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Wear characterization and microstructure evaluation of silicon carbide based nano composite coating using plasma spraying
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 23834–23843
www.materialstoday.com/proceedings
IConAMMA_2017
Wear characterization and microstructure evaluation of silicon carbide based nano composite coating using plasma spraying Muhammed Muneer S a , Nadeera M b* a
b
PG Scholar, Department of Mechanical Engineering, TKM College of Engineering, Kollam - 691005, India Assistant Professor, Department of Mechanical Engineering, TKM College of Engineering, Kollam - 691005, India
Abstract Thin films of various thickness of SiC – Al203 composite is deposited on aluminium alloy 6061 using Plasma spraying process. Wear tests on pin-on-disc tester is conducted to compare the wear characteristics of uncoated and coated samples. The micro hardness tests of coated samples and uncoated samples are compared. The microstructure characterization of the Nano-coated films using Scanning Electron Microscope (SEM) of the samples is studied. The results and studies clearly depicts that the major variations in coating performance can be obtained by exploiting proper plasma spray conditions and optimum percentage of SiC – Al203 in composite coatings. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: Composite Coating; Plasma Spraying; Microstructure Characterization
1.
Introduction
Aluminium alloys are widely used in automotive engines, automotive body parts, bicycle frames, aerospace applications. To meet the different applications listed above, the materials used to fabricate these items must have high strength, modulus of elasticity, abrasion resistance, corrosion resistance and wear resistance. The material should also have a low density, thermal expansion, and thermal conductivity.
* Muhammed Muneer S a. Tel.: +91-9037951519 a. Nadeera M b. Tel.: +91 9847889937 b. E-mail address: [email protected] a, [email protected] b. 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017].
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One of the major drawbacks faced by Aluminium and its alloys was its poor wear resistance. This drawback limited the aluminium and its alloys in some applications. Ceramic based composite Nano coatings enabled the aluminium alloys to overcome its drawback. Various kind of ceramic composite materials can be used, out of which, from studies, Silicon Carbide based composite coating offers a better wear resistance. The Silicon Carbide (SiC) cannot be directly coated on the aluminium alloys. Hence Silicon carbide was reinforced on Aluminium Oxide (Al203) in order to obtain proper Nano composite which satisfy the requirements. The SiC-Al203 composite imparts better wear and abrasion resistance as well as better hardness. The technique used for coating is Plasma Spray technique, due to its large choice of applications, automation capabilities, and high degree of flexibility. Peng-fei et al [1] in 2016 have conducted a study on the TiO2 based ceramic coatings for tribological applications like engine cylinder. As part of the study alumina-titania coatings were deposited on engine cylinder by plasma spraying technique. The wear tests were conducted using reciprocating tribometer at various engine conditions such as load, frequency, temperature etc. The morphology of the sprayed coating was obtained from SEM and it showed that a dense lamellae structure with pores and grey zones which shows the presence of TiO2. Also tribological tests showed that coated specimens have improved wear resistance. F. Mubarok et al [2] in 2015 proposed their work, by Suspension plasma spray (SPS) coating technique like atmospheric plasma spray is used. They deposited SiC coatings by SPS shows identical SiC phase peak, which indicates that the nano-film binder protects SiC particles from decomposition. They carried out the analysis using XPS, which exhibits that SiC particles faced some minor oxidation. All the SiC coatings shows poor mechanical performance because of its low cohesive strength, high porosity, and powdery structure which makes the coatings to grain pull out. This caused because of the absence of sintering process during the spraying process resulting to the low performance of SiC in SPS coatings. Berkath Ali Khan, et al [3] in 2014 has coated Al2O3-TiO2 coating on Al-6082 substrate. The technique used for deposition was thermal spraying in which alumina and titania powders were melted and sprayed on the metal substrate on varying thicknesses. The tribological behavior was tested using pin on disc testing machine and the hardness values were obtained from micro-hardness tests. From the study, it was found that coated samples had excellent wear properties than uncoated specimens. From the study, it was also found that coatings had poor adhesion on the metal surface. In order for that, a bond coat material NiCrAl was also used. The study also involved the SEM analysis of worn out specimen. Ville Matikainen et al [4] in 2014 Studied about the wear resistant coatings from Alumina and Titania composite powders sprayed with HVOF spray processes. The study involved of fused and crushed, and agglomerated and sintered Alumina- 13% Titania powders and was compared with pure Alumina. The coatings were subjected to investigations such as abrasion, erosion, and cavitation resistance to study the effect of the coating on the wear behavior. Improved coating properties are obtained as results when agglomerated and sintered composite coating powder is used in this method. The tests proved that the coatings done by using crushed and fused powder shows better wear resistance. Nenad Radic et al [5] in 2014 conducted a study on TiO2/WO3 composite coating on stainless steel substrates with varying percentages of WO3 using spray pyrolysis. The photo catalytic properties were investigated. The study concluded that the current method is a simple and cheap coating method. Also, this method provides versatility due to adjustable process parameters like spray geometry, substrate temperature, composition, concentration of precursor, liquid and gas flow rates. The results again showed that TiO2 had poor adhesion on metal substrates. Also the photocatalytic properties also improved with increased addition of WO3. A. Lanzutti et al [6] in 2013 has done thermal spray coatings by depositing as per industrial standards and procedures, and the Ni/SiC composite coatings were manufactured at laboratory scale by micro and nano sized ceramic particles. All the sample coatings were characterized for their microstructure, mechanical properties and the wear resistance. The tribological properties were analyzed using a tribometer at various temperatures. The results exhibit good wear resistance and the Ni coated specimen shows better wear resistance than compared to normally coated specimen using thermal spray method.
