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Microstructure and properties of coatings with rare earth formed by DC-plasma jet surface metallurgy
Surface & Coatings Technology 200 (2006) 4741 – 4745 www.elsevier.com/locate/surfcoat
Microstructure and properties of coatings with rare earth formed by DC-plasma jet surface metallurgy Hao Chen a,*, Hui-qi Li a,b, Yu-zong Sun a, Min Li a,b a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China b School of Materials Science, Shandong University of Science and Technology, Qingdao 266510, PR China Received 25 October 2004; accepted in revised form 10 April 2005 Available online 6 July 2005
Abstract A new method of surface modification, DC-plasma jet surface metallurgy, is proposed in order to improve wear resistance of materials. This paper reports on an experiment, in which some metallurgical coatings of iron-based alloy with different contents of La2O3 and CeO2 were prepared on AISI1020 steel using a homemade DC-plasma jet surface metallurgy equipment. A scanning electron microscope (SEM), energy dispersive X-ray (EDX) microanalysis and X-ray diffraction analysis (XRD) were employed to observe the microstructure and analyzed the chemical compositions of coatings. The effect of RE oxide (La2O3 and CeO2) on microstructure and properties of coatings was investigated. The result shows that addition of a proper amount of RE oxide (La2O3 and CeO2) can refine and purify the microstructure, significantly increase microhardness and enhance the wear resistance. D 2005 Elsevier B.V. All rights reserved. Keywords: Rare earth; Microstructure; Wear resistance; DC-plasma jet surface metallurgy
1. Introduction Failures of mechanical parts mostly originate from surfaces. Some machine parts cease to be effective only due to the damage on the surface caused by wear or corrosion. Hence, regular downtimes are needed for repair and replacement. In order to increase the service life of such machine parts, surfacing techniques are often used to improve the surface properties of components. Such as laser cladding is an advanced surface modification technology to synthesize wear and corrosion resistant coating materials for components. However, the efficiency of energy transfer of laser is rather low (range 10% to 25%) and its application is limited by the expense of the equipment and the dependence on the reflectivity of bulk material.
* Corresponding author. Present address: School of Materials Science, Shandong University of Science and Technology, Qingdao 266510, PR China. Tel.: +86 532 6057927; fax: +86 532 6057927. E-mail address: [email protected] (H. Chen).
DC-plasma jet surface metallurgy is a high new technique developing fast and an effective material processing method that produces a coating with many unique advantages, such as minimized dilution of the powders material from the base metal, metallurgically bonding between coating and substrate, a fine microstructure and improved surface properties of the coated workpieces [1,2]. It is a kind of rapid, non-equilibrium metallurgical process, which is similar to powder metallurgy. The process cannot be restricted by consistency, melting point and density of constituents; therefore, prealloy powders are not needed. And it can obtain some special structures and phases in random powder and random match. Rare earth (RE) elements have been applied successfully in many fields, such as metallurgy and chemical engineering. Previous study showed the positive effects of RE in flame spraying and surface chemical treatment [3,4]. However, there is still little published work on the application of RE in DC-plasma jet surface metallurgy. With the purpose of making use of excellent characteristic
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.061
SCT-11475; No of Pages 5
4742
H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
of RE to improve the microstructure and properties of composite coatings, this paper presents the experimental research on micro-structural evolution in high power plasma jet surface metallurgy of Fe-based alloy powder with RE oxide. The microstructure, microhardness and wear resistance of coatings were investigated. And the effect of the RE on the microstructure of coatings was studied so as to offer an experimental basis to expand a more promising application field of RE.
