Thermal Shock Resistance and Thermal Insulation Capability Of

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Article Thermal Shock Resistance and Thermal Insulation Capability of Laser-Glazed Functionally Graded Lanthanum Magnesium Hexaluminate/Yttria-Stabilised Zirconia Thermal Barrier Coating

Muhammed Anaz Khan 1, Annakodi Vivek Anand 2, Muthukannan Duraiselvam 3, Koppula Srinivas Rao 4, Ramachandra Arvind Singh 5,* and Subramanian Jayalakshmi 5,*

1 Department of Mechanical Engineering, MLR Institute of Technology, Hyderabad 500043, India; [email protected] 2 Department of Aeronautical Engineering, MLR Institute of Technology, Hyderabad 500043, India; [email protected] 3 Department of Production Engineering, National Institute of Technology, Tiruchirappalli 620015, India; [email protected] 4 Department of Computer Science and Engineering, MLR Institute of Technology, Hyderabad 500043, India; [email protected] 5 Institute of Laser Optoelectronics and Intelligent Manufacturing, College of Mechanical and Electrical Engineering, Wenzhou University, Wenzhou 325035, China * Correspondence: [email protected] (R.A.S.); [email protected] (S.J.) Citation: Anaz Khan, M.; Vivek Anand, A.; Duraiselvam, M.; Srinivas Abstract: In this work, functionally graded lanthanum magnesium hexaluminate (LaMgAl11O19)/yttria- Rao, K.; Arvind Singh, R.; stabilised zirconia (YSZ) thermal barrier coating (FG-TBC), in as-sprayed and laser-glazed conditions, Jayalakshmi, S. Thermal Shock Resistance and Thermal Insulation were investigated for their thermal shock resistance and thermal insulation properties. Results were Capability of Laser-Glazed compared with those of a dual-layered coating of LaMgAl11O19 and YSZ (DC-TBC). Thermal shock ◦ Functionally Graded Lanthanum tests at 1100 C revealed that the as-sprayed FG-TBC had improved thermal stability, i.e., higher Magnesium cycle lifetime than the as-sprayed DC-TBC due to its gradient architecture, which minimised stress Hexaluminate/Yttria-Stabilised concentration across its thickness. In contrast, DC-TBC spalled at the interface due to the difference

Zirconia Thermal Barrier Coating. in the coefﬁcient of thermal expansion between the LaMgAl11O19 and YSZ layers. Laser glazing Materials 2021, 14, 3865. https:// improved cycle lifetimes of both the types of coatings. Microstructural changes, mainly the formation doi.org/10.3390/ma14143865 of segmentation cracks in the laser-glazed surfaces, provided strain tolerance during thermal cycles. Infrared rapid heating of the coatings up to 1000 ◦C showed that the laser-glazed FG-TBC had better Academic Editor: Petrica Vizureanu thermal insulation capability, as interlamellar pores entrapped gas and constrained heat transfer across its thickness. From the investigation, it is inferred that (i) FG-TBC has better thermal shock Received: 31 May 2021 resistance and thermal insulation capability than DC-TBC and (ii) laser glazing can signiﬁcantly Accepted: 7 July 2021 Published: 10 July 2021 enhance the overall thermal performance of the coatings. Laser-glazed FG-TBC provides the best heat management, and has good potential for applications that require effective heat management,

Publisher’s Note: MDPI stays neutral such as in gas turbines. with regard to jurisdictional claims in published maps and institutional afﬁl- Keywords: thermal barrier coating; yttria-stabilised zirconia (YSZ); lanthanum magnesium hexalu- iations. minate (LaMgAl11O19); thermal shock resistance; thermal insulation; laser glazing

Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. Thermal barrier coatings (TBC) are multi-layered ceramic coatings, usually used in This article is an open access article gas turbines to impart thermal insulation to turbine components from hot combustion distributed under the terms and gases [1,2]. Typically, a TBC consists of two distinctive layers, namely (i) metallic bond conditions of the Creative Commons coat and (ii) ceramic top coat. The metallic bond coat is coated over turbine components Attribution (CC BY) license (https:// to provide better compliance with the ceramic top coat. The two layers of a TBC have creativecommons.org/licenses/by/ distinct physical, thermal and mechanical properties. Thermal loading conditions are 4.0/).

Materials 2021, 14, 3865. https://doi.org/10.3390/ma14143865 https://www.mdpi.com/journal/materials Materials 2021, 14, 3865 2 of 20

a major factor that determines the material selection for these two layers [3]. Turbine components such as combustor liners, blades, vanes and nozzles coated with TBCs are required to withstand high thermal loads and render thermal insulation, so as to achieve (i) higher engine efficiency, (ii) emission reduction, and (iii) cooling requirements. Myoung et al. [4] observed improvement in thermal durability upon air cooling thick ZrO2-8% Y2O3 TBCs coated on Ni-superalloy. The magnitude of thermal drop is influenced by factors such as heat transfer coefficients, heat flux, internal cooling, coating thickness and thermal conductivity. Ceramic top coats are expected to impart (a) low thermal conductivity, to enhance thermal insulation, (b) high strain tolerance under cyclic loading, to improve lifetime, and (c) stable microstructure, to minimise deleterious temperature effects such as phase transformations, grain growth and sintering. Yttria-stabilised zirconia (YSZ) is a widely used thermal barrier coating material. However, YSZ as a material has severe limitations, such as (i) ageing, (ii) post-sintering, and (iii) detrimental phase transformation (at temperatures >1200 ◦C) [5,6]. These limitations cause early failure of YSZ coatings. In YSZ, tetragonal to monoclinic phase transformation occurs during service and is the major reason for coating failure. Gu et al. [5] reported that YSZ-Y3Al5O12 (YAG) composite coatings can suppress monoclinic phase transformation. Freidrich et al. [6], in their work on YSZ, observed that above 1100 ◦C, the high oxygen ion conducting nature of zirconia caused increased diffusion of oxygen through the dense ceramic coating, resulting in oxidation of metallurgical interlayer. This consequently led to chipping of the ceramic coating, which limited its long-term high-temperature application [6]. To overcome the limitations of YSZ, (a) doping it with oxide stabilisers (e.g., MgO, Y2O3, Sc2O3, In2O3, CeO2, SnO2 and TiO2) has been investigated [7], and (b) other new materials such as those containing pyrochlore [8], fluorite [9], and perovskite [10] have been developed. Among the new materials, the hexaluminates (MMeAl11O19, M = La, Pr, Nd, Sm, Eu, Gd, Ca, Sr; Me = Mg, Mn, Fe, Co, Ni, Cu, Zn), which have a magnetoplumbite structure, exhibit improved structural and thermal stability up to 1400 ◦C. Hexaluminates have low thermal conductivity [6]. Among hexaluminates, lanthanum magnesium hexaluminate, LaMA (LaMgAl11O19) has good thermo-chemical stability [11], and also has an identical cyclic lifetime similar to that of YSZ [12]. The composition of LaMA is able to prevent post-sintering densification, as was reported by Freidrich et al. [6]. Additionally, high- temperature ageing in LaMA occurs more slower than other commercial zirconia-based TBCs, as was reported in [6]. This makes LaMA a promising material for TBC applications. Conventional double-layer coatings are susceptible to cracking due to thermal stress mismatch and lower fracture toughness, which reduce their lifetime. Functionally graded thermal barrier coatings that have a multi-layered architecture are designed and developed with the aim of enhancing coating compliance and reducing thermal stress mismatch between the two layers, namely, the ceramic layer and metallic bond coat [13]. Functionally graded thermal barrier coatings have composite layers of two different ceramic materials. These coatings are designed such that their top layer is made from ceramic material that has a lower coefficient of thermal expansion, and its weight ratio with the other selected ceramic materials decreases in the subsequent underlying layers. As a consequence of such an architecture in functionally graded thermal barrier coatings, their physical and mechanical properties vary gradually across their coating thickness. Functionally graded thermal barrier coatings have improved thermal cycle lifetime and adhesion strength compared to conventional double-layer structures [12,14–18]. Kim et al. [15] investigated thermoelastic characteristics in TBCs with a graded layer between the top coat and bond coat. By using the finite element method (FEM), they identified that the functionally graded layer can considerably improve cycle lifetime. Kirbiyik et al. [16] synthesised multi-layered ceria and yttria stabilised zirconia (CYSZ)/Al2O3 ceramic TBCs, both in double-layered and functionally graded architectures. It was observed that the functionally graded architecture improved bonding strength between layers, and provided better thermal cycle performance than single-layered and double-layered coatings. Gok et al. [17] conducted thermal cycling experiments on multi-layered and functionally graded Gd2Zr2O7/CYSZ thermal barrier Materials 2021, 14, 3865 3 of 20

