P. KRAJÒÁKOVÁ et al.: INFLUENCE OF MULTIPLE ELECTRON-BEAM REMELTINGS ON THE ... 289–293 INFLUENCE OF MULTIPLE ELECTRON-BEAM REMELTINGS ON THE CHARACTERISTICS OF HVOF AND CGDS SPRAYED CoNiCrAlY COATINGS VPLIV VE^KRATNEGA PRETALJEVANJA Z ELEKTRONSKIM SNOPOM NA LASTNOSTI S HVOF IN CGDS NAPR[ENIMI CoNiCrAlY PREVLEKAMI Petra Krajòáková, Larissa Gouvêa, Jan ^upera, Vít Jan, Ivo Dlouhý, Zdenìk Spotz Institute of Materials Science and Engineering, NETME Centre, Brno University of Technology, Faculty of Mechanical Engineering, Technická 2, Brno 616 69, Czech Republic krajnakova@fme.vutbr.cz Prejem rokopisa – received: 2017-06-30; sprejem za objavo – accepted for publication: 2017-12-21 doi:10.17222/mit.2017.092 Nanostructured CoNiCrAlY bond coatings were deposited onto a Ni-based alloy (Inconel 718) by both HVOF and CGDS spraying techniques. Subsequently, the deposits were remelted by an electron beam up to depth of about 100 μm, which resulted in the removal of defects on the substrate to the bond coat interface. This paper examines the influence of the parameters used for EB remelting, including multiple remelting on the microstructural changes, phase modification and the final state of the coatings. The amount of porosity in the coatings and the surface roughness has been evaluated. Scanning electron microscopy and X-ray diffraction were performed in order to characterize the phase modification before and after the applied treatment. The results indicated that multiple remelting improved the coating in terms of porosity, surface roughness decrease, mechanical strength and chemical homogeneity. This study also demonstrates that the CGDS deposition represents a promising alternative for CoNiCrAlY bond coat manufacturing. Keywords: bond coat, CoNiCrAlY, cold gas dynamic spraying, HVOF spraying, EB remelting Nanostrukturirane prevleke na osnovi CoNiCrAlY so avtorji prispevka nanesli na podlago iz Ni superzlitine (Inconel 718) z dvema metodama napr{evanja; z zelo hitrim napr{evanjem kovinskih delcev v toku me{anice plinastega goriva in kisika (HVOF; angl.: High Velocity Oxygen Fuel) in tehniko dinami~nega napr{evanja v hladnem plinu (CGDS; Cold Gas Dynamic Spraying). Sledilo je takoj{nje pretaljevanje napr{enih prevlek z elektronskim snopom do globine pribli`no 100 μm, kar naj bi odstranilo napake nastale med napr{evanjem in izbolj{alo povezavo prevlek s podlago. V ~lanku avtorji opisujejo vpliv parametrov uporabljenega pretaljevanja z elektronskim snopom, vklju~no z vplivom ve~kratnega pretaljevanja na mikrostrukturne spremembe, fazne modifikacije in kon~no stanje prevlek. Ovrednotili so poroznost izdelanih prevlek in poroznost njihove povr{ine. Izvedli so analize z vrsti~nim elektronskim mikroskopom (SEM; angl.: Scanning Electron Microscopy) in rentgensko difrakcijo (angl.: X-Ray Diffraction), da so lahko dolo~ili fazne transformacije pred in po obdelavi prevlek. Rezultati preiskav so pokazali, da ve~kratno pretaljevanje izbolj{a kakovost prevlek v smislu zmanj{anja njihove poroznosti in povr{inske hrapavosti, izbolj{a kemijsko homogenost in trdnost. [tudija je prav tako pokazala, da izdelava prevlek na osnovi CoNiCrAlY z CGDS postopkom lahko predstavlja alternativo drugim postopkom napr{evanja te vrste prevlek. Keywords: opla{~enje, CoNiCrAlY prevleke, CGDS - dinami~no napr{evanje v hladnem plinu, HVOF - zelo hitro napr{evanje kovinskih delcev v toku me{anice goriva in kisika, pretaljevanje z elektronskim snopom 1 INTRODUCTION Increasing demands for higher gas-turbine engine performance have led to the development of thermal barrier coating (TBC) systems applied to the engine’s hot-components. TBCs typically consist of an underlying McrAlY (M=Ni, Co or both) bond coat with yttria partially stabilized zirconia ceramic top coat. Bond coats are typically manufactured using air-plasma spraying (APS), low-pressure plasma spraying (LPPS), high-velo- city oxygen fuel spraying (HVOF). 1–3 In recent years cold gas dynamic spraying (CGDS) has emerged as a promising coating process. CGDS uses kinetic energy rather than thermal energy to produce the coatings. In this process, fine powder particles are acce- lerated at supersonic speeds and undergo severe plastic deformation upon impacting the substrate to form a coating. 