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Characteristics of plasma cladding Fe-based alloy coatings with rare earth metal elements
Materials Science and Engineering A 452–453 (2007) 619–624
Characteristics of plasma cladding Fe-based alloy coatings with rare earth metal elements Limin Zhang ∗ , Dongbai Sun, Hongying Yu Beijing Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China Received 7 September 2006; received in revised form 22 October 2006; accepted 26 October 2006
Abstract The characteristics of plasma cladding Fe-based alloy coatings with rare earth (RE) oxide CeO2 and La2 O3 have been investigated. Fe-based alloy powders with different contents of CeO2 and La2 O3 were cladded onto a steel substrate. The clad coatings were examined and tested for microstructural features, chemical compositions, phase structures and hardness of the clad coatings. A scanning electron microscope (SEM) equipped with energy dispersive analysis X-rays energy dispersive spectrum (EDS) was employed to observe the microstructure and analyze the chemical compositions of the clad coatings. The crystallographic phases formed during plasma cladding were characterized by an X-ray diffractometer. And the microhardness was tested by HVS-1000 digital microhardness tester. The results showed that when the addition content of CeO2 and La2 O3 was 0.5 wt.%, the microstructure of the coatings could be refined and purified. And the dilution of clad material from the substrate was reduced. Moreover, some new compounds such as LaNi4 Si and Ce2 Ni22 C3 and hard crystallographic phase, Cr3 Si, formed in the coatings and the microhardness values were higher than those of other coatings. The mechanism of the effects of RE oxide on the coating was discussed in the paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Plasma cladding; Rare earth; Dilution; Microstructure
1. Introduction Cerium and lanthanum, as rare earth (RE) elements, have been applied successfully in many fields, such as metallurgy [1] and chemical engineering [2,3]. An area of current interest is the modification of RE to surface engineering. Previous studies have showed the positive effects of RE in flame spraying, surface chemical treatments, laser alloying and laser cladding. Arenas et al. [4] extended the use of lanthanides to improve the corrosion resistance properties of galvanic layers by using a mass-analysed ion implanter. Sun et al. [5] found that the addition of rare earth made the co-deposition of Al2 O3 particles with chromium become possible using electro-deposition method. Haugsrud [6] have been studied the oxide behavior of Ni and Ni coated with superficial oxide coatings including SiO2 , CeO2 and La2 O3 in the temperature ranging 773–1473 K in 10−4 to 1 atm oxygen, and the oxidation kinetics and the surface kinetics have been determined. Wang et al. [7] investigated the effects of rare earth oxide CeO2 and La2 O3 on the microstructure and
wear resistance of laser-clad nickel-based alloy coatings. Yi et al. [8] showed the application of rare earth (RE) in spray-welding materials, and the iron base fluxing alloy powder was adopted because of its lower price and good wear resistance. However, there is still little published work on the application of RE in plasma cladding. Plasma cladding is an effective material processing method that produces a surface layer with many unique advantages such as high efficiency, good fusion bonding between coating and substrate, a fine microstructure and low porosity and improved surface properties of the coated workpieces [9]. Also the plasma cladding technique allows the deposition of a wider compositional spectrum of metallic and composite coatings since the coating materials used are in powder form and not in wire or ascast rod form [10]. In this paper, the effect of CeO2 and La2 O3 on the microstructure of plasma clad Fe-based alloy coatings was studied so as to offer an experimental basis to expand a more promising application field of RE. 2. Experimental procedures
∗
Corresponding author. Fax: +86 10 62332567. E-mail address: [email protected] (L. Zhang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.142
With the commercial low carbon steel as the substrates, the steel specimens were made into 100 mm × 100 mm × 10 mm.
