Cr 3+ Photoluminescence piezo-spectroscopy (CPLPS) has been developed as a non-destructive inspection technique for the measurement of residual stresses within the thermally grown oxide (TGO) in thermal barrier coatings (TBCs). In this study, plasma spray - physical vapor deposition (PS-PVD) process was used to deposit yttria stabilized zirconia (YSZ) topcoat with quasi-columnar structures. Evolution of the microstructures and residual stress distribution in such kind structured TBCs before and after thermal cycle test on burner rigs were investigated. The accumulated tensile stress in the as-sprayed ceramic topcoat changed to compressive state after 100 cycles, and then gradually increased. In addition, the mapping compressive stresses in the TGO measured through the ceramic topcoat surface decreased rapidly firstly and then essentially maintain at a relatively stable value with further testing. Moreover, the pre-oxidation of the bondcoat could significantly affect the stress distribution in the TGO, in contrast, no obviously influence on the stresses in the YSZ topcoat.

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).

In this study, alumina coatings are deposited by atmospheric plasma spraying and sealed using an aluminum phosphate solution containing a small fraction of alumina nanoparticles. Potentiodynamic polarization, electrochemical impedance spectroscopy, and salt spray tests are used to evaluate the corrosion behavior of both the as-sprayed and sealed coating samples. Besides improving corrosion resistance, the sealing treatments are also shown to increase the electrical resistance of the coatings, making them better electrical insulators.

This study assesses the thermal stability of YSZ coatings produced from nanostructured feedstock by means of atmospheric and suspension plasma spraying. Free-standing YSZ coatings were isothermally treated for 24 h at different temperatures (1200-1600 °C) and at 1550 °C for 20 to 100 h. Afterwards, the coatings were examined to determine the effect of heating on phase composition, microstructure, morphology, and hardness. No evidence of tetragonal-monoclinic phase transformation was detected in the coatings that had been treated for 24 h, even at 1600 °C, but in coatings treated for different periods of time at 1550 °C, a phase transformation occurred after 40 h. Overall, the suspension plasma sprayed coating showed the greatest degree of change due to thermal aging.

In the current work, yttria stabilized zirconia splats and coatings were deposited onto superalloy substrates at different temperatures using axial suspension plasma spraying (ASPS). The splats were characterized by scanning and transmission electron microscopy. As the substrates were heated from room temperature to 300 °C, splat morphology changed rapidly. Examination of the morphological and crystallographic features of splats revealed that cooling rate during impacting and spreading might be lower than that of splats deposited by conventional atmospheric plasma spraying. Based on these observations, a possible mechanism is proposed to explain the cauliflower microstructure.

This study evaluates the thermal cycling performance of thick thermal barrier coatings (TTBCs). YSZ topcoats with segmentation cracks were deposited by suspension plasma spraying (SPS) on Ni-base superalloy substrates with the aid of a CoNiCrAlY bond coat applied by HVOF spraying. The as-sprayed SPS coatings were characterized based on surface morphology, cross-sectional microstructure, and phase composition. Thermal cycling tests were then carried out on a burner rig that heated the coating surface to 1523 K, followed by quenching to 423 K using compressed air. The SPS coatings exhibited longer thermal shock life than atmospheric plasma sprayed (APS) YSZ, which is attributable to improved strain tolerance due to the presence of vertically segmented cracks.

Columnar structured coatings, due to their high strain tolerance, are well suited for thermal shock applications and thermal cycling service. In this study, plasma spray-physical vapor deposition (PS-PVD) was used to produce YSZ coatings with quasi-columnar microstructure. Thermal cycling and thermal shock tests were performed, and coating microstructure, phase composition, and residual stresses were evaluated before and after testing. Coatings obtained prior to process optimization, on average, made it to 623 cycles at 1250 °C before 10% of the coating surface showed signs of spallation.

Yttria stabilized zirconia coatings were deposited by plasma spraying and heat treated in air at 1100 °C for 50-200 h. Residual stresses in the ceramic topcoat and the thermally grown oxide (TGO) layer were measured before and after thermal exposure. After 50 h of exposure, tensile stress in the as-sprayed topcoat changed to compressive, which then increased with additional exposure time up to 150 h. The average compressive stresses in the cross-section of the TGO layer are shown to be higher than those on the surface of the oxide. In addition to shedding light on the nature and evolution of stresses in plasma-sprayed thermal barrier coating (TBC) systems, the results of the study also provide insights on crack initiation and propagation in the ceramic topcoat and at the topcoat-TGO-bond coat interface and its role in TBC failures.

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.

