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Wear and friction analysis of polyester coatings with solid lubricant
Surface & Coatings Technology 204 (2010) 2593–2599
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Wear and friction analysis of polyester coatings with solid lubricant M. Zouari, M. Kharrat ⁎, M. Dammak Ecole Nationale d'Ingénieurs de Sfax, Laboratoire des Systèmes Electromécaniques, B.P. 1173, 3038 Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 21 May 2009 Accepted in revised form 1 February 2010 Available online 6 February 2010 Keywords: Friction Wear Coatings Polyester Solid lubricant
a b s t r a c t The friction and wear behaviors of polyester coatings incorporating graphite particles and deposited on an aluminum substrate by a spraying technique were analyzed in this study. Four volume ratios of the graphite particles in the polyester matrix were considered in the range of 0% to 35%. The coatings tribological behaviors were analyzed using high chromium steel ball antagonist. The friction experiments were conducted using a reciprocating tribometer. For each of the considered coating, evolutions of the friction coefficient, the weight losses and the wear scar morphology with the number of sliding cycles were analyzed. The microstructure of polyester coatings and the wear scar micrographs were characterized using an optical microscope and a scanning electron microscope (SEM). Severe wear mechanism is activated for the polyester coating without graphite particles. Incorporating graphite particles in the polyester coating enhances the development of a specific third body which reduces friction and wear. The best friction and wear properties are obtained for highest ratio were the third body was formed on the entire sliding stripe. © 2010 Elsevier B.V. All rights reserved.
1. Introduction For many industrial applications there is a continuous demand for lightweight engineering materials. Especially in the automotive industry lightweight material alternatives replacing steel are needed to realize weight savings in future products without sacrificing the room and performance that motorist are looking for. Industrial engineers are looking at a variety of alternatives, including lightweight metals, plastics and various composite materials. All of these materials, however, have a variety of challenges, including cost and manufacturability. The use of aluminum alloys is well positioned, and it is generally applied in components like suspension pieces, engine blocks or small body panels [1–6]. A more widespread use of light metal alloys in tribological applications, like guide bars, bearing plates, seat supports or bushings, need powerful functional surface coatings to provide wear protection as well as compressive strength. Direct contacts of uncoated light metal substrates with sliding or oscillating counterparts result in severe wear, seizing and high friction coefficients even under lubricated conditions. There are different surface treatment processes for aluminum alloys which are commercially available to increase the resistance of corrosion and wear, they include anodic oxidation or electrodes nickel plating with codeposited PTFE (Polytetraflouroethylene) and silicon carbide particles. But the use of these technologies does not match the requirements of many tribological applications
⁎ Corresponding author. Tel.: + 216 74 241 733; fax: + 216 74 246 347. E-mail address: [email protected] (M. Kharrat). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.001
like sliding motion under very high surface loadings. Other issues include the relatively high costs and the technology process required for the existing surface treatment processes. An important driving factor for new developments in the field of tribology is the environmental concerns related to the effects of lubricants and grease on the ecological systems. Most lubricants contain environmentally harmful chemical additives. A significant amount of these lubricants are released in to the environment, either on purpose or by accident. Therefore there is a steady demand for materials and surface coatings with dry friction capability and solid lubricant ability [7–15]. Solid lubricants are generally defined as solid materials with inherent lubricating properties which are firmly bonded to the surface of the substrate by some methodology [16,17]. As a result of using these materials there will be no need for lubricant or amount of lubricants used will be reduced drastically. Polymer matrix composites with friction and wear reducing phase are developed for such applications [18,19]. The commonly used friction and wear reducing agents are PTFE, graphite powder and MoS2 (Molybdenum disulfide) powder [20,21]. The growing interest in polymer coatings has opened new challenges towards film properties improvement especially wear endurance and adhesion strength. In this sense, many studies aimed to establish correlations between the evolution of main film properties (mechanical properties, durability, adhesion and wear resistance…) and the different stages of curing process. Barletta et al. tried to characterize the adherence and wear endurance of epoxy and polyester based powder coatings under different baking procedure by using the scratch test [16,22,23]. Shaffer and Rogers used the squaring test (ASTM D3359-02) in order to analyze the impact resistance and adhesion strength for different coatings materials [24].
