- Email: [email protected]
B4C composite coatings
Materials & Design Materials and Design 28 (2007) 2177–2183 www.elsevier.com/locate/matdes
Short communication
Wear behaviour of plasma-sprayed AlSi/B4C composite coatings Ozkan Sarikaya a, Selahaddin Anik a, Erdal Celik
b,*
, S. Cem Okumus c, Salim Aslanlar
d
a Sakarya University, Faculty of Engineering, Department of Mechanical Engineering, Esentepe Campus, Sakarya, Turkey Dokuz Eylul University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Buca, 35100 Izmir, Turkey Sakarya University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Esentepe Campus, Sakarya, Turkey d Sakarya University, Faculty of Technical Education, Department of Mechanical Engineering, Esentepe Campus, Sakarya, Turkey b
c
Received 19 December 2005; accepted 17 July 2006 Available online 27 September 2006
Abstract This paper describes the wear behaviour of AlSi/B4C composite coatings with 0–25 wt% B4C particles for diesel engine motors. These coatings were successfully fabricated on AlSi substrates using an atmospheric plasma spray technique. The produced samples were characterized by means of an optical microscope, scanning electron microscope and microhardness tester. The obtained results pointed out that an increase of B4C particles in AlSi coatings was caused on the rising of the microhardness values and the decrease of the thermal expansion coefficient of the coatings. The friction and wear experiments were performed under dry conditions using a ball-on-dics configuration against WC/Co counter material for different loads. It was concluded that wear resistance of the coatings produced using B4C powders is greatly improved compared with the substrate material. The highest wear resistance of the coatings were also determined in the 20% B4C coating. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The driving force behind the development of metal matrix composites (MMCs) has been the attractive mechanical and physical properties and enhanced elevated temperature capabilities. In addition, aluminum matrix composites reinforced with ceramic particles or whiskers have received considerable attention because they can be formed by standard metalworking practices. Apart from improved mechanical properties, other controlling attributes, such as coefficient of thermal expansion and wear resistance, are greatly improved by the addition of ceramic particles such as Si3N4, SiC, Al2O3 and B4C [1,2]. Of these second particles, boron carbide (B4C) has good mechanical properties, high wear and heat resistance. Furthermore, boron carbide has a very high hardness (HV980N = 3500), high melting point (2450 C), high spe-
*
Corresponding author. Tel.: +90 232 412 7473; fax: +90 232 412 7452. E-mail address: [email protected] (E. Celik).
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.07.004
cific heat (970 J kg 1 K 1) and melting enthalpy (1867 kJ kg 1) combined with a low density (2520 kg m 1) and exhibits high abrasive capacity, excellent thermal stability, remarkable chemical inertness high abrasive capacity which appears to be an interesting agent for aluminium based composites [3–6]. The AlSi/B4C system is of interest because of the potential for enhancement of mechanical properties and thermal stability of the matrix by the incorporation of B4C particles. Besides, B4C has lower density than other common commercial reinforcements, such as SiC, Al2O3, resulting in composites with higher specific stiffness [2]. Al-based ceramic reinforced composites have been fabricated through several methods such as pressing, casting, infiltration and plasma spray. Of these techniques, plasma spray, one of the coating processes which is a widely used method among the thermal spray processes, is extensively used to improve the surface characteristics of materials [7–10]. Due to this fact, the coating properties are enhanced if a distinct second phase material such as Si3N4, SiC, Al2O3 and B4C particles are incorporated into the coating
2178
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
microstructures. In addition, the microstructure of the coating is a function of the melting of particles in the plasma which is comprised of millions of individually solidified micron-sized particles. The coating process is affected by the particle temperature, velocity, size, position in the plume and environmental interactions as well as substrate conditions [11–13]. The poor particle melting and low velocity at impact lead to high levels of porosity and poor intergranular bonding [14]. In our previous work [15], the fabrication of AlSi/B4C composite coatings on AlSi piston alloys was investigated for Diesel engine motors. In the present study, the influence of B4C on the wear resistance of AlSi/B4C coatings was scrutinized under dry conditions using a ball-on-disc configuration against WC/Co counter body material for different loads. For characterization of these coatings scanning electron microscopy (SEM) with equipped energy dispersive spectroscopy (EDS), optical microscopy, and microhardness and thermal expansion machines as well as wear tester were utilized to correlate a relationship between wear properties and microstructure. 