Evolution of porosity in suspension thermal sprayed YSZ thermal barrier coatings through neutron scattering and image analysis techniques

Daniel Tejero-Martin,Mingwen Bai,Jitendra Mata,Tanvir Hussain

doi:10.1016/j.jeurceramsoc.2021.04.020

Abstract

Porosity is a key parameter on thermal barrier coatings, directly influencing thermal conductivity and strain tolerance. Suspension high velocity oxy-fuel (SHVOF) thermal spraying enables the use of sub-micron particles, increasing control over porosity and introducing nano-sized pores. Neutron scattering is capable of studying porosity with radii between 1 nm and 10 μm, thanks to the combination of small-angle and ultra-small-angle neutron scattering. Image analysis allows for the study of porosity with radii above ∼100 nm. For the first time in SHVOF 8YSZ, pore size distribution, total porosity and pore morphology were studied to determine the effects of heat treatment. X-ray diffraction and micro-hardness measurements were performed to study the phase transformation, and its effects on the mechanical properties. The results show an abundant presence of nano-pores in the as-sprayed coatings, which are eliminated after heat treatment at 1100 °C; a transition from inter-splat lamellar to globular pores and the appearance of micro-cracks along with the accumulation of micro-strains associated with the phase transformation at 1200 °C.

Highlights

Yttria-stabilised zirconia (YSZ) currently represents the most used material within thermal barrier coatings (TBC) for gas turbine engine components [1,2]
It is evident from the results that the combination of SANS and ultra-small-angle neutron scattering (USANS) allows the study of pores with a radius between ~1 nm and ~10 μm, effectively encompassing the nano-sized features expected from suspension thermal spray and traditional micron-sized features
We have demonstrated that 8YSZ coatings deposited using Suspension high velocity oxy-fuel (SHVOF) thermal spray present a large amount of nano-sized porosity, contrary to more traditional powder-based thermal spray techniques such as atmospheric plasma spraying (APS), and a mixture of elongated, inter-splat pores and spherical pores

Summary

Introduction

Yttria-stabilised zirconia (YSZ) currently represents the most used material within thermal barrier coatings (TBC) for gas turbine engine components [1,2]. Since the porosity in thermal sprayed coatings is strongly correlated to the size distribution of the feedstock particles, mostly due to unmelted feedstock particles and gaps in between adjacent splats [10], there has been a growing interest for submicron feedstock particle distribution. Such a reduction in size allows finer grains, higher strength and durability, reduced porosity sizes [11] and enhances the thermal and mechanical properties of the coating [6,12]

Objectives

Methods

Results

Discussion

Conclusion