Despite their light weight, 2.3 times lighter than Al, polymers are limited to application with low thermal, wear, and abrasion demands. The enhancement of the functional surfaces of the polymers using thermal spraying techniques is a challenging task due to the thermal degradation of polymers, the low wettability, and the disparate atomic properties. The twin-wire arc spraying (TWAS) process comprises two contradictory features. Almost all spraying particles are in a molten state on the one hand, and on the other hand, the spray plume has the lowest heat output among the different thermal spraying techniques. Therefore, it is a promising spraying technique for the required surface improvement. The surface of the 3D-printed parts was metalized using two successive layers. The first layer is a TWAS coating made of low-melting ZnAl 4 to avoid thermal degradation and provide a bond coat. The topcoat is also applied using a TWAS process and was made out of Ni-WC-Co as cored wires. The top hard coating has improved the wear resistance of the polymers by 14.6 times. The erosion of the coated and uncoated specimens was determined using a low-pressure cold gas spray gun. Ni-WC-Co coating led to more than five times higher erosion resistance.

Internal diameter (ID) coating by means of thermal spraying for the wear and corrosion protection of components is currently experiencing growing interest in science and industry. While high-kinetic spray processes (such as HVOF, HVAF or warm spraying) in combination with cermet materials (e.g. WC-Co or Cr3C2-NiCr) are well established for this purpose in traditional coating of external OD (outer diameter) surfaces, they have hardly been used in the ID (internal diameter) area so far. Even though a few special ID spray guns with compact design and low combustion energy are by now available on the market, only little is known about the effects and interactions of the spray parameters on the particle behavior and the coating properties. Due to the mentioned gun specifications and the usually required short spray distances for ID coating, fine spray powders < 15 μm must be used to ensure sufficient melting and acceleration of the particles. In this study warm spraying of fine WC-12Co powders (-10 + 2 μm) using a novel spray gun “ID RED” (Thermico, Germany) was investigated. Statistical design of experiments (DoE) was employed to analyze and to model the influence of varying spray parameter settings on the in-flight particle behavior and the corresponding coating properties.

Grinding wheels are usually manufactured by powder metallurgical processes, i.e. by moulding and sintering. Since this requires the production of special moulds and the sintering is typically carried out in a continuous furnace, this process is time-consuming and cost-intensive. Therefore, it is only worthwhile for medium and large batches. Another influencing factor of the powder metallurgical process route is the high thermal load during the sintering process. Due to their high thermal sensitivity, superabrasives such as diamond or cubic boron nitride are very difficult to process in this way. In this study, a novel and innovative approach is presented, in which superabrasive grinding wheels are manufactured by thermal spraying. For this purpose, flat samples as well as a grinding wheel body were coated by low-pressure (LP) cold gas spraying with a blend of a commercial Cu-Al2O3 cold gas spraying powder and nickel-coated diamonds (8-12 μm). The coatings were examined metallographically in terms of their composition. Afterwards, the grinding wheel was conditioned for the grinding application and the topography was evaluated. This novel process route offers great flexibility in the combination of binder and hard material as well as a costeffective single-part and small-batch production.

Metal structures in offshore facilities are usually protected from corrosion using Zn-Al coatings even though they are subjected to collective stress conditions. This paper evaluates a post-treatment called machine hammer peening and its effect on surface finish, induced residual stresses, and near-surface microstructure of thermally sprayed ZnAl4 coatings. As expected, coating roughness was reduced from about Rz = 53.5 μm in the as-sprayed condition to 10.4 μm after treatment and coating densification was revealed in the near-surface zone. Residual stresses, which were surprisingly compressive in the as-sprayed condition, were likewise affected by the peening process, reaching a maximum of 200 MPa. The influence of peening direction and other such parameters were also investigated as part of the study.

This study investigates the effect of preheating on the dynamic flowability of HVOF powders, including conventional WC-Co, nano WC-Co, WC-FeCrAl, and Cr 3 C 2 -NiCr. The results show that the flowability of WC-Co powders can be significantly improved with a two-hour preheat at 200 °C. One explanation for the improvement is that moisture absorbed by the powder is released during pretreatment, but further study is required as it was found that dynamic density influences flow behavior as well.

