Microstructure and Sliding Wear Resistance of Plasma Sprayed Al₂O₃-Cr₂O₃-TiO₂ Ternary Coatings from Blends of Single Oxides

Abstract

Al₂O₃, Cr₂O₃, and TiO₂ 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 different advantages, binary compositions are often used, while ternary compositions are not studied yet. Atmospheric plasma spraying (APS) of ternary compositions in the Al₂O₃-Cr₂O₃-TiO₂ system was studied using blends of plain powders with different ratios and identical spray parameters. Coatings from the plain oxides were studied for comparison. For these powder blends, different 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 Cr₂O₃.

1. 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 Al₂O₃-Cr₂O₃-TiO₂ 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. For avoiding the formation of metallic chromium for Cr₂O₃ sintering and crushing is also common. For the commercially available binary compositions, the variety of the manufacturing methods are significantly broader. This includes mechanical blends of separately fused and crushed single oxide powders, jointly fused and crushed powders, but also agglomerated and sintered powders. Depending on the oxide particle or grain size, there are large differences in the homogeneity of mixing of the metallic elements in these methods. The homogeneity is lowest in case of the powder blends from single oxide powders and highest in the case of agglomerated and sintered powders, where finely dispersed oxide powders are mixed in the manufacturing process. Suspensions as feedstock become increasingly important, but are currently limited to single oxides [5]. Suspensions with two components are under investigation [6].

Each of the single oxides shows a specific material behavior during spraying, which is detrimental to the processing and/or coating properties [1,2].

For alumina, the detrimental phase transformation from α-Al₂O₃ (corundum) existing in the feedstocks to metastable phases, predominantly γ-Al₂O₃, in the coatings is well known [7,8,9,10]. The reason is the high cooling rate and nucleation of undercooled melt. Some content of the remaining α-Al₂O₃ in the coating is usually explained by the occurrence of non-molten particles [7]. An increase of the α-Al₂O₃ content in the coatings is described as an important measure to improve the coating properties, such as wear, electrical, and corrosion resistance. Except the addition of other oxides (e.g., Cr₂O₃ [8,9,10]), there are several technological measures for this, as the selection of special spray process conditions [11], use of suspensions [1,5] or heat post-treatments [12] as well as plasma-electrolytic oxidation of arc-sprayed and flame-sprayed aluminum coatings [13]. However, each of these technological measures has certain limitations and the stabilization of the α-Al₂O₃ by a tailored powder would be favored.

Chromia has a low deposition efficiency due to oxidation from Cr₂O₃ to volatile CrO₃, which immediately reconverts to Cr₂O₃ when cooling down [1,2,14]. Formation of Cr(VI)-oxyhydrates in wet atmospheres is another detrimental reaction [14,15]. Although the appearance of hexavalent chromium is below the maximum allowable concentrations under normal process conditions, increased safety regulations are permanently under discussion. By adding Al₂O₃ [16] or TiO₂ [1,2] to Cr₂O₃ and forming respective solid solutions, both the formation of hexavalent chromium is suppressed and the deposition efficiency is increased.

Since titania TiO₂ readily loses oxygen in a reducing environment such as during the fusion step of feedstock manufacturing, fused and crushed feedstock powders are non-stoichiometric and preferably designated as TiO_x. The oxygen content can also change during the spray process. The oxygen defects are often disordered in coatings under the strong nonequilibrium conditions of the thermal spray process, but also often forms ordered-structures for certain O/Ti ratios (Magnéli-phases) in feedstock powders, manufactured with lower cooling rates. Due to a eutectic with an oxygen content corresponding to x of about 1.78 in the Ti-O phase diagram (in the two-phase region of the Magnéli-phases Ti₄O₇ and Ti₅O₉). The temperature of appearance of a melt is decreased from 1857 °C for TiO₂ down to 1679 °C at the eutectic point [2,17].

It was found that the addition of a second oxide can improve the coating properties [1,2]. There are some indications in the literature that the coating properties can be further improved by ternary compositions and by the addition of the third oxide of the system. Examples of such potential improvements can be the stabilization of target phases, an acceleration of the formation of a compound and improved sintering properties. This can relate to the feedstock powder manufacturing step and/or the spray process.

