The aim of this work is to develop a model that predicts coating thickness based on thermal spray robot trajectories and the thermal history of the workpiece. To test the model, an alumina layer is deposited on a steel substrate by air plasma spraying. Robot path coordinates are stored in a text file and used to compute substrate temperature fields by solving the transient heat conservation equation during torch displacement. The contributions of the impinging plasma jet and molten particle stream are accounted for in the model. The distribution of matter in the particle spray is used to simulate coating formation and determine coating thickness.

Plasma spray-physical vapor deposition (PS-PVD) has been developed over the past years to produce columnar structured deposits for thermal barrier applications. The non line-of-sight process makes it possible to deposit coatings on complex components in a single production step. The fundamentals of PS-PVD have been well studied and the knowledge obtained is being used to scale up the process. This paper presents initial results on concepts that have been implemented to increase production rates as well as process efficiency.

Base metal surfaces are typically prepared for thermal spraying by chemically cleaning followed by grit blasting. In common practice, HVOF coatings are applied within 2-4 h or the entire cleaning process is repeated to prevent in-service adhesion failures. This study was initiated to assess the effect of long wait times on the adhesion strength of WC-Ni coatings. Test coupons were chemically cleaned, grit blasted, and then set out in shop conditions for up to 48 hrs. Some were also subjected to different forms of contamination including dirty fingerprints, grease, penetrating oil, surface rust, tape residue, and paint. Pull test and metallographic examination results show that none of the contaminants had an influence on the adhesion strength of the HVOF coatings.

In this study, twin wire arc spraying is used to deposit dense Inconel skins on 40 PPI nickel foam sheets. A design-of-experiments approach is used to investigate the effects of grit blasting on the surface characteristics of paste-filled foam. In-flight behavior of molten droplets and its effect on coating properties is assessed at three spray distances. The Inconel coatings are evaluated based on SEM and EDS analysis, roughness measurements, and adhesion testing. The results indicate that acceptable adhesion, porosity, and oxide content can be achieved over a small range of grit blasting parameters.

The hypothesis of local thermal equilibrium (LTE) in thermal plasma has been widely accepted. Most of the simulation models for the arc plasma torch are based on the hypothesis of LTE and its results indicate a good validity to mimic the pattern of the plasma flow inside the plasma torch. However, due to the LTE hypothesis, the electrical conductivity near the electrodes is significantly low because of the lower gas temperature. Consequently, it is difficult for the flow of electrical current to pass between the anode and cathode. Therefore, a key subject for a model depending on the LTE assumption is to deal with the low electrical conductivity near the electrodes. In this study, two models, determining the electrical conductivity at the vicinity of the electrodes with two different assumptions, were employed to calculate the flow patterns inside a non-transferred DC arc plasma torch. A comparison of the gas temperature, velocity, voltage drop and the heat energy of the plasma arc between the two models were carried out. The results indicate that plasma arc inside the plasma torch fluctuates as simulated by both of the two models. It seems that the model can obtain comparable accuracy compared with the experimental results if the plasma gas electrical conductivity is determined by a nominal electron temperature.

The use of fine feedstock powder can extend the feasibility and scope of HVOF coatings to new fields of applications. Especially for the purpose of near-net-shape coatings they facilitate short spraying distances, homogeneous layer morphologies, and smooth coating surfaces. However, the small particle sizes also lead to several challenges. One major issue is the in-flight behavior which is distinctly affected by the low mass and relatively large surface of the particles. In this paper, the in-flight and coating characteristics of WC-CoCr 86-10-4 (-10 +2 μm) were investigated. It was determined that the fine powder feedstock shows a high sensitivity to the gas flow, velocity, and temperature of the spray jet. Because of their low mass inertia, their velocity, for example, is actually influenced by local pressure nodes (shock diamonds) in the supersonic flow. Additionally, the relatively large surface of the particles promotes partial overheating and degradation. Nevertheless, the morphological and mechanical properties of the sprayed layer are hardly affected. In fact, the coatings feature a superior surface roughness, porosity, hardness, and wear resistance.