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E. Sa´nchez et al [7] in 2008 has conducted experiment on Al2O3-13%TiO2 coatings were deposited on stainless steel substrates by Plasma Spray Method. Microstructure characterization has confirmed its nanostructured nature. The microstructure comprised two clearly differentiated regions. Wear hardness values analyzed. Vickers micro hardness values of uncoated were in average which is slightly lower than the values obtained by nanostructured coating. The wear resistance of conventional coatings seems to be lower than as compared to nanostructured coatings. K. Ghosh et al [8] in 1997 made a composite with aluminium and Silicon carbide to coat via Plasma Spraying. As the concentration of the Silicon carbide was changed, the size of the composite was varied. The coating was done with axial feed plasma torch. As the SiC content increased, bonding strength of the coatings found to decrease. The wear resistance got decreased with the increase in Silicon carbide particle size and with the Silicon carbide content in the composite coatings. 2. Experimental Procedure 2.1 Materials Aluminium alloy 6061 is used as base substrate. In this study, Silicon Carbide (SiC) and Aluminium Oxide (Al203) was selected as the reinforcement for the coating. The average particle size of Silicon Carbide powder was 50-60 nm and average size of Aluminium oxide was 250-300nm. 2.2 Sample Preparation Aluminium alloy 6061 in ingot form was cut into small samples of required dimensions using power hacksaw cutting machine and vertical milling machine. The dimensions of the cut specimens are as follows: For Pin-on-disc wear test, Pins having diameter = 6mm and length = 30mm and for Micro Hardness test and SEM analysis, small blocks of 10mm x 10mm x 10mm was taken. Before deposition of coating, the surfaces of samples were polished using emery papers of grit sizes 600, 800, 1000 and the surface was cleaned and degreased using acetone. 2.3 Preparation of Coating Powder The coating of SiC- Al203 powder was done in three compositions by weight as shown in Table 1. Table 1. Compositions of Coating Powder (By Weight) Composition
SiC
Al2O3
1
25%
75%
2
50%
50%
3
75%
25%
The powder samples are prepared at these compositions by using a blending machine to ensure proper mixing of the Nano powder of both Silicon Carbide as well as Alumina. The machine rotates at about 1200RPM. Blending time for each composition is about 45 minutes.
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2.4 Spraying process
Fig.1. Typical Plasma Spray Equipment (Courtesy: FST Plasma Spray Equipment)
Aluminum alloy 6061 used as the base substrate for coating in this work. SiC-Al203 composite powder are used as coating materials and premixed using a blending machine to form three different compositions shown in Table 1. Firstly, NiCrAl bond coat of 10-20μm thickness was deposited to promote the best attainable adhesion between coating and substrate. Second layer coating was done using Aluminum oxide (Al203) and the process is referred as Blasting. Blast coat thickness is about 25-35 micrometer. The top coat was coated with three different compositions of SiC-Al203 as listed above in Table 1. All the coating process was conducted by plasma spray process with a PRAXAIR SG-100 plasma spray gun. The plasma spray process parameters used for this experiment are listed in Table 2.
