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(N) P Load, (Credit: G.D. Quinn.) (Credit: G.D. Quinn.) Distance from edge, d (mm) Figure 2. Ceramic test piece in the edge-chipping machine with a sharp conical 120° indenter. Figure 3. Edge chip data trend. uses a microscope to precisely locate McCormick and Almond arrived at quickly at low loads. Other indenters, an indentation site, such as is shown three important conclusions. such as the Rockwell C indenter, are in Figure 2. Alternatively, a conven- • Flake geometries are remarkably more blunt, and much greater force is tional universal testing machine can similar and appeared to be independent needed to initiate starter cracks that be used. However, precise alignment of of the distance, force, or material. The eventually will grow to form the chip. the indentation site and posttest mea- larger the distance from the edge and There are about a dozen teams around surement of the edge distance is more flaking load, the larger the chip, but the the world now working on this meth- difficult. McCormick and Almond shape is invariant. odology for various applications, such developed the test with rounded-tip • The force versus distance trends as quality control, material rankings, Rockwell C-type indenters (120° coni- are usually linear. materials development, and engineering cal shape with a tip radius of 0.2 mm). • The chipping resistance increases design evaluations. Some of these are However, subsequent studies also use with increased KIc or increased GIc, described below. Vickers, Knoop, and sharp conical 120° which are fundamental material proper- indenters. ties that characterize a material’s resis- Ceramic engine parts The greater the distance from the tance to fracture. A diesel engine manufacturer’s mate- edge, the more force it takes to make a Morrell and Gant7,8 continued the rials selection process identified zirco- chip. Thus, data usually are plotted as NPL work. Subsequent work by vari- nia as superior to traditional tool-steel the load versus the distance, as shown ous groups (e.g., Gogotsi et al.9–11) has for fuel injector plungers, which might in Figure 3. Often the data follow a shown that sometimes the force–dis- seize in short times if there is water linear trend, but, sometimes, a power- tance data follow a linear trend and in the fuel. The manufacturer had an law trend is a better fit, depending on sometimes they do not. The best trend option to make fuel injector plungers the material and the type of indenter. to fit the data depends upon the materi- with zirconias stabilized with vari- The slope of the line resulting from a al and the type of indenter. Some sharp ous additives (Figure 4(a)). Prototype linear fit constitutes the “edge tough- indenters cause starter cracks to form testing of zirconia parts showed that ness parameter” and is designated Te or M. Steep slopes, i.e., large values of Te, indicate a material is resistant to edge chipping—large loads are needed to induce chip fracture even at small dis- tances from the edge. Conversely, small cal properties, such as fracture tough- (Pins courtesy of Cummins Engine Co.) Force (N)- values indicate that the materialerelease rate, GIc,2,4–6 can be related to T chips easily. Other important mechani ness, KIc, or the critical strain energy Te, also. The plot also identifies the “edge strength” parameter, SE(0.5), Figure 4. Comparative results for two make a chip at a distance of 0.5 milli- pin. The darker ceria-doped TZP zirco- (Credit: G.D. Quinn.)zirconias for a diesel engine fuel injector which is defined as the force it takes to meters from the edge. nia has superior edge chip resistance. Edge distance (mm) American Ceramic Society Bulletin, Vol. 92, No. 1 | www.ceramics.org 25


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