Composition of the initial reaction mixtures in the Ti–Al–Si3N4–C system
The introduction of doping elements Al and Si into the coatings allows one to attain a combination of the high characteristics of the hardness and wear resistance with a relatively low friction coefficient. One important factor in increasing of the durability of different products is the provision of thermal stability and oxidation resistance at high temperatures [1, 2]. Therefore, the problem of the development of hard wear-resistant coatings with high thermal stability, heat resistance, and corrosion resistance is very urgent.
In this work, the possibility of synthesizing of promising composite materials based on TiCyNz, Ti5Si3, and TiAl3 from the reactionary mixtures in the Ti–Al–Si3N4–C system is shown. As the initial components of the reactionary mixtures we used powders of titanium, aluminum, technical carbon (ash) of above-mentioned grades, and silicon nitride (TU 88-1-143-88). The mixtures composition was determined from the accounting of the complete transformation of the initial reagents and the formation of the product with phase composition described by the general formula
where x is the mixture parameter taking values in the range from 10 up to 50 wt %.
The experimental compositions of the reactionary mixtures for the synthesis of the composite ceramic materials, depending on the mixture parameter, are presented in Table 1.
|x , wt %||Content of the initial components in green mixture, wt %|
The values of the adiabatic combustion temperature (Tc ad) of the reactionary mixtures in the Ti–Al–Si3N4–C system and the equilibrium composition of the synthesis products at this temperature calculated using the “THERMO” software depending on the mixture parameter are listed in Table 2.
As the mixture parameter increases, the adiabatic combustion temperature decreases monotonically. In this case the content of ceramic phases (titanium carbide, nitride, and silicide) decreases and the fraction of metal melts increases. At x = 40 and 50%, phases of titanium aluminide and aluminum nitride appear. It should be noted that the equilibrium phase composition given in Table 2 shows the state of the system immediately after the combustion under the condition that the combustion temperature equals the adiabatic value. As the sample is cooled, the evolution of the microstructure and the phase composition of the product inevitably take place (the so-called secondary structure formation). For this reason, the composition of the final material should be differing. We can expect the mutual solubility of TiC and TiN with the formation of titanium carbonitride most probably, of the nonstoichiometric composition (taking into account the excess of titanium in the system).
|Calculated composition of final products at the adiabatic temperature, %|
From the results of a thermodynamic calculation, we can make an important conclusion that silicon nitride completely transforms during SHS. It decomposes onto the elements, which react with titanium forming the nitride and silicide phases. Since Si3N4 is a refractory compound, this phase is often considered as inert additive not entering into any reactions. However, due to the higher chemical affinity of titanium with nitrogen and silicon, silicon nitride can be used as a reagent.
The experimental values of the combustion parameters for mixtures with x = 10; 20 and 28,1 % at the initial temperature equal to room temperature are presented in Table 3.
|Тc, К||Uc, cm/s|
At initial room temperature and x = 10, 20, and 28.1%, the combustion proceeds in the self-oscillation mode, while we failed to initiate the combustion at all at higher x (40 and 50%).
It is evident from Tables 2 and 3 that the experimental combustion temperature is lower than the calculated adiabatic temperature by 300–400 K on average, which is associated with heat losses for heating of the surrounding environment. It should be noted that, at T0 = 293 K (20°C), the sample with x = 28.1% is incompletely combusted, so, it does not allow us to measure the combustion temperature.
The dependence of the combustion rate, measured by photodiode light indicator directly during the process of forced SHS pressing, from the composition of the initial reaction mixtures is shown in Fig. 1. It is seen that Uc is almost invariable while the mixture parameter is varying in the range of 10–28.1 %. With further increase of the parameter X (40 and 50%) the combustion rate decreases significantly.
The combustion rate of the three-layered briquettes under the conditions of the quasi-isostatic compression is considerably higher than the combustion rate of homogeneous cylindrical samples in the reaction chamber. Obviously, that one of the causes of this phenomenon is the additional heat coming from the “chemical heater”. We also cannot exclude the influence of the convective heat and mass transfer, which can intensify the heat transmission in the billet pores under the pressing conditions.
The results of an X-ray phase analysis of the compact synthesis products based on TiCyNz, Ti5Si3, and TiAl3 are presented in Table 4. It can be seen that the phase composition and their quantitative ratio change when the mixture parameter is varied. For x = 10, 20, and 28.1%, the predominant phase is titanium carbonitride TiCyNz, which is formed due to the chemical interaction between titanium, carbon and nitrogen, which is evolved during the decomposition of silicon nitride. As the mixture parameter increases, the TiCyNz content in the synthesis products decreases from 55 to 48 %. In addition, we identified the phases of the intermetallic compound TiAl3 and titanium silicide Ti5Si3.
|Phase in the samples composition||Mixture parameter x, wt %|
At x = 28.1%, also the Ti3SiC2 phase presents in amounts to 10%, while at x = 40 %, its content increases up to 39 %. The phase composition of the products at x = 40 % also includes the TiAl3 intermetallic compound (39%) and the Ti5Si3 and TiCyNz phases (13 and 9%, respectively).
