Microstructure and Sliding Wear Resistance of Plasma Sprayed Al 2 O 3 -Cr 2 O 3 -TiO 2 Ternary Coatings from Blends of Single Oxides

: Al 2 O 3 , Cr 2 O 3 , and TiO 2 are most commonly used oxide materials for thermal spray coating solutions. Each oxide shows unique properties comprising behavior in the spray process, hardness, corrosion, and wear resistance. In order to exploit the di ﬀ erent advantages, binary compositions are often used, while ternary compositions are not studied yet. Atmospheric plasma spraying (APS) of ternary compositions in the Al 2 O 3 -Cr 2 O 3 -TiO 2 system was studied using blends of plain powders with di ﬀ erent ratios and identical spray parameters. Coatings from the plain oxides were studied for comparison. For these powder blends, di ﬀ erent deposition rates were observed. The microstructure, roughness, porosity, hardness, and wear resistance were investigated. The formation of the splats from particles of each oxide occurs separately, without interaction between the particles. The exception are the chromium oxide splats, which contained some amounts of titanium. The predominant oxide present in each blend has a decisive inﬂuence on the properties of the coatings. While TiO x causes a low coating porosity, the wear resistance can be increased by adding Cr 2 O 3 .


Introduction
Thermally sprayed oxide ceramic coatings have an outstanding importance in many technological areas. Coatings sprayed from the single oxides and some commercially available binary compositions of the Al 2 O 3 -Cr 2 O 3 -TiO 2 system have multi-functional properties and are widely used as wear and as (sealed) corrosion resistant coatings. Depending on the composition, they are electrically insulating or conductive [1,2].
Plasma spraying and, in particular, conventional atmospheric plasma spraying (APS), is the most widely used thermal spray process for oxide coating manufacturing. Water stabilized plasma spraying (WSP) is a special high-energy process for coating manufacturing and is applied for special purposes, such as manufacturing of ceramic tubes. Coatings with excellent properties can be deposited by high velocity oxy-fuel spraying (HVOF), but often lower powder feed rates and deposition efficiencies are considered as drawbacks. More detailed descriptions of the spray processes used for oxide coating manufacturing are given elsewhere [2][3][4].
Most commonly feedstock powders with a typical particle size in the range 15-45 µm are used. The main feedstock powder manufacturing method for the single oxides is fusing and crushing. Table 1. Commercial single oxide feedstock powder grades, their particle size given by the supplier, and their granulometric data (own measurement). All powders were produced by fusing and crushing.  ACT  50  25  25  47  35  18  CAT  25  50  25  21  63  16  TAC  25  25  50  25  36  39 Feedstock powders were investigated with a scanning electron microscope (SEM) LEO 1455VP (Zeiss, Jena, Germany) with an acceleration voltage of 25 kV. By using a backscattered electron detector (BSD), the material contrast is visualized by different grey levels. In addition, phase composition was studied by X-ray diffraction (XRD) using a Bragg-Brentano geometry operating with Cu K α radiation with a D8 Advance diffractometer (Bruker AXS, Billerica, MA, USA) in a range of 2θ = 15 • -120 • with a step size of 0.02 • and 3 s/step.

Material
Low carbon steel (S235) samples with a diameter of 40 mm were used as substrates, which were grit blasted with alumina (EK-F 24) (3 bar, 20 mm distance, 70 • angle) and cleaned in an ultrasonic ethanol bath before applying the coating. The coatings were produced by atmospheric plasma spraying using an F6 torch (GTV, Luckenbach, Germany) and the spraying parameters, according to Table 3. In order to ensure good comparability, all coatings in this work were sprayed with this parameter set. The parameter set was chosen in such a way that a shift of the composition of the powder blends during processing was avoided. Interruptions of the coating process ensured that the substrate did not heat up to more than 200 • C. The cross sections of the coatings were prepared by the standard metallographic procedure. The analysis of the microstructure was conducted using an optical microscope GX51 (Olympus, Shinjuku, Japan) equipped with a SC50 camera (Olympus, Shinjuku, Japan) as well as by SEM with the same device used for powder analysis. The coating thickness was ascertained at 10 evenly distributed points. To determine the porosity, five images of the coating were evaluated by the image analysis method with the software ImageJ. Furthermore, the hardness of the coatings was measured on the cross sections using a Wilson Tukon 1102 device (Buehler, Uzwil, Switzerland). For this purpose, 10 Vickers indentations with a test load of 2.94 N were examined. The XRD patterns of the coatings were recorded with 3003 TT diffractometer from GE Inspection Technologies in a range of 2θ = 15 • -80 • with a step size of 0.03 • and 4 s/step. The high-resolution microstructure and the local chemical composition of individual splats in the coatings from blends were determined using a FESEM Ultra (Zeiss, Jena, Germany) equipped with an EDS detector X-Max80 and using a voltage of 13 kV. At least five measurements for the three typical individual splat compositions in each coating were performed. The calibration for the quantitative EDS analyses were performed using a cobalt standard.
The processing of the coatings by grinding and polishing was investigated. All coated samples were simultaneously ground on grinding wheels up to a number of 1200 for 3 min at 300 rpm and 25 N. Polishing was then carried out with diamond suspensions down to 1 µm at 200 rpm and a slightly reduced force in order to obtain comparable surfaces for the wear tests. The roughness of as-sprayed and ground coatings was recorded with a tactile profilometer (Jenoptik, Jena, Germany) with five measuring tracks. The characterisation of the sliding wear behavior of the coatings was carried out using a ball on disc test (Tetra, Ilmenau, Germany) using the parameters summarized in Table 4. For each coating, three wear tracks were generated. The evaluation of the wear tracks with regard to the wear depth and the wear volume was carried out using a 3D profilometer MikroCAS (LMI, Teltow, Germany).

