- Email: [email protected]
Erosive wear of plasma electrolytic oxidation layers on aluminium alloy 6061
Wear 301 (2013) 434–441
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Erosive wear of plasma electrolytic oxidation layers on aluminium alloy 6061 ˜ o a, R.D. Mercado-Solis a,n, R. Cola´s a,b, A. Pe´rez a, J. Talamantes c, A. Velasco c M. Trevin a
´nica y Ele´ctrica, Universidad Auto ´s de los Garza, Nuevo Leo ´noma de Nuevo Leo ´n, San Nicola ´n, Me ´xico Facultad de Ingenierı´a Meca ´n Investigacio ´n y Desarrollo en Ingenierı´a y Tecnologı´a, Universidad Auto ´noma de Nuevo Leo ´n, Apodaca, Nuevo Leo ´n, Me´xico Centro de Innovacio c ´n, Me´xico Nemak, S.A. de C.V. Garcı´a, Nuevo Leo b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 September 2012 Received in revised form 3 December 2012 Accepted 8 December 2012 Available online 29 December 2012
Plasma electrolytic oxidation (PEO) is a process that converts the surface of light metals and alloys into a ceramic layer. In aluminium alloys, layers produced by PEO are mainly composed of oxides and other compounds which are harder, therefore, more suitable for tribological applications than the underlying base metal. The aim of this work was to study the erosive wear of PEO layers of different thicknesses and to compare it with the bare aluminium alloy 6061 base metal. PEO was conducted to produce layers 100, 125 and 150 mm thick that were characterised by SEM, XRD, microhardness and stylus profiling prior to testing. Multiple impact angle (201 to 901) erosion tests were performed in a sandblast type machine that simulates the sand blowing process to produce sand cores for castings. The erodent material was resin-bonded silica sand, jetted by compressed air at 0.69 and 1.38 bar producing average particle impact velocities of 6 and 10 m/s, respectively. In most cases, PEO layers wore less than the bare aluminium alloy. For the 6 m/s impact velocity, the 150 mm layer experienced quantitatively lower wear rates than the 125 mm and the 100 mm. On the contrary, for the 10 m/s impact velocity the 150 mm layer showed higher wear than the 125 mm and the 100 mm. Optical, scanning electron and confocal white light microscopy were used to elucidate the erosive wear mechanisms taking place; which are discussed in this paper. & 2012 Elsevier B.V. All rights reserved.
Keywords: Plasma electrolytic oxidation (PEO) Aluminium alloy 6061 Erosive wear
1. Introduction Aluminium alloys are materials of choice in many engineering applications because of their excellent combination of high specific strength, ductility, thermal conductivity and corrosion resistance. Nevertheless, due to their relative softness and low wear resistance, aluminium alloys are not generally employed in tribological applications. Plasma electrolytic oxidation (PEO) is a novel electrochemical process that converts the surface of light metals (aluminium, titanium and magnesium) into a thick oxide layer which is appreciably harder than the underlying base metal. In principle, PEO is similar to hard anodizing, but higher voltages are applied in the former to exceed the dielectric breakdown potential, thus giving rise to localised plasma discharges across the oxide layer. Due to the high temperatures and pressures involved in the reactions, a continuous process of melting, solidification, sintering and densification of the oxide layer takes place as it grows [1,2]. In aluminium alloys, PEO layers are typically 100 mm thick comprising a two sub-layer structure: an inner (compact) sublayer that is adjacent to the base metal, and an outer (porous)
n
Corresponding author. Tel.: þ52 8114 9203 61. E-mail address: [email protected] (R.D. Mercado-Solis).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.011
sub-layer. The inner sub-layer is a dense mixture of crystalline g-Al2O3 and a-Al2O3 phases possessing good mechanical strength, strong adhesion and high hardness ( 41500 HV), while the outer sub-layer mainly contains g-Al2O3 and is less hard than the inner layer [2,3]. The physical and mechanical properties of the layers suggest that PEO is a suitable surface treatment for tribological applications requiring high surface hardness and low bulk densities. From the tribological point of view, the main advantage foreseen in plasma electrolytic oxidation of aluminium is the possibility for designing new tribosystems that will benefit from the attractive combination of the lightness, ductility and thermal conductivity of the bulk aluminium with the high hardness of its surface. In this regard, several studies have reported improved sliding and abrasive wear performance of PEO layers on pure aluminium [4] and aluminium alloys [5–9]. Nevertheless, erosive wear investigations are still limited, for there is virtually no background information besides references [9] and [10]; though, these references are not related to aluminium alloy 6061, a material of high technological importance in automotive and aerospace applications due to its good mechanical properties, workability and weldability. It was reported in both papers that the PEO layers outperformed the erosion resistance of the bare aluminium surface and equalled that of bulk alumina and alumina-sprayed coatings. However, more work still needs to be done to better understand the erosive wear behaviour of PEO
˜o et al. / Wear 301 (2013) 434–441 M. Trevin
435
layers on aluminium alloys and to further enhance their tribological performance. The aim of this work was to study the erosive wear of PEO layers of various thicknesses and to compare it with the bare aluminium alloy 6061 base metal.
