Manufacturing of steel components is often done at high temperatures (HT) posing a serious challenge to components such as forming tools. Thermal spray coatings provide a cost-effective solution for surface protection under HT, corrosive environments and severe wear conditions. Thermally sprayed coatings based on cubic hard materials such as TiC and TiCN can provide an alternative to widely used Cr3C2-NiCr. While the latter possess a superb oxidation resistance and wear resistance at HT, they are prone to degradation in the presence of Mn, an element commonly alloyed in many modern steel grades such as TWIP (twinning-induced plasticity steel). In this study, a (Ti,Mo)(C,N)-29% Ni hardmetal feedstock powder was prepared by agglomeration and sintering. Coatings were deposited using a high velocity air-fuel (HVAF) spray process. The coating was benchmarked against a standard Cr3C2-NiCr coating obtained with the same spray process. Our work comprises analyses of the feedstock powder along with the resulting coating microstructure after deposition and heat treatment. Further, the HT sliding behavior against TWIP steel using a HT pin-on-disc tribometer at 700°C was investigated. The results showed a clear benefit of the TiCN-based coating, with almost no wear detected, while the Cr3C2-coating showed a significant wear loss. Based on these results, the TiCN-based coating is regarded as potential solution for prospective forming applications of modern high Mn steels, such as TWIP.

Aluminum titanate (Al 2 TiO 5 ) is a congruently melting compound in the binary Al 2 O 3 -TiO 2 system, which decomposes below 1200 °C. Its properties (e.g. thermal conductivity, CTE) differ significantly from those of Al 2 O 3 and TiO 2 . Thus it is of special interest to study the stability of Al 2 TiO 5 in the spray process and its influence on the coating properties. A commercial fused and crushed Al 2 O 3 -40%TiO 2 powder, which was found to be substoichiometric, was selected as the feedstock material for the experimental work, as the composition is close to stoichiometric Al 2 TiO 5 . Part of that powder was heat-treated in air at 1150° and 1500°C in order to vary the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including metallographically prepared cross sections. A powder having Al 2 TiO 5 as the main phase was not possible to be prepared due to inhomogeneous distribution of Al and Ti in the original powder. Plasma spraying was performed with a TriplexPro-210 (Oerlikon Metco) using Ar-H 2 and Ar-He plasma gas mixtures with 41 and 48 kW plasma power. Coatings were studied by XRD, SEM of metallographically prepared cross sections, and microhardness HV1. Moreover, the results show a clear influence of the Al 2 TiO 5 content in the feedstock powder on the phase composition of the coatings.

Wear protection is one of the major applications of thermally sprayed hardmetal coatings. This paper presents the latest results of a systematic study on the influence of Cr3C2-NiCr feedstock powder characteristics on coating microstructures and economic parameters like deposition rate and deposition efficiency. Four commercial Cr3C2-NiCr powders with spherically shaped particles but different structural features were characterized and deposited by a liquid-fueled and a gas-fueled HVOF and a HVAF process. Deposition rates and efficiencies were determined; all coatings were analyzed in as-sprayed condition and selected samples were heat-treated at 800 °C in argon atmosphere. The effects of the feedstock powders and spray processes on the coating characteristics (microstructure, hardness, Young’s modulus and diffusion processes during heat treatment) were studied.

This paper presents the results of a study on the static friction properties of (Ti,Mo)(C,N)-Ni hardmetal coatings deposited by HVOF spraying. Coating samples were sprayed on quenched and tempered CrMo steel friction rings using an experimental feedstock powder. Friction surfaces were characterized based on the geometry and distribution of prominent peaks and static coefficient of friction was measured. Test results show that the hardmetal coatings have good potential for use in frictionally engaged joints, but more work is needed to establish a correlation between coating properties and friction behavior.

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.

This study assesses the potential use of thermally sprayed dicalcium diiron pentaoxide (Ca 2 Fe 2 O 5 ) for thermoelectric generators. Ca 2 Fe 2 O 5 coatings up to 2 mm thick were produced by atmospheric plasma spraying and examined. Compared to the bulk material, the coatings exhibit lower thermal and electrical conductivity. The Seebeck coefficient could not be measured, and the thermoelectric performance was inadequate. The limitations derive not only from the thermal spray process, but also the material itself.

