## 3 Results and Discussion
Analysis of the diffraction patterns presented in Figure 6 shows that the increase of condensation temperature leads to the change in the structural phase of the Ta coatings. Figure 5: AFM images of the Ta coatings on the aluminum temperature gradient substrate. Condensation temperature: (**a**) 300–350 \({}^{\circ}\)C, (**b**) 390–420 \({}^{\circ}\)C, and (**c**) approximately 450 \({}^{\circ}\)C.  
Intensity redistribution of the diffraction peaks indicates changing in the mass fraction of phases. In particular, with the rise in condensation temperature, an increased intensity of peaks corresponding to Al is observed, which is associated with multiplying of surface cracks on the coatings. With rising of the condensation temperature, a decrease in the number of reflections, as well as their intensity corresponding to pure Ta, is observed. The intensity of reflections from the Ta\({}_{2}\)Al intermetallic compound also decreases with increasing condensation temperature and practically disappears at 450 \({}^{\circ}\)C [7]. It should be noted that XRD data correlates with the data obtained during the study of the elemental composition. Figure 7 shows that Al content increases and Ta content decreases with rising of the condensation temperature of the tantalum on an aluminum substrate. As indicated above, the relative increase of Al content is associated with crack formation and diffusion of Al from the substrate into the Ta-coating when raising the condensation temperature. More detailed analysis of diffraction patterns shows that a full width at half maximum (FWHM) of main Ta diffraction maximums rise steadily with the increase in condensation temperature, pointing to the deposition of Ta coatings with a high degree of crystalline grain dispersion at high temperatures in the reactive-diffusion zone [7]. Such a complicated situation is expected during condensation of tantalum, a refractory element, - on an aluminum surface. According to the references and the results of personal studies [8], the formation of metastable or stable intermediate chemical compounds, characteristic for this system, cannot be  
Figure 6: X-ray diffraction (XRD) patterns of a Ta coating on a gradient aluminum substrate. Condensation temperature: 300–350 \({}^{\circ}\)C (red), 390–420 \({}^{\circ}\)C (blue), and approximately 450 \({}^{\circ}\)C (green). Figure 7: Dependence of the elemental composition (at %) of Ta gradient coating on an aluminum substrate formed at different temperatures. ruled out upon reaching the appropriate concentration conditions of the components, notwithstanding low condensation temperatures. It was found that with the simultaneous occurrence of solid-phase and reactive-diffusion processes, the formation of intermediate chemical compounds is possible by passing from one state to another through a metastable phase of a transitional composition [9; 10; 11]. Under conditions of great hypothermia and nonequilibrium crystallization, characteristic for vapor-phase technology, the formation of non-controlled, unpredictable metastable phases is possible. The situation at the substrate-coating interface is constantly changing in accordance with the change in technological parameters and the thickness of the condensed film. Therefore, the metastable compounds can be formed or disappear interacting with each other or with other reagents. Based on the above mentioned, it must be assumed that the formation of a highly dispersed structure is a result of conversion of the Ta\({}_{2}\)Al to another chemical compound, being in an X-ray finely dispersed phase. This is also suggested by the fact that with the increase in condensation temperature the microhardness of the coating is continuously increased. Figure 8 shows the dependence of microhardness of Ta coatings and of the physical broadening of the X-ray spectral line (110) (full width at half maximum FWHM) on condensation temperature in the diffraction patterns of Ta.