Teeth and caries formation
Dental enamel has a complex hierarchical structure consisting of nano-scale apatite crystals packed into micro-scale prismatic clusters. We have shown that the development of incipient caries lesions (the earliest stage of tooth decay) in enamel is a complex chemical process that partially removes apatite and dramatically weakens the enamel structure. Chemical treatments using fluoride can reverse the loss of apatite (remineralization) giving a stronger mixed fluorohydroxyapatite phase, but the presence of porosity is essential for the diffusion of ions. Once the porosity is lost the ability to remineralize is diminished.
Bone nanomechanics and genetic mutations
Bones from knockout mouse models have been used to examine changes in mechanical properties, using nanoindentation, and chemistry, using micro-Raman, when key bone proteins are absent. We have shown the importance of non-collagenous bone proteins in maintaining healthy bone, and that each of the proteins has a distinctly different role. Osteopontin regulates apatite growth and its absence causes the organic to inorganic ratio to change which affects the bone’s hardness and elastic modulus. Osteocalcin generally indicates healthy bone and its absence affects carbonate substitutions in the apatite, which affects hardness, but not elastic modulus. Fibrillin-2 affects the formation of elastin and its absence (Beal's Syndrome) in bones is not uniform. Rather, the elastin acts as a “brace” going around cortical bone found near the center of long bones. Without fibrillin-2 the elastic modulus in these bone regions drops.
Hard materials and residual stress
Hard ceramics such as boron carbide and silicon carbide are of great interest as structural materials, for instance in ballistic applications. The presence of residual stresses due to machining and multiple metastable phases in bulk samples results in low uniformity which can affect their performance. Micro-Raman spectroscopy has proven to be particularly useful for studying these materials. Using analysis of Raman peak position, height, width and the relative sizes of peaks we have been able to distinguish both residual stresses and phases. Confocal micro-Raman has enabled the changes in stresses and phases with depth below the surface to be studied. The results have shown that low residual stresses do not correlate with a relative absence of multiple phases and defects. Further to this the sub-surface damage due to machining can increase as the quality of the finish (low roughness) increases, with substantial stresses often present many microns below the surface for mirror-finishes .
Nanomechanics and surface effects
I have been interested for many years in using nanoindentation to study the effects of surface chemistry and thin films on the mechanical properties of metals, ceramics, semiconductors and, most recently, organic crystals. These studies have examined the impact of: immersing materials in organic liquids; adding self-absorbed monolayers (SAMs); varying environmental humidity; and immersing in aqueous solutions. The results have helped to give a fundamental insight into the often elusive area of “chemo-mechanical effects”. The key effect that is always present is a modification to the geometry of nanoscale contacts due to the surface chemistry/coating. This changes the stress field which in turn makes the analysis of data prone to misinterpretation.