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Microstructure and wear behavior of Fe-based amorphous HVOF coatings produced from commercial precursors
SCT-21702; No of Pages 7 Surface & Coatings Technology xxx (2016) xxx–xxx
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Microstructure and wear behavior of Fe-based amorphous HVOF coatings produced from commercial precursors G.Y. Koga a,⁎, R. Schulz b, S. Savoie b, A.R.C. Nascimento b, Y. Drolet b, C. Bolfarini a, C.S. Kiminami a, W.J. Botta a a b
Universidade Federal de São Carlos, Departamento de Engenharia de Materiais, Rod. Washington Luis, km 235, CEP 13565-905, São Carlos, SP, Brazil Hydro-Quebec Research Institute, 1800 Boul. Lionel Boulet, Varennes, QC J3X 1S1, Canada
a r t i c l e
i n f o
Article history: Received 29 June 2016 Revised 18 October 2016 Accepted in revised form 21 October 2016 Available online xxxx Keywords: Thermal spray coatings Amorphous alloys Commercial precursors Steel Wear testing
a b s t r a c t Wear resistant highly amorphous Fe60Cr8Nb8B24 (at.%) coatings of about 280 μm thickness were prepared through high velocity oxygen fuel (HVOF) thermal spray process onto API 5L X80 steel substrate. Feedstock powders were produced by gas atomization with low purity precursors by modifying AISI 430 stainless steel with additions of niobium (Fe-Nb) and boron (Fe-B). It was found that the coatings were mostly amorphous with some embedded FeNbB and Fe2B borides. The formation of a large fraction of amorphous phase was attributed to the high cooling rates of molten droplets combined with a proper powder composition. The average Vickers hardness of the coating (HV0.3 = 838 ± 23) was about four times higher than that of the API 5L X80 substrate (HV0.3 = 222 ± 5). The excellent wear resistance of the amorphous coating in the pin-on-disc measurements was attributed to its large fraction of amorphous phase (~66%) with reinforcing hard Fe2B and FeNbB borides, high Vickers hardness, low oxygen content (b 0.41%), and relatively low porosity (~5). The wear rates of the amorphous coatings were about two orders of magnitude lower than that of the API 5L X80 steel substrate (1.0 × 10−5 and 8.5 × 10−4 mm3·N−1·m−1, respectively). API 5L X80 steel exhibited dominant adhesive wear at low sliding speed and oxidative wear at high sliding speed. HVOF coatings presented oxidative wear regardless of the sliding speed. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Among the bulk metallic glasses (BMGs) developed in many different alloy systems, Fe-based amorphous alloys have drawn great attention due to their unique physical and chemical properties associated with the amorphous phase and the relatively low cost of iron. Compared with traditional crystalline metallic materials, Fe-based amorphous alloys exhibit high strength, toughness and hardness, and superior corrosion resistance, attributed to their disordered structures as well as the chemical homogeneity [1–5]. All of these properties make this family of alloys potential engineering materials in a wide range of structural applications, which withstand aggressive work conditions. Since bulk glassy alloys were synthesized in Fe-based systems, great efforts have been devoted to find and produce BMGs by using commercial base materials [6–9]. However, despite the use of some low purity precursors, their development has been based on small additions of high cost elements (Y and Er) [7,10,11], high purity of some raw materials [12], and strict processing characteristics (e.g., high vacuum and high cooling rates), which restrict their industrial applications.
⁎ Corresponding author. E-mail address: [email protected] (G.Y. Koga).
For instance, it has been reported that Fe-based BMGs can be produced in Fe-C-Si-B-P-Cr-Mo-Al [13], Fe-Cr-Co-Mo-C-B-Y [14], Fe-CoZr-Mo-W [15], and cast iron based amorphous alloy [8] systems by using industrial stainless steel, low purity elements, and ferrous-alloys; hence reducing significantly the production cost. Nevertheless, these bulk Fe-based glasses are generally complex consisting of five or more components and they have additions of some high purity elements or special process is required to provide high cooling rates in an inert environment [16]. Thus, it is still desirable to improve the glass-forming ability (GFA) of Fe-based alloys in order to enhance their ability to form bulk glassy steels under conventional industrial conditions, using, for example, low vacuum furnace, conventional casting methods, commercialgrade raw material, etc. [17]. Progress has been made in this direction. For instance, Cheney and Vecchio [18] have successfully obtained amorphous alloys in the Fe-Cr-Nb-B system (Fe62Cr8Nb8B22 and Fe63Cr4Nb7B26) by adding small amounts of Nb and B to selected stainless steel. However, splat casting technique was used in this case. Thermal spraying is an interesting technological process to produce amorphous alloys coatings thanks to the high cooling rates (101 ‐ 107 K·s−1) that can be achieved and its low cost compared to other more conventional rapid solidification processes. In addition, metallic coatings can be applied to larger and complex parts and, therefore, can widen the application fields by overcoming the volume restrictions
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imposed by the critical cooling rates [19–21]. Thermal spraying includes several particle deposition techniques, such as flame spraying, arc spraying, plasma spraying, high velocity oxygen fuel (HVOF) [22]. Among them, HVOF sprayed coatings have been widely used to produce Fe-based amorphous protective coatings owing to their superior corrosion and wear resistance compared to the other methods [23–28]. In the present work, the nominal Fe60Cr8Nb8B24 (at.%) alloy was used to produce amorphous coatings through HVOF thermal spray process. The substrate chosen was the API 5L X80 steel. To demonstrate the good GFA and the industrial production potential of the amorphous alloy, the feedstock powers were obtained by gas atomization using only conventional ferrous-alloys additions (Fe-Nb and Fe-B) to adjust the composition of a commercial AISI 430 stainless steel. The wear resistance of the coatings and the substrate (API 5L X80) was evaluated and results were discussed.
Table 2 Spraying parameters employed in the gas atomization processing for powder production. High pressure nitrogen gas atomization Gas flow rate – N2 (m3·min−1) Mass flow rate (kg·min−1) Gas to metal ratio Temperature (°C) Nozzle diameter (mm)
3.84 5.90 0.62 1700 6.0
composition and the microstructure of the substrate are presented in Table 1 and Fig. 1, respectively. Sieved as-atomized particles were used as feedstock powders in HVOF thermal spray process to coat degreased and sand blasted 120 × 30 × 5 mm plates and Ø 63.5 × 6.6 mm discs. The detailed HVOF spraying parameters are listed in Table 3. The structure and morphology of the coatings were examined by scanning electron microscopy (SEM) Philips XL30 FEG equipped with energy dispersive spectroscopy (EDS), confocal microscopy (CM) LEXT 3D Measuring LASER microscopy OLS4100 Olympus, and X-ray diffraction (XRD) analysis as described before. Thermal behavior of the detached coatings was investigated by differential scanning calorimeter (DSC), Netzsch 404, at heating rate of 0.67 K·s− 1. To extract the samples, the coated substrate was held against a grinding wheel which cut it from the substrate towards the deposit. Cutting was stopped near the coating/substrate interface and then the specimen was bent as sharply as possible first to one side then to the other, until the specimen breaks. This procedure generates a shearing stress between the basis metal and the deposit which tends to separate part of the coating from the substrate. Finally, the peeled samples were obtained by using a sharp chisel at the back of the coating overhang. Cross-sectional samples were mounted in conducting resin and examined in the SEM using backscattered electron signal to form images. The surface average roughness measurements were performed in a portable surface roughness tester (Mitutoyo SJ-201P) in 3 different regions. A total of 20 cross-sectional images in different regions of the coatings were taken by ZEISS optical microscope (magnification of 500 ×). Image processing was carried out using AnalySIS Pro software to evaluate the average porosity levels. The oxygen content in the detached coatings was analyzed using the instrument mentioned before. The boron content of the coatings was measured by a Varian AA240FS atomic absorption spectrometer (Mulgrave, Australia). The samples were prepared by dissolving the detached coatings in a 0.002 M H2SO4 solution by using ultrapure water obtained from Milli-Q® purification system (Millipak-40 Filter Unit 0.22 μm NPT, Bedford, MA, USA). Reproducibility of data was ensured by repeating the test in 3 solutions prepared with different detached coatings obtained from different regions.
