3.1. XRD of Feedstock Powders and DGS Deposited Coatings
Figure 1a–c show the results of XRD analysis for steel, St-Fe
3C and St-SiC feedstock powders and the corresponding DGS deposited coatings.
Figure 1a shows clearly that the steel powder mainly contains one major phase of FeCrMo. The wide peaks in the XRD pattern of steel powder indicate the existence of an amorphous structure. Usually, the powder produced using gas atomization possesses some degree of amorphization [
13,
35,
36]. The steel coating deposited via DGS was characterized by an almost fully amorphous structure with some small peaks belonging to the FeCrMo crystalline phase as shown in
Figure 1a.
The St-Fe
3C initial powder exhibited more complex phase composition compared to pure steel powder as presented in
Figure 1b. The St-Fe
3C powder consists of the γ-Fe, and Fe
3C phases, in addition to the FeCrMo phase. The phase composition of St-Fe
3C coating consists the same γ-Fe, Fe
3C, and FeCrMo peaks, confirming that the DGS deposition process had no significant effect on the phase composition of the initial powder. In addition to the major phases, some negligible traces of WC were observed in the XRD pattern of St-Fe
3C coating. It shows that formation of some amorphous phases may have occurred during DGS deposition as well.
It is clear from
Figure 1c that the initial powder of St-SiC consists of two major phases, SiC and FeCrMo. It is worth mentioning that all peaks were sharp, which was an indication of crystallinity in the initial powder. In addition, it seems that the intensity of the SiC and other detected phases could overpower the major peaks that belong to steel as seen in
Figure 1a,b. The XRD analysis of the coating revealed that a large amount of amorphous phase was formed during the DGS deposition process. Previous studies have also confirmed formation of such amorphous phases possibly due to rapid cooling during thermal spraying processes where they could not detect any distinguished peaks from crystalline phases in deposited coatings of similar materials [
36,
37]. Despite SiC and FeCrMo phases, some peaks from FeC were also detected in the XRD pattern of St-SiC coating.
3.2. Microstructure (Feedstock Powder and Coatings)
The morphology and particle size distribution of the steel powder are shown in
Figure 2a. The dominant powder geometry was spherical; however, some elongated particles were also observed in the microstructure that are indications of gas atomized particles [
36]. An average particle size was measured to be 29.4 ± 17 μm as shown in
Figure 2b.
The actual chemical composition of the powders was measured using EDS analysis. Since the initial feedstock powders in this study were produced by mixing different materials, to better distinguish particles and composition distribution, the results of point detection are listed in
Table 2. It is worth mentioning that the EDS was not capable of identifying oxygen in the coating composition due to the detection limit of the EDS equipment.
The SEM images of an unetched cross-section of the steel coating are presented in
Figure 3. The lamellae-shaped microstructure, which is typical for thermally sprayed coatings, is present. The change of contrast is clear in
Figure 3 at different layers of the microstructure, which is an indication of differences in phases or chemical compositions. The brightest areas (
Table 2, points two, and four to seven) were enriched by tungsten, the amount of which was varied from 5.02 to 12.58 wt.%. While the gray areas (
Table 2, points one and three) were characterized by a higher amount of Fe compared to the other areas. The darkest regions in the microstructure are identified as pores. The measured porosity of steel coating was 4.95 ± 0.70 Vol.%, indicating the deposited coating was dense and of high quality.
The morphology of the pores varied from round shape to irregular ones (
Figure 3b). It has been found that the sphere-shaped pores were mostly due to the gas entrapment during impact, flattening and solidification of splats [
4], and the elongated pores were formed mostly between splats along with microcracks. Generally, the development of microcracks has been closely related to the high internal stress in the coating due to high in-flight particle velocity or high temperature gradient during the solidification process [
3,
4].
Figure 4 presents the morphology and particle size distribution of St-Fe
3C powders. Most of the powder particles are in spherical shape, and there is a small amount of elongated ones. The particle size distribution was characterized by a relatively wide profile as shown in
Figure 4b. The average powder size of 21.6 ± 10 μm was slightly smaller than the steel powder size (29.4 ± 17 μm). The chemical composition analysis performed on the points F, G and H indicated similar composition as the steel powders as shown in
Figure 4a and
Table 3. However, traces of Fe carbides were also detected at points H and I as listed in
Table 3.
Figure 5 shows the SEM images of the unetched cross-section of the St-Fe
3C coating. The DGS resulted in formation of well-defined layered structure identified by different contrasts (
Figure 5a,b. The difference in chemical composition of the phases was identified using EDS analysis. The point one region was found to be enriched by Fe (~97 wt.%) with a small amount of Cr and C (
Table 3), i.e., this phase could represent iron carbide. The light gray regions (points two and three) basically represent the steel powders according to the measured chemical composition (
Table 3). Region marked as three was enriched by W identified by lighter contrast compared to region two as shown in
Figure 5b. The middle gray regions were characterized by increased iron content of more than 70 Wt.%. The formation of iron-enriched regions may be due to the interaction of molten steel and iron carbide powders during the deposition process. In addition to some unmelted iron carbide as marked in
Figure 5a, the pure WC was detected in the microstructure as well (
Table 3, point seven). The size of these WC particles varied from 1 to 20 μm (inset in
Figure 5b). Again, black regions were identified as pores in (
Figure 5a,b). These pores were characterized by irregular shapes with an average size of 5 ± 1.0 μm and there were some with elongated shapes along the splat boundaries. The porosity of the St-Fe
3C coating was 2.24 ± 033 vol.%, which is approximately half of that of the steel coating. It appears that small Fe
3C particles could fill the voids and pores in the microstructure. Iron is the common element, which could result in a higher diffusion rate and, consequently, formation of a strong bond between matrix and reinforcement phases.
The SEM micrograph of St-SiC powder and particle size distribution are shown in
Figure 6a,b. The initial powder contained two clearly distinguished types of particles, spherical shapes in light gray color, and irregular shapes in dark gray. The light gray particles were mixtures of several elements (points K, N and O in
Table 4), while the small dark gray particles were found to be pure silicon carbide (points M and L in
Table 4). The particle size distribution was concentrated in the range from 2 to 30 μm and the average size of the particles was 13.9 ± 5 μm (
Figure 6b), indicating the influence of smaller SiC particles. The inset of
Figure 6b is the particle size distribution of SiC particles in St-SiC feedstock powder.
The microstructure of St-SiC coating contains unmelted particles surrounded by matrix as labeled in
Figure 7a. The unmelted particles appear to be in spherical shapes mainly consisting of steel particles. Note here that the matrix was characterized by uniform atomic number contrast in comparison with a matrix of steel and St-Fe
3C coatings, where more variations of gray scale were observed (
Figure 3 and
Figure 5).
The EDS analysis at points two, four, five, and seven revealed negligible differences in actual chemical composition (
Table 4). The dark regions (points one, three, and six) were identified as SiC in irregular shapes with an average size of 2.95 ± 1.72 μm, close to the average size of 2.53 ± 1.4 μm for SiC in the inset of
Figure 6b. The porosity of the St-SiC coating was 1.07 ± 0.44% vol.%, which is approximately half of that of the St-Fe
3C coating.