Dataset for "The Role of Ultra-Thin Passive Films and Organic Adsorbates on Aluminum Chips for a Friction-Induced Solid-State Recycling Process"

Aluminum chips (EN AW-6060, LxWxH 1.99 ± 0.08 x 1.6 ± 0.07 x 0.1 ± 0.01 mm) were etched with 0.05 M NaOH and coated with ultra-thin layers octadecy phosphonic acid (ODPA) or methyl diphosphonic acid (MDPA) prior to a friction induced recycling procedure by immersion in 1mM solutions respectively. Sample preparation Cleaning. Al-chips were cleaned using a JEL RRM Mini-II roe wheel mixer. The chips were filled in a suitable flask, and roughly three times their weight in isopropyl alcohol (IPA) (p.A.) is added. They were then washed for 10 minutes at a speed of 20 rpm, after which the solvent was exchanged for a fresh one and the procedure was repeated. The wet chips are then dried in an oven at 140 °C. Etching. A portion of the chips were etched in a 0.05 m NaOH bath (2 L per 450 g of chips, diluted in DI-water from 1 m NaOH (Titripur®, Supelco) for 2 or 10 minutes under manual stirring, after which they were first washed several times with DI water. This was followed by a washing step with IPA to displace the water before drying. The same procedure was used in case of the model substrates. PA-Adsorption. For each state, 150 g of chips were filled in 1 L screw-cap bottles and immersed in 0.7 L of 1 mM phosphonic acid solution which were prepared in bulk. Consequently, 0.7024 g of n‑octadecylphosphonic acid (ODPA, Alfa Aesar) were dissolved in 2.1 L of ethanol (p.A.) and 0.2464 g of methylenediphosphonic acid (MDPA, Thermo Fisher Scientific) were dissolved in 1.39 L of ethanol (p.A.) along with 10 mL of pure water (VWR Chemicals) as solubilizer. The immersed chips were left in their respective baths for 22 h, after which the PA-solutions were decanted, and the chips were placed in crystallizing dishes (Ø = 190 mm). They were then washed twice with ethanol (p.A.) after which the excess ethanol was decanted, and the dishes were placed in an oven at 120 °C for 1.5 h. Preparation of Model Substrates. Model substrates for contact angle analysis and PM-IRRAS measurements were cut into rectangles (20 x 25 mm), manually sanded with 1000 grit SiC-sandpaper and then polished using an Al2O3 (0.03 µm) dispersion to a mirror finish. In order to emulate the cleaning and drying step of the chips, they were then immersed in IPA for 10 min and subsequently dried at 140 °C. Treatment regarding etching and SAM-adsorption was kept identical to the chips, with slight variations in handling due to the different shape. Model substrates for electrochemical measurements were cut to the same dimensions as above but were only sanded to emulate the rougher surface of the chips. Sanded chips were sonicated in ethanol for 10 min, rinsed with ethanol and then blown dry. This was followed by the exact treatment of the polished substrates. Characterization PM-IRRAS spectra of the aluminum flat model substrates were obtained using a Bruker Vertex 70 spectrometer equipped with an LN-MCT detector. IPA-cleaned chips served as a reference for all DRIFTS measurements. Baseline correction was done in OPUS using a polynomic line shape with 0 iterations and pins at 3800 and 2500 cm‑1. All spectra were recorded at an energy resolution of 4 cm-1. XPS measurements were performed using an Omicron NanoTechnology ESCA+ system with a monochromatic Al Kα (1486.7 eV) x-ray source and a hemispherical energy analyzer. The source-analyzer angle was 102°, while the take-off angle of the detected photoelectrons was set to 30° with respect to the surface plane. A pass energy of 100 eV, a step size of 0.5 eV, and a dwell time of 0.1 s were used for the survey spectra. A pass energy of 20 eV, a step size of 0.1 eV, and a dwell time of 0.5 s were used for the high resolution spectra. Peaks were fitted using a Shirley-type background. For O 1s, all components were constrained to the same FWHM. Except for Al 2p, GL(30) line shapes were used. Here, the metallic components were assigned an asymmetric A(0.35,0.6,0)GL(30) line shape. For P 2p, the 3/2 peak was assigned twice the area of its 1/2 counterpart. AFM measurements to determine the roughness of the model substrates performed on a Bruker Dimension Icon atomic force microscope operated in ScanAsyst mode with ScanAsyst-Air cantilevers (nominal tip radius 2 nm). For each sample, 3 separate locations with an area of 5x5 µm were recorded at a resolution of 512 x 512 pixels. Static water contact angle measurements by means of the sessile-drop method were performed using an optical contact angle goniometer (Dataphysics OCA-20) with a dosing volume of 5 µL and a dosing rate of 1 µL/s. The data was evaluated using the SCA20 software. Droplets were fitted using the Young-Laplace equation. The baseline was set manually. Per sample, 9 measurements were taken in a 3x3 pattern. Electrochemical impedance spectroscopy (EIS) measurements were recorded using a Gamry Instruments Interface 1000E. An Ag/AgCl reference electrode was used along with a Na2SO4 (0.1M) electrolyte and a gold wire as counter electrode. The measured area was 0,79 cm2. The EIS measurement ranged from 100 kHz to 0.1 Hz at an AC voltage of 10 mV vs open circuit potential (OCP). Prior to the measurement, the OCP was recorded for 30 min. The equivalent circuit used is as follows: Rely-(Rp-CPEb//(Rb))//CPEp. Alternatively, a depiction can be found in the respective folder. Analysis of the frictional behavior of the aluminum chips was done by pouring 50 g of each state through a funnel with a lower diameter of 2.5 cm mounted at a drop height of 10 cm. Images of the resulting heaps were taken from a fixed position. Their outlines were redrawn manually for easier comparison. Processing of Al-chips. The chips were processed after a steady temperature range was reached. 140 g of chips were processed continuously for each state. During the process, the temperature was recorded tactilely at two points on the tool insert. The measurement was carried out using K-type thermocouples (G/G-24KK-IEC) from Therma Thermofühler GmbH, Lindlar. Tensile tests were performed on a ZwickRoell universal testing machine Z100 with tensile specimens of form F according to DIN 50125. A strain rate of 0.008 s-1 and a clamping length of 30 mm were used.