Journal of Thermal Spray Technology最新文献

In this study, a novel attempt was made to deposit 86WC-10Co-4Cr cermet layer on 1.2709 tool steel substrate prepared by selective laser melting (SLM) process using detonation spraying method. The laser power of 350 W, scan speed of 25 mm/s, hatch spacing of 0.15 mm, and layer thickness of 50 µm were used to fabricate samples at 45° build orientation. The calculated volumetric energy density for fabricating the samples was 1867 J/mm³. The SLM printed samples were annealed and air cooled at 500 °C in a box furnace with a dwell time of 6 h. The average particle size of the powder measured before and after the ball milling process was 237.49 and 43.06 μm, respectively. A coating thickness of 100 μm was targeted using the detonation spraying process. An average spray loss of 13% was observed during cermet coating. The results were compared between the as-built specimen, the heat treated specimen and the 86WC-10Co-4Cr coated specimen. The average Vickers microhardness of the coated sample was found to be 81.04 and 48.96% superior to the as-built and heat treated samples. The average contact angles measured from the as-built, heat treated and coated samples were 82.5°, 64° and 93.9°, respectively, indicating the superior hydrophobic surface in the coated sample. The coated sample offered reduced abrasive wear, improved corrosion inhibition, and better 2D and 3D surface roughness properties than the as-built and heat treated samples, which promises its further use in cermet-based rapid tooling applications.

In this study, the creep constants involved in the creep constitutive equation of MCrAlY bond coatings were identified using a high-temperature creep indentation test developed by our group. CoNi, Co, and Ni were selected as M components. The effects of the M components and spraying processes of atmospheric plasma spraying (APS) and high-velocity flame spraying (HVOF) on the creep constants were also investigated. As a result, the creep exponent (n) was in the range of 6-8, regardless of the difference in the M components and thermal spraying processes. On the other hand, the creep coefficient (k) was the largest in CoNiCrAlY (HVOF), and it was clear that it has the capability of large stress relaxation owing to creep deformation between the ceramic top coating and substrate. SEM observations revealed that the dispersed phase in the matrix was lost with increasing temperature, which led to a reduction in dislocation obstacles and an increase in creep deformation in the CoNiCrAlY bond coatings.

Understanding the microscopic damage process of Al₂O₃ particle-reinforced stainless steel composite coatings under impact loading is vital for the design of impact-resistant coatings, but also complex and challenging due to their inferior toughness. In this work, the stress/strain fields and coating cracks of HVAF-sprayed monolayer and Al₂O₃ particle (two different sizes)-reinforced stainless steel composite coatings under falling ball impact were analyzed by means of finite element simulation and experimental verification. The results showed that three types of cracks, including circular cracks, cone cracks and radial cracks, were generated in the coating during impact, which were mainly induced by the tensile stress at the edge of the impact crater, the shear stress inside the coating, and the equivalent plastic strain on the interface of the coating/substrate, respectively. Compared to the monolayer coating, the stress concentration of the composite coating under impact was dispersed by the Al₂O₃ particles (mainly around the particles). The crack propagation was hampered and deflected by the interface between the particles and the matrix, and the particle fracture would dissipate the impact energy. It was also found that the stress amplitude around the larger Al₂O₃ particles was smaller and the probability of crack initiation was lower, resulting in better impact resistance of this coating. The comparison of the simulation results with the impact experimental results verified that the impact damage of the coating could be effectively predicted by finite element simulation.

Residual stress is an important factor that affects the properties of cold spray coatings. The study performed 2D single-, 3D single-, and 3D multiple-particle finite element simulations based on the Eulerian method, and hole drilling measurements to study residual stress in cold-sprayed copper coatings. The study examined how the basic modeling parameters affect residual stress in the 2D system. It found that the mesh size, substrate dimension, and simulation duration change the residual stress; while material failure does not have much effect because failure is localized. It also found that thermal softening, which occurs when the initial temperature and/or plastic deformation increase, helps to reduce the residual stress. In addition, the residual stress along the axisymmetric axis of the particle changes from being compressive at first to being compressive-tensile-compressive along the through-depth thickness direction with the interfacial stress being tensile that mainly originates from friction shear of the interfacial layers. For the 3D single-particle simulations, the average residual stress to the through-depth thickness shows tension to compression transition and is similar to the 2D cases. For the 3D multiple-particle simulations, the residual stresses in the coatings/substrate are all compressive because of the repeated and strong impact of the particles, which agree with the experimental measurements. The study also found that the compressive stresses decrease when the coating thickness increases because of more plastic deformation-induced thermal softening. This paper clarifies the effects of Eulerian-based finite element parameters on simulating residual stress in cold spray and will serve as a valuable reference for future studies.

