Low Frictional MoS 2 / WS 2 / FineLPN Hybrid Layers on Nodular Iron

: The paper presents the new concept of low frictional hybrid composite coatings on nodular cast iron. The structure of it is multilayer and consists of MoS 2 and / or WS 2 nanoinclusions embedded in the iron nitrides’ zone and relatively deep hard di ﬀ usion zone. It o ﬀ ers a low friction coe ﬃ cient as well as high wear resistance of coated parts. The details of technology as well as the mechanism of layer’s growth have been presented and discussed. The presented technology may be an interesting alternative for chromium-based galvanic coatings of piston rings made of nodular iron using Cr 6 + .


Introduction
Nodular cast iron is the most popular material used for piston rings. The extremely high and intricate thermal and mechanical load of them [1][2][3][4] require the application of advanced solutions, both for a bulk material [5] and surface layer [6,7], as well as for a lubrication regime [1,2,8,9]. In order to improve cooperation between the piston ring and the cylinder sleeve, various coatings are applied. Such coatings include chromium and/or chromium-molybdenum galvanic coatings [10] as well as flame [11] or plasma sprayed [12][13][14][15][16], laser cladded, and PVD/CVD ones [17][18][19]. The PVD coatings of machining tools [20,21], which are still developed, are generally not applicable for piston ring improvement, due to an adhesive nature of the interface between a coating and the original material. The most common protection of piston rings against scuffing and wear are chromium-based galvanic coatings. However, they are also the most dangerous ones for the employees as well as the natural environment, since Cr 6+ is used [22].
The nitriding process is also widely used for surface treatment of rings made mainly of steel [17,23]. It improves the friction coefficient of the surface layer as well as its resistance against hydrogen wear both at dry and lubricated regime [24][25][26][27][28]. More intricate is the issue of cast iron nitriding, due to the presence of graphite precipitation in the microstructure [29,30] Recently, the new non-equilibrium, low-pressure nitriding process (FineLPN) has been developed. It may be used for creation of fully controlled phase structure of nitrided case both on steel and cast iron due to dedicated neural network computer support [31].
Layered materials, like MoS 2 , WS 2 , etc., are well known as efficient solid lubricants [32,33]. They may be used in frictional contacts also at extremely high contact pressures as well as at elevated temperatures. The critical issue with the application of them for the improvement of piston rings frictional performance is the necessity to incorporate of them into a hard and strong matrix of a surface layer. Similar trials of manufacturing of multiphase, gradient surface layers have been presented in numerous papers [10,14,17,18]. The new concept of low frictional hybrid composite coatings [34] on nodular cast iron has been presented in the paper. The structure of it is multilayer and it consists of MoS 2 and/or WS 2 nanoinclusions embedded in iron nitrides zone and relatively deep hard diffusion zone. It offers low friction coefficient, as well as high wear resistance and fatigue strength of coated parts. The details of the multi-stage new technology, as well as mechanism of the layer's growth, have been presented and discussed.

