Resumen de Desarrollo y optimización de recubrimientos hvof base wc-cocr para aplicaciones aeronáuticas
Material engineering has always played a defining role in industry by improving materials and tribological properties, increasing lite time use and thus ultimately reducing material costs. Over the last two decades, material engineering studies have focused on material design and composition due to the introduction of more restrictive laws in several strategic sectors such as automotive, aeronautical or aerospace industry. Moreover, the increasing environmental and worker safety regulations (JS0-14000 and 2000- 53-CE) on contaminant treatments , e.g. for electrolytic hard chrome (EHC), force companies to develop "clean" alternatives. Thus thermal spray technologies, especially high-velocity oxyl-fuel (HVOF) thermal spraying, are now mostly used for that purpose.
The present PhD thesis focused on the development and optimization of thermal projection HVOF coatings onto WC-CoCr matrix asan alternative to conventional EHC coatings. Todo so, two different gun-fuel combinations were optimized; the DJH-2600 gun (Sulzer Metco) using gas (hydrogen) as fuel, and the Wokajet-400 system (Sulzer Metco) using liquid (kerosene), the latter having two different configuration guns: STD "Standard" and ST "Small Target" . An additional objective of this investigation was to be able to fulfill the tight requirements enforced by the aeronautical sector.
A wide range of experimental assays has been performed to achieve an in-depth characterization of the WC-CoCr coating properties produced under different conditions. First, the thermal and kinetic properties of the HVOF guns have been studied. In particular, the effect of each process onto the speed and temperature of the sprayed particles was determined. Second, the effect of the speed and temperature onto various key properties of the coatings, such as the microstructure, roughness, thickness, porosity, Almen deflection, hardness, wear and corrosion resistance, were evaluated and ranked. Notably all tested HVOF thermal spray coatings presented superior tribological and mechanical performance over conventional EHC coatings.
This comprehensive characterization shows for the first time the influence of various key parameters, such as the type of fuel, configuration and stoichiometry of the combustion, thermal-kinetic conditions of the sprayed particles, on the quality of HVOF coatings.
lt was demonstrated that in-flight particle temperature and velocity play an important role in the formation of the coating microstructure and have an effect on the coating quality, such as porosity, hardness and wear resistance. The results showed that the kerosene-fuel gun resulted in a greater velocity and a lower temperature of the particles than the hydrogen-fuel one. For both fuels, the amount of porosity and surface roughness tended to decreases with increasing the particle temperature and velocity. Moreover, in the coatings sprayed with the Wokajet-400 gun was observed a tendency to a wear rate reduction with the hardness increase. However, in the coatings sprayed with the DJ-2600 gun, the wear resistance is related to the carbide degradation and particle temperature.
Key conclusions from this work highlight that all the HVOF thermal spray coatings tested presented superior tribological and mechanical performance to hard chrome and confirm that HVOF-sprayed WC-CoCr coatings are a reliable alternative to electrolytic hard chrome plating in the industry to coat mechanical components.