2. Experimental The grain size of B4C particles ranged between 3 and 10 lm and the mean diameter was 4.7 lm. The grain sizes of AlSi powders were in the range of 15–100 lm and the mean diameter was 45 lm. The density of the B4C powder was 2385.6 kg m 1 and the composition of the powder from the manufacturer’s specification was as follows: 95% B4C, 1% B, 1% C,1% Fe 1% iron oxide and 1% Al. AlSi and B4C powders were blended in ratios of 95:5, 90:10, 85:15, 80:20 and 75:25, respectively. Each of the five mixtures was then milled to form composite powders. Resultant composite powders were used for the one layer deposition by plasma spraying. Prior to the coating process, the samples were grid-blasted using silicon carbide grits in order to produce a rough surface for good bonding. After that, layers were formed successively using the five types of composite powders through plasma spray technique. The coatings were prepared by atmospheric plasma spraying using an atmospheric plasma spray system, type Metco 3MB (Metco Inc.). A gun-sample distance of 150 mm, a current of 500 A and a voltage of 70 V were used in all experiments. A mechanical powder feeder of 3 MP type with a pneumatic vibration device, having a capacity of about 40 g min 1 was used. In the spraying processes, the argon and hydrogen gases were used with pressures of 100 l/min and 15 l/min, respectively. Microstructure examinations were carried out for the specimens before and after wear testing. Microscopic observation of the coatings was performed using an optical microscope (Olympus B 071) and a scanning electron microscopy (SEM; JEOL JSM 40) equipped with energy-dispersive X-ray spectroscopy operated at 20 keV, before and after the wear tests. Each specimen was mounted in conductive resin, grinded with SiC paper and finally polished with 1 lm diamond slurry. The investigation of the distribution of the reinforced particles in the composite coatings was carried out by optical microscopy. Vickers microhardness indentations were taken on sections of coated samples using a Wilson Tukon type tester, parallel and normal, using a load of 100 g at least six indentations on each material. Indentation parameters were set as loading time of 15 s. Thermal expansion measurements were carried out using a Linseis type dilatometer in the temperature ranges of room temperature to 500 C at a constant ramping rate of 10 C min 1. The data were then analysed and values of fractional length change Dl/l0 and the coefficient of linear thermal expansion a(T) were determined for the temperature range of 24 C and 550 C.
The friction and wear experiments were conducted under dry conditions using a ball-on-disc configuration against WC/Co counter body for different loads. Prior to the wear tests, the disc samples with dimension of 5 mm · 25 mm were coated with pure AlSi and AlSi/B4C composite powders. The details of the wear tester configurations are described elsewhere [15]. The friction and wear tests were carried out at room temperature under dry conditions. Each specimen was cleaned in an ultrasonic bath with acetone for 5 min before and after the testing. The wear volume was calculated from the surface profile using a profilometer. The loads used in this experiment were 10, 20 and 40 N, and the sliding speed was a constant rate of 0.1 ms 1. The wear tests were performed at sliding distance of 62.8 m. After the wear, the surface debris of coatings were examined using scanning electron microscopy.
3. Results and discussion Fig. 1 shows cross-sectional optical micrographs taken at a magnification of 200· from the coatings fabricated by plasma spraying of AlSi composite powders with 5%, 10%, 15%, 20% and 25% B4C particles. The B4C particles are of irregular shape, with sharp corners and are uniformly distributed in the coatings. The big black areas in Fig. 1 represent porosity in the reinforced coatings. It is evident from these figures that no microcracking appeared in any coating. Figs. 2 and 3 depict wear volume and rate versus sliding distance for AlSi substrate and AlSi composite coatings as a function of B4C additives under 10 N, 20 N and 40 N loads. It is clear from Figs. 2 and 3 that the wear volume and rates decreased with the increase of the B4C particles. Besides, as wear of the specimens is increased, the measured friction coefficient reduced. The wear volume and wear rate were independent on coating thickness and loads of both sphere and disc. A comparison of friction coefficients of AlSi/B4C composite coatings with three different loads are given in Fig. 4, which shows that friction coefficient decreased with the increase of the B4C particles. That is to say, as B4C content in AlSi increases, wear resistance of the composite coatings increases compared to on AlSi substrate. Of these coatings, the AlSi/20% B4C showed the best wear behaviour. Inasmuch as graphite in the B4C structure possesses an important task as solid lubricate during the sliding/friction wear test [16], it provides a significant decrease in the friction coefficients of the coatings as seen from Fig. 4. It was found that the wear mechanism of the coatings depends on the normal load. The wear of the AlSi substrate and AlSi coating revealed plastic deformation owing to the absence of B4C particles. Furthermore, the B4C reinforced coatings were clearly observed to be both plastically deformed and to show splat pull-out, as shown in Fig. 5. The friction coefficient of B4C reinforced coatings is lower than that of the AlSi substrate and the coating without additive. The lowest friction coefficient among the particle-reinforced coatings is observed in Al–Si/20% B4C composite samples. The wear resistance of materials has often been correlated with hardness [17], and play an important role in the wear tests [18]. Fig. 6 shows the variation of the Vickers microhardness as a function of the B4C additives in the
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
2179
Fig. 1. Cross-sectional optical micrographs of the coatings fabricated by plasma spraying of (a) AlSi + 5% B4C, (b) AlSi + 10% B4C, (c) AlSi + 15% B4C, (d) AlSi + 20% B4C, and (e) AlSi + 25% B4C powders. Magnification is 200·.
