Effect of carbon on tribological property of plasma carburized TiAl based alloy
LIU Xiao-ping(刘小萍)1,2, TIAN Wen-huai(田文怀)1, GUO Chao-li(郭朝丽)2,
HE Zhi-yong(贺志勇)2, XU Zhong(徐 重)2
1. Department of Materials Physics and Chemistry, University of Science and Technology Beijing,
Beijing 100083, China
2. Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China
Received 28 July 2006; accepted 15 September 2006
Abstract：Plasma carburization at two different methane-to-argon gas ratios (5:5 and 6:5) was carried out on the cast TiAl based alloy of Ti-46.5Al-2.5V-1Cr (mole fraction, %) in order to enhance its wear resistance. The results show that after carburization under both carburizing atmospheres, Ti2AlC and TiC are the main carbides in the carburized layer and the value of surface hardness reaches more than HK 822, but for the carburized TiAl treated at CH4?Ar of 5?5, the surface carbon concentration is higher and the carburized depth is slightly thicker than that of alloy carburized at CH4?Ar of 6?5. The result of the ball-on-disk test against hardening-steel counter bodies shows that the wear resistance of the TiAl based alloy carburized under two different carburizing atmospheres is improved compared with non-carburized TiAl. The tribological property is related to the carbon content, and the carburized layer obtained at CH4:Ar of 5:5 possesses a stable friction coefficient, lower volume loss or wear rate and narrow wear scar. The characteristic of the carburized layer was examined by using optical microscopy, glow discharge spectrum and micro-hardness tester.
Key words: TiAl based alloy; plasma carburization; carburizing atmosphere; carbon concentration; tribological property
1 Introduction
Much progress of γ-TiAl based alloys has been made with high room-temperature ductility and high- temperature oxidation resistance above 800 ℃ after decades of research[1-2]. Recently, some efforts have been made to improve the tribological properties of the TiAl based alloys because the poor wear resistance restricts their application of components which are subjected to surface contact and sliding wear in service, for example, exhaust values in car engines and gas turbine blades in aero engines[3]. Various surface modifications, including carburization[4-5], nitriding [6-7], ion implantation[8], laser surface alloying[9] and so on, have been commonly used to improve the wear resistance of TiAl based alloys. Among these surface treatment methods, carburization, especially plasma carburization is currently regarded to be a simple and efficient approach. HUANG et al[4] applied the solid carburization in TiAl based alloy and found that its oxidation resistance is improved due to the Ti3AlC, Al4C3 and Al2C4Ti2 distributing along the carburized layer. WANG et al[5] have reported that carbide in the TiAl surface treated using liquid carburizing agent is TiC and the carburized surface hardness is about HV 940. As we know, the tribological properties are mainly related to the carbides in the modified layer, which is affected by the carburizing atmosphere. Both of them, however, didn’t systemically deal with the wear behavior of the carburized TiAl based alloys, which is important for their applications. In this study, the hard layer with Ti2AlC and TiC was fabricated on the TiAl surface using plasma carburization. The characteristic of the carburized layer formed at different methane-to-argon ratios as well as their tribological properties were investigated in order to optimize the carburizing parameters.
2 Experimental
2.1 Preparation of samples
The alloy used in the present study is a TiAl cast alloy with Ti-46.5Al-2.5V-1Cr(mole fraction, %), which was made from hot isostatically pressed ingot melted using induction shell melting technique. TiAl substrates with the dimensions of 3.8 mm ×13 mm ×13 mm were polished by using 1000# emery papers and cleaned in acetone solution previous to surface treatment. Plasma carburization was performed in a discharge maintained by a d.c. power supply using methane and argon gas mixture with two methane-to-argon ratios (CH4:Ar) of 5:5 and 6:5, respectively. Samples were heated at 980 ℃ for 3 h and cooled down to room temperature in the chamber. The thicknesses and chemical composition of the carburized layer were analyzed by the GDA750 glow discharge spectrum (GDS). The phase formation of the plasma carburized surface was characterized by X-ray diffraction (XRD). Micro-hardness was measured using the LECOM-400-H1 digital micro-hardness tester with a test load of 0.1 N.
2.2 Wear test
The dry sliding wear test was carried out in a WTM-1E wear tester in which non-carburized and carburized TiAl disks were cleaned with acetone. The counterpart material, a ball having 3 mm in diameter and about HK 765 in hardness, were made of ASTM W1-111/2 steel. The schematic demonstration of ball-on-disc wear machine is shown in Fig.1. The samples were rotated at a speed of 1 000 r/min for 10 min under a load of 1.5 N. The friction coefficient was recorded during each test and the wear volume of the worn specimens was estimated by measuring the width as well as the depth of wear track on the surface. The wear rate was calculated by dividing the volume loss by sliding distance and applied load. The worn surface of the samples was observed using an Axiovert 25 CA optical microscope.
Fig.1 Schematic diagram of balll-on-disc wear tester
3 Results
The effect of the carburizing atmosphere on the carbon concentration of plasma carburized TiAl surfaces
is shown in Fig.2, where the curves of TiAl-1C and TiAl-2C represent the carbon contents of the carburized layer formed at the methane-to-argon ratios of 5:5 and 6:5, respectively. After different carburizations, the carbon contents of carburized cases reach 22%(mole fraction) for TiAl-1C and 15%(mole fraction) for TiAl- 2C. The concentration distribution of carbon for the TiAl-1C layer nearly has the same gradient with TiAl-2C layer. The carburized depths of TiAl-1C and TiAl-2C layers are approximately 3 μm and 4.5 μm, respectively. The structures of the carburized TiAl surfaces determin- ed by XRD analysis are composed of Ti2AlC and TiC. In addition to the carbides, other phases like TiAl, TiAl2, Ti3Al5 and TiAl3 are also formed in the carburized sur- face[10].
