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Thermally sprayed WC-based hardmetal coatings offer high hardness, good sliding wear and abrasion performance and find large applications in mechanical engineering, valve construction, or offshore applications. WC-Co coatings are mainly produced by high-velocity oxy-fuel spraying (HVOF) from conventional spray feedstock powders. In this work, suspension-HVOF spraying (S-HVOF) was used to produce dense-structured WC-12Co coatings and their microstructural, mechanical and tribological properties were investigated. Significant work was devoted to the development of appropriate aqueous suspensions starting from commercially available fine WC and Co raw powders feedstock. Suspension spraying was carried out using gas-fuelled HVOF TopGun system; for comparison purposes, liquid-fuelled HVOF K2 was employed to spray WC-12Co coatings starting from commercially available spray powder. Microstructural characterization, x-ray diffraction and microhardness of the coatings were evaluated. Oscillating sliding wear tests were conducted against sintered alumina and WC-6Co balls. The sliding wear performances of the WC-Co sprayed coatings were discussed in term of their microstructure, phase composition and coating-ball test couples.

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.

For more than one decade, suspension spraying has been gaining interest for the development and manufacture of coatings for wide range of applications such as photocatalysis, thermal barrier coatings, wear resistance, biomaterials or superhydrophobicity. Even though these coatings are already promising, a modification of their surface properties can further improve their functionality and lifetime in order to meet the growing requirements on the coating demands. Direct laser interference patterning (DLIP) is a versatile laser technology which allows fabricating microstructures with high flexibility and therefore designing surface properties on a large diversity of materials, including metals, polymers, ceramics and coatings. In this contribution, DLIP technology was applied to texture the surface of suspension sprayed high velocity oxy-fuel (HVOF) TiO2coatings. In particular, a nanosecond-pulsed laser has been used to produce cross-like patterns and the effect of laser parameters on the coating properties was investigated. The coatings were characterized by means of scanning electron microscopy, Raman spectroscopy and UV–Vis–IR measurements, and the influence of the laser treatment on the photocatalytic activity and wettability behavior has been investigated. The results showed that the DLIP technology is suitable for designing and tailoring of the surface properties of the suspension sprayed coatings.

In this work, different Al2O3-TiO2 coatings, i.e., Al2O3, Al2O3-13 wt% TiO2 and Al2O3-40 wt% TiO2, produced by various thermal spray processes, namely: (i) atmospheric plasma spraying (APS), (ii) suspension plasma spraying (SPS), and (iii) suspension high-velocity oxy fuel spraying (S-HVOF), were investigated. For spray purposes, micrometer-sized powders and water-based suspensions of fine submicrometer-sized powders were used. The study aimed to investigate the influence of spray technology and spray feedstock characteristics, mainly chemical composition, i.e., TiO2 content, on the properties of the resulting coating. The sprayed coatings were characterised in terms of selected mechanical and wear properties, including fracture toughness, Vickers and instrumental hardness, Young's modulus, sliding wear resistance, and cavitation erosion resistance. The study showed that the sliding wear rate decreased with the increasing hardness, Young's modulus, and the content of the α-Al2O3 phase in the coating. The dominant wear mechanism was fatigue-induced brittle delamination, followed by the adhesive smearing of the wear debris. The dense microstructures characterised by a high hardness and an increased content of α-Al2O3 favored the resistance of coatings to cavitation erosion. Cavitation erosion was initiated at the microstructural discontinuities like pores or microcracks, resulting in damage by brittle failure with cracking and spallation, which ended up forming large cavitation craters.

