**2. Materials and Methods**

As a substrate in this work, we used monocrystalline Si (100) with the size of 10 mm × 10 mm × 0.38 mm. The diamond–iron system preparation is schematically shown in Figure 1. The samples were ultrasonically cleaned with acetone prior to diamond layer deposition.

**Figure 1.** A schematic illustration of the experimental procedure: (**a**) Diamond deposition; (**b**) Iron deposition; (**c**) Annealing.

Polycrystalline diamond coatings were deposited in the self-made hot filament chemical vapor deposition reactor using a hydrogen–methane mixture (ratio of H2:CH4 = 50:1) with a total gas flow rate 106 mL/min (Bronkhorst EL-FLOW, Bronkhorst High-Tech B.V., Ruurlo, The Netherlands). The distance between tungsten filaments (Ø = 0.16 mm) and substrates was 10 ± 0.5 mm. The substrate temperature during the deposition was maintained at 800 ± 25 ◦C using an infrared thermal imager (ULIRvision TI170, ULIRvision Technology Co., Ltd., Zhejiang, China). The pressure in the reactor during the deposition was maintained at 20 ± 1 Torr (Pfeiffer Vacuum CMR 372, Pfeiffer Vacuum, Annecy, France) and the current at 6.5 ± 0.01 A per filament, see Figure 1a. The total diamond film thickness was 2 ± 0.1 μm.

After diamond layer synthesis, samples were placed in the vacuum chamber. The base pressure in the evacuated chamber was 8 × <sup>10</sup>−<sup>6</sup> Torr. Prior to Fe deposition, samples were cleaned by an Ar<sup>+</sup> ion source with an energy of 3.5 keV.

Fe films were deposited by the two-step thermal evaporation process, see Figure 1b:


After that, samples were annealed under vacuum conditions during 30 min at fixed temperatures in the range of 400–800 ◦C, see Figure 1c. In order to determine the interaction behavior between CVD diamond and evaporated Fe films, we used cross-sectional scanning electron microscopy, XRD, Raman spectroscopy, and EDX measurements. This combination of techniques allows for clarification of the interdiffusion process in the CVD diamond–Fe system at elevated temperatures and for the determination of the temperature starting point of the diamond graphitization. The coatings cross-sectional morphology was studied using a scanning electron microscope (Vega3, TESCAN, Brno, Czech Republic). Structural characteristics of the coatings were studied using X-ray diffraction (Shimadzu XRD 6000, Shimadzu, Kyoto, Japan) in the Bragg–Brentano configuration with Cu Kα (λ = 0.154 nm) radiation in the range of 2θ = 20◦–90◦, with a sampling pitch equal to 0.01◦, and an integration time of 1 s. Raman spectra were recorded using a NanoScan Technology Centaur IHR spectrometer (NanoScan Technology, Dolgoprudny, Russia) with a 514.5 nm source laser.