Feedstock Powder Characterisation
The typical angular morphology of the fused and crushed powder particles is shown in the SEM micrograph in Figure 1. The granulometric parameters d 10 , d 50 , and d 90 , which are also given in Table 1, confirm only minor differences of their particle sizes, which makes them suitable for blending.

Feedstock Powder Characterisation
The typical angular morphology of the fused and crushed powder particles is shown in the SEM micrograph in Figure 1. The granulometric parameters d10, d50, and d90, which are also given in Table  1, confirm only minor differences of their particle sizes, which makes them suitable for blending.  According to the XRD measurements, as shown in Figure 2, the alumina and chromia feedstock powders consist only of α-Al 2 O 3 (corundum) and Cr 2 O 3 (eskolaite), respectively. In the SEM investigation of the chromia powder, small amounts of metallic chromium were detected, which do not appear in the XRD pattern. A large number of peaks in addition to those of rutile were detected for the TiO x powder. These additional peaks, which are not labelled in the pattern in Figure 2, belong to substoichiometric titanium oxide phases. The presence of the (110) peak of rutile at 2θ = 27.4 • indicates that local concentrations of oxygen are closed to stoichiometry, while, in other regions, disordered oxygen-deficient structures exist. According to the gravimetric measurements in another study, x in TiO x powder is about 1.9 [19].
Coatings 2020, 10, x FOR PEER REVIEW 5 of 12 According to the XRD measurements, as shown in Figure 2, the alumina and chromia feedstock powders consist only of α-Al2O3 (corundum) and Cr2O3 (eskolaite), respectively. In the SEM investigation of the chromia powder, small amounts of metallic chromium were detected, which do not appear in the XRD pattern. A large number of peaks in addition to those of rutile were detected for the TiOx powder. These additional peaks, which are not labelled in the pattern in Figure 2, belong to substoichiometric titanium oxide phases. The presence of the (110) peak of rutile at 2θ = 27.4° indicates that local concentrations of oxygen are closed to stoichiometry, while, in other regions, disordered oxygen-deficient structures exist. According to the gravimetric measurements in another study, x in TiOx powder is about 1.9 [19].

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 TiOx coating has the highest as-sprayed Rz 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 TiOx 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 Cr2O3rich 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.
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 TiOx 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. 20 30 40 50 60 70 80

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 Rz value with approximately 70 ± 2 µm. The Cr 2 O 3 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 Cr 2 O 3 and 3.2 ± 0.2 µm for Al 2 O 3 . 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 Cr 2 O 3 -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.  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 TiOx 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 TiOx 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 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. 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 TiOx 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 TiOx 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. 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 Al 2 O 3 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. 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 Cr 2 O 3 contain a significant proportion of titanium in the range between 0.9 at %. and 2.2 at %. It is also noticeable that these Cr 2 O 3 lamellae contain more oxygen than expected (slightly above 60 at %). Variations in the grayscales in these  Figure 5b,d,f.
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 TiOx coating. The lowest hardness was found for the ACT coating.
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 −4 mm³·N −1 ·m −1 ), which is more than twice than that of the TiOx 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 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 Cr 2 O 3 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.
Coatings 2020, 10, x FOR PEER REVIEW 8 of 12 Figure 6. EDS analysis of composite coatings: chemical composition of the material areas marked in Figure 5b,d,f.
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 TiOx coating. The lowest hardness was found for the ACT coating.
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 −4 mm³·N −1 ·m −1 ), which is more than twice than that of the TiOx 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 results of the sliding wear tests are also shown in Figure 7. The Al 2 O 3 coating shows the highest wear rate (8.2 ± 0.1 × 10 −4 mm 3 ·N −1 ·m −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-Al 2 O 3 (corundum), Cr 2 O 3 (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-Al 2 O 3 , γ-Al 2 O 3 , Cr 2 O 3 (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 TiO 2 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-Al 2 O 3 from powder to coating was detected.
Coatings 2020, 10, x FOR PEER REVIEW 9 of 12 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 TiOx 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. ACT