Failure is an inevitable consequence with thermal barrier coatings and failure modes are complicated due to irregular microstructure in the coating layers and wide range of external conditions. In this study, three-point bend tests are used to monitor damage evolution in YSZ-CoNiCrAlY TBCs on superalloy and stainless steel substrates. Coating samples, consisting of the bond coat and topcoat, were deposited by atmospheric plasma spraying on test specimens measuring 80 x 6 x 4 mm. The long, narrow specimens were subjected to three-point bend testing, using acoustic emission sensors to detect the formation and propagation of cracks in the coatings and plastic deformation in the substrates. The investigation results indicate that variations in acoustic emission signals correspond well with changes observed in the stress-strain curves of the coatings and substrates and that failure mechanisms can be systematically analyzed based on the amplitude, frequency, and energy of the acoustic emission signals. A detailed description of the actual failure process is provided.

In the present work, YSZ coatings were deposited on graphite substrates by plasma spray-physical vapor deposition (PS-PVD) in order to study the influence of spray distance on microstructure and durability. Four coating samples were examined in detail via SEM and XRD analysis. The results show that the as-sprayed YSZ has a dense lamellar-columnar microstructure with low porosity. Both monoclinic and tetragonal zirconia were detected in the coatings along with ZrO 2 -x, the latter indicating that oxygen loss occurred at short spraying distances. Coating hardness and Young’s modulus were also measured and were found to vary with spraying distance as well.

In the present study, Al 2 O 3 and Al 2 O 3 -Y 2 O 3 composite coatings were deposited by atmospheric plasma spraying and compared based on microstructure, hardness, and sliding wear behavior. The mass ratio of α-Al 2 O 3 /γ-Al 2 O 3 was found to be 0.53 in the composite, compared to 0.08 in the alumina layer, which explains why the Al 2 O 3 -Y 2 O 3 coatings are harder and better conductors of heat. Tribological tests show that the friction coefficients of the composite coating samples are more stable and the wear rates lower, which may be related to the formation of a graphite transferred film on abraded surfaces.

In this work, finite element modeling is used to investigate the influence of segmentation cracks on stress distribution and failure in thermal barrier coatings deposited by atmospheric plasma spraying. The results indicate that the presence of segmentation cracks does not improve thermal insulation, but it may be beneficial in regard to thermal shock resistance, depending on crack density, and residual stress around crack tips, depending on crack length. It may also improve strain tolerance, which is affected by crack density as well as length. A model is proposed to explain the mechanism of failure in thick TBCs exposed to thermal shock. Damage caused by thermal shock can be attributed to the propagation of segmentation cracks and the formation of horizontal cracks at the bond coat-topcoat interface.

This investigation evaluates the microstructure and properties of thermal barrier coatings produced by suspension plasma spraying. A YSZ suspension was injected axially into a plasma jet and deposited on a superalloy substrate with a CoNiCrAlY bond coat. SEM examination revealed a columnar microstructure with a network of vertical segmentation cracks and horizontal branching cracks. In furnace cycle testing, the TBCs withstood 166 thermal shock cycles with failure attributed to partial spallation of the columnar segments initiating at the edge and center of the coatings. The TBCs were also subjected to burner rig tests to assess thermal insulation properties and to heat treatments up to 1600 °C to evaluate thermal stability based on phase composition, grain size, and microhardness.

Rare-earth zirconates with a pyrochlore structure have attracted great attention for potential application in thermal barrier coatings to further improve the performance and durability of gas turbines. In present work, the Sm 2 Zr 2 O 7 coating was deposited by air plasma spraying technology, and its microstructure and phase composition were examined. The as-sprayed Sm 2 Zr 2 O 7 coating exhibited anion-disordered while cation-ordered fluorite-type structure. Degree of ordering was considerable enhanced after high temperature aging, and transformed to ordered pyrochlore-type structure after thermal aging at temperatures above 1200 ºC. The typical lamellar structure for the as-sprayed Sm 2 Zr 2 O 7 coating gradually decreased with increasing thermal aging temperature, which was caused by microcrack healing at high temperatures.

Gadolinium zirconate (Gd 2 Zr 2 O 7 , GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3mol% yttria partially-stabilized zirconia (3YSZ) incorporation. The fracture toughness of the composite of 90mol%GZ-10mol% 3YSZ (GZ-YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ-YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock resistance of GZ-YSZ composite coating was improved significantly, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of nanostructured 3YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ and GZ-YSZ composite coatings were also discussed.

Al 2 O 3 and Cr 2 O 3 coatings were deposited by atmospheric plasma spraying. The tribological properties of coatings against copper alloy were evaluated with a block-on-ring configuration under dry friction conditions at room temperature. Microstructure of powders and coatings were observed using scanning electron microscopy (SEM) and optical microscope (OM). Some thermo-physical properties of coatings were measured. Results showed that the Cr 2 O 3 coating exhibits cracking failure in the sliding condition with a normal load of 500N and a sliding velocity of 0.84 m/s. This failure of the plasma sprayed Cr 2 O 3 coating is explained in terms of its low thermal conductivity.