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Effects of different bionic units coupling on the sliding wear of gray cast iron
Surface & Coatings Technology 309 (2017) 96–105
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Effects of different bionic units coupling on the sliding wear of gray cast iron Haifeng Zhang a,b, Ti Zhou a,⁎, Hong Zhou a, Zhikai Chen a, Wanshi Yang a, Luquan Ren c a b c
The Key Lab of Automobile Materials, The Ministry of Education, Jilin University, Changchun 130025, PR China The College of Mechanical and Automotive Engineering, Changchun University, Changchun 130022, PR China The Key Lab of Terrain Machinery Bionics Engineering, The Ministry of Education, Jilin University, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 18 August 2016 Revised 13 November 2016 Accepted in revised form 14 November 2016 Available online 15 November 2016 Keywords: Bionic strip unit Bionic pit unit Laser Wear Gray cast iron
a b s t r a c t To enhance the sliding wear resistance of gray cast iron under starved lubrication conditions, two kinds of bionic units were generated by using a pulsed Nd: YAG laser and a drill on the surface of the specimens. The microstructure and microhardness of the bionic strip unit were examined. Comparing the wear resistance of five groups of specimens (an untreated specimen; a specimen with a surface that was completely melted by the laser; a specimen containing only bionic pit unit; a specimen containing only bionic strip unit and a specimen coupling two kinds of bionic units), the results indicate that the wear resistance of the bionic unit coupling specimen is better than that of the others. The result of a stress analysis of the specimen surface shows that the bionic strip unit plays a supporting role and can reduce the stress concentration on the edge of the bionic pit unit. On the other hand, the bionic pit unit can supply lubricating oil to the wear gap, change the moving model of abrasive grains and improve the lubrication performance. The two kinds of units can protect and promote each other under starved lubrication conditions, improve the resistance to sliding wear. © 2016 Elsevier B.V. All rights reserved.
1. Introduction A machine tools guide is one of the most important components in a machine tool, because its mechanical accuracy directly determines the quality of the tool. Gray cast iron (GCI) has been used for machine tool guides for a long time because of its superior wear resistance, low cost, excellent machinability, etc. [1–4]. In addition, GCI can be used to absorb shock that occurs during the work of the machine tools, because of its microstructure that the amount of flake graphite spreads well in an iron matrix. The dominant failure types of a machine tool guide are abrasive wear and adhesion wear [5], which directly affect the life span of the tool. Therefore, it is necessary to improve the wear resistance of GCI. Many researchers have focused on improving the wear resistance of cast iron which surface was melted by a laser [6–9], furthermore, reinforced particles such as the W-, or Ni-based surface film was fabricated by a high energy beam like a gas tungsten arc and a laser [10,11]. Living creature is a continuing source of inspiration in engineering field. The perfect performances on the surface of some animals are derived from the coupling of their different materials, structures and morphologies [12]. Such coupling with different elements brings inspiration to design and study on the bionic coupling surface designed with multiple factors which can interact on each other in engineering. Previously, ⁎ Corresponding author. E-mail address: [email protected] (T. Zhou).
http://dx.doi.org/10.1016/j.surfcoat.2016.11.056 0257-8972/© 2016 Elsevier B.V. All rights reserved.
we fabricated the trails by laser technology or other methods on the relatively soft substrate materials [13,14]. We called the trails as bionic unit which has a better performance than the substrate. Researchers have become aware that many types of bionic coupling elements, such as structures, functions and materials, which are the result of the biological evolution of animals and plants as they adapt to the living environment, could be applied in the field of engineering. Zhou et al. have applied the bionic principle on tool and die steel surfaces to process a series of bionic units using a laser; this has enhanced resistance to thermal fatigue [13– 16], improving the abrasive wear resistance [17–19] and strengthening the mechanical properties [20,21]. In fact, animals and plants almost always apply the coupling effects of various elements. However, most of the current research in this area has concentrated on studying the functions of a single coupling element; little work has been done on the combined effect of two or more coupling elements to solve a problem. In this paper, two kinds of bionic units are coupled on the surface of GCI. Their different characteristics protect and promote each other to improve sliding wear resistance under starved lubrication conditions. 2. Experiments 2.1. Experimental materials GCI codename HT250, which is widely used for machine tool guides, is applied as the base material. Fig. 1 shows the micrograph of the GCI, the microstructure of which consists of flake graphite (G) surrounded
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Group, DK77, China). All surfaces of the specimens were polished using progressively finer grades of silicon carbide impregnated emery paper to remove all the surface irregularities and machining marks, then cleaned with anhydrous ethanol before being drilled and laser processed. The roughness (Ra) of the specimens was 200 μm. A pulsed Nd: YAG laser with 1.06 μm wavelength and rated power of 300 W was employed to create the BSU using a circular pattern Gaussian beam. Fig. 4 shows the schematic of the experimental set-up of the laser processing. The laser head was mounted vertically in the Z-direction and was stationary, while a four-axis displacement machine with numerical control system (G code) carries out the spatial displacement. During the laser process, the samples were placed on the displacement machine. Movement along X and Y axes was used to process the bionic strip units while that along Z-axis was to adjust the desired defocusing amount. The specimens were fabricated using this equipment at room temperature and using argon as the shielding gas in order to preventing the oxidation of BSU. Table 2 provides the parameters of the laser melting. After the fabrication process, the surface and side-face of the specimens were mechanically polished using the emery paper and then cleaned with anhydrous ethanol.
