Chuanxian Ding

Bond strength of plasma-sprayed hydroxyapatite/Ti composite coatings

One of the most important clinical applications of hydroxyapatite (HA) is as a coating on metal implants, especially plasma-sprayed HA coating applied on Ti alloy substrate. However, the poor bonding strength between HA and Ti alloy has been of concern to orthopedists. In this paper, an attempt has been made to enhance the bonding strength of HA coating by forming a composite coating with Ti. The bioactivity of the coating has also been studied. HA/Ti composite coatings were prepared via atmospheric plasma spraying on Ti-6Al-4V alloy substrates. The bond strength evaluation of HA/Ti composite coatings was performed according to ASTM C-633 test method. X-ray diffractometer and scanning electron microscopy were applied to identify the phases and the morphologies of the coatings. The bioactivity of HA/Ti composite coating was qualified by immersion of coating in simulated body fluid (SBF). The obtained results revealed that the addition of Ti to HA improved the bonding strength of coating significantly. In the SBF test, the coating surface was covered by carbonate-apatite, which was testified by X-ray photoelectron spectroscope, indicating good bioactivity for HA/Ti composite coating. The bioactivity of the coating has not been reduced by the addition of Ti.

Apatite formed on the surface of plasma-sprayed wollastonite coating immersed in simulated body fluid.

Wollastonite coatings on titanium alloys substrates were prepared by plasma spraying and incubated in simulated body fluids for different periods to investigate the nucleation and growth of apatite on their surface. Surface structural changes of the specimens were analyzed by XRD and IR technologies. SEM and EDS were used to observe surface morphologies and determine the composition of wollastonite coatings before and after immersion in simulated body fluid. The changes in the concentrations of calcium, silicon and phosphorus in the simulated body fluids due to the immersion of the specimens were measured by inductively coupled plasma atomic emission spectroscopy. The results obtained showed that hydroxycarbonate apatite can be formed on the surface of the coating soaked in SBF for 1 day. With longer immersion periods, the coating surface was covered by hydroxycarbonate apatite, which indicated that the wollastonite coating possesses good bioactivity.

Tribological properties of nanostructured and conventional WC–Co coatings deposited by plasma spraying

Abstract Nanostructured and conventional WC–Co coatings were deposited by vacuum plasma spraying. The wear and friction properties of the two coatings against alumina under dry friction conditions were comparatively studied. It was found that the wear resistance of the nanostructured WC–Co coating is superior to that of conventional WC–Co coatings, especially under high load conditions. The improved wear resistance of the nanostructured coating is attributed to its higher hardness and toughness. The wear mechanism of the nanostructured WC–Co coating is plastic deformation with slight surface fracture, whilst that of a conventional WC–Co coating is the initial removal of a binder phase followed by fragmentation or uprooting of carbide grains. Their tribological properties are discussed in relation to the microstructure of the two coatings. It is concluded the decomposition is a fatal factor influencing the wear resistance of thermal sprayed nanostructured WC–Co coatings.

Bioactivity of plasma sprayed dicalcium silicate coatings

Dicalcium silicate coatings on titanium alloys substrates were prepared by plasma spraying and immersed in simulated body fluids for a period of time to investigate the nucleation and growth of apatite on the surface of the coatings. Surface structural changes of the specimens were analyzed by XRD and IR technologies. SEM and EDS were used to observe surface morphologies and determine the composition of dicalcium silicate coatings before and after immersion in simulated body fluid. The plasma sprayed dicalcium silicate coating was bonding tightly to the substrate. The coating was mainly composed of beta-Ca2SiO4 and glassy phase. A dense carbonate-containing hydroxyapatite (CHA) layer was formed on the surface of the plasma sprayed dicalcium silicate coating soaked in SBF solution for 2 days. In addition, a silica-rich layer was also observed between CHA layer and coatings. With an increase in the immersion time, the CHA layer gradually became thicker. The results obtained indicated that the plasma sprayed dicalcium silicate coating possesses excellent bioactivity.

In vivo evaluation of plasma sprayed hydroxyapatite coatings having different crystallinity.

In this paper, hydroxyapatite (HA) coatings having the crystallinities of 56% and 98% were deposited by the plasma spraying and vapor-flame treatment process. The phase composition and crystallinity of the coatings were investigated by X-ray diffraction and infrared spectra. The dissolution behavior of the coatings in tris-buffer solutions was examined. The results obtained indicated that the coating having the high crystallinity showed the lower dissolution as compared to the low crystallinity coating. The bone bonding ability of HA coatings were observed in vivo by implanted in dogs femur. After 3 months implantation, the high crystallinity coating showed the higher shear strengths and remained integrated, whereas the separation of the coating fragments was clearly observed in the coating having low crystallinity.

Plasma sprayed wollastonite/TiO2 composite coatings on titanium alloys.

