- Email: [email protected]
micro-indenter
Materials Science and Engineering A 371 (2004) 222–228
Characterization of lubricated worn surfaces using a nano/micro-indenter X.Y. Wang∗ , H. Zhang, D.Y. Li Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6G 2G6 Received 31 July 2003; received in revised form 24 November 2003
Abstract Evaluation of wear behavior under lubricant condition could be time consuming, since the material loss may be so small that no significant difference in the protection efficiency between two different lubricants could be determined. Attempts were made to explore potential application of a nano/micro-indenter in characterization of lubricated worn surfaces. In this work, indentation tests were performed on three worn surfaces of a stainless steel experienced unlubricated wear, wear in an anti-wear lubricant oil and in a degraded lubricant oil, respectively. The three worn surfaces showed different responses to indentation. Irregular load–depth curves, such as those with kinks, were observed, and the probability for occurrence of such irregularity depended on the lubrication condition. Different fluctuations in the mechanical behavior of these worn surfaces were observed. The different responses of the worn surfaces to indentation were investigated by examining their morphologies in combination with a modeling study employing the finite element method (FEM). It was demonstrated that a good lubricant led to a relative homogeneous worn surface with less defects, which could be well characterized using the nano/micro-indentation test. © 2003 Elsevier B.V. All rights reserved. Keywords: Wear; Lubrication; Micro-indentation; Homogeneity; FEM
1. Introduction Wear is a complicated process, involving deformation, generation of imperfections, cracking and formation of surface scales such as oxide, amorphous layer, and transferred layer, etc. The surface condition varies significantly under different lubrication conditions. Lubrication is an effective method to reduce friction and protect materials from wear. Various approaches are used to evaluate the lubrication efficiency of a lubricant, including wear test, wear debris analysis, friction measurement and worn surface examination using optical and electron microscopes [1]. These parameters can be monitored simultaneously when one evaluates the efficiency of a lubricant [2]. In situ observation of frictional seizure, wear and surface features during lubricated sliding is also possible using an X-ray microscope system [3]. There are also other attempts made to evaluate the efficiency of a lubricant against wear [4,5].
∗ Corresponding author. Tel.: +1-780-492-5157; fax: +1-780-492-2881. E-mail address: [email protected] (X.Y. Wang).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.11.049
In recent years, the nano/micro-mechanical probing technique has attracted increasing interest from tribologists, lubrication engineers and materials engineers. The nano/micro-indenter provides an effective tool to characterize a surface by determining changes in its surface mechanical properties on nano/micro scale under light loads [6,7]. During wear, the homogeneity of a surface may decrease to some extent, which could be reflected by changes in surface mechanical properties. The nano/micro-mechanical probe is a promising tool to evaluate such changes with high sensitivity. The protection efficiency of a lubricant against wear could thus be evaluated by determining changes in the mechanical behavior of a worn surface. In this work, surfaces of 304 stainless steel samples worn respectively in air and in two different lubricants were examined using a nano/micro-mechanical indenter, with the aim of exploring potential application of nano/micro-indentation in characterizing worn surfaces under lubricant and unlubrcant conditions. The efficiency of a lubricant against wear could be reflected by such characterization. The finite element method (FEM) was employed to gain insight into the indentation process of a crack-containing surface layer in order to understand the response of a worn surface to indentation.
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
223
2. Experimental details
Table 1 Friction coefficients under different wear conditions
A commercial 304 stainless steel (304SS) (C 0.08 wt.%, Mn 2.0 wt.%, Si 1.0 wt.%, P 0.04 wt.%, S 0.03 wt.%, Cr 19.0 wt.%, Ni 9.0 wt.%) was cut to make rod-shaped specimens with 10 mm in diameter and 5 mm in thickness. Testing surface of the specimens was polished with different grid papers and finally a slurry containing alumina particles of 0.05 m. Lubricated wear tests were carried out using a tribometer. During the wear test, a ceramic ball with diameter of 6 mm slid on a specimen along a circle of 1.5 mm in diameter with a velocity of 40 mm/s under applied loads of 1, 2 and 5 N, respectively, for 2000 laps. Friction coefficient was recorded simultaneously during the test. The tribometer had a special container, allowing a wear test performed in a lubricant. In this work, a mineral oil with anti-wear additives (anti-wear oil) and a degraded anti-wear circulating oil (degraded oil) were used, which were provided by Imperial Oil Ltd. (Canada). Worn tracks were examined using an optical microscope and a scanning electron microscope (SEM), respectively. Micro-cracks beneath the worn surface were observed at cross-section using SEM. Chemical compositions of all worn surfaces were analyzed using an energy dispersive spectrometer (EDS). The profile of wear tracks was determined using a profilometer, so that the material loss after wear test could be measured. Micro-indentation tests were performed on surfaces worn under different wear conditions using a micro-mechanical probe (Fischer Technology Inc.). Local mechanical behavior was evaluated from a load–depth curve obtained during the micro-indentation test. In order to obtain reliable results, for each applied load, more than 10 indentation tests were performed at randomly selected locations on a worn surface.
