coating material. Compressed air sprays the molten droplets and accelerates them towards the 
substrate (Fig. 1). Conductive metals that can be drawn into wires can be sprayed using this method. 
Typical particle speeds are 150 m/s. Electric arc spraying is widely used in industry since it is a 
cheap and reliable process. 
The mechanical properties of sprayed coatings are not identical with that of their bulk material since 
their microstructure is different. Due to the spraying process, the (partially) molten particles are 
deformed on impact; thus sprayed coatings have a lamellar microstructure and show an anisotropic 
behaviour (Fig. 2). The rapid cooling and solidification of the particles on the substrate leads to 
significant residual stress which may result in intra- and transgranular microcracks. Interlamellar 
pores arise due to particle shrinkage while cooling down. The adhesion between different lamellae 
may be reduced because of oxide films and residual stress. Pores with sharp edges and inclusions 
from the grit blasting pre-treatment may act like notches, brittle phases (esp. oxides) may lower the 
strength [1, 2]. Choosing the right process and optimised parameters, the mechanical properties can 
be significantly improved. Preheating the substrate reduces the quenching effect of the particles and 
thus the residual stress. The amount of brittle phases may be reduced e.g. by using an inert transport 
gas in order to inhibit excessive oxidation [3]. Generally it can be stated that high energetic spraying 
processes are the most suitable to obtain coating with superior mechanical properties; very high 
particle speeds result in very little porosity (< 2 %) and the strength is comparable with that of 
forged specimens [4]. These techniques can not be applied directly onto foams since the spray 
particles would destroy the foam instead of building up a coating. 
open nozz' 
| 
5 x 4 
N\A 1 
Fig. 1: Schematic drawing of Fig 2: Schematic drawing of a 
electric arc spraying thermally sprayed coating 
3 Experimental 
Commercial aluminium foams with closed cells and a closed foaming skin were coated using a 
commercial arc spraying device (TAFA 9000 arc jet). The foams had a density pgam Of 
approximately 0,5 g/cm’. The thickness of the foaming skin was in the range of 50-200 um. The 
coating materials were wires of an aluminium alloy (AICuSiMn) and NiAl5. AlICuSiMn is an age- 
hardenable alloy. NiAl5 is used because of its good adhesion onto aluminium due to the formation 
of reaction products. The suitability of the substrate preparation had to be investigated. The quality 
of the interface and the coatings had to be optimised. The substrates were cleaned and roughened by 
grit-blasting or grinding to obtain a sufficient adhesion. Metallographic analysis of cross sections 
using light microscopy, Vicker's microhardness and chemical analysis to determine the oxygen 
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