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Wear behavior of plasma sprayed composite coatings with in situ formed Al2O3
Materials and Design 30 (2009) 4516–4520
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Wear behavior of plasma sprayed composite coatings with in situ formed Al2O3 H. Mindivan a,*, C. Tekmen b, B. Dikici c, Y. Tsunekawa b, M. Gavgali d a
Ataturk University, Department of Metallurgy Engineering, 25240 Erzurum, Turkey Toyota Technological Institute, Materials Processing Lab., 2-12-1, 468-8511 Nagoya, Japan Yuzuncu Yil University, Ercis Technical Vocational School of Higher Education, 65400 Van, Turkey d Ataturk University, Department of Mechanical Engineering, 25240 Erzurum, Turkey b c
a r t i c l e
i n f o
Article history: Received 5 March 2009 Accepted 12 May 2009 Available online 19 May 2009
a b s t r a c t In the present study, the wear behavior of in situ formed Al2O3 reinforced hypereutectic Al–18Si matrix composite coatings have been investigated. These coatings were successfully fabricated with mechanically alloyed Al–12Si and SiO2 powder deposited on aluminum substrates by atmospheric plasma spraying (APS). The produced samples were characterized by means of microscopic examinations, hardness measurements and wear tests. The obtained results pointed out that the amount of in situ formed Al2O3 particles increased with increasing spray distance and decreasing in-flight particle velocity and temperature, which was accompanied by an improvement in hardness and wear resistance. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Al–Si alloys have been utilized extensively in the recent past due to superior properties such as low coefficient of thermal expansion, good specific strength, high wear resistance and fluidity [1]. The incorporation of Al2O3 or SiC particles in these alloys is gradually increasing especially in automotive industry where wear is the dominant process such as pistons, cylinder heads and connecting rods [2–4]. Aluminum based composites fabricated by conventional ex situ techniques are known to suffer from inhomogeneous distribution of reinforcements in the matrix and poor wettability of the reinforcement [3]. On the other hand, recently in situ plasma spraying (IPS) is a promising technology for deposition of in situ formed thermodynamically favorable phases such as Mg2Si, MgAl2O4, NiAl3 [5], Ni–Al [6], Al2O3 [7] and TiB2–Al2O3 [8] through the reaction between selective powders by using high velocity oxygen fuel (HVOF), direct current (DC) and radio frequency (RF) plasma spraying methods. The main advantages of in situ processes are good particle–matrix bonding, low fabrication cost, high thermodynamic stability, good high temperature performance and effective load transfer [9,10]. Since the ceramic material Al2O3 exhibits high hardness, melting point and wear resistance [2,3], SiO2 is an attractive oxide material for in situ reinforcing Al–Si alloys with Al2O3 [5,7]. In the literature [5,7,8], there are many researches concerning the preparation and processing of the in situ composites containing Al2O3 particles as the reinforcement produced by in situ plasma spraying method. However, due to short processing time in IPS,
* Corresponding author. Tel.: +90 442 231 4589; fax: +90 442 236 0957. E-mail address: [email protected] (H. Mindivan). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.05.018
in situ reaction intensity strongly depends on plasma spray parameters and in-flight particle characteristics and studies on in situ reaction intensity (alumina and silicon) on the wear of a composite coating produced by in situ plasma spraying method of hypereutectic Al–Si alloy with SiO2 are scarce. The objective of the present work is to investigate the effect of spray distance and in-flight particle characteristics on the sliding wear behavior of plasma sprayed composite coatings with in situ formed Al2O3. 2. Experimental procedure Mechanically alloyed Al–12Si/SiO2 composite powder (Fig. 1) was deposited onto an aluminum substrate by atmospheric plasma spraying to obtain a composite coating consisting of in situ formed Al2O3 reinforced Al–18Si matrix alloy according to the reaction of [7]. Coating experiments were carried out by using atmospheric plasma spraying apparatus (Sulzer Metco, 9M) under conditions given in Table 1. In-flight particle diagnostic was performed using Accuraspray-g3 (Sulzer Metco) where the sensor head was positioned at 200 mm from the plume center line. A more detailed explanation of its working principles can be found in [11].