coatings. It was found that the functionally graded coating had lifetimes almost double those of the single-layered coatings. Surface modification techniques have also been developed to increase the lifetime of TBCs. As an example, by optimising the coating parameters (such as material feed rate, spray distance, etc.), segmentation cracks can be induced in the top coat to provide better coating compliance [19]. However, in this case, the crack geometries become irregular and cannot be controlled during coating process. Several post-treatment processes have been developed to create segmentation cracks in the top coat. Laser glazing is one such advanced process, in which the top coat is remelted to a depth of few hundred microns by scanning a laser beam over the top coat. As a consequence, rapid resolidification within the treated depth induces a controlled network of segmentation cracks, in the direction perpendicular to the coating surface [20,21]. Glazing also reduces surface roughness [20,21]. Segmentation cracks are vertical cracks in the top coat, with a length at least half that of the thickness of the top coat [19]. The formation of segmentation cracks in a controlled manner relieves thermal stress (which is usually induced during thermal cycles) and improves coating compliance. Reduction in residual stress due to presence of segmentation cracks improves strain tolerance of coatings (i.e., accommodation of large thermal strains without failure [19]). In addition, they act as barrier for the propagation of delamination cracks (i.e., parallel cracks [19]). Segmentation cracks thus enhance thermal shock resistance [19–24]. Laser glazing thus improves the structural integrity of coatings [21,25–27]. Ghasemi et al. [27] laser glazed the top coat of YSZ-based nanostructured TBCs. They found that laser glazing eliminates surface porosities and reduces surface roughness of the coatings. Lee et al. [25] laser glazed plasma-sprayed CYSZ thermal barrier coatings. Their tests on thermal cyclic performance revealed a twofold increase in the lifetime of laser-glazed coatings when compared to their as-sprayed counterparts. The thermo-mechanical behaviour of laser- glazed TBCs is hence an important aspect that greatly influences the heat management performance of TBCs, and so requires detailed investigation. The present investigation examines (i) the performance of two types of TBC architecture, namely, dual-layered and functionally graded architectures on the heat management capability of as-sprayed LaMgAl11O19/YSZ coatings. Thermal shock resistance and thermal insulation capability of the two types of archtiectures are evaluated to decipher the difference in their heat management capabilities; and (ii) the effect of laser glazing on the heat management capability of the coatings. The coatings were applied on Hastealloy (a nickel superalloy) surfaces.

2. Experimental Procedure 2.1. Test Substrate and Coating Materials 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 × 25 mm × 5 mm were machined and grit blasted (average surface roughness, Ra~3 to 4 microns). Prior to coating, the coupons were degreased in an acetone bath. To synthesise LaMgAl11O19 (LaMA), a high-temperature solid-state reaction strategy was followed (Equation (1)). ◦ La2O3 oxide powder was preheated at 973 C for 2 h, as it absorbs moisture and converts to lanthanum hydroxide [5,28]. Commercially available La2O3, Al2O3 and MgO were blended in a ball mill with 2:11:1 molar ratio.

2MgO + 11Al2O3 + La2O3 → 2LaMgAl11O19 (1)

Next, the blended powder was ball milled for 5 h and was subsequently heated in a ceramic tubular furnace at 1000 ◦C for 7 h. Heating temperature was progressively increased to 1650 ◦C, for 10 h. Eventually, the synthesised LaMA powder was ball milled and dried to obtain free-ﬂowing powder (average particle size: 45–130 µm). Commercially available 8 wt.% YSZ was used for preparing the composite coatings. Materials 2021, 14, x FOR PEER REVIEW 4 of 22

and dried to obtain free-flowing powder (average particle size: 45–130 µm). Commercially Materials 2021, 14, 3865 available 8 wt.% YSZ was used for preparing the composite coatings. 4 of 20

2.2. Coating Architecture 2.2. CoatingTwo different Architecture coating architectures were followed: (i) double-layer structure (DC- TBC) having two separate layers of YSZ and LaMA above the bond coat and (ii) five ce- Two different coating architectures were followed: (i) double-layer structure (DC-TBC) ramic layers above the bond coat with varying weight fraction of YSZ and LaMA (FG- having two separate layers of YSZ and LaMA above the bond coat and (ii) ﬁve ceramic TBC). Bond coat was made of NiCrAlY (composition: Ni-22Cr-10Al-1.0Y (wt.%)). Coat- layers above the bond coat with varying weight fraction of YSZ and LaMA (FG-TBC). ings were deposited via atmospheric plasma spray (APS) process (machine model: Sulzer Bond coat was made of NiCrAlY (composition: Ni-22Cr-10Al-1.0Y (wt.%)). Coatings were Metco 9MP, Sulzer Metco India Limited). Optimised parameters of atomic spray process deposited via atmospheric plasma spray (APS) process (machine model: Sulzer Metco used to deposit the coatings are given in Table 1. The process parameters were selected 9MP, Sulzer Metco India Limited). Optimised parameters of atomic spray process used basedto deposit on initial the coatings trials and are from given information in Table1 .based The process on previous parameters studies were [29–33]. selected Total basedthick- nesson initial of the trials deposited and from coatings information was 480 based µm. on Architectures previous studies of DC-TBC [29–33]. Totaland FG-TBC thickness are of shownthe deposited as schematics coatings in wasFigure 480 1.µ m. Architectures of DC-TBC and FG-TBC are shown as schematics in Figure1. Table 1. Optimised parameters of atomic spray process used to deposit the coatings.

Table 1. Optimised parameters of atomic spray process used to deposit the coatings. Carrier $$$ Stand-Off $$$ Primary $$$ Secondary $$$ Gas, Ar $$$ Coating Type Current (A) Voltage (V) DistanceStand-Off $$$ Gas,Primary Ar $$$ Gas,Secondary H2 $$$ Carrier (slpm) $$$ Coating Type Current (A) Voltage (V) Distance(mm) (slpm)Gas, Ar (slpm)Gas, H2 Gas, Ar (mm) (slpm) (slpm) (slpm) BondBond Coat Coat 550 550 75 75 110 110 35 35 14 14 2.3 2.3 CeramicCeramic $$$ 650650 61 61 120 120 65 65 12 12 2.6 2.6 TopTop Coat Coat slpm:slpm: standard standard litres perper minute. minute.

FigureFigure 1. 1. CoatingCoating architectures: architectures: ( (aa)) dual-layered dual-layered archit architecture,ecture, DC-TBC DC-TBC and and ( (bb)) functionally functionally graded graded architecture, architecture, FG-TBC.

