Development of Suspension Feedstocks for Thermally Sprayed Zn2TiO4 Coatings

By adjusting the thermal spraying suspension technology, coatings with excellent microstructure, surface morphology, and phase composition can be obtained to meet the application needs in mechanical, electrical or friction fields. The use of suspensions as feedstock material allows a high degree of flexibility with regard to the chemical composition of the sprayed coatings. Moreover, suspension thermal spraying (STS) is a promising technique for the production of coatings, the use of which was previously limited by expensive starting materials. A mixture of less expensive starting materials in the suspension and an "in situ" reaction to the desired product during the spraying process make this possible. Zn2TiO4 coatings are one example where the high costs of blended oxide powders as feedstock material hinder the market introduction, whereas their outstanding electrical properties and photocatalytic activity are of great interest for various industrial applications. In this work, single oxides ZnO and TiO2, Zn acetate salt as ZnO precursor, as well as a Zn2TiO4 powder were used to develop tailored aqueous suspension feedstocks suitable for thermal spraying. To follow the formation of the compositions in the system ZnO-TiO2, differential thermal analysis (DTA) measurements were performed. Preparation routes of stable suspensions and suspension-solution mixtures with low sedimentation rates, low viscosities and good flowabilities are discussed. Microstructures and phase compositions of sprayed coatings are shown, and the “in situ” formation of Zn2TiO4 phase during Suspension High Velocity Oxygen Fuel Spraying (S-HVOF) is demonstrated. This work shows the high potential of suspension feedstocks from single oxide raw materials to obtain Zn2TiO4 sprayed coatings.


Introduction
Since the 1990s, the field of thermal spraying has been increasingly concerned with the production of thin coatings to close the gap between thermal-sprayed and vapor-deposited coatings and to achieve significantly better properties than conventionally sprayed coatings (Ref 1). This goal can be achieved with nano-structured powders, but it is difficult to directly inject them in the conventional spraying process due to the poor flowability. When injected in the plasma jet, the particles with a low mass and a small moment of inertia are mostly entrained by the gas flowing in the edge zone because of a thermophoresis effect. To solve these problems, a liquid carrier for the powders is used to prevent dust formation and enable an adjustable flow behavior with a better feeding (Ref [1][2][3]. Commonly used suspension media are water, ethanol, propanol, or a mixture of different liquids of these. After the suspension enters the flame, fragmentation into droplets occurs depending on the type of the surface tension of the suspension medium, the injection pressure and velocity, the injection type and solid concentration. Subsequently, the suspension medium vaporizes, which is mainly influenced by the enthalpy of the used liquid, followed by a nanoparticle agglomeration and sintering. Finally, partially, fully or non-melted powder particles are formed as well as resolidified molten droplets. They strike the substrate, form splats, and the continuous stacking of splats creates the coating ( Ref 4).
The use of a suspension as spray feedstock imposes certain requirements on its properties: good flowability, low viscosity, high stability of the particles against agglomeration and sedimentation, as well as no corrosive effect on hardware components. The use of suspensions with high solids content enables the production of coatings with high deposition rate. Due to economic and safety considerations, water is more preferred as the liquid medium over alcohol ( . Various dispersing agents and techniques are used to prepare stabilized and homogeneous suspensions by high surface charges of the particles producing electrostatic repulsion or by adsorption of polymeric molecules on the particles surface producing electrosteric or steric repulsion (Ref 9,10). The agglomeration of particles in suspensions can also be avoided by an adjustment of the pH value far from the isoelectric point of the material. Moreover, the morphology of the raw materials as well as the presence of impurities have a strong influence on the suspension behavior ( Ref 9,11). Table 1 presents the main requirements on suspension properties and options to fulfil them. Besides suspensions, solutions of precursor salts can be used as liquid feedstock for sprayed coatings. The desired material is formed in the flame during the spraying process in several steps. At first, droplet fragmentation occurs, which, inter alia, depends on the type of the solvent surface tension, the injection pressure and velocity, the injection type and precursor concentration. Affected by chemical reactions among the precursor material, the fragmentation is followed by a crust formation around the droplets due to precipitation of the precursor and solvent vaporization. Fragmentation is continued in the third step and supplemented by a melting of the outer crust. Finally, full-melted precipitated material in combination with sintered solute particles and unfragmented hard shells are formed (Ref 4,14,15). Often inorganic salts in aqueous solutions or metalorganic compounds in alcoholic solutions are used ( Ref 3,12,13). The solubility properties of the precursors, the resulting pH value and the used solvent are key factors.
The use of suspensions or solution precursors enables the formation of coatings with chemical compositions that are very difficult or expensive to be obtained from feedstock spray powders, or with compositions, which are not commercially available, e.g., binary or ternary ceramic oxides coating systems (Ref 9, 10). The present work focusses on the development and characterization of appropriate water-based suspensions and precursor solutions to produce Zn 2 TiO 4 coatings by thermal spraying. Zn 2 TiO 4 is a very expensive raw material with coarse particles. Although Zn 2 TiO 4 powder and Zn 2 TiO 4 coatings show very interesting properties for many industrial In this work, new effective routes for the preparation of zinc titanate coatings are proposed, using on the one hand the route of Zn 2 TiO 4 powder suspension and on the other hand suspensions from single oxides -ZnO and TiO 2which are available in different particle sizes, crystal structures (TiO 2 anatase and rutile) and at lower cost.
Additionally, a hybrid route using suspension and solution precursors is described. The ''in situ'' formation of Zn 2-TiO 4 in the spraying process using the binary suspension (ZnO ? TiO 2 ) is analyzed as well as the layer formation optimized regarding the feedstock composition. It is investigated which properties of the starting products and the suspension are relevant for the layer formation and which parameters are particularly important.

