ANTIFRICTION PROPERTIES OF NITRIDE-OXIDE COATINGS IN THE PRESENCE OF LUBRICANT

ABSTRACT

The article presents the results of studies of the antifriction properties of nitride oxide coatings obtained by gas nitriding and subsequent oxidation in water vapor. Optimal structural and phase compositions of the coating were established in terms of ensuring better surface treatment and maximum wear resistance under sliding friction conditions in the presence of a lubricant. It is shown that a thin surface oxide layer with polishing properties on the surface of the nitride layer accelerates the achievement of operational roughness during sliding friction in the presence of a lubricant. The type and thickness of the surface oxide layer, which gives a damping quality to the nitride layer located above the internal nitriding zone, have been established.

АННОТАЦИЯ

В статье представлены результаты исследований антифрикционных свойств нитрид-оксидных покрытий, полученных газовым азотированием и последующим оксидированием в парах воды. Были установлены оптимальные структурные и фазовые составы покрытия с точки зрения обеспечения лучшей обработки поверхности и максимальной износостойкости в условиях трения скольжения в присутствии смазочного материала. Показано, что тонкий поверхностный оксидный слой с полирующими свойствами на поверхности нитридного слоя ускоряет достижение эксплуатационной шероховатости при трении скольжения в присутствии смазки. Были установлены тип и толщина поверхностного оксидного слоя, который придает демпфирующие свойства нитридному слою, расположенному над внутренней зоной азотирования.

Keywords: nitriding, oxidation, nitride-oxide coating, wear resistance, lubricant, run-in period, friction, roughness, plasticity.

Ключевые слова: азотирование, оксидирование, нитрид-оксидное покрытие, износостойкость, смазка, приработка, трение, шероховатость, пластичность.

Introduction

Nitriding processes and their varieties are widely used in world mechanical engineering for surface hardening of machine parts and mechanisms operating under conditions of corrosion and wear. At the last stage of processing, a surface diffusion layer is formed on the surface of the products, consisting of compositions of a surface nitride layer and an internal nitriding zone, in which the surface nitride zone is the most responsible for the operational properties of corrosion resistance [1-4].

The combination of nitriding processes with other methods of chemical and thermal treatment makes it possible to obtain modified nitrided layers on steel products, which significantly increase the physical and mechanical properties of the surface, primarily wear resistance and corrosion resistance, providing better adhesive strength with a matrix of a hardened metal or alloy [5-7].

As a result of the combined process of gas nitriding in dissociated ammonia at the first stage and followed by oxidation in water vapor at the second stage (nitrooxidation), a modified diffusion composite nitride–oxide coating is formed, consisting of a composition of nitride phases and a surface oxide layer, which has high anticorrosive properties and the best antifriction performance on medium carbon steels [8].

During isothermal exposure in water vapor, due to diazotization of the nitride layer.is formed an oxide film on its surface. At the same time, dissociation of the high-nitrogen nitride occurs by transition to the low-nitrogen phase. The participation in the diffusion processes of oxygen from the saturating atmosphere during oxidation and carbon during decarbonization of the steel matrix leads to the formation of a modified composition of nitrides: carbonitride Fe₃(NC) (ε'-phase), oxycarbonitride Fe₃(NCO) (ε"-phase) and γ'-phase Fe₄(NCO), and the presence of oxygen atoms and carbon in the γ'-phase expands its area of homogeneity [9,10].

MATERIALS AND METHODS

The wear resistance of a diffusive nitride oxide coating obtained after nitriding and oxidation in water vapor was studying in the steel 45 samples treated in various nitrooxidation modes. In this case, certain ratios of carbonitride (ε'-phase), oxycarbonitride (ε"-phase) and γ'-phases were obtained in the nitride layer with the formation of a thin dense oxide film 1-3 microns thick on their surface, consisting practically of Fe₃O₄ (magnetite).

