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Studies of magnetic properties of iron-based coatings produced by a high-velocity oxy-fuel process
Materials Chemistry and Physics 92 (2005) 419–423
Studies of magnetic properties of iron-based coatings produced by a high-velocity oxy-fuel process M. Cherigui a,∗ , S. Guessasma a , N. Fenineche a , R. Hamzaoui b , O. El-Kedim b , C. Coddet a b
a LERMPS—UTBM (Site de S´ evenans), F-90010 Belfort, Cedex, France Nanomaterials Research Group (NRG)—UMR 5060 CNRS-UTBM, Site de S´evenans, F-90010 Belfort, Cedex, France
Received 4 October 2004; received in revised form 28 December 2004; accepted 9 January 2005
Abstract The paper studies the effect of high-velocity oxy-fuel thermal spraying parameters, in particular spray distance and oxygen flow rate, on coating porosity and magnetic properties of FeSi and FeSiB deposits using the artificial neural network methodology. The magnetic properties correlated to coating porosity were obtained using an optimized network structure. The predicted results permitted to point out the role of porosity for varying the coercivity and saturation magnetization and the stability of magnetic properties with respect to the considered spray parameters. © 2005 Elsevier B.V. All rights reserved. Keywords: Magnetic materials; Coatings; Computer modelling and simulation; Magnetic properties
1. Introduction Magnetic materials are elaborated by various techniques, in particular rapid quenching and mechanical alloying. The major problem of these techniques is their limited field of application. An alternative can be found to solve this limitation. Indeed, thermal spraying can be considered as one of the processes, allowing the realization of deposits according to various techniques conferring specific functionalities to the parts to be covered by integrating a broad material range (metallic, ceramic and polymeric alloys and composite materials). The high-velocity oxy-fuel (HVOF) technique has been widely used mainly due to its high flame velocity (up to Mach 5) and moderate temperature (below 3000 ◦ C). The moderate flame temperature is especially suitable for coating deposition of materials with low melting temperatures [1]. FeSi alloy is selected for its good magnetic properties. In the dynamic mode, the soft magnetic materials dissipate energy in the form of heat. FeSi alloys can also decrease ∗
Corresponding author. Tel.: +33 3 84 58 3243; fax: +33 3 84 58 3286. E-mail address: [email protected] (M. Cherigui).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.01.047
magnetic losses. These losses decompose to loss by hysteresis and loss by Foucault current, from where the interest of these alloys in engines, generators and transformers [2]. The choice of FeSiB is justified by the thermal stability of the residual amorphous phase enriched in boron and the good magnetic properties related to the presence of silicon, where the boron increases the possibility of having an amorphisation phase [3]. Moreover, FeSiB alloys have been widely used as a magnetic core material for electric generator construction [3–5]. In order to quantify the role of the porosity of coating on the magnetic properties of FeSi and FeSiB, a model of data processing is considered based on the artificial neural network methodology. Such a methodology is an adequate tool for the study of complex processes with parameter interdependencies [6]. In addition, this technique proved to be applicable in the domain of materials science [7] and especially in the case of thermal spraying [8]. In this context, a network structure is used to relate HVOF process parameters to both porosity and magnetic properties of FeSi and FeSiB coatings. The predicted magnetic properties are then correlated to the porosity level for each material, taking into account the interdependency revealed by the optimized network structure.
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M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423 Table 1 Spraying parameters Fuel flow rate “methane” (SLPM) Oxygen gas flow rate (SLPM) Oxygen rate in the mixture (fuel/O2 ) Nitrogen carrier gas flow rate (SLPM) Powder feed rate (g min−1 ) Spray distance (mm) Scanning step (mm) Torch-substrate relative velocity (m min−1 ) Scanning velocity (mm s−1 ) Substrate thickness (mm) Spray gun Deposit thickness (m) Number of passes
Fig. 1. X-ray diffraction patterns of the: (a) FeSi and (b) FeSiB powders.
145 290 0.5 20
145 350 0.41 20 35 250–300 12 245 50 0.8 CDS 8944, 3 psi 200 30
The XRD patterns revealed the basically cubic structure Fe3 Si (DO3) which is characteristic for the low-silicon content FeSi alloys. These results agree with those obtained for rapidly quenched Fe100 − x Six alloys [5] and mechanically alloyed FeSi [9]. Concerning FeSiB powder, it has noticed that the XRD pattern is similar to that of FeSi.
