Effect of slider materials on the nano-characteristics of the mechanical mixed layer of worn wrought Al-alloys

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1 Jurnal Nanosains & Nanoteknologi ISSN Vol. 2 No.1, Februari 2009 Effect of slider materials on the nano-characteristics of the mechanical mixed layer of worn wrought Al-alloys M.J. Ghazali Advanced Materials Research Group Department of Mechanical & Materials Engineering, Universiti Kebangsaan Malaysia, Bangi, UKM, Malaysia. mariyam@eng.ukm.my Diterima Editor : 9 Mei 2008 Diputuskan Publikasi : 19 Mei 2008 Abstract The work hardening study of four commercial wrought aluminium alloys; AA, AA3004, AA and AA against two different sliders (M2 steel and an alumina) under dry sliding conditions at 1m/s over the load range 23N to 140N was carried out. A ferrous slider was used, to promote a mechanical mixed layer (MML), whereas an alumina slider to minimise the formation of an MML due to its inert behaviour, so that the true effect of the work hardening induced by wear can be analysed. It was found that in Al/M2 system, the presence of major alloying elements in the Al-alloy that have high solubility in steel promoted a thick mechanically mixed layer. The solubility of these elements in α-fe is in the order of Si, Mn, Cu, Mg, which roughly approximates the thickness of the MML formed, while the Fe content of the MML also scaled in this order. The MMLs with high Fe content tended to be comprised of fragmented particulate, while a low Fe content tended to be associated with a more homogenous MML. As for the Al/Al 2 O 3 system, the MML was derived from fracture of the slider, and also from transfer and re-transfer of the aluminium alloy. In contrast to the Al/M2 tests, none of the alloying elements in the aluminium alloy were expected to chemically react with the Al 2 O 3, but the thickness of the MML appears to be controlled by different factors to that against the M2 slider. It was found that the average nano-hardness of the MML layer in the Al/Al 2 O 3 decreased with load. Unlike alloys worn against M2 slider, the nano-hardness of the MML in this work was associated with the wear rate. In the case of slider type, the nano-hardness of precipitate-hardened A against Al 2 O 3 showed lower hardness than those MML worn against M2 slider. Keywords: nanocharacteristic, wrought Al-alloys, mechanical mixed layer (MML) 1. Introduction Greater attention has been paid to aluminium based metal matrix composites, which have been primarily developed to improve certain properties of alloys. However, most of the research undertaken to date has concentrated mainly on the types of reinforcement and resulting properties. There has never been a systematic investigation into the role of the matrix, as to whether a precipitation hardened alloy matrix, such as 2014, is superior to a work hardening alloy matrix, such as. In addition to work hardening, adhesion between contact surfaces usually results in transfer and mechanical mixing. This appears to occur for the boundary lubricated condition as well as in dry wear [1]. Moreover, the result of matrix/reinforcement bonding in aluminium based MMCs is often undesirable as it may induce reduced ductility and fracture toughness [2]. Thus, in determining the effect of different combinations of matrix alloy and reinforcement, one must first study the performance of the matrix material itself. Transfer between the slider and aluminium alloys may result in the formation of a mechanically mixed layer (MML), particularly for ferrous alloys [3-6]. Although the formation of an MML is known to modify wear behaviour, the exact manner in which it does is not known. The principal objectives of this investigation are to determine and 32 characterise the effect of different slider materials on the formation of the MML and how the alloy matrix composition affect the wear resistance of these materials at nanoscale. 2. Experiments The dry sliding tests were performed between four different wrought alloys and two types of rotating slider; M2 steel and alumina. The aluminium alloys were selected to give a solid solution strengthened alloy (), a model dispersion strengthened alloy with relatively pure Al matrix (3004) and two precipitation hardened alloys ( and ). The chemical composition of each alloy, in weight percentage, is shown in Table 1. The, and alloys were produced by the powder metallurgy route. The pre-alloyed powders with mean particle size (d 50 ) about 25µm were produced by inert gas (argon) atomization and were then cold pressed and extruded into diameter rods. However, the 3004 (dispersion hardened) alloy was in a hot-rolled slab form. The sliders were selected in order to vary the degree of chemical interaction with the Al-alloys. Prior to wear testing, the slider was mounted in a lathe and metallographically polished to 1µm diamond finish prior to each test in order to give, as far as possible, a constant surface finish. Also, Vickers micro-hardness testing was used to determine the hardness of both sliders. The