The tests were carried out on a serial SMC-2 test rig according to the block scheme (steel 45 with nitride oxide coating) - roller (steel 45, HRC 40-42) [7]. The content of elements in steel: C=0.46 %; Si=0.22 %; Mn=0.60 %; Cr=0.04 %; Ni=0.04 %; P=0.25%; S=0.25% (by weight). The hardness and roughness of the friction surfaces of the samples were selected in such a way as to ensure plastic contact on the friction surfaces in the friction zone, in the mode of sliding friction with lubrication.

Industrial I-20 oil was used as a lubricant, lubrication was carried out by dipping the disc into the bath. At the same time, as a result of the rotation of the disk, an oil bump was created in front of the sample, so that the friction zone received a uniformly lubricating material.

The duration of the test was 6 hours, the sliding speed V=0.864 m/s.

Linear wear of the samples was determined by the method of artificial bases based on the intensity of wear. During the tests, the following were recorded: linear wear of samples with reinforcing coatings and the coefficient of friction in the contact zone [11].

The phase composition of nitride oxide layers was determined at the DRON-3 installation by X-ray diffraction analysis, in cobalt filtered radiation at diffraction angles in the range 2θ=20-130 degrees and sample rotation speeds of 1 and 2 degr./min. The study of the element content on the surface of the samples was carried out using the ZEISS DSM 950 electron-optical system. The surface roughness was determined on the profilograph-profilometer Model 201.

The experiments were carried out on nitrided and nitrooxidized samples – pads in a friction pair with a roller made of hardened steel 45. The pads were treated with nitriding at the first stage of treatment in dissociated ammonia at a temperature of 580 ° C for 3 hours (Option A) and part of the samples after nitriding without removing from the furnace at a temperature of 580 °C were oxidized in water vapor for 0.5 hours (Option B). Nitrided samples were also treated under these conditions, followed by oxidation in water vapor at a temperature of 550 °C for 1.0 hours (Option C) by the formation of a diffusion modified coating on their surface with a different ratio of nitride phase compositions in the nitrided layer (Table 1).

To study the effect of the thickness of the nitride and oxide layer on the wear process and other physical and mechanical properties, a nitride oxide coating obtained by nitriding at the first stage of the process at a temperature of 580 °C for 5 hours, followed by oxidation at the second stage for 1.0 hours at a temperature of 550 °C (Table 1, Option D) was studied.

After oxidation of the nitride layer, the proportion of the γ' phase in the nitride layer was 50 % in variants A and B, 75 % in variants C and D of the total sum of the phase compositions in the nitrided layer.

Table 1

Characteristics of diffusion layers on steel 45

Results and Discussion

X-ray diffraction analysis revealed that after nitriding in the atmosphere of dissociated ammonia (Option A), the phase composition of the nitrided layer consists of compositions of a surface nitride layer consisting of ε-, ε'-, γ'-nitrides and an internal nitriding zone Fe_α. After oxidation in water vapor, an oxide layer forms on the surface of the nitride layer due to its deazotization. The structure of the composite diffusion coating will consist of a thin oxide layer and a nitride layer behind it. Due to the strong oxidizing ability of oxygen during the period of isothermal exposure in water vapor with the formation of an oxide layer, oxygen doping of nitride phases occurs simultaneously with the further formation of oxygen-modified nitrides with a low nitrogen content. As a result of oxidation of nitrides, oxynitrides Fe₃(NO), Fe₄(NO) are formed, and oxycarbonitrides Fe₃(NCO), Fe₄(NCO) are present in the carbon zones of the steel matrix. When oxidized on the surface of nitride phases at a temperature of 580 °C (in the above eutectoid temperature for the "Fe-O" system), a surface oxide film is formed consisting of sequentially arranged oxides of Fe₂O₃, Fe₃O₄ and FeO (Table 1, Option B), when oxidized at below the eutectoid a temperature of 550 °C the oxide film consists only of Fe₃O₄ oxide (Table 1, Option C).

