Evolução microestrutural de Ni e Ni-SiC sinterizados após compactação uniaxial ou processamento termoplástico

Abstract

The addition of SiC to nickel or its alloys results in sintered materials with a set of interesting properties for the petroleum, automotive, and aerospace industries, including high toughness and mechanical, corrosion, and wear resistance over a wide range of service temperatures, associated with low cost of starting materials and processing. In addition to traditional powder metallurgy, thermoplastic processing offers an interesting alternative for forming and microstructural development by the dissociation of SiC and formation of sintered materials containing graphite precipitates in (Ni,Si) matrices. Thus, the present work aims to study Ni-SiC combinations produced from uniaxial compaction or thermoplastic processing. Pure nickel and Ni mixtures with 3 wt.% SiC were drum pelleted, with 2.5 wt.% paraffin, uniaxially compacted at 600 MPa and sintered in a conventional furnace at 10 ºC/min up to 1200 ºC for 1 h. Samples obtained by thermoplastic processing were produced from nickel and mixtures containing 1 wt.% SiC with 50% polymeric binders, homogenized in a conical twin-screw co-rotating mixer and molded in a capillary rheometer. After thermal extraction of the binders, they were sintered at 5°C/min up to 1200 ºC for 4 h. XRD, FT-IR, microstructural characterization by MO, SEM/EDS, density in green and of the sintered samples, and Vickers microhardness tests were performed. The nickel samples, produced via thermoplastic processing, showed larger pore sizes at concentrated points, higher diametral shrinkage, and lower Vickers hardness. However, there was no interference of the production techniques on the densities of the sintered nickel samples. The addition of 3 wt.% SiC influenced the densification and also the hardness of the compacted samples. The samples containing 1 wt.% SiC and produced by thermoplastic processing did not show significant microstructural changes. The microscopic and XRD analyses suggest that both techniques used produced Ni-Si type materials with graphite precipitation, a typical microstructure of self-lubricating materials.