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Table 1 Parameters of plasma cladding Current (A) Voltage (V) Scanning velocity (mm/min) Feeding gas flow (Ar, m3 /h) Plasma gas flow (Ar, m3 /h) Protective gas flow (Ar, m3 /h) Plasma arc length (mm)
250–300 40–50 400 0.6 0.6 1.6 27
The surface of the steel required no previous surface treatments. Fe-based self-fluxing alloy powder, having a size of 106–180 m, was chosen as the plasma cladding material. The chemical composition of the powder in wt.% are: 30 Ni, 18 Cr, 2 C, 1 B and 0.8 Si with the rest of Fe. The mixture of the CeO2 and La2 O3 , was added into the Fe-based alloy powder in different ratio with 0, 0.15, 0.3, 0.5 and 1 wt.%, respectively. Plasma cladding was carried out with the plasma cladding equipment produced by Shandong University of Science and Technology and the self-feeding powder was adopted. A lot of experiments have been performed to quantify the effects of the plasma clad parameters on the properties of the coating on a microscopic scale, i.e. the thickness, the powder-clad efficiency, the volumetric dilution ration and the crack formation. With the range of plasma parameters, the best results have been obtained and showed in Table 1. After plasma cladding, the specimens were cut along with the vertical of the plasma torch scanning direct and made into the metallographic specimens which were etched using mixed HNO3 /HCl solution with a volume ratio of 3 to reveal the microstructure of the clad coating. The structural and morphological properties of plasma-clad coating were characterized by several techniques. XRD was carried out to identify phase formation using Cu K␣ radiation at a scanning speed of 2◦ min−1 on a Rigaku Dmax-RB X-ray diffractometer. Observation of the coating morphology was performed in a Cambridge S-360 scanning electron microscope (SEM). Tracor Northern TN550 energy dispersive spectrum (EDS) was used to analyze the composition of the clad coating. The microhardness was measured by HVS-1000 digital microhardness tester with a test load of 0.98 N and a dwelling of 20 s. 3. Results and discussion 3.1. Microstructure observation The microstructure of the coating is shown in Fig. 1. It can be seen that the main microstructure of the coating is in dendrite form. The morphology of dendrites is modified by the addition of CeO2 and La2 O3 . When the content of the RE is optimal, the microstructure of the coating is finer than that of without RE, which can be observed from the Fig. 1d. However, it should also be pointed out that when the RE concentration is too high, for example, higher than 0.5 wt.%, the grain size of the clad coating becomes larger as shown in Fig. 1e in which the CeO2 and La2 O3 content is 1 wt.%. When the RE concentration is too low, the effects cannot be visible, as shown in Fig. 1b, and the grain shape
just appears variation while the size of the grain is similar with that of without RE. Therefore, in this paper the suitable RE oxide concentration is 0.5 wt.% and the microstructure of coating is finer than that of others. The grain boundary is broken down and many granular precipitations occur, which are beneficial to the properties of the clad coating. The refining effect of CeO2 and La2 O3 on microstructure of clad coatings is mainly due to the characteristics of the Ce and La. The Ce and La are surface active elements with a rather large atomic radius (radius of cerium and lanthanum are 0.1824 and 0.1877 nm, respectively). The electronegativity of Ce and La is low (1.05 and 1.1 for Ce and La, respectively). It means that Ce and La can form positive ions and react easily with other elements, such as oxygen, sulfur, silicon and nitrogen, then some stable compounds such as Ce2 Ni22 C3 and LaNi4 Si form during plasma cladding. Some compounds as well as CeO2 and La2 O3 itself may become particles of heterogeneous nucleation. It can increase the number of crystal nuclei and hinder the growth of grains during crystallization of the molten pool. Furthermore, the Ce and La can also reduce surface tension and interfacial energy. And their chemical activity can also decrease critical nucleation work. Therefore, the number of nucleation particles increase. All of these factors result in significant increase in nucleation rate, thus the microstructure of the coating can be refined. On the other hand, since the atomic radius of Ce and La is rather large, the existence of Ce and La within the solid solution would surely cause great distortion of lattice, which would increase the energy of the system. In order to retain the lowest Helmholtz free energy, an enrichment of Ce and La over the grain boundary where the atomic arrangement is irregular, would be required. Thus, Ce and La in clad coating distribute mostly over the grain boundary. When the grains grow, the Ce and La atoms and compounds over the grain boundary would make a dragging effect on the movement of grain boundary, and the growth of grain will be suppressed. Therefore, the gain size is further refined. The chemical activity of Ce and La results in the formation of some high melting point compounds with O, S and N, during cladding process. Part of compounds may float on the liquid phase before solidification and slag may occur on the surface of clad coating. Thus, the inclusion content within the coatings is decreased, and the coatings are purified by deoxidation and desulfuration. As stated above, Ce and La have obvious effects of refining grain size and purifying clad coatings. It should be emphasized that it is suitable to add a certain amount of Ce and La. And too much Ce and La would not beneficial to improve the microstructure and properties. 3.2. Chemical composition analysis of clad coating Table 2 shows the amounts of the Si, Cr, Fe, Ni, La and Ce analyzed by EDS in clad coatings near the bonding zone between coating and the substrate. The percentage by weight of Ni and Cr in coating containing CeO2 and La2 O3 is higher than that of the coating without RE while that of Fe is in reverse. It indicates that CeO2 or La2 O3 has a unique effect to reduce dilution of clad
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Fig. 1. The microstructure of plasma clad coatings, the contents of the CeO2 and La2 O3 in (a–e) are 0, 0.15, 0.3, 0.5 and 1 wt. %, respectively.