Fig. 1. Microstructure of HT250 gray cast iron.
by pearlite (P). The ultimate strength was 133 ± 3 MPa, the yield strength was 115 ± 3 MPa. The composition of the GCI is listed in Table 1. 2.2. Specimen preparation and bionic prototype The structure of a bivalve mollusk shell was chosen as one bionic prototype because of its strong wear resistance, which evolved in a beach environment (Fig. 2(a)). Though the shells suffered significant abrasive wear from the mixture of water, mud and sand, they were still well preserved. Observation shows that the nacreous layer of the shell is made of an aragonite layer (hard) and an organic layer (soft). Moreover, the structure of alternately soft and hard layers is one important factor that could enhance the wear resistance and toughness of the nacreous layer [22,23]. Inspirited by the shell structure, a bionic strip unit (BSU) was processed on the specimen surface using laser melting. The purpose during fabrication was to ensure the BSU and matrix had optimal hard and soft alternate structure. The distribution of the BSU was designed as a grid, because a grid has the best wear resistance among the three kinds of distributions (convex, stria and grid) in the same wear environment [5]. As shown in Fig. 3(c), the angle between BSUs and the direction of wear is designed as 45°. The distance between the BSUs is designed as 8 mm. The pit morphology on head of a dung beetle was chosen as the other bionic prototype, because the pit morphology easily stores air when the beetle's head suffers extrusion and friction from the soil (Fig. 2(b)). The pit morphology could reduce the atmospheric pressure, thereby reducing the friction. According to the pit morphology, the bionic pit unit (BPU) was processed on the specimen surface using a drill. The pit is a cone with a diameter of 1.8 mm. The angle of the cone is 120°. The distance between BPUs is 8 mm (Fig. 3(d)). In order to compare the wear resistance, five groups of specimens were prepared (Fig. 3): an untreated specimen (SI) (Fig. 3(a)), a specimen with a surface that was completely melted with the laser melted parameter (SII) (Fig. 3(b)), a specimen containing only BPU (SIII) (Fig. 3(c)), a specimen containing only BSU (SIV) (Fig. 3(d)) and a specimen coupling two kinds of bionic units (SV) (Fig. 3(e)). All specimens were cut to the same size, 30 mm (L) × 20 mm (W) × 6 mm (D), using a wire electrical discharge machine (Huadong
Table 1 Chemical compositions of HT250 (wt%). Elements
C
Si
Mn
P
S
Cu
Cr
Fe
Composition (wt%)
3.25
1.57
0.92
0.06
0.059
0.5
0.27
Bal.
2.3. Experimental methods The BSU was cut along the vertical direction of the strip. The cross sections of the BSU were obtained for microstructural analysis and measurement of the microhardness; the equipment used was a JSM-5600LV scanning electron microscope and a Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA). A Wyko NT9100 Optical Profiler (manufactured by Veeco, Ltd., USA) was employed for observing and recording the surface morphology and roughness of the specimens after they were wear tested. Phases formed in the BSU were identified by D/max-RC X-ray diffraction (XRD) with Cu Kα radiation operated at a voltage of 40 kV, a current of 40 mA, and a scanning rate of 40/min. In the process of sliding wear, the value and distribution of the von Mises stress on the surfaces of the specimens were estimated by the finite element method. Sliding wear tests were performed using a self-made wear tester (Fig. 5). The eccentric wheel was put in motion along with the rotation of gear reducer by electromotor, which makes the moving part of the simple go on straight line reciprocating movement along with the connecting rod, and the load is located at the top of the moving part. The speed of the wear test was controlled by the frequency converter, which was linked to the electromotor. The pressure of wear test was adjusted by the load's adding or subtracting. The friction pair was a highfrequency induction quenching GCI with an average hardness of 50HRC. The average speed of the wear test was 7 cm/s. The time of wear was 24 h under a load of 120 N at room temperature. The setting of these experimental parameters was the purpose that tried to simulate the working environment of a machine tools guide. Usually, the surface of the machine tools guide is no longer added with the lubricant on the work. Therefore, the surfaces of specimens and friction pairs were covered with the lubrication oil at the beginning of the wear. Due to no longer add the lubrication oil, starved lubrication was formed in the process of wear. In such a test, it is incorrect to record the mass loss of the specimens, since the GCI adsorbs lubricating oil in the wear process under starved lubrication conditions. In present paper, the indentation method was utilized to demonstrate the degree of wear, in which the D-value of measured indentation areas before and after the wear test reflected the volume of mass loss. The microhardness measurement was carried out by a Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA) at a load of 0.2 kg and a dwell time of 10 s. Before the wear test, the microhardness was measured; namely, the indentations were manufactured with a 500 N load in multiple different regions, and the positions and areas of those indentations (A1) were recorded. After the wear test, the areas of the same indentations were measured again (A2). The thickness was obtained by
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Fig. 2. Bionic prototype: (a) shell of bivalve mollusk; (b) head of dung beetle.
calculating the difference areas of the indentations at the beginning and end of wear. In order to reduce the margin of error, several indentations were made on each specimen, and then the average value of the thickness loss was calculated. Fig. 6 shows the schematic of the indentation method; the indentation is the rectangular pyramid at an angle of 136° to the opposite area. A1 and A2 are the magnitude of the areas before and after the wear test, respectively. H denotes the thickness of the metal removal, which can be expressed as follows: H ¼ tg22 °
pffiffiffiffiffiffi pffiffiffiffiffiffi A1 − A2 =2
3. Results and discussion 3.1. Microstructure Fig. 7 shows the SEM micrograph of the etched cross section of the BSU. The cross section of the unit consists of two characteristic microstructural zones: a melted zone and a transition zone. Because of the high temperature in the laser spot zone, the material was melted in this zone. After the laser spot was moved away, the melted material solidified at a high cooling rate, and then the melted zone was formed. The obvious difference between the melted zone and the matrix was that
Fig. 3. Sketch of the five groups of specimens: (a) untreated; (b) all of surface was melted by laser; (c) contained only BSU; (d) contained only bionic pit unit; (e) bionic coupling unit.
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As shown in Fig. 8, X-ray diffraction was used for comparing the phase compositions of the untreated specimen and the BSU before and after wear. There is the same phase of ferrite and cementite (Fe3C). The difference between the BSU and the untreated specimen is that the untreated specimen contains graphite (G), which agrees with the above discussion. In addition, new phases of martensite (M) and residual austenite appears in the BSU. Such results had been also reported in the previous research [24]. Because of the considerable cooling rate, there was not enough time for austenite grains to grow and undergo a phase transformation. Fig. 9 shows the microstructure of the BSU that the grains were fine and compactly aligned together in this area. The needle-like structures of irregular shapes were the pearlite that transformed from the austenite. The dendritic structures on the grain boundary were cementite. Therefore, the SEM micrograph shows an abnormal ledeburite structure (Ld′). These results agree well with the previous research [9]. 3.2. Microhardness
Fig. 4. Schematic of the laser process.