2180
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
Wear volume (x10-3 mm3)
250
10 N 20 N 40 N
200
150
100
50
0 substrate
0
5
10
15
20
25
Fig. 5. SEM micrograph of the worn surface of AlSi/20 % B4C composite coatings.
B4C content (%)
Fig. 2. Variation of the wear volume for AlSi substrate and AlSi/B4C composite coatings under 10 N, 20 N and 40 N load.
180
160
Microhardness (HV)
10 N 20 N 40 N
200
-3
-1
Wear rate (x10 mm .m )
250
-3
150
140
120
100
100 80
50
substrate
0
5
10
15
20
25
B4C content (%) 0 substrate
0
5
10
15
20
25
Fig. 6. Microhardness distribution of AlSi substrate and AlSi/B4C composite coatings.
B4C content (%) 70
Fig. 3. Variation of wear rate for AlSi substrate and AlSi/B4C composite coatings under 10 N, 20 N and 40 N load.
substrate 0 % B 4C
60
5 % B 4C 10 % B4C
CTE x10-6/oC
0.48 0.46
10 N 20 N 40 N
Friction coefficient
0.44 0.42 0.40
15 % B4C
50
20 % B4C 25 % B4C
40
30
0.38
20 0.36 0.34
10 0
0.32
100
200
300
400
500
600
o
Temperature ( C) 0.30 0.28 substrate
0
5
10
15
20
25
Fig. 7. Thermal expansion coefficients of AlSi substrate and AlSi/B4C composite coatings.
B4C content (%)
Fig. 4. Variation of the friction coefficient for AlSi substrate and AlSi/ B4C composite coatings at a sliding distance of 62.8 m under 10 N, 20 N and 40 N load.
coatings. Vickers microhardness values of all coatings were performed by measuring at least six different areas. The microhardness of the coatings ranged from 89 HV to 175
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
2181
Fig. 8. SEM micrographs of the worn surfaces which have no cracking under 40 N load: (a) AlSi substrate, (b) AlSi /0% B4C, (c) AlSi/5% B4C, (d) AlSi/ 10% B4C, (e) AlSi/15% B4C, (f) AlSi/20% B4C and (g) AlSi/25% B4C coatings on AlSi substrate.
2182
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
HV. The hardness value of the substrate is nearly the same as of an AlSi coating without B4C additives. Because the microhardness of B4C is significantly higher than that of the AlSi, microhardness values of the composite coatings increased with the increase of B4C particles. AlSi/25% B4C coating had the highest microhardness values while AlSi coatings had the lowest one. Stresses in the plasma sprayed coatings formed when cooled down from the deposition temperature to room temperature. These are related to temperature gradients in the substrate and coating [17]. The level of residual stress may be estimated by the thermal expansion mismatch between coating and substrate. The low thermal conductivity and more complicated thermal history of thick coatings tend to generate stress profiles [18]. Residual stresses may be compressive if the thermal expansion coefficient of the substrates is larger than that of the coating. When the slider moved onto a void on the surface, wear particles were generated around it. As tensile stresses in the coating increased with an increased friction coefficient, the columnar grain of the coating fractured at a critical stress. This is analogous to the high tensile stress caused by friction, along with internal stress, that leads to surface crack propagation and consequently to coating fracture. The compressive stress caused by a moving slider overlaps with the internal stress, resulting in interfacial cracking with a subsequent coating fracture and detachment [19,20]. The thermal expansion properties of B4C reinforced composite coatings as a fuction of temperature from 25 to 500 C are shown in Fig. 7. For the six coatings, that thermal expansion coefficient gradually increases with increasing temperature. Thermal expansion coefficient values varied between 14.8 and 21 · 10 6 K 1 due to an increase in the content of B4C in the coatings. The increase rate of the thermal expansion coefficient decreases with temperature. The coefficient of linear thermal expansion decreases with increasing boron carbide content, e.g. the thermal expansion coefficient of the aluminum alloy is larger than that of additive coatings. Therefore, the compressive stress in the coating will be high if there is a good bonding at the interface. In order to reveal the wear mechanism of the composite coatings, an examination of the wear surfaces was carried out using SEM. Fig. 8 shows the worn surface of the AlSi substrate and the AlSi/B4C composite coatings during a sliding test under a normal force of 40 N. The worn surface reveals the removal of wear particles and some splat pullout in the wear track. It is clear from these figures that signs of plastic flow can be observed and no cracking is visible. The failure process is obviously dominated by plastic deformation and abrasion. Some porosity in the coating surfaces produced by the plasma spray process was identified, and it may be caused by crack formation. 4. Conclusions AlSi metallic and AlSi/B4C composite powders were sprayed on AlSi substrate by plasma spray technique for
