Si diffusion coating on steels by SiH 4 /H 2 treatment for high temperature oxidation protection
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I. INTRODUCTION
Addition of suitable amounts of Si to the bulk of wrought and cast metals and alloys improves many surface properties of the metal. High silicon alloys resist oxidation and carburization at high temperatures1 and corrosion in aqueous inorganic acids.2'3 Iron requires about 12-14% Si for optimum performance; high Cr ferrous alloys, about 4-8% Si.4 Duriron™, a commercially available cast Si alloy used in extremely corrosive aqueous media, contains 14.5 wt. % Si. Unfortunately, such high levels of Si impair the desirable bulk physical properties of metals. These alloys lose strength and toughness, becoming brittle and difficult to machine. However, if the silicon can be confined only to the surface in a coating, improved surface properties are achieved without modifying the bulk properties of the metal. In fact, higher Si levels are allowable in a surface layer than are feasible or desirable in the bulk alloy. Two types of Si coatings are possible: overlay and diffusion coatings. An overlay coating is a layer of Si deposited on the substrate surface. It is characterized by an abrupt discontinuity in composition and thermal/ mechanical properties between the surface layer and the substrate. On the other hand, a diffusion coating is a broad layer formed from both the deposited Si and the substrate components. Diffusion produces a zone containing both elements (Si and a metal from the substrate) with a gradual transition from a high silicon concentration at the surface down to the original silicon concentration in the base metal. A Si diffusion coating consists of metal silicides and/or metal-silicon solid solutions. The diffusion coating adheres better because of the gradual change in physical properties from the coating to the substrate associated with the gradual change
in composition. In addition, it provides a reservoir of Si in the bulk that can migrate to the surface under certain conditions if the coating is damaged. Available processes for producing silicon diffusion coatings on the surfaces of metals5"8 include molten metal/salt baths, pack cementation, slurry/sinter, ion implantation, and/or chemical vapor deposition (CVD). Advantages of CVD include uniform coating of the substrate, potentially low treatment temperatures, minimum cleaning of parts after treatment, in situ treatment of fabricated parts, ease of surface cleaning, and capability for thermal treatment after deposition. CVD processes using silicon halides, SiHnCl(4_n) (n = 0 to 3), have long been known to form Si diffusion coatings on Fe.9'10 More recently, researchers at Brown Boveri and Cie, A. G., studied11"14 the treatment of Ni and high Ni alloys with Si halides at -1000 °C. Ni silicides, solid solutions, and other species formed, but the use of halides resulted in substrate loss via the formation of volatile metal halides. Processes for siliconizing metals using a silicon hydride at reduced pressures at 900 to 1000 °C were patented in the early 1900s.1516 More recently, Caillet and co-workers17"19 produced diffusion coatings c
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