*3.2. Coating Characterisation*

It was shown that, with the selected parameter set, it was possible to produce well adhering and dense coatings from all single oxides and their ternary blends. The results of the roughness measurements for the as-sprayed coatings shown in Figure 3 reveal clear differences. In the case of single oxides, the TiO*x* coating has the highest as-sprayed *R*z value with approximately 70 ± 2 μm. The Cr2O3 coatings show the lowest roughness with approximately 33 ± 2 μm. Coatings produced from the powder blends show roughness values between these limits. The as-sprayed Rz value of the ACT, CAT, and TAC coatings are 50 ± 4 μm, 49 ± 3 μm, and 55 ± 6 μm, respectively. By grinding and polishing the roughness of the samples, it can be reduced by more than 90%. As can be seen in Figure 3, the Rz values after polishing are in the range of 1.3 ± 0.2 μm for Cr2O3 and 3.2 ± 0.2 μm for Al2O3. The roughness of the TiO*x* coating can even be reduced by almost 98% by polishing and indicates particularly good machinability. The coatings from the blends show a similar behavior. The Cr2O3-rich CAT coating with 2.3 ± 0.5 μm have the lowest roughness. After polishing, all coatings have a comparable roughness, which makes them suitable for wear testing.

**Figure 3.** Roughness values of as-sprayed and polished coated samples.

The results of coating thickness and porosity after 10 passes are shown in Figure 4. The coating thickness gives an indication of the deposition rate of the single oxides and the blends. In the case of the individual oxides, the highest coating thickness (469 ± 16 μm) was measured for the TiO*x* coating, while the chromia coating has the lowest coating thickness. A similar observation was made by others [17]. For the coatings from the blended powders, the titanium oxide-rich TAC coating with 430 ± 18 μm shows significantly higher thickness than the ACT and CAT coatings, where the coating thickness is 342 ± 14 μm and 369 ± 16 μm, respectively.

**Figure 4.** Thickness and porosity of plain oxide coatings (A, C, T) and coatings from blends (ACT, CAT, and TAC).

When studying the porosity, it is noticeable that different levels exist between the individual oxides as a result of the identical spray parameters. While the porosity of the Al2O3 coating is about 5.0% ± 1.0%, the TiO*x* coating with a porosity of 1.6% ± 0.1% is significantly denser. As illustrated in Figure 4, the alumina-rich ACT and chromium oxide-rich CAT coatings have similar porosities of 3.7% ± 0.8% and 3.0% ± 0.5%, respectively. Only the TAC coating with a high content of TiO*x* shows a lower porosity of 1.7% ± 0.2%.

Low magnification SEM images of the cross sections of the coatings from the blends are shown in Figure 5a,c,e. The characteristic microstructure of thermal spray coatings, characterized by unmolten particles, pores, microcracks, and a lamellar structure, is observed. Splats from the individual oxide particles are clearly distinguishable. The high magnification images in Figure 5b,d,f show that the coatings of each blend consist of individual splats of a comparable grayscale denoted as I, II, and III in each of the images.

The results of the measurements of local chemical composition of these individual splats by EDS are compiled in Figure 6. The dark areas (SEM area I) in all coatings have a composition of approximately 60 at % oxygen and 40 at % aluminum, while the elements titanium and chromium were not detected. The bright areas (SEM area II) assigned to Cr2O3 contain a significant proportion of titanium in the range between 0.9 at %. and 2.2 at %. It is also noticeable that these Cr2O3 lamellae contain more oxygen than expected (slightly above 60 at %). Variations in the grayscales in these lamellas were found to be caused by variations of the oxygen content. The areas III with an intermediate grayscale relate to the titanium oxide splats.

**Figure 5.** SEM images (BSD detector) of APS Al2O3-Cr2O3-TiO2 composite coatings: top row: ACT-coating: (**a**) overview image, (**b**) detailed image with marked areas for the EDS measurement. middle row: CAT-coating: (**c**) overview image, (**d**) detailed image with marked areas for the EDS measurement. bottom row: TAC-coating: (**e**) overview image, (**f**) detailed image with marked areas for the EDS measurement. The EDS measurements of the marked areas are shown in Figure 6.

The results of the hardness measurements are presented in Figure 7 and show that all single oxide coatings have high hardness values above 1000 HV0.3, while the Cr2O3 coating has the highest hardness of 1250 ± 79 HV0.3. The coatings from powder blends do not reach the hardness of the plain chromia and alumina coatings. Only the chromium oxide-rich CAT coating has a hardness of 1074 ± 65 HV0.3, which is higher than the hardness of the TiO*x* coating. The lowest hardness was found for the ACT coating.

**Figure 6.** EDS analysis of composite coatings: chemical composition of the material areas marked in Figure 5b,d,f.

**Figure 7.** Hardness and sliding wear rates of plain oxide coatings (A, C, T) and composite coatings (ACT, CAT, TAC).

The results of the sliding wear tests are also shown in Figure 7. The Al2O3 coating shows the highest wear rate (8.2 ± 0.1 × 10−<sup>4</sup> mm3·N−1·m<sup>−</sup>1), which is more than twice than that of the TiO*x* coating. The wear resistance of the chromium oxide coating is so high that no meaningful wear rate was measured. The coatings from the blends also show a high wear resistance. The ACT and TAC coatings have a very similar wear rate. The chromium oxide-rich CAT coating proves to be more wear resistant.

The presentation of the XRD patterns, displayed in Figure 8, is limited to the range 2θ = 18◦–82◦. For the powder blends, the presence of α-Al2O3 (corundum), Cr2O3 (eskolaite), and different titanium oxide phases, as expected from the pattern of the individual oxides (see Figure 2) was found. The XRD patterns of the coatings from the blends, shown in Figure 8, reveal the presence of α-Al2O3, γ-Al2O3, Cr2O3 (eskolaite), and different titanium oxide phases. A shift of the peak positions relative to the standards did not occur. Due to the inhomogeneous distribution of oxygen, the intensity of the suboxide peaks is low. A quantitative determination of the phase is not possible. A change in the intensity of some peaks was observed. For the TAC coating, the TiO*x* peaks of 2θ = 26◦–30◦ lose significantly in intensity in the coating compared to the powders. The intensity of the TiO2 rutile peaks at 2θ = 27.4◦ (110) and 2θ = 54.2◦ (211) increases markedly. For all coatings, especially for the CAT and TAC coatings, a decrease of peak intensity of α-Al2O3 from powder to coating was detected.

**Figure 8.** Diffraction patterns of the powder blends and corresponding coatings: (**a**) ACT, (**b**) CAT, and (**c**) TAC.