Diesel engine applications. The coating thickness was about 600 lm. It was possible to fabricate AlSi alloy matrix composites reinforced with B4C particles. It was found that an increase of the amount of boron carbide particles on the coatings caused decrease of the thermal expansion coefficient. The microhardness of the coatings was also substantially improved by the addition of boron carbides. The friction coefficient of AlSi/20% B4C coating is lower than that of the substrate and other composite coatings. The friction coefficient of a unreinforced coating is somewhat higher than that of the AlSi. The friction coefficient of AlSi/20% B4C coating is about 30% lower than that of the AlSi substrate. The results indicated that low friction corresponds to low wear. The failure for substrate and AlSi coating without B4C particles is related with plastic deformation. The AlSi/B4C coatings also exhibit plastic deformation and abrasive wear. Acknowledgements The authors are grateful to Professor Dr. Fevzi Yilmaz, president of the plasma laboratory of Sakarya University, and Ebubekir Cebeci, technician of this laboratory, for the coating process. References [1] Gui M, Kang SB. Aluminum hydrid composite coatings SiC and graphite particles by plasma spraying. Mater Lett 2001;51:396. [2] Hu HM, Lavernia EJ, Harrigan WC, Kajuch J, Nutt SR. Microstructural investigation on B4C-Al-7093 composite. Mater Sci Eng A 1993;297:94. [3] Lee KB, Sim HS, Cho SY, Kwon H. Reaction product of Al–Mg/B4C composite fabricated by pressureless infiltration technique. Mater Sci Eng A 2001;302:227. [4] Yuhua Z, Aiju L, Yansheng Y, Ruixia S, Yingcai L. Reactive and dense sintering of reinforced-toughened B4C matrix composites. Mater Resear Bull 2004;39:1615. [5] Meyer FD, Hillebrecht H. Synthesis and crystal structure of Al3BC, the first boridecarbide of aluminium. J Alloys and Compounds 1997;252:98. [6] Chao M-J, Niu X, Yuan B, Liang E-J, Wang D-S. Preparation and characterization of in situ synthesized B4C particulate reinforced nickel composite coatings by laser cladding. Surf and Coat Technol, in press. [7] Matejicek J, Sampath S. In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: Part 1: apparatus and analysis. Acta Mater 2003;51:863. [8] Nakamura T, Wang Z. Simulations of crack propagation in porous materials. Trans ASME 2001;68:242. [9] Chraska P, Dubsky J, Kolman B, llavsky J, Forman J. Study of phase changes in plasma sprayed deposites. ASM Int JTTEE5 1992;1–4:301. [10] Khor KA, Dong ZL, Gu YW. Plasma sprayed functionally graded thermal barrier coatings. Mater Lett 1999;38:437. [11] Ghafouri-Azar R, Mostaghimi J, Chandra S. Modeling development of residual stresses in thermal spray coatings. Comp Mater Sci 2006;35:13. [12] Ustel F, Soykan S, Celik E. Avci. Plasma spray coating technology. J Metall 1995. [13] Shorowordi KM, Haseeb ASMA, Celis JP. Velocity effects on the wear, friction and tribochemistry of aluminum MMC sliding against phenolic brake pad. Wear 2004;256:1176.
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183 [14] Polat A, Sarıkaya O, Celik E. Effects of porosity on thermal loadings of functionally graded Y2O3-ZrO2/NiCoCrAlY coatings. Mater Des 2002;23:641. [15] Sarikaya O, Anik S, Aslanlar S, Okumus SC, Celik E. Al–Si/B4C Composite Coatings on Al–Si Substrate by Plasma Spray Technique. Mater Des [submitted for publication]. [16] Ahmed MMI, Abd El-Karım RA, El-Raghy SM, El-Refaire FA. J Mater Sci 1991;26:517. [17] Wang Y, Jiang S, Wang M, Wang S, Xiao TD, Strutt PR. Abrasive wear characteristics of plasma sprayed nanostructured alumina/ titania coatings. Wear 2000;237:176.
2183
[18] Matejicek J, Sampath S. In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: Part 1: apparatus and analysis. Acta Mater 2001;49:1993. [19] Sarıkaya O, Celik E. Effect of Residual Stress on Thickness and Interlayer of thermal barrier ceramic MgO–ZrO2 Coatings on AlSi Alloys and Ni Substrates Using Finite Element Method. Mater Des 2002;23:645. [20] Kocabicak U, Mimaroglu A, Sarikaya O, Mete OH. Comparison of the developed thermal stresses in Al2O3–SG, ZrO2–12%Si + Al and ZrO2–SG coating systems subjected to thermal loading. Mater Des 1999;20:287.