The phase composition of the synthesis products with x = 50% has the strongest distinctions when compared with other samples under study. The presence of silicon nitride in almost the same amount as in the initial green mixture, as well as the TiAl2 intermetallic compound, indicates the incompleteness of the chemical reactions as a result of the incomplete combustion. The main phase is an intermetallide TiAl3 with a content of 47 %. In addition, a small amount of nonstoichiometric titanium carbide (7%) with a lattice period of 0.4309 nm and titanium silicide Ti5Si3 (8%) are found.
Generalization of the data of an X-ray phase analysis allows us to conclude that increase in the mixture parameter from 10 to 50% lead to decrease in the content of the ceramic phases TiCyNz and Ti5Si3 and to increase in the content of the metallic phase TiAl3.
Figure 2 shows the microstructures of the materials with various mixture parameters (magnification ×10000).
At x = 10%, the structure consists of titanium carbonitride grains (the average size of ~1 μm) and TiAl3 and Ti5Si3 binder phases. As x increases from 10 to 20%, the titanium carbonitride grains become finer to 0.5 μm. From a comparison of microstructures of the samples at x = 28.1 and 40 % can be seen that, there is no changing in the grain size of TiCyNz due to x increasing. In addition to titanium carbonitride, titanium aluminide and titanium silicide, the structure of this samples also contains the Mn+1AXn phase [3-5] of the Ti3SiC2 composition in the form of ~300 nm thick characteristic layers. The phase interfaces in the sample with x = 50 % are strongly spread, this is associated with the incompleteness of the chemical reactions during synthesis.
The physical and mechanical properties of the obtained materials, namely, the hardness, ultimate bending strength, and elasticity modulus, as well as the hydrostatic and true (measured using a helium pyknometer) densities, residual porosity, and ultrasonic rate in the bulk material, are given in Table 5.
|x, wt %||ρhydr., g/cm3||ρt, g/cm3||Pres, %||С, m/s||HV, GPa||σbend, MPa||E, GPa|
At x = 50%, the obtained material has an increased brittleness, so its strength properties were not measured.
It is evident from the measured data of the ultrasonic rate that the sample with the mixture parameter of 28.1 % has fewer defects, while the highest defect concentration is observed for the composition with x = 50 %. These results completely agree with the characteristics of the residual porosity and strength. Since the residual porosity of the sample with x = 28.1 % is 0.5 %, while for other compositions (excluding x = 50%), it varies in the limits 2–4 %.
Based on the results of hardness measurement, we can see that, with increasing X from 10 to 50 % the value of HV decreases from 10.3 to 7.4 GPa. This is associated with a decrease in the content of the hard carbonitride phase. The obtained values of hardness are fully comparable with the hardness of the carbide and nitride based ceramics, as well as of the classic hard alloys . The sample with x = 28.1 % has the highest strength. No direct dependence between the elasticity modulus, residual porosity, and mixture parameter is found.
The results of heat resistance tests for the materials based on TiCyNz, Ti5Si3, and TiAl3 are presented in Fig. 3. The values of their specific oxidation rate in air at T = 1173 K and τ = 30 h are given in Table 6.
|x, wt %||Specific oxidation rate, g/(m2×s)||Δm/S, g/m2|
Oxidation process follows the parabolic law when the growth of the oxide film is limited by the diffusion of oxygen through the oxide layer. The material synthesized at x = 40% has the lowest oxidation rate (4.1×10–5 g/(m2×s)), which is explained by a high content of highly heat resistant TiAl3 and Ti3SiC2 phases. It should be noted that oxidation rates of other samples in the system under study are very close to this best result, except for the sample with a mixture parameter of 50 %.
Developed ceramic materials based on titanium carbonitride, titanium silicide, and titanium aluminide (except material with X = 50 %) were used for production by forced SHS pressing technology of experimental disc and segmented planar targets for ion-plasma deposition (magnetron sputtering) of multifunctional nanostructured coatings. The disk targets are shown in Fig. 4.
The modern views about the features of the synthesis of few interesting classes of the systems based on titanium carbonitride, silicide, aluminides, and Mn+1AXn phase are considered in this work.
The experimental works described in the chapter were carried out due to financial support from the Federal Target Program “Scientific and scientific-and-pedagogical personnel of an innovative Russia” for 2009–2013 (State Contracts no. 02.740.11.0133, and 02.740.11.0859), as well as by the Program of creation and development of the National University of Science and Technology “MISIS”.