Iron-based alloys doped with copper can be attributed to anti-friction materials [1–5]. A decrease in the coefficient of friction for low-carbon steel  and cast iron  was noted when copper is added. The decrease in the friction coefficient depend on the concentration of copper-containing particles ranging in size from several nanometers to tens of micrometers. Such particles can be formed, both during prolonged isothermal annealing of hardened steel [6–9] and during crystallization of cast iron-carbon alloys [1–4]. Doping with copper leads to changes in the structure and mechanical properties of the hypereutectoid steel . Adding of 3 wt.% Cu to steel is accompanied by an increase in the microhardness of perlite from 380 to 430 HV. Moreover, the Brinell hardness increases from 340 to 390 HB. For copper containing alloys, the dispersion of the ferritic cement mixture is inherent. An additional factor contributing to the growth of the perlite hardness is the isolation in the ferrite gaps of nano-dimensional particles of the ε-phase. In sliding friction with lubrication, the wear resistance of hypereutectoid steel containing 8.97% copper is by ~23% higher than that of antifriction cast iron АЧС‑1. During laser cladding alloying elements are introduced into a thin layer of coating, without affecting the entire volume of the product. In this case, the number of alloying elements by mass can be reduced by 100–1 000 times.
The purpose of this paper is to determine the effect of the copper oxide nano-particles in the composition of the iron-based charge on the intensity of wear and the score resistance of coatings obtained by laser cladding. SURFACE TREATMENT EQUIPMENT AND RESEARCH METHODS In the experimental studies, the laser complex by IES of RAS was used . The samples were made of 20X steel with dimensions of 15 Ч 20 Ч 80 mm. Iron-based powder ФБХ6–2 was chosen for cladding. Copper oxide nano-powder of 3–9% by weight was added to the welding powder filler [12–13]. The variable factors in the planning of experiments  were as follows: radiation power P = 700–1000 W, processing speed V = 5–9 mm / s and beam diameter d = 2–3 mm. The response of the experiment was the height of the weld bead H (mm), the width of the bead B (mm), and the depth of the heat-affected zone (HAZ) Z (mm). As an additional discrete factor, the beam scanning with a fixed frequency f = 220 Hz was considered. A resonance-type scanner was used with an elastic element with the mirror attached. Cladding was performed at the maximum and minimum values of the factor levels, which are designated z+ and z-, respectively. The upper and lower levels of the factors were chosen based on preliminary experiments, where the formation of cladded layers was visually observed. The metallographic studies of the cladded coatings were carried out using microhardness tester PMT‑3 at a load of 0.98 N, metallographic microscope Altami MET 1C and digital microscope AM413ML. The structure and chemical composition of the cladded layers were studied using TESCAN VEGA 3 SBH scanning electron microscope with an energy-dispersive analysis system employing the modes of reflected and secondary electrons. To determine the score resistance of the hardened samples, a universal friction machine MTU‑01 was used. The tests were carried out according to the "plane-ring" scheme. A comparative analysis was carried out between the samples with the ФБХ6–2 powder charge cladding and the samples cladded with copper oxide nano-powder 3–9% additives into the charge. The ring was made of ШХ‑15 steel with an internal diameter of 24.5 mm and an outer diameter of 30.5 mm, with a hardness of 60–62 HRC. Lubricant grease was used as the lubricant. The sliding speed and pressure on the sample varied discretely within 0.1–1.1 m / s and 1–4 MPa, respectively. The test time was 2 hours. After the tests, the samples were degreased with acetone in accordance with GOST 2786–84 and dried in an oven at 70 °C for 30 minutes. RESULTS OF EXPERIMENTAL STUDIES In order to take the discrete scanning factor into account, the experiments were performed in two series – with and without scanning. The levels of factors, as well as the dependence of the coded variables on the natural quantities, are given in Table. 1. According to the recommendations , an algebraic linear polynomial was taken as a mathematical model y = b0 + b1 x1 + ... + bk xk + b1, 2 x1 x2 ... +bk–1, k xk–1 xk, where bi are linear regression coefficients; y is the system response; k is the number of factors in the experiment. The obtained mathematical models were tested for adequacy by the Student and Fisher criteria. The analysis of the equations obtained shows a directly proportional relationship between the radiation power and the height of the bead and the inverse relationship between the velocity of the beam and the height of the cladded layer. The width of the bead increases with increasing power and diameter of the beam and decreases with increasing processing speed. Scanning leads to an increase in width with a simultaneous decrease in the height of the bead and increases the cladding productivity by 1.3–1.9 times. Analysis of the