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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. [ABSTRACT FROM AUTHOR]

This paper discusses the influence of using 13 wt% of TiO 2 addition to Al 2 O 3 powder and selection of plasma spray parameters on morphology, microstructure, phase composition, mechanical properties, and, particularly, tribological behaviour and cavitation erosion performance of produced coatings. Two types of powders were used to spray the coatings by means of atmospheric plasma spraying (APS): Al 2 O 3 (AT0) and Al 2 O 3 + 13 wt% TiO 2 (AT13). The stand-off distance and torch scan speed were two spray parameters considered in this work. The aim of the study was to investigate how the addition of titania to alumina in agglomerated and sintered nanostructured powders could improve the coating microstructure and functional properties. The results demonstrated that addition of TiO 2 in the powder provides denser microstructure of the coating, improves the contact between splats, prevents the coating from severe fragmentation and spallation during wear, and prolongs the cavitation lifetime. In general, AT0 coatings exhibit slightly better wear resistance than AT13 equivalents. On the other hand, it was found that AT13 coatings present superior cavitation erosion resistance when compared with AT0. • Al 2 O 3 and Al 2 O 3 -TiO 2 coatings were deopisted by APS method • AT0 and AT13 coatings had a good wear resistance (generally less than 40 x 10-6 mm3/Nm) as well as cavitation erosion resistance • Mechanisms of the sliding wear and cavitation erosion were described. [ABSTRACT FROM AUTHOR]

In this work suspension-high velocity oxygen fuel spraying (S-HVOF) and suspension plasma spraying (SPS) were applied to produce Al 2 O 3 coatings using home-made and commercial available Al 2 O 3 water-based liquid feedstocks containing 40 wt% of submicrometer-sized raw powders. The aim of this study was to investigate the differences in the microstructure, mechanical properties and tribological performance of the suspension sprayed coatings with these two techniques. The microstructure of SPS coatings was slightly porous with fine particles of sintered, non-melted powders entrapped between the well melted lamellas. On the contrary, the S-HVOF coatings were very homogeneous in terms of morphology and of very low porosity in range of 1 to 4 vol%. With initial α–Al 2 O 3 powders, it was possible to spray α-phase rich coatings, both in the case of SPS and S-HVOF, as determined by X-ray diffraction, with up to 47 vol% of α–Al 2 O 3 in the S-HVOF coating and about 62 vol% in SPS one. The microhardness, adhesion strength and dry sliding wear resistance testing showed a good performance of coatings sprayed by using liquid feedstocks, especially the S-HVOF sprayed ones. Finally, the influence of feedstock characteristics and spray process conditions on the mechanical and tribological properties was analyzed. • Suspension plasma spraying (SPS) and suspension-based high velocity oxygen fuel spraying (S-HVOF) of alumina coatings. • Comparison of home-made and commercial water-based suspensions. • Studies on morphology, microstructure, porosity, phase composition, roughness, microhardness and wear resistance. • Strongly refined structure is crucial for tribological performance. [ABSTRACT FROM AUTHOR]

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 Al2−xTi1+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. [ABSTRACT FROM AUTHOR]

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 2k + 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. [ABSTRACT FROM AUTHOR]