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In-situ TiB2–Al2O3 formed composite coatings by atmospheric plasma spraying: Influence of process parameters and in-flight particle characteristics
Surface & Coatings Technology 203 (2009) 1649–1655
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
In-situ TiB2–Al2O3 formed composite coatings by atmospheric plasma spraying: Influence of process parameters and in-flight particle characteristics Cagri Tekmen ⁎, Yoshiki Tsunekawa, Masahiro Okumiya Toyota Technological Institute, Materials Processing Lab, 2-12-1, Hisakata Tempaku, Nagoya 468-8511, Japan
a r t i c l e
i n f o
Article history: Received 11 April 2008 Accepted in revised form 15 December 2008 Available online 1 January 2009 Keywords: In-situ Plasma spray In-flight Reactive Mechanical alloying
a b s t r a c t In the present study, mechanically alloyed Al–12Si, B2O3 and TiO2 powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). The effects of mechanical alloying (MA) time and plasma parameters (arc current and primary/secondary/carrier gas flow rate) on in-situ reaction intensity and in-flight particle characteristics (temperature and velocity) have been investigated. It has been observed that MA time has a remarkable effect on powder morphology and relative amount of in-situ formed TiB2 and γ-Al2O3. Inflight particle diagnostic measurements demonstrate that among the plasma parameters arc current has the strongest effect on in-flight particle velocity and temperature. Also, results indicate that in-flight particle velocity is more dominant than temperature on the relative amount of in-situ formed phases. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, a new processing method, in-situ plasma spraying (IPS) has been developed to produce composite coatings with in-situ formed thermodynamically favorable phases through the reaction between selective powders. Composite coatings with in-situ formed Mg2Si, MgAl2O4, NiAl3 [1], Al2O3 [2], TiB2–Al2O3 [3] and Ni–Al [4] phases have been successfully fabricated by thermal spray processes. However, due to short processing time in IPS, in-situ reaction intensity strongly depends on in-flight particle temperature and velocity. Therefore, the effects of plasma spray parameters on in-flight particle characteristics have been a major research subject, and there is a tremendous interest in on-line measurements of particle temperature and velocity [5–11], as well as process modeling [12,13]. In our previous study [3], we successfully produced Al–Si based composite coatings with in-situ formed TiB2–Al2O3 through a mechanically alloyed (MA) Al–12Si/TiO2/B2O3 composite powder. In this study, the effects of MA time, arc current and primary/secondary/ carrier gas flow rate on in-flight characteristics and in-situ formed phase intensity are investigated.
purity) and B2O3 powder (N99.99% purity) with an average particle size of 44 and 50 µm, respectively, were used as starting raw materials. Composite powder was prepared under argon atmosphere in a glove box and mechanically alloyed at 100 rpm for 24, 48, 72 and 96 h with a 20:1 of ball-to-powder weight ratio. A detailed microstructural characterization of the MA composite powder and XRD analysis of the raw materials are given in the previous study [3]. After the MA process, composite powder was sieved to a size range of 38–100 µm. Blasted 30 × 30 × 5 mm pure aluminum (A1050) plate was prepared as a substrate. Coating experiments were carried out by using a DC plasma gun (Sulzer Metco, 9MB) with a 7 mm inner diameter (ID) nozzle under conditions given in Table 1. Powders were injected perpendicularly to the plasma jet axis through a 2 mm ID port located 3 mm downstream of the nozzle exit (Fig. 1). Also, powder size analysis was carried out by using an image analyzer (NIS-Elements D 3.00, Nikon) by performing 100 measurements for each condition, in order to determine the effect of MA time on particle size distribution.
