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【Abstract】Mo2FeB2 based cermets were prepared by spark plasma sintering（SPS） The microstructure evolution and phase transformation were studied by using scanning electron microscopy （SEM） and X-ray diffraction analysis （XRD） in comparison with conventional vacuum sintering （CVS）.It was found that the SPS remarkably accelerated phase transformation and decreased the end sintering temperature.A large amount of Mo2FeB2 hard phase occurred for the sintering temperature of 750℃ by SPS， and no Mo2FeB2 phase was detected for the sintering temperature below 900℃ by CVS. Almost full densification was obtained for the sintering temperature of 1050℃ by SPS， However， the microstructure was uneven， and a significant exudation of the liquid phase through the clearance between SPS punches and die was present in this stage.
【Key words】Mo2FeB2 based cermets；Phase transformation；Spark plasma sintering；Microstructure
0.Introduction
Ternary boride based cermets and their application as engineering materials have been intensively studied[1-2]. At present， Mo2FeB2 based cermets are attracting much attention because of cheap raw material， simple preparation method and superior mechanical properties[3-4].
It is widely accepted that the mechanical properties of the cermets can be improved greatly by reducing the grain sizes. However， it is very difficult to produce the cermets with grain size of below 0.2μm by using conventional vacuum sintering and even hot isostatic pressing.Recently， interest has been growing in the use of spark plasma sintering（SPS） to fabricategrained materials. There were some successful examples for SPS to obtain innovative materials[5-7]. However， the preparation of Mo2FeB2 based cermets by SPS has not been reported up to date. Therefore， the present work was aimed at preparation of Mo2FeB2 based cermet by SPS and investigated the phase transformation during SPS and microstructure characteristics by SPS.
1.Experimental procedure
Commercially available Mo（4μm）， carbonyl Fe（3.5μm）， FeB（45μm）， Cr（45μm），Ni（3μm）and C（5.5μm） powders were used as raw materials. These powders were mixed in a planetary ball-mill in ethanol together with cemented carbides balls for 24h at a speed of 150rpm（rotations per minute）.After milling， the slurries were dried at 80℃ in an infrared stove， and then sieved through 200mesh. After that， the powders were compacted by SPS，which was carried out in a vacuum chamber using an SPS apparatus（Dr.Sinter 1050， Sumimoto Coal Mining， Japan）.The powders were inner placed into a 25mm diameter graphite die and then heated to different sintering temperatures at a heating rate of approximately 350℃/min. A pressure of 50MPa was applied from the start and retained to the sintering temperature and released during the cooling portion of a complete sintering cycle.The samples were soaked for 2min at the sintering temperature.In comparison with SPS，some green compacts with dimensions 39mm×8mm×8mm was sintered in vacuum at the same sintering temperature with a heating rate of approximately 10℃/min. Density measurements were conducted using the Archimedes technique. The crystal structure of cermets was determined using the X-ray diffraction with Cu Kα radiation （D8ADVANCEXX-ray diffractometer）. The microstructure was observed by a scanning electron microscope （SEM， QUANTAN200/FEI and JSM-7500） in the second electron mode.
2.Results and discussion
2.1 XRD analyses
The XRD patterns of mixture powders at various sintering temperatures by SPS were presented in Fig.1. It can be seen that no obvious phase transformation occurred for Mo for the sintering temperature below 600℃.However， a large amount of Mo2FeB2 hard phase was obtained for the sintering temperature of 750℃ with a notable decreasing of Mo peak and Fe2B peak. The results suggested that the rapid formation of Mo2FeB2 was by the reaction 2Mo+2Fe2B=Mo2FeB2+3Fe， which was consistent with those reported by T Ide[8]. At a higher temperature of 900℃， no Mo was available， which showed that Mo completely transformed to the Mo2FeB2 phase.
Fig.1.XRD patterns of mixture powders at various sintering temperatures by SPS
Fig.2.XRD patterns of mixture powders at various sintering temperatures by CVS
In order to compare with the results by SPS， Fig.2 shows the XRD patterns of mixture powders at the corresponding temperature by CVS. It was found that no Mo2FeB2 was available for the sintering temperature of 750℃， and just a small amount of Mo2FeB2 was detected for the sintering temperature of 900℃.In addition， some amount of Mo still existed although the sintering temperature was above 1050℃. The results showed that SPS remarkably accelerated phase transformation， which may be attributed to the higher sintering driving force of SPS.
