Residual Stress in HVOF Thermally Sprayed WC-10Co-4Cr Coating in Landing Gear Application
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Residual Stress in HVOF Thermally Sprayed WC-10Co-4Cr Coating in Landing Gear Application M. Gui1 • R. Eybel1 • S. Radhakrishnan1 • F. Monerie-Moulin2 • R. Raininger1 P. Taylor2
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Submitted: 22 April 2019 / in revised form: 24 June 2019 Ó ASM International 2019
Abstract Type ‘‘N’’ Almen strips were HVOF thermally sprayed WC-10%Co-4Cr coating with spraying parameters used for landing gear coating production. The Almen strips were coated at variable passes and different cooling conditions. Deflections of Almen strip specimens were measured, and the coating residual stresses were calculated. With the same facility and spraying parameters, in situ coating property sensor was also used to continuously monitor the curvature of beam specimens being coated during and after coating deposition to evaluate coating deposition stress and final residual stress. The experiments, together with micro-hardness test, reveal that peening action occurred in the current HVOF spraying is limited, and the residual stress in the coating is dominated by quenching stress and cooling stress. In this study, the substrate temperature of specimens was adjusted by cooling air flow and torch recess time in between spraying passes. The coating residual stress demonstrates to apparently relate the substrate temperature because it significantly affects the cooling stress. Since the spraying parameters are frozen in industrial coating production, the cooling condition is a feasible approach for tailoring the coating residual stress. Keywords Almen strip cooling condition HVOF thermal spraying residual stress WC-Co coating
& M. Gui [email protected] 1
Safran Landing Systems Canada Inc., Toronto, ON, Canada
2
Safran Landing Systems Bidos, Oloron Sainte-Marie Cedex, France
Introduction HVOF-sprayed WC-Co coatings show superior wear resistance and adequate corrosion resistance. As an alternative to electroplating hard chrome, the coatings have been increasingly used in aircraft landing gears (Ref 1-4). The residual stress in the HVOF-sprayed WC-Co coatings is found affecting the mechanical behavior and durability of the coatings, e.g., fatigue resistance and coating spallation. In accordance with SAE Aerospace Material Specification AMS2448, the residual stress shall be compressive and is evaluated by the deflection of type ‘‘N’’ Almen strip as 0.075 to 0.30 mm, i.e., 146 to 586 MPa calculated by Stoney equation, when it is sprayed to 0.127 mm thick coating. Aircraft landing gears work in reciprocal stress circumstance, so the fatigue performance of the material used for making landing gear parts is significant. HVOF spraying process can induce a compressive residual stress in the WC-Co coatings (Ref 5-9). However, the Cr plating process generates a high tensile residual stress and micro-cracks in the coating (Ref 10). Compressive residual stress tends to retard the nucleation and propagation of crack in the coatings in fatigue condition. So high-strength steel specimens with WC-Co coatings deposited by HVOF process de
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