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Edge chip testing of ceramics (b) (c) (a) (Credit: (a) G.D. Quinn; (b, c) R. Danzer.) Figure 5. (a) Ceramic engine valve. (b) Chipped value head, tested as shown in (c). edge chipping was a possible problem. 300,000-year-old flake knife to those in Controlled edge-chipping experiments a modern cemented carbide. on cylindrical components compared The arrowheads shown in Figure the performance of two candidate 6 are obsidian, a volcanic glass, and zirconias. Figure 4(b) shows that only were formed by controlled chipping, a few experiments were necessary to or “knapping.” Obviously, obsidian show that a ceria-stabilized tetragonal occurs only in locales near volcanoes, zirconia polycrystal (TZP) had superior but the samples show clearly the flaked- edge-chip resistance to a magnesia- off regions that shape the arrowheads. stabilized transformation-toughened More commonly, ancient arrows, zirconia (TTZ). spears, and knives were knapped from Valves are another engine compo- local lithic materials. The stone was nent application for ceramic materials, shaped by the removal of a single flake such as the silicon nitride valve shown at a time. (Courtesy of A. Tsirk.) in Figure 5. Danzer et al.12 measured The archeological literature men- chip resistance of the silicon nitride tions that heat treatment below 500°C candidate materials and experimented improves the workability of lithic with various edge bevel shapes. They materials, but, heretofore, this has not Figure 6. Obsidian glass arrowheads. showed that judicious edge beveling been verified quantitatively. To address The stone changed color from amber can increase dramatically the amount the question, J. Quinn, Bradt, and to dark red, similar to that in fragments of force necessary to cause a chip to Hatch applied the edge-chipping test found at archeological sites. There were form. This work reinforced Almond to yellow Bald Eagle Jasper found in significant microstructural and phase and McCormick’s work3 on the effect Pennsylvania.14 This is a well-studied changes, which are described in Ref. 14. of edge shape on the measured chipping amber-colored lithic used to make cut- The edge toughness (slope of the lines) resistance. ting tools in prehistoric times. The of the as-quarried jasper was 52 percent J. Quinn and Mohan also found authors cut rectangular specimens from greater than the heat-treated jasper, that the direction of the load applied a large nodule. One specimen was as shown in Figure 7. In other words, by the indenter matters.13 They used heat-treated at 350°C for 12 hours. the heat-treated jasper had improved test coupons with 90° edges to show that chips formed with smaller forces when the load angles toward the edge. Conversely, applied force directed toward the bulk, required larger loads before chipping occurred. Lithics Force (N)Archaeological discoveries reveal that edge chipping was among the ear- liest of manufacturing processes. Edge- chip testing is a way of applying scien- tific methodology to understanding the Figure 7. Top view of edge chips in innovators. Almond and McCormick3 Eagle Jasper. The color change from (Credit: G.D. Quinn.)as-quarried and heat-treated Baldshowed the similarities of flakes in a the heat treatment is amazing. Distance (mm) activities of prehistoric “engineers” and 26 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 1


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