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Bio-inspired wearable characteristic surface: Wear behavior of cast iron with biomimetic units processed by laser
Applied Surface Science 253 (2007) 9513–9520 www.elsevier.com/locate/apsusc
Bio-inspired wearable characteristic surface: Wear behavior of cast iron with biomimetic units processed by laser Hong Zhou a,*, Na Sun a, Hongyu Shan a, Dianyi Ma a, Xin Tong a, Luquan Ren b b
a Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun, Jilin 130025, PR China Key Laboratory of Terrain-Machine Bionics Engineering of Ministry of Education, Jilin University, Changchun, Jilin 130025, PR China
Received 14 March 2007; received in revised form 9 June 2007; accepted 11 June 2007 Available online 16 June 2007
Abstract Stimulated by the cuticles of soil animals, an attempt to improve the wear resistance of compact graphite cast iron (CGI) with biomimetic units on the surface was made by using a biomimetic coupled laser remelting (BCLR) process. The microstructure and microhardness of biomimetic units were examined. The wear behaviors of biomimetic specimens as functions of laser input energy and biomimetic unit shape were investigated under dry sliding condition, respectively. The results indicated that the biomimetic specimens had better wear resistance than the untreated specimens. The wear resistance of the biomimetic specimens increases with the increase of laser input energy due to the increase of the depth and the width of biomimetic units as well as the increase of the microhardness. The specimen with grid biomimetic units had the best resistance, the stria took the second place and the convex showed the worst. The application of laser remelting provided desirable microstructural changes in biomimetic units, which generated the intensified particles effect for improving the wear resistance. The adhesive wear was the dominative wear mechanism for the biomimetic specimens. # 2007 Elsevier B.V. All rights reserved. Keywords: Biomimetic; Cast iron; Microstructure; Wear behavior; Laser
1. Introduction As compact graphite cast iron (CGI) has high friction coefficient and low wear rate under dry sliding condition, it is one kind of materials that are widely used in vehicle brakes [1]. However, since the development of railway transportation tends to be high-speed and heavy-duty in recent years, the service life of disk brakes decreases significantly. How to improve the wear resistance of CGI to meet the requirement of demanding tribological application is always one of the problems of concern to material researchers. In the previous study, improving the mechanical properties of brake materials were mainly done by adding appropriate alloying elements [2–4], or improving the surface quality [5–7]. According to Zhang et al., the wear resistance of CGI was improved with the increase of phosphorous, vanadium or chromium contents after conducting a series of wear tests using self-made dry sliding tester. The results indicated that the CGI with phosphorous addition * Corresponding author. Tel.: +86 431 8509 4427; fax: +86 431 8509 5592. E-mail address: [email protected] (H. Zhou). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.014
possessed the combination of the high friction coefficient and low wear mass loss [2]; the addition of 0.3 wt.% vanadium to CGI reduced its wear mass loss and improved the wear loss stability when the contact pressure and sliding velocity changed [3]; 0.5 wt.% chromium CGI presented the optimal tribological properties [4]. Sahin et al. [5,6] investigated the effect of boronizing on CGI by box-boronizing method and found that the weight loss decreased with boride layer thickness. The wear properties of CGI with Co-based coatings processed by a high power laser (2 kW continuous Nd:YAG) were examined under dry sliding condition by Ocelı´k et al. [7]. It was found that the good wear resistance of this type of alloy was obtained only at high temperatures (up to 525 8C). Nevertheless, owing to the difficulty of choosing suitable elements with proper chemical compositions, it is difficult to enhance the wear resistance only in one aspect without sacrificing the mechanic property of the whole materials. Furthermore, some complex processes and the additions of elements for these materials are commonly at the cost of increasing the production expense greatly. Nature provides a whole host of superior multifunctional structures that can be used as inspirational systems for the
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design and synthesis of new, technologically important materials and devices. Since the 1980s, Ren et al. [8,9] has been dedicating to the study of the cuticle morphologies and principles of soil animals such as dung beetles, black ants, and pangolins and found that there were generally five kinds of simple structures on the cuticles, including convex, concave, stria, bristle and squama. They are called ‘non-smooth construction units’, which have been found to provide excellent anti-wear properties against soil. Recent works in our research group also found that when applied biomimetic principle on the die and tool steel surfaces to form a series of biomimetic units by laser, there had a considerable effect not only on resisting to thermal fatigue [10,11], but also on improving the abrasive wear resistance [12,13]. Based on the results above, it was decided to explore whether similar benefits could be obtained in resistance to dry sliding wear of CGI treated by biomimetic coupling laser remelting (BCLR) process, without changing the special properties of substrate materials such as excellent machinability, the ability to resist galling and excellent vibration damping. In the present work, we chose the laser facility to manufacture and process the biomimetic units on the surfaces of CGI to form a novel bio-inspired wearable surface. Wear tests were carried out on a block-on-ring tester with a biomimetic CGI block against a GCr15 steel ring under dry sliding wear. Experiments were focused on the effects of laser input energy and biomimetic unit shape on the wear behavior of the bio-inspired wearable surfaces under dry sliding condition. 2. Experimental 2.1. Materials Specimens for wear tests were cut from disk brakes for the high-speed train with the sizes of 14 mm 10 mm 10 mm. The materials of the disc brakes are the CGI (250 HV), whose chemical compositions are listed in Table 1. Fig. 1 is an optical micrograph of the CGI substrate, which consists of vermiculate graphite surrounded by ferrite and some amount of pearlite. 2.2. Preparation of biomimetic specimens Mimicking the cuticles of dung beetles [8] (Fig. 2) the various biomimetic units were processed by the BCLR method using a Nd:YAG laser of 1064 nm wavelength and maximum power 100 W and a Gaussian distribution of the energy in the beam. The processing parameters were laser pulse duration 5.0 ms, frequency 14 Hz and scanning speed 0.71 mm/s. A series of experiments were carried out using different laser input energies and different biomimetic unit shapes, respectively. The laser input energies were 25, 75, 125 and 175 J/cm2 Table 1 Chemical compositions of compact graphite cast iron (CGI) Element
C
Si
Mn
P
S
Re
Mg
Fe
Composition (wt.%) 3.56 2.56 0.71 0.03 0.03 0.02 0.02 Balance
Fig. 1. An optical microstructure of the CGI substrate.