Al—12Si þ SiO2 ! Al2 O3 þ Al—18Si The wear behavior of the coatings has been investigated with respect to the relative amount of the phases depending on spray distance, in-flight particle velocity and temperature values. Relative amounts (RA) of the phases were calculated through the relative intensity (I) ratios by using the strongest peaks obtained from the X-ray diffraction (XRD) patterns (Eq. (1)). A detailed information regarding powder composition, structure and morphology,
H. Mindivan et al. / Materials and Design 30 (2009) 4516–4520
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Fig. 1. Powder morphology (a) as-received Al–12Si and (b) mechanically alloyed Al–12Si/SiO2.
Table 1 Spray parameters. Primary gas flow rate (l/min): Ar Secondary gas flow rate (l/min): H2 Carrier gas flow rate (l/min): Ar Current (A) Spray distance (mm) Nozzle inner diameter (mm) Powder feed rate (g/min)
40 10 6 400 100/125/150 7 8
mechanical alloying conditions, differential thermal and X-ray diffraction analysis of the composite powder and Al–12Si/SiO2 sprayed coating is given elsewhere [7].
RA:% phase ¼
Iphase 100 ðIAl þ ISi þ IAl2 O3 Þ
ð1Þ
In the scope of the present study, the characterization of the coatings was made by microscopic examinations, hardness measurements and wear tests. Microscopic examinations were conducted on the cross-sections of the coated samples by an optical microscope, after grinding and polishing in standard manner. Microhardness measurements were carried out by using a HSV-30 Shimadzu Vickers hardness tester under 200 g load covered all of the microstructure constituents (including the matrix and c-Al2O3 phase). Hereafter, the coatings sprayed at 100, 125 and 150 mm spray distances were called Sample A, Sample B and Sample C, respectively. Wear performance of the coatings was examined on a reciprocating wear tester, which was designed according to the ASTM G133 standard. Wear tests were carried out by rubbing a 10 mm Al2O3 ball on the surfaces of the coatings at normal atmospheric conditions (room temperature and 66% relative humidity). During the wear tests, a 10 mm diameter Al2O3 ball was reciprocally sliding on the surface with a speed of 0.023 m/s and frictional force was continuously recorded using a computer. The amplitude of this motion, total sliding distance and normal load were 12 mm, 36 m and 1.5 N, respectively. After the wear test, semi-elliptic wear
tracks formed on the surfaces of the coatings were examined by a Veeco Dektak 32 surface profilometer and a scanning electron microscope (JEOL 6400). An optical microscope was used to examine the wear scars formed on the Al2O3 balls. 3. Results and discussion 3.1. Microstructure observations A typical cross-section of the coating is given in Fig. 2. It seems that the sprayed coating exhibits a wavy layered structure and consists of rarely unmelted particles and pores, with an average thickness of 200 lm where dark gray areas (marked with arrows) corresponds to the in situ formed c-Al2O3 phase. Relative amounts of the phases calculated from XRD results by using Eq. (1), mean in-flight particle velocity and temperature values, hardness and reciprocating wear tests conducted on the coatings are listed in Table 2. 3.2. Surface hardness The hardness of coatings is strongly relative to the distribution and the amount of the phases. It has been reported that the spray distance, in-flight particle velocity and temperature have a strong influence on the amount of Al2O3 and Si phases [7]. It can be seen from Table 2 that the hardness increases with the increase in Al2O3 and decrease in Si relative amounts. It should be mentioned that, the relatively low hardness of the coating sprayed at 100 mm spray distance may be attributed to the relatively high Si and low Al2O3 amount. At relatively short spray distances the decrease in solidification rate may cause the precipitation and growth of Si particles from the supersaturated solid solution [12,13]. On the other hand, the decrease in spray distance will shorten the dwelling time of the particles in plasma plume and consequently hinder in situ Al2O3 formation. It is only known that increasing the Si content results in an increase of the strength of hypoeutectic alloys and a decrease of the strength of hypereutectic alloys [14]. Among the examined
Fig. 2. Cross-section microstructure of the coating sprayed at 100 mm spray distance: (a) typical appearance and (b) c-Al2O3 in situ formed phase.
