Research on the microstructure and properties of iron based alloy coatings based on gradient transition

2.1. Gradient materials

In the preliminary calculation of this paper, Norem02 alloy, a typical iron-based alloy material for nuclear-grade valves, and F304 stainless steel, a valve material, were selected as calculation materials. The two material compositions are shown in Table 1. Using JMatPro software, the equilibrium phase, carbide, characteristic parameters (density, linear expansion coefficient, thermal conductivity, Young's modulus, Poisson's ratio and specific heat capacity) and Physical and chemical properties (strength, hardness). According to the evaluation, the two mixtures of 30%F304+70%Norem02 and 60%F304+40%Norem02 can be used as candidate gradient materials. Mix F304 and Norem02 powders prepared in proportion, and grind them in a ball mill for 16 hours to obtain a mixture with a particle size of 50 mesh as the test material. The composition of the test materials is shown in Table 2.

2.2. Coating preparation

APS-3000K plasma spraying machine is used to spray coating on F304 stainless steel base material with thickness of 6 mm. The coating arrangement is shown in Fig. 1. The F000 sample does not contain a gradient layer and all use Norem02 powder. F330 contains one gradient material F3, and F630 contains two gradient materials F6 and F3. Before spraying, the surface of the substrate is first degreased, and then the surface is roughened by sandblasting. Spray current is at 450 A, spray voltage 60 V, spray distance 110 mm, powder feeding speed 25 g/min, the main gas flow rate 40 L/min, and the gas pressure 0.6 MPa, the secondary air flow rate 1.0 L/min and the gas pressure 0.6 MPa. The single layer thickness of the coating is 0.2 mm, and each sample is sprayed with 3 layers, with a total coating thickness of 0.6 mm. The morphology of the actual prepared sample is shown in Fig. 2.

3.1. Microstructure observation

A fast-moving wire electric discharge cutting machine was used to cut the sample for microstructure observation perpendicular to the scanning direction of the sprayed layer. After the samples were ground and polished, the chemical wetting method was used to display the structure, using aqua regia as the substrate etchant, and nitric acid solution as the coating etchant. A Zeiss Axio Observer A3 inverted microscope and a Zeiss Sigma 300 field emission scanning electron microscope were used to observe the microstructure, and a Rigaku Rapid IIR micro-area XRD diffractometer was used to analyze the phase.

The cross-sectional morphologies of F000, F330 and F630 coatings are shown in Fig. 3, Fig. 4 and Fig. 5, respectively. The coating has a wavy layered structure as a whole, consisting of highly flattened deformation particles overlapping with each other. There is a micro-metallurgical combination between the coating particles, the particles are basically spread out, and the coating is relatively dense.

Fig. 1Coating layout and sample number

Fig. 2Morphology of sprayed sample

Fig. 3Microstructure of F000 sample

a) Fusion line area

b) Second layer

c) Third layer

Fig. 4Microstructure of F330 sample

a) Fusion line area

b) Second layer

c) Third layer

Fig. 5Microstructure of F630 sample

a) Fusion line area

b) Second layer

c) Third layer

There is a relatively obvious black substance between the substrate and the first layer of coating (Fig. 3(a), Fig. 4(a), Fig. 5(a)), and there are also small black oxides between the coatings (Fig. 3(b), Fig. 4(b), Fig. 5(b)). Energy spectrum analysis shows that they are iron oxides. Although the surface of F304 was sandblasted before spraying, during the spraying process, part of the oxide is involved in the interior of the particle during the particle surface and internal convection, while another part of the oxides gathered and solidified on the particle surface to form a thin oxide layer. The molten particles mixed with oxides spread and solidify on the substrate, forming coatings containing oxide inclusions between flat particles and within the flat layer. There are still a small amount of incompletely melted raw material powder in some coatings (Fig. 3(c), Fig. 5(c)). These powders have insufficient deformation and are in an unmelted (Fig. 5(c)) or semi molten state.

3.2. Hardness measurement and analysis

The hardness measurement was conducted using the MH-5 digital microhardness tester, and the measurement results are shown in Fig. 6. Among them, measuring point 1 is located on the spraying surface, and measuring points 2 to 20 are located on the vertical cross-section of the coating. Measure point 2 at a distance of 50um from the coating surface, and measure every 50 um thereafter. From Fig. 7, it can be seen that the surface hardness of the coating is significantly higher than that of the coating cross-section. When there is no gradient transition material (F000 sample), there is a sharp decrease in hardness between the coating and the substrate, while when gradient materials are used, the difference in hardness between the coating and the substrate is significantly reduced. Comparing the gradient transitions of F3+F3 and F6+F3, the hardness of the F330 transition layer is slightly higher than that of F630.

The hardness of the coating cross-section is related to the type and quantity of carbides in the coating. During the preliminary simulation calculation, the content of carbides in F3 was significantly higher than that in F6. Under the same other conditions, the carbide content was the reason why the hardness of the F330 gradient layer was slightly higher than that of F630.

Fig. 6Hardness measurement results of different deposited layers

3.3. Wear resistance test

Conduct high-temperature wear test using HT-1000 high-temperature friction and wear testing machine. The test time was $t=$ 30 min, the rotation speed was $w=$64.9 r/min, the friction track radius was $r=$3.5 mm, and the load was $F=$15 N. The sample was heated from room temperature to 350 °C and 400 °C for 7 minutes and 8 minutes, respectively, before entering the insulation stage. For SiN balls with a diameter of 6.0mm, there is no lubrication. The wear rate is calculated according to Eq. (1):

where: $W$ – wear rate, mm³/ (N·m); $F$ – riction load, N; $V$ – wear volume, mm³; $L$ – friction distance, mm.

The friction and wear results shown in Fig. 7. Fig. 8 are the cross-sectional profile of the friction and wear track of the sample at 400 °C high temperature. From Fig. 7 and Fig. 8, it can be seen that at 400 °C, the wear rate of coatings containing gradient materials is similar to that of coatings without gradient materials; At 350 °C, the wear rate of non gradient coatings is slightly lower than that of coatings with gradient transitions. It shows that the wear resistance of the coating can be improved after the gradient coating is adopted. According to literature [8], the abrasion resistance of plasma spraying coating is not only related to the macro hardness, but also related to the plasticity, brittleness and cohesive strength of the coating. A coating with a good combination of toughness and brittleness has higher wear resistance. The mechanism by which this gradient material affects the wear resistance of the coating is a topic that needs to be studied in the future.

Fig. 7High temperature friction and wear test results

Fig. 8Cross-sectional profile of worn track at 400 °C