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Influence of oxides addition on the reaction of Fe2O3–Al composite powders in plasma flame
Journal of Alloys and Compounds 579 (2013) 1–6
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Influence of oxides addition on the reaction of Fe2O3–Al composite powders in plasma flame Yong Yang ⇑, Dian-ran Yan, Yan-chun Dong, Xue-guang Chen, Lei Wang, Zhen-hua Chu, Jian-xin Zhang, Ji-ning He Key Lab. for New Type of Functional Materials in Hebei Province, Hebei University of Technology, Tianjin 300132, China School of Materials Science and Engineering, Hebei University of Technology, 300132 Tianjin, China
a r t i c l e
i n f o
Article history: Received 18 April 2013 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 16 May 2013 Keywords: Coating materials Composite materials Plasma spraying Solid state reaction Oxides FeAl2O4
a b s t r a c t The production of FeAl2O4 matrix composite coatings based on Fe2O3–Al–MexOy systems by reactive plasma spraying was achieved. The influence of oxides addition (Al2O3, Cr2O3, Fe3O4 and SiO2) on the reaction of Fe2O3–Al composite powders in plasma flame was investigated. The results showed that chemical reactions occurred in Fe2O3–Al–MexOy composite powders during plasma spraying. The reaction products were different from that of the equilibrium reaction condition. The as-fabricated composite coatings were mainly composed of FeAl2O4, a-Fe and Al2O3. Introduction of the oxide additives affected the reaction process of Fe2O3–Al system. Oxides addition reduced the reaction heat release of Fe2O3–Al system and improved the initial reaction temperature. The main phase FeAl2O4 in the five composite coatings was actually mixed hercynite containing different proportion of Fe and Al elements. The normal hercynite contained more Al and the inverse hercynite contained more Fe. The addition of the additives was beneficial to improve the content of the normal hercynite. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ceramic matrix composite coating shows enhanced toughness and wear resistance comparing with monolithic ceramic coating and has been used to tailor the surface properties of metal parts due to its high hardness, excellent wear, corrosion, chemical and thermal resistance [1–6]. Spinels are double metal oxides (AB2O4) which contain interstitial metal cations in lattice sites having two distinct types of symmetry with respect to the oxygen anions: tetrahedral and octahedral [7]. Because the ratio of occupied octahedral-to-tetrahedral sites is 2/1, spinels with their B cations in octahedral sites (and A cations in tetrahedral sites) are classified as normal spinels. Those with their A cations only in octahedral sites are classified as inverse spinels. Spinels, with a high melting temperature and a weak thermal conductivity, are applied as protective coatings against chemical attack from fused metal or glass [8]. They are also very interesting materials for their stability in presence of borates and silicates [9]. The better corrosion and thermal shock resistance of refractories have been reported to be influenced positively by presence of in situ spinel [10]. Hercynite (FeAl2O4) provides an excellent combination of physical and chemical properties. It has high ⇑ Corresponding author. Address: No. 29 Guangrong Road, Hongqiao District, Tianjin 300132, China. Tel.: +86 22 60204810; fax: +86 22 26564810. E-mail address: [email protected] (Y. Yang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.045
ductility (fracture–mechanical strength) and flexibility against cracking and spalling [11]. The preparation of FeAl2O4 is gaining importance due to its potential applications as magnetic materials, anode materials for high temperature pyroelectrolysis, wear resistant coatings and anti-corrosion coatings [12–15]. Deevi et al. [16] fabricated FeAl2O4 matrix composite coating by reactive plasma spraying of aluminum on carbon steel substrate with a layer of iron oxide on the surface. FeAl2O4, Al2O3, Fe, Fe–Al intermetallic, FeO, Fe3O4 and unreacted Al were found in the coating. Fe2O3–Al mixture is a classical thermite system which was formerly used for the synthesis of ceramic–reinforced metal matrix composites, alumina coatings in situ inside pipes, iron-aluminates, and nanocomposites [17–20,12,21]. In our recent investigation, in situ FeAl2O4 matrix composite coating was prepared by plasma spraying Fe2O3–Al composite powders [22,23]. The results indicated that the crack extension force of the FeAl2O4 matrix composite coating was 20% higher than that of the nanostructured Al2O3–3%TiO2 coating and 100% higher than that of the microstructured Al2O3–3%TiO2 coating. It is known that the microstructure of the coating material is dependent on the reaction products of Fe2O3–Al thermite system in plasma flame, which were different from that of the conventional Fe2O3–Al thermite system in equilibrated combustion synthesis process due to the high temperature, high heating rate, short reaction time, high depositing velocity and high cooling rate of the plasma spraying processing. Therefore, controlling the reaction products of thermite system is very
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important for preparing high performance composite coatings by reactive plasma spraying. In the present investigation, metal oxides (MexOy) additives (Al2O3, Cr2O3, Fe3O4 and SiO2) were added into Fe2O3–Al composite powders and FeAl2O4 matrix composites coatings were prepared by reactive plasma spraying. The aim of this work is the production of FeAl2O4 matrix composites coatings based on the Fe2O3–Al–MexOy systems by the reactive plasma spraying process and the investigation of the role of the metal oxides addition on the formation mechanisms of reaction products and the structural characterization of the FeAl2O4 phase. 2. Materials and methods Fe2O3 powder (analytical grade, Tianjin Third Chemical Reagent Co., Ltd., China) and Al powder (99.9% grade, Anshan Iron and Steel Fine Aluminum Powder Co., Ltd., China) were used as raw materials for preparing sprayable feedstock (composite powders). a-Al2O3, Cr2O3, Fe3O4 and SiO2 were used as additives, and the role of which on the mechanisms of reaction products formation were investigated. Table 1 shows the components of composite powders. The codes ‘‘FA’’, ‘‘FAAl’’, ‘‘FACr’’, ‘‘FAFe’’ and ‘‘FASi’’ denote ‘‘Fe2O3–Al’’, ‘‘Fe2O3–Al–Al2O3’’, ‘‘Fe2O3–Al–Cr2O3’’, ‘‘Fe2O3–Al–Fe3O4’’ and ‘‘Fe2O3–Al–SiO2’’, respectively. The raw materials Fe2O3 powder, Al powder and additives powders were wet-mixed by 99-1A electromotion blender using absolute alcohol as the mixing media and polyvinyl alcohol as binder, and the powder mixture was then dried at 150 °C and sieved through the sieve of 200–300 mesh. The ASTM 1045 mild steel specimens were used as metal substrate. A bond coating of Ni–10 wt.%Al alloy with thickness about 50–100 lm was deposited onto the substrate in order to increase the adhesive strength between the composite coating and the substrate. The as-prepared Fe2O3–Al–MexOy composite powders were then deposited onto the bond coating for about 300 lm in thickness. All the coatings were deposited using GDP-2 type 50 kW plasma spraying device (Jiu Jiang Spraying Device Company, China). The plasma spraying parameters were shown in Table 2. The phase constitution of the as-prepared coatings was characterized by X-ray diffraction (XRD, Philips X’-Pert MPD) with Cu Ka radiation. For analyzing the effect of additives on the reaction of Fe2O3–Al composite powders, HENVEN HCT-2 integrated thermal analyzer was used in this study to investigate the thermal change of the five kinds of composite powders from 25 °C to 1300 °C. The heating rate was 10 °C/min, and Ar gas was used to protect Al powders against oxidation.