Diffusive combined nitride-oxide coatings, rather than coatings applied to the surface of a metal or alloy, combine high adhesion to the base material and certain performance properties. At the same time, the wear resistance of parts in sliding friction pairs depends on the chemical composition, structure and structure, stress state and adhesive strength of the surface layers [12].

An oxide film with a thickness of 1-3 microns and a microhardness of 452-465 MPa on the surface of a nitride sublayer, which has a microhardness of 830-940 MPa, forming strong adhesive bonds and with a sharp change in hardness along the cross section of the coating creates a positive gradient of mechanical properties, which prevents setting provides conditions of external friction (Table 1).

The content of elements in the oxide layer and the types of forming oxides on the friction surface to a depth of 1-3 mkm in nitrided and nitrooxidized samples were studied (Table 2). After surface hardening, not only iron oxides FeO, Fe₃O₄ and oxides of alloying elements containing in the composition of steel, such as MnO, Cr₂O₃, SiO₂, are involved on the surface of the friction zone. During oxidation, an oxide film of the base metal is formed on the surface of nitrides – an oxide structure of the type I and solid solutions of oxygen in alloying elements (an oxide structure of the type II). Although Ni is present in the steel composition, NiO was not detected in the study area. The presence of oxides of alloying elements in the composition of the surface oxide film together with base metal oxides in the presence of a lubricant can successfully have a shielding effect on the oxide film, protecting it from setting of interacting surfaces especially during their run-in at the initial stage of wear. Diffusion nitride-oxide coatings on the surface of parts of friction units with high antifriction characteristics and not prone to setting are very important for ensuring the operability of the friction pair, especially during the initial wear period. During the run-in period, the adaptability of the surface occurs with the distribution of local pressures in the contact zone due to changes in the roughness of the surfaces and with an increase in the actual contact area on the friction surfaces

Table 2

The content of basic and alloying elements, types of oxides in the surface friction zone

A positive gradient of mechanical properties during nitriding can be created due to the soft zone of the high-nitrogen nitride of the ε-phase, based on the low-nitrogen γʹ-phase. During nitrooxidation, the presence of an oxide film on the surface of a low-nitrogen layer consisting of carbonitride, oxycarbonitride and oxynitride phases is fully ensured. These phases form a high adhesive strength with an internal nitriding zone [12,13].

One of the criteria for achieving maximum wear resistance of nitrided coatings is to ensure high plasticity of the surface, with an increase in which the shear strength decreases. The plasticity of nitrided layers after oxidation due to the formation of an oxide layer on their surface of the nitride layer and the formation of low-nitride compositions of modified phase’s increases to 82 % than nitride layers obtained after nitriding, which is 74 %. (Table 1).

Oxidation of nitride layers by filling the micropores of the nitride sublayer increases the plasticity of the surface layers, which is one important factor for increasing the wear resistance of surface coatings. Especially when obtaining a dense oxide layer consisting of a single magnetite (Fe₃O₄) with a thickness of 1-3 microns on the surface of the nitride layer, acting as a barrier film preventing deazotization during isothermal exposure in water vapor leads to phase changes in the nitride layer. Phase transformations are mainly due to the dissociation of the high-nitrogen nitride of the ε-phase with its transition to the low-nitrogen γ'-phase. An increase in the proportion of the γ' phase in the nitride layer leads to an increase in hardness, as a result of which the coating acquires damping properties, which is important under lubrication conditions, which allows to preserve the intermediate film consisting of base metal oxides and oxides of alloying elements. The alloying of nitride phases with oxygen and carbon from the steel matrix gives new qualities to the nitride layers forming a modified oxycarbonitride ε"-phases whose properties are close to the γ'-phase.