One of the greatest obstacles for a wide distribution of thermal spraying techniques is the lack of online control over the spraying process. The thermally sprayed coatings are optimized by an empirical modification of the spraying parameters and the subsequent correlation of these parameters to the obtained coatings. Some intrinsic parameters, such as the fluctuations in twin wire arc spraying and wear in the atomization nozzle, are not adjustable. Even though they have an enormous impact on the obtained coating quality, they are often scientifically neglected for reasons of simplification. In this work, acoustic emission analysis is utilized to study the effect of uncontrollable parameters on acoustic signals. In order to enable an easy determination of the changes in the acoustic signals, the acoustic sensors were mounted on the spraying nozzle as well as on the substrate. At increased current, a lower acoustic emission is recorded. A correlation between uncontrollable parameters, the acoustic signals, and the obtained coating quality was observed. This research contributes to the online control of the spraying process.

Owing to the arc ignition in twin wire arc spraying (TWAS) the wire tips are heated in three different zones. The outer part of the wire tips (contact zone) is heated directly by the arc ignition (zone I). The wires material in this zone becomes fully liquefied. The heat propagation phenomenon raises the temperature of the area immediately adjacent (towards the fed wires) generating a doughy material (zone II). Next to the doughy zone (towards the fed wires) the transferred heat softens the wire material causing a permanent deformation (zone III). The deformation is due to the exerted aerodynamic forces of the atomization gas pressure. A high speed imaging system was used to observe the melting behavior, metal break up, and particle formation under different operating conditions. The liquidus metal in zone I is directly atomized in the form of smaller droplets. Their size is a function of the specific properties of the molten metal and the exerting aerodynamic forces. The doughy area (zone II) is the origin of the extruded metal sheets at the anode and cathode side. The extruded metal sheets in case of cored wires are shorter than the ones observed by solid wires. The extruded metal sheets support the re-ignition of the arc and therefore enhance the process stability in twin wire arc spraying. In this study the effects of adjustable parameters and powder filling on melting behavior, particle formation and process instability were revealed and a comparison between solid and cored wires was made. The findings can improve the accuracy of TWAS process modeling and enhance the atomization of metal droplets through the adoption of specific nozzle geometry modifications.

Inconel alloy coatings have been obtained by means of Cold Gas Spray technology with a high efficiency. Alumina has been sprayed simultaneously achieving an homogeneous distribution along the deposit with a good cohesion between particles. Furthermore, the alumina particles were found to improve the spraying feasibility. A proper optimization of the spraying conditions has been carried out in order to obtain the lowest porosity as possible and good embedment of the alumina. Then, the mechanical properties as well as tribological and oxidation behavior have been characterized. Alumina behaves as reinforcement leading to an amelioration of the abrasive and friction performance. Regarding the oxidation and corrosion behavior, the coatings proved to withstand reasonably well up to 800°C and salt fog tests (5% NaCl solution at 35 °C) indicate durability up to 1000 h.

Off-line programming software is widely used to simulate the robot-assisted work environment. RobotStudio, a product of ABB, offers off-line programming technology for industrial robot applications. Several software products that extend RobotStudio capabilities have been developed to meet special purposes. One such product is the Thermal Spray Toolkit. With the help of TST, RobotStudio can load the mesh information generated by ANSYS analysis software and then create the trajectory of robot spray operations with it. A method of arranging the index of grid nodes generated by ANSYS software is described in this article. The results show that TST can be successfully used to create a smooth robot trajectory, which corresponds to the requirements of industrial applications.

