Introduction

Construction materials that can operate at elevated wear levels and in various media are now attracting increased attention. For example, materials based on TiC with a metal binder that compensates for the high brittleness of the carbide have been called cermets and have broad applications because of their improved physicomechanical properties (hardness, microhardness, flexural and crushing strength, impact resistance, etc.) [1,2,3,4,5]. Cermets have also been used as wear-resistant, cutting, and high-temperature materials [6, 7]. Cermet powders can be used as materials for application of wear-resistant coatings [8,9,10]. Casting [11], sintering [12,13,14], isostatic hot pressing [15, 16], and selective laser sintering [17, 18] are the most well-known powder metallurgy methods for producing cermet items.

Self-propagating high-temperature synthesis (SHS) is the most common known method of producing metal-ceramic composites based on TiC with an added metal binder [19,20,21,22,23,24]. It consists of conducting an exothermic reaction as a combustion wave propagating through a layer to form synthesis products as compounds. The combustion of transition- metal powders in a reactive gas or their mixtures with nonmetals (carbon, boron) is an effective method for synthesizing carbides, nitrides, and carbonitrides. Cermets based on TiC have characteristically high melting points, hardness, corrosion resistance, etc. A metal binder, e.g., NiCr, Ni, Mo, etc., is added to TiC-based powders to decrease the brittleness of the resulting item and increase the adhesion upon application of protective coatings [25]. The SHS method produces powders and sinters that are then dispersed for further use. The advantages of SHS are the low energy consumption, high reaction rate, and purity and uniformity of the product obtained after one processing cycle as compared to traditional powder metallurgy, where syntheses require from 5 to 10 h. These features of SHS are due to the melting of the separate components of the starting mixture during the synthesis, which leads to self-dispersion of the reagents and improved uniformity of the reaction products.

The aim of the present work was to research the materials science of the combustion products from SHS from starting Ti–C–NiCr (10 – 40 wt.%) mixtures under Ar and in a vacuum.

Experimental

The starting reagents were commercial powders of Ti (60 μm, 99.1%), C (1 μm, 99.1%), and NiCr (70 μm, 99.9%) calculated to form after the synthesis TiC (60 – 90 wt.%) with NiCr binder (10 – 40 wt.%) (Table 1).

TABLE 1. Composition of Starting Components, wt.%
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The powders were mixed beforehand and dried. Then, they were pressed into cylindrical blanks of height 30 and diameter 25 mm and mass 35 g. Argon (Ar) at a pressure of 3 atm in the chamber or a vacuum of 10–3 mbar was used as a protective medium during the combustion in SHS mode. The cylindrical blanks had a relative density of 0.55 – 0.67 depending on the compression force of the starting powders. The range of relative density was selected to reach the maximum calculated combustion temperature in them. Figure 1 shows a diagram of the combustion in SHS mode in the protective media. Pressed blanks 4 were placed into reactor 5, where the protective medium was created. A tungsten spiral initiated the combustion from the end of the blank. After the combustion front passed, the cooled sample was removed from the chamber.

Fig. 1.
figure 1

Diagram of combustion in SHS mode in vacuum and in Ar: Ar tank (1 ), thermocouple (2 ), tube furnace (3 ), starting blank (4 ), reactor body (5 ), PC with data collection system (6 ), manometer (7 ).

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X-ray phase analysis (XPA) used an ARL X’TRA x-ray powder diffractometer with angular resolution 2θ = 20 – 80° in steps of 0.02° and a copper anode with average wavelength 1.5418 Å. XPA was performed with added Si powder to avoid instrumental error of the recorder.

The microstructure of the obtained items was studied on an LEO-1450 scanning electron microscope in combination with an INCAEnergy energy-dispersive spectroscopy microanalyzer (EDS System).

Results and Discussion

The obtained samples (sinters) were further dispersed into powders of 2 μm for XPA (Fig. 2). It was found that the materials synthesized in both media consisted of the two phases TiC and NiCr. Additional strengthening phases (Ti0.8Cr0.2C and Cr3C2) were not detected, e.g., like for materials prepared by SHS-extrusion [26,27,28], which in turn improved the physicomechanical properties of the obtained items. The main characteristic peaks of NiCr in the powder patterns were observed to shift to lower angles for 20 – 40 wt.% compositions and to higher angles for the 10 wt.% composition for the syntheses in Ar (Fig. 3). This shifting was not observed for the syntheses in vacuo. The shifting of the diffraction peaks could be explained by different combustion temperatures of the starting blanks in the studied media that affected phase and structure formation in the synthesized materials. The combustion temperature decreased if the amount of binder in the starting mixture increased because a part of the reaction heat was spent on melting the binder [29]. The greater the combustion temperature of the starting blank was, the more the diffraction peaks shifted.

