Wear behaviour of thermal flame sprayed FeCr coatings on plain carbon steel substrate

Wear behaviour of thermal flame sprayed FeCr coatings on plain carbon steel substrate

Journal of Materials Processing Technology 190 (2007) 204–210 Wear behaviour of thermal flame sprayed FeCr coatings on plain carbon steel substrate B...

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Journal of Materials Processing Technology 190 (2007) 204–210

Wear behaviour of thermal flame sprayed FeCr coatings on plain carbon steel substrate B. Uyulgan, E. Dokumaci, E. Celik, I. Kayatekin, N.F. Ak Azem, I. Ozdemir, M. Toparli ∗ Dokuz Eyl¨ul University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Buca, Izmir 35160, Turkey Received 15 August 2006; received in revised form 13 February 2007; accepted 23 February 2007

Abstract The principle aim of this study is to investigate the wear behaviour of FeCr coatings on Ni-based bond deposited plain carbon steel substrate for several applications in power generation plants. For this purpose, FeCr and Ni-based powders were sprayed on plain carbon steel substrates using a thermal flame spray technique. Fabricated layers were characterized by using a X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), microhardness and surface roughness testers. FeCr coatings were subjected to sliding wear against AISI 303 stainless steel counter bodies under dry and acidic environments. A pin-on-plate type of apparatus was used with normal loads of 49 and 101 N and sliding speed of 1 Hz. XRD results revealed that FeCr, Fe, Cr, Fe–Cr–Ni, ␥-Fe2 O3 and Fe3 O4 phases are exist in the coating. In addition, some inhomogenities such as oxides, porosity, cracks, unmelted particles and inclusions were observed by SEM. The surface morphologies of FeCr samples after wear experiments were examined by SEM and EDS. It was found that friction coefficients of the coatings in dry condition are higher than that in acidic environment. © 2007 Elsevier B.V. All rights reserved. Keywords: Thermal flame spray; FeCr coating; Friction coefficient; Wear

1. Introduction The thermal spray process is being widely diffused for industrial applications in order to improve surface protection against corrosion and wear effects for several applications in power generation plants [1]. For these applications, the high strength materials may be layered by means of heat sources such as lasers, plasma torches, electric arcs and combustion flames, plasma spray, detonation gun and high velocity oxy-fuel (HVOF). All these techniques permit the deposition of coating materials generally ductile and improved corrosion and wear resistance. Nevertheless, more traditional electric arc wire spray and combustion flame spray are still widely used [2]. The flame spray deposition technique has a number of disadvantages compared to the other spraying process, such as plasma spray, detonation gun and HVOF, including a bigger grain size microstructure, pore size and crack length, but it also has distinct advantages, such as its being more economical, easier to handle and more adapt-



Corresponding author. Tel.: +90 232 412 74 63; fax: +90 232 412 74 52. E-mail address: [email protected] (M. Toparli).

0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.02.044

able to manufacturing processes with short series or recovery of pieces [3,4]. The flame spray method uses the heat from the combustion of a fuel gas (usually acetylene or propane) with oxygen to melt the coating material, which can be fed into the spraying gun as a powder, wire or rod. In the flame spraying process, powder is directly fed into the flame by a stream of compressed air or inert gas (argon or nitrogen). The molten particles are accelerated and deposited onto a substrate by the flame gases [5,6]. Upon impact, every molten particle flattens on the surface and rapidly solidifies in a highly non-equilibrium conditions forming what is termed a splat. Splat piling up on the substrate surface build up a layered structure, which constitutes the final coating. The main process parameters are the particle size and temperature, their velocity, the angle and rate of deposition (continuous or intermittent), the spraying distance and temperature of the substrates. Qualitatively, one would expect that a high energy concentration (e.g. high temperature and velocities) of the impinging particles could be more effective in the deposition. On the other hand, in order to avoid undesired chemical transformations, excess heating of the samples should be prevented by keeping the kinetic fraction of the particle energy predominant with respect to the heat they