2. Experimental procedure
Table 1 Spray conditions
Al–12Si (Metco 52C-NS) powder manufactured by gas atomization process with a particle size range of 45–106 µm, TiO2 powder (N99%
Plasma current (A) Primary gas flow rate (Ar, l/min) Secondary gas flow rate (H2, l/min) Carrier gas flow rate (Ar, l/min) Powder feed rate (g/min) Spray distance (mm) Spray time (s)
⁎ Corresponding author. Tel.: +81 52 809 1857; fax: +81 52 809 1858. E-mail address: [email protected] (C. Tekmen). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.12.016
300/400/500/600/700 35/40/45 7/10/13 2/4/6 8 150 5
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diffractometer (XRD) with CoKα radiation. DTA analysis was performed with a heating rate of 10 °C/min in air up to 1000 °C. Relative amounts (RA) of in-situ formed phases were calculated through the relative intensity (I) ratios by using the strongest peaks obtained from XRD patterns (Eq. (1)). Iphase x 100 RA:kphase = IAl + ISi + ITiB2 + IAl2 O3 + ITiðAl2:4 Si0:6
ð1Þ
In-flight particle diagnostic was performed using Accuraspray-g3 (Sulzer Metco). The sensor head was positioned at 200 mm from the plume center line (Fig. 1). In-flight particle velocity and temperature measurements were based on time-shift cross-correlation between two signals and twin wavelength pyrometer principle, respectively. A more detailed explanation of its working principles can be found in [14]. A value between 0 and 1 of the cross-correlation verifies the validity of the temperature measurement (the higher the value the higher the accuracy). In this study, during the measurements the correlation number (coefficient) was between 0.6–0.7 which is above the critical value (0.6) given by the manufacturer and ensures that the twin wave length pyrometer is working properly [15]. Microstructural characterization and elemental distribution of coated samples were examined by using a scanning electron microscope (SEM), optical microscope and energy dispersive spectrometer (EDS). Vickers macro and micro hardness measurements were carried out by using HSV-30 Shimadzu and Akashi hardness tester under 200 and 25 g load, respectively. Fig. 1. Schematic illustration of the plasma spraying and in-flight particle diagnostic system.
3. Results and discussion 3.1. Effect of MA time
In order to determine the effect of MA time on the reaction intensity and phase formation, prepared composite powder was analyzed by using a differential thermal analyzer (DTA) and X-ray
The change in powder morphology and size distribution with MA time can be seen clearly from Figs. 2 and 3, respectively. After 24 h MA,
Fig. 2. Composite powder morphology after a) 24, b) 48, c) 72 and d) 96 h MA.
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Fig. 5. DTA curves of 24, 48, 72 and 96 h MA composite powder. Fig. 3. The effect of MA time on powder size distribution.
the composite powder has a characteristic layered (flake-type) structure (Fig. 2a). With continued mechanical alloying a remarkable broadening in particle size range is observed due to the fracture of fragile flakes (Fig. 2b). After 72 h MA, it has been observed that smaller particles are welded into larger pieces but the particle size range is still relatively high (Fig. 2c). After 96 h MA, very fine and very large
Fig. 4. XRD patterns of 24, 48, 72 and 96 h MA composite powder.
particles reached an intermediate size with owing a round-type morphology and narrow particle size distribution (Fig. 2d). MA time also affected experimental studies such that by experiencing
Fig. 6. XRD patterns of coatings sprayed with 24, 48, 72 and 96 h MA composite powder at 400 A arc current and 40, 10 and 4 l/min flow rate of primary, secondary and carrier gas, respectively.
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Fig. 7. In-flight particle velocity versus temperature measured at different spray conditions.
difficulties in feeding 24 h MA composite powder into the plasma flame due to its flake-type morphology (Fig. 2a). XRD patterns of 24, 48, 72 and 96 h MA composite powder are given in Fig. 4. It is noticeable that above 48 h MA, very low intensity and broad diffraction peaks appear at 41.2° and 61.2° which indicate the presence of a relatively small amount and size of TiB. It is well known that nanocrystalline, supersaturated solid solutions, amorphous or metastable nanostructured phases may form through the solid-state reaction by mechanical alloying and high-energy ball milling [16–18]. TiN/TiB2/Ti-silicide composite powders have been successfully synthesized from mixtures of Ti, BN and Si3N4 powders by high-energy ball milling through a mechanically activated selfsustaining reaction [19]. Also, it has been reported that a small fraction of TiB obtained as a reaction product through the direct reaction between Ti and B by discharge assisted mechanical milling process [20]. As shown in DTA curves (Fig. 5), the reactivity of the samples increases as MA time increases. In the DTA plot one endothermic and two exothermic peaks can be seen. Former one indicates the melting of primary Al which is found to be shifted to lower temperatures (~ 575 °C) as compared to bulk Al melting point (~ 660 °C). It is well known that the dissolution of silicon into the aluminum lattice during the MA process decreases the melting temperature of aluminum. Latter exothermic peaks observed in the temperature range of 614–684 °C and 884–915 °C corresponds to the formation of Al2O3 and TiB2, respectively. With the increase in MA time more Al surface comes into intimate contact with TiO2 and B2O3 thus enhances the intensity of the reaction as also observed by XRD analysis (Fig. 4). Also, as seen from the DTA curve of 24 h MA composite powder, alumina formation occurred at relatively low temperature (~ 614 °C) compared to other samples. At lower MA time, the non-contacted (with TiO2 and B2O3) Al surface exposed to air is relatively more, thus alumina formation might be induced with the oxygen in air.
