MoSi2 Composites Research Articles

Microstructures and compression flow behaviour of a rapidly solidified MoSi2 composite containing HfB2 dispersoids

Microstructures and compression flow behaviour of a rapidly solidified MoSi2 composite containing HfB2 dispersoids

In-Situ Processing of Mosi2-Base Composites

AbstractThree different methods of preparing Mosi2 and composites reinforced with ceramic fibers by reactive in-situ processing are described. Reactive powder sintering (co-synthesis) of elemental powders, chemical vapor infiltration/deposition and reactive vapor infiltration are examined. Monolithic Mosi2, SiC particle-reinforced Mosi2 or fibrous Mosi2 composites reinforced with Nicalon fiber were prepared. The advantages and disadvantages of these processes relative to more traditional processing techniques such as HIPing of prealloyed powders, mechanical alloying and a reported in-situ displacement reactions are discussed.

Fabrication and Testing of Plasma-Spray Formed MoSi2 and MoSi2 Composite Tubes

AbstractPlasma-spray forming has been used to fabricate thick-wall tubes of Mosi2 and Mosi2 containing concentric layers of A12O3. This process is being investigated as a potential fabrication method for producing tubular components of Mosi2 and Mosi2 composites for use in high temperature fuel-burner applications. Results will be reported on the spray forming method used to produce tubes of various sizes. The room temperature strength of pure Mosi2 tubes in the as-deposited condition, and after heat-treating at 1500 °C for 2 hours in vacuum, will also be reported. The strength of plasma sprayed Mosi2 tubes were measured via diametral compression of 0-ring and C-ring sections in air at room temperature. Qualification of the strength distribution was based on Weibull statistical theory.

Role of matrix/reinforcement interfaces in the fracture toughness of brittle materials toughened by ductile reinforcements

Crack interactions with ductile reinforcements, especially behavior of a crack tip at the interface, have been studied using MoSi2 composites reinforced with Nb foils. Effects of fracture energy of interfaces on toughness of the composites have also been investigated. Variation of interfacial bonding was achieved by depositing an oxide coating or by the development of a reaction prod- uct layer between the reinforcement and matrix. Toughness was measured using bend tests on chevron-notched specimens. It has been established that as a crack interacts with a ductile re- inforcement, three mechanisms compcte: interfacial debonding, multiple matrix fracture, and direct crack propagation through the reinforcement. Decohesion length at the matrix/reinforcement interface depends on the predominant mechanism. Furthermore, the results add to the evidence that the extent to which interfacial bonding is conducive to toughness of the composites depends on the criterion used to describe the toughness and that ductility of the ductile reinforcement is also an important factor in controlling toughness of the composites. Loss of ductility of the ductile reinforcement due to inappropriate processing could result in little improvement in tough- ness of the composites.

Plasticity Enhancement of MoSi2 at Elevated Temperatures Through the Addition of TiC

ABSTRACTMonolithic MoSi2 and MoSi2-TiC particulate composites with 10 vol % and 15 vol % TiC were tested in compression between 950°C and 1200°C. The MoSi2-TiC composites can be deformed plastically at lower temperatures than MoSi2 can before brittle fracture occurs. The composites exhibit much lower strain hardening rates and attain zero strain hardening rate at much lower strains than monolithic MoSi2. The differences between the composites and monolithic MoSi2 in plasticity and in strain hardening behavior is attributed to efficient dislocation generation into the matrix from sources at the MoSi2-TiC interfaces.

