J. Cent. South Univ. (2019) 26: 2643-2650

DOI: https://doi.org/10.1007/s11771-019-4201-9

Tribological properties of Fe(Cr)-B alloys at high temperature

CUI Gong-jun(崔功军), YANG Zhen-wei(杨振伟), WANG Wen-jie(王文杰), GAO Gui-jun(高贵军)

College of Mechanical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: This work is aimed to study the effect of boron on wear resistance of Fe-Cr-B alloys containing different boron contents (0 wt%, 5 wt%, 7 wt% and 9 wt%) from room temperature (RT) to 800 °C in order to explore their applications as high-temperature wear resistant mechanical parts. Additionally, the wear mechanism of alloys is evaluated. The tribological properties of alloys are systematically studied by using a ball-on-disc tribometer at 10 N and 0.20 m/s from RT to 800 °C sliding against Si₃N₄ ceramic ball. The boron element greatly improves the wear resistance of specimens as compared with that of unreinforced specimen. The friction coefficients of specimens decrease with increasing of testing temperature. The wear rates of Fe-Cr-B alloys decrease firstly and then raise with the increase of boron content. The specific wear rates of specimens with boron are 1/10 of the unreinforced specimen. Fe-21wt%Cr-7wt% B keeps the best tribological properties at high temperature.

Key words: Fe(Cr)-B alloy; composite; high-temperature tribological property; friction; wear resistance

Cite this article as: CUI Gong-jun, YANG Zhen-wei, WANG Wen-jie, GAO Gui-jun. Tribological properties of Fe(Cr)-B alloys at high temperature [J]. Journal of Central South University, 2019, 26(10): 2643-2650. DOI: https://doi.org/10.1007/s11771-019-4201-9.

1 Introduction

The serious wear of machine components is a key question in many industrial fields at high temperature [1-3]. The wear resistance of materials mainly depends on the microstructure and mechanical properties at high temperature. Therefore, wear-resistant material is an essential for improving the efficiency of tribological system of equipment. Fe matrix materials are inexpensive engineering material for many applications. Especially, white cast iron is widely used as the wear resistant materials [4]. However, the fracture toughness of white cast iron is low, and the usage of white iron alloys is restricted [5]. In some cases, white cast iron and Fe-B alloys are comparable in the design of wear resistance.

Fe-B matrix materials are attracting much attention due to their excellent mechanical properties, heat stability and corrosion resistance properties [6-8]. Many researchers expected to exploit other applications as wear resistant mechanical parts such as gears, bearing, and sleeve through different technological means. FU et al [9] investigated the friction and wear properties of nano-eutectic Fe₈₃B₁₇ fabricated by self-propagating high-temperature synthesis technique at dry sliding condition. Nanostructured Fe₈₃B₁₇ alloy possessed better wear resistance than that of coarse grained Fe-B alloy at dry sliding condition. The Mn element was studied to improve the wear resistance of Fe-3wt%B alloy [10]. The fracture toughness of Mn₂B played an important role in wear resistance of alloys. The results indicated that Fe-3wt% B alloy with addition of 2.0 wt% Mn showed the lowest wear rate at dry sliding condition. Additionally, the different wear-resistant boride layers were prepared on the surface of steels [11-13]. The dominate phases of boride coatings were α-Fe, FeB and Fe₂B. The main idea of improving wear resistance of coatings was to increase surface hardness of materials through the formation of Fe(B) phase within the matrix. Because the hardness of coatings (about 20 GPa) was much higher than that of subtracts, the interface had large stress field. Meanwhile, the Fe₂B and FeB phases had the brittle nature, thus, when the coatings suffered from stress impact causing spalling. GOK et al [14] studied the wear properties of boride layer of AISI H13 hot-work tool steel at room temperature and 500 °C. The hardness of boride layer was 5 times higher than that of untreated specimen. The wear rate of treated specimens was one time lower than that of untreated specimens at 500 °C. However, the friction coefficients of treated and untreated specimens reached up to 1.25 at room temperature and 500 °C. Nowadays, the investigations of tribological properties of bulk and coating of Fe-B alloys focus on dry-sliding and lubricating conditions [8, 9, 15]. However, the high- temperature friction and wear properties of Fe-B alloy has not received much attention.