3. Results and discussion 3.1. Thermal analysis of the influence of oxides addition on the reaction of Fe2O3–Al composite powders For analyzing the effect of additives on the reaction of Fe2O3–Al composite powders, HENVEN HCT-2 integrated thermal analyzer was used in this study to investigate the thermal change of the five kinds of composite powders from 25 °C to 1300 °C. Fig. 1 shows the DTA spectra of the five composite powders. The trend of the DTA spectra of the five composite powders was similar, but there was a certain distinction for each spectrum. There were clear endothermic peaks at 661 ± 0.5 °C on the DTA spectra of the five composite powders, which was consistent with the melting point of pure aluminum (660.32 °C) [24]. That indicated that Al powder in the composite powder particles melted first in the heating process. When the heating was continued, there were exothermic peaks appeared at nearly 900 °C in the DTA spectra of each composite powder, but there were differences for the corresponding temperature of the exothermic peaks. The thermite reaction temperature of
Fe2O3 and Al is known to be about 900–1500 °C under equilibrium condition, and the actual reaction temperature will change with the contact degree of Fe2O3 and Al [25]. The raw materials for preparing composite powders are micron Al powder and submicron Fe2O3 powder. The fine powder particles have high surface energy and more fully contact each other, so the thermite reaction temperature would be reduced accordingly. It could be inferred that the exothermic peaks around 900 °C were caused by the self-propagating exothermic reaction between the fine Al powder and Fe2O3 powder. However, there were differences on the exothermic peaks of the DTA spectra due to the addition of Al2O3, Cr2O3, Fe3O4 and SiO2 into the Fe2O3–Al system, which indicated that the additives did produce certain effects on the reaction between Fe2O3 and Al. The order of the peak temperature and the inflexion temperature of the exothermic peaks (the corresponding temperature of the maximum reaction rate and the initial reaction temperature, respectively [26]) of the five composite powders is FACr < FAFe < FA < FASi < FAAl. The peak temperatures of the exothermic peaks of FACr and FAFe composite powders were similar to that of FA composite powders, while the peak temperatures of the exothermic peaks of FASi and FAAl composite powders increased nearly 30 °C compared to the other three. That indicated that the addition of Al2O3 and SiO2 into the Fe2O3–Al system increased the contact distance between Al powder and Fe2O3 powder, which resulted in dilution effect on the Fe2O3–Al system, thereby the temperature of the combustion reaction between Fe2O3 and Al was improved. Fig. 2 shows the DSC spectra of the five composite powders. The order of the exothermic peak area of the five composite powders is FA > FACr > FAFe > FASi > FAAl, which indicated that the addition of additives reduced the reaction heat release of the reaction system. The exothermic reaction between Al and Cr2O3 could occur, while the reaction heat release of Cr2O3–Al system is lower than that of Fe2O3-Al system [27]. Therefore, the reaction heat release of FACr composite powders decreased slightly compared with FA composite powders due to the addition of Cr2O3 in Fe2O3–Al system. Comparing with FA composite powders, the exothermic peak of the DSC spectrum of FAFe composite powders was gentler, which indicated that Fe3O4 reduced not only the reaction heat release but also the reaction rate of Fe2O3–Al system. It could be inferred that Fe3O4 may be an intermediate product of the reaction between Fe2O3 and Al. The exothermic peaks of the DSC spectra of FAAl composite powders and FASi composite powders lagged obviously behind than that of FA composite powders, which was attributed to the inhibition of the reaction between Fe2O3 and Al to some extent by the additives Al2O3 and SiO2. SiO2 could react with FeO to form Fe2SiO4, which would release a certain amount of heat. Therefore, the reaction heat release of FASi composite powders was slightly larger than that of FAAl composite powders. The results of Figs. 1 and 2 show that the addition of additives would reduce the reaction heat release of Fe2O3–Al system and improve the initial reaction temperature. The possible reaction process of the composite powders can be inferred as follows. First, Al powder melted near 660 °C, and the molten Al diffused into the interior of the composite powder particles and contacted more fully with Fe2O3. Next, when the temperature of the composite powder particles reached the temperature range of 800–850 °C,
Table 1 The components of composite powders. Code
Fe2O3 (wt.%)
Al (wt.%)
Al2O3 (wt.%)
Cr2O3 (wt.%)
Fe3O4 (wt.%)
SiO2 (wt.%)
FA FAAl FACr FAFe FASi
75 67.5 67.5 67.5 67.5
25 22.5 22.5 22.5 22.5
– 10 – – –
– – 10 – –
– – – 10 –
– – – – 10
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Y. Yang et al. / Journal of Alloys and Compounds 579 (2013) 1–6 Table 2 The plasma spraying parameters of the coatings. Coatings
Current (A)
Voltage (V)
Primary gas (Ar) flow rate (L min1)
Secondary gas (H2) flow rate (L min1)
Spray distance (mm)
Ni–Al coating Composite coating
500 500
70 60
100 80
20 20
100 100
Fig. 1. DTA spectra of the composite powders: (1#) FA, (2#) FAAl, (3#) FACr, (4#) FAFe, (5#) FASi.
in the present study, which would lead to reaction products Al2O3 and Fe under equilibrium condition according to Eq. (1) [17],
2Al þ Fe2 O3 ¼ Al2 O3 þ 2Fe
Fig. 2. DSC spectra of the composite powders: (1#) FA, (2#) FAAl, (3#) FACr, (4#) FAFe, (5#) FASi.
the self-propagating reaction between Fe2O3 and Al began and a lot of heat was released. Introduction of the additives affected the reaction process of Fe2O3–Al system.
