Measuring Residual Strain and Stress in Thermal Spray Coatings Using Neutron Diffractometers

Abstract

During thermal spray coating, residual strain (or stress) is formed within the coating and substrates due to many processes (quenching stress, peening effect, deposition temperature, lamella structure) and micro-structural phase changes. It is also known that the residual stress values of thermally sprayed coatings are dependent upon the measurement method. Neutron diffraction technique can provide a non-destructive through-thickness residual strain analysis in thermally sprayed components with a level of detail not normally achievable by other techniques. Despite this advantage, the number of studies involving neutron diffraction analysis in thermal spray coatings remain limited, partly due to the limited number of neutron diffraction strain measurement facilities globally, and partly due to the difficulty is applying neutron diffraction analysis to measure residual strain in the complex thermal spray coating micro-structure. This paper provides a comprehensive guide to researchers planning to use this technique for thermal spray coatings, and reviews some of these studies. ENGIN-X at the ISIS spallation source in the UK is a neutron diffractometer (time-of-flight) dedicated to materials science and engineering with high resolution and versatile capabilities. The focus is on the procedure of using ENGIN-X diffractometer for thermal spray coatings with a view that it can potentially be translated to other neutron diffractometers. Neutron sources worldwide (e.g. Africa, Asia, Australia, Europe, and North America) have been used to measure strains in various materials, and here, we present few examples where thermal spray coatings have been characterized at various neutron sources worldwide, to study the residual strains and micro-structures.