Thermally sprayed ceramic coatings are applied for the protection of surfaces that are exposed mainly to wear, high temperatures, and corrosion. In recent years, great interest has been garnered by spray processes with submicrometric and nanometric feedstock materials, due to the refinement of the structure and improved coating properties. This paper compares the microstructure and tribological properties of alumina coatings sprayed using conventional atmospheric plasma spraying (APS), and various methods that use finely grained suspension feedstocks, namely, suspension plasma spraying (SPS) and suspension high-velocity oxy-fuel spraying (S-HVOF). Furthermore, the suspension plasma-sprayed Al2O3 coatings have been deposited with radial (SPS) and axial (A-SPS) feedstock injection. The results showed that all suspension-based coatings demonstrated much better wear resistance than the powder-sprayed ones. S-HVOF and axial suspension plasma spraying (A-SPS) allowed for the deposition of the most dense and homogeneous coatings. Dense-structured coatings with low porosity (4 vol.%) and good cohesion to the metallic substrate, containing a high content of α–Al2O3 phase (56 vol.%) and a very low wear rate (0.2 ± 0.04 mm3 × 10−6/(N∙m)), were produced with the S-HVOF method. The wear mechanism of ceramic coatings included the adhesive wear mode supported by the fatigue-induced material delamination. Moreover, the presence of wear debris and tribofilm was confirmed. Finally, the coefficient of friction for the coatings was in the range between 0.44 and 0.68, with the highest values being recorded for APS sprayed coatings.

In this work suspension-high velocity oxygen fuel spraying (S-HVOF) and suspension plasma spraying (SPS) were applied to produce Al2O3 coatings using home-made and commercial available Al2O3 water-based liquid feedstocks containing 40 wt% of submicrometer-sized raw powders. The aim of this study was to investigate the differences in the microstructure, mechanical properties and tribological performance of the suspension sprayed coatings with these two techniques. The microstructure of SPS coatings was slightly porous with fine particles of sintered, non-melted powders entrapped between the well melted lamellas. On the contrary, the S-HVOF coatings were very homogeneous in terms of morphology and of very low porosity in range of 1 to 4 vol%. With initial α–Al2O3 powders, it was possible to spray α-phase rich coatings, both in the case of SPS and S-HVOF, as determined by X-ray diffraction, with up to 47 vol% of α–Al2O3 in the S-HVOF coating and about 62 vol% in SPS one. The microhardness, adhesion strength and dry sliding wear resistance testing showed a good performance of coatings sprayed by using liquid feedstocks, especially the S-HVOF sprayed ones. Finally, the influence of feedstock characteristics and spray process conditions on the mechanical and tribological properties was analyzed.

Corrosion properties of nanostructured coatings deposited by Suspension High Velocity Oxy-Fuel (S-HVOF) via an aqueous suspension of milled WC-Co powder were compared with conventional HVOF sprayed coatings. Microstructural evaluations of these coatings included X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDX). The corrosion performance of AISI440C stainless steel substrate and the coatings was evaluated in a 3.5 % wt. NaCl aqueous solution at ~25oC. The electrochemical properties of the samples were assessed experimentally, employing potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The potentiodynamic polarization results indicated that coatings produced by S-HVOF technique show lower corrosion resistance compared to the coatings produced by HVOF-JK (HVOF Jet-Kote) and HVOF-JP (HVOF JP5000) techniques. Results are discussed in terms of corrosion mechanism, Bode and Nyqvist plots, as well as equivalent circuit models of the coating-substrate system.

Compositions in the system Cr2O3-TiO2-Al2O3 are among the most used ceramic materials for thermally sprayed coating solutions. Cr2O3 coatings present good sliding wear resistance; Al2O3 coatings show excellent insulation behavior and TiO2 striking corrosion properties. In order to combine these properties, coatings containing more than one oxide are highly interesting. The conventional spraying process is limited to the availability of binary feedstock powders with defined compositions. The use of suspensions offers the opportunity for tailor-made chemical compositions: within the triangle of Cr2O3-TiO2-Al2O3, each mixture of oxides can be created. Criteria for the selection of raw materials as well as the relevant aspects for the development of binary suspensions in the Cr2O3-TiO2-Al2O3 system to be used as feedstock for thermal spraying are presented. This formulation of binary suspensions required the development of water-based single-oxide suspensions with suitable behavior; otherwise, the interaction between the particles while mixing could lead up to a formation of agglomerates, which affect both the stability of the spray process and the coating properties. For the validation of this formulation procedure, binary Cr2O3-TiO2 and Al2O3-TiO2 suspensions were developed and sprayed using the S-HVOF process. The binary coatings were characterized and discussed in terms of microstructure and microhardness.