2 J. Nano Saintek. Vol. 2 No. 1, Feb average result for the M2 slider was 810Hv 20kgf ± 27, whereas the alumina shows the hardness of approximately 1624Hv 30kgf ± 284. Under unlubricated sliding conditions, the wear test is performed at four different loads; 23, 42, 91 and 140N, which were applied via cantilever beam. A fixed sliding speed of ~1m/s was used under ambient laboratory conditions (~22.5 o C and ~30% relative humidity). Lancaster wear equation or specific wear rate; K was used for the evaluation of wear, which requires the volume of material removed by wear processes [7]; V K ' = (1) SW where V = volume loss, S = sliding distance, W = load Detailed characterization by scanning electron microscopy (SEM) was carried on the sub-surfaces (longitudinal cross sections) of the worn alloys. To quantify the work hardening, a depth sensing nanoindentation test was carried out with the indentation tests operated under 600 mn load. Table 1. Chemical composition of as-received wrought aluminium alloys (%wt) IADS Designation Si Fe Cu Mn Mg Zn Cr Ti Ni Al < ± 4.42 ± 0.94 ± 1.35 ± 0.02% 0.04% 0.02% 0.02% <0.02 <0.02 <0.01 <0.02 Balance 3004 < ± < ± <0.02 <0.02 <0.02 <0.02 < % 0.02% Balance < ± 0.12 ± 4.95± < ± < % 0.02% 0.04% 0.02% <0.01 <0.02 Balance 0.31 ± 0.05 ± 1.01 ± 1.05 ± < % 0.02% 0.02% 0.02% <0.02 <0.02 <0.01 <0.02 Balance 3. Results and Discussions Figure 1 gives the specific wear rate (K') as a function of load for both cases (Al/M2 and Al/Al 2 O 3 ). The K' values were found in the range of 0.31x10 4 to 4.23x10-4 mm 3 /Nm and 0.37 to 3.01 x 10-4 mm 3 /Nm for Al/M2 and Al/Al 2 O 3 systems respectively, and were generally within the severe wear regime. For all alloys, the specific wear rate values dropped with load, indicating improved wear resistance at higher loads. The severity of the deformed appearances in worn alloys was found to scale with wear variables, i.e, sliding distance and applied load. In sliding wear, the evolution involves plastic deformation at the surface and sub-surface, the formation of material transfer and debris, environmental-reaction and the mechanical mixing of both contacted surfaces [8,9] ). Due to substantial hardness of the sliding slider, most of the generated energy will be dissipated within the near surface of the pin through plastic deformation [9]. In general, the severe wear involves massive surface damage and large-scale material transfer to the slider. Such appearance was observed in the present study, especially when higher loads (91N and 140N) were applied (for example; Fig. 2(a) for Al/M2 and Figure 2(b) for Al/Al 2 O 3 ). At a microscopic level, Ruff and co-worker [10] suggested that these damages are caused by high raking angle of contacted asperities, resulting increased penetration that lead to extreme surface damage. Examinations on the worn surface also showed the presence of inhomogeneous profiles, having smoother zones that correspond to areas that had recently been in contact with the slider, whereas the rougher zones are regions where surface delamination had occurred. This is indicative of the fact that considerable plastic deformation might have occurred on the surface. Based on these observations, it can be postulated that the wear mechanism of aluminium alloys in both cases was plastic deformation dependent. Specific Wear Rate (mm 3 /Nm) x M2/ M2/3004 M2/ M2/ Al2O3/ Al2O3/ Load (N) Al2O3/3004 Al2O3/ Figure 1 The relationship between specific wear rate (mm 3 /Nm) and applied load. The smooth and dotted lines represent alloys worn on M2 and alumina respectively. The wear mechanisms observed are consistent with Archard 11) adhesive wear characterised by plastic ploughing and transfer of material from the slider. However, much of the surface was covered in a mechanically mixed layer (MML), which is now known to be an important aspect of the wear of Al alloys against ferrous and alumina sliders [12,13] ). With respect to friction and wear behaviour, numerous authors [12,14-16] have concluded that the tribological behaviour is influenced by the mechanical, physical and chemical properties of these near-surface materials.