3.2. Influence of oxides addition on the reaction of Fe2O3–Al composite powders in plasma flame and its reaction products Fig. 3 shows the XRD patterns of the coatings prepared by plasma spraying Fe2O3–Al–MexOy composite powders. It can be seen from Fig. 3a that FA coating is mainly composed of FeAl2O4 and a-Fe, and there are also small amounts of AlFe, AlFe3, Al2O3, Fe2O3 and FeO phases in the coating. That indicated that thermite reaction between Fe2O3 and Al in the composite powders took place during plasma spraying. The molar ratio of Fe2O3/Al is 1:2
ð1Þ
However, it had been pointed out that the chemical composition and phase constitution of the reaction products of Fe2O3–Al thermite system were mainly dependent on the reactants composition, reaction extent and cooling condition [21]. It is well known that the plasma spraying is a non-equilibrium process, which is characterized by high temperatures (10000 K), high velocity (about 200 m s1) and extremely high cooling rate (about 106– 108 K s1) [28,29]. Therefore, the reaction products (FeAl2O4, Fe, AlFe, AlFe3, Al2O3 and FeO), which were also reported in other non-equilibrium processing of Fe2O3–Al thermite system [12,21], were different from that of the equilibrium reaction condition (Al2O3 and Fe). It should be noted that FeAl2O4 phase indexed for the coatings in Fig. 3 was actually a mixed hercynite, namely (Fe1xAlx)(FexAl2x)O4 [30]. The position of the diffraction peaks of (Fe1xAlx)(FexAl2x)O4 will offset with the change of variable x (0 < x < 1). In the case of no special instructions, which were expressed with FeAl2O4. Comparison of the XRD patterns in Fig. 3, it can be seen that chemical reactions occurred in four composite powders with additives (FAAl, FACr, FAFe and FASi) in the plasma spraying process. In addition to the same main phases FeAl2O4 and a-Fe, which were also presented in FA coating, there were new phases formed in the corresponding coatings. That indicated that the additives (Al2O3, Cr2O3, Fe3O4 and SiO2) had impact on the reaction products of the composite powders (Fe2O3–Al) in plasma flame. Fig. 3b shows that the main phases of FAAl coating (FeAl2O4 and a-Fe) are the same as that of FA coating. However, the diffraction peaks of Al2O3 increased evidently. The diffraction peaks of Fe and FeO rose, and there was also Fe3O4 phase present. Fig. 3c shows that FACr coating is mainly composed of FeAl2O4 and a-Fe, and there
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Fig. 3. XRD patterns of the coatings prepared by plasma spraying Fe2O3–Al–MexOy composite powders: (a) FA, (b) FAAl, (c) FACr, (d) FAFe and (e) FASi.
are new phases FeCr2O4 and Cr formed in FACr coating comparing with FA coating, and the diffraction peaks of AlFe, Al2O3, Fe3O4 and FeO decreased. Fig. 3d shows that FAFe coating is mainly composed of FeAl2O4 and a-Fe, and the diffraction peaks of a-Fe, AlFe and Al2O3 in FAFe coating decreased comparing with FA coating, and the diffraction peaks of Fe2O3 and Fe3O4 increased, and there are only a few of FeO in FAFe coating. Fig. 3e shows that FASi coating is also mainly composed of FeAl2O4 and a-Fe, and the diffraction peaks of a-Fe, AlFe and Al2O3 in FASi coating decreased comparing with FA coating, and the diffraction peaks of Fe2O3 disappeared, and only a few of FeO were presented in FASi coating. 3.2.1. Fe2O3–Al system According to the above results, a qualitative mechanism for the Fe2O3–Al reaction in plasma flame could be proposed. First, Fe2O3 was deoxidized into FeO and Fe3O4 by Al, and this part of Al was
oxidized to Al2O3, which reacted with FeO to form FeAl2O4. Another part of Al continued to deoxidize Fe3O4, FeO or Fe2O3 and finally a-Fe was displaced. Some Al in the Al-rich particles reacted with Fe to form AlFe or AlFe3. The oxidized Al2O3 was continuously consumed due to its participation in the reaction to form FeAl2O4, and therefore few Al2O3 was retained in the composite coating. A small amount of Fe2O3 did not react and remained in the composite coating. 3.2.2. Fe2O3–Al–Al2O3 system A qualitative mechanism for the effect of Al2O3 on the Fe2O3–Al reaction in plasma flame could be proposed from the analysis of Fig. 3b, Figs. 1 and 2. The Al2O3 additive was not involved in the formation reaction of FeAl2O4. The dilution effect of Al2O3 to the Fe2O3–Al composite powders caused the decrease of FeAl2O4 and the increase of the intermediate reaction product FeO. The Al2O3
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additive was melted in plasma flame and re-crystallized after depositing on the substrate or previous layers, and severe lattice distortion occurred due to the rapid cooling of plasma spraying process [31]. 3.2.3. Fe2O3–Al–Cr2O3 system It can be seen from Fig. 3c, Figs. 1 and 2 that there are new phases FeCr2O4 and Cr presented in FACr coating, and the diffraction peaks of AlFe, Al2O3, Fe3O4 and FeO decreased. That indicated that the additive Cr2O3 could react with Al to generate Cr and Al2O3. Al was consumed due to the reaction between Cr2O3 and Al and did not have too much residual, which therefore resulted in the decrease of AlFe. Equilibrium thermodynamic calculation shows that the priority order of reduction of oxides by Al is Fe2O3 > Fe3O4 > FeO > FeAl2O4 > Cr2O3 [32]. However, there was no Cr2O3 detected in FACr coating, but a mass of FeAl2O4, some FeCr2O4 and a few iron oxides were present in FACr coating. This result indicated again that the reaction of the reactants and their reaction products in plasma spraying process were different from that under equilibrium conditions. In the reaction process of FACr composite powders, some FeO reacted with Cr2O3 to form FeCr2O4, which resulted in the decrease of FeO in the FACr coating. 3.2.4. Fe2O3–Al–Fe3O4 system It can be seen from Fig. 3d, Figs. 1 and 2 that the diffraction peaks of a-Fe, AlFe and Al2O3 in FAFe coating decreased comparing with FA coating, and the diffraction peaks of Fe2O3 and Fe3O4 increased, and there are only a few of FeO in FAFe coating. The above results indicated additive Fe3O4 and/or its decomposition product FeO may be directly involved in and promote the formation of FeAl2O4, which led to decreasing of the diffraction peak of the reactant Al2O3 related to the formation of FeAl2O4. There was more Al in the composite powders involved in the formation of FeAl2O4, while less Al involved in the reduction formation of a-Fe, which led to the decrease of the diffraction peak of a-Fe in FAFe coating. 