3.1. Powder Characteristics
In this experiment, the Fe-based powder selected is the gas-atomized amorphous powder. Figure 1
shows the micro-morphology of powder. The surface of the powder particles is relatively smooth, which ensures favorable fluidity for thermal spray processing. The powder particles size was observed by SEM. The result showed that the equal sizes of the powder particles were 10.0–50.0 μm (Figure 1
a). EDS reveals the primary element distributions of the powders as shown in Figure 1
b and Table 2
. It is obvious that the proportion of Fe in powder is the highest; in addition, the proportion of Mo and Cr is 20% and 19%, respectively. Figure 1
c shows the XRD patterns of the powders. Except for a broad amorphous diffraction peak at 44°, there are no peaks of crystalline phases. The broad amorphous diffraction peak illustrates high amorphous phase content of the powders.
3.2. The Microstructure of the As-Sprayed Coating and Annealed Coatings
displays the typical microstructure of a cross-section of the as-sprayed coating and annealed coatings. As seen from the SEM images, all of the coatings have a dense lamellar structure with some finite pores. The reasons that the as-sprayed coatings contain finite pores are mainly associated with spraying process. In the spraying process, the high speed influence of semi-fused particles on the coatings accompanies rapid concretion. During the rapid cooling process, the surface of semi-molten particle shrinks, resulting in the appearance of some finite pores. And the junction of particles can also produce micropores due to rapid cooling. Nevertheless, it is clear that the pores decreased with increasing annealing temperature, this is because the un-melted particles undergo diffusion under the action of sintering.
In addition, cracks can be seen from the cross-sections of the annealed coatings; after heat treatment, nanocrystalline grains can be confirmed in the annealed coatings, and much more nanocrystalline grains exist in the annealed coatings can form crystal defects, which usually may generate the cracks [24
]. It is obvious in from Figure 2
that the Fe-based amorphous/ nanocrystalline coatings have an inhomogeneous structure with black and gray zones. The EDS proved that the black area was an Fe rich area with a small quantity of Cr and Mo. In addition, the Fe-rich phase became blurry with the increase of annealing temperature. Many researchers also proved the inhomogeneous gray area containing iron oxide phase, and the content of the oxide phase increased with annealing temperature increasing [17
]. The variation of microstructure indicated that sintering, diffusion and oxidation occurred during heat treatment.
3.3. XRD and TEM Analysis
shows the XRD patterns of the coatings before and after annealing at 350, 450, 550 and 650 °C for 1 h, respectively. It is clear that the amorphous structures of the coatings did not change much after annealed at 350, 450 and 550 °C. All diffraction peaks of the coatings are low and broad, and it implies that high amorphous phase content in the coatings. After being annealed at 650 °C for 1 h, a crystallization phase appeared, and the broad amorphous diffraction peak disappeared in the coating. After heat treatment at 650 °C, the (Fe, Cr), Cr3
Si and FeO phases were identified.
In order to further study the microstructure of the as-sprayed coating and annealed coatings in detail, a TEM analysis of the coatings was implemented. Figure 4
a is a bright field TEM pattern of the as-sprayed coating, where the selected area electron diffraction (SAED) pattern has a wide diffraction ring. It shows that the amorphous phase is the main one in the as-sprayed coating. After annealing at 350 °C, in Figure 4
b, the SAED of the coatings is a wide diffraction ring which is almost identical to the as-sprayed coating. The amorphous microstructure of the coating has not changed significantly. Figure 4
c shows the SAED pattern of the coating after being annealed at 450 °C, which still holds a diffraction halo ring. This indicated that the amorphous coating has excellent thermal stability. After being annealed at 550 °C for 1 h, the phase composition slightly changes, the amorphous phase is no longer the sole phase in the coating, and some fine nanocrystalline were identified. After annealing, crystalline spots at 550 °C can be proved from the SAED pattern of bright field the TEM image in Figure 4
d, which shows the diffraction ring of the amorphous phase around some sporadic diffraction spots. This showed that the annealing temperature was crystallization temperature and crystallization had appeared. The TEM micrograph and SAED images of the coating annealed at 650 °C for 1h reveal two disparate morphologies. The amorphous phase still exists, but quite a few crystalline spots were found, which confirmed that the coating annealed at 650 °C underwent the precipitation of crystalline phases.
There are some reasons why the amorphous transition temperature occurs at 650 °C. Firstly, in the spraying process, an intermittent spraying method is adopted to ensure that the coating surface is not in a high temperature state for a long time, so that the coating can be quickly cooled and the occurrence of crystallization is inhibited. Secondly, compared with HVOF technology, SPS technology has a higher heat source, which enables the powder to be fully melted and forms a flake structure when the liquid drops hit the substrate. Therefore, it can achieve a higher cooling rate during the solidification process of the droplet, which ensures a higher degree of amorphization [28
]. Thirdly, adding molybdenum and chromium is a very effective way for depressing the fusing temperature and enhancing the amorphous forming ability of the Fe-based alloys [29
]. In addition, the diffusion of atoms is controlled by the heat treatment temperature. At low temperatures, the diffusion of atoms is of a short-range and the amorphous phase does not change significantly. At 650 °C, atoms in the amorphous phase undergo long-range diffusion to form nanocrystalline structures.