To increase wear resistance, it is important to work on the surface, especially in the presence of lubrication, during which there is an increase in the actual contact area in the friction zone, which is accompanied by a decrease in specific pressures in the contact zone during the initial wear period to a steady state. The study of the K burn-in intensity coefficient on hardened samples, depending on the load in the range from 50 N to 300 N, shows that the K value has higher values in the presence of an oxide layer than samples treated using the classical gas nitriding method (Table 1, Option A). In particular, surface run-in occurs more intensively with a large thickness of the oxide layer (Table 1, Option B).

Especially the oxide layer has good wettability, which is necessary in conditions of sliding friction in the presence of a lubricant. Diffusion nitride oxide coatings, the oxide film in which provides a positive gradient of surface properties, good surface plasticity, better wettability and uniformity of the oxide film, can be used to control the process of running-in of the rubbing surface in sliding friction nodes both under dry friction and in the presence of lubrication [14].

Table 3 shows the values of the wear intensity and friction coefficients of nitride-oxide coatings having different phase compositions at varying pressures.

Table 3

Changes in the antifriction characteristics of wear resistance, coefficient of friction and surface roughness of nitride oxide coatings depending on the load on the counter body

The results of the tests carried out to determine the comparative antifriction characteristics of nitride and nitride oxide coatings showed that under conditions of external sliding friction in the presence of lubrication, the wear intensity of a nitride oxide coating with an oxide layer consisting of Fe₃O₄ (Table 3, Variants B and D) had the best wear intensities compared with nitrided ones (Table 3, Variant A) and with nitride oxide coatings with a surface oxide film consisting of a mixture of Fe₃O₄ and FeO (Table 3, Option D).

In nitride oxide coatings with the studied nitrided sublayers consisting of compositions of low-nitride phases, a decrease in friction coefficients is observed, and the temperature in the contact zone increases with an increase in the applied load from 60 °C to 85 °C, which at such temperatures does not occur structural and phase changes in the nitride sublayer during friction.

The transition from the initial state of the friction surfaces to the steady state is accompanied by complex irreversible phenomena occurring in a thin surface layer, which determines the transition of surface quality characteristics from the initial state to the operational mode. Therefore, the study of surface roughness in the initial state and in the friction process is important from the point of view of predicting the durability of friction pairs.

The study of the surface profilogram and the values of changes in the parameters of R_a, as well as the morphology of the treated surface show that the oxide layer during the burn-in process has a good polishing effect on the hardened surface, accompanied by an acceleration in achieving operational roughness during the friction process. The polishing effect of the oxide layer is strongly manifested with an increase in the contact load (Fig. 1).

 a)        b)

Figure 1. Friction surfaces during the run–in period of samples processed according to option: a – B; b - C.

In the process of friction, however, the oxide layer is worn off the surface and the nitride layer begins to wear out. After the wear of the oxide layer, the antifriction parameters change, the wear intensity decreases, the coefficient of friction tends to its minimum value and a uniform period of steady wear begins.

Figure 2 shows a scan from the friction surface and the elementary composition at the point of the "Spectrum 23", in which O, Si and Mn are detected. The presence of oxygen in the surface film and in the composition of the lubricant in the contact zone leads to the formation of secondary oxide structures, which, according to the author of the work [14], depending on the combination of secondary structures in friction pairs, the wear resistance of the rubbing unit changes in the best direction. On the surface, a positive gradient of properties is achieved by combining type I oxide structures, i.e. solid oxygen solutions, on the surface of one of the friction pairs and type II oxide structures – a chemical compound of oxygen on the other.

 а)     б)

Figure 2. A scan of the friction surface (a) and the elementary composition of the surface layer at the point "Spectrum 23" (b).

The decrease and stabilization of the coefficient of friction at the time of formation of oxide structures in sliding friction pairs has been experimentally confirmed for many materials, including iron and steel, and iron nitrides interact more actively with oxygen than the metal base.

