Hardness of bioactive glass film deposited on titanium alloy by pulsed laser ablation

Abstract

Certain glasses which contain specific amounts of SiO2, Na2O, CaO and P2O5 (bioglasses) are of a limited number of materials forming strong chemical bonds with bone in vivo, while remaining stable under the harsh conditions encountered in the human body [1–7]. These glasses contain usually less than 60 mol% SiO2, and are of a high Na2O content and a high CaO/P2O5 ratio [5]. Such a composition produces a surface that becomes highly reactive when exposed to a water-containing environment, e.g. of the human body fluids [5]. Monolithic bioglasses are too brittle to be used in structural applications, e.g. as dental root implants. Poor fracture toughness and strength limited the use of these bulk materials to non-load-bearing implants. One way to overcome this problem is to coat a metallic implant with bioglass. The coating provides the bone bonding capacity while the metal substrate provides the structural support. For this reason, various methods were tested to coat metallic implants with glass. The success achieved by these methods is variable. The most commonly used techniques are enameling, electrophoresis, sol-gel, and plasma spraying. The main disadvantage of these methods is poor adhesion strength of the coating to substrate. Besides, electrophoresis and sol-gel methods are substrate specific. Pulsed laser technique has recently been used for fabrication of numerous organic and inorganic films and coatings, as well as for their surface modification [8–11]. Over the last decade laser-induced processes were effectively introduced into advanced biomaterials research and development. The advantage of pulsed laser ablation deposition (PLAD) results from the following. Through the use of a laser radiation on a target, it is possible to vaporize a high refractory material altering the solid only at the zone of the laser spot. The ablated material forms dense plasma. Plasma plume spreads out transporting atoms, ions, clusters and particles to the substrate to form a thin coating. High temperature reached on the target, strongly contributes to maintain the starting materials stoichiometry even for multi-component systems with species of very different vapor pressures. PLAD allows to optimize physical properties of the coating which can be monitored by simply tuning the various deposition parameters: laser irradiation wavelength, energy of the laser pulse, pulse duration, gas composition and pressure in deposition chamber, type of the target and temperature of substrate. PLAD provides strong bonding of the coating with substrate, low residual stress imposed on the coating, and controlled roughness of the surface. Strong bonding of the coating to the substrate is due to the high energy of impacted species. Moreover, PLAD is a versatile technique and can be applied effectively for deposition of multilayered coatings. At the same time, PLAD is well matched with other coating methods, such as plasma spray and biomimetics. This allows to optimize the coatings parameters for different kinds of implants. The structural functions, which an implant should perform in service, are dependent on mechanical properties of the coating, the hardness being one of the most important characteristics. Due to the high reactivity of bioglass with respect to the humid environment, the hardness can be varying along the coating thickness. This may particularly allow to evaluate the hydration depth of the bioglass. In this letter, a study of the PLAD of a commercial Bioglass® on Ti-6A1-4V alloy substrate and hardness of the coating is reported. The original Bioglass® 45S5 has a nominal composition (at.%): Na 38.31, Ca 21.21, Si 36.37, P 4.11, by excluding the oxygen content to permit a comparison with energy dispersive X-ray (EDS) analysis. Before starting the PLAD process, EDS revealed the bioglass target compositions is as follows (at.%): Na 30.3, Ca 23.2, Si 43.8, P 2.8. The compositions difference could probably be due to hydratation and/or losses of volatile compounds. Further, by analyzing small areas at high magnification, small inhomogeneities in the elemental composition were found, but the discrepancy does not exceed 3%. The substrate material for the PLAD coating was a commercial Ti-6A1-4V alloy. Surface of