material from base metal. It is beneficial to the coating properties maintaining the composition of the clad material. With increasing of RE oxide concentration, the percentage by weight of Cr in the coating grows higher while that of Ni is in reverse. The percentage by weight of Si is irregular. As shown in Table 2, the percentage by weight of Si is high when the concentration of the CeO2 and La2 O3 is 1 or 0.3 wt.% and that of Si is low when the concentration of the CeO2 and La2 O3 is 0.5 or 0.15 wt.%.
Therefore, it can be known that Si in the coating must forms the segregation. It is desirable in plasma cladding to minimize the dilution of clad material from base metal. Due to dilution, the chemical composition of the coating would be different from that of clad material and the properties of the coating would be affected. The addition of Ce and La could increase the latent heat of melting of alloy, which causes the lowering of liquidus temperature
Table 2 Results of the EDS test near the bonding zone of the clad coatings
a (without RE) b (with 0.15 wt.% RE) c (with 0.3 wt.% RE) d (with 0.5 wt.% RE) b (with 1 wt.% RE)
Si
Cr
Fe
Ni
La
Ce
0.3608 0.4389 0.2693 0.4070 0.2617
3.8005 3.8024 4.3037 4.4144 5.1853
92.8677 88.7281 88.7555 85.0335 82.2590
2.9711 6.0348 6.2313 9.2209 11.2004
0.0000 0.5576 0.1044 0.4233 0.5414
0.0000 0.43831 0.3358 0.4909 0.452
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and the lifting of solidus temperature. The solidification range would become narrow and time of solidification be shortened. Therefore, the diffusion between coating and substrate would be weakened. As a result, the diffusion of Ni and Cr from the
coating to substrate is lessened and the dilution of clad material from substrate is reduced. In addition, the absorption of Ce and La on the grain boundary would hinder diffusion of other elements, which also results in the reduction of dilution.
Fig. 2. X-ray diffraction spectra of plasma clad coatings.
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Table 3 Diffraction angle of X-ray diffraction peaks Ingredient
Diffraction angle (2θ)
a (without RE) b (with 0.15 wt.% RE) c (with 0.3 wt.% RE) d (with 0.5 wt.% RE) e (with 1 wt.% RE)
43.519 43.520 43.530 43.521 43.500
50.580 50.580 50.585 50.600 50.479
74.562 74.560 74.571 74.572 74.559
90.500 90.479 90.523 90.360 90.360
95.979 95.982 95.982 95.981 95.979
3.3. Phase structure of plasma clad coatings The results of X-ray diffraction are shown in Fig. 2. The addition of CeO2 and La2 O3 results in the formation of compounds such as LaNi4 Si and Ce2 Ni22 C3 in the coating. When the addition of CeO2 and La2 O3 is 0.3 or 0.5 wt.%, the compound Cr3 Si formed in the coating. Cr3 Si, an intermetallic compound, is the production of chromium alloyed with silicon and has a much higher hardness (11.8 GPa) and elastic modulus (350 GPa) than unalloyed chromium. Cr3 Si has been examined as a potential high temperature material and wear resistant material [11]. There does not appear Cr3 Si in the clad coating without RE oxide, while the addition of 0.5 or 0.3 wt.% CeO2 and La2 O3 results in the formation of compound Cr3 Si. The addition of CeO2 and La2 O3 is lower than 0.3 wt.%, the compound Cr3 Si vanish. It is indicated that the proper addition of CeO2 and La2 O3 promotes chromium alloyed with silicon, increasing the high temperature and wear resistant properties of the clad coatings. The diffraction angle 2θ of stronger diffraction peaks of the coatings are shown in Table 3. It can be seen that the diffraction angle 2θ of the coatings with less than 0.5 wt.% CeO2 and La2 O3 are larger than that of the coating without CeO2 and La2 O3 . And the diffraction angle 2θ of the coating with 1 wt.% CeO2 and La2 O3 is smaller than that of the coating without RE. According to Bragg’s law of X-ray diffraction, 2dh k l sin θ = λ, where λ is the wavelength of X-ray and a constant for a certain diffraction condition, dh k l the interplanar spacing of h k l crystal plane and θ is the diffraction angle, it can be referred that the interplanar spacing dh k l of crystal plane decrease with increasing the diffraction angle 2θ. The interplanar spacing of crystal plane in cubic system is as follows: a dhkl = √ h2 + k 2 + l 2 The lattice constant, a, decreases with decreasing of interplanar spacing dh k l for a certain h k l crystal plane. In brief, the addition of CeO2 and La2 O3 , when the concentration of which is less than 0.5 wt.%, could increase the diffraction angle and decrease interplanar spacing and the lattice constant.