Surface
3. Results and discussion 3.1. Friction and wear under different wear conditions
Friction coefficient
1 0.8
dry condition
0.6 0.4 in the degraded oil
0.2
in the anti-wear oil
0 0
500
1000
1500
2000
1
2500
Laps
Fig. 1. Typical changes in friction coefficient of surfaces worn under different conditions (applied load: 5 N).
2
5
Dry wear 0.080 ± 0.181 0.803 ± 0.149 0.802 ± 0.136 Wear in degraded oil 0.141 ± 0.018 0.178 ± 0.014 0.182 ± 0.016 Wear in anti-wear oil 0.120 ± 0.021 0.145 ± 0.014 0.151 ± 0.018
cated sliding, sliding in the degraded oil lubricant and in the anti-wear oil lubricant, respectively. Stable coefficients of friction under different conditions are given in Table 1. As demonstrated, the oil lubricants markedly decreased friction, and the anti-wear oil was more efficient than the degraded oil to reduce friction. Corresponding material losses under the conditions were evaluated by measuring profiles of wear tracks. The material losses are illustrated in Fig. 2. It was demonstrated that the material loss caused by wear in the anti-wear oil was the smallest, followed by that in the degraded oil and the unlubricated wear led to the largest material loss. 3.2. Homogeneity of worn surface The indentation tests were performed in randomly selected positions on a worn surface. A typical morphology of a surface worn under unlubricated condition and corresponding load–depth curves for different locations are shown in Fig. 3. As shown, the worn surface is by no means homogeneous, demonstrated by large variations in the local response to indentation. Mechanical properties such as hardness and elastic behavior of a worn surface could be evaluated from the load–depth curve. Hardness is related to the indentation depth. Under a fixed load, the smaller the indentation depth, the harder is the material. The elastic behavior may be evaluated using the ratio (η) of the recoverable deformation energy to the total deformation energy [8]. The recoverable deformation energy is represented by the area enclosed by the unloading curve and the maximum depth, and the area enclosed by the loading curve and the maximum depth represents the total deformation energy. A higher η value corresponds to a better elastic behavior of a material. Therefore,
5 Material loss (X10 µm3)
Sliding wear tests under different lubrication conditions were carried out. Curves of friction coefficients versus numbers of laps under a load of 5 N are shown in Fig. 1, which were obtained under different conditions, including unlubri-
Load (N)
14
unlubrcated wear wear in the degraded oil wear in the anti-wear oil
12 10 8 6 4 2 0 0
1
2 3 4 5 Applied load (N)
6
Fig. 2. Material losses under different wear conditions.
224
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
Fig. 3. Morphology of an unlubricated worn surface and corresponding indentation curves obtained from different domains.
the maximum depth and the ratio (η) could be used to evaluate the mechanical behavior of a worn surface. During lubricated sliding wear, work hardening, deformation, fracture and possible formation of surface film could lead to marked variations in mechanical properties. It is not unreasonable to consider that a good lubricant may reduce the variation or fluctuation in local mechanical properties and retain a relatively homogeneous worn surface. Thus, the fluctuation in local mechanical properties could be used to evaluate the homogeneity of a worn surface. Typical load–depth curves of surfaces worn under a maximum load of 200 mN, respec-
tively, in the anti-wear oil, the degraded oil and air are presented in Fig. 4. Load–depth curves of an as-received sample are also illustrated in the figure for comparison. One may see that the load–depth curves of the as-received specimen (Fig. 4d) are very close because of its homogeneous surface, while those of the surface worn under unlubricated condition (Fig. 4c) are very scattered. Comparing the load–depth curves of surfaces worn respectively, in the anti-wear oil (Fig. 4a) and the degraded oil (Fig. 4b), the former showed less fluctuation than the latter. Indentation tests were also performed under different maximum loads, 20, 30, 50, 100,
Fig. 4. Indentation curves obtained from different sample surfaces: (a) worn in the anti-wear lubricant oil; (b) worn in the degraded lubricant oil; (c) dry worn surface; (d) as-received sample surface.