Table 2 Laser parameters used in laser melting. Single pulse laser energy (J)
12
Pulse duration (ms) Frequency (Hz) Scanning speed (mm·s−1) The spot diameter (mm)
8 5 0.5 1.2
the flake graphite completely disappeared in the melted zone. The material around the melted zone made a solid phase transformation without melting because it reached the phase transition temperature, but did not meet the melted temperature. The carbon in matrix diffused rapidly with the high temperature, the pearlite transformed into the austenite in this zone. However, the austenite does not have enough time to grow up and homogenize due to a high cooling rate. The austenite transformed to the martensite and residual austenite when they were quenched to the room temperature. In addition, a little non-dissolved graphite was observed in this zone.
Microhardness plays a significant role, because it is considered a good indication of wear resistance. The matrix was melted by the laser irradiation and then solidified rapidly, resulting in the change and improvement in the hardness. Therefore, the microhardness of the surface and the cross-section were measured 10 times using a loading of 0.2 kg with a holding time of 10 s respectively. Fig. 10(a) shows the microhardness curve of the cross-section of the BSU from the surface to the bottom of the melted zone. It could be seen that the microhardness of the melted zone is higher than that of the matrix. Fig. 10(b) indicates the microhardness curve of the surface of the BSU. The improvement in microhardness can be attributed to the refinement of the grains, the dissolution of graphite, the formation of a large number of network carbides and the appearance of martensite. 3.3. Wear resistance 3.3.1. Effect of the bionic unit Fig. 11 is the chart of the loss of thickness of the five groups of specimens obtained by the indentation method after six times of the wear test, which shows the difference in wear resistance. The result indicates that the wear resistance of SIII was the worst, and instead of being improved, the wear resistance of SIII decrease compared to SI. This does not mean that BPU can only reduce the wear resistance, but, the degree of enhancing wear resistance is lower than the degree of reducing the
Fig. 5. The sketch of a self-made wear tester.
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Fig. 6. The sketch of the indentation method.
wear resistance. In the wear process, the BPU can store the lubricating oil, and can trap the abrasive grains [25] and reduce the wear area. These factors all can increase the wear resistance. However, BPU also brings the influence of the other side. The edges of BPU will cause stress concentration. The stress concentration areas are subjected to severe wear. This should be the main reason for BPU reducing the wear resistance. The wear resistances of SII and SIV are better than it of SI. It indicates that BPU can improve the wear resistance against the matrix. The reasons for improving the wear resistance of BPU are obtained by discussing the microstructures and the microhardness of it at above. In addition, we also noticed the difference of X-ray diffraction pattern of BPU at before and after wear (Fig. 8(b) and (c)). The residual austenite of BPU decreased after wear. It shows that the retained austenite in BPU is induced to transform into martensite. These results agree well with the previous research [26–28]. This part of martensite is beneficial to improve the wear resistance. The wear resistance of SIV was better than that of SII, which indicated that a structure of alternately soft and hard layers was better than a structure of completely hard layers for wear resistance. BSU has better wear resistance and plasticity due to high hardness [6,8] and good toughness [21]. The matrix near the BUS can effectively absorb energy, reduce impact load and dissipate heat [30]. The different function of
Fig. 7. SEM structure of BSU processed by laser.
BSU and matrix and their interaction are the main factors that the wear resistance of SIV was better than that of SII. The wear resistance of SV was the best, which demonstrated that the BPU contributed to wear resistance in SV. The wear resistance of the
Fig. 8. X-ray diffraction patterns taken from the surfaces of the specimens. (a) Untreated specimen; (b) bionic strip unit; (c) bionic strip unit after the wear.
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Fig. 9. SEM microstructure of BSU in melted zone.
specimen coupling the two kinds of bionic units is improved compared with it coupling single kind of bionic unit. The performance of BPU enhancing the wear resistance is more than that reducing the wear resistance due to the surface structure. BPU has the two advantages that storing lubricating oil and trapping abrasive grain. These advantages not only enhance the wear resistance of the matrix, but also improve the wear resistance of BSU. The surface structure with two kinds of the bionic unit not only reduces the disadvantage of the BPU, but also promotes mutual assistance between the two kinds of the bionic units on the wear resistance.
3.3.2. Worn surfaces Fig. 12 showed the worn surfaces of specimens after a lubrication sliding wear test under the same wear test conditions. Fig. 12(a) showed the worn surface morphology of SI and the surface shape curve of the line that was perpendicular to the direction of the wear. A large number of deep scratches that looked like furrows could be seen running parallel to the wear direction, and there were irregular grooves of various depths. The surface roughness (Ra) was 329.26 nm, as measured by the Wyko. The irregular grooves emerged because the flake graphite was removed from the gray iron during the wear process. Because of the low degree of hardness and the weak shear resistance of SI, hard micro-peaks on the surface of the friction pair and abrasive grains on the friction surface could cut into the surface during the wear process. This indicated that the wear type of the specimens was abrasive wear. The worn surface morphology of the matrix of the other specimens was similar to that of SI. Fig. 12(b) showed the worn surface morphology of SII. Compared with SI, the surface was smoother,
Fig. 11. Lost thickness of the specimens. (An untreated specimen (SI), a specimen with a surface that was completely melted with the laser melted parameter (SII), a specimen containing only BPU (SIII), a specimen containing only BSU (SIV) and a specimen coupling two kinds of bionic units (SV).)
the depth of scratches was much shallower and the width of scratches was narrower. Moreover, the irregular grooves disappeared in SII. The surface roughness (Ra) of SII was 65.28 nm. It had the positive effect that the grooves disappeared after the surface was melted by the laser. The groove could cause a defect, because it could carry the stress concentration to the edge of the specimen and produced a crack at the tip during the wear process. The melted zone had properties of grain refinement, a high degree of hardness and strong deformation resistance; therefore, the hard micro-peaks or abrasive grain would have had difficulty cutting the surface of the unit, which indicated an obvious improvement in the wear resistance. Fig. 12(c) showed the worn surface morphology of SIV at the edge of the BSU. The bright area was the BSU, while the other area was the matrix with the irregular grooves. The morphology curve showed that the width and depth of the scratches on the BSU and matrix were significantly different, which was similar to the comparison of Fig. 12(a) and (b). This indicated that the wear resistance of the BSU was higher than that of the matrix under the same wear conditions.
