Multicomponent coatings such as CrAlSiN, TiAlSiN, and CrAlSiWN are of practical interest on account of their mechanical properties, wear resistance, and resistance to oxidation [1–4].

The CrAlSiN coating consists of (Cr, Al)N crystallites in a matrix of amorphous silicon nitride [1]. Al atoms may dissolve in CrN, replacing the Cr atoms at lattice points; that obstructs the motion of dislocations [5]. Si atoms tend to segregate along the grain boundaries in the form of amorphous SixNy. That strengthens the grain boundaries and reduces the crystallite size [6]. As a result, the coatings are very hard, and they have distinctive tribological properties [7–9]. In addition, the coating is resistant to oxidation and chemically stable thanks to the protective oxide layer consisting mainly of Al2O3 [10]. Despite numerous studies of multicomponent coatings, there is little information regarding the change in chemical composition of the CrAlSiN coating over the depth.

Coefficient of friction of the CrAlSiN coating is relatively high [11]. That hinders its use without lubricants. However, researchers have not attempted to identify the additives most effective for use with the CrAlSiN coating.

In the present work, to establish the properties of the CrAlSiN coating, we investigate the variation in chemical composition over the depth by means of X‑ray photoelectron spectroscopy (XPS), and we investigate the interaction of the coating with adsorbent molecules by attenuated total internal reflection IR spectroscopy (ATR-IR spectroscopy).

To obtain ion-plasma coatings, we use the PLATIT π80 vacuum system (Switzerland). Before coating application, the sample surface is cleaned by means of argon ions for 5 min. Coatings are applied by means of two arc evaporators (rated heating power 10 kW at 300–450°C, voltage 20–30 V, arc discharge current 60–130 A). The bias potential at the substrate is 100–150 V; the vacuum is (1.3–4.7) × 10−2 mbar. The time of coating application is 180 min.

Tribological assessment of the samples employs a TRB system (Anton Paar, Austria), using a reciprocating module with a cylinder–plate configuration. The AISI 52100 chrome-plated steel cylinder (diameter 6 mm, length of generatrix 6 mm) is rigidly attached in a holder. The plate moves at a frequency of 10 Hz, with amplitude 1 mm. The load on the cylinder is 10 N. Since the cylinder is immobile, this is a test of slipping friction. The test duration is 100 000 cycles. The frictional drag is measured by a tensosensor and continuously recorded.

The surface is investigated by X-ray photoelectron spectroscopy on a system produced by SPECS Surface Nano Analysis (Germany). The exciting source is monochromatic AlKα radiation with energy 1486.6 eV. The spectral resolution of the system is 0.6 eV. The standard selected for precise determination of the binding energy employs the Fermi level and the line of the С 1s carbon level (energy 285 eV). The vacuum is 8 × 10−10 mbar. The sample dimensions are 10 × 10 × 2 mm. The interaction of organic molecules with the surface of the CrAlSiN coating is investigated by means of a Nicolet 380 FTIR spectrometer.

X-ray photoelectron spectroscopy establishes the chemical compounds in which specific elements are present at the surface. That permits prediction of the coating’s hardness and wear resistance. In Fig. 1, we show review spectra obtained from the surface of the CrAlSiN coating (1) and spectra obtained from the surface of the sample after ionic etching with argon (2−5), which are used to investigate the change in coating composition over the depth. In Fig. 1b, we show the electron spectra in the energy range 0–200 eV. It follows from Fig. 1 and Table 1 that the coating for the first 60 min of etching consists of chromium, aluminum, silicon, nitrogen, oxygen, and carbon (for 20 min). The etching rate with an accelerating potential difference of 3 kV is around 1.85 Å/s. Ionic profiling continues until iron lines from the steel substrate appear in the review spectrum of the coating.

Fig. 1.
figure 1

Review spectra obtained by X-ray photoelectron spectroscopy from the initial surface of the CrAlSiN coating (1) and spectra obtained from the surface of the sample after etching with Ar+ ions for 10 (2), 30 (3), 60 (4), and 90 (5) min, in the energy range 0–1400 eV (a) and 0–200 eV (b).

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Table 1. Elemental surface composition of CrAlSiN coating
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In Fig. 2, we show the XPS spectra of chromium atoms. Components А (Cr 2p3/2) and А1 (Cr 2p1/2) in the electron spectrum Cr 2р correspond to chromium in CrN (binding energy 575.7 eV). For the spin state of the chromium compound, on account of the 2p spin-orbital splitting, the information regarding its chemical state from the electron lines of components А (Cr 2p3/2) and А1 (Cr 2p1/2) is duplicated. Therefore, the components with subscript 1 will be disregarded. Component B at 577 eV corresponds to chromium oxide [12, 13].

