SPRAYING PARAMETERS STUDY OF HVOF COATINGS, BASED ON CRC. Šárka HOUDKOVÁ, Zdeněk ČESÁNEK, Michaela KAŠPAROVÁ, Jan SCHUBERT

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1 SPRAYING PARAMETERS STUDY OF HVOF COATINGS, BASED ON CRC Šárka HOUDKOVÁ, Zdeněk ČESÁNEK, Michaela KAŠPAROVÁ, Jan SCHUBERT Výzkumný a zkušební ústav Plzeň s.r.o., Tylova 1581/46, Plzeň, CZ, houdkova@vzuplzen.cz Abstract To obtain the superior quality coatings, it is necessary to provide the optimization procedure of all sprayed coatings. Although the spraying parameters of the group of materials (etc. Co-based alloys, WC-based cermets coatings and so) are use to be similar, the differences in alloy elements or matrix composition can make a difference in the most appropriate spraying parameters. Even for the powder with similar composition and declared manufacturing technology, the powder can change in a way, that influence the final coatings properties, and small change in spraying parameters (flame temperature, velocity, spraying distance and so) could be necessary to keep the coating properties. For that reason, the optimization should be done in the case of change of powder supplier to ensure the coatings quality. In the paper, the optimization procedure and some basic mechanical properties such as hardness, microhardness and abrasive wear resistance of two CrC-based HVOF cermet coatings with different matrix material (NiCr and CoNiCrAlY alloy) are compared and the influence of the matrix material is described. Keywords: Coating, Cr 3 C 2 -NiCr, Cr 3 C 2 -CoNiCrAlY, HVOF, spraying parameters, abrasive resistance, hardness 1. INTRODUCTION The technology of High Velocity Oxygen-Fuel (HVOF) spraying is well established technology of thick coating deposition. The available range of flame temperatures and velocities makes the HVOF technology among other thermal spray technologies (such as atmospheric or vacuum plasma spraying, flame or electric arc spraying) the most suitable for spraying of cermet materials. The high flame velocity in combination with the medium temperatures enables to spray cermet coatings with low amount of oxides and low porosity and with inner compressive stress. The most often used cermet materials can be divided into two groups the cermet based on WC carbides and the cermet based on Cr 3 C 2 carbides. While the WC-based cermets are well known for their excellent wear resistance, but their usability is restricted by temperature, the CrC-based cermets are used in the hot environments. In this paper, the attention is paid to the CrC-based cermet HVOF coatings. The Cr 3 C 2 carbide is usually combined with 25% of NiCr matrix. Such a composition is largely used in the industry where the high temperature resistance combined with sliding, abrasive and erosion wear resistance is required (e.g. power industry). In many cases, it serves as an alternative to the environmentally danger hard chromium [1,2]. Up to now, many papers were focused on the microstructure and [3] of Cr 3 C 2-25%NiCr HVOF sprayed coating. Much attention was paid to their high temperature erosion resistance, in relation to the potential of the Cr 3 C 2-25%NiCr coating to counter the erosion of turbine components [1,4-6], to the corrosion resistance [2,7] or hot corrosion behavior in an aggressive environments [8,9]. While erosion resistance is usually found to be excellent, the results of corrosion resistance testing are fluctuating. The porosity and strength of splat boundaries as well as the boundaries between carbides and matrix are responsible for penetration of corrosive media through the coating. Moreover, the latest work of Mathews [6] shown, that in certain conditions the oxidation of Cr 3 C 2-25%NiCr coating is more pronounced than expected due to preferential oxidation of the Cr 3 C 2 carbides matrix boundaries. The same observation was made in the service life of the Cr 3 C 2-25%NiCr coatings sprayed in VZU Plzeň.

