This work investigates the influence of powder characteristics on thermal spray coatings produced from titanium suboxide powders. The powders used were prepared from commercially fused and ground titanium dioxide reduced in a mixture of hydrogen and argon gas. Two particle sizes were produced and subsequently applied by atmospheric plasma and HVOF spraying. Originally melted and crushed powder fractions served as a reference. All deposits showed high wear resistance, particularly the HVOF-sprayed layers. Paper includes a German-language abstract.

Titanium suboxides form a class of ceramic materials possessing such technically interesting properties as electrical conductivity and solid lubrication. Consequently, these materials have a high potential for application as thermally sprayed coatings. In this paper the preparation and characterisation of two spray powders of different composition by agglomeration and sintering with a narrow range of the value n in Ti n O 2n-1 is described. Powders were characterised by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), thermogravimetric measurements (TG), helium pycnometry, nitrogen adsorption and mercury intrusion techniques. The sprayability was tested by plasma spraying. The coating structures were studied by optical microscopy, XRD, TG and TEM. Although the powders were only partially oxidized as a result of the spray process, the crystallographic structure was changed significantly, according to XRD and TEM investigations. As an alternative method of preparation of titanium suboxide spray powders, the reduction of a fused and crushed spray powder with hydrogen is described. Powder particle shape and size distribution are not changed in this process.

Titanium dioxide (TiO 2 ) has emerged as an excellent photocatalyst material for environmental purification about two decades ago but only recently few works have focused on the photocatalytic properties of sprayed titanium oxide coatings. So far, the role of oxygen deficiency which can appear as a result of the spray process or by use of titanium suboxide powders on the photocatalytic activity has not been investigated. Also the possible influence of the shear plane structure of titanium suboxides (Magnéli phases) on the photocatalytic activity was not taken into consideration. In the present work, the photocatalytic properties of three powders and coatings sprayed from these powders by APS and VPS are investigated: (1) a commercial fused and crushed titanium oxide powder, (2) an agglomerated and sintered titanium suboxide powder consisting predominantly of Magnéli phases Ti 6 O 11 and Ti 5 O 9 , (3) an agglomerated and sintered powder consisting of Ti 2 Cr 2 O 7 and Ti 6 Cr 2 O 15 (Magnéli phases in the TiO 2 - Cr 2 O 3 phase diagram). The phase compositions of the powders and the coatings were investigated by X-ray diffraction. Neither for the spray powders nor for the coatings any ability to reduce the NOx concentration by the photocatalysis was found. From this it is concluded that both oxygen deficiency as well as Magnéli phase structure are not responsible for photocatalytic properties of materials in the Ti-O phase diagram.

Extensive research activities were conducted over the last few years on coatings made of titanium oxide, an established material for thermally sprayed coating solutions. Multiple existing and potential applications are closely connected with the existence of different titanium dioxide modifications and the formation of suboxides. This provides a basis for discussions on the Ti-O phase diagram as well as the properties and conditions of formation of relevant phases. Coating microstructures, phase compositions and mechanical properties are discussed as a result of interactions of different spray powders in different spray conditions of atmospheric and vacuum plasma spraying (APS and VPS), as well as of high-velocity oxyfuel (HVOF) spraying. The discussion on applications is focused on electrically conductive coatings, coatings with photocatalytic properties and coatings for wear applications.

Titanium oxide is established as an oxide material for thermally sprayed coating solutions and has received increasing interest over the last few years. Scientific and technological research focuses on electrical conductivity, solid lubrication and photocatalytic properties of titanium oxide (TiOx) coatings of differing stoichiometry. The aim of this study was to investigate the effect of oxygen loss by reduction with hydrogen occurring in the conditions of vacuum plasma spraying (VPS) from commercial titanium oxide feedstock on coating microstructure, hardness, phase composition, abrasion wear resistance and electrical resistivity. The X-ray diffraction pattern showed the presence of rutile, with peaks decreasing in intensity with increasing hydrogen content in the plasma-forming gas. The intensities of the peaks showed significant deviations from those of the standard. An increase in hydrogen flow rate did not influence the coating microstructure, hardness or abrasion wear resistance, but it caused the electrical resistivity to decrease. VPS coatings prepared from commercial fused and crushed powder show a resistivity in the range of 0.01-0.1 Ohm*cm, which corresponds exactly to the range published in the literature. Comparison with results for APS- and HVOFsprayed coatings reveals that VPS coatings yield the best combination of abrasion wear resistance and electrical resistivity when commercial titanium oxide spray powder is used.

