- Email: [email protected]
Catalytic behavior of LaFeO3 pervoskite oxide during low-pressure gas nitriding
Applied Surface Science 506 (2020) 145045
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Catalytic behavior of LaFeO3 pervoskite oxide during low-pressure gas nitriding
T
⁎
Chengsong Zhanga, , Yun Wanga, Xing Chena, Hongtao Chenb, Yeqiong Wuc, Yixue Wangd, Lina Tange, Guodong Cuia, Dazhi Chena a
School of Material Science and Engineering, Southwest Jiaotong University, Chengdu 610031, PR China School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, PR China c Chengdu Hangli Industrial Co., Ltd., Chengdu 611930, PR China d School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China e Shanghai Aerospace Equipments Manufacturer Co., Ltd., Shanghai 200000, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Low-pressure gas nitriding LaFeO3 perovskite oxide Rare earth Catalytic behavior
The LaFeO3 perovskite oxide has been proved to be an effective catalyst in the gas nitriding. However, its catalytic efficiency was sensitive to the nitriding parameters. In order to clarify the catalytic conditions of the LaFeO3 oxide, the low-pressure gas nitriding was carried out in the present work. The effects of gas pressure and ammonia flow rate on catalytic behavior of the LaFeO3 oxide were discussed. The catalytic feature and conditions of the LaFeO3 oxide was identified. The microstructure and phase composition of nitrided layers were analyzed by the optical microscope, transmission electron microscopy, scanning electron microscope and X-ray diffraction, respectively. The microhardness profiles of nitrided layers were measured by micro-hardness tester to evaluate catalytic efficiency of the LaFeO3 oxide. The X-ray photoelectron spectroscopy was applied to reveal the catalytic behavior. The results show that the LaFeO3 perovskite oxide exhibited an excellent catalytic activity via accelerating dissociation of ammonia molecules and diffusion of nitrogen atoms. The dehydrogenation reaction of ammonia molecules was the rate-determining step for the rare earth nitriding, which was greatly affected by gas pressure and ammonia flow rate. The LaFeO3 catalyst shows an excellent durability of catalytic activity in low pressure and low flow rate conditions.
1. Introduction Gas nitriding is an effective thermo-chemical treatment that could dramatically improve the surface properties of steels [1]. However, the extensive application of gas nitriding was restricted due to its low efficiency. Conventional gas nitriding at 530 °C for 15 h only generates a nitrided layer with the thickness of 258 μm [2]. Increasing the nitrided temperature is a common method for a thicker nitrided layer, but it inevitably leads to performance degradation [3,4]. In order to improve the nitriding efficiency, many methods have been developed, aiming at activating the nitrided surface or accelerating the diffusion rate of nitrogen atoms. The nitriding efficiency of iron and steels could be improved by surface nanocrystallization with surface mechanical attrition treatment (SMAT) [5,6]. The significantly increased density of dislocations provided massive diffusion routes for nitrogen atoms [7]. A surface pre-treatment by cathode sputtering has a similar effect as SMAT [8]. Hu et al. reported that the thickness of nitrided layers could
⁎
be increased through pre-oxidation treatment [9]. The oxides formed on the nitrided surface could increase the surface free energy and nitrogen potential. [10]. All these new technologies mentioned above need a pretreatment before nitriding, which make the procedure tedious. Especially, SMAT pretreatment will result in a rough nitrided surface due to the severe plastic deformation on the surface [11]. In order to simplify the process, altering the process parameters or adding catalytic agents are applied to improve the nitriding efficiency. A pressurized gas nitriding with significantly accelerated nitriding efficiency was reported because of the faster reaction rates and the optimized nitrogen potential on nitrided surfaces under higher pressure [12,13]. Meanwhile, the rare earth (RE) elements are usually used as catalyst for thermo-chemical treatments. The RE reagents can be easily added into the furnace with the nitriding atmosphere and present an obviously catalytic function [14–16]. Although the effectiveness of RE reagents has been proved in many literatures [17–19], the catalytic mechanism has not been understood yet. Many researchers attribute the
Corresponding author. E-mail address: [email protected] (C. Zhang).
https://doi.org/10.1016/j.apsusc.2019.145045 Received 15 September 2019; Received in revised form 14 November 2019; Accepted 10 December 2019 Available online 12 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
catalytic effect to the micro-alloy function of RE elements which accelerate the absorption, transfer and diffusion of nitrogen atoms on the treated surface [20,21], however, the catalytic RE species is not detected on the nitrided surfaces and in the nitrided layers. Until now, all the suggested catalytic mechanisms of RE elements are just simple phenomenological inferences based on the existed experimental data. It is hard to clarify in which process and how do the catalytic RE species work in detail. Fortunately, in our previous studies, an ABO3-type perovskite oxide, i.e. LaFeO3, was proved to have catalytic effect during nitriding [21,22]. The catalytic mechanism of the LaFeO3 perovskite oxide was attributed to the promotion of the decomposition of ammonia gas and the dissolution of nitrogen atoms caused by the oxygen vacancies in the LaFeO3 oxide [21]. Meanwhile, we also found that the catalytic efficiency of the LaFeO3 oxide was sensitive to the ammonia flow rate and was weaken under higher gas flows. It indicated that the catalytic efficiency of the LaFeO3 oxide was strongly dependant on the nitriding potential in atmosphere. In order to clarify the suitable catalytic conditions for the LaFeO3 oxide, the low-pressure gas nitriding was carried out with the LaFeO3 coating at different ammonia flow rates. In present work, the nitrogen potential was controlled by gas pressure and ammonia flow rate. The effects of gas pressure and ammonia flow rate on catalytic behavior of the LaFeO3 oxide were evaluated. The catalytic mechanism was discussed. Thus, the optimized catalytic condition for the LaFeO3 oxide would be summarized.
Fig. 1. Schematic illustration of low-pressure gas nitriding used in this study.
Table 1 Detailed parameters of low-pressure gas nitriding.
2. Experimental 2.1. Materials and pretreatments AISI 4140 steel, with the chemical compositions of (wt. %) 0.41C, 0.22Si, 0.67Mn, 1.06Cr, 0.02Ni, 0.16Mo, 0.02Cu, 0.01P, 0.001S and Fe in balance, was used in this work. The AISI 4140 steel was austenitized at 860 °C for 30 min and then quenched in oil to obtain the homogeneous substrate. After that, the steel bar was cutted into the size of ϕ18 × 4 mm and the samples were mechanically polished using silicon carbide papers from 240 to 400 grade. All polished samples were ultrasonically cleaned in the ethyl alcohol and then dried before coating LaFeO3 film. The LaFeO3 coating was synthesized by the sol-gel method [23]. The sol-gel precursor solution of LaFeO3 was prepared in the same way as our previous work [21], and coated on the clean surfaces by a spincoating method before gas nitriding [24].
Specimens
Nitriding temperature (°C)
Nitriding time (h)
NH3 (L/ min)
Pressure (MPa)
LPN-1 LPN-2 LPN-3 LPN-4 LPN-5 LPN-6 LPN-7 LPN-8 LPN-9 LPN-10 LPN-11 LPN-12 LPN-13 LPN-14 LPN-15 LPN-16 LPN-17 LPN-18 LPN-19 LPN-20
550
2
0.1 0.2 0.5 1.0 0.1 0.2 0.5 1.0 0.1 0.2 0.5 1.0 0.1 0.2 0.5 1.0 0.1
0.01
1 4 8 16
0.02
0.05
0.1
0.01
7001F, Japan) equipped with an energy dispersive X-ray analyzer (EDS). The phase compositions in the nitrided layer were detected by using X-ray diffraction (XRD, X’pert pro, Netherlands) with Cu-Kα radiation. The glancing angle was from 20 to 100° and the accelerating voltage was set to 40 kV. Microhardness profiles of nitrided layers were measured by a microhardness tester (HVS-1000, China) under an indentation load of 100 gf for 15 s. In order to insure the accuracy of the microhardness testing, four microhardness data were tested in each position at same depth and the average values were used. The depth of effective hardened layers (EHL) was evaluated by using the same method in our previous work [21]. To clarify the catalytic mechanism and behavior of the LaFeO3 oxide in low-pressure, X-ray photoelectron spectroscopy (XPS, Thermo Scientifc K-Alpha spectrometer) was applied to analyze the chemical state of elements on the nitrided surface. A transmission electron microscopy (TEM, JEM-2100, Japan) was utilized for understanding the existed state of the LaFeO3 oxide on the nitrided surface. The phase composition in the micro area was identified by selected area electron diffraction (SAED). In order to protect the LaFeO3 coating on the nitrided surface, a thin foil with thickness of 40 μm was prepared by mechanically polishing and nitrided with the LaFeO3 coating. Then, the nitrided thin foil was reduced by ion thinning and observed by TEM. To avoid the embrittlement of the nitrided thin foil, the nitriding process was carried out at 550 °C for 1 h.
2.2. Low-pressure gas nitriding The low-pressure gas nitriding (LPN) treatments were conducted in a vacuum quartz tube furnace at 550 °C for 1–16 h with and without LaFeO3 coating. The Schematic illustration of LPN treatments was shown in Fig. 1 and the detailed parameters were listed in Table 1. Firstly, the pretreated samples were heated to 550 °C in air and the heating rate was set to 14 °C/min. During the heating process, the LaFeO3 coating would in-situ form on the surface of samples. Then the chamber was evacuated to lower than 0.01 MPa by a rotary pump when the temperature reached to 550 °C. After that, the ammonia with the flow rate of 0.1 to 1 L/min was induced into the furnace and the gas pressure was set to 0.01–0.1 MPa by controlling the vacuum valves. Finally, all specimens were slowly cooled to room temperature in ammonia after gas nitriding. The samples without coatings were nitrided under the same conditions for comparison. 2.3. Characterization After LPN treatments, the cross-sectional morphologies of nitrided layers were observed by using an optical microscopy (Olympus GX51F, Japan). The corresponding surface morphologies and elemental distributions were observed by scanning electron microscope (SEM, JSM2
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 2. Microhardness profiles of nitrided layers treated at 550 °C for 2 h with and without LaFeO3 coating.
