TECHNOLOGY OF CREATION COATINGS BASED ON FeCrAlY.

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1 TECHNOLOGY OF CREATION COATINGS BASED ON FeCrAlY Šárka Houdková b, Roman Splítek b, Michaela Kašparová a, Matěj Hruška b, Fosca Di Gabriele c a Research and Testing Institute, Plzeň, Czech Republic, kasparova@vzuplzen.cz b University of West Bohemia, Plzeň, Czech Republic, houdkov@ntc.zcu.cz, rsplitek@ntc.zcu.cz, maslej@ntc.zcu.cz c Research Centre Rez (UJV group), Rez, Czech Republic, gab@cvrez.cz Abstract FeCrAlY based coatings are used for their protective properties against high temperature corrosion in engineering industry. Due to its capability of resisting high corrosive medium, up to 1000 C, it offers good solutions for many applications from heavy engineering to oil rig structure in sea water. For this anticorrosion properties this material was took into account as a protective application in atomic reactors against liquid metal corrosion. As the most suitable and innovative technology, the thermal spraying followed by laser remelting was chosen. While the thermal spraying enables to deposit the FeCrAlY alloy on the parts surface in the desired thickness, the laser remelting eliminates the voids and inner oxides, which could later cause the degradation of the layer. This paper is focused on optimization of deposition parameters for HVOF method in the case of thermal spraying atypically thin coating (50-70 µm) of FeCrAlY. The second part of experiment is focused on searching parameters for laser remelting leading to creation of homogenate structure with high anticorrosion properties. For the microstructure evaluation, the optical and scanning electron microscopy was used, and the EDX analyses were used for elements analysis. Due to laser remelting the absence of voids and internal oxides was achieved. The choice of appropriate laser treatment parameters can provide the metallurgical bonding between the coating and the surface with low dilution. Key words: FeCrAlY, HVOF, laser remelting, SEM, EDX 1. Introduction FeCrAlY material offers many applications from protective coatings against corrosion in heavy machinery to experimental anticorrosion layers on surgical implants [1]. One of the interesting applications of FeCrAlY is metal foam based on this material [2]. This foam is investigating for sound absorption properties. Metal foams have been proposed for use in jet engines as acoustic treatment over rotors and fan blades. But the most wide-spread application is protective coating on feritic/martensitic steel [3]. In Generation IV nuclear reactors, heavy liquid metals are considered as a cooling system. Steel T91 was chosen for this application for its good tolerance against neutron radiation and high temperatures [4]. However in the environment of liquid metal (lead, lead-bismuth) the use of T91 is limited. If the temperature is higher than 500 C or the amount of oxygen in the cooling medium is out of a very specific range, the corrosion resistance will descend significantly. For improving the corrosion endurance, the thin layer of FeCrAlY has been chosen and investigated. In this paper we report process of searching for parameters of HVOF (High Velocity Oxygen Fuel) thermal spray coating process and following laser remelting. Parameters of spraying and laser were optimized in order to achieve a thin coating without pores or internal oxides. Moreover, the perfect melting of the interface between coating and substrate, gave to the specimens the structure of one, homogeneous material. Coatings were then tested in a loop with flowing liquid lead at various temperatures and oxygen contents. 2. Methodology 2.1 Materials The chemical composition of FeCrAlY sprayed powder is in the Tab 1. The coatings were sprayed on the two types of grid-blasted substrates: i) standard carbon steel ČSN ; ii) ferritic - martenzitic steel designated as T91. The carbon steel substrate was used for the optimization procedure of HVOF spraying