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.

Traditional fused and crushed (F&C) titania feedstock powders have relatively poor flow and are limited in the feed-rates that can be achieved before deposition efficiencies (DEs) start to decline. In addition, the coatings made with F&C powders generally have high stiffness and limited fracture toughness. Such coatings bear the risk of cracking/delaminating particularly in the case of thick coatings. To address these issues, a new agglomerated and sintered (A&S) titania feedstock powder has been developed and compared in side-by-side spray tests to the traditional F&C titania powder of comparable particle size. The spray behavior, achievable feed-rates and DEs, as well as the resulting coating characteristics were evaluated from the view point of their application as thick, electrically conductive coatings. The new A&S powder yields up to approximately 200% improvement in DE while producing coatings with more suitable microstructure, lower electrical resistivity and higher thickness. Furthermore, for a given set of process parameters, the DEs obtained with this new powder show little sensitivity to the powder feed-rates, thus allowing spraying at higher feed-rates without compromising coating DEs. This feature of the powder has significant commercial advantages for thick coatings when combined with high throughput guns such as TriplexPro-200. Preliminary results of particle diagnostics towards understanding of the fundamental principles behind these improvements are also discussed.

In this work, completely ceramic heating elements have been developed by the combination of conductive and insulating thermally sprayed oxide coatings. These heating elements with a total thickness of less than 1 mm have been directly applied on metallic substrates. APS- and HVOF-sprayed Al 2 O 3 and spinel (MgAl 2 O 4 ) coatings were employed for insulation. A comparative analysis of the insulating properties (dielectric strength, electrical resistivity) of these coatings is presented. The HVOF-sprayed spinel coatings show better dielectric breakdown strength and higher electrical resistance stability. TiO x , TiO 2 -10%Cr 2 O 3 and TiO 2 -20%Cr 2 O 3 powders have been used to prepare the electrical conductive coatings. The thermal and oxidation stabilities at high temperature, as well the electrical properties have been investigated. Addition of Cr 2 O 3 reduced the oxidation rate of titanium oxide and increased the operational temperature of the heating coating. A ceramic heater consisting of spinel coating as insulator and TiO 2 - 20Cr 2 O 3 as conductor was sprayed on a metallic roller and the electrical stability during the long-term (300h) thermo-cycling (from RT to 300°C) was successfully tested.

This work shows that with computer-controlled detonation spraying, the phase composition of coatings can be changed relative to that of the feedstock powders. New phases can appear in substantial quantities due to chemical reactions of reduction, oxidation, and nitridation as well as interfacial interactions between phases in composite powders. The key advantage of computer control is that it precisely regulates the quantity and stoichiometry of explosive gas mixtures. It has thereby been found that TiO 2 experiences partial reduction to titanium suboxides and that chemical reactions with nitrogen are also possible. It has also been found that when nitrogen is present, titanium aluminides, Ti 3 Al and TiAl, are likely to form nitrides in the sprayed coatings. Interfacial reactions between the phases of a composite have been studied, and in the case of the Ti 3 SiC 2 -Cu system, it has been found that deintercalation of Si can be prevented by maintaining relatively cold spraying conditions. At higher temperatures, coatings of an unusual phase composition form in which carbon-deficient TiCx inclusions are distributed in the Cu matrix as modified by the dissolution of silicon. The formation of new phases affects coating microstructure development and results in new microstructural features.

The plasma spray process is used to create titanium oxide coatings under the current stoichiometry of titania and titanium suboxides. This study used feedstock powder with Magnéli phases TinO 2n-1 , slightly reduced titania TiO 2-x , and rutile. A factorial design of experiments approach was used to better understand the influence of operational parameters on coating quality, in particular, the electric resistivity and the degree of oxidation of the titanium oxide during the spraying. Firstly, arc current intensity and stand-off distance were studied; the results show strong correlations between particle temperatures and the electric resistivity of the coating. Then, different plasma compositions were used in order to understand the influence of hydrogen in the formation of titanium sub-oxides. The hardness of the most significant coatings was analyzed.

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.

