Improvement in Slurry Erosion and Corrosion Resistance of Plasma-Sprayed Fly Ash Coatings for Marine Applications

An Investigation into the Relative Efficacy of High-Velocity Air-Fuel-Sprayed Hydroxyapatite Implants Based on the Crystallinity Index, Residual Stress, Wear, and In-Flight Powder Particle Behavior

Due to its resemblance to the bone, hydroxyapatite (HA) has been widely used for bioactive surface modification of orthopedic implants. However, it undergoes significant thermal decomposition and phase transformations at a high operating temperature, leading to premature implant failure. This investigation uses high-velocity air-fuel (HVAF) spray, an emerging low-temperature thermal spray technique, to deposit HA over the Ti-6Al-4V substrate. Coating characteristics, such as the crystallinity index and phase analysis, were measured using X-ray diffraction, Raman analysis, and Fourier transform infrared spectroscopy, residual stress using the sin2ψ method, and tribological performance by a fretting wear test. The coating retained an over 90% crystallinity index, a crystallite size of 41.04 nm, a compressive residual stress of -229 ± 34.5 MPa, and a wear rate of 1.532 × 10-3 mm3 N-1 m-1. Computational in-flight particle traits of HA particles (5 to 60 μm) were analyzed using computational fluid dynamics; it showed that 90% of particles were deposited at a 700 to 1000 m/s velocity and a 900 to 1450 K temperature with a 2.1 ms mean residence time. In-flight particle oxidation was minimized, and particle impact deformation was maximized, which caused severe plastic deformation, forming crystalline, compressive residual stressed coatings. The thermal decomposition model of low-temperature HVAF-sprayed HA particles helped to understand the implants' crystallinity index, residual stress, and tribological characteristics. Hence, this experimental and computational analysis shows that the HVAF process can be a promising candidate for biomedical applications for having strong and durable implants.

Design of High-Entropy Alloy Coating for Cavitation Erosion Resistance by Different Energy-Induced Dynamic Cyclic Behaviors

The dispute over the effect of cavitation heat on material surface intensifies the fuzziness of cavitation erosion (CE) mechanism and limits the development of protective materials. Here, an anti-CE Al₁₀Cr₂₈Co₂₈Ni₃₄ high-entropy alloy (HEA) coating with single face-centered cubic (FCC), prepared by high-velocity oxy-fuel (HVOF) spraying technologies, was designed by inducing mechanical and thermal energy-induced behaviors to transform or counteract each other. The results showed that, on the one hand, this coating underwent the refinement of the average grain size from 1.22 to 1.02 μm, the increase in dislocation density from 1.28 × 10^-10 to 1.83 × 10^-10 m^-2, and the martensitic transformation from FCC to body-centered cubic (BCC) under the cavitation load; on the other hand, cavitation heat could indeed induce grain growth and realize structural relaxation, which confirmed that cavitation heat acting on the material surface at temperatures theoretically above 1206.28 K also played a significant role in the CE mechanism. That is, the surface microstructure of this coating was always in a dynamic cycle during the CE process. Therefore, the coating achieved the simultaneous absorption of mechanical impact energy and thermal energy released by the bubble collapse while effectively avoiding the overenrichment of crystal defects and finally exhibited a CE resistance 2 times better than that of the classical AlCrCoFeNi HEA coating. This design concept of inducing different energy restraints or neutralization through the special response behaviors of surface microstructure provides a completely new way for the development of CE-resistant materials.