Cr2O3-based coatings offer high hardness, excellent sliding wear performance, and corrosion resistance. Therefore, they are widely applied in the paper industry, as well as for pumps and mechanical sealing systems. Compared to the conventional spray processes, the technology of suspension-HVOF spraying (S-HVOF) allows the production of dense, finely structured coatings with smoother surfaces and improved mechanical properties by using submicron-scaled raw materials. This work investigates the microstructure and performances of Cr2O3 and Cr2O3-15%TiO2 coatings obtained by S-HVOF starting from water-based suspensions. For the development of the suspensions with binary composition, two routes were used to produce ready-to-spray suspensions: (a) mixture of two stable suspensions in the desired ratio, and (b) dispersion of an appropriate alloyed material in the solvent. In order to evaluate the potential of suspension spraying over the conventional APS and HVOF processes, the mechanical properties, corrosion, and sliding wear resistances of the S-HVOF coatings were compared with those of the coatings produced from feedstock spray powders. From the experimental results, it was observed that, in most of the cases, the suspension-sprayed coatings showed denser microstructures, enhanced mechanical properties, wear resistance, and superior corrosion performances.

The development of CMAS (CaO–MgO–Al2O3–SiO2) -resistant thermal barrier coatings is an urgent issue, as the operating temperatures of aero-engines exceed the melting point of the CMAS. Application of alumina as top layer on 7YSZ coatings has shown promising CMAS resistance behavior. It reacts with the CMAS and crystallizes into new stable phases, which seals the CMAS penetration paths in the top layer to save the underlying 7YSZ layer from infiltration. This paper deals with the study of the erosion behavior of porous thermally sprayed Al2O3 sacrificial layer on top of EB-PVD 7YSZ coatings before and after the CMAS infiltration and investigates the combined erosion/corrosion regime. The TBC system has been infiltrated with three different CMAS compositions having a different Ca/Si ratio which resulted in different reaction scenarios and products. The change in the erosion behavior of the coating system as a function of CMAS infiltration depth was investigated and an erosion model has been proposed. The key factors that dictate the erosion resistance of such alumina layers were found to be the thickness and morphology of the reaction layer, the porosity of the non-reacted alumina layer underneath the reaction layer as well as the coating adhesion mechanism between alumina and 7YSZ.

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications. Published version

Research and development work for about one decade have demonstrated many unique thermal spray coating properties, particularly for oxide ceramic coatings by using suspensions of fine powders as feedstock in APS and HVOF processes. Some particular advantages are direct feeding of fine nano- and submicron-scale particles avoiding special feedstock powder preparation, ability to produce coating thicknesses ranging from 10 to 50 µm, homogeneous microstructure with less anisotropy and lower surface roughness compared to conventional coatings, possibility of retention of the initial crystalline phases, and others. This paper discusses the main aspects of thermal spraying with suspensions which have been taken into account in order to produce these coatings on an economical way. The economic efficiency of the process depends on the availability of suitable additional system components (suspension feeder, injectors), on the development and handling of stable suspensions, as well as on the high process stability for acceptance at industrial scale. Special focus is made on the development and processability of highly concentrated water-based suspensions. While costs and operational safety clearly speak for use of water as a liquid media for preparing suspensions on an industrial scale, its use is often critically discussed due to the required higher heat input during spraying compared to alcoholic suspensions.

Sliding wear evaluation of nanostructured coatings deposited by Suspension High Velocity Oxy-Fuel (S-HVOF) and conventional HVOF (Jet Kote (HVOF-JK) and JP5000 (HVOF-JP)) spraying were evaluated. S-HVOF coatings were nanostructured and deposited via an aqueous based suspension of the WC–Co powder, using modified HVOF (TopGun) spraying. Microstructural evaluations of these hardmetal coatings included X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDX). Sliding wear tests on coatings were conducted using a ball-on-flat test rig against steel, silicon nitride (Si 3 N 4 ) ceramic and WC–6Co balls. Results indicated that nanosized particles inherited from the starting powder in S-HVOF spraying were retained in the resulting coatings. Significant changes in the chemical and phase composition were observed in the S-HVOF coatings. Despite decarburization, the hardness and sliding wear resistance of the S-HVOF coatings was comparable to the HVOF-JK and HVOF-JP coatings. The sliding wear performance was dependent on the ball-coating test couple. In general a higher ball wear rate was observed with lower coating wear rate. Comparison of the total (ball and coating) wear rate indicated that for steel and ceramic balls, HVOF-JP coatings performed the best followed by the S-HVOF and HVOF-JK coatings. For the WC–Co ball tests, average performance of S-HVOF was better than that of HVOF-JK and HVOF-JP coatings. Changes in sliding wear behavior were attributed to the support of metal matrix due to relatively higher tungsten content, and uniform distribution of nanoparticles in the S-HVOF coating microstructure. The presence of tribofilm was also observed for all test couples.