Due to their superior wear resistance and oxidation behavior, Stellite coatings are widely used in industrial applications where they are exposed to high temperature. Common processes for applying Stellite coatings include high-velocity oxyfuel spraying, laser cladding, and plasma transferred arc welding. Although Stellite welding consumables are available, there are few studies on arc-sprayed Stellite coatings in the literature. This work investigates the microstructural characteristics of an arc-sprayed deposit produced using a CoCr-based cored wire with 4.5 wt% W. The deposit is examined both in its as-sprayed state and after high-temperature exposure. Microstructure formation is assessed via SEM and EDX analysis, phase transformation processes are determined by XRD analysis, and friction and wear properties are measured. The findings are presented and discussed and compared with those obtained from conventional CoCr-based coatings.

In this study, WC-Co coatings were deposited on additively manufactured 316L stainless steel substrates by HVOF spraying. Prior to spraying, the SLM parts were exposed to various mechanical pretreatments, before and after which their surface topography and residual stress state were assessed. After spraying, Vickers indentation tests were conducted to assess interfacial bond strength between the coating and substrate. To differentiate between topographical effects and residual stress related phenomena, stress-relief heat treatments were conducted at various points in the investigation.

The ongoing development of new HVOF spray guns for internal diameters is driving demand for finer spray powders. Fine spray powders (< 20 µm) are necessary to achieve short spray distances, but they also create new challenges. The first steps in the thermal spray process chain are powder preparation and feeding. If these steps are not stable, no sufficient coating quality can be obtained. This present work compares volumetric and fluidization powder feeding methods and investigates the feeding behavior of agglomerated and sintered WC-Co(Cr) powders with particle fractions of -5+15 µm, -20+5 µm, and -10+2 µm. Particle size fraction was measured ex situ by laser diffraction and particle outflow from the injectors was recorded in-situ by means of particle image velocimetry.

Current developments in different industrial sectors show an increasing demand on thermally sprayed internal diameter (ID) coatings. But up to now, the research focus is mainly on conventional processes such as arc spraying and plasma transferred wire arc spraying (PTWA), especially for cylinder liner surfaces. However, efficient HVOF and APS torches are meanwhile available for ID applications. Thus, in the present work, the focus of research is on the ID spraying of bond coats (BC) and thermal barrier coatings (TBC) for high temperature applications. An HVOF-ID gun IDCoolFlow mono with a N 2 injection was used to spray dense BCs (MCrAlY). The TBCs (YSZ) were sprayed by applying an SM-F100 Connex APS torch. Initially, flat steel samples were used as substrates. The morphology and properties of the sprayed ID coating systems were investigated with respect to the combination of different HVOF and APS spray parameter sets.

Additive manufacturing (AM) has already been evolved into a promising manufacturing technique. In order to achieve the performance of conventionally manufactured components, additively manufactured components must meet at least the same mechanical and physical requirements. Due to the layer-wise building process, the properties of additively manufactured components differ from that of bulk materials. Within the scope of this study, selective laser melting (SLM) was employed to manufacture specimens which serve as substrates for a subsequent coating process. An Inconel 718 (IN718) alloy served as AM feedstock. Mechanical posttreatments were applied to the AM samples and rated with respect to the successive thermal spraying process. The produced AM samples were examined in their initial state as well as under post-treated conditions. In this report, the resulting surface roughness was analyzed. Different AM samples were coated by means of high velocity oxy-fuel (HVOF) spraying and atmospheric plasma spraying (APS). The interface between the thermally sprayed coating and the AM substrate was metallographically investigated. Adhesion tests were conducted to scrutinize the bond strength of the coating to the AM substrate.

Diamond is the hardest known material and hence is suitable for a large, diverse field of applications for industrial processing tools. Different types of diamonds are used for these tools. For example, large diamonds are frequently used to industrially process minerals, while fine diamonds are used to process glass or carbon fibre reinforced plastic, as well as for grinding and lapping processes. A major challenge when processing diamonds is the process temperature as diamonds show decomposition effects at higher temperatures. Thermally spraying processes, which have the advantage of having only a small thermal influence on diamonds due to the short dwelling time of the particles in the flame, are going to be investigated as an alternative processing method for diamond-reinforced coatings. Additionally, these processes are flexible regarding the application process of contour accurate coatings. This paper gives an insight into the relationship between spray process parameters and the diamond decomposition concerning the particle temperature during the spraying process. For this purpose, the process parameters are varied and the resulting coatings are characterized, analyzing their influence on the diamonds in the coating. One focus of this paper is the scanning electron microscopical investigation of a thermally induced carbonating of the diamonds.

