This study aims to better understand stress fields in composite coatings produced by plasma spraying. To that end, Al 2 O 3 -TiO 2 coatings are deposited under conditions that result in architectures differing in pore content, crack density, and crack orientation. SEM images of the coatings are divided into discrete stress domains that are analyzed by finite elements. FEA simulations show that network architecture has a significant influence on stress fields and that secondary phases have a particularly negative effect. The paper also proposes a generic method for stress analysis based on representative volume elements and points out its advantages and limitations.

Elastic properties of 20 and 40 µm thick deposits of yttria fully stabilized zirconia (YSZ), fabricated by vacuum plasma spraying (VPS) and air plasma spraying (APS) with modified injection system were investigated at room temperature by nanoindentation, and 4 point flexion test and at 800°C by 4 point bend test. The data was correlated with structural analysis of different YSZ deposits. At room temperature, E values of VPS YSZ deposit decreased from 237 ± 6 to 105 ± 5 GPa on increasing nanoindentation load from 1 mN to 450 mN. The results indicated change from intrinsic to defect-dependent E values with increasing load. Despite lower porosity of VPS deposit (6 ± 1%) compared to that of APS (24 ± 1%), E values, measured by flexion test at room temperature and at 800°C, of former were 35 ± 1 and 16 ± 1 and of latter were 55 ± 1 and 18 ± 1 GPa respectively. The interlamellar sliding, parallel to applied load, was considered as prime reason of lower rigidity of deposits.

Complex multi-scale pore network architecture characterized by a multi-modal pore size distribution and connectivity develops during the manufacturing of ceramic thermal spray coatings from intralamellar and interlamellar cracks generated when each lamella spreads and solidifies to globular pores resulting from lamella stacking defects. This network significantly affects the coating properties and their in-service behaviors. The De Hoff’s stereological analysis permits the quantification of the 3-D distribution of spheroids (i.e., pores) from the determination of their 2-D distribution estimated by image analysis when analyzing the coating structure from a polished plane. Electrochemical impedance spectroscopy aims at electrochemically oscillating a material surface by frequency variable current and potential and at analyzing the complex impedance. When a coating covers the material surface, the electrolyte percolates through the more or less connected pore network to locally corrode the substrate. The resistive and capacitive characteristics of the equivalent electrical circuit will depend upon the connected pore network architecture. Al 2 O 3 -13TiO 2 coatings were atmospherically plasma sprayed using several sets of power parameters, the arc current intensity, the plasma gas total flow rate and the plasma gas composition, namely to scan their effects on the pore network architecture. In parallel, particle characteristics upon impact, especially their related dimensionless numbers such as Reynolds, Weber and Sommerfeld criterion, were determined. Analyses permitted the identification of (i) the major effects of power parameters on the pore architecture and (ii) the related formation mechanisms.

In previous works, laser irradiation was associated to thermal spraying in the so-called MELTPRO process to improve the performance and the durability of TBCs. This study aims at presenting results concerning the thermal behavior of TBC's manufactured according to this process. Two types of severized thermal cycling were implemented: (i) “oxidizing” test: isothermal shocks were performed at different temperatures, ranging from 900°C to 1050°C down to 0°C; (ii) “quite non-oxidizing” test: thermal shocks were implemented from a temperature of 1100°C down to 50°C. Moreover, thermal annealing at 1100°C were performed to compare sintering phenomenon. TBC microstructure and its evolution during heat treating were characterized using image analysis, Knoop micro-indentation and XRD analysis. MELTPRO process was shown to increase twofold the lifetime of TBCs during isothermal shock tests. This is attributed to the fact that the columnar structure and the pore-crack architecture of remelted coatings improve the compliance property and decrease the permeability of TBCs. XRD analyses show that, in the Y-PSZ TBCs, the main phase is the metastable tetragonal (t’) phase both for as-sprayed and MELTPRO processed coatings. Moreover, remelted TBCs show a higher phase stability than as-sprayed TBCs during thermal shock tests: it seems that the remelted coatings have higher phase stability thanks to their pore architecture, which lead to a better compliance in relation to the thermo mechanical stresses, and so to a decrease in the stress variations undergone by the structure during the thermal cycles. During thermal annealing, it seems that MELTPRO processed coatings are less affected by sintering than as-sprayed coatings. Sintering phenomenon primarily concerns inter-lamellar cracks of non remelted areas. Besides, whatever the coating manufacturing process, the main remaining phase is the metastable tetragonal phase whatever the heat treating duration.

