Residual stress and adhesion of thermal spray coatings: microscopic view by solidification and crystallisation analysis in the epitaxial CoNiCrAlY single splat

A new approach is proposed to achieve an in-depth understanding of crystallisation, residual stress and adhesion in epitaxial splats obtained by Combustion Flame Spray. Modelling of the fundamental process mechanisms is achieved with the help of experimental observations providing details with a sub-micrometre spatial resolution. At this scope, High Angular Resolution Electron Backscatter Diffraction and Transmission Electron Microscopy analysis are employed to provide insights into crystallisation and residual stress levels, while FIB-milled microcantilever beam bending is used for fracture strength measurements in the case of single splats. A comparison to fully-developed coatings is achieved by employing the X-ray Diffraction technique and pull-off methods for residual stress and fracture strength, respectively. The methodology is applied to metallic CoNiCrAlY material sprayed onto a Ni-based superalloy substrate. The establishment of different crystallisation regions: epitaxial and polycrystalline, is the result of variations in the heat flux direction at the solidification front. Significant dislocation density is also reported, indicating the relevance of impact dynamics and plastic deformation mechanisms. The comparison with fully-developed coatings suggests a reduction in inter-splat bonding at splat overlapping

Zirconia catalysed acetic acid ketonisation for pre-treatment of biomass fast pyrolysis vapours.

Crude pyrolysis bio-oil contains significant quantities of carboxylic acids which limit its utility as a biofuel. Vapour phase ketonisation of organic acids contained within biomass fast-pyrolysis vapours offers a potential pre-treatment to improve the stability and energy content of resulting bio-oils formed upon condensation. Zirconia is a promising catalyst for such reactions, however little is known regarding the impact of thermal processing on the physicochemical properties of zirconia in the context of their corresponding reactivity for the vapour phase ketonisation of acetic acid. Here we show that calcination progressively transforms amorphous Zr(OH)4 into small tetragonal ZrO2 crystallites at 400 °C, and subsequently larger monoclinic crystallites >600 °C. These phase transitions are accompanied by an increase in the density of Lewis acid sites, and concomitant decrease in their acid strength, attributed to surface dehydroxylation and anion vacancy formation. Weak Lewis acid sites (and/or resulting acid-base pairs) are identified as the active species responsible for acetic acid ketonisation to acetone at 350 °C and 400 °C, with stronger Lewis acid sites favouring competing unselective reactions and carbon laydown. Acetone selectivity is independent of acid strength

In-situ studies on martensitic transformation and high-temperature shape memory in small volume zirconia

Recently, the shape memory effect with significant recoverable shape deformation has been discovered in small volume single-crystal zirconia. The application potential for such shape memory ceramics has spurred in-depth exploration of the martensitic transformation crystallography and high temperature shape memory effect. In this work, the martensitic transformation of micron-sized zirconia has been studied in both a pristine as-produced condition and after micromechanical compression, using synchrotron scanning micro X-ray diffraction (μXRD). The pristine zirconia was found to transform into the monoclinic phase via a different crystallographic path than the compressed zirconia, resulting in distinct monoclinic variant preferences. The characteristic martensitic transformation temperatures were also found to be higher after uniaxial compression than in the pristine condition. Such observations provide insight on the differences between stress-induced and thermally-induced martensitic transformation. Moreover, we have observed a full cycle of the shape memory effect with large recoverable strain (7%) in micron-sized zirconia at a temperature of 400 °C. Our findings provide an important guideline to tailor the high-temperature shape memory properties of zirconia for further scientific research and engineering applications

On the size-dependent phase transformation in nanoparticulate zirconia

We have studied the structures of zirconia nanoparticles formed by plasma-spraying an organo-metallic precursor. Inspection of the particles in the TEM reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, while larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the zirconia particles coarsen and undergo a phase transformation when the particle size is of the order of 18 nm, for reasons associated with the surface energy, and the occurrence of this phase transformation produces a sudden change in the driving force for coarsening. Grain size distributions below the critical size for the transforrnation are log-normal, but as the transformation occurs, the size distribution changes to a markedly less skewed form. The development of this distribution is followed to establish whether it grows self-similarly, or returns to log-normality once normal driving forces are restored after the phase transformation is complete