**1. Introduction**

Modern civilization expects from material engineering scientists to produce lightweight and durable materials that meet the high strength and quality requirements set for innovative constructions made by the automotive and aerospace industries. Under certain structural load conditions, the increase of strength and stiffness of the materials contributes to reducing construction dimensions and consequently also the mass. Because the global oil resources are constantly declining, and renewable energy sources are not effective enough yet, use of lightweight and durable materials becomes a necessity. This type of materials is highly desirable in car, aircraft and space vehicle production because its use has many benefits, such as lower fuel consumption, higher capacity and speed. Insufficient strength and stiffness of the constructions made of metals and alloys led to the development of metal-matrix composites (MMC). The composites of this type achieve high strength and ductility thanks to the metallic matrix, while the stiffness is provided by the reinforcement, which consists of particles-perchance fibers-metallic or ceramic with high stiffness. The microstructure of this materials consists of soft matrix and hard phases which provides increase in abrasion resistance also at high temperatures. Metal-matrix composites can be designed to have specific properties, such as low thermal expansion coefficient and high thermal conductivity so that these materials are suitable for use in applications for installation of electronic microcircuits. Metal-matrix composite materials are widely used in car and air applications nowadays [1–3].

In the 1970s, technologies for producing high-strength carbon fibers were developed. They began to be used for the preparation of advanced composites used for producing rocket engine nozzles, projectile cores, thermal shields, isolators and thermal radiators. Since 1970, carbon fibre reinforced composites have been widely used in the production of aircraft brakes, space constructions, military and commercial airplanes, lithium-ion batteries and sports equipment. Research in the field of carbon materials has been revolutionized by the discovery of carbon nanotubes (CNT) by Sumio Iijima in 1991. The carbon nanotubes (CNTs) have unique mechanical properties compared to carbon fibers, e.g. stiffness to 1000 GPa, strength of 100 GPa and thermal conductivity to 6000 W/(m·K) [4,5]. In recent years, a number of studies have been carried out using CNT carbon nanotubes as reinforcement of various materials: polymers, ceramics and metals, with the majority of research involving polymer composites [6,7], ceramic composites in second place [8,9], and only recently have been published several papers on composites with metallic matrix reinforced with carbon nanotubes (CNT) [10,11]. This is quite surprising considering the fact that most construction materials used in the contemporary world are metals. Publications on this topic concern various aspects such as fabrication [12–15], microstructure [16,17], modelling of mechanical properties and the chemical interaction between carbon nanotubes (CNTs) and metals [4,18–21]

Nanotechnology had a strong influence on the direction of research in the field of surface engineering and related production technology of surface layers and coatings [22,23]. Nowadays, it is possible to use welding methods for producing not only conventional tribological coatings with specific frictional characteristics (high or low coefficient of friction) and resistance to wear, erosion or corrosion but also for producing coatings with unprecedented properties, often intended for special applications and working in difficult conditions, e.g., nanocomposite coatings with high hardness and high resistance to dynamic loads, coatings with frictional characteristics that adapt to changing operating conditions (temperature, humidity), thermal barrier coatings or biocompatible coatings [24,25]. Often, high-quality nanostructure coatings are used on parts of car engines made of aluminum alloys, on copper alloys intended for propellers of vessels, or on heat-resistant intermetals. In surface engineering technology, the implementation of this type of coating is possible by thermal spraying, where the applied metallic layer is bonded to the substrate adhesively or mechanically without melting the base material [24]. The main advantage of thermal spraying technology is a minimal thermal influence on the sprayed materials. Even in the case of laser cladding technologies characterized by the lowest heat input of all the cladding technologies, the substrate material is always partially melted, as well as the additional material, usually in a form of metallic or composite powder. The carbon nanotubes, due to the small dimensions, have very low heat capacity. Additionally, they have high absorption of laser radiation. For this reason, the introduction of carbon nanotubes into the melt pool during laser cladding is basically impossible, because overheating and decomposition of nanotubes [26–33].

Pioneers in the field of thermal spraying processes for composite coatings of aluminum-carbon nanotubes (CNT) were a research group from Florida International University, who successfully deposited carbon nanotubes in the Al-Si matrix in the powder plasma spraying process [34].

S. R. Bakshi and others [10] made multi-layer nanocomposite coatings of aluminum-carbon nanotubes (CNT) in the cold gas spraying process. In order to obtain a good dispersion of carbon nanotubes in Al-Si microparticle eutectic powders, spray drying was used. Spray-dried powders containing 5 wt.% carbon nanotubes (CNTs) were mixed with pure aluminum powder to obtain total nominal carbon nanotube (CNT) compositions in the coating material of 0.5 wt.% and 1 wt.%. As a result of cold spraying, coatings with a thickness of 500 µm were obtained in which the carbon nanotubes were evenly distributed in the matrix. The carbon nanotubes were of shorter length because during the deposition process they fractured as a result of impact and shear between the Al-Si particles and the aluminum matrix.

A. K. Keshri and others [11] compared impact on carbon nanotubes (CNTs) of various heat sources used during thermal spray processes—plasma spraying (PS), high-velocity oxy fuel spraying (HVOF), cold spraying (CS) and plasma spraying of liquid precursor (PSLP). Carbon nanotubes (CNTs) have been successfully preserved as reinforcements in composite metal and ceramic coatings in all thermal spray processes with the exception of PSLP.

There is no data in the literature regarding tribological properties of powder flame-sprayed (PFS) aluminum coatings reinforced with carbon nanotubes (CNT). The purpose of this article is to present the state of knowledge in this area of research and present the possibility of using powder flame spray technology (PFS) for the production of composite coatings with a metallic matrix reinforced with carbon nanotubes (CNT).

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

### *2.1. Aim of Study*

The conducted studies were aimed at comparing the structure, chemical composition, hardness and resistance to abrasive and erosive wear of powder aluminum flame-sprayed coatings reinforced with Nanocyl NC 7000 carbon nanotubes in amount of 0.5 wt.%, 1 wt.% and carburite in an amount of 0.5 wt.% with a reference coating made of aluminum powder EN AW 1000 series (Metallisation Ltd., West Midlands, UK) on non-alloy S235J0 steel. Carburite as aluminum matrix reinforcement was used in order to compare tribological properties of this composite coating with coating reinforced with CNTs with equal weight participation of carbon material. The scope of research included:

