Silicon Carbide Whisker-reinforced Alumina Research Articles

Development of yttria and ceria toughened alumina composite for cutting tool application

Yttria and ceria toughened alumina (YCTA) ternary ceramic composite were developed for cutting tool application. Alumina, ceria and yttria powders were mixed with polyvinyl alcohol and uniaxially pressed. The green compacts were sintered, subsequently hot isostatically pressed (HIPed) and finish machined in the form of cutting tool inserts. The densities of the green compacts and the sintered and the HIPed specimens of YCTA were evaluated. The mechanical properties of the sintered and the HIPed specimens of YCTA were also evaluated. Machining studies were conducted using the newly developed YCTA inserts on grey cast iron work piece. For comparison, the commercially available zirconia toughened alumina (ZTA) inserts and silicon carbide whisker reinforced alumina (SCWRA) inserts were also used for the machining studies. The performance of YCTA inserts is compared with that of the ZTA and SCWRA inserts.

Analysis of the weight loss during sintering of silicon-carbide whisker-reinforced alumina composites

Good densification of Al 2O 3-SiC whisker composites usually requires use of the expensive hot-pressing technique. In this work, composites with 12 vol% whisker content have been made by pressureless sintering at temperatures above 1700 °C. To evaluate the effect of the sintering environment, tests were conducted either with samples placed in different powder beds, or by gas pressure sintering. The highest densities are obtained by the use of an alumina powder bed, or by gas pressure sintering. The latter method leads to densities of 95% of theoretical. The densities achieved are strongly dependent on the chemical stability of the species at the sintering temperature. This stability was evaluated by measuring the weight loss after sintering. Then, an analysis of the sintering process with reference to chemical engineering techniques was conducted. The analysis shows the relevance of the selected sintering cycles.

Chip formation analysis in high speed machining of a nickel base superalloy with silicon carbide whisker-reinforced alumina

High speed turning tests were performed on a heat resistant alloy (Inconel 718), using SiC (20%) whiskers reinforced ceramic tools. The main aims of these tests were the following: (1) mapping cutting speed-feed and machined volume in order to find a region free from tool breakage; (2) analysing tool wear and chip formation mechanisms; and (3) from the experimental results modelling analytically both chip formation processes and tool wear mechanism. Tool and chip were observed at the SEM and EDAX semiquantitative analyses were carried out to evaluate micro-welds on the chip and areas of welded or scattered material over the tool. Micro-hardness mapping was carried out on the longitudinal cross-section of the chips to monitor its dependence by process parameters. Variable wear mechanisms along the tool-chip contact length that were attributed to variations in plastic deformation energy were observed. There variations were analytically modelled in orthogonal cutting conditions. Longitudinal and tranverse shear planes into the chip were also observed. The causes and the mechanisms of wear, chip formation and the hardening of work material were deduced. The presence of whiskers pull out mechanisms due to temperature effects in the tool-chip interface were also observed.

Cyclic fatigue-crack propagation in a silicon carbide whisker-reinforced alumina composite: role of load ratio

The characteristics of subcritical crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R=Kmin/Kmax); results are compared with similar subcritical crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-crack growth has been measured over six orders of magnitude from ∼10−11–10−5 m cycle−1 at load ratios ranging from 0.05–0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (ΔK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, ΔKTH, for example, is found to be increased by over 80% at R=0.05 compared to R=0.5. These results are rationalized in terms of a far greater dependency of growth rates on Kmax(da/dN ∞ Kmax30) compared to ΔK(da/dN ∞ ΔK5), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of crack advance underlying such behaviour are discussed in terms of timedependent crack bridging involving either matrix grains or unbroken whiskers.

Microstructural Evaluation of Silicon Carbide Whisker Reinforced Alumina Fabricated with Carbon-Coated Whiskers

The presence of interfacial films at the whisker-matrix interface can significantly influence the fracture toughness of ceramic composites. The film may alter the interface debonding process though changes in either the interfacial fracture energy or the residual stress at the interface. In addition, the films may affect the whisker pullout process through the frictional sliding coefficients or the extent of mechanical interlocking of the interface due to the whisker surface topography.Composites containing ACMC silicon carbide whiskers (SiCw) which had been coated with 5-10 nm of carbon and Tokai whiskers coated with 2 nm of carbon have been examined. High resolution electron microscopy (HREM) images of the interface were obtained with a JEOL 4000EX electron microscope. The whisker geometry used for HREM imaging is described in Reference 2. High spatial resolution (< 2-nm-diameter probe) parallel-collection electron energy loss spectroscopy (PEELS) measurements were obtained with a Philips EM400T/FEG microscope equipped with a Gatan Model 666 spectrometer.

Method for preparing configured silicon carbide whisker-reinforced alumina ceramic articles: Tiegs, T.N. (United States Department of Energy, Washington, DC, USA) US Pat 4 652 413 (24 March 1987)

Method for preparing configured silicon carbide whisker-reinforced alumina ceramic articles: Tiegs, T.N. (United States Department of Energy, Washington, DC, USA) US Pat 4 652 413 (24 March 1987)