J. Cent. South Univ. Technol. (2011) 18: 1334-1339

DOI: 10.1007/s11771-011-0842-z

Effects of h-BN content on properties of Ni-Cr/h-BN composite

WEI Xiao-feng(韦小凤), WANG Ri-chu(王日初), FENG Yan(冯艳),

PENG Chao-qun(彭超群), ZHU Xue-wei­­(朱学卫)

School of Materials Science and Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: Ni-Cr/h-BN self-lubricating composities were prepared by powder metallurgy (P/M) method. The effects of hexagonal boron nitride (h-BN) content on the mechanical and tribological properties of the Ni-Cr/h-BN composites were investigated. The corresponding frictional models were established to analyze the formation of the lubricant h-BN films on the surfaces of the Ni-Cr/h-BN composites. The results show that, when the content of h-BN increases from 5% to 15% (mass fraction), the bending strength of the Ni-Cr/h-BN composite decreases from 96.670 MPa to 17.319 MPa, and the hardness (HB) decreases from 33 to 14. The friction coefficient of the Ni-Cr/h-BN composite decreases firstly from 0.385 to 0.216, and then increases to 0.284, while the wear rate decreases firstly from 4.14×10^{-9 kg/(N·m) to 1.35×10-9 kg/(N·m), then increases to 2.36×10-9 kg/(N·m). The best comprehensive mechanical and tribological properties can be obtained between 10% and 12% h-BN addition.}

Key words: Ni-Cr/h-BN composite; hexagonal boron nitride; solid lubricating film; friction coefficient; wear rate

1 Introduction

Metal-based self-lubricating composites have found wide applications in many machines which require lubrication over a wide range of temperature [1-2]. The most common solid lubricant is MoS₂, whose highest service temperature in air is limited to about 400 °C [3-4]. Another common lubricant is graphite, which generally works only up to about 600 °C [5-7]. At a higher temperature, MoS₂ and graphite will be partly decomposed and oxidized during the friction process [8-9], and an effective lubricating film cannot be  formed. This leads to almost linearly decreased tribological properties of the self-lubricating composites. In order to solve this problem, it is urgent to develop self-lubricating composites with stabilized solid lubricants at high temperatures.

The solid lubricant hexagonal boron nitride (h-BN) is called white graphite because of its color and a similar structure as graphite [10]. It also has good wear performance, excellent lubrication property and good thermal stability. The melting point (sublimation temperature) of h-BN is 3 000 °C [11] and the oxidation temperature is 900-1 200°C [12]. It has been widely used in high-temperature lubrication-sealing materials of aerospace engine in recent years [13]. It is an excellent additive of the high-temperature self-lubricating materials [14-15]. However, there are many problems coexisting with the good lubrication performance. The composites with h-BN as lubricant has poor sintering performance, and the effects of the h-BN addition on the mechanical properties and the tribological properties of the composites contradict with each other. Therefore, there are few reports about the composites containing h-BN as solid lubricant.

Ni-Cr based self-lubricating composites with h-BN as the solid lubricant were prepared in this work. The effects of the h-BN content on the self-lubricating behaviors were discussed, and the corresponding frictional contact models were proposed.

2 Experimental

The raw materials used in this work were alloyed Ni-20Cr powder of 99% purity, and hexagonal boron nitride (h-BN) of 99.7% purity. After fully mechanically mixed in a three-dimensional mixing machine, the mixture of the alloyed Ni-20Cr powder with 5%, 8%, 10%, 12% and 15% (mass fraction) of h-BN were cold pressed into 50 mm × 10 mm × 3 mm and 20 mm ×   12 mm × 6 mm strips at 300 MPa pressure. Then, the strips were sintered in a furnace with hydrogen. The sintering temperature is 1 150 °C, and the sintering time is 60 min.

The hardness and bending strength of the Ni-20Cr/h-BN composites were examined on a HW187.5 Brinell sclerometer and a CCS-44100 electronic tester, respectively. The dry friction and wear test were carried out on a M-2000 ring-blocks sliding friction machine, with a coupled GCr40 steel ring and a normal load of  76 N. The speed of the steel ring is 400 r/min (0.42 m/s). The constitution of the composites was analyzed with X-ray diffractomers (XRD). Metallographic structure and morphologies of the worn surfaces were observed with a POLYVER-MET optical microscope and a Sirion 200 scanning electron microscope (SEM), respectively.

