Ultrafine-grained commercially pure titanium and microstructure response to hydroxyapatite coating methods
Abstract
Commercially pure titanium (cp-Ti) is an ideal biomaterial as it does not evoke an inflammatory foreign body response in the body. However, the low strength of cp-Ti prevents the use in most orthopaedic load bearing applications. Therefore, many metal orthopaedic implants are commonly made of higher strength metal alloys that are less biocompatible. Nanostructured materials exhibit superior mechanical properties compared to their conventional grain sized counterparts. Severe plastic deformation (SPD) of metals has been shown to produce nanostructured materials. SPD by machining is a single-step deformation route that refines the grain microstructure, to develop an ultrafine grained (UFG) microstructure. UFG cp-Ti strips were developed with induced shear strains of up to 4.0 using a machining-based process. Both Vickers microhardness evaluation and microstructural analysis were used to characterize the as-received (annealed) and machined states. For induced shear strains between 1.9 and 4.0 in grade 2 cp-Ti the hardness was increased from 188 ± 7 kg/mm2 in the as-received state to between 244 ± 6 and 264 ± 12 kg/mm 2 in the as-machined state, corresponding to an increase in hardness between 31 and 41%. The microstructural analysis revealed a grain size reduction from 34 ± 11 μm in the as-received state to ∼ 100 nm for machined grade 2-Ti. A complete annealing study suggested that recovery/recrystallization occurs between 300 and 400°C, with a significant hardness drop between 400 and 600°C, while grain growth is continuous, starting at the lowest annealing temperature of 300°C. Hydroxyapatite (HA) is commonly applied to orthopaedic devices to promote bone growth. Machined Ti strips were coated with HA using conventional plasma spray as well as two alternative low-temperature application routes (sol-gel with calcination and anodization with hydrothermal treatment) to evaluate the thermal influence on the UFG-Ti substrate. Plasma spray produced a thick (20 to 70 μm) HA crystalline coating, sol-gel followed by calcination did not produce crystalline HA, while anodization with the proper hydrothermal treatment yielded a homogenous crystalline HA coating 5 to 15 μm thick based on the anodization condition. Mechanical and microstructural evaluation of the UFG-Ti substrates revealed that both the plasma spray and anodization followed by hydrothermal treatment (220 – 225°C) did not affect the substrate grain size or hardness, while the thermal processing and calcination treatment at 313 – 446°C for the sol-gel method caused recovery and grain growth, as well as a significant decrease in the hardness of the Ti-substrates.
Degree
Ph.D.
Advisors
Trumble, Purdue University.
Subject Area
Biomedical engineering|Industrial engineering|Materials science
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