3.4. Influence of Annealing Temperature on the Corrosion Resistance
shows the polarization curves of the samples after open-circuit tests in a 3.5% NaCl solution. It can be seen that there are considerable differences in corrosion potential (Ecorr
) and corrosion current densities (Icorr
) between different samples (from Table 3
). The as-sprayed coating shows lower Icorr
and higher Ecorr
than other annealed samples. The Ecorr
of the coatings decreased with increasing annealing temperature, but the Icorr
of the coatings increased.
There are three reasons for these phenomena. Firstly, although the quantity of pores decreased with the increasing annealing temperature, more and more puny cracks are formed (this can be inferred from the Figure 2
). Generally, cracks can help the corrosion medium enter into the coatings, which will lead to faster corrosion rate. The as-sprayed coating with less cracks offers a better barrier to impede the electrolyte attack on the basis material [31
]. Secondly, the amorphous content of Fe-based amorphous coatings decreased after heat treatment. The presence of a nanocrystalline phase created grain boundaries, segregates and crystalline defects, which plays a critical role of corrosion initiation sites [32
]. Thirdly, coatings annealed at different temperature accelerated the existence of loose structure oxides on the coatings’ surface and inside. Generally speaking, the oxides increased with increasing annealing temperature, the vast oxides can loosen the bonding force of the coatings [34
], and the corrosion medium via oxide pore entering into coatings.
shows the microstructure on the surface of all coatings immersed in a 3.5% NaCl solution for 540 min. A small number of holes is observed at the as-sprayed coating surface after the as-sprayed coating is immersed in a 3.5% NaCl solution for 540 min in Figure 6
a. Compared with the surface of the as-sprayed coating, the coating annealed at 650 °C shows distinct cracks (Figure 6
e). The surface morphology of the coating gets worse with increasing annealing temperature. Analyzing the reasons, some important changes occur on the surface of the coating and inside during the heat treatment process, such as coating oxides, grain boundaries or crystalline defects. This series of changes easily delivers paths for a corrosion medium to enter into the coating. The channel effect increases with the increase of the annealing temperature.
In order to clearly understand the influence of heat treatment on corrosion resistance, the effect of heat treatment on the mass loss of the sample immersed in a 3.5% NaCl solution for 540 min was studied (Figure 7
). The result shows that the rate of mass loss increases rapidly with the increasing of annealing temperature. The sample has the largest mass loss when the temperature reached 650 °C. The mass loss of as-sprayed coating is lower than other annealed coatings. The mass loss of the as-sprayed coating and the annealed coatings at different temperatures are 4, 7, 8, 11 and 18 mg, respectively. The mass loss of the coating annealed at 650 °C (18 mg) is 4.5 times that of the as-sprayed coating. The reason for this phenomenon is that the number of grain boundaries, segregates and crystal defect increased with increasing annealing temperature [22
]. Consequently, the mass loss of samples increased with the increase of heat treatment temperature.
Compared with other research results [17
], in this study, coatings fabricated by high Cr-content powder shows lower mass loss, lower corrosion rate and better corrosion resistance. This phenomenon can be explained by the following reasons. Firstly, the coating shows the best corrosion resistance in connection with the formation of the passive film and slight passivation zone. The Cr added into the thermal spraying powders, under the action of heat treatment produced more chromium oxides, and the chromium oxide increased with increasing annealing temperature. A plentiful passive film successfully reduces the corrosion rate. This account can be confirmed by other researches [10
]. In addition, compared with HVOF technology, the SPS technology has a higher heat source temperature to make powders melt more sufficiently and then spread to form a tabular-lamellar structure when droplets strike on the substrate. This compact lamellar structure can effectively resist the entry of corrosive media. Thus, high Cr-content coatings in this study fabricated by SPS show lower mass loss in a 3.5% NaCl solution and better corrosion resistance than others.
3.5. Influence of Annealing Temperature on the Microhardness
shows the microhardness data of the as-sprayed coating and annealed coatings. The as-sprayed coating exhibited the lowest microhardness of about 721 HV among all coatings, while the microhardness values of the coatings annealed at 350, 450 and 550 and 650 °C were 848, 889, 982 and 1018 HV, respectively. The reasons why the maximum value of hardness appeared in 650 °C is as follows. Firstly, after heat treatment, the amorphous phase is partially transformed into a nanocrystalline structure, which disperses in the coating and strengthens it [35
]. Secondly, when the annealing temperature is 650 °C, the rich-Cr, Mo phases and the second phase of Cr3
Si form in the coating; these products play the role of solid solution strengthening [36
]. Thus, it can explain that the annealing temperature plays a significant role in improving the microhardness of the coatings. In Figure 8
, we clearly show the relation of the microhardness and annealing temperature.