This addition can be made in different ways, e.g., during the feedstock manufacturing process (joint processing in fused and crushed as well as agglomerated and sintered powder manufacturing) or by blending single oxide powders. The latter case is the simplest one, and blended powders have a high importance in the current industrial practice of thermal spraying. This is valid for commercially available blends in the Al₂O₃-TiO₂ system, such as Al₂O₃-40%TiO₂ and Al₂O₃-13%TiO₂ [18,19], but also for blending at the production site [20]. In general, an interaction between components of the blends will occur only during the spray process and is, in general, expected to be low due to the large particle size. However, an intensive interaction during the spray process was found in the case of an Al₂O₃-40%TiO₂ blend [21]. Another example are blends of alumina-rich compositions of the Al₂O₃-Cr₂O₃ system, which can lead to a stabilization of the α-Al₂O₃ during WSP [10]. Blends of the binary Cr₂O₃-TiO₂ system were investigated as well [17,20]. However, ternary compositions of the Al₂O₃-Cr₂O₃-TiO₂ system were not studied neither as powder blends, pre-alloyed powders, nor suspensions.

In this work, the interaction of single oxide ternary powder blends of the Al₂O₃-Cr₂O₃-TiO₂ system during APS and their sliding wear properties are studied. Coatings from the single oxides are investigated for comparison.

2. Materials and Methods

Commercial fused and crushed Al₂O₃, Cr₂O₃, and TiO_x powder grades, compiled in

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.

Material	Supplier	Particle Size	Granulometric Data
			d10	d50	d90
Al2O3	Saint Gobain Coating Solutions, Avignon, France	−45 + 15 µm	21	34	56
Cr2O3	GTV, Luckenbach, Germany	−45 + 15 µm	19	34	54
TiOx	Ceram, Albbruck-Birndorf, Germany	−45 +20 µm	22	39	61

, were used in this work. The particle size distribution was determined by laser diffraction analysis in a Cilas 930 device (Cilas, Orléans, France). To prepare the powder blends, dried powders were mixed using a tumble mixer, according to the compositions given in

Table 2. Compositions of the powder blends.

Designation of the Blend/Coating	Components (at %)			Components (wt %)
	Al2O3	Cr2O3	TiO2	Al2O3	Cr2O3	TiO2
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.

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. The parameters of the APS process with the F6-torch.

Argon	Hydrogen	Current	Spraying Distance	Traverse Speed	No. of Passes	Powder Feed Rate
41 L/min	11 L/min	600 A	110 mm	0.4 m/s	10	30 g/min

. 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. Ball-on-disk test parameters.

Force	Radius	Speed	Cycles	Wear Distance	Counter Body
					Material	Diameter
10 N	5 mm	0.05 m/s	15916	500 m	Al2O3	6 mm

. 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).

3. Results

3.1. 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-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 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].

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 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 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.

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 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 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.

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 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.

4. 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 Al₂O₃-Cr₂O₃-TiO₂ system, two high melting components (Al₂O₃ and Cr₂O₃) are combined with one lower melting oxide TiO_x. Thus, the liquid phase occurs at lower temperatures compared to TiO₂ and the addition of TiO_x to Cr₂O₃ and Al₂O₃ decreases the coating porosity. For Al₂O₃-rich and Cr₂O₃-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-Al₂O₃ to γ-Al₂O₃ as described in Section 1 occurs for all powder blends. The additions of Cr₂O₃ and TiO₂ 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 TiO₂ or near stoichiometric TiO_x as a result of the thermal spray process.

Recently, a surprising high dissolution of titanium atoms in γ-Al₂O₃ in APS coatings obtained from a commercial Al₂O₃-40% TiO₂ 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 Cr₂O₃-rich solid solution (Cr,Ti)₂O₃ 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 Cr₂O₃ coating was the highest, it is not surprising that the hardness is increased with a rising content of Cr₂O₃. The Cr₂O₃-rich CAT coating shows the highest hardness for the ternary blends. As expected, the Al₂O₃-rich ACT coating and the plain Al₂O₃ coating show higher hardness values than TiO₂-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 Al₂O₃ coatings can be significantly reduced by adding TiO_x and Cr₂O₃. Both coatings have a higher wear resistance than the plain Al₂O₃ coating. Possible reasons could be the improved toughness of the coating expected by adding TiO_x [21] and/or addition of Cr₂O₃, which has, by far, the highest wear resistance. Except for the Al₂O₃ 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 Cr₂O₃ 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 Cr₂O₃ content positively influences the sliding wear resistance.

5. Summary and Conclusions

It was shown that firmly adhering coatings can be produced by atmospheric plasma spraying from powder blends containing Al₂O₃, Cr₂O₃, and TiO₂ using an identical spray parameter set. Investigations with XRD have shown that the phase transformation from α-Al₂O₃ to γ-Al₂O₃ also occurs in the powder blends and is not influenced by the addition of Cr₂O₃ 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 Cr₂O₃ content. By using powder blends, the sliding wear rate can be improved when compared to plain Al₂O₃ 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.