2. Experimental procedure 2.2. Coating preparation 2.1. Powders Two types of nanocrystalline powders were used (in at.%): Fe–6.5Si and Fe–6.5Si–18.5B as alloys with average granulometries of 45 and 40 m, respectively. These nanocrystalline powders were produced from FeSi microcrystalline powder with an average particle size of 65 m using mechanical alloying during 48 h. FeSiB powder was obtained from Fe, Si and B elementary powders using the same process. The ball milling was carried out using a planetary high-energy ball milling (Retsch PM 400). Steel balls (diameter: 20 mm) and 50 ml volume jar were used. Four jars were mounted on a planar disc. With the rotation of the disc, the vials moved in a circular and opposite direction compared to the disc rotation. The rotation speed of disc was equal to 400 rpm and the rotation speed of jar was equal to 800 rpm. The grain size of the resulting powders was estimated using X-ray diffraction (XRD) and applying the Scherrer formula. They were about 10 and 12 nm for FeSiB and FeSi, respectively (Fig. 1).
A Sulzer Metco CDS 8944 system was used for coating preparation. Methane was used as a fuel gas with a flow rate of 145 SLPM (standard litre per minute). The flow rate of oxygen was either 290 or 350 SLPM. Powder carrier gas was nitrogen with a flow rate of 20 SLPM. Two spray distances were used, 250 and 300 mm. The powders were coated on copper substrates using sheet sizes of 70 mm × 25 mm × 0.8 mm. The experiments were carried out using an air-cooling system. Table 1 shows the spray parameters. 2.3. Characterization Optical microscopy was used to calculate the porosity level by image analysis, using a NIH image free software. Ten images were used to assess mean and standard deviation of the porosity level. Magnetic measurements were realized using a hysteresismeter Bull M2000 SIIS, which enabled to draw the hysteresis loop of the considered samples. It per-
Table 2 Magnetic measurements of the coatings Material
Spray distance (mm)
Oxygen flow rate (SLPM)
Coating porosity (%)
FeSi FeSiB FeSi FeSiB FeSi FeSiB FeSi FeSiB
250 250 250 250 300 300 300 300
290 290 350 350 290 290 350 350
4.48 3.208 4.518 3.408 4.608 3.508 4.628 3.608
± ± ± ± ± ± ± ±
0.53 1.10 0.55 1.00 0.53 0.90 0.51 1.10
Coercivity (Oe) 15.008 26.008 15.068 26.408 15.158 26.508 15.218 26.508
± ± ± ± ± ± ± ±
2.10 3.40 2.12 3.50 2.06 3.60 2.10 3.30
Saturation magnetization (A m2 kg−1 ) 1170 1600 1165 1595 1160 1590 1155 1580
± ± ± ± ± ± ± ±
11 9 10 10 11 11 10 12
M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423
mitted also to calculate the magnetic properties, particularly coercivity and saturation magnetization. Table 2 shows the experimental porosity levels and magnetic properties for the studied conditions. The coating porosity varied from 3.9 to 4.6%, the coercivity from 21 to 27 Oe and the saturation magnetization from 1376 to 1600. The variation recorded for each property represented 15, 29 and 17% from the average value, respectively. 2.4. Statistical analysis
421
3. Results and discussion 3.1. Coating microstructure Based on previous experiences using a single parameter variation, the most significant spray parameters were adjusted to minimize the oxygen content as well as the porosity of the coatings. Fig. 2 a and b shows the cross-section of dense FeSi and FeSiB coatings, respectively. Denser coatings were obtained in the case of FeSiB as FeSi deposit structure presents a large amount of porosity. In addition, the presence of several non-molten particles in the FeSi deposit is clearly observed as shown in Fig. 1a. The porosity values are about 4.5 and 3.5% for FeSi and FeSiB, respectively.
