**1. Introduction**

Nodular cast iron is the most popular material used for piston rings. The extremely high and intricate thermal and mechanical load of them [1–4] 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 [12–16], laser cladded, and PVD/CVD ones [17–19]. 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 scu ffing 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 Cr6+ 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 coe fficient of the surface layer as well as its resistance against hydrogen wear both at dry and lubricated regime [24–28]. 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 MoS2, WS2, etc., are well known as e fficient 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 MoS2 and/or WS2 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.

#### **2. 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.


**Table 1.** Standardised Chemical Composition of S14 Nodular Cast Iron [% by Weight].

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 MoS2 or MoS2 + WS2 suspended in 10 mL of C3H7OH + 1 mL of C5H11OH 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 N2 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.


**Table 2.** Technological Details of Compared Processes Options.

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

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.

#### **3. 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 MoS2 and optionally WS2 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.

**Figure 1.** (**a**) External appearance of samples after spray coating and preliminary annealing. Sintering parameters: temperature: 300 ◦C, time: 2 h ramp 5 ◦C/min withstand for 1 h in N atmosphere; (**b**) Exemplary cross section of final microstructure of hybrid layer – option A.

SEM cross sections and EDS pictures are presented in Figures 2 and 3 for layers options A and B, respectively.

**Figure 2.** SEM Cross Sections and corresponding EDS maps of S, Mo and W distribution for layer option A.

<sup>10</sup>ΐm Sulfur S L΅1 Molybdenum Mo L΅1 Tungsten W L΅<sup>1</sup>

<sup>10</sup>ΐm Sulfur S L΅1 Molybdenum Mo L΅1 Tungsten W L΅<sup>1</sup>

**Figure 3.** SEM Cross Sections and corresponding EDS of S, Mo and W distribution maps for layer option B.

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.

**Figure 4.** (**a**) Comparison of friction coefficients obtained during tribological tests; (**b**) Comparison of the maximum depth of frictional tracks.

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

according to the parameters A. That low frictional effect is the result of layered microstructure of numerous and relatively deep distributed MoS2 and WS2 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 MoS2 and WS2 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 Cr6+ from the manufacturing of piston rings made of nodular iron.
