Ï¡ÓнðÊô(Ó¢ÎÄ°æ) 2015,34(02),101-106

Preparation and scratch test of AlMgB₁₄ modified by TiB₂

College of Materials Science and Engineering, Taiyuan University of Technology

College of Mechanical and Electrical Engineering, Qingdao Binhai University

Institute of Chemical and Biological Technology, Taiyuan University of Science and Technology

Õª Òª£º

In this paper, the Al Mg B14 and Al Mg B14¨CTi B2 composites were synthesized by means of mechanical alloying and the field-activated and pressure-assisted synthesis process. The effect of temperature and pressure on the purity and property of products was discussed. The results show that the process of preparing Al Mg B14 bulk materials is optimized as follows: synthesis temperature1,400¨C1,500 ¡ãC, heating rate 100 ¡ãC min-1, axial pressure60 MPa, heat preservation 8¨C10 min, optimum starting powders' ratio Al: Mg: B = 0.1915:0.1363:0.6722, and adding excessive 3 wt% Al. The abrasion resistance of Al Mg B14 composites with varying amounts of Ti B2 was studied using single-point diamond scratch tests with loads ranging from 10 to 100 N in 10 N increments. The scratch width increases almost linearly with the applied load and decreases with Ti B2 proportion increasing up to 70 wt%.With its advantages of fast heating, short reaction time,energy conservation, and high purity, this method offers a new way to synthesize Al Mg B14 and Al Mg B14¨CTi B2 composites.

ÊÕ¸åÈÕÆÚ£º23 October 2012

»ù½ð£ºfinancially supported by the National Natural Science Foundation of China (No. 50975190);

Preparation and scratch test of AlMgB₁₄ modified by TiB₂

Lei Zhuang Yang Miao Wen Liu Qing-Sen Meng

College of Materials Science and Engineering, Taiyuan University of Technology

College of Mechanical and Electrical Engineering, Qingdao Binhai University

Institute of Chemical and Biological Technology, Taiyuan University of Science and Technology

Abstract£º

In this paper, the AlMgB14 and Al Mg B14¨CTiB2 composites were synthesized by means of mechanical alloying and the field-activated and pressure-assisted synthesis process. The effect of temperature and pressure on the purity and property of products was discussed. The results show that the process of preparing AlMgB14 bulk materials is optimized as follows: synthesis temperature1,400¨C1,500 ¡ãC, heating rate 100 ¡ãC min-1, axial pressure60 MPa, heat preservation 8¨C10 min, optimum starting powders' ratio Al: Mg: B = 0.1915:0.1363:0.6722, and adding excessive 3 wt% Al. The abrasion resistance of AlMgB14 composites with varying amounts of TiB2 was studied using single-point diamond scratch tests with loads ranging from 10 to 100 N in 10 N increments. The scratch width increases almost linearly with the applied load and decreases with TiB2 proportion increasing up to 70 wt%.With its advantages of fast heating, short reaction time,energy conservation, and high purity, this method offers a new way to synthesize AlMgB14 and AlMgB14¨CTiB2 composites.

Keyword£º

AlMgB14; Field-activated and pressureassisted synthesis; Temperature; Pressure; Property;

Author£º Qing-Sen Meng e-mail: mengqingsen@263.net;

Received£º 23 October 2012

1 Introduction

The ultrahard material of Al Mg B₁₄has attracted wide attention in recent years because of its high hardness,excellent abrasive resistance, chemical inertness, low friction coefficient, and cheap raw materials. It is a new member of hard materials after diamond, c BN, and B₄C.Al Mg B₁₄-based composites are mainly used in special machines in industries of military projects, metal cutting,mining, forestry, petroleum, agriculture, etc, for its ability to withstand a wide temperature range [1¨C3].

The boride Al Mg B₁₄was prepared by several methods.Okada et al. [4] and Higashi et al. [5] prepared single crystals of Al Mg B₁₄from metal salts. The boride was also prepared by high-energy milling of the elements followed by hot pressing or sintering [6]. David et al. [7] used metal hexaborides as precursors for the preparation of Al Mg B₁₄employing the pulsed electric current sintering method(PECS).

