Thread: Can emissivity be used as a differentiating criteria for metals?

In the non destructive testing, we have to distinguish flawed/wrong composition samples/raw materials from the good ones without damaging them. I want to ask if emissivity of surfaces can be used as a differentiating criteria? I mean, will the difference between surface emissivities of good and bad samples be that much different? Is it done anywhere before?

Originally Posted by TheStrider

In the non destructive testing, we have to distinguish flawed/wrong composition samples/raw materials from the good ones without damaging them. I want to ask if emissivity of surfaces can be used as a differentiating criteria? I mean, will the difference between surface emissivities of good and bad samples be that much different? Is it done anywhere before?

It would inform, but wouldn't be dispositive. The problem is that emissivity is a function of work function, which is largely a function of surface properties, so it is basically insensitive to the properties of the bulk.

Thanks!
But will the properties of a surface change with composition? I mean, a sample of 20% Cu & 80% Al and a sample of 50% Cu and 50% Al will have significantly different surface properties? And hence emissivities?

Originally Posted by TheStrider

Thanks!
But will the properties of a surface change with composition? I mean, a sample of 20% Cu & 80% Al and a sample of 50% Cu and 50% Al will have significantly different surface properties? And hence emissivities?

Absolutely, which is why I said emissivity measurements will inform. However, work function is also strongly affected by surface topography, the presence of tiny amounts (monolayers) of adsorbed species, the phase of the moon, etc. In the old vacuum valve/tube days, tiny amounts of thorium were discovered to lower the work function of tungsten filaments so much that emissivity increased by orders of magnitude. Later, oxides of barium and strontium were used to lower the work function still further, leading to valves that worked with a red glow, rather than at a white-hot incandescence.

The emissivity value per se can only tell you that something has changed, but not what that something is, nor whether that something is harmful. You can't even infer the surface concentration of contaminants without knowing additional information.

Ya emissivity can be use to find defects on metal surfaces, in NDT(non destructive testing or NDE or NDE) we use electromagnatic radiation, x rays. 3D xrays, ultrasonic and eddy current to finding defects over metal and plastic surfaces. Just like to find pore in Welding or creak in welding. in NDT we produces a x ray like pattern or image to see defects, emissivity is also an relative property of material which compare energy emission in term of radiation but it not same for all the materials, depends upon the color of a body, wavelength and and angle of emissivity.
Why we not use emissivity ?
slow process to finding creaks, also the emission of radiation depends upon temperature of a body and color of a body.
i hope its help you somehow.
thanks