3.4. Stress analysis and lubrication mechanism 3.4.1. Finite elements analysis In order to analyze the stress on the surface of the specimens during the wear process, ANSYS software was employed to simulate the von
Fig. 10. Microhardness of BSU: (a) microhardness on the section of BSU from the surface to bottom; (b) microhardness on the surface of BSU.
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Fig. 12. Worn surface morphology of the specimens: (a) the worn surface morphology of SI; (b) the worn surface morphology of SII; (c) the worn surface morphology of SIV at the edge of the BSU.
Mises stress distribution and change. Fig. 13 showed the von Mises stress cloud chart on the surface of SI–SV and the model with the specimen and friction pair. The type of analysis used was contact analysis, the purpose of which was to simulate the change of morphology and material that influences the size and distribution of stress on the surface of specimens. Fig. 13(f) showed the stress state and sliding direction of the model in the sliding wear process. Fig. 13(a–e) showed the stress cloud chart of the five groups of specimens (SI–SV) on the wearing surface. The shape of the specimens and the sliding direction could influence the stress distribution on the surface, and the amount of stress on the edge of the specimen was significant. The stress cloud charts of SI and SII were similar to a certain degree, due to their having the same surface morphology; however, different materials on the surfaces of these two specimens resulted in their having different stress distributions. Fig. 13(c) showed the stress concentration on the edge of the BPU, and indicated that the main reason for the decrease in wear resistance
(in contrast with SI) was the result of a different surface morphology. Because of the change in the microstructure of the material, the stress resistance was increased in the zone of the BSU, while it was reduced on the edge of the BSU. This indicated that the BSU played a supporting role during the wear process due to its high wear resistance. Fig. 13(e) showed the stress cloud chart of the coupling of two bionic units. The stress that two kinds of bionic units coupled together was well distributed on the surface of the specimen. The stress of two units coupled was lower than that of S III and SIV at the concentration of stress. This demonstrated that the supporting role of the BSU reduced the stress concentration on the edge of the BPU. 3.4.2. Lubrication and couple mechanism The wear gap filled with lubricating oil at the beginning of the lubrication sliding wear in SI and SII. However, during the sliding wear
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Fig. 13. Stress cloud chart in wear process: (a) stress cloud chart on the surface of SI; (b) stress cloud chart on the surface of SII; (c) stress cloud chart on the surface of SIII; (d) stress cloud chart on the surface of SIV; (e) stress cloud chart on the surface of SV; (f) stress cloud chart on the specimen and friction pair.
process, the lubricating oil in the wear gap would decrease, resulting in a decrease in lubrication. The BPU could store lubricating oil, and it could also supply oil when the oil in the gap was reduced. Further, the BPU could improve abrasion resistance. The BPU could also trap the abrasive gains [24] and change their moving model [29] (Fig. 14). The abrasive grains could cut into the surface of the specimen to produce scratches, which resulted from the squeezing of two surfaces during the sliding wear process. The BPU prevented the abrasive grains from being subjected to this squeezing; at the same time, abrasive grains arrived at the edge of the BPU, and the point where abrasive grains contacted the surface of the specimen, and contacted other points pasted to the surface of the friction pair, torque was generated that rotated the abrasive grains. Therefore, the moving model of abrasive
grains changed from sliding to rolling. In the rolling process, the sharp angle of the abrasive grain was abraded, which resulted in the forming of spherical or cylindrical abrasive grains, which improved the lubrication effect. Depending on the results of stress analysis by finite element method, the BSU played a supporting role. The BSU protected the matrix from more stress, reduced the stress on the edge of the BPU, and provided favorable conditions for the lubrication of the BPU. By the analysis of the lubrication mechanism, the BPU could supply the preferable lubrication. The lubrication of the BPU protected the BSU effectively. Two kinds of units protected and promoted each other in the wear process and, used together, they improved the resistance against sliding wear.
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Fig. 14. The lubrication principle chart of BPU.
4. Conclusions (1) The BSU consists of a melted zone and a transformed zone. New phases of martensite (M) and residual austenite appeared in the BSU, excepting the original phase of ferrite, cementite (Fe3C) and graphite (G). The SEM micrograph shows a modified ledeburite structure. The microhardness has been greatly improved because of the change in the microstructure. (2) The wear resistance of the specimen with the bionic strip units located in the grid is better than that of the specimen's surface melted all by laser. The wear resistance of SV was the highest. The wear mechanism of the specimen was abrasive wear. Compared with SI, the width and depth of scratches were narrower and shallower on the BSU. (3) The bionic unit changes the distribution of stress on the surface of the specimens. The BSU endures more stress and plays a supporting role. There is a stress concentration on the edge of the BPU. Upon coupling two kinds of bionic units, the supporting role of the BSU reduces the stress concentration on the edge of the BPU, and the stress is well distributed. (4) The BPU can store lubricating oil and can supply oil when the oil in the gap is diminished. The BPU generated torque that rotated the abrasive grains and changed the abrasive grain moving model. Two kinds of bionic units can protect and promote each other during the wear process, thus improving the resistance to sliding wear together. Acknowledgment This article was supported by Project 985 – High Performance Materials of Jilin University and the Project 985 – Bionic Engineering Science and Technology Innovation and National Natural Science Foundation of China (No. 51275200). References [1] M. Ramadan, M. Takita, H. Nomura, Effect of semi-solid processing on solidification microstructure and mechanical properties of gray cast iron, Mater. Sci. Eng. A 417 (2006) 166–173. [2] T. Yamazaki, T. Shibuya, C.J. Jin, T. Kikuta, N. Nakatani, Lining of hydraulic cylinder made of cast iron with copper alloy, J. Mater. Process. Technol. 172 (2006) 30–34. [3] T. Willidal, W. Bauer, P. Schumacher, Stress/strain behaviour and fatigue limit of grey cast iron, Mater. Sci. Eng. A 413-414 (2005) 578–582.