3 Results and discussion

3.1 Effects of h-BN content on microstructures of Ni-Cr/h-BN composites

The optical micrographs of the Ni-Cr/h-BN composites with different h-BN contents are shown in Fig.1. It can be found that the h-BN phases (black phases in the images) are surrounded by the Ni-Cr matrix (white phases in the images). These two phases distribute homogeneously in Figs.1(a) and (b). When the content of h-BN increases, as seen in Figs.1(c) and (d), the h-BN particles aggregate severely, and large pores appear. Accordingly, the continuity of the Ni-Cr matrix is gradually weakened.

In the Ni-Cr/h-BN composites, it is clear that no reaction occurs between the Ni-Cr matrix and the h-BN. The Ni-Cr matrix and the h-BN are stable during the sintering process. They form a composite with the Ni-Cr as the matrix, and h-BN as the solid lubricant. Obviously, this phenomenon is good for the Ni-Cr/h-BN composite to play a self-lubricating function. However, the bonding strength of the interface between the Ni-Cr matrix and the h-BN is poor. As a result, the function of the solid lubricant h-BN in the Ni-Cr/h-BN composite is similar to the pores in the composite for the mechanical properties.

With the increase of h-BN content, the solid lubricant h-BN distributes increasingly unevenly. This is due to its flake structure, and poor liquidity and dispersion, as shown in Figs.1(c) and (d). The density of the composite decreases accordingly. This indicates that the h-BN content should be limited to an appropriate level in consideration of the density of the composite.

The X-ray diffraction pattern of the Ni-Cr/h-BN composite containing 10% h-BN is shown in Fig.2. The X-ray diffraction patterns of other Ni-Cr/h-BN composites in the present work are similar to this one. This illustrates that there are only diffraction peaks for the Ni-Cr solid solution and h-BN, while no peaks corresponding to new reaction products are detected. Thus, it can be deduced that the obtained composite is composed of Ni-Cr and h-BN. The results further confirm the formation of Ni-Cr/h-BN composites and are in good agreement with the optical microscopic structure analysis.

3.2 Effects of h-BN content on physical and mechanical properties of Ni-Cr/h-BN composites

The effects of the h-BN content on the density and the porosity of the Ni-Cr/h-BN composites are shown in Fig.3(a). The graph confirms that with the increase of h-BN content, the density of the Ni-Cr/h-BN composite decreases gradually and the porosity increases accordingly. This may be due to the poor wettability between the Ni-Cr matrix and the solid lubricant h-BN, and little sintering action between the Ni-Cr matrix and the h-BN during the sintering process. This means that the h-BN powders hinder the sintering process of the Ni-Cr matrix. With the increase of the h-BN content, the hindering effect becomes more remarkable. Therefore, the density of sintered Ni-Cr/h-BN decreases and the porosity increases with increasing h-BN content.

Fig.1 Optical micrographs of Ni-Cr/h-BN composites with different contents of h-BN: (a) 8%; (b) 10%; (c) 12%; (d) 15%

Fig.2 XRD pattern of Ni-Cr/10%h-BN composite

Fig.3 Effects of h-BN content on physical and mechanical properties of Ni-Cr/h-BN composites: (a) Physical properties;  (b) Mechanical properties

The effects of the h-BN content on the hardness and the bending strength of the Ni-Cr/h-BN composites are given in Fig.3(b). It is shown that, with the increase of h-BN content from 5% to 15%, the bending strength of the Ni-Cr/h-BN composites decreases from 96.670 MPa to 17.319 MPa, and the hardness (HB) decreases from 33 to 14. This changing trend is due to the effects of pores on the properties.