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2.2. Apparatus and experimental procedure
Table 1 Chemical composition and main mechanical properties of aluminum substrate. Chemical composition (wt.%)
E (GPa)
Mg Si Mn Ni Fe Cu Cr Zn Pb 70 0.36 0.65 0.56 b 0.1 0.72 5.83 0.13 0.25 0.37
Rm (MPa)
A (%)
402
12
In this study, a polymer composite material, with graphite particles which are used as friction and wear reducing phase, was deposited on an aluminum substrate using a spraying process. Coatings with different graphite volume ratios were studied against sliding friction using a steel ball antagonist and a reciprocating tribometer. 2. Experimental details 2.1. Materials The substrate used for the surface coating was an ISO 2014 aluminum-based alloy machined to the dimension of 30 × 15 × 20 mm3. The chemical composition and main mechanical properties of this aluminum alloy are listed in Table 1 [25]. The substrate surfaces were roughened by sandblasting to a surface finish Ra ≈ 2 µm. Synthetic graphite powder supplied by ALDRICH was used in this study. The particles size not exceeds 20 µm. Specific amount of fine graphite particles with four different volume ratios (0%, 15%, 25% and 35%) was homogeneously dispersed in a polyester resin supplied by REICHHOLD company. To enable the mixture spraying onto the substrate surface with a spry gun, the viscosity was reduced by adding acetone with a volume ratio of 25%. For the coating cure, specimens were then kept 48 h in an oven at 40 °C. The mean measured value of the coating hardness was 167HV. For each specimen, the thickness of the coating film was measured using ultrasounds technique. The measured thickness values vary between 86 µm and 161 µm. To carry out friction test on the obtained coating, high chromium steel ball (100Cr6) with 15 mm in diameter and 0.02 µm in Ra surface roughness was used as the antagonist. Before the friction test, the surface for each specimen was cleaned with ethanol.
2.2.1. Friction test Fig. 1 represents the reciprocating tribometer used for the coating layer friction and wear analysis. The apparatus allows to have a contact between the steel ball and the coating surface under a constant normal load Fn = 10 N. Tangential cyclic motion was then applied to the coated aluminum specimen using crank system which is driven by an electric motor with an electronic speed regulator. The tangential motion amplitude and frequency were adjusted to 5 mm and 1 Hz respectively. A load cell located between the coated aluminum specimen and its holder allows to measure the tangential force. The output of this load cell was continuously stored by using a data acquisition system. 2.2.2. Adhesion test and weight losses Squaring test was carried out on the coated specimen by using an apparatus of the type BRAIVE . All these tests are carried out under a pressure of 1.4 bars. Through the use of a sharp instrument, we aim at incising the coating until reaching the substrate in order to set the squaring. Then we will examin the incisions and the square's behavior to be presented by their caracteristic values. To evaluate the test result, we can examine the squared surface coating with the naked eye and try to classify them in accordance with a table of reference (Table 2). After each friction test, the weight losses of the sample was carried out using a balance to 10− 4 g of precision. 2.2.3. Optical microscope (OM) and scanning electron microscope (SEM) The observation of the wear scars of the coatings after the friction test is carried out using a binocular optical microscope type LEICA DMILM . This microscope allows a maximum magnification (500). It is equipped with three filters making it possible to vary the prime coat thus the detection of the surface defects becomes easier. This microscope is equipped with a numerical camera allowing the transmission of the photographs on computer supports to be able to treat them. Scanning electron microscope was also used to caracterise the wear scars morphology of the coatings after the friction tests. The
Fig. 1. Reciprocating friction and wear test apparatus.
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Table 2 Table of references of squaring test.
• ISO value: 0/value ASTM: 5 B The incisions are clear, smooth and without bur; no part is detached.
• ISO value: 1/value ASTM: 4 B In the intersections of network, one observes some small glares of the coating representing a surface is removed from approximately 5%.
• ISO value: 2/value ASTM: 3 B The coating is removed along the incisions and/or the intersections of the network: separated surface represents between 5 and 15% of the surface considered of the squares.
• ISO value: 3/value ASTM: 2 B The coating is detached along the incisions or in the squared parts per pieces, broad band or entirely. Surface stripped between 15 and 35%.
• ISO value: 4/value ASTM: 1 B The coating is detached per sometimes whole pieces: surface stripped between 35 and 65%.
• ISO value: 5/value ASTM: 0 B Relate to all the degrees of stripped surface not being able more to be described by the ISO value of 4, with more than 65% of stripped surface.
