INFLUENCE OF SUBSTRATE ROUGHNESS AND TEMPERATURE ON THE ADHESION COHESION OF ALUMINA COATINGS/

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.