The Brinell hardness is a characterization of resistance to local plastic deformation. In the process of measuring Brinell hardness, the pressure keeps on the Ni-Cr matrix, the h-BN and the pores simultaneously. In other word, the Brinell hardness is a kind of comprehensive hardness of the composite [16]. With the increase of h-BN content, the porosity increases significantly, and the effective volume to resist the indentation obviously decreases. Accordingly, the ability to resist material surface deformation decreases; therefore, the value of hardness is lower.

The pores are the causes of the local stress concentration and microcrack. They can reduce the bending strength of the materials made by P/M method. Therefore, the pores are often the direct factor that causes the composite failure under external force. According to the theory of MAUSNER [17], the relationship between bending strength and porosity can be expressed as follows:

                                                             (1)

where σ₀ is the bending strength of the densified material (MPa); σ is the bending strength of the materials made by P/M method (MPa); ε is the porosity (%); B is a constant coefficient which depends on the material and the experimental conditions, and generally is 4-7.

From Eq.(1), it can be found that, with the increase of the porosity, the bending strength of the materials made by P/M method decreases. This is in good agreement with the effects of the h-BN content on the bending strength of the Ni-Cr/h-BN composites.

3.3 Effects of h-BN content on friction and wear properties of Ni-Cr/h-BN composites

Fig.4 demonstrates the duplex effects of the h-BN content on the friction and the wear properties of the Ni-Cr/h-BN composites. With the increase of the h-BN content from 5% to 15%, the friction coefficient of the Ni-Cr/h-BN composite decreases firstly from 0.385 to 0.216, and then increases to 0.284, while the wear rate decreases firstly from 4.14×10^{-9 kg/(N·m) to 1.35×10-9 kg/(N·m), then increases to 2.36×10-9 kg/(N·m).} The lowest value of wear rate exists at the content of h-BN of 10%, and the lowest value of friction coefficient exists at the content of h-BN of 12%. Considering the comprehensive tribological properties of the Ni-Cr/h-BN composite, the optimum h-BN content should be controlled between 10% and 12%.

Fig.4 Effects of h-BN content on friction coefficient and wear rate of Ni-Cr/h-BN composites

Microstructures of the worn surfaces of the Ni-Cr/h-BN composites are given in Fig.5. It is shown that the solid lubricating film formed on each composite is consistent with its friction and wear properties. The composite containing 8% h-BN produces only an isolated island-like film, with a large amount of spalling pits (see Fig.5(a)). This indicates that no effective self-lubrication film is formed. When the h-BN content increases to 10%, a considerably integrated lubricating film is formed (see Fig.5(b)) along with much fewer spalling pits. Therefore, both friction coefficient and wear rate decrease significantly. The most continuous lubricating film further forms on the whole surface of the 12% h-BN composite (see Fig.5(c)), and the friction coefficient decreases to the lowest level. But, the stability and strength of the film decrease remarkably. From the magnified images (see Fig.6) of the flaked film, it can be seen that, the film on the surface of the composite containing 12% h-BN is easily subjected to local flaking during friction. As a result, the wear rate increases again. When the h-BN content increases to 15%, however, severe spalling occurs on the composite surface. The film integrity reduces (see Fig.5(d) and Fig.6(d)), resulting in a remarkable increase in the friction coefficient and the wear rate again. It is obvious that the integrity (or surface coverage) of the formed lubricating film also varies with the h-BN content. It can be used to characterize the friction and wear behavior of the composite.

With the increase of the h-BN content, an almost linear reduction in friction is observed with increasing the film coverage and then increased friction is observed as the film is unstable and subjected to severe flaking. It is certain that the optimum h-BN concentration for the lowest friction corresponds to the comprehensive property of the continuous h-BN film and appropriate mechanical property of the composite.

For further investigation of the mechanism, the formation mechanism model of the lubricating film is established to describe the principles of the thin solid lubrication film of the Ni-Cr/h-BN. In the sliding friction process, the embedded solid lubricant particles are extruded due to the friction heat and deformation force, and then a lubricating film forms on the surface of the friction pair, as shown in Fig.7. When the h-BN concentration is too low to form a continuous lubricating film on the composite, the friction between the composite and the dual materials can be regarded as being approximate in a boundary lubrication state, where some metal-metal direct contact occurs between the exposed Ni-Cr particles and the dual materials surface. According to BOWDEN and TABOR’s theory [18-19], the friction force F can be expressed as

                                                 (2)

where A is the total contact area of the friction pair, n is the percentage of lubricating film coverage, and τ_f and τ_b represent the shearing strength of the lubricating film and the matrix, respectively.