pressure and ammonia flow rate. The gas pressure has more pronounced effect than that of the ammonia flow rate, which indicates that the nitriding potential is mainly determined by the gas pressure during the low-pressure gas nitriding. Under each ammonia flow rate, there is a fold point for the gas pressure (see dash lines in Fig. 3). When the gas pressure surpasses the fold point, the depth of EHL and the surface hardness closes to a constant. It indicates that the equilibrium state or saturated state of nitrogen concentration on the nitrided surfaces have achieved. It is worthy to note that the fold line moves to low gas pressure with the addition of the LaFeO3 oxide, which demonstrates that the LaFeO3 oxide can accelerate the decomposition of ammonia and increase the nitriding potential rapidly to reach the equilibrium state. As a result, the LaFeO3 coated samples commonly show a thicker EHL and a higher surface hardness than the uncoated ones under the same conditions. Here we use the increase rate of the EHL depth, η, to evaluate the catalytic efficiency of the LaFeO3 oxide, i.e. η = (DRE − D0)/ D0 , where DRE and D0 are the EHL depth for samples with and without LaFeO3 coating, respectively. The catalytic efficiency of the LaFeO3 oxide as a function of the gas pressure and ammonia flow rate is shown in Fig. 4. The total map can be divided into two areas to distinguish the effectiveness of the LaFeO3 catalyst. The invalid/weak area is defined as the catalytic efficiency lower than 0.1 by considering the error of
3. Results 3.1. Catalytic efficiency of the LaFeO3 oxide during low-pressure gas nitriding In order to evaluate the catalytic efficiency of the LaFeO3 oxide during low-pressure gas nitriding, the depth of EHL was measured by the microhardness testing. The microhardness profiles of nitrided layers with and without LaFeO3 coating are compared in Fig. 2. The LaFeO3 coated samples generally exhibit a thicker nitrided layer with higher hardness than the uncoated ones, demonstrating its effective catalytic activity. But the difference in microhardness profiles between the LaFeO3 coated sample and the uncoated one reduces and even disappears with the increase of the gas pressure and ammonia flow rate. It indicates that the LaFeO3 oxide is effective for specific conditions of low pressures and low ammonia flow rates. It is known that nitriding potential is dependant on the rate of ammonia decomposition which is controlled by the gas pressure and flow rate [25]. Therefore, there are reasons to believe that the catalytic efficiency of the LaFeO3 oxide is related to the nitriding potential in atmosphere. The depth of EHL and surface hardness plotted as a function of the gas pressure and ammonia flow rate are shown in Fig. 3. Both the depth of EHL and the surface hardness increases rapidly along with the gas 3
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 3. The dependence of (a) the depth of effective hardened layers and (b) the surface hardness on the gas pressure and ammonia flow rate for the uncoated and LaFeO3 coated samples nitrided at 550 °C for 2 h.
microhardness measurements. Except the area above, the LaFeO3 oxide exhibits a catalytic activity in a wide range of gas pressures and ammonia flow rates. The strongly catalytic activity mainly occurs in the area of lower gas pressures (< 0.02 MPa) and lower ammonia flows (< 0.8 L/min). With the increase of the gas pressure, the catalytic efficiency decreases rapidly and the active range for the ammonia flow rate reduces to a narrow one. It indicates that the catalytic efficiency of the LaFeO3 oxide is sensitive to the nitriding potential controlled by the gas pressure.
3.2. Phase composition of the low-pressure gas nitrided layers Fig. 5 displays XRD patterns of surface layers nitrided at 550 °C for 2 h with and without LaFeO3 coating under various gas pressures and ammonia flow rates. The LaFeO3 oxide was successfully synthesized through sol-gel method during heating process. For the samples nitrided at pressure of 0.01 MPa, all nitrided surfaces mainly consist of α′Fe and ε-nitrides (see Fig. 5a and b). For higher gas pressure (0.02 MPa) and ammonia flow rate (0.2 L/min), the LaFeO3 oxide makes α′-Fe phase disappearing and nitrides increasing. When the gas pressure and ammonia flow rate reach to 0.1 MPa and 1.0 L/min respectively, the single ε phase is formed uniformly on the nitrided surfaces (see Fig. 5d).
Fig. 4. Dependence of the catalytic efficiency of the LaFeO3 oxide on the gas pressure and ammonia flow rate (nitriding conditions: 550 °C for 2 h).
4
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 5. Comparison of XRD patterns of nitrided layers treated at 550 °C for 2 h with and without LaFeO3 coating under different gas pressures and ammonia flow rates of (a) 0.01 MPa-0.1 L/min, (b) 0.01 MPa-0.5 L/min, (c) 0.02 MPa-0.2 L/min and (d) 0.1 MPa-1.0 L/min.
to the higher nitrogen concentration induced by the LaFeO3 oxide (marked by white arrows in Fig. 6a). The LaFeO3 coated sample has a thicker CL as compared with the uncoated one in the catalytic area given in Fig. 4. The difference in the thickness of CL between the uncoated and LaFeO3 coated sample gradually disappears with the increase of the gas pressure and ammonia flow rate, which corresponds to the reduction of catalytic efficiency of the LaFeO3 oxide. The microstructural characteristic is consistent with the corresponding phase composition on the nitrided surface.
It indicates the nitriding potential can be significantly improved by increasing the gas pressure and ammonia flow rate. By comparing the difference in phase compositions between nitrided surfaces with and without LaFeO3 coating, it can be found that the LaFeO3 oxide brings a high nitrogen concentration on the nitrided surfaces. Such high nitrogen concentration results in the increase in the amount of ε nitrides (see Fig. 5a and b) and promotes the phase transition from γ′ to ε nitrides (see Fig. 5c). All these findings in phase compositions are consistent with the results of surface hardness, i.e. the amount of nitrides is proportional to the surface hardness.
3.4. Surface morphologies of the low-pressure gas nitrided layers 3.3. Cross-sectional microstructure of the low-pressure gas nitrided layers In order to clarify the catalytic behavior of the LaFeO3 oxide in detail, the morphologies of nitrided surfaces with and without LaFeO3 coating were observed and the elemental distributions were compared as shown in Fig. 7. We select two typical samples for analyze. One is located in the excellent effective area (sample LPN-1) and the other is in the invalid/weak effective area (sample LPN-16). The cracked surface morphology is the same as our previous work, which results form the shrinkage of the sol-gel coating during heating process [21]. For the uncoated samples, the oxygen and nitrogen distribute homogeneously on the nitrided surfaces. But for the LaFeO3 coated samples, the oxygen co-exists with the lanthanum which mainly concentrates in the LaFeO3 coatings. Besides there is only a trace of lanthanum can be detected in the cracks. The nitrogen distribution does not perfectly match the surface morphology. Nitrogen element covers throughout the nitrided surface. In addition, the nitrogen concentration in the LaFeO3 coating is higher than that in the cracks, which indicates the migration of nitrogen atoms from LaFeO3 coatings to cracks during the nitriding process. Comparing the LaFeO3 samples located in the effective area with that
Fig. 6 shows the typical cross-sectional microstructures of nitrided layers treated at 550 °C for 2 h under various gas pressures and ammonia flow rates for the uncoated and LaFeO3 coated samples. The lowpressure gas nitrided layers are composed of a compound layer (CL) and a diffusion layer (DL) underneath. However, the growth rate of CL will be slow down dramatically under the low-pressure condition. The uncoated sample is free of CL when nitrided at low-pressure of 0.01 MPa and low ammonia flow rate of 0.1 L/min (see Fig. 6a). A thin and continuous CL begins to form with increasing the ammonia flow rate to 1.0 L/min (see Fig. 6b) or elevating the gas pressure to 0.02 MPa (see Fig. 6c). Keeping on increasing the gas pressure and ammonia flow rate to 0.1 MPa and 1.0 L/min respectively, the CL grows rapidly and the thickness of CL reaches to about 12 μm (see Fig. 6d). For the LaFeO3 coated samples, more nitrides can be formed on the nitrided surface as compared with the uncoated samples at the same conditions (see discontinuous CL in Fig. 6a). Meanwhile, there are obvious precipitations of nitrides on the grain boundaries for the LaFeO3 coated samples due 5
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 6. Cross-sectional microstructures of nitrided layers of the uncoated and LaFeO3 coated specimens treated at 550 °C for 2 h under different gas pressures and ammonia flow rates of (a) 0.01 MPa-0.1 L/min, (b) 0.01 MPa-1.0 L/min, (c) 0.02 MPa-0.2 L/min and (d) 0.1 MPa-1.0 L/min.
4.1. Catalytic behavior of the LaFeO3 oxide
located in the invalid area, it can be found that the nitrogen distribution tends to be uniform with the reduction of catalytic efficiency of the LaFeO3 oxide. It demonstrates the catalytic efficiency of the LaFeO3 also depends on the elemental distribution on the nitrided surface. Fig. 8 shows the microstructure of the nitrided surface with LaFeO3 coating observed by TEM. It can be seen that the microstructure mainly consists of two characteristic areas, i.e. a uniform grey area and an area with a massive of flocculent shadows. The SAED patterns confirm that these two areas are composed of single LaFeO3 oxide (area A) and α′-Fe phase (area B), respectively. It is worthy to note that the SAED pattern of the LaFeO3 oxide exhibits a superstructural feature due to the ordering of oxygen vacancies [26,27]. It implies there are abundant oxygen vacancies in the LaFeO3 oxide. Meanwhile, the calculated interplanar spacing by SAED patterns is slightly smaller than the referenced value in the standard PDF card, which is also an evidence for the existence of oxygen vacancies. It is also worthy to note that there is an interface between the LaFeO3 oxide and the α′-Fe matrix. The interfacial area (area C in Fig. 8a) is formed by mechanical contact without any characteristics of chemical bonding. The EDS measurements are used to detect the elemental migration across the interface. The results show that only iron and oxygen can be detected in the interfacial area (see Fig. 8f). The oxygen content in the interfacial area is lower than that in the LaFeO3 oxide (see Fig. 8d), which verifies the migration of oxygen atoms from the LaFeO3 oxide to the matrix. The lanthanum atoms cannot diffuse out of the LaFeO3 oxide. It implies that the rare earth element works only on the nitrided surface and cannot affect the diffusion of nitrogen atoms in the deeper layer. The diffusion of oxygen atoms also further explains the existence of oxygen vacancies in the LaFeO3 oxide.