2 and for the primary laser treatment; for the spraying and laser treatment of the final samples the T91 substrate were used (composition in Tab. 2). Table 1 Chemical composition of FeCrAlY powder Element Fe Cr Al Y Ni Co wt% Bal Table 2 Chemical composition of steel T91, wt%. Element Fe C Cr Ni Mo Mn Si V P N Al Cu Nb wt% Bal HVOF spraying The FeCrAlY coatings were sprayed by HVOF TAFA JP 5000 spraying equipment, using procedure standard in VZÚ Plzeň. Seven sets of spraying parameters (Tab.3) were used to evaluate the influence of equivalent ratio (the ratio between the amount of oxygen and fuel, representing the flame temperature) and combustion pressure (representing the flame velocity) on the coatings microstructure with particular respect to the amount of oxygen in the coatings. The thickness of coatings for parameters optimization procedure was about 400 µm. Table 3 Deposition parameters used for first part of experiment (P is the combustion pressure, Ø is the equivalent ratio) P [psi]/ Ø The second sets of the coating samples were sprayed on the T91 substrate. The thickness of the coating was between µm. This, for thermally sprayed coatings atypically low thickness, was sprayed due to the requirement of customer. The low thickness was achieved by applying only two spraying passes, in contrast to usual 6-7. The higher amount of porosity in the low-thickness coatings microstructure compared to usual was observed, because of the lower impingement effect connected with lower number of spraying passes. 2.3 Laser remelting The laser remelting process was realized on direct CW (continuous wave) diode laser COHERENT ISL4000L with maximal output 4.3 kw and 808 mm wavelength. The FeCrAlY coating was remelted in a vacuum chamber with argon atmosphere, max. 1.2 bar pressure, with continual pumping of the creating vapors. The dimension of laser spot was 6 x 1 mm. The tested parameters of laser treatment are summarized in the Tab 4. Table 4 Parameters of laser treatment Substrate Mark Power Speed Chamber pressure Ar pressure material [W] [cm/min] [bar] [bar] T , T , T91 T , T91 T , T91 T , The technology of laser remelting was applied to create a metallurgical bonding between FeCrAlY coating and steel substrate due to the requirement of minimal substrate heat affection. Relatively small samples volume was the reason for overheating of the samples using constant laser process parameters. On the other hand, the lower laser power was not sufficient for melting of the coating material. The remelting of the coating was obtained in a narrow range of parameters, combining the laser beam profile, spot energy distribution, process speed and overlap of individual tracks.

3 The wide of remelted track 1 mm was found optimal, the wider lead to higher roughness of remelted surface and increased the non-homogeneity of remelting process. The overlap of the individual tracks was 60%. The process speed was the only variable that enables to influence the amount of heat, transferred to the samples. The process speed corresponding to parameters referred as T91 3 was chosen as the best. The highest process speed was used 400 cm/min for first 20 passes, the 425 cm/min for the rest to decrease the amount of heat transfer into already heated samples. 2.4 SEM and EDX analyses The coatings microstructure and their content of elements were evaluated by Scanning electron microscopy on FEI Quanta 200 microscope equipped with an energy dispersive X-ray microanalysis (EDX) system. The beam voltage of microscope was set to 30kV. Diameter of beam focused on sample was few tens of nanometres and analysed area at EDX was approximately 1 μm 3. Quanta 200 is equipped by EDAX NEW XL-30 Silicon doped by Lithium detector. 3. Results and Discussion 3.1 Microstructure of layer and content of oxygen In order to find optimal deposition parameters, different combinations of combustion pressure and flame temperature were investigated. Modification of those parameters can change the microstructure of coating and its mechanical properties. It is made by different temperature and speed of spraying particles which affect the coverage of surface and homogeneity of the coating itself. The evaluation of mechanical properties and deposition efficiency of the spraying process showed only the mild dependency of the evaluated parameters on the spraying parameters. Based on the mechanical properties measurements, the coating sprayed by parameters 5 was found to be the best. Nevertheless, the amount of oxygen in the coatings was the most important parameter in this case. Simultaneously, it was also the most sensitive parameter regarding the change of spraying parameters. In the Fig. 1 the microstructures of coatings, sprayed by parameters 1 and 5 (ref. to Tab.3) are shown. While the general amount of pores is comparable in both coatings, the amount of oxides in the microstructure is significantly lower in the coating 1. The lower oxidation rate is caused by lower flame temperature during spraying. On the other hand, low temperature and low combustion pressure is also responsible for a low rate of melting and spreading of the splats. In the consequence, the low intersplat cohesion can be expected. Such a microstructure can be considered as problematic if the coating should serve in as-sprayed condition, but in the case of following laser post-treatment it relevancies less critical. Fig. 1 Cross-section of FeCrAlY coating on steel parameters 1(a), parameters 5(b) The EDX analyses confirm the significant change in content of oxygen (drop from 0.96% to 0.05%wt.), as it can be seen in the Tab. 5.

