Nozzle geometry has a profound effect on HVOF spraying, influencing combustion gas dynamics as well as particle behavior. Nozzle dimensions are also important in cold gas-dynamic spraying (CGDS), particularly the length of the nozzle which affects gas flow temperature and speed. In this study, numerical simulations and experiments were conducted to determine how the length of the entrance convergent section of gun nozzles affects HVOF spraying. Process changes that occur inside the nozzle (as predicted by simulation) were correlated with coating properties. An Al2O3-TiO2 powder was used for the experimental studies. Changes in nozzle length had a significant impact on deposition efficiency, microstructure, hardness, and particle velocity. These relationships (as measured and calculated) were then applied to the nozzle design for the CGDS method.

During the impact and solidification of thermal spray droplets on a substrate, the density increases when the droplet solidifies. Depending on the material, the changes in density could be significant. For example, aluminum oxide's density changes by 66%, while the changes are 12% and 19% for nickel and copper, respectively. For zirconia, this change is 24%. The effect of such densification on the dynamic of the droplet impact and the formation of porosity could be dramatic. In this study, the effect of shrinkage of a molten droplet during solidification on droplet impact is numerically investigated for several materials. Results for the impact of molten alumina, nickel, copper, and zirconia droplets on both smooth and rough surfaces are presented. The results of variable density cases are compared with those assuming constant density. The effect of thermal shrinkage is particularly vital in the interaction of two impacting droplets. The shrinkage promotes the formation of additional pores.

As applications of thermal spray processes are expanding, the importance of computer-aided design systems and computer-aided engineering systems for these processes has been growing. The principal objective of this study is to propose a new analytic method for the prediction of coating thickness and deposition efficiency. This method is called the particle tracing method and is based on the Monte Carlo simulation method. In order to evaluate the validity of this model, several tests were carried out. The same stainless steel 316L layers coated by the HP/HVOF process (TAFA JP-5000) were used throughout each test. First, spray patterns were observed which had formed on flat-plate specimens from various spray gun angles. Coating thickness distributions on several curved planes were consequently investigated. Finally, the coating process for a blade of a compressor in a gas turbine was simulated. In the right of the results of these experiments, it is summarized that the calculated values of the coating thickness obtained by our method are in good agreement with experimental values. The accuracy is within 10% of the maximum thickness value in each specimen, except for at the edge of the work-piece. In conclusion, the particle-tracing method can be applied to the fundamental analytic model in the CAD or CAE system for thermal spray processes.

This paper presents a study of the residual stress and microstructural properties of thick, spray-formed components, produced using the High Velocity Oxy-Fuel (HVOF) thermal spraying process. The forming material used is Tungsten carbide cobalt (WC-Co), a material which is more usually processed using expensive press and sinter technology. The aim of this study is to examine the effect of production parameters on the formation of thick components. In order to fabricate thick specimens, certain problems have to be overcome. More specifically these problems include the minimizing residual stresses, which cause shape distortion in the components and maining the integrity of the coating on a microstructural scale. The dependence of residual stress, and sprayed material characteristics on spraying distance, and powder feed rate conditions is presented. Results show that cylindrical WC-Co components up to a thickness of 9mm can successfully be produced, by careful control of these parameters. This represents a significant improvement on maximum thickness values previously reported for WC-Co [1,2].

This paper compares two types of hydroxyapatite (HA) composite coatings, HA/Ti-6Al-4V and HA/Y-ZrO2. The powders used in the study were prepared using a slurry process then deposited by plasma spraying. The resulting coatings were characterized based on their microstructure, mechanical properties, and biocompatibility. Both composite coatings performed better than pure HA coatings in tensile adhesion and indentation tests. Testing also revealed that the HA/Y-ZrO2 coatings had favorable strength and fracture toughness and that the HA/Ti-6Al-4V coatings had good affinity to living tissue and sufficient mechanical strength.

Experiments have shown that the mechanical properties of plasma-sprayed coatings depend to a large extent on the details of the spraying process, in particular, they are strongly dependent on the details of the solidification and deformation history of the individual droplets which are in turn highly affected by the substrate conditions such as its temperature, material, and surface thermal contact resistance. In this study, droplet-substrate interactions are investigated through a complete numerical solution of droplet impact and solidification for a typical thermal spray process. The energy equation is numerically solved for both droplet and substrate regions; the solution is based on the Enthalpy Method for the liquid and solidified parts of the droplet, and the conduction heat transfer in the substrate. The numerical solution for the complete Navier-Stokes equations is based on the modified SOLA-VOF method using rectangular mesh in axisymmetric geometry. The developed model is suited for investigating droplet impact and simultaneous solidification permitting any desired condition at the substrate. The splat shape, the solidification front, and the temperature profile in the entire droplet and substrate regions are obtained at any desired time elapsed after the impact. Through these results, the nucleation and growth of solidification and droplet-substrate interactions are extensively studied.