Traditionally, large moulds for manufacturing of CFRP (carbon fiber reinforced plastics) parts are machined from a solid metal block making this way of manufacturing very energy and time consuming. Using wire arc spraying thin-walled moulds can be produced by spraying onto an original mould and separating the coating. In order to create a reliable and high quality product the manufacturing process needs to be highly reproducible. Thus the spraying process requires monitoring and control, which can be done using artificial neural networks (ANN). In our approach, for monitoring the process the diagnostic system PFI (Particle Flux Imaging) is used to characterize the spray particle stream, which is essentially achieved by fitting an ellipse to an image of the particle stream. Comparing deviations from a reference ellipse recorded for an “optimal” coating process provides data that can subsequently be used for process control. Investigations performed by the method of design of experiments (DOE) show a very strong correlation of the parameters pressure, current, and voltage with certain parts of the PFI data: for example the semi-minor axis of the ellipse depends linearly on voltage and current but quadratic on pressure. These results can further on be used to control the coating process by ANN. This paper discusses the application of this method and its feasibility for industrial use.

Thermal spraying of oxide ceramic suspensions containing fine and ultrafine powder particles is a new approach for manufacturing ceramic coatings exhibiting a refined microstructure. Suspension sprayed coatings clearly differ from conventionally sprayed coatings regarding microstructure phase composition and resulting mechanical properties. Several industrial applications may take advantage in future; among these are thermal barrier structures, thermal shock protection, solid electrolytes, catalytically active surfaces and wear resistant coatings. Two methods, namely Suspension Plasma Spraying (SPS) and High Velocity Suspension Flame Spraying (HVSFS) are suitable to process suspensions but lead to rather different coating structures due to differences in the achievable particle velocities and temperature. Generally, HVSFS can lead to more dense coatings with low porosity values. With SPS on the other hand, coatings with a high volume fraction of porosity featuring a homogeneous pore structure are achievable. The presentation will compare SPS and HVSFS regarding the spray process, achieved properties of the oxide coatings and potential applications.

Industrial robots are widely used in thermal spraying operations in order to assure process accuracy and reproducibility, as well as to increase worker´s safety. Robot path planning for the coating of free form geometries keeping constant the kinematic parameters can lead to complex robot paths and in some cases to unreachable robot configurations. An external rotational axis in coordinated movement with the robot can be used to manipulate complex shaped components during the spraying operation and to simplify the robot movement. Moreover, the robot kinematics, which define the spraying path, spray angle, spray distance and spray velocity, are key parameters to control the heat and mass transfer to the substrate during the coating deposition, and they influence therefore the development of residual stresses. The present work describes the off-line planning of robot trajectories for thermal spraying operations using an external rotational axis taking into consideration the heat transfer to the component.

High-velocity suspension flame spraying (HVSFS) is used for direct processing of submicron and nano-scaled particles to achieve dense surface layers in supersonic mode with a refined structure, from which superior properties are expected. The application of solutions as a carrier fluid for nano-particles in thermal spray systems is a new approach that requires some thermo-physical and chemical optimization. Three dimensional modeling and analysis of the combustion and gas dynamic phenomena of the three-phase HVSFS process is performed in this study for an industrial TopGun-G torch, based on a numerical model for a conventional HVOF process. Parameter analysis of the solution mixture (proportion between aqueous and organic solvent) in a suspension is performed as well as analysis of the variation of the combustion chamber depending on the torch design, leading to more homogeneous flow properties for an improved HVSFS torch.

Efficient transport is a key requirement for a powerful economy and will continue to gain importance leading to the necessity for high performance transport vehicles. A highly loaded major component of trains is the chassis. With increasing load, new materials and concepts have to be developed in order to increase the performance of these components. One approach to improving chassis components like bearings for chassis components is the deployment of thermally sprayed coatings by means of HVOF and HVSFS, which can significantly improve their wear resistance, prolong their life time and simultaneously reduce frictional losses. Using the example of spherical bearings, several HVOF sprayed metallic, cermetic and ceramic coatings have been tested for their ability to improve the properties of the components. In addition to metallographic investigations, extensive mechanical tests on test rigs have been carried out to determine the performance of the coatings under realistic conditions.