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Fig. 2. Examples of adhesion test applied on; (a): polyester, (b): polyester with 15% by volume of graphite, (c): polyester with 25% by volume of graphite and (d): polyester with 35% by volume of graphite.
Fig. 3. Optical micrographs of the coating surface; (a): polyester, (b): polyester with 15% by volume of graphite, (c): polyester with 25% by volume of graphite, and (d): polyester with 35% by volume of graphite.
examinations were carried out on an apparatus Philips XL 30. This microscope is equipped with an analyzer of energy dispersion X (EDX). Before the SEM observation samples were gold coated. 3. Results and discussion 3.1. Coatings adhesion and structure Polyester coatings adhesion was evaluated according to Standard Test Methods for Measuring Adhesion by Tape Test. The result of the four coatings tested showed that only polyester without solid lubricant has no removed area (Fig. 2a). For the polyester coatings which contain 15% and 25% graphite volume ratio, the failure obtained was less than 5% of the total area (Fig. 2b and c). The highest removed area was observed on the polyester coating containing the high graphite volume ratio (35%) where failure attained 5% of the total area (Fig. 2d).
Fig. 4. The evolution of the friction coefficient versus the number of sliding cycles for polyester coatings charged with graphite.
M. Zouari et al. / Surface & Coatings Technology 204 (2010) 2593–2599
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example, the friction coefficient drops from 0.71 to 0.43 when the graphite volume ratio increases from 0% to 35%. No significant difference on the friction behavior was obtained between 15% and 25% graphite volume ratios. However, a much large friction drop was obtained with the 35% graphite volume ratio. For polyester coatings containing graphite lubricant, the friction coefficient undergoes a slight quasi-linear increase until 500 sliding cycles after which it remains almost constant. 3.3. Weight losses
Fig. 5. The evolution of the weight losses versus the number of sliding cycles for polyester coatings charged with graphite.
The optical micrographs of the coating surface that contains the four considered volume ratios of graphite particles is shown in Fig. 3. It can be seen that the graphite particles are homogeneously dispersed in the polyester matrix. 3.2. Friction coefficient The evolution of the friction coefficient with the number of sliding cycles for the four considered volume ratios of graphite particles are shown in Fig. 4. The effect of the graphite friction reducing phase is well appreciated in this figure. In fact, after 1000 sliding cycles for
For the four considered graphite volume ratios, evolutions of the weight losses with the number of sliding cycles are reported in Fig. 5. The effect of the graphite wear reducing phase is well underlined in this figure. Indeed after 1000 sliding cycles for example, the wear losses fall from 3 mg to about 0.6 mg when the graphite volume ratio increases from 0% to 35%. As for the friction behavior, no significant difference for the wear behavior was obtained between the 15% and 25% graphite volume ratios. Nevertheless, a much large wear decrease was obtained for the highest graphite volume ratio. 3.4. Wear mechanisms The analysis of the wear mechanisms has been developed on the basis of optical micrograph of the wear scar (Fig. 6). For the 0% graphite volume ratio (Fig. 6a), the wear scar width increases as the number of sliding cycles increases and the great part of wear particles is removed from the contact stripe. Severe wear mechanism is activated in this case as it was confirmed by the high mass losses
Fig. 6. Typical optical micrographs of the coating worn surfaces; (a): polyester with 0% by volume graphite, (b): polyester with 15% by volume graphite, (c): polyester with 25% by volume graphite, and (d): polyester with 35% by volume graphite (arrow indicates the sliding direction).
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Fig. 7. SEM images of the coating worn surfaces after 1000 sliding cycles; (a): polyester with 0% by volume graphite, (b): polyester with 15% by volume graphite, (c): polyester with 25% by volume graphite, and (d): polyester with 35% by volume graphite.
