Laser crystallization of amorphous sputter-deposited quasicrystalline coatings

Quasicrystalline (QC) (e.g., Al70Fe10Cu10Cr10) materials offer a unique combination of relatively high hardness, low surface energy/low friction, and low thermal conductivity. This desirable suite of properties is reportedly due to their nonconventional translational symmetry and aperiodic atomic ordering. These materials can be formed only after a high-temperature (>700 degreesC) anneal, and they exhibit brittle behavior in bulk form. Coating deposition alleviates the low toughness issue, but the requirement for a high-temperature anneal degrades the mechanical properties of conventional engineering substrates. To address this latter issue, controlled laser surface treatment of sputter-deposited QC coatings (on Al, Ti, and bearing steel alloys) was performed to convert the amorphous (a) structure into a crystalline (c) phase. Characterization of both a-QC and c-QC films included energy dispersive spectroscopy (EDS) and X-ray diffraction for composition/ structure, C-Brale indentation for relative toughness, and ball-on-disk (BOD) friction/wear tests. Laser treatment was successful in converting the amorphous structure to the crystalline phase, without significant reduction (<10% for Ti-6Al-4V) in substrate hardness, and it was shown that laser pulse energy influences the final surface finish of the c-QC surface. Laser crystallization was observed to increase the indentation resistance/adhesion of c-QC films on coated Ti-6Al-4Vand 52100 steel. Friction/wear tests of c-QC films showed reductions in coefficients of friction, compared to non-coated substrates, of similar to40%, similar to20-25%, and similar to25-30%, respectively, for coated 2024-T3Al, Ti-6Al-4V, and AISI 52100 steel substrates. Reductions in wear damage for c-QC-coated surfaces, compared to non-coated surfaces, were also observed. (C) 2004 Elsevier B.V. All rights reserved.