Abstract
Bones of humans and animals combine two unique features, namely: they are brittle yet have a very high fracture toughness linked to the tortuosity of the crack path and they have the ability to repeatedly heal local fissures such that full recovery of overall mechanical properties is obtained even if the local bone structure is irreversibly changed by the healing process. Here it is demonstrated that Ti2AlC MAX phase metallo-ceramics also having a bone-like hierarchical microstructure and also failing along zig-zag fracture surfaces similarly demonstrate repeated full strength and toughness recovery at room temperature, even though the (high temperature) healing reaction involves the local formation of dense and brittle alumina within the crack. Full recovery of the fracture toughness depends on the healed zone thickness and process zone size formed in the alumina reaction product. A 3-dimensional finite element method (FEM) analysis of the data obtained from a newly designed wedge splitting test allowed full extraction of the local fracture properties of the healed cracks.
Highlights
Bones of humans and animals combine two unique features, namely: they are brittle yet have a very high fracture toughness linked to the tortuosity of the crack path and they have the ability to repeatedly heal local fissures such that full recovery of overall mechanical properties is obtained even if the local bone structure is irreversibly changed by the healing process
The projected crack length was estimated from the cumulative acoustic energy EAE and calibrated against the final crack length as measured with scanning electron microscopy (SEM); see supplementary information Fig. S3
Our findings demonstrate that the proposed toughening mechanisms, i.e., interlocking, crack bridging and crack deflection, apply when brittle oxide ceramic filler material formed within the former crack still occurs in successive fracture and healing cycles
Summary
Bones of humans and animals combine two unique features, namely: they are brittle yet have a very high fracture toughness linked to the tortuosity of the crack path and they have the ability to repeatedly heal local fissures such that full recovery of overall mechanical properties is obtained even if the local bone structure is irreversibly changed by the healing process. It is demonstrated that Ti2AlC MAX phase metallo-ceramics having a bone-like hierarchical microstructure and failing along zig-zag fracture surfaces demonstrate repeated full strength and toughness recovery at room temperature, even though the (high temperature) healing reaction involves the local formation of dense and brittle alumina within the crack. Human bone, which is the main structural material in our body, is brittle and is continuously exposed to minor or mediocre overloads yet has to function even up to 100 years Bone meets this requirement by a hierarchical and laminated s ubstructure[6,7,8,9,10], making the brittle material fracture in a ‘tough’ manner, together with ‘repeatable’ full recovery of strength and toughness by autonomous self-healing reactions involving reactive repair and remodelling s tages[11,12]. As a result of their laminated structure the fracture behaviour of MAX phase is not unlike that of (fresh) bone
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