Table 2. Plasma Spray Process Parameters Gun PRAXAIR SG-100 Current (DC)
500
Current (DC)
75
Primary Gas Flow Rate (Argon, lpm)
100
Carrier Gas Flow Rate (Hydrogen, lpm)
100
Powder Feeder (grams/min)
37
Powder Flow Rate (grams/min)
100
Spray Distance (mm)
60
3. Characterization Techniques Certain characterization techniques such as pin-on-disc test, Micro hardness and Scanning Electron Microscope (SEM) studies were conducted for the study of wear, mechanical characteristics and microstructure evaluation of
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SiC-Al203 composite coating on AA6061 under ASTM standards (ASTM E92-17, ASTM E384-16, ASTM E99-04, ASTM F1372-93). There were four test samples including an uncoated and three coated samples. 3.1 Wear Test For the wear test on the pin-on-disc testing machine, cylindrical pins with ASTM G99 standard having 6 mm diameter and 30 mm length standard were used with parameters as shown in Table 3. The samples were coated on the plane surface normal to the longitudinal axis. Table 3. Wear Test Parameters Parameters
Values
Load
3Kg
Sliding Velocity
2 m/s
Sliding Distance
1000m
Counter face material
Hardened ground Steel
Wear Track radius
100mm
Sliding time
8 min 51 sec
3.2 Micro Hardness Test Table 4. Micro Hardness Test Conditions Parameters
Variables
Machine
MITUTOYO - HM100, JAPAN
Load
30Kgf
Test Duration
10Secs
For the micro hardness test, each test specimen was prepared as a square block in the size of 10mm x 10mm x 10mm according to ASTM E92 standard as shown in Table 4. The test was conducted on vicker’s micro hardness tester with a load of 30kgf for a time of 10 seconds. 3.3 Microstructure Characterization The microstructure and the composition of the coatings were studied using Scanning Electron Microscope (SEM). The microstructure was characterized by using SIGMA FSEM equipment. For the microstructural study of the coatings, each test specimen was prepared as a square block in the size of 10mm x 10mm x 10mm with ASTM F1372-93 standard. Since the microstructural study of the coatings was conducted, no sample preparation procedure was done. SEM analysis was done to confirm the quality of the SiC-Al203 composite Nano coating on AA6061.
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4. Results and Discussions The Final results obtained after various tests and Characterization are as follows: 4.1 Wear Test
Wear (Micro meters)
Wear Rate - Comparison 1200 1000 800 600 400 200 0 -200 0
100
200 300 400 Duration of Sliding ( Sec )
500
WEAR - PURE ALUMINIUM 6061
WEAR - 25%SiC + 75%Al203
WEAR - 50%SiC + 50%Al203
WEAR - 75%SiC + 25%Al203
600
Fig.2. Wear rate comparison graph
The Fig.2 shows the comparison of wear rate of all the uncoated and coated specimens. The wear test is conducted for about 510 secs for all the specimens. From the tests, it is observed that Uncoated specimen of Aluminium 6061 has the highest wear rate which is followed by the 75% SiC + 25% Al203 specimen and then followed by the 25% SiC + 75% Al203 specimen. The least wear rate is obtained in 50% SiC + 50% Al203 composition specimen. This is because, as the SiC content in the composite increases, the binding property of the composite increases but only up to 50%. After 50% of increase in SiC content, the binding property of the SiC decreases due to the reduction of Al203 content, which holds the SiC content in the SiC - Al203 composite matrix. 4.2 Micro Hardness Table 5. Micro Hardness Test Values Composition
Average Hardness Value (HV)
Pure Aluminum 6061
65
25%SiC + 75%Al203
150
50%SiC + 50%Al203
169
75%SiC + 25%Al203
190
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Fig.3. Comparison - Average hardness value
The obtained micro hardness test values are shown in Table 5. For pure aluminum 6061, the micro hardness value obtained is 65 HV. For the 25%SiC + 75% Al203 composite coated sample, the micro hardness value obtained is 150HV. For 50%SiC + 50% Al203 composite coated sample, the micro hardness value obtained is 169HV.For the 75%SiC + 25% Al203 composite coating, the micro hardness value obtained is 190HV. The Fig 3. shows the comparison of Average hardness value. The values obtained after micro hardness test clearly depicts that the hardness values significantly increase with increase in the Silicon Carbide content. This concludes that, larger the Silicon Carbide content, higher will be the Hardness value and Vice-Versa. 100% Silicon carbide cannot be coated on aluminum substrate due to its poor binding property. Hence Alumina is required for proper adhesion of the SiC on the substrate. 4.3 Microstructure Characterization Scanning Electron Microscope (SEM) images were analysed to confirm the quality of the SiC- Al203 coating on the AA6061 substrate. The fig.4 demonstrates the Scanning Electron Microscope image of the plain uncoated substrate. It can be noted that the surface roughness was not uniform throughout the surface area and it may have led to the low adhesion of SiC- Al203 coating.
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Fig.4. SEM image of uncoated sample
The fig.5 shows SEM image of coated substrate 25% SiC + 75% Al203. The SiC in the coatings has a distribution, composition, and morphology similar to that in the powders sprayed. It can be clearly seen that whitish particle embedded in a grey matrix. The white particles indicate SiC particles and the round grey matrix constitutes for Al203. The SiC was distributed as individual particles in the matrix as shown in Fig 5, with limited clustering. At 25% SiC + 75% Al203 composition as shown in the Fig 5, the SiC particles were not uniformly distributed due to its lower percentage composition when compared to the other two substrates. It is clearly seen that there is redundancy of an oxide binder in the coating.
Fig.5. SEM Image of 25% SiC + 75% Al203 Coated Sample
The fig.6 shows the SEM image of the coated specimen at 50%SiC + 50% Al203 composition. It can be clearly seen that abrasive hexagonal shaped SiC particles were distributed uniformly. The attachment of the layers is enhanced in the coating since the binder is now located within the vicinities of the SiC particles. Also, the mixing of SiC- Al203 particles was well which in turn affects the strength of the coatings. This deposit contains equal amount of SiC and
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Al203. It can be seen that SiC particles are dispersed in alumina (round shaped) matrix evenly. This ensures proper binding property between SiC/ Al203. The success of these coatings is due to the nano-film binders of Al203 that aid matrix formation and protects SiC particles from decomposition. The Plasma spray technique is superior for retaining its binding.