Fig. 8. The effect of in-flight particle a) velocity and b) temperature on the relative amount of in-situ formed γ-Al2O3 and TiB2.
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Fig. 9. Cross section microstructure, elemental distribution and line profile of the coating sprayed with 96 h MA composite powder at 400 A arc current and 40, 10 and 4 l/min flow rate of primary, secondary and carrier gas, respectively.
XRD results of the coatings sprayed at 400 A arc current with composite powders prepared at different MA times are given in Fig. 6. A remarkable decrease in Si (111) peak intensity has been observed after the coating process for all samples. The decrease and disappearance of Si peak may be attributed to the formation of Ti (Al2.4Si0.6). Since the atomic radius of Al (0.125 nm) and Si (0.110 nm) is smaller than Ti (0.140 nm) Al and Si might be dissolved into Ti to form a Ti(AlSi) solid solution. In our previous study, it has been assumed that during mechanical alloying the crystal structure of Si may change from cubic to tetragonal and therefore the peak obtained at 46.5° was stated as Si, however, a detailed XRD analysis indicates that this peak corresponds to Ti(Al2.4Si0.6). On the other hand, a and γ-Al2O(440) diffracnoticeable increase in in-situ formed TiB(100) 2 3
tion peak intensities was observed as MA time increases. The further increase in TiB2 and decrease in Ti(Al2.4Si0.6) peak intensity with the increase in MA time suggesting that Ti(AlSi) solid solution was decomposed owing the released heat during the exothermic formation of Al2O3 and TiB2. 3.2. Effect of plasma parameters In-flight particle diagnostic measurements were carried out by using 96 h MA composite powder, and the effects of arc current and primary/secondary/carrier gas flow rate on in-flight particle temperature and velocity is given in Fig. 7. Results reveal that for each spray condition the particle temperature (min.: ~ 2310 °C) is
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always above the melting temperature of the constituents of the composite powder. It can be said that, the most effective parameter on in-flight particle characteristics is the arc current. The increase in arc current increases the number of gas ionization and plasma gas expansion velocity, which will result in an increase in particle temperature and velocity, respectively [21,22]. However, the significant decrease in particle temperature obtained at 700 A may be attributed to the shorten dwell time due to relatively high particle velocity which may reduce the heat energy absorbed by in-flight particle [23]. Also, results demonstrate that hydrogen and argon gas flow rate is more effective on in-flight particle temperature and velocity, respectively. The difference in their effect may be attributed to the difference in their molecular weight [9]. The higher molecular weight of argon leads to a higher increase in the mass flow rate of plasma and results in a higher acceleration. On the other hand, the effect of hydrogen gas flow rate on in-flight particle temperature may be attributed to the increase in heat energy transferred from the plasma jet to the particle due to the higher thermal conductivity and enthalpy of hydrogen plasma. Both in-flight particle velocity and temperature increase slightly with the increase in carrier gas (argon) flow rate. Guessasma et al., attributed such an increase to the drifting of particles deeper into the jet core due to the increase in particle injection velocity [24]. However, it should be taken into account that the effect of carrier gas flow rate may also affect the amount and size of the particles injected into the plasma flame. In order to determine the effects of in-flight particle velocity and temperature on the relative amounts (calculated by using Eq. (1)) of in-situ formed γ-Al2O3 and TiB2, coating experiments were carried out with different conditions as given in Fig. 7 by using 96 h MA composite powder. As seen from Fig. 8, in-flight particle velocity is more effective than temperature on the relative amount of in-situ formed phases. The increase in the amounts of in-situ formed phases obtained at relatively low in-flight particle velocities may be attributed to relatively high dwell time outside the plasma flame which provides more time for the reaction between the composite powder constituents. Since the minimum measured temperature is already above the melting point of the phases, it is reasonable that the further increase in temperature may not considerably affect the in-situ reaction intensity. The highest relative amount of γ-Al2O3 and TiB2 was obtained in the coating sprayed with 400 A arc current, 40, 10 and 4 l/min flow rate of primary (Ar), secondary (H2) and carrier gas (Ar), respectively. In-flight particle velocity and temperature for this optimum condition was measured as 97 m/s and 2352 °C, respectively.