(a)
plasma torch
2. Experimental details
(b)
2.1. Materials Low-carbon steel (AISI1020) was used as a substrate material for the DC-plasma jet surface metallurgy. The dimensions of the substrate were 100 mm 40 mm 10 mm. Fe-based alloy powder was used as the coating material. The particles of the Fe-based alloy powder were less than 150 Am in size and were spherical in shape. Table 1 shows the chemical composition of Fe-based alloy powder. In order to improve the microstructure and properties of coatings, La2O3 and CeO2, having a purity of over 99% and each accounts for 50%, were added being to the Fe-based alloy powder in different rations ranging from 0 to 0.6 wt.%. 2.2. Experimental procedure DC-plasma jet surface metallurgy basically consists of the powder, with which the coating is to be formed, meeting the plasma jet at the same times as the surface, thus causing both to melt simultaneously and then rapidly solidify to form a dense coating, metallurgically bonded to the base material. The test equipment was carried out by means of a homemade set-up for transferred arc plasma jet surface metallurgy equipment. Argon used in the process served the functions of both plasma gas and shielding gas. Fig. 1 shows the DC-plasma jet surface metallurgy equipment used in the experiment and a schematic diagram of plasma jet metallurgical process. The surface metallurgy processing parameters were: output power: 10 kW, working current: 300 A, beam diameter: 5 mm, scanning speed: 500 mm/min, sending powder gas flow rate: 0.6 m3/h, protection gas flow rate: 1.5 m3/h and ionic gas flow rate: 0.6 m3/h. The analysis specimens were cut from the transverse intersection along the vertical direction of plasma jet scanning, polished with fine diamond paste and etched Table 1 Chemical composition of Fe-based alloy (wt.%) C
Cr
Ni
B
Si
Fe
3.2 – 4.1
18 – 24
3.2 – 3.8
0.35 – 0.45
4.5 – 4.7
Bal.
argon
plasma torch
Metallurgical coating substrate
Fig. 1. (a) A homemade DC-plasma jet surface metallurgy equipment. (b) Schematic diagram of the plasma jet metallurgical process.
with aqua regia (hydrochloric acid: nitric acid = 3:1) for 1 – 2 min. The microstructure was observed using XJP100 optical microscope and LEO1450 scanning electronic microscope. The composition was measured by energy dispersive X-ray (EDX) microanalysis. The phase structure of the coatings was determined by X-ray diffraction analysis (XRD). A Vickers’ hardness tester with a load of 0.1 kg measured the microhardness along the depth in cross-section. Room temperature wear resistance of the surface metallurgical coatings was evaluated on a MLS-225 block-on-wheel dry sliding wear tester. The block specimens were made of quartz sand. The wear tests were operated at a load of 70 N and a rotational speed of 400 rpm/min without lubrication. The wear weight loss was measured using a precision electronic balance with an accuracy of 0.1 mg.
3. Results and discussion 3.1. Microstructure characterization of coatings The SEM microstructure features of the coatings with different contents of RE oxide are shown in Fig. 2. It can be seen that the microstructure of the coatings consists of dendrite grains and eutectic. The microstructure, the morphology and distribution of dendrites are modified with the addition of RE oxide. The addition of RE oxide reduces the secondary dendrite spacing, as shown in Fig. 2(b). Due to the high-speed heating and cooling, it is
H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
(a)
(b)
50µm
4743
(c)
50µm
50µm
Fig. 2. Microstructure of coatings with different content of RE (a) 0% RE, (b) 0.2% RE and (c) 0.5% RE.
possible to obtain metastable structures without being restricted by the equilibrium phase diagram. A similar microstructure can be found in laser-clad layer [5]. But the primary dendrite in the coatings is coarser than that in layers prepared by laser cladding because of high heat input and slower cooling rate during the DC-plasma jet surface metallurgy. It should point out that, when the addition of RE is too great, for example, more than 0.3 wt.%, the effect is not good on the contrary, as shown in Fig. 2(c). The grain size of the coatings becomes larger and the second dendrite spacing becomes wider. The reason can be attributed to generate the chemical compounds of RE and boron, thereby reduced the alloying effect of boron. In molten pool, boron as a strong deoxidation element directly influences the degree of deoxidation and also influences such physics chemical properties as the viscosity and surface tension of liquid metal and slag. Fig. 3 shows an X-ray diffraction spectrum of the coating with the addition of RE oxide. It is difficult to identify the phases present in the coatings because the spectrums of some phases are close to each other and even overlapping with each other. According to the indexed results of the diffraction peaks in terms of JCPDS cards, the phases constituents in the coating include g-Fe, a-Fe, Cr1.65Fe0.35B0.96 and LaBO3. XRD
Fig. 3. XRD spectrum of the coating.