Fig.6. SEM Image of 50%SiC + 50%Al203 Coated Sample
Fig. 7 shows the higher magnification of SiC/ Al203 (75:25) deposit. SiC is much higher than compared to Al203 the SiC particles cover the inferior Al203 there by masking the properties of Al203 by a limit. The uniformity of the coatings improved with increase in the volume percentage of SiC in the composites. As the content of the Al203 decreases, the binding property between the particles reduces and thereby increases the wear rate. Also, as the SiC content increases, the hardness increases correspondingly.
Fig.7. SEM Image of 75%SiC + 25%Al203 Coated Sample
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5. Conclusions The performance of a new anti-wear coating is evaluated by its response under several physical and mechanical conditions as it becomes essential for selecting the coating of proper composition as per the required application. Therefore, in the present work, SiC-Al203 coating is deposited on the aluminium alloy 6061 via Plasma spray method at different SiC- Al203 compositions. Various characterization techniques such as Pin-on-disc test, Micro hardness, and microstructural characterization were also conducted. A brief glance on the results obtained from this work showed a positive response to the deposition of SiC- Al203 coating on the aluminium alloy 6061. The following conclusions can be drawn from the present investigation: 1) SiC- Al203 composite coating have been successfully deposited on the Aluminium alloy 6061 by using Plasma spray method. The Plasma spray method which was conducted manually was found to be very economical and effective due to the uniform deposition. 2) The adhesion of the coating has improved by addition of Alumina (Al2O3) i.e. the SiC particles were embedded into Al2O3 matrix to obtain good adhesion of the coating. 3) The wear tests showed that wear rate of the coated specimens has significantly improved when compared to uncoated specimens and the coating with SiC/Al203 (50:50) composition shows minimum wear rate. 4) Micro hardness test showed that the coating at SiC/Al203 (75:25) composition exhibits high hardness value i.e. the maximum achievable hardness was 190 HV when compared with other values for the specified load range. 5) It also showed that the SiC and Al203 has a considerable effect on the mechanical properties such as hardness and also on wear rate. 6) Microstructural evaluation using Scanning Electron Microscope revealed that SiC particles were uniformly distributed for the coating at SiC/Al203 (50:50) composition. 7) The results obtained from this study can be used effectively for the development of components that require better wear and mechanical properties. References [1] Peng-fei and Guo-zheng, Tribological behaviors of internal plasma sprayed TiO2-based ceramic coating on engine cylinder under lubricated conditions, Tribology International 102· June 2016. [2] F. Mubarok, N. Espallargas, Suspension Plasma Spraying of Sub- Micron Silicon Carbide Composite Coatings, Journal of Thermal Spray Technology, June 2015, Volume 24, Issue 5, pp. 817–825. [3] Berkath Ali Khan C A, Dr. Anil Kumar C, Dr. Suresh P M, International Journal of Innovative Research in Science,Engineering and Technology., 2014; Vol. 3, Issue 6. [4] Ville Matikainen, Kari Niemi, Heli Koivuluoto and Petri Vuoristo, Abrasion, Erosion and Cavitation Erosion Wear Properties of Thermally Sprayed Alumina Based Coatings, Coatings 2014, 4, 18-36; doi:10.3390/coatings4010018. [5] Nenad Radic and Stevan Stojadinovic, TiO2/WO3 photocatalytic composite coatings prepared by spray pyrolysis, Surface and Coatings Technology Volume 258, 15 November 2014, Pages 763–771. [6] A. Lanzutti, M. Lekka, E. Marin, L. Fedrizzi, Tribology in Industry Vol. 35, No. 2 (2013) 113‐122. [7] Sánchez, E.; Bannier, E.; Cantavella, V.; Salvador, M.D.; Klyatskina, E.; Morgiel, J.; Grzonka, J.; Boccaccini, A.R, Deposition of Al2O3TiO2 Nanostructured Powders by Atmospheric Plasma Spraying, Journal of Thermal Spray Technology. Sep2008, Vol. 17 Issue 3, p329-337. [8] K. Ghosh, T. Troczynski, and A.C.D. Chaklader, Aluminum-Silicon Carbide Coatings by Plasma Spraying, JTTEE5 7:78-86, ASM International, Submitted 26 Dec 1996; in revised form 14 Oct 1997. [9] Ahmed Ibrahim, Abdel Salam Hamdy, Microstructure, Corrosion, and Fatigue Properties of Alumina-Titania Nanostructured Coatings, Journal of Surface Engineered Materials and Advanced Technology, Vol.1 No.3, October 18, 2011
ScienceDirect Materials Today: Proceedings 5 (2018) 23834–23843
www.materialstoday.com/proceedings
IConAMMA_2017
Wear characterization and microstructure evaluation of silicon carbide based nano composite coating using plasma spraying Muhammed Muneer S a , Nadeera M b* a
b
PG Scholar, Department of Mechanical Engineering, TKM College of Engineering, Kollam - 691005, India Assistant Professor, Department of Mechanical Engineering, TKM College of Engineering, Kollam - 691005, India
Abstract Thin films of various thickness of SiC – Al203 composite is deposited on aluminium alloy 6061 using Plasma spraying process. Wear tests on pin-on-disc tester is conducted to compare the wear characteristics of uncoated and coated samples. The micro hardness tests of coated samples and uncoated samples are compared. The microstructure characterization of the Nano-coated films using Scanning Electron Microscope (SEM) of the samples is studied. The results and studies clearly depicts that the major variations in coating performance can be obtained by exploiting proper plasma spray conditions and optimum percentage of SiC – Al203 in composite coatings. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: Composite Coating; Plasma Spraying; Microstructure Characterization
1.