In the present work, the high-temperature tribological properties of Fe(Cr) matrix alloys containing different contents of B element were studied by high-temperature disc-on-ball tribotester. The alloys were fabricated by powder metallurgy technique (P/M). The effect of B element addition on the friction and wear properties were investigated, and to explore the wear mechanisms in details.

2 Experimental procedure

In this study, Fe matrix materials were fabricated by powder metallurgy technique (P/M). The pre-alloyed Fe-Cr-B powder were prepared by using high temperature sintering technology at 1000 °C. The sintering alloyed powder was milled by using a high energy ball mill, then the pre- alloyed powders were sintered in a vacuum furnace. The component of each composites was given in Table 1, and the sintered bulk alloys were denoted as CB0, CB5, CB7 and CB9, respectively. The dominate phases of samples containing B elements was α-Fe, FeCr and Fe₂B (see Figure 1). The hardness of alloys increased with an increase in boron element content. The physical mechanical properties, prepared detail and microstructure were given in other literature [16].

Table 1 Chemical components of sintered alloys

Figure 1 SEM image of typical microstructure of specimen containing boron element

The high-temperature tribological properties were evaluated by using a high-temperature wear tester with a ball-on-disc configuration [17]. The upper ball was fixed, and the lower disk specimens rotated in high temperature environment. The Si₃N₄ ceramic ball was chosen as counterpart with a hardness 15 GPa and diameter of 6 mm. The specimens were processed into disk with a dimension of 18 mm×18 mm×4 mm. The testing surfaces of specimens were polished by abrasive paper. The wear test was conducted under the load of 10 N and sliding speed of 0.20 m/s. Meanwhile, the turning radius is 5 mm with a testing time 30 min, and the measured temperatures of specimens were room temperature (25 °C), 200, 400, 600 and 800 °C. Each test was carried out three times in order to ensure the reproducibility of results and the mean values were obtained.

The morphologies of worn tracks and element types of alloys were analyzed by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The cross sectional profile of worn track was measured by a 2207-surface profiler after wear test, and then the worn volume of specimens was calculated V=AL. The wear rate of specimens (W=V/(SF)) was calculated by a function of worn volume divided by normal load (F) and sliding distance (S), and the specific wear rate was expressed as mm³/(N·m).

3 Results and discussion

3.1 High-temperature tribological properties of specimens

The vibrations of friction coefficients of Fe matrix specimens at 10 N and 0.20 m/s from room temperature to 800 °C against ceramic ball are shown in Figure 2. From Figure 2, it can be found that the friction coefficients of different specimens decrease slightly as temperature rises from room temperature to 800 °C. Meanwhile, the friction coefficients of different specimens are found to be in the range of 0.5-0.7. Generally speaking, the specimens with- and without B element do not show obvious differences of friction coefficients when the testing temperatures are less than 400 °C. However, with increasing temperature, the friction coefficient of specimen CB0 is higher than those of specimens with addition of B element, whereas the friction coefficients of specimens CB5, CB7 and CB9 show similar values and trend. It proves that COFs of specimens are independent on the content of B element with the rise of temperature.

Figure 2 Variation of friction coefficients of sintering alloys with different temperatures at 10 N and 0.20 m/s

