The introduction of laser processing into the processes of forming and modifying materials opens up new prospects for creating unique structures with specified properties. Of particular interest is laser crystallization of amorphous quasi-crystalline coatings applied by spraying, since it allows local modification of the material structure, obtaining functional elements with increased hardness, corrosion resistance and other useful characteristics.
The spraying method makes it possible to obtain thin films with a uniform composition and high adhesion to the substrate. However, as a result of rapid cooling, amorphous structures are formed that do not have the full potential of quasicrystals. Laser processing, in turn, allows for controlled heating and cooling of the material, initiating phase transitions and crystallization.
Quasicrystals (e.g., AlFe10Cu10Cr10) are characterized by a unique combination of high hardness, low surface tension/low friction, and poor thermal conductivity. These properties are thought to be due to their unusual translational symmetry and non-periodic arrangement of atoms. These materials are typically produced by high-temperature annealing (above 700 °C), and in bulk form they exhibit brittleness. Coatings are used to address the problem of low strength, but high-temperature annealing negatively affects the mechanical properties of standard structural materials used as substrates. To overcome this limitation, controlled laser surface treatment of deposited quasi-crystalline coatings (on Al, Ti, and bearing steels) has been applied to transform the amorphous (a) structure into the crystalline (c) phase. Energy dispersive spectroscopy (EDS) and X-ray diffraction for composition and structure, C-Brale for relative hardness, and ball-on-disc (BOD) friction and wear tests were used to characterize the a-QC and c-QC films. Laser treatment ensured the transition from the amorphous structure to the crystalline phase without significant reduction (<10% for Ti–6Al–4V) in substrate hardness.
It was found that the laser pulse energy affects the final surface finish of c-QC. It was observed that laser crystallization increased the indentation resistance and adhesion of c-QC films on Ti–6Al–4V and 52100 steel substrates. Friction and wear tests of c-QC films demonstrated a decrease in friction coefficients by approximately 40%, 20–25%, and 25–30% compared to uncoated 2024-T3Al, Ti–6Al–4V, and AISI 52100 steel substrates, respectively. A decrease in wear of c-QC-coated surfaces was also observed compared to uncoated ones.
The laser crystallization process includes several stages: absorption of laser radiation, heating of the material to the crystallization temperature, nucleation and growth of crystalline phases, and subsequent cooling. Laser radiation parameters such as power, pulse duration, scanning speed determine the temperature field in the material and, consequently, the structure of the forming layer.
Research shows that optimal selection of laser processing parameters allows obtaining quasicrystalline coatings with a high degree of ordering and improved performance characteristics. By varying the energy and scanning speed of the laser, it is possible to control the crystallite size, orientation and phase composition, which opens up opportunities for creating materials with specified properties for various applications, including protective coatings, sensors and catalysts.
Author: F. Kustas, P. Molian, A. Sadhu Kumar, M. Besser, D. Sordelet
Institute: Engineered Coatings, Inc., P.O. Box 4702, Parker, Colorado 80134-4702, USA, Iowa State University, Ames, Iowa 50011, USA, Department of Energy’s Ames Laboratory, Ames, Iowa 50011, USA