Refractory ceramic coatings: processes, systems and wettability/adhesion

Abstract

Refractory materials such as ceramics and refractory metals/alloys are widely used for structural applications in severe environments where more efficient use of energy is a prime need. Owing to properties such as high strength and refractoriness, low thermal expansion and electric conductivity these materials may be used as turbocharger rotors, valves, valve seats and turbocharger turbine wheels in gas turbine engines, tubes in industrial heat exchangers, fuel elements in nuclear reactors, fusion reactor wall material, tool materials in machining and casting and as electronic packaging. In these applications, however, these materials often experience extreme mechanical, thermal and chemical loading conditions and are expected to perform effectively beyond existing limits. The existing limits of performance for these materials can be extended only via coating technology, which presently appears to be the most suitable technical and economic approach. Thus there remains a critical need to develop a suitable and reliable refractory coating system and coating technique.In view of the above requirements, the present paper embarks on a brief review of refractory coatings. The review begins with a set of criteria required for an effective refractory coating. The criteria include general thermophysical and mechanical properties for the refractory coating only and properties of the coating-substrate as a system. The review continues with a section on methods of coating. The classification of methods of coating is based on categories such as gaseous, solution, molten/semi-molten and solid state processes. Each category is explained with conventional and novel techniques and coatings that can be synthesized using these techniques. These categories are supplemented also with examples of applications. A major emphasis has been put on refractory materials in the following section on coating material systems. The most important materials systems have been grouped as oxides and nitrides, carbides, borides, silicides and aluminides. The general properties that make them useful as refractory coatings, along with specific materials and applications, have been reviewed. Wettability and adhesion between coating and substrate are very important for the coating-substrate system to perform effectively and reliably. The last section discusses the fundamentals of wettability and work of adhesion, based on which wetting in metal-metal, metal oxide, metal-carbon/carbide systems has been discussed. Finally, the review sets the motion of thoughts with a description on future developments.

Oxidation of ZrB[sub 2]-Based Ceramics in Dry Air

“…6,7 The use of a ceramic-type additive ͑i.e., silicon nitride͒ in a ZrB 2 -based material showed overall improvement in mechanical properties when compared to literature on similar materials. 13,14 As this class of advanced ceramics is designed to be exposed frequently to very hot and hostile environments, high-temperature stability, especially resistance to oxidation, is a key criterion for actual applications. The introduction of second phases ͑i.e., SiC, ZrC͒ has succeeded in obtaining structurally stronger ZrB 2 -based composites with improved resistance to ablation/oxidation.…”

Deposits in Gas-fired Rotary Kiln for Limonite Magnetization-Reduction Roasting: Characteristics and Formation Mechanism

“…The results of phase analysis show that fayalite is the most important phase composition and the most important binder phase of the deposits. Alkali metal oxides and salts are known to act corrosively on refractory brick [37]; moreover, the liquid phase may react and bond with the refractory brick [19,38]. These may be two important reasons for the deposits formed and adhered on kiln wall.…”

Dispersion and Stabilization of TiN and TiC Nanoparticles in Organic Suspensions

“…Among techniques to process TiN and TiC coatings, such as physical (PVD) or chemical vapor deposition (CVD) or nitridation of TiO 2 should be highlighted. PVD includes techniques such as cathodic arc evaporation [1], ion beam deposition [2,3], ion implantation [4], plasma [5] and laser methods [6,7]. In those techniques, TiN nanocrystals grow forming typically elongated particles in the range of several hundreds of nanometers and coatings with hardness of approximately 27 GPa are obtained.…”