Journal of Thermal Spray Technology最新文献

In cold spray, successful bonding occurs when particle impact velocities exceed the critical velocity. The description of the critical velocity includes temperature upon impact and material properties, relying on tabulated data of bulk material. However, rapid solidification of powder particles during gas atomization results in higher strengths than reached by respective bulk materials, causing an underestimation of the critical velocity. Thus, a readjustment of the semiempirical calibration constants can supply a more accurate prediction of the requested spray conditions for bonding. Using copper and aluminum as examples, experimentally determined particle strengths for various particle sizes were 43% and 81% higher than those of the corresponding soft bulk materials. Cold spraying was performed over a wide range of parameter sets, achieving deposition efficiencies (DE) ranging from 2% to 98%. DEs were plotted as a function of particle impact velocities and temperatures, as calculated by a fluid dynamic approach. By using DEs of 50%, the critical velocities of the different powders and the corresponding semiempirical constants were determined. The results reveal material-dependent differences in the mechanical pre-factor. This allows a more precise description of individual influences by particle strengths on critical velocities and enhances the understanding and prediction of coating properties.

In this study, we developed a ~ 2 mm thick deposit of CoCrFeNiMn high entropy alloy (HEA) from cold spray. After cold-spraying, annealing at 600, 800 and 1000 °C for 5 hrs was conducted to improve and consolidate the microstructure. The influence of the annealing treatment on the microstructure, hardness and tensile strength of the HEA deposit was studied. The results showed that annealing treatment increased the fraction of metallurgical bonded areas due to diffusion, which resulted in enhanced mechanical performances of the deposit. The examined fractured surfaces of the tensile test samples revealed that the annealing treatment changed the failure behavior of the as-sprayed deposit from mostly particle-particle interface failure to void coalescence (ductile failure). Interestingly, a distinct microstructure was observed for the deposited annealed at 600 °C; a partially recrystallized microstructure with a small volume fraction of Cr-rich phase formed along grain boundaries, whereas fully recrystallized microstructure at higher two temperatures. The strengthening effect of partial recrystallisation, with a small volume fraction of the Cr-rich phase led to a greater reduced modulus and tensile strength (~196.7 GPa and 51.7 MPa) of the deposit annealed at 600 °C when compared with that annealed at 800 °C (~182.5 GPa and 43.6 MPa). It is believed that the small volume fraction of the Cr-rich phase partly constrained the deformation of the surrounding FCC HEA matrix during mechanical loading, leading to better mechanical properties as compared to the deposit annealed at 800 °C.

Cold spray deposition of SiCp/Al composite coatings shows great potential in the field of material protection. However, the strengthening effect of single-scale reinforcement on the composite coating’s performance is limited. To further enhance the mechanical properties of the composite coating, a dual-scale reinforcement model with both micron and nanoparticles was adopted. The addition of nanoparticles further enhances the individual scale advantages and coupling effects of SiC particles, resulting in a composite coating with excellent comprehensive properties, thus meeting the combined requirements for strength and wear resistance. Micro-nano-SiCp/6061Al composite coatings were designed and prepared using high-pressure cold spray technology. The preparation process, microstructure, and property changes of the micro-nano-reinforced composite coatings were systematically studied. The results indicate that cold spray can successfully produce micro-nano-dual-scale SiCp/6061Al composite coatings. The SiC/Al nano-composite coating exhibits a dense structure with micron and nano-SiC particles uniformly dispersed throughout the 6061Al matrix. Compared to single micron-reinforced SiCp/6061Al composite coatings, the addition of nano-SiC particles significantly strengthen the 6061Al matrix. The hardness of cold-sprayed micro-nano-reinforced SiC/6061Al composite coatings increased by 21.9% and the wear resistance has been improved substantially, while the wear rate reduced by 41.92%. With the content of nano-SiC particles increasing, the hardness and wear resistance of the micro-nano-reinforced SiC/6061Al composite coatings initially increase and then decrease. When the mass fraction of nanoparticles reaches 5%, the hardness peaks at 100.64 Hv, while the wear rate decreases to 1.0390 × 10⁻⁴mm³/N m. The proposed cold spray method for preparing dual-scale SiC/6061Al composite coatings could provide data support for future applications of SiC particle-reinforced aluminum matrix composite coatings.

