Norman L. Hecht

Oxidation behavior of hot isostatically pressed silicon nitride containing Y2O3

Oxidation in the presence of air and water vapor at high temperatures wasstudied for Si3N4 ceramics containing Y2O3 and Al2O3 as sintering aids.The test environments for this study consisted of air with 0, ∼1.2,and 6.4 v/o H2O at temperatures from 1000°C to 1350°C. Theoxidation exposure times were up to 500 hours. The presence of water vaporenhances oxidation and crystallization of the oxidation phases. Weight losswas observed for the oxidation in air or dry air because of Na contaminationduring the fabrication processing. The effect of applied stress on the growthof oxide scale is minimal, however, the applied stress resulted in deeperpenetration of oxygen and pit formation in the oxide phase.

Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y2O3

The effect of water vapor on the mechanical behavior of Si3N4 ceramics was studied. Strength measurements by flexural dynamic fatigue tests were made at temperatures from 1038°C to 1350°C and at actuator speeds of 8.4 × 10−2 and 8.4 × 10−5 mm/s (∼200 and ∼0.2 MPa/s). Step stress rupture tests were also performed at 1288°C and 1150°C. Water vapor had a beneficial effect on the flexural strength due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases. Time-to-failure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores were the primary mechanism for slow crack growth for NT 164 Si3N4.

The Plasma Arc

Plasma is a gaseous cloud composed of free electrons, positive ions, neutral atoms and molecules. It has been referred to as “the 4th state of matter” because of its unique properties. Its behavior involves complex interactions between electromagnetic and mechanical forces.

The Thermal-Mechanical Behavior of Selected Si3N4 and SiC Ceramics

This paper presents the results of a long term program initiated in December of 1984 to investigate the effects of environment (temperature, atmosphere, and stress) on the mechanical behavior of eight Si3N4 and three SiC ceramics being considered for heat engine applications. Microstructure, chemistry, and physical properties were determined. The mechanical behavior of these materials was investigated from room temperature to 1400°C by employing tests for flexural and tensile strength, dynamic, static and cyclic fatigue, and fracture toughness. The results obtained from these evaluations showed that the thermal mechanical behavior was quite varied, depending on the composition and processing methods employed. Batch to batch differences were also found to cause variances in the property values measured. Insights gained from this work about the failure mechanisms and potential service life are also discussed.Copyright

Characterization of Plasma Effluent

The effluent which can be generated by the arc plasma jet is by its very nature well suited to the evaluation of materials in high temperature environments. The various facilities and types of tests which use the plasma for this purpose have been discussed in the previous chapter. All of the tests discussed have a common requirement that the conditions to which the test model is exposed must be well defined.

Material Evaluations Utilizing the Plasma Jet

It is only natural that a device such as the arc plasma jet, which is capable of generating on a continuous basis an effluent which has far higher temperatures and produces higher heat fluxes than any other known technique, should get widespread use in materials testing. Test facilities utilizing the unique capabilities of the plasma jet are in extended use for many different and widely varying types of material evaluations. Among the more common of these are ablation, reentry simulation, thermal shock, thermal stress, dynamic oxidation and rocket exhaust simulation. The techniques and degree of sophistication of these evaluations range from the simplest of screening tests to highly detailed simulations of conditions which would exist in an actual application.

Materials and Applications for Plasma Spraying

Flame sprayed coatings have found wide utilization as protective and decorative coatings. In addition, this process has been effective for fabricating complex thin wall components. Numerous applications for metal, ceramic and organic coatings are described in the literature. A summary of the applications for plasma sprayed materials is presented in Table 5.