Wollastonite/TiO2 composite coatings were prepared using plasma spraying technology onto Ti-6Al-4V substrate. The composite coatings exhibit obvious lamellar structure with alternating wollastonite coating and TiO2 coating. No obvious cracks exist on the interface between coatings and substrate. In the case of composite coatings, the primarily crystalline phases of the coatings are wollastonite and rutile, indicating wollastonite and TiO2 did not react during plasma spraying process. Some of rutile in the powders transforms into anatase due to plasma spraying. The mean bond strength of the composite coatings is higher than 30 MPa. The Vickers microhardness of coatings increase with the increase in the content of TiO2. Wollastonite/TiO2 composite coatings were soaked in simulated body fluid to examine their bioactivity. Carbonate-containing hydroxyapatite (CHA) layer was formed on the surface of the wollastonite and W7T3 coatings soaked in simulated body fluid, while was not formed on the surface of the TiO2 and W3T7 coatings after immersion. In addition, a rich-silica layer appeared at the interface of CHA and wollastonite and W7T3 coatings. In order to investigate the cytocompatibility of the coatings, osteoblast was seeded onto the surface of the coatings. The scanning electron microscopy observation showed that the addition of wollastonite promote the proliferation of osteoblast. It is enough to prove that the wollastonite and wollastonite/TiO2 composite coatings possess excellent cytocompatibility.

Characterization of alumina-3 wt.% titania coating prepared by plasma spraying of nanostructured powders

Abstract Nanostructured and conventional alumina–3 wt.% titania coatings were deposited by air plasma spraying (APS). The microstructure and phase composition of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties including hardness, adhesion strength, crack extension force (GC) and sliding wear rate were measured. Equiaxed α-Al2O3 grains were observed in the nanostructured Al2O3–3 wt.% TiO2 coating and the diameter of α-Al2O3 grains were about 150 to 700 nm in size. The microhardness of both kinds of coating was similar and about 820 HV0.2. However, the adhesion strength and crack extension force of the nanostructured coating increased by 33 and 80%, respectively, as compared with those of the conventional coating. The wear rate of the nanostructured coating was lower than that of the conventional coating. The results were explained in terms of characteristics of the powders and microstructure of the coatings.

Atmospheric plasma sprayed coatings of nanostructured zirconia

Abstract Nanostructured zirconia coating has been fabricated by atmospheric plasma spraying. The microstructure of the coating has been characterized with SEM and XRD, and the microhardness has been measured. The results show ZrO 2 coating possessed two kinds of structure. One was the poorly consolidatedly structure, which was composed of nanosized particles. The other was an overlapping structure, which consisted of micrometer size particles. The former was the main structure in the coating. The zirconia coating had the same phase composition as the starting powder and the thickness of coating was quite heterogeneous. The as-sprayed nano zirconia coating had a similar microhardness to sintered ZrO 2 , which was much higher than the conventional couterpart.

UV-irradiation-induced bioactivity on TiO2 coatings with nanostructural surface

Titania (TiO2) coatings with nanostructural surface prepared using plasma spraying technology were irradiated by ultraviolet light in simulated body fluids to improve their bioactivity. The in vitro bioactivity of the coatings was evaluated by investigating the formation of apatite on their surfaces in simulated body fluids. Bone-like apatite was observed to precipitate on the UV-irradiated TiO2 coating with nanostructural surface after it was immersed in simulated body fluid for a certain period, but not on the as-sprayed and UV-irradiated TiO2 coatings without nanostructural surface. The results indicate that the nano-TiO2 surface can be activated by UV-irradiation to induce its bioactivity. The ability of apatite formation on the nano-TiO2 surface was improved with the increase of UV-irradiation time. The in vivo results reveal that the as-prepared TiO2 coating with nanostructural surface cannot induce the formation of new bones during the implantation period, but the UV-irradiated TiO2 coating with nanostructural surface could do so during an implantation time longer than 2 months. Our results indicate that the osseointegration ability of the plasma-sprayed TiO2 coating with nanostructural surface can be improved by UV irradiation.

Microstructure, bioactivity and osteoblast behavior of monoclinic zirconia coating with nanostructured surface

A monoclinic zirconia coating with a nanostructural surface was prepared on the Ti-6Al-4V substrate by an atmospheric plasma-spraying technique, and its microstructure and composition, as well as mechanical and biological properties, were investigated to explore potential application as a bioactive coating on bone implants. X-ray diffraction, transmission electron microscopy, scanning electron microscopy and Raman spectroscopy revealed that the zirconia coating was composed of monoclinic zirconia which was stable at low temperature, and its surface consists of nano-size grains 30-50 nm in size. The bond strength between the coating and the Ti-6Al-4V substrate was 48.4 + or - 6.1 MPa, which is higher than that of plasma-sprayed HA coatings. Hydrothermal experiments indicated that the coating was stable in a water environment and the phase composition and Vickers hardness were independent of the hydrothermal treatment time. Bone-like apatite is observed to precipitate on the surface of the coating after soaking in simulated body fluid for 6 days, indicating excellent bioactivity in vitro. The nanostructured surface composed of monoclinic zirconia is believed to be crucial to its bioactivity. Morphological observation and the cell proliferation test demonstrated that osteoblast-like MG63 cells could attach to, adhere to and proliferate well on the surface of the monoclinic zirconia coating, suggesting possible applications in hard tissue replacements.