Figure 3 presents the specific wear rates of Fe matrix alloys with testing temperature at 10 N and 0.20 m/s. It can be seen that the wear rate of specimen CB0 increases as temperature increases, and the wear rate is in the order of 10^-4 mm³/(N·m) as the testing temperature increases from room temperature to 600 °C. At 800 °C, the wear rate of specimen CB0 is about 7.8×10^-2 mm³/(N·m). CB0 suffers from severe wear at high temperature. On the whole, the wear rates of specimens CB5, CB7 and CB9 are in the order of 10^-5 mm³/(N·m). Generally, the specific wear rates of specimens with B element addition are 1/10 of that of unreinforced specimens (CB0) in the range of 25 °C to 800 °C sliding against ceramic balls. It indicates that B addition effectively improves the high-temperature wear resistance of Fe matrix alloys. The specimens CB5, CB7 and CB9 show similar wear rate trend. Overall, from 25 °C to 400 °C, the wear rates of samples increase, and then decrease when the temperature rises up to 800 °C. Meanwhile, the wear rates decrease with the increase of B content. With the increase of B element content to 9%, the wear rate of specimen increases. According to the results, the specimen CB7 keeps the lowest wear rates as compared with the cases of other specimens at testing temperatures.

The friction coefficients of different specimens greatly depend on the testing temperature [18, 19]. At high temperature, the metal elements react with oxygen and form single or composite oxides on the worn surfaces (Cr₂O₃, Fe₂O₃, Fe(BO₂)₂ and Cr₂(FeO₂)₆) (see Figure 4). With the increase of temperature, the generation rate of oxides speeds up. These oxides are repeatedly grinded by tribo- couples on the surfaces. Finally, the oxides glaze layer forms in wear tracks at high temperature and changes the contact mode of metal-to-metal, which acts as protective film and provides lubricating effect for specimens [20, 21]. However, the oxides protective layer is impossible to form at low temperature (see Figure 4). And therefore, the friction coefficients of different specimens decrease with the rise of temperature. The wear resistance of alloys is considered to be based on their hardness (W=k(LP/H)) [22]. The high hardness can improve the wear resistance of materials during wear process. The specimen CB0 has low hardness as compared with those of other alloys due to the presence of α-Fe and Fe(Cr) phases [16]. Meanwhile, the low hardness of alloy is not enough to support stable oxides glaze layer in wear track in order that the fresh material exposes on worn surfaces, and the material is accelerated oxidation, thereby aggravating the friction coefficients and wear rates of specimen CB0 at high temperature. At 400 °C, the hardness of specimens containing B element decreases, and the oxides content is low or the rate of oxides formation is not enough to provide lubricating effect for specimens, resulting in the high wear rates of specimens at 400 °C [21]. The hardness of specimens CB5, CB7 and CB9 increases with the increase of B element content. Therefore, the wear rates of specimens decrease with the increase of B content. However, with the B content further increasing up to 9%, the amount of Fe₂B increases in matrix. Fe(B) phase has the brittle nature causing fracture due to the impact of applied load during sliding, which causes the increase of wear rate. Generally speaking, the specimen with 7 wt% B has the best tribological properties at temperature ranging from room temperature to 800 °C.

Figure 3 Variation of specific wear rates of sintering alloys with different temperatures at 10 N and 0.20 m/s

Figure 4 XRD patterns of specimen with worn surfaces at 400 °C and 800 °C

3.2 Evaluation of worn surfaces

Figure 5 gives SEM images of worn surfaces of specimen CB0 at different testing temperatures. At room temperature and 400 °C, the delamination and plastic deformation are noted on the worn surfaces. It indicates that the wear process of sample is dominated by delamination and deformation. At 800 °C, the oxides glaze layer is found on the worn surfaces and the glaze layer is loose. However, the part of oxides glaze layer peels off in order that the protective layer does not fully cover the surfaces of wear track (see Figure 5(c)), resulting in a transition from low wear to high wear. The wear mechanism of specimen CB0 is mainly oxidative wear at 800 °C.