Introduction
Aluminium alloys are widely used in automotive engines, automotive body parts, bicycle frames, aerospace applications. To meet the different applications listed above, the materials used to fabricate these items must have high strength, modulus of elasticity, abrasion resistance, corrosion resistance and wear resistance. The material should also have a low density, thermal expansion, and thermal conductivity.
* Muhammed Muneer S a. Tel.: +91-9037951519 a. Nadeera M b. Tel.: +91 9847889937 b. E-mail address: [email protected] a, [email protected] b. 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017].
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One of the major drawbacks faced by Aluminium and its alloys was its poor wear resistance. This drawback limited the aluminium and its alloys in some applications. Ceramic based composite Nano coatings enabled the aluminium alloys to overcome its drawback. Various kind of ceramic composite materials can be used, out of which, from studies, Silicon Carbide based composite coating offers a better wear resistance. The Silicon Carbide (SiC) cannot be directly coated on the aluminium alloys. Hence Silicon carbide was reinforced on Aluminium Oxide (Al203) in order to obtain proper Nano composite which satisfy the requirements. The SiC-Al203 composite imparts better wear and abrasion resistance as well as better hardness. The technique used for coating is Plasma Spray technique, due to its large choice of applications, automation capabilities, and high degree of flexibility. Peng-fei et al [1] in 2016 have conducted a study on the TiO2 based ceramic coatings for tribological applications like engine cylinder. As part of the study alumina-titania coatings were deposited on engine cylinder by plasma spraying technique. The wear tests were conducted using reciprocating tribometer at various engine conditions such as load, frequency, temperature etc. The morphology of the sprayed coating was obtained from SEM and it showed that a dense lamellae structure with pores and grey zones which shows the presence of TiO2. Also tribological tests showed that coated specimens have improved wear resistance. F. Mubarok et al [2] in 2015 proposed their work, by Suspension plasma spray (SPS) coating technique like atmospheric plasma spray is used. They deposited SiC coatings by SPS shows identical SiC phase peak, which indicates that the nano-film binder protects SiC particles from decomposition. They carried out the analysis using XPS, which exhibits that SiC particles faced some minor oxidation. All the SiC coatings shows poor mechanical performance because of its low cohesive strength, high porosity, and powdery structure which makes the coatings to grain pull out. This caused because of the absence of sintering process during the spraying process resulting to the low performance of SiC in SPS coatings. Berkath Ali Khan, et al [3] in 2014 has coated Al2O3-TiO2 coating on Al-6082 substrate. The technique used for deposition was thermal spraying in which alumina and titania powders were melted and sprayed on the metal substrate on varying thicknesses. The tribological behavior was tested using pin on disc testing machine and the hardness values were obtained from micro-hardness tests. From the study, it was found that coated samples had excellent wear properties than uncoated specimens. From the study, it was also found that coatings had poor adhesion on the metal surface. In order for that, a bond coat material NiCrAl was also used. The study also involved the SEM analysis of worn out specimen. Ville Matikainen et al [4] in 2014 Studied about the wear resistant coatings from Alumina and Titania composite powders sprayed with HVOF spray processes. The study involved of fused and crushed, and agglomerated and sintered Alumina- 13% Titania powders and was compared with pure Alumina. The coatings were subjected to investigations such as abrasion, erosion, and cavitation resistance to study the effect of the coating on the wear behavior. Improved coating properties are obtained as results when agglomerated and sintered composite coating powder is used in this method. The tests proved that the coatings done by using crushed and fused powder shows better wear resistance. Nenad Radic et al [5] in 2014 conducted a study on TiO2/WO3 composite coating on stainless steel substrates with varying percentages of WO3 using spray pyrolysis. The photo catalytic properties were investigated. The study concluded that the current method is a simple and cheap coating method. Also, this method provides versatility due to adjustable process parameters like spray geometry, substrate temperature, composition, concentration of precursor, liquid and gas flow rates. The results again showed that TiO2 had poor adhesion on metal substrates. Also the photocatalytic properties also improved with increased addition of WO3. A. Lanzutti et al [6] in 2013 has done thermal spray coatings by depositing as per industrial standards and procedures, and the Ni/SiC composite coatings were manufactured at laboratory scale by micro and nano sized ceramic particles. All the sample coatings were characterized for their microstructure, mechanical properties and the wear resistance. The tribological properties were analyzed using a tribometer at various temperatures. The results exhibit good wear resistance and the Ni coated specimen shows better wear resistance than compared to normally coated specimen using thermal spray method.