Fig. 10. The effect of MA time on the macro-hardness of the coating sprayed at 400 A arc current and 40,10 and 4 l/min flow rate of primary, secondary and carrier gas, respectively.
439Hv) consists mainly of Ti, Al and Si, which indicate towards Ti (Al2.4Si0.6) phase. Region “B” (with a micro-hardness of 1170HV) consists of only Al and O, which indicates to γ-Al2O3. Region “C” (with a micro-hardness of 662HV) consists of Ti and Si. These results indicate that the composite coating exhibits a structure where relatively hard in-situ formed phases are embedded in a soft matrix, which makes them potential candidates for wear-resistance applications. 4. Conclusions This study has investigated the effects of mechanical alloying time, arc current and primary/secondary/carrier gas flow rate on in-flight particle characteristics and relative amount of in-situ formed γ-Al2O3 and TiB2. The major results are summarized as follows: 1- Increasing the MA time from 24 to 96 h changes the powder morphology from flake-type to round-type structure, narrows the particle size distribution and enhances powder flowability. Also, the reactivity between the constituents of the composite powder increases as MA time increases. 2- The most effective plasma parameter on in-flight particle characteristics is determined as arc current. However, hydrogen and argon gas flow rate is more effective on in-flight particle temperature and velocity, respectively. Also, it has been found that the in-flight particle velocity is more dominant than temperature on the relative amount of in-situ formed phases.
3.3. Microstructural characterization References Microstructural characterization was performed on coatings sprayed with 96 h MA composite powder. A typical cross section microstructure, elemental distribution and line profile of the coating are shown in Fig. 9. Since TiO2, B2O3, B and Ti were not detected by XRD analyses, the regions with relatively high intensity of oxygen and titanium correspond to in-situ formed γ-Al2O3 (dark gray color in BSE) and TiB2 (bright white color in BSE), respectively. Also, line EDS analysis demonstrates Al–Ti–Si and Ti–Si rich regions, which may correspond to Ti(Al2.4Si0.6). As well known, solid solubility extensions have been achieved by mechanical alloying and rapid solidification processes (RPS) such as plasma spraying [25–28]. Macro hardness measurements demonstrate that the highest hardness value is obtained in coatings sprayed with 48 h MA powder (Fig. 10), where the Ti(Al2.4Si0.6)(112) peak intensity is relatively high (Fig. 6). It can be said that, the coating hardness depends more on the intensity of Ti(Al2.4Si0.6) rather than γ-Al2O3 or TiB2. Fig. 11 shows the EDS analysis of the micro-hardness tested regions. The region denoted with “A” (with a micro-hardness of
[1] I. Ozdemir, I. Hamanaka, M. Hirose, Y. Tsunekawa, M. Okumiya, Surf. Coat. Technol. 200 (2005) 1155. [2] C. Tekmen, M. Yamazaki, Y. Tsunekawa, M. Okumiya, Surf. Coat. Technol. 202 (2008) 4163. [3] C. Tekmen, Y. Tsunekawa, M. Okumiya, Surf. Coat. Technol. 202 (2008) 4170. [4] S. Kumar, V. Selvarajan, Chem. Eng. Process. 45 (2006) 1029. [5] Z. Salhi, S. Guessasma, P. Gougeon, D. Klein, C. Coddet, Aerosp. Sci. Technol. 9 (2005) 203. [6] C. Li, B. Sun, Mater. Sci. Eng., A 379 (2004) 92. [7] S. Sampath, X. Jiang, A. Kulkarni, J. Matejicek, D.L. Gilmore, R.A. Neiser, Mater. Sci. Eng., A 348 (2003) 54. [8] M. Krauss, D. Bergmann, U. Fritsching, K. Bauckhage, Mater. Sci. Eng., A 326 (2002) 154. [9] J.C. Fang, W.J. Xu, Z.Y. Zhao, H.P. Zeng, Surf. Coat. Technol. 201 (2007) 5671. [10] G. Shanmugavelayutham, V. Selvarajan, T.K. Thiyagarajan, P.V.A. Padmanabhan, K.P. Sreekumar, R.U. Satpute, Curr. Appl. Phys. 6 (2006) 41. [11] H. Xiong, L. Zheng, L. Li, A. Vaidya, Int. J. Heat Mass Transfer 48 (2005) 5121. [12] S. Guessasma, Z. Salhi, G. Montavon, P. Gougeon, C. Coddet, Mater. Sci. and Eng., B 110 (2004) 285. [13] L. Wang, J.C. Fang, Z.Y. Zhao, H.P. Zeng, Surf. Coat. Technol. 201 (2007) 5085. [14] G. Mauer, R. Vaben, D. Stover, J. Therm. Spray Technol. 16 (3) (2007) 414. [15] Sulzer Metco – Accuraspray-g3, Product Manual 40857 MAN EN 03. [16] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1. [17] C. Suryanarayana, E. Ivanov, V.V. Boldyrev, Mater. Sci. and Eng., A 304–306 (2001) 151. [18] K. Wieczorek-Ciurowa, K. Gamrat, Mater. Sci.-Pol. 25 (2007) 219.
