The present work is a continuation of the investigations of vacuum-diffusion gas-discharge processes used to form modified surface layers of metals under the mutual influence of the autonomous parameters of treatment: temperature, the composition of a saturating medium, pressure in a gas-discharge chamber, and the duration of saturation. On the basis of the obtained experimental results, we formulate our ideas concerning the possibility of practical application of this principally new technological process based on the optimization of the combination of its autonomous modes of saturation, which would promote the formation of surface structures improving the reliability and durability of machine parts under the operating conditions. In this case, as a basis, we use an absolutely new energy model whose principal specific feature can be described as follows: The main role in the hardening of the surface layers of metals is played by the subprocesses that can be regarded as the most appropriate under the specific operating conditions of the products [1].

The analysis of the literature sources [2,3,4] demonstrates that all known theoretical models of ionic nitriding are, to a great extent, hypothetical. Many of these models have no analytic substantiation and the criteria for optimization of the nitrided surfaces (and, hence, the procedures of control over nitriding required to get the desired operational characteristics) are absent. The accumulation of a great amount of technological data that are not based on the general theoretical foundation does not promote the development of practical applications of the procedure of ion-plasma hardening of the products and frequently leads to the opposite results because the accumulated results are not based on the actual operation conditions.

In the planned cycle of works, we formulate absolutely new concepts of the theory of diffusion gasdischarge processes of ionic nitriding in metals based on the priority of the energy approaches [1]. From this viewpoint, the indicated processes were considered neither in Ukraine, nor abroad, despite the fact that the proposed approach opens absolutely new possibilities for the production of diffusion ion-nitrided layers with new properties and extends the sphere of their application in the field of hardening of the metal surfaces.

Methods of Investigations

We performed ion nitriding in glow discharge by using a specially designed installation at the Podillya Scientific Physicotechnological Center of the Khmel’nyts’kyi National University [1].

In our investigations, we chose the following materials extensively used for the production and the repair of elements of the technological equipment of food industry: 20, 45, 45Kh, U8A, 38KhMYuA, and 12Kh18N10T steels. 38KhMYuA steel was taken to study the influence of alloying elements on the physicomechanical characteristics of the surfaces after ionic nitriding in glow discharge.

We applied the methods of metallographic, electron-microscopic, and X-ray diffraction analyses and the method of ferromagnetic resonance.

The structure of cross section of the diffusion layer and the distribution of microhardness over its thickness were studied on etched microsections. The submicroscopic structure of the nitride zone was analyzed in a REM-200 scanning electron microscope. The X-ray phase diffraction analysis of the surface layers of the specimens was carried out in a DRON-200 diffractometer with the use of K α -radiation.

The residual stresses were found by the Davidenkov method by measuring the deflection of plane specimens in the course of etching of the nitrided layer [6].

Results of Investigations and Discussion

The intensity of vacuum-diffusion ionic nitriding in glow discharge is 1.5–2 times higher than the intensity of furnace nitriding, which determines new properties of the ion-nitrided layers [3]. According to [1, 5], the equilibrium between the processes of spraying and deposition of nitrogen ions is attained for 4 h. Therefore, our main investigations of the properties of nitrided surfaces were performed for the indicated duration of the process.

Note that the changes in microhardness across the thickness of the diffusion layer characterize the fractions and distributions of the structural and phase components of the coating and may serve as the measure of the energy capacity of the surface layers [5, 7]. Therefore, we studied the influence of the parameters of ionic nitriding on the indicated characteristic (Figs. 1a, b).

Fig. 1.
figure 1

Dependences of the microhardness of nitrided zone on temperature T (solid lines), nitrogen content C (dashed lines), and the pressure of gaseous mixture P (dash-dotted lines) for: (1) 38KhMYuA, (2) 45Kh, (3) 45, and (4) 20 steels (a) and the distributions of microhardness HV1 over the thickness of the diffusion nitrided layers for specimens made of 45Kh steel for the following durations of nitriding: (1) 1 h; (2) 2 h; (3) 3 h; (4) 4 h (b).

