Cold-spray (CS) technology is a "solid-phase particle deposition process without melting;" however, it has been established as an additive manufacturing technology that can be applied beyond the framework of one field of thermal spraying. The scope of application of this technology has expanded to include ceramics and polymers. There are other solid particle deposition processes besides CS, such as aerosol deposition (AD), which differ in the material type, size, impact speed and temperature of the target particles. We can expect that there is a common intrinsic mechanism through which solid-phase particles are joined and deposited in the solid phase. This review summarizes previous studies on the mechanism of cold-spray deposition and bonding, which can be understood as a mechanochemical phenomenon in part, and it is driven by the deformation of the particles and the resulting change in the chemical state of the particle surface, and stabilization by contact in a short time. When we understand these issues correctly, the optimal mechanical conditions (material size and collision conditions) for joining particles of various materials will be systematically understood, and it will be possible to perform different fabrication processes from thin films to additive manufacturing without melting various materials.
In the next generations of turbine engines, improving the heat resistance and reducing the weight are the key essential solutions to increasing the thermal efficiency, reducing fuel consumption and CO2 emissions. The silicon-based ceramics (ex. SiCf/SiC ceramics matrix composites: CMCs and monolithic Si3N4) are the leading candidates for these applications because of the extreme light weight and superior heat resistance compared to the current Ni-based alloys. The main challenge of the Si-based ceramics is its oxidation and volatilization of the silica in the high temperature combustion gas environment with the water vapor, and thereby its rapid recession. The most promising approach is protecting the surface of the CMCs from the water vapor attack by an external environmental barrier coating layer (EBC). Since the early 1990′s a lot of efforts have been done and no single material can satisfy all the EBC requirements. Thereby, the current EBC trends are directed to the development of multilayered EBCs, with different functions as a significant solution to prevent the CMC recession and to maximize its performance for next engine generations. This paper discusses the history, current status and future trends of EBC development not only in the world but also in Japan. Furthermore, it introduces the future prospects of fine particle spraying in EBC developments.
Optimal deposition parameters for the aerosol deposition of a β-SiAlON coating and the microstructure change of an EBC after heat exposure in air are investigated. Dense and crystalline SiAlON coating having developed texture, where the (0001) plane is declined approximately 10° from the coating plane is formed. The deposition rate increases with the gas flow rate when the rate is ranging from 12 to 16 L/min. Further increase of the gas flow rate decreases the deposition rate. Regarding the 15 μm thick mullite coating deposited on SiAlON substrate heat exposed at 1573 K over 30 h, delamination of the coating occurs due to the oxidation of SiAlON. 30 μm thick mullite coating prevents the oxidation. As for the EBC deposited on Si-SiC substrate, delamination occurs at Si-SiC/SiAlON interface by the oxidation of SiC during heat exposure at 1573 K. At the bonded region during heat exposure, SiAlON prevent the mullite coating to become (SiO2+mullite) two-phase state by supplying Al to the mullite. Residual Si at the substrate move to SiAlON and mullite coating under heat exposure at 1673 K. Structure of EBC is maintained by using SiC substrate in which the Si will not move to the coating during exposure
A thermal barrier coating (TBC) is an indispensable technology for improvement of the efficiency of advanced gas turbines. However, in the TBC system, thermally grown oxide (TGO) forms at the interface between the top-coat and bondcoat during opration. Delamination or spallation at the interface occurs by the TGO formation and growth. Therefore, formation control of the TGO is important to improve the delamination resistance of the TBC. Previously, we succeeded in improving the delamination resistance of TBC by adding cerium (Ce) to the CoNiCrAlY alloy as a bond coat (BC) which assist the formation of the inward TGO. The inward TGO reduces the thermal stress experienced by TBC through the formation of vertical cracks initiated by the inward TGO. The aim of our study is to improve the delamination resistance of TBC and develop TBC with inward TGO. For this purpose, TBC with CeO2 and ZrO2 added to the BC material were prepared. To perform TBC specimens with internal oxide in the BC, BC materials with several particle sizes and several BC spraying methods were used. The internal oxide amountand delamination resistant property of these TBC specimens were evaluated with SEM observation and four-point bending test.