|Januaris Pane||1. Postgraduate Program of Physics, Faculty of Mathematic and Natural Science, Universitas Sumatera Utara, North Sumatera 20155-Indonesia 2. Pendidikan Fisika, FKIP, Universitas HKBP Nommensen Medan,|
|Dedi Holden Simbolon||1. Postgraduate Program of Physics, Faculty of Mathematic and Natural Science, Universitas Sumatera Utara, North Sumatera 20155-Indonesia. 2. Department of Primary School Teacher Education, FKIP, Uni|
|Hubby Izzudin||Research Center for Advanced Material, National Research and Innovation Agency, Kompleks PUSPIPTEK Serpong, South Tangerang, Banten 15310-Indonesia|
|Ahmad Afandi||Research Center for Advanced Material, National Research and Innovation Agency, Kompleks PUSPIPTEK Serpong, South Tangerang, Banten 15310-Indonesia|
|Bambang Hermanto||Research Center for Advanced Material, National Research and Innovation Agency, Kompleks PUSPIPTEK Serpong, South Tangerang, Banten 15310-Indonesia|
|Kerista Sebayang||Postgraduate Program of Physics, Faculty of Mathematic and Natural Science, Universitas Sumatera Utara, North Sumatera 20155-Indonesia|
|Syahrul Humaidi||Postgraduate Program of Physics, Faculty of Mathematic and Natural Science, Universitas Sumatera Utara, North Sumatera 20155-Indonesia|
|Marhaposan Situmorang||Postgraduate Program of Physics, Faculty of Mathematic and Natural Science, Universitas Sumatera Utara, North Sumatera 20155-Indonesia|
Advanced power generation will be operated at higher temperatures and pressure to achieve higher efficiency and reduce CO2 emission. This may significantly impact the use of carbon steel that previously has been used in boiler fabrication. In this study, a flame spraying technique was applied to develop a highly resistant coating of MoSi2 added FeCrAlTiY on ST41 steel to improve its oxidation resistance. Four variations of MoSi2 concentration as 0, 10, 20 and 30 in mass% were prepared to investigate the effect of its addition on the cyclic oxidation resistance of FeCrAlTiY coating at 700oC for 8 cycles. The phase composition and microstructure of the coating before and after the oxidation test were analyzed using XRD and SEM, respectively. While the element distribution along the coating was characterized using an EDX. According to the results, partially and fully melted particles, oxides and pores are present in the coatings. It becomes more porous with the increase of MoSi2 concentration. The oxidation test results indicate that the FeCrAlTiY with 10 mass% MoSi2 addition exhibits the lowest mass gain (0.217 mg/mm2) compared to that of MoSi2-free coating (0.261 mg/mm2) and FeCrAlTiY coating with 20 and 30 mass% MoSi2 (0.297 and 0.308 mg/mm2, respectively). As the MoSi2 concentration increases, its addition leads to the deterioration of FeCrAlTiY coating oxidation resistance. The results suggest that FeCrAlTiY-10 mass% MoSi2 is the most resistant coating to cyclic oxidation at 700oC in air and can be applied as a protective coating in advanced power generation.
Carbon Steel; FeCrAlTiY; Flame Spraying; MoSi2; Oxidation
In order to achieve higher efficiency and reduce CO2 emissions, the working fluid of the boiler will be operated at higher temperatures and pressures. In the next decade, Advanced ultra-supercritical power plants (A-USC) are expected to enter operation and will approach 50% net electricity generation efficiency with the use of advanced metal alloys capable of withstanding steam temperatures and pressures over 700oC and 350 bar (Tramošljika et al., 2021). The usage of carbon steels that are widely used for a wide range of industrial applications, such as boiler and pressure vessel fabrication, may become limited when applied at high temperatures and oxidative atmospheres. The Fe element tends to be easily oxidized to form thick Fe-oxide layers. Accordingly, surface treatment is required (Simbolon et al., 2020).
