Fabrication and characterization of aluminum-based powders multi-reinforced with nano-and microparticles

. Multi-reinforced powders were obtained by high-energy ball milling in a planetary mill. The process of obtaining heterogeneous powders consisted of two stages. At the first stage, a nanocomposite powder of AlMg6 + 0.3 wt.% C 60 was obtained. In the second stage, 10, 30, 50, and 70 wt.% Al 2 O 3 were added to the obtained nanocomposite powder and processing continued. Methods such as scanning electron microscopy, X-ray diffraction, and particle size analysis were used to characterize the obtained powders. It is shown that after the first stage of processing, the particles of the composite powder are characterized by an irregular shape. C 60 reinforcing particles in the form of nanosized agglomerates were fixed on the surface of aluminum powder particles. After the second stage of processing, the particle size of the powder mixture decreased from 17.8 to 12.3 μm, while the proportion of Al 2 O 3 particles increased from 10% to 70% by weight. It is shown that the synthesized heterogeneous powders were a mechanical mixture consisting of complex composition agglomerates and micro-sized ceramic particles. Complex composition agglomerates were formed from nanocrystalline matrix material particles and C 60 , and also with Al 2 O 3 microparticles embedded in them, as well as located on the surface. The concentration of Al 2 O 3 particles on the surface and inside the agglomerates increased with increasing weight fraction of ceramic particles in the mixture. It has been found that the introduction of 10-70 wt. % Al 2 O 3 into the powder mixture increases the microhardness of the powder particles by approximately 16-23%. The resulting multi-reinforced powder mixtures can be utilized for coating deposition using the cold gas dynamic spraying method.


Introduction
Surface layer engineering is one of the key methods of improving the operational properties of machine parts.Within this concept, an important place is occupied by the development and research of new coatings that provide specified tribological surface properties.One of the promising techniques for effectively applying tribological coatings based on aluminum matrices is cold gas dynamic spraying [1][2][3][4][5][6][7][8].However, gas-dynamic spraying of powders with a particle size smaller than 2 μm is difficult due to their deceleration in a compressed layer that occurs when a supersonic gas flow impinges on an obstacle.In addition, during the acceleration of powder particles in a gas flow, a strong velocity dispersion occurs for particles of different sizes, while the nanosized fraction can fly away without reaching the coating [9,10].Therefore, the use of polydisperse powder for gas-dynamic spraying, which is a simple mechanical mixture of nano-and microsized particles, seems to be ineffective.Taking this into account, the formation of tribological coatings multi-reinforced with nanoand microparticles requires the use of agglomerated powders.The application of such powders for gas-dynamic spraying makes it possible to obtain coatings with a high level of properties [11].The presence of nanosized C60 particles increases the mechanical properties of the matrix material [12][13][14][15][16], increasing the strength of fixing microsized ceramic particles in the matrix.
The method of mechanical processing in a planetary ball mill can be successfully used for obtaining such powders, providing the synthesis of multiparticle powders of the granulometric composition required for gas-dynamic spraying [17,18].The obvious advantages of this method include the possibility of implementing the processes of reinforcement distribution, nanostructuring and dispersion of matrix material within one technological cycle [19].
The present work is aimed at obtaining and studying heterogeneous powders based on aluminum alloy, multi-reinforced with nano-and microparticles, suitable for gas-dynamic spraying.

Equipment and research methods
The process of obtaining the multi-reinforced powders consisted of two stages.At the first stage, a nanocomposite powder based on AlMg6 aluminum alloy reinforced with 0.3 wt.% fullerenes C60 was obtained.For this purpose, the initial components were co-processed in a planetary ball mill (AGO-2U) at 1800 rpm for 60 min.At the second stage, 10, 30, 50, or 70 wt.%Al2O3 powder with an average particle size of 18.5 μm was added to the obtained AlMg6/C60 nanocomposite powder.For this purpose, the mixtures were placed in a ceramic milling container and processed in a FRITSCH PULVERISETTE 6 planetary ball mill at 600 rpm for 15 min.
The surface morphology of the synthesized powders was studied by scanning electron microscopy (SEM) using a LEO 1450 VP equipped with an INCA 300 detector for energy dispersive X-ray spectroscopy.The granulometric composition was determined on the device Microsizer-201C.X-ray diffractometer Bruker D8 ADVANCE was used to determine the structural-phase composition of powders.The sizes of coherent scattering domains were calculated using the Selyakov-Sherrer equation.Volume-averaged sizes of the coherent scattering domains were determined based on the assumption of spherical crystallites.The microhardness of the powder particles was measured using the kinetic indentation method with a Micro-Combi Tester CSM Instruments on cross-sectioned specimens.The measurements were conducted using a Vickers indenter with a load of 0.1 N and a dwell time of 10 s.A minimum of 10 measurements were performed.