Short communication
Wear behaviour of plasma-sprayed AlSi/B4C composite coatings Ozkan Sarikaya a, Selahaddin Anik a, Erdal Celik
b,*
, S. Cem Okumus c, Salim Aslanlar
d
a Sakarya University, Faculty of Engineering, Department of Mechanical Engineering, Esentepe Campus, Sakarya, Turkey Dokuz Eylul University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Buca, 35100 Izmir, Turkey Sakarya University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Esentepe Campus, Sakarya, Turkey d Sakarya University, Faculty of Technical Education, Department of Mechanical Engineering, Esentepe Campus, Sakarya, Turkey b
c
Received 19 December 2005; accepted 17 July 2006 Available online 27 September 2006
Abstract This paper describes the wear behaviour of AlSi/B4C composite coatings with 0–25 wt% B4C particles for diesel engine motors. These coatings were successfully fabricated on AlSi substrates using an atmospheric plasma spray technique. The produced samples were characterized by means of an optical microscope, scanning electron microscope and microhardness tester. The obtained results pointed out that an increase of B4C particles in AlSi coatings was caused on the rising of the microhardness values and the decrease of the thermal expansion coefficient of the coatings. The friction and wear experiments were performed under dry conditions using a ball-on-dics configuration against WC/Co counter material for different loads. It was concluded that wear resistance of the coatings produced using B4C powders is greatly improved compared with the substrate material. The highest wear resistance of the coatings were also determined in the 20% B4C coating. 2006 Elsevier Ltd. All rights reserved.
1. Introduction The driving force behind the development of metal matrix composites (MMCs) has been the attractive mechanical and physical properties and enhanced elevated temperature capabilities. In addition, aluminum matrix composites reinforced with ceramic particles or whiskers have received considerable attention because they can be formed by standard metalworking practices. Apart from improved mechanical properties, other controlling attributes, such as coefficient of thermal expansion and wear resistance, are greatly improved by the addition of ceramic particles such as Si3N4, SiC, Al2O3 and B4C [1,2]. Of these second particles, boron carbide (B4C) has good mechanical properties, high wear and heat resistance. Furthermore, boron carbide has a very high hardness (HV980N = 3500), high melting point (2450 C), high spe-
*
Corresponding author. Tel.: +90 232 412 7473; fax: +90 232 412 7452. E-mail address: [email protected] (E. Celik).
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.07.004
cific heat (970 J kg 1 K 1) and melting enthalpy (1867 kJ kg 1) combined with a low density (2520 kg m 1) and exhibits high abrasive capacity, excellent thermal stability, remarkable chemical inertness high abrasive capacity which appears to be an interesting agent for aluminium based composites [3–6]. The AlSi/B4C system is of interest because of the potential for enhancement of mechanical properties and thermal stability of the matrix by the incorporation of B4C particles. Besides, B4C has lower density than other common commercial reinforcements, such as SiC, Al2O3, resulting in composites with higher specific stiffness [2]. Al-based ceramic reinforced composites have been fabricated through several methods such as pressing, casting, infiltration and plasma spray. Of these techniques, plasma spray, one of the coating processes which is a widely used method among the thermal spray processes, is extensively used to improve the surface characteristics of materials [7–10]. Due to this fact, the coating properties are enhanced if a distinct second phase material such as Si3N4, SiC, Al2O3 and B4C particles are incorporated into the coating
2178
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
microstructures. In addition, the microstructure of the coating is a function of the melting of particles in the plasma which is comprised of millions of individually solidified micron-sized particles. The coating process is affected by the particle temperature, velocity, size, position in the plume and environmental interactions as well as substrate conditions [11–13]. The poor particle melting and low velocity at impact lead to high levels of porosity and poor intergranular bonding [14]. In our previous work [15], the fabrication of AlSi/B4C composite coatings on AlSi piston alloys was investigated for Diesel engine motors. In the present study, the influence of B4C on the wear resistance of AlSi/B4C coatings was scrutinized under dry conditions using a ball-on-disc configuration against WC/Co counter body material for different loads. For characterization of these coatings scanning electron microscopy (SEM) with equipped energy dispersive spectroscopy (EDS), optical microscopy, and microhardness and thermal expansion machines as well as wear tester were utilized to correlate a relationship between wear properties and microstructure. 