A martensitic stainless steel, X17CrNi15-2, was used in this study, which is a favorable material in liquid-end manifold of reciprocating pump due to its excellent hardness, tensile strength and corrosion-resistance [7]. The material properties of the X17CrNi15-2 are shown in Table 1.

The tested specimens used are standard SEN B3 precracked specimens, with dimensions in accordance with ISO 12108-2018 requirements [8]. These specimens were fabricated from a forged block with dimensions of 400 × 500 × 800 mm,

of which the hardness, i.e. ~ 45HRC, is obtained by head treatment. A pre-crack of 5.5 mm in length was machined using the electric discharge machining method [9] [10]. All of the SEN B3 specimens have the same initial crack length a₀ = 11.85 mm, the thickness B = 40 mm, and the width W = 80 mm. The configuration of the SEN B3 precracked specimen is shown in Figure 1.

The ceramic coating deposition surface treatment [6] was carried out, where some of the test specimens were immersed in a ceramsite solution containing TiO₃, Na₂B₄O₂ and other materials at 150˚C and cured at room temperature. This technique can enhance wear resistance of the specimen surfaces, without the decrease in corrosion resistance of the stainless steel.

The test rig, used for the testing of the SEN B3 specimens, is INSTRON1343 electro-hydraulic servo universal testing machine, as shown in Figure 2. Besides the framework and the base, the test rig consists of two servo-hydraulic actuators to provide axial load, which is suitable for the tension-tension testing of the specimens. The automated control of the test rig was achieved by a digital controller on a computer. In-phase sinusoidal load profiles, with a frequency of 50 Hz, and an axial load with a stress ratio of 0.1, were applied to the fatigue crack growth tests. A crack-opening-displacement (COD) technique was used to measure crack lengths in SEN B3 specimens for the determination of fatigue crack growth rates. A total of six tests were carried out: three tests for parent material specimens and three tests for the ceramic coated specimens. A constant maximum axial load, i.e. 60 kN, was applied in all cases.

According to ISO 12108-2018 [8], the stress-intensity factor K, of SEN B3 specimens, can be calculated using the applied forces and the corresponding crack lengths obtained from the fatigue crack growth tests, using Equation (1):

where F is the applied force, B is the specimen thickness, W is the specimen thickness measured from the reference plane to the specimen edge, g (a/W) is stress-intensity factor geometry function, which can be obtained using Equation (2):

where α = a/W. a is the crack length and W is the specimen width, as shown in Figure 1.

Hence, the stress-intensity factor range ΔK, can be calculated by Equation (3):

where K_max and K_min are the maximum and minimum stress intensity factors, corresponding to maximum and minimum applied forces, respectively.

Figure 3 shows the change in crack length along the test cycles for the parent and coated material specimens. There were no significant scatters observed through the fatigue life, N_f, for coated material specimens as compared to the parent material specimens.

The fatigue life, N_f, varies from 139,000 to 224,000 for the parent material specimens which is about 181,000 for the coated material specimens. The test matrix for the SEN B3 specimens is shown in Table 2.

The fracture surfaces for a selection of the two types of specimens are shown in Figure 4. It is found that no crack tunneling occurs for both the parent material specimen and the coated material specimens.

The experimental data were processed based on the standard of ISO 12108-2018 [8] and invalid data were excluded. The crack growth rates were obtained by the use of the seven-point incrementing polynomial method. The crack growth rate, da/dN (in mm/cycle) are characterized by the Paris law [11], a well-known power law equation. As a result, the crack growth rate, da/dN, can be correlated with the stress intensity factor range, ΔK, which is shown in Equations (4) and (5), for parent material and coated material specimens, respectively. The experimental data can be well fitted in the characteristic equations, as shown in Figure 5.

for the parent material SEN B3 specimens and

for the coated material SEN B3 specimens.

The microstructure of a typical ceramic-coated layer was observed on the cross-section area of the tested specimen, which is shown in Figure 6. It can be seen that the cross-section consists of three distinct regions, i.e. the ceramic-coated region, the transition region and substrate region. The thickness of the ceramic-coated layer is about 200 μm. Figure 7 presents the hardness profile

which is carried out at the top of the coating surface and across the substrate region. The maximum hardness value of the ceramic-coated layer is about 870 HV while the hardness for the X17CrNi15-2 substrate is only 270 HV. The hardness profile shows that the ceramic-coated surface results in an increase in hardness.

In order to determine the mechanisms and chemical compositions associated with each phase of the fatigue damage process, SEM and EDS analyses were carried out on both parent material and coated material specimens. Figure 8 shows the SEM images of fatigue fracture surfaces of the parent material specimen at different stages, namely crack initiation, crack propagation and final fracture. It is found that the fatigue behaviour dominates the crack initiation and propagation stages for the parent material specimen. The fracture mechanism of the final fracture stage is dimple fracture, as shown in Figure 8(c). The SEM images of fatigue fracture surfaces of the parent material specimen at different stages as shown in Figure 9. Similar results are obtained for the coated specimen during the crack initiation and propagation stages. However, the final fracture for the coated specimen is caused by the combination of fatigue, ductile fracture and cleavage, as shown in Figure 9(c). Figure 10 shows the EDS analyses of the parent material and coated material specimens at the final fracture stage. It is found that the parent material specimen shows higher levels of C at matrix regions along with Fe- and O-rich regions, compared with the coated material specimen.