to form the stria biomimetic units on the specimens’ surfaces for investigating the influence of the laser input energy on the wear resistance, respectively. Then the laser input energy 125 J/ cm2 was fixed, while the shapes on the sides of the biomimetic specimens were convex, stria and grid for investigating the influence of unit shape on the wear resistance, respectively. The biomimetic unit distance for all the specimens is 2 mm. A schematic illustration of biomimetic specimens is shown in Fig. 3. After the laser processing, transverse sections were obtained and the standard method of metallography was followed to prepare the samples for the microstructure analysis and the microhardness measurement, using a JSM-5600LV scanning electron microscope and a Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA) with a 25 g load, respectively. 2.3. Wear tests Dry sliding wear tests were performed using an MM-200 block-on-ring wear tester (made by the Xuanhua Testing Machine Factory). Its schematic diagram is Fig. 4. All the friction and wear tests were carried out at room temperature. A stationary CGI block with biomimetic units slid on a rotating GCr15 steel ring. The GCr15 steel (austenized at 840 8C, oil quenched and tempered for 2 h at 170 8C) had an average hardness of 61–63 HRC and the sizes of 16 mm 50 mm 10 mm (inside diameter outside diameter thickness). A normal load of 8 kg was used for wear tests. The rotational speed of the ring was 400 rpm and the sliding time was 30 min. Before each test, the surface of the steel ring was polished to a roughness of about 0.1 mm, while biomimetic specimens were ultrasonically cleaned in anhydrous alcohol and dried before and after wear tests. The mass loss was measured by a sensitive electronic balance with an accuracy of 0.0001 g. The difference in mass of three test blocks before and after the experiment gave the average mass loss. To assist the analysis of wear mechanisms, the worn surfaces of block specimens were examined by a JSM-5600LV scanning electron microscopy.
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Fig. 2. SEM micrograph of typical shape of dung beetle cuticles: (a) convex shape of head and (b) stria shape of elytron.
3. Results and discussion 3.1. Microstructure The starting microstructure of the CGI consists of vermiculate graphite surrounded by ferrite and some amount of pearlite. After the BCLR process, a modified layer called biomimetic unit was obtained (see Fig. 5), which consists of two characteristic microstructure zones, i.e. a melted zone and a transition zone. 3.1.1. Melted zone Fig. 6 shows the corroded cross-section microstructures of the biomimetic units processed by laser with various input energies of 25, 75, 125 and 175 J/cm2, respectively. Compared with the original structure, the unit structure is refined and complete dissolution of graphite and fusion of metal are evident. Some dendritic networks and some needlelike structures of irregular shapes in random locations are observed in the melted zone, which can be found in all the biomimetic specimens. According to the XRD analysis of the melted layer (Fig. 7) and SEM observation (Fig. 6c) it is found that the melted zone is composed of ledeburite (martensite + cementite), also containing a small amount of residual austenite. Similar microstructures were also found in the previous work [14,15].
Fig. 3. A schematic illustration of biomimetic specimens.
Fig. 4. A schematic illustration of the block-on-ring wear tester.
The metallurgy mechanism can be interpreted as follows. The surface of the biomimetic specimen is rapidly melted after laser irradiation. Since the melting occurs only at the surface over a relatively short duration and the bulk of the substrate remains cool, the liquid metal in the molten pool will solidify and recrystallize at a high cooling rate. At the beginning of solidification, the primary austenite forms and then crystallizes as the cellular/dendrite structure in the direction towards the surface, which corresponds to the contrary direction of the heat transfer. The last liquid will be enriched in carbon and form cementite surrounding the austenite dendrites [16]. When the temperature reaches the eutectic point, the eutectic reaction
Fig. 5. SEM structure of biomimetic unit processed by laser with an input energy of 125 J/cm2.
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Fig. 6. SEM structures of the melted zone of the biomimetic units processed by laser with various input energies: (a) 175 J/cm2, (b) 125 J/cm2, (c) 75 J/cm2 and (d) 25 J/cm2.
occurs. The solidification structures are the primary austenite and the fine ledeburite (eutectic austenite + cementite). During the rapid cooling, the primary austenite and eutectic austenite could transform mostly to martensite with some residual austenite. As a result, the final structure of the melted zone is fine-grained ledeburite (martensite + cementite) and residual austenite. However, the ledeburite in the present study is distinctly different from the conventional one such as in a white iron, indicating a considerable amount of a high cooling rate during solidification; because the former comprises martensite and cementite, the latter is a matrix containing pearlite and cementite.
3.1.2. Transition zone Fig. 8 shows the microstructures of the transition zones of the biomimetic units processed by laser with various input energies of 25, 75, 125 and 175 J/cm2, respectively. Note that the morphologies of the transition zone are extremely different from the melted zone. In this zone, only partial dissolution of the graphite occurred. The microstructure shows that there is martensite and undissolved graphite in this zone. It may be explained as follows: there is high degree of superheating and austenite nucleation in the transition zone due to the thermal influence from the melted zone. Thus, the pearlite, into which the additional carbon from graphite diffuses during the laser heating, transforms into a carbon rich austenite microstructure. Meanwhile, the ferrite transforms into an austenite microstructure, in which the carbon diffuses rapidly at high temperature [15,16]. Nevertheless, the austenite does not have enough time to grow up and homogenize at a high cooling rate. Accordingly, the product is the inhomogeneous fine austenite. During the rapid cooling, the martensite transformation is inevitable. The final structures are martensite and undissolved graphite. 3.2. Microhardness
Fig. 7. XRD analysis of untreated specimen and biomimetic specimen processed by laser with an input energy of 75 J/cm2.
Microhardness is of particular importance in assessing wear resistant coatings, because it is considered as a good indication of coating integrity and wear resistance [17]. The great difference in microstructures between the untreated and the biomimetic specimens suggests that there may be a corresponding improvement in mechanical properties. Table 2 illustrates the depth, the width and the average microhardness of biomimetic units after the BCLR treatment
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Fig. 8. SEM structure of the transition zone of the biomimetic units processed by laser with various input energies: (a) 175 J/cm2, (b) 125 J/cm2, (c) 75 J/cm2 and (d) 25 J/cm2.
using different input energies. The biomimetic units’ dimensions were the average of five measurements of each specimen. The reported microhardness values were the average of 5–10 measurements along the direction of laser irradiation (from the top of the unit to the bottom), which were obtained at different locations on the polished and etched cross-sections of the biomimetic units for each specimen. The average microhardness of the substrate is 250 HV, while that of the biomimetic units in the melted zone varies from 540 to 680 HV, and in the transition zone varies from 460 to 520 HV. The low input energy (25 J/cm2) results in a narrow melted zone (0.485 mm) and of a little penetration (0.100 mm) with a relatively low hardness value in the melted zone (540 HV), so does the transition zone (460 HV); while the high input energy (175 J/cm2) produces a deep (0.270 mm) and wide melted zone (0.861 mm) with a relatively high hardness value in the melted zone (680 HV). It can be attributed to the solution of carbon, the formation of martensite, the refinement of structure and the large proportion of cementite.
