The roughness is affected significantly by the type of coating. The
coated surface of a given item of equipment is usually machined after
coating to reduce its roughness. In our study instead, machining was not
carried out to study what could be the worst case scenario of a given
surface of difficult access, and to analyze the possible effect of
roughness of coated layer.
The lowest roughness of the Nano coating would be a consequence of the
finer splat’s architecture. The NM coating presented the highest
roughness. It can be explained by the particle’s accommodation.
Particle’s packing is defined by filling the voids between larger
particles with smaller ones, whose voids will be filled again with even
smaller particles and so on. However, the Nano particles agglomerates
itself, as could be seen in the powder analysis, and do not fill so well
the voids between the Micro particles. It hinders the proper packaging,
which reflects in the roughness of the layer.
The fatigue nucleation is encouraged by the roughness increase [18].
The fatigue strengths of the three coatings are comparable to each
other, although they are lower than the substrate (Figure 8). The
fatigue limits found for layers Nano, NM and Micro were, respectively,
302 MPa, 297 MPa and 311 MPa. Taking the standard deviation of 25 MPa
stipulated by the applied method, there is no significant difference
between the fatigue limits of the three coatings. The substrate fatigue
limit of 400 MPa exceeds those of all the coatings. Factors attributed
to the critical stress for fatigue nucleation, positive residual
stresses and roughness, can adversely affect the coatings fatigue
performance.