attained after 1000 sliding cycles (Fig. 5). Incorporating graphite particles in the polyester coating changes the wear mechanism (Fig. 6b, c, and d). In case, we assist to the development of a third body which cover the sliding stripe as the number of sliding cycles increases. Graphite was a potential candidate of lubricants, which could also form a transfer film on the sliding counterpart [26,27]. Graphite had a layer structure (carbon layer) in which the atoms were arranged in a hexagonal unit cell within each layer [28]. These layers are linked by weak Van der Waals bonds, which can be easily broken by shear force under sliding conditions. This is way the mass losses have been seriously reduced for the 35% graphite volume ratio as compared with the 0% graphite volume ratio. For the 35% graphite volume ratio, the third body develops on the whole sliding stripe (Fig. 6d) while only the central part of the sliding band is covered by wear particles for the 15% and 25% graphite volume ratio (Fig. 6b and c). That is why similar friction and wear behavior have been obtained for coating with 15% and 25% graphite volume ratio. For a better understanding of the wear mechanisms, the analysis of the friction zone by scanning electron microscope (SEM) were done after 1000 cycles. After the metallization of the wear scar, the SEM micrographs obtained for the polyester coatings containing the four considered graphite volume ratio are represented in Fig. 7. For the 0% graphite volume ratio we can note a deformation of the surface asperities by the antagonist which generate the detachment of particles and the formation of cavities (Fig. 7a). This can affirm that an adhesive wear mechanism is activated. The severity of this wear mechanism is attenuated as graphite volume ratio increases. Cavities development was quasi absent in the case of the polyester coating containing 35% graphite volume ratio (Fig. 7d). 4. Conclusions In this study polyester coatings incorporating different graphite volume ratios were deposited on aluminum substrate using the
spraying technique. Coatings were studied against sliding friction by using a high chromium steel ball antagonist and a specific reciprocating tribometer. The following conclusions can be drawn: • The best adhesion is obtained in the case of polyester coatings without solid lubricants. The particles of solid lubricants decrease the adhesion of our coatings; • the incorporation of graphite particles in the polyester coating reduces considerably friction and wear; • similar friction and wear behavior have been identified for polyester coatings with 15% and 25% graphite volume ratio; • the best friction and wear properties are obtained for polyester coating with 35% graphite volume ratio; • for higher graphite volume ratio, plenteous graphite flakes spread symmetrically on the sliding surface, which reduced the ‘direct contact’ between the steel counterpart and composites; and • optical microscope and scanning electron microscope analysis of the polyester coatings wear scars revealed an adhesive wear mechanism for the four considered graphite volume ratios. References [1] C. Winandy, Editor's Note in Automotive Light Metals 1–2, First Global Media Group, New Jersey, USA, December 2000, p. 9. [2] S. Bahadur, D. Gong, Wear 158 (1992) 41. [3] J. Bijwe, J. Indumathi, J.J. Rajesh, M. Fahim, Wear 249 (2001) 715. [4] Z. Zhang, C. Breidt, L. Chang, K. Haupert, K. Friedrich, Composites Part A 35 (2004) 1385. [5] M.H. Cho, S. Bahadur, A.K. Pogosian, Wear 258 (2005) 1825. [6] J.A. Williams, J.H. Morris, A. Ball, Tribol. Int. 30 (1997) 663. [7] C. Schwarts, S. Bahadur, Wear 251 (2001) 1532. [8] Q. Zhao, S. Bahadur, Wear 217 (1998) 62. [9] S. Bahadur, L. Zhang, J.W. Anderegg, Wear 203 (1997) 464. [10] J.V. Vort, S. Bahadur, Wear 181 (1995) 212. [11] K.S. Bhabani, J. Bijwe, Wear 253 (2002) 787. [12] R. Gadow, D. Sherer, Surf. Coat. Technol. 151–152 (2002) 471. [13] J. Xu, M.H. Zhu, Z.R. Zhou, Thin Solid Films 475 (2004) 320. [14] M. Barletta, V. Tagliaferri, Surf. Coat. Technol. 200 (14–15) (2006) 4282.
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G.W. Sawyer, K.D. Freudenberg, P. Bhimaraj, L.S. Schadler, Wear 254 (2003) 573. M. Barletta, G. Bolelli, A. Gisario, L. Lusvarghi, Prog. Org. Coat. 61 (2008) 262. M.S. Shi, Manuf. Technol. (1996) 53. Y.S. Jin, C.H. Zhou, Wear 205 (1997) 77. N.M. Renevier, J. Hamphire, V.C. Fox, J. Witts, T. Allen, D.G. Teer, Surf. Coat. Technol. 142–144 (2001) 67. [20] J.-P. Bonino, S. Vaillant, R. El Berrechi, P. Barthes, F. Martin, P. Bacchin, J. Ferret, Proceedings of VII International Conference on Chromium Plating No. 27, SaintEtienne, 2004. [21] T. Larsen, T.L. Andersen, B. Thorning, A. Horsewell, M.E. Vigild, Wear 265 (2008) 203.