3.3. Wear resistance 3.3.1. Effect of the laser input energy The wear resistance specimens are divided into five groups. One group was untreated, while the others were processed to stria using different laser input energies. Three specimens of each group were chosen for wear tests. The average mass losses of biomimetic specimens as a function of laser input energy are presented in Fig. 9. It can be seen that the wear mass loss of the biomimetic specimens decreases more than 48% compared with that of the untreated specimens in the same sliding way against the steel blocks. The biomimetic specimens display different wear resistance, which increases by 0.92, 1.56, 1.79 and 2.17 times of that of untreated specimens, respectively, as the laser input energies are 25, 75, 125 and 175 J/cm2, respectively. That is, the wear mass loss decreases with the increase of the laser input energy. The lower wear mass loss for biomimetic specimens may originate from the existence of the biomimetic units, which is equivalent to many uniformly distributed intensified particles in
Table 2 Dimensions and average hardness of biomimetic units processed at different conditions Specimen No.
Input energy (J/cm2)
Scanning speed (mm/s)
Unit depth (mm)
Unit width (mm)
Melted zone
Transition zone
Depth (mm)
Width (mm)
Average hardness (HV)
Average hardness (HV)
1 2 3 4 5
25 75 125 175 –
0.71 0.71 0.71 0.71 –
0.141 0.283 0.362 0.389 –
0.568 0.883 0.984 1.023 –
0.100 0.185 0.245 0.270 –
0.485 0.787 0.835 0.861 –
540 600 660 680 250
460 490 520 520 250
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that the biomimetic specimens have better wear resistance than the untreated specimens. Various shapes of the biomimetic units affect the wear resistance of the specimens. The more the intensified particles of biomimetic units are, the better the wear resistance of the specimens is. The convex biomimetic specimens have the least intensified particles, which results in the worst wear resistance. The various shapes of biomimetic units lead to the difference in the roughness of the biomimetic specimens, which causes the different wear resistance of the specimens. 3.4. Worn surfaces Fig. 9. Wear mass loss as a function of laser input energy.
the substrate, for the reduction of grain sizes of the microstructure, the formation of martensite with a high concentration of carbon, and the great dislocation density [10,18,19] in biomimetic units all enhance the hardness and improve the wear resistance. In addition, a series of biomimetic units augment the surface roughness, which decreases the contact area of the sliding pairs and reduces the wear rate. 3.3.2. Effect of the biomimetic unit shape Another noticeable effect is the shape of biomimetic units. According to the schematic illustration mentioned in Section 2.2, three shapes of biomimetic units were processed, which were convex, stria and grid. Three specimens of each shape were chosen for the wear tests. Fig. 10 shows the average wear mass loss results of the various shapes of biomimetic specimens when the laser input energy was 125 J/cm2. Note that the wear resistance of the various biomimetic specimens is better than that of the untreated ones. The wear resistance varies with the biomimetic unit shape. The wear resistance of the biomimetic specimens with grid biomimetic units is the best, which improves by more than 75% than the untreated specimens; the second is the stria, which improves by 64%; and the third is the convex, which improves by 46%. It may result from the following reasons: Despite the various shapes of the biomimetic specimens, the biomimetic units are tantamount to intensified particles in the substrate. Therefore, they can improve the wear resistance so
Fig. 10. Wear mass loss as a function of biomimetic unit shape.
Fig. 11 shows the SEM micrographs of the worn surfaces of the untreated specimen and the biomimetic specimens after a dry sliding wear test cycle of 30 min under the same wear test conditions against the bearing ring GCr15. Arrowheads indicate the sliding direction and loop areas represent the biomimetic units. As can be seen in Fig. 11a, some materials are lifted and sheared off in the wear process; the wear grooves with irregular shapes are in various depths. Besides, macroscopic observation reveals that some red-brown and black wear debris, whose chemical nature is a-Fe2O3 and Fe3O4 [20], clearly appear on the worn surfaces and some materials adhere to the steel counterparts. These phenomena indicate that there is severe plastic deformation on the surface of the untreated specimen; some materials are pulled out and transfer to the counterpart surface resulting in adhesive wear. Deep grooves on the wear surfaces are generated as a result of cutting (abrasive) action, produced by the wear debris. Some of the debris is the iron oxides, which primarily originated from the contacting surface of the GCr15 bearing steel counterparts and were oxidized during the dry sliding wear process. The slight oxidation wear occurs with the rising of the friction surfaces temperature. Therefore, the wear mechanism of the untreated specimen is the adhesive wear, the abrasive wear and the oxidation wear. Fig. 11b–d compare the morphologies of the worn surfaces of the biomimetic specimens with different kinds of the biomimetic units under the same sliding condition. It is seen that (1) in the substrate zones, all the three types of biomimetic specimens present an adhesive characterization, but the extent of the surface plastic deformation is different because of the various unit shapes. The adhesive characterization in the substrate zone of biomimetic specimen with convex unit (Fig. 11b) is the severest, whereas that of the biomimetic specimen with the grid unit is the mildest (Fig. 11d). (2) In the biomimetic unit zones, the worn surface of the convex unit (Fig. 11b) is characterized by adhesion in the middle part of this zone, whereas the adhesive characterization in the stria unit zone (Fig. 11c) is not obvious. There are no significant characteristic features of adhesive wear except for some large island-like transferred layers in some part of the grid biomimetic unit zone (Fig. 11d). (3) Deep grooves are found in both the substrate zones and the biomimetic zones in the biomimetic specimens with convex unit (Fig. 11b) and with
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Fig. 11. SEM micrographs of the worn surfaces: (a) untreated specimen, (b) specimen with convex biomimetic units, (c) specimen with stria biomimetic units and (d) specimen with grid biomimetic units.
stria unit (Fig. 11c), but only slight plowing is observed on the worn surface in the substrate zone of biomimetic specimen with grid unit (Fig. 11d). (4) Compared with the untreated specimen (Fig. 11a), the extent of the abrasive wear on the substrates of the biomimetic specimens (Fig. 11b–d) is lower and there is less
red-brown and black wear debris on the steel counterpart observed by macrography. The dominative wear mechanism of the biomimetic specimens is the adhesive wear, whereas the abrasive and oxidation signs are not obvious. It is proved that biomimetic units can resist the removal of materials and retard,
Fig. 12. SEM micrographs of the worn surfaces of stria biomimetic specimens processed by laser with various input energies: (a) 25 J/cm2, (b) 75 J/cm2, (c) 125 J/ cm2 and (d) 175 J/cm2.
