Porous Apatite Coating on Various Titanium Metallic Materials via Low Temperature Processing

Nowadays, hydorxyapatite (HA) is widely used as bioceramics in reconstructive surgery, in dentistry and as drug delivery materials due to the good biocompatibility and osteoconductivity [Hench, 1998]. One of the limitations for the usage of these materials is their low mechanical strength. Thus, many researchers focuses on the development of new biomaterials, that combine the osteoconductive characteristics of bioactive ceramics with sufficient strength and toughness for load-bearing applications. The combination of high strength of the metals with osteoconductive properties of bioactive ceramics makes HA coated metallic implants, which titanium (Ti) or its alloys was mainly used, very attractive for the loaded-bearing applications in orthopedic and dental surgery[Long & Rack, 1998]. A plasma spraying method has been conventionally employed for the HA coating. However, this method has some problems (e.g. a poor coating-substrate adherence, lack of HA crystallinity) for the long-term performance and lifetime of the implants [Aoki, 1994]. Therefore, new HA coating methods hava attracted great interests in recent years for replacing the high temperature techniques like plasma spraying. Hydrothermal hot-pressing (HHP) method is a possible processing route for producing a ceramic body at relatively low temperatures (under 300 ̊C) [Yamasaki et al., 1986]. The compression of samples under hydrothermal conditions accelerates densification of inorganic materials. It is known that the water of crystallization in calcium hydrogen phosphate dihydrate (CaHPO4・2H2O; DCPD) is slowly lost below 100 ̊C [Peelen et al., 1991]. If the released water can be utilized as a reaction solvent during the HHP treatment, it is to be expected that the joining HA to metal can be achieved simultaneously under the hydrothermal condition, in addition to the synthesis and solidification of HA through the chemical reaction as follows [Hosoi et al., 1996]: