In steam turbines, blades operate in a high-speed wet steam environment and are often damaged by combined erosion from liquid droplets and solid particles. To reveal the mechanism of composite modification via high velocity oxy-fuel spraying (HVOF) and laser shock peening (LSP) on improving blade erosion resistance, an accelerated erosion experimental method was designed in this work. Five different processes were proposed, including UT, LSP, UT-HVOF, LSP-HVOF, and HVOF-LSP. The results indicate that compared with UT specimens, LSP treatment induces high compressive residual stress in the surface layer of 1Cr12Ni3Mo2VN stainless steel, which leads to shallower compound erosion pits. Compared with UT-HVOF and LSP-HVOF specimens, the HVOF-LSP specimen has the lowest coating porosity and the highest surface microhardness of 1500 HV0.5, representing an increase of 14.5% and 8.7% respectively. This demonstrates that LSP post-treatment can enhance the load-bearing capacity of HVOF coatings effectively. Microstructural analysis further reveals that the HVOF-LSP specimen presents the shallowest erosion pits and the longest penetration lifetime of the WC coating. Accordingly, the HVOF-LSP treatment can effectively improve the service life and protection performance of materials under accelerated erosion conditions, providing a technical reference for the long-term service of turbine blades. 1. Introduction Surface erosion damage is recognized as one of the dominant failure modes of turbine blades [ 1, 2, 3]. Low-pressure last-stage blades operate in an environment with high steam humidity, where numerous fine water droplets impact the blade surface at high velocities and induce typical liquid droplet erosion [ 4]. At the initial stage, this erosion appears in the form of surface pitting and cratering [ 5]. Meanwhile, solid particle impurities entrained in the high-velocity droplets also strike the blade surface along with the high-speed steam flow [ 6], generating microscopic scratches on the material surface. These scratches, coupled with the liquid droplet erosion, further exacerbate surface degradation [ 7]. Accelerated spallation of the surface material gradually evolves localized surface damage into fatigue fracture, which may eventually cause blade failure [ 8]. Currently, passive anti-erosion strategies are predominantly adopted by mainstream turbine manufacturers worldwide [ 9, 10, 11], with research primarily focusing on blade surface protection technologies. These strategies mainly include Stellite alloy cladding [ 12, 13], high-frequency hardening [ 14, 15], laser hardening [ 16, 17], and protective coating deposition [ 18]. Hlushkova et al. [ 19] investigated the effects of three surface treatment methods on the erosion resistance of turbine blades, including high-frequency current strengthening, T15K6 alloy electrospark alloying, and 15H11MF electrospark alloying. The results showed that high-frequency current strengthening achieved the greatest reduction in blade erosion rate. Short-pulsed laser shock waves can induce grain refinement in the surface layer and generate deep compressive residual stresses, which substantially improve the fatigue life as well as the wear and erosion resistance of metallic materials [ 20]. Ahmad et al. [ 21] evaluated the water droplet erosion resistance of various blade steels, and confirmed that laser-hardened X5CrNiCuNb 16-4 blade steel possesses the optimal erosion resistance. Zhou et al. [ 22] realized surface nanocrystallization of nickel-based alloy blades via LSP. The nanohardness exhibited excellent thermal stability, which effectively enhanced the high-temperature fatigue performance of turbine blades. Gao et al. [ 23] reported that LSP combined with high-frequency induction quenching significantly improves the cavitation erosion resistance of the surface layer of 14Cr12Ni3Mo2VN steel. Mednikov et al. [ 24, 25] adopted pulsed laser surface treatment in combination with protective coatings to construct surface textures, further enhancing the water erosion resistance of blades. Coating technology is widely acknowledged as an effective and reliable surface protection approach [ 26, 27, 28, 29]. Li et al. [ 30] reported that WC-10Co-4Cr coatings possess superior water droplet erosion resistance compared with Stellite coatings, and their performance is less sensitive to impact angle. Lamana et al. [ 31] found that increasing cobalt content enhances the fracture toughness and cavitation erosion resistance of WC-based coatings. Wu et al. [ 32] found that substrate preheating significantly improves the metallurgical bonding strength at the coating–substrate interface. A favorable balance among stress relief, microstructure refinement, and tribological performance can be achieved at a preheating temperature of 600 °C. Wang et al. [ 33] deposited TiN, CrN and multilayer Ti/TiAlN coatings on Ti6Al4V alloy substrates via physical vapor deposition (PVD). The results indicated that the interface between the Ti interlayer and TiAlN layer can disperse tensile stress and crack tip stress, thereby restraining crack propagation and delivering