3.2.5. Fe2O3–Al–SiO2 system It can be seen from Fig. 3e, Figs. 1 and 2 that FASi coating is also mainly composed of FeAl2O4 and a-Fe. a-Fe, AlFe and Al2O3 in FASi coating decreased comparing with FA coating, Fe2O3 disappeared, and only a few of FeO were presented in FASi coating. According to Fe2O3–SiO2 phase diagram and FeO–SiO2 phase diagram [33], SiO2 could only react with FeO to form Fe2SiO4, while Fe2O3 could only form eutectic mixture with SiO2. FeO was consumed due to the formation of Fe2SiO4, which promoted the decomposition of Fe2O3. In view of the above reasons, no iron oxides were detected in FeASi coating. 3.3. Influence of oxides addition on FeAl2O4 phase Enlarging the diffraction peaks of FeAl2O4 phases in the XRD patterns of Fig. 3, it was found that there was some difference for the diffraction angles of FeAl2O4 phases. The diffraction peaks of {3 1 1} crystal face of FeAl2O4 in the composite coatings are shown in Fig. 4. It was already mentioned above that FeAl2O4 indexed for the coatings in Fig. 3 was actually a mixed hercynite, namely (Fe1xAlx)(FexAl2x)O4. The diffraction angles of (Fe1xAlx)(FexAl2x)O4 will offset with the change of variable x (0 < x < 1). It could be, therefore, inferred that the offset of the diffraction peaks in Fig. 4 was attributed to the different relative content of the normal hercynite phase and the inverse hercynite phase in the mixed hercynite. Supposing that the hercynite is a uniform continuous phase in each composite coatings, and combining the XRD diffraction data with the cubic crystal diffraction direction formula [34], the lattice constant (a) of the mixed hercynite can be obtained. The value of lattice constant (a) reflects the relative content change
Fig. 4. The diffraction peaks of {3 1 1} crystal face of FeAl2O4 in the composite coatings: (1#) FA, (2#) FAAl, (3#) FACr, (4#) FAFe, (5#) FASi.
of the mixed hercynite (the normal hercynite phase and the inverse hercynite phase) in each composite coatings, and the effect of the additives on the formation of FeAl2O4 could be estimated. The calculation process will not consider the influence of the stress on the diffraction peaks due to the same spraying parameters for the five composite coatings. The diffraction peaks data of {3 1 1} crystal face of FeAl2O4 in the composite coatings were selected for calculating the lattice constants, which are shown in Table 3. The lattice constant of (Fe0.781Al0.219)(Al1.781Fe0.219)O4 is a = 0.8229 nm and the lattice constant of (Fe0.899Al0.101)(Al1.899Fe0.101)O4 is a = 0.8151 nm [30], from which it could be inferred that the smaller x is, the smaller the lattice constant of the mixed hercynite is and the lower the Fe riched inverse hercynite is. It can be seen from Table 3 that the addition of the additives could influence the lattice constant value of the hercynite phase, and the order of which is aFA > aFASi > aFAFe > aFAAl > aFACr. It could be inferred that the content order of the Al riched normal hercynite in the mixed hercynite is FeAl2O4FACr > FeAl2O4FAAl > FeAl2O4FAFe > FeAl2O4FASi > FeAl2O4FA. The content of the normal hercynite in the mixed hercynite of FACr coating is the highest. The reason may be that FeO was consumed due to the reaction between FeO and Cr2O3, which reduced the content of the Fe riched inverse hercynite. There is more Al in FAAl composite powders than that in FA composite powders due to the addition of Al2O3 additive. Moreover, FeAl2O4 could form solid solution in Al2O3 and improve the content of Al in the reaction products. That resulted in the increase of the Al riched normal hercynite. Although there is more Fe in FAFe composite powders than that in FA composite powders due to the addition of Fe3O4 additive, FeO generated by the decomposition of Fe3O4 could pro-
Table 3 The lattice constant of FeAl2O4 in the composite coatings. Coatings
2-Theta (deg.)
sin h
k (nm)
h, k, l
a (nm)
FA FAAl FACr FAFe FASi
36.000 36.360 36.387 36.347 36.139
0.309008 0.311994 0.312218 0.311887 0.310162
0.1542 0.1542 0.1542 0.1542 0.1542
3, 3, 3, 3, 3,
0.8269876 0.8190722 0.8184850 0.8193552 0.8239124
1, 1, 1, 1, 1,
1 1 1 1 1
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mote the formation of FeAl2O4. That, therefore, increased the content of the normal hercynite. For FASi coating, the additive SiO2 could react with FeO and thereby reduced the Fe riched inverse hercynite. 4. Conclusions (1) Thermite reaction between Fe2O3 and Al took place during plasma spraying. The reaction products were different from that of the equilibrium reaction condition. Introduction of the oxide additives affected the reaction process of Fe2O3– Al system. (2) The possible reaction process of the Fe2O3AlMexOy composite powders is that Al powder melted near 660 °C first, and the molten Al diffused into the interior of the composite powder particles and contacted more fully with Fe2O3. Next, when the temperature of the composite powder particles reached the temperature range of 800–850 °C, the self-propagating reaction between Fe2O3 and Al began and a lot of heat was released. Oxides addition reduced the reaction heat release of Fe2O3–Al system and improved the initial reaction temperature. (3) The composite coatings were mainly composed of FeAl2O4, a-Fe and Al2O3. The additives influenced the reaction products of the Fe2O3–Al system in plasma flame. The content of Al2O3 in FAAl coating increased evidently due to the addition of Al2O3 additive. There were new phases FeCr2O4 and Cr formed in FACr coating. The content of FeAl2O4, Fe3O4 and Fe2O3 increased and a-Fe decreased in FAFe coating. a-Fe, AlFe and Al2O3 in FASi coating decreased and Fe2SiO4 was formed due to the reaction between SiO2 and FeO. (4) The main phase FeAl2O4 in the five composite coatings was actually mixed hercynite containing different proportions of Fe and Al elements. The normal hercynite contained more Al and the inverse hercynite contained more Fe. The addition of the additives was beneficial to improve the content of the normal hercynite. The content order of the normal hercynite in the mixed hercynite is FeAl2O4FACr > FeAl2O4FAAl > FeAl2O4FAFe > FeAl2O4FASi > FeAl2O4FA.
Acknowledgments The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (51072045, 51102074 and 51272065), China Postdoctoral Science Foundation (20110490979) and Project (20101317120005) supported by Doctoral Program Specialized Research Foundation for Universities, China and Outstanding Youth Fund for Science and Technology Research of Universities in Hebei Province, China (Y2012003).