Field-activated and pressure-assisted synthesis (FAPAS)is a new technique for synthesizing materials under multiphysics coupling fields (electric, mechanical, thermal, and chemical). Meng et al. [8] and Munir [9] investigated that FAPAS provides activation in the form of current directly applied to the starting powders and to the region in a die containing the powder. The advantages of the simultaneous application of an electric field (current) and a uniaxial pressure to consolidate or synthesize and consolidate materials were discussed in a recent review [9]. Additional activation can be obtained through milling of the powders prior to the synthesis reaction. Mechanical activation (MA)can change the reactivity of the milled powders by changing the thermodynamic driving force of the system,by increasing the solubility of an element in another, and by increasing the reaction interfaces [9, 10].

In this paper, the results from an investigation on the simultaneous formation and consolidation of Al Mg B₁₄and Al Mg B₁₄¨CTi B₂composites by FAPAS were presented. The abrasion behavior of the newly processed material was studied using the single-point diamond scratch test [11,12].

2 Experimental

2.1 Experimental procedure

The starting materials were elemental aluminum, magnesium, and boron powders. The purity of aluminum powder was 99.95 %, and the particle size was in the range of 1¨C5 lm. The magnesium powder was 99.99 %pure, and the particle size was 74 lm. The purity of boron powder was 99.5 %, and the particle size was in the range of 5¨C15 lm. The mass of Al, Mg, B powders was calculated in line with the mole ratio of Al Mg B₁₄.In order to reduce the formation of spinel phase Mg Al₂O₄,thecalculationofmoleratiowas(Al Mg B₁₄)_0.8634(Mg Al₂O₄)_0.1367and extra added Al powder was 3 wt%. So the mass ratios of Al, Mg, and B powders were 0.1915, 0.1363, and 0.6722. Because oxygen contamination of the preliminary powders was the main concern in producing a single-phase product(i.e., avoid the spinel phase Mg Al₂O₄), the storing and handling of powders were done in a glove box under argon atmosphere [13]. The mechanically alloyed AlMg B₁₄powder was then mixed with Ti B₂and milled for an additional 30 min. Ti B₂typically possessed a purity and particle size of 99.9 % and 2¨C5 lm.

The FAPAS apparatus is depicted in Fig. 1. In this experiment, an Al₂O₃tube (2 mm in thickness, 20 mm in diameter) was used to isolate the die from the powders so that the electric current flowed through the powders entirely,which would bring about high-density current sintering, as shown in Fig. 1 [8].

The powder mixture (total mass, 12 g) which contains aluminum, magnesium, and boron powders was subjected in a planetary jar and milled for 10 h. The jars were sialon lined and the balls were zirconia (10 mm diameter). A charge ratio (the ball to powder mass ratio) of 14 was used. The powder mixture was cold pressed into a cylinder with a diameter of 20 mm and then sintered in a FAPAS apparatus.

The single-point scratch test was determined using a multifunction tester of materials (MFT-4000). The test equipment consisted of a Rockwell ¡®¡®C¡¯¡¯ 120¡ã spherocone diamond indenter which was secured to the end of a vertical spindle. The indenter traverse speed was fixed at 5 mm min^-1, while the normal load was varied from10 to 100 N in 10 N increments. The tests were done in laboratory air (23 ¡ãC and 40 %¨C50 % relative humidity).In order to examine the subsurface damage, selected specimens were coated with a thin layer of gold and then placed in a scanning electron microscope (SEM) for observation.

Fig. 1 Schematic diagram of FAPAS sintering apparatuses

2.2 Principle of FAPAS

During the process of FAPAS, low melting point particles melted firstly under the effect of outer heat sources, Joule heating, and the combustion reactive head distributed to high melting point particles through the capillary effect.Then, the effect of electric field and pressure promoted the interfacial bonding of the particles and combustion reactions, and led to the rapid diffusion and combination of the elements. Meanwhile, the impressed pressure made the particles easy to slide, promoted particles rearrangement,and improved particle stacking structure. The high-temperature capillarity formed by the reaction heat and the Joule heat caused the dissolution and reprecipitation of the solid particles. With the combustion going, the reaction heat led to the existence of large quantities of liquid phases and improved the sintering densification rate via the mass transfer process of dissolution¨Cprecipitation and flow of liquid phases. Bulk diffusion and grain boundary diffusion were strengthened by the effect of the electrical current¡¯s heating and vertical pressure, and thus a high-quality solid sintering was synthesized under low temperature and short time [7, 14¨C16].

