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Development of oxide dispersion strengthened (ODS) bondcoat for gas-turbine components Introduction Gas-turbine components work in very severe conditions, caused by mechanical stresses, high temperatures, oxidation and hot corrosion. To protect the base material the state of the art suggests the use of a multilayered thermal barrier coating, based on an external ceramic layer and an inner bondcoat. The ceramic layer (usually Yttria-stabilized zirconia, YSZ, deposited by Air Plasma Spray, APS) acts as a thermal barrier to protect the component against the high temperatures, whereas the bondcoat (usually a Co,Ni-CrAlY coating deposited by High Velocity Oxygen Fuel HVOF) protects the material from oxidation and hot-corrosion. During the working life the formation of a thermally grown oxide layer (TGO) between the topcoat and the bondcoat take place. This system usually fails due to the complete spallation of the ceramic layer, caused by the mismatch between the ceramic layer and the TGO grown above the bondcoat. Therefore in order to improve the behavior of this system new coatings have been investigated, in order to enhance the oxidation and hot corrosion resistance and to reduce the mismatch among those different layers. The present investigation aims to improve the bondcoat behavior at high temperatures by using oxide dispersion strengthened (ODS) bondcoat to replace the traditional M-CrAlY coatings (M=Ni and/or Co). An ODS material consists of nano or microstuctured oxide particles embedded in a matrix based on a ductile material. The present investigation deals with the study of ODS-materials based on a metal matrix (standard CoNiCrAlY powders for thermal-spraying processes) and alumina powders as a strengthening media. The ODS-rich particles are generally obtained by mechanical milling (alloying). Oxide particles and metal powders were mixed together and then milled for several hours, using a milling machine. During the milling process, due to the ball-powder collisions, the hard phase is embedded inside the metal matrix, forming a homogenous powder. The particle size distribution and the morphology of the particles depend on the milling parameters and on the features of the starting powders. The first part of this work deals with the optimization of the milling procedure in order to obtain suitable powders for the thermal-spraying process. Different kind of Al 2O3 powders and milling parameters are used in order to achieve a homogenous microstructure and a proper particle size distribution. The second part deals with the coating deposition and with the characterization tests, necessary to investigate the effect of the oxide dispersed particles on the high temperature behavior of the bondcoats. ODS systems are usually designed to improve the mechanical properties of the selected component, such us the wear resistance or the high temperature creep resistance. The novelty of this approach lies in the use of ODS composite materials to improve the high temperatures behavior of gas-turbine components. Methods For the production of the dispersion strengthened powders, a commercial CoNiCrAlY powder (Amdry 995C, -90 + 45 μm, Sulzer-Metco), with a nominal composition (in wt. %) of Co–32Ni–21Cr–8Al–0.5Y, was used as starting matrix material. The strengthening material employed in this investigation was aluminum oxide. In order to investigate the effect of the particle size on the microstructure of the milled powders and in order to reach an appropriate distribution of the hard phase, different kinds of alumina were used: AKP-30 (α-Al2O3, -0.5 + 0.3 μm, Sumitomo Chemicals Co.,Ltd., Japan), Martoxid MR-70 (α-Al2O3, -3 + 0.1 μm, Martinswerk GmbH, Germany) and Amperit 740.0 (α-Al2O3, -22 + 5 Figure 1 - scheme of the milling process μm, H.C. Starck GmbH, Germany). Different amount of alumina (10, 20 and 30 wt. %) were added to the standard bondcoat powder, to evaluate the high temperature behavior and properties of coatings based on different formulations. A Simoloyer CM01 (Zoz, Wenden, Germany) high-energy milling machine equipped with a 0.7 l chamber was used for the milling process. The milling media consisted of 100Cr6 stainless steel balls, with an approximate diameter of 5 mm. The ball-to-powder mass ration used in this study was 10:1, selected according to previous investigations. The chamber was filled with 1400 g of milling balls and 140 g of powder mixture. In order to achieve a high relative