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Fig. 11. Cross section microstructure, Vickers micro hardness indentation, elemental distribution and line profile of the coating sprayed with 96 h MA composite powder at 400 A arc current and 40, 10 and 4 l/min flow rate of primary, secondary and carrier gas, respectively. [19] J.W. Kim, H.S. Chung, S.H. Lee, K.H. Oh, J.H. Shim, Y.W. Cho, Intermetallics 15 (2007) 206. [20] A. Calka, D. Oleszak, J. Alloys Compd. 440 (2007) 346. [21] S. Guessasma, G. Montavon, C. Coddet, Surf. Coat. Technol. 192 (2005) 70. [22] J.C. Fang, W.J. Xu, Z.Y. Zhao, H.P. Zeng, Surf. Coat. Technol. 201 (2007) 5671. [23] C. Tekmen, Y. Tsunekawa, M. Okumiya, Surf. Coat. Technol. 203 (2008) 223.
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
In-situ TiB2–Al2O3 formed composite coatings by atmospheric plasma spraying: Influence of process parameters and in-flight particle characteristics Cagri Tekmen ⁎, Yoshiki Tsunekawa, Masahiro Okumiya Toyota Technological Institute, Materials Processing Lab, 2-12-1, Hisakata Tempaku, Nagoya 468-8511, Japan
a r t i c l e
i n f o
Article history: Received 11 April 2008 Accepted in revised form 15 December 2008 Available online 1 January 2009 Keywords: In-situ Plasma spray In-flight Reactive Mechanical alloying
a b s t r a c t In the present study, mechanically alloyed Al–12Si, B2O3 and TiO2 powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). The effects of mechanical alloying (MA) time and plasma parameters (arc current and primary/secondary/carrier gas flow rate) on in-situ reaction intensity and in-flight particle characteristics (temperature and velocity) have been investigated. It has been observed that MA time has a remarkable effect on powder morphology and relative amount of in-situ formed TiB2 and γ-Al2O3. Inflight particle diagnostic measurements demonstrate that among the plasma parameters arc current has the strongest effect on in-flight particle velocity and temperature. Also, results indicate that in-flight particle velocity is more dominant than temperature on the relative amount of in-situ formed phases. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, a new processing method, in-situ plasma spraying (IPS) has been developed to produce composite coatings with in-situ formed thermodynamically favorable phases through the reaction between selective powders. Composite coatings with in-situ formed Mg2Si, MgAl2O4, NiAl3 [1], Al2O3 [2], TiB2–Al2O3 [3] and Ni–Al [4] phases have been successfully fabricated by thermal spray processes. However, due to short processing time in IPS, in-situ reaction intensity strongly depends on in-flight particle temperature and velocity. Therefore, the effects of plasma spray parameters on in-flight particle characteristics have been a major research subject, and there is a tremendous interest in on-line measurements of particle temperature and velocity [5–11], as well as process modeling [12,13]. In our previous study [3], we successfully produced Al–Si based composite coatings with in-situ formed TiB2–Al2O3 through a mechanically alloyed (MA) Al–12Si/TiO2/B2O3 composite powder. In this study, the effects of MA time, arc current and primary/secondary/ carrier gas flow rate on in-flight characteristics and in-situ formed phase intensity are investigated.
