COMPARISON OF NiCrBSi COATINGS, HVOF SPRAYED, RE-MELTED BY FLAME AND BY HIGH-POWER LASER

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1 COMPARISON OF NiSi COATINGS, HVOF SPRAYED, RE-MELTED BY FLAME AND BY HIGH-POWER LASER Šárka HOUDKOVÁ, Marek VOSTŔÁK, Matěj HRUŠKA, Jan ŘÍHA, Eva SMAZALOVÁ, Zdeněk ČESÁNEK, Jan SCHUBERT, University of West Bohemia, New Technology Research Centre, Univerzitní 8, Plzeň, CZ, Abstract The possibility of increasing the mechanical properties of some of thermally sprayed coatings by applying the thermal post treatment is known, even though not very often use. The NiSi alloy belongs to the group of materials that profits considerably from re-melting of previously sprayed coating. The technologies of remelting differ, usually in dependence on the available equipment. The most often are furnace or flame remelting. Although these technologies are not expensive, they cannot be effectively controlled and their results are not fully repeatable. To remove this disadvantages, the high power lasers can be use to re-melt the coating with precisely controlled heat input. In the paper, the comparison of microstructure and mechanical properties of as-sprayed, flame re-melted and laser re-melted NiSi coating is presented. The advantages of using high-power lasers are described with respect to the functional properties of evaluated coating. Keywords: NiSi, HVOF, coating, laser re-melting, flame re-melting, microstructure, mechanical properties 1. INTRODUCTION The NiSi coatings are usually used as a wear, corrosion and oxidation resistant coatings that can be applied at high temperature conditions up to 800 C. Their application range is wide, including glass mould industry, paper industry, petrol industry or chemical industry. They have been applied for example on the fan blades, hot working punches, piston rings, valves, rollers in steel making, fossil-fuel-fired boilers, waste incineration boilers pump shafts at so on [1-4]. Often they are used as a replacement of environmentally harmful hard chromium coatings [1,5,6]. The technology of coating deposition includes different kinds of thermal spray technologies: oxygen-fuel flame spraying [7-9], atmospheric plasma spraying [1,5,6,10,11] and HVOF spraying [3,4]. The sprayed NiSi coatings exhibit the typical microstructure resulting from the spraying technology: pores, oxides and unmelted particles. Their amount depends on the used technology. While the flame spraying results in poor coating quality, the HVOF gives coatings with low porosity and minor unmelted particles. Thanks to the unique coating material composition, the NiSi can be fused after the spraying process to increase the functional properties, namely the corrosion resistance. While chromium is responsible for corrosion and oxidation resistance and thermal stability, boron and silicon is decreasing the melting temperature and help to the re-melting process, the presence of carbon enables to create the carbides, that can increase the hardness and wear resistance of the coating [6]. The re-melting process is realized most often by flame [7,8], in the furnace [3], or lately by energy of high-power lasers [1,4,6,8,11]. While the flame re-melting process enables to decrease the amount of porosity of the coating, homogenize the coating microstructure and increase the related corrosion resistance, it is not able to reach the substrate material and create the metallurgical bonding between substrate and the coating. The re-melting in the furnace has the biggest disadvantage in influencing the substrate bulk material: the temperature necessary for NiSi re-melting is 1040 C [9]. The technology of laser re-melting has a potential to eliminate above

2 mentioned disadvantages. It can be well controlled to reach locally the temperature necessary for melting the coating as well as to create the metallurgical bonding between the coating and substrate and simultaneously avoid the influencing of the substrate material. On the other hand, the laser re-melting process brings a risk of creation of the cracks, caused by the high thermal gradient in the small influenced area [11]. The presence of cracks is crucial regarding the corrosion resistance, the property of interest in the re-melting process. The attention has to be paid to precise process parameter adjustment. The pre-heating and controlled cooling should take a place to avoid the crack formation and coated component deformation [3,8,12]. To accomplish the list of possible NiCrSiB deposition technologies, the direct laser cladding has to be mentioned [2,7,8,12]. It has the advantage of the direct creation of metallurgical bonding combined with deposition of porous-free coating. The similar risk of cracking during solidification appears as in the case of laser re-melting. In this paper, the HVOF sprayed NiSi coating is compared to the coating, sprayed by HVOF and remelted by flame and by HPDD laser. The microstructure of the coatings was evaluated by optical and scanning electron microscopy, the phase composition of original powder material and as-sprayed and remelted coatings was compared by means of XRD measurements, and basic mechanical properties were measured to describe mutual differences. 2. EXPERIMENTAL 2.1 Coatings preparation The NiSi coating was sprayed by HP/HVOF TAFA JP5000 spraying equipment in VZU Plzen onto grit blasted substrate. For spraying, the FST powder was used. Its nominal chemical composition is: 15%Cr; 4%Fe; 4,25% Si, 3%B; 0,7%C; Ni rest. The grit blasting was realized by Al 2 O 3, F22 (with a grain size 0,8-1 mm). The dimensions of substrate (200 x 100 x 10 mm) were chosen to ensure sufficient heat dissipation during re-melting process. For the flame re-melting the GTV 6P-II gun was used. The constant distance 100 mm from coated substrate was kept. No specific traverse speed or offset was used. The HPDD 4kW laser Coherent HighLight ISL-4000L; 808 ± 10 nm wave length was used for laser remelting. The laser re-melting parameters were chosen based on the previously done optimization process (will be published elsewhere). Parameters of laser re-melting process were as follows: The specific energy 93 J/mm 2, traverse speed 70 mm/min, spot size 10 x 6 mm, 2 mm overlap. To prevent the high thermal gradient, responsible for cracking of the coating, the pre-heating of the substrate to the 250 C was used. 2.2 Experimental methods The microstructure of the coatings were evaluated on the cross sections (grinded and polished using automatic Leco grinding and polishing equipment) by optical microscope Nicon Epiphot 200, by digital optical 3D microscope Hirox KH7700, and SEM Quanta 200 from FEI equipped by EDAX NEW XL-30 Silicon doped by Lithium detector. To highlight the microstructure features, the metallographic etching by etchant (1HCl:10HNO 3 :10H 2 O) was done. The phase composition of original powder, HVOF as-sprayed coating, HVOF sprayed plus flame re-melted and HVOF sprayed plus laser re-melted coatings was evaluated and compared by means of XRD. The measurement was carried out on an automatic powder diffractometer Panalytical X Pert Pro. This instrument uses a Copper X-ray tube ( 1 = 0, nm) and an ultra-fast semiconductor detector PIXcel with high resolution ability. For the measurement the symmetrical Bragg-Brentano ( - ) geometry was used. The measuring range was The surface roughness was measured by surftester Mitutoyo SJ-201P, according to DIN EN ISO The reported values are average from at least five measurements. The surface hardness HR15N was measured