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[24]
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effects of different bionic units coupling on the sliding wear of gray cast iron Haifeng Zhang a,b, Ti Zhou a,⁎, Hong Zhou a, Zhikai Chen a, Wanshi Yang a, Luquan Ren c a b c
The Key Lab of Automobile Materials, The Ministry of Education, Jilin University, Changchun 130025, PR China The College of Mechanical and Automotive Engineering, Changchun University, Changchun 130022, PR China The Key Lab of Terrain Machinery Bionics Engineering, The Ministry of Education, Jilin University, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 18 August 2016 Revised 13 November 2016 Accepted in revised form 14 November 2016 Available online 15 November 2016 Keywords: Bionic strip unit Bionic pit unit Laser Wear Gray cast iron
a b s t r a c t To enhance the sliding wear resistance of gray cast iron under starved lubrication conditions, two kinds of bionic units were generated by using a pulsed Nd: YAG laser and a drill on the surface of the specimens. The microstructure and microhardness of the bionic strip unit were examined. Comparing the wear resistance of five groups of specimens (an untreated specimen; a specimen with a surface that was completely melted by the laser; a specimen containing only bionic pit unit; a specimen containing only bionic strip unit and a specimen coupling two kinds of bionic units), the results indicate that the wear resistance of the bionic unit coupling specimen is better than that of the others. The result of a stress analysis of the specimen surface shows that the bionic strip unit plays a supporting role and can reduce the stress concentration on the edge of the bionic pit unit. On the other hand, the bionic pit unit can supply lubricating oil to the wear gap, change the moving model of abrasive grains and improve the lubrication performance. The two kinds of units can protect and promote each other under starved lubrication conditions, improve the resistance to sliding wear. © 2016 Elsevier B.V. All rights reserved.
1. Introduction A machine tools guide is one of the most important components in a machine tool, because its mechanical accuracy directly determines the quality of the tool. Gray cast iron (GCI) has been used for machine tool guides for a long time because of its superior wear resistance, low cost, excellent machinability, etc. [1–4]. In addition, GCI can be used to absorb shock that occurs during the work of the machine tools, because of its microstructure that the amount of flake graphite spreads well in an iron matrix. The dominant failure types of a machine tool guide are abrasive wear and adhesion wear [5], which directly affect the life span of the tool. Therefore, it is necessary to improve the wear resistance of GCI. Many researchers have focused on improving the wear resistance of cast iron which surface was melted by a laser [6–9], furthermore, reinforced particles such as the W-, or Ni-based surface film was fabricated by a high energy beam like a gas tungsten arc and a laser [10,11]. Living creature is a continuing source of inspiration in engineering field. The perfect performances on the surface of some animals are derived from the coupling of their different materials, structures and morphologies [12]. Such coupling with different elements brings inspiration to design and study on the bionic coupling surface designed with multiple factors which can interact on each other in engineering. Previously, ⁎ Corresponding author. E-mail address: [email protected] (T. Zhou).
http://dx.doi.org/10.1016/j.surfcoat.2016.11.056 0257-8972/© 2016 Elsevier B.V. All rights reserved.
we fabricated the trails by laser technology or other methods on the relatively soft substrate materials [13,14]. We called the trails as bionic unit which has a better performance than the substrate. Researchers have become aware that many types of bionic coupling elements, such as structures, functions and materials, which are the result of the biological evolution of animals and plants as they adapt to the living environment, could be applied in the field of engineering. Zhou et al. have applied the bionic principle on tool and die steel surfaces to process a series of bionic units using a laser; this has enhanced resistance to thermal fatigue [13– 16], improving the abrasive wear resistance [17–19] and strengthening the mechanical properties [20,21]. In fact, animals and plants almost always apply the coupling effects of various elements. However, most of the current research in this area has concentrated on studying the functions of a single coupling element; little work has been done on the combined effect of two or more coupling elements to solve a problem. In this paper, two kinds of bionic units are coupled on the surface of GCI. Their different characteristics protect and promote each other to improve sliding wear resistance under starved lubrication conditions. 2. Experiments 2.1. Experimental materials GCI codename HT250, which is widely used for machine tool guides, is applied as the base material. Fig. 1 shows the micrograph of the GCI, the microstructure of which consists of flake graphite (G) surrounded
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Group, DK77, China). All surfaces of the specimens were polished using progressively finer grades of silicon carbide impregnated emery paper to remove all the surface irregularities and machining marks, then cleaned with anhydrous ethanol before being drilled and laser processed. The roughness (Ra) of the specimens was 200 μm. A pulsed Nd: YAG laser with 1.06 μm wavelength and rated power of 300 W was employed to create the BSU using a circular pattern Gaussian beam. Fig. 4 shows the schematic of the experimental set-up of the laser processing. The laser head was mounted vertically in the Z-direction and was stationary, while a four-axis displacement machine with numerical control system (G code) carries out the spatial displacement. During the laser process, the samples were placed on the displacement machine. Movement along X and Y axes was used to process the bionic strip units while that along Z-axis was to adjust the desired defocusing amount. The specimens were fabricated using this equipment at room temperature and using argon as the shielding gas in order to preventing the oxidation of BSU. Table 2 provides the parameters of the laser melting. After the fabrication process, the surface and side-face of the specimens were mechanically polished using the emery paper and then cleaned with anhydrous ethanol.