Raw materials
For suspension preparation, a commercially available ZnO (Zinc oxide ReagentPlus, Sigma-Aldrich, Germany) and three TiO 2 raw powders were used. The powder characteristics are listed in Table 2 and 3. TiO 2 R320 (Sachtleben, Germany) (hereinafter referred to as TiO 2 (A)) and TiO 2 from Alfa Aesar, Germany (TiO 2 (B)) was present as a rutile phase. Titanium(IV) oxide EMPROVEÒ ESSENTIAL (SAFC, Germany) (TiO 2 (C)) was present as anatase phase. Additionally, Zn acetate salt (Zn(CH 3 COO) 2   The average particle sizes D 50,3 in Table 2 and 3 were measured via laser-light diffraction (Mastersizer 2000, Malvern Panalytical GmbH, Germany). The specific surface area was determined by N 2 adsorption measurements (ASAP 2020, Micromeritics, USA). The powder density was analyzed via helium pycnometry (Pentapycnometer, Quantachrome Instruments, USA). For XRD analyses, a D8 Advanced from Bruker AXS, Germany, with a symmetrical Bragg-Brentano geometry was used. The measurements were carried out using Cu-Ka radiation and a position sensitive detector (PSD, LynxEye XE-T) in the range of 5-90°2theta. The qualitative phase analysis was carried out with the Software Diffrac.Eva (Bruker AXS) using the JCPDS 2021 database. The phases were quantified using Rietveld refinement analysis with the Software TOPAS V6 (Bruker AXS).