excellent water droplet erosion resistance. By employing HVOF, Wang et al. [ 34] fabricated two types of WC-10Co-4Cr coatings with different particle packing densities on 300M steel substrates. The results revealed that the coating prepared from tungsten carbide powder with a lower particle packing density achieves better erosion resistance. However, most previous studies have mainly focused on the water droplet erosion performance of single coatings or individual LSP treatments [ 35]. The erosion mechanism under the coexistence of solid particles and high-speed water droplets has not been fully clarified. Furthermore, existing research has largely concentrated on single-process treatments [ 9], while investigations on the anti-erosion performance of the combined LSP and HVOF hybrid process remain relatively limited [ 36]. Accordingly, it is of great significance to explore the influence of the combined LSP and HVOF process on the erosion resistance of 1Cr12Ni3Mo2VN stainless steel, which is widely applied to low-pressure last-stage turbine blades. In this work, a hybrid process integrating LSP and HVOF is proposed. The mechanical properties of specimens under five different process schemes are experimentally investigated, and the erosion resistance of 1Cr12Ni3Mo2VN stainless steel and WC coatings under the combined process was explored in equivalent accelerated erosion tests. 2. Materials and Methods 2.1. Materials The feedstock powder used for HVOF was WC-10Co-4Cr thermal spray powder (Luoyang Golden Egret Geotools Co., Ltd., Luoyang city, China), and the mass fractions of its constituent elements are listed in Table 4. The microstructure of the powder was characterized by scanning electron microscopy (SEM), as shown in Figure 1. It can be observed that the powder particles vary in size, with particle diameters ranging from 15 to 45 μm. The particles exhibited regular morphology and high sphericity, along with a rough and porous surface structure. Moreover, the powder presents uniform distribution and good dispersibility, possessing favorable physical characteristics for the formation of dense coatings. 2.2. Methods 2.2.1. LSP Treatment As shown in Figure 2, the LSP experiments were carried out using a LAMBER-12 high-power pulsed Nd:YAG laser manufactured by Beijing Zhuolei Laser Technology Co., Ltd., Beijing city, China Two groups of LSP experiments were designed, as illustrated in Figure 3. Figure 3a presents the single LSP treatment performed on the UT specimens (LSP). A regular spot array was formed on the stainless steel surface, and compressive residual stress was induced in the surface layer. Figure 3b shows the LSP treatment applied to the coating surface of HVOF specimens (HVOF-LSP). A regular spot array was generated on the coating surface, and compressive residual stress was introduced into both the coating and the surface layer of the stainless steel substrate. Both LSP treatments were performed following the path shown in Figure 3c. The peened area for both groups were 10 mm × 10 mm, fully covering the entire surface of the specimens. During the LSP experiments, a 0.1 mm thick black adhesive tape was attached to the treated surface of all specimens as an absorbing protective layer, and flowing deionized water with a thickness of approximately 2 mm was used as the confining layer. To ensure experimental repeatability, six identical specimens were prepared for the LSP process, of which three were used for subsequent HVOF. In addition, three duplicate specimens were prepared for the HVOF-LSP process. Considering the relatively thin coating thickness, a laser energy of 2 J was adopted in the experiments to avoid coating cracking caused by excessive laser power. The other LSP process parameters are listed in Table 5. 2.2.2. High Velocity Oxy-Fuel Spraying The HVOF experiment was conducted using a JP8000 HVOF system manufactured by Praxair Surface Technologies, Inc., (Danbury, CT, USA)., as shown in Figure 4. In the HVOF flame jet, WC powder particles are heated, melted and accelerated to supersonic speed. The molten particles impact the substrate at high velocity, forming a dense coating through deformation, stacking and rapid cooling. Three groups of HVOF specimens were designed in this fashion. Six duplicate specimens were prepared for the untreated substrate with HVOF spraying (UT-HVOF), and another three duplicate specimens were fabricated for HVOF spraying after laser shock peening pre-treatment (LSP-HVOF). According to the process parameters listed in Table 6, a total of twelve specimens were treated by HVOF, with a coating thickness of 200 μm per specimen. 