2. Experimental procedure 2.1. Production of feedstock powders The Fe60Cr8Nb8B24 (at.%) alloy is regarded as having high potential for industrial applications thanks to its high GFA, relatively low cost of the alloying elements, and the possibility to adjust stainless steel composition using minor commercial grade additions [5,18]. Metallic powders with nominal composition of Fe60Cr8Nb8B24 (at.%) were produced by gas atomization process using commercial AISI 430 stainless steel and conventional Fe-Nb and Fe-B ferrous alloys, Table 1. The raw materials were melted in an induction furnace under argon atmosphere and sprayed at 1700 °C by using N2 atomization gas. Table 2 shows the parameters used for the production of metallic powders. The as-atomized powders were sieved to select particles with size ranges between 20 to 53 μm. The morphology and the composition of the feedstock powders were characterized by scanning electron microscopy (SEM) with wavelength-dispersive X-ray spectroscopy (WDS). Phase constituents were identified by using X-ray diffraction (XRD) analysis performed on an X-ray diffractometer Rigaku Geigerflex ME210GF2, with Cu-Kα radiation. The thermal stability was examined in a differential scanning calorimeter (DSC), Netzsch 404, at heating rate of 0.67 K·s−1. The oxygen content was analyzed by Nitrogen/Oxygen determinator LECO TC-436 DR. Amorphous Fe60Cr8Nb8B24 (at.%) ribbons were produced by meltspinning with copper wheel rotating at a speed of 50 m·s−1 in an argon atmosphere. In this case, high purity Fe (99.97%), Cr (99.997), Nb (99.8) elements and commercial Fe-B were used as raw materials. The obtained amorphous ribbons were used as reference in the study of amorphization. 2.2. Production of coatings by high velocity oxygen fuel (HVOF)
2.3. Wear measurements and characterization of worn surface The chosen substrates in this work were extracted from API 5L X80 line pipes (the American Petroleum Institute specification API 5L covers steel line pipes normally used in the petroleum and gas industries which may present aggressive wear environments). The chemical
Vickers hardness measurements in different cross-section regions were performed on the coatings and API 5L X80 steel substrates using a Vickers diamond indenter (Newage Testing Instrument model Auto-
Table 1 Chemical compositions of the precursor materials: AISI 430 stainless steel, Fe-B and Fe-Nb master alloys and the nominal composition of the Fe-Cr-Nb-B alloy (in this case in wt.%, for comparison with the commercial materials). The composition of the API 5L X80 steel used as substrate is also given. Alloy/wt.%
C
Si
Mn
Cr
Ni
S
P
Nb
Co
N
B
Fe
AISI 430 Fe-B Fe-Nb Fe-Cr-Nb-B API 5L X80⁎
0.06 0.30 0.10 0.15 0.07
0.20 0.57 1.10 0.53 0.21
0.74 – – 0.31 1.64
17.62 – – 8.88 0.10
0.37 – – 0.15 0.25
0.17 – 0.10 0.09 –
0.02 – 0.10 0.03 0.01
0.03 – 66.40 15.77 0.04
0.03 – – 0.01 –
0.03 – – 0.01 –
– 16.50 – 5.49 –
Bal. Bal. Bal. Bal. Bal.
⁎ Additional elements: Al = 0.02%, Ti = 0.01%, Mo = 0.13%, Cu = 0.02%.
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3. Results and discussion 3.1. Characterization of feedstock powders and coatings obtained by HVOF
Fig. 1. Optical micrograph of the API 5L X80 steel substrate after polishing and etching (Nital 2%), which shows the refined grain structure typical of a microalloying pipeline steels.
C.A.M.S. Computer-Assisted Microhardness System) with a load of 300 g and dwell time of 15 s. An average value was obtained from at least ten individual measurements from each specimen. The size of the Vickers hardness indentations was large enough to cover several different phases of the coatings and thus the reported Vickers hardness values represent the overall hardness of the coating. Wear tests were performed using pin-on-disc type sliding-wear equipment. The tests were conducted at room temperature and under dry sliding conditions. Pin-on-disc tests were performed on (Ø 63.5 × 6.6 mm) polished disc. The measurements were conducted at constant load of 2 kgf, and sliding distance of 103 m. Wear track radius, R, and the sliding speed, v, were: a) R = 28 mm, v = 80 cm/s, b) R = 25 mm, v = 30 cm/s, c) R = 22 mm, v = 10 cm/s, and d) R = 19 mm, v = 4 cm/s. The wear rate, K, was calculated using the formula K = V/(F.L), where V (mm3) is the total volume of material removed, F(N) is the normal load and L(m) is the total sliding distance. The average cross-sectional surface of the ring was obtained from measurements taken at four different equidistant points along the wear track ring using a Dektak-150 surface profiler (Bruker Corporation, San Jose, CA, USA). Alumina spheres of 1/ 4 inch in diameter (Ø 6.35 mm) were used as counterface pins. Four measurements at each sliding speed were conducted and the values presented are mean values and standard deviations. Morphologies of worn surface were characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and the loose debris characterized by X-ray diffraction.
Fig. 2 shows the SEM image of the feedstock powders used in the HVOF thermal spray process. It can be seen that most of the particles produced by gas atomization are in the desired size range (20 to 53 μm) after proper sieving. Most of them present spherical or nearspherical morphology and smooth surface due to the good fluidity during atomization. WDS analyses conducted on 6 different particles (see Table 4) confirm that the chemical composition of the feedstock powders is consistent with the nominal Fe60Cr8Nb8B24 (at.%) alloy, despite the presence of some impurities from the low purity raw materials. Also, the powder obtained by gas atomization presented low oxidation levels (0.07%). The boron content of the HVOF coatings measured by atomic absorption spectroscopy (5.06 ± 0.06 wt.% B) is in agreement with the measured boron content by WDS, ~ 5 wt.%, of the feedstock powders. Considering that the risk of loss of boron during deposition process is higher than the loss of Fe, Cr, and Nb, the final composition of the coating is considered to be similar to the composition of the feedstock powders. The presence of a large fraction of amorphous phase in the feedstock powders and coatings produced by HVOF was confirmed by XRD results depicted in Fig. 3. XRD patterns of both feedstock powders and coatings exhibit a broad halo peak, but they also present some crystalline peaks in comparison with the XRD patterns of the as-spun ribbons obtained by melt-spinning. These results suggest that the cooling rates of the gas atomization process and HVOF thermal spray route were lower than that of the melt-spinning. The crystalline peaks observed in the XRD patterns of the coatings are associated to Fe2B, FeNbB, and α-Fe solid solution phases. Since localized reheating may occur due to deposition of successive molten droplets, which release latent heat in solidification, some crystallization can occur during the HVOF process [29]. However, it is interesting to note in Fig. 3 that the diffractograms of feedstock powders and the resulting coatings showed crystalline peaks of almost the same intensities; hence it appears that the HVOF thermal spray processing did not induce pronounced crystallization. In spite of the occurrence of some crystalline peaks observed in the XRD patterns of the feedstock powders and HVOF coatings, the DSC curves in Fig. 4 show that the amorphous content in these samples is high. Compared with the as-spun ribbons which can be considered as fully amorphous, the amorphous phase content of the samples can be estimated through the following equation: Vf = ΔHsample/ΔHribbon, where ΔHsample is the crystallization enthalpy of the feedstock powders or coatings and ΔHribbon is the crystallization enthalpy of the as-spun ribbons. Even though the morphologies of the analyzed samples in DSC were different (powder, peeled coatings, and cut ribbons), the obtained curves can be used for semi-quantitative calculations of the
Table 3 High velocity oxygen fuel thermal spray processing parameters. A Praxair Surface Technologies TAFA model JP-8000 HVOF system and a 5220 HVOF gun mounted on a robot arm were used for deposition. HVOF parameters Substrate Powder size range (μm) Oxygen flow (slpm) Kerosene flow (L·h−1) O2/kerosene equivalence ratio Powder carrier Torch velocity (mm·s−1) Standoff distance (mm) Step size (mm) Number of passes Coating thickness (μm)
API 5L X80 steel 20 to 53 896.8 22.1 1.3 Argon 500 330.2 10 5 280
Fig. 2. Secondary electron SEM image of Fe60Cr8Nb8B24 (at.%) feedstock powders obtained by gas-atomization after proper sieving.
