Based on the mechanics of discrete contact of rough surfaces of machine parts, quantitative criteria are proposed that determine the boundaries of fretting wear regimes. The physical foundation of these criteria is the ratio of the amplitude of tangential oscillatory displacements of contacting bodies and characteristic linear quantities associated with the dimensions of the real contact areas of the contacting surfaces. It is convenient to characterize fretting wear regimes using maps on which the boundaries of the areas of the corresponding regimes are shown in the coordinates: oscillation amplitude/normal contact load (or nominal contact pressure). The influence of the wear regime on the life of one of the common types of connections subject to fretting wear, namely electrical contacts, has been shown experimentally. Using a map of fretting wear regimes, the influence of the amplitude of tangential oscillations and normal load on the dependence of the contact electrical resistance on the number of oscillation cycles under different regimes was analyzed. The fundamental difference between this dependence in the fretting wear regime and in the normal reciprocating sliding of contacting surfaces is shown. The proposed criteria of fretting wear regimes and their visual representation in the form of wear regime maps allow predicting the wear mechanism of a given contact joint. Understanding the wear mechanism allows for a reasonable choice of materials for contacting parts and contact joint parameters (contact pressure, contact rigidity) to ensure the reliability of friction joints, including electrical contacts, under fretting wear conditions.

10.26456/pcascnn/2024.16.643 Abstract. The parameters of the structure of a technical surface at the nanoscale level, determined by the profile method, are considered. The possibility of applying the parameters determined in accordance with domestic and international standards for the roughness profile (microscale level) to the characteristics of the surface at the nanoscale level, which goes beyond the standards, is shown. This possibility is provided by the model of the profile of a technical surface that underlies modern standards as an implementation of a broadband random normal process. An experimental verification of the normality of the process was carried out by comparing the experimental values of the integral probability distribution function of the profile ordinates with theoretical values that obey the normal distribution. The experimentally obtained values of the nine most important parameters, as well as their some relationships between them, are presented, confirming the reliability of the surface profile model as a normal random process. The use of standard surface profile parameters ensures to avoid errors and misunderstandings associated with ambiguous interpretation of one or another parameter.

Gas laser cutting technology has several advantages over other methods of material separation. Reducing the roughness of the cut surface is practically an important task, since the roughness parameters have a significant effect on the operational properties of functional surfaces. The article presents the results of a comprehensive assessment of the microgeometry of the surface of a gas laser cut and studies of the microstructure of a two-layer steel material, high-speed steel grade Р9M4K8, deposited on 30KhGSA structural steel. The influence of the modes of gas laser cutting (cutting speed and laser radiation power) on the parameters of roughness, phase composition, and microhardness of the cut surface layers is investigated. It is shown that the roughness of the cut surface is comparable to the roughness of surfaces after machining and under certain conditions can be less than after mechanical or oxygen cutting. It is shown that the roughness parameters change depending on the distance to the upper edge of the cut. Correlation ratios of the main parameters of surface roughness after gas laser cutting have been determined and compared with similar ratios for surfaces after machining. It is also shown that the height parameters of roughness significantly depend on the cutting modes. No noticeable effect of the chemical composition of steels on the roughness of the cut surface was revealed. Analysis of the microstructure of the surface of steels after gas laser cutting showed that in the surface layer of the cut to a depth of more than 200 microns, the steels were re-hardened. In the hardening zone, the surface layer has an increased microhardness: for high-speed Р9M4K8 steel HV0.2 = 8760 ± 150 MPa, for 30KhGSA steel, HV0.2 = 6740 ± 90 MPa. The results make it possible to select the technological parameters of gas laser cutting that provide an optimal combination of microgeometry and mechanical properties of the cut surface, which makes it possible to reduce or eliminate subsequent processing.

We substantiate the applicability of the stationary random process model to the subroughness (nanoroughness) profile of technical surfaces. The normality of the distribution of the nanoroughness-profile ordinates is verified by means of Pearson and Lilliefors goodness-of-fit tests. We calculate the autocorrelation functions and the spectral densities of the processes under study, as well as the correlation interval and the effective spectrum width. The examples show the possibility of calculating the surface nanoroughness-profile parameters on the basis of the autocorrelation function. The calculated parameter values coincide with those experimental within the error.

The results of experiments continuing the study of the features of the temperature effect on the frictional characteristics of metal friction pairs are presented. The features lie in the ambiguity of this effect associated with the prehistory of the contact, that is, with the sequence of temperature changes that preceded this state. The object of research is a friction pair of gold and gold–copper alloy (80% Au, 20% Cu). The following temperature effects are investigated experimentally during the heating of the contact from room temperature to the maximum temperature and cooling of the contact from the maximum temperature to room temperature: the temperature effect on the maximum force of static friction, on the coefficients of static Amontons and Coulomb friction, and on the specific force of static friction. The experiments are carried out according to the ball–plane friction scheme: a spherical sample with a radius of curvature of 0.5 mm made of the Au80Cu20 alloy was in contact with a flat sample of gold (Au99.99). Normal contact load varied in the range of 0.5–4 mN, and the temperature varied in the range of 21–130°C. It is shown that the experimental dependence of the friction force on the normal force in contact is adequately described by linear regression equations: single-term (Amontons’ law) and binomial (Coulomb’s law). The coefficients of linear regression models (Amontons and Coulomb friction coefficients) are statistically significant at p = 0.05, and, at the same time, they are independent of the normal load. The values of the proportionality coefficients (friction coefficients) in the binomial regression dependence of the friction force on the normal force are on average 12% less than the analogous coefficients in the single-term regression dependence. The nature of the temperature dependences of the static friction forces and the Amontons and Coulomb friction coefficients sensibly coincides with heating and cooling of the contact; hysteresis does not appear in these dependences. The results are of theoretical and practical importance for improving the quality and reliability of sliding electrical contacts that are heated as a result of Joule heat.

A study of the microtopography of the surfaces of steel samples after gas laser cutting has been carried. Profi le records of the investigated surfaces indicate the presence of two types of surface deviations from the geometric shape: waviness and roughness. At the same time, waviness is not present on all investigated surfaces. For different modes of the laser cutting process, the height and spacing parameters of roughness and waviness were determined. It is shown that the parameters of surface roughness after gas laser cutting are comparable to those of surfaces after such types of machining as turning and milling. Regression equations are obtained for some roughness parameters, which show the dependence of these parameters on the modes of the gas laser cutting process.

The temperature effect on the frictional characteristics of a silver-silver friction pair was experimentally studied. These characteristics are as follows: Amontons' and Coulomb's friction coefficients and specific friction force parameters. The applicability of the Coulomb's friction law to the description of the friction force dependence on the normal load in the studied temperature range is substantiated. In addition, the experiments showed that the contact frictional characteristics are significantly affected by its temperature prehistory and the temperature variation direction during the experiment-from room to maximum or from maximum to room.