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[22] M. Barletta, Surf. Coat. Technol. 203 (2009) 1863. [23] M. Barletta, L. Lusvarghi, F. Pighetti Mantini, G. Rubino, Surf. Coat. Technol. 201 (2007) 7479. [24] S.J. Shaffer, M.J. Rogers, Wear 263 (2007) 1281. [25] JIS Handbook, Non-Ferrous Metals and Metallurgy, Japanese Standards Association, 1995, p. 706. [26] S.C. Kang, D.W. Chung, Wear 254 (1–2) (2003) 103. [27] N.K. Myshkin, M.I. Petrokovets, A.V. Kovalev, Tribol. Int. 38 (11–12) (2005) 910. [28] D.D.L. Chung, J. Mater. Sci. 37 (8) (2002) 1475.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Wear and friction analysis of polyester coatings with solid lubricant M. Zouari, M. Kharrat ⁎, M. Dammak Ecole Nationale d'Ingénieurs de Sfax, Laboratoire des Systèmes Electromécaniques, B.P. 1173, 3038 Sfax, Tunisia
a r t i c l e
i n f o
Article history: Received 21 May 2009 Accepted in revised form 1 February 2010 Available online 6 February 2010 Keywords: Friction Wear Coatings Polyester Solid lubricant
a b s t r a c t The friction and wear behaviors of polyester coatings incorporating graphite particles and deposited on an aluminum substrate by a spraying technique were analyzed in this study. Four volume ratios of the graphite particles in the polyester matrix were considered in the range of 0% to 35%. The coatings tribological behaviors were analyzed using high chromium steel ball antagonist. The friction experiments were conducted using a reciprocating tribometer. For each of the considered coating, evolutions of the friction coefficient, the weight losses and the wear scar morphology with the number of sliding cycles were analyzed. The microstructure of polyester coatings and the wear scar micrographs were characterized using an optical microscope and a scanning electron microscope (SEM). Severe wear mechanism is activated for the polyester coating without graphite particles. Incorporating graphite particles in the polyester coating enhances the development of a specific third body which reduces friction and wear. The best friction and wear properties are obtained for highest ratio were the third body was formed on the entire sliding stripe. © 2010 Elsevier B.V. All rights reserved.
1. Introduction For many industrial applications there is a continuous demand for lightweight engineering materials. Especially in the automotive industry lightweight material alternatives replacing steel are needed to realize weight savings in future products without sacrificing the room and performance that motorist are looking for. Industrial engineers are looking at a variety of alternatives, including lightweight metals, plastics and various composite materials. All of these materials, however, have a variety of challenges, including cost and manufacturability. The use of aluminum alloys is well positioned, and it is generally applied in components like suspension pieces, engine blocks or small body panels [1–6]. A more widespread use of light metal alloys in tribological applications, like guide bars, bearing plates, seat supports or bushings, need powerful functional surface coatings to provide wear protection as well as compressive strength. Direct contacts of uncoated light metal substrates with sliding or oscillating counterparts result in severe wear, seizing and high friction coefficients even under lubricated conditions. There are different surface treatment processes for aluminum alloys which are commercially available to increase the resistance of corrosion and wear, they include anodic oxidation or electrodes nickel plating with codeposited PTFE (Polytetraflouroethylene) and silicon carbide particles. But the use of these technologies does not match the requirements of many tribological applications
⁎ Corresponding author. Tel.: + 216 74 241 733; fax: + 216 74 246 347. E-mail address: [email protected] (M. Kharrat). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.001
like sliding motion under very high surface loadings. Other issues include the relatively high costs and the technology process required for the existing surface treatment processes. An important driving factor for new developments in the field of tribology is the environmental concerns related to the effects of lubricants and grease on the ecological systems. Most lubricants contain environmentally harmful chemical additives. A significant amount of these lubricants are released in to the environment, either on purpose or by accident. Therefore there is a steady demand for materials and surface coatings with dry friction capability and solid lubricant ability [7–15]. Solid lubricants are generally defined as solid materials with inherent lubricating properties which are firmly bonded to the surface of the substrate by some methodology [16,17]. As a result of using these materials there will be no need for lubricant or amount of lubricants used will be reduced drastically. Polymer matrix composites with friction and wear reducing phase are developed for such applications [18,19]. The commonly used friction and wear reducing agents are PTFE, graphite powder and MoS2 (Molybdenum disulfide) powder [20,21]. The growing interest in polymer coatings has opened new challenges towards film properties improvement especially wear endurance and adhesion strength. In this sense, many studies aimed to establish correlations between the evolution of main film properties (mechanical properties, durability, adhesion and wear resistance…) and the different stages of curing process. Barletta et al. tried to characterize the adherence and wear endurance of epoxy and polyester based powder coatings under different baking procedure by using the scratch test [16,22,23]. Shaffer and Rogers used the squaring test (ASTM D3359-02) in order to analyze the impact resistance and adhesion strength for different coatings materials [24].