Figure 5 SEM images of worn surface of CB0 at room temperature (a), 400 °C (b) and 800 °C (c)

The worn surfaces of specimens CB7 and CB9 with different temperatures at 10 N and 0.2 m/s are given in Figures 6 and 7. At room temperature, the wear tracks of specimens CB7 and CB9 show some spalling pits (see Figures 6(a) and 7(a)). With the addition of boron, the toughness of materials

Figure 6 SEM images of worn surfaces of CB7 at room temperature (a), 400 °C (b) and 800 °C (c)

Figure 7 SEM images of worn surfaces of CB9 at room temperature (a), 400 °C (b) and 800 °C (c)

reduces. Fatigue break occurs for the material on the surface due to the reciprocating load repeatedly function, resulting in the presence of spalling pits. It indicates that the wear mechanism of specimens is fatigue wear at room temperature. As the temperature rises to 400 °C, the oxides layer does not form on the worn surfaces, and some spalling pits are found. Generally speaking, the worn surfaces of specimens is smoother than those of room temperature (see Figures 6(b) and 7(b)). The hardness of materials decreases due to the effect of high temperature, and the fracture toughness is improved. The tendency of fatigue spalling reduces, which indicates that the wear mechanism of specimens is slight fatigue wear during the sliding process. The SEM morphologies of specimens CB7 and CB9 at 800 °C are presented in Figures 6(c) and 7(c). It is clear that the oxides glaze layer forms on the worn surfaces at 800 °C. Meanwhile, the worn surfaces are completely covered by oxides glaze layer, which indicates that the high hardness of specimens can support the oxides glaze layer at high temperature. This glaze layer can act as lubricating film and decrease the friction coefficients and wear rates of specimens [21-23]. As a result, the wear mechanism of specimens is characterized by oxidation at 800 °C.

Figure 8 illustrates the worn surfaces of coupled Si₃N₄ balls of specimens CB7 at different temperatures. It can be seen that the tribo-couples suffer from wear during the sliding process at different temperatures. The wear area decreases with the increase of temperature. This means that the wear of ceramic balls decreases at high temperature. Meanwhile, the transferred materials are found on the worn surfaces with increasing of temperature. These transferred materials including oxides cover the worn surfaces of tribo-couples in order to decrease the contact area of counterparts, and avoids wear. This accords with the results of tribological behavior at different testing temperatures.

Figure 8 SEM images of coupled ceramic balls of specimen CB7 at different temperatures:

4 Conclusions

The high-temperature tribological properties of Fe-Cr-B alloys were tested from room temperature to 800 °C. The effect of boron on the friction coefficient and wear rate were studied.

The friction coefficients of specimens are independent of the boron content at different testing temperatures, which greatly depend on testing temperature, and the friction coefficients decrease with the increase of temperature. The wear rates of specimen without boron is higher than those of specimens with boron. The wear rates of specimens with boron element decrease firstly and then increase as the boron content rises. The wear rates of the specimens containing boron are 1/10 of that of unreinforced specimens (CB0) in the range of 25 °C to 800 °C. It attributes to the high hardness and the formation of stable oxides glaze layer on the worn surfaces which consists mostly of single and composite oxides. This oxides glaze layer decreases the direct contact area of ball and samples, which plays an important part in improving the wear resistance of samples at high temperature. All things considered, 7 wt% B is a key content at which there is a transition of high-temperature tribological properties.

At room temperature and 400 °C, the wear mechanism of CB5, CB7 and CB9 is dominated by fatigue wear. However, at 800 °C, the worn surfaces is characterized by oxidative wear.

References

[1] AHMADI A, SADEGHI F, SHAFFER S. In-situ friction and fretting wear measurements of Inconel 617 at elevated temperatures [J]. Wear, 2018, 410-411: 110-118. DOI: 10.1016/j.wear.2018.06.007.

[2] ZHU X S, FU Y D, LI Z F, LENG K. Wear resistance of TC4 by deformation accelerated plasma nitriding at 400 °C [J]. Journal of Central South University, 2016, 23: 2771-2776. DOI: 10.1007/s11771-016-3339-y.

[3] JIN G, CAI Z B, GUAN Y J, CUI X F, LIU Z, LI Y, DONG M L, ZHANG D. High temperature wear performance of laser-cladded FeNiCoAlCu high entropy alloy coating [J]. Applied Surface Science, 2018, 445: 113-122. DOI: 10.1016/j.apsusc.2018.03.135.

