Silicide Coatings Research Articles

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 2. Stability and Heat Resistance of Silicide Coatings on Tungsten and Molybdenum at 1500-2000°C

The kinetics of diffusion redistribution of phases within the system WSi2 ― W on heating tungsten silicide in air in the temperature range 1500-2000°C is studied. The stability and heat resistance of silicide coatings on tungsten is mainly governed by the diffusion of silicon towards the interphase boundaries W ― W5Si3, W5Si3 ― WSi2, and WSi2 ― SiO2, formation at them of diffusion barriers of lower silicide W5Si3, and also a protective SiO2 film at the outer boundary of the silicide coating. It is established that the transition rate for the higher to the lower tungsten silicide WSi2 → W5Si3 is on average four times slower than the transition rate for MoSi2 → Mo5Si3. It is shown that an increase in silicon concentration in the WSi2 surface layer stimulates formation of diffusion barrier compounds at interphase boundaries. This leads to an increase in the stability of the phase composition and heat resistance of a silicide coating on metals. In particular at 1700°C the transition rate for molybdenum silicide on tungsten MoSi2 → (Mo, W)5Si3 is about twenty times slower than the transition rate for MoSi2 → Mo5Si3, and less by a factor of about eleven than the transition rate for WSi2 → W5Si3. Here there is also an increase in the heat resistance of silicide coatings on tungsten and molybdenum. It is shown that the SiO2 film on tungsten silicide does not lose its protective properties up to 2000°C.

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 1. Effect of Subsurface Layer on Process of Phase Redistribution in System WSi2 ― W

The effect of a subsurface layer of metal silicide on the phase composition stability of high-temperature MeSi2-type coatings on refractory metals was investigated. Using the system WSi2 ― W as a prototype it was experimentally determined that a subsurface layer of the tungsten disilicide, and the distribution profile of silicon in the diffusion zone upon high-temperature heating have a substantial effect on the formation of a barrier layer of lower silicide which determines the stability of the system as a whole. It is proposed that the search for more stable silicide coatings on refractory metals should be directed toward the creation of diffusion barriers on not only the inner interface MeSi2 ― Me, but also the external surface of the coating. The process of coating formation must be accompanied by the formation of a metal silicide on the external surface with the highest possible concentration of silicon.

Silicide Coatings for Niobium: Mechanisms of Chromium and Silicon Codeposition by Pack Cementation

In the present work, a pack cementation (PC) process was carried out to codeposit Cr and Si on a Nb substrate. Three masteralloy compositions (Cr 36.4 Si 63.6 (A), Cr 44.5 Si 55.5 (B) and Cr 14.3 Si 85.7 (C)) were tested in the presence or absence of an activator (CrCl 3 or SiCl 4 ). The study of deposition mechanisms was undertaken through thermodynamic and dynamic calculations. Si appears to be deposited due to disproportionation reactions of lower halides, SiCl 2 and SiCl 3 , while Cr seems to be transported by a molecular species containing Cr and Si, not yet identified.

Thermodynamic Calculations as the Basis for CVD Production of Silicide Coatings

The exponential increase in computing power realized over the past three decades has required devices of ever-decreasing dimensions. With these reductions in feature size, each generation of circuit technology creates a new set of materials-science challenges, as exemplified by the use of silicide-based diffusion barriers.The use of transition-metal silicides in the semiconductor industry began in the early 1980s with (in retrospect) fairly thick films, simple compositions, and minimal microstructural requirements. The implementation of thin-film binary silicides in integrated circuit (IC) applications required the development of appropriate modeling techniques to select the deposited metal, its gaseous precursor, and the Silicon precursor. These vectors were used to define experimental conditions to yield desired films. Chemical-vapor-deposition (CVD) experiments were simulated and CVD phase diagrams were used to describe the changing film properties with different thermodynamic conditions.In the early 1990s, research began to focus on CVD of ternary Systems (Ta-Si-N, Ti-Si-N). Modeling these complex Systems required optimization of the thermodynamic data and careful evaluation of the ternary phase diagrams.Current-generation materials are deposited in extremely thin layers (ULSI, or ultralarge-scale integration), composed of multiple elements from a variety of gas sources, an d have tailored micro-structures. As described in this article, early thermodynamic modeis helped develop deposition techniques for early-generation silicides. As the technological requirements increased, the modeling method s evolved in parallel, yielding continued insight into the relevant processes. Current work on CVD modeling couples thermodynamic calculations with heat and mass transfer. Incorporating both kinetic and thermodynamic effects, these methods provide a more realistic description of the CVD reactor.

