Optimisation of powder processing for non-magnetic Fe-Mn and Fe-Mn-Ag biodegradable scaffolds

Abstract

Biodegradable metallic implants have been gaining interest in the biomedical sector. The similar mechanical properties of these implants to bone make them suitable to treat bone defects. Fe-based alloys, specifically Fe-Mn and Fe-Mn-Ag, fabricated with powder metallurgy techniques, have been suggested for this application as it would degrade at a similar rate to the regeneration rate of human bone. The addition of Mn will lead to the formation of an austenite phase, whose anti-ferromagnetic characteristics would make the implant MRI-compatible. Also, the addition of Ag is hypothesised to increase the corrosion rate through micro-galvanic corrosion. This work investigates the effect of processing Fe-35Mn powder mixtures using high energy ball milling with the aim of achieving mechanical alloying and obtaining smaller, and more spherical powders. Several milling runs were conducted to study the effect of individual variables, such as milling time, milling speed, process control agent (PCA), and pause intervals with respect to the resultant particle size, circularity, and the degree of mechanical alloying. The optimal process parameters were found using the ‘Taguchi Method’. Obtaining smaller and spherical powder morphology would improve the sintering process since the larger surface areas of the smaller powders would help accelerate diffusion. It was found that milling time was the most significant variable in reducing the particle size. Reducing the pause interval down to 10 minutes resulted in further improvements of the morphology. On the other hand, the powder was only mechanically alloyed at a high milling speed of 340 RPM. Pressureless sintering was applied to differently processed powder mixtures. A tube furnace held at 1200ºC for 3 hours and using a N2-5H2 atmosphere (to prevent oxidation) was used. Five powder mixtures were investigated, including: (i) an unmilled Fe-35Mn powder, (ii) milled Fe-35Mn powder having the smallest and most spherical morphology, (iii) milled Fe-35Mn-5Ag powder, (iv) mechanically alloyed Fe-35Mn powder with slightly bigger and less spherical morphology than powder groups (ii) and (iii), and finally (v) alloyed Fe-35Mn-5Ag powder. Shrinkage was observed in the milled coupons and the pore sizes were three times smaller than the coupons made from un-milled powder. During immersion testing in Hank’s solution, the degradation rate of milled and alloyed coupons was at least double that of coupons from un-milled powder. These results show that the powder metallurgy techniques have a significant potential to produce biodegradable implants using Fe-Mn alloys.