It is known that the gas nitriding process contains the following three steps, (I) the adsorption of ammonia molecules on the surface, (II) the formation of active nitrogen atoms by surface reactions, and (III) the diffusion of nitrogen atoms towards the inner layers [28]. The slowest step among three steps above is the key fact to control the growth of nitrided layers. Therefore, in order to understand the catalytic behavior of the LaFeO3 oxide, the adsorption and dissociation of ammonia as well as the diffusion of nitrogen atoms should be clarified by analyzing the chemical state of elements and atomic bonds on the nitrided surface. Fig. 9 shows the high-resolution XPS spectra of Fe 2p3/ 2 , La 3d5/2, N 1s and O 1s obtained from nitrided surfaces of LPN-1 samples with and without LaFeO3 coating before and after argon ion sputtering. The ion sputtering is used to evaluate the strength of atomic bonds on the nitrided surface. All peaks in XPS spectra are identified according our previous work and references [21,29–34]. It can be seen that the peaks for all metallic atoms will move to the low binding energy after sputtering. The shift of peak corresponds to the change of chemical bonding state [35,36]. It also indicates that the metallic atoms are active sites for adsorption and dissociation of ammonia molecules during gas nitriding. When the metallic atoms bond with non-metallic atoms (see Fe-N and Fe-O bonds in Fig. 9d and f), the binding energies are fixed without any shift for the uncoated sample due to the strong bonding strength. But the binding energy for the adsorbed NHx species still tends to low energy level along with the iron atoms, which suggests the ammonia molecules are mainly adsorbed on the iron sites. For the LaFeO3 coated samples, both the Fe-N and Fe-O peaks shift to low binding energy (see Fig. 9e and g). The chemical shift of Fe-N and Fe-O bonds after sputtering indicates the weakened strength for the corresponding bonds. It is true of that the rare earth element contributes to all these changes. It is worthy to note that the diffusion rate of nitrogen atoms in the lattice of iron significantly depends on the interaction between the iron and nitrogen atoms. The weakened Fe-N bonds undoubtedly decrease the activation energy of diffusion for nitrogen atoms, which is beneficial for the rapid migration of nitrogen atoms. The step (III) will not be the rate-determining step of the rapid nitriding for the LaFeO3 coated samples any more. Meanwhile, the hydroxyl can be detected in the XPS spectra of O 1s (see Fig. 9f and g), which indicates the dehydrogenation of ammonia molecules occurs on the nitrided surface. The dehydrogenation reaction finally results in the formation of active nitrogen atoms [37]. The LaFeO3 coated sample exhibits a higher percentage of hydroxyl (52.66%, calculated by the area ratio of hydroxyl to total peak of oxygen) than the uncoated one (42.52%). This indicates a more intense dissociation of ammonia
4. Discussions Based on the results above, a phenomenon can be observed, i.e. the LaFeO3 oxide presents an excellent catalytic activity during low-pressure gas nitriding. That cannot only increase the surface hardness but also improve the EHL depth. Although the catalytic mechanism of the LaFeO3 oxide has been proposed in our previous work [21], it is incomplete and not suitable for the low-pressure conditions. Therefore we make effort to discuss and reveal the catalytic mechanism of the LaFeO3 oxide during low-pressure gas nitriding in detail. The effects of the gas pressure and ammonia flow rate on the catalytic efficiency of the LaFeO3 oxide are also discussed. It is a good extension for our previous work.
6
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 7. SEM images and elemental distribution of nitrided surfaces treated at 550 °C for 2 h with and without LaFeO3 coating. Two kinds of typical samples located in the effective area and invalid/weak area (as shown in Fig. 4) are selected for comparison.
of nitrogen atoms.
molecules occurred on the LaFeO3 coated surface than that on the uncoated surface. On the other hand, the weakened Fe-O bonds for the LaFeO3 coated sample will induce massive oxygen vacancies in the LaFeO3 oxide. Those excess oxygen atoms diffuse to the nitrided surface to accelerate the dehydrogenation process. Therefore, the step (II) is also not the barrier of rapid nitriding for the LaFeO3 coated sample. For the adsorption of ammonia molecules, although the percentage of the adsorbed NHx species for the LaFeO3 coated samples is lower than that for the uncoated samples, it does not mean that the adsorption of ammonia molecules on the LaFeO3 coated surface is weak due to the unknown ratio of physical adsorption of ammonia molecules. Considering the excellent catalytic effect of the LaFeO3 oxide, it can be concluded that the adsorption of ammonia molecules is not the key fact to determine the catalytic efficiency of the LaFeO3 oxide. The adsorption of ammonia molecules is sufficient under present nitriding conditions for both the uncoated and LaFeO3 coated samples. Therefore, the catalytic effect of the LaFeO3 oxide mainly reflects in the aspects of accelerating the rates in the decomposition of ammonia molecules and the diffusion
4.2. Catalytic feature and conditions for the LaFeO3 oxide It is clear that the rare earth element can vary the chemical state of Fe-N and Fe-O bonds. The XPS peaks of Fe-N and Fe-O bonds shift to high binding energy with the rare earth addition (see the spectra before sputtering in Fig. 9d-g), which is attributed to the Fermi level moving up in energy [38]. The rising Fermi level will lead to the oxygen and nitrogen ions to deviate from their equilibrium state to the active state due to the loss of electrons, which indeed accelerate the dehydrogenation of NHx species and diffusion of nitrogen atoms. We can also find a certain rule via comparing the XPS spectra of O 1s for the samples with and without LaFeO3 coating, i.e. the XPS spectra of O 1s with higher binding energy and higher percentage of hydroxyl corresponds to the excellent catalytic efficiency for the LaFeO3 coated samples. Fig. 10 gives the high-resolution XPS spectra of O 1s obtained from nitrided surfaces with and without LaFeO3 coating under different 7
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 8. TEM images of the nitrided surface with LaFeO3 coating treated at 550 °C for 1 h. (a) Bright field image of the nitrided surface with LaFeO3 coating, (b)-(c) SAED patterns of area A and area B in (a) respectively, (d)-(f) EDS spectra for area A, B and C in (a) respectively.
conditions of gas pressures and ammonia flow rates. It can be seen that the LaFeO3 coated sample nitrided at low gas pressure and low ammonia flow rate presents such typical catalytic feature of the XPS spectra of O 1s (see Fig. 10a). The catalytic feature disappears with the increase of the gas pressure and ammonia flow rate due to the sufficient supplement of hydrogen atoms to bring the oxygen ions back to their equilibrium state. The gas pressure and ammonia flow rate mainly affect the formation of hydroxyl to determine the catalytic efficiency of the LaFeO3 oxide. Meanwhile, the gas pressure and ammonia flow rate can also affect the morphologies of nitrided surfaces. Fig. 11 displays the typical morphologies of nitrided surfaces with LaFeO3 coating after treated at 550 °C for 2 h under different gas pressures and ammonia flow rates. It clearly shows that the surface morphologies vary obviously with the gas pressure and ammonia flow rate. The LaFeO3 coating formed during heating process exhibit a smooth surface before nitriding (see Fig. 11d). After nitriding under conditions of 0.01 MPa–0.1 L/min, there are massive nitrides preferential formed on the LaFeO3 coating and a rough surface is obtained. Then the nitrides grow rapidly with the increase of gas pressures and ammonia flow rates and fill into the cracks gradually. The catalytic efficiency of the LaFeO3 oxide disappears with the decrease of roughness on the nitrided surface
(see Fig. 11b and c). Based on the discussions above, it can be concluded that the catalytic efficiency of the LaFeO3 oxide mainly depends on the rate of the dehydrogenation reaction which is also affected by the gas pressure and ammonia flow rate. On the other hand, the low gas pressure and low ammonia flow rate provide a rough surface that is beneficial for the adsorption of ammonia molecules.
4.3. Durability of the catalytic activity of the LaFeO3 oxide The LaFeO3 oxide has been proved to be an excellent catalyst in the low-pressure gas nitriding with low ammonia flow rate. In order to evaluate the durability of the catalytic activity of the LaFeO3 oxide, the nitriding duration is prolonged to 16 h. Fig. 12 shows the microhardness profiles and growth kinetics of nitrided layers treated at 550 °C for different durations with and without LaFeO3 coating. It can be seen that the LaFeO3 coated samples present an overwhelming advantage in the hardness and depth of EHL as compared with the uncoated ones. The high catalytic efficiency for the LaFeO3 oxide is maintained up to 16 h or more. The depth of EHL for the LaFeO3 coated sample can reach to 431 μm which is twice as thick as that for the uncoated sample. The remarkable durability of the catalytic activity of the LaFeO3 oxide is 8
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 9. High-resolution XPS spectra of (a)-(b) Fe 2p3/2, (c) La 3d5/2, (d)-(e) N 1s and (f)-(g) O 1s obtained from nitrided surfaces of LPN-1 samples with and without LaFeO3 coating before and after argon ion sputtering. The sputtering time is 1 min.
5. Conclusions
attributed to the optimized nitriding potential on the nitrided surface controlled by the gas pressure and ammonia flow rate. The low gas pressure and ammonia flow rate can maintain the rough nitrided surface for a long duration, which is necessary for the rapid nitriding. It is worthy to note that the low-pressure gas nitriding with the LaFeO3 catalyst addition is a novel technology with many economical and ecological advantages. It is performed under low pressure and low ammonia flow rate indicating the low gas consumption and low emission of exhaust gases as the ZeroFlow gas nitriding [39]. Therefore, the LaFeO3 oxide has a great potential in applications of the rapid and deep nitriding.
In this paper, the LaFeO3 perovskite oxide was apply in the lowpressure gas nitriding to clarify the best catalytic conditions. The effects of gas pressure and ammonia flow rate on the catalytic efficiency of the LaFeO3 perovskite oxide have been investigated and its catalytic mechanism has been understood clearly. A few conclusions were summarized as follows: (1) The LaFeO3 oxide exhibited an excellent catalytic activity in the low-pressure gas nitriding, which could obviously increase the depth of effective hardened layers and the surface hardness as well as improve the formation of iron nitrides. (2) The catalytic efficiency of the LaFeO3 oxide was sensitive to the 9
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 10. High-resolution XPS spectra of O 1s obtained from nitrided surfaces with and without LaFeO3 coating treated at 550 °C for 2 h under different conditions of (a) 0.02 MPa-0,2 L/min and (b) 0.1 MPa-1.0 L/min.
