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22SHS OF CERAMIC NITRIDE-CARBIDE NANOPOWDER COMPOSITIONS OF Si3N4-SiC AND AlN-SiC WITH THE USE OF SODIUM AZIDE AND
HALIDE SALTS
Yu. V. Titova*", A. P. Amosov", D. A. Maidan", G. S. Belova", and I. A. Uvarova"
aSamara State Technical University, Samara, 443100 Russia
*e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10174
Powders of refractory nitride (AlN, Si3N4) and carbide (SiC, TiC) compounds are widely used for the manufacture of non-oxide structural ceramics with low weight, high hardness, wear resistance, heat resistance, corrosion resistance [1, 2]. Traditionally, such ceramics are produced by reaction sintering or hot pressing of powders. But single-phase ceramics from individual refractory compounds can be badly sintered, poorly processed, be too brittle, have a large coefficient of friction, etc. To solve these problems, several approaches are used. First, composite ceramics of several phases are used [1-3]. For example, the addition of titanium carbide particles to silicon nitride increases the hardness and fracture toughness due to the higher hardness of TiC compared to Si3N4 and due to stresses arising because of the different thermal expansion coefficients resulting in the curvature of cracks caused by internal stresses. The hardness of Si3N4-5% TiC composites is 17 HPa, the fracture toughness is 6.5 MPa m1/2 [4]. Such composites are used for the production of cutting tools and wear-resistant parts, as well as ceramics with improved thermal conductivity and electrical conductivity [5]. To obtain dense ceramics with high dielectric properties, SiC is added to AlN [6]. The AlN-SiC composition has an increased bending strength with an increase in the SiC content, and the maximum bending strength is observed in the AlN-30% SiC composite containing P-SiC fibers [7]. Secondly, the transition to nanostructured ceramics is used, since it has been repeatedly shown that the reduction of the powder size, the transition to nanopowders and the manufacture of nanostructured ceramics can significantly improve its properties [8, 9]. But traditional methods of sintering can no longer be used here, as they lead to the consolidation of nanopowders and increase their size. A modern alternative to this traditional method is, for example, the spark plasma sintering (SPS) method, which is most widely used for sintering nanopowders. Third, both approaches are used simultaneously, i.e. nanostructured ceramic composites. A study on Si3N4/SiC nanocomposites reported improved bending strength [10]. It should be noted that if a component is introduced into the initial mixture of ceramic powders in the form of large particles or agglomerates of small particles, the compaction of the composite and its strength after sintering deteriorate [3]. To obtain high strength of the composite, it is necessary that the particles of all components are very small and evenly distributed over the volume of the composite [3]. It is also necessary to take into account that very small particles of the components can be uniformly distributed in the volume of the composite not by mechanical mixing of the powders, but by chemical synthesis of these particles in the volume of the composite in the implementation of in-situ processes [3].