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Table 4 Nominal and measured compositions of the feedstock powders. Chemical composition at.%
Fe
Cr
Nb
B
Impurities
Nominal alloy Measured compositiona
60 55 ± 4
8 8.4 ± 0.6
8 6.8 ± 0.3
24 22 ± 5
– Bal.
a Averages and standard deviations calculated from WDS measurements took in 6 different particles.
amorphous content. As a result, the estimated amorphous fraction of the feedstock powders and HVOF coatings is about 85% and 66%, respectively. In addition, the supercooled liquid region (SLR), ΔTx, defined by the difference between the onset temperature of crystallization (Tx) and the glass transition temperature (Tg) is, approximately, 60 K for all samples. The large ΔTx indicates high thermal stability of the supercooled liquid against crystallization, which suggests high glass forming ability of the Fe60Cr8Nb8B24 (at.%) alloy. Moreover, the deviation from nominal composition of the feedstock powers (as depicted in Table 4) and of the resulting coatings produced by HVOF did not affect the glass forming ability of the resulting alloy. The XRD and DSC results suggest that highly amorphous Fe-based alloy coatings can be fabricated using low purity raw materials. In this case, the suitable modification of AISI 430 stainless steel by the addition of commercial iron-alloys (Fe-Nb and Fe-B) provided high GFA, which demonstrates the effectiveness of the addition of minor low-cost alloying elements in promoting bulk glass formation. Additionally, the HVOF processing parameters also contributed to the achievement of coatings with high fraction of amorphous phase and low oxidation levels. The oxygen content of the powders and the coatings are 0.07 and 0.41%, respectively. Therefore, it can be concluded that only slight oxidation occurred during the spraying process. SEM observations of the cross-section regions of the coatings are shown in Fig. 5. Some common microstructural features of metallic coatings produced by HVOF spray process can be seen in Fig. 5(a). Unmelted and partial-melted particles are identified along the 280 μm thick as-sprayed coating by their near spherical morphology. Two phases can be identified through EDS analysis in Fig. 5(b): Nb-rich bright particles (point A), and (Fe,Cr)-rich grey lamellae and un-melted particles, point B. Probably, these phases are related to FeNbB and matrix phases, respectively. Fig. 5 also shows the existence of micropores (porosity = 5.7 ± 0.6%). The porosity is divided in several types of pores, depending upon their position [30]: 1) at the junction of lamellar splats; 2) within the matrix of the coating; 3) within the un-melted powder particles; 4) near the un-melted or partially melted powder. The preheating of powders in the supercooled liquid region prior to spraying, and the use of higher energy (kinetic and thermal energy) may be interesting to assure more intimate contact at the splat
Fig. 3. XRD patterns of the melt-spun ribbons, feedstock powders, and as-deposited coatings for the Fe60Cr8Nb8B24 (at.%) nominal alloy.
Fig. 4. DSC curves of the as-spun ribbons, feedstock powders, and HVOF coatings.
boundary, coating densification, and a general increase of the deposition efficiency. The surface topography of the as-sprayed HVOF coatings is shown in Fig. 6. High average surface roughness (Ra = 8.2 ± 0.2 μm) is revealed in Fig. 6(a). Featureless areas correspond to the molten material, which splashed against the substrate. Un-molten droplets can be seen on the surface as spherical protrusions (Fig. 6(b)), with diameters between 20 to 53 μm. The roughness of HVOF coatings can be decreased by applying small feedstock powders and high spraying energy in terms of fuel/oxygen ratio which lead to an increased fraction of fully melted
Fig. 5. Backscattered electron images of the cross-section region of the HVOF coatings: (a) thickness and microstructural constituents, (b) FeNbB and matrix phases. The numbers from 1 to 4 and the letters A and B were included to specify the pore types and the chemical contrasts, respectively.
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that of the substrate API 5L X80. The profile curve of the worn surface of the API 5L X80 steel, Fig. 7(a), shows pronounced grooves compared to the HVOF coatings, Fig. 7(b). As a result, HVOF amorphous coating can be an effective approach to protect API substrates withstanding wear environment and extending their life-time. The coatings exhibit a high hardness over 800 HV0.3, which is about four times higher than that of the API 5L X80 steel substrate (HV0.3 = 222 ± 5) and comparable to other Fe-based amorphous coatings [33– 36]. The dense random packed atomic configurations of the amorphous structure can effectively resist to the plastic deformation caused by the applied load and lead to high hardness values [3]. Moreover, the presence of high resistant borides (Fe2B and FeNbB) embedded in the amorphous matrix also enhances the hardness of the HVOF as-sprayed coatings. Hard borides are beneficial to increase the wear resistance of alloys. However, the effectiveness of the protection against wear promoted by hard M2B type borides and FeNbB particles depends on their volume fraction as well as the distance between them [37]. The increase of the volume fraction of hard borides such as Fe2B and FeNbB and the decrease of the distance between each other provide more protection to the matrix. The effect of the sliding speed on the wear behavior of the amorphous coating and the substrate was also evaluated. It can be seen (as depicted in Table 5) that the wear rates of the API 5L X80 decrease with the increase of the sliding speed. XRD analysis of the debris of API 5L X80 steel collected after pin-on-disc measurements (Fig. 8)
Fig. 6. Topography of as-deposited HVOF coatings surface observed by confocal microscopy (a) and (b) at different magnification.
particles [31]. However, it has been shown that small particles (size smaller than 20 μm) are not suitable for processing with the HVOF thermal spray technique because of their small mass inertia [32]. Moreover, inappropriate fuel/oxygen ratios can increase the overheating which, in turn, decreases the amorphization and increase the oxidation during spraying. While achieving the required quench rate is not difficult, there can be problems with surface roughness and with melting in one region causing partial crystallization of a neighboring amorphized area [33]. 3.2. Wear resistance Table 5 summarizes the results obtained from pin-on-disc measurements carried out on the API 5L X80 substrate and HVOF coatings. Surfaces of the as-sprayed coatings were polished prior to the pin-on-disc tests, and therefore, have a similar surface roughness to API 5L X80 steel samples. It can be seen from Table 5 that the wear rates of the HVOF Fe-based coatings are about 2 orders of magnitude lower than Table 5 Pin-on-disc results for the amorphous coating obtained by HVOF and for the substrate API 5L X80 steel. ΔD/D is the relative change in diameter of the alumina ball after wear tests. Material
ΔD/D (%)
Sliding speed (cm·s−1)
Eroded volume (mm3)
Wear rate (mm3·N−1·m−1)
API 5L X80 (Substrate)
−0.5
HVOF Coating
-2.4
4 10 30 80 4 30
33.3 ± 0.3 17.0 ± 0.3 10.4 ± 0.1 7.0 ± 0.3 0.14 ± 0.01 0.27 ± 0.07
(1.67 ± 0.01) × 10−3 (8.5 ± 0.2) × 10−4 (5.20 ± 0.07) × 10−4 (3.5 ± 0.2) × 10−4 (6.9 ± 0.7) × 10−6 (1.4 ± 0.3) × 10−5
Fig. 7. Surface-profilometer images from worn surface after pin-on-disc measurement for: a) API 5L X80 steel, and b) HVOF coating.