The combustion reaction of aluminum, magnesium, and boron powders is as follows:

Merzhanov and Borovinskaya [17] put forward the thermodynamic criterion judging self-sustainability of SHS combustion wave according to reaction adiabatic temperature T_ad; aluminum and magnesium powders could carry on self-sustained combustion reaction when T_ad£¾1,800 K, so as to reach the effect of concentrative heat liberation and fast sintering.

The formation of the Al Mg B₁₄was accomplished under a vacuum of 0.01¨C0.02 Pa. The sample was initially heated to 600 ¡ãC by Joule heating with a current of about 1600 A under a pressure of 15 MPa. The temperature was then increased to 1,500 ¡ãC at a rate of 100 ¡ãC min^-1, the pressure was increased to 60 MPa, and the parameters were kept at this level for 10¨C15 min until the end of the experiment.

3 Results and discussion

3.1 Effect of synthesis parameters on density

3.1.1 Effect of temperature on density

Synthesis temperature was one of the key parameters. In order to investigate the effect on density, the mixture was milled for a further 12 h and then synthesized using the FAPAS method under a pressure of 60 MPa at 1200, 1400,1500, and 1,600 ¡ãC.

X-ray analyses were then performed on the samples to determine phase composition. Figure 2 shows the results for 1,200, 1,400, and 1,600 ¡ãC. As can be seen from Fig. 2, when the milled powders were heated in the FAPAS to 1,200 ¡ãC, the formation of the higher boride(Al Mg B₁₄) can be clearly seen, but the spinel phase (Mg Al₂O₄) is still the major phase present. Moreover, the reaction is not complete, as seen by small peaks for the reactant elements. The reaction becomes nearly complete when the temperature of synthesis is 1,400 ¡ãC; the higher boride phase Al Mg B₁₄is the major phase. Al Mg B₁₄decomposes when the temperature is higher than1,600 ¡ãC. This temperature is consistent with published results [18, 19].

Fig. 2 XRD patterns for samples at 1,200, 1,400, and 1,600 ¡ãC

Table 1 Density, porosity, and Al Mg B₁₄:Mg Al₂O₄major peak ratio for samples prepared at 20, 40, and 60 MPa at 1,500 ¡ãC  ÏÂÔØԭͼ

Table 1 Density, porosity, and Al Mg B₁₄:Mg Al₂O₄major peak ratio for samples prepared at 20, 40, and 60 MPa at 1,500 ¡ãC

The samples are mainly composed of boron and a few Al Mg B₁₄when the temperature is lower than 1,200 ¡ãC, as shown in Table 1. The spinel phases of Mg Al₂O₄and AlMg B₁₄form at 1,200 ¡ãC and higher temperature.

3.1.2 Effect of axial pressure

The samples were prepared at 1,500 ¡ãC under a pressure of 20, 40, and 60 MPa. The major peak ratio(Al Mg B₁₄:Mg Al₂O₄) of samples is shown in Table 1.

The preparing process of Al Mg B₁₄bulk materials is optimized as follows: synthesis temperature 1,500 ¡ãC,heating rate 100 ¡ãC min^-1, axial pressure 60 MPa, and heat preservation 8¨C10 min.

3.2 Microstructure analysis

TEM and EDS were used to examine the microstructure of Al Mg B₁₄hot-pressed disks. Most of the phases are observed to be smaller than 1 lm, as seen in Fig. 3. EDS analysis shows that darker regions marked by Arrow A in Fig. 3contain primarily Al, Mg, and B as seen in Fig. 4a, and are thus thought to be predominately single-phase Al Mg B₁₄.The brighter regions marked by Arrow B contain Al, Mg,and O (Fig. 4b), and are thus thought to contain Mg Al₂O₄(spinel). Mg Al₂O₄is formed as a result of the reaction of oxygen either dissolved in the constituents, present as an adsorbed gas, or in the voids between particles.

