Thermo-Physical Properties Measurement of Advanced TBC Materials with Pyrochlore and Perovskite Structures

Thermal barrier coatings (TBCs) serve as thermal insulator in the hot region of an aircraft engine. Besides this, it also protects the underlying metal surface from the harsh corrosive and eroding environment. The associated lower thermal conductivity of TBC ceramic materials plays an important role in the improvement of thermal efficiency of the engine in term of increased combustion temperature and power. The thermal conductivity of the conventional yttria stabilized zirconia (YSZ) and three advanced ceramic materials with perovskite (CaZrO3) and pyrochlore structure (La0.75Nd0.25)2Zr2O7 & Nd2Ce2O7) have been determined using differential scanning calorimetry (DSC). With thin metallic disk on the ceramic samples of different heights were heated / scanned using a standard DSC apparatus. The results were evaluated for the thermal conductivity measurement using well established procedure /calculations. The analyzed results were compared with that of other techniques given by other researchers and found to be in good agreement with an error of 10-15%. The result of coefficient of thermal expansion (CTE) that was measured using a dilatometer up to 1273°K has also given.


Introduction
Thermal barrier coatings (TBCs) not only protects the underlying metal surface from the harsh corrosive and eroding environment, but also it serves as thermal insulator in the hot region of an aircraft engine. The purpose of high temperature thermal stability and the need for lower thermal conductivity enhancing the turbine inlet temperature (TIT) is the drive for the improvement in the existing materials associating research and development of new more durable materials. As an outcome, number of high potential materials such as pyrochlores, perovskites and hexa-aluminates have been researched and explored for various required properties [1][2][3][4]. The associated lower thermal conductivity of TBC ceramic materials plays an important role in the improvement of thermal efficiency of the engine in term of increased combustion temperature. Although the state of art 8% yttria stabilized zirconia (YSZ) is generally used as a standard TBC in aircraft engines and power generation turbines due to its excellent thermal and mechanical properties (e.g., low thermal conductivity, higher TEC, fracture toughness and thermal fatigue life) [5][6][7], it has limitation of application temperature which is below 1200 o C due to tetragonal to monoclinic polymorphic phase transformation. Therefore, the need of advanced gas turbines operating at +1500°C drove the newer research for more efficient ceramics showing higher compatibility with bond-coat, high temperature structural stability and lower thermal conductivity. The ceramics with perovskite and pyrochlore crystal structure have earned great attention for advanced TBC applications due to their higher melting point, high temperature structural stability (up to melting point of substrate) and low thermal conductivity [8][9][10][11][12][13][14][15][16][17][18][19]. In the present study, the thermal conductivity of the standard 8% yttria stabilized zirconia (YSZ) and three advanced ceramic materials with perovskite and pyrochloretype crystal structurehave been determined using differential scanning calorimetry (DSC). These materials are: (i) calcium zirconate (CaZrO 3 , perovskite type orthorhombic crystal structure), (ii) neodymium doped lanthanum zirconate (La 0.75 Nd 0.25 ) 2 Zr 2 O 7 , pyrochlore type crystal structure) and (iii) Neodymium cerate (Nd 2 Ce 2 O 7 , fluorite type crystalstructure). Crystal structure of all of them is stable at higher temperature (+1600°C) with comparatively lower thermal conductivity than YSZ [20]. They also have very promising thermo physical properties required for advanced TBCs [21][22][23][24][25][26][27][28][29][30].
Many techniques are used for thermal conductivity measurements of low conductivity material and TBC ceramics such as laser flash method [31][32][33], infrared thermographictechnique [34][35][36][37], differential scanning calorimeter (DSC) [38,39] and modulated temperature differential scanning calorimeter (MTDSC) [40][41][42]. In the present study, the thermal conductivity of the sintered samples of the TBC ceramics was measured using DSC with thin metallic disk on ceramic sample. It is comparatively simpler and economical method as a simple DSC equipment is used without need of expensive modulated temperature facility. In this method, a thin copper disk on the ceramic samples of different heights were heated / scanned using a standard DSC apparatus. The wellestablished theory and procedure of Camirand,C.P. [38] was used to evaluate the results for thermal conductivity measurement.The analyzed results were also compared with that of other techniques given by other researchers. The thermal expansion coefficient (TEC) is also an important property of the TBC ceramics. A higher TEC of the ceramic topcoat is generally required for better compatibility with bondcoat for better thermo mechanical fatigue life. It was measured using a high temperature dilatometer.
Sample Preparation.For thermal conductivity measurements, five samples of each four TBC ceramic material enlisted below was prepared with different thicknesses (0.5 to 2mm) and 3mm diameter by using different weights in the die during green compaction. The procedure used in the preparation of the samples is given in the flow chart given in Fig. 1a Fig. 1b shows the sintering cycle adopted during sample preparation. The extremely slow heating rate (1°C/min) at the initial stage and hold at 300°C is to drive out the binder from the green compact without inducing crack in the sample. The remaining heating and cooling rate was kept 3°C/min which is also slow enough to prevent the cracking in the ceramic samples. Fig. 1c shows stereo microscope image of the sintered samples of Nd 2 Ce 2 O; the variation in the thickness of the samples is evident. The metallic pure copper disk samples were prepared from the 0.3 mm sheet using 3mm dia punching die. After measuring the weight and dimension of the ceramic samples and copper disk, DSC scans were taken by placing the metallic disk on the ceramic sample in one crucible of the DSC apparatus and leaving the other one empty. For thermal expansion measurement, cylindrical ceramic samples of four TBC materials with 25mm height and 9mm dia were prepared using the similar procedure given in the Fig. 1.
Thermal Conductivity Measurement. The theory of measurement of the thermal conductivity of a ceramic using DSC is based upon melting of a thin metallic disk onto the ceramic samples of different heights or thicknesses; the variation in the thickness of ceramic samples affect the heat flow from DSC furnace to metal disk, effecting the slope of melting curve with a time-base shift in its differential power, dP.
After measuring the size and the dimensions of the metallic copper disk and ceramic sample, the metallic disk was placed over the ceramic sample as per schematic shown in the Fig. 2a. Fig. 2b shows thermal resistances at contact regions in the cross sectional view. When the metallic sample is place on the top of the ceramic sample, the total thermal resistance R T is: Where R 1 is thethermal resistance at Cu disk to ceramic sample interface, R 2 is the thermal resistance at DSC crucible to ceramic sample interface and R c is the thermal resistance of ceramic sample.
Before running the actual sample, a zero-line curve was obtained initially with the empty furnaces of DSC which was subsequently subtracted from the actual test curves.
A fast scan ~15 °C/ min was carried out to minimize the convection effects in the furnace with the typical melting curve of pure metal (Fig.3).
The slope of the increasing part of the dP / dT curve is inversely proportional to the total thermal resistance, R T . The thermal resistance of the ceramic sample, R c is proportional to the thickness-to-area ratio of ceramic sample.
Where t and A is the thickness and area of the ceramic sample, respectively, and Here, λis the thermal conductivity of ceramic sample. The R 1 + R 2 is a constant resistance and it was kept at minimum by making the ceramic and metallic samples with very flat and smooth surface. Before each scan, a preliminary slow scan was also run in order to completely fuse the metallic sample onto the ceramic sample; it help to reduce the thermal resistance at metallic Cu disk to ceramic sample. Total thermal resistance was calculated using Eq.2 from five scans of ceramic samples with different known thicknesses and constant surface area (ca. 3mm) and plotted against respective t/A ratio of the samples as shown in the Fig. 4. The inverse of slope of this linear plot derives the thermal conductivity as per equation (4). The results are given in the Table 1.