2.2.3. Accelerated Erosion Test To simulate the surface damage behavior and coating protection effect of the final-stage steam turbine blades under the impact environment of high-speed water droplets and solid particles, a DPFIM-A dual-phase flow apparatus was used to reproduce the liquid–solid composite erosion behavior. Five groups of specimens with different treatments were mounted on the erosion test setup, and the principle of the simulated erosion process is shown in Figure 5. A high-pressure waterjet carrying high-speed solid particles was directed onto the specimen surface to simulate the combined erosion effect of high-speed water droplets and solid particles on the final-stage blades. To achieve accelerated erosion and shorten the test period under laboratory conditions, the solid particles were alloy steel shots with an average size ranging from 200 μm to 700 μm, which were mixed with water and sprayed vertically onto the specimen surface. The detailed experimental parameters are listed in Table 7. To monitor the evolution of erosion morphology over time, the surface morphology of each specimen was measured once per minute of erosion, for a total erosion duration of 4 min, resulting in four surface morphology measurements. Three repeated tests were conducted for each group of specimens. Scanning electron microscopy (SEM) was employed to characterize the microscopic morphology of erosion damage. A Keyence VK-X1000K 3D laser confocal microscope (KEYENCE, Osaka, Japan) was used for layer-by-layer scanning to acquire the three-dimensional morphology of specimens and the cross-sectional profiles of erosion pits. To ensure the stability and repeatability of experimental results, three parallel specimens were prepared for each process and tested repeatedly under identical experimental conditions. The measured results exhibit low data dispersion and satisfactory repeatability among the three parallel samples of the same process. Accordingly, representative data were selected for subsequent analysis in this study. 3. Results 3.1. Cross-Sectional Micromorphology of the Coating Figure 6 presents the cross-sectional micromorphology of WC-10Co-4Cr coatings prepared with three different HVOF processes. It can be seen that all coatings exhibit a typical HVOF deposition morphology with an obvious layered stacking structure. Remarkable mechanical interlocking characteristics are observed at the interface between the coating and the 1Cr12Ni3Mo2VN stainless steel substrate, showing a distinct zigzag morphology. This interfacial characteristic originates from the intense impact of high-velocity sprayed particles on the substrate surface, followed by plastic deformation and anchoring adhesion. The EDS elemental mapping results show that the coating–substrate interface is well-defined, with clear separation between W (a1/b1/c1) and Fe (a2/b2/c2) and no large-scale elemental intermixing. The line scan profiles (a3/b3/c3) further reveal that the transition zone between W and Fe is very narrow, with a mutual diffusion distance of less than 5 μm. This indicates that no significant metallurgical bonding occurs at the interface, and the coating–substrate bonding is primarily mechanical in nature, accompanied by a narrow heat-affected zone. As can be seen from Figure 7, the main body of the coating is composed of the hard WC ceramic phase, which provides the coating with high hardness and dominant wear resistance. The Co-Cr metallic binder phase is continuously and uniformly distributed around the WC particles. With its excellent plastic deformation capability, it effectively improves the coating toughness, enhances the bonding strength between WC particles, and endows the coating with certain corrosion resistance. The uniform combination and synergistic effect of the two phases at the microscale are the key to the excellent comprehensive performance of the WC-10Co-4Cr coating. 3.2. Coating Porosity Porosity is one of the key microstructural indicators for evaluating the quality and service performance of thermal-sprayed coatings. A lower porosity generally corresponds to higher coating density and better mechanical integrity. Figure 8 shows the SEM micrographs of WC-10Co-4Cr coatings prepared by three different processes. It can be seen that pores (marked in blue) are mainly distributed at the layered structural interfaces of the coating with varied sizes and morphologies. Quantitative analysis of the metallographic images was carried out using ImageJ software (version 1.54p). The EDS analysis was performed using AZtecOne software (version AZtec 6.3, Oxford Instruments). The measured porosities of the coatings fabricated by UT-HVOF, LSP-HVOF and HVOF-LSP processes are 1.17%, 1.14%, and 1.08%, respectively. The porosities of the three coatings are less than 2%. That indicates that, within the process parameter scope of this study, different pre-treatment or post-treatment schemes exert no significant effect on coating porosity. Nevertheless, pores concentrated at the interlayer interfaces can still act as stress concentration sites. They adversely affect the coating’s resistance to crack initiation and propagation, and further degrade the overall mechanical properties of the coating. 