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transport. In flame spray processing of metallic powders, the formation of high oxide content phases leads to better tribological characteristics and the oxide content (likely to increase with increasing spray distance) results in harder coatings with better anti-wear characteristics (especially under low contact pressure conditions) [2]. In recent years, FeCr alloys have received much attention as high-temperature oxidation and wear resistance materials for the requirement of the utilities to increase the thermal efficiency of fossil fired power generation plants. A number of high strength 9–12% Cr steels have been developed for application as construction materials in such advanced power plants [7]. In addition, the FeCrNi and FeCrAl coatings have been successfully applied in the contexts of corrosion, wear and oxidation resistance at elevated temperatures [8]. Notably, Cr addition to Fe material causes secondary hardening when its content exceeds 10% and decreases stacking fault energy of austenite, both factors contributing to increase the wear resistance. Thermal-sprayed coatings of FeCr are being employed to decrease the coefficient of sliding frictional behaviour between various sliding components [9]. There is a lack of information regarding as the wear behaviour of FeCr coating in acidic condition. In the present paper, wear behaviour of FeCr/Ni-based coatings produced on plain carbon steel substrates by thermal spray technique is reported for several applications in power generation plants. With this regards, these layers were characterized by SEM, EDS, microhardness and surface roughness. The effect of load, distance and condition on the wear behaviour of FeCr coatings was scrutinized by using a combination of microstructural and mechanical results. Wear tests with AISI 303 stainless steel (pin)/specimen (plate) configuration scheme in the case of dry and acid environment under normal loads of 49 and 101 N have been performed for different thermal spray coatings. 2. Experimental aspects The plain steel substrates were cut with nominal dimensions (mm) of 13 mm × 13 mm × 4 mm. Before flame spray process, the surface was typical for machining, with a rust- and impurity-free average surface roughness Ra ∼ 1.5 ␮m. Before the coating process, substrates were ultrasonically cleaned and grit-blasted with 35 grit Al2 O3 (alumina) abrasive to eliminate grease and oxide. Flat substrates were used in the experiments, thereby running the risk of forming large surface undulations as a result of the manual nature of the process. The grit-blasted substrates were coated with a bond layer using Ni-based powders to improve adhesion of the FeCr coating to the substrate. The chemical composition of FeCr powder is: Fe = 87.60 and Cr = 12.40. The chemical composition of Ni-based powder is listed in Table 1. Metco 442 combustion powder thermal flame spray system was used to coat the surface of the substrate. FeCr and Ni-based powder particles were sprayed at powder feet rate of 4 lb/min and optimum spray distance from gun to sample was 150 mm. Dry air, oxygen and acetylene pressures were, respectively, chosen as 3, 0.4 and 0.7 bar to melt and spray powders.

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XRD analysis of as-sprayed coatings was performed by means of a Rigaku diffractometer, employed at ambient temperature with an intensity scanner versus different angle between 10◦ and 90◦ (step size of 0.050◦ , scanner velocity = 3 s/step) using a Cu K␣ irradiation (wavelength, λ = 0.15418 nm), a voltage of 40 kV and a 30 mA filament current. The cross-section of all samples was sandpapered and polished before microstructural observations and microhardness measurements. A JSM 6060 (JEOL) Model Scanning Electron Microscope was used to examine the microstructure of the cross-sectional area of the samples. Thickness of the coating and some inhomogenities such as oxides, porosity, cracks, unmelted particles, semi melted particles and inclusions were determined by SEM including EDS attachment. In addition, SEM image analysis programme was used in order to determine the volume percentage of porosity. Microhardness tests were carried out using Dynamic Ultra-Microhardness Tester with DUH-W20 (SHIMADZU) Model. Vickers hardness measurements were taken on transversal cut of the coatings, scanning from the surface through not affected zones of the substrate during the coating. Wear tests were performed by using a universal wear friction tester (PLINT 88) with a pin-on-plate type apparatus. An AISI 303 steel pin of 5 mm in diameter was used as a counter body. Surface of each FeCr coated samples was subjected to sliding wear test in ambient air at room temperature in dry and acidic (1% H2 SO4 + ethanol) environment. Both coated plate and counter body were cleaned in acetone and dried in air prior to wear test. Normal loads applied during the tests were 49 and 101 N. The sliding speed, stroke and sliding distance were 1 Hz, 6 and 9.960 mm, respectively. More details regarding as wear experiments can be found in Refs. [1,6,10]. Wear loss and friction coefficients were evaluated for each sample by using the wear test machine. The effect of environment, distance and load on the wear loss and friction coefficient was determined. Before and after wear test, surface roughness of FeCr coated samples was measured by using a SJ-301 stylus type Mitutoyo Surface Roughness Measurement Tester. Wear morphology of coated samples were examined before and after wear tests by using SEM. Semi quantitative analysis was performed by using EDS after wear experiments.