velocity between the milling media within the mill and in order to cool down the machine, the rotation speed was varied during the process: every 4 minutes the chosen revolution speed was decreased down to 800 rpm for 1 minute. The ground revolution speed was varied from 1200 to 1600 rpm, to investigate the effect of the milling speed on the particle size and microstructure (fig. 1). The progress of the milling process was investigated by taking powder samples every hour for a maximum milling time of 6 hours. After the milling process powders were investigated by means of optical microscopy and particle size distribution measurement, in order to select the optimal milling parameters and powder formulations. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) will be also performed, to detect chemical composition and crystalline phases of the powders. The most promising powders were then deposited on steel substrates by using 2 different thermal-spraying techniques: High Velocity OxyFuel (HVOF) (DJ-2600, Sulzer Metco) and Air Plasma Spray (APS) (Triplex Pro, Sulzer Metco). The best spraying parameters were used to deposit a thin ODSbondcoat layer on a standard bondcoat (based on CoNiCrAlY Amdry 9954 powder) previously deposited by HVOF (and heat treated in a Figure 2 - topcoat + thin ODS-bondcoat + standard bondcoat vacuum atmosphere) on an Inconel substrate. XRD, SEM (and Energy Dispersive X-Ray Spectroscopy, EDS), optical microscopy and roughness measurements were performed. Subsequently half of the surface of the samples based on the thin ODS-bondcoat layer deposited on the traditional bondcoat was coated with a standard YSZ topcoat (fig. 2). These samples were then put inside a oven set at 1100°C (air atmosphere) and cooled down for 1 hour by taking them out from the furnace each 23 hours, to investigate the oxidation and the thermal shock resistance. Results and discussion CoNiCrAlY Amdry 995C (80 wt.%) + Al2O3 MR-70 (20wt.%), effect of the milling time In order to investigate the effect of the milling time on the particle morphology, the powder batch based on the CoNiCralY powder Amdry 995C with the addition of the MR-70 alumina powder, was milled up to 6 hours, using the standard cycle previously described (fig. 1). The powder was analyzed every hour by means of optical microscope and particle size distribution measurement. Figure 3 shows the percentile results (D10, D50, D90) of the six powder batches (ConiCrAlY-Al2O3 80-20 wt. %) as a function of the milling time. It can be noticed that the particle size decreases increasing the milling time, up to 3 hours. After this time the trend is less regular, because fragmentation and agglomeration phenomena occurred at the same time: after 4 hours the agglomeration phenomena are more relevant, whereas after 5 and 6 hours powders are more brittle (cause the metal matrix is work-hardened) and fragmentation phenomena prevail. Increasing the milling time made also the particle size distribution curve more narrow. SEM images (fig. 4) show the evolution of the particle microstructure during the milling process. The lamellar structure of powders milled for 1-2 hours was gradually Figure 3 - Percentile results (D10, D50, D90 values) as a function of the milling time. converted into a more homogeneous microstructure, in which the hard phase is better embedded inside the metal matrix. Figure 4 - Optical microscope images (cross-section) of the powder 995C-M20, after 1 hour milling (left) and after 6 hours (right). CoNiCrAlY Amdry 995C (80 wt.%) + Al2O3 MR-70 (20wt.%), effect of the milling speed In order to evaluate the effect of different milling parameters on the particle size and morphology new powders were milled varying the rotation speed (1200 and 1400 rpm, up to 4 hours). The particle size distribution curves of powders milled at 1200 and 1400 rpm show that the variation of the milling speed affect the particle size, because the energy generated by the ball-particle collisions is lower. In both batches even after 3 hours the curves are bimodal, indicating the presence of alloyed and unalloyed particles. The optical microscope images confirm what previously assumed revealing a high amount of pure metal lamellae and partially alloyed particles even at the end of the milling process (4 hours). CoNiCrAlY Amdry 995C (80 wt.%) + Al2O3 MR-70 (10, 30wt.