Surface Engineering in Sheet Metal Forming
Abstract: In recent years, surface engineering techniques have been developed in order to improve the tribological performance in many industrial applications. In sheet metal forming processes, the usage of liquid lubricants can be decreased by using self lubricated tribo surfaces which will result in more environmentally friendly workshops. In the present work two different concepts, i.e. the deposition of thin organic coatings on the steel sheet and PVD coatings on the tool, have been evaluated. The sheet materials investigated include Zn and 55%Al-Zn metal coated steel sheet, which in general are difficult materials to form under dry conditions since they are sticky and thus have a high tendency to adhere to the tool surface. The PVD coatings include CrN, TiN and various DLC coatings. The work comprises tribo testing and post test characterisation using surface analytical techniques in order to evaluate the tribological properties of the tribo surfaces. The tribological tests of different tribo couples were conducted by using modified scratch testing and ball-on-disc testing. From these test results different friction and wear mechanisms have been identified. The deposition of thin organic coatings on the steel sheet metal has been found to be promising in order to control the friction and to avoid metal-metal contact resulting in galling. However, it has been found that the tribological characteristics of organic coated steel sheet are strongly influenced by coating chemical composition, the substrate surface topography and the coating thickness distribution. The performance of the PVD coatings depends mainly on the chemical composition and topography of the coated surface. By choosing PVD coatings such as diamond like carbon (DLC) low and stable friction coefficients can be obtained in sliding contact against Zn. Surface irregularities such as droplet-like asperities may cause an initial high friction coefficient. However, after a running in process or by polishing the PVD coating low friction coefficients can be obtained resulting in a stable sliding contact. The combination of imaging (optical profilometry, LOM, SEM) and chemical analytical techniques (EDS, AES, ToF-SIMS) gave valuable information concerning the friction and wear properties of the tribo surfaces investigated.
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