Silicide Coatings Research Articles

Deposition of boride, nitride, and silicide coatings with desired crystal morphology and perfection

The influence of the relative supersaturation, elastic strain energy of the growing phase, and adhesion energy on the morphology and orientation of boride, nitride, and silicide coatings is estimated using X-ray diffraction, electron microscopy, and secondary-ion mass spectrometry data. A diagram is constructed from which growth conditions for phases with given morphology and structural perfection can be found.

Effect of silicide coatings on the mechanical properties of alloy 5VMTs

The paper examines the effect of silicide coatings and testing media on the strength and plasticity of alloy 5VMTs between 25 and 1100°C. It is established that uncoated samples tested in argon have higher strength and plasticity over the entire temperature range of interest than coated samples tested in argon and air. The major cause of decrease in the mechanical properties of coated samples is the silicide layer whose properties depend on testing temperature.

Formation of Cr-modified silicide coatings on a Ti–Nb–Si based ultrahigh-temperature alloy by pack cementation process

Cr-modified silicide coatings were prepared on a Ti–Nb–Si based ultrahigh temperature alloy by Si–Cr co-deposition at 1250 °C, 1350 °C and 1400 °C for 5–20 h respectively. It was found that both coating structure and phase constituents changed significantly with increase in the co-deposition temperature and holding time. The outer layers in all coatings prepared at 1250 °C for 5–20 h consisted of (Ti,X) 5Si 3 (X represents Nb, Cr and Hf elements). (Ti,X) 5Si 4 was found as the only phase constituent in the intermediate layers in both coatings prepared at 1250 °C for 5 and 10 h, but the intermediate layers in the coatings prepared at 1250 °C for 15 and 20 h were mainly composed of (Ti,X) 5Si 3 phase that was derived from the decomposition of (Ti,X) 5Si 4 phase. In the coating prepared at 1350 °C for 5 h, single (Ti,X) 5Si 3 phase was found in its outmost layer, the same as that in the outer layers in the coatings prepared at 1250 °C; but in the coatings prepared at 1350 °C for 10–20 h, (Nb 1.95Cr 1.05)Cr 2Si 3 ternary phase was found in the outmost layers besides (Ti,X) 5Si 3 phase. In the coatings prepared at 1400 °C for 5–20 h, (Nb 1.95Cr 1.05)Cr 2Si 3 ternary phase was the single phase constituent in their outmost layers. The phase transformation (Ti,X) 5Si 4 → (Ti,X) 5Si 3 + Si occurred in the intermediate layers of the coatings prepared at 1350 and 1400 °C with prolonging co-deposition time, similar to the situation in the coatings prepared at 1250 °C for 15 and 20 h, but this transformation has been speeded up by increase in the co-deposition temperature. The transitional layers were mainly composed of (Ti,X) 5Si 3 phase in all coatings. The influence of co-deposition temperature on the diffusion ability of Cr atoms was greater than that of Si atoms in the Si–Cr co-deposition processes investigated. The growth of coatings obeyed inverse logarithmic laws at all three co-deposition temperatures. The Si–Cr co-deposition coating prepared at 1350 °C for 10 h showed a good oxidation resistance due to the formation of SiO 2 and Nb, Cr-doped TiO 2 scale after oxidation at 1250 °C for 10 h.

Microstructure and High Temperature Oxidation Performance of Silicide Coating on Nb-Based Alloy C-103

The microstructure and oxidation resistance of NbSi2 coating formed on Nb-based alloy C-103 by a pack siliconization process have been studied. The as-formed coating consists of an outer NbSi2 layer and an inner Nb5Si3 layer. A NbSi2–Nb5Si3 two-phase zone is also present between the above two layers. Weight-change data obtained under isothermal and cyclic oxidation in air at 1100 and 1300°C, suggests that the coating gives oxidation protection up to about 4 h. The oxide scale that formed on the coating during oxidation exposure consists of an outer glassy silica layer and an inner Nb2O5-silica mixed layer. Nb2O5 phase is also present in the outer silica scale in the form of elongated particles. Oxidation protection is achieved primarily by the presence of the glassy silica layer on the surface. Spallation of this layer during thermal cycling causes significant reduction in the protective life of the coating.