Fig. 3. Microhardness profiles of plasma clad coatings.
pool because of the convection, and then the microstructure in the surface is worsening and the microhardness is relatively low. In the middle of coating the microhardness values are highest. The microhardness values decrease at the bonding of coating and substrate. At the bonding of coating and substrate, the microhardness values become low due to dilution. The data also show that the addition of CeO2 and La2 O3 leads to varying of microhardness values of the clad coating. When the addition of the CeO2 and La2 O3 is 0.15 wt.%, the microhardness values almost have no difference from that of without CeO2 and La2 O3 . With increasing the addition of CeO2 and La2 O3 , the average microhardness values appear higher than that of without CeO2 and La2 O3 . The average microhardness of the coating with 0.5 wt.% CeO2 and La2 O3 is highest. But when the addition of CeO2 and La2 O3 is 1 wt.%, the microhardness values are lower than that without rare earth oxide. It can be known from the Xray diffraction spectra that the compound, Cr3 Si, which has a much higher hardness than unalloyed chromium, formed in the plasma clad coatings with 0.3 and 0.5 wt.% CeO2 and La2 O3 , and the microstructure is finer than others yet. All these factors make the hardness values increase. 4. Conclusion
3.4. Hardness of plasma clad coating Fig. 3 presents the microhardness profile along the depth direction of the plasma clad coating. The microhardness profile presents graded distribution. On the surface of the coating, the microhardness values are between 500 and 700 HV0.1 . During the plasma cladding, the slag and impurity float from the molten
(1) The smooth, no crack and porosity clad coating is obtained by plasma cladding. The addition of rare earth oxide CeO2 and La2 O3 is of great significance on plasma-clad Fe-based alloy coatings. (2) The optimum addition of CeO2 and La2 O3 in this study is 0.5 wt.% and the microstructure is refined and purified.
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The dilution of clad material from the substrate is also reduced. (3) Some new phases, LaNi4 Si and Ce2 Ni22 C3 , appear in the clad coating containing Ce and La, which can cause variations of structure of the coatings. The hard crystallographic phase, Cr3 Si, appears in the coatings when the addition content of CeO2 and La2 O3 is 0.3 or 0.5 wt.%. (4) According to Bragg’s law of X-ray diffraction, it can be found that when the concentration of CeO2 and La2 O3 is not more than 0.5 wt.%, the diffraction angle increases while interplanar spacing and the lattice constant decrease. (5) The average microhardness of the coating with 0.3 or 0.5 wt.% CeO2 and La2 O3 is higher than that of coating without CeO2 and La2 O3 . The average microhardness values of the coating with 0.5 wt.% CeO2 and La2 O3 is highest. But when the addition content of the CeO2 and La2 O3 is 1 wt.%, the average microhardness is lower than that of other coatings. Acknowledgements The authors are grateful to Beijing fund government of China for financial support for this work. They would also like to thank
corrosion and protection center of University of Science and Technology Beijing for its help in experimental procedure used in the work. References [1] R. Van Deun, K. Binnemans, Mater. Sci. Eng. C 18 (2001) 211– 215. [2] S.Y. Zhang, F.M. Gao, C.X. Wu, J. Alloys Compd. 275–277 (1998) 835–837. [3] Y.-H. Huang, Z.-G. Xu, C.-H. Yan, et al., Solid State Commun. 114 (2000) 43–47. [4] M.A. Arenas, J.J. de Damborenea, A. Medrano, et al., Surf. Coat. Technol. 158–159 (2002) 615–619. [5] K.-N. Sun, X.-N. Hu, J.-H. Zhang, et al., Wear 196 (1996) 295–297. [6] R. Haugsrud, Corros. Sci. 45 (2003) 1289–1311. [7] K.L. Wang, Q.B. Zhang, M.L. Sun, et al., Appl. Surf. Sci. 174 (2001) 191–200. [8] W. Yi, C. Zheng, Peng Fan, et al., J. Alloys Compd. 311 (2000) 65–68. [9] C. Zhao, F. Tian, H.-R. Peng, J.-Y. Hou, Surf. Coat. Technol. 155 (2002) 80–84. [10] M. Li, H.-d. Li, Y.-z. Sun, Q. Ji, H.-q. Li, The 14th Congress of International Federation for Heat Treatment and Surface Engineering Proceedings-III, Shanghai, China, 2004, pp. 23–26. [11] J.W. Newkirk, J.A. Hawk, Wear 251 (2001) 1361–1371.
