Titanium Powder Sintering for Preparation of a Porous FGM Destined as a Skeletal Replacement Implant

Abstract

This contribution refers to basic research into a functionally graded material (FGM) as a permanent skeletal replacement implant with improved properties. The latter concerns elastic adaptation and structural compatibility in order to achieve faster bone tissue ingrowth and better long-term stability. Target is a sintered porous coating on a compact substrate with a perpendicular gradient of the porosity and, hence, of the Young's modulus. To achieve optimal porosity and pore size, coarse Ti powder fractions with diameters of up to 1000 μm were chosen. Additionally, an acceleration of the sintering progress was attempted using fine-grained secondary powder components, such as silicon or titanium hydride. Preliminarily, beam-like model specimens without gradient were produced to elucidate how powder characteristics and processing influence the general sinter behaviour, porosity, strength, and elastic properties. Silicon-assisted liquid phase sintenng (LPS) resulted in a substantial increase of the neck diameter (≤ 500 μm) and the strength of the sinter contacts. Milling the powder mixture was found to he superior to liquid-dispersed approaches in regard of uniform distribution of Si. The Young's modulus determined by ultrasound velocity measurements ranged from 5 to 80 GPa, covering typical values of cortical bone (12÷23 GPa). In dependence on porosity, a correlation was perceivable as predicted by application of the effective medium approach (EMA). Specimens with porosity gradients were successfully fabricated and characterized using acoustic microscopy and quantitative description of the microstructural geometry.