2. Experimental procedure A series of rectangular test specimens (100 50 10 mm) of aluminium alloy 6061 were surface-treated by plasma electrolytic oxidation (400–600 VAC, 50 Hz, electrolytes KOH and Na2SiO4). The process time was varied in order to obtain oxide layers 100, 125 and 150 mm thick. The chemical composition analysis (%wt.) of the aluminium alloy 6061 base metal was Al/1.0Mg/0.7Si/ 0.16Cu/0.10Fe/0.04Cr/0.01Zn. Surface and sub-surface structures of the PEO layers were observed using a scanning electron microscope under the backscattered electrons mode. In order to confirm the phases present in the oxide layers prior to wear testing, XRD analyses were ˚ for performed using a CuKa radiation source (l ¼ 1.54059 A) diffraction angles 2y from 51 to 901, step size 0.051 and dwell time 2 s. The average surface roughness (Ra) of the oxide layers was measured using a stylus roughness tester with diamond tip radius of 2 mm and a 0.75 mN measuring force. The reported values correspond to the average of six Ra measurements, 10 mm long. The Ra values of the layers were measured for reporting purposes only and for comparisons with previously reported values for similar types of PEO layers. Vickers micro hardness measurements (50 g, 15 s) of the layers and the base metal were performed on a polished cross-section of the samples (the averages of five measurements were reported). Since no differences in hardness as a function of layer thickness were expected beforehand, hardness was not considered as an experimental variable in this work. Rather, hardness was measured for reporting purposes only and for comparisons with previously reported values for similar types of PEO layers and to provide a sensible property comparison between the soft base metal and the much harder oxide layer. Erosive wear tests were performed at room temperature using the sandblasting-type machine shown schematically in Fig. 1. A pressurised mixture of dry air ( 40 1C dew point) and erodent is created inside the sealed chamber and subsequently jetted onto the test specimen located 50 mm away from the tip of the nozzle. In this study, impingement angles of 201, 301, 401, 601 and 901 were evaluated at pressures of 0.69 and 1.38 bar producing average particle velocities of 6 and 10 m/s, respectively, measured via frame-by-frame video imaging. The inner diameter
Fig. 2. Silica sand mesh 50, roundness factor 0.91.
Table 1 Sieve analysis of the silica sand.
mm 425 300 212 150 106 75 53
Mesh
%
40 50 70 100 140 200 270
7.29 33.17 35.93 17.95 5.13 0.47 0.06
of the nozzle was 12.7 mm and the mass flow rate of the erodent was 0.769 and 1.141 kg/s for the 6 and 10 m/s impact velocities, respectively. The test specimens were weighed on an analytical balance (0.1 mg readability) before and after each test. Thus, erosion was calculated from the mass loss of the eroded material (mg) divided by the total mass of erodent (kg) [11]. In each test, a total of 5 kg of resin-bonded silica sand (0.8% wt. resin) were employed as erodent. The typical morphology of the silica sand is shown in Fig. 2, while the average size distributions (from sieve analyses) are presented in Table 1. Results demonstrated that nearly 94% of the particles were mesh size 100 (150 mm) or coarser and about 0.5% were fine aggregates. The roundness factor equation [11] was used to estimate the roundness of the silica sand grains: F¼ 4pA/p2; where F is the roundness factor, A is the projected surface area of the grain, and p is the perimeter of a circle of area equal to A. The more the value of F approaches 1, the rounder the grain. In this study, the average roundness factor of the sand grains was 0.91. Finally, in order to elucidate the erosive wear mechanisms, the scars were observed by optical, scanning electron and confocal white light microscopy, and further analysed by X-ray diffraction to establish possible structural variations of the surface.
3. Results and discussion
Fig. 1. Schematic of the sandblast type erosive wear test machine.
The typical surface and sub-surface structures of the PEO layers under study are shown in Fig. 3. For all three thicknesses, the two sub-layer structure was identified. The inner sub-layer presented a relatively compact structure without observable flaws whereas the outer sub-layer exhibited a large proportion of porosity and crater-like structures, which are known to be inherent to the PEO conversion process [12].
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˜o et al. / Wear 301 (2013) 434–441 M. Trevin
Fig. 3. Typical surface and sub-surface microstructures of the PEO layers: (a,b) 100 mm; (c,d) 125 mm; (e,f) 150 mm.
Fig. 4. X-ray diffraction data of the PEO layers before testing.
X-ray diffraction data of beam intensity versus diffraction angle 2y is presented in Fig. 4. These results indicate that the oxide layer is mainly composed of mullite, gamma alumina (g-Al2O3), alpha alumina (a-Al2O3) and amorphous alumina (2y 151 to 351). The low signal-to-noise ratio observed in the diffractogram may be attributed to the presence of the amorphous phase, which has been reported to comprise up to 30% of the oxide layer [13]. The average surface roughness (Ra) was 5.39, 6.39 and 6.66 mm for the 100, 125 and 150 mm layers, respectively. These Ra values are in agreement with previously reported data for similar PEO layers [10]. The microhardness value (HV50) for the aluminium alloy 6061 was 10973 whereas for the PEO layers 1556 711. These hardness values are in good agreement with some previously reported data for similar PEO layers [10].
