Biomimetic nanostructured coatings on nano-grained/ultrafine-grained substrate: Microstructure, surface adhesion strength, and biosolubility

We describe here the structural morphology, nucleation and growth, surface adhesion response and biosolubility in simulated physiological fluid of nanohydroxyapatite (nHA)-based coatings (nHA, chitosan-nHA, collagen-nHA) on nano-grained (NG)/ultrafine-grained (UFG) 316L austenitic stainless steel bulk substrate. The NG/UFG stainless steel was processed by a novel procedure involving controlled phase reversion of strain-induced martensite. Electron microscopy studies indicated that the morphology of nHA on NG/UFG stainless steel is characterized by a vein-type interconnected structure consisting of fibrils that closely mimics the fibrous structure present as part of the hierarchical structure of the bone. Interestingly, the dimensions of the vein-type interconnected structure were similar to the grain size of NG/UFG stainless steel substrate. This structural morphology was retained in crystallized chitosan-nHA coating, but was modified when collagen is codeposited with nHA. The mechanisms of deposition are discussed based on the structural characteristics of coatings. Adhesion response of coating as determined by scratching tests suggested superior adhesion strength of nHA coating on NG/UFG substrate as compared to that on CG substrate and is attributed to grain size effect and wettability of the substrate.