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The analysis of the accumulated results of investigations enables us to formulate the following general regularities of changes in the microhardness of the nitrided zone depending on the parameters of the mode of nitriding:

  1. (1)

    the maximum hardness increases with the content of carbon and the amount of alloying elements in the base (Fig. 1a; curves 1–4); the influence of the alloying elements (especially, of the nitrideforming elements) on the microhardness is much more pronounced than the influence of carbon (curves 1–3);

  2. (2)

    as the nitriding temperature increases, the microhardness of the nitrided zone decreases and the rate of decrease becomes higher as the degree of alloying and the carbon content of the base increase;

  3. (3)

    the level of microhardness increases with the content of nitrogen and the pressure of the gas medium but the rate of increase is much lower than the rate of decrease in the case of growing temperature;

  4. (4)

    the increase in the duration of nitriding for constant temperature, pressure, and the composition of gaseous mixture does not affect microhardness and promotes its smoother transition to the hardness of the base with simultaneous extension of the zone with higher levels of microhardness (Fig. 1b).

The maximum microhardness of the nitrided zone is obtained for 38KhMYuA steel because the nitrides of alloying elements have a higher hardness, dispersion, and heat resistance as compared with iron nitrides. Moreover, the decrease in microhardness observed as the nitriding temperature increases is explained by a decrease in the degree of discreteness of nitrides. Since iron nitrides are characterized by the low heat resistance and high rate of coagulation [3], the level of hardness of carbon steels insignificantly depends on the nitriding temperature (Fig. 1a, curve 4).

According to the results obtained in [3], the dispersion of inclusions in the form of nitrides of alloying elements exerts the crucial influence on the level and distribution of microhardness over the thickness of the nitrided layer. Thus, the investigations of the fine structure of 45Kh nitrided steel in the REM-200 scanning electron microscope showed that, as the nitriding temperature increases from 833 to 873°K, the surface area of nitrides becomes, on the average, 38 times larger and, hence, the level of microhardness decreases.

The obtained results are confirmed by the data of X-ray diffraction analysis. It is known that the maximum microhardness of the nitrided layer corresponds to the formation of coherent nuclei of the nitride phase, which lead to the most pronounced distortions of the crystal lattice of the matrix whose mean value is characterized by the width and intensity of the diffraction maxima of the α -phase. As the nitriding temperature and the size of nitride particles increase, the coherence is violated and the distortion of the crystal lattice of the matrix and the hardness of the layer decrease (Fig. 1a).

The alloying elements increase the solubility of nitrogen in the alloyed α-phase and, hence, promote a decrease in the diffusion coefficient of nitrogen and in the thickness of the diffusion layer. The thickness also decreases with the increase in the carbon content of the matrix. In this case, the concentration of nitrogen in the surface layer of the ε -phase insignificantly varies. Thus, the fractions of the ε -phase on 20 and 45 steels are equal to 19 and 21%, respectively.

The analysis of the influence of the parameters of ionic nitriding on the thicknesses of the nitride zone h N and the diffusion layer h and microhardness HV1 revealed the possibility of efficient control over these characteristics, mainly by changing the temperature of the process. Hence, the nitriding temperature is the most universal energy parameter of the process (Fig. 2). In this case, the most intense growth of the diffusion layer was observed for 45 carbon steel (Fig. 2; curve 2 of the dependence h = f (T)). The least intensity of growth was detected for the 38KhMYuA alloyed steel (curve 3). This fact is explained by the low mobility of nitrogen in the nitrides of alloying elements, which block diffusion processes in the solid solution [3]. On the contrary, the depth and the intensity of growth of the nitride zone h N increase with the degree of alloying with nitrideforming elements (with the exception of aluminum and silicon which somewhat decrease the solubility of nitrogen) [3]. Then iron nitrides and alloying elements inhibit the diffusion of nitrogen into the bulk of the metal and the development of the zone of internal nitriding.

Fig. 2.
figure 2

Dependences of the depth of diffusion layer h (thick solid lines) and the thickness of nitride zone h N (thin solid lines) on the temperature, nitrogen concentration in the saturating mixture (dashed lines), and pressure P (dash-dotted lines) for: (1) 45Kh, (2) 45, and (3) 38KhMYuA steels.

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As the concentration of nitrogen in gaseous mixture increases to 75%, the thickness of the nitride zone h N also increases. However, the subsequent increase in the nitrogen concentration leads to a decrease in h N. This is possibly explained by the deterioration of cleaning of the metal surface caused by the lower kinetic energy of nitrogen ions as compared with argon ions [1] (Fig. 2; h N = f (C,N2)).

