Abstract 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.

Abstract Laser-assisted cold-spray has been recognized for over a decade as a technique capable of depositing high quality coatings. By laser heating (and hence softening) the surface being coated, deposition can occur at particle velocities lower than those normally associated with the cold spray process. This can be used to increase deposition rate. However, it can also be used to facilitate the deposition of higher hardness material combinations, normally more out of the reach of the conventional cold spray process. Laser heating can also reduce the requirements of the process on gas usage and gas heating for a given combination, making it more cost-effective. In the work reported below, the capability of a novel co-axially laser-assisted system (COLA) to deposit higher hardness materials, relevant to a range of different industrial applications, has been evaluated. This system can be retrofitted to conventional cold spray equipment.

Abstract Thermally sprayed hard coatings, including tungsten carbide and chromium carbide cermets and other hard metallic materials, were studied in two types of wear tests. Surfaces of the coatings were worn by coarse and hard quartz sand in a rubber-wheel dry abrasion wear test, and by fine and soft kaolin abrasive in a wet slurry abrasion wear test. The aim of the work was to study how the surfaces retain their high polished finish and gloss, and the type of wearing of different coatings and materials. The results showed that coatings with hard tungsten carbides were worn preferentially by removal of the the binder material. Cermet coatings with softer chromium carbides, and with another types of uniform microstructures showed more uniform wear and better retained their glossy finish.

Abstract The operating temperature of the combustion chamber of heavy-duty gas turbines has been continually increased in order to improve engine power and efficiency. Part of this improvement is due to the use of ceramic thermal barrier coatings (TBCs), which reduce thermal flow from the combustion gas to the metallic structural part of the components. The most traditional deposition technique for bond coats on combustion chamber components is Air Plasma Spray; whereas this technology is the optimum one for ceramic coatings, due to air entrapment in the plasma jet significant oxidation of Al is produced during bond coats deposition. The thermal design of a thick porous TBC able to reduce the heat flow to the minimum allowable by the materials temperature limits is performed. Paper includes a German-language abstract.

Abstract In this paper, the industrial application study is carried out to optimize the robot-controlled plasma spraying process of the thermal insulation layer for the hot gas flow of a GE-Frame-6B gas turbine. Due to the need for the off-line programming procedure, a precise 3D model of the automated coating system and the workpiece is created. In preliminary tests without a workpiece, basic spraying parameters such as spraying distance and spraying angle are optimized to achieve a sufficient layer quality. In the spray experiments, yttria stabilized zirconia Thermal Barrier Coating is used as the coating material and (NiCr)-6Al (Metco 443NS) as the bondcoat. The microstructure is examined using a light microscope, and microhardness measurements are also carried out. The results of the injection experiments are used to set the injection parameters and the travel path for the real component. Paper includes a German-language abstract.

Abstract The HVOF spraying process is used to produce NiCoCrAlYRe layers, which serve as adhesive layers for plasma-sprayed thermal insulation layers. This paper investigates the influence of the HVOF spray parameters on the structure and properties of the layers. The parameters are optimized to produce high layer densities with low degrees of oxidation and the same surface roughness for subsequent TBC coatings. High-temperature oxidation of HVOF-sprayed NiCoCrAlY Re layers by means of TBC layers is investigated at temperatures of 950 and 1000 deg C in time cycles of up to 3000 hours. A vacuum-sprayed layer with the same composition is examined as a reference. Oxidized layers are characterized, and peculiarities such as oxide layer thickness, proportion of the NiAl phase, and imperfections at the boundary layer are measured. It turned out that the less expensive HVOF layer offers the same protective properties as a VPS layer. Paper includes a German-language abstract.

Abstract The atmospheric plasma spraying process and the high velocity oxy-fuel (HVOF) spraying process are used to produce ceramic layers from titanium suboxide powder. Titanium suboxide powder is made from commercially fused and ground powders by reduction with hydrogen. Two powder particle sizes, which were produced for both atmospheric plasma spraying and for the HVOF spraying process, were used in tests. Originally melted and crushed powder fractions served as a reference powder. Furthermore, hardness, wear resistance, and electrical resistance were determined. All titanium oxide layers showed high wear resistance, with HVOF-sprayed layers exhibiting higher wear resistance than the layers produced with the atmospheric plasma spraying process. Atmospheric plasma-sprayed layers, on the other hand, are characterized by higher electrical conductivity. Paper includes a German-language abstract.

