Mechanical Adhesion in Adhesively Bonded Metal-Joints with Retentive Laser-Structured Adherends Paul Ludwig Geiss and Sascha Gramsch-Kempkes University of Kaiserslautern, Faculty Mechanical and Process Engineering, Workgroup Materials and Surface Technologies (AWOK) Kaiserslautern, 67663, Germany [email protected] Introduction The Ciba Laser Pretreatment process (CLP) [1] is one of the earlier examples for using environmentally friendly laser technologies to clean and modify adherend’s surfaces prior to adhesive bonding. Laser-structured base retention brackets have been used successfully in orthodontic applications for several years to enhance the durability of the brackets adhesion under the detrimental service conditions of the oral cavity. In this specific area of application the goal of the laser structuring process is to create beads of molten and resolidified metal, protruding from the adherend’s surface and thus facilitating a strong mechanical interlocking between the dental adhesive and the brackets base plate. The objective of this research study is to investigate the structure property relationship between the shape and geometrical alignment of individual laser retentions and their ability to increase the peel strength and durability of structural adhesive joints. The T-peel specimen were made of X5CrNi18-10 stainless steel with a thickness of 1 mm. Laser structuring was done in pulsed mode operation with a pulse frequency of 17.500 Hz and an average pulse duration of 5 µs. Non isotropic surface patterns (Fig. 2) were placed at different angles on the specimen’s surfaces to be able to differentiate between the influence of the dimensions of the retentive structures perpendicular to the surface and their orientation in relation to the direction of peeling (Fig. 3). 100 mm Experimental 100 mm Figure 2. Example of a non-isotropic surface pattern. Laser structuring was done with a Trumpf TruMark™ 3020 Nd:YAG laser. Fig. 1 illustrates the dimensions of the T-peel specimen that were used in this study. The size was chosen to take account for the maximum working area of the laser unit. s r L2 La LK L1 L2 L b L = 106 mm L1 = 10 mm L2 = 12,75 mm La = 59,5 mm LK = 85 mm b = 15 mm Figure 1. Size of the T-peel specimen. d = 3 mm r = 3 mm s = 1 mm Figure 3. Orientation of anisotropic laser patterns on the surface of the T-peel specimen. The variation of laser patterns was mainly governed by the number of pulses per cavity and the pitch clearance B (125 µm and 175 µm) (Fig. 4). Additional parameter variations included the choice of shielding gases (At=air, Ar=Arcal™, La=Lasal™ and Va=Varigon™) (Fig. 5) and the application of an electrochemical anodic treatment (EP) prior to adhesive bonding of the specimen. Pm,5% -fractile [N/mm] pm,5%-Fraktil [N/mm] 150 mm B=125 µm 8 B=175 µm 7 6 5 4 3 2 1 0 Figure 4: Examples and denotation of laser patterns with different pitch clearances B. 2R milled surface Edelkorund grit blasted Figure 6. Influence of the type of shielding gas on the peel resistance of T-peel specimen. 50 mm 50 mm Arcal (Ar) Figure 5. Influence of the type of shielding gas (At=air, Ar=Arcal™) on the shape of the laser patterns. All T-peel specimen were bonded with a two component epoxy based adhesive and cured at 80 °C in a convection chamber. Testing was done at a separation speed of 50 mm/min. Results and Discussion In Figure 6 the peel resistance of T-peel specimen bonded in the initial condition without further surface preparation (milled surface, 2R) and after grit blasting with corundum is given as a reference. The peel resistance is expressed as the 5% p-fractile probabilistic distribution of the average peel resistance. 8 7 Pm,5% -fractile [N/mm] pm,5%-Fraktil [N/mm] Air (At) Figure 7 illustrates the influence of the type of shielding gas on the 5% p-fractile probabilistic distribution of the average peel resistance values. Although the employment of shielding gas increases the expansion of the individual retentions perpendicular to the surface, the peel resistance of the T-peel specimen prepared at ambient atmospheric conditions is superior. 6 5 4 3 2 1 0 BAt125 BAr125 BLa125 BVa125 Figure 7. Influence of the type of shielding gas on the peel resistance of T-peel specimen. Accelerated corrosive aging according to DIN EN ISO 9227 leads to a significant reduction of the peel resistance in Fig. 8. Again the specimen that had been laser-treated under atmospheric condition with a pitch clearance of 125 µm exhibit a superior performance indicated by a residual strength after aging of almost 75 % of the initial peel resistance. The specimen with the denotation BAt125EP and BAt175EP had been subjected to an electrochemical anodic polishing procedure after laser treatment. initial Ppm,5% -fractile [N/mm] [N/mm] m,5%-Fraktil 9 4 weeks strength ungealtert 4 saltspray Wochen SST 8 weeks 16 weeks 8 saltspray Wochen SST 16saltspray Wochen SST 8 7 6 5 4 3 2 1 BAt175 EP BAt125 EP BAr175 BAr125 BAt175 BAt125 2R EP 0 Figure 8. Influence of aging under salt spray conditions on the peel resistance of T-peel specimen. The electro-polishing treatment of specimens BAt125EP and BAt175EP after laser structuring serves to unify the chemical properties of the adherent surface in reference to the initial condition without significantly changing the size and shape of the laser pattern. Conclusions Environmentally friendly single step surface pretreatment methods prior to adhesive bonding are of increasing importance for a variety of industrial applications. In this study the generation of retentions by laser surfacestructuring has led to adhesive joints with a high level of peel resistance and excellent durability under corrosive aging conditions. The main reason for their good performance is assigned to the mechanical interlocking of structures consisting of molten and resolidified metal, protruding from the adherend’s surface. Electrochemical posttreatment causes a slight reduction of the initial strength and durability of the laser-structured peel specimen. This indicates that the sub-micron-scale morphology of the surfaces and their chemical properties caused by the laser treatment carries an accessory effect on the performance of adhesive joints which will be a subject of subsequent research studies. References 1. 2. 2. R. Broad, J. French, J., Int. J. of Adhesion and Adhesives, 1999, 2-3, pp 193-198. DIN EN ISO 9227, corrosion tests in artificial atmospheres - salt spray tests (ISO 9227:2012)