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Fig. 8. XRD pattern of the API 5L X80 debris collected after pin-on-disc measurements.
shows that they were composed by α-Fe and Fe3O4 which suggest that the mechanisms were adhesive wear at low speed and oxidation wear at high speed. Fig. 9a,b show the morphology of the worn surfaces of the API 5L X80 substrate. As shown in Fig. 9(a), the wear mechanism at v = 4 cm·s−1 is adhesive wear which involves considerable plastic deformation and formation of numerous deep furrows. For v = 30 cm·s−1, Fig. 9b, the wear mechanism is adhesive wear together with oxidation wear, as confirmed by the EDS analysis performed on the dark region 1 compared to the bright region 2. The wear rates of HVOF coatings increase with the increase of sliding speed. The volumes of debris from HVOF coatings after pin-on-disc measurements were insufficient, due to their high wear resistance, to conduct XRD analysis. SEM micrographs of the worn surface of HVOF coatings, Fig. 9c,d, indicate similar wear mechanism regardless of the sliding speed. Examination of the morphology of the tracks indicates that the HVOF coatings experienced little plastic deformation with formation of only few plastic furrows and no visible cracking or fracture were observed after the measurements. Plastic deformation patterns on the worn surfaces of highly amorphous Fe-based coatings have been observed [35] and they have been attributed to friction heating which induces crystallization of the amorphous alloys [38]. EDS measurements (Fig. 9c,d) taken from bright regions 4 and 6 compared to the chemical results from dark regions 3 and 5 indicate different
oxidation degrees due to the continual process of oxide removal and oxidation of the exposed surface of the HVOF coating. Despite the insufficient amount of wear debris from the HVOF coating for XRD analysis, SEM and EDS measurements were carried out on the debris located on the alumina spheres after wear test, Fig. 10. The SEM image shows that worn-off debris exhibit a flake-like morphology and the EDS analyses reveal that the debris contains oxygen and elements present in the composition of the BMG (Fe, Cr, and Nb), suggesting materials transfer and oxidation phenomena. The content of light elements such as boron could not be determined by EDS. The analyses of the worn surface, Fig. 9, and debris, Fig. 10, point out that the predominant type of surface-degradation mechanism is the formation and subsequent removal of the oxide layers due to the presence of metallic elements such as Fe, Cr and Nb which are prone to easy oxidation. 4. Conclusions • Fe60Cr8Nb8B24 (at.%) amorphous coatings were produced by HVOF thermal spray process using only commercial precursors. It was demonstrated that highly amorphous metallic coatings can be obtained from primarily industrial grade materials (AISI 430 stainless steel and iron-alloys, Fe-B and Fe-Nb) in a system with relatively few components. • HVOF thermal spraying parameters assured high fraction of amorphous phase (~66%) together with good quality of the coating (high hardness, 838 ± 23 HV0.3, and low oxidation level, ~ 0.41%). The microstructure of the coatings was composed by an amorphous matrix with a small fraction of embedded α-Fe and Fe2B phases. Some micrometric FeNbB borides were also observed heterogeneously distributed in the 280 μm thick coatings. • API 5L X80 substrate exhibited dominant adhesive wear at low speed and oxidation wear at high speed. HVOF amorphous coatings presented predominantly oxidative wear regardless of the sliding speed. The oxidative wear regimes of HVOF coatings led to mild wear compared to the severe adhesive and oxidative wear of the API 5L X80 steel substrate. • HVOF amorphous coatings presented wear rates (1.0 × 10− 5 mm3·N−1·m−1) about 2 orders of magnitude lower than the API 5L
Fig. 9. Backscattered electron images and EDS analyses of the worn surfaces after pin-on-disc measurements of a) API X80 steel at v = 4 cm·s−1, b) API X80 steel at v = 30 cm·s−1, c) HVOF coating at v = 4 cm·s−1, and d) HVOF coating at v = 30 cm·s−1. S.D. (sliding direction).
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Fig. 10. Backscattered electron images of alumina sphere after pin-on-disc measurement in HVOF coating at 30 cm·s−1 and EDS analyses of the debris. S.D. (sliding direction).
X80 steel substrate (8.5 × 10−4 mm3·N−1·m−1) in pin-on-disc tests. The high wear resistance of the HVOF amorphous coatings is probably attributed to their large amorphous content together with the hard boride phases which form a wear resistant metal-ceramic composite.
