Jianhong He

A review on nanostructured WC-Co coatings

Abstract Unique mechanical properties of nanostructured materials motivate a worldwide interest to synthesize nanostructured coatings. Nanostructured WC–Co coatings have been extensively investigated because of the importance of WC–Co coatings in industrial applications, usually with respect to wear resistance requirements. The present paper has reviewed the studies in this field, including synthesis of nanostructured WC–Co powder, concern on decomposition of nanostructured WC particles, influence of spraying parameters on microstructure and mechanical properties of nanostructured coatings. Significant achievements in this field are: (1) It is possible to produce nanostructured WC–Co powders without non-WC/Co phases in quantities although a high percentage of non-WC/Co phases is frequently reported in conventional WC–Co powder; (2) By controlling agglomerate size of feedstock powder, fuel chemistry and fuel–oxygen ratio, the decomposition of WC particles can be reasonably eliminated so as to synthesize near nanostructured WC–Co coating with a low amount of non-WC/Co phases; and (3) Increased hardness, toughness and wear-resistance can be obtained in near nanostructured WC–Co coatings that are properly synthesized.

Mechanisms of microstructure evolution during cryomilling in the presence of hard particles

Abstract The present study was undertaken to provide insight into the mechanisms that govern the evolution of microstructure in Ni powder during cryomilling with nitride particles. The AlN particles are distributed in Ni powder particles after cryomilling, and the particles with initial size of 2 μm are fractured into smaller size, 50∼300 nm, during cryomilling. The distribution of particles is uniform, and some extremely small particles, size range of ∼20 nm, are also observed by TEM after cryomilling. With addition of AlN particles, the Ni powder particle size after cryomilling is reduced, and contamination of iron and gaseous atoms, N and O, is increased. For the grain size of Ni, the present results show that, in the presence of 2 wt% (5 vol%) AlN particles, the Ni grain size is reduced to 37 nm after 8 h of cryomilling. In contrast, the grain size of Ni cryomilled under identical conditions but without particles exceeded 100 nm. In terms of volume fraction, the results show an increase in the rate of grain size refinement with increasing volume fraction of AlN particles for the range studied, i.e. 1.2–5.0 vol%. The grain size is also reduced to 25 nm with increasing impeller speed up to 340 rpm, which provides higher kinetic energy, and longer cryomilling time of 20 h. This observation is rationalized on the basis of a mechanism involving the interactions of dislocations with hard, non-deformable nitride particles, and thermally induced dislocation generation due to the thermal expansion coefficient difference between the Ni matrix and the nitride particles.

Precipitation phenomenon in nanostructured Cr3C2–NiCr coatings

Abstract Precipitation in the nanostructured Cr 3 C 2 /NiCr coatings was investigated. Ultrafine Cr 2 O 3 particles with an average size of 8.3 nm were observed using transmission electron microscopy in the nanostructured Cr 3 C 2 /NiCr coatings exposed to elevated temperatures. In addition to the precipitation of oxide particles, the phase transformations in the original NiCr amorphous phase, which was always observed in the as-sprayed nanostructured Cr 3 C 2 /NiCr coatings, were also identified. Internal oxidation was thought to be responsible for the precipitation of the dispersed oxide particles. The results of microhardness and scratch-resistance tests showed that microhardness of the conventional coating slightly increased only with an increase in the exposure temperature, while that of the nanostructured coating increased significantly from 1020 to 1240 HV 300 . Compared with the conventional Cr 3 C 2 /NiCr coatings, the scratch-resistance and coefficient of friction were found to be increased and reduced respectively in the nanostructured coatings. Heat treatment led to further increase in scratch-resistance and further decrease in coefficient of friction of the nanostructured coatings. The increases in microhardness and scratch-resistance and decrease in coefficient of friction of the nanostructured coatings were attributed to a high density of oxide nanoparticles precipitating within the coating as the exposure temperature increased.

