Claims:The scope of the invention is defined by the following claims:

Claim:
1. A component comprising:
a) A base metal nickel based super alloy coated with multilayer thermal coating (FG-TBC) comprises as-sprayed and laser glazed functionally graded lanthanum magnesium hexaluminate (LaMgAl11O19)/Yttria-stabilized Zirconia (YSZ).
b) The FG-TBC coating materials YSZ and LaMgAl11O19 were plasma spray coated which does not react with contaminant composition in the environment.
c) The high-temperature stability was attained in FG-TBC using infrared rapid heating technique at 1000 °C
2. As mentioned in claim 1, surface morphology of the coated samples was modified using laser glazing.

3. According to claim 1, the varying composition of LaMgAl11O19/YSZ minimized the thermal expansion coefficient mismatch across the coating thickness and improved the thermal stability.

4. As mentioned in claim 1, the completely revamped microstructure induced through laser glazing further improved the lifetime of the coatings. The segmentation cracks over the glazed surface provided strain tolerance during thermal cycles.
5. As mentioned in claim 1, the failure of as-sprayed DC-TBC coating occurs along the interface and was attributed to the higher thermal gradient across the coating thickness that resulted in accumulation of stress.
6. According to claim 1, the laser glazed coatings have better thermal shock resistance and thermal insulation than the as-sprayed coatings, which is attributed to: (i) reduction in accumulation stresses, (ii) presence of segmented microcracks in the glazed coating (iii) sealing of surface pores (iii) constrained heat transfer due to entrapped gas in interlamellar pores. , Description:Field of Invention
The present invention relates to high-temperature stability of as-sprayed and laser glazed lanthanum magnesium hexaluminate (LaMgAl11O19)/Yttria-stabilized Zirconia (YSZ) based thermal barrier coating having two different coating architectures.
The objectives of this invention
The objective of this invention is to attain high-temperature stability, thermal insulation, and thermal shock resistance by as-sprayed and laser glazed thermal barrier coatings.
Background of the invention
Gas turbines are coated with multilayered ceramic coatings known as ‘thermal barrier coatings’ (TBCs) to impart thermal insulation (~100 °C – 300 °C) to the turbine components from hot combustion gases [A. Feuerstein et al., J.Therm. Spray Technol. 17 (2008) 199–213 and Vijay Kumar et al., Prog. Org. Coat.90 (2016) 54–82]. A typical TBC consists of two distinctive layers: metallic bond coat and ceramic topcoat. The metallic bond coat is coated over the superalloy substrate to provide better compliance with the ceramic topcoat. These layers have discrete physical, thermal and mechanical properties and were selected considering the type of thermal loading conditions [Abdullah Cahit Karaoglanli et al., Progress inGas Turbine Performance, InTech, (2013), 237-268]. The turbine components such as combustor liners, blades, vanes and nozzles coated with TBCs can withstand higher thermal loads, which impart higher engine efficiency and reduces emission and cooling requirement. The improved thermal endurance, along with film cooling imparts a thermal drop by about 300°C [Sang-Won Myoung et al., Surf. Coat. Technol. 215(2013) 46–51]. The magnitude of the thermal drop is determined by factors such as heat transfer coefficients, heat flux, internal cooling, coating thickness and thermal conductivity. Yttria-stabilized zirconia (YSZ) is being commonly used as the ceramic thermal barrier coating material over the years. However, YSZ undergoes ageing and phase transformation at temperatures above 1200 °C [LijianGu et al., J. Eur. Ceram.Soc. 33(15–16) (2013)3325-3333 and C. Friedrich et al. J. Therm. Spray.Techn. 10 (2001)592-598]. To fulfil higher thermal stability requirements, different coating materials were developed, such as conventional YSZ doped with oxide stabilizers [F.M.Pitek et al., Surf. Coat. Technol.201 (2007) 6044–6050], materials having pyrochlore [R.Vaben et al., Int. J. Appl. Ceram. Tec. 1 (2004) 351–361], fluorite [X.Q.Cao et al., Adv. Mater.15 (2003) 1438–1442], perovskite [W.Ma et al., J. Am. Ceram. Soc. 91 (2008) 2630–2635]. Among all the developed materials, the hexaluminates (MMeAl11O19, M=La, Pr, Nd, Sm, Eu, Gd, Ca, Sr; Me=Mg, Mn, Fe, Co, Ni, Cu, Zn) that have magnetoplumbite structure exhibit improved structural and thermal stability up to 1400 °C. This ceramic material has lower thermal conductivity, and its hexagonal plate-like grain structure imparts reduced sintering rate. The lanthanum magnesium hexaluminate (LaMgAl11O19) is one among such hexaluminates that exhibit superior fracture toughness and thermo-chemical stability [Xiaolong Chen et al., Surf. Coat.Tech. 206 (8–9) (2012) 2265-2274] and an identical cyclic lifetime to that of YSZ [X. Q. Cao et al., J. Eur. Ceram.Soc., 28(10) (2008) 1979-1986].

