Extreme High -Speed Laser Cladding (EHLA) is a new process category of laser cladding. In this study, EH-LA layer was characterized by comparing with conventional laser cladding (LC) layer. Basic SUS316L layers, as well as WC-reinforced SUS316L layers, were formed on SUS304 substrates using both LC and EHLA processes. The macroscopic morphology, microstructure, microhardness, wear resistance, and residual stress of the four types of layers were evaluated. As a result, EHLA layers exhibited slightly higher micro-hardness and less wear loss than that of LC layers, despite the presence of more micropores. This can be due to their finer dendritic structures. Furthermore, residual stress of EHLA layer was lower than that of LC layer due to those micropores. Additionally, EHLA can add up to 45 wt.% WC into SUS316L layer without crack formation, resulting in higher wear resistance than that of LC where crack formation occurred at 25 wt.% WC. This enhanced crack resistance in EHLA is believed to be due to the less heat input during deposition.

Cavitation erosion (CE) damage, which occurs in the main parts (made of high chromium cast steel) of hydroelectric power generation machine, is one of the serious problems. It is expected that life time of those parts would be prolonged if the suitable CE-resistant coating is applied on the surface of the cast steel. In this study, WC-cermet coatings (WC-CoCr and WC-Cr 3 C 2 -Ni), which were fabricated by high-velocity oxygen-fuel (HVOF) thermal spraying process, was interested in protecting CE attack to the cast steel. To clarify CE property of the WC-cermet coatings, the ultrasonic vibration tests were conducted, and the amount of volume loss characterized as CE damage was measured. The microstructure and the fracture toughness, which was evaluated by the indentation test method, of the coatings were related with the CE damage. As the results obtained in this study, the fragment which was spalled from the surface after CE test was almost flake-like shape, and its size was from 2µm to 50µm. SEM observation indicated that this fragment included both WC particle and metal binder, which means that WC particle and metal binder was still strongly bonded together. It was also confirmed that the amount of volume loss could relate directly with the fracture toughness KIC rather than Vickers hardness. It was considered that CE damage was progressed into the depth by throwing out the fragment originated from micro crack initiation. Thus, it was required that the CE resistance of the developed coatings could be labelled through the fracture toughness.

Adhesion strength of thermally sprayed coatings is usually measured in accordance with the tensile method specified by ISO 14916. A major limitation of the method, however, is that it cannot measure adhesion strengths greater than that of the glue used to prepare the test specimen. Indentation testing, by virtue of its simplicity and practicality, is a promising alternative in such cases. Collaborative work has been conducted by members of the Japan Thermal Spray Society (JTSS) to establish a standard method for measuring coating adhesion using a conventional Vickers indenter. This paper provides an overview of the experimental and theoretical work that was done and describes the criteria proposed to quantify adhesion strength based on standardized test procedures.

A transpiration cooling system for gas turbine applications has significant benefit for reducing the amount of cooling air and increasing cooling efficiency. In this paper, the porous ceramic coating using a plasma-spraying process, which can infiltrate a cooling gas and control the loss of coolant gas pressure, and the properties of the porous coating material fabricated in this study, such as permeability of the cooling gas, thermal conductivity and adhesion strength, are shown. The mixture of 8wt% yttria stabilized zirconia and polyester powders was employed as the coating, in order to deposit the porous ceramic coating onto the Ni-based super alloy substrate. The ceramic coating deposited with such mixed powder showed superior permeability for cooling gas and thereby to lead to considerable reduction of the substrate temperature based on the transpiration cooling mechanism. The adhesion strength of the porous ceramic coating was almost the same level as that of the ceramic coating utilized in current gas turbine components. The high-temperature exposure test revealed that the permeability of cooling gas did not change though a rigorous experience for a long period of up to 1000hours. It was also confirmed that the coating peeling damage did not occur after the thermal cycling test. It is concluded consequently that the transpiration cooling system proposed here for gas turbine could be achieved using the porous ceramic coating.

A fundamental study of inelastic deformation of freestanding plasma-sprayed thermal barrier coatings (TBCs) has been conducted. Cantilever-type bending tests are carried out to obtain stress-strain curves from freestanding ceramic layers detached from the TBC-coated sample via electrochemical treatment. In order to investigate inelastic deformation in the sample, in-situ scanning electron microscope (SEM) observation is performed by means of a small tensile testing device that can be inserted into the SEM vacuum chamber. The bending test result indicated that the coating deforms with a nonlinear behavior under monotonic loading, in spite of the fact that it is a ceramic material. Spray parameters such as in-flight velocity affected the stress-strain curve significantly. In-situ SEM observation during the bending test revealed that sliding at boundaries between splats plays an important role in inelastic deformation.

The surfaces of gas turbine components are coated by thermal barrier coatings (TBCs) with a plasma spraying technique. Spallation damage is an essential problem, as well known, in usage of TBC. A lot of efforts for TBC interfacial strength examination had been done, however studies examined how residual stress are formed after the process and also the coating stress changes with temperature were limited. In this report, the residual stress prediction model is proposed based on the splat deposition process. Simplified model including the plasma sprayed process is developed based on shear-lag theory. The simplification is given in continuous particle deposition process. That is, continuous particle deposited coating is modeled as a single layer, which is called by "deposition layer" in this study. This deposition layer is assumed to impact directly onto the substrate. The binding layer is also introduced to express multiple cracks caused by quenching stress in splats and sliding deformation at splat boundary. It is shown that the numerical analysis has good agreement with the associated experiments.

It is well known that thermal spray condition affects the coating properties such as porosity, elastic modulus, coefficient of thermal expansion (CTE), coating fracture strength and coating cohesive strength. Therefore, residual stress formed in the sprayed coating and coating stress generated during in-service is dramatically changed with the thermal spray condition. In this study, effect of several kinds of thermal spray conditions on these properties of the coating was examined experimentally. Typical thermal barrier coating system composed of a partially stabilized zirconia (its chemical composition is 8wt%Y 2 O 3 -ZrO 2 ) and CoNiCrAlY bond coating was selected herein. In-flight particle velocity and temperature, and the substrate temperature were changed as the thermal spraying process parameters varied. For the ceramic coating layer, the coating properties such as porosity, Vickers hardness, CTE, elastic modulus, bending fracture strength, fracture toughness of splat boundary and then coating residual stress were measured systematically.