Fig. 11. SEM images of nitrided surfaces with LaFeO3 coating after treated at 550 °C for 2 h under different conditions of gas pressures and ammonia flow rates. (a) 0.01 MPa-0.1 L/min, (b) 0.02 MPa-0.2 L/min, (c) 0.1 MPa-1.0 L/min and (d) the initial morphology of LaFeO3 coating before nitriding.
the rare earth nitriding. The gas pressure and ammonia flow rate mainly affected the catalytic efficiency of the LaFeO3 oxide by varying the rate of the dehydrogenation reaction. (5) The low gas pressure and low ammonia flow rate could maintain the nitrided surface in a rough status, which is beneficial for the adsorption of ammonia molecules as well as the increase of the nitriding potential. The durability of the catalytic activity of the LaFeO3 oxide could be maintained under low gas pressures and low ammonia flow rate, which indicates the LaFeO3 oxide has a great potential in applications of the rapid and deep nitriding.
nitriding potential. The highest catalytic efficiency occurred at low nitriding potential conditions, i.e. lower gas pressure and lower ammonia flow rate. The influence of the gas pressure was more significant than that of the ammonia flow rate. (3) The LaFeO3 oxide performed the catalytic activity on the nitrided surface by accelerating the dissociation rate of ammonia molecules and the diffusion rate of active nitrogen atoms. It can be attributed to the massive oxygen vacancies in the LaFeO3 oxide and the weakened Fe-N and Fe-O bonds induced by rare earth addition. (4) The excellent catalytic efficiency corresponded to the high percentage of hydroxyl on the nitrided surface. The dehydrogenation reaction of ammonia molecules was the rate-determining step for 10
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 12. Microhardness profiles of nitrided layers treated at 550 °C for (a) 1 h, (b) 4 h, (c) 8 h and (d)16 h with and without LaFeO3 coating and (e) growth kinetics of nitrided layers as a function of nitriding duration. The inset in (e) shows the surface morphology of the LaFeO3 coated sample after nitrided for 16 h.
Author contributions
plasma nitriding for AISI4140 steel, Vacuum 109 (2014) 170–174. [10] J. Li, X. Yang, S. Wang, K. Wei, J. Hu, A rapid D.C. plasma nitriding technology catalyzed by pre-oxidation for AISI4140 steel, Mater. Lett. 116 (2014) 199–202. [11] B. Arifvianto, M. Suyitno, Mahardika, Effects of surface mechanical attrition treatment (SMAT) on a rough surface of AISI 316L stainless steel, Appl. Surf. Sci. 258 (2012) 4538–4543. [12] B. Wang, W. Fu, F. Dong, G. Jin, W. Feng, Z. Wang, S. Sun, Significant acceleration of nitriding kinetics in pure iron by pressurized gas treatment, Mater. Des. 85 (2015) 91–96. [13] B. Wang, S. Sun, M. Guo, G. Jin, Z. Zhou, W. Fu, Study on pressurized gas nitriding characteristics for steel 38CrMoAlA, Surf. Coat. Technol. 279 (2015) 60–64. [14] J. Peng, H. Dong, T. Bell, F. Chen, Z. Mo, C. Wang, Q. Peng, Effect of rare earth elements on plasma nitriding of 38CrMoAI steel, Surf. Eng. 12 (1996) 147–151. [15] M. Dai, C. Li, J. Hu, The enhancement effect and kinetics of rare earth assisted salt bath nitriding, J. Alloy. Compd. 688 (2016) 350–356. [16] M.F. Yan, R.L. Liu, Influence of process time on microstructure and properties of 17–4PH steel plasma nitrocarburized with rare earths addition at low temperature, Appl. Surf. Sci. 256 (2010) 6065–6071. [17] M.F. Yan, R.L. Liu, Martensitic stainless steel modified by plasma nitrocarburizing at conventional temperature with and without rare earths addition, Surf. Coat. Technol. 205 (2010) 345–349. [18] X. Wang, M. Yan, R. Liu, Y. Zhang, Effect of rare earth addition on microstructure and corrosion behavior of plasma nitrocarburized M50NiL steel, J. Rare. Earth. 34 (2016) 1148–1155. [19] Y. Wu, M. Yan, Effects of lanthanum and cerium on low temperature plasma nitrocarburizing of nanocrystallized 3J33 steel, J. Rare. Earth. 29 (2011) 383–387. [20] L.N. Tang, M.F. Yan, Effects of rare earths addition on the microstructure, wear and corrosion resistances of plasma nitrided 30CrMnSiA steel, Surf. Coat. Technol. 206 (2012) 2363–2370. [21] X. Chen, X. Bao, Y. Xiao, C. Zhang, L. Tang, L. Yao, G. Cui, Y. Yang, Low-temperature gas nitriding of AISI 4140 steel accelerated by LaFeO3 perovskite oxide, Appl. Surf. Sci. 466 (2019) 989–999. [22] C.S. Zhang, M.F. Yan, Z. Sun, Experimental and theoretical study on interaction between lanthanum and nitrogen during plasma rare earth nitriding, Appl. Surf. Sci. 287 (2013) 381–388. [23] Z. Zhong, K. Chen, Y. Ji, Q. Yan, Methane combustion over B-site partially substituted perovskite-type LaFeO3 prepared by sol-gel method, Appl. Catal. A 156 (1997) 29–41. [24] S.A. Mahadik, F. Pedraza, S.S. Mahadik, Comparative studies on water repellent coatings prepared by spin coating and spray coating methods, Prog. Org. Coat. 104 (2017) 217–222. [25] K. Kiełbasa, R. Pelka, W. Arabczyk, Studies of the Kinetics of Ammonia Decomposition on Promoted Nanocrystalline Iron Using Gas Phases of Different Nitriding Degree, J. Phys. Chem. A 114 (2010) 4531–4534. [26] S. Steinsvik, R. Bugge, J. Gjonnes, J. Tafto, T. Norby, The defect structure of SrTilxFexO3-y (x= 0–0.8) investigated by electrical conductivity measurements and electron energy loss spectroscopy (EELS), J. Phys. Chem. Solids 58 (1997) 969–976. [27] R.H.E. van Doorn, A.J. Burggraaf, Structural aspects of the ionic conductivity of La1xSrxCoO3-δ, Solid State Ionics 128 (2000) 65–78. [28] L. Torchane, P. Bilger, J. Dulcy, M. Gantois, Control of iron nitride layers growth kinetics in the binary Fe-N system, Metall. Mater. Trans. A. 27 (1996) 1823–1835. [29] P.A.W. van der Heide, Systematic x-ray photoelectron spectroscopic study of La1xSrx-based perovskite-type oxides, Surf. Interface Anal. 33 (2002) 414–425.
Chengsong Zhang designed the research framework and wrote the manuscript. Yun Wang prepared the nitrided samples and performed the experiments. Xing Chen, Hongtao Chen and Yeqiong Wu performed the SEM, TEM and hardness tests. Guodong Cui and Dazhi Chen participated in analyzing the experimental results. Yixue Wang and Lina Tang contributed to the technical discussions of results. All authors commented on the manuscript. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51601156, 51601048, 51601117 and U1537201). We appreciate Dr. Zhenye Zhong for his assistance in SEM testing. References [1] N. Limodin, Y. Verreman, T.N. Tarfa, Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth, Fatigue Fract. Eng. Mater. Struct. 26 (2003) 811–820. [2] B. Wang, B. Liu, X. Zhang, J. Gu, Enhancing heavy load wear resistance of AISI 4140 steel through the formation of a severely deformed compound-free nitrided surface layer, Surf. Coat. Technol. 356 (2018) 89–95. [3] M. Karakan, A. Alsaran, A. Çelik, Effect of process time on structural and tribological properties of ferritic plasma nitrocarburized AISI 4140 steel, Mater. Des. 25 (2004) 349–353. [4] M. Fattah, F. Mahboubi, Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel, Mater. Des. 31 (2010) 3915–3921. [5] W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu, K. Lu, Nitriding Iron at Lower Temperatures, Science 299 (2003) 686–688. [6] W.P. Tong, Z. Han, L.M. Wang, J. Lu, K. Lu, Low-temperature nitriding of 38CrMoAl steel with a nanostructured surface layer induced by surface mechanical attrition treatment, Surf. Coat. Technol. 202 (2008) 4957–4963. [7] W.P. Tong, C.Z. Liu, W. Wang, N.R. Tao, Z.B. Wang, L. Zuo, J.C. He, Gaseous nitriding of iron with a nanostructured surface layer, Scripta Mater. 57 (2007) 533–536. [8] J. Baranowska, K. Szczecinski, M. Wysiecki, Increasing of gas nitriding kinetics via surface pre-treatment, Surf. Coat. Technol. 151–152 (2002) 534–539. [9] H. Liu, J. Li, F. Sun, J. Hu, Characterization and effect of pre-oxidation on D.C.
11
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
[30] R. Dudric, A. Vladescu, V. Rednic, M. Neumann, I.G. Deac, R. Tetean, XPS study on La0.67Ca0.33Mn1-xCoxO3 compounds, J. Mol. Struct. 1073 (2014) 66–70. [31] L. Wang, J.Y. Wang, Z.X. Wang, C. He, W. Lyu, W. Yan, L. Yang, Enhanced antimonate (Sb(V)) removal from aqueous solution by La-doped magnetic biochars, Chem. Eng. J. 354 (2018) 623–632. [32] D. Feng, Z. Zhou, M. Bo, An investigation of the thermal degradation of melamine phosphonite by XPS and thermal analysis techniques, Polym. Degrad. Stabil. 50 (1995) 65–70. [33] J. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Systematic XPS studies of metal oxides, hydroxides and peroxides, Phys. Chem. Chem. Phys. 2 (2000) 1319–1324. [34] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. [35] X. Wang, W.T. Zheng, H.W. Tian, S.S. Yu, W. Xu, S.H. Meng, X.D. He, J.C. Han,
[36]
[37] [38]
[39]
12
C.Q. Sun, B.K. Tay, Growth, structural, and magnetic properties of iron nitride thin films deposited by dc magnetron sputtering, Appl. Surf. Sci. 220 (2003) 30–39. Erik Lewin, Mihaela Gorgoi, Franz Schäfers, Svante Svensson, U. Jansson, Influence of sputter damage on the XPS analysis of metastable nanocomposite coatings, Surf. Coat. Technol. 204 (2009) 455–462. G. Lanzani, K. Laasonen, NH3 adsorption and dissociation on a nanosized iron cluster, Int. J. Hydrogen Energy 35 (2010) 6571–6577. E. Beyreuther, S. Grafström, L.M. Eng, XPS investigation of Mn valence in lanthanum manganite thin films under variation of oxygen content, Phys. Rev. B 73 (2006) 155425. L. Maldzinski, J. Tacikowski, 12-ZeroFlow gas nitriding of steels, Thermochemical Surface Engineering of Steels, Woodhead Publishing, Oxford, 2015, pp. 459–483.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Catalytic behavior of LaFeO3 pervoskite oxide during low-pressure gas nitriding
T
⁎
Chengsong Zhanga, , Yun Wanga, Xing Chena, Hongtao Chenb, Yeqiong Wuc, Yixue Wangd, Lina Tange, Guodong Cuia, Dazhi Chena a
School of Material Science and Engineering, Southwest Jiaotong University, Chengdu 610031, PR China School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, PR China c Chengdu Hangli Industrial Co., Ltd., Chengdu 611930, PR China d School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China e Shanghai Aerospace Equipments Manufacturer Co., Ltd., Shanghai 200000, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Low-pressure gas nitriding LaFeO3 perovskite oxide Rare earth Catalytic behavior
The LaFeO3 perovskite oxide has been proved to be an effective catalyst in the gas nitriding. However, its catalytic efficiency was sensitive to the nitriding parameters. In order to clarify the catalytic conditions of the LaFeO3 oxide, the low-pressure gas nitriding was carried out in the present work. The effects of gas pressure and ammonia flow rate on catalytic behavior of the LaFeO3 oxide were discussed. The catalytic feature and conditions of the LaFeO3 oxide was identified. The microstructure and phase composition of nitrided layers were analyzed by the optical microscope, transmission electron microscopy, scanning electron microscope and X-ray diffraction, respectively. The microhardness profiles of nitrided layers were measured by micro-hardness tester to evaluate catalytic efficiency of the LaFeO3 oxide. The X-ray photoelectron spectroscopy was applied to reveal the catalytic behavior. The results show that the LaFeO3 perovskite oxide exhibited an excellent catalytic activity via accelerating dissociation of ammonia molecules and diffusion of nitrogen atoms. The dehydrogenation reaction of ammonia molecules was the rate-determining step for the rare earth nitriding, which was greatly affected by gas pressure and ammonia flow rate. The LaFeO3 catalyst shows an excellent durability of catalytic activity in low pressure and low flow rate conditions.
