applied sciences Article  
The Influence of Low-Pressure Plasma Treatments on the Lap Shear Strength of Laser-Joined AISI 304 Hybrids with Polypropylene and Polyamide 6.6  
[PERSON], [PERSON], [PERSON], [PERSON] and [PERSON],*(c)  
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 3: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 4: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 5: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 6: https://  
+++ ==WARNING: Truncated because of repetitions==
www.mdpi.com/journals/applsci  
++  
laser heats the backside of the metal, transferring the heat through the cross-section of the metal substrate via heat transfer. In both cases, the polymer plasticizes and wets the metal substrate [18]. The schematic of these joining processes is shown in Figure 1. Pressure during the joining process is necessary to ensure good contact between the metal surface and the molten plastic. To improve the bonding strength, different kinds of surface pretreatments were investigated. Some research groups used laser ablation to create grooves in the metal substrate to improve mechanical adhesion. [PERSON] et al. could increase the tensile strength of an AISI 304-polyamide 6 at 2.1 MPa to 11.3 MPa by laser-structuring the steel surface [19]. Furthermore, other investigations revealed that additional adhesion theories alongside mechanical adhesion occur in laser-joined metal and plastic hybrids. XPS measurements showed that a chemical bond between polyamide and AISI 304 exists due to increased peaks of chromium and oxygen, indicating the creation of chromium oxide [16]. [PERSON] and [PERSON] identified a possible chemical bond between the chromium oxide layer of an AISI 304 steel with elements of the used polyethylene terephthalate in their laser-joined hybrids [20]. [PERSON] et al. created aluminum-polyamide 6.6 hybrids via laser-joining. They identified that the chemical bond formed was a carbon-oxygen-aluminum bond. One of the possible reaction sides is the double oxygen bond of the amide group in polyamide 6.6. This indicates that polymers with polar groups like polyamide 6.6 are capable of creating chemical bonds at the interface [21; 22]. X-ray mappings performed by [PERSON] et al. on a laser-joined 1050\(-\)H24 aluminum alloy with polyamide 6.6 with a laser-polished or laser-ablated surface also showed physiochemical bonding between aluminum oxide and polyamide 6.6 at the interface [23]. [PERSON] et al. investigated the bonding mechanisms of laser-joined AISI 304-PMMA hybrids. They used a pulsed laser with an area energy of 20.16 J mm\({}^{-2}\) to achieve a maximum lap shear strength of 4.17 MPa. They identified M\(-\)O and M\(-\)C chemical bonds and mechanical anchoring of the molten plastic on the steel's surface as the main contributions to the adhesive strength [24]. [PERSON] et al. used a low-pressure plasma treatment before joining aluminum and polyamide by means of indirect laser-joining. They used a mixture of oxygen and nitrogen as the plasma gas.