## 1 Introduction  
X-ray photoelectron spectroscopy (XPS) is an analytical technique that has been used for many years to study surfaces. The formation, development and application of the XPS method are described in detail in the widely known books by [PERSON] [1; 2] and in many other monographs and reviews published later [3; 4; 5; 6; 7; 8; 9; 10] and articles published over the past decade [11; 12; 13]. In modern conditions of rapid spectral equipment development and the emergence of high-intensity synchrotron radiation sources, XPS methods have been actively used to study nanostructured systems and materials. The electron exit-depth energy dependence data [14] make it possible to determine the atom distribution and their chemical state variation with depth without sample destruction. This allows us to study the inner structure of coating layers several nanometers thick and makes XPS the preferred non-destructive testing method for interfaces and complex nanosized coatings. The development of the systems of the effective sample charge compensation during the XPS measurement significantly expanded the studied object's range and allowed us to study the finely dispersed materials. In this work, XPS is used to study hidden layers and interfaces of pyrolytic tungsten carbide coatings on the multi-walled carbon nanotube (MWCNT) surface. MWCNTs are modern nanostructured materials with high chemical and heat resistance, conductivity, hardness and strength. In combination with a large surface, this makes MWCNTs a promising material for usage in various applications. Moreover, the MWCNT surface modification with inorganic and organic coatings allows us to change the properties of resulting composites. With a coating thicknesses of about several nanometers, the composites acquire unique physicochemical properties [15]. It is known that carbon nanotubes have properties similar to graphite, in particular, metals poorly wet their surface [16]. This means that without MWCNT surface treatment, it will be difficult to achieve good adhesion of metal-containing particles and coatings. Hybrid materials based on MWCNTs are the subject of intensive investigations [17] because of a large choice of the methods of their synthesis (electrochemical techniques, various deposition methods such as atomic layer deposition, pulsed laser deposition, physical and chemical vapor deposition, etc.). The MWCNT wall's modification with various metal-containing nanoparticles [18; 19] can expand the range of the nanotubes' functional properties [20], giving them the required magnetic [21], catalytic [22] and electronic [23] properties. The Cu\({}_{2}\)O/Cu/MWCNT nanocomposite has successfully proved to be a catalyst for the germanium tetrachloride reduction with hydrogen [24; 25]. The MWCNT surface decoration with iron nanoparticles was used for nanoelectronic device manufacture, data storage in magnetic media and in magnetic resonance imaging [26]. Nanostructured tungsten carbide has received significant attention because of its enhanced tribomechanical properties [27] as well as fuel cell application [28; 29; 30]. In chemical catalysis, tungsten carbide (WC) can be used as a catalyst in hydrogenation, dehydrogenation, isomerization and synthesis of hydrocarbon materials. For example, recently, [PERSON] et al. [31] reported that the hydrogenation ability of WC catalyst can be improved by synthesizing nano-sized WC. The use of the nano-WC catalyst for the hydrocarbons of vacuum residue caused the lowest coke formation and the highest yield of commercial liquid fuel products in comparison with commercially available bulk WC catalyst. Several researchers have demonstrated that a combination of nanostructured tungsten carbide with graphitic carbon--for example, carbon nanotubes--has found wide application in the synthesis of nanocomposites due to their superior physical and chemical properties. MWCNT-based hybrid materials with the surface decorated by tungsten carbides nanoparticles or thin films form a new class of functional carbon nanomaterials.