The surface morphology of specimens significantly affects the measurement accuracy of indentation hardness. Surface undulation leads to dispersion in measured hardness and makes it very difficult to obtain an accurate hardness. In the past, mechanical polishing and increasing the indentation depth were widely performed to decrease the influence of surface morphology. However, both methods have limitations for the hardness measurement of small-scale structures such as thin films or coatings. Thus, obtaining an accurate hardness measurement from one or two simple indentation tests is of great application value. In this study, we introduced a new regression method to eliminate the influence of surface undulation on hardness measurements. We simulated the indentation tests of thin films with undulating surfaces by finite element simulation and then analyzed the regularity of the measured hardness. The numerical simulations validated that the regression method can effectively eliminate the influence of surface undulation and obtain the accurate hardness of materials. This method breaks through the limitations of conventional methods, simplifies the testing workload, and improves measurement accuracy.

The depth dependence of the indentation hardness, i.e., the indentation size effect (ISE), was manifested as an increase or decrease in hardness when penetration depth decreases, which was observed by previous experiments. Most scholars believe it is generated by strain gradient. However, additional opinions argue that other factors, similar to the strain gradient, can also contribute to the ISE. Importantly, if such an ISE occurs in an experiment, one needs to distinguish its causes. In this study, two external factors were considered, i.e., the specimen surface defect and indenter tip irregularity, which can lead to the hardness–depth-dependent relationships of the ISE. Moreover, a theoretical model was established to interpret the two dependent relationships and validated via finite element simulations. The simulation results showed that both external factors lead to significant hardness–depth dependences, in great agreement with theoretical derivations. Moreover, the simulation results demonstrated that the surface defect and indenter tip irregularity have a unified influence on the ISE, which is attributed to a sole depth deviation δ. Lastly, the ISE based on both external factors was eliminated and the real hardness was obtained. Based on this, when an ISE appears, it can be distinguished whether it is caused by external factors or strain gradient.

To fully realize the potential application of spalled thermal barrier coating systems (TBCs) in gas turbine blades, it is essential to evaluate the service behavior of TBCs and the critical spallation size for safety servicing. For this purpose, the evaluation of the localized spallation of TBCs under high-temperature gas was investigated experimentally and numerically. Thermal insulation experiments and a conjugate heat transfer numerical algorithm were used to clarify the over-temperature phenomenon, temperature distributions, the relevant flow characteristics of the high-temperature gas in the localized spallation region of TBCs, and the influencing mechanisms that consider the spallation width were identified. The results suggested that when the spallation width was less than 10 μm, the temperature in the TBCs did not change due to the weak impression of gas. When the spallation width exceeded the security coefficient of about 3 mm, the TBCs were difficult to service safely due to the impact of high-temperature gas. Furthermore, the concept of an over-temperature coefficient was proposed to describe the over-temperature damage and a nonlinear fitting equation was obtained to reveal and predict the evolution of the over-temperature coefficient. The over-temperature coefficient may serve as a valuable metric in determining the performance degradation of TBCs.

The degradation mechanisms for environmental barrier coatings (EBCs) under high-temperature water vapour conditions are vital for the service of aero-engine blades. This study proposes a theoretical model of high-temperature water vapour corrosion coupled with deformation, mass diffusion and chemical reaction based on the continuum thermodynamics and the actual water vapour corrosion mechanisms of an EBC system. The theoretical model is suitable for solving the stress and strain fields, water vapour concentration distribution and coating corrosion degree of an EBC system during the water vapour corrosion process. The results show that the thickness of the corrosion zone on the top of the EBC system depended on water vapour diffusion, which had the greatest influence on the corrosion process. The top corroded area of the rare-earth silicate EBC system was significantly evident, and there was a clear dividing line between the un-corroded and corroded regions.

Residual particles embedded at the bond coat/substrate (BC/SUB) interface after grit blasting can affect the failure behavior of thermal barrier coatings (TBCs) under thermal cycling. This study employed a 2D finite element model combining the cohesive zone method (CZM) and extended finite element method (XFEM) to analyze the effect of interfacial grit particles. Specifically, the CZM was used to simulate crack propagation at the BC/thermally grown oxide (TGO) interface, while XFEM was applied to model the arbitrary crack propagation within the BC layer. Three models were analyzed: no grit inclusion, 20 μm grit particles, and 50 μm grit particles at the BC/SUB interface. This systematic variation allowed isolating the influence of particle size on the location of crack propagation onset, stress distribution, and crack growth behavior. The results showed that grit particles at the SUB/BC interface had negligible influence on the crack propagation location and rate at the BC/TGO interface, due to their spatial separation. However, their presence significantly altered the radial tensile stress distribution within the BC layer. Larger grit particles induced more intense stress concentrations and promoted earlier and more extensive vertical crack propagation within the BC. However, due to plastic deformation and stress redistribution in the BC, the crack propagation was progressively suppressed in the later stages of thermal cycling. Overall, grit particles primarily promoted vertical crack propagation within the BC layer. Optimizing grit blasting to control grit particle size is crucial for improving the durability of TBCs.

High-temperature fasteners are metal parts of gas turbines and steam turbines, which work at high temperatures and under stress for a long time. However, the frequent seizures of fasteners bring great trouble to the normal maintenance of power plants. In this paper, three kinds of dense and controllable CrAlN-based gradient multilayer coatings were prepared on the samples and screws by arc ion plating (AIP) technology. The morphology, composition, structure, nano hardness, adhesion, residual stress, and room temperature tribological performance of the coating were investigated. To evaluate the high-temperature, anti-seizing performance, coated screws were heated to 700 °C for 140 h with a torque of 20 N·m. The results indicate that the CrN/CrAlN multilayer coating shows better comprehensive properties. The characterization of coated screws proved that the coating structures obtained on the screws were similar to the flat samples. However, the as-prepared coating on the screws showed different thickness variation rules, which was related to the clamping method, deposition distance, and screw shape. After a simulation service, the thread of the screw remained intact with similar structure and thinner thickness. The above results indicate that the high-temperature seize prevention of fasteners can be successfully achieved by preparing a CrAlN-based multilayer coating, which is suitable for fasteners with service temperatures below 700 °C.