2.3.2.3. Laser Laser Glazing Glazing DC-TBCDC-TBC and and FG-TBC FG-TBC surfaces surfaces were were laser laser glazed glazed using using a a ytterbium-doped ytterbium-doped fibre ﬁbre laser laser (wavelength:(wavelength: 1080 1080 nm). nm). The The laser laser was was operated operated in in continuous continuous wave wave (CW) (CW) mode mode and and the the beambeam was was kept kept at at a a slightly defocused defocused positi position.on. Diameter Diameter of of the the circular circular laser laser beam beam was was ddspotspot = =0.4 0.4 mm. mm. Defocused Defocused position position of laser of laser was wasused used so as so to as control to control the delivery the delivery of con- of centratedconcentrated energy energy density density and to and eliminate to eliminate the deterioration the deterioration of ceramic of ceramic layers layersupon inter- upon actioninteraction with withthe focussed the focussed beam. beam. For Forthis this purpos purpose,e, initial initial trials trials were were conducted conducted on on single single trackstracks on on the the developed developed coatings, coatings, with with laser laser power power setting setting at at 500 500 W, W, 700 700 W W and and 900 900 W. W. ScanningScanning speed speed was was kept kept constant constant at at 150 150 mm/min. mm/min. FromFrom post-deposition post-deposition surface surface analysis, analysis, the the optimal optimal laser laser power power setting setting was was identified identiﬁed asas 700 700 W W and and this this laser laser power power setting setting was was used used to to glaze glaze the the coatings coatings for for thermal thermal tests. tests. The The percentage of overlap was selected after measuring the glazed layer width of a single track. In the present work, the coated surfaces were glazed through 30% overlapped parallel tracks to achieve uniform remelting across the surface.

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2.4. Surface Characterisation and Phase Analysis Surface roughness of the TBCs was measured using a 3-D surface profilometer (Rtec instruments, San Jose, CA, USA) with vertical resolution less than 0.1 nm and lateral resolution of 100 nm. Presence of defects such as cracks and pores was identified using scanning electron microscope (SEM). Energy dispersive spectroscopy (EDS) was used for elemental analysis on the TBC surfaces. Phase analysis of the as-synthesised, as-sprayed and laser-glazed surfaces was conducted using X-ray diffraction (XRD, Rigaku ULTIMA-IV, Tokyo, Japan) with Cu-kα radiation. After the thermal shock resistance tests, the failure mechanism of TBCs were identified using SEM.

2.5. Thermal Shock Test Thermal shock tests were conducted to determine the resistance of the coatings to thermal spalling, using a high-temperature mufﬂe furnace at 1100 ◦C. Samples were heated for 10 min and were subseqeuntly quenched in water that was maintained at 20 to 25 ◦C. This heating–quenching cycle was repeated to determine the thermal shock resistance. Surfaces of the coated samples were monitored after every test, and the heating–quenching cycles were repeated until 20% of spallation was observed [34,35].

2.6. Infrared Rapid Heating Test Thermal insulation capability of the ceramic coated samples was evaluated using a 150 kW infrared (IR) rapid heater (Figure2). Samples were mounted on sample holder, such that their coated surfaces were exposed towards IR heater. Ni-superalloy substrate Materials 2021, 14, x FOR PEER REVIEW 6 of 22 was taken as the reference sample to characterise the thermal insulation of the coated test coupons. A gap of about 75 mm was maintained between the heater and the test coupons.

Figure 2. SchematicSchematic of of infrared infrared (IR) rapid heater.

3. ResultsType-R and thermocouples Discussion were used to measure surface temperatures. Back wall temperature drop was measured with time. Thermocouples T and T were attached to the 3.1. Surface Topography 1 2 front side of the base reference sample facing the IR heater. Thermocouples T3 and T4 were The plasma spray process involves accelerating ceramic powders towards a target attached on the back side of the base reference sample. T3 and T4 were the controller and surface using high-energy plasma. In the case of TBCs, the size of the ceramic particles redundant thermocouples. Thermocouples T5 and T6 were attached to the back side of the andcoated surface test coupon.roughness Thermal of bond insulation coat influenc providede their by microstructure ceramic layers and in thesurface coated roughness. samples During the APS process, molten and semi-molten particles impinge on the targeted substrate and/or previously deposited ceramic layers at higher temperature and pressure. This causes flattening and solidification of thin splats and results in the formation of anisotropic lamellar structure. The coatings consist of various types of defects, which include globular pores. Process parameters influence the adhesion strength of the APS coatings [36,37]. In the present work, the absence of microcracks between the layers indicates that the selected process parameters were optimal for producing good coatings. Rougher bond coat surfaces facilitate better wettability of the molten splats and improve the adhesion of ceramic material to bond coat. This increases the coating lifetime [38,39]. During the spray process, as the molten splats solidify, the surface becomes rougher. The molten splats, as they reach the target surface, spread over the previously deposited solidified splats [22,23]. This imparts higher surface roughness to the as- sprayed coatings (Figure 3a). Laser glazing reduces the roughness of the ceramic coating (Figure 3b).

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was analysed by measuring the difference in temperature recorded by the thermocouples attached to the back side of the uncoated base reference sample and the coated test coupons. Test specimens were heated to 1000 ◦C at the rate of 25 ◦C/s. Peak temperature was attained in 100 s.

3. Results and Discussion 3.1. Surface Topography The plasma spray process involves accelerating ceramic powders towards a target surface using high-energy plasma. In the case of TBCs, the size of the ceramic particles and surface roughness of bond coat influence their microstructure and surface roughness. Dur- ing the APS process, molten and semi-molten particles impinge on the targeted substrate and/or previously deposited ceramic layers at higher temperature and pressure. This causes flattening and solidification of thin splats and results in the formation of anisotropic lamellar structure. The coatings consist of various types of defects, which include globular pores. Process parameters influence the adhesion strength of the APS coatings [36,37]. In the present work, the absence of microcracks between the layers indicates that the selected process parameters were optimal for producing good coatings. Rougher bond coat surfaces facilitate better wettability of the molten splats and improve the adhesion of ceramic material to bond coat. This increases the coating lifetime [38,39]. During the spray process, as the molten splats solidify, the surface becomes rougher. The molten splats, as they reach the target surface, spread over the previously Materials 2021, 14, x FOR PEER REVIEW 7 of 22 deposited solidified splats [22,23]. This imparts higher surface roughness to the as-sprayed coatings (Figure3a). Laser glazing reduces the roughness of the ceramic coating (Figure3b) .

Figure 3. Three-dimensional surface topographies of FG-TBC: (a) as-sprayed surface and (b) laser-glazed surface. Laser Figure 3. Three-dimensional surface topographies of FG-TBC: (a) as-sprayed surface and (b) laser-glazed surface. Laser glazing reduces roughness of ceramic coating. glazing reduces roughness of ceramic coating. Scanning electron microscopy (SEM) images of the as-sprayed and laser-glazed sur- Scanning electron microscopy (SEM) images of the as-sprayed and laser-glazed faces of FG-TBC are shown in Figure4a–d. The as-sprayed FG-TBC surface shows partially surfaces of FG-TBC are shown in Figure 4a–d. The as-sprayed FG-TBC surface shows and completely melted ceramic powders (Figure4a). The as-sprayed FG-TBC surface is partially and completely melted ceramic powders (Figure 4a). The as-sprayed FG-TBC porous and contains micro-cracks (Figure4a). During the spray process, the entrapped gas surfaceescapes is through porous the and molten contains ceramic, micro-cracks which creates (Figure bubbles 4a). During and results the spray in the formationprocess, the of entrappedopen pores gas over escapes the surface through of the the as-sprayed molten ceramic, coating. which During creates the rapid bubbles solidification and results of thein themolten formation material, of open the induced pores over thermal the surface strain across of the theas-sprayed coating thicknesscoating. During and the the relieving rapid solidificationstrain due to solidificationof the molten cause material, micro-cracks the induced in the as-sprayed thermal strain coating across [27]. However,the coating the thicknesspropagation and of the micro-cracks relieving strain across due the coatingto solidification thickness cause is restricted micro-cracks by the mechanicalin the as- sprayedinterlocking coating of the [27]. overlapped However, resolidified the propagation splats. Aof significantmicro-cracks difference across betweenthe coating the thicknesssurface topography is restricted of by as-sprayed the mechanical and laser-glazed interlocking surfaces of the overlapped can be observed resolidified (Figure splats.4a,b). ADue significant to the laser difference glazing, between the coarser the surface and rougher topography surface of ofas-sprayed the as-sprayed and laser-glazed ceramic is surfacesremelted can and be densifies observed (Figure (Figure4b–d). 4a,b). Due to the laser glazing, the coarser and rougher surface of the as-sprayed ceramic is remelted and densifies (Figure 4b–d).

