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Nanoabrasive wear. Prospects for obtaining new materials
F.Kh. Urakaev, A.P. Chupakhin, T.A. Ketegenov1, and V.S. Shevchenko2
Novosibirsk State University, Novosibirsk, 630090, Russia
1 Institute of Combustion Problems, Almaty, 480012, Kazakhstan 2 Institute of Mineralogy and Petrography SB RAS, Novosibirsk, 630090, Russia
A new method by realization of mechanochemical reactions with the participation of the steel material of milling tools is proposed. The method is based on using abrasive, desirably amorphous additives in order to simplify the interpretation of the results of X-ray powder analysis, e.g. glass, fused quartz, boron, etc. It has been tested for the preparation of metal-oxide-sulfide-carbide nanocomposite powders during mechanical activation of a mixture of quartz as abrasive and: (i) CuO (tenorite) and PbS (galena) with formation of Cu (copper) or Pb (lead) and amorphous iron silicates on a surface of quartz particles; (ii) S (sulfur) into FeS2 (pyrite); (iii) C (carbon) into Fe3C (cementi-te).
1. Introduction
The role of milling tools in mechanochemical processes is usually limited only by verification of the possibility of contamination of the final products of mechanical activation (MA) by the material of milling tools [1-4]. Whether if it is difficult to exclude the phenomenon of abrasive wear of milling tools it is impossible then can we apply of material of milling tools as a full component of the system at studying of mechanical activation processes? This article is devoted to investigation of this problem.
2. Experiment
The steel two-vial ball planetary mill AGO-2 was used as a mechanochemical reactor. The volume of each vial was V = 140 cm3, the number of balls was N = 400, their radius was R = 0.2 cm, the relative velocity of collisions of milling tools was Wa = 11 m/s [5-8]. For more efficient treatment of the samples, all the 4 possible orientations of the mill axis were used: vertical, horizontal, and ±15° towards the latter [8]. The products of MA were investigated by the standard X-ray powder analysis. MA activated samples were annealed isothermally for 2 h at ~700 °C (this point corresponds to the maximal temperature accepted for the crystallization of MA oxide products [9]) in Ar flow at ~ 1 cm3/s in alumina crucibles with graphite plugs and with titanium sponge placed before the crucibles in the heated region of the quartz tube in order to purify argon from possible oxygen admixture. Tenorite or galena in the amount of 0.5-2.5 g was added to the weighed portions of crushed fused quartz in the amount of 3 g. The samples were preliminarily ground and homogenized for 1 h in Fritsch Pulverisette mill equip-
ped with agate furniture. The result of X-ray of one of these samples based on native galena is shown in Fig. 1(a). Not only the presence of galena is observed but also some admixtures (mainly crystalline quartz) inherent in this mineral. The initial tenorite was obtained by thermal decomposition of malachite at ~250 °C for 7 h (the result of X-ray analysis of this black powder is shown in Fig. 2(a)). Sulfur (or carbon- graphite) in amount of 0.4 to 2.5 g (C: 0.1-1.6 g), was added to the weighed portions (3 g) of crushed obj ect-plate glass or quartz tube. The samples also were preliminarily ground and homogenized in Fritsch Pulverisette mill. The results of X-ray of initial glass- and quartz-based samples and sulfur crystals alone (weighed portion of 2 g for it MA time in the mill AGO-2 up to 180 min) show only the presence of orthorhombic sulfur (PDF 83-2285). X-ray of initial samples on the basis of C corresponds PDF 1-640 for graphite in mixes with amorphous abrasive, and in a mix with a-quartz the superposition of reflexes takes place, see also PDF 1-640 and, for example, PDF 75-443 for a-SiO2. The prescribed annealing temperature (660±5 and 960 ± 10 °C) was distinctly lower than the cementite formation (see [10] for details of the phase diagram in the Fe-C system).
3. Abrasive-reactive processing of tenorite and galenite
Preliminary experiments demonstrated both the similarity and a substantial difference in the progress of mechano-chemical processes with the participation of tenorite and galena: in both systems the abrasive wear of steel milling tools takes place, it is more clearly exhibited in the system
© F.Kh. Urakaev, A.P. Chupakhin, T.A. Ketegenov, and V.S. Shevchenko, 2004
with galena; even after short (5-15 min) MA of the system with tenorite, a strong self-lining of milling tools by the treated material occurs, the material starts to change its color from black to fully green after MA time ~ 1h, this color, as well as self-lining, is conserved and deepen during further treatment; to the contrary, self-lining is absent from the system with galena, a black and uniform oily powder is formed after treatment for a short time.