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2.2. Apparatus and experimental procedure
Table 1 Chemical composition and main mechanical properties of aluminum substrate. Chemical composition (wt.%)
E (GPa)
Mg Si Mn Ni Fe Cu Cr Zn Pb 70 0.36 0.65 0.56 b 0.1 0.72 5.83 0.13 0.25 0.37
Rm (MPa)
A (%)
402
12
In this study, a polymer composite material, with graphite particles which are used as friction and wear reducing phase, was deposited on an aluminum substrate using a spraying process. Coatings with different graphite volume ratios were studied against sliding friction using a steel ball antagonist and a reciprocating tribometer. 2. Experimental details 2.1. Materials The substrate used for the surface coating was an ISO 2014 aluminum-based alloy machined to the dimension of 30 × 15 × 20 mm3. The chemical composition and main mechanical properties of this aluminum alloy are listed in Table 1 [25]. The substrate surfaces were roughened by sandblasting to a surface finish Ra ≈ 2 µm. Synthetic graphite powder supplied by ALDRICH was used in this study. The particles size not exceeds 20 µm. Specific amount of fine graphite particles with four different volume ratios (0%, 15%, 25% and 35%) was homogeneously dispersed in a polyester resin supplied by REICHHOLD company. To enable the mixture spraying onto the substrate surface with a spry gun, the viscosity was reduced by adding acetone with a volume ratio of 25%. For the coating cure, specimens were then kept 48 h in an oven at 40 °C. The mean measured value of the coating hardness was 167HV. For each specimen, the thickness of the coating film was measured using ultrasounds technique. The measured thickness values vary between 86 µm and 161 µm. To carry out friction test on the obtained coating, high chromium steel ball (100Cr6) with 15 mm in diameter and 0.02 µm in Ra surface roughness was used as the antagonist. Before the friction test, the surface for each specimen was cleaned with ethanol.
2.2.1. Friction test Fig. 1 represents the reciprocating tribometer used for the coating layer friction and wear analysis. The apparatus allows to have a contact between the steel ball and the coating surface under a constant normal load Fn = 10 N. Tangential cyclic motion was then applied to the coated aluminum specimen using crank system which is driven by an electric motor with an electronic speed regulator. The tangential motion amplitude and frequency were adjusted to 5 mm and 1 Hz respectively. A load cell located between the coated aluminum specimen and its holder allows to measure the tangential force. The output of this load cell was continuously stored by using a data acquisition system. 2.2.2. Adhesion test and weight losses Squaring test was carried out on the coated specimen by using an apparatus of the type BRAIVE . All these tests are carried out under a pressure of 1.4 bars. Through the use of a sharp instrument, we aim at incising the coating until reaching the substrate in order to set the squaring. Then we will examin the incisions and the square's behavior to be presented by their caracteristic values. To evaluate the test result, we can examine the squared surface coating with the naked eye and try to classify them in accordance with a table of reference (Table 2). After each friction test, the weight losses of the sample was carried out using a balance to 10− 4 g of precision. 2.2.3. Optical microscope (OM) and scanning electron microscope (SEM) The observation of the wear scars of the coatings after the friction test is carried out using a binocular optical microscope type LEICA DMILM . This microscope allows a maximum magnification (500). It is equipped with three filters making it possible to vary the prime coat thus the detection of the surface defects becomes easier. This microscope is equipped with a numerical camera allowing the transmission of the photographs on computer supports to be able to treat them. Scanning electron microscope was also used to caracterise the wear scars morphology of the coatings after the friction tests. The
Fig. 1. Reciprocating friction and wear test apparatus.