Suspension Preparation
Because of the different particle size distribution in the raw materials, different preparation routes were developed (Fig. 2). The fine as-received TiO 2 and ZnO powders could be used for preparation of single oxide aqueous suspensions without any previous powder treatment. A dissolver DispermatÒ CV-Plus (VMA, Germany) was used for dispersing the powders and mixing the suspensions. A small amount of a dispersing agent (1 wt.% related to powder mass) was necessary to stabilize the ZnO suspension. For TiO 2 , respectively, the pH value was adjusted to 9. Aqueous suspensions with 50 wt.% solids content were prepared and diluted to 25 wt.% suspension with water adjusted to pH. Binary ZnO-TiO 2 suspensions were prepared by mixing the single oxide suspensions in a ratio that corresponds to the molar ratio of the elements in Zn 2 TiO 4 .
The coarse particles in Zn 2 TiO 4 powder make direct use in suspension formulation impossible. Hence, an additional milling step was necessary for suspension preparation. The powder was mixed in deionized water with adding a small amount of an organic dispersing agent (0.3 wt.% related to powder mass) to create a 50 wt.% suspension. The suspension was milled in a planetary ball mill (Fritsch, Germany) for 2 h by using polyamide grinding cups and 10 mm ZrO 2 milling balls. After 2 h, the balls were changed to 2 mm ZrO 2 milling balls. After milling, the solids content was measured using a MA30 Moisture Analyzer (Sartorius, Germany) and diluted to 25 wt.% by adding deionized water.

Suspension-Solution Preparation
The fine as-received TiO 2 powder TiO 2 (A) was used for preparation of a suspension-precursor solution feedstock The powder was added to an 0.4 M aqueous solution of Zn 2 (CH 3 COO) 2 Á2 H 2 O (ZnAc) in the ratio corresponding to the molar ratio of the elements in Zn 2 TiO 4 and dispersed directly with a dissolver DispermatÒ CV-Plus (VMA, Germany). Figure 2 illustrates the feedstock preparation steps schematically.

Suspension Characterization
High flowability, low viscosity, low sedimentation rate, narrow particle size distribution and high stability are To verify the suitability of the powder mixtures and to simulate the formation of Zn 2 TiO 4 coatings during the spraying process, differential thermal analysis (DTA) was performed by using an Netzsch STA 449 F1 Jupiter (Netzsch, Germany). The initial masses of the suspensions for thermal analysis were about 240 mg (ZnO ? TiO 2 ) and 500 mg (Zn 2 TiO 4 ). The suspensions were dried in alumina crucibles under flowing air. After drying, the crucibles were heated up with 20 K/min up to 1620°C in air. The masses of the samples before heating were about 60 mg (ZnO ? TiO 2 ) and 280 mg (Zn 2 TiO 4 ).

Suspension Spraying and Coating Characterization
Suspension spraying tests were carried out with the HVOF system TopGun (GTV Verschleißschutz GmbH, Germany) using ethylene as fuel gas to validate the sprayability of the developed formulations. The suspensions were fed from an industrially suitable three pressurized-vessels suspension feeder developed by Fraunhofer IWS (Ref 9). Suspension injectors with an internal diameter of 0.25 mm were used for the axial injection of the suspension as a compact jet directly into the combustion chamber of the HVOF gun. The suspension feed rate was kept constant at 25 ml/min. For producing 50 lm thick coatings, the C 2 H 4 flow rate was 60 l/min, O 2 flow rate was 180 l/min and spray distance was 110 mm. Low-carbon steel plates with a thermally sprayed Al 2 O 3 coating were used as substrates. During spraying, the substrates were cooled using air jet systems.
The coating microstructures were examined by optical microscopy and SEM on metallographically polished cross sections. XRD measurements were carried out in the same way as the powder characterization.