An artificial neural network was used to analyze the experimental data. The input parameters were material type, spray distance and fuel rate. The output parameters were porosity level, coercivity and magnetization saturation. The optimization process of the network structure was rigorously identical to that of another work [10]. Such an optimization considered a training and a testing process based on a submitted database. The optimization steps were run for 1000 cycles after which 100% of the experimental sets were learnt correctly (difference between predicted and experimental values was less than 5%). The average error was less than 0.002.
The optimized artificial neural network structure was used to predict the correlation between porosity and spray distance and oxygen flow rate for FeSi and FeSiB coatings. Fig. 3 shows the porosity evolution as a function of spray distance and oxygen flow for FeSi and FeSiB coatings. It is noticed that
Fig. 2. Morphology of the coatings: (a) FeSi and (b) FeSiB.
Fig. 3. Porosity level predicted evolution vs. spray distance and oxygen flow rate for: (a) FeSi and (b) FeSiB coatings.
3.2. Porosity level related to spray parameters
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M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423
the coating porosity remains nearly unchanged when spray distance increases from 160 to 340 mm. The same magnitude can be observed when varying the oxygen rate of the two materials. This increase of porosity with the increase of spray distance can be related to the following considerations: - When the spray distance increases, particles leave the flame and begin to solidify before they impinge the substrate and thus their temperature is low; - The decrease of particle temperature has the consequence to decrease the flattening process as their viscosity is high and wetability is low; - The incomplete flattening process increases the amount of porosity. The oxygen flow rate seems to have no significant effect on coating porosity especially for the FeSi material. Roughly speaking, an oxygen flow rate increase is related to a decrease of the flame temperature for a fixed fuel flow rate [11]. This decrease favours the presence of the no-molten particles in the coating and an increase of porosity if the fuel flow rate is not sufficient.
Fig. 5. Predicted evolution of saturation magnetization vs. spray distance and oxygen flow rate for: (a) FeSi and (b) FeSiB coatings.
3.3. Magnetic properties related to spray parameters
Fig. 4. Predicted evolution of coercivity vs. spray distance and oxygen flow volume for: (a) FeSi and (b) FeSiB coatings.
Fig. 4 shows the predicted coercivity variation when varying spray distance and oxygen flow for FeSi and FeSiB coatings. It seems that the coercivity increases with both oxygen flow rate and spray distance. The increase of coercivity follows the increase of porosity level (Fig. 3), which suggests a correlated variation between magnetic properties and porosity level. In fact, this represents a partial correlation as the coercivity variation is also dependent on the oxygen flow rate. The saturation magnetization exhibits the same trend with respect to spray parameters (Fig. 5). As the porosity is mostly affected by the increase of spray distance, this parameter is considered as a control factor of the magnetic properties. Previous studies permitted to point out the role of the spray distance on the microstructure because of the large sensitivity of particle velocity and temperature [12,13]. In order to investigate more deeply the role of porosity in the modification of magnetic properties, one has to withdraw the spray parameter variables. This is done by collecting the ANN responses for any process parameter combination and plotting the porosity versus the magnetic properties. Fig. 6
M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423
423
saturation magnetization is noticed. Generally, in magnetism studies, a decrease of this parameter is related to an increase in coercivity [15]. 4. Conclusions This study examined the effect of HVOF thermal spraying parameters on the porosity and magnetic properties of coatings using the artificial neural network methodology.
Fig. 6. Predicted evolution of coercivity vs. porosity level for FeSi and FeSiB coatings.
- The spray distance was the control factor for varying the porosity for both FeSi and FeSiB as the increase of this parameter permitted to increase the spray distance. The oxygen flow rate was not sensitive to porosity variation. - Coercivity and saturation magnetization were dependent on both spray parameters. - Structural modifications related to spray distance variation permitted to predict the increase of coercivity with the increase of porosity. - The saturation magnetization is correlated to the coercivity and is thus affected by the porosity level. References
Fig. 7. Predicted evolution of saturation magnetization vs. porosity level for FeSi and FeSiB coatings.