It is well known that surface structure modification as surface hardening (Ismail and Taha, 2014) and coating deposition has been widely used to reduce part production costs (Luo et al., 2014), to improve wear resistance, hardness, or to act as environmental barrier coating (EBC) for critical components, resulting in higher oxidation or corrosion resistance (Singh et al., 2022). Thus, the selection of appropriate coating material and composition becomes particularly important (Saraswati, Nugroho, and Anwar, 2018). The FeCrAlY was receiving attractive attention for high-temperature oxidation and corrosion resistance applications because of the formation of a protective scale in the external layer (Wessel et al., 2004; Bennett and Bull, 1997). In addition, the selection of the appropriate coating method largely determines the final coating properties. Thermal spray technology as HVOF and flame spraying, offers some advantages, including it can be applied for various kinds of materials, creating near net shapes of nanostructured or nanocomposite, and also being easy to use (Simbolon et al., 2020; Sofyan et al., 2010).
In order to solve the aforementioned issues, alloying elements may be added to the mix. Several studies have reported the beneficial effect of some elements as Si and Mo, in improving the properties of metals and alloys. Inoue et al. (2018) reported that an adequate amount of Si addition to stainless steel could promote the formation of SiO2 layer at the substrate/oxide interface. The formed SiO2 can also act as the nucleation site for the formation of Cr2O3 scales (Nikrooz et al., 2012). Saito et al. (1998) also pointed out that the crystalline SiO2 can act as a heterogeneous nucleation site and accelerates the ?-Al2O3 to ?-Al2O3 transformation. While the appropriate amount of Mo content has beneficial effects in improving oxidation resistance (Yao et al., 1999). Many researchers have shown an increased interest in attractive silicides material of MoSi2 for high-temperature resistance applications due to its properties such as high melting temperature, low density, high hardness and excellent oxidation resistance (Wen and Sha, 2018; Zhang et al., 2019).
Based on the points presented above, this paper attempts to consider the addition of MoSi2 to improve the FeCrAlTiY oxidation resistance. The primary aim of the present work is to investigate the effect of MoSi2 addition on the high-temperature oxidation behavior of flame-sprayed FeCrAlTiY coatings deposited on the surface of low carbon steel at 700oC.
FeCrAlTiY-MoSi2 coatings on carbon steel ST41 were prepared by using a flame spray technique. The sample and coating preparation procedures were conducted according to our previous study (Simbolon et al., 2020). Here, we briefly repeated the experiment. The commercial ST41 steel was cut into coupons of about 10 × 10 × 3 mm (see Figure 1a). A hole was made in the top part of the sample to hang it during the coating process. The substrate surface was ground to SiC papers and cleaned in an ultrasonic bath using an ethanol solution. It was dried and then sandblasted using coarse brown fused alumina.
The commercial powders of FeCrAlTiY-Sandvik Materials Technology Ltd. (Fe, 17.2 Cr, 6.6 Al, 0.58 Ti, 0.47 Ni, 0.36 Mn, 0.097 Y, 0.097 Cu) with an average size of about 106 µm and MoSi2-Japan New Metals Co. Ltd. (Mo, 35.8-37.8 Si, ? 0.05 C, ? 0.23 Fe, ? 0.5 O) with the average size of about 5~10 µm were selected as powder coating. To investigate the influence of MoSi2 content on the oxidation resistance of FeCrAlTiY coatings, four different coating compositions such as FeCrAlTiY coating with 0, 10, 20 and 30 mass % MoSi2 were deposited on the all surfaces of ST41 (see Figure 1b) using a Metallisation Flamespray MK74. Before coating, each composition was mixed by rotary milling for 30 min. Here, the pressure of oxygen and acetylene for coating deposition was adjusted to 2.07 and 0.83 bars, respectively and the compressed air pressure was set to 1.34 bars. The distance between the sample and the spray gun was kept constant carefully of about 20 cm.