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Table 2 Table of references of squaring test.
• ISO value: 0/value ASTM: 5 B The incisions are clear, smooth and without bur; no part is detached.
• ISO value: 1/value ASTM: 4 B In the intersections of network, one observes some small glares of the coating representing a surface is removed from approximately 5%.
• ISO value: 2/value ASTM: 3 B The coating is removed along the incisions and/or the intersections of the network: separated surface represents between 5 and 15% of the surface considered of the squares.
• ISO value: 3/value ASTM: 2 B The coating is detached along the incisions or in the squared parts per pieces, broad band or entirely. Surface stripped between 15 and 35%.
• ISO value: 4/value ASTM: 1 B The coating is detached per sometimes whole pieces: surface stripped between 35 and 65%.
• ISO value: 5/value ASTM: 0 B Relate to all the degrees of stripped surface not being able more to be described by the ISO value of 4, with more than 65% of stripped surface.
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Fig. 2. Examples of adhesion test applied on; (a): polyester, (b): polyester with 15% by volume of graphite, (c): polyester with 25% by volume of graphite and (d): polyester with 35% by volume of graphite.
Fig. 3. Optical micrographs of the coating surface; (a): polyester, (b): polyester with 15% by volume of graphite, (c): polyester with 25% by volume of graphite, and (d): polyester with 35% by volume of graphite.
examinations were carried out on an apparatus Philips XL 30. This microscope is equipped with an analyzer of energy dispersion X (EDX). Before the SEM observation samples were gold coated. 3. Results and discussion 3.1. Coatings adhesion and structure Polyester coatings adhesion was evaluated according to Standard Test Methods for Measuring Adhesion by Tape Test. The result of the four coatings tested showed that only polyester without solid lubricant has no removed area (Fig. 2a). For the polyester coatings which contain 15% and 25% graphite volume ratio, the failure obtained was less than 5% of the total area (Fig. 2b and c). The highest removed area was observed on the polyester coating containing the high graphite volume ratio (35%) where failure attained 5% of the total area (Fig. 2d).
Fig. 4. The evolution of the friction coefficient versus the number of sliding cycles for polyester coatings charged with graphite.
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example, the friction coefficient drops from 0.71 to 0.43 when the graphite volume ratio increases from 0% to 35%. No significant difference on the friction behavior was obtained between 15% and 25% graphite volume ratios. However, a much large friction drop was obtained with the 35% graphite volume ratio. For polyester coatings containing graphite lubricant, the friction coefficient undergoes a slight quasi-linear increase until 500 sliding cycles after which it remains almost constant. 3.3. Weight losses
Fig. 5. The evolution of the weight losses versus the number of sliding cycles for polyester coatings charged with graphite.
The optical micrographs of the coating surface that contains the four considered volume ratios of graphite particles is shown in Fig. 3. It can be seen that the graphite particles are homogeneously dispersed in the polyester matrix. 3.2. Friction coefficient The evolution of the friction coefficient with the number of sliding cycles for the four considered volume ratios of graphite particles are shown in Fig. 4. The effect of the graphite friction reducing phase is well appreciated in this figure. In fact, after 1000 sliding cycles for
For the four considered graphite volume ratios, evolutions of the weight losses with the number of sliding cycles are reported in Fig. 5. The effect of the graphite wear reducing phase is well underlined in this figure. Indeed after 1000 sliding cycles for example, the wear losses fall from 3 mg to about 0.6 mg when the graphite volume ratio increases from 0% to 35%. As for the friction behavior, no significant difference for the wear behavior was obtained between the 15% and 25% graphite volume ratios. Nevertheless, a much large wear decrease was obtained for the highest graphite volume ratio. 3.4. Wear mechanisms The analysis of the wear mechanisms has been developed on the basis of optical micrograph of the wear scar (Fig. 6). For the 0% graphite volume ratio (Fig. 6a), the wear scar width increases as the number of sliding cycles increases and the great part of wear particles is removed from the contact stripe. Severe wear mechanism is activated in this case as it was confirmed by the high mass losses
Fig. 6. Typical optical micrographs of the coating worn surfaces; (a): polyester with 0% by volume graphite, (b): polyester with 15% by volume graphite, (c): polyester with 25% by volume graphite, and (d): polyester with 35% by volume graphite (arrow indicates the sliding direction).
