Durability of working bodies of soil-cultivating machines strengthened bycomposite electrolytic coatings (CEC)

The task of this work is to find the optimal ratio between the size of the particles of silicon carbide and their volumetric content in the nickel matrix to provide maximum characteristics of strength and wear resistance of the working bodies of soil-processing machines.

Key words: ground-making machines, composite electrolytic coatings (CEC), durability.

КЕП з розмірами частинок 28/20 та 50/40 мкм дозволяють отримати найбільш зносостійкі покриття. При цьому покриття з частинками 28/20 мкм мають вищу зносостійкість, але покриття з частинками 50/40 мкм є більш технологічними при їх формуванні.

Розмір частинок наповнювача має значний вплив на трибологічні характеристики КЕП, а саме зносостійкість та коефіцієнт тертя. Встановлено, що найвищою зносостійкістю та найменшими коефіцієнтами тертя характеризуються покриття, що мають у якості наповнювача порошки фракцій 28/20 та 50/40 мкм. Трибологічні дослідження показують перспективність і ефективність застосування КЕП для підвищення зносостійкості робочих органів ґрунтообробних машин.

Physical and mechanical characteristics of the CEC based on a nickel matrix containing a silicon carbide (SIC) are largely determined not only by the geometric dimensions of SIC particles, but also their volumetric content in the nickel matrix. Thus, in [1] it is indicated that there is a proportional dependence between the hardness of particles, their number in a nickel matrix and durability. The maximum strength value is achieved with the optimal content of particles in the coating, the excess of which dramatically reduces the physical and mechanical characteristics of the CEC. Therefore, the task of this work is to find the optimal ratio between the size of the particles of silicon carbide and their volumetric content in the nickel matrix to provide maximum characteristics of strength and wear resistance of the working bodies of soil-processing machines.

In [3], it has been shown that the use according to optimal modes of carbaazonation in a non-hydrogen teaching discharge allowed to increase the wear resistance of the cultivator's paws in 1.7...1.8 times.

Surface reinforcement is used to increase wear resistance in cases where there are no increased requirements for the volume strength of parts, but their high surface durability is required.

Laps of cultivators are one of the most mass details of the working bodies of agricultural machines. As a result of small service life, a large number of pieces are spent in the form of spare parts, the production of which requires a significant amount of high-quality metal.

In accordance with the technical requirements, the guarantee experience of the archery legs of the cultivator should be no less than 25 hectares, but as shows the practice of exploiting such working bodies, their work for refusal on various soils does not exceed 14 - 19 hectares.

In general, it can be stated that for most regions of Ukraine, one set of details of the working bodies of soil-cultivating machines is not enough on the current annual cycle (spring + autumn), and therefore it is necessary to continue the searches for new, economically substantiated methods and methods of surface strengthening of the working bodies of soil-cultivating machines.

Thus, the purpose of the study is to develop a technology for strengthening the working bodies of soil- cultivating machines by applying composite electrolytic coatings (CEC).

Materials, electrolytes and electrolysis modes when forming a heart

In the scientific literature, the prospects of use of nickel as a matrix for the next formation of a heap of different composition, filling and properties are noted. Nickel is characterized by high affinity to most types of filler particles. Nickel Based CEC is divided into several types: core, multilayer coatings with increased corrosion resistance in the atmosphere, self-residual coatings, etc.

Choosing a filler for a chest is determined by the requirements for the properties of the coating: high strength of grip with matrix, affinity with matrix material, corrosion resistance in aggressive environments, highly high mechanical characteristics, etc.

Such requirements will some extent satisfy the powders of carbides and, in particular, silicon carbide. The silicon carbide is recommended for the creation of compositions to increase the hardness and wear resistance in friction without lubrication and at elevated temperatures [6, 7, 8], corrosion resistance [7]. The silicon carbide in the nickel matrix improves the coating properties: microhardness increases by 1 ... 2.5, internal stresses decreases in 3 ... 8 times, and corrosion resistance increases in 4 ... 50 times [8]. Coverage with silicon carbide has the best grip with steel compared to other fillers. By strength of grip with the base we have such a row: 487-SiC; 213-TiC; 216-Cr7C3 [6].