The macroscopic appearance of the erosive wear scars of the PEO layers are shown in the images of Figs. 5 and 6 for impact velocities of 10 and 6 m/s, respectively (the flow direction of the erodent was left to right, except for the 901 angle). Selective surface wear was observed in most cases since some zones were more worn than others, resulting in a characteristic island-like wear pattern. The lighter areas correspond to more deeply worn zones within the oxide layer thickness, while the shiny areas correspond to the exposed aluminium (seen more extensively in the 10 m/s impact velocity tests). Fig. 7 provides a direct comparison between the cured erodent materials after testing. For the 10 m/s impact velocity tests (Fig. 7a), numerous loose uncoated particles were observed within the erodent, whereas for the 6 m/s impact velocity tests (Fig. 7b) the particles remained better coated and bound. These
˜o et al. / Wear 301 (2013) 434–441 M. Trevin
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Fig. 5. Erosive wear scars produced at the 10 m/s impact velocity tests. Each image corresponds to an area size 20 15 mm2.
Fig. 6. Erosive wear scars produced at the 6 m/s impact velocity tests. Each image corresponds to an area size 20 15 mm2.
Fig. 7. Typical samples of the cured erodent materials after testing for impact velocities of: (a) 10 m/s; (b) 6 m/s.
observations suggest that, for the 10 m/s impact velocity, a partial depletion of the binder occurred before impacting the test surface due to the high drag forces of the pressurised air, thus leaving some particles uncoated which did not bind to the other particles after the resin was cured. By correlating the three dimensional surface height data of Fig. 8, an almost complete removal of the outer sub-layer became evident, while some portions of the inner layer stood out partially and others were completely worn away, thus exposing the underlying aluminium. XRD data of the worn test samples is presented in Fig. 9. By comparing the diffractograms of Figs. 4 and 9 it became evident that the amorphous phase, the g-Al2O3 phase and the mullite were no longer present after wear, whereas the a-Al2O3 phase persisted and aluminium was detected to a greater extent. Also, the presence of SiO2 (silica sand) was detected at 2y 171, suggesting the embedment of erodent particles within the wearing surfaces. These results in conjunction with the wear scar
observations, suggest that the a-Al2O3 phase contained in the inner sub-layer better resisted erosive wear than the other phases that were originally present in the PEO oxide layer. Experimental data comparisons of erosion versus impact angle for the three PEO layer thicknesses and the bare aluminium alloy are presented in Fig. 10. Ductile type erosive wear behaviour was predominant in the 10 m/s impact velocity tests since the maxima of erosion occurred at impact angle 301 and the minima at 901 (Fig. 10a). According to the 10 m/s erosion data presented in Fig. 10a, for all impact angles, except the 201, the 150 mm thick layers underwent more wear than the other layers and the bare aluminium, whereas the 100 and 125 mm layers exhibited less wear for angles 301 and higher. These results may be explained in terms of the differences in thickness of the outer sub-layers and their relative proportion with respect to the total thickness of the PEO layers. It can be observed in Fig. 3 that the brittle and more porous outer sub-layer comprises about 60% of the total thickness in the 150 mm layer, whereas it is only about 40% and 25% in the
438
˜o et al. / Wear 301 (2013) 434–441 M. Trevin
Fig. 8. Three dimensional surface height data of the erosive wear scars, 10 m/s impact velocity, 601 impact angle: (a) 100 mm; (b) 125 mm; (c) 150 mm. Each image corresponds to an area size 15 12 mm2.
125 mm and 100 mm layers, respectively. Therefore, since the entire outer sub-layer was effectively removed (as presented in Figs. 5–7), then larger volumes of material were therefore eroded from the 150 mm layers, resulting in higher mass losses. At low impact angles (201), the erosive particles will substantially decelerate due to their entrapment within surface pores as it became evident from the XRD analyses of Fig. 9. This resulted in a less effective material removal mechanism than the brittle fracture mode seen at higher impact angles, or the cutting mode which could be expected for less rough surfaces. Because the 150 mm PEO layer was the roughest, its comparatively lower wear at the 201 impact angle may be attributed to its greater potential to entrap erodent particles. For the 6 m/s impact tests no clear tendency towards a ductile or brittle type behaviour was observed (Fig. 10b). The erosion maxima mostly occurred at impact angle 401 (except for the 150 mm layer) and the minima at 901. In contrast with the 10 m/s tests, at 6 m/s the 150 mm layer underwent less wear than the other layers and the bare aluminium, except for impact angle 201, where almost the same erosion was measured for all three layers (Fig. 10b). This behaviour may be explained in terms of the higher resilience of the resin-coated silica sand particles. Since the particles remained coated, they
were capable of admitting some proportion of the impact energy that would otherwise be fully transmitted to the target surface, thus resulting in lower material removal rates. As seen in Fig. 6, more steady erosion of the PEO layers was present in the 6 m/s impact velocity test specimens. Therefore, the lower mass loss in the 150 mm thick PEO layers may be attributed to their greater remnant thickness under the steady erosive wear conditions that were characteristic of the 6 m/s impact speed. Remarkably, for the 401 impact angle, the erosion of the 150 mm layer was nearly half as that of the other three test samples. Similarly, this may be explained in terms of the greater remnant thickness of the 150 mm thick PEO layer under steady erosive wear conditions. Overall, the higher amount of erosion which resulted from the higher impact velocity tests (10 m/s) was mainly due to the fact that the erodent particles possess greater kinetic energy which was transformed into deformation energy upon impact, thus resulting in higher material removal rates. However, the predominance of ductile -type erosive behaviour of such hard and brittle oxides cannot be fully explained from the erosion data alone. The wear scar analyses confirm that a considerable proportion of the brittle phases mullite, amorphous Al2O3 and g-Al2O3 contained within the oxide layer may had been readily removed
˜o et al. / Wear 301 (2013) 434–441 M. Trevin
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Fig. 9. X-ray diffraction data of the PEO layers after testing.
