Measurement of microscopic bridging stresses in an alumina molybdenum composite by in situ fluorescence spectroscopy

The R-curve behavior of an Al2O3 ceramic with 25 vol% of molybdenum-metal p articles added was studied by using fracture-mechanics experiments and in s itu piezospectroscopic measurements of microscopic bridging tractions. Crac ks were propagated by using a crack stabilizer, which allowed stable crack growth in a bending geometry. Microscopic bridging stresses were measured i n situ during fracture propagation by detecting the shift of the Cr3+ fluor escence lines of Al2O3. Laser spots similar to 1 mu m in diameter and simil ar to 10 mu m deep were focused at the ceramic/metal interface of the bridg ing sites, and the closure stresses that acted on the crack faces were reco rded as a function of external load. The maximum stress that was experience d by the stretched metal particles prior to final failure was similar to 0. 4 GPa, The maximum stress magnitude was not markedly different in relativel y small (i.e., <5 mu m) metal particles, failing with large ductility, as c ompared with larger particles which, instead, fractured in semibrittle fash ion. A map of bridging tractions along the crack wake was constructed under a constant stress intensity factor, almost equal to that which is critical for crack propagation. Using this map to theoretically predict the rising R-curve behavior of the composite led to results that were consistent with the fracture-mechanics experiments, thus enabling us to explain the observe d toughening, primarily in terms of a crack-bridging mechanism.