Fundamental Research On Mechanical Properties And Surface Adsorption Of Mg-Ca System

With the development of biomaterials, biodegradable materials are introduced in this paper. The primary idea, development and challenge of biodegradable materials are shown in details. It is well known that Mg and its alloys are one new kind of metal biodegradable materials. The research and application of Mg and its alloys are not completed yet, and many things need to be done before Mg and its alloys can be used as a kind of clinical materials. However, based on the cheap price, good biocompatibility, density and mechanical properties close to natural bone and biodegradation in human body environment without releasing toxic ions, Mg and its alloys have been one of research hotspots in the field of biodegradable materials. And as one kind of potential biodegradable implants, Mg alloys, especially the Mg-Ca alloys, are introduced with its properties and present research situation. It is noticeable that many researches upon the biodegradable Mg alloys are done from the view of experiment, which suggests the lack of theoretical research upon Mg and its alloys as biodegradable implants. The properties like structural stability, mechanical property, phase transformation, corrosion property and the electronic structure are investigated by using First-principle calculation, and the intrinsic mechanism are discussed theoretically in this paper.Calculation results shows that Mg-Ca phases are energetically favorable with negative heats of formation within the entire composition range, and a strong chemical bonding is formed between Mg and Ca atoms. It is also revealed that the composition plays an important effect on mechanical properties of Mg-Ca alloys, and as a result, the MgCa phase with Mg:Ca=1:1would be a good candidate as degradable bone implant in terms of Young’s modulus and ductility. Considering the pressure in the human body, a series of researches upon the phase transformation of Mg-Ca alloys under high pressure have been done. It is found out that Mg3Ca, MgCa and MgCa3have phase sequences of BCC→HCP and FCC→HCP under high pressure, respectively, and Ca plays a dominant role in determining the electronic structures and stable crystal structure of various Mg-Ca phases.Because there is few theoretical research upon the corrosion behaviors of MgCa alloys, we study the atomic chlorine adsorption on the surface of MgCa alloys in order to learn the corrosion behaviors. The surface energies and the work functions of the pure surfaces are calculated as well as the charge distribution and the lattice relaxation of the Mg(0001) surface are specially studied at the same time. The adsorption structures and the adsorption energies of different surfaces adsorbing chlorine atoms in different adsorption sites are systematically studied. Cl atom prefers the "hollow sites" to other sites. For example, for the Mg(0001) surface, the most stable adsorption site among all possible sites is the fcc site, for the Ca(111), it is the hcp site, and for BCC MgCa phase, it is the long site. The adsorption energies of MgCa phase surfaces are between the adsorption energies of pure Mg surfaces and pure Ca surfaces.