Cold spray is a dynamic additive manufacturing process which results in a unique microstructure and mechanical properties. This work investigates cold spray deposited material under high strain-rate dynamic loading, and specifically the influence of post-build heat treatment on the material strength when subjected to incipient spallation. As-deposited and heat-treated samples were characterized and subjected to shock loading with a plate impact apparatus; the free-surface velocity was measured during the experiment, and the samples were recovered for postmortem analysis. The test results show that the as-deposited material has little to no strength under high strain-rate tensile loading and breaks into pieces. After a short heat treatment, the material recovers some of its tensile strength (compared to wrought copper) but does not exhibit the expected damage morphology and void distribution. When the heat treatment time is extended to several hours and the temperature is increased, the material exhibits ramp-like shock rise and damage formation that is widely distributed within the sample. This work contributes to a better understanding of the influence of heat treatment on the microstructure and subsequent material strength properties under high strain-rate loading, which is crucial for applications where cold spray is a technique of interest.

Yttria-stabilized zirconia (YSZ), a typical thermal barrier coating, faces challenges in meeting the stringent service requirements of critical components such as aero-engine blades due to high-temperature phase transitions and susceptibility to sintering. In the short term, optimizing the coating structure provides an effective and cost-efficient solution to this problem. This study deposited a multi-modal YSZ coating using supersonic atmospheric plasma spraying. The evolution of the microstructure and thermal-mechanical properties of the coating during sintering was systematically studied. The results showed that the multi-modal YSZ coating mainly comprised crystalline regions and unmelted particles, which remained stable after sintering at 1200 °C for 100 h. During sintering for less than 20 h, micro-defects such as cracks and pores rapidly healed by forming sintering necks, significantly enhancing hardness and elastic modulus of the coating. After 50 h, rapid sintering of the unmelted particles led to the formation of interfacial cracks between these particles and the crystalline regions. This effectively reduced the coating's thermal conductivity by inhibiting heat transfer, which slowed down sintering behavior and maintained the stability of hardness and elastic modulus.

Two WC-10Co-4Cr and WC-12Co coatings were deposited on the surface of AZ91 magnesium alloy using high velocity oxygen fuel (HVOF) technology aiming to improve the wear resistance and corrosion resistance. The hardness and organization of the surface and cross section of the coating and substrate were compared. The differences in wear resistance and corrosion resistance between the coatings and the substrate were compared. The wear experiments showed that the wear volumes of WC-10Co4Cr, WC-12Co and AZ91 substrate were 2.7 × 10^-3 mm³, 1.1 × 10^-3 mm³ and 6.1 × 10^-1 mm³, respectively. The coatings mainly exhibited abrasive wear and adhesive wear. The better wear resistance of the coatings than the AZ91 substrate is due to the high hardness of the coatings. The corrosion resistance of the coatings was better than that of the substrate, and the corrosion resistance of WC-10Co-4Cr was better than that of WC-12Co. The corrosion currents density of WC-10Co4Cr, WC-12Co and AZ91 substrates are 4.02 μA cm^-2, 34.31 μA cm^-2 and 46.79 μA cm^-2, respectively. The Cr element is favorable for further improving the corrosion resistance.