Structure and oxidation behavior of Si–Y co-deposition coatings on an Nb silicide based ultrahigh temperature alloy prepared by pack cementation technique

In order to improve the oxidation resistance of silicide coatings on Nb silicide based alloys, Y-modified silicide coatings were prepared by co-depositing Si and Y at 1050, 1150 and 1250 °C for 5–20 h, respectively. It has been found that the coatings prepared by co-depositing Si and Y at 1050 and 1150 °C for 5–20 h as well as at 1250 °C for 5 h were composed of a thick (Nb,X)Si 2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X) 5Si 3 inner layer, while the coatings prepared by co-depositing Si and Y at 1250 °C for 10–20 h possessed a thin outer layer composed of (Ti,Nb) 5Si 3 and Ti-based solid solution, a thick (Nb,X)Si 2 intermediate layer and a thin (Nb,X) 5Si 3 inner layer. EDS analyses revealed that the content of Y in the (Nb,X)Si 2 layers of all the coatings was about 0.34–0.58 at.% while that in the outer layers of the coatings prepared by co-depositing Si and Y at 1250 °C for 10–20 h was about 1.39–1.88 at.%. The specimens treated by co-depositing Si and Y at 1250 °C for 10 h were selected for oxidation test. The oxidation behavior of the coating specimens at 1250 °C indicated that the Si–Y co-deposition coating had better oxidation resistance than the simple siliconized coating because the oxidation rate constant of the Si–Y co-deposition coating was lower than that of the simple siliconized coating by about 31%. The scale developing on the Si–Y co-deposition coating consisted of a thicker outer layer composed of SiO 2 and TiO 2 and a thinner SiO 2 inner layer.

Interaction in the MoSi2–W system at 1500 to 1800°C

The paper studies the kinetics of the diffusion redistribution of phases in the MoSi2–W system when tungsten samples with molybdenum silicide coatings are heated in air at 1500–1800°C. It is established that the (Mo x ,W y )5Si3 phase, which represents a molecular solid solution of lower molybdenum and tungsten silicides, forms in an exchange reaction between molybdenum and tungsten at the MoSi2–W5Si3 interface. The MoSi2–W system is much more stable than the WSi2–W and MoSi2–Mo systems.

Molybdenum-based oxidation-resistant MoSi2–Al2O3 and WSi2–Al2O3 coatings

Various studies and comparative tests are conducted on oxidation-resistant unmodified and modified MoSi2–Al2O3 and WSi2–Al2O3 silicide coatings in air at 1800 and 1900°C. Metallographic, x-ray diffraction, electron microprobe, and chemical analyses show that the introduction of alumina into molybdenum and tungsten disilicides improves their hardness and produces fine-grained silicides with high corrosion resistance. The composition with fine alumina particles (O < 0.1 μm) with the total alumina concentration from 3 to 15 wt.% in the coating is most favorable for improving the oxidation resistance of disilicides. It is shown that the high oxidation resistance of MoSi2–Al2O3 and WSi2–Al2O3 coatings is due to the presence of alumina in disilicides, which contributes to the formation of an external high-temperature oxide layer. This layer consists of α-Al2O3 and a mullite sublayer in which Al2O3 crystals are introduced into SiO2.

Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800–1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si 2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X) 5Si 3 transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti) 3Si 5Al 2 phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti) 2Al layer formed in the coating between the (Nb,X) 5Si 3 layer and the substrate, but when aluminizing was performed at 900–1000 °C, the new layer formed was (Nb,Ti)Al 3. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000–1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti) 3Si 5Al 2, (Nb,X)Si 2 and (Nb,Ti)Al 3 was obtained by co-depositing Si and Al at 1050 °C.

Relationship Between Pack Chemistry and Growth of Silicide Coatings on Mo–TZM Alloy

A theoretical model equation has been derived to relate the growth kinetics of silicide coating with the pack chemical composition and other processing conditions for siliconizing of Mo–TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy to improve its oxidation resistance at high temperatures. A series of experiments conducted with varying pack Si and content, time , and temperature confirmed the validity of the model. was the main coating layer formed during the siliconizing process. Optimum processing conditions were derived for doping of Al in to form in the outer layer of the coating.

In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM

Multilayer alumino-silicide and silicide coatings were formed by in situ chemical vapour co-deposition of Al and Si on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy for improving its high-temperature oxidation resistance. MoSi 2 and Mo (Si, Al) 2 layers were formed in the inner and the outer layers, respectively in the case of alumino-silicide coating. Whereas silicide coating consisted of Mo 5Si 3 and MoSi 2 phases in the inner and the outer layers, respectively. 24–100-μm thick coatings were formed by optimizing the pack mixture of Al and or Si, NH 4F and Al 2O 3 powders and conducting the experiments at 1000 °C for 8–36 h. MoSi 2 layer showed a faster growth rate and presence of columnar grains. A small weight gain at the initial stages was observed during the oxidation tests of the coated samples under continuous or cyclic heating at 1300 °C in air. Neither cracks nor peeling of the coating layers were noticed after oxidation tests.