2.2 Microstructure
Fig.3 shows the plots of the average relative density at different sintering temperatures by SPS and CVS. It can be seen that the relative density increased with an increase of sintering temperatures by SPS， and almost full densification was obtained for the sintering temperature of 1050℃. However， a significant exudation of the liquid phase through the clearance between SPS punches and die was also present when the sintering temperature was 1050℃ which showed that liquid phase sintering took place in this stage.
As for the cermets by CVS， the relative density was no obvious increase with an increasing sintering temperature. In addition， a slight decrease of relative density occurred when the the sintering temperature was 750℃ and 900℃，which was attributed to the swelling of the formation of Mo2FeB2[8]. Clearly， the SPS remarkably increased the velocity of densification. Fig.3 Average relative density as a function of sintering temperature.
The fracture surfaces were observed using second electrons in the SEM at various sintering temperatures by SPS， as shown in Fig.4. It was found that the noticeable necks were developed for the sintering temperature of 600℃， which was formed by the diffusion in the solid state. In addition， the relative spheroidized pores were present. At a higher sintering temperature of 750℃， the dimension of pore was decreased， which suggested that the densification increased with an increase of sintering temperature. As for sintering temperature of 900℃， the increased interconnectivity between powder particles and an obvious fine grain facets on the fractured necks occurred. The fine facets were found by energy dispersive X-ray analysis（EDX） to be rich in Mo，Fe，and B and were identified as Mo2FeB2 grains with the aid of the XRD results.Progressively more rearrangement of powder particles was considered to take place as the temperature was increased from 900℃ to 1050℃， and almost full densification was achieved in this stage， which was characterized by the disappearance of noticeable visible pore. As mentioned above， liquid phase sintering took place in this stage， which was responsible for the observed densification.However，the microstructure was uneven， which was characterized by the agglomeration of the binder phase and a higher contiguity of the hard phase.
Fig.4.Fracture surfaces by SPS with at the sintering temperature of：
（a）600℃， （b）750℃， （c）900℃， （d）1050℃
Fig.5.Fracture surfaces by CVS with at the sintering temperature of：
（a）600℃ and （b）1050℃.
In comparison with SPS， Fig.5 shows that the fracture surfaces by CVS at the sintering temperature of 600℃ and 1050℃. It can be seen that no obvious change took place at the sintering temperature of 600℃ and a large amount of pore still existed at the sintering temperature of 1050℃. Clearly， SPS remarkably decreased the end sintering temperature.
3.Conclusion
（1）The SPS remarkably accelerated phase transformation.A large amount of Mo2FeB2 hard phase occurred for the sintering temperature of 750℃ by SPS， and Mo completely transformed to Mo2FeB phase at the sintering temperature of 900℃. No Mo2FeB2 phase was detected for the sintering temperature below 900℃ by CVS.
（2）The SPS obviously decreased the end sintering temperature in comparison with CVS. Almost full densification was obtained for the sintering temperature of 1050℃ by SPS， However， a significant exudation of the liquid phase through the clearance between SPS punches and die was present in this stage. （3）The microstructure was uneven by SPS， which was characterized by the agglomeration of the binder phase and a higher contiguity of the hard phase.
4.Acknowledgements
This work was financially supported by the research initiation funds of China Three Gorges University under project no. KJ2011B010. [科]
【References】
[1]K.Takagi，Y.Yamasaki and M.komai：J.Solid.State.Chem Vol，1997，133：243.
[2]H.Z.Yu，Y.Zheng，W.J.Liu，X.M.Pang and W.H.Xiong：Int.J.Refract.Met.Hard.Mater Vol，2010，28：.338.
[3]H.Z.Yu，Y.Zheng，W.J.Liu，J.Z.Zheng and W.H.Xiong：Int.J.Refract.Met.Hard.Mater Vol，2011，29：724.
[4]H.Z.Yu，Y.Zheng，W.J.Liu，J.Z.Zheng and W.H.Xiong：Int.J.Refract.Met.Hard.Mater Vol，2010，28：286.