Ratio of standard deviation to average hardness (%)
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
225
100 90 80 worn in air worn in the degraded oil worn in the anti-wear oil
70 60 50 40 30 20 10 0 0
100
200
(a)
300
400
500
600
Applied load (mN)
Ratio of standard deviation of average value (%)
60 worn in air worn in the degraded oil worn in the anti-wear oil
50 40 30 20 10 0 0
100
200
and 200 mN in addition to 500 mN. The above mentioned phenomenon was also observed. In order to obtain more quantitative information, hardness and η-value and relative ratios of standard deviations to average values of the properties of different worn surfaces under different loads are shown in Figs. 5 and 6, respectively. One may see that the anti-wear oil lead to smaller ratios than the degraded oil and the ratios of the unlubricated worn surface are the highest (Fig. 6). The results implied that the surface worn in the anti-wear oil was more homogeneous than that worn in the degraded oil; the homogeneity of the unlubricated worn surface was the poorest. 3.2.1. Irregularity of worn surface Further analyzing the load–depth curves, one may obtain more information about a worn surface, e.g. its irregularity. Fig. 7 illustrates three typical load–depth curves. Curve (c) was a normal one; curve (a) came from a surface whose very surface layer was softer, which made the change in indentation depth faster at the beginning of indentation; curve (b) came from a worn surface which showed a sudden increase (kink) in the indentation depth without corresponding increase in load during the indentation process. It is known that wear may destroy the homogeneity of a material surface with formation of micro-cracks and surface irregularities [1]. The shape of curve (a) may be affected by possible formation of a soft surface film, by the existence of micro-cracks or by a scratch groove on the surface. When the indenter
400
500
600
Fig. 6. Relative ratios of standard deviations to average value of (a) hardness and (b) η-value of different surfaces with respect to the indentation load.
tip approached these regions, it could move down quickly without a large indentation load. When the tip reached unaffected layers beneath surface, the increase in the penetration depth slowed down as shown in Fig. 7. Regarding curve (b), the occurrence of kink could be caused by propagation of a subsurface micro-crack or initiation of cracking [9,10]. Initially, the resistance to indentation was large. When the tip penetrated into the subsurface and approached a crack, collapse of the surface layer above the crack might happen, thus resulting in the kink. In the present work, curves (a) and (b) were often observed when indentation was performed on a surface worn in the degraded oil, compared to that worn in the anti-wear oil; and the probability of observing such curves was the highest for the unlubricated worn surface. Probabilities of observing the irregular load–depth curves 250 200 Load (mN)
Fig. 5. Average hardness and η values of different worn surfaces under different maximum loads.
300
Applied load (mN)
(b)
(c)
150
(b)
100 50
(a) 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth ( m)
Fig. 7. Typical load–depth curves obtained during micro-indentation under a maximum load of 200 mN.
226
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228 Table 2 Probabilities of observing of irregular load–depth curves Specimen
Probability (%)
As-received surface Surface worn in air Surface worn in degraded oil Surface worn in anti-wear oil
0 60.00 38.57 25.71
for different samples are given in Table 2. If the as-received sample surface was considered as a perfect surface, one may see that the sliding wear obviously decreased the material surface homogeneity. The unlubricated worn surface had the lowest homogeneity among all worn surfaces; and the surface worn in the anti-wear oil was better than that worn in the degraded oil. In order to better understand the unusual change in load–depth curve and rank the lubricants in terms of their protection efficiency, morphologies of different worn surfaces were observed under a scanning electronic microscope (SEM). Typical SEM images of three surfaces worn respectively in the anti-wear oil, the degraded oil and in the unlubricated condition are shown in Fig. 8(a)–(c). A poor surface with cracks was observed under the unlubricated condition (Fig. 8a). In this case, oxide layer possibly formed due to considerable frictional heating. The lubricated worn surfaces appeared more homogeneous, compared to the surface worn under the unlubricated condition, as shown in Fig. 8(b) and (c). Many more micro-cracks or scratch grooves appeared on the surface worn in the degraded oil than that worn in the anti-wear oil. SEM observations indicated that the anti-wear oil was more efficient than the degraded oil to protect the surface from wear. As a result, the surface worn in the anti-wear oil had lower irregularity than that worn in the degraded oil. These results were consistent with the indentation test. The cross-section of worn surfaces was also observed using SEM. Micro-cracks beneath the worn surface were observed. Fig. 9 illustrates micro-cracks in a surface layer under the unlubricated condition. The existence of micro-cracks in the worn surface
Fig. 8. SEM morphologies of surfaces worn under an applied load of 5 N. (a) A surface worn in unlubricated condition; (b) a surface worn in the anti-wear oil; (c) a surface worn in the degraded wear oil.
Fig. 9. Subsurface micro-cracks were introduced during unlubricated wear under an applied load of 5 N.
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228 F R Symmetry axis
layer could be explained to cause kinks on the load–depth curves such as curve (b) in Fig. 7. This was also demonstrated by modeling an indentation process with existence of micro-cracks (see Section 3.3). Chemical compositions of the worn surfaces were analyzed using EDS, and results of the analysis are given in Table 3. As shown, the oxygen content in the unlubricated worn surface was significantly higher than that in the original sample. Under the lubricated condition, however, the chemical compositions of the worn surfaces did not change much when worn in the anti-wear oil and the degraded oil, respectively. It is thus not easy to evaluate the lubricants from possible changes in the chemical composition of a worn surface. The surface irregularity or the ratio of standard deviation of a mechanical property to its average value appears to be a promising parameter for evaluating and ranking lubricants.
227
Indenter tip
Specimen
W
h d
L
Subsurface crack
Fig. 10. A system for the FEM simulation. Only half space was treated due to the mirror symmetry.