125
150
100
Al
125
150
Al
18 16 Erosion (mg/kg)
Erosion (mg/kg)
100
30 28 26 24 22 20 18 16 14 12 10
14 12 10 8 6 4
20°
30°
40° 60° Impact angle
90°
20°
30°
40° 60° Impact angle
90°
Fig. 10. Erosion versus impact angle for impact velocities of: (a) 10 m/s; (b) 6 m/s.
during the tests. The remnant layer material was mostly the a-Al2O3 phase which is known to possess higher adherence to the base metal than the other phases. Therefore, subsequent impacts inflicted more wear in the already exposed aluminium than in the oxide layer, thus resulting in the observed ductile-type erosive behaviour. The erosion data obtained in this work may not be so directly compared with results found in the literature [9,10,14] because the erodent in this work was not loose sand or slurry, but a mixture of sand and resin binder (more representative of the blowing process to produce sand cores for castings). In this sense, the mixture possesses higher resilience than the loose sand and, therefore, it is capable of admitting some proportion of the impact energy that would otherwise be fully transmitted to the target surface, thus resulting in lower material removal rates. Scanning electron micrographs of typical wear surfaces are presented in Figs. 11 and 12 for impact velocities of 10 and 6 m/s, respectively. In general, due to their considerably high hardness
( 1550HV50) and brittleness, layer fracture, delamination and spalling were the dominant wear mechanisms at higher impact angles (Fig. 11a,b,d,e and g), whereas at lower impact angles lateral lip fracture and chipping were more evident (Fig. 11c,f and i). Fig. 11a shows no remnant of outer sub-layer, while a dominant sub-surface crack parallel to the surface was observed within the inner sub-layer. Some portions of the right side of the layer had already completely spalled off. A large area of exposed aluminium can be observed in Fig. 11b since a considerable volume of oxide flaked off due to layer fracture. Subsequent impacts on the bare aluminium produced lower erosion rates by ploughing the soft surface. Similar oxide fracture can be observed in Fig. 11d. Craters (instead of ploughs) are more commonly developed due to the perpendicular impact of the erodent (bottom-left area of the micrograph). Less severe erosion resulted from the 6 m/s impact velocity with respect to the 10 m/s impact velocity and more uniformly worn surfaces were observed from these tests (Fig. 12).
440
˜o et al. / Wear 301 (2013) 434–441 M. Trevin
Fig. 11. Scanning electron micrographs of the erosive wear scars, 10 m/s impact velocity.
Fig. 12. Scanning electron micrographs of the erosive wear scars, 6 m/s impact velocity.
4. Conclusions In this work, aluminium alloy 6061 was surface-treated by plasma electrolytic oxidation (PEO) to obtain oxide layers 100, 125 and 150 mm thick with the aim of studying their erosive wear behaviour. The PEO layers comprised mullite, gamma alumina (g-Al2O3), alpha alumina (a-Al2O3) and amorphous alumina. The surface microstructure exhibited a large proportion of porosity and crater-like features, which negatively affected its resistance against erosive wear. As expected, the 10 m/s impact velocity resulted in more amounts of wear than the 6 m/s impact velocity. In general, unexpected ductile type behaviour was observed for the 10 m/s impact velocity. This was mainly attributed to the rapid removal of the brittle phases of the outer sub-layer by fracture and the exposure of the aluminium bare metal which then wore steadily at a lower rate than the PEO layers. Less severe
erosion resulted from the 6 m/s impact velocity with respect to the 10 m/s impact and more uniformly worn surfaces were observed from these tests. For the 6 m/s impact velocity, the 150 mm layer experienced quantitatively lower wear rates than the 125 mm and the 100 mm, whereas for the 10 m/s impact velocity the 150 mm layer showed higher wear than the 125 mm and the 100 mm.
Acknowledgements The authors wish to thank Facultad de Ingenieria Mecanica y Electrica (FIME), Universidad Autonoma de Nuevo Leon (UANL), Consejo Nacional de Ciencia y Tecnologia (CONACYT) and Nemak S.A. de C.V. for their support during the execution of this work.
˜o et al. / Wear 301 (2013) 434–441 M. Trevin
References [1] J.A. Curran, T.W. Clyne, Acta Materialia 54 (2006) 1985. [2] J.M. Wheeler, J.A. Curran, S. Shrestha, Surface and Coatings Technology 207 (2012) 480. [3] W.C. Gu, G.H. Lv, H. Chen, G.L. Chen, W.R. Feng, S.Z. Yang, Materials Science and Engineering: A 447 (2007) 158. [4] Yongfeng Jiang, Yingyue Zhang, Yefeng Bao, Ke Yang, Wear 271 (2011) 1677. ˜ o, N.F. Garza-Montes-de-Oca, A. Pe´rez, M.A.L. Herna´ndez-Rodrı´guez, [5] M. Trevin A. Jua´rez, R. Cola´s, Surface and Coatings Technology 206 (2012) 2213. [6] U. Malayoglu, K.C. Tekin, U. Malayoglu, S. Shrestha, Materials Science and Engineering: A 528 (2011) 7451. [7] E. Arslan, Y. Totik, E.E. Demirci, Y. Vangolu, A. Alsaran, I. Efeoglu, Surface and Coatings Technology 204 (2009) 829.
