Characterisation and Testing of Multifunctional Surfaces

Alessandro Godi

Research output: Book/ReportPh.D. thesisResearch

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Abstract

Surface texturing is considered an effective way for reducing friction losses and wear occurrence in mechanical systems. A large number of surfaces with textures artificially engineered has been proposed by researchers worldwide and among them lie a new developed typology: MUFU surfaces, where the acronym stands for multifunctional. Produced by hard-turning followed by a highly controllable Robot Assisted Polishing process, MUFU surfaces feature reservoirs for providing extra-lubrication between the contacting parts as well as uppermost flat regions for ensuring the bearing capability. The introduction of MUFU surfaces is however bound with a series of challenges constituting the topic of the present work. The exploration touches a number of disciplines encompassing metrology, tribology and modelling. The metrological investigation represents the core of the work as further researches are bound to a clear and comprehensive description of the surfaces analysed. Robust filtering methods are adopted, extended, coded and implemented in the commercial software SPIPTM. These methods prove to be extremely suitable in handling the raw data coming out from a measuring instrument and yield a correctly filtered and aligned roughness profile that would be unrealistically distorted if current practice methods were used. Once an aligned profile is obtained, a further operation is introduced: feature separation. The surface features are separated with a newly developed algorithm and analysed independently according to their function. In the present case, the roughness of the plateaus is investigated independently from the valleys, which on their turn are described by the amount of lubricant they can contain.
These methods are applied throughout the whole experimental work in assessing the performances of MUFU surfaces in different applications. In machine elements, characterised by lower normal pressures, a new test rig is designed and developed studying the friction between bodies in pure sliding contact. Tests with this new device display how the employment of MUFU surfaces can reduce friction up to 50% compared to regularly machined surfaces. In metal forming tools, experimental tests are run in different processes. In deep drawing applications MUFU surfaces reduce the likelihood of galling occurrence compared to a highly polished surface. The presence of the valley impede the galling propagation especially in real production conditions and test can run smoothly without failures. In ironing applications the severity of the conditions makes instead the texture being more harmful than useful and is therefore not advised.
Both analytical and numerical models are considered for studying the functionality of the surfaces. The analytical models, depending on the approach adopted, can give underestimations or overestimations of the results obtained with the same texture and an improvements and extension are needed in the future with the modification of the assumptions made. The road to be trodden seems though numerical modelling, whose implementation is still at an early stage. Numerical models are necessary for studying the functionality of MUFU surfaces in metal forming applications. Numerical models exist but they require ameliorations and extensions before they can be reliably used.

Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages290
Publication statusPublished - 2013

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