Several Aspects of Interaction between Chrome and Nanodiamond Particles in Metal Matrix Composites When Being Heated

Abstract

The paper considers the development of a technological scheme for preparing metal matrix nanocomposites based on the interaction between nanodiamond reinforcing particles and a chromium matrix when being heated, forming chromium carbide nanoparticles. These carbides are in situ synthesized ceramic reinforcing nanoparticles. The first stage of preparing composites is to obtain composites with the chromium matrix and nanodiamond reinforcing particles. For this purpose, mechanical alloying is used, i.e., processing in planetary mills. The size of a primary nanodiamond particle is 5 nm, but they are combined in agglomerates that are hundreds of micrometers in size. The time of processing in the planetary mill defines the crushing degree of the agglomerates. In this study, processing was carried out for 0.5 h, 2 h, and 4 h. The second stage for obtaining composites with reinforcing particles of chromium carbides is thermal processing. Explorations using the method of differential scanning calorimetry showed that reducing the size of nanodiamond reinforcing particles (by prolonging the time of processing in the planetary mill) leads to a decrease in the initial temperature of the reaction for developing carbides. The worked-out technique for obtaining composites was patented in the Russian Federation (the patent for invention 2772480).

1. Introduction

The use of various carbon nanoparticles (e.g., fullerenes [1,2], nanodiamonds [3,4,5,6,7,8,9,10,11,12,13,14], nanographite [15,16], etc.) in developing composites with a polymeric or metallic matrix generates substantial interest [17,18,19,20,21,22]. Chemical interaction between metals and carbonic nanoparticles proceeds differently in comparison with macro-materials [23,24,25]. This paper considers the interaction between chrome and nanodiamonds in metal matrix composites with a chromium matrix and nanodiamond reinforcing particles. Chromium carbides [26] are divided into stable and metastable types. Stable chromium carbides include Cr₂₃C₆, Cr₇C₃ (hP80), and Cr₃C₂ (oP20); metastable chromium carbides include Cr₂C, Cr₃C, CrC, Cr₇C₃ (hP20), and Cr₃C₂ (oC20).

These composites generate interest for several reasons. Chromium is a metal that is widely used in engineering. Substantial amounts of chromium are used in coatings, including electrochemical ones. However, when producing electrochemical chromium coatings, the use of carcinogenic materials cannot be avoided. This fact reduces the attractiveness of such technologies. There are other techniques for applying coatings (e.g., gas dynamics [27,28], frictional cladding [29], etc.). However, to improve the efficiency of such coatings, when chrome is used, it is necessary to modify the chromium material to improve its wear resistance, durability, etc. This paper considers such a modification of the chromium material using nanoparticles, providing an opportunity to improve the mechanical properties of the material without degrading the quality of its surface.

2. Materials, Equipment, and Methods

For our explorations, commercially available powders of chromium and nanodiamonds were used (Figure 1). The average particle size of chromium powder was 5–10 µm. The size of primary nanodiamond particles was 5 nm, but the nanodiamonds were combined in agglomerates up to hundreds and thousands of micrometers in size. The content of nanodiamond particles in the explored composites was 15% by volume.

To obtain composite granules, mechanical alloying was applied, i.e., co-processing of the composite components in a planetary mill [30,31,32]. For this purpose, a Pulverisette 6 planetary mono-mill (FRITSCH GmbH, Idar-Oberstein, Germany) was used, with a grinding jar capacity of 80 mL and grinding balls of 5 mm in diameter made of chromium steel. The ratio of the balls’ weight to the processed materials’ weight was 36:1. The rotation speed was 400 rpm. The processing was implemented in an argon atmosphere for 0.5–4 h (the shut-down period for cooling was not included in the total processing time). The initial and processed materials were examined via X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) methods. X-ray diffraction was carried out using a DRON-3 diffractometer (Saint-Petersburg, Russia) and a D8 ADVANCE Bruker diffractometer (Berlin, Germany) in monochromatic Cu Kα radiation (with a diffracted-beam monochromator), using the EVA phase analysis software package and the PDF-2-2006 database of interplanar distances of crystalline phases. The study of thermal effects during sample heating was implemented on a DSC 404 C Pegasus differential scanning calorimeter manufactured by NETZSCH (Selb, Germany). Measurements were performed in platinum–rhodium crucibles with aluminum oxide inserts under a heating rate of 20 °C·min⁻¹. First, in a tightly sealed chamber of the calorimeter, a vacuum was created, and then the chamber was filled with argon; a sample was placed in the chamber before the creation of the vacuum. A dynamic inert atmosphere (argon; blowdown rate: 50 mL·min⁻¹) was maintained during the measurements. A TESCAN VEGA3 (Brno-Kohoutovice, Czech Republic) scanning electron microscope was used for examining the general appearance of granules and their surface. Transmission electron microscopy studies were performed with JEOL JEM 2100F and JEOL JEM 2100 F/Cs transmission electron microscopes (Akishima, Japan).

3. Results and Discussion

Composite granules are generated as the result of co-processing nanodiamonds with chromium powder. Figure 2 demonstrates the general appearance of the composite granules (chromium + 15% nanodiamonds by volume) after various times of processing by mechanical alloying (SEM images). The average particle size of the primary chromium powder was 5–10 µm. During mechanical alloying, the particles of the processed material were joined and cracked, the nanodiamond agglomerates were fragmented, and the components were mixed. The processing time influences the mixing ratio of the materials, the crushing ratio of the nanodiamond agglomerates, and the size of composite granules.

