Tom Troczynski

Water-based sol–gel synthesis of hydroxyapatite: process development

Hydroxyapatite (HA) ceramics were synthesized using a sol-gel route with triethyl phosphite and calcium nitrate as phosphorus and calcium precursors, respectively. Two solvents, water and anhydrous ethanol, were used as diluting media for HA sol preparation. The sols were stable and no gelling occurred in ambient environment for over 5 days. The sols became a white gel only after removal of the solvents at 60 degrees C. X-ray diffraction showed that apatitic structure first appeared at a temperature as low as 350 degrees C. The crystal size and the HA content in both gels increase with increasing calcination temperature. The type of initial diluting media (i.e., water vs. anhydrous ethanol) did not affect the microstructural evolution and crystallinity of the resulting HA ceramic. The ethanol-based sol dip-coated onto a Ti substrate, followed by calcination at 450 degrees C, was found to be porous with pore size ranging from 0.3 to 1 microm. This morphology is beneficial to the circulation of physiological fluid when the coating is used for biomedical applications. The satisfactory adhesion between the coating and substrate suggests its suitability for load-bearing uses.

Sol-gel hydroxyapatite coatings on stainless steel substrates

Thin film hydroxyapatite deposits onto sandblasted 316L stainless steel substrates were prepared using water-based sol-gel technique recently developed in our lab. The coatings were annealed in air at 375 degrees C, 400 degrees C, and 500 degrees C. Phase formation, surface morphology, interfacial microstructure, and interfacial bonding strength of the coatings were investigated. Apatitic structure developed within the coatings while annealing at temperatures > or = 400 degrees C, while those heat-treated at 375 degrees C showed poor crystallinity. The coatings were dense and firmly attached to the underlying substrates, reaching an average bonding strength (as determined through the pull-out test) of 44 MPa. Nano-porous structure was found for the coatings annealed at 500 degrees C, believed to result from grain growth, and causing a slight decrease in the bonding strength. Surface microcracking, although not extensive, occurred after annealing at temperatures > or = 400 degrees C, and was linked to non-uniform thickness of the coating due to roughness of the substrate. A contraction of the coatings as a result of sintering, and phase transition from amorphous (or poor crystalline) to reasonably good crystalline apatite, may be responsible for the loss of structural integrity of the thicker sections of the coatings. It seems quite promising that a dense and adhesive apatite coating can be achieved through water-based sol gel technology after short-term annealing at around 400 degrees C in air.

Structural evolution of sol–gel-derived hydroxyapatite

Structural evolution upon transformation of sol to gel, and gel to final ceramic during the synthesis of hydroxyapatite is investigated using Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), thermal behavior (DTA and TGA), and electron microscopy examination (SEM/TEM). The sol was first thermally aged at 45 C for various time periods up to 120 min. The colloidal sol, which may have an oligomeric structure, was relatively stable against coagulation. Upon drying, the sol particles consolidated into dry gel through van der Waals attraction, and showed X-ray amorphous phosphate structure. The solid gels showed a particulate microstructure, composed of primary particles of about 8-10 nm in diameter. The amorphous gel transformed into crystalline apatite at temperatures > 300 C. The calcined gels showed a nano-scale microstructure, with grains of 20-50 nm in diameter. Through an appropriate heat treatment between 300 and 400d degrees C. the apatite prepared using current process exhibits a nano-scale, low-crystallinity, carbonated apatitic structure, which closely resembles that of human bone apatite.

Development of plasma-sprayed bioceramic coatings with bond coats based on titania and zirconia

Bond coats for plasma-sprayed hydroxyapatite (HAp) coatings on Ti-6A1-4V hip endoprotheses are being developed for improved in vivo performance. Bond coat powders consisting of (i) CaO-stabilized zirconia, (ii) a eutectic composition of titania and non-stabilized zirconia, and (iii) titania were applied by atmospheric plasma spraying (APS) to Ti-6A1-4V-coupons and 100 microm-thick Ti-6A1-4V foils. Subsequently, a thick layer of HAp was sprayed onto the thin bond coats. Peel tests on Ti-6A1-4V foil/bond coat/HAp top coat assemblies revealed that titania and titania/ zirconia bond coats increased the peel adhesion strength in a statistically significant way from 22 N m(-1) (HAp without a bond coat) to >42 and 32 N m(-1), respectively. Microstructural investigations by SEM on cross-sections of coatings leached in simulated body fluid for up to 28 days led to the conclusion that the chemically very stable bond coats act as an improved chemical barrier against in vivo release of metal ions from the implant, as well as an improved adhesive bond by development of very thin well-adhering reaction layers, presumbly composed of perovskite, calcium dititanate, and/or calcium zirconate.

Aging effect on the phase evolution of water-based sol-gel hydroxyapatite.

In a number of recent reports on the synthesis of sol-gel hydroxyapatite, aging of the precursor solution has been found to be critical in developing an apatitic phase. Critical aging time is required to complete reaction between Ca and P molecular precursors to form a desired intermediate complex that permits a further transformation to apatite phase under appropriate thermal treatment. In this investigation, we employed a water-based sol-gel process recently developed to fabricate hydroxyapatite at relatively low temperatures. The aging effect on apatite formation was systematically studied in terms of aging time and temperature. Experimental results show that the aging time is considerably reduced as aging temperature rises. Long-term thermal aging was unfavorable for apatite formation. The optimal aging parameters for apatite formation were experimentally determined, which was further consolidated into a phase evolution map. Aging kinetic was investigated by monitoring the variation of solution pH, following the determination of an apparent activation energy, which has a value as high as 10.35 kcal/mol, for the chemical reaction occurring upon aging. Optimal solution chemistry was elucidated based on the corresponding phase evolution map.

