In this investigation, the behavior of overlapping HVOF-sprayed WC-Co splats is analyzed. Samples are sprayed and documented in several steps with a low density of splats. This method enables to distinguish between several splats in areas where overlapping clusters of splats occur. The splats are sorted according to their chronological order of impact. With this information, cross sections are made of these clusters to study the porous structure, developing in the center and at the boundaries of the splats. An ion-polishing method was employed to avoid the corruption of small pores and structures by mechanical force. It turns out that pores in HVOF coatings are initiated by the overlapping regions at the edges of the splats. Splats impacting directly onto each other show very dense microstructures at the interface in the center region of the impact.

This study investigates the HVOF spraying characteristics of a new WC-FeCrAl powder as compared to a standard WC-Co feedstock. Significantly higher particle temperatures were recorded for the WC-FeCrAl powder during spraying, presumably an effect of phase reactions during particle dwell time in the jet. XRD graphs revealed W 2 C and δ-Fe 2 O 3 formations. Gibbs free energy calculations propose that energy is being released during the formation of these phases. Comparable correlations between in-flight particle measurements and splat morphologies were found for both powders. Coating hardness was also found to be comparable, although porosity was significantly lower in the WC-FeCrAl samples. This is attributed to the smaller carbide grain size of the new powder, which might help explain the lower viscosity of the molten particles at impact. A response surface analysis of XRD measurements indicates that W 2 C formation occurs in the spray jet, and it is assumed that δ-Fe 2 O 3 formation occurs on the surface of the substrate after particle deposition.

During the last 20 years, numerous scientists have studied the formation of thermally sprayed WC-Co coatings. Most of them focused on the direct connection between parameter variations and coating properties, such as the microstructure, wear or hardness. As the formation of single splats is the foundation for any thermal spray coating, this work focuses on the investigation of single splat morphologies. The aim of single splat interpretation is to determine the influence of different spray parameters on the morphological distribution of particles inside the flame. This distribution is indispensable to understand the formation of each coating layer during the process. Unfortunately, most of the methods presently used for generating single splats do not allow an assignment of each splat to its radial position in the flame. A method to create a footprint of a spray jet with an extremely short exposure time was used in this paper. The resulting field of splats enables the assignment of the splats on the specimen to their radial position in the cross-section of the spray jet. The footprints were analyzed by correlating the quantities and morphology of the splats to measurements of the spray jet properties and the splat’s radial position inside the jet.

Numerous factors, such as the spray parameters and non-predictable conditions, during the spraying process influence the properties of thermally sprayed coatings. However, the condition of the particles at the moment of impact on the substrate is the most crucial factor for the morphology and the mechanical properties of a coating. Thus, nowadays particle analyzing systems are employed in order to explain the relation between the process parameters and the properties of a layer. Yet, modeling of thermal spray processes is conducted disregarding particle parameters and only correlating process parameters with layer properties. This article presents a new approach on process modeling, for relating spray parameters to particle-inflight conditions. The modeling of the particle properties in relation to the process parameters shall allow conducting targeted adjustments during the running process, to optimize particle properties. This method will enable to influence coating properties during the spraying process, eliminating the influence of unpredictable environmental or process-related disturbances. In a series of experiments, spray beam properties were measured in an HVOF thermal spray process with agglomerated and sintered WC-Co powder. Spray parameters were correlated to the particle-in-flight conditions, which were measured by utilizing two particle analyzing systems.

Thermal spraying technology still suffers from a lack of reproducibility due to uncontrollable factors during the process. Current methods of process control by means of observing process parameters like gas- and powder flow are insufficient to guarantee a constant quality of coatings, while a direct analysis of the deposited layer is time- consuming and can only be conducted after the process. Furthermore, recently developed mathematical models which correlate process parameters to coating properties are not applicable for all materials. As the particles’ behavior during the process affects the coating properties, a direct process control by the observation of the particles seems expedient. This method is applicable on running processes and thus avoids defective production. In this study, HVOF spraying experiments were conducted. The in-flight particles’ behavior was investigated using an optical diagnostic system, while coating properties were analysed by metallographical methods.