The present study investigates the influence of spray torch handling parameters such as the spray angle, spray distance, track pitch, and gun velocity on the deposition rate and the deposition quality of Atmospheric Plasma Sprayed WC-12Co coatings as well as Twin Wire Arc Sprayed WSC-Fe coatings. Based on results of light microscopical analysis similarities as well as fundamental differences in the sensitivity of the two spray processes, regarding changes in handling parameters, are discussed. Both coating systems show distinct changes of the deposition rate when varying the handling parameters. An empirical model could be determined to describe the coating deposition. This model enables an optimization of path planning processes by reducing the number of optimization loops. However, the coatings show visible changes in the microstructure, which have to be taken into consideration in order to guarantee the production of high quality coatings.

In HVSFS (High Velocity Suspension Spraying), a suspension is axially fed into the combustion chamber of a modified HVOF torch. The particle formation is strongly influenced by the phenomena occurring during the short dwell time in the combustion chamber and expansion nozzle, i.e. break up and evaporation of the liquid jet, particle formation, sintering and melting. Optimization of the spray torch has already led to an improved coating process but is still an important topic for the future. A deeper understanding of the suspension liquid / flame interaction is still necessary to control coating properties. It turns out, that among other process parameters, rheological properties and agglomeration behaviour of the suspension have a strong influence of the particle formation and hence resulting coating properties. As a result of using nano particles HVSF sprayed coatings show partly different microstructures compared to HVOF sprayed coatings. To gain a deeper understanding of the process particles were collected in-flight to get information about the size distribution and melting degree during the spray process. The paper will give an overview of the present state of HVSFS development at the IFKB and present experimental results.

The conventional high-velocity oxy-fuel (HVOF) process has characteristics of high flame velocity and moderate temperature, and is widely used to deposit cements, metals and alloys coatings such as WC-Co, nickel and stainless steel. In this paper, a high pressure HVOF system with combustion chamber pressure up to 3.0MPa, and with characteristics of higher flame velocity and lower temperature was developed. In-flight particle velocity was measured using the DPV-2000 system at combustion chamber pressures from 1.0 to 3.0MPa, and stainless steel 316L powder was deposited at a combustion chamber pressure of 3.0MPa. The influence of spray conditions on the coating microstructure, deposition efficiency and micro-hardness were investigated. It was shown that the combustion chamber pressure has significant influence on particle velocity. Dense coatings composed of unmolten or partially molten particles could be deposited by varying the spray parameters. In the experiment, deposition efficiency up to 90% was achieved at the optimized spray conditions.

It is usually difficult to deposit effectively spray particles with a much limited melting in thermal spraying. In the present study, flame spraying was employed to produce yttria-stabilized zirconia (YSZ) particles at a limited melting. The melting degree of YSZ particles was controlled by flame combustion intensity and flying distance of spray particles within gas flame. The velocity of spray particles was quantitatively measured. The effects of spray distance and acetylene flow rate on the particle velocity, and deposition behavior were examined. The surface morphology of deposited particles was characterized by scanning electron microscopy. The bonding between deposited YSZ particles and YSZ substrate was examined from cross section. The spray particles in different melting states were obtained by changing flame spray parameters. The deposition experiment revealed that the YSZ particles at different melting states from only surface layer melted state, semi-melted state to substantially melted state can be successfully deposited on YSZ substrate by controlling substrate temperature. As a result, YSZ surfaces with different morphologies of particles from a near spherical shape, hat-on-frustum shape and hat-shape with different neck sizes, to a pancake shape and well spread disk shape have been created.

High velocity oxygen fuel has been used as fabrication technique for manufacturing aluminium coatings reinforced with different weight percents of silicon carbide particles on Mg-Zn alloys used as substrates. The aim of the investigation is to improve the tribological performance of the ZE41A magnesium alloy. The parameters of the thermal projection system have been optimized in order to maximize the SiC particles incorporation in the aluminium matrix of the coating. Pin-on-disc tests were developed to characterize the tribological behavior of the different specimens. Minor degradation of the magnesium alloy was achieved after . Composite coatings with thicknesses of about 120 µm, reinforced with about 10 wt.% and with high adhesion to the substrate were achieved. After the coating parameters were optimized, the wear rate of the magnesium with the composite coatings decreased by two orders of magnitude in comparison to that of the uncoated magnesium alloy.

The MCrAlY type coatings provide oxidation protection and good adhesion between the ceramic topcoat and the substrate. The high velocity oxy fuel (HVOF) process is currently being introduced to make the bond coats instead of conventional low pressure plasma spraying due to lower cost. In this research, the METJET-III HVOF liquid fuel system was used to produce MCrAlY coatings and the Tecnar DPV2000 was used to investigate in-flight particle temperature, velocity and diameter. To examine the coating-build, the wipe test was used to reveal the particle deformation on the substrate with different spraying parameters and correlated impact behaviour with particle properties. It was found that increasing of total fuel plus oxygen flow rate, percent stoichiometry and stand-off distances did not dramatically change the particle temperature. Increasing of percent stoichiometry and stand off distance led to reduction in particle velocity while increasing of the total fuel plus oxygen flow rate led to increase in the particle velocity. The average particle velocity below 640 m/s seems to make less splat formation than the average particle velocity above 750 m/s. Compiling results of the coating microstructure, the splat formation and the in-flight particle measurements revealed the effect of spraying parameters on the coating characteristics.

Polypropylene (PP) was flame sprayed onto rough mild steel substrates at room temperature (RT) that was preheated at 70 °C, 120 °C, and 170 °C. Single solidified droplets (splats) were collected and analysed to understand how processing variables influenced the thermal spray coating characteristics. The splat morphology was characterized in detail using optical and scanning electron microscopy (SEM). The splats exhibited a disk-like shape with a large central viscous core and a fully melted wide rim with a thin edge. The splat size increased with increasing substrate temperature. A unique flat microstructure was observed on the surface of the splat deposited onto the RT substrate, whereas a flowing pattern appeared on the splat surfaces deposited onto the preheated substrates and the pattern increased by increasing the substrate temperature. The results of this study revealed improved splat-substrate adhesion by heating the substrate from RT to 170 °C. On the basis of the result, the influence of substrate parameters on splat morphologies was employed to establish a relationship between the microstructural characteristics and processing variables of flame sprayed polymeric coatings.