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.

Dense sintered technical ceramics demand special surface preparation in order to be coated by thermal spraying. Sandblasting results in the damage of the interface region and leads to bonding defects. On the contrary, by varying the laser conditions, different laser structured surfaces were generated, which at Rz roughness values in the range of 40 μm, allowed to achieve thick and well bonding coatings. Therefore, laser ablation is proved to be the best method for surface preparation of ceramic substrates. In the case of porous ceramics (including pre-sintered ceramics) substrate preparation can be omitted, depending on their porosity level. Ceramics with porosity content up to 60 % can be coated by APS, whereas HVOF can be used on ceramics with porosity up to 30 %. On ready-to-get-coated pre-sintered ceramic parts (without substrate preparation), followed by co-firing of the substrates and coatings, the development of new ceramic components is possible.

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 Cr 3 C 2 -NiCr feedstock powder characteristics on coating microstructures and economic parameters like deposition rate and deposition efficiency. Four commercial Cr 3 C 2 -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 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.

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 assesses the challenges of preparing dense technical ceramic substrates for thermal spraying and evaluates the capabilities of laser ablation in comparison with sandblasting. Sintered Si3N4 and AlN substrates were prepared by both methods and surface roughness was measured before and after treatment. Alumina coatings were deposited by suspension-HVOF and atmospheric plasma spraying, and coating cross-sections were analyzed by optical microscopy and SEM. Sandblasting had little or no effect on surface roughness and cracks were observed in coating cross-sections at the near-surface region of the substrate. Laser ablation, on the other hand, significantly increased surface roughness for both ceramics, producing hole patterns that are shown to vary with laser power and pulse timing. In the case of plasma spraying, the best coatings were achieved when the holes in the substrate were less than 100 µm in depth. With suspension sprayed coatings, the best results were obtained on substrates with deeper (> 100 µm) holes.

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.

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 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.

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.

The combination of excellent mechanical, thermal and chemical properties of silicon carbide (SiC) and titanium carbide (TiC) has made these materials very attractive both for structural ceramics applications and for thermal sprayed coatings. To suppress oxidation and to avoid the formation of silicides during spraying of SiC-based composites, feedstock spray powders have been developed containing 32 wt.-% of an alumina-yttrium ceramic binder matrix. The spray powders are prepared by spray-drying and sintering (a&s). Also, TiC-based composite spray powders showing the same matrix material and content have been developed and produced. Thermal spray processing of the described powders by atmospheric plasma spraying (APS) using an F6 APS torch and high velocity oxygen fuel spraying (HVOF) with the Top Gun G acetylene torch is carried out. Both the produced coatings and feedstock powder are characterized by optical microscopy, X-ray diffractometry (XRD) and scanning electron microscopy (SEM) including energy dispersive X-ray analyses (EDXS).

The specific advantages of TiC as a hard material are its low density, high hardness and the high alloyability of the hard phase - binder metal composite. Currently developed agglomerated and sintered core-rim structured TiC-based powders were intensively studied in the last few years for thermal spray coating solutions. In the work described in this paper two different powders with cubic (Ti,Mo)C and (Ti,Mo)(C,N) hard phases and Ni/Co binder, representing the first and second alloying step for the binary TiC-Ni/Co composite, were used together with mechanically mixed NiBSi powder to produce wear resistant coatings by plasma-transferred arc welding (PTA) and laser cladding. Basic process parameters, coating microstructures and properties are described. Coatings with fine grained hard particles were obtained by both processes, while the coating prepared from the nitrogen-containing powder by laser cladding shows a significant smaller hard particle grain size and increased hardness.