Figure 4. SEM images: (a) as-sprayed FG-TBC. Laser-glazed (LG) surfaces obtained using laser power setting of (b) 700 W, (c) 500 W and (d) 900 W.

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Figure 3. Three-dimensional surface topographies of FG-TBC: (a) as-sprayed surface and (b) laser-glazed surface. Laser glazing reduces roughness of ceramic coating.

Scanning electron microscopy (SEM) images of the as-sprayed and laser-glazed surfaces of FG-TBC are shown in Figure 4a–d. The as-sprayed FG-TBC surface shows partially and completely melted ceramic powders (Figure 4a). The as-sprayed FG-TBC surface is porous and contains micro-cracks (Figure 4a). During the spray process, the entrapped gas escapes through the molten ceramic, which creates bubbles and results in the formation of open pores over the surface of the as-sprayed coating. During the rapid solidification of the molten material, the induced thermal strain across the coating thickness and the relieving strain due to solidification cause micro-cracks in the as- sprayed coating [27]. However, the propagation of micro-cracks across the coating thickness is restricted by the mechanical interlocking of the overlapped resolidified splats. Materials 2021, 14, 3865 A significant difference between the surface topography of as-sprayed and laser-glazed7 of 20 surfaces can be observed (Figure 4a,b). Due to the laser glazing, the coarser and rougher surface of the as-sprayed ceramic is remelted and densifies (Figure 4b–d).

Figure 4. SEM images: (a)) as-sprayedas-sprayed FG-TBC.FG-TBC. Laser-glazedLaser-glazed (LG) (LG) surfaces surfaces obtained obtained using using laser laser power power setting setting of of (b )(b 700) 700 W, W,(c) 500(c) 500 W andW and (d) ( 900d) 900 W. W.

Laser glazing improves the surface characteristics of TBC by increasing microhard- ness, sealing surface porosity, reducing surface roughness, minimising the bending modulus of coatings, and by creating a controlled network of segmented cracks over the coatings [21,40,41]. The laser glazing parameters can be varied to obtain significant variation in surface morphology over the glazed surfaces [42]. Both pulsed wave and continuous lasers can be used for surface glazing. Important laser parameters include pulse power, peak power, pulse length, pulse shape, laser beam wavelength, laser scanning speed and the geometry of the laser beam (i.e., depth of focus, spot size) [43]. In the present work, the laser scanning speed was kept at 150 mm/min, which was selected on the basis of preliminary trials. Upon visual inspection of the laser-glazed surface, it was seen that the colour of the coatings changed from a pale grey to a light yellowish glossy surface. This change in colour is known to occur during laser glazing, and indicates optimal laser glazing conditions. Scanning speeds greater than 150 mm/min cause a higher thermal gradient across the coating thickness and a higher rate of thermal stress [44]. Upon interaction with the laser beam, the pores and micro-cracks heal significantly, leading to a homogeneously resolidified net-shaped structure (Figure4b, laser glazed at 700 W). Segmentation micro-cracks occur due to the higher solidification rate imparted by the raster scanning of the laser source at the optimal laser power settings of 700 W. Segmentation cracks are known to influence thermal shock resistance and thermal cycle lifetime in TBCs [19,22,25,45–47]. The partially dense surface topography of FG-TBC glazed at 500 W (Figure4c) indicates that this laser power level was not sufficient to glaze the surface effectively. The presence of macro-cracks on the surface of FG-TBC glazed at 900 W (Figure4d) indicates that this laser power level was not optimal for laser glazing. Based on these observations, a laser power level of 700 W was selected to laser glaze the coatings for their investigation. Figure5a,b show the surface roughness and surface porosity of the as-sprayed and laser-glazed DC-TBC and FG-TBC, respectively. The DC-TBC surface has higher roughness compared to FG-TBC surface, both in the as-sprayed and laser-glazed conditions. For both coating architectures, the as-sprayed surfaces are rougher than the laser-glazed Materials 2021, 14, 3865 8 of 20

surfaces. Partially melted particles (Figure4a) cause higher roughness (Figure5a). DC-TBC and FG-TBC both have higher porosity levels in their as-sprayed conditions (Figure5b) . Porosity in the coatings appears in the form of open pores, interlamellar pores (i.e., interlamellar spaces between splats), and globular pores, causing coating failure [48,49]. Open pores permit diffusion of oxygen ions from flue gas into the metallic bond coat, causing oxide formation (thermally grown oxides, TGOs [49]). TGOs cause coating failure at the bond coat interface. The interlamellar pores that form due to rapid solidification lead to delamination of coating [48]. Globular pores, a result of improper filling of the coating material, stacking inconsistencies, incomplete contact between the splats, and the presence of unmelted particles, are initiation sites for coating failures [39]. Post-processing Materials 2021, 14, x FOR PEER REVIEWtreatments can reduce surface porosity, as is evident from the low porosity levels9 in of the 22

laser-glazed TBCs (Figures5b and6).

Figure 5. ((aa)) Surface roughness roughness (µm) (µm) and ( b) surface porosity (%) of as-sprayedas-sprayed and laser-glazedlaser-glazed coatingcoating surfaces.

Figure 6. OpticalOptical microscopic images of surfaces of: ( a)) as-sprayed as-sprayed and and ( b) laser-glazed FG-TBC. In the right-hand side images, pores appear in grey/black colours. The left-hand side pictures are from thethe right-hand right-hand side images, pores appear inin grey/blackgrey/black colours. colours. The The left-hand left-hand side side pictures pictures are from the image analysis of the respective optical microscopic images, in which different colours represent thethe image image analysis analysis of of the the respecti respectiveve optical optical microscopic microscopic images, in which which different different colours represent pores of different size. pores of different size. 3.2. Microstructure and Phase Analysis The interface between the YSZ and LaMA ceramic layers in the as-sprayed DC-TBC is shown in Figure 7a. Conformal deposition of LaMA over YSZ is evident. EDS analysis of the region (Figure 7a) confirms the presence of LaMA and YSZ elements (Figure 7b,c). The SEM cross-section image of FG-TBC and the corresponding EDS spectra are shown in Figure 8a,b. Elemental mapping of La and Zr taken across the FG-TBC (Figure 8c,d) shows gradual variation of the La and Zr elements across the coating thickness. This confirms the formation of the graded layer across FG-TBC.

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During laser glazing, owing to the rapid melting and resolidification, the open pores on surface close; in other words, they get patched. Additionally, remelting and resolidification densifies the coating material and induces segmentation cracks in the coating [19,49]. The glazed FG-TBC surface thus has a lower surface roughness (3.7 µm) and a lower porosity level (6.1%). Similar behaviour was reported by Ghasemi et al. [27] when nanostructured TBCs containing a YSZ ceramic top coat were laser glazed. A significant reduction in surface roughness after laser glazing was observed. They reported that the surface roughness (Ra) of the as-sprayed coating was 9.2 µm, which upon laser-glazing was reduced to 2.5 µm. Furthermore, they observed that the as-sprayed surface had cracks, voids and pores. Upon laser glazing, they observed an absence of defects, complete resolidification, and a dense microstructure with segmentation cracks [27].