Conclusion

By combining the processes of gas nitriding and steam oxidation, the quality of the resulting nitride layer can be significantly improved by subjecting it to phase transformations during oxidation. As a result of nitrooxidation, the forming surface oxide film has a beneficial effect not only on the corrosion properties of carbon steel products, but also improves the physic-mechanical properties of the diffusion coating, in particular:

- a surface oxide film with a sublayer consisting of compositions of low-nitrogen oxycarbonitride phases provides the best positive gradient of mechanical properties, good surface plasticity and wettability, as well as the intensity of the coating surface during sliding friction under lubrication conditions;

- the oxide layer in the nitride oxide coating prevents the surface from setting during the burn-in process and the presence of oxides of alloying elements in it additionally has a shielding effect relative to the basic oxide Fe₃O₄;

- the oxide film during running-in has a good polishing effect on the modified nitride sublayer in achieving operational surface roughness during friction, as well as reducing the wear rate of the nitride coating and reducing the coefficient of friction in the contact zone of the rubbing surfaces.

As a result of comparative tests, it was found that the nitride oxide coating, consisting of an oxide layer of magnetite _Fe3O4 with a thickness of 1-3 microns, with a nitride sublayer consisting of compositions mainly consisting of oxygen- and carbon-doped nitrides of the γ'-phase and oxycarbonitrides of the ε"-phase, has the best antifriction properties.

References:

1. Yu.M. Lakhtin, Ya.D. Kogan, G.I. Shpis and Z. Byomer. Theory and technology of nitriding. (Metallurgy, Moscow, 1991), pp. 320.
2. D. Руе. Practical Nitriding and Ferritic Nitrocarburizing (ASM International, 2003), pp. 256.
3. S.A. Gerasimov, L.I. Kuksenova, V.G. Lapteva Structure and wear resistance of nitrided structural steels and alloys. -M., Publishing House of Bauman Moscow State Technical University, 2012. pp. 518.
4. Ya.D. Kogan. Research works of the Yu.M. Lakhtin school in the development of modern technologies for surface hardening of machine parts and tools, Metallurgy and heat treat. of met. No5, 2010), pp. 5-14.
5. J. Bosslet & M. Kreutz, “TUFFTRIDE^®-/QPQ^®-Process”, promotional material available online: http://www.durferrit.com/media/pdf/Tenifer_QPQ_eng.pdf (Accessed: 07.10.2012).
6. L.G. Petrova, L.P. Shestopalova. Oxi-nitriding of alloyed steels with forming of nano-scaled oxide film. International Journal of Microstructure and Materials Properties 9(1) pp. 25–37. DOI:10.1504/IJMMP.2014.061057
7. Kh.K. Eshkabilov, Structural-phase transformations during nitro oxidation of steels and surface properties. E3S Web of Conferences 401, 03014 (2023).
8. Kh.К. Eshkabilov Gradient structural-phase states and properties of surface layer after nitro-oxidation. E3S Web of Conferences 401, 03012 (2023).
9. F.Z. Benlanreche and E. Nouicer, Improvement jf surface properties of low carbon steel by nitriding treatment, 6^th International Congress & Exhibition ARMAS2016, Vol.131, No. 1, 2017, pp. 20-23.  
10. Kh.К. Eshkabilov Gradient structural-phase states and properties of surface layer after nitro-oxidation. E3S Web of Conferences 401, 03012 (2023).
11. Kh.K. Eshkabilov. Workability and Microgeometry of Nitride Oxide Diffusion Coating on Structural Steels. AIP Conf. Proc. 3243, 020072 (2024) https://doi.org/10.1063/5.0247629.
12. L.G. Petrova, P.E.Demin. Surface Modification Techniques for Steel Components Working in Wear and Corrosion Conditions. Key Engineering Materials, 2022, 909, pp. 108–114. https://doi.org/10.4028/p-4k0zjs.
13. Yu.M. Lakhtin, Ya.D. Kogan. Structure and strength of nitrided alloys. –M.: Metallurgy, 1982.pp. 176.
14. B.I. Kostetsky. Friction, lubrication and wear in machines. – Kiev, Technika, 1970. pp. 395.