Figure 1 The schematic images of (a) ST41 without coating and (b) FeCrAlTiY-MoSi2 coatings on ST41
The resistance of coatings toward oxidation at high temperatures was evaluated according to the mass gain of the oxidized sample. All coated samples were placed in an alumina crucible separately. The samples were then cyclically oxidized in static air of a muffle furnace at 700oC eight times. One cycle consists of 20 h exposure at 700oC, and 4 h cooled down to room temperature. The mass gain of the samples at each cyclic time was periodically measured using an electronic balance with an accuracy of 0.01 mg. The aforesaid temperature was chosen to represent the operational temperature of A-USC.
The phase composition of
the coatings was determined by using X-ray diffraction (Rigaku Smartlab). In
order to examine the oxide scales formed on the coating surface, the XRD
analysis was performed on the oxidized surface.
The microstructure evolution and compositional analysis of
cross-sectional samples were investigated using a scanning electron microscope
(SEM Hitachi SU3500) and energy dispersive X-ray spectrometer (EDX Horiba),
respectively. In this study, the microstructure of oxidized coated sample was
also compared with the substrate without a coating which was oxidized with the
same conditions as described above.
3.1. XRD and SEM Analysis of FeCrAlTiY and MoSi2 Powders
The X-ray diffraction patterns and the SE SEM images of FeCrAlTiY and MoSi2 powders used in this study are presented in Figure 2 and 3, respectively.
Figure 2 X-ray diffraction patterns of (a) FeCrAlTiY and (b) MoSi2 powders
Figure 3 SE SEM images of (a) FeCrAlTiY and (b) MoSi2 powders
It can be seen that the original FeCrAlTiY and MoSi2 powders show spherical and irregular morphology, respectively.
3.2. Phase Constituent and Microstructure of FeCrAlTiY-MoSi2 Coatings
The phase structures of FeCrAlTiY-MoSi2 coatings on ST41 are shown in Figure 4. The FeCrAlTiY coating without MoSi2 addition is composed by FeCr [DB Card No.: 00-034-0396], Fe(Cr, Al)2O4 [DB Card No.: 00-003-0873] and FeO [DB Card No.: 00-074-1884]. Meanwhile, other new phases such as MoSi2 [DB Card No.: 00-081-2167] and Mo5Si3 [DB Card No.: 00-008-0429] are found in the FeCrAlTiY coating with different content of MoSi2. The intensity of the FeCr phase in the coatings tends to decrease with the addition of MoSi2. In contrast, the oxides of Fe, Cr and Al are likely to form in the coating. This could be due to the fact that the oxides are covering the metallic compounds. The presence of oxides in the coating implies that some coating elements are more favored to oxidize during coating preparation to form FeO and Fe(Cr, Al)2O4. It seems that Al2O3 and Cr2O3 scales were formed firstly during flame spraying because of a strong affinity of Al and Cr for oxygen. The consumption of Al and Cr leads to its coating element depletion. Accordingly, the Fe element was then preferentially oxidized to form FeO. Some amount of FeO was then reacted with Al2O3, Cr2O3 and oxygen, forming Fe(Cr, Al)2O4.
Figure 4 XRD patterns of FeCrAlTiY coatings with (a) 0, (b) 10, (c) 20 and (d) 30 mass% MoSi2
It is worth noting that the Mo5Si3 phase with intensity difference is observed in the FeCrAlTiY-MoSi2 coatings. Its formation may proceed due to the oxidation reaction of the MoSi2 phase through the following reaction,
5 MoSi2 + 7 O2(g) à 7 SiO2 + Mo5Si3 (1)
as reported by previous studies (Zhu et al., 2022; Hansson et al., 2004). Si from the MoSi2 phase was consumed and reacted with oxygen to form SiO2 and Mo5Si3. However, as shown in Figure 4 b, c and d, the SiO2 peaks’ reflection is not observed. This may be attributed to the fact that the oxide is in the amorphous phase, so it is not detected by X-ray diffraction (Chakraborty, 2016).
