Nd-YAG Laser Surface Treatment Of Cr2O3Coatings. Wear Tests In Tribometer LWF-1

Abstract

Superficial laser treatment allows tribological behaviour of certain materials to be improved. This paper presents the results obtained using Nd-YAG laser on a plasma sprayed coating of Cr2O3 fixing the optimal parameters of such treatment. On the other hand, wear resistance tests performed using an LWF-1 machine with pairs steel-ceramic coating, show, as a rule, a better performance of both steel and ceramic coating under dry friction and abrasive condition, mainly due to the change caused by laser treatment on the coating micros true ture. INTRODUCTION Plasma spraying is a technique that allows high quality coatings to be obtained using materials with a high melting point, as it is the case for ceramics. The ceramic coatings thus generated show good wear, corrosion and thermal barrier resistance [1]. However, the high porosity presented by such coatings when sprayed with plasma, prevents them from being used for some applications since their behaviour is not as good as expected [2]. A recent technique, currently being Transactions on Engineering Sciences vol 2, © 1993 WIT Press, www.witpress.com, ISSN 1743-3533 318 Surface Treatment Effects studied by few research groups, e.g. Mordike [3], Cuetos [4], Havra [5], Iwamoto [6] and Fernandez [7], is the laser surface treatment, which improves the behaviour of the coatings. The unique characteristics presented by the laser beam for thermal treatment (accuracy, energy density, and minimum heat input per unit volume), cause minimum thermal effect on the substratum when the coating is treated. This makes of laser a key element for industrial use in this field. In this work Nd-YAG laser equipment is used to melt the plasma sprayed Cr% 03 coatings whose behaviour at wear, before and after the treatment, is studied. EXPERIMENTAL PROCEDURE Wear tests on ceramic coatings have been carried out before and after the laser treatment. The ceramic coating studied is a chromium oxide compound which was projected by the plasma, using F 1140 steel as substratum that had been previously shot blasted to give it the required roughness and cleanness. The spraying was done using METCO 9 MB equipment of 40 KW. The material used for the coating was a type of powder with the following composition: 96% Cr2 Og, 2% TiO2, 2% other oxides. Before the ceramic material was projected, a bond layer was sprayed over the shot blasted substratum. The composition of the bond was: 89.5% Ni, 5.5% Al, 5% Mo. The final average thickness of coating and bond layer together was 0.3 mm. Two types of test specimens were used, block and ring, defined according to the ASTM G77 standard, whose geometry is described on figure 1. The block, whose structure is shown in 2, consists of a steel substratum, a plasma sprayed bondage layer and a ceramic coating, also plasma sprayed. with the foregoing characteristics. The ring was made of SAE 4620 steel. Transactions on Engineering Sciences vol 2, © 1993 WIT Press, www.witpress.com, ISSN 1743-3533 Surface Treatment Effects 319