analysis also shows that some new phases appeared in the metallurgical coatings by the addition of RE oxide, as compared with coatings without RE oxide. In addition,
Fig. 4. EDX analysis of coatings. (a) SEM image of coatings with 0.2% RE, (b) EDX of reign dark zone and (c) EDX of reign bright zone.
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H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
during solidification of metallurgical coatings, if the compound is precipitated from the matrix and distributed uniformly, it can reinforce the matrix through precipitation strengthening. Furthermore, the semi-quantitative analysis carried out in the different zones using EDX shows the composition of the matrix, as shown in Fig. 4. The analysis of multiphase shows systematically the presence of Si, Cr, Fe and La (Fig. 4b,c). It reveals that the intensities of the elements Si, Fe and La in bright zone (Fig. 4c) are higher than those in dark zone (Fig. 4b). In the EDX analysis, it was not possible to quantify the amount Ce in the coating. This indicates that, after plasma jet surface metallurgy to deal with, the coatings only includes microcontent Ce. 3.2. Microhardness distribution Fig. 5 shows the hardness distribution of coatings formed by plasma jet surface metallurgy. It can be seen that the addition of La2O3 and CeO2 can significantly increase the microhardness of metallurgical coatings. The results show that there is an optimum value in the addition of RE. The difference between the hardness of the two coatings (0% RE, 0.2% RE), which could be due to the grain size of coatings with RE, is further refined. Under this experiment condition, the microhardness of coatings with 0.2 wt.% RE oxide is better than that of coatings with 0.5 wt.% and without RE. 3.3. Wear resistance Fig. 6 shows the relative wear loss of the specimens under room temperature dry sliding wear test. The coatings with RE have much higher wear resistance under the present wear test conditions. It was found that a proper amount of RE oxides contributed to increase the hardness and refine
Fig. 5. Effect of RE on microhardness of the coating.
Fig. 6. Wear mass loss of specimens.
the microstructures of the coatings, which led to increase wear-resistance of the coatings.
4. Discussion The radius of RE and iron atom differ 35% around (the radius of cerium, lanthanum and iron is 0.1824, 0.1877 and 0.126 nm, respectively), common chemical heat treatment methods make it impossible to dissolve excessive RE in metal. The most remarkable characteristic of plasma jet surface metallurgy is rapid solidification and crystallization, in this way, can enable saturating the rare earths and dissolve them in the metal coatings. In the molten pool formed by plasma jet surface metallurgy, RE can react with many elements such as oxygen and sulfur. The chemical equation is: 2RE(liquid) + 3[O] + 1 / 2[S2]YRE2O3S(solid). With shifting of plasma jet, the alloy powders and a thin surface layer of substrate material are melted simultaneously. Owing to the protection of argon, partial La2O3 particles dissolve and form La ion, reacted with oxygen and sulfur, and generated a kind of new chemical compound (La2O3S). At the same time, the compounds formed with RE element may float on the liquid phase before solidification and be cleaned off the metallurgical coatings as slag. Thus, the inclusion content within the coatings is decreased and the coatings are purified by deoxidation and desulfuration [6]. On the other hand, some compounds as well as CeO2 or La2O3 itself may become particles of heterogeneous nucleation. The number of crystal nuclei is increased during the crystallization of coatings: it is beneficial to increase the nucleation rate and promote non-spontaneous nucleation [7,8]. The greater the number of crystal nuclei, the finer the crystal grains of the crystal [9,10]. Lastly, Ce and La distribute mostly over the grain boundary. When the grains grow, 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
H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
grain would be suppressed. Therefore, the grain size is further refined [11].
