Effect of post vacuum heating on the microstructural feature and bonding strength of plasma-sprayed hydroxyapatite coatings

Characterization and mechanical behaviour of reinforced hydroxyapatite coatings deposited by vacuum plasma spray on SS-316L alloy

Hydroxyapatite powder reinforced individually with 10wt percentage (wt%) of Al₂O₃ and ZrO₂ (HA + 10wt% Al₂O₃ and HA + 10wt% ZrO₂) was thermally sprayed onto SS-316L substrate with a bond coat of Zirconia by vacuum plasma spray (VPS) technique. The resulted coatings were heat treated at 700°C for 1h to study its effects on microstructural and mechanical properties of the deposited coatings. The characterization of the coatings was carried out using scanning electron microscope, x-ray diffraction, porosity, surface roughness and crystallinity using Rietveld analysis. The results indicated that after post coating heat treatment substantial decrease in porosity was witnessed along with significant improvement in crystallinity. Besides, the hardness across the cross-section of the coatings and bond strength was considerably improved; however the hardness of top coat was reduced owing to the loosening of un-melted and partially melted particles by diffusion process which takes place during heat treatment.

Suspension thermal spraying of hydroxyapatite: microstructure and in vitro behaviour

In cementless fixation of metallic prostheses, bony ingrowth onto the implant surface is often promoted by osteoconductive plasma-sprayed hydroxyapatite coatings. The present work explores the use of the innovative High Velocity Suspension Flame Spraying (HVSFS) process to coat Ti substrates with thin homogeneous hydroxyapatite coatings. The HVSFS hydroxyapatite coatings studied were dense, 27-37μm thick, with some transverse microcracks. Lamellae were sintered together and nearly unidentifiable, unlike conventional plasma-sprayed hydroxyapatite. Crystallinities of 10%-70% were obtained, depending on the deposition parameters and the use of a TiO2 bond coat. The average hardness of layers with low (<24%) and high (70%) crystallinity was ≈3.5GPa and ≈4.5GPa respectively. The distributions of hardness values, all characterised by Weibull modulus in the 5-7 range, were narrower than that of conventional plasma-sprayed hydroxyapatite, with a Weibull modulus of ≈3.3. During soaking in simulated body fluid, glassy coatings were progressively resorbed and replaced by a new, precipitated hydroxyapatite layer, whereas coatings with 70% crystallinity were stable up to 14days of immersion. The interpretation of the precipitation behaviour was also assisted by surface charge assessments, performed through Z-potential measurements. During in vitro tests, HA coatings showed no cytotoxicity towards the SAOS-2 osteoblast cell line, and surface cell proliferation was comparable with proliferation on reference polystyrene culture plates.

Contact nanofatigue shows crack growth in amorphous calcium phosphate on Ti, Co-Cr and Stainless steel

Fatigue testing of load-bearing coated implants is usually very time-consuming and so a new contact nanofatigue test using a nanoindenter has been evaluated. A cube corner indenter provided the fastest indication of failure, through crack formation, compared to a spherical indenter. Contact nanofatigue was performed on a sintered hydroxyapatite and then on amorphous calcium phosphate splats produced on titanium, stainless steel and Co-Cr surfaces, made either at room temperature or on 250°C preheated surfaces. Sintered hydroxyapatite showed continual plastic deformation, but this is not that apparent for splats on metal surfaces. Substrate preheating was found to induce cracking in splats, explained by greater thermal residual stresses. Endurance during contact nanofatigue, measured as time to crack formation, was the lowest for splats on titanium followed by Co-Cr and stainless steel. The splat on titanium showed both cracking and plastic deformation during testing. Good agreement has been reached with previous studies with cracking directed to the substrate without splat delamination. Contact nanofatigue with the nanoindenter easily and quickly identifies cracking events that previously required detection with acoustic emission, and shows good feasibility for mechanical testing of discs and splats produced by thermal spraying, spray forming, laser-ablation, aerosol jet and ink jet printing.