2. Experimental The grain size of B4C particles ranged between 3 and 10 lm and the mean diameter was 4.7 lm. The grain sizes of AlSi powders were in the range of 15–100 lm and the mean diameter was 45 lm. The density of the B4C powder was 2385.6 kg m 1 and the composition of the powder from the manufacturer’s specification was as follows: 95% B4C, 1% B, 1% C,1% Fe 1% iron oxide and 1% Al. AlSi and B4C powders were blended in ratios of 95:5, 90:10, 85:15, 80:20 and 75:25, respectively. Each of the five mixtures was then milled to form composite powders. Resultant composite powders were used for the one layer deposition by plasma spraying. Prior to the coating process, the samples were grid-blasted using silicon carbide grits in order to produce a rough surface for good bonding. After that, layers were formed successively using the five types of composite powders through plasma spray technique. The coatings were prepared by atmospheric plasma spraying using an atmospheric plasma spray system, type Metco 3MB (Metco Inc.). A gun-sample distance of 150 mm, a current of 500 A and a voltage of 70 V were used in all experiments. A mechanical powder feeder of 3 MP type with a pneumatic vibration device, having a capacity of about 40 g min 1 was used. In the spraying processes, the argon and hydrogen gases were used with pressures of 100 l/min and 15 l/min, respectively. Microstructure examinations were carried out for the specimens before and after wear testing. Microscopic observation of the coatings was performed using an optical microscope (Olympus B 071) and a scanning electron microscopy (SEM; JEOL JSM 40) equipped with energy-dispersive X-ray spectroscopy operated at 20 keV, before and after the wear tests. Each specimen was mounted in conductive resin, grinded with SiC paper and finally polished with 1 lm diamond slurry. The investigation of the distribution of the reinforced particles in the composite coatings was carried out by optical microscopy. Vickers microhardness indentations were taken on sections of coated samples using a Wilson Tukon type tester, parallel and normal, using a load of 100 g at least six indentations on each material. Indentation parameters were set as loading time of 15 s. Thermal expansion measurements were carried out using a Linseis type dilatometer in the temperature ranges of room temperature to 500 C at a constant ramping rate of 10 C min 1. The data were then analysed and values of fractional length change Dl/l0 and the coefficient of linear thermal expansion a(T) were determined for the temperature range of 24 C and 550 C.
The friction and wear experiments were conducted under dry conditions using a ball-on-disc configuration against WC/Co counter body for different loads. Prior to the wear tests, the disc samples with dimension of 5 mm · 25 mm were coated with pure AlSi and AlSi/B4C composite powders. The details of the wear tester configurations are described elsewhere [15]. The friction and wear tests were carried out at room temperature under dry conditions. Each specimen was cleaned in an ultrasonic bath with acetone for 5 min before and after the testing. The wear volume was calculated from the surface profile using a profilometer. The loads used in this experiment were 10, 20 and 40 N, and the sliding speed was a constant rate of 0.1 ms 1. The wear tests were performed at sliding distance of 62.8 m. After the wear, the surface debris of coatings were examined using scanning electron microscopy.
3. Results and discussion Fig. 1 shows cross-sectional optical micrographs taken at a magnification of 200· from the coatings fabricated by plasma spraying of AlSi composite powders with 5%, 10%, 15%, 20% and 25% B4C particles. The B4C particles are of irregular shape, with sharp corners and are uniformly distributed in the coatings. The big black areas in Fig. 1 represent porosity in the reinforced coatings. It is evident from these figures that no microcracking appeared in any coating. Figs. 2 and 3 depict wear volume and rate versus sliding distance for AlSi substrate and AlSi composite coatings as a function of B4C additives under 10 N, 20 N and 40 N loads. It is clear from Figs. 2 and 3 that the wear volume and rates decreased with the increase of the B4C particles. Besides, as wear of the specimens is increased, the measured friction coefficient reduced. The wear volume and wear rate were independent on coating thickness and loads of both sphere and disc. A comparison of friction coefficients of AlSi/B4C composite coatings with three different loads are given in Fig. 4, which shows that friction coefficient decreased with the increase of the B4C particles. That is to say, as B4C content in AlSi increases, wear resistance of the composite coatings increases compared to on AlSi substrate. Of these coatings, the AlSi/20% B4C showed the best wear behaviour. Inasmuch as graphite in the B4C structure possesses an important task as solid lubricate during the sliding/friction wear test [16], it provides a significant decrease in the friction coefficients of the coatings as seen from Fig. 4. It was found that the wear mechanism of the coatings depends on the normal load. The wear of the AlSi substrate and AlSi coating revealed plastic deformation owing to the absence of B4C particles. Furthermore, the B4C reinforced coatings were clearly observed to be both plastically deformed and to show splat pull-out, as shown in Fig. 5. The friction coefficient of B4C reinforced coatings is lower than that of the AlSi substrate and the coating without additive. The lowest friction coefficient among the particle-reinforced coatings is observed in Al–Si/20% B4C composite samples. The wear resistance of materials has often been correlated with hardness [17], and play an important role in the wear tests [18]. Fig. 6 shows the variation of the Vickers microhardness as a function of the B4C additives in the
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
2179
Fig. 1. Cross-sectional optical micrographs of the coatings fabricated by plasma spraying of (a) AlSi + 5% B4C, (b) AlSi + 10% B4C, (c) AlSi + 15% B4C, (d) AlSi + 20% B4C, and (e) AlSi + 25% B4C powders. Magnification is 200·.