3 J. Nano Saintek. Vol. 2 No. 1, Feb (a) x y x y (b) (a) (c) Figure 3 Subsurface damage of longitudinal cross sections of (a) A alloys against M2 at 91N, (b) A3004 alloys against M2 at 91N and (c) A3004 alloys against Al 2 O 3 at 23N. Insets represent the high magnified selected area. A corresponding EDS analysis of A in (a) are shown in x and y areas. (b) A correlation between MML thickness and specific wear rate is plotted in Fig. 4 (a) and (b) for both Al/M2 and Al/Al 2 O 3 systems respectively. Figure 2 Digital Elevation Models (DEM) of alloy worn surface against (a) M2 and (b) Al 2 O 3 at 140N showing the texture and depth images. S.D represents the sliding direction. The mixed layer which is commonly known as the mechanically mixed layer (MML), was formed through the incidental mixing between the two materials that occurs at the contact spots under normal pressures. The extent and compositional features of these subsurface zones were found to depend on the conditions of sliding wear, material and environment, which developed quickly under dry sliding wear conditions [17]. The present work has confirmed that an MML was formed in the sliding wear of the Al-alloys against both sliders. The distinctive morphology of the mixed layer has led to a suggestion that its formation was due to a compression of the transfer material and the entrapped debris, which was followed by mechanical mixing during the sliding process. As shown in high magnified images in Fig. 3, the MMLs consisted of a mixture of materials derived from both contacting materials and based on the corresponding EDS results (Fig. 10 (a)); the MMLs were found to contain Al, Fe (for Al/M2 case) and O (which was presumed to be in the form of oxide), which shows that the MML comprised a complex mixture of both surfaces, with some degree of oxidation. MML Thickness ( μm) MML thickness ( μm) Specific Wear Rate (mm 3 /Nm) x Specific Wear Rate (mm 3 /Nm) x Figure 4 The relationship between MML thickness and the specific wear rates of (a) Al alloy/m2 and (b) Al alloy/al 2 O 3 system.

4 J. Nano Saintek. Vol. 2 No. 1, Feb For Al/M2, the specific wear rate was relatively insensitive to MML thickness for the 3004 and, although the specific wear rate decreased in a linear manner with increasing MML thickness. Interestingly, the two alloys where the specific wear rate was relatively insensitive to MML thickness also exhibited MMLs with the least Fe content (Table 2(a)), and the most homogeneous structure (i.e., Fig. 3(b)). Conversely, the exhibited the highest Fe content, the most heterogeneous structure and the greatest influence on the specific wear rate. The thickness of the MML is only one of several potential ways in which the MML can affect wear rate. Clearly for Al/M2 system, the mechanical properties of the MML (in particular hardness and fracture stress) and its adhesion to the substrate are contributing factors. In the Al/M2 system, the exhibited the thickest MML and the highest Fe content. Since this was not replicated by the, rather the reverse, it is clear that it is not the Mg content of the that promotes the formation of a thick MML. Thus, these results imply that stronger adhesion and transfer from the slider is promoted by the Si in the alloy, while a high Mg content in the Al-alloy reduces adhesion. Similarly, the presence of Cu in the also appears to have promoted stronger adhesion than an equivalent amount of Mg, although the Cu was not as potent as the Si. The Mn in the 3004 also promoted a relatively thick MML, but one that was more homogeneous than for the. The solubility of these elements in α-fe is in the order Si, Mn, Cu, Mg, which roughly approximates to the thickness of the MML formed. Thus, the observations are in-line with the Archard theory of adhesive wear, as might be expected. In the Al/Al 2 O 3 system, the original intention of using an Al 2 O 3 slider was to avoid the formation of an MML and therefore determine the effect of the work hardening behaviour of the alloy on the wear resistance. However, this was not the case. SEM observations of the alumina slider suggested that aluminium was transferred from the aluminium alloy pin to the alumina. The transfer layers would have