A numerical model is presented for the computation of heat transfers during the APS thermal spray process. This model includes the contributions of both the impinging plasma jet and that of the particle flux on the substrate heating. The contribution of the impinging plasma jet is taken into account using a computational fluid dynamic model describing the impact of the plasma jet on the substrate. For this part of the work, a two-layer extension to the Chen-Kim k-s model was used allowing the description of both the turbulent plasma jet and that of the flow in the viscous sub-layer formed on the substrate surface. The contribution of the sprayed particles is taken into account considering their distribution in the spray jet. Since this is an important parameter that could affect the model accuracy, measurements of the deposit thickness profiles were first performed using the non-destructive acoustic microscopy method and the corresponding particle flux distribution was then deduced. Heat transfers inside the substrate were then computed using a three dimensional in-house code based on a finite volume approach. In the case studied, the results show that the contribution of the sprayed particles forming the coating is much more focalized than that of the plasma flow itself whereas the substrate nature has a strong influence on the thermal flux dissipation (not presented in the following). These elements are expected to provide useful information concerning the coating adhesion mechanisms and the formation of residual stresses during the coating elaboration.

The plasma-spray process features complex plasma-particle interactions that can result in process variations that limit process repeatability and coating performance. This paper reports our work on the development of real-time diagnostics and control for the plasma spray process. The strategy is to directly monitor and control those degrees of freedom of the process that are observable, controllable, and affect resulting coating properties. This includes monitoring of particle velocity and temperature as well as the shape and trajectory of the spray pattern. Diagnostics that have been developed specifically for this purpose and integrated with a closed loop process controller are described.

Not least because of the multitude of the possibilities which this process has to offer, has thermal spraying established itself in an extremely wide range of industrial sectors. In quest of new applications, special attention is frequently paid to the properties of individual systems. However, the trumps of the process can only be played out in combination with all the components as a whole. In industry nowadays, it is often the costs alone which are hotly disputed, and especially so when it comes to industrial gases. Yet precisely here infinite opportunities present themselves to positively influence the process. Starting with the optimization of costs, followed by the lifetime of the systems, through the variety of coating properties which can be tailored to the application, the influencing variables are endless. Investigations into this potential are already in full swing. Powder manufacturers are testing the many possibilities in their laboratories and have put their heads together with the R&D departments of hardware and gas suppliers in an effort to continually broaden the coating spectrum.

Splats formed during a thermal spray process may be either highly fragmented or intact and disk-like. To predict this change in splat morphology, a dimensionless solidification parameter (Θ), which takes into account factors such as the substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, has been defined. Θ is the ratio of the thickness of the solid layer formed in the splat while it is spreading, to the splat thickness. The value of Θ can be calculated from simple analytical models of splat solidification and spreading. If the solid layer growth is very slow (Θ << 1), the droplet spreads out to a large extent. Once it reaches maximum spread it becomes so thin that it ruptures, producing fragmented splats. If, however, the solid layer thickness is significant (Θ ~ 0.1 – 0.4), the droplet is restricted from spreading too far and does not become thin enough to rupture. Under such circumstances, disk-type splats are expected. When the solid layer growth is rapid (Θ~1), spreading of the droplet is significantly obstructed by the solid layer, producing splats with fingers around their periphery. Predictions from the model are compared with experimental data.

The trend in thermal spraying is more and more towards a globally uniform level of high-grade spray coatings. It is therefore extremely important that auxiliaries such as spray materials or industrial gases undergo precise examination in order to exactly define their influence. Based on their application, these auxiliary materials can always be supplied at the required purity level. In order to guarantee such high levels, the gas industry invests a great deal in analysis and supply concepts which ensure this purity from the tank or cylinder through to the point of delivery. A further point is of particular significance in today’s business world. With ever-increasing raw material prices, it is absolutely essential that spray processes are optimized to the maximum. This is not only made possible by selecting the right system, but also by choosing the right gas and gas mixture.

Under marine and coastal conditions, the degradation by corrosion of low-alloyed steels is generally observed. In order to overcome such important corrosion problems, the use of thermal spray coatings made of noble materials may be an attractive solution. 316 stainless steel thermal spray coating, an iron alloy coating, is often considered for corrosion protection because of its low material cost. Also, the high velocity oxy-fuel (HVOF) is often the selected coating process because it is known to provide coatings with a very low porosity level preventing the corrosive media to reach the substrate. The present paper compares the corrosion behavior of wrought 316 stainless steel with sprayed coatings made of the same alloy on 1020 mild steel. The corrosion behavior of materials is studied under salt fog conditions and with electrochemical techniques in brine simulating the marine environment. The coatings have been sprayed by HVOF under usual conditions. The results of this study demonstrate that the material behavior with regard to corrosion is process dependent . The HVOF sprayed stainless steel coating is much more sensitive to corrosion than wrought stainless steel. Corrosion product appearing on the samples is not only linked to the corrosion of the substrate by diffusion of the corrosive solution through pores but is also generated by intrinsic corrosion of coating itself. An enhanced sensitivity of the coating with regard to corrosion is attributed to the surface of particles or droplets, which are most likely degraded during the spraying process. However, thermal spray coatings having performances as good as wrought stainless steel can be obtained. In the present work, it is demonstrated that coatings obtained using vacuum plasma spray (VPS) have similar corrosion properties than wrought stainless steel in simulated marine environment. The industries considering corrosion protection of their components in marine environments by the use of stainless steel coatings must be aware of the reliability of their coatings. During the usual HVOF spray process, particles or droplets of stainless steel 316 are subject to important modification leading to a loss of performance against corrosion. Oxidation of alloying elements necessary to obtain a good stainless steel most likely occurs. However, the use of vacuum sprayed stainless steel coatings results to efficient protection against corrosion in marine environment.

This paper discusses some of the challenges encountered and overcome in the development of a thermal spray production cell and its integration into a production line for aluminum engine blocks. Examples of thermal spray coatings applied to cylinder bore surfaces are presented and discussed.