Strength recovery by diffusive crack healing

Generic principles of crack-healing ceramics

“…The second term expresses the net force driving either crack advance (G c >G heal ) or regression (G c <G heal ).  is a constant having the status of an activation area for the kinetic crack motion [49] [50], significantly higher activation energies were reported for vapour-solid oxidation reaction (G * <400 kJ/mol), and solid state matter transport through viscous flow of an amorphous inter-granular phase (G * < 500 kJ/mol), surface diffusion (G * < 600 kJ/mol) and grain boundary diffusion (G * < 800 kJ/mol) [10,51,52]. Compared to the repair capacity of polymers and polymer composite materials which are able to regenerate cracks near ambient temperature or supported by UV radiation catalysis [7,53], repair reactions in ceramic materials therefore require elevated temperatures to overcome the high activation energy barrier.…”

“…For the case of surface diffusion controlled crack healing in the initial stage of pore perturbation, Evans and Charles [10] formulated a model for healing kinetics assuming isotropic surface tension and parabolic crack shape. Extending this model for cracks of elliptical cross section, Hickmann and Evans [58] derived a modified power law expression for relating …”

“…Analyses of a number of experimental crack healing data confirmed the general validity of Eq. (6) for characterization of timetemperature healing behaviour in polycrystalline ceramics where thermally activated condensed matter transport dominates crack regression [10,19,20,22,50,[62][63][64][65]. Moreover, viscous flow controlled healing may be described with similar functional form but replacing diffusivity D by viscosity  and C  Reduction of effective flaw size upon healing reaction induces a strength recovery with time.…”

“…This new paradigm in materials design has led to several exciting inter-disciplinary approaches involving chemistry, mechanics, and materials processing to successfully incorporate crack healing functionality in non-biological materials [7]. Ab initio calculations [8], Monte-Carlo simulations [9], and continuum models [10] were applied to describe the healing reaction at various length scales ranging from the atomistic level to the macroscopic component size. Phenomenological rate equations based on experimental work or numerical simulations were derived which describe crack healing and strength recovery for various damage scenarios and repair reaction mechanisms [11].…”

“…The formation of an eutectic melt as well as the local particle rearrangement induced by a phase transition (ZrO 2, tetr →ZrO 2, mcl ) was considered as further healing mechanisms in multi-component and multi-phase ceramic materials [15]. Crack healing was studied on single crystal systems [16,17], polycrystalline materials [10,18] as well as amorphous glasses [18,19]. Most studies were concerned with surface cracks which were introduced by thermal shocking, indenting, impacting, cleaving, inscribing, or stressing of a precrack [20].…”