Yttria partially stabilized Zirconia (Y-PSZ) thermal barrier coatings (TBCs) have numerous applications as insulation layers onto gas turbine components. The main function of TBC is to enhance efficiency by increasing working temperature. These coatings are commonly manufactured using air plasma spraying. The characteristics of TBCs strongly depends on their pore and crack network architecture. Engineering the coating architecture by an adapted process is a prerequisite to modify TBC characteristics. Therefore, air plasma spray and in situ laser irradiation by diode laser processes were combined in this study to modify structural characteristics of TBCs. The coatings were remelted layer by layer during their deposition or alternately (i.e., an as-sprayed layer followed by an in situ remelted layer). This process was named MELTPRO. TBC apparent thermal conductivity was quantified implementing numerical computations based on 2-D real discrete structures. Coating thermal properties were correlated to their pore network architecture characteristics quantified by image analysis (i.e., nature, orientation, percentage, etc.). Moreover, thermal annealing at 1100°C were performed to compare sintering phenomenon of manufactured TBCs. Results show that the MELTPRO process permits: (i) to modify of the pore network architecture of as-sprayed TBCs. The porosity level increases by the in situ remelting process due to the formation of large horizontal cracks (as shown by image analysis). These horizontal cracks behave as thermal resistances and are responsible for the decrease of the coating thermal conductivity of about 30 % compared to the as-sprayed ones; (ii) to architecture differently the pore network for a constant thermal conductivity, because thermal conductivity and porosity level are not explicitly correlated; (iii) to improve the thermal insulation which remains unchanged after thermal annealing treatments: this seems to be due to the fact that the large cracks of remelted TBCs are less sensitive to sintering than inter and intra lamellar cracks of as-sprayed TBCs.

The morphologies of Y-PSZ coatings remelted after (i.e., post-treated) or simultaneously (i.e., in situ) with their deposition were observed by optical and scanning electron microscopy in order to study the behavior of the coatings during laser irradiation. A change in the microstructure, from lamellar to dendritic, was observed in both cases. Moreover, cracks and delaminations are less emphasized for the coatings treated during deposition than for those treated after deposition. Finally, the pore connectivity was evaluated implementing an electrochemical test. Results clearly indicate that the coatings obtained by in situ laser remelting are significantly more impervious than as-sprayed coatings.

Several studies have been undertaken recently to adapt Yttria Partially Stabilized Zirconia (Y-PSZ) thermal barrier coating (TBC) characteristics during their manufacturing process. Thermal spraying implementing laser irradiation appears as one of these possibilities to modify the coating morphology. This study aims at presenting the results concerning in situ (i.e., simultaneous treatment) and a posteriori (i.e., posttreatment) laser treatments implementing a high power laser diode. In both cases, the coatings were atmospheric plasma sprayed (APS). Laser irradiation was achieved using a 3 kW, average power, laser diode, exhibiting an 848 nm wavelength. Experiments were performed to reach two goals. First, laser post-treatments aimed at building a map of the laser processing parameter effects on the coating microstructure, in order to estimate the laser processing parameters, which seem to be suited to the change into in situ coating remelting. Second, in situ coating remelting aimed at quantifying the involved phenomena. In that case, the coating was treated layer by layer as it was manufactured. The input energy effect was studied by varying the scanning velocity (i.e., between 35 and 60 m.min -1 ), and consequently the irradiation time (i.e., between 1.8 and 3.1 ms, respectively).