Fig. 1. Microstructure of HT250 gray cast iron.
by pearlite (P). The ultimate strength was 133 ± 3 MPa, the yield strength was 115 ± 3 MPa. The composition of the GCI is listed in Table 1. 2.2. Specimen preparation and bionic prototype The structure of a bivalve mollusk shell was chosen as one bionic prototype because of its strong wear resistance, which evolved in a beach environment (Fig. 2(a)). Though the shells suffered significant abrasive wear from the mixture of water, mud and sand, they were still well preserved. Observation shows that the nacreous layer of the shell is made of an aragonite layer (hard) and an organic layer (soft). Moreover, the structure of alternately soft and hard layers is one important factor that could enhance the wear resistance and toughness of the nacreous layer [22,23]. Inspirited by the shell structure, a bionic strip unit (BSU) was processed on the specimen surface using laser melting. The purpose during fabrication was to ensure the BSU and matrix had optimal hard and soft alternate structure. The distribution of the BSU was designed as a grid, because a grid has the best wear resistance among the three kinds of distributions (convex, stria and grid) in the same wear environment [5]. As shown in Fig. 3(c), the angle between BSUs and the direction of wear is designed as 45°. The distance between the BSUs is designed as 8 mm. The pit morphology on head of a dung beetle was chosen as the other bionic prototype, because the pit morphology easily stores air when the beetle's head suffers extrusion and friction from the soil (Fig. 2(b)). The pit morphology could reduce the atmospheric pressure, thereby reducing the friction. According to the pit morphology, the bionic pit unit (BPU) was processed on the specimen surface using a drill. The pit is a cone with a diameter of 1.8 mm. The angle of the cone is 120°. The distance between BPUs is 8 mm (Fig. 3(d)). In order to compare the wear resistance, five groups of specimens were prepared (Fig. 3): an untreated specimen (SI) (Fig. 3(a)), a specimen with a surface that was completely melted with the laser melted parameter (SII) (Fig. 3(b)), a specimen containing only BPU (SIII) (Fig. 3(c)), a specimen containing only BSU (SIV) (Fig. 3(d)) and a specimen coupling two kinds of bionic units (SV) (Fig. 3(e)). All specimens were cut to the same size, 30 mm (L) × 20 mm (W) × 6 mm (D), using a wire electrical discharge machine (Huadong
Table 1 Chemical compositions of HT250 (wt%). Elements
C
Si
Mn
P
S
Cu
Cr
Fe
Composition (wt%)
3.25
1.57
0.92
0.06
0.059
0.5
0.27
Bal.
2.3. Experimental methods The BSU was cut along the vertical direction of the strip. The cross sections of the BSU were obtained for microstructural analysis and measurement of the microhardness; the equipment used was a JSM-5600LV scanning electron microscope and a Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA). A Wyko NT9100 Optical Profiler (manufactured by Veeco, Ltd., USA) was employed for observing and recording the surface morphology and roughness of the specimens after they were wear tested. Phases formed in the BSU were identified by D/max-RC X-ray diffraction (XRD) with Cu Kα radiation operated at a voltage of 40 kV, a current of 40 mA, and a scanning rate of 40/min. In the process of sliding wear, the value and distribution of the von Mises stress on the surfaces of the specimens were estimated by the finite element method. Sliding wear tests were performed using a self-made wear tester (Fig. 5). The eccentric wheel was put in motion along with the rotation of gear reducer by electromotor, which makes the moving part of the simple go on straight line reciprocating movement along with the connecting rod, and the load is located at the top of the moving part. The speed of the wear test was controlled by the frequency converter, which was linked to the electromotor. The pressure of wear test was adjusted by the load's adding or subtracting. The friction pair was a highfrequency induction quenching GCI with an average hardness of 50HRC. The average speed of the wear test was 7 cm/s. The time of wear was 24 h under a load of 120 N at room temperature. The setting of these experimental parameters was the purpose that tried to simulate the working environment of a machine tools guide. Usually, the surface of the machine tools guide is no longer added with the lubricant on the work. Therefore, the surfaces of specimens and friction pairs were covered with the lubrication oil at the beginning of the wear. Due to no longer add the lubrication oil, starved lubrication was formed in the process of wear. In such a test, it is incorrect to record the mass loss of the specimens, since the GCI adsorbs lubricating oil in the wear process under starved lubrication conditions. In present paper, the indentation method was utilized to demonstrate the degree of wear, in which the D-value of measured indentation areas before and after the wear test reflected the volume of mass loss. The microhardness measurement was carried out by a Vickers Microhardness Tester (model 5104, manufactured by Buehler Co. Ltd., USA) at a load of 0.2 kg and a dwell time of 10 s. Before the wear test, the microhardness was measured; namely, the indentations were manufactured with a 500 N load in multiple different regions, and the positions and areas of those indentations (A1) were recorded. After the wear test, the areas of the same indentations were measured again (A2). The thickness was obtained by
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Fig. 2. Bionic prototype: (a) shell of bivalve mollusk; (b) head of dung beetle.
calculating the difference areas of the indentations at the beginning and end of wear. In order to reduce the margin of error, several indentations were made on each specimen, and then the average value of the thickness loss was calculated. Fig. 6 shows the schematic of the indentation method; the indentation is the rectangular pyramid at an angle of 136° to the opposite area. A1 and A2 are the magnitude of the areas before and after the wear test, respectively. H denotes the thickness of the metal removal, which can be expressed as follows: H ¼ tg22 °
pffiffiffiffiffiffi pffiffiffiffiffiffi A1 − A2 =2
3. Results and discussion 3.1. Microstructure Fig. 7 shows the SEM micrograph of the etched cross section of the BSU. The cross section of the unit consists of two characteristic microstructural zones: a melted zone and a transition zone. Because of the high temperature in the laser spot zone, the material was melted in this zone. After the laser spot was moved away, the melted material solidified at a high cooling rate, and then the melted zone was formed. The obvious difference between the melted zone and the matrix was that
Fig. 3. Sketch of the five groups of specimens: (a) untreated; (b) all of surface was melted by laser; (c) contained only BSU; (d) contained only bionic pit unit; (e) bionic coupling unit.
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As shown in Fig. 8, X-ray diffraction was used for comparing the phase compositions of the untreated specimen and the BSU before and after wear. There is the same phase of ferrite and cementite (Fe3C). The difference between the BSU and the untreated specimen is that the untreated specimen contains graphite (G), which agrees with the above discussion. In addition, new phases of martensite (M) and residual austenite appears in the BSU. Such results had been also reported in the previous research [24]. Because of the considerable cooling rate, there was not enough time for austenite grains to grow and undergo a phase transformation. Fig. 9 shows the microstructure of the BSU that the grains were fine and compactly aligned together in this area. The needle-like structures of irregular shapes were the pearlite that transformed from the austenite. The dendritic structures on the grain boundary were cementite. Therefore, the SEM micrograph shows an abnormal ledeburite structure (Ld′). These results agree well with the previous research [9]. 3.2. Microhardness
Fig. 4. Schematic of the laser process.
