Zwilżalność warstw Al₂O₃ kształtowanych do zastosowań tribologicznych

Abstract

This doctoral dissertation consists of an extensive literature analysis on the current state of knowledge on Al₂O₃ layers produced on aluminum alloys. The main focus was on: the anodizing process of aluminum alloys, types of electrolytes used in anodizing, types of anodizing, Al₂O₃ layers for tribological applications, technology of the anodizing process and models of oxide layers. Attention has also been paid to the characteristics of aluminum and its alloys. In order to find ways to modify the oxide layers, the focus was on thermo-chemical treatment. The bibliographic review also included an in-depth analysis of surface wettability, contact angles and surface free energy. The experimental part of the work consisted in the production of Al₂O₃ layers on the basis of three research plans: two Hartley's plans and the total plan (after preparation of samples). All the oxide layers were produced by the direct current anodizing method using a three-component electrolyte. Based on the first Hartley Plan, Al₂O₃ layers were produced for three variable anodizing parameters (current density, process time and electrolyte temperature). Hartley's second plan was used to create oxide layers at a constant electrolyte temperature (298 K), treating as variables: current density, process time and compounds used for thermo-chemical treatment after anodizing (distilled water, sodium dichromate, sodium sulfate). Based on the overall plan, samples were made using two anodizing variables (current density, electrolyte temperature) and a constant process time of 20 minutes. For all produced layers, measurements of the thickness, the contact angle of the surface and the calculation of the surface free energy (SFE) were carried out. The layers produced on the basis of the Hartley Plan were used for tribological tests (carried out on the T-17 tester in reciprocating motion, under dry friction conditions) and for stereometric tests. The tribological tests contributed to the determination of the friction coefficient μ and the mass wear of the material. As a result of stereometric tests, amplitude parameters, load-bearing curves and isometric images of the surface were determined. The anodized layers on the basis of the total plan were used for sclerometric studies and their stereometric analysis. The cross-sections of the scratches and the values of the parameters f1 (material swelling) and f2 (crack depth) were determined, on their basis the layer wear process was determined. For selected layers, after the scratch test, photos were also taken using a scanning microscope to assess the type of cracks. Additionally, for selected layers, tests of microhardness, surface morphology with image analysis, nanostructure tests, EDS chemical composition and X-ray diffraction (XRD) tests were carried out to identify the phase composition. The results of the research allowed to determine the influence of the Al₂O₃ layer production conditions on the surface wettability, and thus on the tribological properties. By changing the parameters of layer production and thermo-chemical treatment, it was possible to shape surfaces with extremely different wettability. The layers were produced with hydrophobic properties (the highest contact angle with water was 95.33 ± 3.86°) and with strongly hydrophilic properties (the lowest contact angle with water was 8.62 ± 2.02°). The highest surface wettability was characteristic for the samples subjected to thermo-chemical treatment in sodium dichromate solution (the contact angles for all samples were below 27.44 ± 4.13°). These samples were also characterized by the highest coefficient of friction μ (above 0.2), the lowest value of the wear intensity of the tribopartner material (below 26 μg/km), the highest surface roughness (the highest amplitude parameters) and the highest Svk coefficient (responsible for sliding cooperation). The use of sodium dichromate for thermo-chemical treatment also contributed to increasing the microhardness of the layers by over 1000 MPa, the highest value was achieved for the layer subjected to thermo-chemical treatment in sodium sulphate solution. The layers produced as a result of thermo-chemical treatment in a sodium dichromate solution showed the presence of sodium dihydroxy aluminum carbonate (AlCH₂NaO₅) and sodium chromium oxide (NaCrO₂), a derivative of chromium oxide used in self-lubricating surfaces, in the phase composition.