Characteristics of plasma cladding Fe-based alloy coatings with rare earth metal elements Limin Zhang ∗ , Dongbai Sun, Hongying Yu Beijing Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, China Received 7 September 2006; received in revised form 22 October 2006; accepted 26 October 2006
Abstract The characteristics of plasma cladding Fe-based alloy coatings with rare earth (RE) oxide CeO2 and La2 O3 have been investigated. Fe-based alloy powders with different contents of CeO2 and La2 O3 were cladded onto a steel substrate. The clad coatings were examined and tested for microstructural features, chemical compositions, phase structures and hardness of the clad coatings. A scanning electron microscope (SEM) equipped with energy dispersive analysis X-rays energy dispersive spectrum (EDS) was employed to observe the microstructure and analyze the chemical compositions of the clad coatings. The crystallographic phases formed during plasma cladding were characterized by an X-ray diffractometer. And the microhardness was tested by HVS-1000 digital microhardness tester. The results showed that when the addition content of CeO2 and La2 O3 was 0.5 wt.%, the microstructure of the coatings could be refined and purified. And the dilution of clad material from the substrate was reduced. Moreover, some new compounds such as LaNi4 Si and Ce2 Ni22 C3 and hard crystallographic phase, Cr3 Si, formed in the coatings and the microhardness values were higher than those of other coatings. The mechanism of the effects of RE oxide on the coating was discussed in the paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Plasma cladding; Rare earth; Dilution; Microstructure
1. Introduction Cerium and lanthanum, as rare earth (RE) elements, have been applied successfully in many fields, such as metallurgy [1] and chemical engineering [2,3]. An area of current interest is the modification of RE to surface engineering. Previous studies have showed the positive effects of RE in flame spraying, surface chemical treatments, laser alloying and laser cladding. Arenas et al. [4] extended the use of lanthanides to improve the corrosion resistance properties of galvanic layers by using a mass-analysed ion implanter. Sun et al. [5] found that the addition of rare earth made the co-deposition of Al2 O3 particles with chromium become possible using electro-deposition method. Haugsrud [6] have been studied the oxide behavior of Ni and Ni coated with superficial oxide coatings including SiO2 , CeO2 and La2 O3 in the temperature ranging 773–1473 K in 10−4 to 1 atm oxygen, and the oxidation kinetics and the surface kinetics have been determined. Wang et al. [7] investigated the effects of rare earth oxide CeO2 and La2 O3 on the microstructure and
wear resistance of laser-clad nickel-based alloy coatings. Yi et al. [8] showed the application of rare earth (RE) in spray-welding materials, and the iron base fluxing alloy powder was adopted because of its lower price and good wear resistance. However, there is still little published work on the application of RE in plasma cladding. Plasma cladding is an effective material processing method that produces a surface layer with many unique advantages such as high efficiency, good fusion bonding between coating and substrate, a fine microstructure and low porosity and improved surface properties of the coated workpieces [9]. Also the plasma cladding technique allows the deposition of a wider compositional spectrum of metallic and composite coatings since the coating materials used are in powder form and not in wire or ascast rod form [10]. In this paper, the effect of CeO2 and La2 O3 on the microstructure of plasma clad Fe-based alloy coatings was studied so as to offer an experimental basis to expand a more promising application field of RE. 2. Experimental procedures
∗
Corresponding author. Fax: +86 10 62332567. E-mail address: [email protected] (L. Zhang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.142
With the commercial low carbon steel as the substrates, the steel specimens were made into 100 mm × 100 mm × 10 mm.