3. Results and discussion 3.1. Phase structure Fig. 1 shows the XRD pattern of present phases formed in FeCr coated samples. Fe and Cr phases are the main constituent of the coating. Due to the fact that the coating was deposited on plain carbon steel substrate in air, some oxides including ␥Fe2 O3 and Fe3 O4 with low intensity were determined. That is to say, it can be pointed out that oxide phases are formed during the interacting process of air with melted particles. FeCr, Fe, Cr and Fe–Cr–Ni phases were also found from the XRD analyses of FeCr coated samples. FeCr alloys have received much attention as high-temperature oxidation resistance materials [11]. Furthermore, the XRD pattern shows the polycrystalline nature. The

Table 1 Composition of Ni-based powder Composition

Ni

Cr

Al

Mo

Si

B

Fe

Ti

Weight percentage (wt.%)

73.5

8.5

7

5

2

2

1

1

Fig. 1. XRD pattern of FeCr/Ni-based layers on plain carbon steel substrate.

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Fig. 2. Cross-sectional SEM images of a FeCr coated specimen.

spray process makes the particles to crystallize when they solidify from molten state. As the powder particles enter the hot flame, they get heated to temperatures much above the melting point of the material and the molten drops are propelled through the hot flame and get deposited on the stainless steel substrate [3,5,12]. 3.2. Microstructure Microstructural characterization of thermal spray coatings involves quantitative measurements of geometrical features such as porosity (in the form of voids, cracks and other defects) and analysis of material aspects in coatings such as splat structure, interfaces, phases, etc. [12]. Depending on these properties, microstructural features can be elucidated. Fig. 2 shows a typical cross-sectional SEM microstructure of the flame sprayed FeCr coating on Ni-based powder deposited on plain carbon steel substrate. As shown in Fig. 2, there is a Ni-based bond layer between the substrate and FeCr coating to enhance the adhesive strength and certain properties. As explained in Refs. [2,13,14] in details, in Ni-based bond coating, Ni, Cr and Ti promote resistance to oxidation and high temperature corrosion and increases the hardness of the coating by forming very hard precipitates. The low Al content provides formation of hard film of such as discontinuous ␣-Al2 O3 in the bond layer. The ceramic-ceramic chemical bonding energy between Fe2 O3 and Fe3 O4 in top coating and Al2 O3 is stronger than metal–ceramic adsorptive bond between FeCr and Ni-based layers. Along with Al2 O3 and Cr2 O3 oxides with very low content which form during flame spraying, they act as a strengthening mechanism providing many pinning sources for the laminated layers themselves. In flame spraying of Mo element in Ni-based powder, the formation of an oxide phase leads to improved wear properties. Boron brings down the melting temperature and helps in the formation of hard phases. Silicon is added to increase self-fluxing properties. All coating layers contain porosity, oxides, cracks, unmelted particles and inclusions. The coating surface is very rough and contains large quantity of pores or voids. These voids may be due to the pull-out effect. While polishing, the weakly bound/unmelted/partially melted particles pull out from the coating surface. Unmelted and semimelted particles, varying between 25 and 100 ␮m, are clearly seen in cross-sectional area of the coating. This may peel off from the coating while polishing. Generally speaking, coatings deposited at lower power level

