Study On Particle Flattening, Microstructures And Properties Of HVOF And Low Power Plasma Sprayed Al-Cu-Cr Quasicrystalline Coatings

Quasicrystals have been described as a new class of materials with long-range quasi-periodic translational symmetry and non-crystal orientational order. They have remarkable physical and mechanical properties but can not be applied as structural materials due to their inherent brittleness at ambient temperature. Fabricating quasicrystalline (QC) film or coating onto metallic substrate by vapor deposition or thermal spray, provides a synergetic way of combining the high toughness of metals with the superior properties of QC materials. Thermal sprayed QC coatings have been successfully applied in many fields. However, the coating quality and properties are degenerated, and its application is restricted, due to high porosity, overmany defects, low bond strength, etc. So an in-depth study on formation mechanisms of thermal sprayed QC coating is very practically important.In the present study, the experimental research and theoretical analysis are integrated, and the identical Al65Cu20Cr15 QC powders are sprayed by High Velocity Oxy-Fuel (HVOF) and low power non-transferred plasma respectively. Characteristics of Al65Cu20Cr15 QC particle flattening behavior, the coating phase composition and the variation laws of the QC coating microstructure and properties, are systematically investigated; in order to predict the QC coating microstructure and properties, control coating quality and promote the application of thermal sprayed QC coatings.Particle flattening behavior of Al65Cu20Cr15 QC powders sprayed by High Velocity Oxy-Fuel (HVOF) and low power non-transferred plasma respectively, is very different. Dominating factors on thermal spray particle flattening behavior derive from two main aspects, particle characteristics and substrate properties. In term of particle characteristics, most (≥70%) of low power plasma sprayed Al65Cu20Cr15 QC particles are fully molten droplets with lower flying velocity, and the higher the temperature and melting degree of the particle is, the higher the droplet flattening ratio is. HVOF sprayed particles have very high flying velocity, but the temperature and melting degree of them are lower, and most of them (>80%) are semimolten predominant solid particles, which have unmelted solid core covered by molten liquid surface and are called solid-liquid two-phase particle, and the higher the particle velocity is, the higher the flattening ratio of it. In terms of substrate properties, the flattening of HVOF sprayed particles with high kinetic energy but with low thermal energies is sensitive to substrate mechanical property such as hardness and elastic limit etc., other than thermo-physical properties, for that the heat transfers from HVOF flame to substrate is high. On the contrary, the flattening of low power plasma sprayed particles is more sensitive to substrate thermophysical properties than mechanical property, such as substrate melting point, temperature and thermal conductivity etc.. Furthermore, using substrate with higher hardness in HVOF spraying, and using substrate with lower thermal conductivity or higher temperature in plasma spraying, would increase flattening ratio of particles and improve coating quality.During the low power plasma spraying of Al65Cu20Cr15 QC feedstock powders, moderate increase of plasma enthalpy Hj improved morphology of flattening splats. However, when plasma enthalpy Hj increased excessively, the splats with irregular splash periphery were induced and the volume fraction of the icosahedral QCⅠ-phase decreased, because the higher H2 content in the plasma promoted the excessive vaporization and oxidation of aluminum. The proper value of plasma enthalpy Hj was 2.48 kJ/L, and the corresponding mean flattening ratio and Renolds number for plasma spray molten droplets was about 4.10 and 17,000, respectively. Either decrease of the substrate thermal conductivity or moderate increase of the substrate preheat temperature promotes the increase of the QCⅠ-phase content in the coating. For the molten droplets of the Al65Cu20Cr15 QC feedstock powders with a nominal size variation between 61 to 74μm sprayed onto AISI304 substrate, there was a critical value of the substrate preheat temperature, i.e. transition temperature Tt, and it was about 573 K. When the substrate preheat temperature was higher than Tt, morphologies of most of splats (≥50%) changed transitionally from an extensively splashed pattern to a disc-shape.As confirmed by experimental results, when HVOF sprayed Al65Cu20Cr15 QC particles impinged into deposition surface, unmelted solid of particle was crushed, and previously-deposited coating was plastically deformed and even cracked by subsequent particle impinging, because of high brittleness of quasicrystal and high flying velocity and low melting degree of HVOF sprayed particles. The particles crush and plastic deformation of previously-deposited coating promote to refine grains and decrease porosity of the coating. Hence, the coating microstructure of HVOF sprayed so coarse (61-74μm) Al65Cu20Cr15 QC powders was still fine and dense, though the preferential particle size of feedstock powders for HVOF spraying is 15-40μm.In as-sprayed coatings, splats of low power plasma sprayed Al65Cu20Cr15 QC particles derive from fully molten droplets, while splats of HVOF sprayed particles develop from two sources, melted parts of most semimolten particles and few microsize fully molten droplets, and apparently, the size of the former was much larger than that of the latter. On the other hand, the melting degree of low power plasma sprayed Al65Cu20Cr15 QC particles was much higher, so alloying agent (especially Al) vaporized or oxidized more severely, and the icosahedral QCⅠ-phase content in the coating was less whilst that of such crystalline phases asε,αandθwere more. In summary, the icosahedral QCⅠ-phase content in HVOF sprayed coatings were higher than that of low power plasma sprayed coatings, and had finer and denser micro-structures. And experimental results showed that, microhardness and bond strength of HVOF sprayed coatings were much higher, and antifriction and wear resistant properties were more excellent, but deposition efficiency of powders was lower, which was only 50-80% of that of plasma spraying, and the production cost was much higher.When the Al-Cu-Cr coatings were prepared by HVOF spraying, substituting C3H6 for C3H8 would increase the energy and heat transferring from HVOF flame to particles, and improve morphology of flattening splats and coating quality, due to the former can be gasified more easily than the latter at room temperature. Keeping O2-C3H8 content ratio between 4.2-5.6, moderate increase of flame heat enthalpy promoted the degree growth of particles melting and flattening, and increased coating microhardness and bond strength, and decreased coating porosity and content of the QCⅠ-phase.Experimental results showed that, the antifriction and wear resistant properties and hydrophobic surface property of HVOF sprayed Al65Cu20Cr15 QC coatings were more excellent than those of HVOF sprayed FeAl crystalline coatings.