Effect of physical properties of Cu Ni graphite composites on tribological characteristics by grey correlation analysis Results in Physics 7 (2017) 263–271 Contents lists available at ScienceDirect Re[.]
Results in Physics (2017) 263–271 Contents lists available at ScienceDirect Results in Physics journal homepage: www.journals.elsevier.com/results-in-physics Effect of physical properties of Cu-Ni-graphite composites on tribological characteristics by grey correlation analysis Yiran Wang ⇑, Yimin Gao ⇑, Liang Sun, YeFei Li, Baochao Zheng, Wenyan Zhai State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an, Shaanxi Province 710049, PR China a r t i c l e i n f o Article history: Received 30 October 2016 Received in revised form 25 December 2016 Accepted 26 December 2016 Available online 29 December 2016 Keywords: Switch slide baseplates Gray correlation analysis Cu-Ni-graphite composites a b s t r a c t Cu-Ni-graphite composites are intended for using as switch slide baseplates materials in high-speed railways industrials The tribological characteristics of Cu-Ni-graphite composites with different graphite content were affected by physical properties and were generated variation correspondingly It is difficult to study the correlation degree between tribological characteristics and physical properties by the means of experiments However, grey correlation analysis (GCA) is a suitable mathematic method for researching the correlation with each factor In this study, Cu-Ni-graphite composites were prepared by powder metallurgy and the correlation degree between tribological characteristics (Cu-Ni-graphite composites slid against U75V steel) and physical properties were calculated by GCA method The results showed that through the calculating by GCA method, compared with physical properties of Cu-Ni-graphite composites, the most effective way to reduce the friction coefficient is to improve the relative density as well as the increasing of hardness can also bring down the friction coefficient usefully The best way to reduce wear rate is the increasing of work of rupture and flexural strength, when these two properties have a small amount of ascension, the wear rate would be rapidly decreasing The GCA results could help to improve materials’ performance and give reasonable advices to the following studies Ó 2016 Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/) Introduction Recently, the industrial of high-speed train has been rapidly developed and plenty of new materials are being researched for substituting the parts of traditional railway equipment The railway switch is one of the most important parts in rail transportation and switch slide baseplates is an extremely critical component in railway switch always affects the safety of the train The schematic of the railway switch is shown in Fig It was reported by D.M Bishop that the malfunction of the switch slide baseplates in railway accidents accounted for over 10% in British railway [1] Thus, switch slide baseplates were affirmed to be lubricated by lubricating oil on the surface for reducing the friction coefficient with tongue rail when they work together In China, Q235 steel coating with chromium is used for the switch slide baseplates in the high-speed railway and other rail transit recently However, switch slide baseplates are always located in the outdoor environment and the lubricating oil will separate from their surface during heavy snow or sandstorm Consequently, self⇑ Corresponding authors E-mail addresses: wyr1010@hotmail.com (Y Wang), ymgao@mail.xjtu.edu.cn (Y Gao) lubricating material used as switch slide baseplates is studied for substituting the traditional materials Taylor W.G et al.[2] researched the influence of friction coefficient on tongue rail and switch slide baseplates They found that with the increasing of the friction coefficient, the switch force and the frictional resistance grew substantially Therefore, materials with low friction coefficient and good corrosion resistance are set as the focus of research In the early time, some selflubrication materials were applied to produce switch slide baseplates [1], like PTFE, ceramic materials and porous oiled ironbase materials In addition, self-lubricating polymer materials were also investigated [3–5] Copper matrix self-lubricating composites behave fine tribological performances and corrosion resistance, while it is not yet used for switch slide baseplates now In terms of metal self-lubricating composites, the lubricant particles were added into the metal matrix which could highly promote both the anti-friction and wear-resistance properties [6] Compared with Fe matrix composites and other self-lubrication materials, copper matrix self-lubricating composites also