The potential to improve the tribomechanical performance of HVOF-sprayed WC-12Co coatings was studied by using aqueous WC-12Co suspensions as feedstock. Both as-sprayed and hot-isostatic pressed (HIPed) coatings were studied. Mathematical models of wear rate based on the structure property relationships, even for the conventionally sprayed WC-Co hardmetal coatings, are at best based on the semi-empirical approach. This paper aims to develop these semi-empirical mathematical models for suspension sprayed nanocomposite coatings in as-sprayed and heat-treated (Host Isostatically Pressed (HIPed)) conditions. Microstructural evaluations included Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDX). The nano hardness and modulus of the coated specimens was investigated using a diamond Berkovich nano indenter. Sliding wear tests were conducted using a ball-on-flat test rig. Results indicated that the HIPing post-treatment resulted in crystallization of amorphous coating phases and increase in elastic modulus and hardness. Influence of these changes on the wear mechanisms and wear rate are discussed. Results are also compared with conventionally sprayed High Velocity Oxy-Fuel (HVOF) hard metal WC-Co coatings.

Tribomechanical properties of nanostructured coatings deposited by suspension high velocity oxy-fuel (S-HVOF) and conventional HVOF (Jet Kote) spraying were evaluated. Nanostructured S-HVOF coatings were obtained via ball milling of the agglomerated and sintered WC-12Co feedstock powder, which were deposited via an aqueous-based suspension using modified HVOF (TopGun) process. Microstructural evaluations of these hardmetal coatings included transmission electron microscopy, x-ray diffraction, and scanning electron microscopy equipped with energy dispersive x-ray spectroscopy. The nanohardness and modulus of the coated specimens were investigated using a diamond Berkovich nanoindenter. Sliding wear tests were conducted using a ball-on-flat test rig. Results indicated that low porosity coatings with nanostructured features were obtained. High carbon loss was observed, but coatings showed a high hardness up to 1000 HV2.9N. S-HVOF coatings also showed improved sliding wear and friction behavior, which were attributed to nanosized particles reducing ball wear in three-body abrasion and support of metal matrix due to uniform distribution of nanoparticles in the coating microstructure.

Photocatalytic properties of titania have been studied very intensively for a variety of applications, including air and water purification. In order to clarify the influence of the phase composition and other parameters, thermal spraying with suspensions was applied to produce photocatalytically active titania coatings starting from two commercially available anatase and rutile submicron powders. Aqueous suspensions containing 40% solids by weight were sprayed with an HVOF process using ethylene as the fuel gas. The spray parameters were chosen in order to produce mechanically stable coatings and to preserve a high content of the initial crystalline phases of the powders. The coating microstructures, phase compositions, and surface properties were characterized. The photocatalytic performance was evaluated by degradation of the pink dye Rhodamine B (RB) using two techniques: degradation of an aqueous solution of RB and discoloration of impregnated RB. All the coatings exhibited photocatalytic activity to varying degrees, depending on the phase composition as well as other factors, namely, the coating microstructure, surface morphology, surface hydroxylation, light absorption, and interaction with the pollutant.