The polymer cold spray (CS) process has recently been demonstrated as a promising coating and repair technique for fiber-reinforced polymer composites (FRPs). However, a noticeable variation in coating thickness (herein referred to as checkerboard pattern) often occurs in the initial pass of low-pressure CS deposition. The checkerboard pattern occurs due to the periodic variations in matrix thickness and volume above the subsurface fiber weave pattern. When the initial pass exhibits the so-called checkerboard pattern, the CS deposition for subsequent passes may be negatively affected in terms of deposition efficiency, porosity, adhesion, surface roughness, and thickness consistency. The present work compares results of both numerical simulations and experimental studies performed to reveal the governing mechanisms for and elimination of checkerboarding. Single particle impact numerical simulations are conducted to observe thermomechanical behavior of particles during CS impact on the FRP surface at different regions of the composite material. Complementary experimental CS studies of exemplar powders onto FRPs with various surface interlayer thicknesses are also presented and discussed. Experimental analyses of deposits include microstructural observations to compare against the simulations while also providing practical strategies for the elimination of checkerboarding effects. It is demonstrated that the thickness and volume of the matrix region underneath the impact area are the main contributing factors that govern the CS deposition variations on CFRP substrates. As such, increasing the surface epoxy layer thickness beyond a critical value can reduce the effect of substrate stiffness effects imposed by the subsurface fiber tows, thereby effectively eliminating the checkerboard patterns.

The in-flight behavior of quasicrystal (QC) particles during the high-velocity oxygen fuel (HVOF) process across four distinct operational settings was analyzed using computational fluid dynamics (CFD) simulations. A three-dimensional two-way coupled Eulerian–Lagrangian approach was used to simulate the process. The gas phase was modeled by solving equations governing mass, momentum, energy, and species, alongside the shear stress transport (SST) k-ω turbulence model, while the oxygen-propylene premixed combustion was simulated using the eddy dissipation model. Following the gas flow modeling, the trajectory and thermal evolution of QC particles were tracked within the computational domain, utilizing accurate correlations for drag coefficient and Nusselt number that cover a wide range of Mach, Knudsen, and Reynolds numbers. The analysis revealed that large particles do not melt due to their mass and the low thermal conductivity of QC materials. These particles typically attain impact velocities around 400 m/s. In contrast, smaller particles with diameters less than 20-25 μm reach temperatures of 1200 °C or higher, transitioning into a molten state with impact velocities reaching approximately 600 m/s. Moreover, it was found that approaching stoichiometric conditions with reduced mass flow rates of QC powder resulted in elevated particle temperatures and velocities upon impact, consequently leading to a reduction in porosity. To verify this finding, experiments were conducted under varying oxygen-to-fuel ratios and powder loadings, with subsequent measurement of the coating porosity. An in-flight particle diagnostic system was also used to assess the particle velocity. The numerical study agrees closely with the experimental observations.

In this study, the in situ technique was used to observe crack formation and growth in multilayer suspension plasma spray (SPS) thermal barrier coatings (TBCs). Utilizing synchronized three-point bending (3 PB) and scanning electron microscopy, coupled with digital image correlation, we gained real-time insights into strain field dynamics around cracking zones. This approach allowed us to induce bending-driven failure in both single and multi-layered SPS coatings to explore crack behavior in these cauliflower-like multilayer TBCs. Our observations revealed that columnar gaps facilitate crack initiation and propagation from the coatings’ free surfaces. The triple-layer SPS coating showed a reduced susceptibility to vertical cracking compared to other SPS structures, due to a dense gadolinium zirconate layer on the top. Additionally, the splat structure of the bond coat (BC) layer contributes to crack relative path deflection, which could enhance the fracture toughness of the SPS coatings by dissipating the energy needed for crack propagation. Moreover, it was revealed that grit particles at the BC/substrate interface appear to promote crack branching near the interface, localized coating delamination, and serve as nucleation sites for crack development. Therefore, optimizing the grit-blasting process of the substrate prior to BC layer deposition is essential for minimizing the likelihood of crack formation under operational conditions, thereby enhancing durability and extending the lifespan of the coatings. This study highlights the critical role of in situ observation in unraveling the complex failure mechanisms of multi-layered coatings, paving the way for the design of advanced coatings with improved performance in extreme environments.