Produção de material com gradiente funcional pelo processo de metalurgia do pó para possível aplicação como biomaterial

Abstract

The development of new biomaterials with adequate mechanical resistance, biocompatibility, resistance to wear and corrosion, as well as low elastic modulus, close to that of bone tissue, is essential to increase its useful life when implanted in the human organism. Austenitic stainless steels, especially AISI 316L, are well accepted by the human body and have a relatively low cost compared to titanium and cobalt-chromium alloys. A functional gradation structure can be developed on this steel to improve the response at the tissue-implant interface, controlling the variation in composition, properties and microstructure. Therefore, the objective of this work is to produce a Material with Functional Gradient from the powders of this metal and boron, through the Powder Metallurgy process, associated with its microstructural, physical, mechanical, electrochemical and biological characterization. The powders were evaluated for morphology, granulometry, tapped density and fluidity. The three mixed compositions used to obtain the samples were defined with a 0.2% boron gradation, the first layer having a pure steel composition. These were homogenized in a “Y” shaped mixer, cold uniaxially compacted, vacuum sintered and analyzed microstructurally by optical microscopy. Stainless steel powders showed higher values of beaten density than boron due to the practically spherical morphology of the metal particles, which allows for greater packaging factors than the irregular shape of the boron particles. Both powders did not flow due to the small particle size, confirmed by the particle size analysis, which reported an average diameter of 9.51 ± 3.24 μm for steel and 3.54 ± 2.98 μm for boron. The green bodies suffered “crumbling” after removal of the pressing matrix, with destruction of the projected functional gradient and the presence of visible craters, delamination and cracks along the cross section. Such problems can be associated with the lack of flowability, high crystallinity and small particle size of the steel powders used. Some green bodies, although cracked, maintained their cylindrical geometric shape and presented moderately superior resistance to the other samples after removal of the matrix and transport to the furnace, being, therefore, submitted to sintering. The sintered bodies reached up to the third stage of the process, reached an average porosity of 12%, remained with the cracks from the pressing stage and presented dimensional distortions, signs of oxidation, bubbles and, mainly, absence of the functional gradient projected by the incorrect use of sintering technique. Due to the defects, the sintered samples were not evaluated for apparent density, tensile and compressive strength, elastic modulus, microhardness, corrosion resistance in synthetic human plasma and in vitro cytotoxicity. Thus, it is not possible to say whether the production of a Material with a Functional Gradient by means of Powder Metallurgy, from the mentioned raw materials, makes it feasible to apply it for biological purposes. [resumo fornecido pelo autor]