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confine and prohibit the abrasive action produced by the debris. Moreover, the grid biomimetic specimens have more intensified particles than the stria and the convex ones; hence the grid ones display better wear resistance than the other two. Fig. 12 shows the SEM morphologies of the worn surfaces of the stria biomimetic specimens processed by laser with various input energies under the same dry sliding condition against GCr15 steel for 30 min. As can be seen, the surfaces of the substrates show the adhesive wear and the abrasive wear modes in which the severe plastic deformation, deep grooves and wear debris are found in the sliding direction, whereas those of the stria biomimetic unit zones show less surface damage than the substrates with increasing laser input energies. The low input energy (25 J/cm2, see Fig. 12a) results in severe plastic deformation and deep grooves in both of the substrate zone and the biomimetic unit zone, while the high input energy (175 J/ cm2, see Fig. 12d) results in seldom adhesive characterization in the biomimetic unit zone. By macrography observation, the red-brown and black wear debris on the steel counterpart was degressive with the increasing of laser input energy. According to the explanation mentioned above, the higher laser input energy produces desirable microstructural changes to form a series of harder biomimetic units of greater depth and width, which can improve the wear resistance by retarding, confining and prohibiting the abrasive action produced by the debris and preventing pull-out of the materials by plastically trapping from the wear surface. 4. Conclusions Imitating the cuticles of soil animals, the CGI with biomimetic unit structures processed by the BCLR method is similar to that of the surface of a dung beetle. The experimental study of microstructure, microhardness of biomimetic units and the wear resistance comparisons between biomimetic specimens and untreated specimens lead to the following conclusions: (1) After the BCLR process, there are two microstructure zones in the biomimetic unit: the melted zone and the transition zone. A fine-grained ledeburite (martensite + cementite) with residual austenite is achieved in the melted zone. The transition zone consists of the martensite and undissolved graphite. (2) Wear tests show that the bio-inspired wearable surface can improve the wear resistance of CGI sliding against GCr15 steel rings under dry sliding condition. Both the laser input energy and the shape of biomimetic units influence the wear resistance of the biomimetic specimens. Along with the increase of laser input energy, the depth, the width and the hardness of the biomimetic unit increase, so does the wear resistance. Concerning the shapes of the biomimetic unit, the grid shape biomimetic specimen exhibits the best wear resistance among the three kinds of the biomimetic specimens, the stria times, and the convex worst. (3) On the basis of some strengthening mechanism, such as the refinement of grains and the alteration of microstructure as
well as the great dislocation density in the biomimetic unit zone, the intensified particles effect generated by biomimetic units is the main reason for enhancing the wear resistance. In addition, the contact area of the sliding pairs decreases due to the existence of biomimetic units so that the wear rate reduces. (4) Worn surfaces characterization results indicate that the formation of biomimetic units plays a positive role in reducing wear rate by retarding, confining and prohibiting the abrasive action produced by the debris and preventing pull-out of the materials by plastically trapping from the wear surface. The wear mechanism of the untreated specimens is mainly due to the adhesive wear, the abrasive wear as well as the oxidation wear, whereas the adhesive wear is the main wear mechanism of biomimetic specimens under dry sliding test condition. Acknowledgements The authors would like to acknowledge the National HighTech (863) Program (No. 2002AA331180), the Project 985Automotive Engineering of Jilin University, the Research Fund for the Doctoral Program of Higher Education (No. 20040183026), and the Science and Development Foundation of Jilin for financial support. References [1] G. Cueva, A. Sinatora, W.L. Guesser, A.P. Tschiptschin, Wear 255 (2003) 1256–1260. [2] Y. Zhang, Y. Chen, R. He, B. Shen, Wear 166 (1993) 179–186. [3] B. Shen, Y. Chen, Y. Zhang, Y. Zhang, Chin. J. Iron Steel Res. 9 (1997) 47– 50. [4] H. Kou, B. Shen, Y. Zhang, Y. Chen, Chin. Hot Working Technol. 6 (1997) 14–15. [5] S. Sahin, C. Meric, Mater. Res. Bull. 37 (2002) 971–979. [6] C. Meric, S. Sahin, B. Backir, N.S. Koksal, Mater. Des. 27 (2006) 751–757. [7] V. Ocelı´k, U. de Oliveira, M. de Boer, J.Th.M. de Hosson, Surf. Coat. Technol. 201 (2007) 5875–5883. [8] X. Jia, L.Q. Ren, B.C. Chen, Chin. J. Mater. Res. 10 (1996) 556–560. [9] L.Q. Ren, J. Tong, J.Q. Li, B.C. Chen, J. Agr. Eng. Res. 79 (2001) 239–263. [10] H. Zhou, Z.H. Zhang, L.Q. Ren, Q.F. Song, L. Chen, Surf. Coat. Technol. 200 (2006) 6758–6764. [11] H. Zhou, Y. Cao, Z.H. Zhang, L.Q. Ren, X.Z. Li, Mater. Sci. Eng. A 433 (2006) 144–148. [12] H. Zhou, L. Chen, W. Wang, L.Q. Ren, H.Y. Shan, Z.H. Zhang, Mater. Sci. Eng. A 412 (2005) 323–327. [13] L. Chen, H. Zhou, Y. Zhao, L.Q. Ren, X.Z. Li, J. Mater. Process Technol. 190 (2007) 211–216. [14] J. Grum, R. Sˇturm, Mater. Charact. 37 (1996) 81–88. [15] J. Grum, R. Sˇturm, Appl. Surf. Sci. 187 (2002) 116–123. [16] K.Y. Benyounis, O.M.A. Fakron, J.H. Abboud, A.G. Olabi, M.J.S. Hashmi, J. Mater. Process Technol. 170 (2005) 127–132. [17] L. Pawlowski, The Science and Engineering of Thermal Spry Coatings, John Wiley and Sons, 1995,, p. 211. [18] H. De Beurs, J.A. Hovius, J.Th.M. De Hosson, Acta Metall. 26 (1988) 3123–3130. [19] J.P. Chu, J.M. Rigsbee, G. Banas´, H.E. Elsayed-Ali, Mater. Sci. Eng. A 260 (1999) 260–268. [20] K.-H. Habig, Wear Behavior and Hardness of Materials (L. Yan, Trans.), China Machine Press, Beijing, 1987 , pp. 187–193 (Original work published 1980) (in Chinese).