Nucleation and Growth of MoSi2 Pest During Low Temperature Oxidation

The phenomenon of “MoSi2 pest” has been studied in detail by low temperature oxidation of various polycrystalline MoSi2 monoliths and composites. Single crystalline samples were studied to address the nucleation and growth of pest reaction. Pest of MoSi2 was observed to occur at temperatures between 375 and 500°C. Samples after pest reactions yielded to powdery reaction products of MoO3 whiskers, SiO2 clusters, and residual MoSi2 particles. Although all the samples showed complete disintegration after sufficient oxidation durations were rendered, the kinetics of pest disintegration varied among different samples. By performing interrupted oxidation tests, pest reaction was found to consist of nucleation and growth stages, with the former stage acting as the rate-limiting step. Furthermore, the nucleation of pest reaction was found to be closely related to the formation of MoO3, which was preferentially formed at such defect sites as intergranular boundaries and cracks. Mechanisms leading to the pest disintegration of MoSi2 are discussed in light of the nucleation and growth of pest reaction.

R‐Curve Behavior in Zirconia‐Reinforced Molybdenum Disilicide Composites

R‐curves for hot‐pressed 20 vol% ZrO2 (2.5‐YPSZ and 8‐YPSZ) reinforced MoSi2 composites were evaluated using the indentation/bend technique. Results indicated a modest increase in the material toughness for both composites. Experimental R‐curves were compared with theoretically predicted R‐curves to evaluate the critical transformation stress for the partially stabilized ZrO2 composite. A high critical transformation stress value obtained was indicative of the inhibition of transformation toughening effects due to microstructural inhomogeneity in the partially stabilized ZrO2‐MoSi2 composites.

Hardness and Fracture Toughness of SiC‐Particle‐Reinforced MoSi2 Composites

The indentation technique has been used to evaluate the hardness and fracture toughness of SiC‐reinforced MoSi2 composites made by hot‐pressing. It is seen that the toughness increases with increasing indentation crack length (under increasing load) and a probable mechanism responsible for this behavior is described. It is also observed that there is an optimum volume fraction of SiC particles for which the maximum fracture toughness of the composite can be achieved.

Interface and mechanical behavior of MoSi2-based composites

MoSi2-based composites reinforced with particles, whiskers, and continuous fibers were fabricated using hot pressing and hot isostatic pressing techniques. The microstructure, interface compatability, and interfacial properties between the reinforcements and matrix are discussed. The microstructural parameters which control the mechanical behavior of the MoSi2 composites were characterized. The need for developing a satisfactory reinforced MoSi2 composite for high-temperature structural applications is also addressed.

Spray Forming of MoSi2 and MoSi2-Based Composites

ABSTRACTVacuum plasma spraying has been used to fabricate dense MoSi2 and TiB2-reinforced MoSi2-based composites. The spray-formed deposits have high hardness and fracture toughness as compared to that achieved by other processing techniques. Annealing of the spray formed MoSi2 leads to a decrease in hardness and an increase in fracture toughness. Reinforcement with TiB2 leads to toughening and strengthening of the spray-formed MoSi2.

Development of TiC particle-reinforced MoSi 2 composite

On etudie l'effet de particules de TiC sur la microstructure, les proprietes mecaniques et le comportement a la rupture de MoSi 2 en vue du developpement de materiaux pour hautes temperatures

823. Vapour-phase growth of thin corundum crystals: M M Butcher and E A D WhiteJ. Amer. Ceramic Soc, 48 (9), 1965, 492–493

The corrosion behaviour in closed system of a pressureless sintered ZrB2–20 vol% MoSi2 composite was studied in 1.8 M H2SO4 solution at RT, 40 and 70 °C up to 90 h. The corrosion resistance is quite poor because of the easy leaching of ZrB2 phase. MoSi2 phase and secondary phases such as ZrO2 and Mo5Si3 are less affected by acidic corrosion. After corrosion at RT and 40 °C, both the weight losses and the microstructural features indicate that the corrosion is not uniform through the specimens’ volume. The weight gain data are greatly influenced by the amount of preferential corrosion sites on the samples surfaces (pores or scratches remaining after polishing). At 70 °C, the leaching of ZrB2 is very fast and gas evolution occurs. Pourbaix diagrams indicate that at 70 °C ZrB2 phase is less stable than MoSi2 and the formation of Zr4+ cations.