Fig. 2.
figure 2

Cr 2p XPS spectrum from the initial surface of the CrAlSiN coating (1) and spectra obtained from the surface of the sample after etching with Ar+ ions for 10 (2), 30 (3), 60 (4), and 90 (5) min, in the energy range 570–595 eV.

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As is evident from Table 1, the concentration of the elements varies nonmonotonically over the coating thickness. At the surface in contact with the air, hydrocarbon contaminants are usually present, decreasing the spectral intensity and contrast. After etching for 30 min, the concentrations of oxygen and nitrogen atoms trend in opposite directions: the content of oxygen atoms is decreasing, while the content of nitrogen atoms is increasing. At the surface, the concentration of chromium atoms is around 23 at %. For the first 10 min, their content increases, reaching 30 at %. Such enrichment is explained by the removal of the layer of hydrocarbon contaminants. In a layer of thickness around 200 nm, corresponding to etching for 10–30 min, the concentration of chromium atoms declines (Table 1), and the fine structure of the chromium line changes. That is associated with coating formation in an atmosphere of nitrogen and argon. At that depth, the chromium is present in compounds with nitrogen. The presence of chromium atoms at the surface ensures hardness, strength, and corrosion resistance, but with slight loss of plasticity. Increase in the concentration of aluminum atoms at a depth of 400 nm impairs the deformational strength of the coating, which is associated with increase in hardness and wear resistance of the lower coating levels.

Modification of the metal surface is accompanied by change in adsorption activity of the surface layers. That affects the interaction between the metal and the lubricant. ATR-IR spectroscopy is a convenient method for investigating the adsorption on solid adsorbents and identifying the formation of transfer films and secondary surface structures in friction [13, 14].

The adsorption of different materials on the CrAlSiN coating is of interest. Understanding the specifics of adsorption on the surface permits appropriate choice of lubricant additives capable of producing the strongest boundary layers at the coating.

Most antifrictional additives contain a polar group, which is effectively adsorbed on metal surfaces, and an apolar component, which creates a boundary lubricant layer so as to prevent jamming of the contact surfaces. To study adsorption on a CrAlSiN coating, we use a model compound: ethanol. The coating surface is saturated with alcohol vapor for 5, 10, 20, 30, and 60 min.

In adsorption, ethanol absorption spectrum is practically unchanged. Therefore, this is a physical adsorption process. Hydrogen bonds between the alcohol molecules, which appear in the form of a broad absorption band at 3100–3500 cm−1, indicate the presence of a polymolecular layer adsorbed on the CrAlSiN coating (Fig. 3). The surface spectrum obtained after holding in ethanol vapor for 5 min is identical to that of the initial surface. No increase in the adsorption is seen even after holding for more than 20 min.

Fig. 3.
figure 3

IR spectra from the initial surface of the CrAlSiN coating (1) and after holding in ethanol vapor for 10 (2) and 20 (3) min.

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Thus, it follows from ATR-IR spectroscopy that the strongest bonds with the surface of the CrAlSiN coating are formed by polar molecules containing oxygen, nitrogen, sulfur, and phosphorus atoms, such as complex-forming additives. The use of such additives in oils operating in contact with a CrAlSiN coating improves the interaction between the contact surface and the lubricant, the formation of strong boundary layers, and improved contact performance [15, 16].

To confirm our findings, we conduct tribological tests of a CrAlSiN coating in an environmentally benign transmission oil developed for the tail gear of a helicopter, whose active component is a sulfur-bearing additive [17]. For comparison, we use TSgip mineral oil (Fig. 4).

Fig. 4.
figure 4

Tribological test data for a CrAlSiN coating in TSgip mineral oil (a) and in environmentally benign transmission oil (b): μ, coefficient of friction.

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The results indicate that the coefficients of friction are 30–40% lower in the environmentally benign transmission oil than in the mineral oil. This is confirmed by IR spectroscopic data, according to which polar additives form the strongest boundary layers at the surface of the CrAlSiN coating.

CONCLUSIONS

Analysis of the variation in concentration of various elements over the depth of a CrAlSiN coating indicates that the hardest and most wear-resistant layers lie at around 400 nm.

Polar complex-forming additives containing oxygen, nitrogen, sulfur, and phosphorus atoms are most effectively adsorbed on with the coating surface. If such additives are introduced to oils used with the CrAlSiN coating, strong boundary layers will be formed and the frictional properties at contact will be improved.