D. De Felicis

On the use of copper-based substrates for YBCO coated conductors

It is well known that the recrystallization texture of heavily cold-rolled pure copper is almost completely cubic. However, one of the main drawbacks concerning the use of pure copper cube-textured substrates for YBCO coated conductor is the reduced secondary recrystallization temperature. The onset of secondary recrystallization (i.e., the occurrence of abnormal grains with unpredictable orientation) in pure copper substrate was observed within the typical temperature range required for buffer layer and YBCO processing (600–850 °C). To avoid the formation of abnormal grains the effect of both grain size adjustment (GSA) and recrystallization annealing was analyzed. The combined use of a small initial grain size and a recrystallization two-step annealing (TSA) drastically reduced the presence of abnormal grains in pure copper tapes. Another way to overcome the limitation imposed by the formation of abnormal grains is to deposit a buffer layer at temperatures where secondary recrystallization does not occur. For example, La2Zr2O7 (LZO) film with a high degree of epitaxy was grown by metal-organic decomposition (MOD) at 1000 °C on pure copper substrate. In several samples the substrate underwent secondary recrystallization. Our experiments indicate that the motion of grain boundaries occurring during secondary recrystallization process does not affect the quality of LZO film.

Wear mechanisms and in-service surface modifications of a stellite 6B Co-Cr alloy

In the present paper, the in-service wear mechanisms and cross-sectional microstructural evolution of a Stellite 6B cobalt-based alloy subjected to sliding contact conditions are analysed. The analysis is carried out using focused ion beam (FIB), transmission electron microscopy (TEM), 3D confocal profilometer and nanoindentation techniques. Samples under investigation consisted of a lip-seal adopted in a Tunnel Boring Machine (TBM). In the working conditions, the lip-seal is in sliding contact with a rubber seal in a pressurised oil environment. In those conditions, an unexpected low wear resistance was observed during service. As a consequence, a long machine downtime was necessary to replace the damaged component. Results of FIB-SEM/EDS characterisation showed severe grooving due to the presence of sand (SiO2) particles embedded in the rubber seal in the contact area. In addition, three body abrasive wear is observed immediately outside the contact area. Microstructural evaluation of Stellite 6 B Co-based alloy of the cross-section also showed the presence of a nano-crystalline hardened layer with diffuse presence of stacking faults. The presence of a tribo-film with a complex structure was also clearly observed. It is concluded that wear resistance of such components could be significantly enhanced either by a proper control of abrasive contaminant particles in the lubricant oil or by introducing a proper protective coating (e.g. a thermal sprayed thick coating).

Production of CrN/NbN superlattice coatings by cathode switching reactive cathodic arc evaporation

CrN/NbN superlattice and CrN coatings, approximately 3 pm in thickness, have been deposited on tool steel samples by a modified reactive cathodic arc evaporation technique (CAE) in an industrial-size chamber. Superlattice deposition was achieved by triggering alternatively on Cr and Nb cathodes, obtaining layers of about 4-5 nanometers. X-ray diffraction, scanning and transmission electron microscopy, atomic force microscopy and energy dispersive X-ray analysis were used to determine microstructure and chemical composition of both coatings and coating defects. Diffraction techniques showed for the CrN coating a (220) preferred orientation and a strongly preferred (200) orientation for superlattice samples, with a lattice parameter intermediate between the CrN and the NbN ones. Defect evaluation was focused on shape, dimension, density, clustering and other process-sensitive features. In CrN/NbN superlattice coatings a separation between defects and the surrounding bulk coating was evident, in fact TEM analysis revealed a different microstructure and chemical composition in this area. On the basis of macroparticle formation theory an explanation of the experimental observations is proposed.