In the present work, three different APS alumina coatings were fabricated using three fused and crushed alumina powders of different particle size fine, medium and coarse. The influence of the particle size on thermal properties and micro-structural features of the produced coating were investigated by thermal insulation test and detailed image analysis technique, respectively. The analyzed micro-structural features include the total porosity, pore size (fine, medium, and large) and cracks. All types of cracks were considered in calculations as voids and were evaluated according to their sizes as pores. All spray parameters except the particle size were fixed throughout the spraying process. The results revealed that the fine starting powder has produced the densest coating with the lowest total porosity and that the total porosity increases with an increasing particle size. This was expected as powders of smaller particle size will reach a higher in-flight temperature and velocity than powders of bigger particle sizes as long as the same spray parameters are applied. However, a detailed image analysis investigation on the three produced coatings showed that the fraction of fine pores and cracks versus the total porosity is substantially higher in coatings produced by using fine starting powders than those produced using medium and coarse powders. In this work, a connection between the thermal insulation and the porosity fraction, which includes fine pores and cracks, was revealed.

In order to guarantee their protective function, thermal sprayings must be free from cracks, which expose the substrate surface to e.g. corrosive media. Cracks in thermal sprayings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross-sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology as well as coating defects were inspected using light microscopy on metallographic cross-sections. Additionally, the resulting crack patterns were imaged in 3D by means of X-ray micro-tomography.

The super-elasticity behavior of a NiTi-shape memory alloy (SMA) is very promising regarding cavitation resistance. The need of high vacuum conditions by thermal spraying processes, to avoid oxidation, has always been and still is the main obstacle for the widespread of NiTi as a coating material. This work deals with studying the effect of the different shroud concepts on the obtained oxide content and the phases of the obtained twin wire arc sprayed (TWAS) coatings. The concepts include the use of argon as a shield in gas shroud (GS) as well as the use of an extended air cap attachment as a massive shroud (MS). The use of MS-concept led to a significant decrease in oxide content and therefore was selected to spray pre-alloyed NiTi-SMA wires. The standoff distance between the MS-outlet and the substrate surface shows also an effect on the obtained phases and thus on the behavior of the obtained coatings. At lower standoff distance a pseudo-elastic behavior was obtained and therefore a higher cavitation and wear resistance. The use of argon as atomization and shield gas with a massive shroud could be a cost-effective alternative for vacuum process in case of spraying NiTi-SMA pre-alloyed feedstock materials.

Due to good performance in abrasive and sliding wear and enhanced oxidation behavior, coatings based on Co-Cr-W alloys are widely used in industrial applications, where the material is exposed to high temperature. Within the scope of this study, a Co-based alloy similar to commercial Stellite 6, which additionally contains 20.6 wt.% of vanadium, was deposited by Twin Wire Arc Spraying (TWAS). Multi-criteria optimization using statistical design of experiments (DoE) have been carried out in order to produce adequate coatings. The produced coatings have been analyzed with respect to their tribological behavior at elevated temperatures. Dry sliding experiments were performed in the temperature range between 25°C and 750°C. Oxide phases were identified in the investigated temperature range by X-ray diffraction (XRD) using synchrotron radiation. The V-doped Stellite-based coating possesses a reduced coefficient of friction (COF) of about 0.37 at elevated temperatures (above 650°C), which was significant lower when compared to conventional Stellite 6 coating that serves as reference. In contrast, both produced coatings feature a similar COF under room temperature. X-ray diffraction reveals the formation of cobalt vanadate and vanadium oxides above 650°C. The formation of vanadium oxides exhibits the ability of self-lubricating behavior, thus leading to enhanced tribological properties.

Additive manufacturing (AM) techniques give access to completely new manufacturing processes. AM techniques using metals, ceramics, or plastics feedstock are predestined for lightweight construction and for components with complex shapes or internal functions. AM processing with plastics stands out due to the low density of polymers, a good process capability, and low initial costs. The properties of polymer components are extremely dependent on the utilized plastics and the reinforcements, e.g. in the form of fibres. Furthermore, coatings can improve the properties and enhance the possible range of applications for plastics. In the present study, PLA (polylactic acid) was printed utilizing Fused Layer Modeling (FLM). The surfaces of the PLA samples were directly structured with pits with different widths during printing. Subsequently, the surfaces were coated with ZnAl 2 by means of Twin Wire Arc Spraying (TWAS). Adhesion tests meeting DIN EN 582 were conducted to measure the adhesion of the coating on the structured plastic surface. The results were compared to the adhesion of ZnAl 2 coatings on grit blasted and as-built surfaces. Overall, the surface adhesion was significantly better for the samples with directly structured surfaces. Hence, a direct structuring of the surface during a 3D building process promises to be an outstanding possibility to prepare surfaces prior to coating processes.