Materials and Methods
The substrate material specimens for research were pieces of industrially manufactured piston rings (Φ117.5 mm × 2.68 mm × 4.60 mm) made of S14 grade nodular cast iron. The standardised chemical composition of used cast iron is shown in Table 1. According to the industrial technology, the rings have been hardened to obtain the matrix microstructure of tempered martensite free of carbide precipitations. The ca. 2 µm of micro-particles diameter of tungsten disulphide and molybdenum disulphide have been used as low frictional reinforcing phases. The creation of a low friction surface layer was multi-stage and the process has been conducted as follows. First, the slurry 1 g of MoS 2 or MoS 2 + WS 2 suspended in 10 mL of C 3 H 7 OH + 1 mL of C 5 H 11 OH has been prepared. Then, it has been used for the uniform spray coating of piston rings specimens. After the natural drying, the coated specimens have been annealed at 300 • C for 1 h in an inert N 2 atmosphere to sinter the cladded particles and to adhere them to the metallic substrate. Next, such preliminary prepared green compacts were thermo-chemically treated by FineLPN low-pressure nitriding [31] or alternatively, additionally treated by gas sulphonitriding in an active atmosphere containing ammonia gas and sulphur vapors [35]. Two options A and B of the multi-stage surface engineering process have been conducted and compared. The technological schedule and parameters of them are presented in the Table 2. The cross-section microstructures of specimens were observed having used the optical microscope Nikon MA200 (Nikon Instech Co., Ltd., Tokyo, Japan). The microstructure and chemical composition of surface layers were investigated also by using the scanning electron microscope (SEM) JEOL JSM-6610 LV (JEOL Ltd., Tokyo, Japan) equipped with the energy dispersion spectroscope (EDS) X-MAX 80 Oxford Instruments (Oxford Instruments Group, Abingdon, UK).
Dry friction tribological tests at oscillating movement have been conducted using ball on disc tribometer SRV Optimol Instruments Prüftechnik (Optimol Instruments Prüftechnik GmbH, Munich, Germany). The parameters of tests were as follows: 20 N regular load, 1 mm stroke, 20 Hz frequency, 1800 s total time of test, at temperature of 25 • C. Dry friction coefficient has been registered during the test, and the maximum depth of frictional tracks has been measured after it alike. Three Coatings 2020, 10, 293 3 of 7 specimens were investigated for each option. The average values of frictional coefficient have been determined from all plots' courses (total duration 5400 s). The average values of the maximum depth of frictional tracks as well as the standard deviation of results have been calculated too.

Results and Discussion
The external appearance of piston rings specimens after spray coating and preliminary annealing are shown in Figure 1a. The metallic surface has been homogenously coated by a tight coating of low frictional particles which adhere themselves strong enough, and also to the substrate. An exemplary final microstructure cross-section of the obtained hybrid low frictional layer is presented in Figure 1b. It consists of the following subzones: an externally grown compound zone containing low-friction particles of MoS 2 and optionally WS 2 embedded in iron nitrides ε -3 µm, white iron nitrides zone (−17.8 µm), partially containing FeS fine inclusions (−5.8 µm), and relatively deep (−154 µm) dark diffusive zone in the original microstructure. Such a multizone gradient structure of the surface layer should be beneficial from the point of view of frictional and antiwear properties of piston rings made of nodular cast iron.