2180
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
Wear volume (x10-3 mm3)
250
10 N 20 N 40 N
200
150
100
50
0 substrate
0
5
10
15
20
25
Fig. 5. SEM micrograph of the worn surface of AlSi/20 % B4C composite coatings.
B4C content (%)
Fig. 2. Variation of the wear volume for AlSi substrate and AlSi/B4C composite coatings under 10 N, 20 N and 40 N load.
180
160
Microhardness (HV)
10 N 20 N 40 N
200
-3
-1
Wear rate (x10 mm .m )
250
-3
150
140
120
100
100 80
50
substrate
0
5
10
15
20
25
B4C content (%) 0 substrate
0
5
10
15
20
25
Fig. 6. Microhardness distribution of AlSi substrate and AlSi/B4C composite coatings.
B4C content (%) 70
Fig. 3. Variation of wear rate for AlSi substrate and AlSi/B4C composite coatings under 10 N, 20 N and 40 N load.
substrate 0 % B 4C
60
5 % B 4C 10 % B4C
CTE x10-6/oC
0.48 0.46
10 N 20 N 40 N
Friction coefficient
0.44 0.42 0.40
15 % B4C
50
20 % B4C 25 % B4C
40
30
0.38
20 0.36 0.34
10 0
0.32
100
200
300
400
500
600
o
Temperature ( C) 0.30 0.28 substrate
0
5
10
15
20
25
Fig. 7. Thermal expansion coefficients of AlSi substrate and AlSi/B4C composite coatings.
B4C content (%)
Fig. 4. Variation of the friction coefficient for AlSi substrate and AlSi/ B4C composite coatings at a sliding distance of 62.8 m under 10 N, 20 N and 40 N load.
coatings. Vickers microhardness values of all coatings were performed by measuring at least six different areas. The microhardness of the coatings ranged from 89 HV to 175
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
2181
Fig. 8. SEM micrographs of the worn surfaces which have no cracking under 40 N load: (a) AlSi substrate, (b) AlSi /0% B4C, (c) AlSi/5% B4C, (d) AlSi/ 10% B4C, (e) AlSi/15% B4C, (f) AlSi/20% B4C and (g) AlSi/25% B4C coatings on AlSi substrate.
2182
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183
HV. The hardness value of the substrate is nearly the same as of an AlSi coating without B4C additives. Because the microhardness of B4C is significantly higher than that of the AlSi, microhardness values of the composite coatings increased with the increase of B4C particles. AlSi/25% B4C coating had the highest microhardness values while AlSi coatings had the lowest one. Stresses in the plasma sprayed coatings formed when cooled down from the deposition temperature to room temperature. These are related to temperature gradients in the substrate and coating [17]. The level of residual stress may be estimated by the thermal expansion mismatch between coating and substrate. The low thermal conductivity and more complicated thermal history of thick coatings tend to generate stress profiles [18]. Residual stresses may be compressive if the thermal expansion coefficient of the substrates is larger than that of the coating. When the slider moved onto a void on the surface, wear particles were generated around it. As tensile stresses in the coating increased with an increased friction coefficient, the columnar grain of the coating fractured at a critical stress. This is analogous to the high tensile stress caused by friction, along with internal stress, that leads to surface crack propagation and consequently to coating fracture. The compressive stress caused by a moving slider overlaps with the internal stress, resulting in interfacial cracking with a subsequent coating fracture and detachment [19,20]. The thermal expansion properties of B4C reinforced composite coatings as a fuction of temperature from 25 to 500 C are shown in Fig. 7. For the six coatings, that thermal expansion coefficient gradually increases with increasing temperature. Thermal expansion coefficient values varied between 14.8 and 21 · 10 6 K 1 due to an increase in the content of B4C in the coatings. The increase rate of the thermal expansion coefficient decreases with temperature. The coefficient of linear thermal expansion decreases with increasing boron carbide content, e.g. the thermal expansion coefficient of the aluminum alloy is larger than that of additive coatings. Therefore, the compressive stress in the coating will be high if there is a good bonding at the interface. In order to reveal the wear mechanism of the composite coatings, an examination of the wear surfaces was carried out using SEM. Fig. 8 shows the worn surface of the AlSi substrate and the AlSi/B4C composite coatings during a sliding test under a normal force of 40 N. The worn surface reveals the removal of wear particles and some splat pullout in the wear track. It is clear from these figures that signs of plastic flow can be observed and no cracking is visible. The failure process is obviously dominated by plastic deformation and abrasion. Some porosity in the coating surfaces produced by the plasma spray process was identified, and it may be caused by crack formation. 4. Conclusions AlSi metallic and AlSi/B4C composite powders were sprayed on AlSi substrate by plasma spray technique for