Even after 0.5 h of treatment, the size of the nanodiamond agglomerates was reduced substantially, and they were integrated in the chromium matrix. The sizes of the granules varied from 1 µm to 30 µm (Figure 2a,b). The longer the processing period, the more homogeneous the structure of the composite granules. After processing for 4 h (Figure 2c,d), the fine fraction was reduced; the size of granules was 20–30 µm, but the granules consisted of finer fragments, which is characteristic of the chromium material.

It is impossible to identify crushed nanodiamond agglomerates in mixtures with metals using X-ray diffraction methods (the reason for this is explained in [33,34]). The presence of nanodiamonds can be proven using the synchrotron radiation or transmission electron microscopy methods. Figure 3 shows the electron diffraction pattern obtained when examining the composite (chromium + 15% nanodiamonds by volume) after 2 h of processing using transmission electron microscopy. Diffraction rings from nanodiamonds (111), (211), (311), along with single reflexes from the chromium material (110), (200), (211), etc., can be seen clearly. It is worth noting that the location of the reflexes from chrome (110) coincides with the ring from nanodiamonds (111). The presence of diffraction rings from nanodiamonds points to the fact that at this stage the nanodiamonds were not transformed into the graphitic material, and did not react with chromium, forming carbides, and reducing the size of the chromium particles substantially (chromium was present in the examination zone not in the form of monocrystal, but in the form of polycrystal).

To obtain chromium carbides in industry, thermal processing at temperatures of 1150–1600 °C is used. From previous data, it has been established that using mechanical alloying decreases the necessary level of temperature—for example, to 1000 °C [35]. There is a hypothesis that the starting temperature of the chemical reaction for the formation of carbides will be decreased substantially if the reacting carbonic particles are reduced in size (especially in the nanodimensional area) [23,24,25,36]. To justify this hypothesis in terms of the chromium + nanodiamonds combination, mechanical alloying of the mixture (chromium powder and nanodiamonds) was carried out for various processing times in the planetary mill (0.5 h, 2 h, and 4 h). The longer the processing time, the greater the fragmentation of nanodiamond agglomerates (i.e., carbonic particles will be reduced in size). The sizes of the crushed nanodiamond agglomerates were assessed using transmission electron microscopy. After processing for 0.5 h, most of the crushed agglomerates were 100–200 nm in size. Increasing the processing time to 2 h further reduced the size of the nanodiamond agglomerates to 30–50 nm. After processing for 4 h, the nanodiamond agglomerates were fragmented substantially, reducing them to 10 nm in size. The size of the primary nanodiamond particles was 5 nm.

Thermal effects under heating were examined using differential scanning calorimetry. Experiments showed that the longer the processing time, the lower the starting temperature of the chemical reaction for generating chromium carbides (Figure 4).

As already noted, the processing time in the planetary mill defines the crushing ratio of the nanodiamond agglomerates. Thus, the main reason for this decrease in the initial temperature for generating carbides is undoubtedly the reduction in the size of the carbonic particles (nanodiamonds). For example, for the composite (chromium + 15% nanodiamonds by volume), after being processed for 0.5 h, the initial temperature of the chemical reaction for generating chromium carbides was 630 °C; after being processed for 2 h, the starting temperature of the reaction between chromium and the carbonic material was decreased to 600 °C, while mechanical alloying for 4 h decreased the starting temperature of the chemical reaction for generating chromium carbides to 520 °C.

The decrease in the starting temperature of the chemical reaction for generating chromium carbides, when heating the mixture of the chromium material with carbonic nanoparticles (nanodiamonds), is very important for several reasons. On the one hand, these explorations set the temperature interval of the working efficiency for the chromium + nanodiamonds composite. On the other hand, the temperature conditions for generating chromium carbides are set. The knowledge that carbides can be generated even at 520 °C provides an opportunity to substantially reduce the energy consumption for obtaining these materials and the cost of the used equipment, and to extend the operational life of the equipment.

The synthesis of chromium carbides can be proven with X-ray diffraction methods when the samples are heated. Figure 5 shows the X-ray diffraction pattern of the DSC examinations when being heated to 690 °C. The arrows indicate peaks of chromium; vertical lines represent the chromium carbide Cr₇C₃. The unmarked peaks are associated with chromium oxide (oxygen is adsorbed with air in the initial powder; moreover, argon, which is used for the chamber blowdown, also has oxygen remnants, etc.).

Various annealing temperatures and various contents of carbonic reinforcing particles in the composite lead to the formation of various carbides, including metastable ones.

The study of the electron diffraction patterns (obtained via transmission electron microscopy) of the composite during annealing showed the total absence of diffusion rings from the nanodiamonds. Only reflexes from chromium carbides and chromium were observed. This suggests that all of the nanodiamonds reacted, forming chrome carbides. The composite material consisted of the chromium matrix and reinforcing particles of the chromium carbide. The worked-out technique for obtaining the composite material using in situ synthesis of reinforcing carbide particles directly in the chromium matrix was patented in the Russian Federation [37].

4. Conclusions

In metal matrix composites (initial materials: chromium and nanodiamonds), our specific findings were as follows: (i) the duration of the mechanical alloying process has an influence on the typical size of the remaining nanodiamond agglomerates; (ii) the nanodiamonds do not react with the chromium during the milling phase, but persist as nanodiamonds; (iii) the temperature upon heating at a constant rate of 20 K/min at which the massive conversion from nanodiamonds to chromium carbide begins was reduced from 690 °C (after milling for 0.5 h) to 520 °C (after milling for 4 h)—the smaller the size of the carbonic particles, the lower the starting temperature of synthesis.

5. Patents

Popov, V.A. Composite material and its manufacturing method. Patent of the Russian Federation 2,772,480, Bul.14, 2022.