shows the coercivity response as a function of porosity level response for FeSi and FeSiB coatings. The coercivity exhibits parabolic relationships with the porosity level. The increase of coercivity is explained by the fact that the porosity acts against the continuity of magnetic properties through the coating structure. These are considered as defects anchoring Bloch walls and involving consequently an increase of coercivity [14]. One can conclude that an improvement of the coercivity can be related to a low-porosity content. Fig. 7 shows the predicted variation of the saturation magnetization as a function of porosity level. A low decrease of
[1] H. Li, K.A. Khor, P. Cheang, Biomaterials 23 (2002) 2105. [2] R.M. Bozorth, Ferromagnetism, England, 1951, p. 77. [3] M. Cherigui, H.I. Feraoun, N.E. Fenineche, H. Aourag, C. Coddet, Mater. Chem. Phys. 85 (2004) 113. [4] M. Cherigui, N.E. Fenineche, C. Coddet, Surf. Coat. Technol. 192 (2005) 19. [5] L.K. Varga, F. Mazaleyrat, J. Kovac, A. K`akay, Mater. Sci. Eng. A 304–306 (2001) 946. [6] M.M. Nelson, W.T. Illingworth, A Practical Guide to Neural Nets, third ed., Addison-Wesley, New York, 1991. [7] H.K.D.H. Bhadeshia, Neural networks in materials science, ISIJ Int. 39 (1999) 966. [8] S. Guessasma, G. Montavon, C. Coddet, in: E.F. Lugscheider (Ed.), Proceedings of the International Thermal Spray Conference 2002, DVS-Verlag, D¨usseldorf, 2002, p. 435. [9] M. Abdellaoui, C. Djega-Mariadassou, E. Gaffet, J. Alloys Comp. 259 (1997) 241. [10] S. Guessasma, C. Coddet, Acta Mater. 52 (2004) 5157. [11] B.R. Marple, J. Voyer, J.-F. Bisson, C. Moreau, J. Mater. Proc. Tech. 117 (2001) 418. [12] V.V. Sobolev, J.M. Guilemany, Mater. Lett. 18 (1994) 304. [13] L. Zhao, M. Parco, E. Lugscheider, Surf. Coat. Technol. 179 (2004) 272. [14] M. Nacken, W. Heller, Arch. Eisenhuettenw. 31 (1960) 153. [15] R.M. Bozorth, H.J. Williams, Rev. Mod. Phys. 17 (1995) 72.
Studies of magnetic properties of iron-based coatings produced by a high-velocity oxy-fuel process M. Cherigui a,∗ , S. Guessasma a , N. Fenineche a , R. Hamzaoui b , O. El-Kedim b , C. Coddet a b
a LERMPS—UTBM (Site de S´ evenans), F-90010 Belfort, Cedex, France Nanomaterials Research Group (NRG)—UMR 5060 CNRS-UTBM, Site de S´evenans, F-90010 Belfort, Cedex, France
Received 4 October 2004; received in revised form 28 December 2004; accepted 9 January 2005
Abstract The paper studies the effect of high-velocity oxy-fuel thermal spraying parameters, in particular spray distance and oxygen flow rate, on coating porosity and magnetic properties of FeSi and FeSiB deposits using the artificial neural network methodology. The magnetic properties correlated to coating porosity were obtained using an optimized network structure. The predicted results permitted to point out the role of porosity for varying the coercivity and saturation magnetization and the stability of magnetic properties with respect to the considered spray parameters. © 2005 Elsevier B.V. All rights reserved. Keywords: Magnetic materials; Coatings; Computer modelling and simulation; Magnetic properties
1. Introduction Magnetic materials are elaborated by various techniques, in particular rapid quenching and mechanical alloying. The major problem of these techniques is their limited field of application. An alternative can be found to solve this limitation. Indeed, thermal spraying can be considered as one of the processes, allowing the realization of deposits according to various techniques conferring specific functionalities to the parts to be covered by integrating a broad material range (metallic, ceramic and polymeric alloys and composite materials). The high-velocity oxy-fuel (HVOF) technique has been widely used mainly due to its high flame velocity (up to Mach 5) and moderate temperature (below 3000 ◦ C). The moderate flame temperature is especially suitable for coating deposition of materials with low melting temperatures [1]. FeSi alloy is selected for its good magnetic properties. In the dynamic mode, the soft magnetic materials dissipate energy in the form of heat. FeSi alloys can also decrease ∗
Corresponding author. Tel.: +33 3 84 58 3243; fax: +33 3 84 58 3286. E-mail address: [email protected] (M. Cherigui).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.01.047
magnetic losses. These losses decompose to loss by hysteresis and loss by Foucault current, from where the interest of these alloys in engines, generators and transformers [2]. The choice of FeSiB is justified by the thermal stability of the residual amorphous phase enriched in boron and the good magnetic properties related to the presence of silicon, where the boron increases the possibility of having an amorphisation phase [3]. Moreover, FeSiB alloys have been widely used as a magnetic core material for electric generator construction [3–5]. In order to quantify the role of the porosity of coating on the magnetic properties of FeSi and FeSiB, a model of data processing is considered based on the artificial neural network methodology. Such a methodology is an adequate tool for the study of complex processes with parameter interdependencies [6]. In addition, this technique proved to be applicable in the domain of materials science [7] and especially in the case of thermal spraying [8]. In this context, a network structure is used to relate HVOF process parameters to both porosity and magnetic properties of FeSi and FeSiB coatings. The predicted magnetic properties are then correlated to the porosity level for each material, taking into account the interdependency revealed by the optimized network structure.