exhibit a significant amount of unmelted particles and macro porosity which may influence the mechanical behaviour. As indicated elsewhere [15], it is understandable that at higher power levels and shorter spray distances, particles are at good molten state, impinging with higher velocities on the substrate. At lower power and higher torch to base distance, not only the particles get cooled but also their velocities are reduced. This will lead to greater number of unmelted particles resulting in higher porosity. The volume fraction of porosity of FeCr and Ni-based bond coatings were 4.72 and 5.23%. However, the sprayed composite coatings possess a porous structure, respectively. Especially, plasma sprayed FeCr–TiC composite coatings contain 16–20% porosity [4]. Also, coatings obtained from powders and wires with FeCrB composites are highly porous, because of insufficient time to melt completely of the powder [16]. Moreover, splat structure can be clearly seen from Fig. 2. In thermal spraying, many individual molten droplets impacting on the substrate flatten to form splat which pile one-on-top-ofother creating the deposit. Splat forms according to splashing mechanism which is related to fluid dynamic instabilities during spreading. For thermal spray, molten metallic droplets, splashing behaviour is considerably more complex. There is a strong thermal interaction between the droplet and substrate; the droplet will lose heat and may solidify during the spreading process depending on the droplet/substrate condition where the interface temperature increasing dramatically. Substrate temperature affects the splat morphology through the change in interface nature, surface chemistry, solidification behaviour, wetting and contact of molten droplet with substrate, droplet/substrate mechanical interaction. The splat morphology plays an important role in the nature of the deposit build-up process and consequently affects the microstructure and properties of the sprayed materials [17]. Coating thicknesses of FeCr top and Ni-based bond layers were determined by SEM. The thickness of FeCr top and Ni-based bond layers were found to be 865 and 182 ␮m, respectively, which are reasonable for several applications in power generation plants. Increasing the coating thickness, residual stresses in the coating layers generally arise as well as rapid cooling of individual splats upon contact with substrate and temperature differences between the substrate and deposit in the presence of different thermal expansion coefficient. The generation of residual stress at the interface may cause failure due to plastic deformation, fracture, delamination and/or surface wear [2]. Fig. 3 illustrates the SEM micrographs of flame sprayed FeCr coated surfaces at two different magnifications. As shown in Fig. 3, the surface of FeCr coatings contains unmelted spherical particles, porosities and microcracks that have been formed due to rapid cooling process during spraying process. 3.3. Microhardness Vickers microhardness values of FeCr top, Ni-based bond layers and substrate are shown in Fig. 4. Microhardness values that were taken along the cross-section of the sample were decreased regularly. At the same time, the average value of the

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Fig. 3. SEM images of flame sprayed FeCr coating (a) 2500× and (b) 1000× magnification.

microhardness of FeCr top and Ni-based bond coatings is significantly higher than that of the substrate. However, Cr played a major role in the difference in hardness of FeCr top coating top coating due to its moderate strengthening effect [15]. In addition to Cr content, some inhomogenieties such as porosity, oxides and unmelted particles cause a change in hardness values of the top coating. As indicated in a research of Habib et al. [18], concerning flame sprayed coatings, local variation in hardness results from the variation in particle temperatures and velocity which are inherent in the spray process. Regarding as the local variation in microhardness, it can be noted that when indenter test load is reduced, microhardness increases due to factors, such as elastic recovery, the decreasing effect of grain boundaries, porosity and microcracks in the coating layers. Similar results are also obtained in this study.

