Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C. Part I: Literature review

In the first part of this paper the results of a literature review are presented. An overview of the oxidation behaviour in air and in combustion environments of both pure MoSi₂ and MoSi₂ composites in the temperature range from 400 to 1650°C is given. The second part of this paper reports about our results from oxidation tests with selected MoSi₂ composites (containing 15 vol.‐% Al₂O₃, Y₂O₃, ZrO₂, HfO₂, SiC, TiB₂, ZrB₂, or HfB₂, respectively) from different development stages at temperatures in the pest region as well as up to 1700°C. The third part describes the oxidation behaviour of the optimised MoSi₂ composites developed on the basis of the results from part II.     

Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C. Part II: Initial screening of the oxidation behaviour of MoSi2 composites

In the first part of this paper a literature survey on the oxidation behaviour of MoSi₂ and MoSi₂ composites has been given. The present second part reports about the experimental results from oxidation tests with several MoSi₂ composites containing 15 vol.‐% Al₂O₃, Y₂O₃, ZrO₂, HfO₂, SiC, TiB₂, ZrB₂, or HfB₂, respectively, from different development stages. The tests were conducted at 1600°C in air for 100 hours. It was shown that the production route of the powders has a significant influence on the performance of the materials at high temperatures. This was attributed to contaminations. Additionally, the results indicated that only the composites with SiC, ZrO₂ or HfO₂, respectively, are suitable for application above 1500°C. A more detailed report about the optimised material will follow in the third part of this paper.     

Materials for temperatures above 1500°C in oxidizing atmospheres. Part II: Experimental results on the oxidation of MoSi2 composites and coated RSiC

The topic “Materials for temperatures above 1500°C in oxidizing atmospheres” is discussed in a 3 part publication. In the first part a literature survey had been performed leading to the conclusion that either particle or fiber reinforced MoSi₂‐based materials or RSiC coated with a MoSi₂‐based layer are suitable for applications at such high temperatures. In the present part such material systems are investigated experimentally at 1500 and 1600°C with respect to their oxidation resistance in air. Particular interest was given to the influence of the Mo₅Si₃ content in MoSi₂ and the influence of second phase particles in MoSi₂ consisting of ZrB₂ and SiC. Furthermore, the oxidation resistance of several dip‐coatings on RSiC which were manufactured from a polysiloxane precursor by subsequent pyrolysis were also investigated. Part III which reports about “contact corrosion” between different materials at these temperatures will follow in the next issue.     

MoSi2 oxidation resistance coatings for Mo5Si3/MoSi2 composites

In order to improve the oxidation resistance properties of 30 at.% Mo₅Si₃/MoSi₂ composite at high temperature in air, a molybdenum disilicide coating was prepared on its surface by a molten salt technology. XRD and SEM analysis showed that only tetragonal MoSi₂ phase existed in the coating after being siliconized for 5 h at 900°C. The oxidation film formed on the uncoated sample was not dense, so that oxygen diffused easily through it. The volatilization of MoO₃ resulted in the oxidation film separating from the substrate. The MoSi₂ coating was proved to be an effective method to prevent 30 at.% Mo₅Si₃/MoSi₂ composites from being oxidized at 1200°C. A dense glassy SiO₂ film was formed on the MoSi₂ coating surface, which acted as a barrier layer for the diffusion of oxygen atoms to the substrate. The 30at.% Mo₅Si₃/MoSi₂ composites with a MoSi₂ coating showed much better oxidation resistance at high temperature.     

Materials for temperatures above 1500°C in oxidizing atmospheres. Part III: Contact reaction tests

The topic “Materials for Temperatures above 1500°C in Oxidizing Atmospheres” is discussed in a 3 part publication. In the first part [1] a literature survey had led to two potential material systems consisting either of particle reinforced MoSi₂ or of a MoSi₂ based oxidation resistant coating on creep resistant RSiC. Both systems were verified experimentally in a second publication [2] including the development of optimum oxidation resistant MoSi₂ composite materials and of an optimum coating system on RSiC. The present part deals with the contact corrosion behavior of these silica formers if other refractory materials are used in combination with these material systems as it is often the necessity in practical applications. Therefore, the contact reactions of MoSi₂ with 10 other high temperature oxide based structural materials were investigated at 1600°C in air. The results are mainly documented by microprobe investigations with respect to the contact reaction products formed. It was found that only two material combinations are conceivable for long‐term use in contact with MoSi₂. The results are in agreement with the considerations discussed in part one of this publication [1].     