3.3. Finite element modeling The indentation tests on the worn surfaces showed different types of load–depth curves (Fig. 7). Among them,
Fig. 11. Load–depth curves for surfaces having a underneath crack of 3.875 m in length. The subsurface crack was put in different positions with depths (d) from 0.4 to 1.0 m, respectively. (a) No crack, (b) d = 0.4 m, (c) d = 0.6 m, (d) d = 0.8 m and (e) d = 1.0 m. The broken curve represents the crack length that changed during indentation.
228
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
Table 3 Chemical compositions of different surfaces (at.%) Worn surface
Substrate Dry wear Wear in anti-wear oil Wear in degraded oil
Elements Fi
Cr
Ni
Mn
Si
P
O
others
69.91 49.87 69.42 69.51
20.06 14.45 20.07 20.21
8.08 5.44 7.90 7.69
1.21 0.97 1.37 1.39
0.75 7.05 0.77 0.65
– – – –
– 22.04 – –
– 0.17 – 0.55
one showed a kink on its loading curve. In order to better understand this phenomenon, a finite element model was applied to simulate an indentation process. Fig. 10 schematically illustrates a modeled system. A horizontal crack of length 3.8 m was located beneath surface at different depths, respectively. Computational indentation was performed with an indenter tip of 25 m in radius applied on a specimen surface. The indentation process was modeled in two-dimensional space using ANSYS software (version 6.1). The indenter was considered to be rigid, and a bilinear elastic-plastic material (E = 160 MPa, µ = 0.28, σs = 250 MPa and Et /E = 0.05) was studied. It was assumed that no friction existed between two faces in contact. An indentation process was divided into a number of substeps. During indentation, the indentation depth was gradually increased to a designed value and then decreased back to zero. The J-integral was used to predict fracture for each substep. Whenever the J-integral value equaled or exceeded a critical value, the crack tip propagated and advanced to next node. If the new J-integral value was greater than the critical value, the crack continued to grow, otherwise, it stopped to grow. The indentation force, depth and crack length were then recorded for this substep when the crack was no longer to grow. Such a procedure was repeated until the entire indentation process was completed. Fig. 11 illustrates simulated load–depth curves for a surface having an underneath crack (Fig. 9); a crack was respectively put in several positions, 0.4, 0.8 and 1.0 m from the surface. As shown in Fig. 11a, the indentation curve kept smooth during indentation for a specimen without crack. When a crack existed in the specimen, however, a kink occurred on the loading curve as shown in Fig. 11b–e. Meanwhile, the crack grew as indicated by broken curves in Fig. 11, accompanied with the occurrence of the kink. This indicates that the kink was a result of crack growth. As demonstrated, the crack growth near surface can be induced under low indentation force, while larger force is required when the distance between the crack and surface increases. The above analysis demonstrates that the kink on a load–depth curve may result from the growth of a subsurface crack. For a realistic worn surface, the situation could be more complicated. Nevertheless, this FEM modeling indicates that a damaged surface with a subsurface crack
may produce a kink on indentation curve. This is consistent with experimentally observed phenomenon.
4. Conclusion Potential application of nano/micro-indentation test in characterizing a lubricated worn surface and evaluating the efficiency of a lubricant in protecting the surface from wear damage was investigated. It was demonstrated that a good lubricant could result in low fluctuation in local mechanical behavior or surface irregularity. Attempts were made to investigate possible mechanism responsible for the formation of kinks on load–depth curves by means of SEM observation and FEM modeling. It was demonstrated that the kinks might come from subsurface micro-cracks that were introduced during wear. The research has shown that the nano/micro-mechanical probing method is promising for characterization of lubricated and unlubrcated worn surfaces.
Acknowledgements The authors are grateful for financial support from Imperial Oil Ltd., Natural Science and Research Council of Canada and Alberta Energy Research Institute.
References [1] W. R. Loomis, New Directions in Lubrication, Materials, Wear, and Surface Interactions, Park Ridge, New Jersey, USA, 1985. [2] H.K. Trivedi, M.M. Massey, R.S. Bhattacharya, G.A. Strahl, D. Collum, Tribol. Lett. 10 (6) (2001) 229. [3] M. Chandrasekaran, A.W. Batchelor, N.L. Loh, Wear 243 (1/2) (2000) 68. [4] Y. Zhu, S. Ogano, G. Kelsall, H.A. Spikes, Tribol. Trans. 43 (2) (2000) 175. [5] A. Khurshudov, P. Baumgart, R.J. Waltman, Wear 229 (1999) 690. [6] N.A. Burnham, R.J. Colton, J. Vac. Sci. Tech. A7 (1989) 2906. [7] J.L. Loubet, J.M. Georges, O. Marchesini, G. Melle, J. Tribol. 106 (1984) 43. [8] R. Liu, D.Y. Li, Y.S. Xie, R. Liewellyn, Scripta Mater. 41 (1999) 691. [9] M.V. Swain, T. Mencik, Thin Sol. Film 253 (1994) 204. [10] X. Long, G. Cai, L.E. Svensson, Mater. Sci. Eng. A 270 (1999) 260.