2 To avoid oxidation defects in servis, the two possible ways occurred. First is to optimize the spraying parameters of the Cr 3 C 2-25%NiCr coatings to get the superior coatings properties. It can be done by changing the temperature and velocity of the flame in certain range to decrease the amount of porosity and increase the inner cohesion in the coating. Second way is to change the coatings composition to increase the oxidation and corrosion resistance. While WC carbides cannot be used in the hot environments, the Cr 3 C 2 carbides have to be combined with matrix with alternated composition. The CoNiCrAlY alloy has premium high temperature oxidation resistance [9,10]. It is widely used as a bond coat for plasma sprayed thermal barrier coatings [11]. Even if the Cr 3 C 2-25%CoNiCrAlY powder is commercially available, not many papers dealing with its mechanical or wear properties have been found. In our lab, some preliminary experiments were done to evaluate the mechanical and sliding wear behavior of Cr 3 C 2-25%NiCr [12,13]. It was shown, that it has very good sliding properties, but its inner cohesion is poor compare to Cr 3 C 2-25%NiCr. The poor cohesion can be caused by using not optimized spraying parameters. In this work, the attention is paid to the optimization procedure of both types of coatings: Cr 3 C 2-25%NiCr and Cr 3 C 2-25%CoNiCrAlY. The microstructure and basic properties of coatings sprayed by different spraying parameters are compared, and the parameters giving best results were chosen for both coating. 2. EXPERIMENTAL 2.1 Coatings preparation The two sets of samples were sprayed onto grit blasted substrate. The Amperit powders and were used to spray Cr 3 C 2-25%NiCr and Cr 3 C 2-25%CoNiCrAlY coatings, resp. The spraying parameters optimization procedure is based on varying of oxygen and fuel amount to obtain flame with various temperature (represented by equivalent ratio ) and velocity (represented by combustion pressure p). The spraying parameters are summarized in the Table 1 and 2 and following text for Cr 3 C 2 -NiCr and Cr 3 C 2 -CoNiCrAlY coatings, resp. Tab. 1. Variable spraying parameters of Cr 2 C 2 -NiCr coating designation of the samples p [psi] Constant spraying parameters of Cr 3 C 2 -NiCr: Spraying distance: 360 mm; Carried gas: nitrogen; Carried gas flow: 6,5 l/min; Traverse speed: 250 mm/s; Offset: 6 mm; Barrel length: 100 mm Tab. 2. Variable spraying parameters of Cr 3 C 2 -CoNiCrAlY coating designation of the samples p [psi] Constant spraying parameters of Cr 3 C 2 -CoNiCrAlY: Spraying distance: 420 mm; Carried gas: nitrogen; Carried gas flow: 6,5 l/min; Traverse speed: 250 mm/s; Offset: 6 mm; Barrel length: 150 mm

3 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 using magnification 50x and 100x, SEM Quanta 200 from FEI using magnification 200x, 1000x and 3000x. The thickness of the coatings was measured on the coatings cross-sections by optical microscope. For each coating, at least 5 measurements were done and the average value was calculated. 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 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. The abrasive wear resistance of the coatings was evaluated by Dry Sand/Rubber Wheel test according to ASTM G-65. The reported values are the average from 2 measurements. 3. RESULTS 3.1 Coatings microstructure While Cr 3 C 2 -NiCr coatings microstructure where described many times [14], no profound discussion about the Cr 3 C 2 -CoNiCAlY HVOF coatings microstructure was found. In the case of Cr3C2-NiCr, all sprayed coatings showed low amount of porosity, no appearance of cracks or delamination on the coating substrate boundaries. In the Fig. 1 there are the microstructure of Cr 3 C 2 -NiCr coating, sprayed by the lowest temperature and velocity, compared to the Cr 3 C 2 -NiCr coating, sprayed by the highest temperature and velocity (par. 1 vs par. 15). It can be seen, that the level of temperature induced softening combined with higher kinetic energy led to lower amount of porosity and smaller intersplat pores. The plastic deformation of the binder phase on the splats boundaries is recognizable. The better wear test results (Fig. 3) indicates, that the intersplat cohesion is increased. Fig. 1 SEM of Cr 3 C 2 -NiCr and Cr 3 C 2 -CoNiCrAlY coatings microstructures: a) Cr 3 C 2 -NiCr-Parameters 1; b) Cr 3 C 2 -NiCr-Parameters 15; c) Cr 3 C 2 -CoNiCrAlY-Parameters 12. Based on the observation obtained during Cr 3 C 2 -NiCr optimization, the longer spraying barrel (150 mm instead of 100 mm) was used. The longer spraying barrel leads to the higher temperature of the sprayed powder, which seemed to be beneficial in the case of Cr 3 C 2 -NiCr. Nevertheless, the sets of parameters with lowest equivalent ratio (EP~0.8), representing the sets of parameters with the lowest temperature, was not able to produce any coating. The individual droplets didn t attach to each other, and no coating was deposited. The coating was deposited starting from EP~0.9, even if the inner cohesion of coatings 2, 5 and 8 were poor. (Fig. 2).