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Table 1 Parameters of plasma cladding Current (A) Voltage (V) Scanning velocity (mm/min) Feeding gas flow (Ar, m3 /h) Plasma gas flow (Ar, m3 /h) Protective gas flow (Ar, m3 /h) Plasma arc length (mm)
250–300 40–50 400 0.6 0.6 1.6 27
The surface of the steel required no previous surface treatments. Fe-based self-fluxing alloy powder, having a size of 106–180 m, was chosen as the plasma cladding material. The chemical composition of the powder in wt.% are: 30 Ni, 18 Cr, 2 C, 1 B and 0.8 Si with the rest of Fe. The mixture of the CeO2 and La2 O3 , was added into the Fe-based alloy powder in different ratio with 0, 0.15, 0.3, 0.5 and 1 wt.%, respectively. Plasma cladding was carried out with the plasma cladding equipment produced by Shandong University of Science and Technology and the self-feeding powder was adopted. A lot of experiments have been performed to quantify the effects of the plasma clad parameters on the properties of the coating on a microscopic scale, i.e. the thickness, the powder-clad efficiency, the volumetric dilution ration and the crack formation. With the range of plasma parameters, the best results have been obtained and showed in Table 1. After plasma cladding, the specimens were cut along with the vertical of the plasma torch scanning direct and made into the metallographic specimens which were etched using mixed HNO3 /HCl solution with a volume ratio of 3 to reveal the microstructure of the clad coating. The structural and morphological properties of plasma-clad coating were characterized by several techniques. XRD was carried out to identify phase formation using Cu K␣ radiation at a scanning speed of 2◦ min−1 on a Rigaku Dmax-RB X-ray diffractometer. Observation of the coating morphology was performed in a Cambridge S-360 scanning electron microscope (SEM). Tracor Northern TN550 energy dispersive spectrum (EDS) was used to analyze the composition of the clad coating. The microhardness was measured by HVS-1000 digital microhardness tester with a test load of 0.98 N and a dwelling of 20 s. 3. Results and discussion 3.1. Microstructure observation The microstructure of the coating is shown in Fig. 1. It can be seen that the main microstructure of the coating is in dendrite form. The morphology of dendrites is modified by the addition of CeO2 and La2 O3 . When the content of the RE is optimal, the microstructure of the coating is finer than that of without RE, which can be observed from the Fig. 1d. However, it should also be pointed out that when the RE concentration is too high, for example, higher than 0.5 wt.%, the grain size of the clad coating becomes larger as shown in Fig. 1e in which the CeO2 and La2 O3 content is 1 wt.%. When the RE concentration is too low, the effects cannot be visible, as shown in Fig. 1b, and the grain shape
just appears variation while the size of the grain is similar with that of without RE. Therefore, in this paper the suitable RE oxide concentration is 0.5 wt.% and the microstructure of coating is finer than that of others. The grain boundary is broken down and many granular precipitations occur, which are beneficial to the properties of the clad coating. The refining effect of CeO2 and La2 O3 on microstructure of clad coatings is mainly due to the characteristics of the Ce and La. The Ce and La are surface active elements with a rather large atomic radius (radius of cerium and lanthanum are 0.1824 and 0.1877 nm, respectively). The electronegativity of Ce and La is low (1.05 and 1.1 for Ce and La, respectively). It means that Ce and La can form positive ions and react easily with other elements, such as oxygen, sulfur, silicon and nitrogen, then some stable compounds such as Ce2 Ni22 C3 and LaNi4 Si form during plasma cladding. Some compounds as well as CeO2 and La2 O3 itself may become particles of heterogeneous nucleation. It can increase the number of crystal nuclei and hinder the growth of grains during crystallization of the molten pool. Furthermore, the Ce and La can also reduce surface tension and interfacial energy. And their chemical activity can also decrease critical nucleation work. Therefore, the number of nucleation particles increase. All of these factors result in significant increase in nucleation rate, thus the microstructure of the coating can be refined. On the other hand, since the atomic radius of Ce and La is rather large, the existence of Ce and La within the solid solution would surely cause great distortion of lattice, which would increase the energy of the system. In order to retain the lowest Helmholtz free energy, an enrichment of Ce and La over the grain boundary where the atomic arrangement is irregular, would be required. Thus, Ce and La in clad coating distribute mostly over the grain boundary. When the grains grow, the Ce and La atoms and compounds over the grain boundary would make a dragging effect on the movement of grain boundary, and the growth of grain will be suppressed. Therefore, the gain size is further refined. The chemical activity of Ce and La results in the formation of some high melting point compounds with O, S and N, during cladding process. Part of compounds may float on the liquid phase before solidification and slag may occur on the surface of clad coating. Thus, the inclusion content within the coatings is decreased, and the coatings are purified by deoxidation and desulfuration. As stated above, Ce and La have obvious effects of refining grain size and purifying clad coatings. It should be emphasized that it is suitable to add a certain amount of Ce and La. And too much Ce and La would not beneficial to improve the microstructure and properties. 3.2. Chemical composition analysis of clad coating Table 2 shows the amounts of the Si, Cr, Fe, Ni, La and Ce analyzed by EDS in clad coatings near the bonding zone between coating and the substrate. The percentage by weight of Ni and Cr in coating containing CeO2 and La2 O3 is higher than that of the coating without RE while that of Fe is in reverse. It indicates that CeO2 or La2 O3 has a unique effect to reduce dilution of clad
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Fig. 1. The microstructure of plasma clad coatings, the contents of the CeO2 and La2 O3 in (a–e) are 0, 0.15, 0.3, 0.5 and 1 wt. %, respectively.
