2.2.3. AFM and Profilometry, Roughness Analysis

Two methods were used to assess the surface roughness of the coated sample. In the first method, the Mahr Perthometer M2 profilometer (MAHR, GmbH., Göttingen, Germany) equipment was used, equipped with an NHT 6-100 probe, where ten measurements were made. The test was carried out according to DIN EN ISO 4288 and ASME B46 [31,32], seven segments of 0.8 mm each (cut-off) were used in all measurements. The first and last measurements were discarded due to the initial acceleration and final deceleration processes. To assess the roughness, the following parameters were considered: the arithmetic average of the surface roughness (Ra) and the maximum roughness height (Rmax) according to EN ISO 4287 [33]. The variables are expressed in values of R (Ra and Rmax) since their analysis was performed in 2D. The roughness was confirmed using an atomic force microscope (AFM), VEECO Multimode (VEECO Instruments, Ltd., Woodbury, NY, USA), as the second method. To reach the largest possible area with high precision, an area of 20 × 20 μm² was analyzed and three different analyses were performed. The equipment used was equipped with a 0.007 µm radius probe and with NanoScope 6.13^® software (Bruker, New York, NY, USA).

To understand the morphological pattern of the balls, they were analyzed by non-contact optical profilometry on the Bruker NPFLEX equipment (Bruker Optics Inc, Billerica, Boston, MA, USA) with Vision64^® software (Bruker Optics Inc, Billerica, Boston, MA, USA). Three different analyses were made, taking into account an area of 450 × 600 μm², of the roughness parameters Sa and Sz according to ISO 25178-2:2012 [34]. The 3D techniques provide a better understanding of the surface, so in order to characterize the surface of the balls, the following parameters were analyzed: Sz (maximum topographic surface height) and the arithmetic mean of the surface roughness, Sa. The Sa parameter allows a quick perception of the state of the surface; however, it can be deceptive since it is an average value. On the other hand, the parameter Sz corresponds to the amplitude values between the highest peak and the deepest valley of the five regions of the image, which allows a more comprehensive description of the topography.

PTFE and steel balls 304 and 52,100 were previously cleaned in an ultrasonic acetone bath for five minutes for topographic analysis.