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M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423 Table 1 Spraying parameters Fuel flow rate “methane” (SLPM) Oxygen gas flow rate (SLPM) Oxygen rate in the mixture (fuel/O2 ) Nitrogen carrier gas flow rate (SLPM) Powder feed rate (g min−1 ) Spray distance (mm) Scanning step (mm) Torch-substrate relative velocity (m min−1 ) Scanning velocity (mm s−1 ) Substrate thickness (mm) Spray gun Deposit thickness (m) Number of passes
Fig. 1. X-ray diffraction patterns of the: (a) FeSi and (b) FeSiB powders.
145 290 0.5 20
145 350 0.41 20 35 250–300 12 245 50 0.8 CDS 8944, 3 psi 200 30
The XRD patterns revealed the basically cubic structure Fe3 Si (DO3) which is characteristic for the low-silicon content FeSi alloys. These results agree with those obtained for rapidly quenched Fe100 − x Six alloys [5] and mechanically alloyed FeSi [9]. Concerning FeSiB powder, it has noticed that the XRD pattern is similar to that of FeSi.
2. Experimental procedure 2.2. Coating preparation 2.1. Powders Two types of nanocrystalline powders were used (in at.%): Fe–6.5Si and Fe–6.5Si–18.5B as alloys with average granulometries of 45 and 40 m, respectively. These nanocrystalline powders were produced from FeSi microcrystalline powder with an average particle size of 65 m using mechanical alloying during 48 h. FeSiB powder was obtained from Fe, Si and B elementary powders using the same process. The ball milling was carried out using a planetary high-energy ball milling (Retsch PM 400). Steel balls (diameter: 20 mm) and 50 ml volume jar were used. Four jars were mounted on a planar disc. With the rotation of the disc, the vials moved in a circular and opposite direction compared to the disc rotation. The rotation speed of disc was equal to 400 rpm and the rotation speed of jar was equal to 800 rpm. The grain size of the resulting powders was estimated using X-ray diffraction (XRD) and applying the Scherrer formula. They were about 10 and 12 nm for FeSiB and FeSi, respectively (Fig. 1).
