Jong Jip Kim

Solid particle erosion of SiC and SiC–TiB2 composite hot-pressed with Y2O3

Abstract The solid particle erosion of the hot-pressed monolithic SiC and SiC–TiB 2 particulate composite by angular SiC particles was investigated at room temperature over a range of particle sizes (50–150 μm), velocities (40–100 m/s) and impact angles (30–90°) by weight loss measurements. The erosion of both materials exhibits an anomalous particle size dependence and evidences of plastic deformation together with grain pull-out are observed in the eroded surfaces. The erosion rates of these materials do not increase monotonically with increasing particle size, presumably due to the erodent fragmentation. The velocity exponents are less than 2, which is attributed to the grain pull-out and erodent fragmentation. In addition, the erosion rate is higher for the SiC–TiB 2 composite with higher fracture toughness but lower hardness compared to monolithic SiC indicating that the erosion of these materials is controlled significantly by the plastic deformation.

Solid-particle erosion of hot-pressed silicon carbide and SiC-TiB2 composite

Abstracts are not published in this journal

Effects of galvanostatic treatments on hot corrosion of Ni

Several electrochemical polarization tests have been developed to assess the hot corrosion resistance of gas turbine materials [1-3]. Shores [4] has developed a screening test based on the corrosion current density estimated from the anodic part of the polarization curve after galvanostatic polarization. The corrosion current density of some super-alloys determined by this method correlated well with the penetration rates measured by the burner rig test. Erodos et al. [5] proposed an electrochemical screening test based on the measurement of activation potential after a galvanostatic pretreatment similar to that by Shores. The activation potential after galvanostatic polarization was reported to have a linear correlation with the depth of corrosion determined metallo-graphically after the test. However, these works [4, 5] were conducted in air containing no oxidents such as SO2 or SO3, different from the gas turbine operating environments, and the work by Shores was criticized for low repeatibility [5]. This letter describes the results of a study on the effect of galvanostatic treatment on the corrosion current density of Ni and its scatter measured by the conventional Tafel method, in molten Na2SO 4 in contact with gas atmospheres containing Ar, 02 and SO3. Commercial Ni wire of 99.995% purity was used in this investigation. Some specimens were preoxidized for times of 10 to 180 min in still air at 900 °C. The reference electrode was made of pure Ag wire dipped into a 10 m/o Ag2SO4-Na2SO 4 solution contained in a mullite tube, and the counter electrode was made of pure Au wire. Specimens were exposed to a reagent grade molten Na2SO 4 at 910 °C in Ar, air and Pt-catalysed 0.2% SO2-O 2 gas atmospheres. The flow rate of gases except the air used in static condition was 400 ml/min. For the determination of corrosion current density I~or~, potentials were scanned at a rate of 0.3 mV/s from E~o~r 250 mV to Ecor~ + 250 mV, using a Princeton Applied Research model 273 potentiostat. For the galvanostatic treatment, currents were applied to the specimens for 2 h either at a density of 5 mA cm -2 for the cathodic treatment or at +5 mAcm -2 for the anodic treatment. The results of hot corrosion tests of clean Ni specimens in various conditions are summarized in Table I. Values of Icor~ are remarkably dependent upon the gas atomospheres in contact with the melt. The highest value of Ico~r was observed in specimens tested in Pt-catalysed 0.2% SO2-O2 gas containing both SO3 and O2, and the lowest in Ar gas

Oxidation of hot-pressed silicon carbide

The oxidation of silicon carbide based materials has been the subject of considerable interest in recent years [1]. Especially in sintered and hot-pressed materials, much attention has been paid to the effects of additives on the kinetics and mechanisms of oxidation. In general, the additives and impurities diffuse out to the surface and form glassy products of low viscosity during oxidation at elevated temperatures. This accelerates the diffusion of oxygen through the oxide, and thereby increases the parabolic rate constant or leads to deviations from the parabolic rate law [2, 3]. The additives also lower the crystallization temperature of the silicates and complicate the oxidation rates in long term isothermal experiments, where the rate tends to decrease with time. So far, the role of additives has been reported in several SiC materials containing densification aids, such as B, C, A1203 or TiB2 [3-5]. In the present work, the role of additives is investigated for SiC and SiC-TiB2 particulate composites containing either A1203 or Y203, with an emphasis given to the effects of distributions of additives, such as A1203, Y203 and TiB2, on the oxidation. The chemical compositions of the materials are listed in Table I. Specimens were produced from commercial powders of <0.9 ktm mean particle size. The combined powders were vibromilled in ethanol using SiC balls of 8 mm diameter and bot pressed in graphite dies lined with graphite foil at 20002050 °C at 30-38 MPa under vaccum. In all cases the density of hot pressed pieces exceeded 97% of theoretical. After hot pressing, specimens were cut and mechanically ground to dimensions of 3.0 x 4.0 x 11.0 mm. Oxidation tests were performed in static air, at atmospheric pressure and temperatures between 900 and 1300 °C, and for various times up to 5Oh. The weight of each specimen was measured to an accuracy of 10-5g several times and the result expressed as an average. The oxidation products were characterized by X-ray diffraction (XRD), electron probe X-ray microanalysis (EPMA) and scanning electron microscopy (SEM).