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H. Mindivan et al. / Materials and Design 30 (2009) 4516–4520
Table 2 Relative amounts (RA) of the phases, mean in-flight particle velocity (Vp) and temperature (Tp) values, hardness and reciprocating wear test results. Sample
Spray distance (mm)
RA% Si
RA% c-Al2O3
Vp (m/s)
Tp (°C)
Hardness (HV0.2)
Wear track area (mm2)
Relative wear rate (%)
Coatings
Ball
Coatings
Ball
A B C
100 125 150
9.8 7.9 7.4
3.6 3.9 4.4
132 ± 8 109 ± 7 104 ± 8
2623 ± 35 2589 ± 42 2456 ± 32
158 ± 10 190 ± 8 201 ± 12
26.5 103 3.3 103 3.1 103
0.69 0.29 0.17
100 12.5 11.7
100 42 24
coatings, Sample C exhibited the highest hardness and Sample A exhibited the lowest hardness. A strong interfacial strength between in situ particles and matrix could prevent complete debonding of Al2O3 particles from the aluminum matrix. Al2O3 particles increased the resistance of aluminum matrix to indentation. 3.3. Wear analysis The evaluation of the wear tests was made by measuring the wear track area of the coatings (Fig. 3) and wear scar areas of the Al2O3 balls (Fig. 4). Since the frictional force was continuously recorded during the tests, the variation in friction coefficient was defined as the ratio of frictional force to normal load, during reciprocating wear tests (Fig. 5). Relative wear rate values of the coatings and relevant Al2O3 balls were determined by dividing the wear track areas of the coatings or the wear scar areas of the balls to those of Sample A or the ball used in testing of Sample A. Thus, the wear rates of the Sample A and the ball used to test it were taken as 100%. It is well known that the solid ceramic particles are very hard, which play an impor-
Coefficient of friction
1.04 ± 0.25 0.94 ± 0.08 0.72 ± 0.03
tant role in effectively sustaining the load if they are uniformly dispersed in the matrix and to outcrop on the wear surface of the coating. In this case, in situ Al2O3 formation significantly improved the wear resistance of coatings, while increasing the hardness (Table 2). It has been reported that the wear coefficient is not a strong function of Si amount [15], whereas the hardness and wear resistance increases with the increase in Al2O3 amount [2]. Sample C sprayed at 150 mm spray distance exhibited the highest hardness and the lowest wear rate. The decrease in wear rate was found about 700% for Sample B sprayed at 125 mm spray distance and 750% for Sample C sprayed at 150 mm spray distance. On the other hand, the size of the wear scars formed on the contact surfaces of balls (Fig. 4) is proportional to the size of the wear tracks developed on the surfaces of the coatings being tested (Fig. 3). The decrease of the wear rates of Al2O3 balls was found about 140% and 320% when testing Samples B and C, respectively. In addition to the effect of in situ formed Al2O3 concentration with respect to spray distance, in-flight particle characteristics might also influence the coating properties thus the wear behavior. Although the measured temperature at each condition is above the melting
Fig. 3. 3D-profile images of the wear tracks formed on the surfaces of (a) Sample A, (b) Sample B and (c) Sample C by Al2O3 ball.
H. Mindivan et al. / Materials and Design 30 (2009) 4516–4520
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Fig. 4. Light optical micrographs of the wear scars formed on Al2O3 ball during testing of (a) Sample A, (b) Sample B and (c) Sample C.
Fig. 5. Variation of friction coefficient during wear testing of Sample A, Sample B and Sample C.