purity) and B2O3 powder (N99.99% purity) with an average particle size of 44 and 50 µm, respectively, were used as starting raw materials. Composite powder was prepared under argon atmosphere in a glove box and mechanically alloyed at 100 rpm for 24, 48, 72 and 96 h with a 20:1 of ball-to-powder weight ratio. A detailed microstructural characterization of the MA composite powder and XRD analysis of the raw materials are given in the previous study [3]. After the MA process, composite powder was sieved to a size range of 38–100 µm. Blasted 30 × 30 × 5 mm pure aluminum (A1050) plate was prepared as a substrate. Coating experiments were carried out by using a DC plasma gun (Sulzer Metco, 9MB) with a 7 mm inner diameter (ID) nozzle under conditions given in Table 1. Powders were injected perpendicularly to the plasma jet axis through a 2 mm ID port located 3 mm downstream of the nozzle exit (Fig. 1). Also, powder size analysis was carried out by using an image analyzer (NIS-Elements D 3.00, Nikon) by performing 100 measurements for each condition, in order to determine the effect of MA time on particle size distribution.
2. Experimental procedure
Table 1 Spray conditions
Al–12Si (Metco 52C-NS) powder manufactured by gas atomization process with a particle size range of 45–106 µm, TiO2 powder (N99%
Plasma current (A) Primary gas flow rate (Ar, l/min) Secondary gas flow rate (H2, l/min) Carrier gas flow rate (Ar, l/min) Powder feed rate (g/min) Spray distance (mm) Spray time (s)
⁎ Corresponding author. Tel.: +81 52 809 1857; fax: +81 52 809 1858. E-mail address: [email protected] (C. Tekmen). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.12.016
300/400/500/600/700 35/40/45 7/10/13 2/4/6 8 150 5
1650
C. Tekmen et al. / Surface & Coatings Technology 203 (2009) 1649–1655
diffractometer (XRD) with CoKα radiation. DTA analysis was performed with a heating rate of 10 °C/min in air up to 1000 °C. Relative amounts (RA) of in-situ formed phases were calculated through the relative intensity (I) ratios by using the strongest peaks obtained from XRD patterns (Eq. (1)). Iphase x 100 RA:kphase = IAl + ISi + ITiB2 + IAl2 O3 + ITiðAl2:4 Si0:6
ð1Þ
In-flight particle diagnostic was performed using Accuraspray-g3 (Sulzer Metco). The sensor head was positioned at 200 mm from the plume center line (Fig. 1). In-flight particle velocity and temperature measurements were based on time-shift cross-correlation between two signals and twin wavelength pyrometer principle, respectively. A more detailed explanation of its working principles can be found in [14]. A value between 0 and 1 of the cross-correlation verifies the validity of the temperature measurement (the higher the value the higher the accuracy). In this study, during the measurements the correlation number (coefficient) was between 0.6–0.7 which is above the critical value (0.6) given by the manufacturer and ensures that the twin wave length pyrometer is working properly [15]. Microstructural characterization and elemental distribution of coated samples were examined by using a scanning electron microscope (SEM), optical microscope and energy dispersive spectrometer (EDS). Vickers macro and micro hardness measurements were carried out by using HSV-30 Shimadzu and Akashi hardness tester under 200 and 25 g load, respectively. Fig. 1. Schematic illustration of the plasma spraying and in-flight particle diagnostic system.
3. Results and discussion 3.1. Effect of MA time
In order to determine the effect of MA time on the reaction intensity and phase formation, prepared composite powder was analyzed by using a differential thermal analyzer (DTA) and X-ray
The change in powder morphology and size distribution with MA time can be seen clearly from Figs. 2 and 3, respectively. After 24 h MA,
Fig. 2. Composite powder morphology after a) 24, b) 48, c) 72 and d) 96 h MA.
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Fig. 5. DTA curves of 24, 48, 72 and 96 h MA composite powder. Fig. 3. The effect of MA time on powder size distribution.
the composite powder has a characteristic layered (flake-type) structure (Fig. 2a). With continued mechanical alloying a remarkable broadening in particle size range is observed due to the fracture of fragile flakes (Fig. 2b). After 72 h MA, it has been observed that smaller particles are welded into larger pieces but the particle size range is still relatively high (Fig. 2c). After 96 h MA, very fine and very large
Fig. 4. XRD patterns of 24, 48, 72 and 96 h MA composite powder.
particles reached an intermediate size with owing a round-type morphology and narrow particle size distribution (Fig. 2d). MA time also affected experimental studies such that by experiencing
Fig. 6. XRD patterns of coatings sprayed with 24, 48, 72 and 96 h MA composite powder at 400 A arc current and 40, 10 and 4 l/min flow rate of primary, secondary and carrier gas, respectively.