3.3 Scratch resistance

Scratch tests were conducted in laboratory air with loads ranging from 10 to 100 N in 10 N increments. Figure 5 shows the variation of scratch width with load for different Ti B₂proportions in Al Mg B₁₄. There are two observations that deserve to be mentioned. First, for any material, the scratch width increases almost linearly with load. Second,for any load, the scratch width decreases with Ti B₂proportion increasing up to 70 wt%. The addition of Ti B₂,which is harder than Al Mg B₁₄, would retard the possibility of crack propagating through Ti B₂. At low loads of 20 and30 N, microchips could be seen on the scratched surface,but the critical depth for chip formation is barely reached,as elastic recovery is able to overcome plastic deformation in the grooved track.

Fig. 3 TEM image of sample at 1,500 ¡ãC (Arrow A shows a Al Mg B14crystal and Arrow B showing a Mg Al2O4)

Fig. 4 EDX analysis of regions in Fig. 3: a EDS spectra from Arrow A zones and b EDS spectra from Arrow B zones

Fig. 5 Variation of scratch width with load for Al Mg B14with different Ti B2proportions by weight

3.4 Surface examination

Figure 6 shows the buildup of damage at different magnifications in Al Mg B₁₄with 0, 20, 50, and 70 wt%Ti B₂scratched in air. At a load of 100 N, deformation occurs at surface asperity peaks, and elsewhere the contact is merely superficial. There is local yielding occurring because of a hydrostatic stress state at the indenter tip. A well-defined groove is formed in all of the samples. The groove boundaries are sharp and plastic deformation is observed on the entire surface. In addition to the plowing evidenced by the groove at the top, microfracture of the material on surface peaks is seen in many locations. It appears that microcracking starts at asperity locations because of high stress and high temperature, which is under repeated loading spreads along the surface [20].

Basically, plastic deformation along with fragmented material is seen on the grooved surface of Al Mg B₁₄sample with 0 wt% Ti B₂. On the contrary, profuse cracking occurs on the scratched surface of Al Mg B₁₄-70 wt% Ti B₂. This is due to the presence of brittle phase. Thus, whereas the presence of Ti B₂in Al Mg B₁₄makes it more resistant to scratching, it also makes the material prone to cracking at high loads.

3.5 Fracture toughness

The fracture toughness of the polished samples is calculated by Eq. (4):

where HV is Vickers microhardness, P is load, and l is crack length. The fracture toughness was determined using a load of 1,000 g. The fracture toughness of the samples which were prepared at 1,500 ¡ãC under a pressure of 60 MPa is4.2 MPa m^-1/2. The crack length is 5.95 lm. The average density of the sample is measured using the Archimedes displacement method, yielding a value of 2.62 g cm^-3. The average hardness of the samples is 33.8 GPa.

Fig.6 SEM images of deformation and other surface features in Al Mg B₁₄-(0¨C70)wt%Ti B₂scratched under same load(100 N)a 0 wt%Ti B₂,b 20 wt%Ti B₂,c 50 wt%Ti B₂,and d 70 wt%Ti B₂

4 Conclusion

The process of preparing Al Mg B₁₄bulk materials is optimized as follows: synthesis temperature 1,400¨C1,500 ¡ãC,heating rate 100 ¡ãC min^-1, axial pressure 60 MPa, heat preservation 8¨C10 min, optimum starting powders¡¯ ratio Al:Mg: B = 0.1915:0.1363:0.6722, and adding excessive3 wt% Al. The average hardness of Al Mg B₁₄prepared under these conditions is 33.8 GPa, while the average density is2.62 g cm^-3and the fracture toughness 4.2 MPa m^-1/2. With its advantages of fast heating, short reaction time, energy conservation, and high purity, FAPAS offers a new way to synthesize Al Mg B₁₄. For Al Mg B₁₄¨CTi B₂composites, the scratch width decreases with Ti B₂proportion increasing up to70 wt%. Scratch width increases almost linearly with load for all the materials tested. At the high load of 100 N, a groove with a sharp boundary is formed, and plastic deformation is observed on the entire surface along with some cracks.