Laser surface texturing - Fundamental study and applications

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Abstract

The increased demand in stringent requirements on engineered surfaces in the aerospace and manufacturing industries drove the need for developing advanced surface engineering techniques such as chemical etching, plasma etching, corona discharge and laser surface texturing in order to alter material surface physical and chemical properties. Among these techniques, laser surface texturing has been identified as one of the most efficient and effective surface treatment/ texturing techniques which utilizes laser ablation to meet the demand of practical engineering requirements. This thesis details three practical engineering challenges in the field of paint adhesion, dust adhesion and tribology performance of SPF sheet forming dies in which case the problems and motivation for development came from projects by industrial partner collaborations with BAe Systems, Rolls-Royce and Dyson Ltd. The proposed solutions to these challenges are formulated around laser surface texturing techniques using excimer and femtosecond lasers on three engineering materials which are CFRP, ABS polymer and HR4 nickel alloy.As ablation is the main mechanism used in laser surface texturing techniques in achieving surface property changes, the understanding of laser beam interaction with materials is crucial. The fundamental understanding of laser beam interaction with different materials has been researched since the use of lasers in practical engineering applications by which laser material interaction parameters such as ablation threshold, incubation coefficient and optical penetration depth are of primary interest in addition to the ablation rate. Currently the published literatures are either material specific or laser specific with minimal or zero comparison between different types of lasers and materials as such which limits the understanding of laser beam interaction with materials. In addition, laser beam interaction with polymers has always been done using ultraviolet wavelength lasers. In this thesis, the interaction between two types of lasers and three types of materials which includes metals and polymers are presented through comparison and discussion between different interactions.It has been discovered that the ablation threshold value is lower for ABSinteraction with excimer laser as compared to the interaction with femtosecond laser due to the difference in the ablation mechanism. The optical penetration depth value is higher for ABS interaction with the excimer laser as compared to the interaction with the femtosecond laser due to differences in the photon energy of the laser beam of different wavelength. Two ablation rate curves were identified on ABS interaction with infrared wavelength femtosecond laser beams which has not been reported before. Chemical composition of the laser treated layer changes through chain scission process, creating free radical carbons that reacted with oxygen, nitrogen and water vapour in air creating oxygen and nitrogen rich functional groups which increased with increasing laser fluence and number of pulses.Laser ablation is known for its capability of altering surface morphology and surface chemistry of materials through excitation of electrons causing bond scission or melt where materials are vaporised, ejected or undergo chemical compositional changes. In the case of polymers, addition of oxygen and nitrogen rich functional groups are identified whereas in the case of metals, changes in crystallographic, orientation and oxidation states are identified. Such changes are deemed ideal for applications such as adhesion where it is mainly used for bonding and joining of similar or dissimilar materials. In this research, excimer laser surface treatment showed improvement in CFRP paint adhesion where a better adhesion is achieved than sand-papered surfaces. Paint adhesion of CFRP surfaces is affected by surface contaminants, surface chemical composition and surface roughness where the degree of inuence is in the respective order.In addition, excimer and femtosecond laser surface treated ABS also improved dust adhesion. The main factors that affect the ABS surface dust adhesion performance are surface roughness and surface chemical composition. The increase in surface roughness increases the surface area available for dust to adhere to. In addition, it also increases the drag coefficient of the air flow results in a higher removal force exerted by the air flow onto the dust particles through changes in the localised aerodynamic flow. The increase in polar functional groups increases the adhesion of the dust particles onto the surface due to an induced dipole moment by the charged dust particles.Laser micro-dimpled surfaces have been reported to be effective in reducing friction coefficient and wear rate of surfaces under oil lubricated conformal contact conditions where the dimples act as reservoirs to store lubricant and wear particles. But such surfaces have not been extensively researched under non-conformal contact conditions for different lubricated environments. In addition, there are contradictory results found between published literatures which observe under similar wear environments and conditions but with different dimple geometry. Hence a detailed investigation on dimpled surfaces under non-conformal contact conditions is being carried out. Laser surface texturing of 100 μm size dimples shows a reduction in nickel alloy wear rate under dry and oil lubricated environments. A higher dimple area ratio reduces the wear rate under dry condition with abrasive wear as the main wear mechanism. Under oil lubricated environments, the friction coefficient is dependent on the surface contact pressure, sliding speed and the viscosity of the lubricant and the wear rate is dependent on the film thickness which correlates to the friction coefficient. The wear rate of a dimpled surface is dependent on three factors which are the dimple diameter to contact area diameter ratio, depth of the dimple produced and the density of the dimples.Positive results are obtained in all three engineering applications indicating the feasibility of laser surface texturing techniques in providing suitable material surface properties for these applications.

Keyword(s)

ABS; Ablation; Carbon fibre composite; Dust adhesion; Laser surface texturing; Nickel alloy; Paint adhesion; Tribology

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