Chapter II P: Laser power;
H: Magnetic field;
V. YSZ graded thermal barrier coatings for high temperature applications
5.3 Results of the graded thermal barrier coating
5.2.2 Thermal barrier effect measured with laser flash technique
For measuring the thermal barrier effect of deposited graded coatings, a thermal graphic camera (Thermo Tracer TH9260) and corresponding software (Thermal Movie) were used for the analysis.
Thermal graphic camera can capture IR irradiations and form images. Different color schemes in the images indicate different temperature values. A 1020nm fiber laser (IPG YLS-2000) with a short pulse of 375ms and a power of 100W was used as the heat source for the measurement of the coatings. In order to perform comparisons, a 2µm pure YSZ on SUS substrate, a bare SUS substrate together with a graded coating (1µm SUS/YSZ graded coating + 1µm pure YSZ on top, 2µm in total) were tested. All specimens were irradiated by the same laser pulse on the coating surface and the backside thermal images were captured by the thermal graphic camera. Using the software to process the images, temperature history at the rear surface of each sample can be obtained.
Experimental scheme for measurement is shown in Figure 29. A 375ms fiber laser pulse was irradiated on the coating surface in order to emulate the actual working scenarios as heat fluxes reach the TBC first. A thermal graphic camera was setup behind the sample and recorded the thermal radiation due to heat transfer from the TBC to the rear surface of the specimen, therefore the temperature history (highest temperature) can be obtained through image analysis using the software. SUS is highly reflecting and surfaces of graded sample as well as pure YSZ sample are in white color. To increase the specimens’ absorption and reduce the measurement noise, 500nm thick DLC coatings (black) were deposited on three types of samples before measurement. Also, the amount of energy input for all three samples can be guaranteed to be equal for reliable results. For each sample, three tests were conducted and the averaged temperature curves are shown in Figure 30.
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Figure 29. Schematics of thermal graphic camera setup for temperature measurement.
In Figure 30, temperature history curves for all three types of coatings are plotted. Black curve denotes the pure SUS substrate while red and blue curves denote graded coating and pure YSZ coating, respectively. All three samples were at the room temperature at first, then a 375ms laser pulse was irradiated at around 0.9s on the time scale. The short pulse was used to minimize the cooling during the measurements. Right after the laser pulse, backside temperatures of the samples increased dramatically to their respective maximum values, then naturally decreased back to room temperature over time. In the figure, maximum temperature of SUS substrate is the highest (~448℃) among all three cases, and this is obvious as pure SUS has higher thermal conductivity. Pure YSZ has the lowest maximum temperature of roughly ~383℃ due to the thermal barrier effect of YSZ. Graded coating case lies in between (maximum temperature ~407℃) because of the coexistence of SUS and YSZ content in the SUS/YSZ graded coating region. The tendency seems reasonable considering the mixture rules of physical properties. Due to the limitation of our thermal camera, maximum temperature of only 500℃
is tested, but we believe that similar behaviors also stand for higher temperatures.
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Figure 30. Averaged temperature history curves of three types of samples. Black curve denotes the temperature reaction of a pure SUS substrate after a laser pulse irradiation; red curve denotes the temperature reaction of a 1µm SUS/YSZ graded with a 1µm pure YSZ coating on top with a SUS substrate after a laser pulse irradiation; blue curve denotes the temperature reaction of a 2µm pure YSZ coating on a SUS substrate after a laser pulse irradiation. Laser pulse parameters used for three samples are the same.
For analyzing the thermal properties of the measurements, a method of using half time (t1/ 2, time value at half signal height) and sample thickness (d) for calculating the thermal diffusivity (α) [62] is adopted here. The formula ( drawn in Figure 31) is developed by Parker et al. in 1961 and can be mathematically expressed as =0.1388
(
d2/t1/2)
, where d is in the unit of cm and t1/ 2in the unit of s. The densities ρ of pure SUS 316L and bulk YSZ are 8000 and 5900 kg/m3, and their specific heat cp values are 500 and 600J/kg·K, respectively. From the measured raw data we can get the half time value t1/ 2for SUS, graded and YSZ samples and they are 0.1387, 0.1424 and 0.1427s, respectively.Therefore, all three thermal diffusivity values for three samples can be obtained easily as αSUS=4.0029×10-6, αGraded=3.9773×10-6 and αYSZ=3.9687×10-6 m2/s. Using the diffusivities and density/specific heat values we can now calculate the thermal conductivities easily by getting the product of diffusivity, specific heat and density, described ask=
cp . For three samples, their thermal５２
conductivity values (coating and substrate as one body) are now calculated as kSUS = 16.0116, kGraded = 15.9016, and kYSZ=15.8645W/m·K. With the knowledge the thermal conductivity values (as one body) and use the mixture rule reversely, we can get the conductivity values for the graded coating and pure YSZ (without substrates), and they are kGraded’=4.8573 and kYSZ’=1.1431W/m·K. Therefore we have obtained the thermal conductivity values for pure SUS (16.0116 W/m·K) and pure YSZ (1.1431W/m·K), too. Compare the results with literatures (kSUS’’=21.5 W/m·K and kYSZ’’=1.5~2.05 W/m·K), we found out that our results are a lot smaller, possible reasons are experimental errors and during the measurements, our environment isn’t perfectly isolated so there is heat loss during temperature rising, and that will account for the smaller conductivities. Also, our materials are PLD deposited so some extend of porous structures are expected, this will also contribute to the smaller conductivity values.
Figure 31. Scheme of thermal diffusivity calculation.