# Mechanical Behavior of Bi-Layer and Dispersion Coatings
## 1 Introduction  
The carbon nanotubes (CNTs) belong to highly promising functional materials due to their extraordinary properties as electrical semiconductivity, high mechanical stiffness, tensile strength, high elasticity, significant thermal conductivity, and, in certain conditions, superconductivity [1, 2]. The CNTs may appear in different forms as single-, double-, and multi-wall carbon nanotubes (MWCNTs) [3], nanohorns, and nanobuds [1]. They find applications in different fields as many branches of nanotechnology, nanomedicine, membranes and biosensors, electrochemical, piezoelectric and gas sensors, capacitors, and transistors in electronics [3], and high-performance batteries [4, 5]. In particular, in medicine, they are proposed for drug targeting, cancer diagnosis, and treatment, as antibacterial and antifungal structures [2], and as gene carriers [6]. The most popular development direction of CNTs is their application in multi-component materials. The typical recent examples include cellulose--CNTs for methylene adsorption [7], nitrogen-doped graphene--CNTs for microwave absorption [8], polypyrrole--CNTs for storage devices [9], ZnO--CNTs dispersed in PU for better anti-corrosion resistance [10], cement [11, 12], and Al alloy reinforced with CNTs [13]. The specific optical and electrical properties, high specific surface area, high chemical activity, and significant mechanical behavior are the reasons for the development of a great number of such composite materials [14, 15]. CNTs can also form coatings, layers, or films. They have been, rather seldomly, proposed as single coating, e.g., to enhance the adsorption of albumins on Ti [16], in condensation heat transfer systems [17], to enhance biotribological resistance of Ti alloy [18], to mitigate biofouling [19], and in environmental protection [20]. However, theyare more often used as one of the layers in sandwich (hybrid) coatings, or as one of the components of the coating. The most frequently studied composite coatings are those based on CNTs and CNTs reinforced hydroxyapatite (HAp) [21; 22; 23], and HAp substituted by (Sr + Mg + Zn) [24], Hap-collagen-MWCNTs [25], and Hap-titanium-CNTs [26]. Other research focused on composite coatings such as tantalum oxide-CNTs [27], collagen-CNTs [28], collagen-CNTs-acrylic acid [29], CNTs-polysiloxane [30], and Al\({}_{3}\)Ti-Cu-SiC-CNTs [31]. As concerns the noble metals, it is important to note the electrodes obtained by atomic layer deposition and composed of MWCNTs, TiO\({}_{2}\), and Pt [32]. The possibility of interaction of Ni, Ti, Pd, and Au with CNTs was shown for different forms of carbon nanotubes [33]. Nanochitosan capped gold nanoparticles were also obtained [34]. The important role of nanogold in the immobilization of proteins associated with SARS-CoV-2 was shown by [PERSON] et al. [35; 36]. The use of noble or semi-noble metals is justified by their antibacterial properties. However, if silver is assumed to be a safe element to human cells [37], the higher potential risk comes from the use of nanocopper than two-valent copper ions, related to different mechanisms of their toxic effects [38]. On the other hand, rGO (reduced graphene oxide)--nanoCu scaffolds demonstrated high antibacterial efficiency [39]. The nanogold was shown to eliminate pathogenic bacteria and even cancer cells [40], but no data about the side effects of this element are known. The research studies described in this article are aimed at determining several properties of some new promising multi-component systems for medicine, which are based on MWCNTs. The titanium dioxide is added to the MWCNTs-based coating to enhance its mechanical behavior, while nanocopper is implemented to create the antibacterial activity of titanium implants. The MWCNTs-titanium oxide systems were developed for different purposes as nanocomposites obtained by sol-gel technique [41], by high energy ball milling [42], and by atomic layer deposition of titania on MWCNTs [43].