Characterization of lubricated worn surfaces using a nano/micro-indenter X.Y. Wang∗ , H. Zhang, D.Y. Li Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6G 2G6 Received 31 July 2003; received in revised form 24 November 2003
Abstract Evaluation of wear behavior under lubricant condition could be time consuming, since the material loss may be so small that no significant difference in the protection efficiency between two different lubricants could be determined. Attempts were made to explore potential application of a nano/micro-indenter in characterization of lubricated worn surfaces. In this work, indentation tests were performed on three worn surfaces of a stainless steel experienced unlubricated wear, wear in an anti-wear lubricant oil and in a degraded lubricant oil, respectively. The three worn surfaces showed different responses to indentation. Irregular load–depth curves, such as those with kinks, were observed, and the probability for occurrence of such irregularity depended on the lubrication condition. Different fluctuations in the mechanical behavior of these worn surfaces were observed. The different responses of the worn surfaces to indentation were investigated by examining their morphologies in combination with a modeling study employing the finite element method (FEM). It was demonstrated that a good lubricant led to a relative homogeneous worn surface with less defects, which could be well characterized using the nano/micro-indentation test. © 2003 Elsevier B.V. All rights reserved. Keywords: Wear; Lubrication; Micro-indentation; Homogeneity; FEM
1. Introduction Wear is a complicated process, involving deformation, generation of imperfections, cracking and formation of surface scales such as oxide, amorphous layer, and transferred layer, etc. The surface condition varies significantly under different lubrication conditions. Lubrication is an effective method to reduce friction and protect materials from wear. Various approaches are used to evaluate the lubrication efficiency of a lubricant, including wear test, wear debris analysis, friction measurement and worn surface examination using optical and electron microscopes [1]. These parameters can be monitored simultaneously when one evaluates the efficiency of a lubricant [2]. In situ observation of frictional seizure, wear and surface features during lubricated sliding is also possible using an X-ray microscope system [3]. There are also other attempts made to evaluate the efficiency of a lubricant against wear [4,5].
∗ Corresponding author. Tel.: +1-780-492-5157; fax: +1-780-492-2881. E-mail address: [email protected] (X.Y. Wang).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.11.049
In recent years, the nano/micro-mechanical probing technique has attracted increasing interest from tribologists, lubrication engineers and materials engineers. The nano/micro-indenter provides an effective tool to characterize a surface by determining changes in its surface mechanical properties on nano/micro scale under light loads [6,7]. During wear, the homogeneity of a surface may decrease to some extent, which could be reflected by changes in surface mechanical properties. The nano/micro-mechanical probe is a promising tool to evaluate such changes with high sensitivity. The protection efficiency of a lubricant against wear could thus be evaluated by determining changes in the mechanical behavior of a worn surface. In this work, surfaces of 304 stainless steel samples worn respectively in air and in two different lubricants were examined using a nano/micro-mechanical indenter, with the aim of exploring potential application of nano/micro-indentation in characterizing worn surfaces under lubricant and unlubrcant conditions. The efficiency of a lubricant against wear could be reflected by such characterization. The finite element method (FEM) was employed to gain insight into the indentation process of a crack-containing surface layer in order to understand the response of a worn surface to indentation.
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
223
2. Experimental details
Table 1 Friction coefficients under different wear conditions
A commercial 304 stainless steel (304SS) (C 0.08 wt.%, Mn 2.0 wt.%, Si 1.0 wt.%, P 0.04 wt.%, S 0.03 wt.%, Cr 19.0 wt.%, Ni 9.0 wt.%) was cut to make rod-shaped specimens with 10 mm in diameter and 5 mm in thickness. Testing surface of the specimens was polished with different grid papers and finally a slurry containing alumina particles of 0.05 m. Lubricated wear tests were carried out using a tribometer. During the wear test, a ceramic ball with diameter of 6 mm slid on a specimen along a circle of 1.5 mm in diameter with a velocity of 40 mm/s under applied loads of 1, 2 and 5 N, respectively, for 2000 laps. Friction coefficient was recorded simultaneously during the test. The tribometer had a special container, allowing a wear test performed in a lubricant. In this work, a mineral oil with anti-wear additives (anti-wear oil) and a degraded anti-wear circulating oil (degraded oil) were used, which were provided by Imperial Oil Ltd. (Canada). Worn tracks were examined using an optical microscope and a scanning electron microscope (SEM), respectively. Micro-cracks beneath the worn surface were observed at cross-section using SEM. Chemical compositions of all worn surfaces were analyzed using an energy dispersive spectrometer (EDS). The profile of wear tracks was determined using a profilometer, so that the material loss after wear test could be measured. Micro-indentation tests were performed on surfaces worn under different wear conditions using a micro-mechanical probe (Fischer Technology Inc.). Local mechanical behavior was evaluated from a load–depth curve obtained during the micro-indentation test. In order to obtain reliable results, for each applied load, more than 10 indentation tests were performed at randomly selected locations on a worn surface.