3.3. Mechanical Properties 3.3.1. Microhardness The microhardness distribution along the cross-sectional depth of specimens with five different processes were tested using a FALCON 500 Vickers microhardness tester (INNOVATEST, Maastricht, The Netherlands). The test load was set to 500 g with a dwell time of 10 s. For cross-sectional hardness measurement, three parallel tests were performed at each measuring point, and the average value was adopted. Ten measurement points were selected inward along the longitudinal direction from the surface of each specimen, with a spacing of 40 μm between adjacent points. The microhardness distribution curves of the five processes are shown in Figure 9. As can be seen from the figure, the microhardness of the UT stainless steel substrate remains at 370 HV 0.5 without abnormal fluctuations, indicating that the substrate material exhibited a relatively stable microhardness distribution. After LSP treatment, plastic deformation occurs on the material surface, and the microhardness increased to 440 HV 0.5, representing a relatively small increase. The depth of the LSP-affected layer is approximately 260 μm. The hardness distribution in the WC coating region exhibited typical fluctuating characteristics, and the local microhardness fluctuates around an average value of approximately 1300 HV 0.5, reflecting the microstructure inhomogeneity of the coating. Among them, the HVOF-LSP process exhibited a more prominent hardening effect within the range of 0–120 μm, with a maximum microhardness up to 1500 HV 0.5. This indicates that LSP post-treatment can effectively enhance the load-bearing capacity of the coating. It is noteworthy that the near-surface hardness of the substrate for all three HVOF-processed specimens is higher than that of the untreated substrate. This phenomenon is attributed to the intense and continuous impact of high-velocity molten/semi-molten particles on the substrate surface during HVOF. Such impact is similar to shot peening strengthening, which induces plastic deformation and work hardening in the substrate surface layer. Consequently, the mechanical properties of the substrate surface layer are synergistically optimized simultaneously with coating deposition. Among all groups, the LSP-HVOF specimen exhibits the highest substrate hardness of 440 HV 0.5, which is 7.8% higher than that of the UT-HVOF specimen (approximately 408 HV 0.5). This further improves the erosion resistance of the substrate after coating spalling. 3.3.2. Residual Stress Compressive residual stress can effectively inhibit crack propagation [ 37]. Therefore, a high level of compressive residual stress also plays a positive role in preventing coating cracking. In this study, an HDS-I X-ray stress analyzer (Handan Est Stress Technology Co., Ltd., Handan, China). was used to measure the residual stress. The [211] crystal plane was selected as the diffraction plane, and the stress constant was set to −318 MPa/(°). The ψ angles were chosen as 0° and 25°, respectively, and the scanning range of the diffraction angle 2θ was set from 147° to 161°. To obtain the depth distribution of residual stress, an electrolytic polishing layer-by-layer removal method was adopted. An XF-1 electrolytic polisher (POTECH / Ningbo Jinglüe Ocean Technology Co., Ltd., Ningbo, China) with saturated sodium chloride electrolyte was employed to perform layer-by-layer etching of the WC-10Co-4Cr coating. Before and after each layer removal, the specimen thickness was measured using a precision micrometer to determine the removal depth until the coating was completely stripped and the 1Cr12Ni3Mo2VN stainless steel substrate was exposed. Each measuring point was tested in triplicate, and the average value was calculated with the measurement error recorded. The residual stress at the coating–substrate interface under the five treatment processes is presented in Figure 10. The results indicate that a compressive residual stress of approximately −77.8 MPa remains on the surface of the UT specimen owing to machining grinding and polishing. After LSP treatment, the compressive residual stress in the surface layer of the LSP specimen increases significantly, with a maximum value of −629.4 MPa. With the introduction of thermal spray deposition, laser shock peening pre-treatment, and laser shock peening post-treatment, the compressive residual stress state at the coating–substrate interface shows distinct differences. The average interfacial compressive residual stresses of the UT-HVOF, LSP-HVOF and HVOF-LSP specimens are −77.8 MPa, −595.1 MPa and −122.8 MPa, respectively. The average compressive residual stress of the UT-HVOF specimen is lower than that of the UT specimen. This is attributed to the thermal effect generated during HVOF which induces residual stress relaxation, whereas the depth of the affected layer is relatively shallow. Among these, the LSP-HVOF process with laser shock peening pre-treatment can effectively retain and increase the magnitude of interfacial compressive residual stress, which is beneficial for extending the service time before coating spalling under erosive service conditions. 