Nanostructured coatings

doi:10.1016/S0921-5093(01)01986-4

Materials Science and Engineering: A

Volume 336, Issues 1–2, 25 October 2002, Pages 274-319

https://doi.org/10.1016/S0921-5093(01)01986-4 Get rights and content

Abstract

This review is based essentially on the results in the field of synthesis and characterization of nanostructured coatings obtained by the authors themselves or their colleagues. Characteristics of feedstock powders for synthesizing nanostructured coatings such as particle size and morphology, changes in chemical composition and grain size are summarized. The evolution of microstructure caused by mechanical milling in two typical powder systems, Cr₃C₂–NiCr and Inconel 625, and mechanisms governing the development of the nanostructure are discussed. Mechanical properties and microstructure of several nanostructured coatings are evaluated by using microhardness testing, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. As background information, a review of the agglomeration process for milled powders and of thermal spraying technologies to synthesize nanostructured coatings are also included. In addition, the methodologies used to characterize the performance of the milled powders and nanostructured coatings, as well as the practice techniques for sample preparation, are described in detail.

Introduction

In thermal spraying technology, molten or semi-molten powders are deposited onto a substrate to produce a two-dimensional coating or in some cases, a three-dimensional self-standing material. The microstructure and properties of the material depend on the thermal and momentum characteristics of the impinging particulate [1], which are determined by both the spraying methodology and the type of feedstock materials employed. Powders, rods and wires can be used as feedstock materials. Various coatings are deposited on the surface of a substrate to either provide or improve the performance of materials in industrial applications.

Nanostructured (or nanocrystalline) materials are characterized by a microstructural length scale in the 1–200 nm regime [2]. More than 50 vol.% of atoms are associated with grain boundaries or interfacial boundaries when the grain is small enough. Thus, a significant amount of interfacial component between neighboring atoms associated with grain boundaries contributes to the physical properties of nanostructured materials [3]. Using nanostructured feedstock powders, thermal spraying has allowed researchers to generate coatings having higher hardness, strength and corrosion resistance than the conventional counterparts [4], [5], [6]. The purpose of this paper is to review: (a) the synthesis and characterization of nanostructured feedstock powders; (b) the agglomeration of these powders for use in coatings; and (c) the processing and characterization of nanostructured thermal spray coatings.

Section snippets

Synthesis of nanostructured feedstock powders

Preparation of nanostructured feedstock powders is the first step for synthesis of nanostructured coatings. A number of techniques that are capable of producing nanostructured materials include gas condensation, mechanical alloying/milling, crystallization of amorphous alloys, thermochemical method, spray conversion processing, vapor deposition, sputtering, electro-deposition, and sol–gel processing techniques [7]. Of these techniques, only mechanical alloying/milling and thermochemical

Characterization of nanocrystalline powder synthesized by mechanical milling

It is important to be able to identify powder characteristics, such as particle size, powder morphology, grain size, phase constituents and deformation faults as a function of milling parameters. Unfortunately, a real-time monitoring system for quantifying these characteristics is not available. The construction of the tank permits the removal of powder samples from the tank during milling to conduct analysis of powder characteristics without disturbing the milling. Therefore, the dependence of

Agglomeration

Spherical powder particles, with dimensions ranging from 10 to 50 μm, are typically required for most available thermal spray systems. As discussed in Section 3.1, as-synthesized nanocrystalline powders do not satisfy these requirements for the size and morphology of the particle. Accordingly, an agglomeration procedure is often necessary, even for conventional powders that do not meet these requirements. Spray drying is a popular agglomeration technique [72], [73], [74]. In this technique, a

Thermal spraying of nanostructured coatings

Today, a number of thermal spraying techniques are available. Flame spraying (FS), arc spraying (AS), detonation gun spraying (DGS), continuous detonation spraying (CDS), atmospheric plasma spraying (APS), twin wire arc spraying (TWAS), low pressure plasma spraying (LPPS) or vacuum plasma spraying (VPS), controlled atmosphere plasma spraying (CAPS), high velocity flame spraying (HVFS) and high velocity oxygen fuel spraying (HVOF) are widely used to produce coatings for different industrial

Summary

This article reviews the synthesis and characterization of nanostructured feedstock powders, the agglomeration of these powders used in coatings, and processing and characterization of nanostructured thermal sprayed coatings. The published results show that mechanical milling can be effectively used to synthesize nanostructured powders. Whether a composite or a single-phase starting powder is involved, mechanical milling leads to the formation of nanocrystalline structure under certain milling

Acknowledgements

The authors gratefully acknowledge financial support provided by the Office of Naval Research under grants N00014-94-1-0017, N00014-98-1-0569 and N00014-00-1-0109, and N00014-01-C-0384 as well as many useful discussions with Professor Enrique J. Lavernia at University of California, Irvine.

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ReviewNanostructured coatings

Abstract

Introduction

Section snippets

Synthesis of nanostructured feedstock powders

Characterization of nanocrystalline powder synthesized by mechanical milling

Agglomeration

Thermal spraying of nanostructured coatings

Summary

Acknowledgements

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Physica

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Nanostructured coatings