In this work the microstructural characteristics and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) and conventional HVOF spray processes are presented. The electrical resistance at different relative air humidity (RH) levels (from 6 to 97% RH) and values of dielectric strength were investigated by direct current electrical resistance measurements, electrochemical impedance spectroscopy, and dielectric breakdown tests. Relationships between electrical properties and coating characteristics are discussed. At low humidity levels (up to 40% RH) the electrical resistivities of S-HVOF and HVOF coatings were on the same order of magnitude (10(exp 11) Omega m). At a very high humidity level (97% RH) the electrical resistivity values for the S-HVOF coatings were in the range 10 (exp 7)-10(exp 11) Omega·m, up to five orders of magnitude higher than those recorded for the HVOF coating (orders of magnitude of 10(exp 6) Omega·m). The better electrical resistance stability of the suspension-sprayed Al 2O 3 coatings can be explained by their specific microstructure and retention of a higher content of alpha-Al 2O 3. The dielectric strength E d of suspension-sprayed coatings was found to be 19.5-26.8 kV·mm -1 for coating thicknesses ranging from 60 to 200 µm. These values were slightly lower than those obtained for conventional HVOF coatings (up to 32 kV·mm -1). However, it seemed that the dielectric strength of conventionally sprayed coatings was more sensitive to the coating thickness (when compared with the values of E d determined for S -HVOF coatings) and varied to a greater extent (up to 10 kV·mm -1) when the coating thickness varied in the range 100-200 µm.

Stable water- and water/ethanol suspensions of TiO2 were plasma sprayed on stainless steel substrates. The suspensions were injected using two different systems: external, using an atomizing injector, and internal, performed with a continuous-stream injector inside the plasma torch anode. In order to find the optimal spray parameters, seven experimental runs were performed and the resulted deposits were mainly characterized by means of scanning electron microscopy and X-ray diffraction analysis. The microstructural examinations revealed that the coatings obtained by internal injection of the suspension show two principal characteristics: dense zones with well molten big lamellae and porous zones containing fine nanometric and submicrometric grains. When applying the internal continuous-stream injector, the amount of anatase varied from about 1.6 vol.% to 8.8 vol.%. The amount was with 9.4 vol.% to 15.4 vol.% considerably lower when an external atomizing injector was used. The photocatalytic activity of selected coatings was determined by measuring the degradation of an aqueous solution of dye methylene blue. The photon efficiency of titania deposits, which is a measure of the photocatalytic performance, varied from 0.022% up to 0.032% and did not seem to depend on the amount of anatase phase.

This paper presents an overview of current research activities regarding the properties and functionalities of finely structured alumina (Al2O3) and titania (TiO2) coatings prepared by suspension plasma spraying and suspension HVOF spraying. A selection of new experimental results obtained by the authors is also included. In the case of Al2O3, focus was on the retention of a higher content of the α-phase in the coatings without any post-treatment or alloying. For TiO2, the goal was to preserve the initial anatase phase in order to obtain photocatalytically active titania coatings. Coating microstructures, phase compositions, and functionalities resulting from the interactions between different working parameters are discussed.

In the present work, the elaboration of nanostructured alumina and titania coatings by thermal spraying with liquid precursors is described. Nano- and submicrometer-sized powders were used to prepare aqueous or alcoholic suspensions. The suspensions were sprayed using APS and HVOF processes in order to obtain thin and thick deposits. The paper discusses the coating microstructures as a function of suspension characteristics and spray parameters in both APS and HVOF processes.

Suspension Thermal Spraying is a relatively new thermal spaying technique to produce advanced thermal barrier coatings. This technique enables the production of much different performance thermal barrier coatings than conventional thermal spraying which uses solid powder as a feedstock material. In this work a comparative study is performed on four different types of thermal barrier coatings sprayed with two different thermal spay processes, suspension high velocity oxy-fuel spraying (SHVOF) and suspension plasma spraying (SPS) using two different water-based suspensions. Tests carried out include microstructural analysis with SEM, porosity analysis using weight difference by water infiltration, thermal conductivity measurements using laser flash analysis and lifetime assessment using thermo-cyclic fatigue tests. The results showed that SPS coatings were much porous and hence showed lower thermal conductivity than SHVOF coatings produced with the same suspension. From the thermo-cycling tests it was observed that the SPS coatings showed a higher lifetime than the SHVOF ones.