3.4. Erosion Resistance of 1Cr12Ni3Mo2VN Stainless Steel 3.4.1. Surface Morphology of Erosion Pits Figure 11 shows the evolution of surface morphology with erosion time for the UT and LSP specimens. The results indicate that the eroded surfaces of both specimens exhibit circular erosion pits, and the variation in pit depth follows a parabolic trend. Material loss is most severe at the central region, while the damage is relatively mild at the edge region. This morphological characteristic is primarily attributed to the sudden expansion of the flow channel cross-section: high-speed water droplets and solid particles in the central region impact the specimen surface with the highest kinetic energy. Figure 12 shows the variation curves of erosion pit depth and width over time for the UT and LSP specimens, with measurements conducted along the central axis of the erosion pit. It can be observed that both the pit width/depth of the two specimens continuously increase with the rise in erosion time, indicating that material damage gradually accumulates under continuous particle impact. After 4 min of erosion, the maximum erosion pit width and depth of the UT specimen reached 3012 μm and 2392 μm, respectively, which far exceed the coating thickness of 200 μm. The LSP introduces grain refinement and increased dislocation density in the surface layer of the material, which enhance the resistance to plastic indentation and micro-cutting. Accordingly, the LSP specimen exhibits smaller erosion pit depth and width under the same erosion duration. After 4 min of erosion, the maximum erosion pit width and depth for the LSP specimen are 2935 μm and 1960 μm, respectively, reduced by 2.56% and 18.06% compared with the UT specimen. These results demonstrate that LSP treatment plays a particularly prominent role in suppressing the longitudinal propagation of erosion pits. 3.4.2. SEM Morphology of Erosion Pits Figure 13Figure 14 present the SEM morphologies of UT and LSP specimens of 1Cr12Ni3Mo2VN stainless steel after 3 min of erosion. Severe plastic deformation is observed on both specimen surfaces, forming pressing lips induced by high-speed particle impact. Nevertheless, under the same erosion duration, the surface damage degree of the LSP specimen is remarkably lower than that of the UT specimen. More importantly, microcracks are detected within the pressing lip regions, indicating that the substrate undergoes cumulative plastic deformation under vertical particle impact, which further triggers microcrack initiation. With prolonged erosion time, these microcracks propagate toward the interior of the material. This indicates that the failure of 1Cr12Ni3Mo2VN stainless steel under solid–liquid composite erosion is not dominated by a single mechanism, but follows a mixed damage mode governed jointly by the milling mechanism and impact fatigue mechanism. Moreover, both UT and LSP specimens share the same dominant damage mechanism. 3.5. Erosion Resistance of WC-10Co-4Cr Coating 3.5.1. Surface Morphology of Erosion Pits Figure 15 shows the temporal evolution of surface morphology for WC-10Co-4Cr coatings with three different processes after erosion tests. Morphological analysis reveals that, within the same erosion duration, the surface material damaged area of the three HVOF-coated specimens are noticeably reduced compared with the UT specimens, as shown in Figure 11. Figure 16 presents the cross-sectional profiles, depth, and width curves of erosion pits for specimens with three different coating processes. After 3 min of erosion, the maximum erosion pit depths of the UT-HVOF, LSP-HVOF, and HVOF-LSP specimens are 136 μm, 127 μm, and 109 μm, respectively. All values are less than the coating thickness of 200 μm, indicating that the substrate is not damaged. With the increase in erosion time, the erosion pit depth gradually expanded. After 4 min of erosion, the corresponding erosion pit depths reach 780 μm, 604 μm, and 467 μm, respectively. All WC-10Co-4Cr coating specimens prepared by the three processes undergo critical failure. The coating in the central region is completely penetrated, exposing the 1Cr12Ni3Mo2VN stainless steel substrate, and resulting in the loss of the protective effect of the coating. After coating failure, the substrate damage degree differs significantly among specimens with different processes. Among them, the UT-HVOF specimen exhibits the deepest substrate erosion pit, reaching 780 μm. For the LSP-HVOF specimen, although the coating is penetrated, the substrate surface layer has been strengthened by pre-treatment. The surface layer maintains ultra-high residual stress and hardness, endowing it with relatively good erosion resistance. Consequently, the depth of the central erosion region is lower than that of the UT-HVOF specimen. Due to the optimal erosion resistance of its coating, the HVOF-LSP specimen exhibits the slowest coating penetration, which extends the effective protection duration and results in the minimum erosion pit depth. 