Hybrid plasma spraying combines plasma spraying of dry powders and liquids (suspensions and solutions). Combination of these two approaches allows deposition of microstructures consisting of both conventional coarse and ultrafine splats. Moreover, splats with dissimilar size may have also different chemistry. Such combination is potentially interesting for many fields of thermal spraying, including thermal barrier coatings (TBCs), as novel microstructures may be economically and relatively easily obtained. The technology has recently reached a level, where coatings with interesting hybrid microstructures may be reliably deposited, so that their potential for practical applications may be evaluated. In this study, first experimental TBCs with YSZ-based hybrid topcoat were deposited by hybrid water/argon stabilized plasma (WSP-H) technology. Al 2 O 3 and YAG were selected as secondary phase deposited from suspension as both provide strong materials contrast in scanning electron microscope (SEM) so they can be used as “markers” in the coating microstructure. Samples were exposed to thermal cycling simulating in-service TBC conditions in order to test their thermal shock resistance. Changes of the coating microstructure were studied by SEM analysis and X-ray diffraction.

Environmental degradation of thermal barrier coatings (TBC) by molten deposits such as calcium magnesium alumino-silicates (CMAS) is one of the most vital factors resulting in the failure of thermal barrier coatings, while turbine engine inlet temperatures are kept increasing for higher fuel efficiency. A new phase composite ceramic had been developed and evaluated for the topcoat of a durable thermal barrier coating (TBC) system with low thermal conductivity property and improved erosion resistance. The present work is to continue the effort to exploring the behavior of CMAS resistance of the phase composite TBC at high temperatures. The effects of CMAS attack and thermal exposure on the TBC degradation were investigated in experimental runs. In addition, a YAG-modified layer over the top of the TBC was applied with the attempt to improve CMAS resistance of the TBC system. The evaluation of CMAS resistance was focused on the most important characteristics of coating microstructure, CMAS penetration, and failure mode and test condition factors. The mechanisms for the CMAS infiltration and the TBC damages were discussed based on the analyses of the CMAS corroded samples in details.

Yttrium aluminum garnet (YAG) has desirable properties for a thermal barrier coating (TBC), although there are two production challenges. One, YAG has a relatively low thermal expansion coefficient which leads to large thermal mismatch stresses, and two, amorphous phases are produced by atmospheric plasma spraying. Solution precursor plasma spraying (SPPS) has to potential to solve both problems. First off, it produces no amorphous phases. Secondly, it can produce a cracked microstructure that mitigates the CTE mismatch issue. To judge the adequacy of the properties of SPPS YAG, a summary of the properties of common TBCs is presented. It is shown that the properties of SPPS YAG fall within desirable or usable ranges. Current efforts described in this paper focus on improving the efficiency and rate of deposition.

Recently, hybrid water-stabilized plasma technology (WSP-H) was successfully adapted for the deposition of tailored single-layer coatings using liquid feedstocks (i.e. suspensions or solutions). High enthalpy of the WSP-H torch enables efficient deposition of the coatings with high feed rates and novel microstructures. In this study, concurrent and consecutive deposition of alumina-titania, YAG and YSZ suspensions was carried out in order to demonstrate the possibility of depositing multiphase coatings having layered, intermixed or even functionally graded (FGM) microstructures. Successful deposition of all three types of microstructures proves the feasibility of further development of the next generation of suspension plasma-sprayed coatings using WSP-H technology.

In this study, the spray-dried Al 2 O 3 /Y 2 O 3 composite powder was prepared using commercially available nanosized Al 2 O 3 and Y 2 O 3 feedstock. The mass ratio of Al 2 O 3 to Y 2 O 3 was 2:1. Atmospheric plasma spraying possesses high thermal enthalpy, large temperature gradient and rapid cooling rate. The microstructure and wear resistant performance of plasma-sprayed Al 2 O 3 .Y 3 Al 5 O 12 (YAG) coating were investigated. As-sprayed Al 2 O 3 .YAG coating was chiefly composed of amorphous phase, which may reveal superplastic feature in supercooled liquid region (from 503.0°C to 906.5°C). The as-sprayed Al 2 O 3 .YAG coating exhibits fine plasticity and toughness. Friction and wear tests of the coatings were executed on a MMU-5GL tribological tester using a ring-on-disk arrangement. The coatings were deposited on end flat surfaces of the wear rings. The graphite disks were prepared. The wear tests were conducted at following conditions: relatively high load of 2000N; a rotational speed of 500rpm (equivalent to a sliding velocity of 0.68m/s). Friction coefficients could be obtained real-timely. The thermocouple was applied to measure the worn surface temperatures. The Al 2 O 3 /YAG amorphous coating/graphite pair possessed lower friction coefficient and worn surface temperature, compared with Al 2 O 3 coating/graphite and Al 2 O 3 -Cr 2 O 3 coating/graphite pairs. After wear tests, many network cracks visible to naked eye appeared the surfaces of Al 2 O 3 and Al 2 O 3 -Cr 2 O 3 coatings. However, no cracks were observed on the worn surface of the Al 2 O 3 /YAG amorphous coating. Therefore, plasma-sprayed Al 2 O 3 .YAG coating possesses excellent wear resistance under severe conditions with high pv values (p: contact pressure; v: friction velocity).

