Residual strain measurement in thermal spray cermet coatings via neutron diffraction

The impact and fatigue resistance of overlay coatings is significantly influenced by the residual strain (or stress) field induced during coating deposition, post-treatment and in-service loading. Optimization of residual stress field is therefore critical to the life and performance of components. Non-destructive measurement of these stress fields in relatively thinner (300 to 400 micron) thermal spray coatings however poses a challenge because conventional techniques such as deep hole drilling, x-ray diffraction, synchrotron diffraction, and changes in beam curvature either makes these technique destructive, and/or provides only a very near-surface strain measurement. This particularly complicates the strain analysis in cermet coatings, e.g. WC-Co deposited by the thermal spraying process, where the low penetration depth of x- and synchrotron- diffraction rays can only provide a through thickness measurement of stress profile via the destructive layer removal technique. Recent investigations have therefore concentrated on the use of neutron diffraction technique for such analysis, and this paper reports some of the early findings of the comparison of through thickness strain measurements in relatively thin (400 µm) as-sprayed and post-treated WC-12%Co coatings via neutron diffraction technique. Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray), hence diffraction results from the interaction with the atomic nucleus. Neutrons therefore have greater penetration depth in most engineering materials and therefore provide a non-destructive through thickness strain measurement. Results of strain measurement are discussed with the structure property relationship and contact fatigue performance, and indicate that post-treatment of these coatings results in harmonization of the strain field within the coating, and at the coating substrate interface. This significantly influences the contact fatigue performance by improving both the cohesive and adhesive strength of these coatings. Copyright © 2006 by ASME.