3.5.2. SEM Morphology of Erosion Pits The material removal mechanism of the coating remains consistent during the erosion experiment. Figure 17 presents the SEM morphologies of the UT-HVOF specimen after erosion for 1 min, 2 min, 3 min, and 4 min, respectively. It can be observed that the material loss in the central region becomes increasingly significant as the erosion time increases from 1 min to 3 min. The coating undergoes macroscopic spalling in a lamellar manner, forming spalling pits of varying sizes, while partial coating still remains on the substrate surface. When the erosion time reaches 4 min, the coating completely spalls off, the substrate surface is exposed, and the protective effect of the coating is entirely lost. 4. Mechanism Analysis 4.1. Protective Effect of Laser Shock Peening As shown in Figure 18, the UT specimen is subjected to repeated impact by solid particles during dual-phase waterjet erosion. Severe plastic deformation occurs on the specimen surface layer. When the accumulated strain exceeds the ductility limit of the material, microcracks initiate at the bottom of the deformed region and propagate rapidly, eventually causing material spalling in the form of debris. Under continuous erosion, the erosion pit depth increases quickly, and obvious surface damage is formed. LSP can introduce high compressive residual stress into the surface layer and improve the surface microhardness simultaneously. During the erosion process, the synergistic effect of high microhardness and compressive residual stress in the modified layer promotes the plastic closure of microdefects and the passivation of crack tips. Therefore, subjected to the same erosion time, the LSP specimen presents a smaller surface damage area and shallower erosion pit depth in comparison with the UT specimen. Although LSP can improve the erosion resistance and service life of 1Cr12Ni3Mo2VN stainless steel, the depth of the modified layer is relatively small, resulting in a short effective protection duration. With prolonged erosion time, the modified layer gradually spalls off, and the substrate is directly exposed to harsh service conditions, which accelerates the erosion damage of the matrix. This indicates that the single LSP process offers limited improvement in the erosion resistance of 1Cr12Ni3Mo2VN stainless steel, and further confirms the necessity of adopting a combined HVOF and LSP process to achieve superior protective performance. 4.2. Protective Effect of Coating Combined with LSP Hybrid Process When erosion proceeds within the effective protection period of the coating, specimens with different coating processes exhibit distinct erosion resistance. As shown in Figure 19, stress concentration easily forms at defects such as pores and edges of unmelted particles in the coating, which further induces microcrack initiation. Under the impact of solid particles and high-speed water droplets, microcracks rapidly propagate along weak interlayer interfaces. This causes the coating to fail via large-area lamellar spalling and accelerates the surface damage of the blade substrate. As no pre-treatment is applied to the substrate of the UT-HVOF specimen, the exposed substrate surface lacks a modified layer after coating failure. Accordingly, it delivers the poorest erosion resistance and possesses the maximum erosion pit depth, as displayed in Figure 20a. As shown in Figure 19b, the erosion behavior of the LSP-HVOF specimen is identical to that of the UT-HVOF specimen prior to complete penetration of the coating. Since the substrate of the LSP-HVOF specimen was subjected to LSP pre-treatment before spraying, the surface layer of the substrate possesses high compressive residual stress and forms a hardened layer. Therefore, the substrate still maintains a good erosion resistance after the coating spalls off completely, and the propagation rate of erosion pits is significantly reduced compared with that of the UT-HVOF specimen. Accordingly, LSP pre-treatment can serve as a secondary protective barrier for the substrate. With the increase in erosion time, the surface modified layer of the substrate gradually spalls off, and the erosion resistance of the specimen gradually becomes consistent with that of the UT specimen ( Figure 20b). As shown in Figure 19c, for the HVOF-LSP specimen with LSP post-treatment, laser shock waves can promote tighter bonding between coating particles, close or passivate local internal defects, and effectively reduce the porosity of the coating, which can further improve coating hardness. Meanwhile, compressive residual stress is generated in the coating under the shock of waves, which effectively inhibits the initiation and propagation of microcracks in the coating. This significantly enhances the toughness and microhardness of the coating, thereby delaying the spalling process of the HVOF-LSP coating and prolonging its effective protection duration. In addition, since the thickness of the modified layer exceeds the coating thickness, LSP can also strengthen the substrate, ensuring that the substrate still maintains superior erosion resistance even after the coating is fully penetrated ( Figure 20c). 