Complexity in dynamics and mechanism of supersonic flame formation and effects of processing variables has made the understanding of interaction of particles and flame a difficult task. Lack of such understanding limits the possibilities of controlling the process to obtain desired in-flight particles temperature and velocity and consequent particles state. This problem is even more pronounced in TS systems with no dedicated decoupled temperature and velocity controlled regime. Different approaches based on total volume flow, back pressure and fuel to oxygen ratio have been examined to address the robustness of each approach to control the temperature and velocity. WC-CoCr material was used employing DJ-2600 torch. A guideline to control the in-flight particles temperature and velocity based on process variables is provided. A process map was developed to establish a correlation between process, in-flight particles state, microstructure, properties and performance.

This work deals with the flattening of alumina molten particles, called droplets, on stainless steel substrates either smooth or blasted and preheated at different temperatures. The blasted surface roughness has been limited to Ra= 1.4 µm to image the flattening droplet. Besides flattening and splat cooling, the wettability of melted millimeter-sized alumina drops on the same substrates was measured. The transition temperature, Tt, has been shown to be different between smooth and rough surfaces. For a smooth surface, Tt, is 170°C, and at 200°C 100% of disk shaped splats are obtained. For the rough surface, Tt is close to 300°C with porous splats, becoming almost dense at 450°C. Close to alumina melting temperature, wettability does not vary with the substrate preoxidation, which may not be the case when the temperature is much over the melting one as in plasma spray conditions.

A design of experiments approach was used to characterize process-microstructure relationships in the twin wire arc process using zinc feed stock. Specifically, the effect of arc current, primary atomizing gas pressure, secondary atomizing gas pressure, and standoff distance on deposition efficiency, spray pattern shape, atomization behavior, coating porosity, and coating surface roughness were investigated. All work was conducted using a Praxair 8835 torch spraying 02Z zinc feed stock. It was found that primary and secondary atomizing gas pressure significantly affected spray pattern shape, atomization behavior, coating porosity and coating surface roughness. Arc current significantly affected spray pattern shape. Standoff distance significantly affected deposition efficiency, spray pattern shape, and surface roughness. Discussion will focus on using the relationships identified through this experiment to tune the wire arc process.

The thermal cycle test of Al 2 O 3 sprayed 1%Cr-0.5%Mo steel and 18%Cr-8%Ni steel was performed, and the failure type of thermally sprayed coating on both steels was compared. The results obtained are follows. (1)The thermal cycle fatigue life of thermal sprayed 1%Cr- 0.5%Mo steel was shorter than that of thermal sprayed 18%Cr-8%Ni steel. (2)In the case of thermally sprayed 18%Cr-8%Ni steel that is difficult to oxidize at high temperature, during thermal cycle, a tensile stress occurs in a sprayed coating and a compressive stress occurs in a substrate. Thus the thermal strain occurs at the interface of sprayed coating and substrate. The sprayed coating delaminates when the thermal cycles reach the limit value. (3)In the case of thermal sprayed 1%Cr-0.5%Mo steel that is easy to oxidize at high temperature, during thermally cycle, dense oxide film forms at the interface between the sprayed coating and substrate, and adhesive strength in the interface becomes low. Thus the sprayed coating delaminates under the lower thermal strain.

Alumina is a relatively low-cost material and easily processable by thermal spraying into wear and corrosion resistant coatings. However, thermally sprayed alumina coatings show inferior corrosion resistance versus chromia coatings, particularly in low and high pH electrolytes. Further, alumina possesses decreased mechanical properties in the as-sprayed state. In the present study, the effect of chromia addition on the properties of the plasma and HVOF sprayed alumina coatings were studied. Pure alumina powder and four different Al 2 O 3 -Cr 2 O 3 powders were prepared in two different atmospheres and produced to coatings with APS and HVOF methods. Phase composition of the powders and coatings were studied by X-ray diffraction. Electron microscopy was used for the examination of the microstructure of the powders and coatings. The mechanical properties of the coatings were studied by hardness and abrasion resistance tests. The chemical properties of the coatings were examined by corrosion exposure tests. Results indicate that with chromia addition it is possible to improve the mechanical and chemical properties of the thermally sprayed alumina coatings.