[5]Y Zheng，SX Wang，YL Yan，NW Zhao and X Chen.Int.J.Refract.Met.Hard.Mater Vol，2008，26：306.
[6]P Feng，WH Xiong，LX Yu，Y Zheng and YH Xia.Int.J.Refract.Met.Hard.Mater Vol，2004，22：133.
[7]M Alvarez，JM Sanchez.Int.J.Refract.Met.Hard.Mater Vol，2007，25：107.
[8]T Ide，T Ando.Metall Mater Trans A Vol，1989，20：16.
【Key words】Mo2FeB2 based cermets；Phase transformation；Spark plasma sintering；Microstructure
0.Introduction
Ternary boride based cermets and their application as engineering materials have been intensively studied[1-2]. At present， Mo2FeB2 based cermets are attracting much attention because of cheap raw material， simple preparation method and superior mechanical properties[3-4].
It is widely accepted that the mechanical properties of the cermets can be improved greatly by reducing the grain sizes. However， it is very difficult to produce the cermets with grain size of below 0.2μm by using conventional vacuum sintering and even hot isostatic pressing.Recently， interest has been growing in the use of spark plasma sintering（SPS） to fabricate
1.Experimental procedure
Commercially available Mo（4μm）， carbonyl Fe（3.5μm）， FeB（45μm）， Cr（45μm），Ni（3μm）and C（5.5μm） powders were used as raw materials. These powders were mixed in a planetary ball-mill in ethanol together with cemented carbides balls for 24h at a speed of 150rpm（rotations per minute）.After milling， the slurries were dried at 80℃ in an infrared stove， and then sieved through 200mesh. After that， the powders were compacted by SPS，which was carried out in a vacuum chamber using an SPS apparatus（Dr.Sinter 1050， Sumimoto Coal Mining， Japan）.The powders were inner placed into a 25mm diameter graphite die and then heated to different sintering temperatures at a heating rate of approximately 350℃/min. A pressure of 50MPa was applied from the start and retained to the sintering temperature and released during the cooling portion of a complete sintering cycle.The samples were soaked for 2min at the sintering temperature.In comparison with SPS，some green compacts with dimensions 39mm×8mm×8mm was sintered in vacuum at the same sintering temperature with a heating rate of approximately 10℃/min. Density measurements were conducted using the Archimedes technique. The crystal structure of cermets was determined using the X-ray diffraction with Cu Kα radiation （D8ADVANCEXX-ray diffractometer）. The microstructure was observed by a scanning electron microscope （SEM， QUANTAN200/FEI and JSM-7500） in the second electron mode.
2.Results and discussion
2.1 XRD analyses
The XRD patterns of mixture powders at various sintering temperatures by SPS were presented in Fig.1. It can be seen that no obvious phase transformation occurred for Mo for the sintering temperature below 600℃.However， a large amount of Mo2FeB2 hard phase was obtained for the sintering temperature of 750℃ with a notable decreasing of Mo peak and Fe2B peak. The results suggested that the rapid formation of Mo2FeB2 was by the reaction 2Mo+2Fe2B=Mo2FeB2+3Fe， which was consistent with those reported by T Ide[8]. At a higher temperature of 900℃， no Mo was available， which showed that Mo completely transformed to the Mo2FeB2 phase.
Fig.1.XRD patterns of mixture powders at various sintering temperatures by SPS
Fig.2.XRD patterns of mixture powders at various sintering temperatures by CVS
In order to compare with the results by SPS， Fig.2 shows the XRD patterns of mixture powders at the corresponding temperature by CVS. It was found that no Mo2FeB2 was available for the sintering temperature of 750℃， and just a small amount of Mo2FeB2 was detected for the sintering temperature of 900℃.In addition， some amount of Mo still existed although the sintering temperature was above 1050℃. The results showed that SPS remarkably accelerated phase transformation， which may be attributed to the higher sintering driving force of SPS.
2.2 Microstructure
Fig.3 shows the plots of the average relative density at different sintering temperatures by SPS and CVS. It can be seen that the relative density increased with an increase of sintering temperatures by SPS， and almost full densification was obtained for the sintering temperature of 1050℃. However， a significant exudation of the liquid phase through the clearance between SPS punches and die was also present when the sintering temperature was 1050℃ which showed that liquid phase sintering took place in this stage.