point of the phases (Al, Si, SiO2 and Al2O3), the relatively high inflight particle velocity measured at 100 mm of spray distance may induce the unmelted or semi-melted state owing to insufficient heat exchange with the plasma jet [16]. As a result, the poor
adhesion of these unmelted and semi-melted particles with the matrix may deteriorate the wear resistance of Sample A sprayed at 100 mm spray distance. In Table 2, the standard deviations of the coefficient of friction values represents the fluctuations in the friction coefficient with distance in the steady state condition, which can be attributed to the production of third body particles during wear testing [17]. Suh and Sin [18] indicated that the accumulation of debris in sliding wear usually increases friction. Therefore, the high wear rate of Sample A can be correlated with the wide range of fluctuation in friction coefficient than that of Sample C. Wear rate decreased with decreasing coefficient of friction for Sample C. Fig. 6 depicts the scanning electron micrographs of the wear tracks produced on the surfaces of the coatings. Coating sprayed at 100 mm of spray distance exhibited relatively rough wear contact surface than the other coatings. In the wear tracks, very fine grooves and small dimples (Al2O3 particles), which were probably transferred from the ball during wear testing are present (Fig. 6a). Al2O3 particles were more frequently observed in the wear track of Sample A, which exhibited the highest wear rate. On the other hand, by increasing the spray distance to 125 mm, it seems that the depth of the grooves and the number of dimples decreases
Fig. 6. Scanning electron micrographs of the wear tracks formed on the surfaces of (a) Sample A, (b) Sample B and (c) Sample C.
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H. Mindivan et al. / Materials and Design 30 (2009) 4516–4520
and the worn surface shows rather a smooth appearance as a result of homogeneous wear (Fig. 6b). Therefore, the high wear rate of Sample A can be attributed to heavy third body abrasive effect of relatively coarse Al2O3 particles detached from the ball. Since wear debris particles have been oxidized and are hard, this relative movement may results in abrasive wear, leading to the formation of ripples in the Sample A. Coating sprayed at 150 mm of spray distance appears to be mainly adhesive and the worn surface shows plastically deformed plateaus (Fig. 6c). Also, the splat boundaries can be easily distinguished which indicates that the powder is highly oxidized during the flight in the plasma plume (in-flight) and/or after impinging onto the substrate. Since the oxidation of powder can potentially affect the coating properties and performance [19], it has been observed that it improves the wear resistance. 4. Conclusions This study points out the effect of spray distance and in-flight particle characteristics on the sliding wear behavior of plasma sprayed composite coatings with in situ formed Al2O3. The amount of in situ formed Al2O3 particles by plasma spraying of mechanically alloyed Al–Si/SiO2 composite powder increased with increasing spray distance and decreasing in-flight particle velocity and temperature, which was accompanied by an improvement in hardness and wear resistance. The increment of hardness and wear resistance of the coatings by in situ Al2O3 formation is in the following order; Sample A, Sample B and Sample C. Among the examined coatings, Sample C containing the highest in situ Al2O3 particles achieved hardness and wear resistance increment of about 30% and 750%, respectively. Acknowledgements The authors express sincere appreciation to Prof. Dr. E. Sabri Kayali and Prof. Dr. Huseyin Cimenoglu from the Metallurgical and Material Engineering Department of Istanbul Technical University for the wear tests.
References [1] Basavakumar KG, Mukunda PG, Chakraborty M. Dry sliding wear behaviour of Al–12Si and Al–12Si–3Cu cast alloys. Mater Des 2009;30:1258–67. [2] Kök M, Özdin K. Wear resistance of aluminium alloy and its composites reinforced by Al2O3 particles. J Mater Process Technol 2007;183:301–9. [3] Durai TG, Das K, Das S. Synthesis and characterization of Al matrix composites reinforced by in situ alumina particulates. Mater Sci Eng A 2007;445– 446:100–5. [4] Mindivan H, Kayali ES, Cimenoglu H. Tribological behavior of squeeze cast aluminum matrix composites. Wear 2008;265:645–54. [5] Ozdemir I, Hamanaka I, Hirose M, Tsunekawa Y, Okumiya M. In situ formation of Al–Si–Mg based composite coating by different reactive thermal spray processes. Surf Coat Technol 2005;200:1155–61. [6] Kumar S, Selvarajan V. In-flight formation and characterization