Effect of milling time on the microstructure and mechanical properties of Mo₂FeB₂ based cermets

Yupeng Shen, Zhifu Huang, Lei Zhang

DE State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi Province, PR 

INTRODUCTION: Ternary boride based cermets formed by reaction boronizing sintering strategy, such as Mo₂FeB_2, Mo₂NiB₂ and WCoB based cermets, are widely studied at home and abroad due to their excellent wear properties and corrosion resistance.¹ Particularly, compared with Mo₂NiB₂ and WCoB based cermets, Mo₂FeB₂ based cermets attract a lot of attentions due to their inexpensive raw materials, small wear quantities, remarkable metallurgy bonding ability, and prominent mechanical properties.²

Mo₂FeB₂ based cermets consist of the hard Mo₂FeB₂ phase acting as the reinforced phase and Fe binder phase conducting the toughness.³ Previous studies mostly focused on refining the microstructure and improving mechanical properties of Mo₂FeB₂ based cermets by adding alloy elements, such as manganese, vanadium and carbon.^4-7 However, the fabrication of Mo₂FeB₂ based cermets was rarely concerned. Particularly, the preparation technology of the mixed powders was not investigated systematically. The mixed powders can be prepared by planetary ball milling, which is commonly used for its powerful and relatively simple operation. Planetary ball milling is a complex process and the optimization of a lot of variables needs to be obtained to achieve the desired ball-milling effect.⁸ Therein the milling time is the most important parameter, which has an effect on the final constitution of the powders. In general, the milling time is so chosen as to get a steady state between the fracture and cold welding of the powder particles. It should also be realized that, if the powder is milled for times longer than required, the cycle of preparation of Mo₂FeB₂ based cermets and the level of contamination will increase. Therefore, it is necessary that the powder is milled just for the required duration and not any longer.  
  

In the present study, the effect of milling time on the morphology and distribution of the milled powders of Mo, FeB and Fe was studied. The microstructures and mechanical properties of Mo₂FeB₂ based cermets prepared at different milling times were also investigated. The results can provide important guideline for the following preparation of Mo₂FeB₂ based cermets.

METHODS: The Mo (Mo≥99.9 wt.%), FeB (B=20.05 wt.%) and carbonyl Fe (Fe≥99.9 wt.%) powders were milled in a planetary ball mill for different times from 4 h to 32 h in 4 h increments. Si₃N₄ balls of 5 mm were used for milling and Ethanol was used as the process control agent. The weight ratio of ball-to-powder was 6:1 and the rotation speed of milling was 250 rpm (rotations per minute). The whole milling process was carried out in an argon atmosphere.

After milling, the slurries of milled powders were dried at 50 °C in a rotary evaporator (RE-52A), and then the dried powders were pressed into green compacts at 200 MPa with the dwell time of 120 s. After compaction, the green compacts were sintered in high temperature vacuum tube furnace. The pressure of vacuum was controlled at 10-2-10-1 Pa during sintering. The furnace chamber was heated to the temperatures of 1050 °C with the heating rate of 10 °C / min and duration time of 30 minutes, and then was heated to the temperatures of 1250 °C at the heating rate of 2 °C / min and duration time of 30 minutes. After sintering reaction, the samples were cooled inside the furnace to room temperature.

RESULTS: Fig. 1(a, b and c) shows the microstructure of Mo₂FeB₂ based cermets prepared at different milling time. Obviously, as shown in the figure, the gray phase is Mo₂FeB₂ hard phase and dark one is Fe binder phase. In addition, as milling time is 8 h (Fig. 1a), there are more aggregation phenomena of Mo₂FeB₂

duo to the nonuniform distribution of powders. With milling time increasing, the Mo₂FeB₂ particles distribute more uniformly. As milling time exceeds 24 h (Fig. 1b and c), the aggregation phenomena of Mo₂FeB₂ particles almost disappears. Moreover, as milling time is less than or more than 24 h, the grain coarsening phenomena of Mo₂FeB₂ particles can be observed. Furthermore, Fig. 1d shows the morphology evolution of Mo₂FeB₂ based cermets as a function of milling time. From the figure, it is clear that the volume fraction of Mo₂FeB₂ increases first and then decreases. The cermets prepared at milling time of 24 h exhibit the maximum volume fraction of the Mo₂FeB₂. On the contrary, from this figure, it also can be seen that, the particle sizes of Mo₂FeB₂ hard phase decreases first and then increases with increasing the milling time. The cermets prepared at milling time of 24 h have the smallest grain size of 2.25μm.

Figure 1 - A) Microstructure of cermets prepared at 8 h.  B) Microstructure of cermets prepared at 24 h.  C) Microstructure of cermets prepared at 32 h.  D) Morphology evolution of cermets as a function of milling time.

DISCUSSION: At the milling times of 4 h, 8 h and 12 h, the distribution and size of milled powders is nonuniform and large, respectively. The contact areas among powder particles are relatively smaller. As a result, the reactions of synthesizing Mo₂FeB₂ phase are not complete and there is more Fe2B in the cermets. With milling time increasing from 12 h to 24 h, the reactions of synthesizing Mo₂FeB₂ phase become more and more adequate due to more contact among powder particles. As milling time exceeds 24 h, the milled powders have a higher surface energy due to long-time milling. Thus, powder particles tend to merge and grow up, especially in high temperature. The reactions of synthesizing Mo₂FeB₂ phase are not complete compared with those at milling time of 24 h. According to literature,⁷ in the liquid phase sintering, the growth of Mo₂FeB₂ hard phase is controlled by solution-precipitation mechanism. At milling time of 24 h, the reactions of synthesizing Mo₂FeB₂ phase are carried out adequately. There are a large number of crystal nucleuses of Mo₂FeB₂ in the liquid phase. The growth of crystal nucleuses is inhibited due to the limitation of solution-precipitation. Thus, the smallest grain size of Mo₂FeB₂ hard phase is obtained in the cermets prepared at milling time of 24 h.

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