Abstract This paper presents the production of self-lubricating chromium carbide-25Ni+calcium fluoride multi-component layers using the HVOF spraying process. The spray powders are produced by atomization with subsequent sintering and qualification processes. The microstructure of the spray powder and layers is characterized by SEM methods. The content of added calcium fluoride in the sprayed layer is determined using the EDS method and compared with its content in the spray powder. The wear resistance is investigated using the Gray-Sand-Rubber-Real test. It turned out that atomization is a suitable method for the production of multi-component spray powders that contain solid lubricants. By adding a solid lubricant phase to the hard metal layer, the wear resistance is reduced for certain phase proportions. However, the wear resistance is at an acceptable level if layer properties such as wear resistance, corrosion resistance, and high temperature resistance are combined with one another to obtain suitable friction properties. Paper includes a German-language abstract.

Abstract This paper provides a short summary of the different concepts related to the structure and preparation of titanium carbide (TiC)-containing wettable powders that could be found in the literature. Empirical composite developments of TiC-containing materials for coating applications are not suitable for fully describing the technical potential of these composites. TiC-containing layers can complete the range of thermal coating solutions based on tungsten carbide and chromium carbide. The development of TiC-based powders that are better than the prior state of the art is the result of a long-term process producing permanent improvements. The processability of different TiC-based powders from alloying steps one and two was tested with various high velocity oxygen fuel installations and different fuel gases. TiC-based coatings offer special advantages through tailoring of the composition to suit requirements in applications in which complex stress profiles exist. Paper includes a German-language abstract.

Abstract In this paper, plasma layers sealed by aluminum phosphate and orthophosphoric acid are analyzed analytically by means of electron microscopy. Aluminum phosphate seals are used as thermal insulation layers for aluminum oxide and chromium oxide layers as well as for yttria stabilized zirconia thermal insulation layers based on zirconium oxide. 22 magnesium oxide-zirconium dioxide thermal barrier coatings are sealed with normal orthophosphoric acid. The microstructure of thermal barrier coatings based on zirconium oxide is extensively investigated. In yttria stabilized zirconia layers, the conversion-free tetragonal t'-zirconium oxide is found as the main phase in the lamellae. Seals created with the help of orthophosphoric acid are in amorphous form in the layer. The investigations of the sealing layer interface show a chemical reaction between the phosphate seal and the layer surface. From this, it follows that the sealing of the layer lamellas takes place through chemical reactions. Paper includes a German-language abstract.

Abstract The HVOF spraying process was used to create layers consisting of mechanically mixed chromium(II) carbide + 25% NiCr, NiCr, and CoCr-based metal powder alloys. This paper provides the investigation of additional influences of a metallic phase with tribological properties under friction conditions on the layer. The tribological properties of the layers are characterized under dry friction. The paper investigates the tribological behavior of the layer and the tribological film layers created during dry friction. It analyzes and discusses the influences of temperature, pressure, and friction speed on the tribological behaviour. Paper includes a German-language abstract.

Abstract This paper examines plasma-sprayed aluminum oxide and chromium oxide layers as well as aluminum phosphate-sealed layers by means of thermogravimetric analysis in order to determine both the stability of plasma-sprayed layers and the phosphate reaction as a function of the temperature during the sealing process. The thermogravimetric analysis of aluminum oxide layers showed no changes in weight in the layer during the heat treatment. In the case of chromium oxide layers, on the other hand, the thermogravimetric analysis showed an increase of 1.7 percent by weight due to oxidation of the sprayed layer. The thermogravimetric analysis of sealed layers showed weight losses of approximately 14%, which agreed well with the behavior of the pure seal. In the case of aluminum oxide layers, the behavior differed from that of sealed chromium oxide layers as a result of the reaction between the seal and the aluminum oxide layer. Paper includes a German-language abstract.