been further work hardened and may also have become oxidised, and eventually detached. This detachment process appears to have occurred through fracture at or near the surface of the alumina (as shown by the intergranular facets in Fig. 5). Some of this fractured alumina will then have re-transferred back to the pin along with the severely work hardened aluminium alloy. Thus, the MML appears to have comprised heavily work hardened aluminium, alumina derived from the slider, and possibly aluminium oxide based phases (of a structure not determined), arising from the oxidation of the aluminium alloy. Figure 5. BEI micrograph of the wear scar of the alumina slider against at 140N. An inset represent the high magnified selected area. As reported earlier in Al/M2 system, insensitivity of the wear resistance of both 3004 and alloys towards the MML thickness was also noted. However, the 3004 which had a particularly homogenous MML structure, exhibited scattered data for the MML thickness. It is also significant that the content of Al in the MML layer of 3004 was found to be higher than for the other alloys as indicated in Table 2(b), presumably due to the presence of Al-oxides (due to environment and Al 2 O 3 slider reactions) which may give a detrimental effect on wear rate. Table 2 Average quantitative EDS analysis on MML of Al-alloys against (a) M2 and (b) Al 2 O 3 (a) Element 42N N 23N 140N 23N 140N 42N 140N Mg Al Si Mn Fe Cu (b) Element N 140N 23N 140N 42N 140N 23N 140N Mg Al Mn Cu Si Mg

5 J. Nano Saintek. Vol. 2 No. 1, Feb Table 3 Average nanohardness of MML of Al-alloys against (a) M2 and (b) Al 2 O 3 under 600µN. Average hardness of MML of Al-alloys against (c) M2 and (d) Al 2 O 3 via Knoop microhardness (25gf) (a) Alloy Type Load (N) Nanohardness (GPa) ± ± ± ± ± ± ± (b) Alloy Type Load (N) Nanohardness (GPa) ± ± ± ± ± ± ± Tables 3(a) - (d) show the difference values between the universal hardness taken from the P (force)-h (indentation depth) curves and the conventional Knoop hardness. Both universal and conventional hardness exhibited obvious load size effect; the hardness increases with decreasing indentation depth. Indentation via the conventional method was avoided especially on the mixed layers (MML), since they occasionally exhibited brittle behaviour, where extensive cracks were observed around the indentation marks and no reading could be taken. A distinct indentation load effect was found for the hardness from the P-h curves. It is clear that the absolute hardness values of the MML in both cases are substantially higher than the bulk hardness of the underlying material. In comparison, the MMLs of alloys showed outstanding hardness at both loads with GPa, although at this point the alloys were the ones which exhibited lowest wear loss at these respective loads. An interesting result was obtained in the Al/Al 2 O 3 system, pertains to the soft shear layer observed in worn 3004 alloy at 23N, just beneath the MML layer as shown in Figure 3(c). This shear layer was composed of two-sub-layers identified as a soft layer and a hardened layer. The soft layer with average hardness of 1.99GPa was located beneath the MML formation and was caused by the generation of voids at matrix/dispersion particles interface as a result of high shear strains [18]. Comparing these two layers, the MML layer could be described as the tribolayer due to its intimate mixing between contacting materials during sliding and the shear layer as the refinement layer. It was found that the average of nanohardness of the MML layer in the Al/Al 2 O 3 system decreased with load. In, the nanohardness of the MML was 6.08GPa at 23N. By increasing the applied load to 140N, the nanohardness of the MML dropped to 5.39GPa. (c) Alloy Type Load (N) HK ± ± ± ± ± ± ± ± 5.66 (d) Alloy Type Load (N) HK ± ± ± ± ± ± ± ± This trend also occurs in the and. Unlike alloys worn against M2 slider, the nanohardness of the MML in this work was associated with the wear rate as given earlier in Fig At both loads (23N and 140N), the nanohardness of the MMLs was found in the following order;,, and finally the The alloy with thin and the hardest MMLs exhibited good wear resistance at these loads. In the case of