Fig. 2.
figure 2

XPA patterns of SHS combustion products: vacuum (a) and Ar (b ); Ti–C–10 wt.% NiCr (1 ); Ti–C–20 wt.% NiCr (2 ); Ti–C–30 wt.% NiCr (3 ); Ti–C–40 wt.% NiCr (4 ); Si ( ), TiC ( ), NiCr ( ).

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Fig. 3.
figure 3

Main diffraction peaks of synthesized NiCr compounds: vacuum (a) and Ar (b ); starting NiCr (1 ); Ti–C–10 wt.% NiCr (2 ); Ti–C–20 wt.% NiCr (3); Ti–C–30 wt.% NiCr (4); Ti–C–40 wt.% NiCr (5 ).

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The unit-cell constants of TiC and NiCr were calculated for each synthesized composition (Table 2). Regardless of the synthesis medium (Ar or vacuum), all values were similar for similar synthesis conditions. The a constant decreased and approached the table value for NiCr if the fraction of NiCr in the starting mixture increased. The composition Ti–C–NiCr (40 wt.%) was notable because the a constant of the crystal lattice increased by 0.04 Å for the material synthesized in Ar as compared to that synthesized in vacuo.

TABLE 2. Unit-Cell Constants
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Dependences of unit-cell constant a of TiC on the NiCr content were plotted (Fig. 4). Constant a for standard TiC without NiCr metal binder was noted (card PDF2 32-1383) (Fig. 4a ). Unit-cell constant a decreased with increasing metal-binder content in the material. All values obtained in Ar and vacuum were similar to each other although that for Ti–C–NiCr (40 wt.%) synthesized in Ar did not agree. Figure 4b shows several known compounds of Ni and Cr with various stoichiometries. Constant a approached the value for TiC0.62 with increasing fraction of NiCr binder. These compounds are noted in Fig. 4 to understand to which side the stoichiometry shifted. Calculations according to Vegard’s rule [30] were used to establish the stoichiometry of the formed NiCr compound from combustion in Ar or vacuum for each studied composition (Table 3). Unit-cell constant a decreased if the wt.% of NiCr was increased, like for the unit-cell constant for Ti–C–NiCr (40 wt.%) (Fig. 4b ). This occurred because of various heat-transfer conditions and the combustion temperatures of the chosen compositions during the syntheses in Ar and in vacuo.

Fig. 4.
figure 4

Dependences of unit-cell constant a on NiCr content: TiC (a) and NiCr (b ).

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TABLE 3. Nichrome Stoichiometry
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The structures of the synthesized materials were studied using the Ti–C–NiCr (30 wt.%) as an example (Fig. 5). EDS results in characteristic images of the chemical elements confirmed the XPA results, i.e., the material consisted of TiC grains situated in a NiCr matrix (Fig. 6). The TiC grains were round. The average TiC grain size for the materials synthesized in Ar was 2.4 μm; in vacuo, 1.9 μm. Heat transfer from the reaction zone was greater for the synthesis in vacuo as compared to that in Ar. This prevented growth of TiC grains after the combustion wave passed. Analogous results were obtained for the other compositions.

Fig. 5.
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EDS results in characteristic image of chemical elements.

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Fig. 6.
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SEM images of microstructure of synthesized materials: argon (a) and vacuum (b ).

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Conclusion

The influence of the medium on the structure and phase composition of synthesis products from combustion of Ti–C–NiCr (10 – 40 wt.%) in SHS mode was experimentally established. Unit-cell constant a decreased and approached the table values for TiC and NiCr as the fraction of NiCr in the starting mixture increased. Unit-cell constant a of NiCr increased by 0.04 Å for material synthesized in Ar as compared to that in vacuo, where the unit-cell constant of TiC increased by 0.0037 Å. The size of TiC grains decreased by 1.5 times for compositions synthesized in vacuo as compared to those synthesized in Ar because of greater heat transfer from the reaction zone.

The work was financed by grant No. MD-2909.2021.4 from the RF President for state support of young Russian scientists and doctors of science.