economizers, as well as for justification of their sorting or life extension.

Dry sliding of model samples of C235 steel against 45 steel under alternating current with a density of up to 120 A/cm² using the pin-on-ring configuration was investigated. The possibilities of increasing wear resistance by changing one of the geometric parameters (height) of the sample (pin) were studied and the temperature of the sample holder at different points was investigated. It was shown that this temperature can exceed 150°C in the sample mounting zone. The increase in the temperature of the sample holder and the wear intensity were nonlinear in accordance with a curve close to an exponential function with increasing current density. Sliding at a current density higher than 100 A/cm² occurred under conditions of catastrophic wear. Formation of a transfer layer with a thickness of about 20 μm was observed on the contact surface of the samples. Using X-ray phase analysis, it was found that the transfer layer contains more than 70% FeO. The analysis of wear surfaces allowed us to reveal the mechanism of contact layer deterioration, which was presented as the division of the nominal contact area into two sectors. The boundary between the sectors was quite clear and perpendicular to the sliding direction. The sector directed towards the oncoming contact surface of the counterbody had traces of adhesion, plowing, etc., which usually appear during sliding without current. The other sector had signs of deformation similar to the deformation of a viscous liquid. Traces of adhesion were not observed here. The friction coefficient decreased with increasing current density and did not change its behavior upon transition to the catastrophic wear mode. The presented regularities were observed at a qualitative level regardless of the height of the samples. Only the heating temperatures of the sample holder differed quantitatively. The results of the work will be useful in creating a real friction unit operating in sliding current collection with a high current density.

The structure of the near-surface damaged layer in diamond single crystals after circular grinding is studied using scanning and atomic force microscopy. It is shown that the dimensions and structure of the damaged layer are determined by the fractional composition of diamond grains in the diamond abrasive tool. The structure of the damaged layer of the single crystal is formed by networks of microcracks with open and closed crack mouths dividing the layer into blocks. The near-surface section of the damaged layer consists of blocks (1–3 μm) separated by a network of microcracks. Further on, there is a section of larger blocks, which transitions into a section of blocks located between fragments of the damaged grains of the single crystal. Wear of a diamond single crystal during diamond abrasive grinding occurs as a result of two simultaneously occurring processes: formation of a damaged surface layer during dynamic contact interaction of diamond grain tips on the single crystal surface and its destruction during the penetration and movement of diamond grain tips in the damaged layer. Contact interactions of diamond grain tips with the single crystal surface determine its morphology, the structure of the damaged layer, which affect the color characteristics of the diamond.

Improving the efficiency for grinding the parts of machines and mechanisms provides for the choice of rational processing modes, the characteristics of the abrasive tool and the cutting fluid, as well as the selection of modifying additives impregnated into the body of the grinding wheel. Interrrelations between the chemical composition and the main physicochemical and operational properties of the cutting fluid, which determine its efficiency in increasing the processing productivity and surface quality in the course of grinding have been investigated. The studies have been performed in order to reveal the character of physicochemical phenomena in a contact zone between abrasive grains and metal under processing with the use of an RSZ-2 unit. A mechanism determining the effect of thermoelectric phenomena on the interaction between the abrasive grain and the metal in the presence of a lubricating-and-cooling agent (cutting fluid) in the course of grinding has been revealed. It is shown that the grinding process can be considered as a continuous change in the energy of atomic and molecular interactions in a three-phase metal–lubricant–abrasive grain interfacial system.

The article presents the results of experimental studies of the durability of protective oxide coatings of 12X18N10T stainless steel in a lead coolant environment, when simulating an emergency situation of “intercircuit leakage of a steam generator.” The tests were carried out on the FTM-2023 NSTU stand in a lead coolant with a temperature of up to 430–450°C without circulation. The tube bundle for simulating a steam generator leak during the outflow of the light phase is two rows of 22 × 3.5 pipes welded to a plate with a pitch of 33 mm. The data were obtained for tube bundles with two different types of holes for the outflow of the vapor phase: a slit hole of 13 × 1 mm and a round hole with a diameter of 4 mm. The processing and analysis of the results allowed us to conclude that when a steam-water mixture enters the heavy liquid metal coolant circuit, the oxide coatings become thinner depending on the location of the tube in the bundle and the shape of the discharge hole. There is a noticeable decrease in microhardness (by 49 HV for a tube bundle with a round discharge hole and by 23 HV for a tube bundle with a rectangular discharge hole), which is more pronounced when approaching the discharge hole and depends on the orientation of the tube surface onto which the mixture enters, relative to the leak. The data are recommended for use in assessing the consequences of a large leak in a horizontal steam generator in installations with heavy liquid metal coolant.