Processing and characterization of multilayered and functionally graded CYSZ/Gd2Zr2O7 based new generation thermal barrier coatings

Thermal barrier coatings (TBC) are used on some critical parts such as blades, vanes and combustor chambers of gas turbines to protect metallic substrate, to provide thermal insulation and to improve the efficiency. A traditional TBC system consists of ceramic top coat and metallic bond coat (MCrAlY). On the other hand, thermally grown oxide (TGO) layer forms between the bond coat and the ceramic top coat during operation at high temperature. While the basic mission of the ceramic top coat is to reduce the temperature of substrate, metallic bond coat decreases the coefficient of thermal expansion (CTE) mismatch between the top coat and substrate. Yttria-stabilized zirconia (YSZ) has been widely and commercially used as a ceramic top coat material for TBC in the gas turbine systems. However, an undesirable phase transformation and sintering take place in the YSZ above 1170 °C. A diffusionless phase transformation from metastable tetragonal (t')-phase to tetragonal (t) and cubic (c) phase occurs above that temperature. This tetragonal phase transforms into monoclinic phase with a high volume change during cooling and this phenomenon causes the coating to be damaged. Therefore the usage temperature limit for YSZ is about 1170 °C. On the other hand, the advanced new generation gas turbine engines need to operate at higher turbine inlet temperatures pushing the temperature limits of YSZ. As a result alternative TBC materials should be selected having better thermal properties than YSZ under extreme conditions. Among the rare earth oxide materials, gadolinium zirconate (GZ) seems to be the most efficient one in terms of its low thermal conductivity, high phase stability and a very good resistant to damage by hot corrosion and CMAS attack. Beside all of these advantages, it has two poor properties effecting its thermal cycling performance in negative manner. One of them is its low CTE and another one is its high tendency to react with the TGO layer. In this study, for the purpose of increasing the thermal cycling performance of GZ-based TBC, multilayered (ML) and functionally graded (FG) GZ/CYSZ (ceria and yttria stabilized zirconia) thermal barrier coatings were produced in 2, 4, 8 and 12 layered by high-velocity oxy-fuel (HVOF) and air plasma spraying (APS) processes. Thus, as top coat material, high thermal stability, CMAS, hot corrosion resistance and low thermal conductivity properties of GZ were combined with property of high CTE of CYSZ. Also, single layered GZ, YSZ and CYSZ coatings were produced as reference samples. Commercial GZ, CYSZ, YSZ and bond coat powders (NiCoCrAlY) were used as starting powders. In the all designs, composition of the bottom and topmost layers were 100% CYSZ and GZ, respectively. TBCs containing only GZ or CYSZ in each layer were specified as MLed coatings. However, each layer had different ratio of mixture (wt.%) of CYSZ and GZ in the FGed coatings. Different amounts of GZ powder were mixed with CYSZ powder for producing FGed coatings. GZ and CYSZ powders were weighed according to the composition of each layer and then mixed for 6 h in a turbula type of mixer using zirconia balls. Before coating operations, INCONEL, 316L and Al substrates were subjected to cleaning and grit blasting process. Afterwards, NiCoCrAlY powder was sprayed onto the prepared surface in a total thickness of 90 ± 20 μm by HVOF process. GZ, CYSZ, YSZ and mixture of ceramic top coat powders were sprayed onto the bond coating layer by APS system. In both of the coating designs, individual layers had approximately equal thickness of the layers. Total thickness of the ceramic coatings was about 350 ± 50 μm. Microstructural characterizations were carried out by field emission gun scanning electron microscope (FESEM) attached with Energy Dispersive Spectrometer. An image analysis software (Image J) and back-scattered electron images were used to process porosity measurements. The phase characterizations were performed using X-ray diffractometer within the range of 10 to 90° using Cu Kα radiation. ASTM-C633 standard was used to determine the bonding strength of the coatings. The back surface of the substrate and surface of the ceramic top coat were glued to an apparatus thanks to a high performance epoxy adhesive. The bonding strength experiments were exerted by using a universal testing machine. The values of bonding strength were calculated using the relationship between load and area when the failure occurs on the sample. Fracture types were determined by evaluating surface of the fracture regions. TC performance test of the MLed and FGed TBCs was carried out by using a dynamic heat flux of oxygen-propane flame. Surface of the coatings was heated to 1250 °C ± 50 °C for 1 min. by an oxy-propane flame, and then cooled down (below 150 °C) for 1 min. by using air jet. A cycle consisted of heating + cooling period, and therefore, total time of a cycle was 120 seconds. Maximum temperature on the back surface of substrate was about 890 °C during heating period. The thermal conductivity of the coatings was determined by use of a laser flash thermal properties