Intermetallic Alloys and Intermetallic Matrix Composite Coatings

Dear Colleagues,

Unlike conventional metal alloys, which could be described as a base material to which certain percentages of other elements have been added, intermetallic compounds have a particular chemical formula with a fixed or narrow range of chemical composition. In addition, instead of their atoms being linked with relatively weak metallic bonds, the bonding may be partly ionic or covalent, which gives them an ordered crystal lattice. Some intermetallics can maintain this order until their melting point, which is the main reason why they possess a strong stability at high temperatures.

Extensive research has been dedicated to aluminides, especially as bond coats in Thermal Barrier Coatings (TBCs) to overcome the fundamental barriers to higher temperature operations with the requisite durability in future gas turbines. Commonly used types of bond coats are M–Cr–Al–Y (where M is Ni or Co) or Pt-modified aluminide coatings.

Transition metal aluminides based on Ti, Fe, Ni, Co, and Nb are seen as promising due to their potential use as coatings in aggressive environments. They possess sufficiently high concentrations of aluminum to form a continuous, fully adherent alumina layer on the surface when exposed to corrosive, oxidizing, carburizing, and sulfidizing conditions.

Other intermetallics, such as MoSi₂, also exhibit an outstanding high-temperature resistance of above 1000^ºC. Some of these intermetallics can be reinforced to improve their performances. Examples of this can be found in the case of ZrB₂-MoSi₂ ultra-high-temperature Ceramic Matrix Composites, or even aluminides alloyed with ceramic phases.

Potential scientific research within the last two decades has focused on the use of thermal spray technologies and laser cladding to produce coatings and optimize their microstructures in order to improve their adhesive and cohesive strength, mechanical, corrosion, oxidation, and wear properties. Such coating technologies may imply some oxidation of the raw material during the deposition process, which actually introduce reinforcement phases that can contribute to changes in the thermophysical properties or increase hardness and wear resistance, but that are detrimental to oxidation and corrosion, since they leave aluminum-depleted areas. To improve the wear performance, ceramic hard phases can also be introduced as a feedstock.

As such, the production of different intermetallic, cermet, and ceramic protective coatings can be simple, beneficial, and highly predictable by various conventional synthesis methods with GMA and GTA processes, thermal (D-gun, HVOF, Arc and plasma) spraying, cold spraying, PVD, CVD, ion implantation, and additive manufacturing processes (SLM, DMLS, LENS, among others).

Therefore, the scope of this Special Issue is dedicated to:

• Coatings produced by different processes, including but not limited to thermal spray, laser and plasma processing, PVD, CVD, ion implantation, and additive manufacturing processes (SLM, DMLS, LENS, among others), as well as GMA and GTA welding processes that include different intermetallic, cermet, and transition metal aluminides with hard ceramic-reinforcing phases such as carbide, boride, and oxide particulates.
• Theoretical and experimental research on the deposition mechanisms.
• Understanding metal–ceramic bonding interfaces and dissolution according to temperature-dependent processing. The micro-joint formation that takes place when the surface particles with non-equilibrium solidification are melting leads to eutectic inclusions, even during amorphous phase formation under the extremely harsh formation conditions of the multilayered structure of the protective coatings.
• Raw material powder manufacture for protecting intermetallic-matrix composite (IMC) coatings.
• Mechanical properties and residual stresses of intermetallic and IMC coatings.
• Understanding the degradation mechanisms of coatings through friction, wear, or other dynamic loading conditions.
• Understanding aqueous corrosion, hot corrosion in molten salts, and the high-temperature oxidation mechanisms of coatings.
• Analysis of gas detonation phenomenon, thermodynamic studies, and the numerical modeling of processes in supersonic metallization jet flow.

Prof. Dr. Cezary Senderowski
Dr. Núria Cinca
Guest Editors

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