4745
Acknowledgements This project was supported by the Natural Science Foundation of Shandong Province (No. Y2002F12).
5. Conclusions From the present investigation, the following conclusions may be drawn: (1) Uniform, metallurgical bonded coatings can be obtained by plasma jet surface metallurgy. (2) A proper amount of La2O3 and CeO2 refine the microstructure of metallurgical coatings. The secondary dendrite spacing of the coatings becomes small. However, when the addition of RE is too great, on the contrary, the effect is not good. (3) The wear resistance of the coatings with a proper amount of RE can be enhance by plasma jet surface metallurgy, which may be attributed to the increase of hardness as well as refinement of microstructure.
References [1] H.Q. Li, B.W. Liu, H.D. Li, et al., 14th IFHTSE Congress, 2004, pp. 952 – 955 (Qct). [2] M. Li, H.D. Li, H.Q. Li, et al., 14th IFHTSE Congress, 2004, pp. 23 – 25 (Qct). [3] D. Sheng, Chin. Rare Earths 16 (1995) 36. [4] Y.F. Lian, L.G. Yu, O.J. Xue, Wear 181 – 183 (1995) 436. [5] Q. Li, D. Zhang, T.Q. Lei, Surf. Coat. Technol. 137 (2001) 122. [6] Y.F. Shen, J. Rare Earths (4) (1997) 344. [7] Q.A. Li, C.S. Z.H.Yu, J. Chin. Rare Earth Soc. 13 (1995) 280. [8] Y.X. Sun, Y.Z. Wang, Y.L. He, Z.G. Feng, Surf. Eng. 3 (1995) 43. [9] L. Shang, J. Rare Met. 19 (1995) 132. [10] W. Wang, J. Chin. Rare Earth Soc. 8 (1990) 252. [11] M.Q. Zhang, Heat Treat. Met. (2) (2000) 32.
Microstructure and properties of coatings with rare earth formed by DC-plasma jet surface metallurgy Hao Chen a,*, Hui-qi Li a,b, Yu-zong Sun a, Min Li a,b a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China b School of Materials Science, Shandong University of Science and Technology, Qingdao 266510, PR China Received 25 October 2004; accepted in revised form 10 April 2005 Available online 6 July 2005
Abstract A new method of surface modification, DC-plasma jet surface metallurgy, is proposed in order to improve wear resistance of materials. This paper reports on an experiment, in which some metallurgical coatings of iron-based alloy with different contents of La2O3 and CeO2 were prepared on AISI1020 steel using a homemade DC-plasma jet surface metallurgy equipment. A scanning electron microscope (SEM), energy dispersive X-ray (EDX) microanalysis and X-ray diffraction analysis (XRD) were employed to observe the microstructure and analyzed the chemical compositions of coatings. The effect of RE oxide (La2O3 and CeO2) on microstructure and properties of coatings was investigated. The result shows that addition of a proper amount of RE oxide (La2O3 and CeO2) can refine and purify the microstructure, significantly increase microhardness and enhance the wear resistance. D 2005 Elsevier B.V. All rights reserved. Keywords: Rare earth; Microstructure; Wear resistance; DC-plasma jet surface metallurgy
1. Introduction Failures of mechanical parts mostly originate from surfaces. Some machine parts cease to be effective only due to the damage on the surface caused by wear or corrosion. Hence, regular downtimes are needed for repair and replacement. In order to increase the service life of such machine parts, surfacing techniques are often used to improve the surface properties of components. Such as laser cladding is an advanced surface modification technology to synthesize wear and corrosion resistant coating materials for components. However, the efficiency of energy transfer of laser is rather low (range 10% to 25%) and its application is limited by the expense of the equipment and the dependence on the reflectivity of bulk material.
