Chemical leaching is proposed as a method to control porosity in stainless steel HVOF coatings. The leaching behavior is evaluated as a function of the stainless steel 444 and the pore former (Fe 3 Al) volume fractions and particle size distributions. The resulting porous structures are evaluated by optical microscopy. It was observed that the melting degree of the stainless steel splats was an important factor to retain some mechanical integrity after leaching. A discussion is presented to point out possible routes for the improvement of chemical leaching as a methodology for pore control in thermal spray coatings.

High-Sn bearing alloys have been used for more than a century in many areas of industry. They are typically applied by casting, although thermal spraying is gaining in use, particularly for component repairs. This study evaluates the effects of HVOF spray parameters on the velocity, temperature, size, and trajectory of Babbitt particles and correlates the in-flight characteristics with coating porosity and intermetallic phase distribution. In the experiments, Babbitt layers up to 370 μm thick were deposited on carbon steel substrates while measuring particle properties and deposition efficiency. Coating samples were analyzed by means of optical and electronic microscopy and some were chemically etched to reveal the size and distribution of intermetallic phases. Test results show a significant refinement in intermetallic phase distribution when compared with commercial flame and arc-sprayed coatings.

Protective coatings with high wear, erosion and corrosion resistance are of great importance in many fields of application and in particular, in the electric power generation sector. In this paper, the HP-HVOF (high-pressure high velocity oxy-fuel) technique is used to produce dense rapidly quenched metal-ceramic nanocomposite protective coatings. The powders for the thermal spray process are produced by high energy ball milling using mechanochemical displacement reactions to synthesize ceramic components in-situ at the nanometric scale. Boron nitride solid lubricant is used as a source of nitrogen and boron to precipitate nitride and boride phases in a corrosion resistant iron aluminide metal matrix. The formation of the hard phases during milling and/or thermal treatments is investigated using various analytical methods. The tribological properties of the coatings with and without ceramic additives are reported.