Chang-Jiu Li

Deposition characteristics of titanium coating in cold spraying

Titanium coating was deposited by cold spraying process using nitrogen and helium gases under different temperatures and pressures. The deposition characteristics of the particle in cold spray were studied by the examination of the microstructure evolution of the deposited spot and coating. The effects of the gas type and temperature on the deposition behavior were examined. The microstructure was examined using optical microscopy and scanning electron microscopy. It was found that the pattern of a sprayed spot in cold spray presents a conical shape. The deposition efficiency of spray particles increases with the increase in gas temperature. Two distinguishable top and inner regions exist in the spot deposit and coating, which are characterized by the porous and dense microstructures. The dense microstructure results from the accumulative effect of tamping on the top porous region by the successive impact of following particles. The tamping effect has great influence on the microstructure of the coating in cold spray.

Quantitative characterization of the structure of plasma-sprayed Al2O3 coating by using copper electroplating

Abstract The structure of a plasma-sprayed Al2O3 coating was studied by observing copper deposited in the coating by electroplating. The distribution of copper in the microstructures of the electroplated coating revealed clearly the existence of micropores, a non-bonded area between ceramic lamellae, and microcracks in individual flattened particles in the coating. It becomes possible to determine clearly a lamella in a coating and to examine detailed structures of the coating from a cross-section of a single flattened ceramic particle. The mean thickness of flattened ceramic particles, vertical crack density in the individual particles and mean bonding rate to the coating are introduced to characterize the structure of a ceramic coating. It was recognized that a flattened ceramic particle can be regarded as a disc which has the same thickness. It becomes clear that the average bonding rate between ceramic lamellae is only about 30% for the coating commonly used. Furthermore, it was also recognized that many microcracks perpendicular to the lamellae are distributed uniformly in the coating and almost all the microcracks run across the lamellae.

Examination of the critical velocity for deposition of particles in cold spraying

The critical velocity of copper (Cu) particles for deposition in cold spraying was estimated both experimentally and theoretically. An experimental method is proposed to measure the critical velocity based on the theoretical relationship between deposition efficiency and critical velocity at different spray angles. A numerical simulation of particle impact deformation is used to estmate the critical velocity. The theoretical estimation is based on the critical velocity corresponding to the particle velocity at which impact begins to cause adiabatic shear instability. The experimental deposition was conducted using Cu particles of different particle sizes, velocities, oxygen contents, and temperatures. The dependency of the critical velocity on particle temperature was examined. Results show that the critical velocity can be reasonably measured by the proposed test method, which detects the change of critical velocity with particle temperature and oxygen content. The Cu particles of oxygen content 0.01 wt.% yielded a critical velocity of about 327 m/s. Experiments show that the oxygen content of powder significantly influences the critical velocity. Variations in oxygen content can explain the large discrepancies in critical velocity that have been reported by different investigators. Critical velocity is also found to be influenced by particle temperature as well as types of materials. High particle temperature causes a decrease in critical velocity. This effect is attributed to the thermal softening at elevated temperatures.

The relationship between microstructure and Young’s modulus of thermally sprayed ceramic coatings

An idealized model for the microstructure of thermally sprayed ceramic coatings, consisting of the stacking of lamellae a few micrometres thick, has been used to estimate Young’s modulus of the coating perpendicular to the coating plane. A theoretical relationship between Young’s modulus and the microstructural parameters has been established. There are two components of elastic strain of the coating under tensile stress, one arising from localized elastic strain at the regions of real-bonded area between lamellae, and the other arising from elastic bending of the lamellae between bonded regions. The bending component only becomes significant for a percentage bonding ratio between lamellae of less than 40%. The bending strain contribution depends strongly upon geometrical parameters of the coating microstructure. The estimated Young’s modulus for a typical alumina coating, based on quantitative microstructural data, was about 24% of that for the fully dense material. Taking into account the variable proportion of α-Al2O3 and γ-Al2O3 forms in an alumina coating, the comparison of the estimated Young’s modulus with published data gives reasonable agreement for the coating prepared over a wide range of processes and experimental conditions.

Effect of annealing treatment on the microstructure and properties of cold-sprayed Cu coating

Copper (Cu) coating was deposited by cold spraying, and the electrical resistivity of the coating in both directions parallel and perpendicular to the coating surface was measured to investigate the anisotropy of the coating. Annealing treatment was applied to the coating to examine its effect on the microstructure and properties of the cold-sprayed Cu coating. The examination of coating microstructure evidently revealed that the coating was constituted by the flattened particles, and the interfaces were clearly observed between the deposited particles. The anisotropy in microstructure and electrical resistivity was present in cold-sprayed Cu coating. The electrical resistivity of the as-sprayed coating was higher than that of Cu bulk. Moreover, the electrical resistivity along the direction parallel to the coating surface was lower than that along the perpendicular direction. It was found that annealing treatment led to the enhancement of particle interface bonding and evident recry stallization of the elongated grains and remarkable grain growth as well. The annealed coating presented equiaxed grain structures similar to the annealed Cu bulk with particle interfaces almost disappearing under certain annealing conditions. The coalescence of voids or pores in the coating was clearly observed at high annealing temperatures. Moreover, the annealed coating yielded an electrical resistivity and microhardness comparable to Cu bulk.

