The paper briefly describes major thermal spray techniques used to spray functionally graded coatings such as atmospheric plasma spraying, high velocity oxy-fuel spraying, suspension and solution precursor plasma spraying, and finally low and high pressure cold gas spray method. The examples of combined spray processes as well as some examples of post spray treatment including laser and high temperature treatments or mechanical one, are described. Then, the solid and liquid feedstocks used to spray and their properties are shortly discussed. The reviewed properties of functional coatings include: (i) mechanical (adhesion, toughness, hardness); (ii) physical (porosity, thermal conductivity and diffusivity, thermal expansion, photo-catalytic activity), and; (iii) bioactivity and simulated body fluid (SBF) corrosion. These properties are useful in present applications of functionally graded coatings as thermal barriers, the bioactive coatings in prostheses, photo-catalytic coatings in water treatment, coatings used in printing industry (anilox and corona rolls). Finally, some of the future possible fields of functional thermal sprayed coatings applications are discussed, e.g., to coat polymer substrates or to use the cheap technology of low pressure cold gas spray method instead of expensive technology of vacuum plasma spraying to obtain bond coatings.

Based on the recent analysis of various databases, cold spray (CS), the newest method among thermal spraying technologies, has received the unabated attention of hundreds of researchers continuously since its invention in the 1980s. The significance of CS lies in the low process temperature, which usually ensures compressive residual stresses and allows for the formation of coatings on a thermally sensitive substrate. This paper concerns the low-pressure cold spray (LPCS) variant employed for forming metal matrix composites (MMCs) with high ceramic contents and all-ceramic coatings. At the very beginning, the influence of LPCS process parameters on deposition efficiency (DE) is analysed. In the next part, the most useful feedstock powder preparation techniques for LCPS are presented. Due to the combination of bottom-up powder production methods (e.g., sol-gel (SG)) with LCPS, the metal matrix that works as a binder for ceramic particles in MMC coatings can be removed, resulting in all-ceramic coatings. Furthermore, with optimization of spraying parameters, it is possible to predict and control phase transformation in the feedstock material. Further in the paper, differences in the bonding mechanism of metal–ceramic mixtures and ceramic particles are presented. The properties and applications of various MMC and ceramic coatings are also discussed. Finally, the exemplary direction of CS development is suggested.

The paper investigates the cavitation erosion (CE) and sliding wear (SW) resistance of cold-sprayed Al/Al₂O₃ and Cu/Al₂O₃ composites and studies them in relation to a set of metallic materials such as aluminium alloy (AlCu4Mg1), pure copper (Cu110), brass (CuZn40Pb2) and stainless steel (AISI 304). The coatings were deposited on stainless steel by low-pressure cold spray (LPCS) using Al (40 wt.%) and Cu (50 wt.%) blended with Al₂O₃ (60 and 50 wt.%, respectively) feedstocks. CE resistance was estimated by the stationary sample method according to the ASTM G32 standard. The SW test was conducted using a ball-on-disc tester with compliance to the ASTM G99 standard. Results obtained for the LPCS coatings show that the Cu/Al₂O₃ coating exhibits a denser structure but lower adhesion and microhardness than Al/Al₂O₃. The Al/Al₂O₃ and Cu/Al₂O₃ resistance to cavitation is lower than for bulk alloys; however, composites present higher sliding wear resistance to that of AlCu4Mg1, CuZn40Pb2 and stainless steel. The CE wear mechanisms of LPCS composites start at the structural discontinuities and non-uniformities. The cavitation erosion degradation mechanism of Al/Al₂O₃ relies on chunk material detachment while that of Cu/Al₂O₃ initiates by alumina removal and continues as layer-like Cu-metallic material removal. CE damage of metal alloys relies on the fatigue-induced removal of deformed material. The SW mechanism of bulk alloys has a dominant adhesive mode. The addition of Al₂O₃ successfully reduces the material loss of LPCS composites but increases the friction coefficient. Coatings’ wear mechanism has an adhesive-abrasive mode. In both CE and SW environment, the behaviour of the cold-sprayed Cu/Al₂O₃ composite is much more promising than that of the Al/Al₂O₃.

The study aims to elaborate a neural model and algorithm for optimizing hardness and porosity of coatings and thus ensure that they have superior cavitation erosion resistance. Al₂O₃-13 wt% TiO₂ ceramic coatings were deposited onto 316L stainless steel by atmospheric plasma spray (ASP). The coatings were prepared with different values of two spray process parameters: the stand-off distance and torch velocity. Microstructure, porosity and microhardness of the coatings were examined. Cavitation erosion tests were conducted in compliance with the ASTM G32 standard. Artificial neural networks (ANN) were employed to elaborate the model, and the multi-objectives genetic algorithm (MOGA) was used to optimize both properties and cavitation erosion resistance of the coatings. Results were analyzed with MATLAB software by Neural Network Toolbox and Global Optimization Toolbox. The fusion of artificial intelligence methods (ANN + MOGA) is essential for future selection of thermal spray process parameters, especially for the design of ceramic coatings with specified functional properties. Selection of these parameters is a multicriteria decision problem. The proposed method made it possible to find a Pareto front, i.e., trade-offs between several conflicting objectives—maximizing the hardness and cavitation erosion resistance of Al₂O₃-13 wt% TiO₂ coatings and, at the same time, minimizing their porosity.

Nanocrystalline TiO₂ coatings were successfully deposited on metallic substrate surfaces through low-pressure cold spray. Sol–gel amorphous TiO₂ dry gel was for the first time used as a feedstock powder. Agglomerated morphology of the amorphous powder with high-density packing of particles and reduced pores number was an important factor for successful low-pressure cold spraying. However, in the present study one more factor contributing to anchoring of TiO₂ particles by cold gas spraying onto a substrate was indicated. This factor enabling effective powder deposition and coating production is based on an exotic for conventional ceramic materials’ plastic deformation of amorphous powder. Furthermore, the detailed analysis of SEM, Raman and XRD measurements clearly demonstrated that amorphous titanium dioxide is transformed into crystalline anatase form during the process of low-pressure cold spraying.