Optimizing a Bath for Plasma Electrolytic Nitrocarburizing of Austenitic Stainless Steel to Reduce the Applied Voltage and Evaluation of the Bio-Compatibility of the Coating in Rabbit Tribia

A plasma electrolytic nitrocarburizing process was carried out on austenitic stainless steel in order to enhance surface properties in medical implants applications. An urea-based solution with 10 percent NH4Cl and 3 percent Na2CO3 was used as the electrolyte. The samples were coated using the least possible voltage (150 volts). Cross-section and surface morphology of the coated samples were studied using a scanning electron microscope. Microstructure and composition of the samples were investigated using X-Ray Diffraction analysis and Energy Dispersive X-Ray analysis. The microstructure of the outer layer of the coating consisted of iron and chromium oxides. In order to study the bio-compatibility of nitrocarburized samples and to compare the results with those of untreated samples, a group of each type of samples was placed inside rabbit's tribia bones. After six weeks, the pins were removed from the body and were studied using X-Ray and EDX analyses, to obtain images of the sample's surfaces, both post and prior to the implantation. Studying these images revealed the formation of a layer of tissue on the surface. EDX analysis also showed the presence of a significant amount of phosphorous and calcium with a non-homogenous distribution. These results indicated tissue growth on the surface due to the presence of a porous structure.