**1. Introduction**

One of the problems in the industrial application of moving bodies concerns the mechanical interaction between sliding surfaces and surface chemical reactions or corrosion occurring in reactive environments such as an aqueous media. A simplified description of tribo-corrosion phenomenon is related to a material transformation process due to simultaneous corrosion and wear taking place at contacting surfaces in relative motion [1].

Corrosion resistance is one of the most important factors to be taken into consideration for manufacturing metal products, as the formation of rust can have a devastating impact on the performance. Protection of the metal surfaces with physical vapour deposited coatings is a widely used technique. It could be assumed that such treatment will be even more relevant in future due to boosting of 3D metal printing technologies (additive manufacturing) [2]. Reliable lifetime prediction for a component used in an aqueous corrosive environment requires the identification of corrosion failure modes. Such failure modes can be pitting (if halide ions are present), stress-corrosion caused cracking by hydrogen embrittlement and corrosion fatigue [3].

The effect of mechanical stimulation on chemical degradation of materials and, vice-versa, the influence of corrosion on the mechanical response of contacting materials are of grea<sup>t</sup> concern for modern technologies including power generation, marine, and offshore industries. Materials properties, surface transformations, and electrochemical reactions are important aspects to be considered during materials selection for any specific application as cumulative effects of mechanical and chemical factors can result in unexpected behaviour and catastrophic loss of integrity. However, the chemo-mechanical mechanisms of tribo-corrosion are not ye<sup>t</sup> well-understood and are extremely complex as they involve a grea<sup>t</sup> number of parameters [1,4–6]. A realistic evaluation of materials reliability is further hindered by the experimental difficulties in process characterization. Moreover, the overall rate of material degradation is rarely the sum of just corrosion and wear but is influenced by multiple reactions and transformations that take place during tribo-corrosive interactions. Therefore, an attempt to use Raman spectroscopy as a non-destructive and relatively fast method for understanding processes of tribo-corrosion is of potential benefit [7].

Nowadays the use of protective coatings containing carbon, oxygen, or nitrogen (e.g., carbides, nitrides, carbonitrides, or oxynitrides) is considered to be a practical method for improvement of the performance of metals and alloys [8–10]. Transition metal nitrides ensure the high hardness, acceptable wear, and corrosion resistance when applied as physical vapour deposited (PVD) coatings to enable application under aggressive environments [11,12]. Dominating phase transition changes from cubic to hexagonal have been found in Al*x*Cr1−*<sup>x</sup>*N by increasing x up to about 0.71 [13]; however, this value has not been strictly defined.

In many cases, a ceramic coating cannot be applied directly to an SS substrate due to insufficient bonding efficiency. As the result of this, intensive delamination of a coating can take place. It is especially harmful, if emission of Cr containing particles takes place that can oxidize into a toxic and cancerogenic Cr(VI) [14]. Accordingly, an adhesive interlayer with as possible similar lattice parameters can be applied. In such situation, a process of inter-diffusion between coating and substrate may occur. Cohesive energy densities or solubility parameters should match according to thermodynamic considerations to attain good bonding between a substrate and an adhesive layer [15]. A combination of Fe and Cr satisfy these conditions as both have bcc structures. It is well known that specific interactions between the components enable blending the miscible materials [16]. The ideal work of adhesion properties of the Cr(100)/Fe(100) and Cr(110)/Fe(110) abrupt interfaces has been predicted to be about 5.4 J·m<sup>−</sup>2. Endothermic intermixing occurs at the interface of Cr film and Fe substrate, exhibiting a very strong adhesion caused by strong covalent bonding in addition to metallic cohesion and nearly lack of strain [14]. Intermixing causes a favourable concentration gradient transition zones distinguished by thermodynamic compatibility of a substrate-coating system [17].

Herein, the tribo-corrosive processes occurring at multilayered AlCrN PVD coatings deposited over stainless steel (SS) substrate demonstrating an applicability of Raman spectroscopy for determination of corrosion products and possible coating failures under static and tribologically initiated conditions is reported.

#### **2. Materials and Methods**
