FIB
and
TEM
study of nanometric tribofilm formed on stainless steel during fretting‐impact tribologic tests in simulated Pressurized Water Reactor conditions
Authors: Legras Laurent , Mansot Jean‐Louis , Perillat Guillaume , Cuka Andi Mikosch 
Published: December 2016

Wear is one of the degradation mechanism observed on some component of PWR reactor. It was mainly studied based on wear tests in various conditions and SEM observations of wear surface. In this paper we present results obtained by TEM on wear scars observed on cross‐sectioned samples. Tribologic tests were performed at two temperatures using a fretting impact tribometer on 304L stainless steel tubes in an aqueous solution similar to the primary circuit fluid presents in Pressurized Water Reactors (PWR). The evolution as a function of number of cycles of the surface of the wear scar at 25°C and 75°C was measured by SEM showing that wear kinetic saturated at 25°C and is higher at 75°C (Fig.1). By modifying acceleration voltage from 3kV to 15kV and by using different detectors in SEM, we showed that a scaling oxide with wear scratches is formed in both conditions covering a surface fraction higher at 25°C than at 75°C (darker areas on backscattered images in Fig.2). Cross sections were extracted from unworn surface and from the center of the wear scars obtained after 20000 cycles using a Helios Nanolab Dual Beam FIB .The tribofilm and underlying metal structure were studied by EELS in a XFEG TECNAI OSIRIS TEM equipped with a Gatan Quantum filtered imaging system. A cold work layer coming from the manufacturing process is observed at the surface of the unworn specimen. It consists in a 500 nm thick outer layer constituted of nanometric grains and inner highly strained grains. We observed that during tribologic tests this layer is evolving being recovered and recrystallized. Because of the initial heterogeneity of strain degree at the outer surface of the tube, it is difficult to conclude on a potential different evolution of the cold work layer as a function of temperature. On those cross sections, both scaling oxide layers, appearing amorphous, and stacks of multilayered compound forming bumps are observed. EDXS and EELS mapping were also performed to study the oxide observed by SEM on the wear scars (Fig. 3). At 25°C, the scaling oxide observed on the surface is found to be amorphous and Cr and Fe rich. Chemical shifts of Cr L
2,3
edge and O K Near Edge Structures (NES) evolve from the surface and the interfaces to the centre of this oxide layer whereas Fe L
2,3
edge remains unchanged and characteristic of oxidized Fe. This indicates that oxidation state of Cr is changing at interfaces where oxide is scaling. At 25°C, a continuous nanometric and polycrystalline almost composed of pure Ni metal film is also between this oxide areas and locally at Cr oxide/metal interface. Bumps at the wear scar surface observed by SEM are stacks of nanometric Ni metal and more or less oxidised 304L films. At 75°C, the growth of Cr/Fe amorphous oxide and Ni rich film is also observed, but the Cr/Fe rich oxide seems more porous and the Ni enrichment only reaches 40% and is found to be oxidised. Facing wear kinetics measured during tribologic tests, the modification of the tribofilms composition depending on temperature might be the reason of the difference of wear kinetics observed between 25°C and 75°C. This very new results should be implemented by a deeper analysis (via modelling and test samples) of fines structures of 0 and Cr edges to give a better understanding of the competition between oxidation and mechanical wear processes.