Acknowledgements The authors gratefully acknowledge the financial support of the Brazilian institutions CAPES, FAPESP project number 2013/05987-8, CNPq (grant number 153269/2013-8), and Petrobras. The Hydro Quebec Canada is gratefully acknowledged for its support to the production of the HVOF coatings and pin-on-disc measurements and analysis. References [1] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater. 48 (2000) 279–306, http://dx.doi.org/10.1016/S1359-6454(99)00300-6. [2] M. Ashby, A. Greer, Metallic glasses as structural materials, Scr. Mater. 54 (2006) 321–326, http://dx.doi.org/10.1016/j.scriptamat.2005.09.051. [3] C. Schuh, T. Hufnagel, U. Ramamurty, Mechanical behavior of amorphous alloys, Acta Mater. 55 (2007) 4067–4109, http://dx.doi.org/10.1016/j.actamat.2007.01.052. [4] H. Habazaki, Corrosion of amorphous and nanograined alloys, Shreir's Corros 2010, pp. 2192–2204, http://dx.doi.org/10.1016/B978-044452787-5.00107-4. [5] G.Y. Koga, R.P. Nogueira, V. Roche, A.R. Yavari, A.K. Melle, J. Gallego, et al., Corrosion properties of Fe–Cr–Nb–B amorphous alloys and coatings, Surf. Coat. Technol. 254 (2014) 238–243, http://dx.doi.org/10.1016/j.surfcoat.2014.06.022. [6] A. Inoue, X.M. Wang, Bulk amorphous FC20 (Fe-C-Si) alloys with small amounts of B and their crystallized structure and mechanical properties, Acta Mater. 48 (2000) 1383–1395, http://dx.doi.org/10.1016/S1359-6454(99)00394-8. [7] W.H. Wang, Roles of minor additions in formation and properties of bulk metallic glasses, Prog. Mater. Sci. 52 (2007) 540–596, http://dx.doi.org/10.1016/j.pmatsci. 2006.07.003. [8] M. Shapaan, A. Bárdos, L.K. Varga, J. Lendvai, Thermal stability and glass forming ability of cast iron–phosphorus amorphous alloys, Mater. Sci. Eng. A 366 (2004) 6–9, http://dx.doi.org/10.1016/j.msea.2003.08.053. [9] H.Y. Jung, S. Yi, Enhanced glass forming ability and soft magnetic properties through an optimum Nb addition to a Fe-C-Si-B-P bulk metallic glass, Intermetallics 18 (2010) 1936–1940, http://dx.doi.org/10.1016/j.intermet.2010.03.011. [10] Z.P. Lu, C.T. Liu, Role of minor alloying additions in formation of bulk metallic glasses: a review, J. Mater. Sci. 39 (2004) 3965–3974, http://dx.doi.org/10.1023/B: JMSC.0000031478.73621.64. [11] C.T. Liu, Z.P. Lu, Effect of minor alloying additions on glass formation in bulk metallic glasses, Intermetallics 13 (2005) 415–418, http://dx.doi.org/10.1016/j.intermet. 2004.07.034. [12] Y.H. Zhao, C.Y. Luo, X.K. Xi, D.Q. Zhao, M.X. Pan, W.H. Wang, Synthesis and elastic properties of amorphous steels with high Fe content, Intermetallics 14 (2006) 1107–1111, http://dx.doi.org/10.1016/j.intermet.2006.01.047. [13] H. Li, S. Yi, Fabrication of bulk metallic glasses in the alloy system Fe–C–Si–B–P–Cr– Mo–Al using hot metal and industrial ferro-alloys, Mater. Sci. Eng. A 449–451 (2007) 189–192, http://dx.doi.org/10.1016/j.msea.2006.02.262. [14] P.H. Tsai, A.C. Xiao, J.B. Li, J.S.C. Jang, J.P. Chu, J.C. Huang, Prominent Fe-based bulk amorphous steel alloy with large supercooled liquid region and superior corrosion resistance, J. Alloys Compd. 586 (2014) 94–98, http://dx.doi.org/10.1016/j.jallcom.2013.09.186. [15] Y. Hu, M.X. Pan, L. Liu, Y.H. Zhao, D.Q. Zhao, W.H. Wang, Synthesis of Fe-based bulk metallic glasses with low purity materials by multi-metalloids addition, Mater. Lett. 57 (2003) 2698–2701, http://dx.doi.org/10.1016/S0167-577X(02)01360-5. [16] T.D. Shen, R.B. Schwarz, Bulk ferromagnetic glasses in the Fe–Ni–P–B system, Acta Mater. 49 (2001) 837–847, http://dx.doi.org/10.1016/S1359-6454(00)00365-7.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and wear behavior of Fe-based amorphous HVOF coatings produced from commercial precursors G.Y. Koga a,⁎, R. Schulz b, S. Savoie b, A.R.C. Nascimento b, Y. Drolet b, C. Bolfarini a, C.S. Kiminami a, W.J. Botta a a b
Universidade Federal de São Carlos, Departamento de Engenharia de Materiais, Rod. Washington Luis, km 235, CEP 13565-905, São Carlos, SP, Brazil Hydro-Quebec Research Institute, 1800 Boul. Lionel Boulet, Varennes, QC J3X 1S1, Canada
a r t i c l e
i n f o
Article history: Received 29 June 2016 Revised 18 October 2016 Accepted in revised form 21 October 2016 Available online xxxx Keywords: Thermal spray coatings Amorphous alloys Commercial precursors Steel Wear testing
a b s t r a c t Wear resistant highly amorphous Fe60Cr8Nb8B24 (at.%) coatings of about 280 μm thickness were prepared through high velocity oxygen fuel (HVOF) thermal spray process onto API 5L X80 steel substrate. Feedstock powders were produced by gas atomization with low purity precursors by modifying AISI 430 stainless steel with additions of niobium (Fe-Nb) and boron (Fe-B). It was found that the coatings were mostly amorphous with some embedded FeNbB and Fe2B borides. The formation of a large fraction of amorphous phase was attributed to the high cooling rates of molten droplets combined with a proper powder composition. The average Vickers hardness of the coating (HV0.3 = 838 ± 23) was about four times higher than that of the API 5L X80 substrate (HV0.3 = 222 ± 5). The excellent wear resistance of the amorphous coating in the pin-on-disc measurements was attributed to its large fraction of amorphous phase (~66%) with reinforcing hard Fe2B and FeNbB borides, high Vickers hardness, low oxygen content (b 0.41%), and relatively low porosity (~5). The wear rates of the amorphous coatings were about two orders of magnitude lower than that of the API 5L X80 steel substrate (1.0 × 10−5 and 8.5 × 10−4 mm3·N−1·m−1, respectively). API 5L X80 steel exhibited dominant adhesive wear at low sliding speed and oxidative wear at high sliding speed. HVOF coatings presented oxidative wear regardless of the sliding speed. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Among the bulk metallic glasses (BMGs) developed in many different alloy systems, Fe-based amorphous alloys have drawn great attention due to their unique physical and chemical properties associated with the amorphous phase and the relatively low cost of iron. Compared with traditional crystalline metallic materials, Fe-based amorphous alloys exhibit high strength, toughness and hardness, and superior corrosion resistance, attributed to their disordered structures as well as the chemical homogeneity [1–5]. All of these properties make this family of alloys potential engineering materials in a wide range of structural applications, which withstand aggressive work conditions. Since bulk glassy alloys were synthesized in Fe-based systems, great efforts have been devoted to find and produce BMGs by using commercial base materials [6–9]. However, despite the use of some low purity precursors, their development has been based on small additions of high cost elements (Y and Er) [7,10,11], high purity of some raw materials [12], and strict processing characteristics (e.g., high vacuum and high cooling rates), which restrict their industrial applications.
⁎ Corresponding author. E-mail address: [email protected] (G.Y. Koga).
For instance, it has been reported that Fe-based BMGs can be produced in Fe-C-Si-B-P-Cr-Mo-Al [13], Fe-Cr-Co-Mo-C-B-Y [14], Fe-CoZr-Mo-W [15], and cast iron based amorphous alloy [8] systems by using industrial stainless steel, low purity elements, and ferrous-alloys; hence reducing significantly the production cost. Nevertheless, these bulk Fe-based glasses are generally complex consisting of five or more components and they have additions of some high purity elements or special process is required to provide high cooling rates in an inert environment [16]. Thus, it is still desirable to improve the glass-forming ability (GFA) of Fe-based alloys in order to enhance their ability to form bulk glassy steels under conventional industrial conditions, using, for example, low vacuum furnace, conventional casting methods, commercialgrade raw material, etc. [17]. Progress has been made in this direction. For instance, Cheney and Vecchio [18] have successfully obtained amorphous alloys in the Fe-Cr-Nb-B system (Fe62Cr8Nb8B22 and Fe63Cr4Nb7B26) by adding small amounts of Nb and B to selected stainless steel. However, splat casting technique was used in this case. Thermal spraying is an interesting technological process to produce amorphous alloys coatings thanks to the high cooling rates (101 ‐ 107 K·s−1) that can be achieved and its low cost compared to other more conventional rapid solidification processes. In addition, metallic coatings can be applied to larger and complex parts and, therefore, can widen the application fields by overcoming the volume restrictions
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imposed by the critical cooling rates [19–21]. Thermal spraying includes several particle deposition techniques, such as flame spraying, arc spraying, plasma spraying, high velocity oxygen fuel (HVOF) [22]. Among them, HVOF sprayed coatings have been widely used to produce Fe-based amorphous protective coatings owing to their superior corrosion and wear resistance compared to the other methods [23–28]. In the present work, the nominal Fe60Cr8Nb8B24 (at.%) alloy was used to produce amorphous coatings through HVOF thermal spray process. The substrate chosen was the API 5L X80 steel. To demonstrate the good GFA and the industrial production potential of the amorphous alloy, the feedstock powers were obtained by gas atomization using only conventional ferrous-alloys additions (Fe-Nb and Fe-B) to adjust the composition of a commercial AISI 430 stainless steel. The wear resistance of the coatings and the substrate (API 5L X80) was evaluated and results were discussed.