show the advantage of anti-corrosion properties and well wear-resistance with steel exposed to air [7] It is reported that graphite in copper matrix composites can form lubricating films to reduce friction http://dx.doi.org/10.1016/j.rinp.2016.12.041 2211-3797/Ó 2016 Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 264 Y Wang et al / Results in Physics (2017) 263–271 slide baseplates on tribological characteristics In this work, the data including friction coefficient, wear rate, hardness, relative density, bending strength, flexural modulus and work of rupture were taken into the mathematical model and calculated by GCA method Through quantitative calculation, the correlation degrees between physical properties and tribological characteristics were evaluated and established the relationship between them The GCA results could help to improve material performances and give reasonable advices to the following studies Materials and experiments Material preparation Fig Schematic of the railway switch and switch slide baseplates coefficient and improve its wear resistance during the course of friction [6–12] As a consequence, researchers began regarding graphite as an ideal lubricating phase possesses both advantage of inexpensive price and good anti-friction According to the operational requirement and design, Cu-Ni-graphite composites are intended for using as switch slide baseplates materials in highspeed railways industrials In the previous study, Cu-Ni-graphite composites are utilized to slide against U75V steel without lubrication The materials require both tribological properties and physical properties In order to achieving better tribological properties, more graphite used as lubricating phase was added in composites The wear rate and friction coefficient were decreasing with graphite addition but physical properties were influenced by graphite content in composites However, tribological properties are critical to Cu-Ni-graphite composites applied as switch slide baseplates It is notable that the graphite would benefit for tribological properties but is harmful to the physical properties With variation of the composition in Cu-Ni-graphite composites, their physical properties and tribological characteristics are both changed correspondingly However, the means of experiments could hardly analysis the correlation degree between tribological characteristics and physical properties Moreover, the influence of physical properties of Cu-Ni-graphite composites on tribological characteristics is difficult to be researched by worn morphology and other testing methods under the same operating conditions Gray correlation analysis (GCA) method is a basic method of grey system theory for systems analysis, which based on qualitative analysis and quantitative analysis [13–18] GCA is able to use the quantitative analysis of the development trend of dynamic process, in consequence, reveals the essence of the phenomenon and development trend In other studies [6–12], Cu-graphite composites were only researched by analysis the effect of graphite content on the physical properties and friction performance However, it can be hardly found the study on how physical properties affect the tribological characteristics of composites during dynamic development Therefore, it is necessary to study the effect extent and correlation degree between physical properties and tribological characteristics in Cu-Ni-graphite composites applied as switch slide baseplates Compared with other multivariate statistical analysis methods, just like regression analysis and so on, GCA method has advantages in revealing the essence of the phenomenon and the development trend more correctly It is one of the best suitable methods used for researching the correlation between physical properties and tribological characteristics in composites GCA method can also offer guidance for improving different physical properties by controlling materials composition and other methods The purpose of this paper is determined to study the influence of physical properties of Cu-Ni-graphite composites used as switch According to the operational requirement and design, different mass fractions of dispersed graphite were used as an addition into Cu-Ni-graphite composites and the chemical composition is given in Table The friction pairs of Cu-Ni-graphite composites was U75V steel as tongue rail material, whose components and mechanical properties are illustrated in Table The composites were prepared by powder metallurgy, and the raw powders were 37.5 lm atomized commercial copper powders (99.9% purity) and 45 lm atomized nickel powders (99.9% purity) The flaky graphite (99% purity) consisted of platelets contain the morphology of irregular shaped flakes with