4–6 This technique has been used to produce coatings from various materials with different micro- structures including conventional, polycrystalline, nano- crystalline, amorphous and metastable structures. CGDS operates at much lower temperatures than thermal spray processes, the particles do not get anywhere close to melting temperatures and consequently the achieved coatings exhibit no grain growth. In addition, it can use inert gases that hinder in-process particle surface oxida- tion. 7–10 These advantages make CGDS an interesting alternative for the deposition of bond coats. The present study is a part of an ongoing research project that aims to develop high-performance bond coats by means of CGDS for the manufacturing of TBCs to be applied on hot components of gas-turbine engines. The current experimental work uses CGDS and also a conventional Materiali in tehnologije / Materials and technology 52 (2018) 3, 289–293 289 UDK 620.1:67.017:621.793.7 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(3)289(2018) hot spraying technique (HVOF) to deposit CoNiCrAlY coatings, which are then subsequently remelted by elec- tron beam (EB) to achieve a homogeneous micro- structure. The EB remelting process optimisation of CoNiCrAlY coatings is the final objective of the present work. 2 EXPERIMENTAL PART The starting powder was a conventional CoNiCrAlY powder with a spherical morphology and a particle size distribution ranging from 5 μm to 20 μm with the follow- ing composition: Co-36.91, Ni-34.38, Cr-19.35, Al-6.16, Y-0.98, O-0.74 (w/%). This material is generally used in high-temperature applications such as bond coat where oxidation and hot-corrosion are the key. An Ni- based wrought superalloy – Inconel 718 was used as the sub- strate. It’s chemical composition is Ni-49.84, Fe-19.69, Cr-17.81, Nb-4.6, C-3.03, Mo-2.65, Ti-1, Al-0.73, O-0.64 (w/%). Two sets of CoNiCrAlY coating (70–75-μm thick) were thermally sprayed onto the Inconel 718 substrates by HVOF and CGDS procedures. EB surface treatment was carried out using K26 15-150 Pro-Beam machine working with maximum beam input power 15 kW at 150 kV accelerating voltage and 100 mA current. The EB landing spot was oscillated with 7 mm amplitude on the specimen surface creating a pattern of overlapping loops. The first set of samples was treated by one cycle of the surface treatment, which con- sisted of two EB passes. Two consecutive cycles giving four passes altogether were performed in the second set of samples (Table 1). The phase composition before and after EB remelting was measured by X-ray diffraction (XRD) using a Philips X’Pert diffractometer. The diffractometer was set up in Bragg-Brentano geometry using Cu-K radiation ( = 0,154 nm) equipped with a linear Xcelerator detector. The surface morphology and metallographic sections of the samples before and after EB treatment were observed by scanning electron microscopy (SEM, Carl Zeiss Ultra Plus). Table 1: Double (1 st set) and two times double remelting (2 nd set) conditions; 15 mm/s processing speed EB current for 1 st set EB current for 2 nd set I. (mA) II. (mA) I. (mA) II. (mA) III. (mA) IV. (mA) sample A 2.7 2.5 sample C 2.7 2.5 2.7 2.5 sample B 3 2.7 sample D 3 2.7 3 2.7 3 RESULTS Figure 1 shows SEM images of the as-deposited CoNiCrAlY coatings as manufactured by the HVOF and CGDS technique onto an Inconel 718 substrate. The HVOF coating (Figure 1a) shows a number of pores and P. KRAJÒÁKOVÁ et al.: INFLUENCE OF MULTIPLE ELECTRON-BEAM REMELTINGS ON THE ... 290 Materiali in tehnologije / Materials and technology 52 (2018) 3, 289–293 Figure 2: Microstructure of EB remelted bond coats (sample A) de- posited by: a) HVOF, b) CGDS Figure 1: As-deposited CoNiCrAlY coatings applying different depo- sition techniques: a) HVOF, b) CGDS locations with poor bonding between splats. The CGDS coating (Figure 1b) exhibits a very limited porosity, negligible number of cracks and also a lower surface roughness. The coatings prepared by both methods (HVOF and CGDS) were EB remelted up to a depth of about 85–100 μm. After the EB treatment, the coatings were rapidly solidified. Figures 2 to 5 show SEM micro- structure images of the cross-sections after EB treatment according to different EB conditions as shown in Table 1. Figures 2 and 3 present microstructures after two EB passes, the processing speed was the same but the beam current changed (Table 1). From Figure 2 can be derived that there still is poor bonding of the coating to the substrate; the depth of the remelted area is too small to heal the interface. Cracks and small pores can be seen in both HVOF and CGDS samples. Original as-deposited porosity and splat boundaries are still visible. An in- crease of the beam current as applied for sample B (Figure 3) increased the depth layer affected by remelting from 81 μm to 88 μm for HVOF sample and from 72 μm to 86 μm for CGDS sample. The cracks and pores counts were reduced significantly; the structure is refined for both treatments. The two times double EB remelting according to C and D procedures were per- formed in order to further increase the depth and to improve the microstructural properties of the remelted layer (Figures 4 and 5). The depth of remelted layer P. KRAJÒÁKOVÁ et al.: INFLUENCE OF MULTIPLE ELECTRON-BEAM REMELTINGS ON THE ... Materiali in tehnologije / Materials and technology 52 (2018) 3, 289–293 291 Figure 5: Microstructure of EB remelted bond coats (sample D) de- posited by: a) HVOF, b) CGDS Figure 4: Microstructure of EB remelted bond coats (sample C) de- posited by: a) HVOF, b) CGDS Figure 3: Microstructure of EB remelted bond coats (Sample B) de- posited by: a) HVOF, b) CGDS after treatment C reaches 95 μm (for HVOF) and 90 μm (for CGDS), Figure 4, which is not very deep and still some small cracks and pores in HVOF sample and large ones in CGDS sample are observable. For treatment condition "D" similar findings were found as follow from Figure 5. Here some major defects and pores (in CGDS) are newly formed during surface treatment, while smaller ones (were seen in HVOF) sample. 