Synthesis and characterization of nanostructured Cr3C2NiCr

Pre-alloyed Cr3C2-25 (Ni20Cr) powder was synthesized by mechanical ball milling in Hexane [H3(CH2)4CH3]and the variation of powder characteristics with milling time was investigated using SEM, X-ray and TEM. The average powder size drastically decreased with time during the first four hours of milling; then decreased slightly as milling continued up to 20 hours. For milling times in excess of four hours, the particle size approached 5 microns. X-ray diffraction analysis revealed a larger structural change in the NiCr solid solution powder relative to that experienced by the chromium carbide phases. This result indicated that the NiCr solid solution powder was subjected to heavier deformation than the chromium carbide powder. During the initial stages of milling, the brittle chromium carbide powders are fractured into sharp fragments and embedded into the NiCr solid solution powder. As milling continued a NiCr chromium carbide polycrystal composite powder was formed for times up to 20 hours of milling, transforming the sharp carbide fragments into spherical carbide particles. Conventional cold welding and fracturing processes primarily occurred only among the NiCr powder and composite powders. Milling times of up to 20 hours led to the formation of a poly crystal nanocomposite powder system in which chromium carbides, with average size of 15 nm, were uniformly distributed in NiCr matrix.

Development of nanocrystalline structure during cryomilling of Inconel 625

Nanocrystalline Inconel 625 alloy, with a uniform distribution of grains, was synthesized using cryogenic mechanical milling. Microstructures of the powder, cryomilled for different times, were investigated using transmission electron microscopy (TEM), scanning electron microscopy, and x-ray diffraction. The results indicated that both the average powder particle size and average grain size approached constant values as cryomilling time increased to 8 h. The TEM observations indicated that grains in the cryomilled powder were deformed into elongated grains with a high density of deformation faults and then fractured via cyclic impact loading in random directions. The fractured fragments from the elongated coarse grains formed nanoscale grains. The occurrence of the elongated grains, from development to disappearance during intermediate stages of milling, suggested that repeated strain fatigue and fracture, caused by the cyclic impact loading in random directions, and cold welding were responsible for the formation of a nanocrystalline structure. A high density of mechanical nanotwins on {111} planes was observed in as-cryomilled Inconel 625 powders cryomilled, as well as in Inconel 625 powder milled at room temperature, Ni20Cr powder milled at room temperature, and cryomilled pure Al.

Thermal Stability of Nanostructured Cr 3 C 2 -NiCr Coatings

The thermal stability behavior of nanostructured Cr3C2-NiCr coatings was investigated. The nanostructured Cr3C2-NiCr coatings, synthesized using mechanical milling and high-velocity oxygen fuel (HVOF) thermal spraying, were thermally exposed in air at 473, 673, 873, and 1073 K for 8 h. The results show that microhardness of the conventional coating increased slightly with increasing temperature, while that of the nanostructured coating drastically increased from 1020 to 1240 HV300 for the same temperature increases. Heat treatment led to increases in scratch resistance and decreases in the coefficient of friction for the nanostructured Cr3C2-NiCr coatings. A high density of Cr2O3 oxide particles with average size of 8.3 nm was found in the nanostructured coatings exposed to high temperatures, which is thought to be responsible for the observed increase in microhardness and scratch resistance and the decrease in the coefficient of friction of the nanostructured coatings.

Particle melting behavior during high-velocity oxygen fuel thermal spraying

Particle melting behavior during high-velocity oxygen fuel (HVOF) thermal spraying was investigated using Inconel 625 powders. The powder characteristics and coating properties were investigated using scanning electron microscopy (SEM), x-ray, and microhardness studies. Results indicated that the volume fraction of unmelted particles in the coatings was dependent on the proportion of powder within a specified size range, in these experiments, 30 to 50 µm. This particle size range was primarily determined by the particle temperature, which was measured during spraying. Particle temperature significantly decreased as particle size increased. The microhardness values for the coatings containing unmelted particles were predicted by a simple rule-of-mixtures equation for the case of a low volume fraction of unmelted particles. However, for the condition of high volume fraction of unmelted particles, the measured microhardness values did not compare favorably with the calculated values, probably due to the presence of porosity, which occurred in the form of voids found among unmelted particles. The microstructure and characteristics of the feedstock powder were retained in the corresponding coating under certain spray conditions.

Synthesis and Characterization of Nanocomposite Coatings

The synthesis of nanocomposite coatings is described in this paper. The nanocomposite feedstock powders are synthesized using mechanical milling, and the characteristics of the milled powders, i.e., morphology, agglomeration behavior, powder size, grain size and structural evolution during milling, are analyzed using X-ray diffraction, SEM and TEM. Using high velocity oxygen fuel (HVOF) spraying, the nanocomposite coatings are sprayed, and the microstructures and properties of the resulting coatings are characterized.