Elementary processes which govern the formation of nano grained and ultra fine grained (N&UFG) zones during oscillating sliding high temperature wear (HTW)

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HTW limits the exploitable service life of many technologically important metallic systems, like for example valve/valve seat tribo systems in Diesel engines. When a metallic tribo system is exposed to HTW, tribo layers form in the surface regions of the wear partners. These layers are of central importance for the microstructural understanding of wear resistance. The present projects aims at studying a specific deeper zone of the tribo layer, where a nano crystalline and ultrafine grained structure (N&UFG) develops during HTW. The elementary microstructural processes which lead to the formation of this N&UFG zone need to be understood to appreciate the mechanisms which govern HTW at temperatures above half of the melting point in K. After HTW exposure (of different degrees), specimens from the surface regions will be taken using focused ion beam extraction (FIB). Then the N&UFG regions will be analyzed using scanning transmission electron microscopy (STEM) in combination with a high angular annual dark field (HAADF) detector, a TEM method which is well suited to investigated collective phenomena depending on dislocations and planar defects in sufficiently large microstructural regions. The investigations will be performed using two classical model Al alloys, Al11Zn and Al5Mg, which have been reported to differ in their tendencies to form dislocation induced substructures during creep and during severe plastic deformation. They will be exposed to wear loading as disk materials in the temperature range between 300 and 500°C (under Ar atmosphere and air). A wear resistant pin made out of tool steel (German grade: 100Cr6) will be used as counter body. The scientific objective of the proposed project is to contribute to a better understanding of the elementary processes which govern the formation and evolution of the N&UFG zones of tribolayers which form in metallic materials under conditions of HTW.