Diesel engine applications. The coating thickness was about 600 lm. It was possible to fabricate AlSi alloy matrix composites reinforced with B4C particles. It was found that an increase of the amount of boron carbide particles on the coatings caused decrease of the thermal expansion coefficient. The microhardness of the coatings was also substantially improved by the addition of boron carbides. The friction coefficient of AlSi/20% B4C coating is lower than that of the substrate and other composite coatings. The friction coefficient of a unreinforced coating is somewhat higher than that of the AlSi. The friction coefficient of AlSi/20% B4C coating is about 30% lower than that of the AlSi substrate. The results indicated that low friction corresponds to low wear. The failure for substrate and AlSi coating without B4C particles is related with plastic deformation. The AlSi/B4C coatings also exhibit plastic deformation and abrasive wear. Acknowledgements The authors are grateful to Professor Dr. Fevzi Yilmaz, president of the plasma laboratory of Sakarya University, and Ebubekir Cebeci, technician of this laboratory, for the coating process. References [1] Gui M, Kang SB. Aluminum hydrid composite coatings SiC and graphite particles by plasma spraying. Mater Lett 2001;51:396. [2] Hu HM, Lavernia EJ, Harrigan WC, Kajuch J, Nutt SR. Microstructural investigation on B4C-Al-7093 composite. Mater Sci Eng A 1993;297:94. [3] Lee KB, Sim HS, Cho SY, Kwon H. Reaction product of Al–Mg/B4C composite fabricated by pressureless infiltration technique. Mater Sci Eng A 2001;302:227. [4] Yuhua Z, Aiju L, Yansheng Y, Ruixia S, Yingcai L. Reactive and dense sintering of reinforced-toughened B4C matrix composites. Mater Resear Bull 2004;39:1615. [5] Meyer FD, Hillebrecht H. Synthesis and crystal structure of Al3BC, the first boridecarbide of aluminium. J Alloys and Compounds 1997;252:98. [6] Chao M-J, Niu X, Yuan B, Liang E-J, Wang D-S. Preparation and characterization of in situ synthesized B4C particulate reinforced nickel composite coatings by laser cladding. Surf and Coat Technol, in press. [7] Matejicek J, Sampath S. In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: Part 1: apparatus and analysis. Acta Mater 2003;51:863. [8] Nakamura T, Wang Z. Simulations of crack propagation in porous materials. Trans ASME 2001;68:242. [9] Chraska P, Dubsky J, Kolman B, llavsky J, Forman J. Study of phase changes in plasma sprayed deposites. ASM Int JTTEE5 1992;1–4:301. [10] Khor KA, Dong ZL, Gu YW. Plasma sprayed functionally graded thermal barrier coatings. Mater Lett 1999;38:437. [11] Ghafouri-Azar R, Mostaghimi J, Chandra S. Modeling development of residual stresses in thermal spray coatings. Comp Mater Sci 2006;35:13. [12] Ustel F, Soykan S, Celik E. Avci. Plasma spray coating technology. J Metall 1995. [13] Shorowordi KM, Haseeb ASMA, Celis JP. Velocity effects on the wear, friction and tribochemistry of aluminum MMC sliding against phenolic brake pad. Wear 2004;256:1176.
O. Sarikaya et al. / Materials and Design 28 (2007) 2177–2183 [14] Polat A, Sarıkaya O, Celik E. Effects of porosity on thermal loadings of functionally graded Y2O3-ZrO2/NiCoCrAlY coatings. Mater Des 2002;23:641. [15] Sarikaya O, Anik S, Aslanlar S, Okumus SC, Celik E. Al–Si/B4C Composite Coatings on Al–Si Substrate by Plasma Spray Technique. Mater Des [submitted for publication]. [16] Ahmed MMI, Abd El-Karım RA, El-Raghy SM, El-Refaire FA. J Mater Sci 1991;26:517. [17] Wang Y, Jiang S, Wang M, Wang S, Xiao TD, Strutt PR. Abrasive wear characteristics of plasma sprayed nanostructured alumina/ titania coatings. Wear 2000;237:176.
2183
[18] Matejicek J, Sampath S. In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: Part 1: apparatus and analysis. Acta Mater 2001;49:1993. [19] Sarıkaya O, Celik E. Effect of Residual Stress on Thickness and Interlayer of thermal barrier ceramic MgO–ZrO2 Coatings on AlSi Alloys and Ni Substrates Using Finite Element Method. Mater Des 2002;23:645. [20] Kocabicak U, Mimaroglu A, Sarikaya O, Mete OH. Comparison of the developed thermal stresses in Al2O3–SG, ZrO2–12%Si + Al and ZrO2–SG coating systems subjected to thermal loading. Mater Des 1999;20:287.