As for the cermets by CVS， the relative density was no obvious increase with an increasing sintering temperature. In addition， a slight decrease of relative density occurred when the the sintering temperature was 750℃ and 900℃，which was attributed to the swelling of the formation of Mo2FeB2[8]. Clearly， the SPS remarkably increased the velocity of densification. Fig.3 Average relative density as a function of sintering temperature.
The fracture surfaces were observed using second electrons in the SEM at various sintering temperatures by SPS， as shown in Fig.4. It was found that the noticeable necks were developed for the sintering temperature of 600℃， which was formed by the diffusion in the solid state. In addition， the relative spheroidized pores were present. At a higher sintering temperature of 750℃， the dimension of pore was decreased， which suggested that the densification increased with an increase of sintering temperature. As for sintering temperature of 900℃， the increased interconnectivity between powder particles and an obvious fine grain facets on the fractured necks occurred. The fine facets were found by energy dispersive X-ray analysis（EDX） to be rich in Mo，Fe，and B and were identified as Mo2FeB2 grains with the aid of the XRD results.Progressively more rearrangement of powder particles was considered to take place as the temperature was increased from 900℃ to 1050℃， and almost full densification was achieved in this stage， which was characterized by the disappearance of noticeable visible pore. As mentioned above， liquid phase sintering took place in this stage， which was responsible for the observed densification.However，the microstructure was uneven， which was characterized by the agglomeration of the binder phase and a higher contiguity of the hard phase.
Fig.4.Fracture surfaces by SPS with at the sintering temperature of：
（a）600℃， （b）750℃， （c）900℃， （d）1050℃
Fig.5.Fracture surfaces by CVS with at the sintering temperature of：
（a）600℃ and （b）1050℃.
In comparison with SPS， Fig.5 shows that the fracture surfaces by CVS at the sintering temperature of 600℃ and 1050℃. It can be seen that no obvious change took place at the sintering temperature of 600℃ and a large amount of pore still existed at the sintering temperature of 1050℃. Clearly， SPS remarkably decreased the end sintering temperature.
3.Conclusion
（1）The SPS remarkably accelerated phase transformation.A large amount of Mo2FeB2 hard phase occurred for the sintering temperature of 750℃ by SPS， and Mo completely transformed to Mo2FeB phase at the sintering temperature of 900℃. No Mo2FeB2 phase was detected for the sintering temperature below 900℃ by CVS.
（2）The SPS obviously decreased the end sintering temperature in comparison with CVS. Almost full densification was obtained for the sintering temperature of 1050℃ by SPS， However， a significant exudation of the liquid phase through the clearance between SPS punches and die was present in this stage. （3）The microstructure was uneven by SPS， which was characterized by the agglomeration of the binder phase and a higher contiguity of the hard phase.
4.Acknowledgements
This work was financially supported by the research initiation funds of China Three Gorges University under project no. KJ2011B010. [科]
【References】
[1]K.Takagi，Y.Yamasaki and M.komai：J.Solid.State.Chem Vol，1997，133：243.
[2]H.Z.Yu，Y.Zheng，W.J.Liu，X.M.Pang and W.H.Xiong：Int.J.Refract.Met.Hard.Mater Vol，2010，28：.338.
[3]H.Z.Yu，Y.Zheng，W.J.Liu，J.Z.Zheng and W.H.Xiong：Int.J.Refract.Met.Hard.Mater Vol，2011，29：724.
[4]H.Z.Yu，Y.Zheng，W.J.Liu，J.Z.Zheng and W.H.Xiong：Int.J.Refract.Met.Hard.Mater Vol，2010，28：286.
[5]Y Zheng，SX Wang，YL Yan，NW Zhao and X Chen.Int.J.Refract.Met.Hard.Mater Vol，2008，26：306.
[6]P Feng，WH Xiong，LX Yu，Y Zheng and YH Xia.Int.J.Refract.Met.Hard.Mater Vol，2004，22：133.
[7]M Alvarez，JM Sanchez.Int.J.Refract.Met.Hard.Mater Vol，2007，25：107.
[8]T Ide，T Ando.Metall Mater Trans A Vol，1989，20：16.