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Influence of oxides addition on the reaction of Fe2O3–Al composite powders in plasma flame Yong Yang ⇑, Dian-ran Yan, Yan-chun Dong, Xue-guang Chen, Lei Wang, Zhen-hua Chu, Jian-xin Zhang, Ji-ning He Key Lab. for New Type of Functional Materials in Hebei Province, Hebei University of Technology, Tianjin 300132, China School of Materials Science and Engineering, Hebei University of Technology, 300132 Tianjin, China
a r t i c l e
i n f o
Article history: Received 18 April 2013 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 16 May 2013 Keywords: Coating materials Composite materials Plasma spraying Solid state reaction Oxides FeAl2O4
a b s t r a c t The production of FeAl2O4 matrix composite coatings based on Fe2O3–Al–MexOy systems by reactive plasma spraying was achieved. The influence of oxides addition (Al2O3, Cr2O3, Fe3O4 and SiO2) on the reaction of Fe2O3–Al composite powders in plasma flame was investigated. The results showed that chemical reactions occurred in Fe2O3–Al–MexOy composite powders during plasma spraying. The reaction products were different from that of the equilibrium reaction condition. The as-fabricated composite coatings were mainly composed of FeAl2O4, a-Fe and Al2O3. Introduction of the oxide additives affected the reaction process of Fe2O3–Al system. Oxides addition reduced the reaction heat release of Fe2O3–Al system and improved the initial reaction temperature. The main phase FeAl2O4 in the five composite coatings was actually mixed hercynite containing different proportion of Fe and Al elements. The normal hercynite contained more Al and the inverse hercynite contained more Fe. The addition of the additives was beneficial to improve the content of the normal hercynite. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Ceramic matrix composite coating shows enhanced toughness and wear resistance comparing with monolithic ceramic coating and has been used to tailor the surface properties of metal parts due to its high hardness, excellent wear, corrosion, chemical and thermal resistance [1–6]. Spinels are double metal oxides (AB2O4) which contain interstitial metal cations in lattice sites having two distinct types of symmetry with respect to the oxygen anions: tetrahedral and octahedral [7]. Because the ratio of occupied octahedral-to-tetrahedral sites is 2/1, spinels with their B cations in octahedral sites (and A cations in tetrahedral sites) are classified as normal spinels. Those with their A cations only in octahedral sites are classified as inverse spinels. Spinels, with a high melting temperature and a weak thermal conductivity, are applied as protective coatings against chemical attack from fused metal or glass [8]. They are also very interesting materials for their stability in presence of borates and silicates [9]. The better corrosion and thermal shock resistance of refractories have been reported to be influenced positively by presence of in situ spinel [10]. Hercynite (FeAl2O4) provides an excellent combination of physical and chemical properties. It has high ⇑ Corresponding author. Address: No. 29 Guangrong Road, Hongqiao District, Tianjin 300132, China. Tel.: +86 22 60204810; fax: +86 22 26564810. E-mail address: [email protected] (Y. Yang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.045
ductility (fracture–mechanical strength) and flexibility against cracking and spalling [11]. The preparation of FeAl2O4 is gaining importance due to its potential applications as magnetic materials, anode materials for high temperature pyroelectrolysis, wear resistant coatings and anti-corrosion coatings [12–15]. Deevi et al. [16] fabricated FeAl2O4 matrix composite coating by reactive plasma spraying of aluminum on carbon steel substrate with a layer of iron oxide on the surface. FeAl2O4, Al2O3, Fe, Fe–Al intermetallic, FeO, Fe3O4 and unreacted Al were found in the coating. Fe2O3–Al mixture is a classical thermite system which was formerly used for the synthesis of ceramic–reinforced metal matrix composites, alumina coatings in situ inside pipes, iron-aluminates, and nanocomposites [17–20,12,21]. In our recent investigation, in situ FeAl2O4 matrix composite coating was prepared by plasma spraying Fe2O3–Al composite powders [22,23]. The results indicated that the crack extension force of the FeAl2O4 matrix composite coating was 20% higher than that of the nanostructured Al2O3–3%TiO2 coating and 100% higher than that of the microstructured Al2O3–3%TiO2 coating. It is known that the microstructure of the coating material is dependent on the reaction products of Fe2O3–Al thermite system in plasma flame, which were different from that of the conventional Fe2O3–Al thermite system in equilibrated combustion synthesis process due to the high temperature, high heating rate, short reaction time, high depositing velocity and high cooling rate of the plasma spraying processing. Therefore, controlling the reaction products of thermite system is very
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important for preparing high performance composite coatings by reactive plasma spraying. In the present investigation, metal oxides (MexOy) additives (Al2O3, Cr2O3, Fe3O4 and SiO2) were added into Fe2O3–Al composite powders and FeAl2O4 matrix composites coatings were prepared by reactive plasma spraying. The aim of this work is the production of FeAl2O4 matrix composites coatings based on the Fe2O3–Al–MexOy systems by the reactive plasma spraying process and the investigation of the role of the metal oxides addition on the formation mechanisms of reaction products and the structural characterization of the FeAl2O4 phase. 2. Materials and methods Fe2O3 powder (analytical grade, Tianjin Third Chemical Reagent Co., Ltd., China) and Al powder (99.9% grade, Anshan Iron and Steel Fine Aluminum Powder Co., Ltd., China) were used as raw materials for preparing sprayable feedstock (composite powders). a-Al2O3, Cr2O3, Fe3O4 and SiO2 were used as additives, and the role of which on the mechanisms of reaction products formation were investigated. Table 1 shows the components of composite powders. The codes ‘‘FA’’, ‘‘FAAl’’, ‘‘FACr’’, ‘‘FAFe’’ and ‘‘FASi’’ denote ‘‘Fe2O3–Al’’, ‘‘Fe2O3–Al–Al2O3’’, ‘‘Fe2O3–Al–Cr2O3’’, ‘‘Fe2O3–Al–Fe3O4’’ and ‘‘Fe2O3–Al–SiO2’’, respectively. The raw materials Fe2O3 powder, Al powder and additives powders were wet-mixed by 99-1A electromotion blender using absolute alcohol as the mixing media and polyvinyl alcohol as binder, and the powder mixture was then dried at 150 °C and sieved through the sieve of 200–300 mesh. The ASTM 1045 mild steel specimens were used as metal substrate. A bond coating of Ni–10 wt.%Al alloy with thickness about 50–100 lm was deposited onto the substrate in order to increase the adhesive strength between the composite coating and the substrate. The as-prepared Fe2O3–Al–MexOy composite powders were then deposited onto the bond coating for about 300 lm in thickness. All the coatings were deposited using GDP-2 type 50 kW plasma spraying device (Jiu Jiang Spraying Device Company, China). The plasma spraying parameters were shown in Table 2. The phase constitution of the as-prepared coatings was characterized by X-ray diffraction (XRD, Philips X’-Pert MPD) with Cu Ka radiation. For analyzing the effect of additives on the reaction of Fe2O3–Al composite powders, HENVEN HCT-2 integrated thermal analyzer was used in this study to investigate the thermal change of the five kinds of composite powders from 25 °C to 1300 °C. The heating rate was 10 °C/min, and Ar gas was used to protect Al powders against oxidation.
