Fabrication of HAp–8YSZ composite layer on Ti/TiO2 nanoporous substrate by EPD/MAO method
Highlights
► We fabricated HAp–8YSZ ceramic composite layer for the first time on Ti substrate through EPD/MAO process. ► We use Ti/TiO2 nanoporous layer in order to improve adherence between ceramic top layer and Ti substrate. ► Dense and crack free HAp–8YSZ composite layer was fabricated by well-designed sintering cycle. ► Phase evolutions was well described according to previous works.
Introduction
Titanium and its alloys are proven to be potentially very suitable materials for load bearing in bio implant applications [1]. However, they exhibit poor bioactive properties, and it is recommended that the titanium-based implants be coated with a bioactive material. Recently, Hydroxyapatite (HAp) has been noted as a bioactive material [2], [3] due to its chemical and biological similarity to the body hard tissues and its consequent direct bonding to host bones [4], [5], [6], [7]. When titanium implants are coated by HAp layers, an appropriate connection to the host bone and a favorable biocompatibility and bioactivity are achieved [8]. Since chemical composition and structure of HAp differ from those of the titanium implants, poor adhesion of this coating material to the substrate is anticipated [9]. To enhance the adhesion of HAp layer to Ti substrate, TiO2 layer as a bond coat which has high chemical affinity with HAp and Ti [1], [8], [10] is placed between the main layer and the substrate [4].
Furthermore, high strength and toughness indexes have been observed by loading ZrO2 to HAp coating which is also considered as an effective way to increase the interfacial bonding strength between the matrix and the coating [11], [12]. Meanwhile, micro-arc oxidation (MAO) process is a relatively new method to fabricate ceramic layers on the surface of the titanium, aluminum, magnesium, and zirconium components with a porous structure and adhere the obtained films firmly to the substrate [10]. More details about the MAO process may be found elsewhere [13], [14], [15].
Moreover, electrophoretic deposition (EPD) is known to be an efficient technique due to its high versatility for application of different materials, simplicity in setup, low equipment cost, little restriction of the shape of substrate and capability to form complex shapes and patterns [16], [17]. EPD is a colloidal process through which charged powder particles dispersed in a liquid medium are attracted and deposited on a conductive substrate with an opposite charge by employing a DC electric field [18], [19], [20].
In this report, HAp–8YSZ composite layers were situated by EPD process on Ti/TiO2 substrate, fabricated by MAO method, for the first time and subsequently, the effect of sintering temperature on microstructure and phase features of the layers were investigated.
Section snippets
Experimental procedures
Commercially pure grade 2 (CP-II) titanium foils with dimensions of 30 mm × 15 mm × 0.5 mm were used as the substrate whose surface was cleaned through a multi-step procedure. First, they were chemically etched in diluted HF solution (5 vol.%) for 30 s and then, washed by distilled water. Afterwards, they were ultrasonically cleaned in acetone for 15 min. Finally, the cleaned substrates were washed again by distilled water and dried in air. MAO electrolyte was prepared by dissolving tri-sodium phosphate
Results and discussion
Fig. 1a–d illustrates the surface morphologies of sintered hydroxyapatite/zirconia composite coatings with different ratios of HAp/8YSZ sintered at 1300 °C. Other SEM/EDX results are not depicted here. Crack free microstructure is observed which can be attributed to the good packing of submicron particles obtained by EPD. EDS results confirmed the presence of Calcium, Phosphorous and Zirconium elements which originated from starting materials. By decreasing the amount of 8YSZ, a slight reduction
Conclusions
HAp/8YSZ composites were successfully fabricated by EPD technique onto Ti/TiO2 nanoporous grown layer. XRD patterns revealed β-TCP formation due to the decomposition of HAp which could be attributed to thermal effects and/or 8YSZ existence. Also, CaZrO3 phases were detected in all samples increasing by the amount of 8YSZ. Glassy phase during sintering of HAp/8YSZ was observed in SEM micrographs; the amount of which was maximized in HZ33 sample.
Acknowledgment
The research team would sincerely appreciate Mr. H. Zargar.
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