mean diameter is about 45 lm The procedure of preparing specimens by powder metallurgy can be divided for steps: powder mixture, compaction, sintering, re-pressing and re-sintering All the raw powders were constantly mixed for 15 h at the speed of 200r per minute Then, the mixture was cold-press at 600 Mpa for Next, sintering was carried out at 1073 k for h in vacuum sintering furnace After that, the specimens were re-pressed at 600 Mpa for Then, specimens re-sintering at 1073 k in vacuum for 0.5 h were the final step in the fabrication Tribology characteristics test The mainly tribology characteristics of Cu-Ni-graphite composites sliding against U75V steel without lubrication are friction coefficient and wear rate The wear tests were carried out on a MMU-1 pin-on-disk tribometer (Jinan Hansen Precision Instruments Co., Ltd.) and the schematic diagram of that is shown in Fig It can be infer that Cu-Ni-Graphite composites as disks (bottom) and U75V steel as pins (upper) The specimens as the pins were cut to the dimension of mm mm 20 mm whose one side grinded to a U5 mm half spherical surface and the disks as the counterpart were machined to the size of U44 mm mm by wire cut electric discharge machine The surface of pins and disks were polished and ultrasonically cleaned for 15 aiming to Ra 1.0 lm before experiments The temperature of wear test without lubricant was about 293 K and the normal load was 20 N which was calculated by the real working condition of switch slide baseplates (about Mpa friction load [4]) The sliding distance of test was designed 100 m and sliding speed were 50 r/min (about 0.065 m/s), which would spend 1530 s for the test Each test had been held for times and the friction coefficient was recorded during the experiments automatically by the MMU-1 pin-on-disk tribometer The specimens were washed by ultrasonically cleaning after the wear test The relation Eq (1) was used for calculating to the wear rate DW ¼ ðW W Þ LA ð1Þ 265 Y Wang et al / Results in Physics (2017) 263–271 Table Chemical composition of Cu-Ni-Composites Content/wt.% fi kị ẳ minminẵX kị X i kị ỵ 0:5maxmaxẵX kị X i ðkÞ i i k i Specimen number Graphite Ni Cu G1 G2 G3 G4 G5 G6 Bal where W1 is weight of the disk before test, W2 is weight of the disk after test, L is the sliding distance, A is the contact area k ẵX kị X i kị ỵ 0:5maxmaxẵX kị X i kị 6ị k Finally, the average correlation coefficient and the grey correlation degree are computed by Eq (7), the quantitative analysis could evaluate on development relations between different parameters and their influence ri ẳ N 1X f kị N kẳ1 i 7ị Physical properties test Results and discussions The porosity and bulk density of Cu-Ni-graphite composites with different mass fractions of graphite were measured by Archimedes’ principle The micro hardness of bulks was tested by HXD1000TMC Vickers hardness meters with a lord of 20 N and a dwell time of 15 s The flexural strength of the material was determined by three-point tests on 30 mm mm mm bars with a span of 24 mm and a crossed speed of 0.5 mm/min The flexural strength of the composites was determined using the relation Eq (2): The tribological characteristics of Cu-Ni-graphite composites sliding against U75V steel rf ¼ 3PL 2bd Friction coefficient is a significant data for the switch slide baseplates materials, it usually required less than 0.3 when sliding against the switch rail material, in addition, the lower friction coefficient is benefit for the reliability of the materials [4] Table indicates the average friction coefficient of the Cu-Ni-Graphite ð2Þ where P is the maximum load at fracture, L is the span length, b is the width and d is the depth of the sample Load-displacement curve was recorded during the experiments automatically by the INSTRON-1195 universal materials tester The flexural modulus of elasticity was calculated by the following equations: Ef ẳ L3 m 4bd 3ị 3 IX ẳ bd 12 4ị where Ef is flexural modulus of elasticity, L is the span length, b is the width and d is the depth of the sample, m is gradient of the initial straight line of the load-displacement curve, Ix is the moment of inertia The work of rupture (U) is also calculated by integration of the area under load-displacement curve Gray correlation analysis (GCA) GCA method is dedicated to analyzing the correlation between graphite content in composites and their physical properties The method can be divided by steps: Firstly, data column of friction coefficient (or wear rate) is specified as noted X0(k) and other data noted as Table shown Secondly, the data initial processing and dimensionless treatment are calculated by using Eq (5): Xi ¼ yðkÞ i ¼ 0; 1; 2; 3; y1 ð1Þ ð5Þ Thirdly, the absolute difference value of X0 with Xi is calculated, then the correlation coefficient fi(k) is computed by following equation: Fig Schematic diagram of pin-on-disk tribometer Table Chemical composition and mechanical properties of U75Vsteel Element Content/wt.