Oludele Olusegun Popoola

Characterization of copper layers produced by cold gas-dynamic spraying

The cold gas-dynamic spray method produces coatings or deposits by introducing solid feedstock particles into a supersonic gas stream developed through the use of a converging-diverging (de Laval) nozzle. The particles thus accelerated impact on a substrate surface and develop into a dense deposit through a process believed to be similar to cold compaction. The work reported here explores the internal nature and physical characteristics of copper deposits produced by the cold gas-dynamic spray method using two vastly different starting powders: in one case, a “spongy” copper obtained by a direct-reduction process, and in the second, a denser, more spheroidal particulate produced by gas atomization. Optical and electron microscopies (scanning electron microscopy [SEM] and transmission electron microscopy [TEM]) were used to observe details of microstructure in the feedstock particles and deposits. Young’s modulus and residual stress measurements for the deposits were obtained through mechanical means, and measurements of hardness and electrical conductivity are reported. The internal structure of the cold-spray deposit was influenced by the surface purity of the feedstock material.

In-situ transmission electron microscopy study of phase transformations in KNbO3 perovskite

KNbO3 undergoes the phase transformations cubic (Pm3m) →tetragonal (P4mm2) →orthorhombic (Bmm2) at 438°C and 234°C respectively. The cubic →tetragonal transformation is accompanied by a paraelectric →ferroelectric phase transition in the crystal. It is preceded by the appearance of o microdomains at 440°C and occurs with characteristic features of a first-order displacive transformation temperature hysteresis, temperature-dependent growth and orientation relationship with the orientation relationship {110}c‖{110}t and (001)c|(001)t. On the other hand, the tetragonal →orthorhombic phase transformation takes place at exactly 234°C with a front that swept through the thin foil once the transformation was initiated. It occurs with the orientation relationship [001[c‖[001]o and (100)t‖(100)o. The cubic-to-tetragonal transformation involves a purely dilatational mechanism along the principal axes of the cubic phase while the tetragonal-to-orthorhombic transformation involves a shear along the diagonal of the te...

Fatigue behavior of a thermally sprayed low carbon steel coating

The fatigue behavior of a thermally sprayed low carbon steel coating has been investigated at room and elevated temperatures. The coating was found to have higher fatigue resistance at 300°C than at room temperature. Further studies indicate that the higher fatigue resistance at the elevated temperature was related to a rapid increase in the elastic modulus of the coating with time during the early part of the fatigue life. The increase in the elastic modulus of the coating was shown to result from annealing of the as-sprayed microstructure at elevated temperatures. Following annealing, the weaker oxide-rich region of the microstructure also became more resistant to crack initiation. The strengthened oxide-rich region delayed accumulation of fatigue damage in the coating, leading to improved fatigue resistance of the coating at elevated temperatures.