[4] ZHANG Y, SHIMIZU K, KUSUMOTO K, HARA H, HIGUCHI C. Influence of Ni addition on erosive wear characteristics of multi-component white cast iron at elevated temperature [J]. Wear, 2017, 376-377: 452-457. DOI: 10.1016/j.wear.2016.12.044.

[5] BERNS H. Comparison of wear resistant MMC and white cast iron [J]. Wear, 2003, 254: 47-54. DOI: org/10.1016/ S0043-1648(02)00300-9.

[6] LENTZ J, ROTTGER A, THEISEN W. Hardness and modulus of Fe₂B, Fe₃(C,B) and Fe₂₃(C,B)₆ borides and carbonborides in the Fe-C-B system [J]. Materials Characterrization, 2018, 135: 192-202. DOI: 10.1016/ j.matchar.2017.11.012.

[7] CAMPOS-SILVA I, PALOMAR-PARDAVE M, PEREZ PASTEN-BORJA R, KAHVECIOGLU FERIDUN O, BRAVO-BARCENASD, LOPEZ-GARCIA C, REYES- HELGUER R. Tribocorrosion and cytotoxicity of FeB-Fe₂B layers on AISI 316 L steel [J]. Surface & Coatings Technology, 2018, 349: 986-997. DOI: 10.1016/j.surfcoat. 2018.05.085.

[8] YI D W, XING J D, ZHANG Z Y, FU H G, YANG C Y. Effect of titanium and nitrogen additions on the microstructures and three-body abrasive wear behaviors of Fe–B cast alloys [J]. Tribology Letters, 2014, 54(2): 107-117. DOI:org/10.1007/s11249-014-0314-3.

[9] FU L, YANG J, BI Q L, ZHU S Y, LIU W M. Dry-sliding tribological properties of nano-eutectic Fe₈₃B₁₇ alloy [J]. Tribology Letters, 2009, 34: 185-191. DOI: 10.1007/s11249- 009-9422-x.

[10] JIAN Y X, HUANG Z F, XING J D, GUO X Z, WANG Y, ZHENG L. Effects of Mn addition on the two-body abrasive wear behavior of Fe-3.0 wt%B alloy [J]. Tribology International, 2016, 103: 243-251. DOI: org/10.1016/ j.triboint.2016.07.008

[11] GUNEN A, KANCA E, KARAKAS MS, KOC V. High temperature wear behavior of the surface-modified externally cooled rolls [J]. Surface & Coatings Technology, 2018, 348: 130-141. DOI: 10.1016/j.surfcoat.2018.04.071.

[12] LYU Y Z, SUN Y F, JING F Y. On the microstructure and wear resistance of Fe-based composite coatings processed by plasma cladding with B₄C injection [J]. Ceramics International, 2015, 41: 10934-10939. DOI: 10.1016/ j.ceramint.2015.05.036.

[13] MADAH F, DEHGHANIAN C, AMADEH A A. Investigations on the wear mechanisms of electroless Ni–B coating during dry sliding and endurance life of the worn surfaces [J]. Surface & Coatings Technology, 2015, 282: 6-15. DOI: 10.1016/j.surfcoat.2015.09.003.

[14] GOK M S, KUCUK Y, ERDOGAN A, KANCA E, OGE M, KANCA E, GUNEN A. Dry sliding wear behavior of borided hot-work tool steel at elevated temperatures [J]. Surface & Coatings Technology, 2017, 328: 54-62. DOI: 10.1016/j.surfcoat.2017.08.008.

[15] DYBKOV V I, SIDORKO V R, GONCHARUK L V, KHORUZHA V G, SAMELYUK A V. Microstructure, Growth Kinetics, and Abrasive Wear Resistance of Boride Layers on Fe–30% Cr alloy [J]. Powder Metallurgy and Metal Ceramics, 2013, 51(9, 10): 518-530. DOI:10.1007/ s11106-013-9463-4.