Surface
3. Results and discussion 3.1. Friction and wear under different wear conditions
Friction coefficient
1 0.8
dry condition
0.6 0.4 in the degraded oil
0.2
in the anti-wear oil
0 0
500
1000
1500
2000
1
2500
Laps
Fig. 1. Typical changes in friction coefficient of surfaces worn under different conditions (applied load: 5 N).
2
5
Dry wear 0.080 ± 0.181 0.803 ± 0.149 0.802 ± 0.136 Wear in degraded oil 0.141 ± 0.018 0.178 ± 0.014 0.182 ± 0.016 Wear in anti-wear oil 0.120 ± 0.021 0.145 ± 0.014 0.151 ± 0.018
cated sliding, sliding in the degraded oil lubricant and in the anti-wear oil lubricant, respectively. Stable coefficients of friction under different conditions are given in Table 1. As demonstrated, the oil lubricants markedly decreased friction, and the anti-wear oil was more efficient than the degraded oil to reduce friction. Corresponding material losses under the conditions were evaluated by measuring profiles of wear tracks. The material losses are illustrated in Fig. 2. It was demonstrated that the material loss caused by wear in the anti-wear oil was the smallest, followed by that in the degraded oil and the unlubricated wear led to the largest material loss. 3.2. Homogeneity of worn surface The indentation tests were performed in randomly selected positions on a worn surface. A typical morphology of a surface worn under unlubricated condition and corresponding load–depth curves for different locations are shown in Fig. 3. As shown, the worn surface is by no means homogeneous, demonstrated by large variations in the local response to indentation. Mechanical properties such as hardness and elastic behavior of a worn surface could be evaluated from the load–depth curve. Hardness is related to the indentation depth. Under a fixed load, the smaller the indentation depth, the harder is the material. The elastic behavior may be evaluated using the ratio (η) of the recoverable deformation energy to the total deformation energy [8]. The recoverable deformation energy is represented by the area enclosed by the unloading curve and the maximum depth, and the area enclosed by the loading curve and the maximum depth represents the total deformation energy. A higher η value corresponds to a better elastic behavior of a material. Therefore,
5 Material loss (X10 µm3)
Sliding wear tests under different lubrication conditions were carried out. Curves of friction coefficients versus numbers of laps under a load of 5 N are shown in Fig. 1, which were obtained under different conditions, including unlubri-
Load (N)
14
unlubrcated wear wear in the degraded oil wear in the anti-wear oil
12 10 8 6 4 2 0 0
1
2 3 4 5 Applied load (N)
6
Fig. 2. Material losses under different wear conditions.
224
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
Fig. 3. Morphology of an unlubricated worn surface and corresponding indentation curves obtained from different domains.
the maximum depth and the ratio (η) could be used to evaluate the mechanical behavior of a worn surface. During lubricated sliding wear, work hardening, deformation, fracture and possible formation of surface film could lead to marked variations in mechanical properties. It is not unreasonable to consider that a good lubricant may reduce the variation or fluctuation in local mechanical properties and retain a relatively homogeneous worn surface. Thus, the fluctuation in local mechanical properties could be used to evaluate the homogeneity of a worn surface. Typical load–depth curves of surfaces worn under a maximum load of 200 mN, respec-
tively, in the anti-wear oil, the degraded oil and air are presented in Fig. 4. Load–depth curves of an as-received sample are also illustrated in the figure for comparison. One may see that the load–depth curves of the as-received specimen (Fig. 4d) are very close because of its homogeneous surface, while those of the surface worn under unlubricated condition (Fig. 4c) are very scattered. Comparing the load–depth curves of surfaces worn respectively, in the anti-wear oil (Fig. 4a) and the degraded oil (Fig. 4b), the former showed less fluctuation than the latter. Indentation tests were also performed under different maximum loads, 20, 30, 50, 100,
Fig. 4. Indentation curves obtained from different sample surfaces: (a) worn in the anti-wear lubricant oil; (b) worn in the degraded lubricant oil; (c) dry worn surface; (d) as-received sample surface.
Ratio of standard deviation to average hardness (%)
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
225
100 90 80 worn in air worn in the degraded oil worn in the anti-wear oil
70 60 50 40 30 20 10 0 0
100
200
(a)
300
400
500
600
Applied load (mN)
Ratio of standard deviation of average value (%)
60 worn in air worn in the degraded oil worn in the anti-wear oil
50 40 30 20 10 0 0
100
200
and 200 mN in addition to 500 mN. The above mentioned phenomenon was also observed. In order to obtain more quantitative information, hardness and η-value and relative ratios of standard deviations to average values of the properties of different worn surfaces under different loads are shown in Figs. 5 and 6, respectively. One may see that the anti-wear oil lead to smaller ratios than the degraded oil and the ratios of the unlubricated worn surface are the highest (Fig. 6). The results implied that the surface worn in the anti-wear oil was more homogeneous than that worn in the degraded oil; the homogeneity of the unlubricated worn surface was the poorest. 3.2.1. Irregularity of worn surface Further analyzing the load–depth curves, one may obtain more information about a worn surface, e.g. its irregularity. Fig. 7 illustrates three typical load–depth curves. Curve (c) was a normal one; curve (a) came from a surface whose very surface layer was softer, which made the change in indentation depth faster at the beginning of indentation; curve (b) came from a worn surface which showed a sudden increase (kink) in the indentation depth without corresponding increase in load during the indentation process. It is known that wear may destroy the homogeneity of a material surface with formation of micro-cracks and surface irregularities [1]. The shape of curve (a) may be affected by possible formation of a soft surface film, by the existence of micro-cracks or by a scratch groove on the surface. When the indenter
400
500
600
Fig. 6. Relative ratios of standard deviations to average value of (a) hardness and (b) η-value of different surfaces with respect to the indentation load.