To achieve solid oxide fuel cells (SOFC) at reduced costs, the atmospheric plasma spray (APS) process could be an attractive technique. However, to make dense and thin layers as needed for electrolytes, a suspension is preferably implemented as a feedstock material instead of a conventional powder. Suspensions of yttria-stabilized zirconia particles in methanol have been prepared with various solid loadings and states of dispersion. An external injection system was used to ensure the atomization and radial injection of the suspension into the Ar-H2 plasma under atmospheric conditions. The coatings morphologies were characterized by scanning electron microscopy, and their porosity was evaluated by the Archimedes method. Differences in the microstructure of the deposits were observed depending on the APS operating conditions. Special attention has been dedicated to assess the influence of the suspension as well as the injection on the layer morphology. For this purpose, the atomization has been investigated and efforts have been made to understand relationships among suspension properties, atomization, and coating microstructure.

This paper proposes 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 oxyfuel spraying). Agglomerated spray dried anatase TiO2 powder was used as feedstock material for spraying. Morphology and microstructural characteristics of the coatings were studied mainly by scanning electron microscopy and x-ray diffraction. The photocatalytic behavior of the TiO2-base surfaces was evaluated from the conversion rate of gaseous nitrogen oxides (NOx). It was found that the crystalline structure depended strongly 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 has allowed retention of the original anatase phase and for very reactive TiO2 surfaces to be obtained for the removal of nitrogen oxides.

This paper describes formation of titanium dioxide coatings designed for photocatalytic applications, obtained by suspension plasma spraying (SPS), an alternative of the atmospheric plasma spraying (APS) technique in which the material feedstock is a suspension of the material to be sprayed. Two different TiO2 powders were dispersed in distilled water and ethanol and injected in Ar-H2 or Ar-H2-He plasma under atmospheric conditions. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses were performed to study the microstructure of the titania coatings. Photocatalytic efficiency of the elaborated samples was evaluated from the conversion ratio of different air pollutants: nitrogen oxides (NOx) and sulfur dioxide (SO2). The morphology and crystalline structure of the deposits depended mainly on the nature of the solvent (water or alcohol) used in the preparation of the slurries. Dense coatings were obtained starting from aqueous suspensions and porous deposits were elaborated by plasma spraying of a PC105 alcoholic suspension. A significant phase transformation from anatase to rutile occurred when ethanol was used as a solvent. Different photocatalytic performances were observed as a function of the nature of the liquid material feed-stock, the spraying parameters, and the nature of the pollutant.

Titanium dioxide is one of the most important photocatalysts that allows the environmental purification of various toxic organic compounds in water and removal of harmful air pollutants. In this paper, two techniques of plasma spraying – plasma spraying from an agglomerated nanopowder and liquid suspension plasma spraying – were used to produce thin deposits. The microstructure of coatings was characterized using scanning electron microscopy and X-ray diffraction. The photocatalytic properties of the coatings were evaluated by the decomposition test of nitrogen oxides and compared with the efficiency of the initial agglomerated nanopowder and that of the Degussa P25 powder. Noticeable different behaviours in the photocatalytic tests were observed for coatings performed either from conventional or liquid plasma spraying. The TiO2 deposits realized by spraying of the liquid suspension exhibited higher photocatalytic efficiency. It is found that the plasma spraying of a slurry is an effective method to retain the original anatase phase and prevent the increase of the crystallite size. This new method of modified spraying is proved to be a promising technique to elaborate photocatalytic coatings for the removal of nitrogen oxide pollutants.

TiO2 and TiO2–Al composite coatings were prepared by plasma spraying using a reconstituted nanosized feedstock via a spray drying method. Effects of various spray conditions on the microstructure, porosity, microhardness and wear resistance related to the mechanical performance of coatings were evaluated. The coatings sprayed using relatively low plasma power were composed of two distinct microstructures of well defined lamellar structure, similar to microstructure of conventional plasma sprayed coatings generated by fully melted particles, and embedded nano or sub-micron particles generated by partial/non-molten particles of feedstock materials. The fraction of partial/non-molten particles in coating layers was increased by Al additive. Such a characteristic of blended microstructure of coatings was clearly confirmed from a bimodal distribution of microhardness described by Weibull plots. The optimized addition of Al into TiO2 increased the spraying efficiency, and also, improved mechanical properties such as microhardness and wear resistance.

