Functional Coatings and Their Applications: A Web Perspective

This article is one of eight papers to be presented exclusively on the web as part of the January 2000 JOM-e the electronic supplement to JOM.
	The following article appears as part of JOM-e, 52 (1) (2000), http://www.tms.org/pubs/journals/JOM/0001/Dahotre/Dahotre-0001.html JOM is a publication of The Minerals, Metals & Materials Society

Surface coating is now widely recognized as an enabling technology of major importance in the successful, effective, and efficient exploitation of materials in engineering practice. Involving as it does both the enhancement and upgrading of conventional materials in engineering applications, as well as the creation and synthesis of advanced and novel materials, surface coating may be broadly regarded as embracing activities, such as surface properties optimization, as an integral part of the design process and synthesis and fabrication technologies. While all new coating methods, requirements, and options are being met as challenges by physicists, chemists, and engineers, the existing traditional coating technologies, such as diffusion coating, plating, and vapor deposition, are undergoing their own resurgence.

This month's installment of JOM-e (the journal's web-only supplement) appropriately presents various papers on functional coatings and their applications. The topics dealt with are fundamental issues of wettability, spreading, and interfacial phenomenon; novel techniques, such as high-density infrared (HDI) transient liquid coating (TLC); laser-based advanced coating techniques; and newly revived conventional techniques, including electrodeposition/electroplating and slurry coating. Both ceramic- and metal-based coatings, along with functionally graded coatings, are discussed.

Asthana and Sobczak provide an excellent review on the important and age-old phenomena of wetting and spreadability in coatings. This article reviews the status of high-temperature wettability as manifested in the contact-angle phenomena with a focus on metallic coatings. A large body of useful scientific knowledge related to the wetting and spreading of high-temperature coatings comes from the fundamental studies based on Young's equation. In spite of its considerable success in various fields, however, Young's equation has been a subject of controversy and debate. Factors underlying such debate are first briefly reviewed by the authors, who then provide an overview of the high-temperature capillarity phenomenon.

Using HDI TLC for Cr₂C₃ and WC-reinforced with nickel-based binder for aluminum die-casting processes involving severe thermal cycling is described by Blue et al. Advantages of infrared heating include a clean noncontact heat source, rapid-response energy fluxes with excellent spatial and temporal control, rapid cooling rates, and low or no thermal mass associated with the source.

Papers by Pei and De Hosson on functionally graded coatings (FGCs) produced by laser-powder cladding and by Agarwal and Dahotre on the functionality of laser-engineered composite TiB₂ coatings provide a look at laser use for the synthesis and fabrication of different types of coatings on aluminum and steel substrates. Pei and De Hosson demonstrated the production of FGCs of AlSi₄₀ on cast aluminum alloy via a one-step laser powder cladding process. The process completely avoided the interfacial problems associated with deposition by conventional processes. Coatings produced by this method can be designed independent of the composition of the substrate material. On the other hand, Agarwal and Dahotre have employed a fiber-optically delivered Nd:YAG laser to engineer TiB₂/Fe composite coatings on steel substrate. This approach provided flexibility to fabricate coatings into various configurations of shapes and sizes as well as compositions (TiB₂ vol.% in iron-matrix). The functionality of these composite coatings for applications such as Hall-Heroult cells, tools, dies, etc. has been evaluated based on wear, high-temperature oxidation, and liquid-metal corrosion properties.

The revival and usefulness of older processes, such as electrodeposition, electroplating, and slurry coating are presented in the next three papers. Many processes based on electrodeposition, such as electrophoretic deposition and electrolytic deposition, for ceramic films are reviewed by Zhitomirsky in a paper on ceramic films via cathodic electrodeposition. Characteristics such as rigid control of film thickness, uniformity, and deposition rate; low process cost; suitability for deposition on complex surface geometry and selected area; and impregnation of porous substrate are described.

The cyclic oxidation performance of electrolytically deposited 8-10 mm thick layers of aluminide and platinum-aluminide on nickel-based cast alloy was evaluated by Das et. al in a paper on the comparative evaluation of cyclic oxidation performance of plain aluminide and platinum-aluminide coatings on cast nickel-based superalloy CM-247. Such coatings are designed for enhanced performance of nickel-based components for gas-turbine engine application. The existence of platinum in platinum-aluminide coatings provided enhanced performance in terms of protection against cyclic oxidation at high temperature (>1,000°C) compared to bare nickel-based alloy and plain aluminide-coated nickel-based alloy. This enhanced performance in platinum-aluminide coating is due to the ability of platinum to provide a more adherent protective alumina layer and to delay the formation of spinel during oxidation exposure.

Seal et al. describe the beneficial effect of superficial cerium oxide on four grades of stainless steel in a paper on ceria-based, high-temperature coatings for oxidation prevention. The beneficial effect included a decreased rate of reaction and improved scale adherence due to internal oxidation of silicon that acted as a pegging action for better adherence.