4 Table 5 EDX analysis of FeCrAlY coating sprayed by parameters 1 and 5 Element wt.% Parameter 1 Parameter 5 O 0,05 0,96 Al 8,48 8,28 Cr 14,99 14,99 Fe 76,48 75,77 Due to the lowest amount of oxygen, the coating sprayed by parameters 7 with was chosen for the laser post-treatment. 3.2 Microstructure and composition of elements after laser remelting In the Fig. 2 the cross section of the FeCrAlY coating remelted by parameters referred as T1 (Tab.4) is shown. The sprayed coating thickness varied between initial coating thicknesses 50 to 70 µm. In the figures, the locations of EDX measurement are marked. In the Fig. 2a the location marked as 3 and 2 are in untreated area. One can see splats and un-melted coating. The Fig. 2b picture represents cross-section under laser path. It is clearly visible that there are no distinguishable interface between rest of the coating and substrate after laser treatment. The results of corresponding EDX analyses are summarized in the Tab. 6. Fig. 2 Microstructure and points of EDX analysis (numbers) on substrate. Table 6 Results of EDX from FeCrAlY on substrate. Element wt.% Fe Cr Al O Element wt.% Fe Cr Al O The SEM and EDX results imply that the thickness of remelted layer is too big. The contents of coating elements, presented near the surface, are significantly lower than in the original coating (compared to EDX results measured in location 3). In the Fig. 3, the results of laser treatment using parameters referred as T91 1 are shown. The corresponding EDX measurements are summarized in the Tab 7. On the remelted surface, small areas of oxides were found, probably as a result of oxygen presented in the original coating. The EDX analyses of the dendritic oxide structure showed, that oxides consist mainly from aluminium and yttrium. The depth profile of the element content showed also in this case, that the depth of melting is too big and the dilution of coating with substrate material is higher than required.

5 Fig. 3 Cross-section of coating remelted by parameters T91 1. Frames are areas of EDX analysis. Table 7 Results of EDX from FeCrAlY on T91 substrate, parameters 1 Element wt.% Fe Cr Al Y Element wt.% Fe Cr Al Y The last attempt was done to decrease the amount of heat, affecting the surface. For that purpose, the transition speed of the laser spot across the treated surface was increased. The results of the treatment by parameters referred as T91 3 are shown in the Fig 4. and Tab 8. Using the higher transition speed, the dilution of the coating and substrate material is acceptable. The amount of chromium and aluminium in the surface layer is sufficient to protect the surface against corrosion. Fig. 4: Cross-section of final sample on T91 substrate (parameters T91 3)

6 Table 8 EDX of deep profile from Fig.4a and Fig.4b Element wt.% Fe Cr Al O Element wt.% Fe Cr Al O Preliminary results after exposure in Pb The coatings tested in flowing Pb at different temperatures, exposure times, and oxygen content [5] showed a resistance to damage markedly higher than the base T91 material. The outer areas of Al 2 O 3 did not affect the material, although, they are considered detrimental because of their effect on the thermal conductivity of the material surface. Further investigations are on going to assess the properties of the specimens to harsher experimental conditions. 4. Conclusions The optimization procedure of HVOF spraying of FeCrAlY coating enabled to find spraying parameters that provide the coating with low amount of pores and oxides in the microstructure. The success of coating laser remelting depends on the appropriate choice of power, sufficient for material melting, and transitional speed provided the low degree of dilution between substrate and coating materials. In the consequence of residual oxygen in the coating, the aluminium oxides areas originated along the laser path on the surface sample. The high concentration of Al in the outer oxide decreased its content in the coating and could cause the decrease in corrosion resistance. However, the resistance to damage of the coated surface is markedly superior to the T91 ACKNOWLEDGEMENT The work was carried out thanks to the financial support of the Ministry of Industry and trade, project n. MPO FR-T11/423 The result was developed within the CENTEM project, reg. no. CZ.1.05/2.1.00/ that is cofunded from the ERDF within the OP RDI programme of the Ministry of Education, Youth and Sports. REFERENCE [1] P. Pérez, V.A.C. Haanappel, M.F. Stroosnijder, Formation of an alumina layer on a FeCrAlY alloy by thermal oxidation for potential medical implant applications, Surface and Coatings Technology 139, 2001, [2] S.V. Raj, Microstructural characterization of metal foams: An examination of the applicability of the theoretical models for modelling foams, Materials Science and Engineering A 528, (2011), [3] A. Weisenburger, A. Heinzel, G. Müller, H. Muscher, A. Rousanou, T91 cladding tubes with and without modified FeCrAlY coatings exposed in LBE at different flow, stress and temperature conditions, Journal of Nuclear Materials 376, 2008, [4] Y. Dai, V. Boutellier, D. Gavillet, H. Glasbrenner, A. Weisenburger, W. Wagner, FeCrAlY and TiN coatings on T91 steel after irradiation with 72 MeV protons in flowing LBE, Journal of Nuclear Materials (2011) (article in press). [5] F. Di Gabriele, D. Karník, J. Klečka, internal UJV report, DITI 2302/91, December 2011.