Table 2 Laser parameters used in laser melting. Single pulse laser energy (J)
12
Pulse duration (ms) Frequency (Hz) Scanning speed (mm·s−1) The spot diameter (mm)
8 5 0.5 1.2
the flake graphite completely disappeared in the melted zone. The material around the melted zone made a solid phase transformation without melting because it reached the phase transition temperature, but did not meet the melted temperature. The carbon in matrix diffused rapidly with the high temperature, the pearlite transformed into the austenite in this zone. However, the austenite does not have enough time to grow up and homogenize due to a high cooling rate. The austenite transformed to the martensite and residual austenite when they were quenched to the room temperature. In addition, a little non-dissolved graphite was observed in this zone.
Microhardness plays a significant role, because it is considered a good indication of wear resistance. The matrix was melted by the laser irradiation and then solidified rapidly, resulting in the change and improvement in the hardness. Therefore, the microhardness of the surface and the cross-section were measured 10 times using a loading of 0.2 kg with a holding time of 10 s respectively. Fig. 10(a) shows the microhardness curve of the cross-section of the BSU from the surface to the bottom of the melted zone. It could be seen that the microhardness of the melted zone is higher than that of the matrix. Fig. 10(b) indicates the microhardness curve of the surface of the BSU. The improvement in microhardness can be attributed to the refinement of the grains, the dissolution of graphite, the formation of a large number of network carbides and the appearance of martensite. 3.3. Wear resistance 3.3.1. Effect of the bionic unit Fig. 11 is the chart of the loss of thickness of the five groups of specimens obtained by the indentation method after six times of the wear test, which shows the difference in wear resistance. The result indicates that the wear resistance of SIII was the worst, and instead of being improved, the wear resistance of SIII decrease compared to SI. This does not mean that BPU can only reduce the wear resistance, but, the degree of enhancing wear resistance is lower than the degree of reducing the
Fig. 5. The sketch of a self-made wear tester.
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Fig. 6. The sketch of the indentation method.
wear resistance. In the wear process, the BPU can store the lubricating oil, and can trap the abrasive grains [25] and reduce the wear area. These factors all can increase the wear resistance. However, BPU also brings the influence of the other side. The edges of BPU will cause stress concentration. The stress concentration areas are subjected to severe wear. This should be the main reason for BPU reducing the wear resistance. The wear resistances of SII and SIV are better than it of SI. It indicates that BPU can improve the wear resistance against the matrix. The reasons for improving the wear resistance of BPU are obtained by discussing the microstructures and the microhardness of it at above. In addition, we also noticed the difference of X-ray diffraction pattern of BPU at before and after wear (Fig. 8(b) and (c)). The residual austenite of BPU decreased after wear. It shows that the retained austenite in BPU is induced to transform into martensite. These results agree well with the previous research [26–28]. This part of martensite is beneficial to improve the wear resistance. The wear resistance of SIV was better than that of SII, which indicated that a structure of alternately soft and hard layers was better than a structure of completely hard layers for wear resistance. BSU has better wear resistance and plasticity due to high hardness [6,8] and good toughness [21]. The matrix near the BUS can effectively absorb energy, reduce impact load and dissipate heat [30]. The different function of
Fig. 7. SEM structure of BSU processed by laser.
BSU and matrix and their interaction are the main factors that the wear resistance of SIV was better than that of SII. The wear resistance of SV was the best, which demonstrated that the BPU contributed to wear resistance in SV. The wear resistance of the
Fig. 8. X-ray diffraction patterns taken from the surfaces of the specimens. (a) Untreated specimen; (b) bionic strip unit; (c) bionic strip unit after the wear.
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Fig. 9. SEM microstructure of BSU in melted zone.
specimen coupling the two kinds of bionic units is improved compared with it coupling single kind of bionic unit. The performance of BPU enhancing the wear resistance is more than that reducing the wear resistance due to the surface structure. BPU has the two advantages that storing lubricating oil and trapping abrasive grain. These advantages not only enhance the wear resistance of the matrix, but also improve the wear resistance of BSU. The surface structure with two kinds of the bionic unit not only reduces the disadvantage of the BPU, but also promotes mutual assistance between the two kinds of the bionic units on the wear resistance.
3.3.2. Worn surfaces Fig. 12 showed the worn surfaces of specimens after a lubrication sliding wear test under the same wear test conditions. Fig. 12(a) showed the worn surface morphology of SI and the surface shape curve of the line that was perpendicular to the direction of the wear. A large number of deep scratches that looked like furrows could be seen running parallel to the wear direction, and there were irregular grooves of various depths. The surface roughness (Ra) was 329.26 nm, as measured by the Wyko. The irregular grooves emerged because the flake graphite was removed from the gray iron during the wear process. Because of the low degree of hardness and the weak shear resistance of SI, hard micro-peaks on the surface of the friction pair and abrasive grains on the friction surface could cut into the surface during the wear process. This indicated that the wear type of the specimens was abrasive wear. The worn surface morphology of the matrix of the other specimens was similar to that of SI. Fig. 12(b) showed the worn surface morphology of SII. Compared with SI, the surface was smoother,
Fig. 11. Lost thickness of the specimens. (An untreated specimen (SI), a specimen with a surface that was completely melted with the laser melted parameter (SII), a specimen containing only BPU (SIII), a specimen containing only BSU (SIV) and a specimen coupling two kinds of bionic units (SV).)
the depth of scratches was much shallower and the width of scratches was narrower. Moreover, the irregular grooves disappeared in SII. The surface roughness (Ra) of SII was 65.28 nm. It had the positive effect that the grooves disappeared after the surface was melted by the laser. The groove could cause a defect, because it could carry the stress concentration to the edge of the specimen and produced a crack at the tip during the wear process. The melted zone had properties of grain refinement, a high degree of hardness and strong deformation resistance; therefore, the hard micro-peaks or abrasive grain would have had difficulty cutting the surface of the unit, which indicated an obvious improvement in the wear resistance. Fig. 12(c) showed the worn surface morphology of SIV at the edge of the BSU. The bright area was the BSU, while the other area was the matrix with the irregular grooves. The morphology curve showed that the width and depth of the scratches on the BSU and matrix were significantly different, which was similar to the comparison of Fig. 12(a) and (b). This indicated that the wear resistance of the BSU was higher than that of the matrix under the same wear conditions.