The work presented in this paper addresses some of the challenges of manufacturing thermoelectric (TE) generators by thermal spraying. One of the main obstacles is achieving good coat-on-coat bonding between different types of materials. The coatings must also be mechanically stable and optimized for their respective function. At least four types of materials are required, including electrical insulators, conductors, and thermoelectrically active p- and n-type semiconductors. Four ceramic and three metal feedstock powers were deposited by APS, HVOF, and HVAF spraying using special masking systems, substrate pretreatments, and layer thickness monitoring. After process optimization for each material, multilayer TE generator modules were successfully produced.

This work assesses the influence of powder characteristics on the deposition efficiency, microstructure, and tribological properties of Cr 3 C 2 -NiCr coatings. Four commercial powders prepared by different methods were used for the study. All have a spherical morphology but vary in terms of porosity, carbide grain size, and flowability. The feedstocks were deposited on flat low-carbon steel substrates using a liquid-fueled HVOF torch mounted on an industrial robot. Deposition efficiency was measured along with coating hardness, Young’s modulus, and abrasive wear resistance. In addition, some of the coatings were heat treated and changes in microstructure and hardness were recorded.

This work demonstrates a new single-cathode, multi-anode plasma spray process and compares it with conventional APS and HVOF spraying. Alumina feedstock powders mixed with 13, 40, and 44 wt% titania were deposited under a wide range of spraying conditions following a design of experiments approach. Deposition rate and efficiency were measured and coating characteristics, including microstructure, phase composition, hardness, Young’s modulus, electrical resistivity, and cavitation wear, are compared. The results are presented and the advantages of each process are discussed.

In order to improve both corrosion and wear resistance of standard plasma sprayed alumina coatings chromia alloyed feedstock powders and high velocity oxygen fuel (HVOF) spraying as an alternative process were used in this work. Alumina and alumina-chromia powders with different chromia contents (27% and 50%-mass., but with different amount of (Al,Cr) 2 O 3 solid solution formed) were deposited by TopGun HVOF spraying and atmospheric plasma spraying (APS). The coatings were evaluated by optical microscopy, microhardness measurements, XRD and SEM. Abrasion wear resistance of the coatings was tested with the rubber-wheel abrasion test. Corrosion characteristics of the coatings were evaluated by exposure tests at 85 °C for 310 hours at low and high pH conditions. The amount of dissolved elements in solution, as analysed by ICP, was found to be a direct indication of the corrosion resistance of the coatings. Coatings deposited by TopGun HVOF spraying resulted in a denser microstructure, higher microhardness and significantly improved abrasion wear resistance as compared to corresponding coatings deposited by APS. For the tested conditions, corrosion resistance of both APS and HVOF-sprayed alumina coatings was low but significantly improved by chromia addition for both spray processes. However, the content of chromia addition alone does not allow to draw conclusions on the effectiveness of this measure.

In this paper the characteristics (microstructure, phase compositions) and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) process and conventional HVOF spray method are compared. The electrical resistance (electrical resistivity) and dielectric strength were investigated using DC-electrical resistance measurements, electrochemical impedance spectroscopy (EIS) and dielectric breakdown test. The electrical resistance was determined at room temperature at different relative air humidity (RH) levels, from 6% RH up to 97% RH. Differences in the electrical insulating properties due to the different coating characteristics are discussed. The suspension-sprayed Al 2 O 3 coatings showed better electrical resistance stability at high humidity levels (> 75% RH), which could be explained by a specific microstructure and retention of a higher content of α-Al 2 O 3 . Nonetheless, the values of dielectric breakdown voltage and dielectric strength recorded for suspension sprayed coatings were lower than those of HVOF coatings.

Considerable effort has been made to translate the beneficial properties of bulk Ti(C,N)-based hardmetals to wear resistant thermal spray coatings. Such efforts have focused primarily on as-sprayed coatings. However, past work has shown that hardmetal coatings can undergo significant changes when operated at elevated temperature for extended periods. This work characterised the microstructural changes in a HVOF sprayed (Ti,Mo)(C,N)-Ni coating treated in air at 700°C for up to 30 days. The microstructural development of the carbonitride phase was very subtle. Image analysis indicated that the Mo-rich rim phase underwent the greatest degree of dissolution during spraying and precipitation with heat treatment. Dissolution of the carbonitride phases during spraying led to significant alloying of the Ni binder. Rapid recovery of the Ni binder composition occurred after one day of treatment, but it retained a higher steady state degree of alloying relative to the starting powder.