3.2. Microstructure and Phase Analysis The interface between the YSZ and LaMA ceramic layers in the as-sprayed DC-TBC is shown in Figure7a. Conformal deposition of LaMA over YSZ is evident. EDS analysis of the region (Figure7a) conﬁrms the presence of LaMA and YSZ elements (Figure7b,c). The SEM cross-section image of FG-TBC and the corresponding EDS spectra are shown in Figure8a,b. Elemental mapping of La and Zr taken across the FG-TBC (Figure8c,d) shows Materials 2021, 14, x FOR PEER REVIEW 10 of 22 gradual variation of the La and Zr elements across the coating thickness. This conﬁrms the formation of the graded layer across FG-TBC.

Figure 7. (a) SEM image of DC-TBC (cross-section). (b) EDS spectrum on LaMA. Peaks corresponding to La, Mg, Al and Y Figureare present. 7. (a) (SEMc) EDS image spectrum of DC-TBC of the YSZ/metallic(cross-section). bond (b) EDS coat spectrum interface. Peakson LaMA. corresponding Peaks corresponding to Y, Zr, La, to Al, La, Cr Mg, and Al Ni and are Y are present. (c) EDS spectrum of the YSZ/metallic bond coat interface. Peaks corresponding to Y, Zr, La, Al, Cr and Ni present. are present.

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Figure 8. (a) SEM image ofof thethe cross-sectioncross-section ofof FG-TBC. FG-TBC. ( b(b)) EDS EDS spectrum spectrum across across the the graded graded layer layer showing showing the the presence presence of ofLa, La, Mg, Mg, Zr Zr and and Al Al elements. elements. (c, d(c), Elementald) Elemental maps maps showing showing the the gradual gradual variation variation of La of andLa and Zr acrossZr across the coatingthe coating thickness. thickness. The XRD patterns for the as-synthesised LaMA powder, as-sprayed and laser-glazed surfacesThe areXRD shown patterns in Figure for the9 .as-synthesised Over the as-sprayed LaMA surface,powder, the as-sprayed LaMA amorphous and laser-glazed phase surfacescan be observed are shown as ain major Figure phase 9. Over with the broader as-sprayed peaks. surface, Peaks of the LaAlO LaMA3 are amorphous present in phase the as- cansprayed be observed and laser-glazed as a major samples phase duewith to broader partial decompositionpeaks. Peaks of of LaAlO LaMA3 are oxides present during in the as-sprayedspray process. and Otherlaser-glazed volatile samples intermetallic due to peaks partial are decomposition not observed. of Partial LaMA decomposition oxides during theof LaMA spray oxidesprocess. along Other with volatile volatilisation intermetallic during peaks high-temperature are not observed. synthesis Partial reduceddecomposi- the tionpercentage of LaMA of oxides volatile along intermetallics with volatilisation [11]. For during the laser-glazed high-temperature surface, sy thenthesis narrow reduced peaks theindicate percentage the crystallisation of volatile intermetallics of LaMA oxides. [11]. The For presence the laser-glazed of α-Al2 Osurface,3 peaks the in thenarrow XRD peakspattern indicate of the laser-glazedthe crystallisation surface of indicates LaMA oxides. the partial The decomposition presence of α-Al of2 LaMAO3 peaks oxides. in the XRD pattern of the laser-glazed surface indicates the partial decomposition of LaMA ox- 3.3. Thermal Shock Resistance ides. Thermal shock resistance is an important property of TBCs. The reliability of TBCs under extreme operating conditions is a critical factor. Gas turbine components operate with repeated run–stop cycles, inducing large ﬂuctuations in temperature (i.e., cyclic thermal loads) to the TBCs used for the components. This causes thermal stresses across coatings [34,50–53]. As a consequence, degradation mechanisms manifest, such as sintering effect, thermal expansion, and high temperature friction. Evaluation of the thermal shock resistance of TBCs is therefore vital for their screening and selection for gas turbine components. The number of cycles to failure of DC-TBC and FG-TBC coatings are shown in Figure 10. In the as-sprayed condition, FG-TBC has a higher cycle lifetime than DC-TBC, by 30 cycles. In the laser-glazed condition, FG-TBC has a higher cycle lifetime than DC-TBC, by 77 cycles. Among the FG-TBC, the laser-glazed coating has a higher cycle lifetime than its as-sprayed counterpart, by 65 cycles. These results indicate that the laser-glazed FG-TBC has the best thermal shock resistance.

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FigureFigure 9. 9.X-ray X-ray diffractiondiffraction patterns patterns of of as-synthesised, as-synthesised, as-sprayed as-sprayed and and laser-glazed laser-glazed TBCs. TBCs.

3.3. Thermal Shock Resistance Thermal shock resistance is an important property of TBCs. The reliability of TBCs under extreme operating conditions is a critical factor. Gas turbine components operate with repeated run–stop cycles, inducing large fluctuations in temperature (i.e., cyclic thermal loads) to the TBCs used for the components. This causes thermal stresses across coatings [34,50–53]. As a consequence, degradation mechanisms manifest, such as sintering effect, thermal expansion, and high temperature friction. Evaluation of the thermal shock resistance of TBCs is therefore vital for their screening and selection for gas turbine components. The number of cycles to failure of DC-TBC and FG-TBC coatings are shown in Figure 10. In the as-sprayed condition, FG-TBC has a higher cycle lifetime than DC-TBC, by 30 cycles. In the laser-glazed condition, FG-TBC has a higher cycle lifetime than DC-TBC, by 77 cycles. Among the FG-TBC, the laser-glazed coating has a higher cycle lifetime than its as-sprayed counterpart, by 65 cycles. These results indicate that the laser-glazed FG-TBC has the best thermal shock resistance.

FigureFigure 10. 10. ThermalThermal shock shock resistance, resistance, i.e., i.e., number number of of cycles cycles to to fail failureure of of DC-TBC DC-TBC and and FG-TBC FG-TBC coatings coatings in in their their as-sprayed as-sprayed andand laser-glazed laser-glazed conditions. conditions.

3.3.1.3.3.1. As-Sprayed As-Sprayed TBCs TBCs DifferentDifferent failure failure mechanisms mechanisms were were observed observed for for as-sprayed as-sprayed DC-TBC DC-TBC and and FG-TBC. FG-TBC. In theIn theas-sprayed as-sprayed DC-TBC, DC-TBC, horizontal horizontal cracks cracks form formand propagate and propagate at theat interface the interface between be- YSZtween and YSZ LaMgAl and LaMgAl11O19 layers11O due19 layers to the due difference to the differencein their coefficient in their coefﬁcientof thermal expansion of thermal −6 −1 ◦ (YSZexpansion CTE: 10.2 (YSZ × CTE:10−6 K 10.2−1, room× 10 temperatureK , room to temperature 877 °C; LaMgAl to 87711OC;19 CTE: LaMgAl 5.1311 ×O 1019−6CTE: K−1 [11]).5.13 ×Mismatch10−6 K− 1in[ 11the]). thermal Mismatch expansion in the thermal coefficient expansion causes coefﬁcientthermal stress causes mismatch thermal at stress the interface,mismatch which at the changes interface, the which local volume changes alon theg local the interface. volume along With thethe interface.increase in With the numberthe increase of thermal in the cycles, number spalla of thermaltion of the cycles, coating spallation occurs ofdu thee to coating the propagation occurs due of tohori- the zontalpropagation cracks. of horizontal cracks. SEM images of as-sprayed DC-TBC, taken after different numbers of thermal cycles, are shown in Figure 11a–e. Horizontal cracks are initiated along the YSZ/LaMgAl11O19 interface after 22 cycles. The intensity of these cracks increases with increasing numbers of thermal cycles. The high thickness of both YSZ and LaMgAl11O19 (thickness: 180 µm each) induces a lower thermal gradient across the coating thickness and favours the accumulation of stress. Branching of the micro-cracks after 65 cycles can be observed in Figure 11d. The coalescence of the micro-cracks that can be seen in the sample after 74 cycles (Figure 11e) is caused by the accumulated thermal elastic strain, which is relieved rapidly upon quenching during the thermal shock test.