equations for the HAZ depth shows that the radiation power has the greatest influence on the HAZ depth. An increase in the speed of the beam movement leads to a decrease in the HAZ depth, which is explained by the smaller instantaneous energy absorbed by the cladded material per unit of time. For comparison, the surfaces for the widths of the cladding zones B and Bscan = f (P, V) are constructed for d = 2 mm (Fig. 1). When using a round laser spot for hardening or cladding, the exposure time at its center is determined by the ratio of the beam diameter to the speed of its movement, and at the edges of the spot the exposure time tends to zero. In this case, only a part of the energy of 30–60% is effectively used for hardening or cladding, depending on the degree of defocusing of the laser beam, and the remaining energy is consumed for useless heating of the zones near the hardened or fused path. In high-frequency oscillations of the beam along the normal to the vector of its displacement, this lost energy participates in the cladding process, affecting the productivity of the process. This can be clearly seen from the graphs of the surfaces obtained by the results of experiments for the width of the beads. Since the height of the beads varies insignificantly, it is also possible to visually determine the effectiveness of the process. According to the data obtained, the nature of the linear dependence obtained in the calculations is the same as for the experimental values, however, the calculated function grows somewhat faster. The maximum deviation of the calculated data from the experimental values was 3%. The cladded layer 0.8 mm thick has a high hardness, of the order of 762–806 HV, with underlying HAZ with a hardness of 540–720 HV, the thickness of this layer is 0.8 mm, below are the zones of troostite and sorbitol. The base metal is sorbitol-like perlite and ferrite with a hardness of 190–210 HV. The presence of a flashing zone of the base at a depth of 50–100 µm indicates a high adhesion strength between the coating and the base metal. In the process of coating cladding based on ФБХ6–2 powder, the defects in the form of cracks appear in different modes. To exclude crack formation, the technology of preliminary heating of samples at a temperature of 350 °C for 2 hours was developed, followed by laser beam flashing and holding in the furnace for 2 hours. When cladding a coating containing copper oxide nano-powder, copper oxide is reduced to pure copper with oxygen burnup. Copper oxide was chosen because of the low reflectivity of the laser radiation. The introduction of a soft phase (Al, Cu, V, etc.) is widely used in laser and plasma cladding. These elements contribute to the relaxation of stresses arising in the cladded coatings. Fig. 2 shows the dependence of the sliding speed on the pressure on the sample when tested on a friction machine MTU‑01. The introduction of ФБХ6-2 copper oxide nano-powder into the charge leads to a 1.5 to 2-fold increase in the score resistance of the coating with a gradual change in the sliding speed of 0.1–1.1 m / s and pressure within the range of 1–4 MPa. For coating with addition copper oxide nano-powder, the sliding speed at which the jamming occurs is by 1.2–2.3 times higher than for the cemented sample depending on the concentration of copper oxide in the charge and the contact pressure. The wear rates for the samples cladded with ФБХ6-2 powder with various concentrations of copper oxide are shown in Table 2. It has been established that with increase in the concentration of copper oxide in the charge, the score resistance increases and the wear rate to a copper oxide concentration of 7% decreases and increases at a concentration of 9%. The wear rate and the jamming load on the copper oxide concentration are shown in Fig. 3. Fig. 4 shows photographs of microsections of the welded layers of ПГ-ФБХ6–2 powder with the addition of copper oxide nano-powder, obtained with the help of a scanning electron microscope (SEM). The elemental composition in the upper part of the cladded layer is Fe – 50,13%, Cr – 33,66%, C – 8,46%, Cu – 3,63%, Mn – 1,89%, Si – 1,21%, O – 1.02% (Fig. 4a). As part of the structure, the white zones represent copper agglomerates with a size of 1 to 10 µm, and their total content is 1.4% of the content in the charge. The elemental composition of the fusion zone with the basic metal (Fig. 4b) is shown in Table 3. White zones (spectra 38, 39, 40) represent copper agglomerates of up to 10 µm in size. The fusion zone with the basic metal (spectrum 42) contains 3.4% copper, indicating a uniform distribution of copper from the surface to the basic metal. CONCLUSIONS It was found that the presence of copper in the cladded layers increases the sliding speed at which a score develops up to 3.3 times and the wear rate is up to 30% compared to the cemented sample. With high-frequency scanning of the beam, the effectiveness of the cladding process is 1.3–1.9 times higher than without scanning the beam. The introduction of a copper oxide nano-powder into the charge on the basis of ФБХ6–2 powder improves the hardiness of the cladded coatings 1.5 to 2-fold.