3. Results and discussion 3.1. Thermal analysis of the influence of oxides addition on the reaction of Fe2O3–Al composite powders For analyzing the effect of additives on the reaction of Fe2O3–Al composite powders, HENVEN HCT-2 integrated thermal analyzer was used in this study to investigate the thermal change of the five kinds of composite powders from 25 °C to 1300 °C. Fig. 1 shows the DTA spectra of the five composite powders. The trend of the DTA spectra of the five composite powders was similar, but there was a certain distinction for each spectrum. There were clear endothermic peaks at 661 ± 0.5 °C on the DTA spectra of the five composite powders, which was consistent with the melting point of pure aluminum (660.32 °C) [24]. That indicated that Al powder in the composite powder particles melted first in the heating process. When the heating was continued, there were exothermic peaks appeared at nearly 900 °C in the DTA spectra of each composite powder, but there were differences for the corresponding temperature of the exothermic peaks. The thermite reaction temperature of
Fe2O3 and Al is known to be about 900–1500 °C under equilibrium condition, and the actual reaction temperature will change with the contact degree of Fe2O3 and Al [25]. The raw materials for preparing composite powders are micron Al powder and submicron Fe2O3 powder. The fine powder particles have high surface energy and more fully contact each other, so the thermite reaction temperature would be reduced accordingly. It could be inferred that the exothermic peaks around 900 °C were caused by the self-propagating exothermic reaction between the fine Al powder and Fe2O3 powder. However, there were differences on the exothermic peaks of the DTA spectra due to the addition of Al2O3, Cr2O3, Fe3O4 and SiO2 into the Fe2O3–Al system, which indicated that the additives did produce certain effects on the reaction between Fe2O3 and Al. The order of the peak temperature and the inflexion temperature of the exothermic peaks (the corresponding temperature of the maximum reaction rate and the initial reaction temperature, respectively [26]) of the five composite powders is FACr < FAFe < FA < FASi < FAAl. The peak temperatures of the exothermic peaks of FACr and FAFe composite powders were similar to that of FA composite powders, while the peak temperatures of the exothermic peaks of FASi and FAAl composite powders increased nearly 30 °C compared to the other three. That indicated that the addition of Al2O3 and SiO2 into the Fe2O3–Al system increased the contact distance between Al powder and Fe2O3 powder, which resulted in dilution effect on the Fe2O3–Al system, thereby the temperature of the combustion reaction between Fe2O3 and Al was improved. Fig. 2 shows the DSC spectra of the five composite powders. The order of the exothermic peak area of the five composite powders is FA > FACr > FAFe > FASi > FAAl, which indicated that the addition of additives reduced the reaction heat release of the reaction system. The exothermic reaction between Al and Cr2O3 could occur, while the reaction heat release of Cr2O3–Al system is lower than that of Fe2O3-Al system [27]. Therefore, the reaction heat release of FACr composite powders decreased slightly compared with FA composite powders due to the addition of Cr2O3 in Fe2O3–Al system. Comparing with FA composite powders, the exothermic peak of the DSC spectrum of FAFe composite powders was gentler, which indicated that Fe3O4 reduced not only the reaction heat release but also the reaction rate of Fe2O3–Al system. It could be inferred that Fe3O4 may be an intermediate product of the reaction between Fe2O3 and Al. The exothermic peaks of the DSC spectra of FAAl composite powders and FASi composite powders lagged obviously behind than that of FA composite powders, which was attributed to the inhibition of the reaction between Fe2O3 and Al to some extent by the additives Al2O3 and SiO2. SiO2 could react with FeO to form Fe2SiO4, which would release a certain amount of heat. Therefore, the reaction heat release of FASi composite powders was slightly larger than that of FAAl composite powders. The results of Figs. 1 and 2 show that the addition of additives would reduce the reaction heat release of Fe2O3–Al system and improve the initial reaction temperature. The possible reaction process of the composite powders can be inferred as follows. First, Al powder melted near 660 °C, and the molten Al diffused into the interior of the composite powder particles and contacted more fully with Fe2O3. Next, when the temperature of the composite powder particles reached the temperature range of 800–850 °C,
Table 1 The components of composite powders. Code
Fe2O3 (wt.%)
Al (wt.%)
Al2O3 (wt.%)
Cr2O3 (wt.%)
Fe3O4 (wt.%)
SiO2 (wt.%)
FA FAAl FACr FAFe FASi
75 67.5 67.5 67.5 67.5
25 22.5 22.5 22.5 22.5
– 10 – – –
– – 10 – –
– – – 10 –
– – – – 10
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Y. Yang et al. / Journal of Alloys and Compounds 579 (2013) 1–6 Table 2 The plasma spraying parameters of the coatings. Coatings
Current (A)
Voltage (V)
Primary gas (Ar) flow rate (L min1)
Secondary gas (H2) flow rate (L min1)
Spray distance (mm)
Ni–Al coating Composite coating
500 500
70 60
100 80
20 20
100 100
Fig. 1. DTA spectra of the composite powders: (1#) FA, (2#) FAAl, (3#) FACr, (4#) FAFe, (5#) FASi.
in the present study, which would lead to reaction products Al2O3 and Fe under equilibrium condition according to Eq. (1) [17],
2Al þ Fe2 O3 ¼ Al2 O3 þ 2Fe
Fig. 2. DSC spectra of the composite powders: (1#) FA, (2#) FAAl, (3#) FACr, (4#) FAFe, (5#) FASi.
the self-propagating reaction between Fe2O3 and Al began and a lot of heat was released. Introduction of the additives affected the reaction process of Fe2O3–Al system.