A Sulzer Metco CDS 8944 system was used for coating preparation. Methane was used as a fuel gas with a flow rate of 145 SLPM (standard litre per minute). The flow rate of oxygen was either 290 or 350 SLPM. Powder carrier gas was nitrogen with a flow rate of 20 SLPM. Two spray distances were used, 250 and 300 mm. The powders were coated on copper substrates using sheet sizes of 70 mm × 25 mm × 0.8 mm. The experiments were carried out using an air-cooling system. Table 1 shows the spray parameters. 2.3. Characterization Optical microscopy was used to calculate the porosity level by image analysis, using a NIH image free software. Ten images were used to assess mean and standard deviation of the porosity level. Magnetic measurements were realized using a hysteresismeter Bull M2000 SIIS, which enabled to draw the hysteresis loop of the considered samples. It per-
Table 2 Magnetic measurements of the coatings Material
Spray distance (mm)
Oxygen flow rate (SLPM)
Coating porosity (%)
FeSi FeSiB FeSi FeSiB FeSi FeSiB FeSi FeSiB
250 250 250 250 300 300 300 300
290 290 350 350 290 290 350 350
4.48 3.208 4.518 3.408 4.608 3.508 4.628 3.608
± ± ± ± ± ± ± ±
0.53 1.10 0.55 1.00 0.53 0.90 0.51 1.10
Coercivity (Oe) 15.008 26.008 15.068 26.408 15.158 26.508 15.218 26.508
± ± ± ± ± ± ± ±
2.10 3.40 2.12 3.50 2.06 3.60 2.10 3.30
Saturation magnetization (A m2 kg−1 ) 1170 1600 1165 1595 1160 1590 1155 1580
± ± ± ± ± ± ± ±
11 9 10 10 11 11 10 12
M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423
mitted also to calculate the magnetic properties, particularly coercivity and saturation magnetization. Table 2 shows the experimental porosity levels and magnetic properties for the studied conditions. The coating porosity varied from 3.9 to 4.6%, the coercivity from 21 to 27 Oe and the saturation magnetization from 1376 to 1600. The variation recorded for each property represented 15, 29 and 17% from the average value, respectively. 2.4. Statistical analysis
421
3. Results and discussion 3.1. Coating microstructure Based on previous experiences using a single parameter variation, the most significant spray parameters were adjusted to minimize the oxygen content as well as the porosity of the coatings. Fig. 2 a and b shows the cross-section of dense FeSi and FeSiB coatings, respectively. Denser coatings were obtained in the case of FeSiB as FeSi deposit structure presents a large amount of porosity. In addition, the presence of several non-molten particles in the FeSi deposit is clearly observed as shown in Fig. 1a. The porosity values are about 4.5 and 3.5% for FeSi and FeSiB, respectively.
An artificial neural network was used to analyze the experimental data. The input parameters were material type, spray distance and fuel rate. The output parameters were porosity level, coercivity and magnetization saturation. The optimization process of the network structure was rigorously identical to that of another work [10]. Such an optimization considered a training and a testing process based on a submitted database. The optimization steps were run for 1000 cycles after which 100% of the experimental sets were learnt correctly (difference between predicted and experimental values was less than 5%). The average error was less than 0.002.
The optimized artificial neural network structure was used to predict the correlation between porosity and spray distance and oxygen flow rate for FeSi and FeSiB coatings. Fig. 3 shows the porosity evolution as a function of spray distance and oxygen flow for FeSi and FeSiB coatings. It is noticed that
Fig. 2. Morphology of the coatings: (a) FeSi and (b) FeSiB.
Fig. 3. Porosity level predicted evolution vs. spray distance and oxygen flow rate for: (a) FeSi and (b) FeSiB coatings.
3.2. Porosity level related to spray parameters
422
M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423
the coating porosity remains nearly unchanged when spray distance increases from 160 to 340 mm. The same magnitude can be observed when varying the oxygen rate of the two materials. This increase of porosity with the increase of spray distance can be related to the following considerations: - When the spray distance increases, particles leave the flame and begin to solidify before they impinge the substrate and thus their temperature is low; - The decrease of particle temperature has the consequence to decrease the flattening process as their viscosity is high and wetability is low; - The incomplete flattening process increases the amount of porosity. The oxygen flow rate seems to have no significant effect on coating porosity especially for the FeSi material. Roughly speaking, an oxygen flow rate increase is related to a decrease of the flame temperature for a fixed fuel flow rate [11]. This decrease favours the presence of the no-molten particles in the coating and an increase of porosity if the fuel flow rate is not sufficient.
Fig. 5. Predicted evolution of saturation magnetization vs. spray distance and oxygen flow rate for: (a) FeSi and (b) FeSiB coatings.
3.3. Magnetic properties related to spray parameters
Fig. 4. Predicted evolution of coercivity vs. spray distance and oxygen flow volume for: (a) FeSi and (b) FeSiB coatings.
