Thermal spraying (APS and HVOF) of an agglomerated nanostructured powder, based on the composition of a commercial martensitic steel, is introduced. The nanostructure of the produced powder is examined by means of microscopy and X-ray diffraction. The influence of the two different processes on crucial properties such as porosity, microhardness, adhesion, and wear resistance is studied. High wear resistance is noted for both coatings. The HVOF coating, especially, showed better wear performance in comparison with the APS coating and the bulk martensitic steel. The superiority of the HVOF coating over the APS coating regarding the aforementioned properties is attributed to a higher retention of the nanostructure of the starting powder, higher peening and relatively low oxidation.

An empirical model was built for the prediction of HVOF sprayed NiCr in-flight particle properties based on the spraying parameters. The employment of factorial design in process parameter development, allowed determination of the contribution of the key process variables, such as flame energy (combustion pressure and O 2 /F ratio), spray distance and feed rate, on the resultant particle velocities and surface temperatures. The significance of each parameter was used to compose a simple model which enabled the description of the particles’ in-flight properties. Particles with velocities ranging as much as 300 m/s and temperatures ranging up to 350 °C were used to produce a range of coatings on an in situ curvature sensor enabling the determination of evolving and residual stress. These diverse particle states combined with the effect of flame impingement on the substrate, resulted in coatings of similar thickness, but significantly different stress states. Real time evolving stresses during deposition were interrelated to particle in-flight properties and, consequently, to spraying parameters.

Nanostructured and conventional Al 2 O 3 powders have been plasma sprayed, under the same parameters, to produce coatings on 304 stainless steel. The nanopowder was fabricated by mechanical attrition techniques and composed of agglomerates of nanoparticles, whereas the conventional powder consisted of solid granules. In order to maintain the beneficial effect of the nanostructure in the final coating, a fraction of the powder’s nanostructure should be retained in the coating after spraying. Dissimilarities were observed between the two types of coating, regarding porosity, roughness, deposition efficiency and α-Al 2 O 3 to γ-Al 2 O 3 proportions, which are related with the different percentages of semimolten particles in the coatings. The nanocoatings retained a higher percentage of semimolten particles than the conventional coatings, owing to the higher porosity of the nanoparticle agglomerates, as compared to the conventional solid granules. The molten part of both conventional and nanostructured coatings consisted of γ-Al 2 O 3 of columnar morphology. In order to investigate the mechanism of the melting front advance into the particle interior during their presence in the plasma stream, the particles were sprayed directly into distilled water. The nanoparticles mainly formed hollow spheres, whereas the conventional particles mainly formed compact spheres. The internal porosity of the solidified nanoparticle agglomerates, which affected the overall coating porosity, was linked to the hollow sphere phenomenon.