**1. Introduction**

In medical applications, especially for hip and knee implants, a CoCr alloy is mostly used, due to good mechanical, anticorrosive and tribological characteristics [1–3]. CoCr alloys have also been used as screws in trauma plating systems. In this case, a low osseointegration, compared to Ti alloys, could facilitate an easier removal after the healing of the bone fracture. Due to its higher strength, a CoCr alloy was also used for idiopathic scoliosis applications, where the results proved to be better in the case of the CoCr alloy compared to stainless steel (SS) and a Ti alloy [4]. Another application was the use of a CoCr alloy in implants used for the correction of spine deformities, due to the high rigidity of the CoCr rod compared to SS- and Ti-based ones [5,6]. CoCr was also found to be well adapted in dentistry for its good castability, especially the wrought alloys. Guide wires, clips, orthodontic arch wires and catheters are among the main applications in this field [7,8]. Thus, the decreased corrosion resistance of SS and the low wear resistance of Ti alloys make it di fficult to replace CoCr alloys in a wide range of applications.

Despite these good properties, in clinical practice the implants made of a CoCr alloy exhibited a high rate of failure due to various complications after implantation: (i) high toxicity as a result of the migration of toxic metal ions; (ii) a high amount of wear debris surrounding the peri-implant tissues and body organs, mainly due to the low wear resistance in biological fluids; (iii) low bioactivity abilities; (iv) a non-hydrophilic surface [1,2]. During the friction process, wear debris of CoCr alloys were generated in di fferent size and shapes [9] and migrated to the periprosthetic tissues, leading to a failure of the implants. The wear of hip or knee joints is a complex process that involves many factors, such as the material and geometry of the implant, synovial fluid properties (various protein levels) and the patients' lifestyles and body weight. Thus, the wear particles larger than 0.5–10 μm (round to oval to irregular shapes) play a dramatic role as third-body wear, leading to an intense wear process [9–11]. The main problem with these debris is their high size, as cells (i.e., macrophages, fibroblasts, giant cells, neutrophils, lymphocytes, osteoclasts) will interact with these debris, leading to a chronic inflammatory response [9,12–14]. Another problem can be the release of Co and Cr ions into the synovial fluid and their correlated increased concentration in the blood. Although considered essential elements for the body, their increase in concentration can be detrimental for certain functions [15]. Thus, their long-term exposure can lead to cellular e ffects in the adjacent tissue and even to necrosis [16]. Nevertheless, in order to eliminate the mentioned disadvantages found in CoCr alloy medical implants, several modifications of alloy surfaces were carried out over recent decades by: (i) coating, using various types of techniques (PVD, CVD, ion implantation, plasma spray); (ii) surface structuring (laser processing, sand blasting, acid etching, anodization); (iii) micro arc oxidation; (iv) electrochemical oxidation [1,17]. The PVD method chosen for this study, namely the cathodic arc method, combines both a high degree of ionization of the ejected particles and a high e fficiency of the evaporation process. Even though the initial energies are about 20 eV for light elements and around 200 eV for heavy elements, the final ion velocities (in the range of 1–2 × 10<sup>4</sup> m/s) were found to be independent of the cathode material and ion charge state, due to electron–ion coupling [18,19]. Thus, with an enhanced atom mobility and surface di ffusion, due to the higher energy of the ions, the obtained materials have favorable conditions in order to obtain di fferent coating properties [20].

The present study aimed to analyze TiSiCN as a possible coating solution to improve the corrosion and wear resistance of CoCr alloys used for orthopedic implants. For comparison, TiCN and uncoated CoCr were used as control groups. The TiCN coating was selected as a reference because it has good mechanical properties and good corrosion resistance, and an acceptable wear resistance in dry environments [21–23]. By the addition of Si into TiCN, it was expected to significantly reduce the friction and wear process, as well as to improve the corrosion resistance of the CoCr alloy. It was reported that the addition of Si governs grain refinement and Si-containing coatings present superior friction and wear performance [24]. Furthermore, it was demonstrated that the addition of Si to various materials with biological applications enhances the proliferation and di fferentiation of human osteoblasts, accelerating the osseointegration process [25]. Additionally, Si–N thin films proved to have remarkable properties, which included high thermal stability and chemical inertness, in addition to those already mentioned [26]. A survey of the literature shows that TiSiCN coatings have a superior tribological performance, but tests were performed mainly in conditions used in industrial applications, such as cutting tools and the automotive industry. Their main advantages are low friction, high wear resistance, good mechanical properties such as toughness and high resistance on plastic deformation [27–32]. For medical applications, however, they have not ye<sup>t</sup> been tested. Nevertheless, an alternative solution was the addition of Zr and Cr to the Si–N and Si–C–N matrix for severe wear and corrosive applications [33].

In the present study, the coatings were obtained by the cathodic arc evaporation method on the CoCr substrates under a mixture of CH4 and N2 gases. The investigation included the examination of elemental and phase composition, texture, structure, morphology and mechanical properties (Young modulus, hardness, roughness, stress). Special attention was devoted to the corrosion resistance performed in a 90% DMEM +10% FBS solution, at 37±0.5 ◦C. In order to understand the damaging effect of the corrosion test, the morphology and roughness after corrosion were evaluated. Electrochemical impedance spectroscopy (EIS) was also performed in order to investigate the behavior of the proposed systems. Thus, this method gave an insight into the electrochemical processes which occurred at the material–electrolyte interfaces. The research was conducted in order to find new and improved structures as a better solution to optimize the safety and efficacy of biomaterials.

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