Nano-indentation measurement of oxide layers formed in LBE on F/M steels

Ferritic/martensitic (F/M) steels (T91, HT-9, EP 823) are candidate materials for future liquid lead or lead bismuth eutectic (LBE) cooled nuclear reactors. To understand the corrosion of these materials in LBE, samples of each material were exposed at 535 °C for 600 h and 200 h at an oxygen content of 10 wt%. After the corrosion tests, the samples were analyzed using SEM, WDX and nano-indentation in cross section. Multi-layered oxide scales were found on the sample surfaces. The compositions of these oxide layers are not entirely in agreement with the literature. The nano-indentation results showed that the E-modulus and hardness of the oxide layers are significantly lower than the values for dense bulk oxide materials. It is assumed that the low values stem from high porosity in the oxide layers. Comparison with in-air oxidized steels show that the E-modulus decreases with increasing oxide layer thickness. © 2008 Elsevier B.V. All rights reserved.

Innovative measurement methodology in the study of weathering of automotive coatings

An initial aim of this project was to evaluate the conventional techniques used in the analysis of newly prepared environmentally friendly water-borne automotive coatings and compare them with solvent-borne coatings having comparable formulations. The investigation was carried out on microtuned layers as well as on complete automotive multi-layer paint systems. Methods used included the very traditional methods of gloss and hardness and the commonly used photo-oxidation index (from FTIR spectral analysis). All methods enabled the durability to weathering of the automotive coatings to be initially investigated. However, a primary aim of this work was to develop methods for analysing the early stages of chemical and property changes in both the solvent-borne and water-borne coating systems that take place during outdoor natural weathering exposures and under accelerated artificial exposures. This was achieved by using dynamic mechanical analysis (DMA), in both tension mode on the microtomed films (on all depths of the coating systems from the uppermost clear-coat right down to the electron-coat) and bending mode of the full (unmicrotomed) systems, as well as MALDI-Tof analysis on the movement of the stabilisers in the full systems. Changes in glass transition temperature and relative cross-link density were determined after weathering and these were related to changes in the chemistries of the binder systems of the coatings after weathering. Concentration profiles of the UV-stabilisers (UVA and HALS) in the coating systems were analysed as a consequence of migration in the coating systems in separate microtomed layers of the paint samples (depth profiling) after weathering and diffusion co-efficient and solubility parameters were determined for the UV stabilisers in the coating systems. The methods developed were used to determine the various physical and chemical changes that take place during weathering of the different (water-borne and solvent-borne) systems (photoxidation). The solvent-borne formulations showed less changes after weathering (both natural and accelerated) than the corresponding water-borne formulations due to the lower level of cross-links in the binders of the water-borne systems. The silver systems examined were more durable than the blue systems due to the reflecting power of the aluminium and the lower temperature of the silver coatings.

Measurement of residual stress by load and depth sensing indentation with spherical indenters

A new experimental technique is presented for making measurements of biaxial residual stress using load and depth sensing indentation (nanoindentation). The technique is based on spherical indentation, which, in certain deformation regimes, can be much more sensitive to residual stress than indentation with sharp pyramidal indenters like the Berkovich. Two different methods of analysis were developed: one requiring an independent measure of the material's yield strength and the other a reference specimen in the unstressed state or other known reference condition. Experiments conducted on aluminum alloys to which controlled biaxial bending stresses were applied showed that the methods are capable of measuring the residual stress to within 10-20% of the specimen yield stress. Because the methods do not require imaging of the hardness impressions, they are potentially useful for making localized measurements of residual stress, as in thin films or small volumes, or for characterization of point-to-point spatial variations of the surface stress.