Results and discussion
Figure 1 shows the data of particle size analysis of the obtained powders.Analysis of the data, shows the multimodal nature of the particle size distribution of the powder mixture.Apparently, local maximums in the range up to 8 μm belong to ceramic particles and indicate their fragmentation during processing in a planetary mill.The above data show that the addition of Al2O3 ceramic particles to the AlMg6/C60 powder mixture leads to a decrease in the average particle size of the AlMg6/C60+Al2O3 powder mixture.For example, with the addition of 10 to 70 wt.%Al2O3, the average particle size of the powder mixture decreases from 17.8 to 12.3 μm.At the same time, the particle size of AlMg6/C60 obtained at the first stage could reach 300 μm.
Fig. 2 shows SEM images of powder mixtures containing 10 and 70 wt.%Al2O3.In addition, the insets show the EDS-microanalysis data, which allow identifying ceramic Al2O3 particles on SEM images of powder mixtures.The ceramic particles are characterized by a splinter shape with a smooth fracture surface morphology, which allows their identification on SEM images even visually.The analysis of SEM images makes it possible to conclude that the obtained multireinforced powders are a complex mechanical mixture consisting of agglomerates and micro-sized ceramic Al2O3 particles.Agglomerates are the particles of nanocomposite material AlMg6/C60 with embedded in them, as well as located on the surface of microsized ceramic particles Al2O3 (see Fig. 2c).The concentration of Al2O3 particles on the surface of agglomerates increased with increasing mass fraction of ceramic particles in the mixture.
Fig. 3 shows the XRD results of multi-reinforced powder mixtures.Analysis of the data obtained for different powder compositions shows that the XRD patterns are qualitatively similar.The presence of peaks corresponding to the AlMg6 matrix alloy and ceramic α-Al2O3 particles was noted.The absence of peaks corresponding to C60 fullerenes can also be noted.In general, this is typical for carbon nanostructures in such concentrations [20] and may be caused by the low sensitivity of this method of identification to crystalline matter, the content of which does not exceed 2-3%.It should also be noted that the intensity of α-Al2O3 peaks increases with an increase in its amount in the charge.

Fig. 3. XRD of mechanically synthesized multi-reinforced powder mixtures with different amounts of ceramic particles
Calculation of the size of the coherent scattering region performed for the obtained powders using the Selyakov-Scherrer equation shows that the matrix material has a nanocrystalline structure (average crystallite size ~40-60 nm).Which is formed in the matrix material during high-energy ball milling.The presence of a nanocrystalline structure can also have a positive effect on the tribological properties when using powder for coating deposition.For example, by further increasing the hardness.
Figure 4 presents the microhardness data of synthesized powder particles as a function of Al2O3 content in the powder mixture.Additionally, a dashed line is depicted in Figure 4, representing the microhardness of AlMg6 powder obtained through the same processing route but without the addition of reinforcing particles.Analysis of microhardness variations reveals that the addition of 0.3 wt.% C60 enhances the microhardness of the powder particles by approximately 20% compared to AlMg6 powder obtained through the same processing route but without C60 addition.Furthermore, additional reinforcing of the powder with Al2O3 particles also leads to an increase in the average microhardness from ~313 to 343 HV at 10 and 70 wt.% of reinforcement, respectively.
Thus, the obtained results confirm the effectiveness of utilizing multi-reinforcement to enhance the microhardness of powder particles, which will contribute to the improvement of the mechanical properties of the resulting coatings.

Conclusion
Thus, nanocrystalline powders based on aluminum multi-reinforced with nano-and microparticles were successfully obtained by co-processing in a planetary ball mill.At the first stage, a nanocomposite powder of AlMg6 + 0.3 wt.% C60 was obtained.In the second stage, 10, 30, 50, and 70 wt.%Al2O3 (average particle size 18.5 μm) were added to the obtained nanocomposite powder and processing continued.It is shown that after the first stage of processing, the particles of the composite powder are characterized by an irregular shape.C60 reinforcing particles in the form of nanosized agglomerates were fixed on the surface of aluminum powder particles.After the second stage of processing, the particle size of the powder mixture decreased from 17.8 to 12.3 μm, while the proportion of Al2O3 particles increased from 10% to 70% by weight.The results of X-ray diffraction analysis of the composite powder obtained at the first stage show only the presence of diffraction peaks corresponding to the matrix material.The peaks of the nanosized C60 reinforcement were not recorded on the X-ray diffraction patterns.After the second stage of milling, besides the peaks of matrix material, the presence of Al2O3 peaks was observed.The mechanism of the formation of heterogeneous powder particles was proposed.It is shown that the synthesized heterogeneous powders were a mechanical mixture consisting of complex composition agglomerates and micro-sized ceramic particles.Complex composition agglomerates were formed from nanocrystalline matrix material particles and C60, and also with Al2O3 microparticles embedded in them, as well as located on the surface.The concentration of Al2O3 particles on the surface and inside the agglomerates increased with increasing weight fraction of ceramic particles in the mixture.It has been demonstrated that the introduction of 10-70 wt.% Al2O3 into the powder mixture, along with mechanical processing for as short as 15 minutes, results in a reduction in the average particle size to approximately 12.3-17.8μm and increases the microhardness of the powder particles by approximately 16-23%.Granulometric analysis suggests that these powders can be utilized for the fabrication of tribological coatings through cold gas-dynamic spraying.

E3SFig. 1 .
Fig. 1.Granulometric composition of mechanically synthesized multi-reinforced powder mixtures with different amounts of ceramic particles

E3SFig. 4 .
Fig. 4. Microhardness of mechanically synthesized multi-reinforced powder mixtures with different amounts of ceramic particles