increase by applying higher loads. If dry and acidic conditions are compared with each other, both static and dynamic friction coefficients in acidic environment are lower than that in dry environment. Whereas in dry environment, friction coefficients slightly increase by applying higher load, however in acidic environment this increase occurs significantly. In dry environment, despite the rapid initial increase of static friction coefficient, dynamic friction coefficient shows disordered change. In acidic environment and at 101 N loads, dynamic friction coefficient exhibits a steady state behaviour, but at 49 N loads there is a small amount of decrease at the dynamic friction coefficient. Fig. 5b points out the changes of the amount of wear loss by sliding time for the same coating. When the amount of wear loss was compared for each load levels, it could be said that the applied load plays a significant role on the amount of wear loss. The effect of applied load on wear loss is more dominant than

3.4. Wear The variation of friction coefficient and wear loss of the FeCr coating as a function of sliding time is indicated in Fig. 5. Same load levels were chosen in dry and acidic environments in order to compare the friction and wear behaviour of the coatings. Fig. 5a illustrates the changes of the amount of friction coefficient of FeCr coating as a function of sliding time under dry and acidic conditions and load levels. The effect of environment on friction coefficient is more dominant than that of load levels. In both dry and acidic conditions, friction coefficient

Fig. 4. Microhardness (HV) values of FeCr top and Ni-based bond coatings and the substrates as a function of distance (␮m) from the surface of top coating.

Fig. 5. The variation of (a) friction coefficient and (b) wear loss of FeCr coating as a function of sliding time for 49 and 101 N loads in sliding speed of 1 Hz.

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Table 2 Semi quantitative analysis of the worn samples at 49 N and 101 N loads under dry and acidic conditions Components

Cr Fe Ni Si S

Weight percentage At 49 N loads applied in dry condition

At 101 N loads applied in dry condition

At 49 N loads applied in acidic condition

At 101 N loads applied in acidic condition

13.732 82.829 2.580 0.516 0.342

15.789 80.934 2.617 0.467 0.193

12.584 85.194 1.703 0.308 0.211

7.890 91.078 0.378 0.412 0.242

Table 3 Surface roughness measurements of FeCr coating before and after wear experiments Loads (N)

49 101

Conditions Surface roughness before wear test (␮m)

Surface roughness under dry condition (␮m)

Surface roughness under acidic condition (␮m)

11.21 11.21

8.96 7.58

10.57 9.79

that of environment. In acidic environment, although the wear loss increases rapidly by applying a relatively higher load at the beginning, a steady state condition occurs. On the other hand, in dry environment the amount of wear loss follow a similar wear behaviour with acidic environment for the same load levels initially, but then the amount of wear loss sharply increases with respect to sliding time. As seen from Fig. 5b, wear rate is maximum for 101 N loads in dry condition. Cause of this, in dry condition wear products could not be taken away from the specimen surface and increase the amount of wear loss. In contrast, in acidic environment wear products are rejected from the worn surface by the solution. The results of semi quantitative analysis for 49 and 101 N loads applied under dry and acidic conditions are compared in Table 2. In dry environment the amount of stuck particles coming from stainless steel pin is more than that in acidic environment. As can be seen from Table 2, the weight percentage of Ni in dry condition at 101 N loads is higher than at 49 N loads. Surface roughness measurement of FeCr coated samples taken before and after wear tests are given in Table 3. It is clear from Table 3 that surface roughness of FeCr coating decreases after wear tests carried out under dry and acidic conditions. Sur-

face roughness of as-sprayed coating is 11.21 ␮m. Nevertheless, this value changes after wear test due to applied loads and test conditions. At higher loads, surface roughness decreases owing to the flattening effect of stainless steel pin during wear test. Effect of acidic media can be clearly noticed from Table 3 as it causes the corrosion of FeCr coating surface by forming pits. It is clearly visible in Fig. 3 that the coating surfaces have not a smooth feature owing to atmospheric flame spray system. The particles on the highest point of the rough surface acts as a contact region, flow out of the specimen surface and stick again to surface. Therefore, these particles profoundly influence the tribological behaviour of the coating. Also, microcracks, which are observed on the surface before wear tests, reduce the corrosion and wear resistance of the coating [19]. Micrographs of samples taken after wear test under dry sliding condition are depicted in Fig. 6. Due to plastic deformation, cracks and tears exist on the surface propagate perpendicular to the wear direction. At higher loads, the pin surface is almost in contact with the plate but at lower load there is no full contact between the pin and plate. Because of this, near the worn region as seen in Fig. 6, it is possible to observe unworn depth region under the surface.