The results of the examination of MA products by means of X-ray and thermal annealing demonstrated the occurrence of the following processes and phenomena: abrasive wear of steel milling tools, amorphization, reduction of copper from tenorite and lead from galena, the absence of crystalline phases of iron oxides and sulphides which are other products of the reductive reactions, for example CuO + Fe = Cu + FeO (A r G = -28.0 kcal/mol).
The formation of X-ray amorphous product in the system with tenorite and phenomenon of the iron abrasive wear are shown in Fig. 2(b) (see a peak in the 20 angle region 44-45°). The degree of amorphization increases with increasing MA time; even the shape of iron peak becomes broader and more difficult to distinguish. Only metal copper is the product of the annealing of this sample in argon atmosphere at ~700 °C; the reflections of other possible products of the exchange reaction (iron oxides) are completely absent, see Fig. 2(c).
A similar phenomenon of iron abrasive wear and lead reduction but directly during the MA of the system with galena is shown in Fig. 1(b). Attention should also be paid to the absence of reflections of other possible products of
the reductive exchange reaction, namely, iron sulphides. An increase in the duration of MA to 2 h does not lead to substantial changes of the spectrum, except for the disappearance (amorphization) of the reflections of the admixture of crystalline quartz in galena. The annealing of these products does not change the situation: it results only in insignificant changes of the relative intensities and in substantial narrowing ofthe corresponding reflections. A further increase of the time of MA leads to the predominance of abrasive wear in comparison with all the other processes (see Fig. 1(c)).
Using the X-ray data, we calculated crystalline blocks in the structure of the metal particles obtained in MA of the system with galena. The calculation was performed using a known method [2, 11-13]. The fine lattice parameters were calculated using the half-width of the intensity profile of diffraction peaks shown in Figs. 3(b, c) for the time of MA: 1, 3.5 and 2 h (the X-ray spectrum is not shown). In order to determine the instrumental broadening, the profile of the corresponding reflections after the annealing of these samples was used. The obtained block size in lead was: 83 nm (1 h of MA), 61 nm (2 h of MA) and 46 nm (3.5 h of MA); the corresponding data for the “abrasive” iron particles were 24, 19 and 12 nm.
Thus, nanocomposite metal-oxide-sulphide powders based on quartz matrix were obtained in AGO-2 mill with steel furniture by means of MA of native minerals (tenorite and galena) in mixture with abrasive agent (fused quartz). The method can also find broad application both for obtaining chalcogenide glasses and processing of geological and
Fig. 1. X-ray data for the system fused quartz (3 g) - galena (1.5 g): initial mixture, cf. PDF 1-880 (a); time of MA is 1 h, PDF 4-686 and 6-696 (b); time of MA is 3.5 h (c)
49
45
41
IT
ff
37
33
v
M
\%
Fig. 2. X-ray data for quartz (3 g) - tenorite (1.5 g): initial CuO, PDF 801917 (a); time of MA is 1 h (b); sample (b), annealing, 700 °C, 2 h (PDF 4-836 for Cu) (c)
Fig. 3. The X-ray data for the system glass (3 g) - sulfur (0.4 g), MA for 135 min
Fig. 4. The X-ray data for the system fused quartz (3 g) - sulfur (1.6 g), MA for 135 min
man-made materials, e.g. complex sulfide concentrate [1, 6, 14-16] and technological sulfur.
4. Abrasive-reactive processing of sulfur
The situation (see above in Section 3) changes (Fig. 3) when sulfur MA in the presence of inert glass particles. The structural changes that occur with sulfur are due to the achievement of the necessary conditions at the impact-friction contact between glass particles lined with sulfur, in accordance with the results of modeling [17]. Though the hardness of glass particles is a bit higher than the hardness of steel, the abrasive wear of steel furniture is small.
Crucial changes occur during the activation of samples based on amorphous quartz. The formation of pyrite is shown in Fig. 4 (FeS2, PDF 71-2219). The hardness of quartz particles is much higher than that of steel and in this case amorphization of sulfur and it chemical reaction with iron nanoparticles formed in substantial amount as a result of abrasive wear of steel furniture by treated quartz particles occur concurrently and lead to the synthesis of pyrite.
