Journal of Thermal Spray Technology最新文献

Thermal spray metallic coatings usually present a lamellar structure with limited lamellar interface bonding due to inevitable oxidation involved. Such microstructure not only degrades the mechanical performances of the coatings significantly but also cannot provide effective corrosion protection to the substrate. In the present paper, the recent progresses to generate oxide-free molten metallic droplets by air plasma spraying will be summarized toward to deposition of highly dense metal coatings. It is revealed that by designing the metallic spray powders containing deoxidizers such as boron or carbon based on thermodynamics theory of oxidation, the oxide-free in-flight molten metal droplets can be created in open atmosphere during air plasma spraying. The thermodynamics and kinetics for the in-flight deoxidization are presented for the deoxidizers of boron and carbon. The tests with Ni-based and Cu-based alloy coatings using boron as deoxidizer showed that the oxygen content in the coatings can be reduced to less than 0.6 wt.%. It was demonstrated with NiCrAlY, NiAl and FeAl alloys that contain element Al the coatings with low oxide contents can be deposited by APS using carbon as deoxidizer. The post-impact oxidation is mainly responsible for the introduction of oxide inclusions in these coatings. Besides deoxidizer addition, ultra-high temperature is the other necessary condition for the generation of oxide-free molten droplets. It was revealed that a minimum deoxidizer content is necessary to maintain continuous oxidation protection of alloying elements along with rapid mass transfer mechanism within molten metal droplets during the whole spray distance.

Cold spray is a material deposition technology with a high deposition rate and attractive material properties that has great interest for additive manufacturing (AM). Successfully cold spraying free-form parts that are close to their intended shape, however, requires knowing the fundamental shape of the sprayed track, so that a spray path can be planned that builds up a part from a progressively overlaid sequence of tracks. Several studies have measured track shape using ex situ or quasi-in situ approaches, but an in situ measurement approach has, to the authors’ knowledge, not yet been reported. Furthermore, most studies characterize the track cross section as a symmetric Gaussian probability density function (PDF) with fixed shape parameters. The present study implements a novel in situ track shape measurement technique using a custom-built nozzle-tracking laser profilometry system. The shape of the track is recorded throughout the duration of a spray, allowing a comprehensive investigation of how the track shape evolves as the deposit is built up. A skewed track shape is observed—likely due to the side-injection design of the applicator used—and a skewed Gaussian PDF—a more generalized version of the standard Gaussian PDF—is fit to the track profile. The skewed Gaussian fit parameters are studied across two principal nozzle path parameters: nozzle traverse speed and step size. Empirical relationships between the fit parameters and the nozzle path parameters are derived, and a physics-based inverse relationship between nozzle speed and powder mass deposition rate is obtained. One of the fit parameters is shown to be an effective means of monitoring deposition efficiency during spraying. Overall, the approach presents a promising means of measuring track shape, in situ, as well as modeling it using a more general shape function.

Hydroxyapatite (HA) coatings on titanium substrates are widely investigated for biomedical applications due to their biocompatibility and osteoconductivity. This study explores the impact of HA powder particle size on the mechanical and electrochemical properties of flame-sprayed coatings. HA powders were synthesized via a wet chemistry method and characterized using x-ray diffraction, FTIR, and scanning electron microscopy. Flame spraying was employed to deposit HA coatings of varying particle sizes (0-37 µm, 37-63 µm, 63-104 µm, and 104-125 µm) onto titanium substrates. Mechanical properties such as surface roughness, adhesion strength, wear resistance, and Vickers hardness were evaluated, revealing that coatings with smaller particle sizes exhibited smoother surfaces, higher adhesion strengths, superior wear resistance, and greater hardness. Electrochemical properties were assessed through potentiodynamic polarization and electrochemical impedance spectroscopy in simulated body fluid, demonstrating that coatings with finer particle sizes displayed enhanced corrosion resistance compared to those with larger particles. Overall, this study underscores the critical role of HA powder particle size in optimizing the performance of flame-sprayed HA coatings for biomedical applications.