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[8] G. Sabatini, L. Ceschini, C. Martini, J.A. Williams, I.M. Hutchings, Materials Design 31 (2010) 816. [9] L. Rama Krishna, K.R.C. Somaraju, G. Sundararajan, Surface and Coatings Technology 163–164 (2003) 484. [10] R.C. Barik, J.A. Wharton, R.J.K. Wood, K.R. Stokes, R.L. Jones, Surface and Coatings Technology 199 (2005) 158. [11] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold Ltd, 1992. [12] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Surface and Coatings Technology 201 (2007) 8746. [13] J.A. Curran, T.W. Clyne, Surface and Coatings Technology 199 (2005) 177. [14] J.R. Laguna-Camacho, L.A. Cruz-Mendoza, J.C. Anzelmetti-Zaragoza, A. Marquina-Chavez, M. Vite-Torres, J. Martinez-Trinidad, Progress in Organic Coatings. 74 (2012) 750.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Erosive wear of plasma electrolytic oxidation layers on aluminium alloy 6061 ˜ o a, R.D. Mercado-Solis a,n, R. Cola´s a,b, A. Pe´rez a, J. Talamantes c, A. Velasco c M. Trevin a
´nica y Ele´ctrica, Universidad Auto ´s de los Garza, Nuevo Leo ´noma de Nuevo Leo ´n, San Nicola ´n, Me ´xico Facultad de Ingenierı´a Meca ´n Investigacio ´n y Desarrollo en Ingenierı´a y Tecnologı´a, Universidad Auto ´noma de Nuevo Leo ´n, Apodaca, Nuevo Leo ´n, Me´xico Centro de Innovacio c ´n, Me´xico Nemak, S.A. de C.V. Garcı´a, Nuevo Leo b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 September 2012 Received in revised form 3 December 2012 Accepted 8 December 2012 Available online 29 December 2012
Plasma electrolytic oxidation (PEO) is a process that converts the surface of light metals and alloys into a ceramic layer. In aluminium alloys, layers produced by PEO are mainly composed of oxides and other compounds which are harder, therefore, more suitable for tribological applications than the underlying base metal. The aim of this work was to study the erosive wear of PEO layers of different thicknesses and to compare it with the bare aluminium alloy 6061 base metal. PEO was conducted to produce layers 100, 125 and 150 mm thick that were characterised by SEM, XRD, microhardness and stylus profiling prior to testing. Multiple impact angle (201 to 901) erosion tests were performed in a sandblast type machine that simulates the sand blowing process to produce sand cores for castings. The erodent material was resin-bonded silica sand, jetted by compressed air at 0.69 and 1.38 bar producing average particle impact velocities of 6 and 10 m/s, respectively. In most cases, PEO layers wore less than the bare aluminium alloy. For the 6 m/s impact velocity, the 150 mm layer experienced quantitatively lower wear rates than the 125 mm and the 100 mm. On the contrary, for the 10 m/s impact velocity the 150 mm layer showed higher wear than the 125 mm and the 100 mm. Optical, scanning electron and confocal white light microscopy were used to elucidate the erosive wear mechanisms taking place; which are discussed in this paper. & 2012 Elsevier B.V. All rights reserved.
Keywords: Plasma electrolytic oxidation (PEO) Aluminium alloy 6061 Erosive wear
1. Introduction Aluminium alloys are materials of choice in many engineering applications because of their excellent combination of high specific strength, ductility, thermal conductivity and corrosion resistance. Nevertheless, due to their relative softness and low wear resistance, aluminium alloys are not generally employed in tribological applications. Plasma electrolytic oxidation (PEO) is a novel electrochemical process that converts the surface of light metals (aluminium, titanium and magnesium) into a thick oxide layer which is appreciably harder than the underlying base metal. In principle, PEO is similar to hard anodizing, but higher voltages are applied in the former to exceed the dielectric breakdown potential, thus giving rise to localised plasma discharges across the oxide layer. Due to the high temperatures and pressures involved in the reactions, a continuous process of melting, solidification, sintering and densification of the oxide layer takes place as it grows [1,2]. In aluminium alloys, PEO layers are typically 100 mm thick comprising a two sub-layer structure: an inner (compact) sublayer that is adjacent to the base metal, and an outer (porous)
n
Corresponding author. Tel.: þ52 8114 9203 61. E-mail address: [email protected] (R.D. Mercado-Solis).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.011
sub-layer. The inner sub-layer is a dense mixture of crystalline g-Al2O3 and a-Al2O3 phases possessing good mechanical strength, strong adhesion and high hardness ( 41500 HV), while the outer sub-layer mainly contains g-Al2O3 and is less hard than the inner layer [2,3]. The physical and mechanical properties of the layers suggest that PEO is a suitable surface treatment for tribological applications requiring high surface hardness and low bulk densities. From the tribological point of view, the main advantage foreseen in plasma electrolytic oxidation of aluminium is the possibility for designing new tribosystems that will benefit from the attractive combination of the lightness, ductility and thermal conductivity of the bulk aluminium with the high hardness of its surface. In this regard, several studies have reported improved sliding and abrasive wear performance of PEO layers on pure aluminium [4] and aluminium alloys [5–9]. Nevertheless, erosive wear investigations are still limited, for there is virtually no background information besides references [9] and [10]; though, these references are not related to aluminium alloy 6061, a material of high technological importance in automotive and aerospace applications due to its good mechanical properties, workability and weldability. It was reported in both papers that the PEO layers outperformed the erosion resistance of the bare aluminium surface and equalled that of bulk alumina and alumina-sprayed coatings. However, more work still needs to be done to better understand the erosive wear behaviour of PEO
˜o et al. / Wear 301 (2013) 434–441 M. Trevin
435
layers on aluminium alloys and to further enhance their tribological performance. The aim of this work was to study the erosive wear of PEO layers of various thicknesses and to compare it with the bare aluminium alloy 6061 base metal.