Oxidation of SiOC/MoSi2/SiC Composites Prepared by Polymer Pyrolysis

From the pre-ceramic polymer of polymethylsiloxane (PMS) and powders of MoSi₂, SiC and Si, new ceramic composites that consisted primarily of an amorphous SiOC matrix containing dispersed particles of MoSi₂ and SiC were synthesized. The composites displayed superior oxidation resistance at both high and low temperatures by forming SiO₂ on the surface. The thin, amorphous SiO₂ layer that formed initially gradually to crystallized during oxidation between 1000 and 1300°C. An outer highly porous and an inner superficial SiO₂ layer that formed from the initial stage of oxidation between 400 and 500°C protected the composites from pesting.     

The corrosion behavior of several heat resistant materials in air + 2% Cl2 at 300 to 800 °C. Part 3 – Alumina formers and intermetallics

The corrosion behavior of Alloy 214 and the intermetallics Fe₃Al, TiAl and MoSi₂ was investigated in dry air and air with 2% Cl₂ at temperatures of 300 to 800 °C. The results show that corrosion resistance of Alloy 214 and the two aluminides very much depend on the test temperature. For TiAl massive corrosion starts at 500 °C. Alloy 214 shows corrosion rates similar to chromia formers at temperatures of 650 and 800 °C while the best corrosion resistance of all alumina formers tested is revealed by Fe₃Al. For MoSi₂ only some little (“internal”) oxidation is observed even at 800 °C.     

Oxidation behavior of molybdenum silicides and their composites

A key materials issue associated with the future of high-temperature structural silicides is the resistance of these materials to oxidation at low temperatures. Oxidation tests were conducted on Mo-based silicides over a wide temperature range to evaluate the effects of alloy composition and temperature on the protective scaling characteristics and pesting regime for the materials. The study included Mo₅Si₃ alloys that contained several concentrations of B. In addition, oxidation characteristics of MoSi₂–Si₃N₄ composites that contained 20–80 vol.% Si₃N₄ were evaluated at 500–1400°C.     

Degradation of strength properties and its fracture behaviour of TiB2-TiC-based composite ceramic cutting tool materials at the high temperature

Strength retention is important for tool materials at high temperature because cutting temperature in machining is ranged from room temperature to 1000 °C. A study examining the strength properties and fracture behaviour of TiB₂-TiC-based composite ceramic cutting tool materials is presented at different temperatures. MoSi₂ and SiC additives are considered to investigate their effects on the density, microstructure, strength and failure mechanism of composites. It is found that the addition of SiC contributed more to the high-temperature strength of composites than MoSi₂, but it did not improve the room-temperature strength, despite grain refinement. The TBAVS8 composite has a flexural strength of 800 MPa at room temperature and can retain 75% at 900 °C. At room temperature, the fracture behaviour of composites was dominated by the strong bonding of the Ni binder phase. At high temperatures, the softer Ni binder phase was pinned, and its sliding was inhibited by SiC particles, which decelerated the strength degradation.     

Comparative studies of the oxidation of MoSi2 based materials: Low-temperature oxidation (300–900 °C)

Molybdenum disilicide (MoSi₂) rapidly oxidizes at 400–600 °C, which given enough time, can lead to its disintegration. Above 1000 °C, MoSi₂ exhibits better oxidation resistance due to the formation of a continuous SiO₂ layer (or alumina layer for the materials doped with aluminum). However, during high-temperature service, the protective layer on MoSi₂ could be damaged, e.g. due to erosion, volatilization, and micro-cracks in thermal cycling, or due to exposure to reducing atmospheres. In this study, the oxidation characteristics of MoSi₂ based materials were investigated in air, with the pre-oxidized protective layer removed to simulate such surface damages. Five different, commercially available, MoSi₂ based heating elements, i.e. Kanthal Super (labelled by the manufacturer as KS-1700, KS-1800, KS-1900, KS-ER and KS-HT) were exposed to 300 to 900 °C isothermally, for 12 to 240 h, and their mass changes determined. Scanning Electron Microscopy, Energy-dispersive X-ray spectroscopy, and X-ray diffraction analyzed the microstructure, chemical composition and phase composition of the oxidized samples. It was found that the oxidation behavior of the different materials under investigation depended strongly on their chemical and phase composition, exposure time and temperature. KS-ER, KS-1800 and KS-1700 showed better resistance against the low temperature (300 to 900 °C) degradation for up to 240 h, while KS-HT and KS-1900 underwent significant degradation after 240 h of air exposure within the same temperature range. For rapid comparison of the materials damage sensitivity we propose a novel Cumulative Mass vs Temperature Index CMTI, including both mass gain and loss at temperatures ranging from 300 to 900 °C. The index allows quick ranking of the materials under consideration.     