Fig.5 Morphologies of worn surfaces of Ni-Cr/h-BN composites containing (a) 8%, (b) 10%, (c) 12% and (d) 15% h-BN

Fig.6 Magnified images of flaked film on Ni-Cr/h-BN composites with different contents of h-BN: (a) 8%; (b) 10%; (c) 12%;     (d) 15%

Fig.7 Formation mechanism of lubricating film

Supposing that the normal load is distributed proportionally on the contact area, the friction coefficient μ is then obtained:

                                                       (3)

where μ_f and μ_b are the friction coefficients under the extreme conditions of full solid film lubrication and full base alloy metal-metal contact, respectively.

Equations (2) and (3) indicate that with the increase of film coverage area on the friction surface, which also means the increase of the h-BN content in the composite, the friction coefficient decreases linearly from an extremely high level at full metal-metal contact to very low level at full solid film lubrication. Nevertheless, because the film coverage does not always increase linearly with the increase of the h-BN content, the reduction in friction is not completely linear with the increase of h-BN content up to 12%. The general changing tendencies of both the friction coefficient and the wear rate agree well with the above analysis.

When the h-BN content increases enough to form a continuous and integrated film on the whole friction surface, full film lubrication is achieved, as shown in Figs.5(b), 5(c) and Fig.7(d). It is another boundary lubrication state of self-lubrication, as n=1 in Eqs.(2) and (3). The self-lubricating property of the composite is then mainly determined by the lubricity of the film, which, like other preplaced solid lubricating film, is in turn significantly affected by two factors. The first one is related to the lubricity of the lubricant, such as the μ_f in Eq.(3), which is the main property of the lubricant. The second one is the mechanical properties of the substrate of the film. In the Ni-Cr/h-BN composites, support of the film comes from the load-carrying capacity (hardness and bending strength) of the composite. With further increasing of the h-BN content after full film coverage is achieved, the mechanical properties of the composite are greatly reduced. Therefore, the h-BN film can no longer be effectively supported by the composite, resulting in friction increasing and film flaking again, as shown in Fig.4. The wear rate increases again at 10% of h-BN content, while the friction coefficient increases again at 12% of h-BN content. This illustrates that the decreased mechanical properties have more remarkable effect on the wear rate than on the friction coefficient.

Summing up the above analysis, it can be concluded that an appropriate h-BN content exist theoretically in the Ni-Cr/h-BN self-lubricating composite. In order to obtain the minimum friction coefficient and wear rate, the content of the solid lubricant h-BN should be adjusted to a level where a homogeneous and continuous lubricating film is just formed on the whole frictional surface, meanwhile, the mechanical properties of the composite have not been reduced enough to decrease the self-lubricity of the film remarkably.

4 Conclusions

1) With the increase of h-BN content, the h-BN aggregates severely, and distributes increasingly unevenly.

2) With the increase of h-BN content, the density of the Ni-Cr/h-BN composite decreases gradually the porosity increases accordingly, and both the hardness and bending strength of the Ni-Cr/h-BN composite decrease.

3) The optimum content of h-BN in the Ni-Cr/h-BN self-lubricating composite is between 10% and 12%. At this level, homogeneous and continuous lubricating film is just formed on the whole frictional surface, while the mechanical properties of the composite have not decreased enough to cause a remarkable decrease in the lubricity of the film.

References

[1] SHIAO J S, WANG T Z. Dry self-lubricating composite [J]. Composites Part B, 1996, 27(5): 459-465.

[2] WANG Wen-chao. Application of a high temperature self-lubricating composite coating on steamturbine components [J]. Surface and Coatings Technology, 2004, 177/178: 12-17.