Bio-inspired wearable characteristic surface: Wear behavior of cast iron with biomimetic units processed by laser Hong Zhou a,*, Na Sun a, Hongyu Shan a, Dianyi Ma a, Xin Tong a, Luquan Ren b b
a Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun, Jilin 130025, PR China Key Laboratory of Terrain-Machine Bionics Engineering of Ministry of Education, Jilin University, Changchun, Jilin 130025, PR China
Received 14 March 2007; received in revised form 9 June 2007; accepted 11 June 2007 Available online 16 June 2007
Abstract Stimulated by the cuticles of soil animals, an attempt to improve the wear resistance of compact graphite cast iron (CGI) with biomimetic units on the surface was made by using a biomimetic coupled laser remelting (BCLR) process. The microstructure and microhardness of biomimetic units were examined. The wear behaviors of biomimetic specimens as functions of laser input energy and biomimetic unit shape were investigated under dry sliding condition, respectively. The results indicated that the biomimetic specimens had better wear resistance than the untreated specimens. The wear resistance of the biomimetic specimens increases with the increase of laser input energy due to the increase of the depth and the width of biomimetic units as well as the increase of the microhardness. The specimen with grid biomimetic units had the best resistance, the stria took the second place and the convex showed the worst. The application of laser remelting provided desirable microstructural changes in biomimetic units, which generated the intensified particles effect for improving the wear resistance. The adhesive wear was the dominative wear mechanism for the biomimetic specimens. # 2007 Elsevier B.V. All rights reserved. Keywords: Biomimetic; Cast iron; Microstructure; Wear behavior; Laser
1. Introduction As compact graphite cast iron (CGI) has high friction coefficient and low wear rate under dry sliding condition, it is one kind of materials that are widely used in vehicle brakes [1]. However, since the development of railway transportation tends to be high-speed and heavy-duty in recent years, the service life of disk brakes decreases significantly. How to improve the wear resistance of CGI to meet the requirement of demanding tribological application is always one of the problems of concern to material researchers. In the previous study, improving the mechanical properties of brake materials were mainly done by adding appropriate alloying elements [2–4], or improving the surface quality [5–7]. According to Zhang et al., the wear resistance of CGI was improved with the increase of phosphorous, vanadium or chromium contents after conducting a series of wear tests using self-made dry sliding tester. The results indicated that the CGI with phosphorous addition * Corresponding author. Tel.: +86 431 8509 4427; fax: +86 431 8509 5592. E-mail address: [email protected] (H. Zhou). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.014
possessed the combination of the high friction coefficient and low wear mass loss [2]; the addition of 0.3 wt.% vanadium to CGI reduced its wear mass loss and improved the wear loss stability when the contact pressure and sliding velocity changed [3]; 0.5 wt.% chromium CGI presented the optimal tribological properties [4]. Sahin et al. [5,6] investigated the effect of boronizing on CGI by box-boronizing method and found that the weight loss decreased with boride layer thickness. The wear properties of CGI with Co-based coatings processed by a high power laser (2 kW continuous Nd:YAG) were examined under dry sliding condition by Ocelı´k et al. [7]. It was found that the good wear resistance of this type of alloy was obtained only at high temperatures (up to 525 8C). Nevertheless, owing to the difficulty of choosing suitable elements with proper chemical compositions, it is difficult to enhance the wear resistance only in one aspect without sacrificing the mechanic property of the whole materials. Furthermore, some complex processes and the additions of elements for these materials are commonly at the cost of increasing the production expense greatly. Nature provides a whole host of superior multifunctional structures that can be used as inspirational systems for the
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design and synthesis of new, technologically important materials and devices. Since the 1980s, Ren et al. [8,9] has been dedicating to the study of the cuticle morphologies and principles of soil animals such as dung beetles, black ants, and pangolins and found that there were generally five kinds of simple structures on the cuticles, including convex, concave, stria, bristle and squama. They are called ‘non-smooth construction units’, which have been found to provide excellent anti-wear properties against soil. Recent works in our research group also found that when applied biomimetic principle on the die and tool steel surfaces to form a series of biomimetic units by laser, there had a considerable effect not only on resisting to thermal fatigue [10,11], but also on improving the abrasive wear resistance [12,13]. Based on the results above, it was decided to explore whether similar benefits could be obtained in resistance to dry sliding wear of CGI treated by biomimetic coupling laser remelting (BCLR) process, without changing the special properties of substrate materials such as excellent machinability, the ability to resist galling and excellent vibration damping. In the present work, we chose the laser facility to manufacture and process the biomimetic units on the surfaces of CGI to form a novel bio-inspired wearable surface. Wear tests were carried out on a block-on-ring tester with a biomimetic CGI block against a GCr15 steel ring under dry sliding wear. Experiments were focused on the effects of laser input energy and biomimetic unit shape on the wear behavior of the bio-inspired wearable surfaces under dry sliding condition. 2. Experimental 2.1. Materials Specimens for wear tests were cut from disk brakes for the high-speed train with the sizes of 14 mm 10 mm 10 mm. The materials of the disc brakes are the CGI (250 HV), whose chemical compositions are listed in Table 1. Fig. 1 is an optical micrograph of the CGI substrate, which consists of vermiculate graphite surrounded by ferrite and some amount of pearlite. 2.2. Preparation of biomimetic specimens Mimicking the cuticles of dung beetles [8] (Fig. 2) the various biomimetic units were processed by the BCLR method using a Nd:YAG laser of 1064 nm wavelength and maximum power 100 W and a Gaussian distribution of the energy in the beam. The processing parameters were laser pulse duration 5.0 ms, frequency 14 Hz and scanning speed 0.71 mm/s. A series of experiments were carried out using different laser input energies and different biomimetic unit shapes, respectively. The laser input energies were 25, 75, 125 and 175 J/cm2 Table 1 Chemical compositions of compact graphite cast iron (CGI) Element
C
Si
Mn
P
S
Re
Mg
Fe
Composition (wt.%) 3.56 2.56 0.71 0.03 0.03 0.02 0.02 Balance
Fig. 1. An optical microstructure of the CGI substrate.
