M.L. Lau

Thermal spraying of nanocrystalline inconel 718

Abstract Nanocrystalline Inconel 718 was thermal sprayed utilizing a HVOF (High Velocity Oxygen Fuel) thermal spraying facility. First, nanocrystalline powder was produced by high energy ball milling of Inconel 718 (average particle size, 10μm) under methanol and gaseous nitrogen. The ball milled powder was then processed by HVOF to produce a coating 250 μm thick. The nanocrystalline Inconel 718 coating exhibited a significant increase in hardness (approximately 60%) over that of the Inconel 718 control sample after the thermal spray process. The grain sizes of the as-received, as-ball milled, and HVOF processed materials were 10μm, 25nm, and 32nm, respectively, as determined by X-ray diffraction. A scanning electron microscopy (SEM) analysis of the powders and coating exhibited typical morphologies of as-received and as-ball milled materials. After HVOF spraying, some of the particles appear to have gone through the processing without complete melting.

Thermal Spraying of Nanocrystalline Ni Coatings

The present paper describes the synthesis, characterization, and grain growth behavior of nanocrystalline Ni coatings generated using a novel synthesis approach, namely high velocity oxy-fuel (HVOF) thermal spraying. In the present investigation, the feedstock powders were prepared by mechanical milling in a methanol environment which yielded agglomerates with a flake-shaped geometry and an average grain size of less than 100 nm. The milled powders were then introduced into the HVOF spray system in order to investigate the feasibility of generating a coating with grain sizes in the nanocrystalline range (e.g., <100 nm). Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the milled powders and the as-sprayed coatings. Transmission electron microscopy analysis performed on cross sections of the coating revealed a mixture of fine nanocrystalline grains and elongated coarse grains.

Grain growth behavior of nanocrystalline inconel 718 and Ni powders and coatings

Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled powders, as well as the HVOF-derived coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth by maintaining a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good agreement with the experimental results.

Synthesis and characterization of nanocrystalline CoCr coatings by plasma spraying

Abstract The present paper describes the synthesis and characterization of nanocrystalline CoCr (ASTM F75) coating produced by plasma spraying for possible surgical implant applications. The feedstock powders were synthesized by mechanical milling to produce irregular agglomerates with an average grain size of less than 100 nm. The powders were then introduced into an argon plasma spray to successfully produce a nanocrystalline coating. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the resultant sprayed coatings. Microhardness and porosity measurements were performed on the conventional and the nanocrystalline coatings to characterize and compare the physical and mechanical properties.

Synthesis of nanocrystalline M50 steel powders by cryomilling

Abstract The present paper reports on a study of the synthesis of nanocrystalline high speed tool steel M50 powders (4.5% Mo, 4.0% Cr, 1.0% V, 0.8% C, balance Fe) by cryogenic high energy ball milling (cryomilling). Pre-alloyed M50 steel is spray atomized, and subsequently cryomilled in liquid nitrogen for 25 hours. Elemental Al powder is added prior to cryomilling to promote the formation of nanoscale Al2O3 and AlN dispersoids to improve the thermal stability of the nanocrystalline M50 steel. High resolution transmission electron microscopy (HRTEM) reveals the formation of various carbides (V8C7, Fe3C, and FeC), oxides (Al2O3, MoO3, and V3O7), and a nitride phase (AlN) during cryomilling. Following one hour of heat treatment at 1373 K (0.77 Tm), an average grain size of 70 nm was retained for the M50 steel powders.

Mathematical modeling of particle behavior of nanocrystalline Ni during high velocity oxy-fuel thermal spray

Abstract The present paper describes a mathematical model predicting the particle behavior of nanocrystalline Ni powders during high velocity oxy-fuel (HVOF) spray. The model development was motivated by successful experimental results as described in the following. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 100 nm. The powders were then introduced into the HVOF spray to produce a nanocrystalline coating. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the resultant sprayed coatings. The equations describing the momentum and energy transfer coupled with a modified geometric ratio were used to account for the size and morphological effects of the nanometric agglomerates. Particle velocities and temperatures of the numerical calculations indicated that fractions of the agglomerates did not melt during the HVOF spraying process, which concurs with the observed experimental results.

Microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying☆

Abstract The microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying is described. Stainless steel powders with a particle size in the range of 45–11 μm were mechanically milled for 10 h in liquid nitrogen to produce powders with a nanocrystalline grain size of 21±8 nm and an aspect ratio of 1.68. The cryomilled powders were subsequently sprayed onto a stainless steel substrate by high-velocity oxygen fuel spraying. The resultant coating exhibited a superior microhardness, despite an increased porosity, over that of the conventional coating sprayed with the same parameters. Transmission electron microscopy performed on the cross-sections of the nanocrystalline coating revealed the splat formation with a thickness ranging from 40 to 400 nm. Various oxide phases (Cr 2 O 3 , FeO, Fe 2 O 3 and γ-Fe 2 O 3 ) in the stainless steel matrix were identified using selected area diffraction. This observation suggests that in-flight oxidation may have occurred during spraying and/or during splat formation.

Particle behavior of nanocrystalline 316-stainless steel during high velocity oxy-fuel thermal spray

Abstract The present paper investigates the particle behavior of nanocrystalline 316-stainless steel powders during high velocity oxy-fuel (HVOF) spray. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 50 nm. The powders were then introduced into the HVOF spray to produce a nanocrystalline coating. A mathematical model is developed to study the variation of different agglomerate sizes resulting from the mechanical milling process on the particle behavior during thermal spraying. A generalized Newtonian equation describing the momentum transfer was used, which incorporates a geometric ratio to account for the size and morphological variations of the micron-sized agglomerates with nano-grained structure. Particle velocity from the numerical calculations indicated that the velocity profile depends strongly on the thickness of the particles, which is a function of milling time and milling media.

Synthesis of nanostructured coatings by high-velocity oxygen-fuel thermal spraying

Publisher Summary This chapter discusses the fundamentals of thermal spraying combines particle melting, quenching, and consolidation in a single operation. The focus is on High-velocity oxy-fuel (HVOF). This spraying technology, facilitated by an achievement of metallurgical and chemical homogeneity, is used to fabricate a variety of simple preform shapes in addition to coatings. Thermal spraying was originally developed to produce corrosion-resistant zinc coatings, as well as coatings for other refractory metals. Thermally sprayed coatings are also widely used in the electronics industries, power generation plants, marine gas turbine engines, ceramics industries, and printing industries. HVOF spraying is the most significant development in the thermal spray industry since the development of the original plasma spray. HVOF is characterized by high particle velocities and relatively low thermal energy when compared to plasma spraying. The applications of HVOF have expanded from the initial use of tungsten carbide coatings to include different coatings that provide resistance to wear or erosion/corrosion. The extremely brief exposure of the precursor nanocrystalline particles to the high temperatures of the HVOF process appears to preserve the nanocrystalline structure in most of the particles deposited on the substrate. In general, the quality of the HVOF-sprayed coating is determined by a combination of various processes.

Thermal Spray Processing of Nanocrystalline Materials

Technological advancements in many sectors of modern society depend strongly on the materials science and engineering community’s ability to conceive of novel materials with attractive combinations of physical and mechanical properties. For instance, in the aerospace industry, the ever increasing demand to manufacture lighter aircraft that can travel at higher speeds and can withstand a higher payload capacity has fueled the development of high strength/low density materials with improved damage tolerance and enhanced temperature capabilities. Driven in part by this critical need, research in materials science and engineering has shifted towards the study and application of non-equilibrium processes. The significant departure from thermodynamic equilibrium associated with these types of processes allows material scientists and engineers to develop materials with unusual combinations of microstructure and physical attributes.