1. Introduction Gas nitriding is an effective thermo-chemical treatment that could dramatically improve the surface properties of steels [1]. However, the extensive application of gas nitriding was restricted due to its low efficiency. Conventional gas nitriding at 530 °C for 15 h only generates a nitrided layer with the thickness of 258 μm [2]. Increasing the nitrided temperature is a common method for a thicker nitrided layer, but it inevitably leads to performance degradation [3,4]. In order to improve the nitriding efficiency, many methods have been developed, aiming at activating the nitrided surface or accelerating the diffusion rate of nitrogen atoms. The nitriding efficiency of iron and steels could be improved by surface nanocrystallization with surface mechanical attrition treatment (SMAT) [5,6]. The significantly increased density of dislocations provided massive diffusion routes for nitrogen atoms [7]. A surface pre-treatment by cathode sputtering has a similar effect as SMAT [8]. Hu et al. reported that the thickness of nitrided layers could
⁎
be increased through pre-oxidation treatment [9]. The oxides formed on the nitrided surface could increase the surface free energy and nitrogen potential. [10]. All these new technologies mentioned above need a pretreatment before nitriding, which make the procedure tedious. Especially, SMAT pretreatment will result in a rough nitrided surface due to the severe plastic deformation on the surface [11]. In order to simplify the process, altering the process parameters or adding catalytic agents are applied to improve the nitriding efficiency. A pressurized gas nitriding with significantly accelerated nitriding efficiency was reported because of the faster reaction rates and the optimized nitrogen potential on nitrided surfaces under higher pressure [12,13]. Meanwhile, the rare earth (RE) elements are usually used as catalyst for thermo-chemical treatments. The RE reagents can be easily added into the furnace with the nitriding atmosphere and present an obviously catalytic function [14–16]. Although the effectiveness of RE reagents has been proved in many literatures [17–19], the catalytic mechanism has not been understood yet. Many researchers attribute the
Corresponding author. E-mail address: [email protected] (C. Zhang).
https://doi.org/10.1016/j.apsusc.2019.145045 Received 15 September 2019; Received in revised form 14 November 2019; Accepted 10 December 2019 Available online 12 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
catalytic effect to the micro-alloy function of RE elements which accelerate the absorption, transfer and diffusion of nitrogen atoms on the treated surface [20,21], however, the catalytic RE species is not detected on the nitrided surfaces and in the nitrided layers. Until now, all the suggested catalytic mechanisms of RE elements are just simple phenomenological inferences based on the existed experimental data. It is hard to clarify in which process and how do the catalytic RE species work in detail. Fortunately, in our previous studies, an ABO3-type perovskite oxide, i.e. LaFeO3, was proved to have catalytic effect during nitriding [21,22]. The catalytic mechanism of the LaFeO3 perovskite oxide was attributed to the promotion of the decomposition of ammonia gas and the dissolution of nitrogen atoms caused by the oxygen vacancies in the LaFeO3 oxide [21]. Meanwhile, we also found that the catalytic efficiency of the LaFeO3 oxide was sensitive to the ammonia flow rate and was weaken under higher gas flows. It indicated that the catalytic efficiency of the LaFeO3 oxide was strongly dependant on the nitriding potential in atmosphere. In order to clarify the suitable catalytic conditions for the LaFeO3 oxide, the low-pressure gas nitriding was carried out with the LaFeO3 coating at different ammonia flow rates. In present work, the nitrogen potential was controlled by gas pressure and ammonia flow rate. The effects of gas pressure and ammonia flow rate on catalytic behavior of the LaFeO3 oxide were evaluated. The catalytic mechanism was discussed. Thus, the optimized catalytic condition for the LaFeO3 oxide would be summarized.
Fig. 1. Schematic illustration of low-pressure gas nitriding used in this study.
Table 1 Detailed parameters of low-pressure gas nitriding.
2. Experimental 2.1. Materials and pretreatments AISI 4140 steel, with the chemical compositions of (wt. %) 0.41C, 0.22Si, 0.67Mn, 1.06Cr, 0.02Ni, 0.16Mo, 0.02Cu, 0.01P, 0.001S and Fe in balance, was used in this work. The AISI 4140 steel was austenitized at 860 °C for 30 min and then quenched in oil to obtain the homogeneous substrate. After that, the steel bar was cutted into the size of ϕ18 × 4 mm and the samples were mechanically polished using silicon carbide papers from 240 to 400 grade. All polished samples were ultrasonically cleaned in the ethyl alcohol and then dried before coating LaFeO3 film. The LaFeO3 coating was synthesized by the sol-gel method [23]. The sol-gel precursor solution of LaFeO3 was prepared in the same way as our previous work [21], and coated on the clean surfaces by a spincoating method before gas nitriding [24].
Specimens
Nitriding temperature (°C)
Nitriding time (h)
NH3 (L/ min)
Pressure (MPa)
LPN-1 LPN-2 LPN-3 LPN-4 LPN-5 LPN-6 LPN-7 LPN-8 LPN-9 LPN-10 LPN-11 LPN-12 LPN-13 LPN-14 LPN-15 LPN-16 LPN-17 LPN-18 LPN-19 LPN-20
550
2
0.1 0.2 0.5 1.0 0.1 0.2 0.5 1.0 0.1 0.2 0.5 1.0 0.1 0.2 0.5 1.0 0.1
0.01
1 4 8 16
0.02
0.05
0.1
0.01
7001F, Japan) equipped with an energy dispersive X-ray analyzer (EDS). The phase compositions in the nitrided layer were detected by using X-ray diffraction (XRD, X’pert pro, Netherlands) with Cu-Kα radiation. The glancing angle was from 20 to 100° and the accelerating voltage was set to 40 kV. Microhardness profiles of nitrided layers were measured by a microhardness tester (HVS-1000, China) under an indentation load of 100 gf for 15 s. In order to insure the accuracy of the microhardness testing, four microhardness data were tested in each position at same depth and the average values were used. The depth of effective hardened layers (EHL) was evaluated by using the same method in our previous work [21]. To clarify the catalytic mechanism and behavior of the LaFeO3 oxide in low-pressure, X-ray photoelectron spectroscopy (XPS, Thermo Scientifc K-Alpha spectrometer) was applied to analyze the chemical state of elements on the nitrided surface. A transmission electron microscopy (TEM, JEM-2100, Japan) was utilized for understanding the existed state of the LaFeO3 oxide on the nitrided surface. The phase composition in the micro area was identified by selected area electron diffraction (SAED). In order to protect the LaFeO3 coating on the nitrided surface, a thin foil with thickness of 40 μm was prepared by mechanically polishing and nitrided with the LaFeO3 coating. Then, the nitrided thin foil was reduced by ion thinning and observed by TEM. To avoid the embrittlement of the nitrided thin foil, the nitriding process was carried out at 550 °C for 1 h.
2.2. Low-pressure gas nitriding The low-pressure gas nitriding (LPN) treatments were conducted in a vacuum quartz tube furnace at 550 °C for 1–16 h with and without LaFeO3 coating. The Schematic illustration of LPN treatments was shown in Fig. 1 and the detailed parameters were listed in Table 1. Firstly, the pretreated samples were heated to 550 °C in air and the heating rate was set to 14 °C/min. During the heating process, the LaFeO3 coating would in-situ form on the surface of samples. Then the chamber was evacuated to lower than 0.01 MPa by a rotary pump when the temperature reached to 550 °C. After that, the ammonia with the flow rate of 0.1 to 1 L/min was induced into the furnace and the gas pressure was set to 0.01–0.1 MPa by controlling the vacuum valves. Finally, all specimens were slowly cooled to room temperature in ammonia after gas nitriding. The samples without coatings were nitrided under the same conditions for comparison. 2.3. Characterization After LPN treatments, the cross-sectional morphologies of nitrided layers were observed by using an optical microscopy (Olympus GX51F, Japan). The corresponding surface morphologies and elemental distributions were observed by scanning electron microscope (SEM, JSM2
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 2. Microhardness profiles of nitrided layers treated at 550 °C for 2 h with and without LaFeO3 coating.
