Vol 39, No 6 (2018)

In addition to the friction coefficient and the linear wear intensity of the frictional materials, the vibroacoustic characteristics are of importance, especially for the stationary friction units. Noise and vibration in the friction units lead to the problems connected with the technological operation of materials. This paper presents the results of studies of tribotechnical and vibroacoustic characteristics of highly filled friction composites depending on the content of iron oxide (Fe₂O₃). Based on the results of tribotechnical tests and noise measurement generated by stationary friction, it is established that the optimum tribotechnical and vibroacoustic properties of friction composites are obtained within the range of low concentrations of iron oxide (0.6–1.2 wt %).

Results of theoretical and experimental studies of the pressure distribution in the sliding bearing clearance are presented considering the influence of dissipative processes. The results obtained allow a more accurate description of special features in the formation of pressure in sliding bearing clearance that cannot be taken into account within the framework of the traditional approach based on the Navier–Stokes equation. The theoretical data are supported by the results of pressure distribution measurement in the clearance between the cylindrical racer of the bearing and the shaft fixed in it.

Thermal and tribochemical processes in a phenol formaldehyde composite reinforced with polyoxadiazole fibers and modified with finely dispersed particles of a polyformaldehyde copolymer were studied by mass spectrometry and X-ray photoelectron spectroscopy. Under thermal exposure to polyformaldehyde, the liberated formaldehyde sharply reduces the amount of phenol that is released. In the case of friction, polyformaldehyde is a bifunctional modifier. It forms a discrete surface, which helps reduce the coefficient of friction. The released products can play the role of a “tribochemical lubricant” and, at the same time, a cross-linking agent. Tribological tests of the modified composite showed a decrease in the coefficient of friction (from 0.20 to 0.18) compared with the original composite, and a significant (approximately 2-fold) decrease in the vibration amplitude of the friction coefficient.

A calculation procedure to determine the temperature field during rotary friction welding of metal is proposed. An axisymmetric nonlinear boundary value problem of heat conduction taking into account the frictional heating of two cylinders (specimens) of finite length is formulated. It is assumed that the materials of specimens are thermally sensitive, and the friction coefficient depends on temperature. The solution of the problem is obtained by the finite element method. The numerical analysis was carried out for two identical specimens made of AISI 1040 grade steel.

A percolation model for two-component random materials, namely, the model of a random resistor grid (RRG) or an “ant limit” is used to describe the frictional characteristics of composites such as ultrahigh–molecular-weight polyethylene/carbon fiber. It is shown that the critical index of this model is determined by the structural parameters of the filler, whereas the RRG model describes equally well the results of frictional wear for both abrasive and fatigue mechanisms of wear.

This paper presents the results of the study of the tribological characteristics of compatible materials, the thickness of the protective films, the depth of scratches, and the roughness of the friction surfaces of specimens in model tribosystems for EV-350 and EV-350-50M extruders for manufacturing solid fuel from biomass. It is found that the addition of an alkaline additive agent, namely, sodium hydroxide, which creates a slightly alkaline medium of pH 7.4, decreases the wear rate and increases the service life of a conveyor and dies of an extruder. It is found that, in a slightly alkaline medium of pH 7.4, the roughness of the working friction surfaces is less than the roughness of surfaces in a slightly acidic medium of pH 5 and a strongly alkaline medium of pH 11. The impact rating of different factors on the wear process is determined, which includes, in descending order, abrasiveness, active acidity of a medium, and load. Considering this rating, the selection of compatible materials for extruder tribosystems that increase their service life is justified; such materials are steel 95Kh18 and cast iron ChKh32. It is established that the use of raw materials that have a slightly alkaline medium close to neutral leads to slight traces of corrosion on the working surfaces caused by the formation of passive films on the metal surface, which slow down the oxygen diffusion to the metal surface and catalytically reduce the corrosion rate.

Test results of the dispersive, rheological, and surface active characteristics of aqueous emulsions of the oxyethylated stearic acid derivatives (Stearox-6) and aqueous binary compositions modified by PEG–Stearox-6 are presented. Both synergism and antagonism of the dispersion effect of components being incorporated into the aqueous binary compositions (ABC) during the process of abrasive machining of steel 45 are established. The value of synergism regarding the dispersion action of ABC components is defined by the possibility to form intermolecular complexes between the molecules of amine-based oligomers of different molecular weight (MW) and Stearox-6, and antagonism is determined by their competitive adsorption at the as-formed surface of steel.

Tribological investigations of the coatings based on Al–Sn with Zn plasticizer reinforced by corundum in a friction pair with steel are presented. The values of the friction coefficient depending on the reinforcing content are experimentally determined, and the perspectives of utilizing synthesized coatings in friction pairs are estimated.

The results of the tests of corrosiveness and oxidative stability of vegetable oils are presented. It is shown that linseed and rapeseed oils have high levels of corrosiveness and oxidative stability. It is established that the basic mechanism of the corrosive action of oils on copper is the reaction with the metal of oil oxidation products with atmospheric oxygen. The anomalously high oxidative stability of linseed oil is related to the predominance of its polymerization processes over hydrolysis processes.

A single impact of a solid particle on the surface of rubber in an aqueous medium is simulated using a special installation. An experimental study made it possible to distinguish the stages and the destruction mechanism of the rubber surface. It is established that rubber wear in an aqueous medium and air have fundamental differences. It is revealed that under low loads, the rubber surface is destroyed due to the peeling of plane rubber microfragments. Under harder loading modes, a crack forms in the center of the contact spot. Further propagation and coalescence of the cracks leads to the spalling of fragments of the material.