% Chemical composition Mechanical properties C Si Mn P S V Yield Strength Tensile Strength Tensile Elongation Hardness 0.77 0.71 0.82 0.017 0.008 0.041 P860 Mpa P980 Mpa P9% 320HV 266 Y Wang et al / Results in Physics (2017) 263–271 Table Model sheet of data initialization noting No Friction coefficient Hardness Relative density Flexural strength Flexural modulus Work of rupture k G1 G2 G3 G4 G5 G6 X0 – – – – – – X1 – – – – – – X2 – – – – – – X3 – – – – – – X4 – – – – – – X5 – – – – – – Table Average friction coefficient of Cu-Ni-graphite composites sliding against U75V steel Specimen G1 G2 G3 G4 G5 G6 Average friction coefficient 0.237 0.202 0.186 0.153 0.201 0.206 Table Wear rate of Cu-Ni-graphite composites sliding against U75V steel Specimen G1 G2 G3 G4 G5 G6 Wear rate/105mgmm3 0.7437 0.2546 0.1732 0.1151 0.1345 0.1457 composites sliding against U75V steel as a function of graphite content It can be concern that with the mass friction of graphite increasing in Cu-Ni-graphite composites resulted in the friction coefficient declined from 0.237 for G1 specimen to 0.153 for G4 specimen, while the friction coefficient exhibited a rising trend from 0.153 for G4 specimen to 0.206 for G6 specimen According to the results of previous research, the graphite formed the lubricating films and transferred frictional mechanisms to the boundary lubrication, the more graphite contents would benefit for integrated lubricating films It is infer that the friction coefficient could decrease rapidly Moreover, the addition of graphite also reduced the physical properties, up to wt.% graphite content, the matrix would occur the a large number of ruptures resulted in the failure of lubricating film Consequently, the friction coefficient increased from wt.% to wt.% graphite content The wear rate of Cu-Ni-Graphite with the pair of U75V steel is shown in Table The data illustrate the variation clearly that the increasing amount of graphite can reduce the wear rate of the disks simultaneously The changing of overall trend is same as friction coefficient, wear rate is descend from 0.7437105 mgmm3 for G1 specimen to 0.1151105 mgmm3 for G4 specimen, then the wear rate is ascend to 0.1457105 mgmm3 for G6 specimen as the Table expressed The cause of the variation is identical with friction coefficient; therefore, the formation and failure of graphite lubricating films give an important reason during the friction process Physical properties of Cu-Ni-graphite composites The variations in bulk density and relative density of Cu-graphite composites with different mass fractions are illustrated in Figs and The composites consists of a-phase and graphite phase, consequently, the bulk density is determined by the amount of the above two phases and their density Because graphite has much lower density than copper, the bulk density gradually reduced to a low value (6.21 g/cm3) with the increasing of graphite addition When the addition up from wt.% to wt.%, the relative density decreased from 98.39% to 92.35% The result can be attributed to the poor sintering ability of graphite, even if there was no obvious pore in the microstructure During the sintering, graphite does not react with copper or nickel The mass fractions of graphite grew and more micro pores were emerged in the boundary accompanied with interface area increasing, as a consequence, relative density descended heavily The Vickers hardness of Cu-Ni-graphite composites as the function of graphite content is given in Fig With increasing of the mass fraction of graphite, the Vickers hardness of the composites decreased The Vickers hardness of G1 is about 55.87HV, when G6 decreases to 38.93HV Graphite is the second phase and its crystal structure is hexagonal with low hardness and poor physical properties; usually its shearing stress is very low so that it could serve as solid lubricant At the other hand, graphite is not wetting with most metal, so it is easy to cleavage in composites In addition, more micro holes accumulating at the boundary lead to the Vickers hardness decreasing about 69.7%, when the mass fraction of graphite rises up from 1% to 6% Generally speaking, the more graphite content in composites, the lower hardness is The results of the flexural strength of the Cu-Ni-graphite composites with different mass fractions of graphite are shown in Fig Only the data from G3 specimen to G6 specimen can be found from the diagram by the reason that the G1 and G2 specimen could not be tested because of their good plasticity and toughness which allowed the two specimens