In the present work, TiO 2 (rutile)-2.5vol.%Ag composite powders produced by mechanical milling were detonation sprayed under different atmospheres using acytelene as a fuel. The atmosphere of spraying was set to be reducing or oxidizing by changing the O 2 /C 2 H 2 mole ratio. Reduction of TiO 2 to Ti 3 O 5 occurred in the coatings deposited under a reducing atmosphere (O 2 /C 2 H 2 =1.05) when particles were heated to reach a molten or a semi-molten state. In the coatings sprayed using a stoichiometric O 2 /C 2 H 2 =2.5 mixture, the major phase was rutile. The composition of the atmosphere does not only determine the chemical environment for the sprayed powders, but also influences the temperature conditions. Increasing oxygen content in the explosive mixture led to much higher temperatures of the sprayed particles as was calculated using a previously elaborated model. When titanium dioxide did not reach melting, the coatings were porous with a spongy surface. Coatings formed by fully or partially molten particles possessed a denser structure. Silver particles experienced melting during spraying but remained uniformly distributed in the coatings. This study demonstrated that careful selection of the composition of the spraying atmosphere offers potential of controlling the phase composition and microstructure of the detonation sprayed coatings.

In this work the sliding wear resistance of HVOF and APS-sprayed coatings from two experimental TiO 2 -10-20 wt.%Cr 2 O 3 fused and crushed feedstock powders was investigated. During the spraying process the particle temperatures and velocities were mapped in the cross section of the spray jet at different spray distances. In addition, the coating surface temperature was monitored. Coating microstructures and phase composition were studied by SEM and XRD, respectively. The elastic modulus (E) and hardness (H) values were measured via in-depth sensing indentation. The unidirectional ball-on-disk sliding wear test was performed at room temperature against a tungsten carbide ball (sliding speed 0.05 m/s, normal force 90 N). The increase of the particle temperature and velocity leads to an increase of hardness and Young’s modulus of the coatings, thereby increasing their sliding wear resistance. The sliding wear resistance can be correlated with the H 3 /E 2 ratio better than with the hardness alone. The sliding wear resistance was also sensitive to the coating surface temperature reached during deposition.

In this paper, nano-structured TiO 2 coatings have been successfully deposited onto titanium alloy substrates by atmospheric plasma spraying technology using optimized plasma parameters. A chemical treatment method was employed to induce bioactivity on the TiO 2 surface. The bioactivity of as-sprayed and chemical treated TiO 2 coatings were evaluated by investigating the formation of apatite on their surface after they were soaked in simulated body fluids (SBF) for a period of time. Microstructure and the phase composition of the as-sprayed coating and apatite were analyzed by Field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectrometry (EDS). The results obtained indicate that as-sprayed TiO 2 coating consists of rutile, anatase and suboxide such as Ti 3 O 5 . The surface of nano-TiO 2 coating is covered by nano particles of about 50nm in size. The bonding strength of TiO 2 coating with Ti alloy substrate is as high as 40 MPa. The corrosion resistance performance of nano-coating in SBF is better than that of Ti-6Al-4V alloy. The surface of as-sprayed TiO 2 coating can not induce bone-like apatite formation. Chemical treatment, such as acid and alkali, can improve bioactivity of TiO 2 coating surface.

Titanium dioxide coatings were sprayed by a water stabilized plasma gun (WSP) to form robust self-supporting bodies with a photocatalytically active surface. Agglomerated nanometric powder was used as a feedstock. In one case argon was used as a powder-feeding as well as coating-cooling gas whereas in the other case nitrogen was used. Stainless steel was used as a substrate and the coatings were released after the cooling. Over one millimeter thick self-supporting bodies were studied by XRD, HR-TEM, XPS, Raman spectroscopy, UV-VIS spectrophotometry and photocatalytic tests. Majority of the tests was done at the surface as well as at the bottom side representing the contact surface with the substrate during the spray process. Porosity was studied by image analysis on polished cross sections where also microhardness was measured. Dominant phase present in the sprayed samples was rutile whereas anatase was the main minor component. Hydrogen content in the nitrogen assisted coating was higher, but the character of the optical absorption edge remained the same for both samples. Photoelectron spectroscopy revealed differences in the character of O 1s peak between both samples. The photocatalytic activity was tested by decomposition of acetone at UV illumination, whereas also the end products - CO and CO 2 - were monitored. The nitrogen-assisted coating was revealed as more efficient photocatalyst.