This paper presents the results of three experiments in which coatings are produced by suspension plasma spraying using a water-stabilized hybrid torch. Shadowgraphy is used to optimize the injection of the suspension and visualize liquid fragmentation in the plasma jet. Deposition efficiency is evaluated and the production of coatings with different microstructures is demonstrated for YSZ, YAG, and Al 2 O 3 .

In this investigation, commercial Al 2 O 3 and Y 2 O 3 nanopowders were used to produce a composite powder feedstock that was plasma sprayed on graphite substrates. An Al 2 O 3 /Y 3 Al 5 O 12 (YAG) eutectic crystalline structure was expected in the coating due to the high enthalpy, large temperature gradient, and rapid solidification of the plasma spraying process, but microstructure and phase analysis of as-sprayed deposits revealed only a small amount of crystalline grains in an amorphous coating matrix. After heat treating, the expected structure was obtained and found to have excellent microstructure and microhardness stability at high temperatures.

This study assesses the effect of acid corrosion on the luminescence of YAG:Ce coatings. The feedstock powder is prepared by high-temperature solid phase synthesis and the coatings are deposited by air plasma spraying. Microstructure and phase composition are characterized and the effect of acid immersion duration on luminescent intensity is measured. It is found that the luminescent properties of YAG:Ce 3+ coatings have a tendency to fluctuate with immersion time, which appears to be related to phase composition.

Novel synthesis of thermal spray grade silicon carbide (SiC) feedstock powder is necessary to allow deposition of this material using atmospheric plasma spraying (APS) method. SiC particles with average size of 1.0 µm are treated using co-precipitation techniques to deliver yttrium aluminum garnet (YAG) binder from its solution precursor as a nano-film onto SiC particles surface. The YAG nano-film will protect SiC core from direct interaction with plasma jet thus hindering their decomposition as well as providing matrix phase within the SiC particles vicinities. The modified SiC particles are sintered and crushed and then sieved to separate 25-45 µm and 45-90 µm size powders, which are then plasma sprayed to deposit SiC coatings of about 300 µm in thickness. Both the feedstock and the coatings were analyzed and compared with regards to their phase composition and microstructures.

In this study, gas- and water-stabilized plasma torches were used to spray cesium-doped yttrium aluminum garnet (Ce:YAG) on different substrate materials and in large-area free-standing layers. The coatings were evaluated based on microstructure, crystallinity, and thermal stability, and tests were performed to measure porosity, hardness, phase composition, band-gap energy, and the presence of defects. Some coatings were also heat treated to determine how it changes their spectral response. The results of the examinations and tests are presented in the paper.

Although thermal spraying technique is used in many industries, it suffers from several problems. For example, the hardness of the coatings is lower than that of sintered material for the incompleted cohesiveness. An yttrium aluminum garnet (YAG) laser was used during HVOF spraying to improve the properties of the applied coating. Several carbide powders were used as thermal spray materials, and stainless steel (SUS304) was used as substrate. Coatings were sprayed by hybrid spraying method, which was combined HVOF spraying with YAG laser. The hardness of coatings sprayed by hybrid spraying was higher, and the weight loss in a blast-erosion test was smaller than that of coatings applied by HVOF spraying only. The particles deposited in the coatings obtained by hybrid spraying were very fine. Laser irradiation to the HVOF flame improve the adhesion strength between particles and the deposition of fine carbide particles in the coating. It was considered that mechanism of coating strengthening in hybrid spraying was resulted from strengthening of cohesiveness by heating effect and decreasing of porosity by flattening effect according to observation results of Ni splats sprayed by hybrid spraying method.