Sol–gel BaTiO3 thin film for humidity sensors

Abstract BaTiO3 ceramic thin films were prepared by sol–gel processing and their humidity sensitive characteristics have been investigated. The films were formed on Al2O3 substrates with comb type Au electrodes by spin coating. It was found that the impedance of the thin films decreases with increasing relative humidity (RH). The humidity sensitivity of the sensor depends on the applied frequency of the electric signal and the activation energy for conduction is reduced with water adsorption. The BaTiO3 thin films show a negligible hysteresis under cyclic humidity changes with a good long-term stability and fast response time. It appears that the sol–gel process produces films that can be used as efficient humidity sensors.

Apatite formation on TiO2 anatase microspheres

Abstract Titanium dioxide (TiO 2 ) coatings have been long considered as biocompatible interfaces to promote the physico-chemical bonding between the bone tissues and implant material (e.g., titanium and stainless steel). Monodispersed TiO 2 (anatase, the low temperature polymorph of TiO 2 ) microspheres, produced in the form of colloidal precipitates, were deposited on different substrates and apatite formation was induced on the resulted surface by immersing the coated substrates in simulated body fluid solution. Analytical and microstructural investigations, conducted by X-ray diffraction, energy depressive X-ray spectroscopy and scanning electron microscopy techniques, showed considerable higher rates of apatite formation, in vitro, on the anatase microspheres compared to the sol–gel-derived thin films of the same oxide. We concluded that the particular surface morphology of the packed TiO 2 microspheres, promotes a faster apatite formation in vitro.

The microstructural dependence of wear and indentation behaviour of some plasma-sprayed alumina coatings

Abstract Plasma-sprayed coatings are built up from individual overlapping solidified droplets, and the nature of boundaries between these “splats” influences their mechanical and wear characteristics. This work aims to elucidate the correlation between these characteristics and microstructural features of model coatings produced under well-controlled processing conditions. A series of coatings with different average splat size were sprayed from pure alumina powders of different particle size using an axial-injection plasma-spray torch under conditions optimized to obtain the best microstructures. Phase composition was determined by X-ray diffraction, while SEM with image analysis of polished and fractured surfaces was used to evaluate coating microstructures, and indentation and wear damage. Coatings were characterized by depth-sensing indentation (DSI) at the sub-splat scale and by DSI, micro- and macro-indentation at multiple splat scales. Both abrasion and dry-particle erosion behaviour of the coatings were also investigated. Results reveal the role of microstructural features on the material performance, particularly the critical influence of porosity, splat (grain) size and inter-splat cohesion, on the multiple-splat scale wear and indentation behaviour of the coatings.

Correlations between microstructural parameters, micromechanical properties and wear resistance of plasma sprayed ceramic coatings

Abstract The micromechanical integrity of a ceramic plasma sprayed (PS) coating is determined by the size and distribution of the defects found in the coating, such as porosity, the inter-lamellar microcrack density, the intra-lamellar microcrack density as well as the lamellar, or splat, dimensions. In this work, several micromechanical tests were used to advance our understanding of the relationships between the different microstructural parameters found in PS ceramic coatings. The tests included depth sensing indentation, micro and macrohardness testing, and controlled scratch testing. Abrasive and erosive wear tests were performed on the same set of coatings, including plasma sprayed alumina and chromia, as well as sintered alumina as a reference material. The best correlations were found between the material hardness ( H ), the level of porosity ( P ) and the abrasive wear volume ( W ). Knoop hardness measurements provided the best correlation with wear data, followed by scratch hardness and Vickers hardness. An exponential function of the type W = k / H n was found, where k and n are constants. A similar function describes the correlation of wear volume with the elastic modulus of the coating. Fracture toughness could only be correlated with wear volume when combined with hardness in a function of the type W = k / H 0.5 K c 0.5 . The incorporation into this function of a “microstructural factor” M = P n improves the correlation.

Influence of apatite seeds on the synthesis of calcium phosphate cement

This preliminary study explores the seeding effect (using crystalline hydroxyapatite particles) on the setting time, compressive strength, phase evolution, and microstructure of calcium phosphate cements (CPC) based on monocalcium phosphate monohydrate and calcium hydroxide. Experimental results showed that the setting time varies from 5 to about 30 min, as the seed concentration increased from 0 to 20 wt%. The compressive strength of CPC increased from 4 to 17 MPa, followed by decrease to 12 MPa, for the same range of seeds content. The CPC transformed to predominantly apatitic structure within 24 h for all the samples, with or without the seeds. However, increase of the seed concentration improved the final crystallinity of the apatite phase, suggesting nucleation and growth effects during precipitation of CPC from the precursor solution. The microstructure of the resulting apatitic cement showed a change from essentially featureless (or glass-like) to thin, elongated plate-like morphology, as seeds concentration increased. Correlation between microstructural evolution and corresponding compressive strength of seeded CPC is investigated.