Fig. 6. Surface morphology of FeCr coated samples tested at (a) 49 N and (b) 101 N loads under dry sliding condition from different areas.

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Fig. 7. SEM micrographs of the surface of FeCr coated sample tested at (a) 49 N and (b) 101 N loads under acidic condition from different areas.

Fig. 8. Surface morphology of FeCr coated samples tested at 49 N loads under (a) dry and (b) acidic conditions.

A typical view of the worn surface of coated samples tested at 49 and 101 N loads in an acidic environment are indicated in Fig. 7. In these figures, wear tracks and scratches are less outstanding. The less amount of cracking and flown out of particles has been observed as a result of plastic deformation at the surface under acidic condition. Fig. 7a shows pits on worn surface that occurs as a result of corrosion process by the effect of acidic environment. Since the wear rate of the sample under 101 N loads is higher than that of the sample under 49 N loads, worn surface is smoother and pits clearly observed in Fig. 7b. Micrographs taken from the surface of worn sample under 49 N loads show the pits in detail (see Fig. 8b). In acidic condition, low friction coefficient is obtained by the effect of liquid environment. A decrease of scratches caused by abrasive wear and decrease of tears and cracks caused by plastic deformation of surface under friction load are also observed. These three situations in acidic condition might be thought as an advantage. Nonetheless, corrosion failure occurred on the surface is a disadvantage compared to dry condition. 4. Summary and conclusion The FeCr/Ni-based coating configuration was fabricated on plain carbon steel substrate by the flame spray technique for several applications in power generation plants. XRD results revealed that ␥-Fe2 O3 , Fe3 O4 , FeCr, Fe, Cr, Fe–Ni–Cr phases we formed in the FeCr/Ni8.5Cr7Al5Mo2Si2B2FeTi arthitectured plain carbon steel substrate. Microstructural observations demonstrated that coatings possessed unmelted spherical parti-

cles, porosities, cracks and oxides. These inhomogenities had reverse effect on wear resistance of the FeCr coating. Relatively higher loads decreases the surface roughness value because of the flattening effect of stainless steel pin during the wear test. The effect of environment on friction coefficient is more dominant than the effect of load levels whereas the effect of applied load on wear loss is more dominant than that of environment. In acidic environment both static and dynamic friction coefficients are less than in dry environment. Acknowledgments The authors would like to thank K. Demirkurt at KROMA Inc., Izmir, Turkey, for kindly help in producing the FeCr and Nibased layers and Dokuz Eyl¨ul University Faculty of Engineering for the financial support. References [1] H. Cetinel, B. Uyulgan, C. Tekmen, I. Ozdemir, E. Celik, Surf. Coat. Technol. 174–175 (2003) 1089–1094. [2] G. Bruno, C. Fanara, F. Guglielmetti, M. Malard, Surf. Coat. Technol. 200 (2006) 4266–4276. [3] S. Tondu, T. Schnick, L. Pawlowski, B. Wielage, S. Steinhauser, l. Sabatier, Surf. Coat. Technol. 123 (2000) 247–251. [4] L.T. Duarte, E.M. Paula, E. Silva, J.R.T. Branco, V.F.C. Lins, Surf. Coat. Technol. 182 (2004) 261–267. [5] J.R.T. Branco, S.V. Campos, Surf. Coat. Technol. 120–121 (1999) 476–481. [6] E. Celik, C. Tekmen, I. Ozdemir, H. Cetinel, Y. Karakas, S.C. Okumus, Surf. Coat. Technol. 174–175 (2003) 1074–1081. [7] J. Zurek, E. Wessel, L. Niewolak, F. Schmitz, T.-U. Kern, L. Singheiser, W.J. Quadakkers, Corros. Sci. 46 (2004) 2301–2317.

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