3.2. Influence of oxides addition on the reaction of Fe2O3–Al composite powders in plasma flame and its reaction products Fig. 3 shows the XRD patterns of the coatings prepared by plasma spraying Fe2O3–Al–MexOy composite powders. It can be seen from Fig. 3a that FA coating is mainly composed of FeAl2O4 and a-Fe, and there are also small amounts of AlFe, AlFe3, Al2O3, Fe2O3 and FeO phases in the coating. That indicated that thermite reaction between Fe2O3 and Al in the composite powders took place during plasma spraying. The molar ratio of Fe2O3/Al is 1:2
ð1Þ
However, it had been pointed out that the chemical composition and phase constitution of the reaction products of Fe2O3–Al thermite system were mainly dependent on the reactants composition, reaction extent and cooling condition [21]. It is well known that the plasma spraying is a non-equilibrium process, which is characterized by high temperatures (10000 K), high velocity (about 200 m s1) and extremely high cooling rate (about 106– 108 K s1) [28,29]. Therefore, the reaction products (FeAl2O4, Fe, AlFe, AlFe3, Al2O3 and FeO), which were also reported in other non-equilibrium processing of Fe2O3–Al thermite system [12,21], were different from that of the equilibrium reaction condition (Al2O3 and Fe). It should be noted that FeAl2O4 phase indexed for the coatings in Fig. 3 was actually a mixed hercynite, namely (Fe1xAlx)(FexAl2x)O4 [30]. The position of the diffraction peaks of (Fe1xAlx)(FexAl2x)O4 will offset with the change of variable x (0 < x < 1). In the case of no special instructions, which were expressed with FeAl2O4. Comparison of the XRD patterns in Fig. 3, it can be seen that chemical reactions occurred in four composite powders with additives (FAAl, FACr, FAFe and FASi) in the plasma spraying process. In addition to the same main phases FeAl2O4 and a-Fe, which were also presented in FA coating, there were new phases formed in the corresponding coatings. That indicated that the additives (Al2O3, Cr2O3, Fe3O4 and SiO2) had impact on the reaction products of the composite powders (Fe2O3–Al) in plasma flame. Fig. 3b shows that the main phases of FAAl coating (FeAl2O4 and a-Fe) are the same as that of FA coating. However, the diffraction peaks of Al2O3 increased evidently. The diffraction peaks of Fe and FeO rose, and there was also Fe3O4 phase present. Fig. 3c shows that FACr coating is mainly composed of FeAl2O4 and a-Fe, and there
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Fig. 3. XRD patterns of the coatings prepared by plasma spraying Fe2O3–Al–MexOy composite powders: (a) FA, (b) FAAl, (c) FACr, (d) FAFe and (e) FASi.
are new phases FeCr2O4 and Cr formed in FACr coating comparing with FA coating, and the diffraction peaks of AlFe, Al2O3, Fe3O4 and FeO decreased. Fig. 3d shows that FAFe coating is mainly composed of FeAl2O4 and a-Fe, and the diffraction peaks of a-Fe, AlFe and Al2O3 in FAFe coating decreased comparing with FA coating, and the diffraction peaks of Fe2O3 and Fe3O4 increased, and there are only a few of FeO in FAFe coating. Fig. 3e shows that FASi coating is also mainly composed of FeAl2O4 and a-Fe, and the diffraction peaks of a-Fe, AlFe and Al2O3 in FASi coating decreased comparing with FA coating, and the diffraction peaks of Fe2O3 disappeared, and only a few of FeO were presented in FASi coating. 3.2.1. Fe2O3–Al system According to the above results, a qualitative mechanism for the Fe2O3–Al reaction in plasma flame could be proposed. First, Fe2O3 was deoxidized into FeO and Fe3O4 by Al, and this part of Al was
oxidized to Al2O3, which reacted with FeO to form FeAl2O4. Another part of Al continued to deoxidize Fe3O4, FeO or Fe2O3 and finally a-Fe was displaced. Some Al in the Al-rich particles reacted with Fe to form AlFe or AlFe3. The oxidized Al2O3 was continuously consumed due to its participation in the reaction to form FeAl2O4, and therefore few Al2O3 was retained in the composite coating. A small amount of Fe2O3 did not react and remained in the composite coating. 3.2.2. Fe2O3–Al–Al2O3 system A qualitative mechanism for the effect of Al2O3 on the Fe2O3–Al reaction in plasma flame could be proposed from the analysis of Fig. 3b, Figs. 1 and 2. The Al2O3 additive was not involved in the formation reaction of FeAl2O4. The dilution effect of Al2O3 to the Fe2O3–Al composite powders caused the decrease of FeAl2O4 and the increase of the intermediate reaction product FeO. The Al2O3
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additive was melted in plasma flame and re-crystallized after depositing on the substrate or previous layers, and severe lattice distortion occurred due to the rapid cooling of plasma spraying process [31]. 3.2.3. Fe2O3–Al–Cr2O3 system It can be seen from Fig. 3c, Figs. 1 and 2 that there are new phases FeCr2O4 and Cr presented in FACr coating, and the diffraction peaks of AlFe, Al2O3, Fe3O4 and FeO decreased. That indicated that the additive Cr2O3 could react with Al to generate Cr and Al2O3. Al was consumed due to the reaction between Cr2O3 and Al and did not have too much residual, which therefore resulted in the decrease of AlFe. Equilibrium thermodynamic calculation shows that the priority order of reduction of oxides by Al is Fe2O3 > Fe3O4 > FeO > FeAl2O4 > Cr2O3 [32]. However, there was no Cr2O3 detected in FACr coating, but a mass of FeAl2O4, some FeCr2O4 and a few iron oxides were present in FACr coating. This result indicated again that the reaction of the reactants and their reaction products in plasma spraying process were different from that under equilibrium conditions. In the reaction process of FACr composite powders, some FeO reacted with Cr2O3 to form FeCr2O4, which resulted in the decrease of FeO in the FACr coating. 3.2.4. Fe2O3–Al–Fe3O4 system It can be seen from Fig. 3d, Figs. 1 and 2 that the diffraction peaks of a-Fe, AlFe and Al2O3 in FAFe coating decreased comparing with FA coating, and the diffraction peaks of Fe2O3 and Fe3O4 increased, and there are only a few of FeO in FAFe coating. The above results indicated additive Fe3O4 and/or its decomposition product FeO may be directly involved in and promote the formation of FeAl2O4, which led to decreasing of the diffraction peak of the reactant Al2O3 related to the formation of FeAl2O4. There was more Al in the composite powders involved in the formation of FeAl2O4, while less Al involved in the reduction formation of a-Fe, which led to the decrease of the diffraction peak of a-Fe in FAFe coating. 3.2.5. Fe2O3–Al–SiO2 system It can be seen from Fig. 3e, Figs. 1 and 2 that FASi coating is also mainly composed of FeAl2O4 and a-Fe. a-Fe, AlFe and Al2O3 in FASi coating decreased comparing with FA coating, Fe2O3 disappeared, and only a few of FeO were presented in FASi coating. According to Fe2O3–SiO2 phase diagram and FeO–SiO2 phase diagram [33], SiO2 could only react with FeO to form Fe2SiO4, while Fe2O3 could only form eutectic mixture with SiO2. FeO was consumed due to the formation of Fe2SiO4, which promoted the decomposition of Fe2O3. In view of the above reasons, no iron oxides were detected in FeASi coating. 3.3. Influence of oxides addition on FeAl2O4 phase Enlarging the diffraction peaks of FeAl2O4 phases in the XRD patterns of Fig. 3, it was found that there was some difference for the diffraction angles of FeAl2O4 phases. The diffraction peaks of {3 1 1} crystal face of FeAl2O4 in the composite coatings are shown in Fig. 4. It was already mentioned above that FeAl2O4 indexed for the coatings in Fig. 3 was actually a mixed hercynite, namely (Fe1xAlx)(FexAl2x)O4. The diffraction angles of (Fe1xAlx)(FexAl2x)O4 will offset with the change of variable x (0 < x < 1). It could be, therefore, inferred that the offset of the diffraction peaks in Fig. 4 was attributed to the different relative content of the normal hercynite phase and the inverse hercynite phase in the mixed hercynite. Supposing that the hercynite is a uniform continuous phase in each composite coatings, and combining the XRD diffraction data with the cubic crystal diffraction direction formula [34], the lattice constant (a) of the mixed hercynite can be obtained. The value of lattice constant (a) reflects the relative content change
Fig. 4. The diffraction peaks of {3 1 1} crystal face of FeAl2O4 in the composite coatings: (1#) FA, (2#) FAAl, (3#) FACr, (4#) FAFe, (5#) FASi.