of nickel aluminide powders in a dc thermal plasma jet. Chem Eng Process 2006;45: 1029–35. [7] Tekmen C, Yamazaki M, Tsunekawa Y, Okumiya M. In-situ plasma spraying: alumina formation and in-flight particle diagnostic. Surf Coat Technol 2008;202:4163–9. [8] Tekmen C, Tsunekawa Y, Okumiya M. In-situ TiB2 and Al2O3 formation by DC plasma spraying. Surf Coat Technol 2008;202:4170–5. [9] Ma ZY, Tjong SC. Creep behavior of in-situ Al2O3 and TiB2 particulates mixturereinforced aluminum composites. Mater Sci Eng A 1998;256:120–8. [10] Mandal A, Chakraborty M, Murty BS. Effect of TiB2 particles on sliding wear behaviour of Al–4Cu alloy. Wear 2007;262:160–6. [11] Mauer G, Vaßen R, Stöver D. Comparison and applications of DPV-2000 and Accuraspray-g3 diagnostic systems. J Thermal Spray Technol 2007;16:414–24. [12] Birol Y. Microstructural evolution during annealing of a rapidly solidified Al– 12Si alloy. J Alloys Compd 2007;439:81–6. [13] Tekmen C, Tsunekawa Y, Okumiya M. Effect of plasma spray parameters on inflight particle characteristics and in-situ alumina formation. Surf Coat Technol 2008;203:223–8. [14] Gupta M, Ling S. Microstructure and mechanical properties of hypo/hypereutectic Al–Si alloys synthesized using a near-net shape forming technique. J Alloys Compd 1999;287:284–94. [15] Elmadagli M, Perry T, Alpas AT. A parametric study of the relationship between microstructure and wear resistance of Al–Si alloys. Wear 2007;262:79–92. [16] Wang L, Fang JC, Zhao ZY, Zeng HP. Application of backward propagation network for forecasting hardness and porosity of coatings by plasma spraying. Surf Coat Technol 2007;201:5085–9. [17] Mindivan H, Bakan HI, Cimenoglu H, Kayali ES. Wear behaviour SiC, Cr3C2 and WC doped alumina. Key Eng Mater 2004;264–268:469–72. [18] Suh NP, Sin H-C. The genesis of friction. Wear 1981;69:91–114. [19] Deshpande S, Sampath S, Zhang H. Mechanisms of oxidation and its role in microstructural evolution of metallic thermal spray coatings—case study for Ni–Al. Surf Coat Technol 2006;200:5395–406.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Wear behavior of plasma sprayed composite coatings with in situ formed Al2O3 H. Mindivan a,*, C. Tekmen b, B. Dikici c, Y. Tsunekawa b, M. Gavgali d a
Ataturk University, Department of Metallurgy Engineering, 25240 Erzurum, Turkey Toyota Technological Institute, Materials Processing Lab., 2-12-1, 468-8511 Nagoya, Japan Yuzuncu Yil University, Ercis Technical Vocational School of Higher Education, 65400 Van, Turkey d Ataturk University, Department of Mechanical Engineering, 25240 Erzurum, Turkey b c
a r t i c l e
i n f o
Article history: Received 5 March 2009 Accepted 12 May 2009 Available online 19 May 2009
a b s t r a c t In the present study, the wear behavior of in situ formed Al2O3 reinforced hypereutectic Al–18Si matrix composite coatings have been investigated. These coatings were successfully fabricated with mechanically alloyed Al–12Si and SiO2 powder deposited on aluminum substrates by atmospheric plasma spraying (APS). The produced samples were characterized by means of microscopic examinations, hardness measurements and wear tests. The obtained results pointed out that the amount of in situ formed Al2O3 particles increased with increasing spray distance and decreasing in-flight particle velocity and temperature, which was accompanied by an improvement in hardness and wear resistance. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Al–Si alloys have been utilized extensively in the recent past due to superior properties such as low coefficient of thermal expansion, good specific strength, high wear resistance and fluidity [1]. The incorporation of Al2O3 or SiC particles in these alloys is gradually increasing especially in automotive industry where wear is the dominant process such as pistons, cylinder heads and connecting rods [2–4]. Aluminum based composites fabricated by conventional ex situ techniques are known to suffer from inhomogeneous distribution of reinforcements in the matrix and poor wettability of the reinforcement [3]. On the other hand, recently in situ plasma spraying (IPS) is a promising technology for deposition of in situ formed thermodynamically favorable phases such as Mg2Si, MgAl2O4, NiAl3 [5], Ni–Al [6], Al2O3 [7] and TiB2–Al2O3 [8] through the reaction between selective powders by using high velocity oxygen fuel (HVOF), direct current (DC) and radio frequency (RF) plasma spraying methods. The main advantages of in situ processes are good particle–matrix bonding, low fabrication cost, high thermodynamic stability, good high temperature performance and effective load transfer [9,10]. Since the ceramic material Al2O3 exhibits high hardness, melting point and wear resistance [2,3], SiO2 is an attractive oxide material for in situ reinforcing Al–Si alloys with Al2O3 [5,7]. In the literature [5,7,8], there are many researches concerning the preparation and processing of the in situ composites containing Al2O3 particles as the reinforcement produced by in situ plasma spraying method. However, due to short processing time in IPS,