Abstract Zirconia based, 8Y 2 O 3 -ZrO 2 and 22MgO-ZrO 2 thick thermal barrier coatings (1000µm), were studied with different sealing methods for diesel engine applications. Aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of the TTBCs was sealed with two different methods, phosphate based sealing treatment and laser glazing. The thickness of the densified top layer in all cases was 50-400µm. XRD analysis showed some minor phase changes and reaction products caused by phosphate based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na 2 SO 4 - 40V 2 O 5 deposit was determined in isothermal exposure at 650°C for 200 h. Corrosion products and phase changes were studied with XRD after the test. Short-term engine test was performed for the reference coatings (8Y 2 O 3 - ZrO 2 and 22MgO-ZrO 2 ) and for the phophate sealed coatings. Engine tests were performed at the maximum load of the engine and it was aimed to evaluate the thermal cycling resistance of the sealed coatings. All the coatings passed the engine test, but some vertical cracks were detected in the phosphate sealed coatings.

Abstract Microstructural study of plasma sprayed chromia coatings sealed with aluminum phosphate, was carried out for determining strengthening mechanisms of the sealant. Characterization was accomplished by X-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. The main phase in the coating is the eskolaite type α-Cr 2 O 3 . The overall structure of the coating is lamellar with columnar grains parallel to the lamella thickness. Amorphous aluminum phosphate sealant has penetrated into the coating filling the structural defects such as cracks, gaps and pores between the lamellas. The average composition of the sealant in the coating is 25 at% aluminum and 75 at% phosphorus giving the molar ratio P/Al of 3, that corresponds to metaphosphates Al(PO 3 ) 3 . The aluminum phosphate sealing in the chromium oxide coatings is based on adhesive binding due to the attractive forces between the condensed phosphates and the coating. There were no indications about chemical binding due to reactions between the sealant and the coating in the sealing treatment for chromia coatings.

Abstract The aim of the present work is to study the microstructure of high temperature oxidation and corrosion resistant MCrAlY coatings prepared by high-velocity oxy-fuel (HVOF) spraying and to compare them to vacuum plasma sprayed (VPS) MCrAlY coatings. In this work, CoNiCrAlY, NiCoCrAlYTaReSiHf, and NiCrAlY coatings were prepared on nickel based superalloys by VPS spraying and by two types of HVOF spray process (DJ Hybrid 2600 and JP-5000). The influence of spray parameters, i.e. oxygen flow rate and spray distance, in one of the HVOF processes (DJH 2600) on coating structures and properties were studied also in more detail. The coatings sprayed with different spraying processes were studied in their as-sprayed state and after heat-treatment by optical microscope and scanning electron microscopy (SEM). The results show that selection of appropriate particle size of the spray powder has marked importance in the HVOF process; presence of fine powder particles produce oxides in the coatings and ease clogging of the nozzle in the HVOF gun. The use of appropriate size distribution of the spray powder enables preparation of HVOF MCrAlY coatings with low oxide content and dense structure. The oxide content of HVOF sprayed coatings should be kept as low as possible in order to make the post heat-treatment active in producing a dense microstructure with correct phase structure. The oxide content can be controlled e.g. by flame temperature control by means of oxygen flow rate and by appropriate spray distance selected individually for each powder. Unacceptable high oxide contents in the as-sprayed structures tend to remain unchanged after heat-treatment, whereas well-developed coating microstructure and phase structure are obtained with HVOF sprayed coatings with low oxide content. Microstructures developed in heat-treated VPS and HVOF sprayed coatings are compared.

Abstract On-line monitoring of two thermal spray processes by means of an imaging diagnostic technique capable of measuring several spray particle properties, such as individual and average particle temperatures, velocities and number of particles with spatial distributions, was studied by using the Spray-Watch thermal spray monitor. Aim of the work was to demonstrate the capabilities of this novel monitor in quick optimisation of certain spray gun parameters to obtain desired particle characteristics in-flight, and hence desired coating structure and properties with high deposition efficiency. Examples are given in plasma spraying of Al 2 O 3 and HVOF spraying of NiCoCrAlY.