to form the stria biomimetic units on the specimens’ surfaces for investigating the influence of the laser input energy on the wear resistance, respectively. Then the laser input energy 125 J/ cm2 was fixed, while the shapes on the sides of the biomimetic specimens were convex, stria and grid for investigating the influence of unit shape on the wear resistance, respectively. The biomimetic unit distance for all the specimens is 2 mm. A schematic illustration of biomimetic specimens is shown in Fig. 3. After the laser processing, transverse sections were obtained and the standard method of metallography was followed to prepare the samples for the microstructure analysis and the microhardness measurement, using a JSM-5600LV scanning electron microscope and a Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA) with a 25 g load, respectively. 2.3. Wear tests Dry sliding wear tests were performed using an MM-200 block-on-ring wear tester (made by the Xuanhua Testing Machine Factory). Its schematic diagram is Fig. 4. All the friction and wear tests were carried out at room temperature. A stationary CGI block with biomimetic units slid on a rotating GCr15 steel ring. The GCr15 steel (austenized at 840 8C, oil quenched and tempered for 2 h at 170 8C) had an average hardness of 61–63 HRC and the sizes of 16 mm 50 mm 10 mm (inside diameter outside diameter thickness). A normal load of 8 kg was used for wear tests. The rotational speed of the ring was 400 rpm and the sliding time was 30 min. Before each test, the surface of the steel ring was polished to a roughness of about 0.1 mm, while biomimetic specimens were ultrasonically cleaned in anhydrous alcohol and dried before and after wear tests. The mass loss was measured by a sensitive electronic balance with an accuracy of 0.0001 g. The difference in mass of three test blocks before and after the experiment gave the average mass loss. To assist the analysis of wear mechanisms, the worn surfaces of block specimens were examined by a JSM-5600LV scanning electron microscopy.
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Fig. 2. SEM micrograph of typical shape of dung beetle cuticles: (a) convex shape of head and (b) stria shape of elytron.
3. Results and discussion 3.1. Microstructure The starting microstructure of the CGI consists of vermiculate graphite surrounded by ferrite and some amount of pearlite. After the BCLR process, a modified layer called biomimetic unit was obtained (see Fig. 5), which consists of two characteristic microstructure zones, i.e. a melted zone and a transition zone. 3.1.1. Melted zone Fig. 6 shows the corroded cross-section microstructures of the biomimetic units processed by laser with various input energies of 25, 75, 125 and 175 J/cm2, respectively. Compared with the original structure, the unit structure is refined and complete dissolution of graphite and fusion of metal are evident. Some dendritic networks and some needlelike structures of irregular shapes in random locations are observed in the melted zone, which can be found in all the biomimetic specimens. According to the XRD analysis of the melted layer (Fig. 7) and SEM observation (Fig. 6c) it is found that the melted zone is composed of ledeburite (martensite + cementite), also containing a small amount of residual austenite. Similar microstructures were also found in the previous work [14,15].
Fig. 3. A schematic illustration of biomimetic specimens.
Fig. 4. A schematic illustration of the block-on-ring wear tester.
The metallurgy mechanism can be interpreted as follows. The surface of the biomimetic specimen is rapidly melted after laser irradiation. Since the melting occurs only at the surface over a relatively short duration and the bulk of the substrate remains cool, the liquid metal in the molten pool will solidify and recrystallize at a high cooling rate. At the beginning of solidification, the primary austenite forms and then crystallizes as the cellular/dendrite structure in the direction towards the surface, which corresponds to the contrary direction of the heat transfer. The last liquid will be enriched in carbon and form cementite surrounding the austenite dendrites [16]. When the temperature reaches the eutectic point, the eutectic reaction
Fig. 5. SEM structure of biomimetic unit processed by laser with an input energy of 125 J/cm2.
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Fig. 6. SEM structures of the melted zone of the biomimetic units processed by laser with various input energies: (a) 175 J/cm2, (b) 125 J/cm2, (c) 75 J/cm2 and (d) 25 J/cm2.
occurs. The solidification structures are the primary austenite and the fine ledeburite (eutectic austenite + cementite). During the rapid cooling, the primary austenite and eutectic austenite could transform mostly to martensite with some residual austenite. As a result, the final structure of the melted zone is fine-grained ledeburite (martensite + cementite) and residual austenite. However, the ledeburite in the present study is distinctly different from the conventional one such as in a white iron, indicating a considerable amount of a high cooling rate during solidification; because the former comprises martensite and cementite, the latter is a matrix containing pearlite and cementite.
3.1.2. Transition zone Fig. 8 shows the microstructures of the transition zones of the biomimetic units processed by laser with various input energies of 25, 75, 125 and 175 J/cm2, respectively. Note that the morphologies of the transition zone are extremely different from the melted zone. In this zone, only partial dissolution of the graphite occurred. The microstructure shows that there is martensite and undissolved graphite in this zone. It may be explained as follows: there is high degree of superheating and austenite nucleation in the transition zone due to the thermal influence from the melted zone. Thus, the pearlite, into which the additional carbon from graphite diffuses during the laser heating, transforms into a carbon rich austenite microstructure. Meanwhile, the ferrite transforms into an austenite microstructure, in which the carbon diffuses rapidly at high temperature [15,16]. Nevertheless, the austenite does not have enough time to grow up and homogenize at a high cooling rate. Accordingly, the product is the inhomogeneous fine austenite. During the rapid cooling, the martensite transformation is inevitable. The final structures are martensite and undissolved graphite. 3.2. Microhardness
Fig. 7. XRD analysis of untreated specimen and biomimetic specimen processed by laser with an input energy of 75 J/cm2.
Microhardness is of particular importance in assessing wear resistant coatings, because it is considered as a good indication of coating integrity and wear resistance [17]. The great difference in microstructures between the untreated and the biomimetic specimens suggests that there may be a corresponding improvement in mechanical properties. Table 2 illustrates the depth, the width and the average microhardness of biomimetic units after the BCLR treatment
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Fig. 8. SEM structure of the transition zone of the biomimetic units processed by laser with various input energies: (a) 175 J/cm2, (b) 125 J/cm2, (c) 75 J/cm2 and (d) 25 J/cm2.
using different input energies. The biomimetic units’ dimensions were the average of five measurements of each specimen. The reported microhardness values were the average of 5–10 measurements along the direction of laser irradiation (from the top of the unit to the bottom), which were obtained at different locations on the polished and etched cross-sections of the biomimetic units for each specimen. The average microhardness of the substrate is 250 HV, while that of the biomimetic units in the melted zone varies from 540 to 680 HV, and in the transition zone varies from 460 to 520 HV. The low input energy (25 J/cm2) results in a narrow melted zone (0.485 mm) and of a little penetration (0.100 mm) with a relatively low hardness value in the melted zone (540 HV), so does the transition zone (460 HV); while the high input energy (175 J/cm2) produces a deep (0.270 mm) and wide melted zone (0.861 mm) with a relatively high hardness value in the melted zone (680 HV). It can be attributed to the solution of carbon, the formation of martensite, the refinement of structure and the large proportion of cementite.