material from base metal. It is beneficial to the coating properties maintaining the composition of the clad material. With increasing of RE oxide concentration, the percentage by weight of Cr in the coating grows higher while that of Ni is in reverse. The percentage by weight of Si is irregular. As shown in Table 2, the percentage by weight of Si is high when the concentration of the CeO2 and La2 O3 is 1 or 0.3 wt.% and that of Si is low when the concentration of the CeO2 and La2 O3 is 0.5 or 0.15 wt.%.
Therefore, it can be known that Si in the coating must forms the segregation. It is desirable in plasma cladding to minimize the dilution of clad material from base metal. Due to dilution, the chemical composition of the coating would be different from that of clad material and the properties of the coating would be affected. The addition of Ce and La could increase the latent heat of melting of alloy, which causes the lowering of liquidus temperature
Table 2 Results of the EDS test near the bonding zone of the clad coatings
a (without RE) b (with 0.15 wt.% RE) c (with 0.3 wt.% RE) d (with 0.5 wt.% RE) b (with 1 wt.% RE)
Si
Cr
Fe
Ni
La
Ce
0.3608 0.4389 0.2693 0.4070 0.2617
3.8005 3.8024 4.3037 4.4144 5.1853
92.8677 88.7281 88.7555 85.0335 82.2590
2.9711 6.0348 6.2313 9.2209 11.2004
0.0000 0.5576 0.1044 0.4233 0.5414
0.0000 0.43831 0.3358 0.4909 0.452
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and the lifting of solidus temperature. The solidification range would become narrow and time of solidification be shortened. Therefore, the diffusion between coating and substrate would be weakened. As a result, the diffusion of Ni and Cr from the
coating to substrate is lessened and the dilution of clad material from substrate is reduced. In addition, the absorption of Ce and La on the grain boundary would hinder diffusion of other elements, which also results in the reduction of dilution.
Fig. 2. X-ray diffraction spectra of plasma clad coatings.