In this work, a TiO 2 coating with nanostructured surface was obtained through plasma sprayed nano-sized TiO 2 powder. Its bonding strength onto Ti-6Al-4V substrate is high up to 38 MPa. At same time, we have successfully improved the bioactivity of plasma sprayed TiO 2 coating with nanostructured surface using hydrogen ion implantation and UV illumination. Bone-like apatite can form on the surface of the post-treated TiO 2 coatings after they are soaked in simulated body fluid for a period of time. Introduction of surface bioactivity (bone conductivity) to plasma-sprayed TiO 2 coatings which are generally recognized to have excellent biocompatibility and corrosion resistance as well as high bonding to titanium alloys makes them more superior than many current biomedical coatings such as plasma-sprayed hydroxyapatite.

In this study, the influence of spray parameters on the electrical resistivity of thermally sprayed ceramic coatings from the system Cr 2 O 3 -TiO 2 was investigated. Fused and crushed feedstock powders with contents of 10 wt. % and 20 wt. % chromium oxide were deposited by APS and HVOF. Temperature and velocity of the particles in the spray jet as well as the coating surface temperature were analyzed during the deposition process. Impedance spectroscopy was used to investigate the electrical resistivity of the coatings and the results were correlated to coating microstructure and phase composition. It was found that phase transformations occur during the spray process. In the coatings a high temperature phase (n-phase) and rutile were observed. Though, the ratio of rutile depends on the spray methods employed for coating deposition. The electrical resistivity of coatings obtained by HVOF can be correlated to the content of chromium oxide. Furthermore, the surface temperature of the coating during deposition also shows some influence. Concerning the coatings resulting from APS, the different mixtures of the plasma gases (Ar-H 2 and Ar-N 2 ) are supposed to have the most important influence on the electric resistivity.

Titanium dioxide (TiO 2 ) is one of the most important photocatalyst that allows the environmental purification of water and air by the decomposition of toxic organic compounds and removal of harmful gases. In the photocatalytic applications, TiO 2 can be used in form of powder or coating. In this paper, two techniques of deposition were used to elaborate thin deposits starting from an agglomerated TiO 2 anatase nanopowder: conventional plasma spraying in atmospheric conditions and suspension plasma spraying. The photocatalytic efficiency of the coatings was performed with respect to nitrogen oxides (NOx) and compared with the photocatalytic activity of the TiO 2 Degussa P25 powder. Differences in the photocatalytic efficiencies of the nanocoatings obtained by the two techniques of plasma spraying were obtained. The coatings elaborated by suspension plasma spraying have poor mechanical properties but better photocatalytic efficiencies. This method is a promising technique to elaborate photocatalytic coatings for the removal of different air pollutants.

In this paper a comparative study on the microstructure and photocatalytic performances of titanium dioxide coatings elaborated by various thermal spraying methods (plasma spraying in atmospheric conditions, suspension plasma spraying and high-velocity oxy-fuel spraying) was proposed. Agglomerated spray-dried anatase TiO 2 powder was used as feedstock material for spraying. Morphology and microstructural characteristics of the coatings were mainly studied by scanning electron microscopy and X-ray diffraction. The photocatalytic behavior of the TiO 2 -based surfaces was evaluated from the conversion rate of gaseous nitrogen oxides (NOx). It was found that the crystalline structure strongly depended on the technique of thermal spraying deposition. Moreover, a high amount of anatase was suitable for the photocatalytic degradation of the pollutants. Suspension plasma spraying permitted to retain the original anatase phase and to obtain very reactive TiO 2 surfaces for the nitrogen oxides removal.

TiO 2 nanopowder was used as a feedstock for spraying with the water stabilized plasma (WSP®) in search for superior mechanical properties and wear resistance of titania coatings. It has been proved that good quality coatings can be made even with the high throughput WSP®. Single splats evaluation and the free flight particles were used for the spray optimization. Phase compositions, stoichiometry and selected properties, such as density and elastic modulus, were then studied at the as-sprayed coatings. As for the phase composition of coatings, mainly rutile with possible traces of Magneli phases have been found. A comparison between tension and compression loading shows that values of the Elastic modulus for compression are slightly higher than these for tension, as it is usual in plasma sprayed coatings.