Microstructure and in vitro behaviour of 45S5 bioglass coatings deposited by high velocity suspension flame spraying (HVSFS)

The high-velocity suspension flame spraying technique (HVSFS) was employed in order to deposit 45S5 bioactive glass coatings onto titanium substrates, using a suspension of micron-sized glass powders dispersed in a water + isopropanol mixture as feedstock. By modifying the process parameters, five coatings with different thickness and porosity were obtained. The coatings were entirely glassy but exhibited a through-thickness microstructural gradient, as the deposition mechanisms of the glass droplets changed at every torch cycle because of the increase in the system temperature during spraying. After soaking in simulated body fluid, all of the coatings were soon covered by a layer of hydroxyapatite; furthermore, the coatings exhibited no cytotoxicity and human osteosarcoma cells could adhere and proliferate well onto their surfaces. HVSFS-deposited 45S5 bioglass coatings are therefore highly bioactive and have potentials as replacement of conventional hydroxyapatite in order to favour osseointegration of dental and prosthetic implants.

Kinetics of hydrothermal crystallization under saturated steam pressure and the self-healing effect by nanocrystallite for hydroxyapatite coatings

Hydroxyapatite coatings (HACs) with a low crystalline state were prepared using the plasma spraying process followed by hermetic autoclaving hydrothermal treatment at 100, 150 and 200 degrees C. Experimental evidence confirmed that the HACs became significantly crystallized and the content of amorphous calcium phosphate decreased by performing the autoclaving hydrothermal treatment in an ambient saturated steam pressure system. The obvious chemisorbed hydroxy groups (OH) peak in the X-ray photoelectron spectra detected from the hydrothermally crystallized HAC specimens means that the hydroxyl-deficient state of plasma-sprayed HACs is significantly improved by the abundant replenished OH groups. The HA nanocrystallite observed from scanning electron microscopy and transmission electron microscopy images within hydrothermally treated HACs is the result of nucleation and grain growth through the replenishment of OH groups into the hydroxyl-deficient HA crystal structure. The microstructural self-healing effect is a result of reduction in defects (pores, microcracks and lamellar boundaries) due to new-growth HA nanocrystallite. According to the systematic derivation of the Arrhenius equation, the HA crystallization is a second-order Arrhenius reaction kinetics. Besides the effects of heating temperature and an atmosphere with abundant water molecules, the saturated steam pressure is a crucial factor which significantly improves the crystallization rate constant and further reduces the activation energy for the hydrothermal HA crystallization.

Low Temperature Crystallization and Structural Modification of Plasma-Sprayed Hydroxyapatite Coating with Hydrothermal Treatment

The effect of autoclaving hydrothermal treatment on the characteristics of plasma-sprayed hydroxyapatite (HA) coatings on the Ti-6Al-4V substrate was investigated. The heating temperatures were 100°C, 150°C and 200°C with ambient saturated steam pressure in an autoclave. On the basis of quantitative analysis of crystallinity using x-ray diffraction (XRD), hydrothermal treatment was found to be effective for increasing the crystallinity and phase purity of the HA coatings. The prominent and sharp OH− and PO4 3− peaks detected from x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra demonstrate a superior crystallized integrity of hydrothermal-treated HA coatings through the incorporation of water vapor. Moreover, the significant presence of OH− peak in XPS spectra represents a replenishment of water molecules which tends to reduce the dehydroxylation state of as-sprayed HA coatings. From the observation of microstructures, crystallized HA was found to diminish the spraying defects of hydrothermal HA coating layers, and finely-crystallized HA crystals, with a Ca/P atomic ratio of 1.67, were observed through transmission electron microscopy (TEM). Hydrothermal treatment could induce a low-temperature crystallization process, and the saturated steam pressure is thought to be a factor which reduces the activation energy and accelerates the HA crystallization. Experimental evidence confirmed that the ambient saturated steam pressure plays an important role in lowering heating temperatures and promoting HA crystallization.