No abstract

This work focuses on basic research into a P/M processed, porous-surfaced and functionally graded material (FGM) destined for a permanent skeletal replacement implant with improved structural compatibility. Based on a perpendicular gradient in porosity the Young's modulus of the material is adapted to the elastic properties of bone in order to prevent stress shielding effects and to provide better long-term performance of the implant-bone system. Using coarse Ti particle fractions the sintering process was accelerated by silicon-assisted liquid-phase sintering (LPS) resulting in a substantial improvement of the neck geometry. A novel evaluation for the strength of the sinter contacts was proposed. The Young's modulus of uniform non-graded stacks ranged from 5 to 80 GPa as determined by ultrasound velocity measurements. Thus, the typical range for cortical bone (10-29 GPa) was covered. The magnitude of the Poisson's ratio proved to be distinctly dependent on the porosity. Specimens with porosity gradients were successfully fabricated and characterized using quantitative description of the microstructural geometry and acoustic microscopy.

A dislocation niechanistn is presented which explains the fast rotation of particles as ol)served during thesintering of rows and loose packings of monocrystalline copper spheres. The experillientally determined data agree well with the data determined by calculation. Thep are important for the production of sintered filters. lntroductionOn the basis of thermodynamic considerations SHEWMOW expressed the opinion for the first time that during the sintering of monocrystalline spheres not only their contact is extended, but also a relative niotion among particles should occur. The main driving force of sintering -the dissipation of surface energy -leads to an increase of the contact area. The endeavour to minimize the contact boundary energy works towards a change in the orientation relations of the contact partners. On the assumption that the rearrangement of contact material required for particle motion would proceed through volume diffusion, SHEWMON, gives for the speed of rotation, where D, is the volume diffusion coefficient, Q the atomic volume, T teniperature, k the Boltaniann constant, x -the contact area radius and B -the angle characterizing the difference of orientation of the contacting spheres.The sintering experiments carried out later with monocrystalline sphere-plate niodels have qualitatively confirmed KHEWMON'S conceptions and have shown that low-energy contact grain boundaries originate due to rotational niotions of particles (HERRMANS et al.; SAUTTER et al.; ERB et al.). The speeds, however, at which particles rotate in such a case and which were measured a t similar niodels (MYKI'RA) and at rows of spheres are much higher than those expected according to equation ( 1 ) . Thus, e.g., in monocrystalline rows of copper spheres (Fig. 1 ) during the heating-up to the isothernial sintering temperature (900 "C) as well as at the beginning of the isothermal sintering there were observed speeds of rotation of 2 10-4 rad s-1 (SCHATT et al.), whereas after the substitution of the respective quantities (z 10 pm) into equation (1) one obtains rotational speeds of only & rad s-l. The fast rotation, which cannot be satisfactorily explained by volume diffusion, is ascribed by the authors to the spontaneous formation of zones with a considerably increased dislocation density and to a diffusion-dislocation-controlled flow (SCHATT, FRIEDRICH ; SCHATT et al.; BOIKO et al.), (SCHATT, FRIEDRICH), which can be definitely derived.

No abstract