Results and Discussion
The external appearance of piston rings specimens after spray coating and preliminary annealing are shown in Figure 1a. The metallic surface has been homogenously coated by a tight coating of low frictional particles which adhere themselves strong enough, and also to the substrate. An exemplary final microstructure cross-section of the obtained hybrid low frictional layer is presented in Figure 1b The analysis of SEM pictures and EDS maps confirmed the incorporation phenomenon of low frictional particles MoS2 and WS2 into the structure of outer zone of hybrid layers.
The probable incorporation mechanism of them is the growth of ε iron nitrides during FineLPN and sulphonitriding processes outside of the original metallic surface due to reciprocal diffusion of iron ions through cationic defects in nitrides structure [35]. After the B process, the outer compound zone that contains sulphur and molybdenum is relatively shallow and tight ( Figure 2). The application of additional sulphonitriding process after FineLPN treatment (option A) causes an important thickening as well as structural loosening of composite's outer zone that contains numerous and very fine inclusions of low frictional particles MoS2 and WS2, which are embedded in ε-nitrides porous matrix. That porous composite outer zone based on ε -iron nitrides matrix should be beneficial from a tribological point of view for both dry [25] and liquid friction conditions [26,36]. Additionally, a relatively deep (ca. 150 μm) and relatively hard (up to 700 HV) diffusive zone should decrease the development of frictional contact area and protect a nodular iron against hydrogen wear [24].  The low frictional and antiwear properties for optimized structure of hybrid layer (option A) have been confirmed by dry friction and oscillation tribological tests. The results of them are presented in Figure 4.  The specimens treated in the hybrid process according to the schedule and parameters of option A have shown an extremely low value of dry friction coefficient, 0.13, which is two-fold than the nonoptimum layers, e.g., those treated to the schedule and parameters of option B (Figure 4). Also, the linear wear results after tribological tests have been twice as less for specimens treated according to the parameters A. That low frictional effect is the result of layered microstructure of numerous and 10 μm 10 μm The analysis of SEM pictures and EDS maps confirmed the incorporation phenomenon of low frictional particles MoS 2 and WS 2 into the structure of outer zone of hybrid layers.
The probable incorporation mechanism of them is the growth of ε iron nitrides during FineLPN and sulphonitriding processes outside of the original metallic surface due to reciprocal diffusion of iron ions through cationic defects in nitrides structure [35]. After the B process, the outer compound zone that contains sulphur and molybdenum is relatively shallow and tight (Figure 2). The application of additional sulphonitriding process after FineLPN treatment (option A) causes an important thickening as well as structural loosening of composite's outer zone that contains numerous and very fine inclusions of low frictional particles MoS 2 and WS 2, which are embedded in ε-nitrides porous matrix. That porous composite outer zone based on ε -iron nitrides matrix should be beneficial from a tribological point of view for both dry [25] and liquid friction conditions [26,36]. Additionally, a relatively deep (ca. 150 µm) and relatively hard (up to 700 HV) diffusive zone should decrease the development of frictional contact area and protect a nodular iron against hydrogen wear [24].
The low frictional and antiwear properties for optimized structure of hybrid layer (option A) have been confirmed by dry friction and oscillation tribological tests. The results of them are presented in Figure 4.  The low frictional and antiwear properties for optimized structure of hybrid layer (option A) have been confirmed by dry friction and oscillation tribological tests. The results of them are presented in Figure 4.  The specimens treated in the hybrid process according to the schedule and parameters of option A have shown an extremely low value of dry friction coefficient, 0.13, which is two-fold than the nonoptimum layers, e.g., those treated to the schedule and parameters of option B (Figure 4). Also, the linear wear results after tribological tests have been twice as less for specimens treated according to the parameters A. That low frictional effect is the result of layered microstructure of numerous and 10 μm 10 μm The specimens treated in the hybrid process according to the schedule and parameters of option A have shown an extremely low value of dry friction coefficient, 0.13, which is two-fold than the non-optimum layers, e.g., those treated to the schedule and parameters of option B (Figure 4). Also, the linear wear results after tribological tests have been twice as less for specimens treated Coatings 2020, 10, 293 5 of 7 according to the parameters A. That low frictional effect is the result of layered microstructure of numerous and relatively deep distributed MoS 2 and WS 2 inclusions. They have been incorporated relatively deep into hard ε -iron nitrides matrix. Therefore, they are durable structural sources of solid lubricant for long frictional action in piston rings -engine cylinders systems.
The obtained tribological test results allow to conclude that optimized hybrid layers contain both MoS 2 and WS 2 low frictional inclusions, and are well aerated by double thermo-chemical treatment (FineLPN + sulphonitriding). Thus, such new technology may represent an interesting alternative to replace and exclude the chromium-based galvanic coatings using Cr 6+ from the manufacturing of piston rings made of nodular iron.

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The new hybrid, multistage technology-that consists of following stages: slurry coating, drying, sintering, FineLPN low pressure nitriding, and sulphonitriding thermochemical treatments-may be an interesting alternative for chromium-based galvanic coatings of piston rings made of nodular iron using Cr 6+ .

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The optimum tribological properties of hybrid layers have been obtained for option of two low frictional particles MoS 2 and WS 2 and additional sulphonitriding heat treatment for important thickening, as well as the structural loosening of outer composite zone.

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The extremely low dry friction coefficient (0.13) and low linear wear have been revealed for the optimum hybrid layer, which were ca. twice as less in comparison to the benchmark technological solutions.

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The optimised low frictional effect is the result of a layered microstructure composed of numerous MoS 2 and WS 2 inclusions, which are and relatively deeply distributed in a hard ε -iron nitrides matrix.

Conflicts of Interest:
The authors declare no conflict of interest.