5. Conclusions (1) Within the same erosion duration, the erosion damage degree of 1Cr12Ni3Mo2VN stainless steel specimens treated by LSP is markedly lower than that of the UT specimen, indicating that LSP can effectively improve the erosion resistance of 1Cr12Ni3Mo2VN stainless steel, to a certain extent. (2) During the effective protection stage of the coating, the three types of WC-10Co-4Cr coatings exhibit distinct erosion resistance. Both UT-HVOF and LSP-HVOF coatings fail through large-area lamellar brittle spalling, showing a similarly high damage rate. In contrast, the HVOF-LSP specimen undergoes post laser shock peening treatment, which reduces coating porosity, increases coating hardness, and introduces high compressive residual stress. Accordingly, its protection duration is remarkably prolonged, presenting the best erosion resistance. With identical erosion time applied to all samples, analysis of the experimental data indicates that the comprehensive performance order of the five processes is: HVOF-LSP > LSP-HVOF > UT-HVOF > LSP > UT. (3) The HVOF–LSP process is dedicated to extending the effective service life of the coating, whereas the LSP–HVOF process prioritizes the secondary protection capability of the substrate after coating failure. The two processes exhibit obvious differences in their underlying strengthening mechanisms. As a pre-treatment route, LSP–HVOF induces high compressive residual stress and forms a modified layer on the substrate surface, accompanied by the generation of micro-dimple textures. In the subsequent HVOF procedure, a self-locking mechanically interlocked interface is constructed, thereby enhancing the load-bearing capacity of the substrate. By comparison, the HVOF–LSP process employs LSP as a post-treatment to consolidate and densify the deposited coating, while simultaneously introducing compressive residual stress. This inhibits the initiation and propagation of service-induced cracks in the coating, thereby extending the time until coating spalling. (4) The LSP-HVOF process shows the highest interfacial compressive stress, whereas the HVOF-LSP demonstrates the best erosion resistance. This indicates that the HVOF-LSP process is a combined effect of reduced porosity, restrained crack propagation, and improved hardness, rather than being determined merely by interfacial residual stress alone. Author Contributions Conceptualization, H.L.; Methodology, H.L.; Software, Y.Y.; Validation, Y.Y., J.L. and K.G.; Formal analysis, J.L. and K.G.; Investigation, S.L. and Z.X. (Zhenrong Xie); Resources, S.L.; Data curation, X.Y. and Z.X. (Zhenrong Xie); Writing—original draft, X.Y.; Writing—review and editing, B.G.; Supervision, X.L.; Project administration, Z.X. (Zhilong Xu) and X.L.; Funding acquisition, B.G. and Z.X. (Zhilong Xu). All authors have read and agreed to the published version of the manuscript. Funding This work was supported by the Natural Science Foundation of Fujian Province, China (No. 2024J08066), the Natural Science Foundation of Xiamen City, China (No. 3502Z202372025), the Synergetic Innovation Special Program of Fuxiaquan National Independent Innovation Demonstration Zone (No. 3502ZCQXT2023009), and the Major Science and Technology Project of Xiamen, Fujian, China (No. 3502Z20231001). Institutional Review Board Statement Not applicable. Informed Consent Statement Not applicable. Data Availability Statement The data presented in this study are available on request from the corresponding author due to laboratory confidentiality agreements. Requests to access the datasets should be directed to the corresponding authors. Conflicts of Interest The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. References Krechkovska, H.; Hredil, M.; Student, O.; Tsyrulnyk, O.; Kret, N. Peculiarities of fatigue fracture of high-alloyed heat-resistant steel after its operation in steam turbine rotor blades. Int. J. Fatigue 2023, 167, 107341. [] [ CrossRef] Zainuddin, N.S.; Zamri, W.F.H.W.; Omar, M.Z.; Sajuri, Z.; Syarif, J. Comprehensive insight into the failure mechanisms, modes, and material selection of steam turbine blades. J. Curr. Sci. Technol. 2024, 14, 47. 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Schematic diagram of two different laser shock peening processes. Figure 3. Schematic diagram of two different laser shock peening processes. Figure 4. HVOF equipment. Figure 4. HVOF equipment. Figure 5. Schematic diagram of liquid–solid composite erosion simulation. Figure 5. Schematic diagram of liquid–solid composite erosion simulation. Figure 6. Cross-sectional SEM morphology and EDS analysis of the coating. ( a) UT-HVOF, ( b) LSP-HVOF, ( c) HVOF-LSP. ( a1– c3) correspond to the SEM morphologies and EDS line scanning analyses of ( a– c). Figure 6. Cross-sectional SEM morphology and EDS analysis of the coating. ( a) UT-HVOF, ( b) LSP-HVOF, ( c) HVOF-LSP. ( a1– c3) correspond to the SEM morphologies and EDS line scanning analyses of ( a– c). Figure 7. EDS elemental analysis of the coating at different locations. ( a) Microstructure morphology of the alloy; ( b) and ( c) EDS spectra and elemental compositions corresponding to the marked spots in ( a), respectively. Figure 7. EDS elemental analysis of the coating at different locations. ( a) Microstructure morphology of the alloy; ( b) and ( c) EDS spectra and elemental compositions corresponding to the marked spots in ( a), respectively. Figure 8. Magnified SEM micrographs of the coating and porosity statistics. ( a) UT-HVOF, ( b) LSP-HVOF, ( c) HVOF-LSP, ( d) Comparison of porosity for samples under three different processing