WC-Co alloys are widely used as cutting and mold tools due to their high hardness, but their performance is difficult to evaluate with conventional tests. We have applied the micro slurry-jet erosion (MSE) test, which we have developed, to evaluate the surface strength of WC-Co alloys. The objective of this study is to understand the erosion mechanism of the WC-Co alloy in order to demonstrate the usefulness of the MSE test. Four specimens (Co content: 6 ~ 19 wt.%, WC grain size: 0.6 ~ 0.8 µm) were tested with angular alumina particle with an equivalent sphere diameter of 41 µm as erodent. The erosion debris was collected from the impacted slurry and analyzed with SEM and EDS, together with the observation of the eroded surfaces and cross-section. The erosion rate of the MSE increased with increasing Co content. The erosion debris was categorized into two types according to size, shape and composition of the particles. The erosion mechanism of the WC-Co alloy was determined to be removal of WC or WC and Co due to intergranular fracture. It was concluded that the MSE test has the possibility of evaluation for strength against intergranular fracture caused by impact force.
In situ observation of lubricant films formed with greases was carried out at EHL conditions with a micro-FTIR. Three urea compounds were used as a thickener and 12 mass percent of the thickener was added in PAO, respectively. From the IR spectra, CH and NH stretching vibration peaks were observed from urea greases. The film thickness and the concentration of thickener were estimated from the absorbance of CH and the ratio of absorbance of NH and CH. Two-dimensional distribution of the thickness and the concentration around Hertzian contact was obtained at the resolution of 50 μm. The thickener of urea grease was concentrated on ball and disk surfaces. It was found that the concentrate on thickener at Hertzian contact region was dependent on thickener itself. Time dependence of the thickness and the concentration at Hertzian contact were obtained at the resolution of 45 s under shearing conditions. The film thickness of urea greases at Hertzian contact was greatly decreased by shearing and the film thickness and the concentration were recovered when the condition was changed to rolling. This behavior was supposed with detachment and growth of the adhered film of the thickener. Lubricating characteristics of urea greases were discussed based on the concentration of thickener at Hertzian contact.