Distribution of temperature in a plate with a single-layer coating subjected to intense heating

We establish the distribution of temperature formed under conditions of abrupt changes in the surface temperature near the adhesional contact between the base and the coating in a plate of infinite length made of molybdenum or niobium both sides of which are covered with silicide coatings. The problem of heat conduction for a multilayer plate subjected to thermal cycling is solved by the method of finite integral transformations. It is shown that a silicide coating with a thickness of 60–100 μm leads to the formation of a significant temperature gradient in the base material under conditions of cyclic variation of temperature.

New silicides for new niobium protective coatings

Efforts to improve at high temperature the oxidation resistance of pure niobium or commercial niobium alloys have led to the development of a pack cementation process for the co-deposition of Si, Ti, Cr and Fe. Owing to the knowledge of the quaternary Nb(Ti)–T–Cr–Si phase diagrams (T=Fe or Co or Ni) and of the crystallographic features of phases present in the silicide coatings, new protective coatings have been applied on pure niobium and Cb752 alloy. The results of the crystallographic study of three new silicides isostructural with Nb 3Fe 3CrSi 6, in which Nb is substituted by Ti and Fe by Co or Ni are reported. The oxidation performances of two new coatings mainly consisting of such a silicide are also outlined.

The deposition of aluminide and silicide coatings on γ-TiAl using the halide-activated pack cementation method

The halide-activated pack cementation method (HAPC) was utilized to deposit aluminide and silicide coatings on nominally stoichiometric γ-TiAl. The deposition temperature was 1000°C and deposition times ranged from 2 to 12 hours. The growth rates of the coatings were diffusion controlled, with the rate of aluminide growth being about a factor of 2 greater than that of silicide growth. The aluminide coating was inward growing and consisted of a thick, uniform outer layer of TiAl3 and a thin inner layer of TiAl2, with the rate-controlling step being the diffusion of aluminum from the pack into the substrate. Annealing experiments at 1100 °C showed that the interdiffusion between the aluminide coating and the γ-TiAl substrate was rapid. In contrast to the aluminide coating, the silicide coating was nonuniform and porous, consisting primarily of TiSi2, TiSi, and Ti5Si4, with the rate-controlling step for the coating growth believed to be the diffusion of aluminum into the γ-TiAl ahead of the silicide/γ-TiAl interface. The microstructural evolution of the aluminide and silicide coating structures is discussed qualitatively.

Isothermal and cyclic oxidation resistance of boron-modified and germanium-doped silicide coatings for titanium alloys

Since titanium alloys with an adequate balance of mechanical properties and high-temperature oxidation resistance have not been developed, protective coatings are required. In our previous paper, B-modified and Ge-doped silicide diffusion coatings grown on CP Ti, Ti−24Al−11Nb, Ti−22Al−27Nb, and Ti−20Al−22Nb by the halide-activated, pack-cementation method were described. In this study, isothermal and cyclic oxidation were used to evaluate the oxidation performance of these coatings in comparison to uncoated substrates. The rate-controlling mechanism for isothermal oxidation at high temperature was solid-state diffusion through a SiO2 scale, while the mechanism for low-temperature oxidation involved grain-boundary diffusion through TiO2. Both isothermal and cyclic oxidation rates for the B-modified and Ge-doped silicide coatings were much slower than for pure TiSi2. Oxygen contamination was not detected by microhardness measurements in the coated substrates after 200 oxidation cycles at 500–1000°C for the Ti−Al−Nb alloys, or at 500–875°C for CP Ti. The excellent oxidation resistance for the optimum coating compositions is discussed.

Development and growth of boron-modified and germanium-doped titanium-silicide diffusion coatings by the halide-activated, pack-cementation method

A halide-activated, cementation pack has been developed to codeposit either silicon and boron or else silicon and germanium in a single processing/reaction step to grow Ti-silicide diffusion coatings on commercially pure (CP) titanium, Ti-22Al-27Nb, and Ti-20Al-22Nb. Since boron is nearly insoluble in TiSi2, a TiB2 layer is localized at the surface of the B-modified silicide coatings. The thickness of the TiB2 layer is controlled by the choice of boron activity and halide activator in the pack. Germanium is soluble in the Ti-silicide layers but inhomogeneously distributed in the Ge-doped silicide coating. The germanium content is controlled by choices of the Si-to-Ge ratio and the halide activator in the pack. The growth kinetics for the five-layered B-modified silicide coatings are generally similar to the undoped silicide coatings. The growth mechanism for the five-layered Ge-doped silicide coatings is generally different from the undoped silicides. The growth of dual-layer Ti-boride coatings was also studied.

