We have investigated the effect of thin films of ZrO2 on the mechanical behavior of [001] single-crystalline MoSi2. Previous work in our laboratories has shown there to be a 20-40% reduction in hardness in coated samples relative to the corresponding uncoated samples. We now compare fracture around the indentations as a function of thickness. Thicker ZrO2 coatings reduce radial crack length around the indentation through the intrinsic compressive stress at the film/substrate interface. In addition, ZrO2-coated materials tested in compression at 1250°C exhibit approximately twice the plastic strain of uncoated materials, indicating that reduction in hardness is associated with enhanced plasticity from dislocation activity associated with the film/substrate interface.

Substantial improvements have been reported in high temperature strength and creep resistance, and room temperature fracture toughness of molybdenum disilicide (MoSi2) reinforced with ductile or brittle reinforcements. The influence of Hot Isostatic Pressing (HIP) process parameters on the mechanical properties of MoSi2 based alloys was studied. Monolithic MoSi2 powder and MoSi2 powder blended with either niobium powder or silicon carbide whisker reinforcements were consolidated by HIP at 1200 − 1400°C, 207 MPa, and 1 - 4 hrs. The HIP'ed compacts were characterized for compression strength and creep resistance at 1100-1300°C. Fracture toughness was measured on single edge notched rectangular specimens at room temperature. Mechanical properties were correlated with post-HIP microstructural features.

Using a cylindrical indenter, the indentation creep behavior of hot pressed and HIPed MoSi2-SiC composites containing 0-40% SiC by volume, was characterized at 1000-1200°C, 258-362 MPa. The addition of SiC affects the creep behavior of MoSi2 in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi2 grains; by obstructing or altering both dislocation motion and grain boundary sliding; and by increasing the overall yield stress of the material. Comparisons are made between indentation and compressive creep studies. It is shown that under certain conditions, compressive creep and indentation creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the indenter.

We studied the creep rupture of a series of MoSi2 materials reinforced with SiC particles. Particulate contents were in the range of 0 to 40 volume percent. Temperature and stress ranges were 1050°C to 1200°C and 10 MPa to 50 MPa, which gave failures ranging from 1 hour to 1500 hours. The creep curves show an extensive tertiary regime, accounting for 25-95% of the total lifetime. Tertiary creep increases with increasing stress and temperature. Cavitation occurs throughout the creep life, and tertiary creep is associated with the linkage of cavities into large cracks. The creep life improves with increasing SiC particle content, with a concurrent loss of creep ductility. Significant improvement occurs only when the particle content is greater than 30 volume percent. Our studies suggest that the creep and creep rupture behavior of MoSi2 can be further improved by increasing the content of SiC particles.

High purity molybdenum disilicide was deformed in compression to strains ranging from 5 to >50%. The deformation was accomplished at temperatures in the range 1200°-1400°C and at strain rates from 10−3 to 10−5 sec−1. The strength of this high purity material was found to be at least twice that of MoSi2 produced by the hot pressing of commercial powder. Microstructural examination revealed that subgrain formation resulted from modest strains (≈10%) while dynamic recrystallization was observed following large strains. Transmission microscopy revealed a significant change in the dislocation substructure after straining as the temperature was increased from 1300°C to 1400°C.

Monolithic MoSi2 and MoSi2 reinforced with SiC particulate reinforcements have been produced with widely varying grain sizes. Monotonic compression behavior of these materials has been examined in the temperature range of room temperature to 1200°C. Based on these results, the ductile-to-brittle transition temperature at low deformation rate of MoSi2 and its composite appears to be near 900°C and may decrease slightly with decreasing grain size. Tension and compression creep behavior of these materials were also studied between 1100 and 1400°C. Creep strength of the matrix decreases with decreasing grain size even in the nominally dislocation climb-glide regime. While 20 vol% SiC particulates provide a reasonable amount of creep strengthening of the fine grain MoSi2 matrix, strengthening due to coarser grain size can be significantly higher in these materials. These results parallel those on other intermetallics.

