Effect of Oxidation and Plume Formation on Low Power Nd-Yag Laser Metal Interaction

The effect of oxide formation and plume formation on laser energy absorption by metallic targets has been studied. The change in directional-hemispherical spectral reflectance of metallic targets during single-shot Nd-Yag laser pulse irradiation was measured using an integrating sphere under controlled environments of both oxygen and argon gas. The spectral transmittance of the plume formed over the targets was also measured using a He-Ne probe laser. The metal targets studied included A16061, Cu, 304 stainless steel, and low-carbon steel. Results obtained show that once a target is hot enough to form a vapor plume, in either argon or oxygen, absorption by the plume significantly limits the amount of laser energy available for absorption at the target surface. Prior to plume formation, the amount of laser energy absorbed by the target is determined by the target surface optical properties. Under the conditions of the present study (incident laser flux of the order of 10⁶ W/cm² over 0.5–1.0 ms), in the case of absorbing metals (αλ>0.3), such as stainless and low carbon steel, the intrinsic absorptivity of the metal is high enough that the effect of ambient gas (oxygen or argon) on absorptivity is insignificant. In the case of nonabsorbing metals (αλ<0.3), such as Cu and A16061, the intrinsic metal absorptivity is low enough that the ambient gas does have a significant effect on the target absorptivity. The formation of an oxide layer that occurs in an oxidizing environment generally has the effect of increasing the target absorptivity. The relative magnitude of the absorptivity enhancement due to oxide formation depends on the respective absorptivities of the oxide and metal. For metals that form oxides that are intrinsically absorbing in the solid state, such as Cu₂O or CuO, the enhancement in absorptivity due to oxide formation can be as much as an order of magnitude. For metals that form oxides that are intrinsically nonabsorbing in the solid state, such as MgO or Al₂O₃, the enhancement in absorptivity due to oxide formation is more modest but still significant (40 percent). The enhancement in absorptivity in the latter case (MgO or Al₂O₃) is postulated to be associated either with a thin, absorptive, transition region composed of a mixture of metal and substoichiometric solid oxide just below target surface or with the melting of the oxide at the surface.