3. Results and discussion
3.1. Surface morphology and topography of the Zr treated samples
The morphology of the aluminium samples was investigated by FE-SEM technique before and after Zr treatment. The FE-SEM micrographs are shown in Fig. 1.
Fig. 1 shows that the surface of degreased aluminium sample(before chemical etching) is approximately smooth. The intermetallic particles cannot be clearly seen on the surface of this sample, indicating the presence of aluminium oxide layer. Fine intermetallic particles (appeared on the aluminium sample as white spots) were observed on the surface of aluminium samples which experienced alkaline etching and desmutting processes. This shows that the aluminium oxide layer was effectively destroyed and removed after alkaline etching and desmutting processes. The scallops, which were appeared on the surface of this sample, can be attributed to the local dissolution of the aluminium alloy matrix surrounding the intermetallic particles, causing particles removal from the surface [5,33]. The FE-SEM micrograph of the Zr treated sample shows that the surface morphology of the Zr treated sample did not change significantly compared to the one without conversion coating. This may mean that the Zr film formed on the surface of aluminium was too thin to be recognized by FE-SEM technique.Therefore, the presence of such film on the aluminium surface was studied through EDS analysis. The results obtained from EDS analysis are given in Table 2.
According to Table 2 it can be seen that about 3.9% w/w Zr element was detected on the surface of Zr treated sample indicating the Zr film formation. However, the thickness and the composition of the film could not be studied by EDS technique. But, regarding the previous reports it can be said that the layer form is nanometric which mainly composed of ZrO2·2H2O [2,26–29]. The surface topography of the Zr treated samples was studied by AFM analysis.In this way, the surface roughness of the samples could be calculated. The AFM micrographs and the calculated roughness values are presented in Figs. 2 and 3, respectively.
According to Fig. 3, the surface roughness was increased after chemical etching of the aluminium sample. This can prove that theoxide layer, presented on the aluminium surface, was destroyed and removed after alkaline etching and desmutting processes. Itcan be seen that the Zr treatment of the chemically etched sample further increased the surface roughness. The increase of surface roughness is an appropriate event helping the organic coating to make physical interactions with the aluminum surface.
3.2. Contact angle measurements
The effects of Zr conversion coating on the wettability and surface free energy of the aluminium sample were studied through measuring contact angle and work of adhesion values. To this end,the water contact angle values were measured on the surface of aluminium samples. Moreover, the wettability of the aluminium surface was studied by calculation work of adhesion (WA) and