Bioceramic composite coatings fabricated by Nd-YAG laser cladding on Ti6Al4V substrate

摘要

Hydroxyapatite(HAP) has found extensive applications in orthopedics and dentistry because of its bioactivity and osteoconductivity. It is well known that HAP is almost non-degradable even after a long implant period,, while TCP is more or less bioresorbable. By controlling the Ca/P ratio in the synthesis process of calcium phosphate, ideal biphasic calcium phosphate ceramics can be produced with reasonable solubility and biomechanical strength, and thus, better biomaterials than either pure HAP or TCP ceramics can be fabricated .However, the intrinsic brittleness and poor strength of calcium phosphate ceramic restricts its clinical applications under load-bearing conditions. A bioceramic coating (mainly hydroxyapatite and calcium phosphate bioceramic) was fabricated on the metal substrate with excellent mechanical properties by using a certain processing technology, which is thought of one of the most promising ways to integrate the merits of the metal and bioceramic materials. Undesirable phase and microstructure formation, and poor coating/metal bonding strength restrict the fabrication technique to obtain hydroxyapatite(HAP) and other calcium phosphate ceramic coatings. Laser cladding technology was adopted to fabricate hydroxyapatite and calcium phosphate compounds coating according to the feature that a metallurgical bonding can be formed by laser cladding process. Compared with CO_2 laser, Nd-YAG laser has different wavelength. The wavelength of CO_2 laser is 10.06μm, and wavelength of CO_2 laser is 1.06μm. Metal and ceramic material has quite different absorbance ability towards them (As Figure 1 indicated) and thus they can generate different laser cladding products by these two laser surface processings with different wavelength. This paper presents a new process and mechanism analysis to obtain bioceramic coating on Ti6Al4V substrate by Nd-YAG laser cladding. A bioceramic composite coating including HAP, Ca_2P_2O_7 , calcium titanates and titanium phosphates was successfully obtained by Nd-YAG laser cladding with pre-depositing mixed powders of CaHPO_4·2H_2O and CaCO_3 directly on Ti6Al4V substrate. The absorption of metal to laser is the highest in infrared range, while the mixed powders of CaHPO_4·2H_2O and CaCO_3 is transparent to infrared light but opaque to UV light. Nd-YAG laser transmits mixed powders of CaHPO_4·2H_2O and CaCO_3 and the laser power is absorbed by Ti6Al4V substrate to produce a thin layer of molten region. The mixed powders will not begin to melt and react with each other until Ti6Al4V substrate is melted by Nd-YAG laser energy and hydroxyapatite and other calcium phosphate compounds present in the coating. The pre-deposited mixed powders obtain the reaction energy from the ambient molten Ti6Al4V substrate during Nd-YAG laser cladding, whereas it gets the chemical reaction energy directly from CO_2 laser beam during CO_2 laser processing which easily make the reacting procedure in the coating superheat and difficult to get HAP constituent or make HAP decompose. Scanning electron microscope (SEM) was used to investigate the microstructure of bioceramic composite coating; the chemical composition of the coating is determined by energy dispersive spectrometry (SEM-EDS) and the phases of the coating are characterized by X-ray diffraction(XRD) technology. There are two general kinds of chemical reaction system in the coating during Nd-YAG laser cladding processing. When CaHPO_4·2H_2O and CaCO_3 powders react together, they make calcium phosphate bioceramic products; The microstructure of the bioceramic composite coating is even and minute because of the rapid solidification in laser processing according to G/R ratio. A chemical metallergical bonding is formed between the boceramic composite coating and Ti6Al4V metalsubstrate. It can also be expected that Nd-YAG laser cladding technology can be used as a further modification procedure to enhance HAp/metal interface property.