## 3 Results and Discussion
### Influence of the Plasma Treatment on the Lap Shear Strength  
As written in the Introduction section, low-pressure plasma treatments, or the increase of the surface free energy in general, can increase the lap shear strength of joints bonded with an adhesive. The results of this work show that the influence differs when a thermal joining method is used. In Figure 18, the increased or reduced arithmetic means of the lap shear values are noted as percentages in comparison with the untreated samples. It is evident that most plasma treatments result in a reduction in the lap shear strength. The O\({}_{2}\) treatment of polypropylene led to a 53.27% decrease, with some of the samples failing when subjected to testing. The Ar treatment of polypropylene reduced the lap shear strength by only 30.8%, while the water\(-\)plasma decreased it by 42.7%. The surface free energy of all investigated polypropylene samples was nearly at the same level, and this was also observed for the polyamide 6.6 samples. Although the arithmetic mean values varied among all plasma-treated samples, the entire box plots have similar values. Ar and H\({}_{2}\)O plasma reduced the lap shear strength of the polyamide 6.6 hybrids by 5% and 12.9%, respectively, while the O\({}_{2}\) plasma treatment increased the arithmetic mean by 2.7%. Figure 17: Surface free energy of the low-pressure plasma and untreated plastic samples. The lap shear strength results of the plasma-treated polyamide 6.6 hybrids, despite variations in the arithmetic mean values, are within the range of the untreated samples. This suggests that increasing the surface free energy due to additional polar groups via low-pressure plasma treatment had no influence on the lap shear strength of polyamide 6.6 hybrids. In contrast, the significant reduction in strength of the polypropylene samples is not within the range of the untreated samples in Figure 12. It is possible that the treatment induced the formation of low-molecular-weight oxidized materials, which could melt and form a weak boundary layer at the interface between steel and plastic [34; 35]. To analyze the interaction between the plasma treatment and the failure mechanism, Figure 19 provides the macroscopic overview of two lap-shear-tested overlap joints treated with O\({}_{2}\) plasma for 180 s.  
Figure 19: Macroscopic overview of two overlap joints with O\({}_{2}\) plasma-treated plastics for 180 s: (**a**) polyamide 6.6 hybrid with cold-rolled steel; (**b**) polypropylene hybrid with sandblasted steel. Figure 18: Lap shear strength of the plasma-treated samples. Figure 19a shows the adhesively failed plasma-treated polyamide 6.6 sample. In comparison with the untreated sample, there are no macroscopic differences, and there was no noticeable increase or decrease of residue on the steel surface. It is worth mentioning that the number of cohesive failed samples was higher with a plasma-treated polyamide 6.6, without a significant increase in lap shear strength. On the polypropylene sample in Figure 19b, no residue or any change compared to the untreated sample can observed macroscopically. To identify possible microscopic changes, SEM measurements are shown in Figure 20.  
The polyamide 6.6 residue on the steel sample shown in Figure 20a,b appears more fractured after the low-pressure plasma treatment, suggesting a potential change in the material properties. The increased brittleness may be attributed to the reduction of water content in polyamide 6.6 during the treatment in a low-pressure atmosphere. It is known that the water content of polyamide 6.6 leads to higher ductility [36], and a low-pressure treatment could, indeed, reduce the water content, especially in the surface area. This provides an explanation for the increased brittleness and the subsequent loss of lap shear strength of the Ar and H\({}_{2}\)O\(-\)plasma-treated samples.