* Corresponding author. Tel.: +90 442 231 4589; fax: +90 442 236 0957. E-mail address: [email protected] (H. Mindivan). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.05.018
in situ reaction intensity strongly depends on plasma spray parameters and in-flight particle characteristics and studies on in situ reaction intensity (alumina and silicon) on the wear of a composite coating produced by in situ plasma spraying method of hypereutectic Al–Si alloy with SiO2 are scarce. The objective of the present work is to investigate the effect of spray distance and in-flight particle characteristics on the sliding wear behavior of plasma sprayed composite coatings with in situ formed Al2O3. 2. Experimental procedure Mechanically alloyed Al–12Si/SiO2 composite powder (Fig. 1) was deposited onto an aluminum substrate by atmospheric plasma spraying to obtain a composite coating consisting of in situ formed Al2O3 reinforced Al–18Si matrix alloy according to the reaction of [7]. Coating experiments were carried out by using atmospheric plasma spraying apparatus (Sulzer Metco, 9M) under conditions given in Table 1. In-flight particle diagnostic was performed using Accuraspray-g3 (Sulzer Metco) where the sensor head was positioned at 200 mm from the plume center line. A more detailed explanation of its working principles can be found in [11].
Al—12Si þ SiO2 ! Al2 O3 þ Al—18Si The wear behavior of the coatings has been investigated with respect to the relative amount of the phases depending on spray distance, in-flight particle velocity and temperature values. Relative amounts (RA) of the phases were calculated through the relative intensity (I) ratios by using the strongest peaks obtained from the X-ray diffraction (XRD) patterns (Eq. (1)). A detailed information regarding powder composition, structure and morphology,
H. Mindivan et al. / Materials and Design 30 (2009) 4516–4520
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Fig. 1. Powder morphology (a) as-received Al–12Si and (b) mechanically alloyed Al–12Si/SiO2.
Table 1 Spray parameters. Primary gas flow rate (l/min): Ar Secondary gas flow rate (l/min): H2 Carrier gas flow rate (l/min): Ar Current (A) Spray distance (mm) Nozzle inner diameter (mm) Powder feed rate (g/min)
40 10 6 400 100/125/150 7 8
mechanical alloying conditions, differential thermal and X-ray diffraction analysis of the composite powder and Al–12Si/SiO2 sprayed coating is given elsewhere [7].
RA:% phase ¼
Iphase 100 ðIAl þ ISi þ IAl2 O3 Þ
ð1Þ
In the scope of the present study, the characterization of the coatings was made by microscopic examinations, hardness measurements and wear tests. Microscopic examinations were conducted on the cross-sections of the coated samples by an optical microscope, after grinding and polishing in standard manner. Microhardness measurements were carried out by using a HSV-30 Shimadzu Vickers hardness tester under 200 g load covered all of the microstructure constituents (including the matrix and c-Al2O3 phase). Hereafter, the coatings sprayed at 100, 125 and 150 mm spray distances were called Sample A, Sample B and Sample C, respectively. Wear performance of the coatings was examined on a reciprocating wear tester, which was designed according to the ASTM G133 standard. Wear tests were carried out by rubbing a 10 mm Al2O3 ball on the surfaces of the coatings at normal atmospheric conditions (room temperature and 66% relative humidity). During the wear tests, a 10 mm diameter Al2O3 ball was reciprocally sliding on the surface with a speed of 0.023 m/s and frictional force was continuously recorded using a computer. The amplitude of this motion, total sliding distance and normal load were 12 mm, 36 m and 1.5 N, respectively. After the wear test, semi-elliptic wear
