Local Fracture of Nanolaminates: Edge Chipping Test
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LOCAL FRACTURE OF NANOLAMINATES: EDGE CHIPPING TEST G. A. Gogotsi,a,1 V. I. Galenko,a and L. A. Prikhna
UDC 539.3
b
The fracture resistance of domestic and foreign nanolaminate ceramics was studied by the EF (edge fracture) method. This method provides for indentor chipping of rectangular specimen edges, which permits evaluating such fracture resistance characteristics, as chipping resistance and edge toughness. There is no way of experimental determining the stress intensity factor effected as the penetration of a Vickers indentor into the polished specimen surface and measurement of crack sizes that are formed near the angles of its imprint. The influence of stress concentrations in the indentor–specimen surface contact zone on the fracture resistance of ceramics was assessed. Different test stress concentrations were produced with conical diamond indentors of different tip radii. Studies on domestic titanoaluminous carbide Ti3AlC2 specimens demonstrated that with a reduction in stress concentration levels (indentors with increasingly larger tip radii), its edge toughness grew to a smaller extent than that of Y2O3 and TS Mg-PSZ ceramics. It is indicative of the limited sensitivity of an examined material to stress concentrations on local fracture. The ratio of edge toughness determined with the indentors of maximum and minimum tip radii (in this case, 400 and 11 mm) can serve as a measure of its sensitivity (in comparison to this characteristic for examined materials). Note that the edge toughness of Ti3AlC2 ceramics is almost one-third as large than, e.g., that of Y2O3 ceramics and 1.6 times smaller than that of TS nonlinearly elastic zirconia Mg-PSZ. Keywords: nanolaminates, fracture resistance, edge fracture (EF) method. Introduction. A MAX phase* nanolaminate Ti 3 SiC 2 with a hexagonal lattice, such as in a-graphite and boron nitride, belongs to the class of brittle materials with a lower barrier of fracture onset resistance (Fig. 1 [1]). These materials feature the nanolaminarity of their grains, which suffer from delamination, flexure, and crushing without macroscopic fracture of the test specimen in the stress concentration zone (Fig. 2 [2]) [3–5]. As a result, their fracture does not lead to the nucleation of a normal smooth crack. The studies on such materials appeared to be incapable of applying, e.g., micromechanical tests with the penetration of a Vickers indentor into the polished specimen surface [6], with this, the crack length is measured near the imprint angles, which is used for computing the critical stress intensity factor K Ic . It stems from the fact that in such material as well as in metals, cracks necessary for fracture toughness measurements are not nucleated. Therefore, micromechanical studies made use of the tests with a conical diamond indentor chipping of rectangular * MAX phases are ternary layered compounds with formal stoichiometry M n + 1 AX n (n =1, 2, 3, ...), where M is the transition a-metal, A is the p-element (e.g., Si, Ge, Al, S, Sn, etc.), and X is carbon or nitrogen.
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