Stress development and adhesion behavior in thin ceramic coatings monitored by positron annihilation during bending
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Stress development and adhesion behavior in thin ceramic coatings monitored by positron annihilation during bending R.Escobar Galindo1, A.van Veen1, H.Schut1, N.J.M.Carvalho2, C.Strondl3, J. Th. M. de Hosson2 1 Interfaculty Reactor Institute, Delft University of Technology, Delft, The Netherlands 2 Materials Science Centre and NIMR, University of Groningen, Groningen, The Netherlands 3 Hauzer Techno Coating, Venlo, The Netherlands. ABSTRACT Positron Beam Analysis was for the first time performed in combination with a four-point bending stage. Applications of this novel technique on PVD TiN (5µm thick), WC:H (2.5µm) and CrN (1.5 µm) layers on tool steel, low carbon steel and stainless steel substrates are discussed. It appeared from PBA results that for these systems the coatings did not fail by delamination during bending. SEM confirmed that there was no decohesion but instead, cracking of the coatings was observed. In order to weaken the interface and quantify adhesion properties we propose to induce blistering based on localised gas implantation. INTRODUCTION The problem of ceramic/metal adhesion plays a crucial role in the successful industrial application of tailored materials such as coatings, composites and multi-layered systems. Examples of the effects observed are decohesion, cavity formation and (material) diffusion along the interfaces [1]. In many cases these problems have their origin in the (micro-mechanical) properties of the interface between the substrate and the coating layer, the host matrix and the embedded particles or in the interfaces between the different layers. In particular these effects are known to take place during deformation of the material. For this reason ways are sought for the “in situ” observation of the dynamic evolution of the above mentioned processes during deformation. Positron Beam Analysis provides a non-contact and non-destructive tool to monitor early stages of changes in the stress state of thin films during mechanical testing. Depending on the residual stress of the as-prepared coating, stresses can be released or increased, e.g., from an initial compressive state to a tensile state. Positrons have proven to be very well capable of observing the transition from compressive to tensile state. This transition is detected by the increase of open volume when the stress state changes from negative to positive (tensile). Regarding adhesion, the very first defects associated with delamination can be observed. Positrons have a high affinity to vacancies and vacancy clusters in metals but also in ceramics [2]. Delamination will start with these types of defects, so positrons are thought to be a very suitable technique to study first stages of decohesion. In this study a four point bending apparatus has been mounted in the analysis chamber of a variable energy positron beam. This configuration allows studying in situ stress development and delamination phenomena of different coating/substrate systems. Varying the positron implantation energy, the coating layer or the coating/subst
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