* Corresponding author. Present address: School of Materials Science, Shandong University of Science and Technology, Qingdao 266510, PR China. Tel.: +86 532 6057927; fax: +86 532 6057927. E-mail address: [email protected] (H. Chen).
DC-plasma jet surface metallurgy is a high new technique developing fast and an effective material processing method that produces a coating with many unique advantages, such as minimized dilution of the powders material from the base metal, metallurgically bonding between coating and substrate, a fine microstructure and improved surface properties of the coated workpieces [1,2]. It is a kind of rapid, non-equilibrium metallurgical process, which is similar to powder metallurgy. The process cannot be restricted by consistency, melting point and density of constituents; therefore, prealloy powders are not needed. And it can obtain some special structures and phases in random powder and random match. Rare earth (RE) elements have been applied successfully in many fields, such as metallurgy and chemical engineering. Previous study showed the positive effects of RE in flame spraying and surface chemical treatment [3,4]. However, there is still little published work on the application of RE in DC-plasma jet surface metallurgy. With the purpose of making use of excellent characteristic
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.061
SCT-11475; No of Pages 5
4742
H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
of RE to improve the microstructure and properties of composite coatings, this paper presents the experimental research on micro-structural evolution in high power plasma jet surface metallurgy of Fe-based alloy powder with RE oxide. The microstructure, microhardness and wear resistance of coatings were investigated. And the effect of the RE on the microstructure of coatings was studied so as to offer an experimental basis to expand a more promising application field of RE.
(a)
plasma torch
2. Experimental details
(b)
2.1. Materials Low-carbon steel (AISI1020) was used as a substrate material for the DC-plasma jet surface metallurgy. The dimensions of the substrate were 100 mm 40 mm 10 mm. Fe-based alloy powder was used as the coating material. The particles of the Fe-based alloy powder were less than 150 Am in size and were spherical in shape. Table 1 shows the chemical composition of Fe-based alloy powder. In order to improve the microstructure and properties of coatings, La2O3 and CeO2, having a purity of over 99% and each accounts for 50%, were added being to the Fe-based alloy powder in different rations ranging from 0 to 0.6 wt.%. 2.2. Experimental procedure DC-plasma jet surface metallurgy basically consists of the powder, with which the coating is to be formed, meeting the plasma jet at the same times as the surface, thus causing both to melt simultaneously and then rapidly solidify to form a dense coating, metallurgically bonded to the base material. The test equipment was carried out by means of a homemade set-up for transferred arc plasma jet surface metallurgy equipment. Argon used in the process served the functions of both plasma gas and shielding gas. Fig. 1 shows the DC-plasma jet surface metallurgy equipment used in the experiment and a schematic diagram of plasma jet metallurgical process. The surface metallurgy processing parameters were: output power: 10 kW, working current: 300 A, beam diameter: 5 mm, scanning speed: 500 mm/min, sending powder gas flow rate: 0.6 m3/h, protection gas flow rate: 1.5 m3/h and ionic gas flow rate: 0.6 m3/h. The analysis specimens were cut from the transverse intersection along the vertical direction of plasma jet scanning, polished with fine diamond paste and etched Table 1 Chemical composition of Fe-based alloy (wt.%) C
Cr
Ni
B
Si
Fe
3.2 – 4.1
18 – 24
3.2 – 3.8
0.35 – 0.45
4.5 – 4.7
Bal.
argon
plasma torch
Metallurgical coating substrate
Fig. 1. (a) A homemade DC-plasma jet surface metallurgy equipment. (b) Schematic diagram of the plasma jet metallurgical process.
with aqua regia (hydrochloric acid: nitric acid = 3:1) for 1 – 2 min. The microstructure was observed using XJP100 optical microscope and LEO1450 scanning electronic microscope. The composition was measured by energy dispersive X-ray (EDX) microanalysis. The phase structure of the coatings was determined by X-ray diffraction analysis (XRD). A Vickers’ hardness tester with a load of 0.1 kg measured the microhardness along the depth in cross-section. Room temperature wear resistance of the surface metallurgical coatings was evaluated on a MLS-225 block-on-wheel dry sliding wear tester. The block specimens were made of quartz sand. The wear tests were operated at a load of 70 N and a rotational speed of 400 rpm/min without lubrication. The wear weight loss was measured using a precision electronic balance with an accuracy of 0.1 mg.