Suspension Properties
The particle size distributions, viscosities and sedimentation rates of the developed suspensions are shown in the graphs of Fig. 3.
All suspensions have very small particle size distributions with D 90,3 below 10 lm. The coarse particles of the Zn 2 TiO 4 raw material required a milling of the powder to reduce the particle size and its distribution. After 4 h milling, the Zn 2 TiO 4 powder showed similar particle sizes to the other suspensions.
ZnO-TiO 2 (A) suspension has the narrowest particles size distribution (see Fig. 3a blue graph) with D 90,3 below 2 lm. In contrast, the particle size distribution of the same Fig. 2 Feedstock preparation routes for thermal spraying material is slightly higher in the Zn(CH 3 COO) 2 -TiO 2 (-A) formulation (see Fig. 3b). The higher Zn-ion concentration resulted from the precursor salt leads to an increase in the conductivity to 14 mS/cm compared to 1 mS/cm in the ZnO-TiO 2 (A) suspension. Thus, the electrochemical double layer around the TiO 2 particles is compressed and the electrostatic repulsion is reduced (Ref 25). Hence, the particle stabilization is decreased, and agglomeration can occur. The results of the viscosity measurements (see Fig. 3c) are consistent with this. The viscosity of all suspensions is very low (below 2 mPaÁs) with nearly Newtonian flow behavior. The suspension-solution mixture has a slightly higher viscosity (5 mPa.s) at lower shear rates and a shear thinning behavior despite the lower solids content in the mixture. This is an indication of an agglomeration of the fine TiO 2 (A) particles. During the viscosity measurements, the agglomerates are broken up due to the stirring of the measuring system, the flow resistance decreases and thus, the viscosity decreases. A pseudoplastic flow behavior occurs. These agglomerates cause significant differences in the sedimentation stability. Sedimentation tests of the suspensions (Fig. 3d) show very low sedimentation velocities (4-5 mm/d at 1G gravity). In contrast, a very high value of sedimentation velocity is recorded for the suspension-solution mixture, where very fast settlement of the particles occurs. Because of the low stability, a continuous stirring of the suspension-solution mixture is required during spraying.
All suspensions show suitable properties for STS applications. Particle size distribution and viscosity of the suspension-solution mixture fulfil the requirements on liquid feedstocks. A fast sedimentation of the TiO 2 (-A) particles might cause problems during the spraying process.
To verify the formation of Zn 2 TiO 4 out of the prepared suspensions, DTA measurements of the different binary suspensions and the suspension-solution mixture are performed (Figs. 4 and 6) and compared with measurements of Zn 2 TiO 4 and the phase diagram of the ZnO-TiO 2 system (Fig. 5).
For the Zn 2 TiO 4 suspension only two peaks, melting at * 1560°C and crystallization at * 1545°C are measured in the DTA, corresponding well to the single composition Zn 2 TiO 4 in the phase diagram of the ZnO-TiO 2 system (Fig. 5).
The suspension mixtures of ZnO and TiO 2 show three partially overlaid effects during heating. All reactions take place at temperatures \ 1600°C, as it is shown in the The DTA of the suspension-solution mixture of TiO 2 (A) in the Zn(CH 3 COO) 2 solution shows a slightly different result (see Fig. 6). In this case, two different reactions must be considered, which are shown in the following chemical reactions Eq 1 and 2.
At first, ZnO is formed at temperatures higher than 300°C (Eq 1). CO 2 is released during this process. At temperatures higher than 1550°C, the reaction of ZnO and TiO 2 to Zn 2 TiO 4 proceeds (Eq 2). The CO 2 formation causes a loss of material from the crucible; the composition of the mixture changes. Hence, a different DTA heating curve is measured (Fig. 6). The peak at 1450°C indicates an excess of TiO 2 because of the loss of ZnO. At higher temperatures, a shift of the melting and crystallization temperatures is detected.
According to the DTA, mainly Zn 2 TiO 4 should form in the coatings starting from the suspension of ZnO and TiO 2 (A) and the mixture of TiO 2 (A) in Zn(CH 3 COO) 2 solution.