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E. Sa´nchez et al [7] in 2008 has conducted experiment on Al2O3-13%TiO2 coatings were deposited on stainless steel substrates by Plasma Spray Method. Microstructure characterization has confirmed its nanostructured nature. The microstructure comprised two clearly differentiated regions. Wear hardness values analyzed. Vickers micro hardness values of uncoated were in average which is slightly lower than the values obtained by nanostructured coating. The wear resistance of conventional coatings seems to be lower than as compared to nanostructured coatings. K. Ghosh et al [8] in 1997 made a composite with aluminium and Silicon carbide to coat via Plasma Spraying. As the concentration of the Silicon carbide was changed, the size of the composite was varied. The coating was done with axial feed plasma torch. As the SiC content increased, bonding strength of the coatings found to decrease. The wear resistance got decreased with the increase in Silicon carbide particle size and with the Silicon carbide content in the composite coatings. 2. Experimental Procedure 2.1 Materials Aluminium alloy 6061 is used as base substrate. In this study, Silicon Carbide (SiC) and Aluminium Oxide (Al203) was selected as the reinforcement for the coating. The average particle size of Silicon Carbide powder was 50-60 nm and average size of Aluminium oxide was 250-300nm. 2.2 Sample Preparation Aluminium alloy 6061 in ingot form was cut into small samples of required dimensions using power hacksaw cutting machine and vertical milling machine. The dimensions of the cut specimens are as follows: For Pin-on-disc wear test, Pins having diameter = 6mm and length = 30mm and for Micro Hardness test and SEM analysis, small blocks of 10mm x 10mm x 10mm was taken. Before deposition of coating, the surfaces of samples were polished using emery papers of grit sizes 600, 800, 1000 and the surface was cleaned and degreased using acetone. 2.3 Preparation of Coating Powder The coating of SiC- Al203 powder was done in three compositions by weight as shown in Table 1. Table 1. Compositions of Coating Powder (By Weight) Composition
SiC
Al2O3
1
25%
75%
2
50%
50%
3
75%
25%
The powder samples are prepared at these compositions by using a blending machine to ensure proper mixing of the Nano powder of both Silicon Carbide as well as Alumina. The machine rotates at about 1200RPM. Blending time for each composition is about 45 minutes.
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2.4 Spraying process
Fig.1. Typical Plasma Spray Equipment (Courtesy: FST Plasma Spray Equipment)
Aluminum alloy 6061 used as the base substrate for coating in this work. SiC-Al203 composite powder are used as coating materials and premixed using a blending machine to form three different compositions shown in Table 1. Firstly, NiCrAl bond coat of 10-20μm thickness was deposited to promote the best attainable adhesion between coating and substrate. Second layer coating was done using Aluminum oxide (Al203) and the process is referred as Blasting. Blast coat thickness is about 25-35 micrometer. The top coat was coated with three different compositions of SiC-Al203 as listed above in Table 1. All the coating process was conducted by plasma spray process with a PRAXAIR SG-100 plasma spray gun. The plasma spray process parameters used for this experiment are listed in Table 2.