3. Results and discussion 3.1. Microstructure characterization of coatings The SEM microstructure features of the coatings with different contents of RE oxide are shown in Fig. 2. It can be seen that the microstructure of the coatings consists of dendrite grains and eutectic. The microstructure, the morphology and distribution of dendrites are modified with the addition of RE oxide. The addition of RE oxide reduces the secondary dendrite spacing, as shown in Fig. 2(b). Due to the high-speed heating and cooling, it is
H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
(a)
(b)
50µm
4743
(c)
50µm
50µm
Fig. 2. Microstructure of coatings with different content of RE (a) 0% RE, (b) 0.2% RE and (c) 0.5% RE.
possible to obtain metastable structures without being restricted by the equilibrium phase diagram. A similar microstructure can be found in laser-clad layer [5]. But the primary dendrite in the coatings is coarser than that in layers prepared by laser cladding because of high heat input and slower cooling rate during the DC-plasma jet surface metallurgy. It should point out that, when the addition of RE is too great, for example, more than 0.3 wt.%, the effect is not good on the contrary, as shown in Fig. 2(c). The grain size of the coatings becomes larger and the second dendrite spacing becomes wider. The reason can be attributed to generate the chemical compounds of RE and boron, thereby reduced the alloying effect of boron. In molten pool, boron as a strong deoxidation element directly influences the degree of deoxidation and also influences such physics chemical properties as the viscosity and surface tension of liquid metal and slag. Fig. 3 shows an X-ray diffraction spectrum of the coating with the addition of RE oxide. It is difficult to identify the phases present in the coatings because the spectrums of some phases are close to each other and even overlapping with each other. According to the indexed results of the diffraction peaks in terms of JCPDS cards, the phases constituents in the coating include g-Fe, a-Fe, Cr1.65Fe0.35B0.96 and LaBO3. XRD
Fig. 3. XRD spectrum of the coating.
analysis also shows that some new phases appeared in the metallurgical coatings by the addition of RE oxide, as compared with coatings without RE oxide. In addition,
Fig. 4. EDX analysis of coatings. (a) SEM image of coatings with 0.2% RE, (b) EDX of reign dark zone and (c) EDX of reign bright zone.
4744
H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
during solidification of metallurgical coatings, if the compound is precipitated from the matrix and distributed uniformly, it can reinforce the matrix through precipitation strengthening. Furthermore, the semi-quantitative analysis carried out in the different zones using EDX shows the composition of the matrix, as shown in Fig. 4. The analysis of multiphase shows systematically the presence of Si, Cr, Fe and La (Fig. 4b,c). It reveals that the intensities of the elements Si, Fe and La in bright zone (Fig. 4c) are higher than those in dark zone (Fig. 4b). In the EDX analysis, it was not possible to quantify the amount Ce in the coating. This indicates that, after plasma jet surface metallurgy to deal with, the coatings only includes microcontent Ce. 3.2. Microhardness distribution Fig. 5 shows the hardness distribution of coatings formed by plasma jet surface metallurgy. It can be seen that the addition of La2O3 and CeO2 can significantly increase the microhardness of metallurgical coatings. The results show that there is an optimum value in the addition of RE. The difference between the hardness of the two coatings (0% RE, 0.2% RE), which could be due to the grain size of coatings with RE, is further refined. Under this experiment condition, the microhardness of coatings with 0.2 wt.% RE oxide is better than that of coatings with 0.5 wt.% and without RE. 3.3. Wear resistance Fig. 6 shows the relative wear loss of the specimens under room temperature dry sliding wear test. The coatings with RE have much higher wear resistance under the present wear test conditions. It was found that a proper amount of RE oxides contributed to increase the hardness and refine
Fig. 5. Effect of RE on microhardness of the coating.
