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This thesis reports on the controlled formation of surface rippling structures induced by tip scanning processes on compliant and brittle materials. Periodic surface structures were generated on polymeric and vitreous materials and with different length scales. Two aspects were focused on: the controlling of orientation and periodicity of the resulting structures via proper tuning the scan conditions and the physical mechanisms ruling the early stages of plowing wear causing the rippling effect. Specifically the influence of the scanned area geometric shape on the orientation of the rippling structure was investigated on a polystyrene surface. Nanoripples were induced by scanning the surface with a silicon tip using atomic force microscopy and dedicated scripts. Inside a structured area two ripple orientations can be observed: near boundaries the ripple orientation is determined by boundary orientation and regions away from the boundaries the ripples are aligned in a steady orientation. This steady orientation can be tuned by the distance between the scan lines. In the boundary regions the orientation of the ripples is different from steady orientation. The orientation of the boundaries clearly affected the orientation of the ripples and the tendency of the ripples to align in a steady angle defined by the scan parameters could be significantly modified. Geometric shapes like squares, circles, stars, pentagons and hearts allowed to distinguish the influence of curved and straight boundaries. Straight boundaries with different orientations allowed a detailed analysis of the influence of the angle on the rippling process. Straight boundaries inclined in the direction of the steady state angle of ripple orientation previously defined generate a uniform ripple pattern covering the entire scan area. The aspect of wear originating from the rippled surface was also investigated on similar polystyrene surfaces. As a result of repetitive scan passes spherical particles with diameters up to 250 nm were nucleated and detached from the surface. The particles originate from the crests of the ripples formed in the first scan pass. As proven by the lateral force signal the detachment occurs smoothly without a static friction peak suggesting a crazing mechanism induced by the scanning tip. Once detached from the surface the particles are displaced and piled up along the edges of scanned area. The formation of periodic surface structures was also investigated on a brittle silica glass. By a combination of scratch tests performed with a diamond microtip mounted in a nanoindenter and imaging with atomic force microscopy the existence of a periodic herringbone pattern inside scratch grooves on silica glass was proven. The rippled pattern was induced in the scratch process when the indenter was pulled laterally along the surface resulting in a microscopic scratch groove. The load was varied up to 30 nN and the scan velocity up to 500 µm/s. The resulting periodicity of the structures was found to increase linearly with increasing scratch velocity. The repetition distance was in the range of sub-µm and the corrugation in the range of a few hundred nm, which was well below indentation depth. In both cases, the surface rippling on a polymeric surfaces and the formation of a periodic pattern inside microscratches on a glass surface, the results were found to be consistent with minimalistic theoretical models for stick-slip.:Contents i Abstract iii Zusammenfassung v 1. Introduction 1 1.1. Periodic surface structures – relevance and formation 1 1.2. Surface rippling created by scanning probe lithography 2 1.3. Wear and nanoparticle release 4 1.4. Aim and outline 4 2. Experimental methods and fundamental concepts 6 2.1. Nanolithography 6 2.2. Atomic force microscopy 7 2.3. Nanoindentation and -scratching 10 2.4. Wear 11 2.5. Stick-slip motion 12 2.6. Spin coating 14 3. Surface rippling on polystyrene 15 3.1. Background and motivation 15 3.2. Methods 20 3.2.1. Sample preparation 20 3.2.2. Scanning probe lithography process 20 3.2.3. Imaging of structures and nanoparticles 21 3.3. Effect of boundaries on the orientation of surface rippling 22 3.4. Particle release as a result of surface rippling 31 4. Periodic structures inside scratches on silica glass 37 4.1. Background and motivation 37 4.2. Methods 38 4.2.1. Sample preparation 39 4.2.2. Scratch tests 39 4.2.3. AFM imaging and analysis 39 4.3. Surface rippling induces by scraping with a sharp indenter 40 5. Conclusion and outlook 49 A. Appendix surface rippling on polymers I B. Appendix surface rippling on glass IV Acknowledgements VII References IX |