SiC–MoSi2/ZrO2–MoSi2 coating to protect C/C composites against oxidation

To improve the oxidation resistance of carbon/carbon (C/C) composites in air at high temperatures, a SiC–MoSi₂/ZrO₂–MoSi₂ coating was prepared on the surface of C/C composites by pack cementation and slurry method. The microstructures and phase compositions of the coated C/C composites were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The result shows that the SiC–MoSi₂/ZrO₂–MoSi₂ coating is dense and crack-free with a thickness of 250–300 μm. The preparation and the high temperature oxidation property of the coated composites were investigated. The as-received coating has excellent oxidation protection ability and can protect C/C composites from oxidation for 260 h at 1773 K in air. The excellent anti-oxidation performance of the coating is considered to come from the formation of ZrSiO₄, which improves the stability of the coating at high temperatures.     

Microstructure Characteristics and Oxidation Behavior of Molybdenum Disilicide Coatings Prepared by Vacuum Plasma Spraying

In this study, molybdenum disilicide (MoSi₂) coatings were fabricated by vacuum plasma spraying technology. Their morphology, composition, and microstructure characteristics were intensively investigated. The oxidation behavior of MoSi₂ coatings was also explored. The results show that the MoSi₂ coatings are compact with porosity less than 5%. Their microstructure exhibits typical lamellar character and is mainly composed of tetragonal and hexagonal MoSi₂ phases. A small amount of tetragonal Mo₅Si₃ phase is randomly distributed in the MoSi₂ matrix. A rapid weight gain is found between 300 and 800 °C. The MoSi₂ coatings exhibit excellent oxidation-resistant properties at temperatures between 1300 and 1500 °C, which results from the continuous dense glassy SiO₂ film formed on their surface. A thick layer composed of Mo₅Si₃ is found to be present under the SiO₂ film for the MoSi₂ coatings treated at 1700 °C, suggesting that the phenomenon of continuous oxidation took place.     

Oxidation behavior of MoSi2 and Mo(Si,Al)2 coated Mo–0.5Ti–0.1Zr–0.02C alloy

MoSi₂ and Mo(Si, Al)₂ coatings were prepared on Mo–0.5Ti–0.1Zr–0.02C alloy using pack cementation process. Oxidation studies revealed that Mo(Si, Al)₂ coating had a much superior oxidation resistance in the temperature range from 400 to 900 °C, where pest disintegration of MoSi₂ occurs due to internal oxidation. The growth kinetics of Al₂O₃ layer formed on Mo(Si, Al)₂ coating was much slower than that of SiO₂ layer formed on MoSi₂ coatings during oxidation.     

Preparation and high temperature oxidation behavior of refractory disilicide coatings for γ-TiAl intermetallic compounds

Novel refractory disilicide layers were applied to γ-TiAl to enhance oxidation resistance at 1050 °C. NbSi₂ and MoSi₂ layers were prepared by joining thin Nb and Mo foils to γ-TiAl surfaces, and siliconizing the combinations (Nb/γ-TiAl, and Mo/γ-TiAl) using molten salts. The coatings and their oxidation behavior were characterized using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. Isothermal oxidation tests showed that the oxidation resistance of uncoated γ-TiAl at 1050 °C in air was insufficient, and scale spallation occurred. NbSi₂ coatings were formed and adhered firmly to the γ-TiAl substrate, whereas Mo film detached from the substrate surface causing failure of the MoSi₂ coatings. Oxidation of the NbSi₂-coated γ-TiAl (NbSi₂/Nb/γ-TiAl) at 1050 °C in air showed improved oxidation resistance at exposure times up to 100 h. Microstructural and compositional developments of the coating at prolonged time were discussed. The NbSi₂ coatings provided sufficient oxidation resistance for γ-TiAl at 1050 °C in air, and have potential use in high temperature applications.     

Research on oxidation and embrittlement of Intermetallic Compounds in the U.S.

Research on the oxidation behavior of intermetallic compounds has been conducted in the U.S. for many years. However, until about ten years ago, this work focusses on the compounds which are important in Ni-base superalloys and their coatings: mainly Ni₃Al and NiAl More recent work has been directed at systems which may be used in monolithic form or as the base for composites. Work has concentrated on three types of systems: Ni- and Fealuminides, refractory metal compounds, and titanium aluminides. Work on the Ni- and Fe-aluminides has concentrated mainly on adherence problems and some anomolous behavior. Work on the refractory metal compounds, particularly MoSi₂ and NbAl₃, has dealt with the problem of selectively oxidizing Al or Si from a refractory metal base and various intermediate-temperature forms of degradation, such as “pesting”. It has become increasingly more clear that, for a number of reasons, the titanium aluminides will be the first “new” metallic materials introduced into commercial high temperature applications (probably aircraft gas turbines and automobiles engines) in many years. As a result a very large amount of work is being done on the oxidation behaviour of these compounds. Initial work dealt with oxidation mechanisms at temperatures on the order of 1000°C. However, both oxidation and mechanical property considerations dictate that the alloys will not be used at temperatures much above 750°C. Therefore, current work is being focussed on oxidation mechanisms at lower temperature and on what may be the “Achilles heel” of these materials, environmental embrittlement. This paper summarizes the work being done in the U.S. and highlights work on what the author believes are the more important problems.     