The use of ultrafine powders in the micro-cold spray (MCS) process, also referred to as the aerosol deposition method, typically results in porous and/or poorly adhering films because the particles do not impact at a high enough velocity for sufficient plastic deformation and interparticle bonding to occur. Under typical operating conditions, particles < 100 nm accelerate to high velocities but then are slowed by the stagnant gas in the bow shock that forms just upstream of the substrate. Using larger particles reduces particle slowing, but large particles can cause erosion of the film at high impact velocity, decreasing deposition efficiency. In this study, a pressure relief channel nozzle using helium as a carrier gas is proposed such that high-velocity deposition of yttria-stabilized zirconia particles as small as 10 nm in diameter is possible. This is well below the size range of powders previously used for MCS. The proposed nozzle design increases impact velocities for 10, 20, and 50 nm particles by ~ 880, 560, and 160 m/s, respectively, when compared to a conventional nozzle. Experimental deposition of ultrafine 8YSZ powder shows that the pressure relief channel nozzle results in lower porosity and more uniform deposits, with a ∼ 186% increase in deposition efficiency.

The demand for utilizing the Axial III Plus plasma spray system has prompted the numerical modeling of its arc plasma torch, integral to creating a digital twin of the suspension plasma spray process. The Axial III Plus plasma torch is a highly efficient and reproducible tool with a unique three-torch exit jet arrangement that allows the axial injection of solid/liquid feedstock, not possible with a single cathode/anode–plasma torch setup. In this study, we employ the local thermodynamic equilibrium approximation of the magnetohydrodynamic (MHD) model to simulate plasma flow inside the single gun plasma torch of Axial III, considering electrode–plasma interactions. Describing electric arc dynamics during restrike proves intricate; thus, a restrike model is used relying on cutoff criteria based on a threshold value E_b of the predicted radial electric field at the electric arc fringes. The model successfully replicates typical electric arc behavior and saw-toothed voltage profiles during restrike, notably capturing the characteristics of the Axial III anode’s unique and complicated design variations in electric arc motion and its corresponding arc voltage profile. Analysis extends to studying variations in E_b, which directly influence mean electric arc length, arc voltage, and mean arc spot time, potentially impacting energy generation and losses in the torch. These findings provide a valuable foundation for future simulations of this design, especially with swirl gas injection and ternary gas mixtures.

This study investigated the effect of powder pre-oxidation on the microstructures and mechanical properties of cold-sprayed nickel coatings. The artificially pre-oxidized nickel powders at 200, 300 and 400 °C for 5 h show the resulting oxygen contents of 0.27, 0.36 and 0.41 wt.%, as compared to 0.21 wt.% in the feedstock powder. Microstructurally, the higher oxygen contents of the impact particles significantly increased in both the number and size of the pores in the as-sprayed coatings by using the pe-oxidized powders, as a result of the porosities of 0.7, 1.5 and 3.3% compared to 0.4% by using the as-atomized powder (natural oxidation condition). Mechanically, the increased oxygen contents of powders result in the reduced properties for the as-sprayed Ni coatings, as the microhardness of 263.2 HV_0.1, 245.3 HV_0.1 and 236.3 HV_0.1 and the tensile strength of 94, 76 and 61 MPa by using oxidized powders compared to those of 289.2 HV_0.1 and 208 MPa by using natural oxidation powder. In addition, post-spray heat treatment at 800 °C for 2 h effectively reduces the small-sized pores and nonbonded particle-particle boundaries within the coatings, which is attributed to a combination effect of annealing twins and dislocation slip during heat treatment. As a result, the microhardness significantly decreased to 135.3 HV_0.1, 126.7 HV_0.1, 124.5 HV_0.1 and 114.7 HV_0.1, while the tensile strength is increased to 210, 166, 133 and 117 MPa, respectively.

The grain structure evolution and nanomechanical behavior of cold-sprayed Al coating on Ti substrate with friction stir spot processing (FSSP) were studied by the electron backscatter diffraction and nanoindentation methods. The low-angle boundaries (LAGBs) fraction and the density of the geometrically necessary dislocations (GNDs) decreased from the base zone (BZ) to the stir zone (SZ). The average grain size, the LAGBs fraction and the density of the GNDs were various in different locations of the SZ, which can be attributed to the variety of local shear strain and temperature gradient during FSSP. The B/(stackrel{{-}}{text{B}}) component, the C component, and the A^*₁/ A^*₂ component were mainly developed in the SZ. The highest intensity of the B/(stackrel{{-}}{text{B}}) component appeared in the 3/8D of the SZ, indicating that the plasticized materials flowed downward experienced the highest shear strain. The materials in the heat affected zone (HAZ) underwent static recrystallization, while the continuous dynamic recrystallization (CDRX) and the geometric dynamic recrystallization (GDRX) occurred in the thermo-mechanically affected zone (TMAZ) and SZ. The nano-hardness and elastic modulus of the cold-sprayed Al coating after FSSP were comparable to those of pure Al bulk. The grain size and dislocation density were the main factors affecting the nano-hardness in the SZ.