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Fig. 7. In-flight particle velocity versus temperature measured at different spray conditions.
difficulties in feeding 24 h MA composite powder into the plasma flame due to its flake-type morphology (Fig. 2a). XRD patterns of 24, 48, 72 and 96 h MA composite powder are given in Fig. 4. It is noticeable that above 48 h MA, very low intensity and broad diffraction peaks appear at 41.2° and 61.2° which indicate the presence of a relatively small amount and size of TiB. It is well known that nanocrystalline, supersaturated solid solutions, amorphous or metastable nanostructured phases may form through the solid-state reaction by mechanical alloying and high-energy ball milling [16–18]. TiN/TiB2/Ti-silicide composite powders have been successfully synthesized from mixtures of Ti, BN and Si3N4 powders by high-energy ball milling through a mechanically activated selfsustaining reaction [19]. Also, it has been reported that a small fraction of TiB obtained as a reaction product through the direct reaction between Ti and B by discharge assisted mechanical milling process [20]. As shown in DTA curves (Fig. 5), the reactivity of the samples increases as MA time increases. In the DTA plot one endothermic and two exothermic peaks can be seen. Former one indicates the melting of primary Al which is found to be shifted to lower temperatures (~ 575 °C) as compared to bulk Al melting point (~ 660 °C). It is well known that the dissolution of silicon into the aluminum lattice during the MA process decreases the melting temperature of aluminum. Latter exothermic peaks observed in the temperature range of 614–684 °C and 884–915 °C corresponds to the formation of Al2O3 and TiB2, respectively. With the increase in MA time more Al surface comes into intimate contact with TiO2 and B2O3 thus enhances the intensity of the reaction as also observed by XRD analysis (Fig. 4). Also, as seen from the DTA curve of 24 h MA composite powder, alumina formation occurred at relatively low temperature (~ 614 °C) compared to other samples. At lower MA time, the non-contacted (with TiO2 and B2O3) Al surface exposed to air is relatively more, thus alumina formation might be induced with the oxygen in air.
Fig. 8. The effect of in-flight particle a) velocity and b) temperature on the relative amount of in-situ formed γ-Al2O3 and TiB2.
C. Tekmen et al. / Surface & Coatings Technology 203 (2009) 1649–1655
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Fig. 9. Cross section microstructure, elemental distribution and line profile of the coating sprayed with 96 h MA composite powder at 400 A arc current and 40, 10 and 4 l/min flow rate of primary, secondary and carrier gas, respectively.
XRD results of the coatings sprayed at 400 A arc current with composite powders prepared at different MA times are given in Fig. 6. A remarkable decrease in Si (111) peak intensity has been observed after the coating process for all samples. The decrease and disappearance of Si peak may be attributed to the formation of Ti (Al2.4Si0.6). Since the atomic radius of Al (0.125 nm) and Si (0.110 nm) is smaller than Ti (0.140 nm) Al and Si might be dissolved into Ti to form a Ti(AlSi) solid solution. In our previous study, it has been assumed that during mechanical alloying the crystal structure of Si may change from cubic to tetragonal and therefore the peak obtained at 46.5° was stated as Si, however, a detailed XRD analysis indicates that this peak corresponds to Ti(Al2.4Si0.6). On the other hand, a and γ-Al2O(440) diffracnoticeable increase in in-situ formed TiB(100) 2 3
tion peak intensities was observed as MA time increases. The further increase in TiB2 and decrease in Ti(Al2.4Si0.6) peak intensity with the increase in MA time suggesting that Ti(AlSi) solid solution was decomposed owing the released heat during the exothermic formation of Al2O3 and TiB2. 3.2. Effect of plasma parameters In-flight particle diagnostic measurements were carried out by using 96 h MA composite powder, and the effects of arc current and primary/secondary/carrier gas flow rate on in-flight particle temperature and velocity is given in Fig. 7. Results reveal that for each spray condition the particle temperature (min.: ~ 2310 °C) is
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always above the melting temperature of the constituents of the composite powder. It can be said that, the most effective parameter on in-flight particle characteristics is the arc current. The increase in arc current increases the number of gas ionization and plasma gas expansion velocity, which will result in an increase in particle temperature and velocity, respectively [21,22]. However, the significant decrease in particle temperature obtained at 700 A may be attributed to the shorten dwell time due to relatively high particle velocity which may reduce the heat energy absorbed by in-flight particle [23]. Also, results demonstrate that hydrogen and argon gas flow rate is more effective on in-flight particle