High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate

The two-layer MoSi₂/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200 °C for 100 h and at 1300 °C for 80 h were 0.270 and 0.499 mg/cm², respectively. Compared with the monolithic MoSi₂ coatings, the transformation of MoSi₂ phase in the MoSi₂/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo₅Si₃ layer in the composite coatings was two orders of magnitude lower than that of the Mo₅Si₃ layer in the monolithic coatings at 1300 °C. The microstructural degradation of MoSi₂ in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi₂ coatings at high temperatures.     

Molybdenum silicide based materials and their properties

Molybdenum disilicide (MoSi₂) is a promising candidate material for high temperature structural applications. It is a high melting point (2030 °C) material with excellent oxidation resistance and a moderate density (6.24 g/cm³). However, low toughness at low temperatures and high creep rates at elevated temperatures have hindered its commercialization in structural applications. Much effort has been invested in MoSi₂ composites as alternatives to pure molybdenum disilicide for oxidizing and aggressive environments. Molybdenum disilicide-based heating elements have been used extensively in high-temperature furnaces. The low electrical resistance of silicides in combination with high thermal stability, electronmigration resistance, and excellent diffusion-barrier characteristics is important for microelectronic applications. Projected applications of MoSi₂-based materials include turbine airfoils, combustion chamber components in oxidizing environments, missile nozzles, molten metal lances, industrial gas burners, diesel engine glow plugs, and materials for glass processing. In this paper, synthesis, fabrication, and properties of the monolithic and composite molybdenum silicides are reviewed.     

Rapid solution hardening at elevated temperatures by substitutional Re alloying in MoSi2

The mechanical properties of solidification processed polycrystalline MoSi₂ and ternary (Mo, Re)Si₂ alloys were evaluated by compression testing at elevated temperatures. Rhenium is found to be a potent solid solution hardening addition to C11_b MoSi₂ at temperatures up to 1600°C (highest temperature used in the study). Dislocation microstructures, characterized by electron microscopy, are consistent with the significant hardening exhibited by Re containing alloys. The high hardening rate cannot be explained by the classical substitutional solid solution hardening theories for metals based on atomic size misfit and elastic moduli mismatch. Since rhenium “disilicide” is semiconducting and has a Si-deficient stoichiometry of ReSi_1.75, the addition of Re to MoSi₂ may lead to the formation of constitutional Si vacancies which may pair with Re substitutionals to form point defect complexes. A model that describes the elliptical strain field (tetragonal distortion) around these point defects is used to interpret the rapid hardening by Re in MoSi₂. Small additions of Re may provide the necessary high temperature strength in MoSi₂-based structural intermetallic alloys for very high temperature applications (∼1200–1600°C).     

Materials for temperatures above 1500°C in oxidizing atmospheres. Part I: Basic considerations on materials selection

In the first part of this paper the results of a literature review are presented. Here we provide an overview of the materials that are suitable for long term service in oxidizing atmospheres at temperatures of 1500°C and higher. Criteria are a sufficient stability against the surrounding environment, low vapor pressure of the material or the reaction product formed and sufficient creep resistance. A further aspect is that reactions between contacting materials of different compositions should not lead to the formation of low melting eutectics at the contact areas. The second part reports about results from thermogravimetric measurements on the oxidation resistance of several potential materials. The third part deals with the experimental investigations concerning the “contact‐corrosion” of selected high‐temperature resistant materials (Al₂O₃, ZrO₂, CeO₂, La₂O₃, Y₂O₃, HfO₂, and the Spinel (MgO · Al₂O₃)) in direct contact with a SiO₂‐scale forming material (MoSi₂) at 1600°C in air, since most of the potential materials for such high temperatures are silica formers. Parts II and III will appear in the next 2 issues of this journal.     

Oxidation behavior of ZrB2 composites doped with various transition metal silicides

This paper deals with the oxidation behavior of ZrB₂-based composites sintered with different additives, namely ZrSi₂, MoSi₂, TaSi₂ and WSi₂. The oxidation mechanisms were investigated between 1200 and 1800 °C for 15 min in a bottom loading furnace. The scope of this study is to draw a classification of goodness for the 4 composites depending on the temperature range and understand how each cation influences the oxidation behavior of ZrB₂ by acting either on glass or on ZrO₂ modification. MoSi₂ was the best additive for improving the oxidation resistance of ZrB₂, even up to 1800 °C.