Microstructural evolution in manganese cobaltite films grown on Crofer 22 APU substrates by pulsed laser deposition

Abstract

Abstract The use of pulsed laser deposition to produce high-quality manganese cobaltite spinel coatings on Crofer 22 APU substrates has been investigated, and the microstructures of the coatings have been studied using a combination of X-ray diffraction and electron microscopy techniques. It is shown that deposition from ceramic Mn1.5Co1.5O4 targets at a chamber pressure of 2.8 × 10− 4 mTorr N2 with the substrate pre-heated to 400 °C resulted in smooth, dense polycrystalline coatings. These coatings contain a mixture of rock-salt (Mn,Co)O and FCC Co phases due to reduction of the target material in the plume during deposition. Post-deposition annealing at atmospheric pressure in laboratory air at 800 °C led to re-oxidation of the coatings to give spinel phases with a thin chromia layer at the interface with the substrate. An annealing time of 0.5 h resulted in a coating with a fine uniform mixture of tetragonal and cubic spinel phases. The presence of pores at the location of the prior Co grains and at the original coating surface was used to infer the mechanisms by which the re-oxidation occurs. An annealing time of 14 h led to phase segregation in the coatings with more Co-rich cubic spinel at the surface and at the coating/substrate interface, and more Mn-rich tetragonal spinel in the center of the coating. There is Cr in the cubic phase only, indicating that the phase segregation occurs due to outward diffusion of Cr from the substrate through the chromia layer. There are local variations in the porosity and surface morphology of the coating, which appear to be correlated with the orientation of the underlying substrate grains. These observations indicate that control of grain structure in the substrate may be necessary to promote microstructural stability in such coatings.