The effects of the atmosphere on the surface modification of alumina by pulsed-laser-irradiation

A near-surface thin layer is melted when alumina is pulsed-laser-irradiated in an Ar-4% H-2 atmosphere or in air. A thin layer of amorphous phase forms when the substrates are irradiated in Ar-4% H-2 at 1 to 1.3 J/cm(2) with multiple laser pulses. Amorphous phase is also found in samples laser-irradiated in air and oxygen. After a laser pulse at an energy density of 1.6 J/cm(2) or higher the melt solidifies epitaxially from the unmelted substrate with a cellular microstructure. There is a decrease in the cooling rate of the melt as the laser energy density is increased because more heat must be dissipated. The amorphous phase forms when the heat input due to the laser pulse produces a superheated melt that cools down sufficiently fast to avoid crystallization. Very small particles of aluminum in the laser-melted and subsequently solidified layer are observed only in samples laser-irradiated in an Ar-4% H-2 atmosphere. In this reducing atmosphere, the alumina is possibly reduced to metallic aluminum which is mixed into the melt by the turbulence provoked by the laser pulses. The effects of these metallic particles on copper deposition when the irradiated substrates are immersed in an electroless bath are discussed.