pressure and ammonia flow rate. The gas pressure has more pronounced effect than that of the ammonia flow rate, which indicates that the nitriding potential is mainly determined by the gas pressure during the low-pressure gas nitriding. Under each ammonia flow rate, there is a fold point for the gas pressure (see dash lines in Fig. 3). When the gas pressure surpasses the fold point, the depth of EHL and the surface hardness closes to a constant. It indicates that the equilibrium state or saturated state of nitrogen concentration on the nitrided surfaces have achieved. It is worthy to note that the fold line moves to low gas pressure with the addition of the LaFeO3 oxide, which demonstrates that the LaFeO3 oxide can accelerate the decomposition of ammonia and increase the nitriding potential rapidly to reach the equilibrium state. As a result, the LaFeO3 coated samples commonly show a thicker EHL and a higher surface hardness than the uncoated ones under the same conditions. Here we use the increase rate of the EHL depth, η, to evaluate the catalytic efficiency of the LaFeO3 oxide, i.e. η = (DRE − D0)/ D0 , where DRE and D0 are the EHL depth for samples with and without LaFeO3 coating, respectively. The catalytic efficiency of the LaFeO3 oxide as a function of the gas pressure and ammonia flow rate is shown in Fig. 4. The total map can be divided into two areas to distinguish the effectiveness of the LaFeO3 catalyst. The invalid/weak area is defined as the catalytic efficiency lower than 0.1 by considering the error of
3. Results 3.1. Catalytic efficiency of the LaFeO3 oxide during low-pressure gas nitriding In order to evaluate the catalytic efficiency of the LaFeO3 oxide during low-pressure gas nitriding, the depth of EHL was measured by the microhardness testing. The microhardness profiles of nitrided layers with and without LaFeO3 coating are compared in Fig. 2. The LaFeO3 coated samples generally exhibit a thicker nitrided layer with higher hardness than the uncoated ones, demonstrating its effective catalytic activity. But the difference in microhardness profiles between the LaFeO3 coated sample and the uncoated one reduces and even disappears with the increase of the gas pressure and ammonia flow rate. It indicates that the LaFeO3 oxide is effective for specific conditions of low pressures and low ammonia flow rates. It is known that nitriding potential is dependant on the rate of ammonia decomposition which is controlled by the gas pressure and flow rate [25]. Therefore, there are reasons to believe that the catalytic efficiency of the LaFeO3 oxide is related to the nitriding potential in atmosphere. The depth of EHL and surface hardness plotted as a function of the gas pressure and ammonia flow rate are shown in Fig. 3. Both the depth of EHL and the surface hardness increases rapidly along with the gas 3
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 3. The dependence of (a) the depth of effective hardened layers and (b) the surface hardness on the gas pressure and ammonia flow rate for the uncoated and LaFeO3 coated samples nitrided at 550 °C for 2 h.
microhardness measurements. Except the area above, the LaFeO3 oxide exhibits a catalytic activity in a wide range of gas pressures and ammonia flow rates. The strongly catalytic activity mainly occurs in the area of lower gas pressures (< 0.02 MPa) and lower ammonia flows (< 0.8 L/min). With the increase of the gas pressure, the catalytic efficiency decreases rapidly and the active range for the ammonia flow rate reduces to a narrow one. It indicates that the catalytic efficiency of the LaFeO3 oxide is sensitive to the nitriding potential controlled by the gas pressure.
3.2. Phase composition of the low-pressure gas nitrided layers Fig. 5 displays XRD patterns of surface layers nitrided at 550 °C for 2 h with and without LaFeO3 coating under various gas pressures and ammonia flow rates. The LaFeO3 oxide was successfully synthesized through sol-gel method during heating process. For the samples nitrided at pressure of 0.01 MPa, all nitrided surfaces mainly consist of α′Fe and ε-nitrides (see Fig. 5a and b). For higher gas pressure (0.02 MPa) and ammonia flow rate (0.2 L/min), the LaFeO3 oxide makes α′-Fe phase disappearing and nitrides increasing. When the gas pressure and ammonia flow rate reach to 0.1 MPa and 1.0 L/min respectively, the single ε phase is formed uniformly on the nitrided surfaces (see Fig. 5d).
Fig. 4. Dependence of the catalytic efficiency of the LaFeO3 oxide on the gas pressure and ammonia flow rate (nitriding conditions: 550 °C for 2 h).
4
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 5. Comparison of XRD patterns of nitrided layers treated at 550 °C for 2 h with and without LaFeO3 coating under different gas pressures and ammonia flow rates of (a) 0.01 MPa-0.1 L/min, (b) 0.01 MPa-0.5 L/min, (c) 0.02 MPa-0.2 L/min and (d) 0.1 MPa-1.0 L/min.
to the higher nitrogen concentration induced by the LaFeO3 oxide (marked by white arrows in Fig. 6a). The LaFeO3 coated sample has a thicker CL as compared with the uncoated one in the catalytic area given in Fig. 4. The difference in the thickness of CL between the uncoated and LaFeO3 coated sample gradually disappears with the increase of the gas pressure and ammonia flow rate, which corresponds to the reduction of catalytic efficiency of the LaFeO3 oxide. The microstructural characteristic is consistent with the corresponding phase composition on the nitrided surface.
It indicates the nitriding potential can be significantly improved by increasing the gas pressure and ammonia flow rate. By comparing the difference in phase compositions between nitrided surfaces with and without LaFeO3 coating, it can be found that the LaFeO3 oxide brings a high nitrogen concentration on the nitrided surfaces. Such high nitrogen concentration results in the increase in the amount of ε nitrides (see Fig. 5a and b) and promotes the phase transition from γ′ to ε nitrides (see Fig. 5c). All these findings in phase compositions are consistent with the results of surface hardness, i.e. the amount of nitrides is proportional to the surface hardness.
3.4. Surface morphologies of the low-pressure gas nitrided layers 3.3. Cross-sectional microstructure of the low-pressure gas nitrided layers In order to clarify the catalytic behavior of the LaFeO3 oxide in detail, the morphologies of nitrided surfaces with and without LaFeO3 coating were observed and the elemental distributions were compared as shown in Fig. 7. We select two typical samples for analyze. One is located in the excellent effective area (sample LPN-1) and the other is in the invalid/weak effective area (sample LPN-16). The cracked surface morphology is the same as our previous work, which results form the shrinkage of the sol-gel coating during heating process [21]. For the uncoated samples, the oxygen and nitrogen distribute homogeneously on the nitrided surfaces. But for the LaFeO3 coated samples, the oxygen co-exists with the lanthanum which mainly concentrates in the LaFeO3 coatings. Besides there is only a trace of lanthanum can be detected in the cracks. The nitrogen distribution does not perfectly match the surface morphology. Nitrogen element covers throughout the nitrided surface. In addition, the nitrogen concentration in the LaFeO3 coating is higher than that in the cracks, which indicates the migration of nitrogen atoms from LaFeO3 coatings to cracks during the nitriding process. Comparing the LaFeO3 samples located in the effective area with that
Fig. 6 shows the typical cross-sectional microstructures of nitrided layers treated at 550 °C for 2 h under various gas pressures and ammonia flow rates for the uncoated and LaFeO3 coated samples. The lowpressure gas nitrided layers are composed of a compound layer (CL) and a diffusion layer (DL) underneath. However, the growth rate of CL will be slow down dramatically under the low-pressure condition. The uncoated sample is free of CL when nitrided at low-pressure of 0.01 MPa and low ammonia flow rate of 0.1 L/min (see Fig. 6a). A thin and continuous CL begins to form with increasing the ammonia flow rate to 1.0 L/min (see Fig. 6b) or elevating the gas pressure to 0.02 MPa (see Fig. 6c). Keeping on increasing the gas pressure and ammonia flow rate to 0.1 MPa and 1.0 L/min respectively, the CL grows rapidly and the thickness of CL reaches to about 12 μm (see Fig. 6d). For the LaFeO3 coated samples, more nitrides can be formed on the nitrided surface as compared with the uncoated samples at the same conditions (see discontinuous CL in Fig. 6a). Meanwhile, there are obvious precipitations of nitrides on the grain boundaries for the LaFeO3 coated samples due 5
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 6. Cross-sectional microstructures of nitrided layers of the uncoated and LaFeO3 coated specimens treated at 550 °C for 2 h under different gas pressures and ammonia flow rates of (a) 0.01 MPa-0.1 L/min, (b) 0.01 MPa-1.0 L/min, (c) 0.02 MPa-0.2 L/min and (d) 0.1 MPa-1.0 L/min.
4.1. Catalytic behavior of the LaFeO3 oxide
located in the invalid area, it can be found that the nitrogen distribution tends to be uniform with the reduction of catalytic efficiency of the LaFeO3 oxide. It demonstrates the catalytic efficiency of the LaFeO3 also depends on the elemental distribution on the nitrided surface. Fig. 8 shows the microstructure of the nitrided surface with LaFeO3 coating observed by TEM. It can be seen that the microstructure mainly consists of two characteristic areas, i.e. a uniform grey area and an area with a massive of flocculent shadows. The SAED patterns confirm that these two areas are composed of single LaFeO3 oxide (area A) and α′-Fe phase (area B), respectively. It is worthy to note that the SAED pattern of the LaFeO3 oxide exhibits a superstructural feature due to the ordering of oxygen vacancies [26,27]. It implies there are abundant oxygen vacancies in the LaFeO3 oxide. Meanwhile, the calculated interplanar spacing by SAED patterns is slightly smaller than the referenced value in the standard PDF card, which is also an evidence for the existence of oxygen vacancies. It is also worthy to note that there is an interface between the LaFeO3 oxide and the α′-Fe matrix. The interfacial area (area C in Fig. 8a) is formed by mechanical contact without any characteristics of chemical bonding. The EDS measurements are used to detect the elemental migration across the interface. The results show that only iron and oxygen can be detected in the interfacial area (see Fig. 8f). The oxygen content in the interfacial area is lower than that in the LaFeO3 oxide (see Fig. 8d), which verifies the migration of oxygen atoms from the LaFeO3 oxide to the matrix. The lanthanum atoms cannot diffuse out of the LaFeO3 oxide. It implies that the rare earth element works only on the nitrided surface and cannot affect the diffusion of nitrogen atoms in the deeper layer. The diffusion of oxygen atoms also further explains the existence of oxygen vacancies in the LaFeO3 oxide.