Table 2 Spraying parameters employed in the gas atomization processing for powder production. High pressure nitrogen gas atomization Gas flow rate – N2 (m3·min−1) Mass flow rate (kg·min−1) Gas to metal ratio Temperature (°C) Nozzle diameter (mm)
3.84 5.90 0.62 1700 6.0
composition and the microstructure of the substrate are presented in Table 1 and Fig. 1, respectively. Sieved as-atomized particles were used as feedstock powders in HVOF thermal spray process to coat degreased and sand blasted 120 × 30 × 5 mm plates and Ø 63.5 × 6.6 mm discs. The detailed HVOF spraying parameters are listed in Table 3. The structure and morphology of the coatings were examined by scanning electron microscopy (SEM) Philips XL30 FEG equipped with energy dispersive spectroscopy (EDS), confocal microscopy (CM) LEXT 3D Measuring LASER microscopy OLS4100 Olympus, and X-ray diffraction (XRD) analysis as described before. Thermal behavior of the detached coatings was investigated by differential scanning calorimeter (DSC), Netzsch 404, at heating rate of 0.67 K·s− 1. To extract the samples, the coated substrate was held against a grinding wheel which cut it from the substrate towards the deposit. Cutting was stopped near the coating/substrate interface and then the specimen was bent as sharply as possible first to one side then to the other, until the specimen breaks. This procedure generates a shearing stress between the basis metal and the deposit which tends to separate part of the coating from the substrate. Finally, the peeled samples were obtained by using a sharp chisel at the back of the coating overhang. Cross-sectional samples were mounted in conducting resin and examined in the SEM using backscattered electron signal to form images. The surface average roughness measurements were performed in a portable surface roughness tester (Mitutoyo SJ-201P) in 3 different regions. A total of 20 cross-sectional images in different regions of the coatings were taken by ZEISS optical microscope (magnification of 500 ×). Image processing was carried out using AnalySIS Pro software to evaluate the average porosity levels. The oxygen content in the detached coatings was analyzed using the instrument mentioned before. The boron content of the coatings was measured by a Varian AA240FS atomic absorption spectrometer (Mulgrave, Australia). The samples were prepared by dissolving the detached coatings in a 0.002 M H2SO4 solution by using ultrapure water obtained from Milli-Q® purification system (Millipak-40 Filter Unit 0.22 μm NPT, Bedford, MA, USA). Reproducibility of data was ensured by repeating the test in 3 solutions prepared with different detached coatings obtained from different regions.
2. Experimental procedure 2.1. Production of feedstock powders The Fe60Cr8Nb8B24 (at.%) alloy is regarded as having high potential for industrial applications thanks to its high GFA, relatively low cost of the alloying elements, and the possibility to adjust stainless steel composition using minor commercial grade additions [5,18]. Metallic powders with nominal composition of Fe60Cr8Nb8B24 (at.%) were produced by gas atomization process using commercial AISI 430 stainless steel and conventional Fe-Nb and Fe-B ferrous alloys, Table 1. The raw materials were melted in an induction furnace under argon atmosphere and sprayed at 1700 °C by using N2 atomization gas. Table 2 shows the parameters used for the production of metallic powders. The as-atomized powders were sieved to select particles with size ranges between 20 to 53 μm. The morphology and the composition of the feedstock powders were characterized by scanning electron microscopy (SEM) with wavelength-dispersive X-ray spectroscopy (WDS). Phase constituents were identified by using X-ray diffraction (XRD) analysis performed on an X-ray diffractometer Rigaku Geigerflex ME210GF2, with Cu-Kα radiation. The thermal stability was examined in a differential scanning calorimeter (DSC), Netzsch 404, at heating rate of 0.67 K·s−1. The oxygen content was analyzed by Nitrogen/Oxygen determinator LECO TC-436 DR. Amorphous Fe60Cr8Nb8B24 (at.%) ribbons were produced by meltspinning with copper wheel rotating at a speed of 50 m·s−1 in an argon atmosphere. In this case, high purity Fe (99.97%), Cr (99.997), Nb (99.8) elements and commercial Fe-B were used as raw materials. The obtained amorphous ribbons were used as reference in the study of amorphization. 2.2. Production of coatings by high velocity oxygen fuel (HVOF)
2.3. Wear measurements and characterization of worn surface The chosen substrates in this work were extracted from API 5L X80 line pipes (the American Petroleum Institute specification API 5L covers steel line pipes normally used in the petroleum and gas industries which may present aggressive wear environments). The chemical
Vickers hardness measurements in different cross-section regions were performed on the coatings and API 5L X80 steel substrates using a Vickers diamond indenter (Newage Testing Instrument model Auto-
Table 1 Chemical compositions of the precursor materials: AISI 430 stainless steel, Fe-B and Fe-Nb master alloys and the nominal composition of the Fe-Cr-Nb-B alloy (in this case in wt.%, for comparison with the commercial materials). The composition of the API 5L X80 steel used as substrate is also given. Alloy/wt.%
C
Si
Mn
Cr
Ni
S
P
Nb
Co
N
B
Fe
AISI 430 Fe-B Fe-Nb Fe-Cr-Nb-B API 5L X80⁎
0.06 0.30 0.10 0.15 0.07
0.20 0.57 1.10 0.53 0.21
0.74 – – 0.31 1.64
17.62 – – 8.88 0.10
0.37 – – 0.15 0.25
0.17 – 0.10 0.09 –
0.02 – 0.10 0.03 0.01
0.03 – 66.40 15.77 0.04
0.03 – – 0.01 –
0.03 – – 0.01 –
– 16.50 – 5.49 –
Bal. Bal. Bal. Bal. Bal.
⁎ Additional elements: Al = 0.02%, Ti = 0.01%, Mo = 0.13%, Cu = 0.02%.
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3. Results and discussion 3.1. Characterization of feedstock powders and coatings obtained by HVOF
Fig. 1. Optical micrograph of the API 5L X80 steel substrate after polishing and etching (Nital 2%), which shows the refined grain structure typical of a microalloying pipeline steels.
C.A.M.S. Computer-Assisted Microhardness System) with a load of 300 g and dwell time of 15 s. An average value was obtained from at least ten individual measurements from each specimen. The size of the Vickers hardness indentations was large enough to cover several different phases of the coatings and thus the reported Vickers hardness values represent the overall hardness of the coating. Wear tests were performed using pin-on-disc type sliding-wear equipment. The tests were conducted at room temperature and under dry sliding conditions. Pin-on-disc tests were performed on (Ø 63.5 × 6.6 mm) polished disc. The measurements were conducted at constant load of 2 kgf, and sliding distance of 103 m. Wear track radius, R, and the sliding speed, v, were: a) R = 28 mm, v = 80 cm/s, b) R = 25 mm, v = 30 cm/s, c) R = 22 mm, v = 10 cm/s, and d) R = 19 mm, v = 4 cm/s. The wear rate, K, was calculated using the formula K = V/(F.L), where V (mm3) is the total volume of material removed, F(N) is the normal load and L(m) is the total sliding distance. The average cross-sectional surface of the ring was obtained from measurements taken at four different equidistant points along the wear track ring using a Dektak-150 surface profiler (Bruker Corporation, San Jose, CA, USA). Alumina spheres of 1/ 4 inch in diameter (Ø 6.35 mm) were used as counterface pins. Four measurements at each sliding speed were conducted and the values presented are mean values and standard deviations. Morphologies of worn surface were characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and the loose debris characterized by X-ray diffraction.