In addition, silicon carbide has high mechanical characteristics: microhardness 29 ... 35, elastic modulus E=394, border strength to the rupture -180, to bend -173 ... 225, on compression -800 [8].

The silicon carbide has a low cost and is produced in large quantities in the form of powders packed by fractions.

For the formation of a cavity on a nickel matrix, sulfate or sulfatchloricular nickel electrolytes [6, 7] is used for the most part. The disadvantage of such electrolytes is a low rate of precipitation of nickel: 20...40 with a current density of 0.2...0,5. Increasing current density leads to a deterioration of the quality of the coating.

In this paper, a chloride nickel electrolyte was used, which allows to increase the cathode density of current, increase the rate of depositing nickel [9]. The composition of the electrolyte is given in Table.1.

To the advantages of the selected nickel electrolyte include the stability of the value of pH throughout the time of electrolysis, which eliminates the effects of acidity change on the volume content of the filler particles into a cavity.

Operational, including tribological and especially cavitation-erosionary characteristics of wear resistance of the CEC depend on the strength of the coupling (adhesion) of the coating with the base. The force of adhesion clutch with a steading base (steel 40X) depends on the degree of surface cleaning from oxides and other chemical compounds. Therefore, before applying CEC, samples are degreased by Viennese lime and subjected to anode treatment for 3... in a 20% solution of sulfuric acid with a density of current 2.

The size of the samples on which 10 x 10 x 5 mm test were performed (Fig. 3, Fig. 4). Before testing the sample and the countertoll was washed with gasoline, acetone and alcohol and dried in air. Determined the mass of the sample and counterparted with an error of not more than 0.0005g. With the help of lab scales (the largest limit of weighing 200g, weighing error is not more than 0,0005g). In the test process, friction coefficient and linear frictional wear are recorded during sustainable wear. The end of the work was determined by stabilization of temperature and friction coefficient. Recording coefficient of friction and linear wear of frictional steam during test was performed every 5 minutes. The intensity of wearing samples with coatings was evaluated by loss of mass of the sample and the magnitude of linear friction pair [8].

where: m1 - mass of sample (countertick) to test, mg;

m2 - mass of sample (countertick) after testing, mg;

L - rubbing path, km.

Research results

Physical and mechanical characteristics of the CEC based on a nickel matrix containing a silicon carbide (SiC) are largely determined not only by the geometric dimensions of SiC particles, but also their volumetric content in the nickel matrix. Thus, in [7], it is indicated that there is a proportional dependence between the hardness of particles, their number in a nickel matrix and durability. The maximum strength value is achieved with the optimal content of particles in the coating, the excess of which dramatically reduces the physical and mechanical characteristics of the CEC.

Thus, the coating on a nickel base with particles of sic different fractions and for convenience, we will denote coating by the size of the filler particles. For example, nickel bp with particles SiC fractions 5/10 ^m, denote both Ni + SiC5, fractions 28/20 pm as Ni + SiC28, 50/40 цМ- Ni + SiC50, 100/80 - Ni + SiCi00, and coating with nanoparticles in size about 50 nm - Ni + SiCnano. When conducting work also used amorphous boron dispersion of about 1 ^m.

Studies have shown that the size of the filler particles has a significant impact on the wear resistance of the coating. According to the results of experiments, the largest linear wear with loads of 20 and 40 N have specimens from steel 40X without coating. When applied to a steel layer of galvanic nickel, which has a hardness Нц= 2.4 ... 2.7 GPa, linear friction pair decreases by 1.4 times, weight demolition of the sample - by 2 times, and countertil in 3 ... 7.6 times Compared to test samples without coating (Fig. 5).