[3] LU Jin-jun, YANG Sheng-rong, WANG Jin-bo, XUE Qun-ji. Mechanical and tribological properties of Ni-based alloy/CeF₃/ graphite high temperature self-lubricating composite [J]. Wear, 2001, 249(12): 1070-1076.

[4] LI Jian-liang, XIONG Dang-sheng. Tribological properties of nickel-based self-lubricating composite at elevated temperature and counterface material selection [J]. Wear, 2008, 265(3/4): 533-539.

[5] BHATTACHARYA R S. Low friction and wear surface for application over a wide range of temperature [R]. Dayton: Universal Energy Systems INC Dayton OH, 1997.

[6] WANG Yan-jun, LIU Zuo-min. Tribological properties of high temperature self-lubrication metal ceramics with an interpenetrating network [J]. Wear, 2008, 265(11/12): 231-237.

[7] XIONG Dang-sheng. Lubrication behavior of Ni-Cr based alloys containing MoS₂ at high temperature [J]. Wear, 2001, 251(1-12): 1094-1099.

[8] WU Yun-xin, WANG Fu-xing, CHENG Yin-qian, CHEN Nan-ping. A study of the optimization mechanism of solid lubricant concentration in Ni/MoS₂ self-lubricating composite [J]. Wear, 1997, 205(1/2): 64-70.

[9] DURAN C, EROGLU S. Liquid-phase sintering and properties of Cr₃C₂-NiCr cermets [J]. Journal of Materials Processing Technology, 1998, 74(1/2/3): 69-73.

[10] ZHANG Shi-tang, ZHOU Jian-song, GUO Bao-gang, ZHOU Hui-di, PU Yu-ping, CHEN Jian-min. Preparation and characterization of reactively sintered Ni₃Al-hBN-Ag composite coating on Ni-based superalloy [J]. Journal of Alloys and Compounds, 2009, 473(1/2): 462-466.

[11] ERHAN B, CETIN B. The effect of transition metals on the structure of h-BN intercalation compounds [J]. Journal of Solid State Chemistry, 2004, 177(4/5): 1768-1770.

[12] DONNET C, ERDEMIR A. Historical developments and new trends in tribological and solid lubricant coatings [J]. Surface and Coatings Technology, 2004, 180/181: 76-84.

[13] LI Fan, ZhANG Deng-jun, LI Bao-hou, LUO Shi-min, ZHAO Xiao-feng. A new chemical plating system for hexagonal boron nitride particles [J]. The Chinese Journal of Process Engineering, 2002, 5(2): 425-430. (in Chinese)

[14] ZHANG Shi-tang, ZHOU Jian-song, GUO Bao-gang, ZHOU Hui-di, PU Yu-ping, CHEN Jian-min. Friction and wear behavior of laser cladding Ni/hBN self-lubricating composite coating [J]. Materials Science and Engineering, 2008, 491(1/2): 151-158.

[15] WEI Sheng-ming, WANG Ri-chu, LI Qing-yong, LI Wenxian. Effect of BN on sintering properties of Ni-based abradable seal alloy [J]. Rare Metal and Engineering, 2006, 35(1): 127-130. (in Chinese)

[16] XIANG Ding-han, YAO Zheng-jun, WEN Jian-ping. Experimental investigation on dry frictional behavior of the two self-lubricating composites under heavy loading conditions [J]. Materials Letters, 2005, 59(18): 2352-2356

[17] MAUSNER H M. Handbook of powder metallurgy [M]. New York: Chem Publ Co, 1973: 482-490.

[18] BO

[19] WDEN F P, TABOR D. The friction and lubrication of solids (Part II) [M]. Oxford: Clarendon Press, 1964: 139-147.

[20] HE Jiang-ai, WANG Yu-wei. Wear and anti-wear of materials [M]. Shenyang: Northeastern University Press, 2002: 23-24. (in Chinese)

(Edited by HE Yun-bin)

Foundation item: Project(MKPT-03-182) supported by the Ministry of Science and Technology of China

Received date: 2010-07-23; Accepted date: 2010-11-09

Corresponding author: WANG Ri-chu, Professor, PhD; Tel: +86-731-88836638; E-mail: wrc910103@163.com