Characterization of the materials used in air plasma sprayed thermal barrier coatings

Abstract

The degradation phenomena of the materials composing thermal barrier coatings (TBCs) have been investigated destructively and non-destructively in this dissertation. In order to make a clear explanation, the degradation phenomena of an individual material have been investigated with mainly impedance spectroscopy (IS) and indentation technique. In addition, the interfacial fracture toughness of an air plasma sprayed (APS) TBC has been measured with a four point bending test. A cold pressed and sintered YSZ sample was first used to characterise the electrical and the mechanical properties of the YSZ. The logarithmic scale of the electrical conductivity of YSZ increases with an increase of the ratio of measured hardness to its Young's modulus (H/E) until the densification stops. The variation of the microstructures, eg, grain size changes the electrical properties of YSZ.The electrical and mechanical properties of air plasma sprayed (APS) yttria stablised zirconia (YSZ) thermal barrier coatings (TBCs) were determined using impedance spectroscopy and an indentation technique, respectively. Upon thermal exprosure, sintering and phase transformation occurs in the YSZ TBCs, leading to changes in both the mechanical and electrical properties of the TBCs. After the thermal treatment, the formation of the monoclinic phase in the YSZ TBCs reduced the density of the TBCs and thus affected both the mechanical properties and conductivity of the TBCs. In this study, a relationship between the electrical and mechanical properties of APS TBCs has been established, allowing us to evaluate the properties of TBCs non-destructively with the use of impedance spectroscopy. The electrical properties of a thermally grown oxide (TGO) formed on a Fecralloy substrate was affected by the residual stress of the TGO layer. In addition, IS can distinguish the effect of grains and grain boundaries when Bode plots and modulus spectra, imaginary modulus M" vs real modulus M', were used. The conductivity and the dielectric constant of the grains in the TGO layer decreases with an increase of its compressive residual stress while the dielectric constant of the grain boundaries is almost constant regardless of the TGO residual stress. The calculated dielectric constant of the grains is slightly higher while that of the grain boundaries is slightly lower than the literature value of the dielectric constant of a pure alumina. The interfacial fracture toughnesses of an APS TBC thermally treated at 1050oC for various times were characterised using four point bending tests. The delamination occurred primarily within the TC, just several micrometres above the TGO layer. The interfacial fracture toughness increases first to a maximum after a 50h thermal treatment, and then gradually decreases with further thermal treatment. The increment of the interfacial toughness at the beginning of the thermal treatment is due to sintering while its reduction is due to the monoclinic phase formation during the thermal treatment.

Date of Award	1 Aug 2011
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ping Xiao (Supervisor)