Microstructure and high-temperature wear behavior of laser clad Ni–Ti–Si ternary metal silicide coatings

Ni–Ti–Si ternary metal silicide coatings were fabricated on AISI 304 stainless steel by laser cladding process. The coatings consisted of Ni 16Ti 6Si 7 primary dendrite and interdendritic Fe–Ni–based solid solution γ and exhibited excellent abrasive and adhesive wear resistance under high temperature metallic dry sliding wear conditions. The excellent wear properties were attributed to the high hardness and covalent dominant atomic bond of the metal silicide Ni 16Ti 6Si 7. The dominant wear mechanism of the coating were delamination of the coating and material transfer from the mating surface.

Oxidation Behavior and Pack Siliconized Oxidation-Resistant Coatings of an Nb-Based Ultrahigh Temperature Alloy

The halide-activated pack cementation method was utilized to deposit silicide coatings on a multicomponent Nb-Ti-Si based alloy. The siliconized temperature was 1150 °C and the holding time was 10h. Both the specimens with siliconized coatings and without coatings were oxidized at 1250°C for 5, 10, 20, 50 and 100h respectively. The coating possessed a double layer structure with the composition of (Nb,X)Si2 (X represents Ti, Cr and Hf), and the outer layer was denser. The major structure in the outer layer was composed of columnar crystals perpendicular to the interface between the coating and the substrate, and that in the inner layer was mainly composed of equiaxed crystals. A transitional layer about 5μm thick was found between the coating and the substrate. After oxidation at 1250°C, the major constituents in the scale were SiO2 and TiO2 and the mole ratio of these two phases was about 2:1. The thickness of the (Nb,X)Si2 layer decreased and that of transitional layer increased as the oxidation time prolonged. The siliconized coating exhibited excellent oxidation-resistance at 1250°C within 50 hours.

Synthesis of oxidation resistant silicide coatings on Mo–Si–B alloys

Abstract During Si-pack cementation a MoSi2 outer layer was synthesized on a Mo–Si–B alloy with a three-phase Mo + Mo5SiB2 + Mo3Si microstructure. Following oxidation exposure, the MoSi2 phase layer was replaced by B containing Mo5Si3 (T1) phase in contact with a Mo5SiB2 layer. The synthesized T1 phase provided an excellent oxidation resistance and stability.

Oxidation Resistant Silicide Coatings for Nb&lt;sub&gt;ss&lt;/sub&gt;/Nb&lt;sub&gt;5&lt;/sub&gt;Si&lt;sub&gt;3&lt;/sub&gt; In-Situ Composites

Oxidation protective silicide coatings for the new Nbss/Nb5Si3 in-situ composites were prepared by molten salt method at 900°C. The experiment results indicated that the majority phase in the coating was NbSi2. More Nb5Si3 was formed at the interface between the substrate and NbSi2 layer during the oxidation. The oxidation resistance of the composites was improved by the coating, significantly. The improvement in the oxidation resistance of the composites maybe mainly attributed to the formation of large amount of SiO2 and Al2O3 on surface of coating.

Microstructure, Chemical and Phase Composition of Chromium Silicide Diffusion Coatings on Carbon Steels

The microstructure, chemical and phase composition of chromium silicide diffusion coatings on steels 20, 45, U8, and U10 are studied. It is established that the phases Cr23C6 and Cr7C3 with a silicon content in them of up to 0.25 at.% form at the surface of chromium silicide coatings. The maximum silicon content at 7% in steels 20 and 45 is observed beneath the carbide layer at a depth of 40–50 µm, but for steel U10 it is 1.9% at the carbide layer-matrix boundary. It is shown that the corrosion resistance of chromium silicide diffusion coatings increases markedly compared with uncoated specimens: by a factor of 15–30 depending on the acid used (H2SO4, HCl, H3PO4, HNO3, CH3COOH). There is also an increase in the cavitation resistance of articles with chromium silicide coatings in water and 3% NaCl solution.