Using the X-ray data, we applied the procedure [2, 1114] to calculate crystal blocks and distortions in the structure of the resulting pyrite (and “abrasive” iron particles). The fine lattice parameters were calculated from half-width of the profile of intensity of the diffraction peaks (220) and (440), accepted for pyrite [14]. In order to determine the instrumental broadening, we used the profile of crystal pyrite lines. The obtained size of blocks in pyrite was ~24 nm (for iron particles, ~10 nm), distortion value was ~1 %. Similar results were obtained also in [12], but for more than 110 hours of MA of Fe + 2S powder mixture.
Thus, nanocomposite based on pyrite and amorphous quartz is obtained during the MA of a mixture of quartz and sulfur in AGO-2 planetary mill with steel furniture (it is possible to use scrap of other metals as a milling tools, which results in obtaining nanocomposites based on the corresponding sulfides). The time necessary for the process is 1-2 orders of magnitude shorter than that required for traditional MA of iron and sulfur powders [11-13].
5. Abrasive-reactive synthesis of cementite
Figure 5(a) shows a change in the X-ray data for the amorphous quartz-graphite system after the MA, suggesting amorphization of the system and wear of the steel milling tools (see reflection a-Fe at 20 = 44.68° or PDF 6-696). In the process of MA of the system with a- SiO2 reflexes of the quartz crystals are kept, but with the significant broadening and reduction of intensity of lines in comparison with the initial homogenized sample (compare, for example, with PDF 75-443). The steel milling tools are also wearied (see the corresponding fragments of the X-ray diagram in F igu-re 5(b)).
As was noted above, the MA samples were annealed under conditions that ruled out thermal synthesis of cementite. All the parameters of the X-ray data on annealed samp-
Fig. 5. The X-ray data for the system quartz (3 g) - graphite (1.5 g), MA for 90 min: sample with the fused quartz (a); with a- SiO2 (b); for sample (a), annealing for 2h in the Ar atmosphere at 960 °C (c)
les of the MA quartz-graphite system are identical to standard cementite (see PDF 75-910 or Fig. 5(c)) with a small admixture of nanodimensional iron particles. However, the degree of cementite crystallization from the amorphous phase (Fig. 5(a)) is less prominent for the samples annealed at 660 °C than for those annealed at 960°C (Fig. 5(c)) and for those with a-quartz (Fig. 5(b)). The presence of residual nanodimensional [7] iron particles in the samples is demonstrated by the following observation: The cementite peak at 20 = 44.72° overlaps the basic reflection of a-Fe, but it does not overlap the reflection of iron at 20 = 82.4-82.5°. Hence, the MA of quartz-graphite systems can produce the Fe- Fe3C -C- SiO2 composite.
6. Conclusion
If we admit that the nanodimensional particles produced by the abrasive wear of the milling tools are entirely expended in the generation of the amorphous cementite phase (by the reaction 3Fe + C = Fe3C) or pyrite (Fe + 2S = FeS2), we would have the following composites after the MA for the:
- fused quartz-graphite system - Fe3C (1.23 g)-C(1.42 g)- quartz (3 g);
- a-quartz-graphite system - Fe3C (0.51 g)-C (1.46 g) - quartz (3 g);
- fused quartz-sulfur system - FeS2 (1.46 g)-S (0.82 g) - quartz (3 g).
However, the structure of the composites can be significantly more complex [7]. In particular, the issues of the localization of the MA product phases and their possible impact on the modification of the surfaces of the MA quartz particles by iron and cementite or pyrite remain to be solved.
Absolute (and relative) quantitative characteristics of the wear of the steel milling tools (drum D and balls B) after MA based on the weighing method for studied systems are given below.
For the quartz (3 g) and graphite (1.5 g) system for 90 min MA: (i) fused quartz — D 0.63 g (0.077 %), B 0.52 g (0.50 %), and total wear 1.15 g; (ii) a-quartz — D 0.23 g (0.028 %), B 0.25 g (0.26 %), and total wear 0.48 g.
For the abrasive (3 g) and sulfur (1.5 g) system for 135 min MA: (i) glass as abrasive — D 0.03 g (0.0037 %), B 0.043 g (0.046 %), and total wear 0.073 g; (ii) fused quartz — D 0.21 g (0.028 %), B 0.47 g (0.49 %), and total wear 0.68 g.
The proposed method can also find a broad application both for obtaining new nanocomposite powders and for processing of a numerous class of geological and man-made materials.
This research has been supported in part by the grants of Russian Foundation for Basic Research (Nos. 02-0332109 and 03-03-32271) and Integration Grant of the SB RAS.
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