of the mixed hercynite (the normal hercynite phase and the inverse hercynite phase) in each composite coatings, and the effect of the additives on the formation of FeAl2O4 could be estimated. The calculation process will not consider the influence of the stress on the diffraction peaks due to the same spraying parameters for the five composite coatings. The diffraction peaks data of {3 1 1} crystal face of FeAl2O4 in the composite coatings were selected for calculating the lattice constants, which are shown in Table 3. The lattice constant of (Fe0.781Al0.219)(Al1.781Fe0.219)O4 is a = 0.8229 nm and the lattice constant of (Fe0.899Al0.101)(Al1.899Fe0.101)O4 is a = 0.8151 nm [30], from which it could be inferred that the smaller x is, the smaller the lattice constant of the mixed hercynite is and the lower the Fe riched inverse hercynite is. It can be seen from Table 3 that the addition of the additives could influence the lattice constant value of the hercynite phase, and the order of which is aFA > aFASi > aFAFe > aFAAl > aFACr. It could be inferred that the content order of the Al riched normal hercynite in the mixed hercynite is FeAl2O4FACr > FeAl2O4FAAl > FeAl2O4FAFe > FeAl2O4FASi > FeAl2O4FA. The content of the normal hercynite in the mixed hercynite of FACr coating is the highest. The reason may be that FeO was consumed due to the reaction between FeO and Cr2O3, which reduced the content of the Fe riched inverse hercynite. There is more Al in FAAl composite powders than that in FA composite powders due to the addition of Al2O3 additive. Moreover, FeAl2O4 could form solid solution in Al2O3 and improve the content of Al in the reaction products. That resulted in the increase of the Al riched normal hercynite. Although there is more Fe in FAFe composite powders than that in FA composite powders due to the addition of Fe3O4 additive, FeO generated by the decomposition of Fe3O4 could pro-
Table 3 The lattice constant of FeAl2O4 in the composite coatings. Coatings
2-Theta (deg.)
sin h
k (nm)
h, k, l
a (nm)
FA FAAl FACr FAFe FASi
36.000 36.360 36.387 36.347 36.139
0.309008 0.311994 0.312218 0.311887 0.310162
0.1542 0.1542 0.1542 0.1542 0.1542
3, 3, 3, 3, 3,
0.8269876 0.8190722 0.8184850 0.8193552 0.8239124
1, 1, 1, 1, 1,
1 1 1 1 1
6
Y. Yang et al. / Journal of Alloys and Compounds 579 (2013) 1–6
mote the formation of FeAl2O4. That, therefore, increased the content of the normal hercynite. For FASi coating, the additive SiO2 could react with FeO and thereby reduced the Fe riched inverse hercynite. 4. Conclusions (1) Thermite reaction between Fe2O3 and Al took place during plasma spraying. The reaction products were different from that of the equilibrium reaction condition. Introduction of the oxide additives affected the reaction process of Fe2O3– Al system. (2) The possible reaction process of the Fe2O3AlMexOy composite powders is that Al powder melted near 660 °C first, and the molten Al diffused into the interior of the composite powder particles and contacted more fully with Fe2O3. Next, when the temperature of the composite powder particles reached the temperature range of 800–850 °C, the self-propagating reaction between Fe2O3 and Al began and a lot of heat was released. Oxides addition reduced the reaction heat release of Fe2O3–Al system and improved the initial reaction temperature. (3) The composite coatings were mainly composed of FeAl2O4, a-Fe and Al2O3. The additives influenced the reaction products of the Fe2O3–Al system in plasma flame. The content of Al2O3 in FAAl coating increased evidently due to the addition of Al2O3 additive. There were new phases FeCr2O4 and Cr formed in FACr coating. The content of FeAl2O4, Fe3O4 and Fe2O3 increased and a-Fe decreased in FAFe coating. a-Fe, AlFe and Al2O3 in FASi coating decreased and Fe2SiO4 was formed due to the reaction between SiO2 and FeO. (4) The main phase FeAl2O4 in the five composite coatings was actually mixed hercynite containing different proportions of Fe and Al elements. The normal hercynite contained more Al and the inverse hercynite contained more Fe. The addition of the additives was beneficial to improve the content of the normal hercynite. The content order of the normal hercynite in the mixed hercynite is FeAl2O4FACr > FeAl2O4FAAl > FeAl2O4FAFe > FeAl2O4FASi > FeAl2O4FA.
Acknowledgments The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (51072045, 51102074 and 51272065), China Postdoctoral Science Foundation (20110490979) and Project (20101317120005) supported by Doctoral Program Specialized Research Foundation for Universities, China and Outstanding Youth Fund for Science and Technology Research of Universities in Hebei Province, China (Y2012003).
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