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.

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.

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.

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.

This study evaluates the effect of hammer peening on the wear behavior of cermet coatings. WC-FeCMnSi coatings were produced by twin wire arc spraying and post-treated on a five-axis machining center equipped with pneumatic peen. The surface topography of the peened coatings was examined and compared to as-sprayed and polished samples. Dry sliding friction and abrasive wear tests showed that the treated coatings had lower friction coefficients, but were less wear resistant than non-treated samples. Likewise, strain hardening effects revealed by nanoindentation testing were offset by process-induced cracking of embedded carbides, which contributes to break-outs and third-body wear.

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.

For coating increasingly complex geometries that require a high accuracy with respect to a near net-shape coating distribution, the use of computer-aided path planning algorithms is mandatory to minimize the need for expensive prototyping experiments. During the planning process, coating simulations are typically evaluated frequently, thus both fast and accurate simulations greatly enhance the potential to find optimal path solutions. In this contribution, an efficient approach for computing the coating thickness on complex workpieces is presented, which makes use of the computational capabilities of mainstream graphics hardware to achieve simulation times well within the single-digit range of seconds for average-sized workpieces. Using a semi-automatic, measurement-based calibration routine, this simulation can easily be adapted to different processes. The simulation has been coupled with a path-optimization approach and was successfully utilized to enhance the deposition accuracy in the context of applying wear-resistant coatings to deep-drawing tools by means of a TWAS process.

This paper presents a thickness measurement method that can be used during thermal spraying. The new method is based on photogrammetry and image reconstruction and is able to measure complex 3D shapes with continuous contours. Initial results demonstrate the nondestructive nature of the method as well as its accuracy, versatility, and speed.

In this work, CFD simulations are used to evaluate air cap configurations for twin wire arc spraying (TWAS). Investigators employed a design of experiments (DoE) approach to identify air cap parameters with the greatest impact on gas velocity, jet convergence, and pressure distribution. The ones selected for study are the convergence angle, the length and diameter of the throat, and the distance between the air cap outlet and the point where the wires intersect. In all configurations studied, the spray wires deflected the flow of the primary gas and narrowed the cross-section of the plume along one axis. The effects of each air cap parameter are discussed in the paper along with possible design improvements.

The aim of this study is to determine how various factors, including process parameters and nozzle configurations, affect the shape and size of the spray jet in twin wire arc spraying. In the experiments, steel specimens were sprayed using an iron-based cored wire with a fused tungsten carbide filling. In-flight particle temperature and velocity and fluctuations in voltage and current were measured during spraying. The shape of the thermal spray spot and the 3D footprint of the plume were determined by means of image analysis and tactile surface profiling methods. The results obtained show that spray plume characteristics, and thus particle distribution, are heavily influenced by secondary gas flow, particularly the number, location, and angle of atomization outlet holes in the secondary gas nozzle.

Various approaches are presented in this paper to adapt conventional twin wire arc spraying (TWAS) for the production of smooth and finely structured coatings. Higher particle velocities were achieved by modifying spraying nozzle geometry. New geometries that incorporate a Laval shape produced the highest particle velocities while also eliminating overspray and extending the high-velocity region. This led to a more focused spraying plume and a change in optimal spraying distance, which was found to be more than 200 mm based on coating roughness. Some of the new nozzles exhibited evidence of particle deposition on the inner walls, which can restrict plume flow if not addressed. The problem is related to the position of the Laval throat in the spray plume as well as changes in gas pressure.

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.

The rapid pace of advancement in thermal spraying is in large part due to the availability of diagnostic tools that are used to measure process variables such as particle temperature, particle trajectories, and plume geometry. The work presented here deals with the adaptation of an imaging method that can be used to visualize, quantify, and distinguish between laminar, transitional, and turbulent flow regimes. The method is based on schlieren photography and an image processing procedure that extracts the density field of a flow relative to a background image. In this study, the promising new method known as background-oriented schlieren (BOS) is used to evaluate different nozzle designs for twin-wire arc spraying. The results are presented and compared with CFD simulations, conventional schlieren images, and color photographs of the spray plume.

This study investigates the potential of external powder injection for producing functionally graded coatings by twin wire arc spraying. In spray trials, the position of the injection port was altered along the spray axis and perpendicular to the arc and different powders and carrier gases were used. Real-time images were captured by a high-speed camera during spraying to detect correlations between gas flow rates, hard particle wetting, and atomization of the molten pool. The optimal location for injection was found to be dependent on the size and density of the powder and the flow rate of the carrier gas. In the case of embedding B 4 C in a Fe-based matrix, a strong metallurgical bond was formed, confirming that powder injection is a viable approach for controlling the composition of twin wire arc sprayed coatings.

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.