It is known that the gas nitriding process contains the following three steps, (I) the adsorption of ammonia molecules on the surface, (II) the formation of active nitrogen atoms by surface reactions, and (III) the diffusion of nitrogen atoms towards the inner layers [28]. The slowest step among three steps above is the key fact to control the growth of nitrided layers. Therefore, in order to understand the catalytic behavior of the LaFeO3 oxide, the adsorption and dissociation of ammonia as well as the diffusion of nitrogen atoms should be clarified by analyzing the chemical state of elements and atomic bonds on the nitrided surface. Fig. 9 shows the high-resolution XPS spectra of Fe 2p3/ 2 , La 3d5/2, N 1s and O 1s obtained from nitrided surfaces of LPN-1 samples with and without LaFeO3 coating before and after argon ion sputtering. The ion sputtering is used to evaluate the strength of atomic bonds on the nitrided surface. All peaks in XPS spectra are identified according our previous work and references [21,29–34]. It can be seen that the peaks for all metallic atoms will move to the low binding energy after sputtering. The shift of peak corresponds to the change of chemical bonding state [35,36]. It also indicates that the metallic atoms are active sites for adsorption and dissociation of ammonia molecules during gas nitriding. When the metallic atoms bond with non-metallic atoms (see Fe-N and Fe-O bonds in Fig. 9d and f), the binding energies are fixed without any shift for the uncoated sample due to the strong bonding strength. But the binding energy for the adsorbed NHx species still tends to low energy level along with the iron atoms, which suggests the ammonia molecules are mainly adsorbed on the iron sites. For the LaFeO3 coated samples, both the Fe-N and Fe-O peaks shift to low binding energy (see Fig. 9e and g). The chemical shift of Fe-N and Fe-O bonds after sputtering indicates the weakened strength for the corresponding bonds. It is true of that the rare earth element contributes to all these changes. It is worthy to note that the diffusion rate of nitrogen atoms in the lattice of iron significantly depends on the interaction between the iron and nitrogen atoms. The weakened Fe-N bonds undoubtedly decrease the activation energy of diffusion for nitrogen atoms, which is beneficial for the rapid migration of nitrogen atoms. The step (III) will not be the rate-determining step of the rapid nitriding for the LaFeO3 coated samples any more. Meanwhile, the hydroxyl can be detected in the XPS spectra of O 1s (see Fig. 9f and g), which indicates the dehydrogenation of ammonia molecules occurs on the nitrided surface. The dehydrogenation reaction finally results in the formation of active nitrogen atoms [37]. The LaFeO3 coated sample exhibits a higher percentage of hydroxyl (52.66%, calculated by the area ratio of hydroxyl to total peak of oxygen) than the uncoated one (42.52%). This indicates a more intense dissociation of ammonia
4. Discussions Based on the results above, a phenomenon can be observed, i.e. the LaFeO3 oxide presents an excellent catalytic activity during low-pressure gas nitriding. That cannot only increase the surface hardness but also improve the EHL depth. Although the catalytic mechanism of the LaFeO3 oxide has been proposed in our previous work [21], it is incomplete and not suitable for the low-pressure conditions. Therefore we make effort to discuss and reveal the catalytic mechanism of the LaFeO3 oxide during low-pressure gas nitriding in detail. The effects of the gas pressure and ammonia flow rate on the catalytic efficiency of the LaFeO3 oxide are also discussed. It is a good extension for our previous work.
6
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 7. SEM images and elemental distribution of nitrided surfaces treated at 550 °C for 2 h with and without LaFeO3 coating. Two kinds of typical samples located in the effective area and invalid/weak area (as shown in Fig. 4) are selected for comparison.
of nitrogen atoms.
molecules occurred on the LaFeO3 coated surface than that on the uncoated surface. On the other hand, the weakened Fe-O bonds for the LaFeO3 coated sample will induce massive oxygen vacancies in the LaFeO3 oxide. Those excess oxygen atoms diffuse to the nitrided surface to accelerate the dehydrogenation process. Therefore, the step (II) is also not the barrier of rapid nitriding for the LaFeO3 coated sample. For the adsorption of ammonia molecules, although the percentage of the adsorbed NHx species for the LaFeO3 coated samples is lower than that for the uncoated samples, it does not mean that the adsorption of ammonia molecules on the LaFeO3 coated surface is weak due to the unknown ratio of physical adsorption of ammonia molecules. Considering the excellent catalytic effect of the LaFeO3 oxide, it can be concluded that the adsorption of ammonia molecules is not the key fact to determine the catalytic efficiency of the LaFeO3 oxide. The adsorption of ammonia molecules is sufficient under present nitriding conditions for both the uncoated and LaFeO3 coated samples. Therefore, the catalytic effect of the LaFeO3 oxide mainly reflects in the aspects of accelerating the rates in the decomposition of ammonia molecules and the diffusion
4.2. Catalytic feature and conditions for the LaFeO3 oxide It is clear that the rare earth element can vary the chemical state of Fe-N and Fe-O bonds. The XPS peaks of Fe-N and Fe-O bonds shift to high binding energy with the rare earth addition (see the spectra before sputtering in Fig. 9d-g), which is attributed to the Fermi level moving up in energy [38]. The rising Fermi level will lead to the oxygen and nitrogen ions to deviate from their equilibrium state to the active state due to the loss of electrons, which indeed accelerate the dehydrogenation of NHx species and diffusion of nitrogen atoms. We can also find a certain rule via comparing the XPS spectra of O 1s for the samples with and without LaFeO3 coating, i.e. the XPS spectra of O 1s with higher binding energy and higher percentage of hydroxyl corresponds to the excellent catalytic efficiency for the LaFeO3 coated samples. Fig. 10 gives the high-resolution XPS spectra of O 1s obtained from nitrided surfaces with and without LaFeO3 coating under different 7
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 8. TEM images of the nitrided surface with LaFeO3 coating treated at 550 °C for 1 h. (a) Bright field image of the nitrided surface with LaFeO3 coating, (b)-(c) SAED patterns of area A and area B in (a) respectively, (d)-(f) EDS spectra for area A, B and C in (a) respectively.
conditions of gas pressures and ammonia flow rates. It can be seen that the LaFeO3 coated sample nitrided at low gas pressure and low ammonia flow rate presents such typical catalytic feature of the XPS spectra of O 1s (see Fig. 10a). The catalytic feature disappears with the increase of the gas pressure and ammonia flow rate due to the sufficient supplement of hydrogen atoms to bring the oxygen ions back to their equilibrium state. The gas pressure and ammonia flow rate mainly affect the formation of hydroxyl to determine the catalytic efficiency of the LaFeO3 oxide. Meanwhile, the gas pressure and ammonia flow rate can also affect the morphologies of nitrided surfaces. Fig. 11 displays the typical morphologies of nitrided surfaces with LaFeO3 coating after treated at 550 °C for 2 h under different gas pressures and ammonia flow rates. It clearly shows that the surface morphologies vary obviously with the gas pressure and ammonia flow rate. The LaFeO3 coating formed during heating process exhibit a smooth surface before nitriding (see Fig. 11d). After nitriding under conditions of 0.01 MPa–0.1 L/min, there are massive nitrides preferential formed on the LaFeO3 coating and a rough surface is obtained. Then the nitrides grow rapidly with the increase of gas pressures and ammonia flow rates and fill into the cracks gradually. The catalytic efficiency of the LaFeO3 oxide disappears with the decrease of roughness on the nitrided surface
(see Fig. 11b and c). Based on the discussions above, it can be concluded that the catalytic efficiency of the LaFeO3 oxide mainly depends on the rate of the dehydrogenation reaction which is also affected by the gas pressure and ammonia flow rate. On the other hand, the low gas pressure and low ammonia flow rate provide a rough surface that is beneficial for the adsorption of ammonia molecules.
4.3. Durability of the catalytic activity of the LaFeO3 oxide The LaFeO3 oxide has been proved to be an excellent catalyst in the low-pressure gas nitriding with low ammonia flow rate. In order to evaluate the durability of the catalytic activity of the LaFeO3 oxide, the nitriding duration is prolonged to 16 h. Fig. 12 shows the microhardness profiles and growth kinetics of nitrided layers treated at 550 °C for different durations with and without LaFeO3 coating. It can be seen that the LaFeO3 coated samples present an overwhelming advantage in the hardness and depth of EHL as compared with the uncoated ones. The high catalytic efficiency for the LaFeO3 oxide is maintained up to 16 h or more. The depth of EHL for the LaFeO3 coated sample can reach to 431 μm which is twice as thick as that for the uncoated sample. The remarkable durability of the catalytic activity of the LaFeO3 oxide is 8
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 9. High-resolution XPS spectra of (a)-(b) Fe 2p3/2, (c) La 3d5/2, (d)-(e) N 1s and (f)-(g) O 1s obtained from nitrided surfaces of LPN-1 samples with and without LaFeO3 coating before and after argon ion sputtering. The sputtering time is 1 min.
5. Conclusions
attributed to the optimized nitriding potential on the nitrided surface controlled by the gas pressure and ammonia flow rate. The low gas pressure and ammonia flow rate can maintain the rough nitrided surface for a long duration, which is necessary for the rapid nitriding. It is worthy to note that the low-pressure gas nitriding with the LaFeO3 catalyst addition is a novel technology with many economical and ecological advantages. It is performed under low pressure and low ammonia flow rate indicating the low gas consumption and low emission of exhaust gases as the ZeroFlow gas nitriding [39]. Therefore, the LaFeO3 oxide has a great potential in applications of the rapid and deep nitriding.
In this paper, the LaFeO3 perovskite oxide was apply in the lowpressure gas nitriding to clarify the best catalytic conditions. The effects of gas pressure and ammonia flow rate on the catalytic efficiency of the LaFeO3 perovskite oxide have been investigated and its catalytic mechanism has been understood clearly. A few conclusions were summarized as follows: (1) The LaFeO3 oxide exhibited an excellent catalytic activity in the low-pressure gas nitriding, which could obviously increase the depth of effective hardened layers and the surface hardness as well as improve the formation of iron nitrides. (2) The catalytic efficiency of the LaFeO3 oxide was sensitive to the 9
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 10. High-resolution XPS spectra of O 1s obtained from nitrided surfaces with and without LaFeO3 coating treated at 550 °C for 2 h under different conditions of (a) 0.02 MPa-0,2 L/min and (b) 0.1 MPa-1.0 L/min.