3 on the as-sprayed coatings surfaces using hardness tester Rockwell HT The reported values are average from at least 5 measurements. The coating microhardness HV0.3 was measured on the coatings cross-sections. The reported values are average from at least 7 measurements. 3. RESULTS 3.1 Coatings microstructure The over whole view on the coatings cross section can be seen in the Fig. 1. The porosity of HVOF assprayed coating is low, no cracking or delamination was observed. The flame re-melted coating in its upper part contains a certain amount of bubbles, but the over whole homogeneity is quite good; the splat boundaries disappeared in consequence of fusing process. The roughness of boundary between coating and substrate is preserved; no metallurgical bonding can be expected due to flame re-melting process. The laser re-melted coating contains no cracks, delamination, porosity or bubbles. Moreover the metallurgical bonding between coating and substrate was successfully created. a) b) c) Fig. 1 SEM of NiSi coating a) HVOF as-sprayed; b) HVOF sprayed + flame re-melted; c) HVOF sprayed + laser re-melted. Fig. 2 OM of etched NiSi HVOF as-sprayed microstructure In the Fig. 2, the individual splats are recognized. The small particles within individual splats are expected to be hard borides or carbides, responsible for high hardness. Similar small precipitates were observed in the microstructure of HVOF sprayed flame re-melted coatings. In the microstructure of laser re-melted coatings the evidence of high thermal energy input can be seen (Fig. 3). The microstructure changes along the cross section. Several different areas were identified: on the substrate-coating boundary, a planar front solidification region is observed. It is followed by the area characterized by well developed dendritic structure about 70 µm thick. In the middle part of the coating, the

4 microstructure with fine grains is followed by well organized crystalic structure with bigger grains. Such a microstructure is typical for laser re-melted or laser clad coatings [1,6,7,10,11]. Fig. 2 OM of etched NiSi HVOF as-sprayed laser re-melted microstructure To identify the individual phase composition the EDX measurement was done. The measured spots are indicated in the Fig. 3 and the results of the EDX measurement are summarized in the Table 1. Fig. 3 The SEM of laser re-melted NiSi with indication of EDX measurement The XRD analyses revealed a high number of phases. The complexity of the NiSi alloy enables to create different types of borides, carbides and silicates. The main structure phases were identified as: FeNi (γ-ni),, Ni 3 B,, Ni x Si y, and. The XRD results together with the EDX measurement led to the conclusion, that the black phase in the Fig. 3 (spot 1) is the chromium boride, while the bright areas (spot 3) can be identified as a (γ-ni). Comparing the measured XRD results with the result reported in literature [3,7,11], the phase corresponding to the carbide (Cr 7 C 3 or (Cr, Fe) 7 C 3 ) was not observed. Instead of them, the Si-based phases were identified in our case. The other identified phases were in an agreement with previously reported results [3,7,11].