tip approached these regions, it could move down quickly without a large indentation load. When the tip reached unaffected layers beneath surface, the increase in the penetration depth slowed down as shown in Fig. 7. Regarding curve (b), the occurrence of kink could be caused by propagation of a subsurface micro-crack or initiation of cracking [9,10]. Initially, the resistance to indentation was large. When the tip penetrated into the subsurface and approached a crack, collapse of the surface layer above the crack might happen, thus resulting in the kink. In the present work, curves (a) and (b) were often observed when indentation was performed on a surface worn in the degraded oil, compared to that worn in the anti-wear oil; and the probability of observing such curves was the highest for the unlubricated worn surface. Probabilities of observing the irregular load–depth curves 250 200 Load (mN)
Fig. 5. Average hardness and η values of different worn surfaces under different maximum loads.
300
Applied load (mN)
(b)
(c)
150
(b)
100 50
(a) 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth ( m)
Fig. 7. Typical load–depth curves obtained during micro-indentation under a maximum load of 200 mN.
226
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228 Table 2 Probabilities of observing of irregular load–depth curves Specimen
Probability (%)
As-received surface Surface worn in air Surface worn in degraded oil Surface worn in anti-wear oil
0 60.00 38.57 25.71
for different samples are given in Table 2. If the as-received sample surface was considered as a perfect surface, one may see that the sliding wear obviously decreased the material surface homogeneity. The unlubricated worn surface had the lowest homogeneity among all worn surfaces; and the surface worn in the anti-wear oil was better than that worn in the degraded oil. In order to better understand the unusual change in load–depth curve and rank the lubricants in terms of their protection efficiency, morphologies of different worn surfaces were observed under a scanning electronic microscope (SEM). Typical SEM images of three surfaces worn respectively in the anti-wear oil, the degraded oil and in the unlubricated condition are shown in Fig. 8(a)–(c). A poor surface with cracks was observed under the unlubricated condition (Fig. 8a). In this case, oxide layer possibly formed due to considerable frictional heating. The lubricated worn surfaces appeared more homogeneous, compared to the surface worn under the unlubricated condition, as shown in Fig. 8(b) and (c). Many more micro-cracks or scratch grooves appeared on the surface worn in the degraded oil than that worn in the anti-wear oil. SEM observations indicated that the anti-wear oil was more efficient than the degraded oil to protect the surface from wear. As a result, the surface worn in the anti-wear oil had lower irregularity than that worn in the degraded oil. These results were consistent with the indentation test. The cross-section of worn surfaces was also observed using SEM. Micro-cracks beneath the worn surface were observed. Fig. 9 illustrates micro-cracks in a surface layer under the unlubricated condition. The existence of micro-cracks in the worn surface
Fig. 8. SEM morphologies of surfaces worn under an applied load of 5 N. (a) A surface worn in unlubricated condition; (b) a surface worn in the anti-wear oil; (c) a surface worn in the degraded wear oil.
Fig. 9. Subsurface micro-cracks were introduced during unlubricated wear under an applied load of 5 N.
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228 F R Symmetry axis
layer could be explained to cause kinks on the load–depth curves such as curve (b) in Fig. 7. This was also demonstrated by modeling an indentation process with existence of micro-cracks (see Section 3.3). Chemical compositions of the worn surfaces were analyzed using EDS, and results of the analysis are given in Table 3. As shown, the oxygen content in the unlubricated worn surface was significantly higher than that in the original sample. Under the lubricated condition, however, the chemical compositions of the worn surfaces did not change much when worn in the anti-wear oil and the degraded oil, respectively. It is thus not easy to evaluate the lubricants from possible changes in the chemical composition of a worn surface. The surface irregularity or the ratio of standard deviation of a mechanical property to its average value appears to be a promising parameter for evaluating and ranking lubricants.
227
Indenter tip
Specimen
W
h d
L
Subsurface crack
Fig. 10. A system for the FEM simulation. Only half space was treated due to the mirror symmetry.
