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Nevertheless, in clinical practice zirconia crowns sometimes chip. However, the problem is not with the zirconia core. What actually chips is the thin porce- lain veneer on the crown’s outer surface that makes the restoration appear more natural. (A monolithic white zirconia (Courtesy of S. Scherrer.) crown would look like a piece of Chiclets gum.) Figure 9 shows that a conventional porcelain and an early-generation glass– ceramic were much less chip resistant. Figure 8. Chipping of human teeth and teeth restorations is a common problem. Modern filled composites and workability—it would take less force to pioneered the use of edge chip test- glass–ceramics have improved chipping chip flakes off the heat-treated stone ing of restoration materials for crowns resistances. Figure 10 shows detailed to shape it. Fracture toughness experi- and bridges.16–20 Other groups21–23 now outcomes for a contemporary glass-filled– ments showed that untreated jasper had employ edge chip testing, and it even is resin-matrix composite. The Vickers and an 82-percent higher K than the heat- used to test human dentin24 and enam- sharp conical 120° indenter trends arecI treated jasper, corroborating the edge el.25 (A short review of edge chip test- similar, but the Vickers indenter required chip results. ing applied to dental materials was pre- 28 percent more force to make a 0.5-mil- G. Quinn and Bradt15 have observed sented at ICACC’12 (Daytona Beach, limeter chip. This is one example where the opposite trend in their new work on Fla.) and will be published in the con- the data better fit the power law with the mineral novaculite. Heat treating ference proceedings.26) Although labo- an exponent of 1.8 than they do a linear improves the edge-chipping resistance. ratory-scale tests use specific indenters trend. Chai and Lawn30 have proposed They believe that prehistoric engineers on test blocks with carefully prepared an indentation mechanics model that shaped the tools first, then heat-treated edges, the chips physically resemble predicts power-law behavior, but only them to improve the chipping resis- some types of in-vivo failures,27,28 as for an exponent of 1.5. This is an area of tance of the sharp edges. shown in Figure 8. ongoing research. Figure 9 compares edge-chipping Teeth results for six contemporary restorative Role of edge chip testing expands Janet Quinn, late research scientist materials.29 Not surprisingly, the yttria Edge chipping also is applicable with the American Dental Association, TZP has the greatest chip resistance. to coatings, electronic material sub- Force (N) Force (N) (Credit: G.D. Quinn.) Edge distance (mm) (Credit: G.D. Quinn.) Figure 10. Edge chip results for a glass-filled–resin-matrix composite. The Vickers and sharp conical indenter trends are similar but non- Edge distance (mm) linear. The inset shows the variation of chip shape with indenter. A Figure 9. Edge chip trends for six dental restorative sharp conical indenter made the left two chips (stained green), and a materials. Vickers indenter made the right two. American Ceramic Society Bulletin, Vol. 92, No. 1 | www.ceramics.org 27


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