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.

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.

In this study, a path strategy for robot-based HVOF spraying is developed via deposition simulations. The simulations include the analysis of the spray plume footprint and the consideration of robot dynamics in the path planning strategy. A fine WC-Co powder was used as the feedstock material.

With appropriate process control, near-net-shape coatings can be successfully applied to parts with complex geometries using HVOF spraying and fine cermet powders. This study assesses the influence of critical process parameters on the deposition rate and properties of HVOF coatings produced using fine agglomerated WC-12Co powder (2-10 µm) with nanosized WC particles. Spray angle, standoff distance, gun velocity, track pitch, and backside air cooling pressure were varied during the experiments in order to determine their effect on coating hardness, roughness, and porosity as well as deposition efficiency. Variations in spray parameters were found to have a significant effect on deposition rate and coating properties.

This study investigates the sliding wear behavior of HVOF sprayed coatings derived from conventional, fine, and nanostructured WC-Co powders. The results show that WC-Co coatings produced from fine and nanostructured feedstocks have significantly higher wear resistance and lower friction coefficients than coatings derived from conventional sized powder. This is attributed to scaling effects in the microstructure and phase evolution of the coating material as explained in the paper.

This study investigates the influence of component geometry, particularly the outer radius of curved surfaces, on the hardness and porosity of HVOF-sprayed WC-Co coatings as well as the deposition rate. A fine agglomerated and sintered WC-12Co powder of spherical shape was sprayed on steel substrates. Process parameters were held constant, while the radius of curvature was varied between 5 and 40 mm. The results show that HVOF spraying is better for depositing fine particles on curved surfaces than other spray processes, although the deposition rate decreases compared to flat substrates.

The application of fine powders in the thermal spray technology represents an innovative approach to apply dense and smooth near-net shape coatings on tools with complex geometry. However, this aim can only be achieved, as long as the influence of the handling parameters of the spray process such as the spray angle are sufficiently understood. In this study the influence of the spray angle on the deposition rate as well as on the coating properties (microhardness, roughness and porosity) of HVOF-sprayed, fine-structured coatings is investigated. A fine agglomerated and sintered WC-l2Co powder (agglomerate size: 2-10 pm, WC-particle size: FSS = 400 nm) was used as feedstock material. It has been shown that HVOF spraying of fine powders is less susceptible to an alteration of the spray angle than most other thermal spray processes such as plasma- or arc-spraying. The reduction of the spray angle results in a decrease of the deposition rate, while no significant degradation of the coating properties was found up to 30°. However, at spray angles below 30° the coating strength is negatively affected by the formation of pores and cracks.

Thermal sprayed WC-Co coatings are widely used for various industrial applications due to their high hardness and corresponding wear resistance. Recently, it has been reported by many researchers, that the use of agglomerated and sintered micron powders with submicron or nanosized carbides can provide the deposition of WC-Co coatings with enhanced or even superior mechanical and tribological characteristics. This can only be achieved, as long as optimized coating conditions adapted to the specific thermo-kinetic behavior of such powders are considered. However, the porosity in the coating morphology represents an inherent problem when using powders with conventional agglomerate size (10-50 µm) and high internal porosity. Consequently, a minimum coating thickness is often necessary to provide suitable wear properties, which reduces the shape or dimensional accuracy when applying such coatings to complex surfaces. In addition, a reduction in surface roughness of the coating cannot be accomplished by fine carbides, since large agglomerates are employed. In this study we used two different fine WC-12Co powders in the HVOF process to manufacture nanostructured coatings with high hardness, moderate toughness, low surface roughness and low porosity. The first powder is a fine agglomerated and sintered powder with particle size of -10+2 µm and carbides in the ultrafine range (400 nm) The second one consists in two loose mixtures of fine Co (Fisher grain size FSS = 3.5 µm) with (a) WC (FSS = 3.0 µm) and (b) WC (FSS = 0.8 µm). Statistical design of experiments (DoE) were utilized to determine main effects of spray conditions on coating properties. Mechanical properties, microstructure and the phase development has been correlated to the in-flight particle behavior. Phase analyses were performed by XRD using synchrotron radiation.

Due to their outstanding properties, WC-Co cermet coatings are widely used in the field of wear protection. It has been reported by many researchers, that their macroscopic and tribological behavior is largely determined by the process conditions and the feedstock powder characteristics respectively. Although the manufacturing route for high velocity oxy fuel(HVOF)flame sprayed WC-Co coating is in an advanced state, there is still a lack of knowledge about the complex relationships between the thermo-kinetic in-flight particle behavior, the microstructure formation and the phase evolution during the spray process. However, a fundamental understanding is not only necessary to provide coatings with optimized properties, but also to meet economic aspects with regard to the HVOF process. In this study the dependencies between the HVOF process parameters and characteristics (chamber pressure, thermal energy transferred to the coolant), the thermo-kinetic in-flight particle behavior (temperature, velocity and the spray particle size) with the microstructure formation (carbide content and size, free mean path of the binder phase, porosity), thermal phase reactions and the mechanical properties (hardness, toughness, roughness) were investigated. Statistical design of experiments (DoE) is utilized to enable a systematic analysis of several influencing factors along with their interactions on the coatings properties and to find optimized spray conditions. X-ray diffraction (XRD) applying high-energy synchrotron radiation has been used, not only to provide a phase analysis in deeper regions of the coating structure, but also to achieve more detailed information on phases which are not or hardly detectable by XRD using a conventional X-ray laboratory source. In addition the sliding and abrasive wear properties at optimized coating properties are studied.

HVOF spraying of fine feedstock powder allows the deposition of cermet coatings with outstanding properties, but selecting and optimizing process parameters can be difficult. In this study, investigators employ a design of experiments (DOE) approach to identify the most relevant process parameters in the spraying of 2-8 μm Cr 3 C 2 -NiCr powders. In a screening step, all parameters were assessed in a 12-run Plackett-Burman experimental design and linear models were used to estimate their effect on coating properties and deposition efficiency. The five most influential parameters were then analyzed in a 16-run fractional factorial set of experiments in order to determine their effect, along with all possible two-way interactions, on coating quality.

Deep-drawing is a widely used sheet metal forming process in the aircraft and automotive industry. The manufacturing of modern parts with complicated shapes and curvatures requires forming tools with highest shape accuracy even at complex surface geometries. However, the application of novel, high-strength sheet metals combined with a continuous increase in productivity impose high tribological demands on forming tools and finally lead to increasing wear. In order to minimize the high costs for the repair and maintenance of such tools it is crucial to enhance their service life by an appropriate surface modification, which is able to preserve the high shape accuracy. Conventional coatings obtained by thermal spraying of coarse grained feedstock materials are not suitable to achieve this aim. In this collaborative study, the feeding and HVOF spraying of WC-Co submicron powders (- 8 + 1 µm) have been investigated to manufacture superfine structured, wear resistant near-net-shape coatings with improved macroscopic properties and smooth surfaces. Special equipment for the powder feeding and a novel HVOF flame spraying system designed by Thermico (optimized for the processing of fine-scaled powder fractions) have been employed. Correlations related to the process dynamics at varying HVOF gas compositions, the thermokinetic particle behavior in-flight and corresponding coating properties have been analyzed.