TiO 2 and TiO 2 -Al composites coatings were prepared by plasma spraying using a reconstituted nanosized feedstock via a spray drying method. Effects of various spray conditions on the microstructure, porosity, microhardness and wear resistance related to the mechanical performance of coatings were evaluated. The coatings sprayed at relatively low plasma power were composed of two distinct microstructures of well defined lamellar structure, similar to microstructure of conventional plasma sprayed coatings typically observed from fully melted particles, and embedded nano or sub-micron particles originated from partial/non-molten particles of feedstock materials. The fraction of partial/non-molten particles in coating layers was increased by Al additive. Such a characteristic of blended microstructure of coatings was clearly confirmed from a bimodal distribution of microhardness described by Weibull plots. The optimized addition of Al into TiO 2 improved mechanical properties such as microhardness and wear resistance.

In order to improve the efficiency of gas turbines, thermal barrier coatings (TBCs) have been applied to components in the hot sections of advanced gas turbines. During service, thermally grown oxide (TGO), which consists of an Al 2 O 3 layer and a mixed oxide layer, forms at the interface between the top coating and bond coating. It is supposed that the reason for failures of TBCs, such as cracking, delamination or spalling, is due to decreased bond strength caused by TGO growth or due to the formation of stress concentration sites caused by porosities in the mixed oxide. In this study, to inhibit the growth of TGO, plasma sprayed CoNiCrAlY bond coating was remelted with a YAG laser prior to spraying the top coating. A thin Al 2 O 3 layer formed at the top coating/bond coating interface, and the formation of porous mixed oxide during thermal aging tests was inhibited. Four-point bending tests showed that the bond strength of TBC with remelted CoNiCrAlY was superior to standard TBC.

Laser post-treatments and plasma-laser hybrid spraying processes are increasingly being used to extend the service life of thermal barrier coatings by making them more resistant to thermal shock. Studies show that laser-induced cracking plays a major role in the improvements achieved. The investigation of such modified layers can be difficult, however, because the stresses associated with metallographic procedures can alter the structural features of segmented microcracks and damage the specimen. In this research, laser treated and laser hybrid sprayed thermal barrier coatings are vacuum impregnated with fluorescent epoxy resins in order to study their microstructure and its relationship with thermal shock resistance. All relevant processes are described along with crack formation behaviors. Paper includes a German-language abstract.

Thermal spraying technique has been widely applied for the production of ceramic protection layer on metals that used in various hostile environments. However, all sprayed coatings have defects such as connected pores and unmelted particles, which deteriorate coating properties. To improve the properties of sprayed coatings, a lot of approaches have been undertaken such as laser irradiation, seal sintering with liquid alloys and sol-gel infiltration technique. Lasers are promising technological tools due to its speedy treatment and simplicity of process control. Moreover, laser treatment technology enables not only the post-treatment but also the pre and simultaneous treatment by combining with spraying process. Generally, wide beams of as uniform as possible are preferred for use in laser surface treatment to obtain a uniform depth of melting, alloying or cladding and to cover a large area by partially overlapping of tracks. However, it is not easy to produce a uniformly treated coating by conventional laser treatment method as desired. To obtain a near-uniform beam intensity for practical laser irradiation, a kaleidoscope was installed in a conventional YAG laser. In this research, laser beam properties of YAG laser equipped with a kaleidoscope and its effect on surface modification of plasma sprayed zirconia coatings and WC-Co system coatings prepared by HVOF spraying was investigated.