Description of Prior Art
Conventional double layer coatings are susceptible to cracking due to thermal stress mismatch and lower fracture toughness, which reduce coating lifetime [Hui Dai et al., Mater. Sci. Eng. A. 433 (2006) 1–7]. In order to improve the coating compliance and to reduce the thermal expansion coefficient mismatch between the ceramic layer and metallic bond coat, gradient coatings termed as ‘functionally graded coatings’ (FG-TBCs) were developed [Gupta M et al., J Therm Spray Tech. 27, (2018) 402–411]. These functionally graded coatings have composite layers of two different ceramic materials. The top layer is made from one ceramic material that has lower coefficient of thermal expansion and the bottom layer comprises of a different ceramic material having slightly different coefficient of thermal expansion. The ratio of two ceramic materials varies, as a result, the physical and mechanical properties gradually vary across the coating thickness. Compared with conventional double layer structures, the FG-TBCs have improved thermal cyclic lifetime and adhesion strength [Xiaolong Chen et al., Surf. Coat.Tech. 206 (8–9) (2012) 2265-2274]. By increasing the topcoat thickness progressively, it has been observed that an increase in thickness by 25 µm provides a thermal drop of 4-9 °C [L. Xie et al., Mater. Sci. Eng. A362 (2003) 204-212 and K.W. Schlichting et al., Mater. Sci. Eng. A 342(2003) 120-130].
Summary of the invention
In the present work, two different coating materials, YSZ and LaMgAl11O19 were plasma spray coated over nickel-based super alloy. The coated samples were laser glazed to modify the surface. The high-temperature stability was attained using infrared rapid heating technique at 1000 °C to analyze the thermal drop induced by as-sprayed and glazed coatings. Further, thermal shock resistance of as-sprayed coatings and glazed coatings were evaluated under high-temperature conditions. The thermal shock tested surfaces were analyzed using scanning electron microscopy (SEM) to identify the failure mechanism.
Detailed description of the invention
Hastelloy C-263 superalloy (Ni-Co-Cr-Mo alloy) used for combustion liners of gas turbines was selected as the test substrate. The test coupons of 25 mm x 25 mm x 5 mm were machined and grit blasted to have an average surface roughness (Ra) of 3-4 microns. The sample coupons were then degreased in an acetone bath. For the synthesis of LaMgAl11O19 (LaMA), a high-temperature solid state reaction was followed. La2O3 powders were preheated at 973 °C for 2 hours as it absorbs moisture and converts to lanthanum hydroxide [25]. Commercially available La2O3, Al2O3 and MgO were blended in a ball mill with 2:11:1 molar ratio.
The blended powders were then ball milled for 5 hours and were heated in a ceramic tubular furnace at 1000 °C for 7 hours. The heating temperature was progressively increased to 1650 °C and continued the heating for 10 hours. The synthesised LaMA powders were ball milled and dried to obtain free-flowing powders with an average particle size of 45-130 µm. Commercially available as-synthesized 8 wt. % YSZ was used along with the LaMA to fabricate DC-TBC and FG-TBC.
Two different coating architectures were followed: the first one contains a double-layered structure having two separate layers of YSZ and LaMA above the bond coat. The second architecture contains five gradient ceramic layers above the bond coat having varying weight fraction. The bond coat NiCrAlY have a nominal composition of Ni-22Cr-10Al-1.0Y (wt. %). The bond coat and ceramic layers were deposited using atmospheric plasma spray (APS) with 3 MB plasma spray gun. All coatings were made for a total thickness of 480 µm.
The surfaces of the DC-TBC and FG-TBC were laser glazed using an Ytterbium-doped fibre laser having 1080 nm wavelength. The laser is operated in a continuous wave (CW) mode and the beam is kept at a defocused position. For initial trials, singles tracks were made with different laser power and scanning speed. The laser power of 700 W with 150 mm/min scanning speed was observed as the optimal glazing parameter. The coated surfaces were glazed following 30% overlapping ratio.
The test samples were subjected to thermal shock study (i.e. resistance to thermal spalling) using a high-temperature muffle furnace maintained at 1100 °C. The samples were heated for 10 minutes and were then quenched in water maintained at 20-25 °C. The cycle was repeated to estimate the thermal resistance offered by the coatings. The surface of the coated samples was monitored for the rate of spallation through sequence of optical image processing and the cycles were repeated until 20% of spallation is obtained. Several other researchers have reported this method to analyze the thermal shock resistance of ceramic coatings [20, 26].
A 150 kW infrared (IR) rapid heater was used to study the thermal insulation provided by the ceramic coated samples. The sample was mounted on the sample holder and the coated surface faced towards the IR heater. The base substrate was kept as the reference to characterize the thermal insulation of the coated test coupons. A gap of ~75 mm was maintained between the heater and the test coupons. The surface temperature with respect to time was measured using Type-R thermocouples. In the uncoated base substrate, two thermocouples (T1 and T2) were attached to the front side facing the IR heater and two thermocouples (T3 and T4) were attached at the back side. The T3 and T4 thermocouples act as the controller and redundant thermocouple. In coated test samples, two thermocouples (T5 and T6) are attached at the back side of the test coupon. The thermal insulation provided by the ceramic layer was analyzed by measuring the difference in temperature recorded by the thermocouples attached to the uncoated base substrate with the coated test coupons. The test specimens were heated to 1000 °C at the rate of ~25 °C/sec at atmospheric conditions. The controller system is programmed to attain the peak temperature with a holding time of 100 sec.
6 Claims & 4 Figures

