Detailed investigations of the phase composition and structure of the initial powder of the Fe66Cr10Nb5B19 alloy with a particle size of 45–75 µm, which was used for detonation spraying in the present work, were shown in our previous work [25]. Powder had a spherical shape and high glassy phase (~ 69 wt.%). The glass transition temperature and crystallization temperature of the Fe66Cr10Nb5B19 alloy were determined as Tg = 794 K and Tx = 846 K, respectively. The melting point of the alloy was molten at Ts = 1169°C. The present quaternary alloy shows a high glass-forming ability with a wide supercooled liquid region (ΔT = Tx - Tg) equal to 52 ºC. For example, the alloys Fe81Si1.9B5.7P11.4 [26], Fe67Nb4Si5B14 [27] and Fe48Mo14Cr15Y2C15B6 [28] showed ΔT equal to 32 ºC, 45 ºC and 69 ºC respectively, the latter containing considerable content of expensive alloying elements such as Mo and Y.
In our previous studies [17, 25], the detonation spraying modes of Fe66Cr10Nb5B19 powder alloy with a particle size of 45–75 µm were determined. It was shown that using 50–60% of an explosive charge at a molar ratio O2/C2H2 of 1.1 allowed to obtain high-quality dense coatings with high content of the glassy phase (less 2 wt.% of crystalline phase).
In the presented work, coatings with a thickness of 500 µm were obtained using 40–70% of explosive charge at a ratio O2/C2H2 of 1.1, on carbon steel substrate (grade St3, GOST 380–2005, analog to ASTM A570). During spraying, the substrates were cooled by compressed air. The content of the crystalline phase in the coatings obtained at 40%, 50%, 60%, and 70% of explosive charges was 2.5 wt.%, 2 wt.%, 1.5 wt.%, and 1 wt.%, respectively, as determined from the X-ray diffraction patterns by the Rietveld method using TOPAS 4.2 software (Bruker AXS). Except for the coating obtained at 40% of an explosive charge, where the porosity was ~ 3%, the porosity of other coatings was less than was less 1%. Bonding strength of Fe66Cr10Nb5B19 coatings measured by pin test method was 150 MPa [25].
Previous studies of corrosion resistance of Fe66Cr10Nb5B19 detonation coatings with glassy structure showed that the coatings had a high resistance to atmospheric [29] and electrochemical corrosion [30]. Developed coatings can be recommended as protective coatings in aggressive environments. Due to the high corrosion resistance of detonation coatings, stainless steel (grade 12Cr18Ni10Ti, GOST 2590 − 2006, analog to AISI 321) was chosen as a reference material in present work.
The microhardness of the coatings was measured using a semi-automatic Wolpert 402 MVD microhardness tester at a load of 300 g. The standard wear tests (according ASTM G 133-05) of the Fe66Cr10Nb5B19 detonation coatings and stainless steel were performed at room temperature with a ball-on-flat universal wear machine (UMT-2, Bruker) operating in dry linearly reciprocating sliding mode, using a WC–6Co ball of 6.35 mm of diameter as a counterpart. The counterpart under applied load was slided against the flat specimen of the coatings. Before the wear tests, the coatings and stainless steel surfaces were polished to a mirror finish. The ball and the tested samples were degreased with acetone and dried in the air before testing. Table 1 shows the parameters of dry linearly reciprocating dry sliding conditions. The coefficient of friction (COF) was analyzed for reciprocating dry sliding tests. After the wear tests, the volume loss data were estimated by optical profilometry (Contour GT-K 3D, Bruker) and analyzed with Vision64 software. The microstructure and morphology of the worn surface after wear testing of the coatings were studied by scanning electron microscopy using an EVO50 XVP microscope (Carl Zeiss). The wear resistance (Rw) of the coatings and stainless steel was calculated by the equation [31]:
Table 1
Parameters of dry linearly reciprocating sliding conditions.
Parameters
|
Values
|
Sliding speed, m s− 1
|
5
|
Stroke length, mm
|
5
|
Applied load, N
|
25
|
Sliding distance, m
|
100
|
where N is the applied load, S and Vw are the total sliding distance and the worn volume, respectively.
Nanoindentation and nanoscratch tests were carried out using a scanning nanohardness tester Nanoscan 4D+ (TISNCM, Russia) with a standard three-sided pyramid Berkovich tip (an angle between the axis and a facet of 65° and tip radius 200 nm). For nanoindentation tests normal load 100 µN, loading rate of 10 µN/s and peak load hold time of 2 s were applied. The average nanohardness reported here was obtained from the 10 × 10 grid of indents (100 indents). Nanoscratch tests were conducted under rumping loads from 10 µN to 100 µN at a loading rate of 10 µN/s at a traverse speed of 0.5 µm s− 1 over a scratch length of 100 µm. After the scratch test, the wear tracks were investigated by optical profilometry coupled to Vision64 software for calculation volume loss.