The mechanical behavior of polycrystalline MoSi2 tested in compression at temperatures from 750°C to 950°C has been investigated. This material can be deformed to several percent strain without fracture at temperatures as low as 900°C. This same material prestrained at 1300°C and subsequently tested at lower temperatures can be deformed to substantial strain at 800°C, and even exhibits modest plasticity at 750°C. Both prestrained and unprestrained materials exhibit microcracking combined with dislocation substructures containing high densities of dislocations. Unprestrained MoSi2 exhibits a yield point in this temperature range which does not exist when it is prestrained. The stress-strain curves of the prestrained material in this temperature range are similar qualitatively to those of the unprestrained material at 1100°C and above. These observations suggest that at these temperatures MoSi2 is a dislocation density limited material which can deform by dislocation plasticity processes if a sufficient dislocation density is available.

We have calculated the mechanical properties of brittle composites with spring-network (SN) model. The composites that we studied involve the transformation toughening effects and the accompanying micro-cracking. Our simulation results are consistent with experiments of MoSi2 toughened with ZrO2. By monitoring the stress changes due to the transformation and micro-cracking we are able to separate, for the first time, the contributions from these two competing effects. We also found that the fracture toughness of the composite increases as the modulus, interfacial cohesion of particle increases.

The zirconium silicide intermetallic compounds with the 16H crystal structure are good materials for high temperature structural applications, because of their high melting point and low density. Their shortcoming is low ductility. To increase ductility, the coordination numbers are increased to increase isotropy of bondings, and to have more metallic nature of bonds.

Combination of elements, silicon, zirconium and other atoms for ternary silicides are selected by theoretical consideration of the coordination numbers. As the coordination numbers increase, the atomic bonds increase metallic properties, and their ductility increases. Higher coordination numbers are obtained by substituting yttrium atoms onto zirconium 6g point set sites.

The factors which control the formation of a protective silica (SiO2) layer on structural silicides at high temperature are summarized. The thermodynamic and kinetic conditions under which both silica and a metal oxide can form are also described. Molybdenum disilicide (MoSi2) forms highly protective silica scales and has the best oxidation resistance at high temperatures. Although the preparation method has little influence, the heating rate and the structure of the silica layer have significant effects on the oxidation behavior of MoSi2.

The effect of Nb morphology on the 1200°C cyclic oxidation resistance of MoSi2/2Ov%Nb composites was investigated. Niobium was incorporated into MoSi2 as particles, random short fibers and continuous aligned fibers. After oxidation, it was found all that the composites had lost weight and essentially disintegrated. This was due to spalling of both the Nb2O5 scale and the MoSi2 matrix. The spalling of the matrix was a result of cracks originating from the oxidized Nb and propagating through the MoSi2 matrix. These cracks arose from two sources: (1) the volume expansion associated with the transformation of Nb to Nb2O5 and (2) the difference in thermal expansion between Nb2O5 and MoSi2. Composites with smaller diameter Nb reinforcements did not disintegrate as rapidly as composites with larger sized Nb reinforcements. This was attributed to the effect of reinforcement size on CTE mismatch cracking.

Induction heating of disk shaped specimens was used to study thermal fatigue behavior of hot pressed MoSi2, hot pressed and hot isostatically pressed (HIPed) MoSi2, and a MoSi2-10 vol% TiC particulate composite. Specimens were subjected to 5 s, 30 s, and 300 s heating and cooling cycles between temperature limits of 700°C and 1200°C. Specimens of both the hot pressed and the hot pressed and HIPed polycrystalline MoSi2 material failed by transgranular cracking on the initial temperature ramp of the 5 s and 30 s cycles. Only when the severity of the thermal cycle was reduced (300 s heating/300 s cooling) was thermal fatigue cracking avoided in the HIPed MoSi2. In sharp contrast, the MoSi2-10% TiC composite remained uncracked when subjected to the severe 30 s heating/30 s cooling cycle. The fatigue results are discussed with reference to the initial microstructure of the specimens and the stress-strain history of the specimens which was obtained by a thermoelastic finite element analysis.

