Physical and chemical processes during nitriding of chromium ferrosilicon by filtration combustion

Introduction

The method of self-propagating high-temperature synthesis (SHS) is based on highly exothermic reactions that occur in the form of a combustion wave in a self-propagating mode. The SHS method has undeniable advantages: energy efficiency, short synthesis time, environmental friendliness, and simplicity of equipment [1 – 3].

Currently, a large number of materials, particularly nitrides, have been obtained by the filtration SHS method in a nitrogen environment [4 – 6]. Several nitride materials possess unique physicochemical properties [7 – 9]. They can be used in the production of gas turbine components [10], heat-dissipating radiators [11], cutting tools [12; 13], photocatalysts [14], semiconductors [15], and so on.

The most promising application in the filtration SHS method is the use of accessible and relatively inexpensive ferroalloys. Using ferroalloys in SHS processes can produce nitride material at a relatively low cost without losing product quality [16; 17]. Iron, which is part of the ferroalloys, has a catalytic effect on the nitriding process of other elements included in the initial mixture [18]. Thus, iron increases the intensity and depth of nitriding of the initial material. There is a considerable amount of work dedicated to the filtration combustion of simple ferroalloys. The patterns of nitriding of ferrosilicon [19 – 21], as well as the filtration combustion of ferrochrome and ferrovanadium [22], have been thoroughly studied. The study [23] investigated the combustion of industrial ferrotitanium in nitrogen. The monograph [24] described the nitriding of ferroboron and ferroniobium in a combustion mode.

However, the use of complex ferroalloys in filtration combustion processes is interesting and little studied. Complex ferroalloys are alloys of iron with two or more elements. At present, SHS combustion of ferro-silicoaluminium [25] and ferro-aluminum-silicon-zirconium [2] has been studied.

The aim of this work was to study the combustion processes of chromium ferrosilicon in a self-propagating mode under conditions of natural nitrogen filtration to obtain a nitride-containing composite material based on chromium nitride and silicon nitride.

Materials and methods

Chromium ferrosilicon (CFS) was used as the starting material. X-ray phase analysis showed that this ferroalloy is multiphase and contains CrSi₂, Si, and FeSi₂ (Fig. 1). According to the chemical analysis, the composition of CFS is as follows (wt. %): 49.4 Si, 29.7 Cr, 20.7 Fe, and the rest are oxides. For nitriding in the self-propagating mode, the initial CFS was ground in a ball mill and dried in a vacuum drying oven at a temperature of 150 °C for 3 h.

Nitriding of the initial CFS was carried out in a constant pressure setup with a volume of 3 liters. For the synthesis, the initial powder mixture was placed in a gas-permeable container mounted on a non-conductive stand. An igniting composition was poured over the initial charge. A coil was connected to the igniting composition to conduct an electric pulse from a transformer. After the electric pulse was applied, the combustion reaction of the igniting composition was initiated. Then, the heat released as a result of the combustion of the igniting composition initiated the combustion reaction of the initial CFS powder. After the combustion wave front passed and complete cooling occurred, the unreacted nitrogen was vented, and the nitrided samples were removed for further physicochemical studies.

The phase composition was studied using a Shimadzu XRD-6000 diffractometer. The oxygen and nitrogen contents were determined using a LEKO-ONH 836 instrument. The maximum combustion temperature was measured using the thermocouple method with tungsten-rhenium thermocouples (WRe5-WRe20) on an LA20USB.

Results and discussion

Combustion of chromium ferrosilicon proceeds in a stationary mode. The nitrided samples obtained based on CFS are macrohomogeneous. An image of the nitrided CFS is shown in Fig. 2.

The probable chemical reactions of the interaction of the initial charge based on CFS with nitrogen are given below:

The overall chemical reaction equation is as follows:

Reaction (4) corresponds to the complete nitriding of the initial CFS (with a conversion degree of 1). Due to the rapid processes of SHS, the initial charge is in the reaction zone for a relatively short time and does not fully react with nitrogen. It is theoretically calculated that the maximum amount of absorbed nitrogen by chromium ferrosilicon is 28.99 %.

The nitrided CFS product is a multiphase material containing β-Si₃N₄, α-Fe, CrN, Cr and CrSi₂. The presence of Cr and CrSi₂ indicates the incomplete nitriding reaction of the initial powder (Fig. 3).

Parameters such as the pressure of the gaseous reactant, sample diameter, particle size, and density of the initial material significantly influence the maximum temperature, process, and feasibility of filtration combustion in a self-propagating mode.