Fig. 6. Wear mass loss of specimens.
the microstructures of the coatings, which led to increase wear-resistance of the coatings.
4. Discussion The radius of RE and iron atom differ 35% around (the radius of cerium, lanthanum and iron is 0.1824, 0.1877 and 0.126 nm, respectively), common chemical heat treatment methods make it impossible to dissolve excessive RE in metal. The most remarkable characteristic of plasma jet surface metallurgy is rapid solidification and crystallization, in this way, can enable saturating the rare earths and dissolve them in the metal coatings. In the molten pool formed by plasma jet surface metallurgy, RE can react with many elements such as oxygen and sulfur. The chemical equation is: 2RE(liquid) + 3[O] + 1 / 2[S2]YRE2O3S(solid). With shifting of plasma jet, the alloy powders and a thin surface layer of substrate material are melted simultaneously. Owing to the protection of argon, partial La2O3 particles dissolve and form La ion, reacted with oxygen and sulfur, and generated a kind of new chemical compound (La2O3S). At the same time, the compounds formed with RE element may float on the liquid phase before solidification and be cleaned off the metallurgical coatings as slag. Thus, the inclusion content within the coatings is decreased and the coatings are purified by deoxidation and desulfuration [6]. On the other hand, some compounds as well as CeO2 or La2O3 itself may become particles of heterogeneous nucleation. The number of crystal nuclei is increased during the crystallization of coatings: it is beneficial to increase the nucleation rate and promote non-spontaneous nucleation [7,8]. The greater the number of crystal nuclei, the finer the crystal grains of the crystal [9,10]. Lastly, Ce and La distribute mostly over the grain boundary. When the grains grow, 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
H. Chen et al. / Surface & Coatings Technology 200 (2006) 4741 – 4745
grain would be suppressed. Therefore, the grain size is further refined [11].
4745
Acknowledgements This project was supported by the Natural Science Foundation of Shandong Province (No. Y2002F12).
5. Conclusions From the present investigation, the following conclusions may be drawn: (1) Uniform, metallurgical bonded coatings can be obtained by plasma jet surface metallurgy. (2) A proper amount of La2O3 and CeO2 refine the microstructure of metallurgical coatings. The secondary dendrite spacing of the coatings becomes small. However, when the addition of RE is too great, on the contrary, the effect is not good. (3) The wear resistance of the coatings with a proper amount of RE can be enhance by plasma jet surface metallurgy, which may be attributed to the increase of hardness as well as refinement of microstructure.
References [1] H.Q. Li, B.W. Liu, H.D. Li, et al., 14th IFHTSE Congress, 2004, pp. 952 – 955 (Qct). [2] M. Li, H.D. Li, H.Q. Li, et al., 14th IFHTSE Congress, 2004, pp. 23 – 25 (Qct). [3] D. Sheng, Chin. Rare Earths 16 (1995) 36. [4] Y.F. Lian, L.G. Yu, O.J. Xue, Wear 181 – 183 (1995) 436. [5] Q. Li, D. Zhang, T.Q. Lei, Surf. Coat. Technol. 137 (2001) 122. [6] Y.F. Shen, J. Rare Earths (4) (1997) 344. [7] Q.A. Li, C.S. Z.H.Yu, J. Chin. Rare Earth Soc. 13 (1995) 280. [8] Y.X. Sun, Y.Z. Wang, Y.L. He, Z.G. Feng, Surf. Eng. 3 (1995) 43. [9] L. Shang, J. Rare Met. 19 (1995) 132. [10] W. Wang, J. Chin. Rare Earth Soc. 8 (1990) 252. [11] M.Q. Zhang, Heat Treat. Met. (2) (2000) 32.