In this paper, nickel-clad cubic boron nitride (Ni/c-BN)-reinforced Ni-based composite coating was prepared on the martensitic stainless steel substrate by laser melting technology. The microstructure, phases and properties of the cladding layer were investigated by x-ray diffractometry, scanning electron microscopy, energy-dispersive spectrometry, microhardness test, wear test and potentio-dynamic polarization. The experimental results show that surface quality of the cladding was excellent and free of obvious defects when the Ni/c-BN content was 5 wt.%, while reticulation cracks appeared when the content was higher than 5 wt.%. In addition, compared with the coating without Ni/c-BN, the addition of Ni/c-BN made the coating form a new phase Ni₃B and a change in the eutectic tissue in the coating, which was made up of γ-Ni, Ni₃B and Ni₄B₃. After added Ni/c-BN, the hardness, wear resistance, and corrosion resistance of the coating increased first and then decreased. Excess Ni/c-BN reduced the solid solution B element content in the matrix and accelerates the formation of the crack of the coating, thereby deteriorating the hardness, wear resistance and corrosion resistance. The coating had the best hardness, wear resistance and corrosion resistance when the Ni/c-BN content was 5 wt.%.

Heat exchange tubes require a porous inner surface to maximize their boiling performance. However, in addition to the geometric limitations of long and narrow tubes, producing porous inner surfaces remains challenging for conventional coating technologies. To prepare porous coatings on the inner surface of narrow tubes, a novel continuous wire electrical explosion spraying device was developed. The charging voltage influenced the overheat factor and expansion velocity of the aluminum wire, which simultaneously affected the size, temperature, and velocity of the explosive products deposited inside medium-carbon steel tubes. These effects ultimately impacted the flattening degree and microstructure of the deposited material. Experiments revealed that the porosity, wettability, adhesion, and rate of increase in coating surface area are all superior at a charging voltage of 12.0 kV. Thus, coatings prepared at this charging voltage can effectively improve the heat transfer of the tube. Our study also provides insights into the effects of charging voltage on the microstructure of deposited film, which may be extended to the coatings of other complex components.

A chromium boride–chromium carbide–alumina composite coating was prepared in situ on a Ti-6Al-4V titanium alloy substrate by plasma spraying of Cr₂O₃-Al-B₄C composite powder. The results show that, in the process of plasma spraying the Cr₂O₃-Al-B₄C composite powder, a chemical reaction occurred among Cr₂O₃, Al, and B₄C, and CrB₂, Cr₃C₂, and γ-Al₂O₃ phases with nanosize were formed in situ in the composite coating. Compared with the coating prepared by plasma spraying CrB₂-Cr₃C₂-Al₂O₃ composite powder, the coating prepared by the Cr₂O₃-Al-B₄C composite powder has a more uniform and refined microstructure, lower porosity, higher microhardness, better toughness, excellent scratch resistance, and sliding wear resistance. This is attributed to the densification effect and to the fine grain strengthening and toughening effect of nanocrystals produced by the participation of the in situ chemical reactions in the plasma spraying process of the Cr₂O₃-Al-B₄C composite powder.

Hybrid bond layers (BLs) were designed, fabricated, and evaluated for cold spray metallization of CFRP. The bond layers consisted of metal mesh embedded in a polymer film adhesive co-cured to the CFRP. Efforts were devoted to identifying the critical opening ratio—i.e., the ratio of mesh opening size to powder diameter, for deposition of an adherent coating. Analysis of powder deposited at mesh openings show a transition from erosion (at a mesh opening ratio of 6.4) to mechanical interlocking and formation of a continuous coating with decreasing opening ratio. Selection of opening ratio yielded either (a) a grid of consolidated thin-walled deposits atop mesh wires separated by microchannel openings, or (b) densified coatings of cold-sprayed Ti. The effective opening ratio increased with increasing diameter ratio—i.e., the ratio of wire diameter to powder size, a consequence of eroded wire peripheries at shallow impact angles. These findings inform the design of future hybrid BLs, in concert with the selection of powder size, for cold spray metallization of CFRP.

High-velocity air-fuel (HVAF) spraying can form dense corrosion- and wear-resistant coatings on the surface of TC18 titanium alloy, solving the problem of protecting aircraft landing gear surfaces. In this study, the combustion reaction and discrete phase models for HVAF spraying of WC-12Co powder were established on the basis of computational fluid dynamics. In addition, the evolution law of flame temperature and velocity during spraying was revealed, and the influence of powder particle size and sphericity on particle flight characteristics was investigated. In view of the randomness of the shape distribution and spatial position of the powder particles, a three-dimensional multi-particle full-cycle random polycrystalline impact model has been established based on coupled Eulerian–Lagrangian and Voronoi methods. The evolution laws of the temperature, strain, and stress fields during particle impact have been determined. Calculations showed that the maximum velocity of the spray flame is 1504 m/s, and the maximum temperature is 1960 K. The size and shape of powder particles affect their velocity and temperature, and the velocity of particles with a small diameter and low sphericity (β) is high. The temperature of ellipsoidal particles with β ≤ 0.6 is significantly lower than that of spherical particles with β ≤ 1.0. Due to the introduction of grain inhomogeneity, the stress distribution of the heterogeneous substrate exhibited significant dispersion. The Mises stress of the grains inside the substrate was positively correlated with the microhardness of the material. This study may provide theoretical guidance for optimizing thermal spray processes.