conditions. Figure 8. Magnified SEM micrographs of the coating and porosity statistics. ( a) UT-HVOF, ( b) LSP-HVOF, ( c) HVOF-LSP, ( d) Comparison of porosity for samples under three different processing conditions. Figure 9. Microhardness Distribution Curves of Specimens under Five Treatment Processes. Figure 9. Microhardness Distribution Curves of Specimens under Five Treatment Processes. Figure 10. Residual Stress at Coating–Substrate Interface of Five Treated Specimens. Figure 10. Residual Stress at Coating–Substrate Interface of Five Treated Specimens. Figure 11. Erosion morphologies of 1Cr12Ni3Mo2VN stainless steel: ( a1– a4) UT, ( b1– b4) LSP. Figure 11. Erosion morphologies of 1Cr12Ni3Mo2VN stainless steel: ( a1– a4) UT, ( b1– b4) LSP. Figure 12. Variation curves of erosion pit width and depth. Figure 12. Variation curves of erosion pit width and depth. Figure 13. SEM morphology of erosion pit on UT specimen after 3 min of erosion. ( a) Macroscopic surface morphology; ( a1) High-magnification morphology at the erosion edge; ( a2, a3) High-magnification morphology of the central erosion region. Figure 13. SEM morphology of erosion pit on UT specimen after 3 min of erosion. ( a) Macroscopic surface morphology; ( a1) High-magnification morphology at the erosion edge; ( a2, a3) High-magnification morphology of the central erosion region. Figure 14. SEM morphology of erosion pit on LSP specimen after 3 min of erosion. ( a) Macroscopic surface morphology; ( a1) High-magnification morphology at the erosion edge; ( a2, a3) High-magnification morphology of the central erosion region. Figure 14. SEM morphology of erosion pit on LSP specimen after 3 min of erosion. ( a) Macroscopic surface morphology; ( a1) High-magnification morphology at the erosion edge; ( a2, a3) High-magnification morphology of the central erosion region. Figure 15. Erosion morphology of WC-10Co-4Cr coatings under different processes: ( a1– a4) HVOF, ( b1– b4) LSP-HVOF, ( c1– c4) HVOF-LSP. Figure 15. Erosion morphology of WC-10Co-4Cr coatings under different processes: ( a1– a4) HVOF, ( b1– b4) LSP-HVOF, ( c1– c4) HVOF-LSP. Figure 16. Cross-sectional profiles of erosion pits on WC-10Co-4Cr coatings under different processes: ( a) 1 min, ( b) 2 min, ( c) 3 min, ( d) 4 min. Figure 16. Cross-sectional profiles of erosion pits on WC-10Co-4Cr coatings under different processes: ( a) 1 min, ( b) 2 min, ( c) 3 min, ( d) 4 min. Figure 17. SEM morphologies of the UT-HVOF specimen after erosion: ( a) 1 min, ( b) 2 min, ( c) 3 min, ( d) 4 min. ( a1– d1) show the magnified details of the areas indicated by the yellow boxes in ( a– d), respectively. Figure 17. SEM morphologies of the UT-HVOF specimen after erosion: ( a) 1 min, ( b) 2 min, ( c) 3 min, ( d) 4 min. ( a1– d1) show the magnified details of the areas indicated by the yellow boxes in ( a– d), respectively. Figure 18. Schematic erosion behavior of UT and LSP specimens. Figure 18. Schematic erosion behavior of UT and LSP specimens. Figure 19. Schematic erosion behavior of three processing specimens prior to coating failure. Figure 19. Schematic erosion behavior of three processing specimens prior to coating failure. Figure 20. Schematic erosion behavior of three processing specimens after coating failure. Figure 20. Schematic erosion behavior of three processing specimens after coating failure. Table 1. Mechanical properties of 1Cr12Ni3Mo2VN stainless steel after standard heat treatment. Table 1. Mechanical properties of 1Cr12Ni3Mo2VN stainless steel after standard heat treatment. Mechanical Properties Tensile Strength/MPa Yield Strength/MPa Hardness/HRC Poisson’s Ratio Elongation/% 1205 930 36.5 0.3 20.5 Table 2. Chemical composition of 1Cr12Ni3Mo2VN stainless steel. Table 2. Chemical composition of 1Cr12Ni3Mo2VN stainless steel. Component C Si Mn P S Cr Ni Mo N V (wt.%) 0.14 0.17 0.75 0.016 0.005 12.05 2.56 1.62 0.035 0.28 Table 3. Treatment routes (√ is the process part involved). Table 3. Treatment routes (√ is the process part involved). Specimen Prior Heat Treatment LSP Preprocessing HVOF LSP Post-Processing UT √ LSP √ √ UT-HVOF √ √ LSP-HVOF √ √ √ HVOF-LSP √ √ √ Table 4. Chemical composition of WC-10Co-4Cr powder. Table 4. Chemical composition of WC-10Co-4Cr powder. W Co Cr Ct Cf O Fe Balance 10.24 4.1 5.52 0.03 0.021 0.026 Table 5. Laser shock peening parameters. Table 5. Laser shock peening parameters. Parameters Wavelength/nm Spot Diameter/mm Pulse Width/ns Pulse Energy/J Frequency/Hz Overlap Rate Value 1064 3 10 2 5 50% Table 6. HVOF Parameters. Table 6. HVOF Parameters. Oxygen Flow Rate (m 3/h) Propane Flow (m 3/h) Nitrogen Flow Rate (m 3/h) Acetylene Flow Rate (m 3/h) Spray Distance (mm) Spray Gun Moving Speed (mm/s) Track Rollover Coefficient Powder Feeding Rate (g/min) 1.1 0.1 0.3 0.3 250 200 0.5 0.3 Table 7. Erosion Test Parameters. Table 7. Erosion Test Parameters. Parameters Spraying Distance (mm) Nozzle Inner Diameter (mm) Water Pressure (MPa) Jet Velocity (m/s) Particle Concentration Water Temperature Values 15 1 12 420 10% 30 °C Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). 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