Fig.3 shows the coefficients of friction for carbur- ized and non-carburized TiAl specimens under the load of 1.5 N. The corresponding wear loss and wear rate are given in Fig.4. Obviously, the carburizing atmosphere has effect on the tribological property of TiAl based alloy. As shown in Fig.3, TiAl-1C layer exhibits a long-term friction with stable and the lowest value of friction coefficient (approximately 0.2) after sliding for 10 min, and for TiAl-2C layer, the friction coefficient is the same as TiAl-1C layer at the initial sliding of 3.5 min and then gradually increases to the average value of 0.8 after the wear for 10 min. As for non-carburized TiAl specimens, the high friction coefficient occurs from the very beginning of wear and reaches about 0.9 after wearing for 5 min. The volume loss and wear rate for both carburized and non-carburized TiAl specimens as shown in Fig.4 are consistent with their friction coefficients in that the TiAl-1C layer also shows the lower volume loss (0.01 mm3) or wear rate (2.37×10-5 mm3/(N·m) among all wear samples, while non-carburized TiAl specimens have higher ones, approximately 0.15 mm3 in volume loss and 1.26×10-3 mm3/(N·m) in wear rate. The wear volume of TiAl-2C layer is twice as much as that of TiAl-1C layer and the wear rate increases about three times compared with the TiAl-1C layer.
Fig.2 Carbon distributions of plasma carburized layers
Fig.3 Friction coefficient vs sliding time of carburized and non-carburized TiAl alloys
Fig.4 Comparison of volume loss and wear rate for carburized and non-carburized TiAl specimens
The optical micrographs of the wear tracks generated on non-carburized and carburized TiAl samples are shown in Fig.5. Extensive abrasive and adhesive wears are dominant during the wear tests of the worn surface of non-carburized TiAl specimen where a lot of deep grooves exist on it (see Fig. 5(a)). The TiAl-1C layer exhibits a uniform wear with the low abrasion and slight track of the carburized surface (see Fig.5(b). The worn surface morphology of TiAl-2C layer as shown in Fig. 5(c) seems to be adhesive wear mainly, showing tribochemical reactions on the carburized surface [11].
4 Discussion
The surface carbon concentration decreases with
the increase of methane proportion in the methane-to- argon ratio, which seems in contravention of the carburizing principle. The plasma carburization is different from the traditional carburization because during plasma carburization, there are two kinds of carbons, thermally activated carbon atoms decomposed from CH4 and excited carbon ions. Both of them are absorbed on the substrate surface and then diffuse into it to form interstitial solid solutions or carbides (Ti2AlC or TiC). And a portion of carbon atoms in plasma zone combine with atoms sputtered form substrate to form carbides (TiC) which are captured by the substrate surface under the action of catalysis, resulting in the carbon atoms diffusing into substrate[12]. The amounts of active carbon atoms and ions reaching the substrate are much more than those of vacuum carburization. When methane supply is excessive, over-abundance of carbon leads to formation of inactivated carbon on the substrate surface which deters carbon in plasma zone from diffusing into substrate, causing the low carbon concentration in the substrate surface. This may be the reason for the low carbon content on the TiAl-2C surface.
It is known that the wear resistance of metal materials is measured in terms of the hardness except erosion wear and the wear rate decreases with the hardness increasing. Like the solid solution materials, the hardness of the TiAl specimen is related to the carbon content. The larger the carbon concentration is, the more the carbides and the higher the hardness are. The average value of surface hardness for the carburized TiAl specimen is HK 990 for TiAl-1C and HK 822 for TiAl-3C, while the hardness of the non-carburized TiAl specimen is about HK 500. For the non-carburized TiAl samples, the harder asperities can easily penetrate into the sliding surface of TiAl samples owing to the higher hardness of counterpart (approximately HK 765), causing effective micro-cutting[13]. It is clear that the lower values of friction coefficient and volume loss for both TiAl-1C and TiAl-2C can be attributed to their high surface hardness or the formation of Ti2AlC and TiC. The existence of these carbides can reduce the adhesive as well as melting between two counterparts during wear, enhancing the wear resistance[14]. With respect to the TiAl-2C layer, the wear mechanism is changed from initial mainly abrasive into adhesive and abrasive wears because of its lower hardness as well as thin carburized layer.
Fig.5 Optical morphologies of worn surface for non-carburized TiAl (a), and carburized TiAl at methane-to-argon ratios of 5:5(b) and 6:5(c)
5 Conclusions
1) The plasma carburization can be used to reduce the friction coefficient, volume loss or wear rate of TiAl based alloy, which are influenced by the carbon concentration of the carburized layer.
2) The higher carbon content leads to the formation of Ti2AlC and TiC as well as the high surface hardness.
3) The improvement of tribological property of the TiAl based alloy is relative to the surface hardening by plasma carburization.
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(Edited by PENG Chao-qun)
Foundation item: Project(50671071) supported by the National Natural Science Foundation of China; Project(2006011052) supported by the National Natural Science Foundation of Shanxi Province, China
Corresponding author: LIU Xiao-ping; Tel: +86-351-6010540; E-mail: liuxiaoping@tyut.edu.cn