3.4. Stress analysis and lubrication mechanism 3.4.1. Finite elements analysis In order to analyze the stress on the surface of the specimens during the wear process, ANSYS software was employed to simulate the von
Fig. 10. Microhardness of BSU: (a) microhardness on the section of BSU from the surface to bottom; (b) microhardness on the surface of BSU.
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Fig. 12. Worn surface morphology of the specimens: (a) the worn surface morphology of SI; (b) the worn surface morphology of SII; (c) the worn surface morphology of SIV at the edge of the BSU.
Mises stress distribution and change. Fig. 13 showed the von Mises stress cloud chart on the surface of SI–SV and the model with the specimen and friction pair. The type of analysis used was contact analysis, the purpose of which was to simulate the change of morphology and material that influences the size and distribution of stress on the surface of specimens. Fig. 13(f) showed the stress state and sliding direction of the model in the sliding wear process. Fig. 13(a–e) showed the stress cloud chart of the five groups of specimens (SI–SV) on the wearing surface. The shape of the specimens and the sliding direction could influence the stress distribution on the surface, and the amount of stress on the edge of the specimen was significant. The stress cloud charts of SI and SII were similar to a certain degree, due to their having the same surface morphology; however, different materials on the surfaces of these two specimens resulted in their having different stress distributions. Fig. 13(c) showed the stress concentration on the edge of the BPU, and indicated that the main reason for the decrease in wear resistance
(in contrast with SI) was the result of a different surface morphology. Because of the change in the microstructure of the material, the stress resistance was increased in the zone of the BSU, while it was reduced on the edge of the BSU. This indicated that the BSU played a supporting role during the wear process due to its high wear resistance. Fig. 13(e) showed the stress cloud chart of the coupling of two bionic units. The stress that two kinds of bionic units coupled together was well distributed on the surface of the specimen. The stress of two units coupled was lower than that of S III and SIV at the concentration of stress. This demonstrated that the supporting role of the BSU reduced the stress concentration on the edge of the BPU. 3.4.2. Lubrication and couple mechanism The wear gap filled with lubricating oil at the beginning of the lubrication sliding wear in SI and SII. However, during the sliding wear
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Fig. 13. Stress cloud chart in wear process: (a) stress cloud chart on the surface of SI; (b) stress cloud chart on the surface of SII; (c) stress cloud chart on the surface of SIII; (d) stress cloud chart on the surface of SIV; (e) stress cloud chart on the surface of SV; (f) stress cloud chart on the specimen and friction pair.
process, the lubricating oil in the wear gap would decrease, resulting in a decrease in lubrication. The BPU could store lubricating oil, and it could also supply oil when the oil in the gap was reduced. Further, the BPU could improve abrasion resistance. The BPU could also trap the abrasive gains [24] and change their moving model [29] (Fig. 14). The abrasive grains could cut into the surface of the specimen to produce scratches, which resulted from the squeezing of two surfaces during the sliding wear process. The BPU prevented the abrasive grains from being subjected to this squeezing; at the same time, abrasive grains arrived at the edge of the BPU, and the point where abrasive grains contacted the surface of the specimen, and contacted other points pasted to the surface of the friction pair, torque was generated that rotated the abrasive grains. Therefore, the moving model of abrasive
grains changed from sliding to rolling. In the rolling process, the sharp angle of the abrasive grain was abraded, which resulted in the forming of spherical or cylindrical abrasive grains, which improved the lubrication effect. Depending on the results of stress analysis by finite element method, the BSU played a supporting role. The BSU protected the matrix from more stress, reduced the stress on the edge of the BPU, and provided favorable conditions for the lubrication of the BPU. By the analysis of the lubrication mechanism, the BPU could supply the preferable lubrication. The lubrication of the BPU protected the BSU effectively. Two kinds of units protected and promoted each other in the wear process and, used together, they improved the resistance against sliding wear.
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Fig. 14. The lubrication principle chart of BPU.
4. Conclusions (1) The BSU consists of a melted zone and a transformed zone. New phases of martensite (M) and residual austenite appeared in the BSU, excepting the original phase of ferrite, cementite (Fe3C) and graphite (G). The SEM micrograph shows a modified ledeburite structure. The microhardness has been greatly improved because of the change in the microstructure. (2) The wear resistance of the specimen with the bionic strip units located in the grid is better than that of the specimen's surface melted all by laser. The wear resistance of SV was the highest. The wear mechanism of the specimen was abrasive wear. Compared with SI, the width and depth of scratches were narrower and shallower on the BSU. (3) The bionic unit changes the distribution of stress on the surface of the specimens. The BSU endures more stress and plays a supporting role. There is a stress concentration on the edge of the BPU. Upon coupling two kinds of bionic units, the supporting role of the BSU reduces the stress concentration on the edge of the BPU, and the stress is well distributed. (4) The BPU can store lubricating oil and can supply oil when the oil in the gap is diminished. The BPU generated torque that rotated the abrasive grains and changed the abrasive grain moving model. Two kinds of bionic units can protect and promote each other during the wear process, thus improving the resistance to sliding wear together. Acknowledgment This article was supported by Project 985 – High Performance Materials of Jilin University and the Project 985 – Bionic Engineering Science and Technology Innovation and National Natural Science Foundation of China (No. 51275200). References [1] M. Ramadan, M. Takita, H. Nomura, Effect of semi-solid processing on solidification microstructure and mechanical properties of gray cast iron, Mater. Sci. Eng. A 417 (2006) 166–173. [2] T. Yamazaki, T. Shibuya, C.J. Jin, T. Kikuta, N. Nakatani, Lining of hydraulic cylinder made of cast iron with copper alloy, J. Mater. Process. Technol. 172 (2006) 30–34. [3] T. Willidal, W. Bauer, P. Schumacher, Stress/strain behaviour and fatigue limit of grey cast iron, Mater. Sci. Eng. A 413-414 (2005) 578–582.
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