Size‐induced grain boundary energy increase may cause softening of nanocrystalline yttria‐stabilized zirconia

Abstract

An increase in hardness with reducing grain sizes is commonly observed in oxide ceramics in particular for grain sizes below 100 nm. The inverse behavior, meaning a decrease in hardness below a critically small grain size, may also exist consistently with observations in metal alloys, but the causing mechanisms in ceramics are still under debate. Here we report direct thermodynamic data on grain boundary energies as a function of grain size that suggest that the inverse relation is intimately related to a size‐induced increase in the excess energies. Microcalorimetry combined with nano and microstructural analyses reveal an increase in grain boundary excess energy in yttria‐stabilized zirconia (10YSZ) when grain sizes are below 36 nm. The onset of the energy increase coincides with the observed decrease in Vickers indentation hardness. Since grain boundary energy is an excess energy related to boundary strength/stability, the results suggest that softening is driven by the activation of grain boundary mediated processes facilitated by the relatively weakened boundaries at the ultra‐fine nanoscale which ultimately induce the formation of an energy dissipating subsurface crack network during indentation.

Al excess extends Hall‐Petch relation in nanocrystalline zinc aluminate

“…Figure 5 shows the aforementioned cross‐sections where all samples present a network of sub‐surface cracks which indicate load energy accommodation as in the quasi‐plasticity model 8,9 . The density of cracks apparently reduces as the grain sizes increase in both S‐ZAO and E‐ZAO, suggesting differences in the energy absorption mechanisms for large and small‐grained samples, which is consistent with previous reports 9,19,20,40 . Long lateral cracks are present at small grain sizes in S‐ZAO (S‐H12 and S‐H15).…”

“…The low activation energy for fracture manifests itself as a decrease in hardness following the inverse Hall‐Petch relation. This proposed relationship between grain boundary energy (or grain boundary strength) and Hall‐Petch behavior suggests that the inverse Hall‐Petch relation could be mitigated by avoiding an increase in grain boundary energy, suggesting ‘colossal’ hardening may be achieved in nanoceramics 20 …”

“…This technique uses a complying WC punch designed to deform in the final stages of sintering, causing full densification with minimal grain growth. This technique differs fundamentally from Ryou et al.’s approach that uses a multianvil boron nitride die, 19 which is akin to Bokov et al.’s high‐pressure spark plasma sintering (HP‐SPS) setup 20 that uses diamond‐SiC punches. Since both Ryou et al.…”

Influence of grain boundary and grain size on the mechanical properties of polycrystalline ceramics: Grain‐scale simulations

“…[43][44][45] This may be due to an increase in the proportion of triple junction, which is related to changes in energy dissipation and fracture behavior. Bokov et al 46 found that inverse Hall-Petch relationship is correlated with the increased triple joint population as grain size decreases, and the increased triple joint population affects the excess energy of grain boundary. In terms of thermodynamic, the excess energy of grain boundary correlates with boundary stability.…”

The role of grain boundaries and their triple junctions in strengthening and softening of nanocrystalline ceramics

“…The critical grain size d c for this transition can depend on the structure and energy of GBs and their chemical composition. Indeed, recent experiments on nanocrystalline YSZ ceramics [11] demonstrated a significant increase in the GB energy with a decrease in grain size below the critical grain size for the transition from the direct to the inverse Hall-Petch dependence for hardness. This situation is similar to that for nanocrystalline metallic alloys where high-energy (nonequilibrium) GBs decrease the yield strength and hardness by promoting GB sliding, while reducing GB energies via lowtemperature annealing increases the strength and hardness of such solids [29 -31].…”