Fig. 4 shows the dependence of the amount of absorbed nitrogen and the combustion rate on the diameter of the initial samples. The influence of the diameter was studied in the range of 35 to 65 mm. Combustion of CFS can be initiated with initial sample diameters of at least 35 mm. Increasing the diameter slightly affects the amount of absorbed nitrogen and leads to a decrease in the combustion rate from 0.11 to 0.021 mm/s. This slight change in the amount of absorbed nitrogen is due to the increased difficulty of nitrogen filtration reaching the reaction zone as the diameter increases. At the same time, due to the slowdown in the combustion wave front, the residence time of the initial CFS particles in the reaction zone increases. The slowdown of the combustion wave front is related to the increased volume of the powder mixture, which requires a large amount of heat for heating. When changing the diameter, the maximum amount of absorbed nitrogen was about 22 %, which is 6.99 % less than the theoretically calculated amount of nitrogen absorption. As the diameter of the initial powder mixture increases, the maximum combustion temperature of CFS changes from 2400 to 2650 °С.

In a laboratory setup with a volume of 3 liters, it is preferable to implement combustion of samples with a diameter of 50 mm.

Increasing the pressure accelerates the filtration of gaseous nitrogen and, accordingly, increases the concentration of the reagent gas in the chemical reaction zone. Combustion of CFS at a nitrogen pressure of less than 3 MPa could not be achieved. Increasing the pressure of gaseous nitrogen leads to an increase in the amount of absorbed nitrogen from 18.3 to 22.8 % and the combustion rate from 0.073 to 0.092 mm/s. In the nitrogen pressure range from 5 to 7 MPa, the change in the amount of absorbed nitrogen becomes less pronounced. Increasing the nitrogen pressure above 7 MPa is not advisable because, at 5 MPa, the influence of pressure on the combustion process becomes insignificant (Fig. 5). When implementing CFS combustion at a pressure of 7 MPa, the sample contains 6.29 % less nitrogen than the theoretically calculated value. As the pressure of gaseous nitrogen increases from 3 to 7 MPa, the maximum combustion temperature rises from 2350 to 2600 °С.

Combustion of CFS with particle sizes greater than 100 µm and without fine fractions (less than 63 µm) could not be achieved. As the particle size of the initial material decreases, the amount of absorbed nitrogen increases from 21.9 to 23.5 % and the combustion rate from 0.081 to 0.140 mm/s. The increase in the dispersity of the initial material leads to an increase in the specific surface area capable of reacting (Fig. 6). The reduction in particle size of CFS results in an increase in the maximum temperature from 2400 to 2490 °С.

The increase in the density of the initial powder mixture was achieved by pressing the initial powder into tablets with a diameter and height of 40 mm in molds. Combustion of the pressed powder, which retains the shape of the tablet (ρ = 2.52 g/cm³), could not be achieved. Therefore, only samples with a bulk density (2.23 g/cm³) were used.

Conclusions

The combustion of chromium ferrosilicon proceeds in a stationary mode, resulting in homogeneous nitrided samples without melt droplets or cracks.

Increasing the diameter leads to a decrease in the combustion rate from 0.11 to 0.021 mm/s and slightly affects the amount of absorbed nitrogen. Increasing the nitrogen pressure results in an increase in the amount of absorbed nitrogen (18.3 – 22.8 %) and the combustion rate (0.073 – 0.092 mm/s) of chromium ferrosilicon. Reducing the dispersity of the initial material allows for an increase in the amount of absorbed nitrogen from 21.9 to 23.5 % and the combustion rate from 0.081 to 0.140 mm/s. A slight increase in the density of the initial powder prevents achieving the combustion reaction of chromium ferrosilicon in a nitrogen environment.

The maximum combustion temperature increases with a rise in gaseous nitrogen pressure from 3 to 7 MPa (from 2350 to 2600 °C), the diameter of the initial samples from 35 to 65 mm (from 2400 to 2650 °C), and a decrease in the particle size of the initial charge from less than 100 to less than 40 µm (from 2400 to 2490 °С).

Stable combustion in the self-propagating mode of chromium ferrosilicon powder samples is possible at a nitrogen pressure of at least 3 MPa, a sample diameter of at least 3.5 cm, a particle size of less than 100 µm with the presence of fine fractions (less than 63 µm), and a sample density not exceeding 2.23 g/cm³. It is optimal to carry out nitriding of the initial chromium ferrosilicon under conditions of natural nitrogen filtration at a pressure of 5 MPa, a sample diameter of 5 cm, a particle size of the initial material less than 100 µm, and a bulk sample density of 2.23 g/cm³.

The product of chromium ferrosilicon nitriding contains β-Si₃N₄, CrN, α-Fe, Cr and CrSi₂. The presence Cr and CrSi₂ indicates the incomplete nitriding reaction of the initial chromium ferrosilicon. The nitrogen saturation is 21.9 %, which is 7.09 % less than the theoretically calculated maximum amount of absorbed nitrogen.