M. Mellali et al., INFLUENCE OF SUBSTRATE ROUGHNESS AND TEMPERATURE ON THE ADHESION COHESION OF ALUMINA COATINGS/, Surface & coatings technology, 81(2-3), 1996, pp. 275-286
This article is devoted to the study of the adhesion/cohesion (A/C) of
alumina coatings sprayed on four different substrates: aluminium allo
y, titanium alloy, cast iron and mild steel. Fused and crushed alumina
particles (-45+22 mu m) were sprayed with a custom-made torch with a
nozzle of internal diameter 7 mm working with an are current of 600 A
and a plasma gas mixture of 45 slm Ar, 15 slm H-2, which resulted in a
voltage of 64 V and a thermal efficiency of 52%. The particles were i
njected internally and were almost fully molten (less than 3% alpha-ph
ase in the collected powders after their passage in the plasma jet). T
he influence of substrate roughness and temperature before (preheating
), during and after spraying was systematically studied. R(a) values w
ere 6-8, 10-13 and 14-21 mu m and the substrates were preheated to tem
peratures T-p of 170, 200, 300, 350, 380 and 500 degrees C. The most i
mportant results were the following: for cold substrates A/C increased
with R(a) up to a maximum of 20 MPa for substrates whose expansion mi
smatch with alumina was less than 4.10(-6) K, the highest A/C values (
50-60 MPa) were obtained with T-p between 350 and 500 degrees C provid
ed the substrates were not oxidized at these temperatures the highest
values of A/C were obtained with the lowest R(a) when particle velocit
y and surface temperature upon impact decreased, for example by load e
ffect, A/C values decreased too.