Exploring Thermal Spray Grey Alumina Coating Pore Network Architecture by Combining Stereological Protocols and Impedance Electrochemical Spectroscopy

Abstract

Abstract Complex multi-scale pore network architecture characterized by a multi-modal pore size distribution and connectivity develops during the manufacturing of ceramic thermal spray coatings from intralamellar and interlamellar cracks generated when each lamella spreads and solidifies to globular pores resulting from lamella stacking defects. This network significantly affects the coating properties and their in-service behaviors. The De Hoff’s stereological analysis permits the quantification of the 3-D distribution of spheroids (i.e., pores) from the determination of their 2-D distribution estimated by image analysis when analyzing the coating structure from a polished plane. Electrochemical impedance spectroscopy aims at electrochemically oscillating a material surface by frequency variable current and potential and at analyzing the complex impedance. When a coating covers the material surface, the electrolyte percolates through the more or less connected pore network to locally corrode the substrate. The resistive and capacitive characteristics of the equivalent electrical circuit will depend upon the connected pore network architecture. Al2O3-13TiO2 coatings were atmospherically plasma sprayed using several sets of power parameters, the arc current intensity, the plasma gas total flow rate and the plasma gas composition, namely to scan their effects on the pore network architecture. In parallel, particle characteristics upon impact, especially their related dimensionless numbers such as Reynolds, Weber and Sommerfeld criterion, were determined. Analyses permitted the identification of (i) the major effects of power parameters on the pore architecture and (ii) the related formation mechanisms.