4 In the Fig. 1, the SEM of Cr 3 C 2 -NiCr coating sprayed at the lowest flame temperature and velocity (Par. 1) is displayed to be compared with Cr 3 C 2 -NiCr sprayed at the highest flame lowest temperature and velocity (Par. 14) and to the microstructure of the best Cr 3 C 2 -CoNiCrAlY coating (par. 12). The microstructure of Cr 3 C 2 -CoNiCrAlY Par. 12 is comparable with the microstructure of Cr 3 C 2 -NiCr - Par. 1, but worse than the microstructure of Cr 3 C 2 -NiCr Par. 15, concerning the amount of porosity. The coating microstructure observations are in agreement with the surface roughness, microhardness and wear tests results. Fig. 2 Cr 3 C 2 -CoNiCrAlY coatings microstructures 3.2 Properties of the CrC-based coatings The results of all measured CrC-based coating are summarized in the Fig. 3. Comparing the Cr 3 C 2 -NiCr and Cr 3 C 2 -CoNiCrAlY coating it can be said, that the Cr 3 C 2 -NiCr showed slightly better behavior, even if the differences are not so significant. The biggest disparity is observed in the surface hardness measurement the difference between the most hard Cr 3 C 2 -NiCr (91.6 HR15N - Par. 15) and Cr 3 C 2 -CoNiCrAlY (87.8 HR15N - Par.12) represents almost 9 HRC units (64 HRC vs. 55 HRC, resp.). On the other hand, the microhardness HV0.3 is almost comparable. The most important functional property, the wear resistance, is better in the case of Cr 3 C 2 -NiCr. The intersplat cohesion is one of the substantial properties for high wear resistance. Its Cr 3 C 2 -CoNiCrAlY lower value can be also responsible for lower values of HR15N. The surface roughness indicates the spreading of the particles after impingement. The lower the Ra and Rz values are, the better the coating spreading can be presumed. Also in this case, the Cr 3 C 2 -CrNi coatings are slightly better compare to Cr 3 C 2 -CoNiCrAlY.In both cases, the same tendency of dependence on spraying parameters was observed: the higher temperature and velocity, the better. The limitations can be found in relative deposition efficiency. The further increase of temperature and velocity of impacting droplets can lead to a splashing of coating materials and decreasing of efficiency. Another question arises in relation to the coating phase composition and amount of inner oxides. The high temperature can cause the creation of undesirable carbides decomposition, as well as increase of the amount of oxides on the splat boundaries, that both can deteriorate the coating properties. The functional properties of the both types of CrC-based coatings, sprayed by the best chosen parameters (Par. 15 for Cr 3 C 2 -NiCr and Par. 12 for Cr 3 C 2 -CoNiCrAlY) will be further tested for high temperature oxidation resistance and for resistance against the influence of hot steam environment. In both tests, the intersplat cohesion is one of the most important parameter.

5 a) b) Fig. 3 Properties mechanical properties of a) Cr 3 C 2 -NiCr and b) Cr 3 C 2 -CoNiCrAlY coatings in dependence on spraying parameters.

