Microstructure of Cr 2 O 3 coatings on steel and the effect of silicon
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This paper concentrates on the microstructural features of steel containing 22 wt. % Cr, coated with Cr 2 O 3 by laser processing. It turned out that after laser coating the Cr 2 O 3 powder has completely transformed to Feo.3Cr2.7O4 having the tetragonal distorted spinel structure. Dispersed in the coating are metallic particles with composition FeCr and a bcc structure. The phases in the coating can be explained from the F e - C r - 0 equilibrium phase diagram with the assumption that complete phase equilibrium is reached in the liquid state but not during solidification. The two equilibrium phases, Lo and Lm, that exist in the molten state solidify as two independent liquids. Addition of Si to the ceramic material, either from the steel matrix or from the ceramic powder, results in a dendritic solidification structure of Cr 3 O 4 dendrites and a Si-containing glassy phase. The dendrites are oriented vertically in the coating, resembling the columnar microstructure that is also observed in ZrO 2 thermal barrier coatings. This structure contains fewer microcracks parallel to the interface resulting in a mechanically more stable ceramic coating.
I. INTRODUCTION Ceramic coatings on metals offer a method to improve the mechanical, thermal, and chemical properties of surfaces while maintaining the good bulk properties of the metal. To improve the physical properties at the highest loaded positions, laser processing can play an important role because of the very localized character of the laser melting process. If the laser is combined with an experimental setup feeding ceramic powder at the melt pool, ceramic coatings can be applied locally without changing the bulk of the material. Another advantage of laser coating is the very high temperature which can be attained during laser treatment. This makes it possible to completely melt a ceramic material. Interaction between the laser melt pool and the molten ceramic can result in reactions between the metal and the ceramic, resulting in a better wetting and a stronger interface between the metal and the ceramic. This process does require an appropriate knowledge of the starting materials and the reactions that may take place. II. EXPERIMENTAL A CW-CO 2 -laser (Spectra Physics 820) was used for coating substrate material with a ceramic layer. The operating conditions were 1.0 kW laser power, a spot size of 0.75 mm, and a laser scan velocity of 20 mm/s. The overlap between subsequent laser tracks was 75%. Cr 2 O 3 powder was delivered at the melt pool by means of a homemade powder feeding system optimized for the low feed rates needed for laser coating. The powders used were Cr2O3-powder and Cr2O3-powder containing 142
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J. Mater. Res., Vol. 9, No. 1, Jan 1994
Downloaded: 23 Jan 2015
5 wt. % SiO 2 . The size distribution of both powders peaked around 10 fim. As substrate, a single phase bcc alloy containing 78 wt. % Fe and 22 wt. % Cr (Fe4Cr) and a dual phase duplex SAF2205 steel were used, the chemical composition of which is listed in Tabl
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