One of the promising in-situ processes is the process of self-propagating high-temperature synthesis (SHS) of a wide variety of refractory compounds, including nitrides and carbides [11, 12]. The process of SHS is attractive due to its simplicity and efficiency, and provides great opportunities for regulating the dispersion and structure of the synthesized ceramic powders, bringing them to the nanoscale level. In particular, in the synthesis of nitrides, such capabilities are realized in the azide SHS process, referred to as SHS-Az, in which as a nitriding reagent is
used not nitrogen gas, but sodium azide powder NaN3, as well as halide salts. The successful experience of application of the process of azide SHS to obtain nanopowders of nitride compositions TiN-BN, AlN-BN, Si3N4-TiN using precursors - halide salts of both nitrided elements [13]. The aim of this work is to study the possibility of obtaining ceramic nitride-carbide nanopowder compositions Si3N4-TiC and AlN-SiC by azide SHS. The compositions of initial powder mixtures for the preparation of single-phase powders AlN, Si3N4, SiC and TiC by the azide SHS technology are known [12], from the analysis of which the following equations of chemical reactions can be proposed for the synthesis of target compositions Si3N4-TiC and AlN-SiC:
Si + 20Al + 6NaN3 + (NH4)2SiF6 + 2C = 2SiC + 20AlN + 6NaF + 4H2 (1)
4Si + 20Al + 6NaN3 + (NH4)2SiF6 + 5C = 5SiC + 20AlN + 6NaF + 4H2 (2)
15Si + 6NaN3 + (NH4)2TiF6 + 2C + Ti= 5 Si3N4 + 2TiC + 6NaF + 4H2 (3)
8Si + 4NaN3 + Na2SiF6 + 2C + 2Ti = 3Si3N4 + 2TiC + 6NaF (4)
8Si + 4NaN3 + Na2SiF6 + 4C + 4Ti = 3Si3N4 + 4TiC + 6NaF (5)
The phase composition of the synthesized products was determined on the Dron-3 diffractometer. X-ray spectra were measured using Cu-radiation. The quantitative phase analysis was performed by the full-profile analysis method (Rietveld method) using the program PDXL 1.8.1.0 using the open crystallographic database (COD). The results of the X-ray phase analysis of the products are given in Tables 1 and 2. Table 1 shows that in combustion of mixtures (1) and (2) it is possible to synthesize the target composition AlN-SiC, but the reaction products also include water-insoluble admixtures of Na3AlF6 and free Si. Moreover, the share of the by-product Na3AlF6 increases with increasing carbon content in the initial mixture that can be explained by decrease in combustion temperature. Table 2 shows that in combustion of mixtures (3)-(5) it was also possible to synthesize the target composition Si3N4-TiC. The byproducts of these reactions are: free silicon and silicon carbide, but their total content does not exceed 6 wt %.
Table 1. The ratio of phases in the washed combustion products for AlN-SiC synthesis.
Composition of combustion products, wt %
The composition of the starting mixture
AlN SiC Na3AlF6 Si
Si+20Al+6NaN3+(NH4)2SiF6+2C 90.3 1.3 7.7 0.7
4Si+20Al+6NaN3+(NH4)2SiF6+5C 81.0 2.6 15.5 0.9
Table 2. The ratio of phases in the washed combustion products for Si3N4-TiC synthesis.
The composition of the starting mixture Composition of combustion products, wt %
P-Si3N4 a- Si3N4 TiC Si SiC
15 Si+6NaN3+(NH4)2TiF6+C 63.5 28.4 5.66 2.44 -
8Si+4NaN3+Na2SiF6+2C+2Ti 37.7 38.2 20.3 2.3 1.5
8Si+4NaN3+Na2SiF6+4C+4Ti 35.6 26.6 31.9 1.9 4.0
The study of the surface topography and morphology of the particles of the synthesized compositions was carried out on a scanning electron microscope JSM-6390A Jeol with the prefix Jeol JED-2200. The results are presented in Figs. 1 and 2. From the presented photos, taking into account the results of the energy dispersion analysis, it is clear that the photos of powders before the operation of water washing in distilled water contain mainly sodium fluoride crystals. Analyzing the photos of powders after washing, it can be concluded that aluminum nitride and silicon carbide are synthesized in the form of equiaxial micro-sized particles.
20kV X10.000 1|jrn
(e) (f)
Fig. 2. Morphology of particles of combustion products of mixtures 15Si + 6NaN3 + (NH4)2TiF6 + C (a, b), 8Si + 4NaN3 + Na2SiF6 + 2C + 2Ti (c, d), 8Si + 4NaN3 + Na2SiF6 + 4C + 4Ti (e, f) before and after water washing.
Thus, the method of azide SHS made it possible to obtain promising ceramic nitride-carbide powder compositions Si3N4-TiC and AlN-SiC in one stage using precursors - halide salts of elements to be nitrided and carbidized.
The synthesized composite powders Si3N4-TiC and AlN-SiC are promising for use in sintering the corresponding composite ceramic materials with increased properties, lower brittleness, good machinability, lower sintering temperatures compared to single-phase ceramic materials made of nitrides or carbides. Additional studies are required to obtain nanoscale powder compositions Si3N4-TiC and AlN-SiC.
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