Thermal expansion of different materials
12-05-2018, 05:32 PM,
#1
Thermal expansion of different materials
After reading quite a bit about thermal expansion of different materials I still have some questions.
Aluminium and stainless steel have very different CTE, aluminium being almost double. How come they are combined together and sustain their bond through heating and cooling cycles.

Thanks
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12-05-2018, 06:50 PM,
#2
RE: Thermal expansion of different materials
(12-05-2018, 05:32 PM)pcdmt Wrote: After reading quite a bit about thermal expansion of different materials I still have some questions.
Aluminium and stainless steel have very different CTE, aluminium being almost double. How come they are combined together and sustain their bond through heating and cooling cycles.

Thanks


It is an interesting question.
In fact, stresses that arise in the layer during the temperature change are directly proportional to the CTE difference between the layer and the substrate (Δα), but that's not all. No less important are other parameters that come into the Hooke equation for stress calculation, temperature difference (ΔT) and modulus of elasticity of the layer (Ecoat). The equation looks like this:

σcoat = ± ΔαΔTEcoat

It is often forgotten that modulus of elasticity of the layer is also responsible for the stresses, but it is extremely important just for thermal spraying.
In your example with stainless steel and aluminum you can see that stresses in an aluminum layer on steel substrate factor 3 are smaller than in a steel layer on aluminum substrate. Here we assume that aluminum layer has a modulus of elasticity of compact aluminum (about 70 GPa) and stainless steel layer has a E-modulus of compact stainless steel (about 210 GPa).
In reality, moduli of elasticity of thermally sprayed coatings are significantly lower than moduli of elasticity of compact materials with the same composition. The reason for this is a defective layer structure with microcracks and pores. For example, a plasma sprayed coating of YSZ has an E-modulus of about 10 GPa, and that is a factor of 20 less than E-modulus of compact YSZ ceramic. Even thermally sprayed stainless steel often has an E-modulus of less than 50 GPa.

Conclusion: thermally sprayed coatings on substrates with different CTEs can withstand heating and cooling cycles undamaged if their moduli of elasticity are low.


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