Effect of powder structure on the structure of thermally sprayed WC-Co coatings

Five representative types of WC-Co powders were selected to clarify the dependence of the structure of sprayed coatings on the structure of powders themselves. The WC-Co coatings were sprayed with the Jet-Kote process and plasma spraying as well. The structure of WC-Co coatings was primarily characterized by X-ray diffraction. The X-ray diffraction patterns of the sprayed coatings were illustrated compared with those of powders, which aims at a better understanding of the structure of thermally sprayed WC-Co coatings. The selected coating was also analysed by differential scanning calorimetry (DSC). The decarburizing process, and the effects of powder structure and spray conditions on the crystal structure of sprayed WC-Co coatings are discussed in detail.

Formation of an amorphous phase in thermally sprayed WC-Co

A WC-Co coating was sprayed by the high-velocity oxyfuel process using a feedstock of tungsten carbide clad with cobalt. The structure of the sprayed coating was characterized by x-ray diffraction (XRD), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). It was found that an amorphous phase of Co-W-C ternary alloy observed as a large, broad peak in the XRD pattern can be formed in the as-sprayed WC-Co coating. The DSC, DTA, and XRD analyses revealed that the amorphous phase crystallized at a temperature of around 873 K to metallic cobalt, Co6W6C, and tungsten with appreciable precipitation of free carbon. The heat treatment of as-sprayed WC-Co coating at a high temperature of 1173 K suggests that annealing at a temperature higher than about 1104 K will promote the reaction of tungsten and cobalt with carbon to form the complex carbide C06W6C.

Optimal design of a novel cold spray gun nozzle at a limited space

Numerical analysis for the accelerating behavior of spray particles in cold spraying is conducted using a computational fluid dynamics program, FLUENT. The optimal design of the spray gun nozzle is achieved based on simulation results to solve the problem of coating for the limited inner wall of a small cylinder or pipe. It is found that the nozzle expansion ratio, particle size, accelerating gas type, operating pressure, and temperature are main factors influencing the accelerating behavior of spray particles in a limited space. The experimental results using the designed short nozzle with a whole gun length of <70 mm confirmed the feasibility of optimal design for a spray gun nozzle used in a limited space.

Effect of sprayed powder particle size on the oxidation behavior of MCrAlY materials during high velocity oxygen-fuel deposition

Abstract The NiCrAlY powder particles of different sizes were used for high velocity oxygen-fuel (HVOF) spraying. The oxidation behavior at processing resulting in the oxide content inside sprayed coatings was investigated. Moreover, the powder passing through the HVOF flame was collected for the examination of the in-flight oxidation. The oxygen contents in coatings and collected powders were estimated by chemical analysis. It was found that the oxygen contents in the collected powders and coatings depended on the particle size. With decreasing in particle size, the oxygen content in the collected powders increased exponentially. The oxygen content inside the coating was approximately equal to that in the collected powders for the mean particle size of 50 μm), there was a clear difference in the oxygen contents inside the coating and in the collected powders. This difference can be attributed to the post-impact oxidation of a splat. The results suggested that there are two essential oxidation mechanisms: an in-flight oxidation and a post-impact one. When the small particles are used, the in-flight oxidation determines the oxygen content in the coating. When large particles are used, the post-impact oxidation controls the oxygen content in the coating.

Characterization of microstructure of Nano-TiO2 coating deposited by vacuum cold spraying

Control of the microstructure of TiO2 coatings through preparation methods significantly influences the coating performance. In this study, a vacuum cold-spray process, as a new coating technology, is used to deposit nanocrystalline TiO2 coatings on conducting glass and stainless steel substrates. TiO2 deposits were formed using two types of nanocrystalline TiO2 powders with mean particle diameters of 200 and 25 nm. Coating microstructures were characterized by scanning electron microscopy and x-ray diffraction analysis. Results demonstrate that a thick nanocrystalline TiO2 coating can be deposited by the vacuum cold-spray process. The coating was found to consist of particles stacked as agglomerates that build up to several hundred nanometers. The coating also presents a mesoporous microstructure that could be effective in such applications as photocatalytic degradation and dye-sensitized solar cells.