slider type, the nanohardness of both precipitate-hardened and alloys against Al 2 O 3 showed lower hardness than those MMLs worn against M2 slider. The MMLs of alloy produced after worn against Al 2 O 3 however exhibited the reverse order, where harder MMLs was noted, although its wear loss was much greater than those MMLs produced against M2 slider. Figure 6 shows an example of distinct indentations across the matrix-mml interfaces of against both sliders at 23N (Figs. 4.26(a) and (b)). A B Figure 6. AFM images of nano-indentation of cross section of alloy at 23N against (a) M2 and (b) Al 2 O 3 along the AB path, which consists of matrix-mml. 4. Conclusion The present work illustrates a variety mechanism of wear and more than one mechanism is almost certain to operate. The variations were strongly influenced by the wide range of wear parameters including the alloy compositions and slider materials used. The strong A B (b)

6 J. Nano Saintek. Vol. 2 No. 1, Feb interaction of the aluminium alloy with M2 slider was expected and formed an important part of this investigation. However, it had been hoped that the alumina slider would be inert, and therefore would not promote an MML. However, in the event it was found that the transfer occurred to the alumina slider, followed by retransfer and fracture of some alumina surface, resulting in the formation of an MML. Severe wear was observed for all alloys, with specific wear rates, in the range 0.31x10-4 to 4.23x10-4 mm 3 /Nm for Al /M2 and 0.37x x10-4 mm 3 /Nm for Al /Al 2 O 3. As expected, the wear severity was found to increase with applied load and sliding distance. For all alloys, sliding at or near room temperature produces a work hardened layer with a strain that varies with depth below the worn surface, regardless the slider s material type. For tests against the M2 slider, wear was strongly influenced by alloy composition. Despite being present in relatively small amounts, elements presents in the Al-alloy with high solubility in the slider materials promoted a thick MML for the M2 slider. However, the thickness of the MML did not dictate the wear resistant of each alloy. The presence of a thick and fragmented MML in alloy provided the best wear resistance for Alalloy/M2 system but a thin and homogenous MML, as in the alloy, was more preferable for Al-alloy/Al 2 O 3 system. In both systems (Al/M2 & Al/Al 2 O 3 ), the A exhibited the greatest hardening, while, as expected, the A3004 exhibited very little work hardening. The work hardening, morphology and hardness of the MML differed markedly between materials, in which the optimum Al alloy is the one with the greatest starting hardness, and subsequent work hardening is of secondary importance. [7] J.K. Lancaster, Wear 10, 103 (1967). [8] G. Straffelini and A. Molinari, Wear 236, 328 (1999). [9] D.A. Rigney and J.P. Hirth, Wear 53, 345 (1979). [10] A.W. Ruff, L.K. Ives and W.A. Glaeser, Fundamentals of Friction and Wear of Materials, edited by Rigney, D.A., Metals Park, Ohio: Am. Soc. Met. (1981). [11] J.F. Archard, J. Appl. Phys. 24, 981 (1953). [12] C. Perrin and W.M. Rainforth, Wear , 171 (1997). [13] Y. Wang, W.M. Rainforth, H. Jones and M. Lieblich, Wear 251, 1421 (2001). [14] A.J. Leonard, C. Perrin and W.M. Rainforth, Mater. Sci. Tech. 13, 41 (1997). [15] H.C. How and T.N. Baker, Wear 210, 263 (1997). [16] D.A. Rigney, Mater. Res. Innovat. 1, 231 (1998). [17] S.L. Rice, H. Nowothy and S.F. Wayne, Wear 74, 131 (1981). [18] R.L. Deuis, C. Subramanian, and J. M. Yellup, Comp. Sci. and Tech. 57, 415 (1997). Acknowledgement The author would like to thank Prof. Mark Rainforth at University of Sheffield for his assistance in this study. Financial support from UKM-GUP-BTT is also gratefully acknowledged. References [1] C. Diaz, J.L. Conzalez-Carrasco, G. Caruana and M. Lieblich, Met. Mater. Trans. A 27A, 3259 (1996). [2] K. Razavizadeh and Eyre, T.S., Wear 79, 325 (1982). [3] J. Zhang and A.T. Alpas, Acta Metall. 45, 513 (1997). [4] R. Antoniou and D.W. Borland, Mater. Sci. Eng. 93, 57 (1987). [5] K.N. Tandon and X.Y. Li, Scripta Mat. 38, 7 (1997). [6] D.A. Rigney, ed., in Fundamentals of Friction and Wear of Materials, Metals Park, Ohio: Am. Soc. Met. (1981).