Fig. 4 shows the predicted coercivity variation when varying spray distance and oxygen flow for FeSi and FeSiB coatings. It seems that the coercivity increases with both oxygen flow rate and spray distance. The increase of coercivity follows the increase of porosity level (Fig. 3), which suggests a correlated variation between magnetic properties and porosity level. In fact, this represents a partial correlation as the coercivity variation is also dependent on the oxygen flow rate. The saturation magnetization exhibits the same trend with respect to spray parameters (Fig. 5). As the porosity is mostly affected by the increase of spray distance, this parameter is considered as a control factor of the magnetic properties. Previous studies permitted to point out the role of the spray distance on the microstructure because of the large sensitivity of particle velocity and temperature [12,13]. In order to investigate more deeply the role of porosity in the modification of magnetic properties, one has to withdraw the spray parameter variables. This is done by collecting the ANN responses for any process parameter combination and plotting the porosity versus the magnetic properties. Fig. 6
M. Cherigui et al. / Materials Chemistry and Physics 92 (2005) 419–423
423
saturation magnetization is noticed. Generally, in magnetism studies, a decrease of this parameter is related to an increase in coercivity [15]. 4. Conclusions This study examined the effect of HVOF thermal spraying parameters on the porosity and magnetic properties of coatings using the artificial neural network methodology.
Fig. 6. Predicted evolution of coercivity vs. porosity level for FeSi and FeSiB coatings.
- The spray distance was the control factor for varying the porosity for both FeSi and FeSiB as the increase of this parameter permitted to increase the spray distance. The oxygen flow rate was not sensitive to porosity variation. - Coercivity and saturation magnetization were dependent on both spray parameters. - Structural modifications related to spray distance variation permitted to predict the increase of coercivity with the increase of porosity. - The saturation magnetization is correlated to the coercivity and is thus affected by the porosity level. References
Fig. 7. Predicted evolution of saturation magnetization vs. porosity level for FeSi and FeSiB coatings.
shows the coercivity response as a function of porosity level response for FeSi and FeSiB coatings. The coercivity exhibits parabolic relationships with the porosity level. The increase of coercivity is explained by the fact that the porosity acts against the continuity of magnetic properties through the coating structure. These are considered as defects anchoring Bloch walls and involving consequently an increase of coercivity [14]. One can conclude that an improvement of the coercivity can be related to a low-porosity content. Fig. 7 shows the predicted variation of the saturation magnetization as a function of porosity level. A low decrease of
[1] H. Li, K.A. Khor, P. Cheang, Biomaterials 23 (2002) 2105. [2] R.M. Bozorth, Ferromagnetism, England, 1951, p. 77. [3] M. Cherigui, H.I. Feraoun, N.E. Fenineche, H. Aourag, C. Coddet, Mater. Chem. Phys. 85 (2004) 113. [4] M. Cherigui, N.E. Fenineche, C. Coddet, Surf. Coat. Technol. 192 (2005) 19. [5] L.K. Varga, F. Mazaleyrat, J. Kovac, A. K`akay, Mater. Sci. Eng. A 304–306 (2001) 946. [6] M.M. Nelson, W.T. Illingworth, A Practical Guide to Neural Nets, third ed., Addison-Wesley, New York, 1991. [7] H.K.D.H. Bhadeshia, Neural networks in materials science, ISIJ Int. 39 (1999) 966. [8] S. Guessasma, G. Montavon, C. Coddet, in: E.F. Lugscheider (Ed.), Proceedings of the International Thermal Spray Conference 2002, DVS-Verlag, D¨usseldorf, 2002, p. 435. [9] M. Abdellaoui, C. Djega-Mariadassou, E. Gaffet, J. Alloys Comp. 259 (1997) 241. [10] S. Guessasma, C. Coddet, Acta Mater. 52 (2004) 5157. [11] B.R. Marple, J. Voyer, J.-F. Bisson, C. Moreau, J. Mater. Proc. Tech. 117 (2001) 418. [12] V.V. Sobolev, J.M. Guilemany, Mater. Lett. 18 (1994) 304. [13] L. Zhao, M. Parco, E. Lugscheider, Surf. Coat. Technol. 179 (2004) 272. [14] M. Nacken, W. Heller, Arch. Eisenhuettenw. 31 (1960) 153. [15] R.M. Bozorth, H.J. Williams, Rev. Mod. Phys. 17 (1995) 72.