tracks formed on the surfaces of the coatings were examined by a Veeco Dektak 32 surface profilometer and a scanning electron microscope (JEOL 6400). An optical microscope was used to examine the wear scars formed on the Al2O3 balls. 3. Results and discussion 3.1. Microstructure observations A typical cross-section of the coating is given in Fig. 2. It seems that the sprayed coating exhibits a wavy layered structure and consists of rarely unmelted particles and pores, with an average thickness of 200 lm where dark gray areas (marked with arrows) corresponds to the in situ formed c-Al2O3 phase. Relative amounts of the phases calculated from XRD results by using Eq. (1), mean in-flight particle velocity and temperature values, hardness and reciprocating wear tests conducted on the coatings are listed in Table 2. 3.2. Surface hardness The hardness of coatings is strongly relative to the distribution and the amount of the phases. It has been reported that the spray distance, in-flight particle velocity and temperature have a strong influence on the amount of Al2O3 and Si phases [7]. It can be seen from Table 2 that the hardness increases with the increase in Al2O3 and decrease in Si relative amounts. It should be mentioned that, the relatively low hardness of the coating sprayed at 100 mm spray distance may be attributed to the relatively high Si and low Al2O3 amount. At relatively short spray distances the decrease in solidification rate may cause the precipitation and growth of Si particles from the supersaturated solid solution [12,13]. On the other hand, the decrease in spray distance will shorten the dwelling time of the particles in plasma plume and consequently hinder in situ Al2O3 formation. It is only known that increasing the Si content results in an increase of the strength of hypoeutectic alloys and a decrease of the strength of hypereutectic alloys [14]. Among the examined
Fig. 2. Cross-section microstructure of the coating sprayed at 100 mm spray distance: (a) typical appearance and (b) c-Al2O3 in situ formed phase.
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Table 2 Relative amounts (RA) of the phases, mean in-flight particle velocity (Vp) and temperature (Tp) values, hardness and reciprocating wear test results. Sample
Spray distance (mm)
RA% Si
RA% c-Al2O3
Vp (m/s)
Tp (°C)
Hardness (HV0.2)
Wear track area (mm2)
Relative wear rate (%)
Coatings
Ball
Coatings
Ball
A B C
100 125 150
9.8 7.9 7.4
3.6 3.9 4.4
132 ± 8 109 ± 7 104 ± 8
2623 ± 35 2589 ± 42 2456 ± 32
158 ± 10 190 ± 8 201 ± 12
26.5 103 3.3 103 3.1 103
0.69 0.29 0.17
100 12.5 11.7
100 42 24
coatings, Sample C exhibited the highest hardness and Sample A exhibited the lowest hardness. A strong interfacial strength between in situ particles and matrix could prevent complete debonding of Al2O3 particles from the aluminum matrix. Al2O3 particles increased the resistance of aluminum matrix to indentation. 3.3. Wear analysis The evaluation of the wear tests was made by measuring the wear track area of the coatings (Fig. 3) and wear scar areas of the Al2O3 balls (Fig. 4). Since the frictional force was continuously recorded during the tests, the variation in friction coefficient was defined as the ratio of frictional force to normal load, during reciprocating wear tests (Fig. 5). Relative wear rate values of the coatings and relevant Al2O3 balls were determined by dividing the wear track areas of the coatings or the wear scar areas of the balls to those of Sample A or the ball used in testing of Sample A. Thus, the wear rates of the Sample A and the ball used to test it were taken as 100%. It is well known that the solid ceramic particles are very hard, which play an impor-
Coefficient of friction
1.04 ± 0.25 0.94 ± 0.08 0.72 ± 0.03
tant role in effectively sustaining the load if they are uniformly dispersed in the matrix and to outcrop on the wear surface of the coating. In this case, in situ Al2O3 formation significantly improved the wear resistance of coatings, while increasing the hardness (Table 2). It has been reported that the wear coefficient is not a strong function of Si amount [15], whereas the hardness and wear resistance increases with the increase in Al2O3 amount [2]. Sample C sprayed at 150 mm spray distance exhibited the highest hardness and the lowest wear rate. The decrease in wear rate was found about 700% for Sample B sprayed at 125 mm spray distance and 750% for Sample C sprayed at 150 mm spray distance. On the other hand, the size of the wear scars formed on the contact surfaces of balls (Fig. 4) is proportional to the size of the wear tracks developed on the surfaces of the coatings being tested (Fig. 3). The decrease of the wear rates of Al2O3 balls was found about 140% and 320% when testing Samples B and C, respectively. In addition to the effect of in situ formed Al2O3 concentration with respect to spray distance, in-flight particle characteristics might also influence the coating properties thus the wear behavior. Although the measured temperature at each condition is above the melting
Fig. 3. 3D-profile images of the wear tracks formed on the surfaces of (a) Sample A, (b) Sample B and (c) Sample C by Al2O3 ball.