Fig. 5. Wearing resistance of Ni-SiCСЕС, depending on the size of the filler: a - weight wear of the sample and countertick, mg / km; b - linear friction pair of friction, цт / km

Wearing resistance of samples from the CEC containing the SiC filler of different fractions is higher than in purely nickel coatings. So, for samples with the inclusion of a fraction of 100/80 цтthere is a significant weight and linear wear, which even exceeds the wear of the sample without coating. The hardness of the matrix with this slightly increased and is H^ = 2,8 - 3,0 GPa, that is, increased by 10 - 15 % compared with a clean nickel due to composite strengthening and larger stresses in the matrix. The obtained results are explained by the fact that the size of the particles is compared with the coating thickness (200 - 300pm) and in these conditions, the plastic nickel matrix can not compensate for contact loads on large SiC particles, which leads to fragile destruction and distorting. In addition, the volumetric content of SiC100 particles in the coating is the largest compared to other test samples and is 46 %. Under such a carbide content, significant stresses that the plastic matrix can not be compensated effectively.

A large weight wear of the sample is observed in the application as a filler for small particles of fraction less than 5 pm and nanoparticles 50 nm (Fig. 5). The use of such particles slightly increases the wear resistance of the coating in comparison with galvanic nickel, but not significantly (a decrease in weight loss of the sample is 10 - 20 %©–) because the content of SiC5 particles and SiCnano was 8 and 3 vol%©–respectively, that is, it is less than large particles and such particles are more efficiently strengthening the matrix (disperse hardening occurs) than increasing wear resistance in comparison with larger particles. This is due to the fact that in such compositions, the particle size is smaller than the size of individual contact spots, they can not effectively perceive the load and therefore the main contribution to wear resistance introduces the NI matrix, which has low mechanical properties and due to the emergence of significant stresses possible occurrence of cracks [8].

The highest wear resistance among the above coatings have a cap with the inclusion of fractions 28/20 and 50/40 цшthat have the smallest wear with all loads. The content of the filler in such coatings is, respectively, 24 and 28 vol%. In this case, slightly less wear is covered with the inclusion of a fraction of 28/20 цш. Weight wear of such samples is an order of magnitude smaller than for coatings with smaller and large particles. Compared to galvanic nickel, such coatings have a decrease in wear 12, 9 and 5 times with loads of 20, 40, 60 N, respectively (Fig. 5).

Fig. 6. Dependence of coefficient of friction f from the size of the particles of the filler СЕС:

The highest wear resistance of coatings with inclusion of 28...50 цшmay be due to the distribution of load they perceive. The solid inclusion load is equal to the actual contact pressure when their sizes are smaller or vessel with a single contact spot (2 - 10 цш). For compositions with optimal particle sizes (28 цш) there are no processes of seizing, abrasive and fragile destruction and there is a normal mechano-oxidation process of wear.

In addition to wear resistance, an important tribotechnical parameter is the coefficient of friction f. An analysis of the test results showed that the coefficient of friction significantly affects the load when the SiC filler and the size of the SiC filler and clearly follows such a regularity that with an increase in loading such as friction decreases (Fig. 6). This is due to the fact that with an increase in normal load n, the friction force FTR increases not so significantly and, accordingly, the ratio f = Ftr/ N decreases.

As to the influence of the size of the particles of the filler of the CEC on the friction coefficient F, it can be noted that the friction coefficient decreases and for the range of particles of filler 40 ... 60 цшfriction coefficient is minimal. For coatings with larger filler particles (fraction 100/80) friction coefficient is greater. This can be explained by the fact that for small particles of the contact spot is greater than the size of the particles and friction occurs on a nickel matrix that has a high friction coefficient and is prone to gripping. For particles of larger than 10цш, the size of the contact is smaller than the size of the particles and the main contribution to the friction is introduced by the filler particles, which contributes to the reduction of friction coefficient and increased wear resistance. For large particles it is possible to distort them and the abrasive destruction begins. It can also be noted that for greater loads (60N), the effect of the size of the filler particles on the friction coefficient decreases, while there is a greater difference between the friction coefficients of nickel coatings (F = 0.82) and filling coes (F = 0.73 and below). . Probably, with larger loads of solid particles, the filler begins to perceive more efficiently load, which in turn positively affects the friction processes.

Practical recommendations

1. Formation of a sac with particles SiCnano and SiC5 is carried out on a vertical, and all other particles in a horizontal cathode. With such particles, we obtain the highest corrosion resistance of the coating.