analyser from room temperature to 1105 °C. CMAS and hot corrosion tests were done by using a CO2 laser as heat source at 1250 °C and held for 60 min. Specimen surfaces were coated with the CMAS+hot corrosion products at a concentration of 30 mg/cm2. GZ TBC's were subjected to a laser remelting process to determine the optimum laser surface modification parameters. The effect of laser remelting on the surface roughness, microstructure, grain size, hardness and phase transformation of the GZ coatings were investigated. According to FESEM analysis, typical surface and polished cross-sectional images of the APSed coatings were obtained. It was understood from the images that GZ and CYSZ powders sufficiently melted in the plasma flame, and thereby, GZ and CYSZ layers seem having a well lamination to each other and to the bonding layer. Characteristic microstructural defects of APSed TBCs such as porosities, and also cracks both parallel (at splats boundaries) and normal to the bond coat/ceramic interface were observed in all of the coatings. Porosity level of reference single layered coatings were increased from 9.21 to 15.12 % with the MLed and FGed coating designs. This situation was attributed to the discontinuous coating process due to layer by layer spraying of MLed and FGed designs. After a layer was sprayed, changing of the powder and parameter required a time for spraying another layer, and this caused fast cooling and shrinkage of the sprayed layer. As a result, vertical micro cracks and porosities formed on the layers. Bonding strength values changed between 8.87 and 12.10 MPa. It is concluded that the values of the bonding strength decreased with the increasing porosity level. They were comperable values with the literature. Whereas fracture type on the coating of single layered GZ was fully adhesive mode, generally a complex adhesive/cohesive mode fracture was observed on the MLed and FGed coatings. There was no fracture between the bond coat/substrate. This was due to the high adhesion strength between metallic substrate and bond coat applied by HVOF process. During thermal cycling test, 300 cycles were applied to the samples. First crack started to form at the edge of ceamic top coats. There was no spallation and microstructural cracks on FGed coatings after 300 cycles, while big spallation was observed on single layered reference GZ and multilayered coating having two layers after 165 and 185 cycles, respectively. The results indicated that, thermal cycling performance of GZ single layered coating was improved thanks to MLed and FGed designs. Cubic fluorite type structure with the space group Fm3m was the main phase on powders. There was no phase transformation observed for GZ after plasma spraying and thermal cycling at 1250°C. It was concluded that the main failure mechanism was high thermal stress which is generated due to the sharp difference in the CTE of bonding layer, CYSZ and GZ phases. Thermal conductivity values of MLed and FGed coatings were changed between 1.19 - 1.76 W/mK at 1105 °C and 0.64 – 0.92 W/mK at room temperature. Additionally, thermal conductivity values of YSZ1, CYSZ1 and GZ1 single layered reference samples were 1.76, 1.59 and 1.36 W/mK (at 1150 °C), respectively. The lowest values were achieved for multilayered and functionally graded coatings having highest number of layers. It is possible to say that the thermal conductivity value decreased by increasing the number of layers. This was due to the increasing interfaces between the layers. Also increasing porosity level of the coatings resulted in decreased value of thermal conductivity. Because porosities acted as phonon scattering centers and a reduction took place in thermal conductivity. CMAS and hot corrision resistance of MLed and FGed designs were higher than YSZ type coating. Size and number of the crystalline rod shaped hot corrosion products were smaller than that of YSZ and CYSZ. On the other hand, a rection layer with thickness of ~ 6 µm came into existance on the GZ surface and this layer blocked the penetration of molten products. After laser surface modification, remelted GZ top layers were obtained within laser power density and scan speed from 70-110 MW/m2 and 90-150 mm/s respectively. Optimum laser remelting parameters in terms of melting depth, cross-sectional damage, surface quality and distribution of the crack network were determined as LGZ-5. Throughout a thin (~35 µm) remelted layer; surface roughness decreased from 8.3 µm to 2.9 µm for as-sprayed and laser remelted specimens respectively. A smooth, flat and dense surface having equiaxed continuous segmented crack network that is perpendicular to the surface was obtained, and open porosities were sealed. Fine and equiaxed grain structure were observed and this structure had columnar morphology growing perpendicular to the surface. The grain size of remelted layer decreased from 7.03 µm to 3.69 µm as the process parameters approached to optimum. Hardness of the remelted region was affected from the laser parameters and it increased from 10.66 GPa to 12.73 GPa with decreasing grain size. No phase transformation was observed after laser remelting but the highest XRD peaks changed due to the preferred orientation.