Influence of spray kinematic parameters on high velocity oxy-fuel sprayed WC-Co coatings’ properties applied on complex geometries.

As the regulatory limitations of hard-chrome plating surge, the successful application of thermal- sprayed wear/corrosion resistant coatings on complex geometries becomes critical. Thermal spraying is a line-of-sight method and thus, spraying a complex geometry results to changes in the spray angle, the spray distance and the effective gun traverse speed. Although there has been some research on the effects of these kinematic parameters on the coatings, previous work tends to examine the kinematic parameters in isolation, disregarding of any interplay between them. Yet, the effective particle velocity at impingement is dictated both by spray angle and spray distance while the particle temperature is mainly dictated by spray distance. In addition, the heat and mass transfer to the underlying coating are controlled by the gun traverse speed. These facts suggest that significant synergistic effects are expected when the spray kinematic parameters vary simultaneously, as when a complex geometry is sprayed. This work aims at evaluating the systemic effect of the spray kinematic parameters on WC-Co coatings sprayed by HVOF. Various coating properties are comprehensively examined and discussed, exploring the microstructures, phase composition, mechanical qualities and tribological performance. Significant interplay between the spray kinematic parameters is demonstrated in a number of coating properties, yielding non-linear behaviours. The notable beneficial role of small spray angle inclinations at long spray distances, in regards to deposition rate, microstructure, microhardness and wear resistance is demonstrated. Mechanisms of the particle rebounding, superficial oxidation of the coating, metallic tungsten crystallization, tribofilm formation and wear damage progression are proposed, with respect to the spray kinematic parameters. Finally, an attempt to generalize the insights from this work to any given sprayable geometry takes place in a prototype software tool in Matlab

Influence of spray kinematic parameters on high velocity oxy-fuel sprayed WC-Co coatings’ properties applied on complex geometries.

As the regulatory limitations of hard-chrome plating surge, the successful application of thermal- sprayed wear/corrosion resistant coatings on complex geometries becomes critical. Thermal spraying is a line-of-sight method and thus, spraying a complex geometry results to changes in the spray angle, the spray distance and the effective gun traverse speed. Although there has been some research on the effects of these kinematic parameters on the coatings, previous work tends to examine the kinematic parameters in isolation, disregarding of any interplay between them. Yet, the effective particle velocity at impingement is dictated both by spray angle and spray distance while the particle temperature is mainly dictated by spray distance. In addition, the heat and mass transfer to the underlying coating are controlled by the gun traverse speed. These facts suggest that significant synergistic effects are expected when the spray kinematic parameters vary simultaneously, as when a complex geometry is sprayed. This work aims at evaluating the systemic effect of the spray kinematic parameters on WC-Co coatings sprayed by HVOF. Various coating properties are comprehensively examined and discussed, exploring the microstructures, phase composition, mechanical qualities and tribological performance. Significant interplay between the spray kinematic parameters is demonstrated in a number of coating properties, yielding non-linear behaviours. The notable beneficial role of small spray angle inclinations at long spray distances, in regards to deposition rate, microstructure, microhardness and wear resistance is demonstrated. Mechanisms of the particle rebounding, superficial oxidation of the coating, metallic tungsten crystallization, tribofilm formation and wear damage progression are proposed, with respect to the spray kinematic parameters. Finally, an attempt to generalize the insights from this work to any given sprayable geometry takes place in a prototype software tool in Matlab

Experimental study of high velocity oxy-fuel sprayed WC-17Co coatings applied on complex geometries. Part A: Influence of kinematic spray parameters on thickness, porosity, residual stresses and microhardness

When a complex geometry is rotated in front of the thermal spray gun, the following kinematic parameters vary in a coupled fashion dictated by the geometry: Stand-off distance, spray angle and gun traverse speed. These fluctuations affect the conditions of particle impact with major implications on the coating’s properties. This work aims to probe into the interplay and isolated effect of these parameters on vital coating characteristics in applications requiring variable stand-off distance and spray angles. WC-17Co powders are sprayed via HVOF on steel substrates in a set of experiments that simulates the spray process of a non-circular cross section, while it allows for individual control of the kinematic parameters. Comprehensive investigation of their influence is made on deposition rate, residual stresses, porosity and microhardness of the final coating. It was determined that oblique spray angles and long stand-off distances compromise the coating properties but in some cases, the interplay of the kinematic parameters produced non-linear behaviours. Microhardness is related negatively with oblique spray angles at short distances while a positive correlation emerges as the stand-off distance is increased. Porosity and residual stresses are sensitive to the spray angle only in relatively short stand-off distances

FIB-SEM Sectioning Study of Decarburization Products in the Microstructure of HVOF-Sprayed WC-Co Coatings

The thermal dissolution and decarburization of WC-based powders that occur in various spray processes are a widely studied phenomenon, and mechanisms that describe its development have been proposed. However, the exact formation mechanism of decarburization products such as metallic W is not yet established. A WC-17Co coating is sprayed intentionally at an exceedingly long spray distance to exaggerate the decarburization effects. Progressive xenon plasma ion milling of the examined surface has revealed microstructural features that would have been smeared away by conventional polishing. Serial sectioning provided insights on the three-dimensional structure of the decarburization products. Metallic W has been found to form a shell around small splats that did not deform significantly upon impact, suggesting that its crystallization occurs during the in-flight stage of the particles. W2C crystals are more prominent on WC faces that are in close proximity with splat boundaries indicating an accelerated decarburization in such sites. Porosity can be clearly categorized in imperfect intersplat contact and oxidation-generated gases via its shape

Experimental study of high velocity oxy-fuel sprayed WC-17Co coatings applied on complex geometries. Part B: Influence of kinematic spray parameters on microstructure, phase composition and decarburization of the coatings

The aim of this study is to evaluate comprehensively the effect of spray angle, spray distance and gun traverse speed on the microstructure and phase composition of HVOF sprayed WC-17 coatings. An experimental setup that enables the isolation of each one of the kinematic parameters and the systemic study of their interplay is employed. A mechanism of particle partition and WC-cluster rebounding at short distances and oblique spray angles is proposed. It is revealed that small angle inclinations benefit notably the WC distribution in the coatings sprayed at long stand-off distances. Gun traverse speed, affects the oxygen content in the coating via cumulative superficial oxide scales formed on the as-sprayed coating surface during deposition. Metallic W continuous rims are seen to engulf small splats, suggesting crystallization that occurred in-flight

Experimental study of high velocity oxy-fuel sprayed WC-17Co coatings applied on complex geometries. Part B: Influence of kinematic spray parameters on microstructure, phase composition and decarburization of the coatings

The aim of this study is to evaluate comprehensively the effect of spray angle, spray distance and gun traverse speed on the microstructure and phase composition of HVOF sprayed WC-17 coatings. An experimental setup that enables the isolation of each one of the kinematic parameters and the systemic study of their interplay is employed. A mechanism of particle partition and WC-cluster rebounding at short distances and oblique spray angles is proposed. It is revealed that small angle inclinations benefit notably the WC distribution in the coatings sprayed at long stand-off distances. Gun traverse speed, affects the oxygen content in the coating via cumulative superficial oxide scales formed on the as-sprayed coating surface during deposition. Metallic W continuous rims are seen to engulf small splats, suggesting crystallization that occurred in-flight