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5/5/04

12:31

Page 1807

Residual Stresses in High-Velocity Oxy-Fuel Metallic Coatings T.C. TOTEMEIER, R.N. WRIGHT, and W.D. SWANK X-ray based residual stress measurements were made on type 316 stainless steel and Fe3Al coatings that were high-velocity oxy-fuel (HVOF) sprayed onto low-carbon and stainless steel substrates. Nominal coating thicknesses varied from 250 to 1500
m. The effect of HVOF spray particle velocity on residual stress and deposition efficiency was assessed by preparing coatings at three different torch chamber pressures. The effect of substrate thickness on residual stress was determined by spraying coatings onto thick (6.4 mm) and thin (1.4 mm) substrates. Residual stresses were compressive for both coating materials and increased in magnitude with spray velocity. For coatings applied to thick substrates, nearsurface residual stresses were essentially constant with increasing coating thickness. Differences in thermal expansion coefficient between low-carbon and stainless steels led to a 180 MPa difference in residual stress for Fe3Al coatings. Deposition efficiency for both materials is maximized at an intermediate (⬃600 m/s) velocity. Considerations for X-ray measurement of residual stresses in HVOF coatings are also presented.

I.

INTRODUCTION

THERMAL spray coatings are increasingly used for surface protection, primarily against high-temperature corrosion and wear. Coatings are commonly applied to hot section gas turbine components,[1] to midspan dampers on fan and compressor blades in aero turbines engines,[2] and are being investigated for use in advanced fossil energy plants.[3,4,5] In all cases, the longevity and durability of the coating are of paramount importance to its primary function of protecting the substrate component. Cracking or spallation of the coating compromises its essential function, and may lead to shortened component life compared to an uncoated condition.[2,6] It is therefore critical to understand factors that may lead to premature coating failure, in addition to simply measuring its resistance to a particular aggressive environment. Residual stresses have been shown to play an important role in the cracking, adhesion, and spallation behavior of coatings.[2,7–10] In addition, residual stresses induced in the substrate by the coating process can also play a role in determining the overall component life.[2] Residual stresses in thermally sprayed coatings can be considered as the sum of three components: a tensile quench stress arising from the solidification and rapid cooling of individual spray particles upon impact with the much cooler substrate, a compressive peening stress imparted by impact of high-velocity spray particles, and a thermal mismatch stress incurred during cooling of the coating-substrate couple from the deposition temperature. The sign of the thermal mismatch stress will depend on the relative thermal expansion coefficients (CTEs) of the coating and substrate. Residual stresses in thermal spray coatings have received considerable attention in the