Ceramics indentation-fracture mechanics

Ceramics can often be damaged under low contact loads and, thus, it is important to understand these damage mechanisms. These processes not only give rise to strength degradation but are linked to wear and erosion. Indentation fracture mechanics has been found to be a very useful approach in understanding these contact-damage processes (see Section 8.8). 

The indentation fracture mechanics approach likens abrasive-workpiece interactions for grinding of ceramics to small-scale indentation events. The deformation and fracture patterns observed for normal contact with a Vickers pyramidal indentor under an applied load P are illustrated in Figure 3.1. A zone of plastic deformation is foimd directly imder the indentor. Two principal crack systems emanate from the plastic zone med-ian/radial and lateral cracks. Median/radial cracks are usually associated with strength degradation and lateral cracks with material removal. 

Most past research on grinding mechanisms for ceramics has followed either the indentation fracture mechanics" approach or machining" approach. The indentation fracture mechanics approach would seem to offer the possibility of describing both the material removal process and its influence on strength degradation in terms of the force or depth of cut at an... 

A. L. Yurkov and R. C. Bradt, Load Dependence of Hardness of SIALON Based Ceramics, in Fracture Mechanics of Ceramics, Vol 11, Eds. R. Bradt et al., (Plenum Press, NY, 199S), pp. 369 378. P. M. Sargent, Use of the Indention Size Effect on Microhardness for Materials Characterization, in Microindentation Techniques in Materials Science and Engineering, ASTM STP 889, eds. P. J. Blau and B. R. Lawn, (ASTM, West Conshohocken, PA, 1986), pp. 160 174. 

Niihara K., 1983, A fracture mechanics analysis of indentation-induced Palmquist crack in ceramics, J. Mater. Sci., Lett. 2, 221. 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

Gong, J.H., Guan, Z.D., Jiang. D.C. (1992), Analysis of strength degradation of indented specimens due to thermal shock , in Bradt, R.C., Hasselman, D.P.H., Munz, D., Sakai, M., Shevchenko, V.Y., Fracture Mechanics of Ceramics, Plenum Press, New York, 605-610. 

A method was developed for the examination of boron nitride ceramics by automated ultrasonic inspection [57]. A comparison of sliding and abrasive wear mechanisms in ceramics and cemented carbides shows that a-BN ceramics are quite susceptible to indentation fracture [58]. Many more additional applications are reported in Section 4.1.1.10.8, p. 129. 

B. R. Lawn and D. B. Marshall. Indentation Fracture and Strength Degradation in Ceramics. Fracture Mechanics of Ceramics, Vol. 3. Plenum, 1978, p. 205. 

INDENTATION FRACTURE, ACOUSTIC EMISSION AND MODELLING OF THE MECHANICAL PROPERTIES OF THIN CERAMIC COATINGS... 

S. Sakaguchi, R curve measurement of alumina using straight notched or indented bending bars. Fracture Mechanics of Ceramics (Plenum Press), vol. 11, 39 51, (1996). 

Does cracking influence hardness This question was raised by M. Swain and M. Wittling at the 1995 Fracture Mechanics of Ceramics conference in Karlsruhe, Germany with their paper titled Indentation Size Effects for Brittle Materials Is There A Simple Fracture Mechanics Explanation They contended that median cracks beneath an indentation could enable an indenter to sink deeper into the material and thereby produce larger indentations than if there were no cracking. They suggested that Knoop indentations should be particularly susceptible, but they furnished no Knoop data to support their hypothesis. 

R. K. Govila, Indentation Precracking and Double Torsion Methods for Measuring Fracture Mechanics Parameters in Hot pressed SisN4, J. Am. Ceram. Soc., 63 (5 6) 319 326 (1980). 

Swain M, Wittling M. Indentation size effect for brittle materials Is there a simple fracture mechanics explanation, in Bradt RC, Hasseiman DPH, Munz D, Sakai M, Shevchenko VY (Eds.) Fracture Mechanics of Ceramics. Vol. 11. New York Plenum Press 1996. 

M. Swain, M. Wittling, The Indentation Size Effect for Brittle Materials Is There A Simple Fracture Mechanics Explanation , pp. 379-388 in Fracture Mechanics of Ceramics, Vol. 11 eds. RC Bradt, DPH Hasselman, D Munz, M Sakai, VY Shevchenko, (Plenum Press, NY, 1996). E. Meyer, Untersuchungen liber Hfirteprufung und HUrte Brinell Methoden, Z Ver. Deut. Ing., 52,645 (1908). 

Mechanical Properties. The hardness of the Ba2YCu307 x ceramic and its room temperature fracture toughness as determined in air by the indentation crack length procedure are given in Table I. The value of Young s modulus used in both this calculation as well as the... 

---