High-temperature and electrochemical oxidation of transition metal silicides

Abstract

546.281:541.138 High-temperature and electrochemical oxidation of transition metal silicides, which are widely used in microelectronics as ohmic contacts and protective coatings for high-temperature alloys, are discussed in this review. The process of oxide film formation during annealing or anodizing is extremely important for both applications of silicides. It is discussed for three disilicides: MoSi 2 , WSi 2 , and TiSi 2 . It has been shown that different types of oxide films may form on the disilicide surface depending on the thermodynamic (formation enthalpies of silicides and associated oxides) and kinetic (diffusion coefficients of silicon in silicide and oxygen diffusivity in oxide) factors: onephase (MoSi 2 ), mixed (WSi 2 ), and multilayer (TiSi 2 ) films. The "silicide pest" phenomenon is discussed in terms of thermodynamic, kinetic, and structural factors influencing the pesting. Analysis of electrochemical oxidation mechanisms for silicides in various electrolyte media reveals numerous similarities between anodic and high-temperature oxidation mechanisms. It is shown that slow silicon transport during anodic treatment at room temperature leads to the formation of multiphase mixed oxide films under electrochemical polarization.Keywords: transition metal silicides, kinetics of high-temperature oxidation, formation mechanism, protective oxide films containing SiO 2 , anodic electrochemical oxidation, electrolyte solutions.Binary refractory compounds of elements from groups 4-6 of the Periodic Table with carbon, boron, nitrogen, and silicon--carbides, borides, nitrides, silicides, and associated composites⎯are very promising materials owing to their excellent, as compared with the initial metals, high-temperature oxidation resistance and chemical stability. These materials are widely used in different engineering areas. New composite materials based on refractory compounds with desired properties constantly show up.Transition metal silicides are widely used in modern engineering. They are remarkable for their significant physical and technical properties: relatively low density, metal conductivity, high corrosion resistance, which are needed in engineering areas using high temperatures, rates, loads, and corrosive media. These are aerospace structures, high-temperature heaters, corrosion protective coatings on refractory alloys, etc. [1]. For example, the high melting temperature combined with the corrosion resistance of semiconducting silicides (Cr, Mn, and Fe disilicides) are the very properties required for thermoelectric generators that convert solar energy into electric energy.

Water vapor thermal treatment effects on spark plasma sintered nanostructured ferritic alloy‐silicon carbide systems

“…In addition, Si element is still present in these domains based on Figure C. It has been reported that transition metal silicides have strong corrosion resistance by forming a protective SiO 2 layer at high temperatures . Thus, a small amount of SiO 2 phase is attached to the surface of the silicides for the water vapor‐treated NFA‐SiC samples.…”

“…It has been reported that transition metal silicides have strong corrosion resistance by forming a protective SiO 2 layer at high temperatures. [46][47][48] Thus, a small amount of SiO 2 phase is attached to the surface of the silicides for the water vapor-treated NFA-SiC samples. The SiC matrix has O element based on Figure 2E, and Si and C elements based on Figure 2C,D.…”

Stability and electrical properties of MoSi2‐ and WSi2‐oxide electroconductive composites

“…Similar results can be clearly seen for these composites as well. It is determined that only W 5 Si 3 secondary phase formed in all three composites after sintering, since WSi 2 also has a high‐temperature oxidation process similar to MoSi 2 . This formation was explained differently by the decomposition of WSi 2 into W 5 Si 3 and Si during hot isostatic pressing at 1500°C and 196 MPa .…”

“…By this way, it was determined that rates of the formation of secondary phases (vol% per hour) highly decreased in all composites except for the MoSi 2 –ZrO 2 after annealing for 24 hours, as presented in the Table , indicating their improved thermal stability over time. However, it is important to point out that high fractions of secondary W 5 Si 3 phase up to 38.8 vol% may cause serious issues in real applications under air atmosphere due to its low oxidation resistance . Therefore, MoSi 2 ‐based composites are found to be more thermally stable with the formation of low fractions of Mo 5 Si 3 having reasonable high‐temperature oxidation resistance, and hence, to be more suitable for advanced sensing applications at high temperature and harsh environments.…”

“…), which have been mostly studied for high‐temperature structural applications along with silicon‐based ceramics (SiC, Si 3 N 4 ), offer great advantages for such sensing applications due to their high melting temperatures (>1900°C), very low oxidation and corrosion rates at high temperatures, high‐temperature stability, good mechanical properties, and high‐temperature electrical conductivity . These materials are intermetallic compounds of transition metals in group IV, V, VI and silicon, so the bonding in these metal silicides is more metallic in nature . Among all refractory transition‐metal silicides, molybdenum disilicide (MoSi 2 ) and tungsten disilicide (WSi 2 ) are the most promising candidate materials for advanced sensing applications, since they are more thermally/chemically stable and oxidation resistant at the temperatures between 1000°C and 1600°C under harsh environments.…”