Coatings Characterization
Suspension spraying demonstrated a good sprayability of all suspensions developed starting from single oxides (ZnO ? TiO 2 ) as well as from Zn 2 TiO 4 . Dense-structured S-HVOF coatings with deposition rates of about 7-9 lm/pass are obtained (see Fig. 7). The particles are well melted, and fine lamellar structures (lamella thickness about 1 lm) are formed. Some closed porosity, and unmolten particles are visible. There are no major differences between the structures of the coatings from the unary Zn 2 TiO 4 suspension and the binary (ZnO ? TiO 2 ) suspensions. Lighter gray zones are ZnO-rich phases, and darker gray zones are TiO 2 -rich areas. The gray shades between them are a mixture of ZnO and TiO 2 .
The XRD diffractograms of the suspension-sprayed coatings are shown in Fig. 9, and the phase composition in wt.% is listed in Table 4. In the Zn 2 TiO 4 coating, the initial phase is almost completely preserved in the suspensionsprayed coatings, only a small amount of about 2.4% of TiO 2 anatase phase is identified. In the coatings based on binary suspensions, the ''in-situ'' alloying and formation of Zn 2 TiO 4 phase occur. Moreover, the XRD analysis corresponds well with the results of DTA thermal analysis.
The amount of Zn 2 TiO 4 is about 62 wt.% in the coating obtained from ZnO ? TiO 2 (B) (i.e., suspension with micron-sized rutile powder) and about 71-72 wt.% in the coatings produced from other binary suspensions. Compared to Zn 2 TiO 4 , secondary phases of ZnO (content of about 20-21 wt.%) and TiO 2 (content of about 8 wt.%) are also identified in the coatings of the binary suspensions. The phase composition of the coatings from suspensions with finer titania particles is similar. Due to their higher specific surface area, the particles are more reactive and therefore, lead to the formation of a higher amount of Zn 2 TiO 4 in the coatings. The crystal structure of the submicron TiO 2 powder seems to be not relevant in the Zn 2-TiO 4 formation.
The microstructure of the coating made from the suspension-solution mixture of TiO 2 (A) and Zn(CH 3 COO) 2 is shown in Fig. 10. The coating is likely thin and porous, containing few amounts of Zn 2 TiO 4 , with TiO 2 and ZnO as the main phases. The results of the DTA measurements cannot be confirmed. The presence of single TiO 2 and ZnO indicates that the in-flight time of the particles in the HVOF-flame is just sufficient to ensure the vaporization of the liquid and the decomposition of the ZnAc as described by Eq 1, but is too short to allow the in situ reaction of TiO 2 and ZnO. Thus, an optimization of the suspension and process parameters is necessary.

Conclusions
In this work, four suspensions containing ZnO and TiO 2 , and Zn 2 TiO 4 , respectively, and a suspension-solution mixture of Zn(CH 3 COO) 2 and TiO 2 were developed as feedstocks for thermal spraying. Following conclusions are highlighted:   • Suspensions with appropriate properties for thermal spraying were prepared from the single oxides ZnO and TiO 2 and their mixture as well as from Zn 2 TiO 4 • The binary suspensions formed by mixing of single oxides show low viscosity, low sedimentation rate and good sprayability • A mixture of TiO 2 in a Zn(CH 3 COO) 2 solution resulted in suitable particle sizes and viscosity for thermal spraying. Destabilization and agglomeration of the TiO 2 particles due to the higher salt concentration and conductivity resulted in a fast settling of the solid and problems during the spraying process • Dense-structured coatings with homogeneous distribution of ZnO and TiO 2 were produced. XRD analysis confirmed the presence of Zn 2 TiO 4 as main phase, as expected from DTA measurements • Thermal suspension spraying showed an enormous potential for the use of low-cost binary suspensions of ZnO and TiO 2 to produce coatings with a high content of Zn 2 TiO 4 phase. ''In-situ'' alloying and formation of Zn 2 TiO 4 was demonstrated.
Acknowledgments The results presented were realized within the framework of the research project DFG No. 428362963 funded by DFG -Deutsche Forschungsgemeinschaft (German Research Foundation). The authors acknowledge the financial support. Thanks are due to Pratidhwani Biswal for helping with spraying and Irina Shakhverdova for metallographic preparation and microscopic analysis of the coatings.
Funding Open Access funding enabled and organized by Projekt DEAL.
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