Quasi-crystalline materials (QCM) have a unique combination of properties, such as high hardness, wear resistance, low friction coefficient and good corrosion resistance. This makes them promising for use as protective coatings in various industries. One of the effective methods of applying QCM is high-velocity oxygen-flame spraying (HVOF), which allows obtaining dense and adhesively strong coatings.
In this paper, the formation of quasi-crystalline coatings of the Al–Cu–Cr system by the HVOF method using coarse-grained initial powders was investigated. Spraying parameters such as fuel, oxygen and carrier gas consumption, as well as spraying distance, were varied to optimize the process of forming a quasi-crystalline structure in the coating.
Quasi-crystalline (QC) coatings based on Al–Cu–Cr were fabricated using a DJ2700 high-velocity oxygen-fuel spraying (HVOF) system using coarse-grained starting powders. Low-energy atmospheric plasma spraying (LPAPS) with the same AlCu20Cr15 QC powders and HVOF spraying of Cu180 powders with particle sizes of 44–61 μm were used for contrast deposition experiments. The phase composition, microstructure and hardness of the resulting QC coatings were studied.
X-ray diffraction analysis showed that the starting material and coatings contained predominantly the icosahedral quasicrystal (IQC) I-Al65Cu24Cr11, as well as three minor crystalline phases: α-Al69Cu18Cr13, θ-Al13Cr2 (i.e., Al83Cu4Cr13), and ε-Al2Cu3. Qualitative analysis of the X-ray diffraction patterns revealed that the coating deposited by HVOF spraying contained more IQC and less crystalline phases compared to the coating deposited by LPAPS. Moreover, increasing the input thermal energy resulted in a decrease in the IQC content and an increase in the proportion of crystalline phase in the coating for both HVOF and LPAPS.
Experiments have shown that when HVOF-sprayed QC particles hit the substrate surface, the unmelted portion of the particles was destroyed, and the already deposited coating layer was deformed, compacted, and even cracked under the impact of high-speed particles. This reduced spatter and compacted the sprayed coating. Thus, coatings deposited by the HVOF method from large (61–74 μm) QC powders had a smaller spatter area and a denser microstructure with lower porosity and higher hardness compared to coatings deposited by the LPAPS method from the same source material. It is important to note that the generally preferred particle size of the source material for HVOF is 5–45 μm.
Based on the results of contrast experiments, the necessary and sufficient conditions for the occurrence of impact destruction of particles during thermal spraying were determined: (1) high brittleness of the original material and (2) high speed and low degree of melting of the sprayed particles.
The obtained coatings were studied by X-ray diffraction, scanning electron microscopy and energy-dispersive analysis. It was found that under certain spraying conditions, coatings with a predominance of the quasi-crystalline phase are formed. The grain size and microhardness of the coatings depend on the spraying parameters. Coarse-grained powders make it possible to obtain coatings with a more pronounced quasi-crystalline structure, which may be due to a decrease in overheating and evaporation of elements during spraying.
The HVOF method is promising for obtaining quasi-crystalline Al–Cu–Cr coatings from coarse-grained initial powders. Optimization of spraying parameters allows obtaining coatings with a high content of the quasi-crystalline phase and improved performance characteristics.
Author: Yingqing Fu, Tianxiang Peng, Deming Yang, Chengqi Sun, Yuzhen Chen, Yang Gao
Institute: Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China