In light weight constructions, research focuses more and more on ecological aspects of materials. In this way vulcanized fiber relive a renaissance because it is produced from renewable raw materials. Vulcanized fiber is a composite material, which was approximately discovered in 1855. It is manufactured by parchmentizing of pulp with the use of a zinc chloride solution. This material is well known for its good strength to weight relationship, but it is susceptible to the contact with aqueous fluids and also shows only a low resistance to wear. Therefore, a functionalization of the surface is needed. The functionalization can be realized by thermal spray processes to increase the potential of vulcanized fiber. Especially the pretreatment of the specimen to enable the coating application is one of the key topics of this paper. Vulcanized fiber is a thermal sensitive material. The impact of hot spray materials can cause undesired reactions such as the combustion of fibers on the substrate’s surface. To reduce this effect, twin wire arc spraying and low pressure cold gas spraying processes were used to apply low-melting materials (copper, zinc) onto the specimen. Thereby, the influence of the coating on the vulcanized fiber surface was investigated. In addition, non-destructive testing methods were evaluated to gain information about the vulcanized fiber.

Due to the superposed thermal and mechanical stress profile, thermo-mechanically coupled forming processes require tools and machine components which meet high demands. High forming forces and process temperatures in the contact zone between the tool and the workpiece limit the life span of these tools. A promising approach for protecting such tools is a combination of thermally sprayed coatings and physical vapor deposited layers. This coating system combines the characteristics of the individual layers and leads to superior mechanical, tribological as well as thermal properties under the mentioned coupled stresses. In this study thermally sprayed alumina (Al 2 O 3 ) and yttria-stabilized zirconia (ZrO 2 ) coatings were produced by atmospheric plasma spraying. Therefor different coating porosities were adjusted in order to varied the effect of thermal insulation for the substrate made of AISI H11 (1.2343). After the coating process the surface roughness of the thermal barrier coatings (TBC) were reduced by polishing process in preparation for the PVD top layer. Subsequently, wear and heat resistant hard TiAlSiN and CrAlSiN coatings were deposited on top of the polished TBCs by using magnetron sputtering process. As a reference the PVD coatings were also applied on a nitrided steel samples. Titanium and chromium interlayers were applied by PVD technique in different coating thicknesses (50 – 150 µm) between PVD and thermally sprayed coatings. Afterwards the influence of these metallic interlayers on coating adhesion of PVD coatings were analyzed by performing scratch tests. Hardness and young’s modulus of PV coatings were investigated by means of nanoindentation. The morphology and topography of the coatings were analyzed by scanning electron microscopy, light microscopy and optical three-dimensional surface analyzer. EDX analyses and X-ray diffraction were used to determine the chemical composition of the PVD coatings. Finally the wear resistant of the PVD top layers were determined at different temperatures (20°C, 500°) by using a high temperature tribometer.

Starting in 2010 a new generation of coal fired power plants in Europe operating at a steam temperature of up 620°C was commissioned. During that commissioning process many cracks occurred in welds of T24 material which was extensively used as membrane wall material in nearly all of the new boilers. The cracks were caused by stress corrosion cracking (SCC) only occurring in the areas of the wall being in contact to high temperature water during operation. The question which step of the commissioning process really caused the cracking was not answered completely even several years after the damage occurred. To answer this question and to define parameters which will lead to cracking in high temperature water many tests were conducted. Generally it was found that slow tensile tests in controlled environment are well suited to get information about materials SCC sensitivity in the laboratory. In the present paper, first the influence of the cracking of welded T24 material in acidic environment containing well-defined amounts of H2S is investigated to address the question if a chemical cleaning process prior to the testing might lead to hydrogen induced SCC. As a second step, cracking behaviour in high temperature water is being investigated. Here the influence of the temperature, the oxygen concentration of the water, the deformation speed of the sample, the heat treatment and the condition of the material on the SCC is analysed.

This study assesses the effect of machine hammer peening (MHP) and carbide grain size fraction on the friction and wear behavior of arc-sprayed WC-W 2 C FeCMnSi coatings. SEM examination shows that post-treatment by MHP compresses the coating, reducing both thickness and porosity, particularly in coatings with ultrafine carbides. The treatments also cause cracking, however, especially in carbide phases. Ball-on-disk tests were carried out on as-sprayed and treated samples to determine sliding wear and friction properties, and dry sand rubber wheel tests were used to evaluate abrasion resistance. SEM and EDX analyses before and after wear testing show how coating microstructure and grain size correlate with the friction and wear test results obtained and the given surface treatments.