Since adhesion is one of the most important properties for coatings to ensure its reliability, it is often necessary to quantitatively or semi-quantitatively evaluate the adhesion strength of coatings. Several testing methods to evaluate coating adhesion have been proposed and used. When applying the adhesion test, it becomes important to understand the characteristics of the test method and the physical meaning of the evaluated parameters. It is well known that the joint of the dissimilar elastic materials has the singular stress field at the edge of the interface; thus, we should consider the delamination mechanisms of coatings based on the interfacial mechanics even if no crack occurs at the interface. The linear interfacial fracture mechanics is also applicable in the actual coatings when satisfying with the small-scale yielding condition, although they are elastic-plastic materials. This review summarized the typical testing methods to evaluate the adhesion of coatings. In addition, it is tried to discuss typical parameters for the coating adhesion with physical meaning based on the interfacial mechanics.

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The bonding mechanism of thermal sprayed ceramic coating was not clear due to the complicated profile of blasted surface and microstructure of the coating. Especially, adhesive mechanism on light metal substrate is not clear because of little previous research. The objective of this study is to investigate the effect of surface roughness on the adhesive strength of yttria-stabilized zirconia coatings sprayed by atmospheric plasma spraying on aluminum alloy (JIS A5052) substrate. Tensile adhesive test and interfacial indentation test were carried out for evaluating their bonding properties. The substrates were blasted in three different pressures and seven different blast angles by fused alumina abrasives with three different particle sizes. Our measurement revealed that tensile adhesive strength was decreased with increasing the substrate roughness (root mean square slope RΔq) . Fracture surfaces after tensile strength test are observed by FE-EPMA analyzer and analyzed by X-ray fluorescence analyzer. It was found from the elemental analysis that the fractured part shifted to the substrate with increasing the RΔq. It was concluded that tensile adhesive strength was decreased with increasing the surface roughness due to the low strength substrate.

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SiC ceramic matrix composites (CMC) are potential materials for hot section gas turbine components due to their high heat resistance and low density. However, SiC CMC degrades over time in steam oxidation environments. Therefore, environmental barrier coatings (EBC) are necessary to protect SiC substrates. Yb₂Si₂O₇ is one of the most potent materials for barrier coatings. In this study, self-healing property is further added to Yb₂Si₂O₇ EBC by incorporating SiC in the form of Yb₂Si₂O₇/SiC granulated powders. We investigated the effect of different SiC contents and particle sizes on the long-term oxidation behavior of Yb₂Si₂O₇/SiC coatings. The self-healing property was observed in the microstructural changes of artificial cracks induced by Vickers indentation. The samples were oxidized at 1300℃ , and the structural change before and after the oxidation was evaluated by SEM, XRD, and EDX. The cross-sectional SEM images show that SiC was entirely oxidized in the first 50 h. Meanwhile, the surface SEM results show that small particle size and high SiC content exhibited a significant volume expansion during the oxidation of SiC. In addition, the self-healing property was significantly active in the initial stage of oxidation, but drastically deteriorated as the oxidation progressed.

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This paper investigates a universal law of the maximum particle velocity as a function of the particle diameter and material density in high-velocity oxy-fuel (HVOF) thermal spray process. The universal law is proposed based on the theoretical considerations of the equation of particle motion. The ranges of diameter and material density of the spray particles selected in this paper are 10-60μm and 4,000-16,000 kg/m³. This series of the maximum particle velocity obtained by the universal law were compared with the results obtained by one-dimensional calculation of the gas/particle flows, with the equivalence ratio of the HVOF thermal spray gun to be 0.5, 1.0 and 1.5. The present research revealed that 1) The maximum particle velocity follows simple Eqs. (10) and (12) within the accuracy of ±4%, 2) Equation (12) provides the relative maximum particle velocity for an arbitrary set of material diameter and density, against a maximum particle velocity for another set of reference particle diameter and density, 3) Equations (10) and (12) holds within the range of particle diameter 10-60μm, and particle density 6,000-16,000 kg/m³. They correspond to the Stokes number, St, to be around 7 < St < 100.

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In recent years, it is highly desirable to extend service life of industrial machinery parts and equipment from the view point of environment and manufacturing costs. In particular, the surface treatment which protects components from wear and corrosive environment has become an important and indispensable technology. On the other hand, hard chrome plating has been widely used for a long time as a countermeasure against wear and corrosion, but it is becoming subject to various regulations because it may generate harmful chemical substances in the coating process. For these issues, plating companies are continually researching and developing plating methods which do not threaten safety of workers or pose a physical burden on product users, but these issues seem to remain unsolved. For this reason, thermal spraying technology has become a candidate as a substitution technology of plating. Since thermal spraying is a dry process which does not require waste liquid treatment, it can be said that it is a process with relatively low environmental impact.
In this commentary article, advantages and disadvantages of plating and thermal spraying are introduced by comparing the characteristics of thermal spray coating and plating.

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Thermal spraying is a method of heating a thermal spray material using combustion or electrical energy and spraying molten or semi-molten particles onto the surface of a substrate to form a coating, mainly for heat resistance, wear resistance and corrosion resistance. It is widely used in fields such as aircraft, industrial gas turbines, steelmaking, and automobiles. The origin of thermal spraying is said to be the metal spraying method invented by M.U. Schoop in Switzerland in 1909, and looking back on the history of the development of thermal spray equipment that has supported the progress of application development during this period, its transition and trends. In particular, with regard to plasma and high velocity flame spraying (HVOF) , which are applied for the purpose of improving surface functions, many developments have been made to improve the performance, and the purpose and trends will be described in detail.

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