The elevation of the pressure of nitrogen-argon mixture exerts almost no influence on the thickness of the diffusion layer h but somewhat increases the thickness of the nitride zone h N (the dash-dotted lines in Fig. 2).

The application of the method of active planning of the experiments enables us to develop statistical models of the influence of ionic nitriding on the thickness and microhardness of the hardened layers. As a result of statistical processing of the experimental data on a significance level of 5%, we obtain the following regression equations:

$$ {\displaystyle \begin{array}{c}{y}_1=256.12+18.27{x}_1,\kern0.5em {y}_2=16.72+0.95{x}_1+0.16{x}_2+0.20{x}_3,\\ {}{y}_3=7.30-0.65{x}_1+0.08{x}_2+0.19{x}_3,\end{array}} $$

where y 1 is the total thickness of the diffusion layer h , y 2 is the thickness of the nitride zone h N, y 3 is the level of microhardness HV1, x 1 is the nitriding temperature, x 2 is the concentration of nitrogen in the gas mixture C , and x 3 is the pressure of gas mixture.

The analysis of the developed models confirms the results on ionic nitriding obtained earlier and allows one to determine the thicknesses of the nitride and diffusion zones and the microhardness of the nitride zone according to its known parameters.

In recent years, the investigations of the structural states of surface layers and the analysis of the process of wear of the metals based on the physics of strength and plasticity of crystalline bodies attract significant attention of the researchers [8].

The dependences of the parameters of crystal lattice on the mode of ionic nitriding were established with the help of the X-ray diffraction analysis. These dependences enable us to quantitatively evaluate the characteristics of strength of the surface layers (Fig. 3).

Fig. 3.
figure 3

Dependences of the ratio of lattice parameters (c/a), the concentration of nitrogen in the surface layer (C N) (а), and the parameters c and a (b) on the mode of nitriding (T , C, and P ) of 45Kh steel.

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The results of the X-ray diffraction analysis are corroborated by the data of metallographic analyses, the levels of microhardness and the character of its distribution (Figs. 1 and 2). Thus, the lattice parameter c increases with the nitriding temperature, whereas the parameter a strongly decreases (Fig. 3b). Hence, the ratio c/a increases (Fig. 3a). As a result of the diffusion of nitrogen into the depth of the diffusion layer, its concentration C N on the surface of the nitride layer decreases with the increase in the nitriding temperature and, therefore, the microhardness of the nitride layer also decreases (see Fig. 1a).

The process of weakening of distortions of the crystal lattice (ratio c/a ) with increase in the concentration of nitrogen in the gas mixture (the curve c/a (N2) in Fig. 3a) is simultaneously compensated by the increase in the concentration of nitrogen in the nitride zone C N(N2) (Fig. 3a) and causes an insignificant increase in the microhardness of the surface layer (Fig. 1).

The increase in the pressure of gas mixture has almost no influence on the ratio of the parameters c/a (P) (Fig. 3a), which is confirmed by the character of changes in the parameters c(P) and a(P) (Fig. 3b) and exerts almost no effect on the microhardness of the nitride zone under given conditions of changes in the pressure of the gas mixture in a gas-discharge chamber (Fig. 1a; curve 4).

However, the decrease in the ratio c/a observed as the concentration of nitrogen in the gas mixture (and simultaneously in the nitride layer) increases (Fig. 3а) and also its deviations from the linear dependence within the range 75–90% (shaded triangles in Fig. 3а) require additional explanations. For this purpose, we plotted the dependences of the lattice parameters a and c on the mode of nitriding (Fig. 3b). These dependences enable us to conclude that the parameter a increases with the concentration of nitrogen in the gas mixture, whereas c decreases. When the nitrogen content becomes equal to 75%, the rates of changes in the lattice parameters become inversely proportional, which enables us to describe the dependence of the ratio c/a on the nitrogen content of the gas mixture by a straight line (c/a (N2) in Fig. 3а). At the same time, the established regularities of changes in the parameters c(N2) and a(N2) enable us to substantiate the optimal choice of the mode of ionic nitriding in a gas mixture formed by 75% N2 + 25% Ar.