Formation of silicide coatings on the surface of a TiAl-based alloy and improvement in oxidation resistance

Abstract A simple and inexpensive method to modify the surface of a TiAl-based alloy by liquid-phase siliconizing has been proposed. After siliconizing in Al-12.87 wt.% Si melt, the surfaces of the TiAl-based alloy were covered with a layer of silicides, composed of ternary TiAlSi and binary TiSi compounds. The thickness of the silicide coatings increase with siliconizing temperature (T) and siliconizing time (t), in +accordance with the parabolic law, and the parabolic rate constant (k) can be described by k = 2.24 × 10−5exp(−107110/RT) m2/s. The results of the isothermal oxidation test show that the silicide coatings exhibit an excellent oxidation resistance. Some of the siliconizing products, Ti7Al5Si12 and two kinds of TiSi compounds, were still preserved after oxidation at 1173 K for 100 h, and Al4Ti2SiO12, TiO2 and Al2O3 as well as SiO2 were formed at the surface of the siliconized sample, but the continuous TiO2 scale or the loose mixture layer of TiO2 and Al2O3 existing in the bare TiAl alloy was not present in the coatings.

Metallic tribological compatibility of laser clad Mo 2Ni 3Si/NiSi metal silicide coatings

Novel wear-resistant Mo 2Ni 3Si reinforced metal silicide coatings were fabricated on a substrate of austenitic stainless steel AISI 321 by laser cladding with Mo–Ni–Si alloy powders. Microstructure of the coating was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDS). Wear resistance of the coatings was evaluated under block-on-wheel dry sliding wear test condition at room temperature. Results indicate that the coatings have excellent wear resistance and tribological compatibility to a metallic counterpart under dry sliding wear test conditions coupling with conventional metallic materials because of the high hardness and strong atomic bonds of the ternary metal silicide Mo 2Ni 3Si with MgZn 2 type Laves crystal structure.

Liquid-phase siliconizing by Al–Si alloys at the surface of a TiAl-based alloy and improvement in oxidation resistance

A simple and inexpensive method to modify the surface of a TiAl-based alloy by liquid-phase siliconizing by Al–Si alloys, has been proposed. The Al–12.87wt%Si alloy and the Al–10wt%Si alloy resulted in the main siliconizing products of Ti 7Al 5Si 12 and Al 12Si 3Ti 5, respectively. The increase of the thickness of the silicide coatings obtained by the Al–10wt%Si alloy obeyed the parabolic law, and the parabolic rate constant ( k) can be described as k=5.266 × 10 −7 exp(−115 379/RT) m 2/s. A protective SiO 2-based scale and alumina-based scale were formed at the outermost surface during isothermal oxidation at 1273 K from the Ti 7Al 5Si 12 and Al 12Si 3Ti 5, respectively. In particular, a remarkable effect on the improvement in the oxidation resistance at 1273 K was achieved by the siliconizing condition of 1053 K/20 min + 953 K/20 min using Al–10wt%Si alloy, with a low K p value of 0.7216 g 2/m 4 h, only about 1/1050 of that of the bare alloy. The results of cyclic oxidation test of 1173 K/100 h followed by 1273 K/100 h showed that the alumina-based scale had a good adherence with the inner partially-oxidized silicide coating. The diffusion reactions between the silicide coating and the TiAl substrate were also discussed.

Silicide coatings for protection of niobium base alloys : V. Terentieva. (Moscow Aviation Inst., Moscow, Russia.)

Silicide coatings for protection of niobium base alloys : V. Terentieva. (Moscow Aviation Inst., Moscow, Russia.)

Co-deposition of aluminide and silicide coatings on γ-TiAl by pack cementation process

Thermochemical analyses were carried out for a series of pack powder mixtures for deposition of aluminide and for co-deposition of aluminide and silicide coatings on γ-TiAl by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for depositing adherent and coherent aluminide and silicide coatings. Pack powder mixtures activated by 2 wt% AlCl3 was used to aluminise γ-TiAl at 1000°C. With proper control of pack compositions and coating conditions, an aluminide coating of TiAl3 with a coherent structure free from microcracking was deposited on the substrate surface via inward diffusion of aluminium. The results of thermochemical calculations indicated that co-deposition of Al and Si is possible with CrCl3 · 6H2O and AlCl3 activated pack powders containing elemental Al and Si as depositing sources. Experimental results obtained at 1100°C revealed that CrCl3 · 6H2O is not suitable for use as an activator for co-depositing aluminide and silicide coatings on γ-TiAl. It caused a significant degree of degradation instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer TiSi4 layer and an inner TiAl3 layer were successfully co-deposited at 1100°C and 1000°C using pack powder mixtures activated by AlCl3. IT is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and silicon. Discussion is presented on the issues that need to be considered to ensure the deposition of aluminide and silicide coatings with coherent structure free from microcracking on γ-TiAl by the pack cementation process.