Cadmium Substitution - garteur

GARTEUR LIMITED
B.3.3
Coating Adhesion
The coating adhesion on the grit blasted steel substrate was good. The metallic coatings
behave in a brittle manner (except ED cadmium) resulting in a high crack density at a
plastic strain of 1%. At increasing strain no signs of coating delamination were observed.
Tests conducted on Delta-tone indicated that the surface roughness is a critical factor in
determining the coating adhesion. A smooth surface was found to give the best
adhesion. If the surface roughness is too high, a micro-layer of Delta-tone prevents
complete wetting of the surface and cavities are formed at the coating/steel substrate
interface.
B.4 Conclusions
1. The microstructures, thicknesses and compositions of the coatings supplied for the
Garteur programme have been compared with information given by the coating
producers and with data published in the open literature. It is concluded that
coatings are representative of those generally used in the aerospace and other
manufacturing industries.
2. Resistance measurements indicate that it is possible to achieve good electrical
conductivity with both coated Hi-Lok and coated countersink screws. Tests made on
simple aluminium alloy lap joints indicate that there is electrical contact between the
fasteners and aluminium alloy.
3. All the metal coatings investigated showed good adhesion to steel substrates which
had been grit blasted. Tests made on the Delta-tone coating indicated that the best
adhesion was achieved when the substrate was smooth.
B.5 References
[B1]
W.G.J. t Hart and J.A.M Boogers, “Coatings considered for cadmium substitution
(Garteur AG17 Programme), NLR CR 97107 L (1997)
Page 50
GARTEUR SM/AG17 TP128