College Park, Maryland June 6 - 10 , 2004
W2-A4 (2:30 PM): Residual Stress Determination in Thermally Sprayed Metallic Coatings by Neutron Diffraction
W. Wagner, T. Keller (Paul Scherrer Institut, 5232 Villigen-PSI, Switzerland), N. Margadant (EMPA Swiss Federal Laboratories for Materials Testing and Research, Thun, Switzerland), T. Pirling (Institut Laue Langevin)
Thermally sprayed coatings are nowadays widely applied in industrial technology for surface protection or improvement of surface properties for specific applications. Among others, residual stresses often play a crucial role in the performance and lifetime of such coatings and therefore, their determination is of general interest.
The present study investigates metallic NiCrAlY deposits (chemical composition 67wt. % Ni, 22wt. % Cr, 10wt. % Al, 1wt. % Y) on steel substrates. Neutron diffraction was used to obtain spatially resolved strain and stress profiles in the deposits and the underlying steel substrates. For the neutron diffraction measurements special emphasis was given to a high spatial resolution when entering the surface and crossing the interface to the substrate. Samples of four different spray techniques were analyzed: atmospheric and water-stabilized plasma spraying (APS and WSP), flame spraying (FS) and wire arc spraying (WAS). The results are quantitatively compared with the average in-plane residual stress determined by complementary mechanical profilometry (bending tests). While the stress profiles from the surface to the interface in the deposits are similar for all investigated spray techniques, their absolute values and gradients vary strongly. This is attributed to different quenching stresses from the impinging particles, different thermal histories the deposit/substrate systems undergo during the spraying and subsequent cooling, and also to different coating properties. In the WSP- and WAS-deposits, a gradient in the stress free lattice parameter was observed. Crack formation is found to be a dominant mechanism for stress relaxation in the surface plane.
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