5 Ni 3 B Ni 3 B Tab. 1. The result of EDX measurement of laser re-melted NiSi coating %wt Ni Cr B Si C Fe Ni 3 B NiCr 2 O 4 Intenzita [cts] Int28 powder ZN Pr-Flame-13-1 Pr-HPDD Ni 2 Si FeNi 3 FeNi 3 NiB NiCr 2 O [ ] Fig. 4 XRD spectra of original powder, as-sprayed, flame and laser re-melted coating. Comparing the measured XRD of original powder, as-sprayed, flame and laser re-melted coating similar phases are observed. The difference in the XRD spectra widths and their small shift are given by the different level of crystalinity and by the stress revealing during coating re-melting. In the case of re-melted coatings, a certain amount of oxide NiCr 2 O 4 is observed, originating from re-melting process in the air atmosphere. In case of laser re-melting, the oxide creates a glass-like surface on the top of the coating, which is beneficial for increasing the coating corrosion resistance. 3.2 Coatings properties The comparison between the properties of HVOF sprayed, HVOF sprayed and flame re-melted and HVOF sprayed and laser re-melted can be seen in the Table 2. The thickness of the as-sprayed and flame re-melted coating is in the range of the measurement scatter (connected with the higher roughness of as-sprayed coating) comparable. The increase of laser-melted coating thickness is caused by the dilution the mixing of substrate and coating material on the coatingsubstrate boundary. The decrease of surface roughness for both technologies of re-melting is obvious. In the case of laser remelting, the surface has almost glass-like surface, probably created by thin oxide layer. The superficial hardness and microhardness is the highest for as-sprayed coatings. The reason is in well distributed small precipitates in the microstructure, combined with compressive residual stress, typical for HVOF coatings. During re-melting, the size of the precipitates increases and their distribution is not so homogonous. Simultaneously, the annealing decrease the level of compressive streets, and in the case of

6 laser re-melting, the tensile stress originating from solidification is introducing. The decrees of hardness in consequence of re-melting is typical for re-melting process [3,4,7-9]. Tab. 2. The result of EDX measurement of laser re-melted NiSi coating Surface Surface Surface Thickness Microhardness hardness HRC roughness roughness [µm] HV0.3 HR15N Ra Ra HVOF 456 ± ± ± ± 0, ± 4.33 HVOF + flame remelting 448 ± ± ± ± ± 1.62 HVOF + laser remelting 500 ± ± ± ± ± 1.36 ACKNOWLEDGEMENT This paper was prepared in the framework of and with the aid of instruments in the project CENTEM no. CZ.1.05/2.1.00/ co-financed by ERDF in the operational programme Research and Development for Innovation of the Ministry of Education of the Czech Republic and project no. SGS REFERENCES [1] SERRES, N., HLAWKA, F., COSTIL, S., LANGLADE, C., MACHI, F., Corrosion properties of in situ laser remelted NiSi coatings comparison with hard chromium coatings, J. Materials Processing Technol., 2011, Vol. 211, p [2] FERNÁNDEZ., E. CADENAS, M.,GOMZÁLEZ, R.,NAVAS, C.,FERNANDÉZ, R., de DAMBORENEA, J. Wear behaviour of laser clad NiSi coating, Wear, 2005, Vol. 259, p [3] MIGUEL,J. M.,GUILEMANY, J.M., VIZCAINO, S. Tribological study of NiSi coating obtained by different processes, Tribology International, 2003, Vol. 36, p [4] TUOMINEN, J., VUORISTO, P., MÄNTYLÄ, T., VIHINEN, J., ANDERSSON, P.H. Corrosion behavior of HVOFsprayed and Nd-YAG laser remelted high-chromium, nicel-chromium coatings, Journal of Thermal Spray Technology, 2001, Vol. 11, No. 2, p [5] SERRES, N., HLAWKA, F., COSTIL, S., LANGLADE, C., MACHI, F., CORNET, A. Dry coatings and ecodesign part.1 Environmental performances and chemical properties, Surf. Coat. Technol., 2009, Vol. 204, p [6] SERRES, N., HLAWKA, F., COSTIL, S., LANGLADE, C., MACHI, F., An investigation of the mechanical properties and wear resistance of NiSi coatings carried out by in situ laser remelting, Wear, 2011, Vol. 270, p [7] NAVAS, C.,COLACO, R.,de DAMBORENEA, J., VILAR, R. Abrasive wear behaviour of laser clad and flame sprayed-melted NiSi coatings, Coat. Surf. Technol.,2006, Vol. 200, p [8] GONZÁLES, R., CADENAS, M.,FERNÁNDEZ, R., CORTIZO, J.L., RODRÍGUEZ, E. Wear behavior of flame sprayed NiSi coating remelted by flame or by laser, Wear, Vol. 262, p [9] BERGANT, Z., GRUM, J. Quality Improvement of Flame sprayed, heat treated, and remelted NiSi coatings, Journal of Thermal Spray Technology, 2009, Vol. 18, no. 3, p [10] SERRES, N., HLAWKA, F., COSTIL, S., LANGLADE, C., MACHI, F., CORNET, A. Dry coatings and ecodesign part.2 Tribological performances, Surf. Coat. Technol., 2009, Vol. 204, p [11] SERRES, N., HLAWKA, F., COSTIL, S., LANGLADE, C., MACHI, F., Microstructure of Metallic NiSi Coatings manufactured via Hybrid Plasma Spray and In Situ Laser Remelting Process, J. Therm. Spray Technol., 2011, Vol. 20, no.1-2, p [12] SERRES, N., PORTHA, N., MACHI, F., Influence of salt fog aging test on mechanical resistance of laser clad coatings, Surf.Coat. Technol., 2011, Vol. 205,p