to obtain resistance to bending deformation So cracking would not happen within the scope of the test For others, the flexural strength decreased form 153.22 MPa for G3 specimen to 95.27 MPa for G6 specimen with the content of graphite increasing The variations of the physical properties of composites are highly related with the content of graphite and the microstructure of composites Fig shows the flexural modulus of elasticity and work of rupture (U) of Cu-Ni-graphite composites with different content of graphite The overall trends are as same as flexural strength The work of rupture (U) is the energy dissipation of bending fracture When the fracture mechanism changes from transgranular fracture to the intergranular fracture, the energy dissipation also transformed from plastic deformation work of a-phase to the poor interface energy during the fraction In terms of Cu-Ni-graphite composites, the value of plastic deformation work is much more than mechanical bonding energy, and that is why the work of rupture (U) decreased heavily from G3 specimen to G6 specimen As a result, the work of rupture changed along with the addition of graphite content in composites Flexural modulus of elasticity is the Y Wang et al / Results in Physics (2017) 263–271 267 Fig Bulk density of Cu-Ni-graphite composites with different mass fractions of graphite Fig Relative density of Cu-Ni-graphite composites with different graphite content complex of tensile modulus and compression modulus The most efficient method to improve the flexural modulus is increasing the flexural strength [18] The flexural modulus of the composites can be described by Eq (8) [7] E ẳ ECu V Cu ỵ EGraphite V Graphite ỵ Ehole V hole 8ị where ECu is flexural modulus of matrix; EGraphite is flexural modulus of graphite; Ehole is flexural modulus of hole and its numerical value is in this equation; VCu is volume fraction of matrix; VGraphite is volume fraction of graphite; Vhole is volume fraction of pole, VCu + VGraphite + Vhole = Oppositely, the value of VCu decreased and crystal structure of the matrix did not change with graphite addition ECu in Eq (8) has the same numerical value for each specimen When VGraphite1 < VGraphite2, it can be inferred that ECuVCu1 > ECuVCu2 and EGraphiteVGraphite1 < EGraphiteVGraphite2; while EGraphite ECu and VGraphite VCu, in consequence, (EGraphiteVGraphite2 EGraphite VGraphite1) (ECuVCu1 ECuVCu2); so, (EGraphiteVGraphite2 + ECuVCu2) (ECuVCu1 + EGraphiteVGraphite1), namely E2 E1 Therefore, with the increasing of the graphite content, the flexural modulus of composites declined heavily; furthermore it might affect the flexural strength as well as other physical properties 268 Y Wang et al / Results in Physics (2017) 263–271 Fig Vickers hardness of Cu-Ni-graphite composites with different mass fractions of graphite Fig Flexural strength of Cu-Ni-graphite composites with different mass fractions of graphite Effect of physical properties on tribological characteristics by using GCA method The influence of physical properties on the friction coefficient The GCA method can be divided into steps and it has already been described in the previous chapter The whole computing results are given below Original data of each factor including friction coefficient, hardness, relative density, flexural strengh, flexural modulus and work of ruputure could be summarized in Table The data initialization to dimensionless number of the second step in GCA is given in Table According to the numerical value of dimensionless numbers, the absolute difference values of X0 with Xi were calculated in Table The minimum differential and max- imum differential were also computed and illustrated in Tables and 10 After finished all steps, the correlation coefficient fi(k) calculated by Eq (6) is shown in Table 11, in addition, Table 12 states that the grey correlation degree (ri) based on the quantitative analysis evaluates on the development relations between physical properties and friction coefficient Using the same method, the grey correlation degrees between physical properties and wear rate were computed respectively Table 12 demonstrates the results about grey correlation degrees between physical properties and friction coefficient Compared with another data, r2 is the biggest numeric (up to 0.898), then hardness > work of rupture > flexural strength > flexural modules in correlation degrees The results represented that relative 269 Y Wang et al / Results in Physics (2017) 263–271 Fig Flexural modulus of elasticity and work of rupture (U) of Cu-Ni-graphite composites with different content of graphite Table Original data of each factor NO Friction coefficient Hardness Relative density Flexural strength Flexural modulus Work of rupture k G1 