2. Experimental procedure A series of rectangular test specimens (100 50 10 mm) of aluminium alloy 6061 were surface-treated by plasma electrolytic oxidation (400–600 VAC, 50 Hz, electrolytes KOH and Na2SiO4). The process time was varied in order to obtain oxide layers 100, 125 and 150 mm thick. The chemical composition analysis (%wt.) of the aluminium alloy 6061 base metal was Al/1.0Mg/0.7Si/ 0.16Cu/0.10Fe/0.04Cr/0.01Zn. Surface and sub-surface structures of the PEO layers were observed using a scanning electron microscope under the backscattered electrons mode. In order to confirm the phases present in the oxide layers prior to wear testing, XRD analyses were ˚ for performed using a CuKa radiation source (l ¼ 1.54059 A) diffraction angles 2y from 51 to 901, step size 0.051 and dwell time 2 s. The average surface roughness (Ra) of the oxide layers was measured using a stylus roughness tester with diamond tip radius of 2 mm and a 0.75 mN measuring force. The reported values correspond to the average of six Ra measurements, 10 mm long. The Ra values of the layers were measured for reporting purposes only and for comparisons with previously reported values for similar types of PEO layers. Vickers micro hardness measurements (50 g, 15 s) of the layers and the base metal were performed on a polished cross-section of the samples (the averages of five measurements were reported). Since no differences in hardness as a function of layer thickness were expected beforehand, hardness was not considered as an experimental variable in this work. Rather, hardness was measured for reporting purposes only and for comparisons with previously reported values for similar types of PEO layers and to provide a sensible property comparison between the soft base metal and the much harder oxide layer. Erosive wear tests were performed at room temperature using the sandblasting-type machine shown schematically in Fig. 1. A pressurised mixture of dry air ( 40 1C dew point) and erodent is created inside the sealed chamber and subsequently jetted onto the test specimen located 50 mm away from the tip of the nozzle. In this study, impingement angles of 201, 301, 401, 601 and 901 were evaluated at pressures of 0.69 and 1.38 bar producing average particle velocities of 6 and 10 m/s, respectively, measured via frame-by-frame video imaging. The inner diameter
Fig. 2. Silica sand mesh 50, roundness factor 0.91.
Table 1 Sieve analysis of the silica sand.
mm 425 300 212 150 106 75 53
Mesh
%
40 50 70 100 140 200 270
7.29 33.17 35.93 17.95 5.13 0.47 0.06
of the nozzle was 12.7 mm and the mass flow rate of the erodent was 0.769 and 1.141 kg/s for the 6 and 10 m/s impact velocities, respectively. The test specimens were weighed on an analytical balance (0.1 mg readability) before and after each test. Thus, erosion was calculated from the mass loss of the eroded material (mg) divided by the total mass of erodent (kg) [11]. In each test, a total of 5 kg of resin-bonded silica sand (0.8% wt. resin) were employed as erodent. The typical morphology of the silica sand is shown in Fig. 2, while the average size distributions (from sieve analyses) are presented in Table 1. Results demonstrated that nearly 94% of the particles were mesh size 100 (150 mm) or coarser and about 0.5% were fine aggregates. The roundness factor equation [11] was used to estimate the roundness of the silica sand grains: F¼ 4pA/p2; where F is the roundness factor, A is the projected surface area of the grain, and p is the perimeter of a circle of area equal to A. The more the value of F approaches 1, the rounder the grain. In this study, the average roundness factor of the sand grains was 0.91. Finally, in order to elucidate the erosive wear mechanisms, the scars were observed by optical, scanning electron and confocal white light microscopy, and further analysed by X-ray diffraction to establish possible structural variations of the surface.
3. Results and discussion
Fig. 1. Schematic of the sandblast type erosive wear test machine.
The typical surface and sub-surface structures of the PEO layers under study are shown in Fig. 3. For all three thicknesses, the two sub-layer structure was identified. The inner sub-layer presented a relatively compact structure without observable flaws whereas the outer sub-layer exhibited a large proportion of porosity and crater-like structures, which are known to be inherent to the PEO conversion process [12].
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Fig. 3. Typical surface and sub-surface microstructures of the PEO layers: (a,b) 100 mm; (c,d) 125 mm; (e,f) 150 mm.