Table 2. Plasma Spray Process Parameters Gun PRAXAIR SG-100 Current (DC)
500
Current (DC)
75
Primary Gas Flow Rate (Argon, lpm)
100
Carrier Gas Flow Rate (Hydrogen, lpm)
100
Powder Feeder (grams/min)
37
Powder Flow Rate (grams/min)
100
Spray Distance (mm)
60
3. Characterization Techniques Certain characterization techniques such as pin-on-disc test, Micro hardness and Scanning Electron Microscope (SEM) studies were conducted for the study of wear, mechanical characteristics and microstructure evaluation of
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SiC-Al203 composite coating on AA6061 under ASTM standards (ASTM E92-17, ASTM E384-16, ASTM E99-04, ASTM F1372-93). There were four test samples including an uncoated and three coated samples. 3.1 Wear Test For the wear test on the pin-on-disc testing machine, cylindrical pins with ASTM G99 standard having 6 mm diameter and 30 mm length standard were used with parameters as shown in Table 3. The samples were coated on the plane surface normal to the longitudinal axis. Table 3. Wear Test Parameters Parameters
Values
Load
3Kg
Sliding Velocity
2 m/s
Sliding Distance
1000m
Counter face material
Hardened ground Steel
Wear Track radius
100mm
Sliding time
8 min 51 sec
3.2 Micro Hardness Test Table 4. Micro Hardness Test Conditions Parameters
Variables
Machine
MITUTOYO - HM100, JAPAN
Load
30Kgf
Test Duration
10Secs
For the micro hardness test, each test specimen was prepared as a square block in the size of 10mm x 10mm x 10mm according to ASTM E92 standard as shown in Table 4. The test was conducted on vicker’s micro hardness tester with a load of 30kgf for a time of 10 seconds. 3.3 Microstructure Characterization The microstructure and the composition of the coatings were studied using Scanning Electron Microscope (SEM). The microstructure was characterized by using SIGMA FSEM equipment. For the microstructural study of the coatings, each test specimen was prepared as a square block in the size of 10mm x 10mm x 10mm with ASTM F1372-93 standard. Since the microstructural study of the coatings was conducted, no sample preparation procedure was done. SEM analysis was done to confirm the quality of the SiC-Al203 composite Nano coating on AA6061.
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4. Results and Discussions The Final results obtained after various tests and Characterization are as follows: 4.1 Wear Test
Wear (Micro meters)
Wear Rate - Comparison 1200 1000 800 600 400 200 0 -200 0
100
200 300 400 Duration of Sliding ( Sec )
500
WEAR - PURE ALUMINIUM 6061
WEAR - 25%SiC + 75%Al203
WEAR - 50%SiC + 50%Al203
WEAR - 75%SiC + 25%Al203
600
Fig.2. Wear rate comparison graph
The Fig.2 shows the comparison of wear rate of all the uncoated and coated specimens. The wear test is conducted for about 510 secs for all the specimens. From the tests, it is observed that Uncoated specimen of Aluminium 6061 has the highest wear rate which is followed by the 75% SiC + 25% Al203 specimen and then followed by the 25% SiC + 75% Al203 specimen. The least wear rate is obtained in 50% SiC + 50% Al203 composition specimen. This is because, as the SiC content in the composite increases, the binding property of the composite increases but only up to 50%. After 50% of increase in SiC content, the binding property of the SiC decreases due to the reduction of Al203 content, which holds the SiC content in the SiC - Al203 composite matrix. 4.2 Micro Hardness Table 5. Micro Hardness Test Values Composition
Average Hardness Value (HV)
Pure Aluminum 6061
65
25%SiC + 75%Al203
150
50%SiC + 50%Al203
169
75%SiC + 25%Al203
190
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Fig.3. Comparison - Average hardness value
The obtained micro hardness test values are shown in Table 5. For pure aluminum 6061, the micro hardness value obtained is 65 HV. For the 25%SiC + 75% Al203 composite coated sample, the micro hardness value obtained is 150HV. For 50%SiC + 50% Al203 composite coated sample, the micro hardness value obtained is 169HV.For the 75%SiC + 25% Al203 composite coating, the micro hardness value obtained is 190HV. The Fig 3. shows the comparison of Average hardness value. The values obtained after micro hardness test clearly depicts that the hardness values significantly increase with increase in the Silicon Carbide content. This concludes that, larger the Silicon Carbide content, higher will be the Hardness value and Vice-Versa. 100% Silicon carbide cannot be coated on aluminum substrate due to its poor binding property. Hence Alumina is required for proper adhesion of the SiC on the substrate. 4.3 Microstructure Characterization Scanning Electron Microscope (SEM) images were analysed to confirm the quality of the SiC- Al203 coating on the AA6061 substrate. The fig.4 demonstrates the Scanning Electron Microscope image of the plain uncoated substrate. It can be noted that the surface roughness was not uniform throughout the surface area and it may have led to the low adhesion of SiC- Al203 coating.
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Fig.4. SEM image of uncoated sample
The fig.5 shows SEM image of coated substrate 25% SiC + 75% Al203. The SiC in the coatings has a distribution, composition, and morphology similar to that in the powders sprayed. It can be clearly seen that whitish particle embedded in a grey matrix. The white particles indicate SiC particles and the round grey matrix constitutes for Al203. The SiC was distributed as individual particles in the matrix as shown in Fig 5, with limited clustering. At 25% SiC + 75% Al203 composition as shown in the Fig 5, the SiC particles were not uniformly distributed due to its lower percentage composition when compared to the other two substrates. It is clearly seen that there is redundancy of an oxide binder in the coating.
Fig.5. SEM Image of 25% SiC + 75% Al203 Coated Sample
The fig.6 shows the SEM image of the coated specimen at 50%SiC + 50% Al203 composition. It can be clearly seen that abrasive hexagonal shaped SiC particles were distributed uniformly. The attachment of the layers is enhanced in the coating since the binder is now located within the vicinities of the SiC particles. Also, the mixing of SiC- Al203 particles was well which in turn affects the strength of the coatings. This deposit contains equal amount of SiC and
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Al203. It can be seen that SiC particles are dispersed in alumina (round shaped) matrix evenly. This ensures proper binding property between SiC/ Al203. The success of these coatings is due to the nano-film binders of Al203 that aid matrix formation and protects SiC particles from decomposition. The Plasma spray technique is superior for retaining its binding.