6 4. CONCLUSION The two types of CrC-based HVOF sprayed coatings with different matrix material were evaluated in dependence on the spraying parameters. In both cases, the higher temperature and velocity of the flame gives better coatings. The Cr 3 C 2 -NiCr coating generally achieved better results of all evaluated properties, but the difference was not too pronounced. For both coatings, the optimized parameters were found and will be further used for R&D purposes as well as for commercial uses. ACKNOWLEDGEMENT The paper was prepared thanks to the financial support of project no. TE LITERATURA [1] ESPALLARGAS, N., BERGETZ, J., GUILEMANNY, J.M., BENEDETTI, A.V., SUEGAMA, P.H. Cr 3C 2-NiCr and WC-Ni thermal spray coatings as an alternatives to the hard chromium for erosion-corrosion resistance. Surface and Coatings Technology, 2008, Vol. 202, p [2] GUILEMANNY, J.M.,FERNÁNDEZ, J., DELGADO, J., BENEDETTI, A.V., CLIMENT, F. Effects of thickness coating on the electrochemical behavior of thermal spray Cr 3C 2-NiCr coatings. Surface and Coatings technology, 2002, Vol. 153, p [3] MATTHEWS, S., HYLAND, M., JAMES, B. Microhardness variation in relation to carbide development in heat treated Cr3C2-NiCr thermal spray coatings. Acta Materialia, 2003, Vol. 51, p [4] MATTHEWS, S.J., JAMES, B.,J., HYLAND, M.M. Microstructural influence on erosion behavior of thermal spray coatings. Materials Characterization, 2007, Vol. 58, p [5] MATTHEWS, S.J., JAMES, B.,J., HYLAND, M.M. High temperature erosion of Cr 3C 2-NiCr thermal spray coatings The role of phase microstructure. Surface and Coatings Technology, 2009, Vol. 203, p [6] MATTHEWS, S.J., JAMES, B.,J., HYLAND, M.M. High Temperature-Oxidation of Cr 3C 2-NiCr Thermal Spray Coatings under Simulated Turbine Conditions. Corrosion Science, 2013, accepted manuscript, doi: [7] GUILEMANNY, J.M.,FERNÁNDEZ, J., DELGADO, J., BENEDETTI, A.V., CLIMENT, F. Effects of thickness coating on the electrochemical behavior of thermal spray Cr 3C 2-NiCr coatings. Surface and Coatings technology, 2002, Vol. 153, p [8] CHATCHA, S.S., SIDHU, H.S., SIDHU, B.S. High temperature hot corrosion behavior of NiCr and Cr 3C 2-NiCr coatings on T91 boiler steel in an aggressive environment at 750 C. Surface and Coatings Technology, 2012, Vol. 206, p [9] BACH, F.W., ENGL, L., BAC. C., LUGSCHEIDER, E., PARCO, E., DUDA. T. Evaluation of modern HVOF systems concerning the application of hot corrosion protective coatings, In. International Thermal Spray Conference 2003, Orlando, Florida, 2003, ASM International, Materials Park, OH, USA, 2003 [10] SAEIDI, S., VOISEY, K.T., McCARTNEY, D.G. The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings, Journal of Thermal Spray Technology, 2009, Vol. 18, p [11] ROY, J.M., RICHTER, L., BEAUVAIS, L., JODOIN, B. Oxidation behavior of CoNiCrAlY bond coats produced by APS, HVOF and CGDS at temperatures of 1000 C and 1100 C, In. International Thermal Spray Conference 2011, Hamburg, Germany, 2011, DVS Media GmbH, Düsseldorf, 2011, p [12] HOUDKOVÁ, Š., KAŠPAROVÁ, M., ZAHÁLKA, F. Sliding friction behavior of HVOF sprayed hardmetal coatings under different load conditions, In Metal 2011: 20 th anniversary Int. Conference : Brno, Hotel Voroněž, Česká Republika [CD-ROM]. Ostrava:TANGER:May 2011, s ISBN: [13] HOUDKOVÁ, Š., BLÁHOVÁ, O., KAŠPAROVÁ, M. The mechanical properties of HVOF properties of HVOF sprayed Cr 3C 2-25%CoNiCrAlY determined by indentation, Chemické Listy, 2012, Vol.106, p [14] ZIMMERMANN, S., KREYE, H., Chromium carbide Coatings produced with Various HVOF Spray Systems, In Proceeding of the 9th National Thermal Spray Conf., Cincinnati, ASM International, Materials Park, Ohio, USA 1996, p