High velocity air fuel (HVAF) is a coating deposition method that provides high density and adhesion, making it attractive for forming protective layers on various materials. Quasi-crystalline Al-Cu-Fe-Si alloys, known for their high hardness, corrosion resistance, and unique tribological properties, are promising for use as protective coatings. However, the HVAF process can lead to phase transformations in the deposited material due to high temperatures and cooling rates.
The study of phase transformations in quasicrystalline Al-Cu-Fe-Si coatings sprayed by HVAF is an important task for process optimization and obtaining coatings with specified properties. During the spraying process, quasicrystalline powder particles are subjected to heating, acceleration, and impact on the substrate, which can cause melting, crystallization, and formation of amorphous phases.
Experimental studies have shown that the phase composition of the sprayed coating depends on the HVAF process parameters, such as flame temperature, gas flow rate and distance to the substrate. Under optimal conditions, it is possible to preserve the quasi-crystalline structure in the coating, but at high temperatures, crystalline phases such as Al, Cu, Fe and silicides can form.
To study the phase changes during the deposition process, quasi-crystalline Al-Cu-Fe-Si coatings were produced by high-speed spraying using an air-fuel mixture. The original powder and the resulting coating were analyzed in detail using scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy.
The obtained data indicate that the following phases were present in the atomized Al50Cu20Fe15Si15 powder: Al3Cu2, a small amount of λ-Al13Fe4, a quasicrystalline (QC) structure, an amorphous phase, and β-Al (Cu, Fe, Si). A typical flattened powder particle had a periphery surrounded by an amorphous phase with a thickness of about 1 μm. The interior of the particle consisted of an alloy coated with Al3Cu2 and β-Al (Cu, Fe, Si) phases enriched with silicon.
Another copper-rich β-Al(Cu, Fe, Si) species was identified near the amorphous region and had a composition similar to the original β-Al(Cu, Fe, Si) phase in the powder. Comparison of the peripheral and internal regions of the particle revealed differences in phase composition, which is explained by differences in heating and cooling rates.
Analysis of the coating microstructure using scanning and transmission electron microscopy revealed the presence of nanoscale crystallites embedded in the quasi-crystalline matrix. These crystallites can influence the mechanical and tribological properties of the coating. Studying the phase transformations in HVAF-sprayed Al-Cu-Fe-Si quasi-crystalline coatings allows optimizing the process to produce coatings with high hardness, corrosion resistance, and wear resistance, making them suitable for use in a variety of industrial applications.
Author: Mingwei Cai and Zhong Shen
Institute: College of Mechatronics and Control Engineering, Shenzhen University, Shenzhen 518060, China