Thermal Stability of Titanium/Hydroxyapatite Systems

Nowadays, the pace of development in medicine and, at the same time, in materials engineering has made it possible to develop, design, and study biomaterials for use in orthopedics, dentistry, skin tissues, or drug transport. This situation offers the prospect of improving and extending the lives of patients. Elderly people are at risk of chronic diseases related to the skeletal system, i.e., bone degeneration. There is also an increasing demand for orthopedic implants for knee and hip joints. Implant materials must meet the appropriate requirements to be used in the human body. Over the years, the search has been on for the perfect combination of materials and a structure that can replicate the natural structure of bone tissue after implantation. One of the best solutions is to design a material composed of at least two phases. Creating such a composite allows the best properties of biomaterials to be used to create a new material. One of the most commonly used implant materials for bone implants is metals, especially titanium and its alloys. They are characterized by excellent mechanical properties essential for joint implants. Unfortunately, metallic biomaterials usually do not meet sufficient requirements for biocompatibility. For this reason, a calcium phosphate ceramic, i.e., hydroxyapatite, is introduced into the system to introduce bioactivity into the system. Hydroxyapatite is too brittle to be applied alone, but it can be an excellent solution for implants in combination with titanium. One of the most critical steps in producing such biomaterials is sintering at high temperatures, which is necessary to obtain the proper material structure. This requires knowledge of the behavior of materials at appropriate temperatures. Stoichiometric hydroxyapatite is stable and retains a single-phase apatite structure – P63/m up to and including 1300°C temperatures. Heating it to higher temperatures triggers its partial decomposition, decreasing the size of crystallites, as well as the appearance of significant structural stresses. It is found that even a slight deviation of the Ca/P ratio from the value of 1.67 significantly reduces the thermal stability of hydroxyapatite. Elevated temperature promotes the slow dehydroxylation of hydroxyapatite, which leads to thermal decomposition and conversion to β-tricalcium phosphate (β-TCP). The phenomenon in question affects the mechanical stability of the HAp coating and may impair its performance in vivo, due to the higher solubility of β-TCP compared to HAp. Moreover, when designing a composite created from titanium and hydroxyapatite, knowing how one component affects the other during high-temperature sintering is essential. This chapter is devoted to the thermal stability of the titanium/hydroxyapatite system.