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27SYNTHESIS OF MAX PHASE T^AlN BY SPARK PLASMA SINTERING OF Ti/AlN COMPOSITE POWDERS OBTAINED BY MECHANICAL ACTIVATION
V. G. Gilev*", M. N. Kachenyuk", A. A. Smetkin", and S. A. Oglezneva"
aPerm National Research Polytechnic University, Perm, 614990 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10046
The compound Ti2AlN belongs to the class of refractory materials having a layered structure and is described in general form by the formula Mn + 1AXn, where M is a transition metal, A is an element of group IIIA or IVA of the periodic system, and X is carbon or nitrogen. Various methods are used for the synthesis [1-3].
The starting materials for the synthesis of Ti2AlN were titanium powder TPP-7 with a fraction less than 325 |m and two different AlN powders: AlN -1 (< 58 |m, d(4/3) = 11.46 |m, d50 = 8 |m) and AlN- 2 (< 69 |m, d(4/3) = 20.7 |m, d50 = 17.2 |m). Mechanical activation (MA) of the powder mixtures was carried out using a SAND planetary mill at a mill drum rotation frequency of 280 min-1 in a vacuum (P < 10 Pa) for 0.667, 2, and 3 h. The mass ratio of the grinding bodies and the material being processed corresponded to 7.5:1. To exclude contamination of the material during MA a tooling made of titanium was used.
Spark plasma sintering of powder compositions was carried out using a Sinter SPS-1050b equipment in a graphite mold with a graphite foil shell under a pressure of 30 MPa. The temperature was varied in the range of 900-1400°C; the isothermal aging was carried out in vacuum for 5 min; and the average heating rate was 80°C/min. The results of determining the chemical composition of aluminum nitride powders are given in Table 1.
Table 1. EDS data of AlN powders.
Powders Chemical composition, wt %
Al Ti Fe Cr Mn Cu Ni Z n Mo Zr Rb
AlN -1 99.31 - 0.54 0.06 0.05 0.03 0.02 - - - -
AlN -2 98.02 1.22 0.44 0.06 0.08 0.02 - 0.04 0.03 0.02 0.01
In the AlN-1 powder, the main phase was the hexagonal phase P63mc (space group 186), as well as 7 wt % Al impurity. In addition, according to the XRD analysis, the initial AlN-1 powder has about 7% bayerite Al(OH)3 as an impurity (space group 14). In the AlN-2 powder, there were no these impurities (Al and its hydroxide. The presence of oxygen in powders and mixtures after MA was assessed using a scanning electron microscope (Table 2). It can be seen that the oxygen content is lower in AlN-2 powder and related mixture than in AlN-1 and Ti/AlN-1.
Table 2. EDS data of AlN powders and mixtures after MA for 3h._
Sample Number of measurements Ti Al N O
AlN-1 6 - 61.0 22.1 17.1
AlN-2 7 - 63.7 35.9 0.67
Ti/AlN-1 5 47.8 27.5 7.7 16.1
Ti/AlN-2 4 54.1 31.8 13.7 0.91
iSHS 2019 Moscow, Russia
The degree of homogenization of the Ti/AlN mixture during mechanical activation can be estimated from the results of X-ray phase analysis [3]. When using Cu Ka radiation with a wavelength of 0.154 nm and an energy of 8051 eV, the depth of penetration into Ti and AlN differs by about 7 times. As the MA time increases from 40 to 180 min, the intensity of the Ti lines increases and of the AlN lines decreases. The AlN-1/Ti ratio determined by the method of full-profile analysis decreases with increasing tma and as the composition of the formed composite particles is homogenized. After 3-h MA, this value is 0.56, which is close to the value of 0.428 calculated for the initial composition of the mixture (Table 3).
Table 3. Phase content (wt %) and AlN/Ti ratio in Ti/AlN mixtures after MA._
Duration of MA, h Mixture with AlN-1 Mixture with AlN-2
AlN-1 Ti AlN/Ti AlN-2 Ti AlN/Ti
0.667 64.4 35.6 1.78 — -
2.0 47.9 52.1 0.77 - -
3.0 36.7 65.3 0.56 41.3 58.7 0.70
Calculation of the composition 30 70 0.428 30 70 0.428
The quality of MA mixture with AlN-2 is worse possibly due to the larger particle size. Similar results were obtained for MA Ti/SiC mixtures [4].
Changes in shrinkage rate and vacuum pressure for samples with two types of AlN powder during SPS are shown in Fig. 1.
Fig. 1. Changes in shrinkage, shrinkage rate and vacuum pressure during SPS at (a, b) 1300°C, (c, d) 1350°C, and (e, f) 1400°C for samples with (a, c, e) AlN-1 and (b, d, f) AlN-2.
The presence of aluminum impurities in the composition with AlN-1 leads to an earlier onset of shrinkage. This composition is characterized by the presence of pressure peak in the region of 1100°C, as well as peaks of pressure and shrinkage rates in the region of 1350 and about 1400°C, associated with the decomposition of Ti2AlN. The sample with AlN-2 during SPS at 1350 and 1400°C has only a weak peak on the shrinkage rate curve near 1350 and 1400°C, respectively.
Table 4 presents data on the mass loss of the samples during the SPS (^m), density and open porosity (Po), determined by hydrostatic weighing after boiling in water, microhardness and the MAX phase portion in the composition.
Table 4. The results of the synthesis of Ti2AlN samples at different temperatures of SPS. _Samples with AlN-1 powder_Samples with AlN-2 powder
Tsps,0C Am, % P, g/cm3 Po, % HV50, GPa £ < <N H Am, % P, g/cm 3 Po, % HV50, GPa £ < <N H
900 0 3.29 16.2 18 0 3.45 20
1000 0 3.69 10.7 40 0 3.66 12.4 52
1100 -1.1 4.10 2.4 64 0 4.13 1.07 69
1200 -4.1 4.24 1.9 6.8+1.1 89 -1 4.27 0.05 6.8+0.8 91
1250 4.33 0.09
1300 -5.6 4.26 1.9 6.7±0.7 90 4.336 0.01 4.7±0.3 97±1
1350 -25.4 4.30 2.5 88 -3 4.358 0.15 92
1400 -34.7 4.36 2.8 75 -5.3 4.33 0.48 89
As can be seen in Table 4, the use of more pure AlN powder made it possible to obtain a more pure product, up to 97-98 wt % at 1300°C and a non-porous state. When the temperature rises to 1350-1400oC, a decrease in the proportion of Ti2AlN is observed, which is a consequence of the decomposition of Ti2AlN with the formation of TiN, weight loss and the appearance of porosity. The latter is less pronounced when using AlN-2 powder.
This work was supported by the Ministry of Education and Science of the Russian Federation in the framework of the implementation of the basic part of the state task (no. 11.8353.2017/8.9).
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