In this study, the deposition, microstructure, and resistivity of APS and HVOF sprayed Cr 2 O 3 -TiO 2 coatings is systematically investigated. Commercially available Cr 2 O 3 -rich feedstock powders are used along with five agglomerated and sintered experimental powders on the TiO 2 -rich side. Both processes are found to produce homogeneous, low-porosity coatings with phase compositions that can be changed by adjusting process parameters. Coating hardness and electrical resistivity are found to depend heavily on Cr 2 O 3 content.

This paper describes the properties and behaviors of alumina and titania coatings that have recently been produced using suspension spraying techniques. It examines coating microstructures and phase compositions are shows how they are influenced by different operating parameters and interactions. A selection of new experimental results obtained by the authors is also presented. In the case of Al 2 O 3 , the goal was to retain a high ratio of the thermodynamically stable α-phase. In the case of TiO 2 , the spraying process was optimized to preserve the anatase phase in order to obtain photocatalytically active layers.

This paper presents the results of corrosion investigations performed on thermal spray coatings with different compositions in the Al 2 O 3 -TiO 2 system. The coatings were deposited on stainless steel via APS and HVOF spraying and metallographically prepared cross sections and surfaces were characterized before and after corrosion testing in NaOH and H 2 SO 4 solutions at various temperatures. Opposite of expectations, APS coatings were found to be more corrosion resistant than the denser HVOF-sprayed layers.

Corrosion protection has long been a challenge to thermally sprayed coatings. Alumina is known for its good corrosion properties, although especially thermally sprayed alumina-based coatings corrode if exposed to certain environments. The coating therefore becomes the limiting part for the component lifetime. Besides the corrosion of the coatings themselves, corrosion of the substrate can result from the existing porosity in the coatings. This work shows the influence of corrosion on alumina-based coatings as a function of the TiO 2 content and the coating process used. The effects of corrosion are quantified to enable lifetime prediction and tailoring of protective coatings. The coatings were analyzed using electrochemical methods and were exposed for long periods to acids and bases at 85 °C. Corrosion properties of the coatings were compared with those of bulk parts, prepared by hot-pressing. An electrochemical method was developed to estimate interconnected of porosity as an alternative to long-term corrosion experiments.

Thermally sprayed titanium suboxide (TiOx) coatings are widely used in industrial applications due to their good tribological properties and their electrical conductivity. These properties are mainly dependent on the amount of oxygen in the crystal lattice. Oxygen defects lead to the formation of so-called Magnéli phases. The range of applications is limited by the fact that TiOx tends to reoxidize in many service conditions, especially at elevated temperatures. Also, the extreme conditions in the flame or torch used in the thermal spray process lead to undefined phase changes and defects. In the TiO 2 -Cr 2 O 3 system, Magnéli phases are also formed, but it is assumed that the properties do not change due to oxidation during spraying and subsequent use. This work shows the possibilities of the new coating materials. Powders with different TiO 2 and Cr 2 O 3 contents and prepared by different technologies were used for the investigation of coating properties. Experimental powders with defined phase compositions were prepared. The powders were thermally sprayed and the coatings investigated in terms of phase composition, microstructure, hardness, and abrasion wear resistance.

In result of the spray process normally a phase change from α-alumina (corundum) in the feedstock powder to predominantly α-alumina in the coating takes place. This is a well known but often neglected fact in the preparation and use of alumina coatings. This is of special interest since the high usage properties of thermally sprayed alumina coatings are generally derived from sintered alumina ceramics which consist of corundum. It is expected that the prevention of this phase transformation will significantly change the mechanical, electrical and other properties of thermally sprayed alumina coatings. There is controversial information about the possibility of stabilization of α-alumina by additions of chromia in the literature. In this work the stabilization using different spray processes (water stabilized plasma WSP, gas stabilized plasma APS and HVOF) was studied. Mechanical mixtures of alumina and chromia were used, as well as pre-alloyed powders consisting of solid solutions. The coatings were studied by X-ray diffraction and metallographic cross-sections. It was shown that in the case of the mechanically mixed powders the stabilization predominantly depends on the applied spray process. The stabilization by use of the WSP process was confirmed.