Growth and oxidation resistance of boron-modified and germanium-doped silicide diffusion coatings formed by the halide-activated pack cementation method

Niobium-base and titanium-base alloys and intermetallic compounds possess many attractive properties, but practical application at high temperature is limited by their rapid scaling and embrittlement by oxygen dissolution. Since alloying schemes have not provided an acceptable balance of oxidation resistance and mechanical properties, protective coatings are required. Procedures for codepositing either silicon and boron, or else silicon and germanium, in a single reaction-processing step by a fluoride-activated, pack cementation method have been developed. These B-modified and Ge-doped silicide coatings were grown on commercially pure titanium, Ti−20A1−22Nb, Ti−22Al−27Nb and Nb−40Ti−15A1 alloys by direct conversion of the base metal into a TiSi 2-base coating. For Nb-base alloys, a protective Ge-doped MoSi 2 coating was grown by the diffusive conversion of a Mo layer deposited by physical vapor deposition. These diffusion coatings have been characterized, and their performance was evaluated by cyclic oxidation in air. Microhardness profiles were used to demonstrate the absence of oxygen contamination for the coated substrates. Preliminary fatigue testing of coated substrates is reported.

Growth and oxidation resistance of boron-modified and germanium-doped silicide diffusion coatings formed by the halide-activated pack cementation method

Niobium-base and titanium-base alloys and intermetallic compounds possess many attractive properties, but practical application at high temperature is limited by their rapid scaling and embrittlement by oxygen dissolution. Since alloying schemes have not provided an acceptable balance of oxidation resistance and mechanical properties, protective coatings are required. Procedures for codepositing either silicon and boron, or else silicon and germanium, in a single reaction-processing step by a fluoride-activated, pack cementation method have been developed. These B-modified and Ge-doped silicide coatings were grown on commercially pure titanium, Ti−20A1−22Nb, Ti−22Al−27Nb and Nb−40Ti−15A1 alloys by direct conversion of the base metal into a TiSi2-base coating. For Nb-base alloys, a protective Ge-doped MoSi2 coating was grown by the diffusive conversion of a Mo layer deposited by physical vapor deposition. These diffusion coatings have been characterized, and their performance was evaluated by cyclic oxidation in air. Microhardness profiles were used to demonstrate the absence of oxygen contamination for the coated substrates. Preliminary fatigue testing of coated substrates is reported.

The kinetics of multilayered titanium-silicide coatings grown by the pack cementation method

The halide-activated, pack cementation coating method is a viable technique for forming silicide diffusion coatings that provide oxidation resistance for titanium alloys at high temperature. In this study, growth rates were determined for the five-layered silicide coatings on commercially pure titanium grown at 950 ‡C, 1050 ‡C, and 1150 ‡C using three halide activators. Solid-state titanium-silicon diffusion couple experiments were also made to determine the diffusional growth rates for the five silicide phases. Based on a model for growth of multiple layers, solid-state diffusion controls the growth rates in all of the silicide layers for coatings formed by packs with the less stable AlF3 and CuF2 activators. However, the growth of TiSi2 was influenced by gas-phase transport for coatings formed using the more stable MgF2 activator. The growth rates of the layers (TiSi2, TiSi, Ti5Si4, Ti5Si3, and Ti3Si) are parabolic; by assuming solid-state dif-fusion control, the diffusion coefficients in these phases were calculated.

Oxidation-resistant boron- and germanium-doped silicide coatings for refractory metals at high temperature

Owing to the poor oxidation resistance of titanium alloys at high-temperature, a protective coating is required. Alloying titanium with enough aluminum or silicon to form a protective Al 2O 3 or SiO 2 scale is not a viable approach. In this study, boron- and germanium-doped silicide coatings were grown on commercially pure (CP) titanium and orthorhombic-base Ti-20Al-22Nb and Ti-22Al-27Nb alloys by a halide-activated, pack cementation method. Isothermal and cyclic oxidation testing in air were used to evaluate the coated substrates at 500–1000 °C. Microhardness profiles were made to test for oxygen contamination of the substrate. These coatings both provided excellent oxidation resistance for CP Ti and the TiAlNb alloys.

Preparation of B4C-SiC composites by reactive chemical vapour deposition from a gaseous BCI3−SiCI4−H2phase

This paper focuses on the reactive chemical vapour deposition of composite B4C-SiC coatings onto porous graphite substrates from a BCl3−SiCl4−H2 vapour phase. The mixed layer seems to exhibit promising properties such as oxidation resistance. However, its preparation as a thin film from the vapour is very difficult owing to the complexity of the system. The experimental parameters such as temperature, number of moles H2, and the amounts of BCl3 and SiCl4 in the reactive vapour phase are systematically discussed. The results are compared with a thermodynamic approach. Also, this paper reports investigations on boron and boron silicide coatings. Scanning electron microscopy, X-ray photoelectron spectroscopy and electron microprobe analysis were used to characterize the various coatings. The compositions, microstructures and properties such as oxidation behaviour of the coatings were also studied. Optimal experimental parameters of the process were reached in order to achieve a dense, adherent and continuous...