[16] CUI G J, KOU Z M. The effect of boron on mechanical behavior and microstructure for Fe–Cr matrix alloy prepared by P/M [J]. Journal of Alloys and Compounds, 2014, 586: 699-702. DOI: 10.1016/j.jallcom.2013.10.110.

[17] ZHOU H, ZHANG Y P, HUA X H, YANG Z H. High-temperature anti-wear behavior of alumina-reinforced Ti-Zr-Mo alloy composites [J]. Wear, 2014, 319: 184-190. DOI: 10.1016/j.wear.2014.08.003.

[18] KUNDU S, DAS S K, SAHOO P. A parametric investigation of high temperature friction performance of electroless Ni-P deposits [J]. Materials Today: Proceedings, 2018, 5: 8547-8556. DOI: 10.1016/j.matpr.2017.11.552.

[19] ZHANG X H, MA J Q, FU L C, ZHU S Y, LI F, YANG J, LIU W M. High temperature wear resistance of Fe–28Al–5Cr alloy and its composites reinforced by TiC [J]. Tribology International, 2013, 61: 48-55. DOI: 10.1016/ j.triboint.2012.12.005.

[20] YANG F H, SU B, ZHANG A J, MENG J H, HAN J S, WU Y Z. Tribological properties and wear mechanisms of Mo₂FeB₂ based cermets at high temperatures [J]. Tribology International, 2018, 120: 391-397. DOI: 10.1016/j.triboint. 2017.12.038.

[21] RENZ A, PRAKASH B, HARDELL J, LEHMANN O. High-temperature sliding wear behaviour of Stellite12 and TribaloyT400 [J]. Wear, 2018, 402-403: 148-159. DOI: 10.1016/j.wear.2018.02.013.

[22] WANG L, YANG J, MA J Q, BI Q L, FU L C, HAO J Y, LIU W M. Tribological properties of a nano-eutectic Fe_1.87C_0.13 alloy under water environment [J]. Tribology Letters, 2010, 40(1): 105-111. DOI: 10.1007/s11249-010-9644-y.

[23] CHENG J, LI F, ZHU S Y, HAO J Y, YANG J, LI W S, LIU W M. High temperature tribological properties of a nickel-alloy-based solid lubricating composite: Effect of surface tribo-chemistry, counterpart and mechanical properties [J]. Wear, 2017, 386-387: 39-48. DOI: 10.1016/ j.wear.2017.06.001.

(Edited by HE Yun-bin)

中文导读

高温条件下Fe(Cr)-B合金的摩擦学性能

摘要：本文研究了在室温至800 °C范围内硼元素及其含量(0 wt%, 5 wt%, 7 wt%和9 wt%)对Fe-Cr-B合金抗磨性能的影响，并以此为依据拓宽其作为高温抗磨原件的应用。另外，探讨了合金的高温磨损机理。在室温到800 °C范围内，当滑动速度为0.20 m/s且载荷为10 N时，以氮化硅陶瓷球作为摩擦副，采用球-盘式高温摩擦实验机系统研究了试样的高温摩擦学性能。相比于不含硼元素的试样，添加硼元素试样的抗磨损性能得到了极大的改善。合金的摩擦系数随着测试温度的升高而降低。随着金属中硼含量的增加Fe-Cr-B合金的磨损率开始降低，然后开始升高。含硼的铁基合金的磨损率是不含硼的铁基合金的磨损率的1/10。在高温条件下，Fe-21wt%Cr-7wt%B合金具有最好的摩擦学性能。

关键词：Fe(Cr)-B合金；复合材料；高温摩擦性能；摩擦；抗磨损性能

Foundation item: Projects(51775365, 51405329) supported by the National Natural Science Foundation of China; Project(2015M570239) supported by the China Postdoctoral Science Foundation

Received date: 2018-08-31; Accepted date: 2018-12-27

Corresponding author: CUI Gong-jun, PhD, Associate Professor; Tel: +86-351-6018949; E-mail: cuigongjun@tyut.edu.cn