Due to their promising photocatalytic properties under visible light irradiation, thermally sprayed ZnO-TiO 2 coatings are of interest as substitute for TiO 2 for various industrial applications, like hydrogen production via water splitting or the reduction of organic pollutants in water. Suspension spraying is an effective method to produce coatings in the binary ZnO-TiO 2 system to form Zn 2 TiO 4 in-situ during the spraying process. Aqueous suspensions containing fine dispersed ZnO and TiO 2 particles are mixed at tailored composition and sprayed using the SHVOF and SPS spraying processes. Coatings with homogeneous distribution of elements and different surface structures and phase compositions are obtained. The phase composition is analyzed via XRD. UV-Vis spectroscopy measurements and photocatalytic tests of Rhodamin B degradation are performed. The potential to use appropriate binary suspension feedstock to produce ZnO-TiO 2 -Zn 2 TiO 4 coatings with different microstructures and photocatalytic properties is presented.

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 our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. Significant work was devoted to the development of appropriate aqueous hardmetal suspensions starting from commercially available fine WC and Co raw powders feedstock. This contribution proposes a step forward in the development of the S-HVOF WC-12Co coatings and evaluation of their microstructural and tribological properties. Suspension spraying trials were carried out using gas-fuelled HVOF TopGun system. For comparison purposes, liquid-fuelled HVOF K2 was employed to spray WC-12Co coatings starting from commercial available spray powder. Microstructural characterization, X-Ray diffraction and microhardness of the coatings were evaluated. Oscillating sliding wear tests were conducted against sintered Al 2 O 3 and WC-6Co balls. The sliding wear performances of the WC-Co sprayed coatings were discussed in term of the microstructure, phase composition and coating-ball test couples.

When compared with conventional thermal spraying processes, thermal spraying of suspensions allows to produce coatings with outstanding properties in terms of microstructure, surface topography, and phase compositions, as well as mechanical, electrical or tribological requirements. The use of suspensions as feedstock results in an almost unlimited flexibility in terms of chemical composition of the sprayed coatings. Moreover, thermal spraying of suspensions is a promising technique for processing expensive raw materials. Zn 2 TiO 4 coatings are only one example where the high costs of blended oxide powders as feedstock material hinders the market introduction, whereas outstanding electrical properties and photocatalytic activity of thermally sprayed Zn 2 TiO 4 coatings are of great interest for various industrial applications. In this work, single oxide ZnO and TiO 2 raw materials as well as a Zn 2 TiO 4 feedstock powder were used to develop tailored aqueous suspensions suitable for thermal spraying. To follow the formation of the compositions in the system ZnO-TiO 2 , differential thermal analysis (DTA) and thermal gravimetry (TG) measurements were performed. Preparation routes of stable suspensions with low sedimentation rates, low viscosity and good flowability are discussed. Exemplary microstructures and phase compositions of sprayed coatings are shown. In all sprayed coatings, the Zn 2 TiO 4 phase has been formed during Suspension High Velocity Oxygen Fuel Spraying (S-HVOF). This work demonstrates the potential to develop appropriate cost-efficient suspension feedstocks from single oxide raw materials to obtain Zn 2 TiO 4 coatings.

Cr 2 O 3 coatings and Cr 2 O 3 -based binary coatings offer high hardness, excellent sliding wear performance and corrosion resistance. Therefore, they are widely applied in the paper and textile industry, as well as for pumps and mechanical sealing systems. Compared to the conventional spray processes, the technology of Suspension-HVOF (high velocity oxy-fuel flame spraying of suspensions, S-HVOF) using submicron-scaled raw materials allows the production of dense, finely-structured coatings with smooth surfaces and improved mechanical properties. This work investigates the microstructure and performance of Cr 2 O 3 and Cr 2 O 3 -15wt.%TiO 2 coatings obtained by SHVOF with water-based suspensions. For the development of the binary composition two routes were used to produce ready-to-spray suspensions: i) dispersion of an appropriate alloyed material in the solvent and ii) mixture of two stable suspensions in the desired ratio. In order to evaluate the potential of suspension spraying over the conventional APS and HVOF processes, the mechanical properties and the corrosion resistance of the S-HVOF Cr 2 O 3 and 85Cr 2 O 3 /15TiO 2 coatings were compared with the coatings produced from spray powders. From the experimental results it was observed that the suspension sprayed coatings showed in most of cases denser microstructure, good mechanical properties and superior corrosion performances.

Next to ZrO 2 , compositions in the system Cr 2 O 3 -TiO 2 -Al 2 O 3 are the most used ceramic materials for thermally sprayed coating solutions. Cr 2 O 3 coatings present good sliding wear resistance, Al 2 O 3 coatings show excellent insulation behaviour and TiO 2 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 compositions: within the triangle of Cr 2 O 3 -TiO 2 -Al 2 O 3 each mixture of oxides can be created. The formulation of binary or ternary suspensions is based on the preparation of stable single-oxide suspensions, which will later be mixed to achieve the desired composition. This procedure requires the development of water-based suspensions with suitable behavior, otherwise the ceramic particles and dissolved organic additives may interact and form agglomerates, which are prejudicial to the thermal spray process. This work presents criteria for the selection of raw materials as well as relevant aspects for the development of binary suspensions to be used in thermal spray. For procedure validation, coatings sprayed using Cr 2 O 3 -TiO 2 suspensions and S-HVOF will be shortly presented and analysed in terms of their microstructure and microhardness.

Thermal spray coatings produced from suspensions can be precisely tuned in terms of thickness, surface morphology, microstructure, and properties. This paper discusses the benefits of using suspensions as feedstocks for atmospheric plasma and HVOF spraying, the equipment required, and the technological challenges that remain to be solved.

In this work suspension-HVOF spraying (S-HVOF) was used to prepare dense and mechanical stable Al 2 O 3 sprayed coatings with high contents of α-Al 2 O 3 phase. Aqueous suspensions with various contents of powder (from 25 wt.% up to 50 wt.%) have been developed starting from two commercial α-Al 2 O 3 powders, characterized by different purity and particle sizes. The suspensions have been internally injected in a modified combustion chamber of a HVOF TopGun-torch. Coating microstructures, phase compositions and mechanical properties resulting from the interaction between suspension characteristics and spray parameters are presented. Use of suspensions with high solid contents allowed the production of thick, dense and mechanically stable coatings. The α-Al 2 O 3 was the main phase in the coatings produced through the injection of suspensions containing powders with very high purity.