Figure 5 shows the typical SEM cross-sectional microstructures of FeCrAlTiY-MoSi2 sprayed coatings on low carbon steel using the flame spray technique.
Figure 5 Cross-sectional SE SEM images of FeCrAlTiY coatings with (a) 0, (b) 10, (c) 20 and (d) 30 mass% MoSi2
The thickness of the coating is about 280-350 µm. According to the cross-sectional images, as shown in Figure 5, the coating appears to have typical lamellar structures of thermal spray coating containing fully melted particles and partially deformed particles, oxides and pores. From Figure 5b, c and d, it can also be seen that with the increase of MoSi2 content, the coatings are susceptible to pores formation. This might be related to the high melting point of MoSi2 (Wen and Sha, 2018). The existence of unmelted, partially, and resolidified particles melted particles creates pores within the coating (Khalesi et al., 2021). It can be seen that 30 MoSi2 addition essentially changes the coating microstructure. Pores are also found at the coating/substrate interface. Mostly, the coating porosity and adherence will significantly affect its oxidation resistance. The presence of pores and cracks can act as a short circuit path for inward oxygen diffusion, resulting in reducing the sample oxidation resistance.
To examine the distribution of elements in the FeCrAlTiY-MoSi2 coatings, we performed BSE SEM and EDX elemental maps analysis in the white striped area of Figure 5, with the results as presented in Figure 6. Mostly, the BSE comp image of black, dark and bright areas indicates pore, oxide and metallic phases, respectively. From EDX elemental maps of FeCrAlTiY coating with different content of MoSi2, one of the results show the presence of oxygen distribution in the coating as a mark with green color which reveals that some coating elements are already oxidized during coating preparation, supporting the results of the XRD analysis as explained above. As shown in Figure 6a, the coating is mostly composed of dark areas and bright areas. The dark area consists of Fe, Cr, Al and O (See Figure 6a). It seems to be Fe(Cr, Al)2O4 and FeO phases according to the results of X-ray diffraction analysis. The grey area is composed mainly of Fe and Cr, with a trace amount of
Al that should be the FeCr phase containing Al.
Meanwhile, the microstructure of FeCrAlTiY coatings with 10, 20 and 30 mass% MoSi2 coatings is almost similar to that of MoSi2-free coating that consists mainly of a dark area of Fe, Cr, Al and O, and the grey area of Fe, Cr and Al, for example, as shown in Figure 6(b) for 10 mass% MoSi2 addition. A bright area of Mo and Si is observed in FeCrAlTiY coatings with varying amounts of MoSi2 that are suspected to be MoSi2 and Mo5Si3 phases. Mo5Si3 usually forms at the SiO2 and MoSi2 interface due to Si consumption (Hansson et al., 2004). However, it is important to note that in the external layer of 30 mass% MoSi2 coatings, a continuous oxygen distribution can be distinguished. This is one indication that the surface of FeCrAlTiY-30 mass% MoSi2 coating is covered by an oxide layer that formed during coating preparation. Even if the formation of oxide during coating preparation prefers to be avoided, its formation may give a potential benefit in providing temporary oxidation protection.
3.3. Oxidation Behavior of FeCrAlTiY-MoSi2 Coatings
Figures 7a and b compare the mass gain per unit area and the square of mass gain per unit area of the FeCrAlTiY-MoSi2 coatings as a function of cyclic oxidation time exposed at 700oC for up to 8 cycles in air atmosphere. The results as shown in Figure 7a show a clear trend of an increasing mass gain of FeCrAlTiY-MoSi2 coatings along the oxidation period. In the initial stage of oxidation, the increase of mass gain is high until the oxide scale is developed on the external layer. Subsequently, the increase of mass gain tends to be slow because the growth of the oxide layer is controlled by cations and anions diffusion across the oxide layer. It can be seen in Figure 7b that the square of mass gain per unit area of the samples tends to increase linearly with cyclic oxidation time, suggesting that the oxidation of FeCrAlTiY-MoSi2 coating obeys the parabolic rate law.