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Table 3 Diffraction angle of X-ray diffraction peaks Ingredient
Diffraction angle (2θ)
a (without RE) b (with 0.15 wt.% RE) c (with 0.3 wt.% RE) d (with 0.5 wt.% RE) e (with 1 wt.% RE)
43.519 43.520 43.530 43.521 43.500
50.580 50.580 50.585 50.600 50.479
74.562 74.560 74.571 74.572 74.559
90.500 90.479 90.523 90.360 90.360
95.979 95.982 95.982 95.981 95.979
3.3. Phase structure of plasma clad coatings The results of X-ray diffraction are shown in Fig. 2. The addition of CeO2 and La2 O3 results in the formation of compounds such as LaNi4 Si and Ce2 Ni22 C3 in the coating. When the addition of CeO2 and La2 O3 is 0.3 or 0.5 wt.%, the compound Cr3 Si formed in the coating. Cr3 Si, an intermetallic compound, is the production of chromium alloyed with silicon and has a much higher hardness (11.8 GPa) and elastic modulus (350 GPa) than unalloyed chromium. Cr3 Si has been examined as a potential high temperature material and wear resistant material [11]. There does not appear Cr3 Si in the clad coating without RE oxide, while the addition of 0.5 or 0.3 wt.% CeO2 and La2 O3 results in the formation of compound Cr3 Si. The addition of CeO2 and La2 O3 is lower than 0.3 wt.%, the compound Cr3 Si vanish. It is indicated that the proper addition of CeO2 and La2 O3 promotes chromium alloyed with silicon, increasing the high temperature and wear resistant properties of the clad coatings. The diffraction angle 2θ of stronger diffraction peaks of the coatings are shown in Table 3. It can be seen that the diffraction angle 2θ of the coatings with less than 0.5 wt.% CeO2 and La2 O3 are larger than that of the coating without CeO2 and La2 O3 . And the diffraction angle 2θ of the coating with 1 wt.% CeO2 and La2 O3 is smaller than that of the coating without RE. According to Bragg’s law of X-ray diffraction, 2dh k l sin θ = λ, where λ is the wavelength of X-ray and a constant for a certain diffraction condition, dh k l the interplanar spacing of h k l crystal plane and θ is the diffraction angle, it can be referred that the interplanar spacing dh k l of crystal plane decrease with increasing the diffraction angle 2θ. The interplanar spacing of crystal plane in cubic system is as follows: a dhkl = √ h2 + k 2 + l 2 The lattice constant, a, decreases with decreasing of interplanar spacing dh k l for a certain h k l crystal plane. In brief, the addition of CeO2 and La2 O3 , when the concentration of which is less than 0.5 wt.%, could increase the diffraction angle and decrease interplanar spacing and the lattice constant.
Fig. 3. Microhardness profiles of plasma clad coatings.
pool because of the convection, and then the microstructure in the surface is worsening and the microhardness is relatively low. In the middle of coating the microhardness values are highest. The microhardness values decrease at the bonding of coating and substrate. At the bonding of coating and substrate, the microhardness values become low due to dilution. The data also show that the addition of CeO2 and La2 O3 leads to varying of microhardness values of the clad coating. When the addition of the CeO2 and La2 O3 is 0.15 wt.%, the microhardness values almost have no difference from that of without CeO2 and La2 O3 . With increasing the addition of CeO2 and La2 O3 , the average microhardness values appear higher than that of without CeO2 and La2 O3 . The average microhardness of the coating with 0.5 wt.% CeO2 and La2 O3 is highest. But when the addition of CeO2 and La2 O3 is 1 wt.%, the microhardness values are lower than that without rare earth oxide. It can be known from the Xray diffraction spectra that the compound, Cr3 Si, which has a much higher hardness than unalloyed chromium, formed in the plasma clad coatings with 0.3 and 0.5 wt.% CeO2 and La2 O3 , and the microstructure is finer than others yet. All these factors make the hardness values increase. 4. Conclusion
3.4. Hardness of plasma clad coating Fig. 3 presents the microhardness profile along the depth direction of the plasma clad coating. The microhardness profile presents graded distribution. On the surface of the coating, the microhardness values are between 500 and 700 HV0.1 . During the plasma cladding, the slag and impurity float from the molten
(1) The smooth, no crack and porosity clad coating is obtained by plasma cladding. The addition of rare earth oxide CeO2 and La2 O3 is of great significance on plasma-clad Fe-based alloy coatings. (2) The optimum addition of CeO2 and La2 O3 in this study is 0.5 wt.% and the microstructure is refined and purified.
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The dilution of clad material from the substrate is also reduced. (3) Some new phases, LaNi4 Si and Ce2 Ni22 C3 , appear in the clad coating containing Ce and La, which can cause variations of structure of the coatings. The hard crystallographic phase, Cr3 Si, appears in the coatings when the addition content of CeO2 and La2 O3 is 0.3 or 0.5 wt.%. (4) According to Bragg’s law of X-ray diffraction, it can be found that when the concentration of CeO2 and La2 O3 is not more than 0.5 wt.%, the diffraction angle increases while interplanar spacing and the lattice constant decrease. (5) The average microhardness of the coating with 0.3 or 0.5 wt.% CeO2 and La2 O3 is higher than that of coating without CeO2 and La2 O3 . The average microhardness values of the coating with 0.5 wt.% CeO2 and La2 O3 is highest. But when the addition content of the CeO2 and La2 O3 is 1 wt.%, the average microhardness is lower than that of other coatings. Acknowledgements The authors are grateful to Beijing fund government of China for financial support for this work. They would also like to thank
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