Fig. 2 shows the SEM image of the feedstock powders used in the HVOF thermal spray process. It can be seen that most of the particles produced by gas atomization are in the desired size range (20 to 53 μm) after proper sieving. Most of them present spherical or nearspherical morphology and smooth surface due to the good fluidity during atomization. WDS analyses conducted on 6 different particles (see Table 4) confirm that the chemical composition of the feedstock powders is consistent with the nominal Fe60Cr8Nb8B24 (at.%) alloy, despite the presence of some impurities from the low purity raw materials. Also, the powder obtained by gas atomization presented low oxidation levels (0.07%). The boron content of the HVOF coatings measured by atomic absorption spectroscopy (5.06 ± 0.06 wt.% B) is in agreement with the measured boron content by WDS, ~ 5 wt.%, of the feedstock powders. Considering that the risk of loss of boron during deposition process is higher than the loss of Fe, Cr, and Nb, the final composition of the coating is considered to be similar to the composition of the feedstock powders. The presence of a large fraction of amorphous phase in the feedstock powders and coatings produced by HVOF was confirmed by XRD results depicted in Fig. 3. XRD patterns of both feedstock powders and coatings exhibit a broad halo peak, but they also present some crystalline peaks in comparison with the XRD patterns of the as-spun ribbons obtained by melt-spinning. These results suggest that the cooling rates of the gas atomization process and HVOF thermal spray route were lower than that of the melt-spinning. The crystalline peaks observed in the XRD patterns of the coatings are associated to Fe2B, FeNbB, and α-Fe solid solution phases. Since localized reheating may occur due to deposition of successive molten droplets, which release latent heat in solidification, some crystallization can occur during the HVOF process [29]. However, it is interesting to note in Fig. 3 that the diffractograms of feedstock powders and the resulting coatings showed crystalline peaks of almost the same intensities; hence it appears that the HVOF thermal spray processing did not induce pronounced crystallization. In spite of the occurrence of some crystalline peaks observed in the XRD patterns of the feedstock powders and HVOF coatings, the DSC curves in Fig. 4 show that the amorphous content in these samples is high. Compared with the as-spun ribbons which can be considered as fully amorphous, the amorphous phase content of the samples can be estimated through the following equation: Vf = ΔHsample/ΔHribbon, where ΔHsample is the crystallization enthalpy of the feedstock powders or coatings and ΔHribbon is the crystallization enthalpy of the as-spun ribbons. Even though the morphologies of the analyzed samples in DSC were different (powder, peeled coatings, and cut ribbons), the obtained curves can be used for semi-quantitative calculations of the
Table 3 High velocity oxygen fuel thermal spray processing parameters. A Praxair Surface Technologies TAFA model JP-8000 HVOF system and a 5220 HVOF gun mounted on a robot arm were used for deposition. HVOF parameters Substrate Powder size range (μm) Oxygen flow (slpm) Kerosene flow (L·h−1) O2/kerosene equivalence ratio Powder carrier Torch velocity (mm·s−1) Standoff distance (mm) Step size (mm) Number of passes Coating thickness (μm)
API 5L X80 steel 20 to 53 896.8 22.1 1.3 Argon 500 330.2 10 5 280
Fig. 2. Secondary electron SEM image of Fe60Cr8Nb8B24 (at.%) feedstock powders obtained by gas-atomization after proper sieving.
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Table 4 Nominal and measured compositions of the feedstock powders. Chemical composition at.%
Fe
Cr
Nb
B
Impurities
Nominal alloy Measured compositiona
60 55 ± 4
8 8.4 ± 0.6
8 6.8 ± 0.3
24 22 ± 5
– Bal.
a Averages and standard deviations calculated from WDS measurements took in 6 different particles.
amorphous content. As a result, the estimated amorphous fraction of the feedstock powders and HVOF coatings is about 85% and 66%, respectively. In addition, the supercooled liquid region (SLR), ΔTx, defined by the difference between the onset temperature of crystallization (Tx) and the glass transition temperature (Tg) is, approximately, 60 K for all samples. The large ΔTx indicates high thermal stability of the supercooled liquid against crystallization, which suggests high glass forming ability of the Fe60Cr8Nb8B24 (at.%) alloy. Moreover, the deviation from nominal composition of the feedstock powers (as depicted in Table 4) and of the resulting coatings produced by HVOF did not affect the glass forming ability of the resulting alloy. The XRD and DSC results suggest that highly amorphous Fe-based alloy coatings can be fabricated using low purity raw materials. In this case, the suitable modification of AISI 430 stainless steel by the addition of commercial iron-alloys (Fe-Nb and Fe-B) provided high GFA, which demonstrates the effectiveness of the addition of minor low-cost alloying elements in promoting bulk glass formation. Additionally, the HVOF processing parameters also contributed to the achievement of coatings with high fraction of amorphous phase and low oxidation levels. The oxygen content of the powders and the coatings are 0.07 and 0.41%, respectively. Therefore, it can be concluded that only slight oxidation occurred during the spraying process. SEM observations of the cross-section regions of the coatings are shown in Fig. 5. Some common microstructural features of metallic coatings produced by HVOF spray process can be seen in Fig. 5(a). Unmelted and partial-melted particles are identified along the 280 μm thick as-sprayed coating by their near spherical morphology. Two phases can be identified through EDS analysis in Fig. 5(b): Nb-rich bright particles (point A), and (Fe,Cr)-rich grey lamellae and un-melted particles, point B. Probably, these phases are related to FeNbB and matrix phases, respectively. Fig. 5 also shows the existence of micropores (porosity = 5.7 ± 0.6%). The porosity is divided in several types of pores, depending upon their position [30]: 1) at the junction of lamellar splats; 2) within the matrix of the coating; 3) within the un-melted powder particles; 4) near the un-melted or partially melted powder. The preheating of powders in the supercooled liquid region prior to spraying, and the use of higher energy (kinetic and thermal energy) may be interesting to assure more intimate contact at the splat
Fig. 3. XRD patterns of the melt-spun ribbons, feedstock powders, and as-deposited coatings for the Fe60Cr8Nb8B24 (at.%) nominal alloy.
Fig. 4. DSC curves of the as-spun ribbons, feedstock powders, and HVOF coatings.
boundary, coating densification, and a general increase of the deposition efficiency. The surface topography of the as-sprayed HVOF coatings is shown in Fig. 6. High average surface roughness (Ra = 8.2 ± 0.2 μm) is revealed in Fig. 6(a). Featureless areas correspond to the molten material, which splashed against the substrate. Un-molten droplets can be seen on the surface as spherical protrusions (Fig. 6(b)), with diameters between 20 to 53 μm. The roughness of HVOF coatings can be decreased by applying small feedstock powders and high spraying energy in terms of fuel/oxygen ratio which lead to an increased fraction of fully melted
Fig. 5. Backscattered electron images of the cross-section region of the HVOF coatings: (a) thickness and microstructural constituents, (b) FeNbB and matrix phases. The numbers from 1 to 4 and the letters A and B were included to specify the pore types and the chemical contrasts, respectively.
