Thermally sprayed ceramic coatings are applied for the protection of surfaces that are exposed mainly to wear, high temperatures, and corrosion. In recent years, great interest has been garnered by spray processes with submicrometric and nanometric feedstock materials, due to the refinement of the structure and improved coating properties. This paper compares the microstructure and tribological properties of alumina coatings sprayed using conventional atmospheric plasma spraying (APS), and various methods that use finely grained suspension feedstocks, namely, suspension plasma spraying (SPS) and suspension high-velocity oxy-fuel spraying (S-HVOF). Furthermore, the suspension plasma-sprayed Al₂O₃ coatings have been deposited with radial (SPS) and axial (A-SPS) feedstock injection. The results showed that all suspension-based coatings demonstrated much better wear resistance than the powder-sprayed ones. S-HVOF and axial suspension plasma spraying (A-SPS) allowed for the deposition of the most dense and homogeneous coatings. Dense-structured coatings with low porosity (4 vol.%) and good cohesion to the metallic substrate, containing a high content of α–Al₂O₃ phase (56 vol.%) and a very low wear rate (0.2 ± 0.04 mm³ × 10⁻⁶/(N∙m)), were produced with the S-HVOF method. The wear mechanism of ceramic coatings included the adhesive wear mode supported by the fatigue-induced material delamination. Moreover, the presence of wear debris and tribofilm was confirmed. Finally, the coefficient of friction for the coatings was in the range between 0.44 and 0.68, with the highest values being recorded for APS sprayed coatings.

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.

The Al₂O₃ + TiO₂ coatings are of the interest of surface technology and tribology due to their good wear resistance and enhanced toughness comparing to pure Al₂O₃ coatings. However, the detailed effect of the used feedstock powder, is often neglected. Here, this work focuses on the deposition of Al₂O₃ + 13 wt % TiO₂ coatings by atmospheric plasma spraying (APS) method as well as on their microstructure, phase composition and selected mechanical properties, in the reference to the route of the powder feedstock preparation. The commercial powder Metco 6221 in agglomerated and sintered form was used as a feedstock material during spraying, due to the fact that, so far, sintered or cladded powders are the most studied ones. The 2^k + 1 spray experiment allowed to evaluate the influence of two variables, namely spray distance and torch linear velocity, on the coating microstructure. Afterwards, the coating adhesion was measured by the means of pull-off test. The correlations between Vickers microhardness, fracture toughness (Kc) as well as the coating morphology and phase composition were investigated. Finally, the dry sliding wear resistance was studied by using Ball-on-Disc method.

In this work, the alumina (Al₂O₃) and alumina-titania coatings with different contents of TiO₂, i.e., Al₂O₃ + 13 wt.% TiO₂ and Al₂O₃ + 40 wt.% TiO₂, were studied. The coatings were produced by means of powder and liquid feedstock thermal spray processes, namely atmospheric plasma spraying (APS), suspension plasma spraying (SPS) and suspension high-velocity oxygen fuel spraying (S-HVOF). The aim of the study was to investigate the influence of spray feedstocks characteristics and spray processes on the coating morphology, microstructure and phase composition. The results revealed that the microstructural features were clearly related both to the spray processes and chemical composition of feedstocks. In terms of phase composition, in Al₂O₃ (AT0) and Al₂O₃ + 13 wt.% TiO₂ (AT13) coatings, the decrease in α-Al₂O₃, which partially transformed into γ-Al₂O₃, was the dominant change. The increased content of TiO₂ to 40 wt.% (AT40) involved also an increase in phases related to the binary system Al₂O₃-TiO₂ (Al₂TiO₅ and Al_2−xTi_1+xO₅). The obtained results confirmed that desired α-Al₂O₃ or α-Al₂O₃, together with rutile-TiO₂ phases, may be preserved more easily in alumina-titania coatings sprayed by liquid feedstocks.

The ceramic coatings based on mixture of Al ₂ O ₃ and TiO ₂ have better properties in comparison to the pure alumina ones. Among many techniques, plasma spraying is very useful method of ceramic coatings manufacturing. In this paper, the results of microscopic, mechanical and tribological properties investigations of Al ₂ O ₃ + 13 wt% TiO ₂ coatings manufactured by atmospheric plasma spraying are presented. The cylinder substrates made from stainless steel (X5CrNi18-10) had a diameter equal to 25 mm and thickness equal to 2 mm. The plasma spray experimental parameters included three variables: (i) type of injection system (external or internal), (ii) size of corundum particles for sandblasting and (iii) torch linear speed. The results confirm, that type of injection system is a dominant parameter. Internal injection results in better degree of particles melting, what influences on wear resistance performance, as well as higher values of bond strength.