Cr 3 C 2 -NiCr coatings are commonly used to provide abrasion and erosion wear resistance on the surface of components, in particular for corrosive and atmospheric high-temperature environments. For these classical and new applications the knowledge of the thermophysical properties is highly important. In the present work the dependence of the heat conductivity on temperature of two HVOF-sprayed Cr 3 C 2 -25NiCr-coatings prepared by a liquid-fuelled HVOF-process from two different feedstock powders from room temperature up to 700 °C was determined. Thermal diffusivities, density functions, specific heat capacities and coefficient of thermal expansion (CTE) were measured in order to compute the heat conductivity for the coatings. All measurements were performed twice (as-sprayed and after a first thermal cycle) in order to take into account the structural and compositional changes. XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Heat conductivities (average of the two coatings) ranging from about 11 W/(mK) at 50°C up to about 20 W/(mK) at 700°C were determined. Differences between the two coatings were clearly detectable. The heat conductivity of the Cr 3 C 2 -NiCr coatings is significantly lower than determined previously for a WC-17%Co coating.

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.

Young’s modulus is not only one of the important mechanical properties of thermally sprayed coatings but also a sensitive indicator of the coating’s microstructural defects. The ceramic coatings studied in this work were based on Al 2 O 3 , TiO 2 and Cr 2 O 3 and prepared by APS (Atmospheric Plasma Spraying), HVOF (High Velocity Oxy Fuel spraying) and suspension-HVOF spraying. The Young’s modulus was systematically studied by laser acoustic surface waves, which is a non-destructive, fast and reliable technique, using two different devices: a tabletop tester for samples and small components (LAwave) and a hand-held portable device. For oxide coatings, it was observed that the results distinctly vary depending on the spray technology. A comparison with the values derived from the instrumented indentation test shows that for most investigated ceramic coatings lower values were measured by laser acoustics. This is due to the influence of the pores and defects in the coatings. The LAwave results are expected to be close to the effective modulus of the material, due to the larger material volume evaluated during the test, which takes the coating defects into consideration.

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.

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.

The metastable anatase structure of titanium dioxide (TiO 2 ) is photocatalytically active and thus able to purify air and water. This makes anatase TiO 2 a desirable material for instance in environmental technology. The present work provides a comparative study on the photocatalytic activity of TiO 2 coatings, prepared by different spray methods: conventional atmospheric plasma spraying, HVOF spraying, suspension spraying and cold spraying. The spray experiments were performed with anatase powder as feedstock and the spray parameters were optimized in order to obtain coatings with high photocatalytic performance. With respect to spray technique and spray conditions, different amounts of the photocatalytic active anatase phase could be retained. The photocatalytic activity of the coatings was evaluated by degradation of dichloroacetic acid. The results show that the photocatalytic activity is highly dependent on the amount of preserved anatase structure obtained under the different spray techniques.

The development of new hardmetal coating applications such as fatigue-loaded parts, structural components and tools for metal forming is connected with improvement of their performance and reliability. For modelling purposes the knowledge of thermophysical, mechanical and other material data is required. However, this information is still missing today. In the present work the thermophysical data of a WC-17Co coating sprayed with a liquid-fuelled HVOF-process from a commercial agglomerated and sintered feedstock powder from room temperature up to 700 °C was determined as an example. The dependence of the heat conductivity on temperature was obtained through measurement of the coefficient of thermal expansion, the specific heat capacity and the thermal diffusivity. Heat conductivities ranging from 29.2 W/(mK) at 50°C to 35.4 W/(mK) at 700 °C were determined. All measurements were performed twice (as-sprayed and after the first thermal cycle) in order to take into account the structural and compositional changes. Extensive XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Bulk samples obtained by spark plasma sintering from the feedstock powder were studied for comparison.

In the present work, APS and HVOF processes have been used to prepare alumina (Al 2 O 3 ) and magnesium aluminate spinel (MgAl 2 O 4 ) coatings designed for electrical insulating applications. The microstructures and the phase compositions of the sprayed coatings were evaluated by microscopic and XRD analysis. The electrical characteristics electrical resistance, electrical resistivity and dielectric breakdown strength were investigated using different methods: direct current (DC) measurements, electrochemical impedance spectroscopy (EIS) and dielectric breakdown testing. The electrical resistance was measured at room temperature at different humidity levels. Differences in the insulating properties due to the different natures of the coating materials, microstructures and the measurement methods used for electrical characterisation are discussed.