The study of linear correlation between surface roughness parameters and adhesion of flame sprayed coatings

Adhesion is one of the reasons for limiting the use of thermally sprayed coatings. Many factors influence the adhesion of thermally sprayed coatings, for example: quality of the workmanship of surface preparation, method of thermal spraying, processing parameters, and the temperature of the preliminary preheat substrate, properties of the substrate and coating materials. Another one is the surface roughness. Attempts are made to use surface roughness parameters (e.g. Ra, Rz, Rq, Rku, Rsk) for prediction of adhesion of thermally sprayed coatings. However, most often these attempts end in failure. This article discusses the task of assessing the linear correlation between the twelve parameters of roughness or material ratio of various surface profiles and strength bond of coatings. For analysis, the following parameters were selected: Rz, Ra, Rq, Rsk, Rt, RSm, Rp, Rv, Rku, Rmr (50%), Rk, Rpk, Rvk. The steel substrates (S185) were machined by abrasive machining. The surface roughness of the steel substrate ranged Rz = 1.89-5.34 mm. Nickel-based coatings were applied by flame spraying by the Casto Dyn DS 8000 torch. Adhesion of coatings was measured by pull – off method. The flame sprayed coatings were characterized by adhesion to the steel substrate in the range of from 5.53 MPa to 8.28 MPa. The highest value of the correlation coefficient (Pearson – R)between parameters Rp, Rpk, and adhesion of thermally sprayed coatings were found. The coefficient R was equalto the 0.83, and the coefficient of determination R2 = 0.7. The lowest value of the correlation coefficient was found for the Rku roughness parameter amounting to 0.37. In the measurements carried out, it can be concluded that the prediction of adhesion of the coatings according to the roughness of the substrate, should be carried out by use the parameters characterizing the peaks height of roughness profile.