temperature and velocity, respectively. The difference in their effect may be attributed to the difference in their molecular weight [9]. The higher molecular weight of argon leads to a higher increase in the mass flow rate of plasma and results in a higher acceleration. On the other hand, the effect of hydrogen gas flow rate on in-flight particle temperature may be attributed to the increase in heat energy transferred from the plasma jet to the particle due to the higher thermal conductivity and enthalpy of hydrogen plasma. Both in-flight particle velocity and temperature increase slightly with the increase in carrier gas (argon) flow rate. Guessasma et al., attributed such an increase to the drifting of particles deeper into the jet core due to the increase in particle injection velocity [24]. However, it should be taken into account that the effect of carrier gas flow rate may also affect the amount and size of the particles injected into the plasma flame. In order to determine the effects of in-flight particle velocity and temperature on the relative amounts (calculated by using Eq. (1)) of in-situ formed γ-Al2O3 and TiB2, coating experiments were carried out with different conditions as given in Fig. 7 by using 96 h MA composite powder. As seen from Fig. 8, in-flight particle velocity is more effective than temperature on the relative amount of in-situ formed phases. The increase in the amounts of in-situ formed phases obtained at relatively low in-flight particle velocities may be attributed to relatively high dwell time outside the plasma flame which provides more time for the reaction between the composite powder constituents. Since the minimum measured temperature is already above the melting point of the phases, it is reasonable that the further increase in temperature may not considerably affect the in-situ reaction intensity. The highest relative amount of γ-Al2O3 and TiB2 was obtained in the coating sprayed with 400 A arc current, 40, 10 and 4 l/min flow rate of primary (Ar), secondary (H2) and carrier gas (Ar), respectively. In-flight particle velocity and temperature for this optimum condition was measured as 97 m/s and 2352 °C, respectively.
Fig. 10. The effect of MA time on the macro-hardness of the coating sprayed at 400 A arc current and 40,10 and 4 l/min flow rate of primary, secondary and carrier gas, respectively.
439Hv) consists mainly of Ti, Al and Si, which indicate towards Ti (Al2.4Si0.6) phase. Region “B” (with a micro-hardness of 1170HV) consists of only Al and O, which indicates to γ-Al2O3. Region “C” (with a micro-hardness of 662HV) consists of Ti and Si. These results indicate that the composite coating exhibits a structure where relatively hard in-situ formed phases are embedded in a soft matrix, which makes them potential candidates for wear-resistance applications. 4. Conclusions This study has investigated the effects of mechanical alloying time, arc current and primary/secondary/carrier gas flow rate on in-flight particle characteristics and relative amount of in-situ formed γ-Al2O3 and TiB2. The major results are summarized as follows: 1- Increasing the MA time from 24 to 96 h changes the powder morphology from flake-type to round-type structure, narrows the particle size distribution and enhances powder flowability. Also, the reactivity between the constituents of the composite powder increases as MA time increases. 2- The most effective plasma parameter on in-flight particle characteristics is determined as arc current. However, hydrogen and argon gas flow rate is more effective on in-flight particle temperature and velocity, respectively. Also, it has been found that the in-flight particle velocity is more dominant than temperature on the relative amount of in-situ formed phases.
3.3. Microstructural characterization References Microstructural characterization was performed on coatings sprayed with 96 h MA composite powder. A typical cross section microstructure, elemental distribution and line profile of the coating are shown in Fig. 9. Since TiO2, B2O3, B and Ti were not detected by XRD analyses, the regions with relatively high intensity of oxygen and titanium correspond to in-situ formed γ-Al2O3 (dark gray color in BSE) and TiB2 (bright white color in BSE), respectively. Also, line EDS analysis demonstrates Al–Ti–Si and Ti–Si rich regions, which may correspond to Ti(Al2.4Si0.6). As well known, solid solubility extensions have been achieved by mechanical alloying and rapid solidification processes (RPS) such as plasma spraying [25–28]. Macro hardness measurements demonstrate that the highest hardness value is obtained in coatings sprayed with 48 h MA powder (Fig. 10), where the Ti(Al2.4Si0.6)(112) peak intensity is relatively high (Fig. 6). It can be said that, the coating hardness depends more on the intensity of Ti(Al2.4Si0.6) rather than γ-Al2O3 or TiB2. Fig. 11 shows the EDS analysis of the micro-hardness tested regions. The region denoted with “A” (with a micro-hardness of
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