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SEM images of as-sprayed DC-TBC, taken after different numbers of thermal cycles, are shown in Figure 11a–e. Horizontal cracks are initiated along the YSZ/LaMgAl11O19 interface after 22 cycles. The intensity of these cracks increases with increasing numbers of thermal cycles. The high thickness of both YSZ and LaMgAl11O19 (thickness: 180 µm each) induces a lower thermal gradient across the coating thickness and favours the accumulation of stress. Branching of the micro-cracks after 65 cycles can be observed in Figure 11d. The coalescence of the micro-cracks that can be seen in the sample after 74 cycles (Figure 11e) is Materials 2021, 14, x FOR PEER REVIEW 14 of 22 caused by the accumulated thermal elastic strain, which is relieved rapidly upon quenching during the thermal shock test.

FigureFigure 11. SEMSEM images images of of as-spr as-sprayedayed DC-TBC DC-TBC after after ( (aa)) 22, 22, ( (bb)) 38, 38, ( (cc)) 52, 52, ( (dd)) 65 65 and and ( (ee)) 74 74 heating– heating– quenchingquenching cycles.

ComparedCompared to DC-TBC, DC-TBC, the the as-sprayed as-sprayed FG-T FG-TBCBC showed showed a a different different failure failure mechanism mechanism (Figure(Figure 12a–c).12a–c). The The graded graded layers layers of FG-TBC of FG-TBC effectively effectively prevent prevent the accumulation the accumulation of stress of andstress provide and provide better betterthermal thermal insulation insulation across acrossthe coating. the coating. The thermal The thermal insulation insulation of the coatingsof the coatings is influenced is inﬂuenced by theirby microstructure their microstructure and crystal and structure crystal structure [54]. FG-TBC [54]. has FG-TBC a top µ layerhas a that top layerconsists that of consists 100% LaMgAl of 100%11 LaMgAlO19 (thickness:11O19 (thickness: 120 µm), which 120 m),has whichlower hasCTE, lower and its weight percentage decreases with subsequent underlying layers. The graded architecture reduces the propensity for the propagation of defects (such as micro-cracks) to the subsequent layers and to the substrate [55]. Due to this gradient architecture, stress accumulation in FG-TBC is lower than that in DC-TBC, which has a dual-layered architecture; consequently, FG-TBC has higher thermal shock resistance than DC-TBC (Figure 10).

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CTE, and its weight percentage decreases with subsequent underlying layers. The graded architecture reduces the propensity for the propagation of defects (such as micro-cracks) to the subsequent layers and to the substrate [55]. Due to this gradient architecture, stress accumulation in FG-TBC is lower than that in DC-TBC, which has a dual-layered Materials 2021, 14, x FOR PEER REVIEW 15 of 22 architecture; consequently, FG-TBC has higher thermal shock resistance than DC-TBC (Figure 10).

Figure 12.FigureSEM images12. SEM of images as-sprayed of as-sprayed FG-TBC FG-TBC after (a) after 53, ( b(a)) 7853, and (b) 78 (c) and 106 ( cycles.c) 106 cycles.

In TBCs, there areIn TBCs, two different there are crack two different propagation crack mechanisms: propagation (i)mechanisms: cracks that (i) propa- cracks that prop- gate along the coatedagate along surface, the i.e.,coated parallel surface, to thei.e., coatedparallel surface, to the coated termed surface, inter-splat termed cracks; inter-splat cracks; and (ii) cracks thatand (ii) are cracks oriented that acrossare oriented the thickness across the of th coatings,ickness of i.e., coatings, perpendicular i.e., perpendicular to to the the coated surface,coated termed surface, intra-splat termed intra-splat cracks [27 cracks,56]. Reports[27,56]. Reports have shown have shown that parallel that parallel cracks cracks provide betterprovide thermal better compliancethermal compliance and insulation and insulation than intra-splat than intra-splat cracks [57 cracks]. In the[57]. In the asas-sprayed FG-TBC,sprayed the cracksFG-TBC, initiate the cracks along initiate the highly along stressed the highly brim stressed region andbrim propagate region and propagate across the coatingacross thickness. the coating The thickness. intensity ofThe these intensity cracks of increasesthese cracks with increases the number with the of number of thermal cycles (Figurethermal 12 cycles). Coatings (Figure were 12). partly Coatings purged were along partly with purged the top along coat and with spalled the top coat and within the ceramicspalled layer. within the ceramic layer.

3.3.2. Laser-Glazed3.3.2. TBCs Laser-Glazed TBCs Laser-glazed coatingsLaser-glazed have highercoatings thermal have higher shock thermal resistance shock than resistance their as-sprayed than their as-sprayed counterparts (Figurecounterparts 10). Laser (Figure glazing 10). Laser causes glazing the remelting causes the and remelting resolidfication and resolidfication of the of the coating surfacescoating and induces surfaces segmentation and induces cracks. segmentation During lasercracks. glazing, During the laser higher glazing, ther- the higher mal gradients andthermal non-uniform gradients resolidification and non-uniform (i.e., resolidi rapidfication solidification (i.e., rapid ~10 7solidificationK/s) favour ~107 K/s) fa- accumulation ofvour thermal accumulation stress across of thermal the treated stress depth across [ 20the,46 treated]. The sheardepth force[20,46]. across The shear force the molten layeracross is accumulated the molten duelayer to is the accumulated induced surface due to tensionthe induced [20,47 surface]. Gravitational tension [20,47]. Gravi- force stabilises thetational induced force shear stabilises force the in induced the remelted shear zone.force in Therefore, the remelted the accumulatedzone. Therefore, the accu- thermal stress tomulated which segmentation thermal stress cracks to which are segmentation subjected will cracks be relatively are subjected lower will than be thatrelatively lower experience by non-segmentationthan that experience cracks. by Thus,non-segmentation it can be observed cracks. that Thus, laser-glazed it can be coatingsobserved that laser- have a higher cycleglazed lifetime coatings than have the a as-sprayed higher cycle coatings lifetime (Figure than the 10 as-sprayed). Similar observationscoatings (Figure 10). Sim- ilar observations have been reported previously, when plasma-sprayed ceria-yttira-stabilised TBCs were laser glazed [25]. The results showed that the thermal cycling lifetime of laser-glazed TBCs increased twofold [25]. Kadhim [33] treated surfaces of yttria partially stabilised zirconia (YPSZ) by laser sealing. Compared to the as-sprayed surface, laser surface processing effectively modified the surface layer by sealing the porosity and reducing