Fig.6. SEM Image of 50%SiC + 50%Al203 Coated Sample
Fig. 7 shows the higher magnification of SiC/ Al203 (75:25) deposit. SiC is much higher than compared to Al203 the SiC particles cover the inferior Al203 there by masking the properties of Al203 by a limit. The uniformity of the coatings improved with increase in the volume percentage of SiC in the composites. As the content of the Al203 decreases, the binding property between the particles reduces and thereby increases the wear rate. Also, as the SiC content increases, the hardness increases correspondingly.
Fig.7. SEM Image of 75%SiC + 25%Al203 Coated Sample
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5. Conclusions The performance of a new anti-wear coating is evaluated by its response under several physical and mechanical conditions as it becomes essential for selecting the coating of proper composition as per the required application. Therefore, in the present work, SiC-Al203 coating is deposited on the aluminium alloy 6061 via Plasma spray method at different SiC- Al203 compositions. Various characterization techniques such as Pin-on-disc test, Micro hardness, and microstructural characterization were also conducted. A brief glance on the results obtained from this work showed a positive response to the deposition of SiC- Al203 coating on the aluminium alloy 6061. The following conclusions can be drawn from the present investigation: 1) SiC- Al203 composite coating have been successfully deposited on the Aluminium alloy 6061 by using Plasma spray method. The Plasma spray method which was conducted manually was found to be very economical and effective due to the uniform deposition. 2) The adhesion of the coating has improved by addition of Alumina (Al2O3) i.e. the SiC particles were embedded into Al2O3 matrix to obtain good adhesion of the coating. 3) The wear tests showed that wear rate of the coated specimens has significantly improved when compared to uncoated specimens and the coating with SiC/Al203 (50:50) composition shows minimum wear rate. 4) Micro hardness test showed that the coating at SiC/Al203 (75:25) composition exhibits high hardness value i.e. the maximum achievable hardness was 190 HV when compared with other values for the specified load range. 5) It also showed that the SiC and Al203 has a considerable effect on the mechanical properties such as hardness and also on wear rate. 6) Microstructural evaluation using Scanning Electron Microscope revealed that SiC particles were uniformly distributed for the coating at SiC/Al203 (50:50) composition. 7) The results obtained from this study can be used effectively for the development of components that require better wear and mechanical properties. References [1] Peng-fei and Guo-zheng, Tribological behaviors of internal plasma sprayed TiO2-based ceramic coating on engine cylinder under lubricated conditions, Tribology International 102· June 2016. [2] F. Mubarok, N. Espallargas, Suspension Plasma Spraying of Sub- Micron Silicon Carbide Composite Coatings, Journal of Thermal Spray Technology, June 2015, Volume 24, Issue 5, pp. 817–825. [3] Berkath Ali Khan C A, Dr. Anil Kumar C, Dr. Suresh P M, International Journal of Innovative Research in Science,Engineering and Technology., 2014; Vol. 3, Issue 6. [4] Ville Matikainen, Kari Niemi, Heli Koivuluoto and Petri Vuoristo, Abrasion, Erosion and Cavitation Erosion Wear Properties of Thermally Sprayed Alumina Based Coatings, Coatings 2014, 4, 18-36; doi:10.3390/coatings4010018. [5] Nenad Radic and Stevan Stojadinovic, TiO2/WO3 photocatalytic composite coatings prepared by spray pyrolysis, Surface and Coatings Technology Volume 258, 15 November 2014, Pages 763–771. [6] A. Lanzutti, M. Lekka, E. Marin, L. Fedrizzi, Tribology in Industry Vol. 35, No. 2 (2013) 113‐122. [7] Sánchez, E.; Bannier, E.; Cantavella, V.; Salvador, M.D.; Klyatskina, E.; Morgiel, J.; Grzonka, J.; Boccaccini, A.R, Deposition of Al2O3TiO2 Nanostructured Powders by Atmospheric Plasma Spraying, Journal of Thermal Spray Technology. Sep2008, Vol. 17 Issue 3, p329-337. [8] K. Ghosh, T. Troczynski, and A.C.D. Chaklader, Aluminum-Silicon Carbide Coatings by Plasma Spraying, JTTEE5 7:78-86, ASM International, Submitted 26 Dec 1996; in revised form 14 Oct 1997. [9] Ahmed Ibrahim, Abdel Salam Hamdy, Microstructure, Corrosion, and Fatigue Properties of Alumina-Titania Nanostructured Coatings, Journal of Surface Engineered Materials and Advanced Technology, Vol.1 No.3, October 18, 2011