Special Issue: Mechanical Properties of Advanced Multifunctional Coatings

Coatings are found almost anywhere in the modern world. The typical examples are architectural wall coatings and automotive paints, which serve to both protect substrates and offer a decorative appearance [1]. In our daily life, we are surrounded by objects with coatings, such as paper books, the lenses of glasses, computer screens, CPUs inside laptops, etc. Coating technology, as one type of surface engineering, is related to the friction, wear, corrosion, abrasion, and fatigue of tools and components. They have a very wide range of applications in industries including automotive, aerospace, nuclear, military and defense, power generation, tool and die, metal forming, agriculture, and food processing. According to the industry market analysis from American Coatings Association, the total value of the global coatings market was estimated to be approximately USD 112 billion in 2014.

There has always been a lot of interest in advanced multifunctional coatings because of the tremendous and unexpected progress in their synthesis, characterization, and properties. A variety of different fields have also used them, such as aeronautics, transportation, biomedicine, equipment for electrical and electronic use, etc. Numerous excellent reviews and research articles have covered many topics in this area, such as dynamic coatings [2], thermal barrier coatings for gas-turbine engine applications [3], nanostructured coatings [4,5], all-nanoparticle thin-film coatings [6], microstructural design of hard coatings [7], anticorrosive coatings [8], advanced multifunctional coatings for vibration control of machining [9], etc. Multifunctional coatings’ mechanical properties play a crucial role in their interaction with external forces and environmental factors. Yet, sophisticated characterization and modeling tools are required to gain a comprehensive understanding of the mechanical properties of these coatings.

This Special Issue aims to provide a forum for researchers to share current research findings and to promote further research into the mechanical behavior of advanced multifunctional coatings, including experimental modeling and theoretical calculations. This Special Issue eventually collects 13 original research articles on the most recent works not limited to the relevant research area of advanced multifunctional coatings, including interesting mechanical behaviors at surfaces and interfaces.

Thermal barrier coatings (TBCs) are layered materials with low thermal conductivity and high thermal stability sprayed onto metal substrates to provide thermal protection in the most demanding high-temperature environment [3,10]. As the most complex coating system, TBCs mainly consist of a ceramic top coating (TC), a metallic bonding coating (BC), and a metal substrate. The structure, mechanical properties, and failure mechanisms of TBCs would need to be understood more in order to improve them.

Thermal oxidation stress of TBCs has been successfully investigated with Cr³⁺ photoluminescence piezospectroscopy, but results for data processing lack systematization and quantitative analysis, especially regarding peak positions. Numerous spectra representing different uniaxial loadings are obtained by Lu et al. [11] through numerical experiments and calibration tests. Based on a comparison of fitting results and discussion of the generation mechanism, the Lorentzian function-rather than the more commonly utilized Psd-Voigt function—is considered to be the most relevant method for the application of Cr³⁺ photoluminescence piezospectroscopy to TBCs because it has an adequate degree of sensitivity, stability, and credibility for quantitative stress analysis.

It is one of the major causes of thermal barrier failure that residual stresses are introduced during manufacturing and service processes inside the TBC top coating. It is essential, therefore, to measure the residual stress in the top coating in a non-destructive and accurate manner in order to assess the lifetime of the TBC. Researchers have used terahertz time-domain spectroscopy (THz-TDS) to measure internal stresses in nonmetals by determining or calibrating the material’s optical stress coefficient. In the work by Wang et al. [12], a THz-TDS-based calibration of the stress optical coefficient is performed for analyses of stresses in the ceramic layer of TBC. Spectra of TBC specimens are analyzed via the unimodal, multimodal, and barycenter methods of fitting THz time-domain spectra to different uniaxial compression loadings obtained with a reflection-type THz-TDS system. When compared to the two other methods, the barycenter method had a higher level of accuracy and stability. This work provides a solid foundation to employ THz-TDS for analyzing the stress inside the top coating of actual TBC structures.