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have been reported previously, when plasma-sprayed ceria-yttira-stabilised TBCs were Materials 2021, 14, x FOR PEER REVIEW 16 of 22 laser glazed [25]. The results showed that the thermal cycling lifetime of laser-glazed TBCs increased twofold [25]. Kadhim [33] treated surfaces of yttria partially stabilised zirconia (YPSZ) by laser sealing. Compared to the as-sprayed surface, laser surface processing effec- tivelysurface modified roughness, the and surface enhanced layer by the sealing thermal the shock porosity resistance and reducing due to the surface presence roughness, of seg- andmentation enhanced cracks the [33]. thermal shock resistance due to the presence of segmentation cracks [33]. SEM images of laser-glazed DC-TBCDC-TBC afterafter 54,54, 68,68, 85 and 93 heating–quenching cyclescycles are shownshown inin FigureFigure 1313a–d.a–d. The segmentation cracks in the laser-glazed coatings accom- modate thermal stresses andand improveimprove thethe strainstrain tolerance.tolerance. The difference inin CTECTE betweenbetween thethe dualdual layers induces thermal strain alongalong the coating, causing formation and propaga- tiontion ofof delaminationdelamination cracks.cracks. The glazedglazed coatingcoating surfacesurface becomesbecomes densifieddensified afterafter 5454 cyclescycles ((FigureFigure 1313),), andand thethe coatingcoating spalls.spalls. GuoGuo etet al.al. [[23]23] reportedreported significantsignificant improvementimprovement inin thermalthermal shockshock resistanceresistance ofof plasma-sprayedplasma-sprayed YSZ duedue toto thethe presencepresence ofof segmentationsegmentation cracks. They observed that the coatings failed byby spallingspalling andand delaminationdelamination [[23].23]. Laser-glazed FG-TBCFG-TBC has has a a higher higher thermal thermal shock shock resistance, resistance, i.e., i.e., a higher a higher cycle cycle lifetime, life- thantime, itsthan as-sprayed its as-sprayed counterpart counterpart and laser-glazed and laser-glazed DC-TBC DC-TBC (Figure (Figure 10). The 10). better The shockbetter resistanceshock resistance of laser-glazed of laser-glazed FG-TBC FG-TBC is due to is (i) due the to gradient (i) the architecturegradient architecture and (ii) the and beneficial (ii) the effectbeneficial of laser effect glazing. of laser To elucidate,glazing. To (i) gradientelucidate, architecture (i) gradient prevents architecture the accumulation prevents the of stressaccumulation and reduces of stress the crack and reduces propagation the crack rate acrosspropagation the graded rate thicknessacross the of graded the coating thickness [33]. Inof addition,the coating it provides[33]. In addition, strain tolerance it provides during strain heating–quenching tolerance during cycles. heating–quenching (ii) Laser glazing cy- densifiescles. (ii) Laser the structure, glazing densifies eliminates the open structure, pores, andeliminates thus prevents open pores, the diffusion and thus of prevents oxygen ionsthe diffusion into the metallic of oxygen bond ions coat. into This the reduces metallic the bond propensity coat. This of oxide reduces formation the propensity and coating of failureoxide formation thereof. In and addition, coating thefailure formation thereof. of In segmentation addition, the cracks formation in the of top segmentation coat upon lasercracks glazing in the top provides coat upon strain laser tolerance glazing during provides heating–quenching strain tolerance during cycles. heating–quench- With prolonged testing cycles. cycles, With the cracks prolonged propagated test cycles, across the cracks the coating propagated thickness across and the spalled coating the thickness glazed layer,and spalled as shown the glazed in Figure layer, 14a–c. as shown The gradient in Figure ceramic 14a–c. layerThe gradient is below ceramic the spalled layer region is be- (notlow the shown spalled here). region (not shown here).

Figure 13.13. High-magniﬁcationHigh-magnification SEMSEM images images of of laser-glazed laser-glazed DC-TBC DC-TBC after after (a )( 54,a) 54, (b) ( 68,b) 68, (c) 85,(c) and85, and (d) 93(d) heating–quenching 93 heating–quenching cycles. cycles.

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Figure 14.Figure High-magnificationHigh-magniﬁcation 14. High-magnification SEM images SEM of images laser-glazed of laser-glazed FG-TBC after FG-TBC ( a)) 73, after (b )( a 126,) 73, and (b) ((126,c) 173173 and heating–quenchingheating–quenching (c) 173 heating–quenching cycles. cycles.

3.4.3.4. Thermal Thermal3.4. InsulationThermal Insulation Insulation Capability Capability Capability TBCsTBCs with TBCs both with the both typestypes the ofof architectures,typesarchitectures, of architectures,in in both both as-sprayed as-sprayed in both as-sprayed and and laser-glazed laser-glazed and laser-glazed condi- con- con- ◦ ◦ ditions,tions, sustained sustainedditions, thesustained the IR rapidIR rapi the heatingd IRheating rapi testd test (1000heating (1000C test at °C 25 (1000 atC/s) 25 °C°C/s) without at 25without °C/s) spallation. withoutspallation. Acquired spallation. Ac- Ac- quiredback wall backquired temperature wall back temperature wall of thetemperature TBCs of the is shownTBCs of the is in TBCsshown Figure is in15 shown .Figure All traces in 15. Figure haveAll traces 15. three All have distinct traces three re-have three distinctgions, namely, regions,distinct (i) namely,regions, the commencement (i)namely, the commencement (i) the region, commencement wherein region, the wherein region, temperature whereinthe temperature increases the temperature atincreases a lower increases atrate a lower duringat ratea thelower during beginning rate the during ofbeginning test the (until beginning of 25 test s), (until (ii) of incubationtest 25 (untils), (ii) time,25 incubation s), wherein(ii) incubation time, the temperaturewherein time, whereinthe the temperatureincreasestemperature linearly increases (25 increases s linearly to 65 s), (25linearly and s to (iii) 65 (25 stabilised s) s, andto 65 (iii) s) time,, stabilisedand wherein (iii) stabilised time, the wherein temperature time, whereinthe temper- reaches the temper- aturestable reaches values.ature stable Tworeaches base values. stable reference Two values. base samples Two reference base (Hastealloy) reference samples were samples(Hastealloy) tested (Hastealloy) to studywere tested the were accuracy to testedstudy of to study the datathe analyser accuracy in recording of the data the analyser drop in temperature.in recording Anthe identicaldrop in temperature. back wall temperature An identical back the accuracy◦ of the data analyser in recording the drop in temperature. An identical back of 998 Cwall was temperature observed for of both 998 °C the was samples. observed This for means both thatthe samples. the drop This in the means back that wall the drop wall temperature of◦ 998 °C was observed for both the samples. This means that the drop intemperature the backin thewall was back temperature 2 wallC, which temperature was is considered 2 °C, was which 2 to°C, is be whichconsidered negligible. is considered to be negligible. to be negligible.

Figure 15. Back wall temperature plot of infrared rapid heating of DC-TBC and FG-TBC in their Figure 15.Figure Back wall15. Back temperature wall temperature plot of infrared plot of rapidinfrared heating rapid of heating DC-TBC of andDC-TBC FG-TBC and inFG-TBC their as-sprayed in their as-sprayed and laser- and laser- glazed conditions.glazed conditions. as-sprayed and laser-glazed conditions. Back wall temperature drops of 67 ◦C and 83 ◦C were observed for the as-sprayed DC- Back wallBack temperature wall temperature drops of drops 67 °C ofan 67d 83°C °C an wered 83 °Cobserved were observed for the as-sprayed for the as-sprayed TBC and FG-TBC coatings, respectively. Pores are known to impede thermal conduction DC-TBC DC-TBCand FG-TBC and FG-TBCcoatings, coatings, respectively. respectively. Pores are Pores known are to known impede to thermalimpede conduc-thermal conduc- and thereby enhance the thermal insulation of coatings [58,59]. In FG-TBC, which has tion and tionthereby and enhancethereby enhancethe thermal the insulationthermal insulation of coatings of coatings[58,59]. In [58,59]. FG-TBC, In FG-TBC, which has which has multiple layers of YSZ-LaMA, mechanical interlocking of splats increases the roughness multiplemultiple layers of layers YSZ-LaMA, of YSZ-LaMA, mechanical mechanical interlocking inte rlockingof splats ofincreases splats increases the roughness the roughness across the ceramic layers and imparts increased levels of porosity between the layers (i.e., across theacross ceramic the layersceramic and layers imparts and increasedimparts increased levels of levelsporosity of porositybetween betweenthe layers the (i.e., layers (i.e., interlamellar pores) [59]. Increased porosity induces increased thermal insulation. For interlamellarinterlamellar pores) [59]. pores) Increased [59]. Increased porosity porosityinduces increasedinduces increased thermal insulation.thermal insulation. For this For this this reason, FG-TBC has a higher back wall temperature drop. Reduction in the back wall