3.3. Wear resistance 3.3.1. Effect of the laser input energy The wear resistance specimens are divided into five groups. One group was untreated, while the others were processed to stria using different laser input energies. Three specimens of each group were chosen for wear tests. The average mass losses of biomimetic specimens as a function of laser input energy are presented in Fig. 9. It can be seen that the wear mass loss of the biomimetic specimens decreases more than 48% compared with that of the untreated specimens in the same sliding way against the steel blocks. The biomimetic specimens display different wear resistance, which increases by 0.92, 1.56, 1.79 and 2.17 times of that of untreated specimens, respectively, as the laser input energies are 25, 75, 125 and 175 J/cm2, respectively. That is, the wear mass loss decreases with the increase of the laser input energy. The lower wear mass loss for biomimetic specimens may originate from the existence of the biomimetic units, which is equivalent to many uniformly distributed intensified particles in
Table 2 Dimensions and average hardness of biomimetic units processed at different conditions Specimen No.
Input energy (J/cm2)
Scanning speed (mm/s)
Unit depth (mm)
Unit width (mm)
Melted zone
Transition zone
Depth (mm)
Width (mm)
Average hardness (HV)
Average hardness (HV)
1 2 3 4 5
25 75 125 175 –
0.71 0.71 0.71 0.71 –
0.141 0.283 0.362 0.389 –
0.568 0.883 0.984 1.023 –
0.100 0.185 0.245 0.270 –
0.485 0.787 0.835 0.861 –
540 600 660 680 250
460 490 520 520 250
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that the biomimetic specimens have better wear resistance than the untreated specimens. Various shapes of the biomimetic units affect the wear resistance of the specimens. The more the intensified particles of biomimetic units are, the better the wear resistance of the specimens is. The convex biomimetic specimens have the least intensified particles, which results in the worst wear resistance. The various shapes of biomimetic units lead to the difference in the roughness of the biomimetic specimens, which causes the different wear resistance of the specimens. 3.4. Worn surfaces Fig. 9. Wear mass loss as a function of laser input energy.
the substrate, for the reduction of grain sizes of the microstructure, the formation of martensite with a high concentration of carbon, and the great dislocation density [10,18,19] in biomimetic units all enhance the hardness and improve the wear resistance. In addition, a series of biomimetic units augment the surface roughness, which decreases the contact area of the sliding pairs and reduces the wear rate. 3.3.2. Effect of the biomimetic unit shape Another noticeable effect is the shape of biomimetic units. According to the schematic illustration mentioned in Section 2.2, three shapes of biomimetic units were processed, which were convex, stria and grid. Three specimens of each shape were chosen for the wear tests. Fig. 10 shows the average wear mass loss results of the various shapes of biomimetic specimens when the laser input energy was 125 J/cm2. Note that the wear resistance of the various biomimetic specimens is better than that of the untreated ones. The wear resistance varies with the biomimetic unit shape. The wear resistance of the biomimetic specimens with grid biomimetic units is the best, which improves by more than 75% than the untreated specimens; the second is the stria, which improves by 64%; and the third is the convex, which improves by 46%. It may result from the following reasons: Despite the various shapes of the biomimetic specimens, the biomimetic units are tantamount to intensified particles in the substrate. Therefore, they can improve the wear resistance so
Fig. 10. Wear mass loss as a function of biomimetic unit shape.
Fig. 11 shows the SEM micrographs of the worn surfaces of the untreated specimen and the biomimetic specimens after a dry sliding wear test cycle of 30 min under the same wear test conditions against the bearing ring GCr15. Arrowheads indicate the sliding direction and loop areas represent the biomimetic units. As can be seen in Fig. 11a, some materials are lifted and sheared off in the wear process; the wear grooves with irregular shapes are in various depths. Besides, macroscopic observation reveals that some red-brown and black wear debris, whose chemical nature is a-Fe2O3 and Fe3O4 [20], clearly appear on the worn surfaces and some materials adhere to the steel counterparts. These phenomena indicate that there is severe plastic deformation on the surface of the untreated specimen; some materials are pulled out and transfer to the counterpart surface resulting in adhesive wear. Deep grooves on the wear surfaces are generated as a result of cutting (abrasive) action, produced by the wear debris. Some of the debris is the iron oxides, which primarily originated from the contacting surface of the GCr15 bearing steel counterparts and were oxidized during the dry sliding wear process. The slight oxidation wear occurs with the rising of the friction surfaces temperature. Therefore, the wear mechanism of the untreated specimen is the adhesive wear, the abrasive wear and the oxidation wear. Fig. 11b–d compare the morphologies of the worn surfaces of the biomimetic specimens with different kinds of the biomimetic units under the same sliding condition. It is seen that (1) in the substrate zones, all the three types of biomimetic specimens present an adhesive characterization, but the extent of the surface plastic deformation is different because of the various unit shapes. The adhesive characterization in the substrate zone of biomimetic specimen with convex unit (Fig. 11b) is the severest, whereas that of the biomimetic specimen with the grid unit is the mildest (Fig. 11d). (2) In the biomimetic unit zones, the worn surface of the convex unit (Fig. 11b) is characterized by adhesion in the middle part of this zone, whereas the adhesive characterization in the stria unit zone (Fig. 11c) is not obvious. There are no significant characteristic features of adhesive wear except for some large island-like transferred layers in some part of the grid biomimetic unit zone (Fig. 11d). (3) Deep grooves are found in both the substrate zones and the biomimetic zones in the biomimetic specimens with convex unit (Fig. 11b) and with
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Fig. 11. SEM micrographs of the worn surfaces: (a) untreated specimen, (b) specimen with convex biomimetic units, (c) specimen with stria biomimetic units and (d) specimen with grid biomimetic units.
stria unit (Fig. 11c), but only slight plowing is observed on the worn surface in the substrate zone of biomimetic specimen with grid unit (Fig. 11d). (4) Compared with the untreated specimen (Fig. 11a), the extent of the abrasive wear on the substrates of the biomimetic specimens (Fig. 11b–d) is lower and there is less
red-brown and black wear debris on the steel counterpart observed by macrography. The dominative wear mechanism of the biomimetic specimens is the adhesive wear, whereas the abrasive and oxidation signs are not obvious. It is proved that biomimetic units can resist the removal of materials and retard,
Fig. 12. SEM micrographs of the worn surfaces of stria biomimetic specimens processed by laser with various input energies: (a) 25 J/cm2, (b) 75 J/cm2, (c) 125 J/ cm2 and (d) 175 J/cm2.