The data of X-ray diffraction analysis enable us to conclude that, independently of the composition of gaseous atmosphere, the ε - (Fe2−3N), γ′- (Fe4N), and α -phases are formed in the surface layer. In this case, the phase composition and the fractions of phase structures in the nitrided layer can be regulated by changing the parameters of the mode nitriding T , C , and P (Fig. 4).

Fig. 4.
figure 4

Dependences of the phase composition of the diffusion layer of 45Kh steel on the mode of ion nitriding (T , C, and P ): (a) content of the ε -phase; (b) content of the γ ′-phase; (c) content of the α -phase, %.

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The analysis of the obtained dependences shows that the temperature of ionic nitriding exerts a strong influence on the amounts of certain phases. Thus, the amounts of the γ ′ - and α-phases in the surface layer increase with temperature (Fig. 4b, c), whereas the amount of the ε -phase decreases (Fig. 4a). Moreover, the amount of ε -phase increases with the content of nitrogen and the pressure of gas mixture (Fig. 4a) and, at the same time, the amount of the γ ′ -phase decreases (Fig. 4b). In this case, the content of the α-phase remains practically invariable (Fig. 4c). Its amount depends solely on the temperature of ionic nitriding in glow discharge, which is a function of the energy characteristics of the process (current density and voltages on the electrodes of the gasdischarge chamber).

In view of the strong difference between the physicochemical properties of the phase-structural components of the nitrided layer and the possibility of regulation of their amounts and fractions, we get an efficient tool for the creation of layers with prescribed characteristics of the surfaces.

The results of numerous investigations [1,2,3, 6] demonstrate that the structure of the layer and its phase composition specify the operating characteristics of the products. Thus, in particular, the high-hardness components formed on the basis of the ε -phase (Fe2N) are used in open friction couples (gear wheels, sprockets of chain drives, conveyor pins, etc.). For closed friction couples (plane bearings, gear wheels, gear boxes, etc.), it is necessary to create a nitrided layer with developed nitride zone formed by low-nitrogen plastic nitride phases γ and γ′ (Fe3N and Fe4N), which are well running in, localize plastic strains, and inhibit the development of fracture processes in the component. The main contribution to the increase in the fatigue strength under alternating loads is made by the zone of internal nitriding (α -phase). It is also worth noting that the α –phase {but of the second kind, i.e., corresponding to the formation of the nitrides of alloying elements (TiN, ZrN, and HfN) [3]} also increases the heat resistance.

For the reliable operation of products in corrosive media at high temperatures, it is reasonable to create diffusion layers with surface nitride zones. Due to its high corrosion resistance, the ε -phase blocks the transportation of oxygen atoms into the matrix and prevents the adsorption decrease in strength (Rehbinder effect) both under static conditions and under the action of alternating stresses [3, 6].

The subsequent X-ray diffraction investigations of the phase structural components of ion-nitrided layers would enable us to better comprehend the mechanisms of their formation, to evaluate their quantitative and qualitative influence on the physicochemical properties of nitrided surfaces, and (which is especially important) to modify their properties depending on the operating conditions of the machine parts.

Conclusions

On the basis of the results of our investigations and the analysis of literature sources, we substantiated the significance and urgency of the analysis of fine mechanisms of the ionic nitriding of metals in glow discharge for their practical application by the optimal combination of the parameters of technological mode with priority of the energy approaches.

The formulated general regularities of the variations of microhardness in the nitride zone depending on the autonomous parameters of the mode of nitriding are formulated and are substantiated by results of the X-ray diffraction analysis of the phase composition of nitrided surfaces.

The structure of the nitrided layer and its phase composition determine the physicomechanical characteristics of the surfaces of machine parts. For this reason, we obtained and analyzed the dependences of the thicknesses of the nitride zone h N and diffusion layer h on the parameters of nitriding, temperature, composition of the gas mixture, and its pressure. We also developed statistical models of the relationship between the parameters of nitriding and the thicknesses of the nitride and diffusion zones and their microhardnesses. We performed the X-ray diffraction analysis of the phase structural components of the nitrided layers, analyzed the changes in the lattice parameters c and a for nitride particles depending on the concentration of nitrogen in the nitride zone C N , and studied the phase composition of the diffusion layer on 45Kh steel as a function of the parameters of nitriding (T , C , and P).