3.3. Finite element modeling The indentation tests on the worn surfaces showed different types of load–depth curves (Fig. 7). Among them,
Fig. 11. Load–depth curves for surfaces having a underneath crack of 3.875 m in length. The subsurface crack was put in different positions with depths (d) from 0.4 to 1.0 m, respectively. (a) No crack, (b) d = 0.4 m, (c) d = 0.6 m, (d) d = 0.8 m and (e) d = 1.0 m. The broken curve represents the crack length that changed during indentation.
228
X.Y. Wang et al. / Materials Science and Engineering A 371 (2004) 222–228
Table 3 Chemical compositions of different surfaces (at.%) Worn surface
Substrate Dry wear Wear in anti-wear oil Wear in degraded oil
Elements Fi
Cr
Ni
Mn
Si
P
O
others
69.91 49.87 69.42 69.51
20.06 14.45 20.07 20.21
8.08 5.44 7.90 7.69
1.21 0.97 1.37 1.39
0.75 7.05 0.77 0.65
– – – –
– 22.04 – –
– 0.17 – 0.55
one showed a kink on its loading curve. In order to better understand this phenomenon, a finite element model was applied to simulate an indentation process. Fig. 10 schematically illustrates a modeled system. A horizontal crack of length 3.8 m was located beneath surface at different depths, respectively. Computational indentation was performed with an indenter tip of 25 m in radius applied on a specimen surface. The indentation process was modeled in two-dimensional space using ANSYS software (version 6.1). The indenter was considered to be rigid, and a bilinear elastic-plastic material (E = 160 MPa, µ = 0.28, σs = 250 MPa and Et /E = 0.05) was studied. It was assumed that no friction existed between two faces in contact. An indentation process was divided into a number of substeps. During indentation, the indentation depth was gradually increased to a designed value and then decreased back to zero. The J-integral was used to predict fracture for each substep. Whenever the J-integral value equaled or exceeded a critical value, the crack tip propagated and advanced to next node. If the new J-integral value was greater than the critical value, the crack continued to grow, otherwise, it stopped to grow. The indentation force, depth and crack length were then recorded for this substep when the crack was no longer to grow. Such a procedure was repeated until the entire indentation process was completed. Fig. 11 illustrates simulated load–depth curves for a surface having an underneath crack (Fig. 9); a crack was respectively put in several positions, 0.4, 0.8 and 1.0 m from the surface. As shown in Fig. 11a, the indentation curve kept smooth during indentation for a specimen without crack. When a crack existed in the specimen, however, a kink occurred on the loading curve as shown in Fig. 11b–e. Meanwhile, the crack grew as indicated by broken curves in Fig. 11, accompanied with the occurrence of the kink. This indicates that the kink was a result of crack growth. As demonstrated, the crack growth near surface can be induced under low indentation force, while larger force is required when the distance between the crack and surface increases. The above analysis demonstrates that the kink on a load–depth curve may result from the growth of a subsurface crack. For a realistic worn surface, the situation could be more complicated. Nevertheless, this FEM modeling indicates that a damaged surface with a subsurface crack
may produce a kink on indentation curve. This is consistent with experimentally observed phenomenon.
4. Conclusion Potential application of nano/micro-indentation test in characterizing a lubricated worn surface and evaluating the efficiency of a lubricant in protecting the surface from wear damage was investigated. It was demonstrated that a good lubricant could result in low fluctuation in local mechanical behavior or surface irregularity. Attempts were made to investigate possible mechanism responsible for the formation of kinks on load–depth curves by means of SEM observation and FEM modeling. It was demonstrated that the kinks might come from subsurface micro-cracks that were introduced during wear. The research has shown that the nano/micro-mechanical probing method is promising for characterization of lubricated and unlubrcated worn surfaces.
Acknowledgements The authors are grateful for financial support from Imperial Oil Ltd., Natural Science and Research Council of Canada and Alberta Energy Research Institute.
References [1] W. R. Loomis, New Directions in Lubrication, Materials, Wear, and Surface Interactions, Park Ridge, New Jersey, USA, 1985. [2] H.K. Trivedi, M.M. Massey, R.S. Bhattacharya, G.A. Strahl, D. Collum, Tribol. Lett. 10 (6) (2001) 229. [3] M. Chandrasekaran, A.W. Batchelor, N.L. Loh, Wear 243 (1/2) (2000) 68. [4] Y. Zhu, S. Ogano, G. Kelsall, H.A. Spikes, Tribol. Trans. 43 (2) (2000) 175. [5] A. Khurshudov, P. Baumgart, R.J. Waltman, Wear 229 (1999) 690. [6] N.A. Burnham, R.J. Colton, J. Vac. Sci. Tech. A7 (1989) 2906. [7] J.L. Loubet, J.M. Georges, O. Marchesini, G. Melle, J. Tribol. 106 (1984) 43. [8] R. Liu, D.Y. Li, Y.S. Xie, R. Liewellyn, Scripta Mater. 41 (1999) 691. [9] M.V. Swain, T. Mencik, Thin Sol. Film 253 (1994) 204. [10] X. Long, G. Cai, L.E. Svensson, Mater. Sci. Eng. A 270 (1999) 260.