Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
(OP)
I am testing different thermal barrier coatings using a high temperature flame, a thermocouple sealed inside a boss on the specimen's back (not visible in the attached sketch), and a circulating water bath; the test specimen quenches in the water periodically.
The baseline is an uncoated, cast iron specimen. Theoretically I would expect the thermocouple temperature measurements with the uncoated specimen to be higher than measurements with coated specimens when the flame temperature is constant. Yet I observe the opposite. What am I overlooking?
The baseline is an uncoated, cast iron specimen. Theoretically I would expect the thermocouple temperature measurements with the uncoated specimen to be higher than measurements with coated specimens when the flame temperature is constant. Yet I observe the opposite. What am I overlooking?





RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
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RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
In case it was not obvious, the thermal barrier coating is represented by the tan surfaces.
Attached is another image showing the back.
Would it be logical to conclude that the uncoated specimen always has a higher heat loss regardless of specimen geometry?
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
Could you explain the water quench?
Are you taking into account the energy required to vaporize the water after the quench? The amount of water on the specimen may be different coated vs. uncoated.
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
I fixed the specimen on an armature which holds the specimen aloft while the flame targets the front surface for a set period, after which the armature plunges the specimen just for a few seconds into a fixed volume water bath which recirculates. The cycle then repeats.
All surfaces of the specimen are submerged, both when coated and not.
I was unable to attach a photo of the set-up.
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
A 2" diameter 1" thick object has the annoying property that the distance to your thermocouple is the same as the distance to the circumference. So, by the time you measure a temperature shift, the rim of the block has also warmed up. That would seem to be an extra layer of complexity in an otherwise complex problem.
I'm unclear whether your test accurately reflects the intent of your thread title. Ostensibly, a "shock" is a high amplitude, short duration event. Therefore, a shock barrier either reduces the amplitude, or increases the duration, or both.
Since it's supposed to be a shock, the flame is ostensibly a pulse? Have you looked at the time characteristics of your thermocouple measurements? You may want to thin down your sample to maybe 1/4", which might minimize the impact of the block's thermal boundaries.
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RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
Your comments about the specimen geometry and thermal conduction prompt me to mention that an infrared gun measuring the bare metal surface temperature along the width near the coated surface registers a LOWER value than the thermocouple measuring the temperature inside a boss from the back.
Attached is a plot of three tests, the baseline and two coated specimens, to address your comments and questions about shocking the specimen.
Thanks for your recommendation to reduce the specimen thickness.
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
Your experiment seems really really complicated, with far too many interdependent variables and everything is transient.
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
As the plot shows, the initial slope is about the same across all specimens, so that variable is unhelpful for a comparative analysis. My focus, therefore, is comparing peak temperatures.
If I were only interested in the relative peak temperatures, does it matter whether the secondary surfaces are shielded or not so long as the environmental conditions are constant across specimens?
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
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RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
That is what you are seeing.
Your baseline sample cools more than the coated samples during the quench. The difference is most pronounced at about time = 250 seconds.
As a result the coated samples get hotter.
So the answer to your question is "No coating is better, because in this cycle, with this test sample configuration the barrier coatings slow down the cooling during the quench portion of the cycle."
RE: Measuring Temperature Effect of Thermal Barriers in Thermal Shock Test
Is your sample representative of the actual application? If so, then you need to be looking at a different material/thickness, and possibly add cooling features like fins to the structure.
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