%), effect of the Al2O3 amount The comparison among samples containing different amount of alumina (10,20 and 30 wt. %) reveals that a higher amount of the hard phase brought to a finer powder (after a 4 hours milling cycle), because increasing the alumina concentration made the particles more brittle and promoted the fragmentation phenomena (fig. 5), whereas the higher fraction of ductile phase within the powder batch containing only 10 wt.% of alumina promoted the agglomeration phenomena during the milling process. Optical microscope pictures (fig. 6) clearly shows the different amount of aluminum oxide inside Figure 5 - Percentile results of particle size distribution measurements performed on the metal matrix of the two different batches. Figure 6 - Optical microscope images of powders containing different amount of Al2O3, milled for 4 hours: 10wt.% of alumina (left) and 30wt.% of alumina (right). Coating deposition The most promising powders were chosen and sprayed on different substrates. In order to evaluate the effect of the alumina amount powders containing 20 and 30 wt. % of the selected oxide were deposited with two different torches (HVOF and APS). Figure 7 – SEM images (back-scattered electrons detector) of coatings deposited by HVOF: the sample on the left contains 20 wt% of alumina, the sample on the right contains 30 wt.%. Sample name M-12-169-DJ-tc M-12-173-DJ-tc M-12-221-tc M-12-223-tc Spraying gun HVOF HVOF APS APS Al2O3 amount (wt.%) 20 30 20 30 Topcoat (YSZ) APS APS APS APS Table 1 – features of the sprayed samples: Inconel 738 substrate + standard bondcoat (Amdry 9954) + thin ODS layer + standard YSZ topcoat. The coatings (deposited by HVOF) containing 20 wt.% of alumina are denser and more homogenous than coating based on a higher amount of this oxide (fig. 7). The thin ODS layers sprayed above the standard bondcoat are less dense than the previous samples, because due to the lower number of passes the peening effect (i.e. the hardening and the increasing of the density caused by the collisions of the new sprayed particles on the already deposited coating) is less relevant. The coating deposited by APS are thinner and more heterogeneous; compared to the HVOF samples the deposition efficiency is lower. The thin ODS layers deposited on the Figure 8 - I topcoat; II thin ODS layer; standard bondcoat; IV substrate standard bondcoat have a poor quality and they don’t cover the whole surface of the sample. The 4 samples based on the thin ODS bondcoat were coated with a standard topcoat (YSZ) and then their high temperatures resistance were determined by an oxidation test (fig. 9). The preliminary results of the oxidation and high temperatures resistance are listed in table 2. The samples based on the ODS bondcoats deposited by APS failed after a limited number of cycle (a single cycle consists of 23 hours at 1100°C and 1 hour cooling). This probably because since the ODS coatings were not perfectly deposited, oxygen diffused inward and reacted with the standard bondcoat and with the substrate. Furthermore the weak adhesion between the topcoat and the bondcoat could have increased the spallation phenomena, making the topcoat failure easier. The samples based on the ODS coating deposited by HVOF exhibited a better resistance, due to their higher thickness and better microstructure. This test indicated also that the coating containing 30 wt.% of alumina (deposited by HVOF) showed a better resistance. Therefore it may be assumed that a higher amount of Figure 9 – Coated samples before the isothermal oxidation alumina improve the behavior of the coatings at high test: (1) M-12-169-DJ-tc; (2) M-12-173-DJ-tc; (3) M-12-221-tc; (4) M-12-223-tc temperatures. The comparison between these samples and standard bondcoat without alumina will allow the comprehension of this strengthening mechanism. Sample name M-12-169-DJ-tc M-12-173-DJ-tc M-12-221-tc M-12-223-tc N° of cycles before the first sign of damage 19 22 4 8 N° of cycles before the complete failure 24 52 15 15 Table 2 – Results of the oxidation test. Future perspectives Future investigations will concern both the optimization of powders and spraying process, in order to improve the behavior of these ODS coatings at high temperatures. Further characterization analysis on the milled powders and coatings will be useful to investigate the microstructure of the new samples and to determine the presence of oxidation phenomena that may occur during the spraying process. The most promising candidates will be tested to evaluate their furnace cycling oxidation resistance, in order to investigate the effect of the oxide phase on the coating properties.