The mechanical properties, adhesion and roughness of polymer coatings depend on many factors, including
the unevenness of the substrate surface. Nevertheless, the influence of the substrate surface roughness is
related to the coating type and substrate material and the used deposition method. Therefore, the effect
of the surface roughness of a structural steel substrate on the mechanical properties of a PEEK coating is
ambiguous. The indentation tests conducted show that, at a specific load of the indenter, the roughness of the
steel substrate surface does not significantly affect the Vicker’s hardness of the tested PEEK coatings. The
average Vicker’s hardness and elastic modulus are approximately 300 MPa and 5.6 Gpa, respectively, at the
lowest of the applied loads, regardless of the surface roughness level of the steel substrate. Nevertheless, the
surface roughness of the steel substrate after fine grinding of Ra = 0.21 μm, compared to the polished one with
Ra = 0.005 μm, meant that adhesion improved, and the scratch hardness increased by approximately 130 to
370 [MPa] of the PEEK coating.