Abstract A novel technique and an instrument for on-line, quantitative imaging diagnostics and process control in thermal spraying have been developed and tested in laboratory and industrial conditions. In-flight spray particles are imaged by their natural luminosity with a short-exposure, digital CCD camera system. Particle images are processed using digital image processing techniques in a PC computer. The number of particles per frame and the spatially resolved particle velocities are calculated from the images. Spectrally resolved image information is further used to determine pyrometric two-color particle temperature. A number of different modes of data presentation have been developed. The developed instrument can be used to determine particle number, velocity and temperature distributions of individual in-flight particles from the imaged region of interest of the plume. Dividing the imaged area into smaller sections, spatial distributions of these parameters can be studied. SprayWatch system provides a technically simple, easy to operate, single imaging instrument, which can provide a visual overview of the spray plume in combination with quantitative evaluation of the most important spray particle parameters. In this paper examples of using the monitoring system with plasma and HVOF spraying are presented. Preliminary test results of using a semiconductor laser generated light sheet to detect cold particles is also demonstrated.

Abstract Three different types of polyethylene powders were flame sprayed onto pre-heated steel substrate previously coated by electrostatic spray system with a thin epoxy primer layer. Properties of the polyethylene (PE) powders, including powder density, particle size and melt flow rate (MFR) were measured in order to study their influence on the mechanical properties of the coating. The spray experiments started with optimization of spraying parameters. The main variables were pre-heating temperature of the substrate, temperature increase during spraying (influenced by the spraying distance), and thickness of the PE coatings. The laboratory tests performed for the coatings were coating characterization by microscopy and mechanical testing. Porosity and thickness of the coatings were determined by optical and stereo microscopy studies from polished cross-sectional samples. Hardness, impact strength, peel strength, and adhesive strength of the coatings were also investigated. Also some hot water sinking and heat cycling tests were performed. As a result from the present studies it can be concluded that powder properties have great influence on the mechanical properties of the final coating.

Abstract Titanium carbide cermet spray powder was produced by the SHS process (Self-propagating High-temperature Synthesis) using elemental Ti, C, Mo and prealloyed CrNiMo powders as starting materials. The powder was characterised (particle size distribution, phase structure, morphology) and the internal structure of each cermet particle was found out to be dense consisting of fine distribution of carbides embedded in a metallic matrix. The particle size range suitable for thermal spraying was obtained by sieving and air classifying. The coatings were prepared by HVOF spraying (DJH2600 and DJH2700). The dry abrasion wear resistance was evaluated by the rubber wheel abrasion wear test and electrochemical corrosion behaviour by open circuit potential measurements. According to the XRD analysis the amount of retained carbides in the coatings is high and the carbide phase has a spherical shape also in the coatings. The microstructure of coatings obtained is dense and the coatings possess good properties in wear and corrosion tests. WC-Co-Cr and Cr3C2-NiCr powders were used for comparison.

Abstract Hardmetal-like coatings of the TiC-Ni system are potential for use as wear, corrosion and heat resistant coatings in various operation conditions. Our previous works [1-12] have shown that these materials are well sprayable using different thermal spray processes such as plasma, D-Gun and HVOF spraying. Since HVOF spraying is today the most important process used to apply carbide based coatings, this study was carried out in order to evaluate more systematically the sprayability of these novel spray powders and the influence of HVOF spray parameters on some coating properties. Coating samples were prepared by using DJ Hybrid gun with propane as a fuel gas, and a CDS gun with hydrogen fuel gas. Oxygen flow rate was varied in both cases for changing the flame temperature. Microstructure, phase composition, hardness, and abrasion wear resistance of the coated samples were investigated. The results showed that both HVOF processes used give satisfactory coating properties and that the use of high oxygen flow rates is beneficial for improving the wear resistance of the coatings. Powders with fine particle size are beneficial in the DJ Hybrid process; the use of coarse powders results in coatings with somewhat higher wear rates. The optimum spray condition for the TiC-Ni system powders differs from that typically used for conventional WC-Co and Cr3C2-NiCr powders by a higher flame temperature.