3.1 X-ray diffraction measurements XRD patterns of the CoNiCrAlY coatings by both techniques before and after remelting are presented in Figure 6. Material before the EB-remelting consists of only one FCC phase, that can be identified as Ni-based solid solution (at both HVOF and CGDS). After EB-remelting of the material new phases like Al 2 O 3 ,A l 4 Ni 3 (at both HVOF and CGDS) and AlYO 3 , Al 5 Y 3 O 12 for HVOF sample appeared. During the EB treatment, although it is done in vacuum, aluminium ox- ide has formed. This might be due to the reaction of the oxygen that has adsorbed to the individual powder parti- cles and aluminium at high temperatures during EB remelting. The AlYO 3 appears in the form of droplets on the modified surface after the EB remelting. These drop- lets consist mainly of aluminium oxide with a small yt- trium-rich grains. 7 4 DISCUSSION The influence of the parameters used in electron- beam remelting including the effect of multiple remelt- ing passes on the microstructural changes and state of the coatings have shown the capability of the technique. When comparing the defectness of bond coat and its interface to substrate for HVOF and CGDS this finding appears to be more important for the HVOF and pro- bably also similar high-temperature coating techniques. Mainly the weak interface to substrate appears to be critical issue as shown, e.g., in 11 . According to recent investigations the qualitative improvement of TBCs function will need to fix several issues but the bond coat to substrate interface belongs to the main challenge. Taking into account thermally grown oxide layer during component/TBC operation this location leads very often to BC/TBC delamination and thus loss of the key TBC functions. Prevention to the delamination crack initiation by interface removal as suggested by EB remelting procedure could lead to substantial enhancement of the BC and TBC functional properties. The potential of a new-generation electron beam technology treatment on the surface and interface modification of CoNiCrAlY bond coats on Inconel substrates was demonstrated. Through the treatment, a good quality transient interface can be obtained for coatings deposited via both the HVOF and CGDS technologies at dissimilar thicknesses. The changes in the microstructure and chemical gradient (from a distinct step into transient) of the interface could prevent the formation of detrimental phases upon subjecting the component to elevated temperatures. Further to that, porosity of the bond coats can be reduced in the treatment and the initial splats structure was not retained, thereby producing bulk-like material. The out- comes can bring partial changes in technology, e.g. a vacuum thermal treatment of the component after the bond coat application could be replaced by electron- beam remelting. Such a procedure will bring improve- ment of the bond coat to substrate interface resulting in enhancement of BC adhesion, even when the production costs will be smaller. 5 CONCLUSIONS The results obtained show that the application of the electron-beam surface modification including the bond coat structure and interface to substrate is associated with certain advantages. These are mainly associated with removal of the interface boundary between the bond coat and the substrate and the homogenisation of the bond coat microstructure. In addition, under selected parameters the EB treatment provided a smooth surface and a low porosity level. The technology parameter window for the successful application of EB remelting has been shown to be relatively narrow. The under- standing of observed negative phenomena like pores is still limited and needs to be further investigated. P. KRAJÒÁKOVÁ et al.: INFLUENCE OF MULTIPLE ELECTRON-BEAM REMELTINGS ON THE ... 292 Materiali in tehnologije / Materials and technology 52 (2018) 3, 289–293 Figure 6: X-ray diffraction patterns of the bond coat deposited by: a) HVOF, b) CGDS technique (before and after EB remelting accord- ing to Table 1 Acknowledgment The works have been supported by the financial support from the project NETME plus centre (Lo1202), project of Ministry of Education, Youth and Sports under the “national sustainability program”, also supported by specific research BUT project: FSI-S-17-4708. 6 REFERENCES 1 V. Kumar, K. Balasubramanian, Progress update on fallure mecha- nism of advanced thermal barrier coatings: A review, Progress in Organic Coatings, 90 (2016), 54–82, doi:10.1016/j.porgcoat.2015. 09.019 2 J. J. Tang, Y. Bai, J. C. Zhang, K. Liu, X.Y. Liu, P. Zhang, Y. Wang, L. Zhang, G.Y. Liang, Y. Gao, J.F. 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