Method for removing silicide coatings in a medium of low-melting metals

We propose a method for removing silicide coatings from the surfaces of workpieces made of refractory metals. The entire volume of the silicide layer is borated in a melt based on low-melting metals. The boride layer formed as a result is characterized by a large number of defects, low mechanical strength, and high etching susceptibility. Therefore, it can easily be removed by any existing method.

Formation of eutectic silicide coatings on molybdenum

General features of structural changes in silicide coatings on molybdenum during vacuum annealing in the temperature range between 1850 and 1920°C have been studied by electron microscopy, metallography, and x-ray phase analysis. The kinetics of eutectic MoSi2-Mo5Si3 formation and structure are studied. It is found that fusion of the eutectic compound coating makes it possible to reduce defects substantially and thus improve surface layer quality.

High temperature oxidation behaviour of laser surface alloyed iron-silicon coatings on iron

Iron silicide (FesSi) coatings of composition Fe-16 at% Si and Fe-25 at% Si have been produced by laser surface alloying on pure iron substrates with predeposited silicon powder. The coatings showed excellent adherence without any pores or cracks. They were compositionally and microstructuraly homogeneous, exhibiting two to three times higher surface hardness values than pure iron. High temperature oxidation resistance of the coatings were tested in the temperature range 1173-1273 K in static pure oxygen ( P ~ , = 13.3 k ~ a ) . Their superiority compared to bulk alloys and CVD coatings is discussed.

Protection of Nb- and Ta-based alloys against high temperature oxidation

The European space shuttle HERMES is designed to employ fastening elements made of Nb- or Ta-base alloys in the assembly of hot structures. In order to protect these parts against oxidation during repeated re-entries (projected temperature-time cycle: 1300°C/20 min) protective coatings will have to be employed. We have investigated three classes of oxidation-protective coatings: modified silicide coatings, noble metal coatings and silicon-carbide multilayer systems. Different types of coating processes (electron-beam ion-plating, chemical vapour deposition, slurry deposition) have been adapted to allow uniform deposition of the coatings onto the completely shaped fastening elements. In the case of silicide coatings, different conditions of the heat-treatment used to form the coating have been studied in detail. In order to simulate re-entry conditions, cyclic oxidation tests in air have been carried out at temperatures up to 1450°C and pressures up to 10 5 Pa. The morphology, microstructure and composition of coated specimens have been analysed in all stages of the manufacturing process (as-deposited, heat-treated) as well as after oxidation tests employing optical and electron microscopy, EDX, XRD and microhardness measurements. The best results have been obtained for Si20Cr20Fe-coated fasteners made of Cb752 (Nb10W2·5Zr) which in cyclic oxidation tests (1300°C/20 min/ 10 5 Pa) exhibited lifetimes up to 40 h. Even then, the end of life occurred invariably by way of local flaws. Preliminary tests on assembled fasteners indicate the criticality of the procedure of mounting the coated parts.

Microstructure and Mechanical Properties of Nitrided Molybdenum Silicide Coatings

AbstractMo-Si-N films with a high nitrogen concentration were produced by sputter-deposition in the presence of nitrogen plasma. The chemical composition was determined with Rutherford backscattering and nuclear reaction analysis. The ratio of Mo to Si was 1:2 in the coatings with a nitrogen concentration of 50 %. The microstructure of the as-deposited coatings on a silicon substrate was amorphous and no crystallization was found after annealing up to 1000 °C, although some relaxation was observed in X-ray diffraction. This was confirmed in highresolution transmission electron microscope examination. The hardness of the Mo-Si-N films was 18.8 GPa as determined with a nanoindenter. This is significantly higher than that of MoSi2 films, 11.2 GPa. The hardness of the Mo-Si-N films increased to 24.4 GPa after annealing at 800 °C, which is approximately the same as the hardness of the high-temperature tetragonal phase of MoSi2, 25.5 GPa. Similarly, the modulus of the as-deposited Mo-Si-N film was higher (257 GPa) than that of the MOSi2 film (222 GPa). However, only a slight increase in the modulus of the Mo-Si-N film was found after annealing at 800 °C, whereas the modulus of the crystallized tetragonal MoSi2 was 382 GPa. No cracking was found in the Mo-Si-N films even after annealing at 1000 °C.

CVD Mo, W and Cr oxycarbide, carbide and silicide coatings on SiC yarn

SiC yarn has been coated with molybdenum oxycarbide and tungsten oxycarbide without degrading the fibre. Conversion of the coatings to the metals has been accomplished by heat treatment in nitrogen. These metal coatings have then been converted to suicide coatings by heating in SiCl4. An SiC/WOC minicomposite has been formed by longer deposition of tungsten oxycarbide on a silicon carbide yarn.