Plasma coating at atmospheric pressure

As well as increasing a bonded joint's resistance to ageing and chemicals, adhesion promoters can also strengthen the bond and extend the service life of an activated surface. Normally adhesion promoters are applied following a surface treatment. For high quality pre-treatments, plasma processes are frequently used. Plasma processes at atmospheric pressure have a number of specific advantages, including the option of in-line treatment, high treatment speeds and the possibility of site-specific treatment. The Fraunhofer Institute for Manufacturing Technology and Applied Materials Research (IFAM) in Bremen studied the deposition of adhesion-promoting layers using plasma at atmospheric pressure. The coating is produced by plasma polymerisation of silicon organic molecules. A plasma source from Plasmatreat GmbH, which was modified at the Fraunhofer IFAM to allow for layer deposition, was used for plasma coating at atmospheric pressure. The adhesion-promoting effect of this coating was investigated by depositing the plasma polymer on two aluminium alloys: 6016 (not anodised) and AlMgSi3. After this the samples were bonded using a heat curing, one-component epoxy adhesive and exposed to a salt spray test. On the untreated samples the bond failed almost completely after the salt spray test. In contrast the plasma-coated samples still retained a lap shear strength of around 15 MPa. Therefore, the adhesion-promoting plasma coating meets the requirement to improve the ageing resistance of a bond in two respects. It increases the bond strength and ensures that the bond between the adhesive and the surface of the substrate is no longer the strength limiting factor. The following assumptions can be made about the function of the plasma polymer based on its adhesion-promoting effect as shown in the results described above: Improvement in the adhesion of the adhesive to the surface resulting from the larger number and increased strength of the adhesive contacts and the Lewis acid-base interactions (for example, covalent bonds). Protection of the substrate surface against corrosion and other damage A change in the degradation mechanism. For example, damaging species are directed by the plasma polymer primarily into the adhesive layer and not onto the surface of the substrate.