Fig. 4. X-ray diffraction data of the PEO layers before testing.
X-ray diffraction data of beam intensity versus diffraction angle 2y is presented in Fig. 4. These results indicate that the oxide layer is mainly composed of mullite, gamma alumina (g-Al2O3), alpha alumina (a-Al2O3) and amorphous alumina (2y 151 to 351). The low signal-to-noise ratio observed in the diffractogram may be attributed to the presence of the amorphous phase, which has been reported to comprise up to 30% of the oxide layer [13]. The average surface roughness (Ra) was 5.39, 6.39 and 6.66 mm for the 100, 125 and 150 mm layers, respectively. These Ra values are in agreement with previously reported data for similar PEO layers [10]. The microhardness value (HV50) for the aluminium alloy 6061 was 10973 whereas for the PEO layers 1556 711. These hardness values are in good agreement with some previously reported data for similar PEO layers [10].
The macroscopic appearance of the erosive wear scars of the PEO layers are shown in the images of Figs. 5 and 6 for impact velocities of 10 and 6 m/s, respectively (the flow direction of the erodent was left to right, except for the 901 angle). Selective surface wear was observed in most cases since some zones were more worn than others, resulting in a characteristic island-like wear pattern. The lighter areas correspond to more deeply worn zones within the oxide layer thickness, while the shiny areas correspond to the exposed aluminium (seen more extensively in the 10 m/s impact velocity tests). Fig. 7 provides a direct comparison between the cured erodent materials after testing. For the 10 m/s impact velocity tests (Fig. 7a), numerous loose uncoated particles were observed within the erodent, whereas for the 6 m/s impact velocity tests (Fig. 7b) the particles remained better coated and bound. These
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Fig. 5. Erosive wear scars produced at the 10 m/s impact velocity tests. Each image corresponds to an area size 20 15 mm2.
Fig. 6. Erosive wear scars produced at the 6 m/s impact velocity tests. Each image corresponds to an area size 20 15 mm2.
Fig. 7. Typical samples of the cured erodent materials after testing for impact velocities of: (a) 10 m/s; (b) 6 m/s.
observations suggest that, for the 10 m/s impact velocity, a partial depletion of the binder occurred before impacting the test surface due to the high drag forces of the pressurised air, thus leaving some particles uncoated which did not bind to the other particles after the resin was cured. By correlating the three dimensional surface height data of Fig. 8, an almost complete removal of the outer sub-layer became evident, while some portions of the inner layer stood out partially and others were completely worn away, thus exposing the underlying aluminium. XRD data of the worn test samples is presented in Fig. 9. By comparing the diffractograms of Figs. 4 and 9 it became evident that the amorphous phase, the g-Al2O3 phase and the mullite were no longer present after wear, whereas the a-Al2O3 phase persisted and aluminium was detected to a greater extent. Also, the presence of SiO2 (silica sand) was detected at 2y 171, suggesting the embedment of erodent particles within the wearing surfaces. These results in conjunction with the wear scar
observations, suggest that the a-Al2O3 phase contained in the inner sub-layer better resisted erosive wear than the other phases that were originally present in the PEO oxide layer. Experimental data comparisons of erosion versus impact angle for the three PEO layer thicknesses and the bare aluminium alloy are presented in Fig. 10. Ductile type erosive wear behaviour was predominant in the 10 m/s impact velocity tests since the maxima of erosion occurred at impact angle 301 and the minima at 901 (Fig. 10a). According to the 10 m/s erosion data presented in Fig. 10a, for all impact angles, except the 201, the 150 mm thick layers underwent more wear than the other layers and the bare aluminium, whereas the 100 and 125 mm layers exhibited less wear for angles 301 and higher. These results may be explained in terms of the differences in thickness of the outer sub-layers and their relative proportion with respect to the total thickness of the PEO layers. It can be observed in Fig. 3 that the brittle and more porous outer sub-layer comprises about 60% of the total thickness in the 150 mm layer, whereas it is only about 40% and 25% in the
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Fig. 8. Three dimensional surface height data of the erosive wear scars, 10 m/s impact velocity, 601 impact angle: (a) 100 mm; (b) 125 mm; (c) 150 mm. Each image corresponds to an area size 15 12 mm2.