In this study, APS and HVOF processes have been used to prepare alumina (Al2O3) and magnesium spinel (MgAl2O4) coatings designed for insulating applications. The electrical characteristics, i.e., dielectric strength and electrical resistance (electrical resistivity) were investigated using different methods: dielectric breakdown test, direct current (DC) measurements, and electrochemical impedance spectroscopy (EIS). The electrical resistance was measured at room temperature at different relative humidity (RH) levels (from 6% RH to 95% RH) as well as at 200 °C. The coating microstructure, phase composition, and water vapor sorption were studied. Differences in the electrical insulating properties due to the different coating system characteristics are discussed. Of the coatings and conditions investigated in this study, the HVOF spinel coatings showed superior dielectric breakdown strength and electrical resistance stability at high humidity levels.

In this article, the results of corrosion investigations performed on thermally sprayed ceramic coatings with different compositions in the Al2O3-TiO2 system (Al2O3, Al2O3-3%TiO2, Al2O3-40%TiO2, and TiO x ) are presented. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high-velocity oxy-fuel (HVOF) spraying processes and characterized by means of optical microscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD). The corrosion properties were investigated in 1 N solutions of NaOH and H2SO4, at room temperature, 60 °C, and 85 °C, as well as in hydrothermal conditions with deionized water at 100 °C and 200 °C. The corrosion stability of the coatings depended on coating characteristics (spraying method, microstructure, and crystalline phase composition) and the corrosive environment (media, test temperature, and duration). In contrast to expectations, APS-sprayed coatings were found to be more corrosion-resistant than the HVOF-sprayed coatings. Addition of TiO2 to Al2O3 increased the corrosion stability, especially for the HVOF-sprayed coatings. In this work, TiO x coatings were found to be more corrosion-resistant than the Al2O3-based coatings.

This work presents a study of the microstructures and photocatalytic behaviour of titanium oxide coatings obtained by thermal spraying of agglomerated nanopowders and suspensions. Fine TiO2 Degussa P25 nanopowder, generally considered as the reference material in photocatalytic applications, was used as the material feedstock. HVOF process and suspension thermal spraying were used to prepare photocatalytic titania coatings. The coatings were mainly characterised by means of SEM and X-ray diffraction. The photocatalytic performance was evaluated based on decolouration of the pink dye Rhodamine B and degradation of gaseous acetaldehyde. A lower degree of pollutant degradation was found for deposits prepared by HVOF spraying of granules due principally to the low content of the photocatalytically active phase, i.e. anatase. Complete photocatalytic degradation of the organic compounds was recorded for the suspension-sprayed coatings. Based on the current results, suspension thermal spraying appears to be the better choice for preparing photocatalytically active titanium oxide surfaces for the removal of organic pollutants.

Molten calcium-magnesium-aluminum-silicate (CMAS) mineral particles cause significant degradation of thermal barrier coatings (TBCs) in aero-engines. One approach to protect the TBC coating against the CMAS attack is the application of a sacrificial coating on top of the TBC coating. In this work, Al2O3 coatings were deposited on EB-PVD 7YSZ layers using suspension plasma spraying (SPS) and suspension high velocity oxy-fuel spraying (SHVOF), in order to produce sacrificial topcoats with two different microstructures and porosity levels. The coating systems were tested under CMAS attack with one natural volcanic ash and two artificial CMAS powders by conducting infiltration tests at 1250 °C in the time intervals between 5 min and 10 h. It was found that the porosity and morphology of suspension sprayed alumina topcoats, the chemical composition of the deposits and the infiltration conditions strongly influence the CMAS infiltration, reaction kinetics and formation of the reaction products. While the porous SPS coatings offer limited resistance against CMAS infiltration, the dense SHVOF coatings show promising CMAS sealing behavior. Among the formed reaction products, only (Fe, Mg) Al spinel acted as an efficient barrier against CMAS infiltration. However, the formation of uniform spinel layers strongly depends on the pore morphology of the sacrificial coating and the CMAS chemistry.