I have coated SUH 3 and SS-AISI-321 metals with a 50-micron NiCr layer followed by a 100-micron layer of YSZ, a 50-micron NiCr layer followed by a 100-micron layer of Lanthanum Zirconate, and a 50-micron NiCr layer followed by a 100-micron layer of Gadolinium Zirconate using the plasma spray technique. The linear thermal expansion dilatometer test for both coated samples was conducted up to 400 degrees for SUH 3 and 700 degrees for SS-AISI-321. The CTE value of uncoated SUH 3 and YSZ, LZ, and GZ coated SUH 3 increases. However, the CTE value of uncoated SS-AISI-321 increases, while that of YSZ, LZ, and GZ coated SS-AISI-321 decreases. Why does the CTE of YSZ, LZ, and GZ coated SS-AISI-321 decrease? And why does the CTE of YSZ, LZ, and GZ coated SUH 3 increase?
We can assume that the thermal expansion coefficients of both base materials (SUH 3 and AISI 321) without a coating increase with temperature. By the way, such a measurement (without coatings) must be carried out in such research work. Normally, a coating has little effect on thermal expansion of a coated component if the component is much larger (thicker) than the coating. Only in the case of comparable thicknesses of the base material and the coating, the coatings can cause different effects with temperature changes. For example, the coated sheets can become convex or concave during heating. Such effects with small thicknesses of the base material can greatly falsify your measurement and must be avoided at all costs. If your dilatometry sample bends during heating because it was only coated on one side, you measure a decrease in the coefficient of thermal expansion when it actually increases.
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