G2 G3 G4 G5 G6 X0 0.237 0.202 0.186 0.153 0.201 0.206 X1 55.87 52.55 44.38 43.42 41.74 38.93 X2 98.39 96.73 95.53 94.28 93.04 92.35 X3 – – 153.22 139.59 107.54 95.27 X4 – – 9767.57 6881.06 5969.41 3940.15 X5 – – 306.66 277.45 248.18 205.03 Table Data Initialization for each factor k X0 X1 X2 X3 X4 X5 G1 G2 G3 G4 G5 G6 1.1504854 0.9805825 0.9029126 0.7427184 0.9757282 1.43514 1.3498587 1.1399949 1.1153352 1.0721808 1.07 1.0474283 1.0344342 1.0208988 1.0074716 – – 1.6082712 1.4652042 1.1287919 – – 2.4789843 1.7463954 1.515021 – – 1.4956836 1.3532166 1.210457 Table The absolute differential value of X0 with Xi k G1 G2 G3 G4 G5 G6 D1 D2 D3 D4 D5 D1 = X0(k)-X1(k) D2 = X0(k)-X2(k) D3 = X0(k)-X3(k) D4 = X0(k)-X5(k) D5 = X0(k)-X5(k) 0.2846546 0.3692762 0.2370822 0.3726168 0.0964527 0.0804854 0.0668457 0.1315216 0.2781803 0.0317434 – – 0.7053586 0.7224857 0.1530637 – – 1.5760717 1.003677 0.5392928 – – 0.5927709 0.6104982 0.2347289 density has the maximum correlation with friction coefficient It can be asserted that the reducing of friction coefficient can take action by means of improving relative density of Cu-Ni-graphite composites, in other words, when relative density of composites has a small amount of ascension the friction coefficient would significantly decrease Furthermore, the tribological properties will get promoted The interpretation of it might that the reduction of relative density could emerge more porosity in composites; the pores facilitated the generation of worn debris and seriously hin- dered the formation of lubricating films The failure tendency of lubricating films would be decreasing with the increasing the relative density, therefore, the long-existed lubricating films might decline the numeric of friction coefficient As a result, the most usefully method to decline friction coefficient is to promote the relative density The correlation degree of r1 is the second largest numeric in Table 12 It means that hardness affects the variation of friction coefficient most strongly by contrast to the rest of physical proper- 270 Y Wang et al / Results in Physics (2017) 263–271 Table The minimum differential value of X0 with Xi Formula Minimum differential min min min 0 0 0 X0(k)-X1(k) X0(k)-X2(k) X0(k)-X3(k) X0(k)-X4(k) X0(k)-X5(k) X0(k)-Xi(k) Table 10 The maximum differential value of X0 with Xi Formula Maximum differential max max max max max max 0.3726168 0.2781803 0.7224857 1.5760717 0.6104982 1.5760717 X0(k)-X1(k) X0((k)-X2(k) X0(k)-X3(k) X0(k)-X4(k) X0(k)-X5(k) max X0(k)-Xi(k) ties The enhancement of hardness can greatly improve the capacity of resist deformation during friction process, besides that, the tendency of crack emerging and failure in lubricating films decreased dramatically Consequently, according to results of GCA, increasing the hardness of Cu-Ni-graphite composites is another effective way to reduce the friction coefficient The values of flexural strength is identical to work of rupture and they are almost equally numeric (0.722 and 0.726) in correlation degree It means that flexural strength has the correlation with friction coefficient that is not too much than relative density or hardness but affects friction coefficient in some ways The matrix would be improved and might form the lubricating films more easily by changing flexural strength Work of rupture is identical to flexural strength and it’s proved that energy dissipation of bending fracture is connected with bonding strength just like flexural strength In terms of flexural modulus, they might be influenced by ECu, VCu, EGraphite and VGraphite ECu is the most important factor for the flexural modulus as discussed above, However, VGraphite and EGraphite are really insignificant It is well known that graphite plays an important role in lubricating films, therefore flexural modulus have almost no effect on friction coefficient and the correlation degree is the lowest compared with other physical properties Effect of physical properties on the wear rate The correlation degree between physical properties and wear rate is manifested in Table 13 The result represents that the numerical sort is work of rupture > flexural strength > hardness > relative density > flexural modules in correlation degrees It can be claimed that work of rupture become the most susceptible factor for variation of wear rate When Cu-Ni-graphite composites slid against U75V steel, crushing and cutting action from U75V steel occured to contacting surface of Cu-Ni-graphite composites Cracks formation and propagation happened in worn surface, at the same time, the worn debris peeled off from the surface in composites The promotion of work of rupture could increase capacity of resistance to bending fracture, in addition, the wear rate can reduce