125 mm and 100 mm layers, respectively. Therefore, since the entire outer sub-layer was effectively removed (as presented in Figs. 5–7), then larger volumes of material were therefore eroded from the 150 mm layers, resulting in higher mass losses. At low impact angles (201), the erosive particles will substantially decelerate due to their entrapment within surface pores as it became evident from the XRD analyses of Fig. 9. This resulted in a less effective material removal mechanism than the brittle fracture mode seen at higher impact angles, or the cutting mode which could be expected for less rough surfaces. Because the 150 mm PEO layer was the roughest, its comparatively lower wear at the 201 impact angle may be attributed to its greater potential to entrap erodent particles. For the 6 m/s impact tests no clear tendency towards a ductile or brittle type behaviour was observed (Fig. 10b). The erosion maxima mostly occurred at impact angle 401 (except for the 150 mm layer) and the minima at 901. In contrast with the 10 m/s tests, at 6 m/s the 150 mm layer underwent less wear than the other layers and the bare aluminium, except for impact angle 201, where almost the same erosion was measured for all three layers (Fig. 10b). This behaviour may be explained in terms of the higher resilience of the resin-coated silica sand particles. Since the particles remained coated, they
were capable of admitting some proportion of the impact energy that would otherwise be fully transmitted to the target surface, thus resulting in lower material removal rates. As seen in Fig. 6, more steady erosion of the PEO layers was present in the 6 m/s impact velocity test specimens. Therefore, the lower mass loss in the 150 mm thick PEO layers may be attributed to their greater remnant thickness under the steady erosive wear conditions that were characteristic of the 6 m/s impact speed. Remarkably, for the 401 impact angle, the erosion of the 150 mm layer was nearly half as that of the other three test samples. Similarly, this may be explained in terms of the greater remnant thickness of the 150 mm thick PEO layer under steady erosive wear conditions. Overall, the higher amount of erosion which resulted from the higher impact velocity tests (10 m/s) was mainly due to the fact that the erodent particles possess greater kinetic energy which was transformed into deformation energy upon impact, thus resulting in higher material removal rates. However, the predominance of ductile -type erosive behaviour of such hard and brittle oxides cannot be fully explained from the erosion data alone. The wear scar analyses confirm that a considerable proportion of the brittle phases mullite, amorphous Al2O3 and g-Al2O3 contained within the oxide layer may had been readily removed
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Fig. 9. X-ray diffraction data of the PEO layers after testing.
125
150
100
Al
125
150
Al
18 16 Erosion (mg/kg)
Erosion (mg/kg)
100
30 28 26 24 22 20 18 16 14 12 10
14 12 10 8 6 4
20°
30°
40° 60° Impact angle
90°
20°
30°
40° 60° Impact angle
90°
Fig. 10. Erosion versus impact angle for impact velocities of: (a) 10 m/s; (b) 6 m/s.
during the tests. The remnant layer material was mostly the a-Al2O3 phase which is known to possess higher adherence to the base metal than the other phases. Therefore, subsequent impacts inflicted more wear in the already exposed aluminium than in the oxide layer, thus resulting in the observed ductile-type erosive behaviour. The erosion data obtained in this work may not be so directly compared with results found in the literature [9,10,14] because the erodent in this work was not loose sand or slurry, but a mixture of sand and resin binder (more representative of the blowing process to produce sand cores for castings). In this sense, the mixture possesses higher resilience than the loose sand and, therefore, it is capable of admitting some proportion of the impact energy that would otherwise be fully transmitted to the target surface, thus resulting in lower material removal rates. Scanning electron micrographs of typical wear surfaces are presented in Figs. 11 and 12 for impact velocities of 10 and 6 m/s, respectively. In general, due to their considerably high hardness
( 1550HV50) and brittleness, layer fracture, delamination and spalling were the dominant wear mechanisms at higher impact angles (Fig. 11a,b,d,e and g), whereas at lower impact angles lateral lip fracture and chipping were more evident (Fig. 11c,f and i). Fig. 11a shows no remnant of outer sub-layer, while a dominant sub-surface crack parallel to the surface was observed within the inner sub-layer. Some portions of the right side of the layer had already completely spalled off. A large area of exposed aluminium can be observed in Fig. 11b since a considerable volume of oxide flaked off due to layer fracture. Subsequent impacts on the bare aluminium produced lower erosion rates by ploughing the soft surface. Similar oxide fracture can be observed in Fig. 11d. Craters (instead of ploughs) are more commonly developed due to the perpendicular impact of the erodent (bottom-left area of the micrograph). Less severe erosion resulted from the 6 m/s impact velocity with respect to the 10 m/s impact velocity and more uniformly worn surfaces were observed from these tests (Fig. 12).
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Fig. 11. Scanning electron micrographs of the erosive wear scars, 10 m/s impact velocity.
Fig. 12. Scanning electron micrographs of the erosive wear scars, 6 m/s impact velocity.
4. Conclusions In this work, aluminium alloy 6061 was surface-treated by plasma electrolytic oxidation (PEO) to obtain oxide layers 100, 125 and 150 mm thick with the aim of studying their erosive wear behaviour. The PEO layers comprised mullite, gamma alumina (g-Al2O3), alpha alumina (a-Al2O3) and amorphous alumina. The surface microstructure exhibited a large proportion of porosity and crater-like features, which negatively affected its resistance against erosive wear. As expected, the 10 m/s impact velocity resulted in more amounts of wear than the 6 m/s impact velocity. In general, unexpected ductile type behaviour was observed for the 10 m/s impact velocity. This was mainly attributed to the rapid removal of the brittle phases of the outer sub-layer by fracture and the exposure of the aluminium bare metal which then wore steadily at a lower rate than the PEO layers. Less severe
erosion resulted from the 6 m/s impact velocity with respect to the 10 m/s impact and more uniformly worn surfaces were observed from these tests. For the 6 m/s impact velocity, the 150 mm layer experienced quantitatively lower wear rates than the 125 mm and the 100 mm, whereas for the 10 m/s impact velocity the 150 mm layer showed higher wear than the 125 mm and the 100 mm.
Acknowledgements The authors wish to thank Facultad de Ingenieria Mecanica y Electrica (FIME), Universidad Autonoma de Nuevo Leon (UANL), Consejo Nacional de Ciencia y Tecnologia (CONACYT) and Nemak S.A. de C.V. for their support during the execution of this work.
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