Keith R. Karasek

Solid particle erosion of a graphite-fiber-reinforced bismaleimide polymer composite

Abstract Erosion rate measurements and scanning electron microscopy observations were made for the steady state erosion of a graphite-fiber-reinforced bismaleimide polymer composite and compared with results obtained on the unreinforced matrix polymer. For erodents with high kinetic energies, the material removal rate of the composite is dominated by the brittle graphite fibers, resulting in erosion rates much higher than those of the matrix polymer.

Solid-particle erosion of bismaleimide polymers

Abstract Solid-particle erosion of bismaleimide (BMI) polymers of various compositions was studied and compared with measured mechanical properties. For angular aluminum oxide erodents of mean diameter 42, 63, 143 or 390 μm impacting normal to the surface at 60 m s −1 , the erosion rate was found to be a strong function of the size of the impacting particle. Material removal occurred primarily by a process of brittle-fracture-induced damage. However, the results suggest that for the BMI specimens, degradation and plasticity occurred in addition to fracture, and that the occurrence of these phenomena reduced the erosion rates.

Composition and microstructure of silicon carbide whiskers

The bulk and surface chemistries of four sets of commercially available SiC whiskers made by three manufacturers were determined. The oxygen content varied significantly, ranging in the bulk from 1.9 to 0.6 at.% and on the surface from 35 to 15 at.%. Surface analysis as obtained by X-ray photoelectron spectroscopy also indicated that the oxygen species differed significantly with whisker supplier; each of three of the whisker sets contained a surface species that is very similar to that found in a Si-O-C glass, while one whisker surface appeared to have a silica-rich surface. Surface carbon concentrations varied significantly, while silicon concentrations did not. Scanning transmission electron micrographs indicate significant morphological variations (i.e., twinning, branching, kinks, surface roughness, etc.) occur in all of the whisker types.

CHARACTERIZATION OF RECENT SILICON CARBIDE WHISKERS

SiC whisker surface chemistry and morphology can strongly impact composite processing and properties. We report here the surface chemistry and morphology of SiC whiskers received during late 1987 and 1988 from five sources. Comparisons are made with previously characterized whiskers.

Solid-particle erosion of in situ reinforced Si3N4

Abstract Steady-state solid-particle erosion has been investigated on in situ reinforced Si 3 N 4 and the “equivalent” fine-grained hot-isostatically-pressed Si 3 N 4 whose R-curve behaviors are quite different, having K IC values in the long-crack limit of 8.3 and 5.6 MPa m p 1 2 respectively. Experiments were carried out at 20 °C, using SiC abrasives, whose diameters ranged from 42 to 1035 μm, varying the angle of impact from 15 to 90° and the velocity from 50 to 100 to 150 m s −1 . The erosion rates of the two materials were, within a factor of two, the same, indicating that the long-crack-length-limit toughness is not an indication of erosion resistance, for the range of particle sizes and velocities studied.

Analysis of grain boundaries for reaction-bonded silicon nitride with yttria addition

Silicon nitride is often the developmental ceramic of choice for high-temperature engine components. Sintering this ceramic, however, is very difficult due to the covalent nature of the bonding. Complicating the matter, Si3N 4 tends to dissociate at temperatures above 1700 ° C. Dissociation can be impeded by application of a nitrogen overpressure, while the general sintering problem is overcome by the addition of small amounts of various oxides. These sintering aids react with the silicon nitride and each other to form liquid phases, allowing sintering at temperatures between 1600 and 1900°C. Common additives include magnesia, yttria, alumina, and their various combinations. Normal additive content is in the range of 1 to 10% by weight. Although these sintering aids produce highly dense structures, they also lead to the formation of secondary phases along grain boundaries. The resulting materials are very strong at room temperature, but they rapidly lose their strength at high temperature (> 1000°C) because high-temperature mechanical properties such as creep are controlled by the grain boundary phases. The grain-boundary phases, in turn, are determined by the quantity and combination of sintering aids, by the firing cycle, and by the densification process (e.g. hot-pressed or reaction-bonded). Thus, by correlating the microstructure with the physical properties and processing/composition with microstructure, the effect of processing and compositional factors upon the physical properties can be determined and controlled. Although MgO was initially used as a sintering aid in the past, Y203 and YaO3-A1203 yield greater hightemperature strength [1, 2]. Yttria is an attractive additive because at low temperatures it reacts with the SiO2 (and some Si3N4) on the surface of the Si3N4 to form a liquid, greatly enhancing the sintering rate [3]. At higher temperatures when densification is complete, the liquid reacts with more Si 3 N 4 to give a highly refractory bonding phase along the grain boundaries. Tsuge et al. [4] showed that crystallization of the grain-boundary phase could improve high-temperature strength. Investigations of Si3N4-Y203 and Si3Na-Y203SiO2 phase diagrams [3, 5, 6] have been used to predict and explain the various grain-boundary constitutents and resulting properties. Using hot-pressed materials, Lange and co-workers [5, 7] found that the phases within the Si3N4 Si2N20 Y2Si207 compatibility triangle (in the Si3N4 Y203 SiO2 phase diagram) were extremely oxidation resistant, while Si3Y203N4, YSiOzN (K-phase), YxoSiyO23N4 (H-phase) and Y48i207N2 (J-phase) readily oxidize at 1000 ° C. Richer-

Dissolution of sintered silicon nitride bulk specimens for elemental analysis

Heating bulk, sintered silicon nitride samples in an aqueous hydrofluoric-hydrochloric acid mixture is shown to decompose the silicon nitride. Subsequent addition of sulphuric acid and volatilization of fluorides permits total dissolution of the bulk specimens for analysis. The elemental compositions that were determined by inductively coupled plasma atomic emission and atomic absorption spectrometries agreed with the nominal sample composition and confirmed analyses performed by scanning transmission electron microscopy. Neutron activation determinations on the same samples are not believed to be as accurate as the spectrometric determinations. Furthermore, the precision of the neutron activation measurements were less satisfactory, especially for key elements such as yttrium.

Transient and Steady-State Erosion of In Situ-Reinforced Si3N4

Relative to most other materials, silicon nitride is very erosion resistant. However, the resulting surface flaws degrade strength - a serious concern for component designers. An in situ reinforced silicon nitride, GS-44, was eroded in a slinger apparatus. Both transient (extremely low level) and steady-state erosion regimes were investigated. Alumina particles with effective average diameters of 140 μm and 63 μm were used at velocities of 50, 100, and 138 m/s. The biaxial tensile strength of the eroded specimens was measured. The strength decreased by about 15 percent after a very small erodent dosage and then remained virtually constant with further erosion.