In the current study, the tribological properties of TiC-based coatings paired with polycrystalline alumina under unlubricated sliding conditions were investigated in order to demonstrate the technological and engineering potential of such coatings. (Ti, Mo)(C, N)-Co coatings were prepared from an agglomerated and sintered spray powder by HVOF spraying using JP-5000 equipment. Cr 3 C 2 -NiCr coatings were studied for comparison. Sliding wear tests were performed over the temperature range from 23°C to 800°C with sliding speeds in the range 0.3-3 m/s, a wear distance of 5000 m and a normal force of 10 N. Wear rates of coatings and sintered alumina counterparts were measured separately. Compared with Cr 3 C 2 -NiCr coatings, (Ti, Mo)(C,N)-Co coatings showed significantly lower total wear rates, corresponding to those found in the region of mixed/boundary lubrication. With few exceptions, the coefficients of friction were found to be lower for (Ti, Mo)(C,N)-Co coatings than for Cr 3 C 2 -NiCr coatings. After tests were performed, the coating microstructures were studied by optical microscopy and SEM. The oxide scales formed on the coating surfaces were investigated by SEM and X-ray diffraction. Comparison of the total wear rates of the couples consisting of sintered alumina and a (Ti, Mo)(C,N)-Co coating with those of other alumina-ceramic and hardmetal-hardmetal tribological systems demonstrates the immense potential of TiC-based coatings for sliding wear applications.

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.

Advanced TiC-based spray powders contain molybdenum and nitrogen as alloying elements and consist of core-rim-structured (Ti,Mo)(C,N) hard phases embedded in Ni, Co or mixed Ni/Co binder phases. For the investigations of the present study, four powders with binder phase contents of 29 and 39 mass-%, corresponding to about 20 and 27 vol.-%, respectively, were prepared by agglomeration and sintering. Coating samples were sprayed using JP-5000 equipment and two spray parameter sets. During coating characterization, special emphasis was placed on the changes in chemical (carbon loss and oxygen uptake) and phase compositions during spraying and their correlation to the coating microstructures and properties. It was found that the decreases in carbon and nitrogen contents were practically independent of the spray parameters for all of the four powders, whereas the oxygen uptake was different for each of the powders. The effect of the oxygen content on different coating properties is discussed in detail. All coatings investigated in this work showed excellent properties. It is anticipated that TiC-based coatings sprayed from these types of advanced powders can complement thermal spray coating solutions based on WC and Cr 3 C 2 .

CaZrO 3 is a material for thermally sprayed ceramic coatings to which so far only a little attention was paid. This material has a high melting point, good thermal stability and a coefficient of thermal expansion close to that of steel. In this paper water stabilized plasma spraying (WSPR) and atmospheric plasma spraying (APS) were used to prepare CaZrO 3 coatings. The spraying feedstock was prepared from fine CaZrO 3 powder by agglomeration (spray drying) and sintering. Powders with three different particle sizes (- 45 + 20 µm, - 63 + 45 µm and -90+63 µm) were used in the experiments. The coarse fractions were used for WSP spraying, while the fine one was sprayed with the APS process. Plasma sprayed materials were studied from the point of view of phase changes and influence of the powder size on structure of coatings. The changes of phase composition were studied by X-ray diffraction on coatings as well as on free flight particles. Formation of a cubic phase with a reduced content of CaO in comparison to CaZrO 3 was observed. Its formation is probably connected with evaporation of CaO during spraying. This cubic phase is similar to the phase obtained by spraying of ZrO 2 +5%CaO. Plasma sprayed coatings were characterized by light and scanning electron microscopy (SEM) and by density and porosity. Coefficients of thermal expansion of plasma sprayed layers from CaZrO 3 were measured.

This paper provides a short summary of the information that can be found in the literature on the preparation and structure of titanium carbide (TiC) wettable powders. From the work that has been done, it can be concluded that TiC-based coatings can be tailored for applications where complex stress profiles are encountered. Paper includes a German-language abstract.

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.

Oxide-bonded silicon nitride (OBSN) powders have been developed to address thermal spray problems associated with high temperatures. This paper examines how such powders perform when applied via detonation gun (DGS) and atmospheric plasma spraying (APS) with axial powder injection. All coatings were characterized using optical microscopy and X-ray diffraction with additional tests being performed on DGS coatings. For the first time, relatively dense Si3N4-rich coatings with an oxide binder phase were produced, and some of the DGS coatings were found to be sufficiently wear resistance for industrial use.