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confine and prohibit the abrasive action produced by the debris. Moreover, the grid biomimetic specimens have more intensified particles than the stria and the convex ones; hence the grid ones display better wear resistance than the other two. Fig. 12 shows the SEM morphologies of the worn surfaces of the stria biomimetic specimens processed by laser with various input energies under the same dry sliding condition against GCr15 steel for 30 min. As can be seen, the surfaces of the substrates show the adhesive wear and the abrasive wear modes in which the severe plastic deformation, deep grooves and wear debris are found in the sliding direction, whereas those of the stria biomimetic unit zones show less surface damage than the substrates with increasing laser input energies. The low input energy (25 J/cm2, see Fig. 12a) results in severe plastic deformation and deep grooves in both of the substrate zone and the biomimetic unit zone, while the high input energy (175 J/ cm2, see Fig. 12d) results in seldom adhesive characterization in the biomimetic unit zone. By macrography observation, the red-brown and black wear debris on the steel counterpart was degressive with the increasing of laser input energy. According to the explanation mentioned above, the higher laser input energy produces desirable microstructural changes to form a series of harder biomimetic units of greater depth and width, which can improve the wear resistance by retarding, confining and prohibiting the abrasive action produced by the debris and preventing pull-out of the materials by plastically trapping from the wear surface. 4. Conclusions Imitating the cuticles of soil animals, the CGI with biomimetic unit structures processed by the BCLR method is similar to that of the surface of a dung beetle. The experimental study of microstructure, microhardness of biomimetic units and the wear resistance comparisons between biomimetic specimens and untreated specimens lead to the following conclusions: (1) After the BCLR process, there are two microstructure zones in the biomimetic unit: the melted zone and the transition zone. A fine-grained ledeburite (martensite + cementite) with residual austenite is achieved in the melted zone. The transition zone consists of the martensite and undissolved graphite. (2) Wear tests show that the bio-inspired wearable surface can improve the wear resistance of CGI sliding against GCr15 steel rings under dry sliding condition. Both the laser input energy and the shape of biomimetic units influence the wear resistance of the biomimetic specimens. Along with the increase of laser input energy, the depth, the width and the hardness of the biomimetic unit increase, so does the wear resistance. Concerning the shapes of the biomimetic unit, the grid shape biomimetic specimen exhibits the best wear resistance among the three kinds of the biomimetic specimens, the stria times, and the convex worst. (3) On the basis of some strengthening mechanism, such as the refinement of grains and the alteration of microstructure as
well as the great dislocation density in the biomimetic unit zone, the intensified particles effect generated by biomimetic units is the main reason for enhancing the wear resistance. In addition, the contact area of the sliding pairs decreases due to the existence of biomimetic units so that the wear rate reduces. (4) Worn surfaces characterization results indicate that the formation of biomimetic units plays a positive role in reducing wear rate by retarding, confining and prohibiting the abrasive action produced by the debris and preventing pull-out of the materials by plastically trapping from the wear surface. The wear mechanism of the untreated specimens is mainly due to the adhesive wear, the abrasive wear as well as the oxidation wear, whereas the adhesive wear is the main wear mechanism of biomimetic specimens under dry sliding test condition. Acknowledgements The authors would like to acknowledge the National HighTech (863) Program (No. 2002AA331180), the Project 985Automotive Engineering of Jilin University, the Research Fund for the Doctoral Program of Higher Education (No. 20040183026), and the Science and Development Foundation of Jilin for financial support. References [1] G. Cueva, A. Sinatora, W.L. Guesser, A.P. Tschiptschin, Wear 255 (2003) 1256–1260. [2] Y. Zhang, Y. Chen, R. He, B. Shen, Wear 166 (1993) 179–186. [3] B. Shen, Y. Chen, Y. Zhang, Y. Zhang, Chin. J. Iron Steel Res. 9 (1997) 47– 50. [4] H. Kou, B. Shen, Y. Zhang, Y. Chen, Chin. Hot Working Technol. 6 (1997) 14–15. [5] S. Sahin, C. Meric, Mater. Res. Bull. 37 (2002) 971–979. [6] C. Meric, S. Sahin, B. Backir, N.S. Koksal, Mater. Des. 27 (2006) 751–757. [7] V. Ocelı´k, U. de Oliveira, M. de Boer, J.Th.M. de Hosson, Surf. Coat. Technol. 201 (2007) 5875–5883. [8] X. Jia, L.Q. Ren, B.C. Chen, Chin. J. Mater. Res. 10 (1996) 556–560. [9] L.Q. Ren, J. Tong, J.Q. Li, B.C. Chen, J. Agr. Eng. Res. 79 (2001) 239–263. [10] H. Zhou, Z.H. Zhang, L.Q. Ren, Q.F. Song, L. Chen, Surf. Coat. Technol. 200 (2006) 6758–6764. [11] H. Zhou, Y. Cao, Z.H. Zhang, L.Q. Ren, X.Z. Li, Mater. Sci. Eng. A 433 (2006) 144–148. [12] H. Zhou, L. Chen, W. Wang, L.Q. Ren, H.Y. Shan, Z.H. Zhang, Mater. Sci. Eng. A 412 (2005) 323–327. [13] L. Chen, H. Zhou, Y. Zhao, L.Q. Ren, X.Z. Li, J. Mater. Process Technol. 190 (2007) 211–216. [14] J. Grum, R. Sˇturm, Mater. Charact. 37 (1996) 81–88. [15] J. Grum, R. Sˇturm, Appl. Surf. Sci. 187 (2002) 116–123. [16] K.Y. Benyounis, O.M.A. Fakron, J.H. Abboud, A.G. Olabi, M.J.S. Hashmi, J. Mater. Process Technol. 170 (2005) 127–132. [17] L. Pawlowski, The Science and Engineering of Thermal Spry Coatings, John Wiley and Sons, 1995,, p. 211. [18] H. De Beurs, J.A. Hovius, J.Th.M. De Hosson, Acta Metall. 26 (1988) 3123–3130. [19] J.P. Chu, J.M. Rigsbee, G. Banas´, H.E. Elsayed-Ali, Mater. Sci. Eng. A 260 (1999) 260–268. [20] K.-H. Habig, Wear Behavior and Hardness of Materials (L. Yan, Trans.), China Machine Press, Beijing, 1987 , pp. 187–193 (Original work published 1980) (in Chinese).