A rock possesses complex structural properties in its medium space, making it one of the most basic materials for engineering coatings. However, it has still proven difficult to quantify the microstructure of rock coatings, such as the degree of connectivity and aggregation. In their first paper, an innovative new complex network theory is proposed by Gao et al. [13] to understand the 3D structure of pore networks in sandstone, which cannot be quantified by traditional methods such as Euler number and fractal dimension. Results from numerical simulations indicate that a scale-free network model is more suitable than random models for describing the pore network in sandstone. According to this research, the pore network in sandstone is uniform and its connectivity has the potential to enhance permeability. Using this method, researchers can examine crack grid distribution in tunnel coatings as well as learn more about the pore connectivity characteristics of sandstone. In their second paper from the same group [14], a dual-porosity network model of rock is proposed based on the Barabasi and Albert (BA) scale-free theory. The porosity degree distribution, average path length, throat length distribution, and other parameters of the rock model are used to quantitatively describe the connection between rock pores and throats. A trend analysis is performed for the permeability of the coating model using microscopic parameters. The dual-pore network model was shown to be capable of matching very well the structural distribution characteristics of different types of rocks, and thus can be used to describe the coating’s structure very effectively. In this way, the mechanical and physicochemical properties of rock can be analyzed theoretically, providing support for the rational design of rock coatings.

The fiber-matrix interface is widely recognized as the most important determinant of composite structural stability. With an appropriate interface, load can be transferred effectively and crack propagation can be prevented. There has been considerable effort dedicated to studying how the interface affects mechanical properties of SiC–SiC composites. Through a combination of experimental measurement and theoretical analysis, Jin et al. [15] investigate the tensile properties of SiC–SiC composites reinforced by domestic Hi–Nicalon type SiC fiber at high temperature and illuminate how the interface coating affext the mechanical property variations. The experimental results show that tensile strength dramatically drops above 1200 C, and fiber-pull-out fracture mode changes to fiber-break. Based on the subsequent FEM (finite element method) analysis, the authors claim that the variation in residual radial stress at the fiber-matrix interface coating is responsible for the reduction in tensile strength and change of fracture mode. The residual radial stress at the fiber-matrix interface coating changes from tensile to compressive once the temperature exceeds the preparation temperature for the composites, increasing the interface strength as the temperature increases.

A novel method called groove HCECB (hot casting plus explosion compression bonding) is developed by Sun et al. [16] and applied to weld 6061 aluminum alloy with Q235a steel. The experimental results demonstrate that the aluminum-steel composite plate can be directly combined by the HCECB method with no defects (such as melting layer, holes, and cracks) observed in the interface. The detailed SEM (scanning electron microscope) and EDX (Energy Dispersive X-Ray) analysis reveal that with tight bonding, and almost no melt, the composite plate has an interface consisting of flatness and microwave. According to the tensile and shear test results, the shear strength is greater than 80 MPa, which is the required bond strength for aluminum-steel composite plates.

In the experimental work by Li et al. [17], Cr-DLC (Cr-containing hydrogenated amorphous diamond-like carbon) films are synthesized by pulsed-dc (direct current) magnetron sputter with a PEM (plasma emission monitor) system using a large-size industrial Cr target. They examine how the Cr atom plasma emission intensity is related to the element concentration, the cross-sectional morphology, the deposition rate, the microstructure, mechanical properties, and tribological properties of Cr-DLC films. Various methods are employed to characterize and analyze the mechanical and tribological behaviors, including nanoindentation, scratch test instruments, and ball-on-disk reciprocating friction/wear testers. Based on the results of the experiment, the PEM system has been successfully applied to magnetron sputtering in order to deposit Cr-DLC with a more stable process.