Figure 7 The mass gain per unit areas and square of mass gain per unit areas of FeCrAlTiY MoSi2 coatings after exposure at 700oC for 8 cycles in air
The oxidation kinetic of FeCrAlTiY coatings varied depending on MoSi2 concentration. For 100% FeCrAlTiY coating, the mass gain of the sample is about 0.261 mg/mm2. The 10 mass% MoSi2 addition leads to lowering the mass gain of FeCrAlTiY coating to 0.217 mg/mm2. In contrast, the addition of 20 and 30 mass% MoSi2 appears to enhance the mass gain of FeCrAlTiY coating into 0.297 and 0.308 mg/mm2, respectively. The mass gain of 30 MoSi2 addition is smaller compared to the other components in the first cycle. It then gradually increases for up to eight cycles, resulting in a larger mass gain than other coatings. This is probably due to the formation of the external oxide layer on the FeCrAlTiY-30MoSi2 coating plays a role in providing temporary protection in the initial stage of oxidation. However, after a certain time of exposure, the oxide layer was degraded, promoting accelerated oxidation. As a result, the mass gain of 30 mass% MoSi2 coating is higher than the other samples. The results, as presented above, suggest that 20 and 30 mass% MoSi2 additions are considerably worse to the oxidation resistance of FeCrAlTiY coating. Meanwhile, the addition of 10 mass% MoSi2 has a beneficial effect in improving the oxidation resistance of FeCrAlTiY coating.
3.4. Phase Constituent and Microstructure of FeCrAlTiY-MoSi2 Coatings After Oxidation
Figure 8 shows the XRD patterns of FeCrAlTiY-MoSi2 coatings fabricated by a flame spray technique oxidized at 700oC for 8 cycles. The XRD measurement was performed on the surface of the oxidized coating.
Figure 8 XRD patterns of FeCrAlTiY coatings with (a) 0, (b) 10, (c) 20 and (d) 30 mass% MoSi2 after exposure at 700oC for 8 cycles in air
The main phase was detected in the FeCrAlTiY coating after oxidation, namely Fe2O3 [DB Card No.: 00-080-2377]. The reflection of coating peaks is not detected in Figure 8a. This is due to a thickening of the oxide layer in the external layer of FeCrAlTiY coating. As a result, X-ray diffraction could not reach the coating surface. On the other hand, there is no significant difference in the coating structure of FeCrAlTiY coating with 10, 20 and 30 mass% MoSi2 after exposure at 700oC for 8 cycles. The coatings form Fe2O3 and Fe(Cr, Al)2O4 after oxidation. The peak coating reflection as FeCr, MoSi2 and Mo5Si3 can still be observed, as shown in Figure 8 b, c and d. But it tends to decrease with the increase of MoSi2 addition in the coatings. This evidence suggests that the oxide scale of FeCrAlTiY coating without MoSi2 addition is thicker than that of MoSi2 addition coatings. Moreover, as MoSi2 content in the coating increase from 10 to 30 mass%, the thickness of the oxide layer is likely to increase.
The microstructure transformation of FeCrAlTiY-MoSi2 coatings after the oxidation test at 700oC for 8 cycles can be observed in Figure 9. It can be seen in 100% FeCrAlTiY coating a thick oxide layer with a thickness of about 130 µm is formed on the coating surface, resulting in a high sample mass gain compared to the 10 mass% MoSi2 coatings. A severe crack formation is also observed in the formed oxide layer and at the oxide/coating interface. In addition, similarly to the other coating composition, pores are still found within the coating layer. The presence of cracks and interconnected pores can act as freeways or paths for the oxidizing gases to penetrate and/or cation diffusion in the coating, oxidizing it and accelerating its degradation (Simbolon et al., 2020; Esmaeil, Nicolaie, and Shrikant, 2019a; Esmaeil, Nicolaie, and Shrikant, 2019b).