dramatically Therefore, work of rupture has the most relevant to wear rate Just like work of rupture, Flexural strength could also affect the variation of wear rate intensely The actions can be taken for decreasing the wear rate by increasing flexural strength Hardness has a medium numeric in the data of correlation degree shown in Table 13 It can be infer that the correlation between hardness and wear rate is lower than work of rupture and flexural strength Consequently, it is difficult to decreasing wear rate by improving the hardness effectively Relative density is also the non-essential factors to change the wear rate and exhibited the lower correlation degree It might be that the pores are area of stress concentration that the crack incubation would emerge around them Therefore, the promoting of relative density may reduce the probability of cracks and then wear rate decreasing r4 is the minimum value among all the data, it may be deduced that flexural modulus affect wear rate slightly and it can hardly decrease wear rate by the way to change the flexural modulus Conclusion 1) The effect of physical properties of Cu-Ni-graphite composites on tribological characteristics was calculated by grey correlation analysis Through the calculating by GCA method, compared with physical properties of Cu-Nigraphite composites, the most effective way to reduce the friction coefficient is to improve the relative density 2) The increasing of hardness can also bring down the friction coefficient usefully Flexural strength and work of rupture affected the variation of friction coefficient to some extent but not too much The flexural modules of Cu-Ni-graphite Table 11 The correlation coefficient fi(k) of each factor k f1(k) f2(k) f3(k) f4(k) f5(k) G1 G2 G3 G4 G5 G6 0.7346349 0.6809191 0.7687269 0.6789593 0.8909509 0.9073305 0.921807 0.8569729 0.7390958 0.9612781 – – 0.527681 0.5216978 0.8373565 – – 0.3333333 0.4398226 0.5937006 – – 0.5707068 0.5634728 0.7704957 Table 12 The grey correlation degree between physical properties and friction coefficient r1 r2 r3 r4 r5 0.7923652 0.8977474 0.7216838 0.5917141 0.7261688 Table 13 The grey correlation degree between physical properties and wear rate r1 r2 r3 r4 r5 0.8264666 0.8102996 0.8711289 0.7653486 0.8812438 Y Wang et al / Results in Physics (2017) 263–271 composites have the lowest numeric in correlation degree by GCA calculation, as a consequence, could hardly affect the changing of friction coefficient 3) The best way to reduce wear rate is the increasing of work of rupture as well as flexural strength is another significant physical properties with lager numeric of grey correlation degree (ri) in wear rate In other words, when the work of rupture and flexural strength of materials has a small amount of ascension, the wear rate would be rapidly decreasing 4) According to correlation degree, the promoting of hardness and relative density has lower effect on decreasing wear rate Compared with other physical properties, the influence of flexural modules on wear rate is the lowest; moreover, there is no need to reduce wear rate by changing flexural modules Acknowledgements This work was supported by National Natural Science Foundation of China (51272207, 51501139), the Science and Technology Project of Guangdong Province in China (2015B010122003, 2015B090926009) References [1] Bishop DM, Chambers J Plastic dry bearings in switch slide baseplates J Rep Proc Permanent Way Institution 1990;108:155–73 271 [2] Tayler WG System serves up smoother switching Railway Track Struct 1984;6:66–7 [3] Li XB, Gao YM, Xing JD, et al Wear reduction mechanism of graphite and MoS2 in epoxy composites Wear 2004;257(3):279–83 [4] Shen W, Cui DF, Shi YJ Research and manufacture of self-lubricating switch glide China Railway Sci 1998;19:103–10 (in Chinese) [5] Luo YY Research on the switch slide baseplates under the conditions of dry friction J Shanghai Tiedao Univ 1998;19:37–42 (in Chinese) [6] Moustafa SF, El-Badry SA, Sanad AM, et al Friction and wear of copper– graphite composites made with Cu-coated and uncoated graphite powders Wear 2002;253(7):699–710 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The effect of physical properties of Cu- Ni- graphite composites on tribological characteristics was calculated by grey correlation analysis Through the calculating by GCA method, compared with physical. .. ECuVCu1 > ECuVCu2 and EGraphiteVGraphite1 < EGraphiteVGraphite2; while EGraphite ECu and VGraphite VCu, in consequence, (EGraphiteVGraphite2 EGraphite VGraphite1) (ECuVCu1 ECuVCu2);... of Cu- Ni- graphite composites with different mass fractions of graphite Effect of physical properties on tribological characteristics by using GCA method The influence of physical properties on