During the last twenty years, micropitting has been particularly problematic on gear surfaces. As a surface fatigue phenomenon, it occurs in rolling and sliding contact that are operating in boundary lubrication (BL) or elastohydrodynamic lubrication (EHL) regimes. Generally, asperities on the surface are usually the source of micropitting, which starts to form in the dedendum of the driver and driven. However, Zhao et al. [18] find that for some gears with interference fit connections of their conical surface, micropitting on the pinion occurs in the addendum. Through a 3D–TCA method (GATES: Gear Analysis for Transmission Error and Stress) based on ISO/TR 15144-1:2014, this study attempts to predict the occurrence of micropitting and try to understand the key influential factors to affect micropitting location. Combined with FEM modeling, they demonstrate that instead of profile shift coefficient, the start of tip relief, and lead slope deviation, the difference of profile slope deviation between pinion and wheel may affect the micropitting location.

Nanoindentation is a widely used experimental tool to investigate mechanical responses of small volumes of materials at a micro-length scale [19]. Zhang et al. [20] apply the nanoindentation technique combined with AFM (atomic force microscope) measurement to investigate the surface morphology, average grain sizes, load-penetration depth curves, and hardness of the three SiO₂ thin films with different thicknesses (500, 1000 and 2000 nm-thick). A detailed analysis of the dependence of the hardness of the SiO₂ thin films on thickness is presented. It is found that the average intrinsic hardnesses of the 500, 1000, and 2000 nm-thick SiO₂ thin films are 11.9, 10.7, and 10.4 GPa, respectively. As the film thickness and grain size increase, the average intrinsic hardness of SiO₂ thin films decreases, in a similar manner to the Hall-Petch relationship.

Anti-reflective (AR) coatings are made up of multiple layers of metal oxides that reduce and eliminate reflection from the front and rear surfaces of ophthalmic lenses. This allows more light to pass through the lens, improving the vision of the wearer as well as decreasing the glare we see in photos. Using nanoindentation hardness as a measure of practical scratch resistance for mechanically tunable anti-reflective hard coatings, Price et al. [21] present a fundamental understanding of the relationship. It has been shown that FEM is an effective technique for analyzing the hardness of multilayer films.

Piirsoo et al. [22] carry out nanoindentation tests on the atomic layer deposited amorphous Al₂O₃-Ta₂O₅ double- and triple-layered films with thickness of 70 nm. It is found that the sequence of the oxides from surface to substrate along with the layer thickness influenced the hardness. However, the elastic modulus of amorphous Al₂O₃-Ta₂O₅ nanolaminates does not depend on the layer structure and fell between 145 and 155 GPa for all the laminates.

Computational modeling such as CFD (continuum level) and MD/MC simulations (atomistic level) can be complementary to experimental investigations. In their work, a simulation model within the framework of computational fluid dynamics (CFD) is presented by Cui et al. [23] to reveal the behavior of resin filling in the process of UV-NIL (ultraviolet nanoimprint lithography) in the moth-eye nanostructure. This model takes the boundary slip effect into account. The model allows researchers to investigate how various process parameters may influence resin filling behavior, such as resin viscosity, inlet velocity, and resin thickness. Comparison with the experimental results indicate good consistency between the simulation and the experimental results, corresponding to the ability to use the simulation model to examine the resin filling behavior of the moth-eye nanostructure in the UV-NIL process.

Xu et al. [24] perform computational simulations combined with grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) to study the adsorption/desorption isotherms of argon thin films confined between commensurate/incommensurate contacts in boundary lubrication. By employing the so-called mid-density scheme to the hysteresis loops, the equilibrium structures associated with n_c and n_c + 1 layers can be obtained. Here, the critical layer number, n_c, corresponds to the monolayer thickness at which the confinement-induced liquid-like to solid-like nucleation occurs. By comparing equilibrium structures predicted by GCMC/MD simulations and those predicted by LVMD (liquid-vapor molecular dynamics) simulations, one can determine the correct values of chemical potentials. Simulations using GCMC/MD can be used to construct equilibrium structures at varying thicknesses that may be suitable as an initial stage for studies using advanced sampling methods.