Fig. 11. SEM images of nitrided surfaces with LaFeO3 coating after treated at 550 °C for 2 h under different conditions of gas pressures and ammonia flow rates. (a) 0.01 MPa-0.1 L/min, (b) 0.02 MPa-0.2 L/min, (c) 0.1 MPa-1.0 L/min and (d) the initial morphology of LaFeO3 coating before nitriding.
the rare earth nitriding. The gas pressure and ammonia flow rate mainly affected the catalytic efficiency of the LaFeO3 oxide by varying the rate of the dehydrogenation reaction. (5) The low gas pressure and low ammonia flow rate could maintain the nitrided surface in a rough status, which is beneficial for the adsorption of ammonia molecules as well as the increase of the nitriding potential. The durability of the catalytic activity of the LaFeO3 oxide could be maintained under low gas pressures and low ammonia flow rate, which indicates the LaFeO3 oxide has a great potential in applications of the rapid and deep nitriding.
nitriding potential. The highest catalytic efficiency occurred at low nitriding potential conditions, i.e. lower gas pressure and lower ammonia flow rate. The influence of the gas pressure was more significant than that of the ammonia flow rate. (3) The LaFeO3 oxide performed the catalytic activity on the nitrided surface by accelerating the dissociation rate of ammonia molecules and the diffusion rate of active nitrogen atoms. It can be attributed to the massive oxygen vacancies in the LaFeO3 oxide and the weakened Fe-N and Fe-O bonds induced by rare earth addition. (4) The excellent catalytic efficiency corresponded to the high percentage of hydroxyl on the nitrided surface. The dehydrogenation reaction of ammonia molecules was the rate-determining step for 10
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
Fig. 12. Microhardness profiles of nitrided layers treated at 550 °C for (a) 1 h, (b) 4 h, (c) 8 h and (d)16 h with and without LaFeO3 coating and (e) growth kinetics of nitrided layers as a function of nitriding duration. The inset in (e) shows the surface morphology of the LaFeO3 coated sample after nitrided for 16 h.
Author contributions
plasma nitriding for AISI4140 steel, Vacuum 109 (2014) 170–174. [10] J. Li, X. Yang, S. Wang, K. Wei, J. Hu, A rapid D.C. plasma nitriding technology catalyzed by pre-oxidation for AISI4140 steel, Mater. Lett. 116 (2014) 199–202. [11] B. Arifvianto, M. Suyitno, Mahardika, Effects of surface mechanical attrition treatment (SMAT) on a rough surface of AISI 316L stainless steel, Appl. Surf. Sci. 258 (2012) 4538–4543. [12] B. Wang, W. Fu, F. Dong, G. Jin, W. Feng, Z. Wang, S. Sun, Significant acceleration of nitriding kinetics in pure iron by pressurized gas treatment, Mater. Des. 85 (2015) 91–96. [13] B. Wang, S. Sun, M. Guo, G. Jin, Z. Zhou, W. Fu, Study on pressurized gas nitriding characteristics for steel 38CrMoAlA, Surf. Coat. Technol. 279 (2015) 60–64. [14] J. Peng, H. Dong, T. Bell, F. Chen, Z. Mo, C. Wang, Q. Peng, Effect of rare earth elements on plasma nitriding of 38CrMoAI steel, Surf. Eng. 12 (1996) 147–151. [15] M. Dai, C. Li, J. Hu, The enhancement effect and kinetics of rare earth assisted salt bath nitriding, J. Alloy. Compd. 688 (2016) 350–356. [16] M.F. Yan, R.L. Liu, Influence of process time on microstructure and properties of 17–4PH steel plasma nitrocarburized with rare earths addition at low temperature, Appl. Surf. Sci. 256 (2010) 6065–6071. [17] M.F. Yan, R.L. Liu, Martensitic stainless steel modified by plasma nitrocarburizing at conventional temperature with and without rare earths addition, Surf. Coat. Technol. 205 (2010) 345–349. [18] X. Wang, M. Yan, R. Liu, Y. Zhang, Effect of rare earth addition on microstructure and corrosion behavior of plasma nitrocarburized M50NiL steel, J. Rare. Earth. 34 (2016) 1148–1155. [19] Y. Wu, M. Yan, Effects of lanthanum and cerium on low temperature plasma nitrocarburizing of nanocrystallized 3J33 steel, J. Rare. Earth. 29 (2011) 383–387. [20] L.N. Tang, M.F. Yan, Effects of rare earths addition on the microstructure, wear and corrosion resistances of plasma nitrided 30CrMnSiA steel, Surf. Coat. Technol. 206 (2012) 2363–2370. [21] X. Chen, X. Bao, Y. Xiao, C. Zhang, L. Tang, L. Yao, G. Cui, Y. Yang, Low-temperature gas nitriding of AISI 4140 steel accelerated by LaFeO3 perovskite oxide, Appl. Surf. Sci. 466 (2019) 989–999. [22] C.S. Zhang, M.F. Yan, Z. Sun, Experimental and theoretical study on interaction between lanthanum and nitrogen during plasma rare earth nitriding, Appl. Surf. Sci. 287 (2013) 381–388. [23] Z. Zhong, K. Chen, Y. Ji, Q. Yan, Methane combustion over B-site partially substituted perovskite-type LaFeO3 prepared by sol-gel method, Appl. Catal. A 156 (1997) 29–41. [24] S.A. Mahadik, F. Pedraza, S.S. Mahadik, Comparative studies on water repellent coatings prepared by spin coating and spray coating methods, Prog. Org. Coat. 104 (2017) 217–222. [25] K. Kiełbasa, R. Pelka, W. Arabczyk, Studies of the Kinetics of Ammonia Decomposition on Promoted Nanocrystalline Iron Using Gas Phases of Different Nitriding Degree, J. Phys. Chem. A 114 (2010) 4531–4534. [26] S. Steinsvik, R. Bugge, J. Gjonnes, J. Tafto, T. Norby, The defect structure of SrTilxFexO3-y (x= 0–0.8) investigated by electrical conductivity measurements and electron energy loss spectroscopy (EELS), J. Phys. Chem. Solids 58 (1997) 969–976. [27] R.H.E. van Doorn, A.J. Burggraaf, Structural aspects of the ionic conductivity of La1xSrxCoO3-δ, Solid State Ionics 128 (2000) 65–78. [28] L. Torchane, P. Bilger, J. Dulcy, M. Gantois, Control of iron nitride layers growth kinetics in the binary Fe-N system, Metall. Mater. Trans. A. 27 (1996) 1823–1835. [29] P.A.W. van der Heide, Systematic x-ray photoelectron spectroscopic study of La1xSrx-based perovskite-type oxides, Surf. Interface Anal. 33 (2002) 414–425.
Chengsong Zhang designed the research framework and wrote the manuscript. Yun Wang prepared the nitrided samples and performed the experiments. Xing Chen, Hongtao Chen and Yeqiong Wu performed the SEM, TEM and hardness tests. Guodong Cui and Dazhi Chen participated in analyzing the experimental results. Yixue Wang and Lina Tang contributed to the technical discussions of results. All authors commented on the manuscript. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51601156, 51601048, 51601117 and U1537201). We appreciate Dr. Zhenye Zhong for his assistance in SEM testing. References [1] N. Limodin, Y. Verreman, T.N. Tarfa, Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth, Fatigue Fract. Eng. Mater. Struct. 26 (2003) 811–820. [2] B. Wang, B. Liu, X. Zhang, J. Gu, Enhancing heavy load wear resistance of AISI 4140 steel through the formation of a severely deformed compound-free nitrided surface layer, Surf. Coat. Technol. 356 (2018) 89–95. [3] M. Karakan, A. Alsaran, A. Çelik, Effect of process time on structural and tribological properties of ferritic plasma nitrocarburized AISI 4140 steel, Mater. Des. 25 (2004) 349–353. [4] M. Fattah, F. Mahboubi, Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel, Mater. Des. 31 (2010) 3915–3921. [5] W.P. Tong, N.R. Tao, Z.B. Wang, J. Lu, K. Lu, Nitriding Iron at Lower Temperatures, Science 299 (2003) 686–688. [6] W.P. Tong, Z. Han, L.M. Wang, J. Lu, K. Lu, Low-temperature nitriding of 38CrMoAl steel with a nanostructured surface layer induced by surface mechanical attrition treatment, Surf. Coat. Technol. 202 (2008) 4957–4963. [7] W.P. Tong, C.Z. Liu, W. Wang, N.R. Tao, Z.B. Wang, L. Zuo, J.C. He, Gaseous nitriding of iron with a nanostructured surface layer, Scripta Mater. 57 (2007) 533–536. [8] J. Baranowska, K. Szczecinski, M. Wysiecki, Increasing of gas nitriding kinetics via surface pre-treatment, Surf. Coat. Technol. 151–152 (2002) 534–539. [9] H. Liu, J. Li, F. Sun, J. Hu, Characterization and effect of pre-oxidation on D.C.
11
Applied Surface Science 506 (2020) 145045
C. Zhang, et al.
[30] R. Dudric, A. Vladescu, V. Rednic, M. Neumann, I.G. Deac, R. Tetean, XPS study on La0.67Ca0.33Mn1-xCoxO3 compounds, J. Mol. Struct. 1073 (2014) 66–70. [31] L. Wang, J.Y. Wang, Z.X. Wang, C. He, W. Lyu, W. Yan, L. Yang, Enhanced antimonate (Sb(V)) removal from aqueous solution by La-doped magnetic biochars, Chem. Eng. J. 354 (2018) 623–632. [32] D. Feng, Z. Zhou, M. Bo, An investigation of the thermal degradation of melamine phosphonite by XPS and thermal analysis techniques, Polym. Degrad. Stabil. 50 (1995) 65–70. [33] J. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Systematic XPS studies of metal oxides, hydroxides and peroxides, Phys. Chem. Chem. Phys. 2 (2000) 1319–1324. [34] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. [35] X. Wang, W.T. Zheng, H.W. Tian, S.S. Yu, W. Xu, S.H. Meng, X.D. He, J.C. Han,
[36]
[37] [38]
[39]
12
C.Q. Sun, B.K. Tay, Growth, structural, and magnetic properties of iron nitride thin films deposited by dc magnetron sputtering, Appl. Surf. Sci. 220 (2003) 30–39. Erik Lewin, Mihaela Gorgoi, Franz Schäfers, Svante Svensson, U. Jansson, Influence of sputter damage on the XPS analysis of metastable nanocomposite coatings, Surf. Coat. Technol. 204 (2009) 455–462. G. Lanzani, K. Laasonen, NH3 adsorption and dissociation on a nanosized iron cluster, Int. J. Hydrogen Energy 35 (2010) 6571–6577. E. Beyreuther, S. Grafström, L.M. Eng, XPS investigation of Mn valence in lanthanum manganite thin films under variation of oxygen content, Phys. Rev. B 73 (2006) 155425. L. Maldzinski, J. Tacikowski, 12-ZeroFlow gas nitriding of steels, Thermochemical Surface Engineering of Steels, Woodhead Publishing, Oxford, 2015, pp. 459–483.