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Fig. 7. SEM images of the coating worn surfaces after 1000 sliding cycles; (a): polyester with 0% by volume graphite, (b): polyester with 15% by volume graphite, (c): polyester with 25% by volume graphite, and (d): polyester with 35% by volume graphite.
attained after 1000 sliding cycles (Fig. 5). Incorporating graphite particles in the polyester coating changes the wear mechanism (Fig. 6b, c, and d). In case, we assist to the development of a third body which cover the sliding stripe as the number of sliding cycles increases. Graphite was a potential candidate of lubricants, which could also form a transfer film on the sliding counterpart [26,27]. Graphite had a layer structure (carbon layer) in which the atoms were arranged in a hexagonal unit cell within each layer [28]. These layers are linked by weak Van der Waals bonds, which can be easily broken by shear force under sliding conditions. This is way the mass losses have been seriously reduced for the 35% graphite volume ratio as compared with the 0% graphite volume ratio. For the 35% graphite volume ratio, the third body develops on the whole sliding stripe (Fig. 6d) while only the central part of the sliding band is covered by wear particles for the 15% and 25% graphite volume ratio (Fig. 6b and c). That is why similar friction and wear behavior have been obtained for coating with 15% and 25% graphite volume ratio. For a better understanding of the wear mechanisms, the analysis of the friction zone by scanning electron microscope (SEM) were done after 1000 cycles. After the metallization of the wear scar, the SEM micrographs obtained for the polyester coatings containing the four considered graphite volume ratio are represented in Fig. 7. For the 0% graphite volume ratio we can note a deformation of the surface asperities by the antagonist which generate the detachment of particles and the formation of cavities (Fig. 7a). This can affirm that an adhesive wear mechanism is activated. The severity of this wear mechanism is attenuated as graphite volume ratio increases. Cavities development was quasi absent in the case of the polyester coating containing 35% graphite volume ratio (Fig. 7d). 4. Conclusions In this study polyester coatings incorporating different graphite volume ratios were deposited on aluminum substrate using the
spraying technique. Coatings were studied against sliding friction by using a high chromium steel ball antagonist and a specific reciprocating tribometer. The following conclusions can be drawn: • The best adhesion is obtained in the case of polyester coatings without solid lubricants. The particles of solid lubricants decrease the adhesion of our coatings; • the incorporation of graphite particles in the polyester coating reduces considerably friction and wear; • similar friction and wear behavior have been identified for polyester coatings with 15% and 25% graphite volume ratio; • the best friction and wear properties are obtained for polyester coating with 35% graphite volume ratio; • for higher graphite volume ratio, plenteous graphite flakes spread symmetrically on the sliding surface, which reduced the ‘direct contact’ between the steel counterpart and composites; and • optical microscope and scanning electron microscope analysis of the polyester coatings wear scars revealed an adhesive wear mechanism for the four considered graphite volume ratios. References [1] C. Winandy, Editor's Note in Automotive Light Metals 1–2, First Global Media Group, New Jersey, USA, December 2000, p. 9. [2] S. Bahadur, D. Gong, Wear 158 (1992) 41. [3] J. Bijwe, J. Indumathi, J.J. Rajesh, M. Fahim, Wear 249 (2001) 715. [4] Z. Zhang, C. Breidt, L. Chang, K. Haupert, K. Friedrich, Composites Part A 35 (2004) 1385. [5] M.H. Cho, S. Bahadur, A.K. Pogosian, Wear 258 (2005) 1825. [6] J.A. Williams, J.H. Morris, A. Ball, Tribol. Int. 30 (1997) 663. [7] C. Schwarts, S. Bahadur, Wear 251 (2001) 1532. [8] Q. Zhao, S. Bahadur, Wear 217 (1998) 62. [9] S. Bahadur, L. Zhang, J.W. Anderegg, Wear 203 (1997) 464. [10] J.V. Vort, S. Bahadur, Wear 181 (1995) 212. [11] K.S. Bhabani, J. Bijwe, Wear 253 (2002) 787. [12] R. Gadow, D. Sherer, Surf. Coat. Technol. 151–152 (2002) 471. [13] J. Xu, M.H. Zhu, Z.R. Zhou, Thin Solid Films 475 (2004) 320. [14] M. Barletta, V. Tagliaferri, Surf. Coat. Technol. 200 (14–15) (2006) 4282.
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