Tribological behavior of in-situ nanosized TiB₂ particles reinforced Mg matrix composites

Peng Xiao, Yimin Gao, Feixing Xu, Cuicui Yang

State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an, Shaanxi Province 710049, PR China

INTRODUCTION: Magnesium and magnesium alloys, as energy-saving and lightest metallic materials, have attracted remarkable interests in aerospace and automotive industries [1]. However, relatively lower strength and poor wear resistance of Mg alloys have limited its further application [2]. In recent years, metal matrix composites reinforced with ceramic particles have been developed to improve mechanical properties and wear resistance of metallic materials [3, 4]. Several researchers have reported that Mg matrix composites containing homogenous hard ceramic particles such as SiC, CaB₆  and feldspar particles, exhibit an improved hardness and strength, which is contributed to excellent wear resistance compared with the unreinforced Mg alloys [5, 6]. However, most of previous research works on dry sliding wear behavior of composites have been mainly concentrated on the Mg matrix composites reinforced with micro sized particles. It has been reported that nano sized particles reinforced Mg matrix composites possess good comprehensive mechanical properties and superior wear resistance compared with Mg alloys or composites reinforced with high contents of micro sized particles [7, 8]. Besides, in-situ particles reinforced Mg matrix composites show a great attention due to its advantages such as clean interface and nanoscale reinforcements formed controllably [9]. But so far, few works in open literatures on dry sliding wear behavior has been performed on in-situ nano sized particles reinforced Mg matrix composites.

What’s more, TiB₂ ceramic particles have been widely used as reinforcements in metal matrix composites due to lower density, higher hardness, good corrosion resistance and wear resistance [10]. In previous reports, nano TiB₂/Mg composites has been fabricated by adding the Al-TiB₂ master alloys synthesized by in-situ technique [11]. In this work, the mechanical properties and tribological behavior of 2.5 wt.% TiB₂/AZ91 composites are investigated. The microstructure evolution of subsurface and effect of nano TiB₂ particles on the wear resistance of AZ91 matrix alloy under various wear conditions are discussed in detail.

METHODS:  Dry sliding wear tests were carried out in a pin-on-disc configuration at room temperature. Pin specimens cut from as-cast ingots have a dimension of 5×5 mm² in cross-section and 12 mm in height, and the hardened AISI 52100 bearing steel (65 HRC) disc with a size of Φ44 mm×5 mm acted as the counterface. The wear tests were conducted at three different sliding speeds of 0.25, 0.5 and 0.75 m/s under the normal loads of 12.5, 25 and 37.5 N with a constant sliding distance of 2000 m. Before each wear test, the pin samples and steel discs were ground and polished to the surface roughness of Ra 0.19 μm. In order to evaluate the wear rate, the mass of each pin sample was measured before and after wear tests using an electronic analytical balance with the precision of ±0.01 mg. Three tests were conducted to obtain an average of mass loss.  

RESULTS:  Fig. 1 (a) and (c) show the SEM micrograph of worn surface for AZ91 matrix alloy and TiB₂/AZ91 composites at the sliding speed of 0.75 m/s under normal load of 37.5 N, respectively. It is clear that the wear damage of AZ91 matrix alloy, such as delamination, craters, grooves and removal of materials, is much more serious than that of composites as shown in Fig. 1 (a) and (c). This is consistent with that wear rate of composites is less than that of AZ91 matrix alloy for all sliding speeds and normal loads (not shown). Fig. 1 (b) and (d) show the SEM micrograph of cross section for AZ91 matrix alloy and TiB₂/AZ91 composites at the sliding speed of 0.75 m/s under normal load of 37.5 N, respectively. From Fig. 1 (b), large quantities of cracks are found in

the subsurface of AZ91 matrix alloy, indicating relative lower strength of AZ91 alloy can not support the MML(mechanical mixed layer) and thus these cracks nucleate, propagate and lead to MML or subsurface delamination during dry sliding wear.

It is obvious that the micro-hardness of cross section of composites worn surface is much higher than that of AZ91 matrix alloy shown in Fig. 1 (f).

Figure 1 -  SEM micrograph of worn surface for (a) AZ91 matrix alloy and (c) TiB₂/AZ91 composites; SEM micrograph of cross section for (b) AZ91 matrix alloy and (d) TiB₂/AZ91 composites at the sliding speed of 0.75 m/s under normal load of 37.5 N；(f) Micro-hardness of subsurface layers for AZ91 matrix alloy and TiB₂/AZ91 composites at the sliding speed of 0.75 m/s under normal load of 37.5 N.

DISCUSSION: At high cyclic normal loads and sliding speeds, the working hardening in the subsurface is pronounced according to the reports [4, 6]. It is noteworthy that the micro-hardness of MML (distance lower than 50 μm) for composites is increased sharply compared with AZ91 matrix alloy. In other words, the capacity of work hardening for Mg matrix composites containing nanosized TiB₂ particles is improved remarkably compared to unreinforced AZ91 matrix alloy. This is ascribed to the presence of fine and dispersed TiB₂ particles in composites, which leads to a high obstacle effect for dislocations moving during the deformation. Similar results have been done in report [8] that nano-Al₂O₃ particles reinforced AZ31 composites possess excellent wear resistance compared to pure Mg and AZ31 alloy at high speeds and normal loads due to the higher work hardening rate. Therefore, the higher hardness and strength of subsurface in composites due to high work hardening rate is enough to support the subsurface and beneficial to the increase of wear resistance. Meanwhile, the absence of cracks in MML of composites (Fig. 1 (d)), as well as larger MML thickness of 35.9 μm than that of 25.2 μm thickness in AZ91 matrix alloy, also confirm the above results.

By comparison, the improvement of wear resistance for TiB₂/AZ91 composites can be explained as follows. First, the addition of TiB₂ particles reduces the abrasive wear at lower sliding speeds and normal loads [3] due to the higher hardness and strength of the composites as discussed in Table 1 of section 3.3 (not shown). Besides, the higher work hardening rate of composites at high sliding speeds and normal loads results in a high surface hardness with a protective effect on MML, which is contributed to the reduction of delamination and excellent wear resistance.

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