Brief description of Drawing
In the figures which are illustrate exemplary embodiments of the invention.
Figure 1 SEM images of (a) as-sprayed FG-TBC and glazed surface using (b) 500 W (c) 700 W and (d) 900 W of laser power
Figure 2 SEM images of the cross-sectioned samples of the as-sprayed DC-TBC after (a) 22 (b) 52 (c) 74 cycles and as-sprayed FG-TBC after (d) 53 (e) 78 (f) 106 cycles.
Figure 3 SEM images of the glazed DC-TBC after (a) 54 (b) 85 (c) 93 cycles and glazed FG-TBC after (d) 73 (e) 126 and (f) 176 cycles.
Figure 4 Back wall temperature plot of infrared rapid heating of DC-TBC and FG-TBC coatings in the as-sprayed and laser glazed conditions.

Detailed description of the drawing
As described above the present invention relates to the high-temperature stability, thermal insulation, and thermal shock resistance offered by as-sprayed and laser glazed thermal barrier coatings.
SEM micrographs of as-sprayed and laser glazed surfaces of FG-TBC are shown in Figure 1 (a-d). As can be seen from Figure 1a, the surface of as-sprayed FG-TBC has partially and completely melted ceramic powders. After laser irriadiation, coarser and rougher surface of the as-sprayed ceramic surfaces gets remelted and densify (Figure 1 b, c & d). The surface topography of FG-TBC glazed at 500W and 150 mm/min (Figure 1 b) were observed to be partially dense and was inferred that the laser power of 500W and 150 mm/min was not sufficient to glaze the surface. Whereas, FG-TBC glazed at 700 W exhibit partially melted surface along with the presence of surface cracks (Figure 1 c). Thermal strain induced higher heating and cooling rate across the treated depth and favoured the formation of cracks. The presence of macro-crack in Figure 1 d indicated that the laser power of 900 W was also not optimal for laser glazing.
The cross-sectional images of as sprayed DC-TBC and FG-TBC after different number of thermal cycles are shown in Figure 2 (a-f). In DC-TBC, Horizontal cracks are initiated along the YSZ/ LaMgAl11O19 interface after 22 number of cycles. The intensity of these cracks increases with the number of thermal cycles as evidenced from their cross-section. Also, the larger thickness of discrete YSZ and LaMgAl11O19 induces higher thermal gradient across the coating thickness and favors the accumulation of stress. The as-sprayed FG-TBC exhibited a different failure mechanism. The graded layers prevented the accumulation of stress and provide improved thermal insulation across the coating.

The cross-sectional images of the laser glazed DC-TBC and FG-TBC after different numbers of thermal cycles are shown in Figure 3 (a-d). The presence of surface cracks in glazed layer improved the coating lifetime. The presence of surface cracks over glazed layer improved the coating lifetime. These micro cracks facilitate the contraction and expansion of ceramic layers during the thermal loading and improve the thermal strain tolerance.

The acquired back wall temperature for all test coupons is shown in Figure 4. The graph has three regions; commencement region, incubation time and stabilised time. A significant reduction in the back wall temperature was observed of the as-sprayed coatings. The laser glazing transforms the surface topography. The rapid re-melting and re-solidification heals the conventional surface damages. The densification of micro-cracks and closing of surface pores over the surface, seal the interlamellar pores between the splats and trap the gas. This reduces the convection and constrains the transfer of heat