## 3 Results and Discussion  
SEM images of surface morphology and cross-sections of the tantalum coatings deposited at the condensation temperature in the range of 300-450 \({}^{\circ}\)C are presented in Figures 2-4. It is obvious that all research samples have thickness not exceeding 1 \(\upmu\)m. Many surface cracks are observed. Figure 2 shows that the coating, formed in the lowest temperature zone (300-350 \({}^{\circ}\)C), is characterized with initial crack formation. It is also confirmed by a cross-section image of the specimen showing a violation of the integrity of the condensed tantalum layer in relatively smaller areas. Figure 1: Scheme of the vacuum chamber of the electron beam installation. As the condensation temperature rises up to 390 \({}^{\circ}\)C cracks open and the surface continuity is violated. Cross section images show that the discontinuity area of Ta coating is broadened (Figure 3). With subsequent increase in temperature to approximately 450 \({}^{\circ}\)C the surface is covered with tantalum spheres. Accordingly, the cross section shows broadening in the discontinuity area of Ta coating (Figure 4). Atomic force microscopy (AFM) images of the Ta coatings on the aluminum substrate, formed from three temperature zones (\(t_{\text{Cond}}\) = 300-350 \({}^{\circ}\)C, \(t_{\text{Cond}}\) = 390-420 \({}^{\circ}\)C, \(t_{\text{Cond}}\) \(\approx\) 450 \({}^{\circ}\)C), are shown in Figure 5.  
Figure 4: Surface microstructure (200 \(\upmu\)m) and (5 \(\upmu\)m) and cross-section view of tantalum coating on a gradient aluminum substrate (\(t_{\text{Cond}}\) \(\sim\) 450 \({}^{\circ}\)C). Figure 3: Surface microstructure (200 \(\upmu\)m) and (5 \(\upmu\)m) and cross-section view of tantalum coating on a gradient aluminum substrate (\(t_{\text{Cond}}\) = 390–420 \({}^{\circ}\)C). Figure 2: Surface microstructure (200 \(\upmu\)m) and (5 \(\upmu\)m) and view of tantalum coating on a gradient aluminum substrate (\(t_{\text{Cond}}\) = 300–350 \({}^{\circ}\)C). The investigation of the topography of Ta coatings using atomic force microscopy was provided on AFM-microscope Ntegra Spectra Solar (NT-MDT, Moscow, Russia) using a semi-contact regime. The AFM image revealed that grain size decreases gradually with the increase of condensation temperature. It was revealed that the coatings of all investigated samples, taken from different temperature zones, despite the cracks, have excellent adhesion to the substrate (1 on a 4 scale, where 1 is the best rating). The results indicate that cracking of the Ta coating may be caused by the difference in the physical and mechanical properties between the condensed tantalum layer and the substrate, causing strains and residual stresses in the boundary zone between the coating and the substrate, ultimately resulting in decomposition of the condensed film integrity. As for the residual stresses in Ta coatings, they can occur for various reasons; one of them is the difference between the thermal expansion coefficients of the substrate material and the coating. Thus, the thermal expansion coefficient values for Ta vary from \(6.6\times 10^{-6}\) C\({}^{-1}\) to \(8.0\times 10^{-6}\) C\({}^{-1}\) in the temperature range of 20-500 \({}^{\circ}\)C and 20-1500 \({}^{\circ}\)C respectively, while for Al it is equal to \(25.6\times 10^{-6}\) C\({}^{-1}\) in the temperature range of 20-300 \({}^{\circ}\)C.  
The magnitude of thermal stresses also depends on the deposition temperature and rises steadily with the increase of condensation temperature. This is the reason for multiple surface cracks on the investigated Ta coatings when raising the condensation temperature. In accordance with XRD analysis of the Ta coatings on an aluminum substrate, formed in the 300-350 \({}^{\circ}\)C temperature range, a multiphase system consisting of pure Ta (coating), Al (substrate) and intermetallic compound Ta\({}_{2}\)Al with tetragonal cells is revealed.