Discussion
Spraying of blends containing large powder particles has the disadvantage that only one component can be sprayed with a truly optimized parameter set. For ternary blends, this is naturally even more difficult as for binary ones. By this reason, for this study, an identical parameter set was used, which represents a compromise for the processability of all oxides. In case of the blends of the Al 2 O 3 -Cr 2 O 3 -TiO 2 system, two high melting components (Al 2 O 3 and Cr 2 O 3 ) are combined with one lower melting oxide TiO x . Thus, the liquid phase occurs at lower temperatures compared to TiO 2 and the addition of TiO x to Cr 2 O 3 and Al 2 O 3 decreases the coating porosity. For Al 2 O 3 -rich and Cr 2 O 3 -rich powder blends (ACT and CAT), a part of the large particle fraction is not melted to the extent that it can contribute to coating build up due to the higher melting temperature. As indicated by the coating thickness after 10 passes (see Figure 4), the highest deposition efficiency is found for the TAC coating.
The XRD investigations show that the typical phase transformation from α-Al 2 O 3 to γ-Al 2 O 3 as described in Section 1 occurs for all powder blends. The additions of Cr 2 O 3 and TiO 2 for the given spray conditions do not have an effect on this phase transformation, which aligns with the literature data [2]. In addition, reduced peak intensities related to sub-stoichiometric phases and an increase of the rutile peaks intensity at 2θ = 27.4 • (110) and 2θ = 54.2 • (211) can be an indication for oxygen gain during the processing and formation of stoichiometric TiO 2 or near stoichiometric TiO x as a result of the thermal spray process.
Recently, a surprising high dissolution of titanium atoms in γ-Al 2 O 3 in APS coatings obtained from a commercial Al 2 O 3 -40% TiO 2 powder blend was observed [19]. Surprisingly, this was not observed in this study for any of the ternary blends.
In the literature, the formation of a Cr 2 O 3 -rich solid solution (Cr,Ti) 2 O 3 is described for binary suspensions [14], while, for blends of large particles, contradictory results are reported [17,20]. In this study, the EDS measurements have shown the presence of small amounts of titanium in the chromium oxide lamellae. The titanium content in the chromia splats of the ACT and TAC coatings was below 2 at % but is slightly higher in the TiO x -rich TAC coating. The higher titania content increases the number of interfaces between titanium and chromium oxide splats. Therefore, more titanium diffuses into the chromium oxide lamellae.
Since the hardness of the Cr 2 O 3 coating was the highest, it is not surprising that the hardness is increased with a rising content of Cr 2 O 3 . The Cr 2 O 3 -rich CAT coating shows the highest hardness for the ternary blends. As expected, the Al 2 O 3 -rich ACT coating and the plain Al 2 O 3 coating show higher hardness values than TiO 2 -rich coatings. However, the titanium oxide-rich TAC coating has a similar hardness compared to the ACT coating. The lower porosity of the TAC coating is assumed to be responsible for this effect.
The sliding wear rate of Al 2 O 3 coatings can be significantly reduced by adding TiO x and Cr 2 O 3 . Both coatings have a higher wear resistance than the plain Al 2 O 3 coating. Possible reasons could be the improved toughness of the coating expected by adding TiO x [21] and/or addition of Cr 2 O 3 , which has, by far, the highest wear resistance. Except for the Al 2 O 3 coating, coating hardness and sliding wear rates show some correlation especially for coatings from the blends. Whereas ACT and TAC coatings have similar hardness values and wear rates, the higher content of Cr 2 O 3 in the CAT coating leads to significantly higher hardness values and a reduction in the wear rate. It should also be taken into account that the sliding wear rates of the plain oxide could be influenced by the identical spray parameter set, which is applied to the deposition of all coatings in this study.
All investigations reveal that the individual properties of the plain oxide strongly influence the properties of the coating from the blends. Therefore, for example, increasing the TiO x content leads to denser coatings and higher deposition rates as well as higher as-sprayed roughness. The increase of the Cr 2 O 3 content positively influences the sliding wear resistance.

Summary and Conclusions
It was shown that firmly adhering coatings can be produced by atmospheric plasma spraying from powder blends containing Al 2 O 3 , Cr 2 O 3 , and TiO 2 using an identical spray parameter set. Investigations with XRD have shown that the phase transformation from α-Al 2 O 3 to γ-Al 2 O 3 also occurs in the powder blends and is not influenced by the addition of Cr 2 O 3 or TiO x . Furthermore, for titania, a gain of oxygen content was found. EDS measurements have shown the existence of small amounts of titanium in the chromium oxide lamellaes. The investigations have shown that the respective dominant single oxide has a significant influence on the coating properties. Whereas a high TiO x content leads to higher deposition rates, higher as-sprayed roughness, and low porosities, the hardness and wear resistance of the coatings can be improved by increasing the Cr 2 O 3 content. By using powder blends, the sliding wear rate can be improved when compared to plain Al 2 O 3 coatings. Thus, the use of powder blends presents a promising approach to adapt or extend the property profile of plain oxide coatings. The reactivity between the materials involved needs to be further investigated in order to exploit further improvement potentials.