Key Engineering Materials Vol. 778
Thermal Expansion Measurements. Samples were tested using a dilatometer up to 1200°C with 10°C/ min heating rate. Fig. 5a shows the sintered sample of the Nd 2 Ce 2 O 7 revealing its cyliderical shape (φ9mm x 25mm).The percentage linear change (dL/L ο %, where dL is change in length and L ο is original length) wasplotted against the temperature as shown in the Fig. 5b.  The Thermal expansion coefficient (TEC) of TBC materials was also measured using the following equation: Where L o is the length of the sample at T o , dL o is the change in length at T o , dL 1 is the change in length at T 1 . The results of the TEC are given in the Table 2. The neodymium cerate (Nd 2 Ce 2 O 7 ) showed the highest expansion which may be attributed to the increase in the structure disorder due to presence of comparatively higher atom size (Nd) in rare earth (RE) cerate (RE 2 Ce 2 O 7 ) causing in formation of ionic vacancy clustering [46]. The TEC of the calcium zirconate (CaZrO 3 ) was also higher than that of yttria stabilized zirconia (YSZ). While the TEC of the neodymium doped lanthanum zirconate (La 0.75 Nd 0.25 ) 2 Zr 2 O 7 ) was comparable with the YSZ. the TEC of the lanthanum zirconate (La 2 Zr 2 O 7 ) is lower (TEC, 9.1 x10 -6 /°K at 1000°C) [47] than 8% YSZ (10.7 x10 -6 /°K at 25-1000°C) [28] which shows may induce the thermal stresses in the coating during operation [28,48]. The doping of neodymium in the lanthanum zirconate (La 2 Zr 2 O 7 ) caused the marked improvement in the TEC, making it compatible choice with YSZ (for engineering double ceramic layered or composition gradient thermal barrier coatings).

Summary
DSC is comparatively simple method to measure the thermal conductivity of ceramics materials with 10-15% variations from conventional thermal conductivity measurement method. The higher difference in the case of Nd 2 Ce 2 O 7 material may not be in agreement as much lower thermal conductivity value (~1W/m°K) of the Nd 2 Ce 2 O 7 was also found in the literature [45].In comparison to the YSZ (a conventional TBC material), the CaZrO 3 , (La 0.75 Nd 0.25 ) 2 Zr 2 O 7 and Nd 2 Ce 2 O 7 showed lower thermal conductivity and higher co-efficient of thermal expansion (TEC); being relatively stable at higher temperature (≥ 1500°C), these materials owned higher potential for application in advanced turbine engines.