INFLUENCE OF DC PLASMA PREHEATING ON OXIDE LAYERS FORMED BY FURNACE HEATING ON LOW CARBON STEEL SUBSTRATES AND RESULTING ADHESION/COHESION OF ALUMINA COATINGS

摘要

In plasma spraying the preheating of low carbon steel substrates (X40) with a rough surface resulting from grit blasting, has received an increased interest because it promotes the adhesion and cohesion (A/C) of alumina coatings. Earlier works have reported the generation of a duplex Fe_3O_44/Fe_2O_3 layer after preheating by the d.c. plasma jet. Improved A/C values were obtained for an increase in Fe_3O_4 content in the oxide layer. Much improved adherence values were achieved with alumina spayed on Fe_(1-x)O layers. In this case, the Adhesion/cohesion values obtained on smooth substrate (Ra ～0.1 μm) were good (～ 40 MPa) and close to those obtained on rough ones (Ra ～ 5 μm). First the time evolution of the Fe_(1-x)O layer thickness and morphology produced in a controlled atmosphere furnace was studied. Then the influence of the plasma preheating of the wuestite layers was studied by SEM, and XRD analyses. XRD analysis results indicated a slight variation in the oxide phases depending on the preheating cycles. Fe_3O_4 was found to increase when a decomposition of Fe_(1-x)O into Fe_3O_4 was observed.