H. Mindivan et al. / Materials and Design 30 (2009) 4516–4520
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Fig. 4. Light optical micrographs of the wear scars formed on Al2O3 ball during testing of (a) Sample A, (b) Sample B and (c) Sample C.
Fig. 5. Variation of friction coefficient during wear testing of Sample A, Sample B and Sample C.
point of the phases (Al, Si, SiO2 and Al2O3), the relatively high inflight particle velocity measured at 100 mm of spray distance may induce the unmelted or semi-melted state owing to insufficient heat exchange with the plasma jet [16]. As a result, the poor
adhesion of these unmelted and semi-melted particles with the matrix may deteriorate the wear resistance of Sample A sprayed at 100 mm spray distance. In Table 2, the standard deviations of the coefficient of friction values represents the fluctuations in the friction coefficient with distance in the steady state condition, which can be attributed to the production of third body particles during wear testing [17]. Suh and Sin [18] indicated that the accumulation of debris in sliding wear usually increases friction. Therefore, the high wear rate of Sample A can be correlated with the wide range of fluctuation in friction coefficient than that of Sample C. Wear rate decreased with decreasing coefficient of friction for Sample C. Fig. 6 depicts the scanning electron micrographs of the wear tracks produced on the surfaces of the coatings. Coating sprayed at 100 mm of spray distance exhibited relatively rough wear contact surface than the other coatings. In the wear tracks, very fine grooves and small dimples (Al2O3 particles), which were probably transferred from the ball during wear testing are present (Fig. 6a). Al2O3 particles were more frequently observed in the wear track of Sample A, which exhibited the highest wear rate. On the other hand, by increasing the spray distance to 125 mm, it seems that the depth of the grooves and the number of dimples decreases
Fig. 6. Scanning electron micrographs of the wear tracks formed on the surfaces of (a) Sample A, (b) Sample B and (c) Sample C.
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and the worn surface shows rather a smooth appearance as a result of homogeneous wear (Fig. 6b). Therefore, the high wear rate of Sample A can be attributed to heavy third body abrasive effect of relatively coarse Al2O3 particles detached from the ball. Since wear debris particles have been oxidized and are hard, this relative movement may results in abrasive wear, leading to the formation of ripples in the Sample A. Coating sprayed at 150 mm of spray distance appears to be mainly adhesive and the worn surface shows plastically deformed plateaus (Fig. 6c). Also, the splat boundaries can be easily distinguished which indicates that the powder is highly oxidized during the flight in the plasma plume (in-flight) and/or after impinging onto the substrate. Since the oxidation of powder can potentially affect the coating properties and performance [19], it has been observed that it improves the wear resistance. 4. Conclusions This study points out the effect of spray distance and in-flight particle characteristics on the sliding wear behavior of plasma sprayed composite coatings with in situ formed Al2O3. The amount of in situ formed Al2O3 particles by plasma spraying of mechanically alloyed Al–Si/SiO2 composite powder increased with increasing spray distance and decreasing in-flight particle velocity and temperature, which was accompanied by an improvement in hardness and wear resistance. The increment of hardness and wear resistance of the coatings by in situ Al2O3 formation is in the following order; Sample A, Sample B and Sample C. Among the examined coatings, Sample C containing the highest in situ Al2O3 particles achieved hardness and wear resistance increment of about 30% and 750%, respectively. Acknowledgements The authors express sincere appreciation to Prof. Dr. E. Sabri Kayali and Prof. Dr. Huseyin Cimenoglu from the Metallurgical and Material Engineering Department of Istanbul Technical University for the wear tests.
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