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that of the substrate API 5L X80. The profile curve of the worn surface of the API 5L X80 steel, Fig. 7(a), shows pronounced grooves compared to the HVOF coatings, Fig. 7(b). As a result, HVOF amorphous coating can be an effective approach to protect API substrates withstanding wear environment and extending their life-time. The coatings exhibit a high hardness over 800 HV0.3, which is about four times higher than that of the API 5L X80 steel substrate (HV0.3 = 222 ± 5) and comparable to other Fe-based amorphous coatings [33– 36]. The dense random packed atomic configurations of the amorphous structure can effectively resist to the plastic deformation caused by the applied load and lead to high hardness values [3]. Moreover, the presence of high resistant borides (Fe2B and FeNbB) embedded in the amorphous matrix also enhances the hardness of the HVOF as-sprayed coatings. Hard borides are beneficial to increase the wear resistance of alloys. However, the effectiveness of the protection against wear promoted by hard M2B type borides and FeNbB particles depends on their volume fraction as well as the distance between them [37]. The increase of the volume fraction of hard borides such as Fe2B and FeNbB and the decrease of the distance between each other provide more protection to the matrix. The effect of the sliding speed on the wear behavior of the amorphous coating and the substrate was also evaluated. It can be seen (as depicted in Table 5) that the wear rates of the API 5L X80 decrease with the increase of the sliding speed. XRD analysis of the debris of API 5L X80 steel collected after pin-on-disc measurements (Fig. 8)
Fig. 6. Topography of as-deposited HVOF coatings surface observed by confocal microscopy (a) and (b) at different magnification.
particles [31]. However, it has been shown that small particles (size smaller than 20 μm) are not suitable for processing with the HVOF thermal spray technique because of their small mass inertia [32]. Moreover, inappropriate fuel/oxygen ratios can increase the overheating which, in turn, decreases the amorphization and increase the oxidation during spraying. While achieving the required quench rate is not difficult, there can be problems with surface roughness and with melting in one region causing partial crystallization of a neighboring amorphized area [33]. 3.2. Wear resistance Table 5 summarizes the results obtained from pin-on-disc measurements carried out on the API 5L X80 substrate and HVOF coatings. Surfaces of the as-sprayed coatings were polished prior to the pin-on-disc tests, and therefore, have a similar surface roughness to API 5L X80 steel samples. It can be seen from Table 5 that the wear rates of the HVOF Fe-based coatings are about 2 orders of magnitude lower than Table 5 Pin-on-disc results for the amorphous coating obtained by HVOF and for the substrate API 5L X80 steel. ΔD/D is the relative change in diameter of the alumina ball after wear tests. Material
ΔD/D (%)
Sliding speed (cm·s−1)
Eroded volume (mm3)
Wear rate (mm3·N−1·m−1)
API 5L X80 (Substrate)
−0.5
HVOF Coating
-2.4
4 10 30 80 4 30
33.3 ± 0.3 17.0 ± 0.3 10.4 ± 0.1 7.0 ± 0.3 0.14 ± 0.01 0.27 ± 0.07
(1.67 ± 0.01) × 10−3 (8.5 ± 0.2) × 10−4 (5.20 ± 0.07) × 10−4 (3.5 ± 0.2) × 10−4 (6.9 ± 0.7) × 10−6 (1.4 ± 0.3) × 10−5
Fig. 7. Surface-profilometer images from worn surface after pin-on-disc measurement for: a) API 5L X80 steel, and b) HVOF coating.
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Fig. 8. XRD pattern of the API 5L X80 debris collected after pin-on-disc measurements.
shows that they were composed by α-Fe and Fe3O4 which suggest that the mechanisms were adhesive wear at low speed and oxidation wear at high speed. Fig. 9a,b show the morphology of the worn surfaces of the API 5L X80 substrate. As shown in Fig. 9(a), the wear mechanism at v = 4 cm·s−1 is adhesive wear which involves considerable plastic deformation and formation of numerous deep furrows. For v = 30 cm·s−1, Fig. 9b, the wear mechanism is adhesive wear together with oxidation wear, as confirmed by the EDS analysis performed on the dark region 1 compared to the bright region 2. The wear rates of HVOF coatings increase with the increase of sliding speed. The volumes of debris from HVOF coatings after pin-on-disc measurements were insufficient, due to their high wear resistance, to conduct XRD analysis. SEM micrographs of the worn surface of HVOF coatings, Fig. 9c,d, indicate similar wear mechanism regardless of the sliding speed. Examination of the morphology of the tracks indicates that the HVOF coatings experienced little plastic deformation with formation of only few plastic furrows and no visible cracking or fracture were observed after the measurements. Plastic deformation patterns on the worn surfaces of highly amorphous Fe-based coatings have been observed [35] and they have been attributed to friction heating which induces crystallization of the amorphous alloys [38]. EDS measurements (Fig. 9c,d) taken from bright regions 4 and 6 compared to the chemical results from dark regions 3 and 5 indicate different
oxidation degrees due to the continual process of oxide removal and oxidation of the exposed surface of the HVOF coating. Despite the insufficient amount of wear debris from the HVOF coating for XRD analysis, SEM and EDS measurements were carried out on the debris located on the alumina spheres after wear test, Fig. 10. The SEM image shows that worn-off debris exhibit a flake-like morphology and the EDS analyses reveal that the debris contains oxygen and elements present in the composition of the BMG (Fe, Cr, and Nb), suggesting materials transfer and oxidation phenomena. The content of light elements such as boron could not be determined by EDS. The analyses of the worn surface, Fig. 9, and debris, Fig. 10, point out that the predominant type of surface-degradation mechanism is the formation and subsequent removal of the oxide layers due to the presence of metallic elements such as Fe, Cr and Nb which are prone to easy oxidation. 4. Conclusions • Fe60Cr8Nb8B24 (at.%) amorphous coatings were produced by HVOF thermal spray process using only commercial precursors. It was demonstrated that highly amorphous metallic coatings can be obtained from primarily industrial grade materials (AISI 430 stainless steel and iron-alloys, Fe-B and Fe-Nb) in a system with relatively few components. • HVOF thermal spraying parameters assured high fraction of amorphous phase (~66%) together with good quality of the coating (high hardness, 838 ± 23 HV0.3, and low oxidation level, ~ 0.41%). The microstructure of the coatings was composed by an amorphous matrix with a small fraction of embedded α-Fe and Fe2B phases. Some micrometric FeNbB borides were also observed heterogeneously distributed in the 280 μm thick coatings. • API 5L X80 substrate exhibited dominant adhesive wear at low speed and oxidation wear at high speed. HVOF amorphous coatings presented predominantly oxidative wear regardless of the sliding speed. The oxidative wear regimes of HVOF coatings led to mild wear compared to the severe adhesive and oxidative wear of the API 5L X80 steel substrate. • HVOF amorphous coatings presented wear rates (1.0 × 10− 5 mm3·N−1·m−1) about 2 orders of magnitude lower than the API 5L
Fig. 9. Backscattered electron images and EDS analyses of the worn surfaces after pin-on-disc measurements of a) API X80 steel at v = 4 cm·s−1, b) API X80 steel at v = 30 cm·s−1, c) HVOF coating at v = 4 cm·s−1, and d) HVOF coating at v = 30 cm·s−1. S.D. (sliding direction).
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Fig. 10. Backscattered electron images of alumina sphere after pin-on-disc measurement in HVOF coating at 30 cm·s−1 and EDS analyses of the debris. S.D. (sliding direction).
X80 steel substrate (8.5 × 10−4 mm3·N−1·m−1) in pin-on-disc tests. The high wear resistance of the HVOF amorphous coatings is probably attributed to their large amorphous content together with the hard boride phases which form a wear resistant metal-ceramic composite.
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