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reason, FG-TBC has a higher back wall temperature drop. Reduction in the back wall tem- temperatureperature of the of theas-sprayed as-sprayed TBCs TBCs compared compared with with the thebase base reference reference material material shows shows the thethermal thermal insulation insulation capability capability of the of ceramic the ceramic coatings. coatings. Back wall Back temperature wall temperature drops dropsof 102 of°C 102and◦ C117 and °C 117were◦C observed were observed for the forlaser- theglazed laser-glazed DC-TBC DC-TBC and FG-T andBC FG-TBC coatings, coatings, respec- respectively.tively. Under Underlaser-glazed laser-glazed conditions, conditions, i.e., due i.e., to duethe densification to the densification of top coat, of top the coat, interla- the interlamellarmellar pores entrap pores entrapgas and gas contribute and contribute toward towardss enhancing enhancing thermal thermal insulation insulation [59]. Guo [59 et]. Guoal. [60] et studied al. [60] studiedthe effect the of effectsplat ofinterfaces splat interfaces on the thermal on the conductivity thermal conductivity of YSZ coatings, of YSZ coatings,by finite byelement finite elementsimulations simulations and experiments. and experiments. They identified They identified that interlamellar that interlamellar pores porescause causelowering lowering of thermal of thermal conductivity conductivity of TBCs, of TBCs, i.e., i.e., the the interlamellar interlamellar pores pores promoted thermalthermal insulation. On On similar similar lines, lines, Wei Wei et et al. al. [61] [61 conducted] conducted simu simulationslations and and studied studied the theeffect effect of lamellar of lamellar interspaces interspaces on thermal on thermal conductivity conductivity of TBCs. of TBCs.They opined They opinedthat interla- that interlamellarmellar pores porestrap gas trap molecules gas molecules and limit and limitthe conduction the conduction of heat of heat flow flow [60,61]. [60,61 They]. They re- reportedported that that the the presence presence of of interlamellar interlamellar pore poress can can contribute contribute up up to to 70% of reduction inin thermalthermal conductivityconductivity [[61].61]. In the present case, laser-glazed FG-TBCFG-TBC showedshowed better thermal insulation capability due to (i) increased formation of interlamellar pores owing to the multiple layers of YSZ- LaMA [[5]5] and (ii) densificationdensification of thethe toptop coat,coat, whichwhich leadsleads toto thethe entrapmententrapment ofof gasgas byby interlamellarinterlamellar pores,pores, preventing preventing heat heat conduction conduction across across the the coating coating thickness thickness (schematically (schemati- showncally shown in Figure in Figure 16). Thus,16). Thus, by this by mechanism,this mechanism, heat heat transfer transfer is suppressed, is suppressed, and and thermal ther- insulationmal insulation is enhanced is enhanced in laser-glazed in laser-glazed FG-TBC FG-TBC (Figure (Figure 15). 15).

FigureFigure 16.16. SchematicSchematic representationrepresentation ofof thethe heatheat insulationinsulation mechanismmechanism inin laser-glazedlaser-glazed FG-TBC.FG-TBC. LaserLaser glazingglazing densiﬁesdensifies thethe toptop coat.coat. InterlamellarInterlamellar spacespace betweenbetween splatssplats (pores)(pores) entrapsentraps gas,gas, constrainsconstrains heatheat ﬂowflow andand preventsprevents heatheat transfertransfer acrossacross thethe coatingcoating thickness.thickness.

In thermalthermal barrierbarrier coatings, thermal conductionconduction is known to occur by phonon transmission. Ceramic oxides in thermal barrier coatingscoatings have lattice imperfections thatthat scatterscatter phonons [[60].60]. Scattering of phonons hinders theirtheir freefree ﬂowflow acrossacross coatingcoating thicknessthickness andand consequently lowerslowers thermal thermal conductivity. conductivity. In In YSZ, YSZ, the the addition addition of yttria of yttria to zirconia to zirconia requires re- 2− 2− Oquiresvacancies O2− vacancies in order in toorder retain to retain the electrical the electrical neutrality neutrality of ionic of lattice. ionic lattice. The O Thevacancy O2− va- andcancy yttria and scatteryttria scatter the incoming the incoming phonons phonon acrosss across the lattice the lattice structure structure [27,60 ,[27,60,62–66].62–66]. This phenomenonThis phenomenon of phonon of phonon scattering scattering induces indu thermalces thermal insulation, insulation, as there as are there insufﬁcient are insuffi- free electrons (phonons are less effective in conducting heat energy compared to free electrons). cient free electrons (phonons are less effective in conducting heat energy compared to free YSZ and hexaluminate (LaMA) have low thermal conductivities (YSZ: 1.3 W/mK [66]; electrons). YSZ and hexaluminate (LaMA) have low thermal conductivities (YSZ: 1.3 hexaluminates: 0.8 to 2.6 W/mK [6]). These ceramics, taken in combination to synthesise W/mK [66]; hexaluminates: 0.8 to 2.6 W/mK [6]). These ceramics, taken in combination to thermal barrier coatings with a functionally graded architecture, can provide good thermal synthesise thermal barrier coatings with a functionally graded architecture, can provide insulation capability, as is evident from the results in the present investigation. good thermal insulation capability, as is evident from the results in the present investigation.

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4. Conclusions

Lanthanum magnesium hexaluminate (LaMgAl11O19)/yttria-stabilised zirconia (YSZ) thermal barrier coatings were prepared with two architectures: (i) functionally graded coating (FG-TBC) and (ii) dual-layered coating (DC-TBC). The influence of the architecture type on the thermal shock resistance and thermal insulation capability of the TBCs was examined. TBCs were subjected to laser glazing, in order to determine the effect of glazing on their thermal shock resistance and thermal insulation capability. The main conclusions drawn from the investigation are as follows: • Surface topography: Laser glazing significantly altered the surface topography of both coating architectures, such that the roughness and porosity of DC-TBC and FG-TBC on their surfaces was reduced. Densification of the top coat material due to laser glazing caused these reductions. • Thermal shock resistance: FG-TBC has better thermal shock resistance, i.e., higher cycle lifetime, than DC-TBC, in both the as-sprayed and laser-glazed conditions. (a) As-sprayed DC-TBC spalled along the YSZ/LaMA interface due to thick dual layers and lower thermal gradient that caused stress accumulation along the interface. In as-sprayed FG-TBC, the functionally graded architecture reduced stress concentration, which increased cycle lifetime. (b) Laser-glazed FG-TBC has higher thermal shock resistance than laser-glazed DC-TBC due to (i) the formation of segmentation cracks, (ii) improved strain tolerance, and (iii) closure of surface pores. • Thermal insulation capability: FG-TBC has better thermal insulation capability, i.e., higher back wall temperature drop, than DC-TBC, in both the as-sprayed and laser- glazed conditions. The multiple layers in FG-TBC cause increased formation of interlamellar pores. Laser-glazed FG-TBC showed better thermal insulation capability due to densification of the top coat, causing the entrapment of gas in interlamellar pores, which constrains heat transfer across the coating thickness. • Laser-glazed FG-TBC has the best heat management, in terms of both thermal shock resistance and thermal insulation capability. It has good potential for applications that require effective heat management, such as in gas turbines.

Author Contributions: Conceptualisation, M.A.K.; methodology, M.A.K., A.V.A., R.A.S. and S.J.; software, M.A.K. and S.J.; validation, S.J. and R.A.S.; formal analysis, M.A.K., R.A.S., and S.J.; investigation, M.A.K.; resources, M.A.K., M.D. and K.S.R.; data curation, M.A.K., R.A.S. and S.J.; writing—original draft preparation, M.A.K., S.J. and R.A.S.; writing—review and editing, S.J. and R.A.S.; visualisation, S.J. and R.A.S.; supervision, M.D.; project administration, M.D. and K.S.R.; funding acquisition, M.D. All authors have read and agreed to the published version of the manuscript. Funding: The work was financially supported by Defence Research and Development Laboratory (DRDL), Government of India. Project number: DRDL/24/08P/12/0513/41812. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Conflicts of Interest: The authors declare no conflict of interest.

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