The stability of MoSi2 was studied at 1600°C in combustion products with an incoming gas to air ratio of 1:6.7 and compared to results in a 1:10 environment. Oxidation was investigated using periodic weight measurements and characterization using XRD, SEM, and EDS. Passive oxidation was observed; MoSi2 was oxidized by H2O and CO2 to form Mo5 Si3 and SiO2. The amorphous silica product formed a surface layer and reduced the oxidation rate as it coarsened. MoO3(g) did not form which was in agreement with the thermodynamic (SOLGASMIX-PV) prediction that it would only form in the presence of molecular oxygen. A good agreement was observed between the measured and calculated weight gains based on the surface layer thickness.

Electrochemical corrosion of Mo and MoSi2 in a molten soda-lime-silicate glass environment was studied using a two electrode cell with Pt as the counter electrode. XRD, SEM, and EDS were used to characterize the corroded specimens. When Mo was given a cathodic bias (∼ 0.8V), the specimen surface oxidized to MoO2(s). With reverse bias, Mo ions were interpreted to have been removed into the glass melt. For MoSi2, Mo5 Si3 interfacial layers were observed for both cathodic and anodic corrosion. Mo5 Si3 debris particles (∼4 µm) were seen floating into the glass en masse under a cathodic bias, but not when the bias was reversed. This was interpreted to be a result of an inherent negative charge of debris particles.

Mo-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.

High-temperature silicides have been used in mechanical structures, heating elements, and electronic, CMOS applications. Their stability in high temperature oxidizing environments and excellent electrical conductivity may also make them useful as high temperature thin-film sensors in harsh environments. We have investigated sputter deposited MoSi2, ReSi2, TaSi2, TiSi2, and WSi2 thin films and characterized their performance as thermoelements and stability up to 1200 °C. A multilayer technique was developed to ensure constant silicide stoichiometry during oxidation thereby maintaining a constant Seebeck coefficient. In addition techniques were developed to suppress the formation of metal oxides from the silicides. The results indicated excellent stability of Seebeck coefficient up to 1200 °C. These results are compared with the problems of thin film instability in the Seebeck coefficient found in noble metal thermocouples. Potential applications for temperature and heat transfer measurements will be discussed.

Compositionally tailored MoSi2/SiC composites with silicon carbide content ranging from 0 to 60 volume percent were synthesized through a novel processing scheme involving the mechanical alloying of elemental molybdenum, silicon, and carbon. The effects of important processing parameters such as the nominal powder composition and the processing temperature on the microstructural evolution during mechanical alloying and subsequent heating are described based on the results obtained from DTA and XRD.

For the application of immersed combustion-radiant tubes in glass melters, a Mo-MoSi2 functionally gradient composite was developed. A gradient profile mixture with varying amounts of powders of these two compounds was hot pressed at 1650°C and 17.4 MPa. Mo5Si3, and Mo3Si reaction products were detected by XRD in the hot pressed composite. Limited cracking was apparent in regions where pure Mo regions applied tensile stresses to regions rich in Mo3Si product. Cyclic heat treatments in a reducing atmosphere showed no signs of further CTE mismatch induced cracking, but did indicate a propensity to creep under load at ∼ 1400°C. A functionally gradient material with MoSi2 on all exterior surfaces showed excellent oxidation protection to the Mo center.

This paper reviews the problems that have limited the applications of refractory metals and their alloys. It also presents examples from recent work that has been done to overcome these problems. The work discussed concerns the refractory metals niobium, tantalum, molybdenum, and tungsten and alloys based on these elements. Recent results on intermetallic compounds that contain a refractory metal and composites that are made of refractory metals and their intermetallics are also presented.

Refractory metal alloys based on Mo, W, Re, Ta, and Nb (Cb) find applications in a wide range of aerospace applications because of their high melting points and high-temperature strength. In this paper, we present recent progress in the understanding and applications of these alloys. Recent studies to improve the oxidation and mechanical behavior of refractory metal alloys, and particularly Nb alloys, will also be discussed. Some Re structures, for extremely high temperature applications (> 2000°C), made by CVD and P/M processes, are also illustrated. Interesting work on the development of new W alloys (W-HfC-X) and the characterization of some commercial refractory metals, e.g., K-doped W, TZM, and Nb-l%Zr, continues. Finally, recent developments in high temperature composites reinforced with refractory metal filaments, and refractory metal-based intermetallics, e.g., Nb3A1, Nb2Be17, and MoSi2, are briefly described.