Study of the Surface Composition on Alumina-NbC Composites

Abstract

The aim of this work is to analyse an alumina-NbC based composite ceramic made from a polymeric precursor (polysiloxane), alumina and metallic niobium. The materials have a fixed concentration of 60 wt% of polymer and 40% of a mixture of niobium and alumina. These materials are mixed and sintered at 1450 C for 6 h. Alumina based composites have been proposed as excellent materials for use as cutting tools, so knowledge of the superficial composition is extremely important because it is directly related to the hardness and abrasion resistance. Analysis of the surface composition was carried out by electron spectroscopy. It should be emphasized that there may be a meaningful difference between the surface and the interior composition due to a eventual processes such as element segregation to the surface and/or diffusion of elements from the surface to the sample bulk. The analysis was performed by XPS and Auger for three niobium concentrations 10, 20 and 40 wt%, and the results show the appearance of niobium on the surface only at the Nb composition of 10 wt%; this appears to be due to a process of niobium atom migration to the interior of the sample or one involving niobium bonds. For the 10 wt% sample after sintering, the formation of NbC on the surface and the presence of niobium Auger peaks were observed. However, for concentrations larger than 10 wt%, oxides and sub-oxides (NbO, NbO 2 , etc.) were formed which may result in the absence of niobium peaks in the spectra. The analysis of the ratio of niobium to carbon atoms at the surface shows a value of 0.1 which reveals that the quantity of niobium and carbon is not sufficient for ideal formation of niobium carbide (NbC). Under these conditions it is verified that there is only a slight formation of niobium carbide on the surface, which is harmful to the hardness of the material. The excess niobium tends to diffuse towards the interior of the sample and react with the oxygen forming sub-oxides.

The influence of (Ti,W)C and NbC on the mechanical behavior of alumina

“…However, hot-pressed alumina-based ceramic composite reinforced with refractory carbides and others elements have been developed as technological alternatives to cemented carbides [2][3][4][5] . Studies published in the literature relevant to alumina ceramics reinforced with (Ti(C,N), WC, TiB 2 /SiCwhisker, TiC and MgB 2 particles have shown increase of the wear resistance, hardness, fracture toughness and thermal shock resistance 3,[6][7][8][9][10] . The aluminabased composite materials present good mechanical properties, in order to replace WC-Co in some special applications, such as machine cast iron and other metals and alloys.…”