Investigation of techniques for determining the residual stresses in WC-17Co thermal sprayed coatings and studying the effect of residual stress on its abrasion resistance
Date
2013-07-23
Authors
Oladijo, Oluseyi Philip
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Abstract
The aim of this project was to investigate the techniques for determining the residual stresses
in WC-17Co thermal sprayed coatings and to study the effect of residual stress on abrasion
resistance on different substrates. The choice of the substrate was due to their different
coefficients of thermal expansion (aluminium, super-invar, 304L stainless steel, mild steel
and brass). Coatings of about 200μm were successfully deposited on all the substrates.
Coatings were deposited by a high velocity oxyl-fuel spraying system (HVOF), and
characterized by evaluating the coating phases and wear resistance, and the residual stresses
were determined via non-destructive methods. Investigations involved both as-sprayed and
their annealed counterparts to ascertain the effect of heat treatment.
Non-destructive determination of residual stresses in the WC-Co coated systems was
exceptionally challenging in that the coatings were only 200 microns thick. The best suited
techniques for investigation of WC were diffraction-based strain scanning using penetrating
radiation such as thermal neutrons (most penetrating), high energy synchrotron X-rays (100
keV enables 20 micron penetration) and laboratory X-rays (limited to 5 micron penetration).
Laboratory X-rays (Necsa, using Co radiation), thermal neutrons (ANSTO, Australia) and Xray
synchrotron (ESRF, France) were successfully employed to resolve the stress conditions.
The neutron investigations enabled two approaches for the determination of the in-surface
stresses, direct measurements (good results for the low neutron attenuation substrates), and
indirect determination using stress balance conditions inferred from the through thickness
depth profiles measured in the substrates (applicable to all the higher neutron attenuating
substrates). Investigations were expanded to the study of the influence of annealing at 40% of
the respective substrate melting temperatures. For each substrate, the through thickness stress
profile differences between the grit-blasted reference material (final before the HVOF
coating) and the grit-blast coated samples were used to determine the elastic contributions
purely ascribed to the coating process. This required exceptional positional resolution neutron
diffraction investigations (positional accuracies better than 0.01 mm). There were both small
compressive and low tensile stresses on the as-sprayed coated samples. After annealing, the
stresses became substantially more compressive. The near-surface trends of the grit-blasted
substrates were completely relaxed after annealing, with thermal stresses being the dominant
mechanism for residual stress induced due to the large difference in the coefficients of
thermal expansion (CTE) between the WC coatings and the substrates.
The three-body abrasive tests were carried out on the parent materials (substrate), grit blasted
substrates, as-sprayed coatings and heat treated coatings. The contributions of each coating
process were established. There was about 45-50% reduction in the wear of grit-blasted
samples compared to the substrates, whilst 80-95% reduction in the wear of the coatings
compared to substrates was found. The wear resistance is due to many different contributions,
but there was a strong correlation between the residual stress and wear resistance of the coating.