Low temperature pressureless “immediate sintering” of a novel nanostructured WC/Co/NiCrSiB-alloy cemented carbide

Abstract

A novel nanostructured cemented carbide formed from WC-5%Co-20%BNi2 brazing alloy is described. During sintering, the BNi2 alloy is infiltrated into a green compact of WC-5%Co at 1050-1100 °C for 2-60 minutes. Perfect wetting behavior and a zero contact angle are achieved after only 40 s. Relative densities of 98.5% and 100% and microhardness values of above 1500HV1 and 1800HV1 are obtained after 2 and 30 minutes, respectively. A change in mean particle size of 0.6 µm in the precursor to a bimodal distribution of 350-400 nm and 10-20 nm is explained by a solution/reprecipitation mechanism.Keywords: nanostructured cemented carbide, liquid phase sintering, immediate sintering, BNi2 brazing alloy, zero contact angle.Cemented carbides, which have been used widely as cutting tools for almost a century, are usually formed by sintering tungsten carbide (WC) particles with a metallic binder [1]. Their favorable properties, which include cold and hot hardness, toughness, bending strength and a low coefficient of thermal expansion, are typically achieved by using a Co binder to wet the WC particles [1][2][3][4][5]. Research into alternatives for Co is motivated by its cost, the undesirable health effect of Co particles that may be released by corrosion or wear and the low corrosion/oxidation resistance of WC-Co [6][7][8]. Logical alternatives include the other ferrous family elements Fe and Ni, which both exhibit solubility for WC and have good mechanical properties. The use of Fe as a binder usually leads to the formation graphite for thermodynamic reasons [9]. Ni has been shown to eliminate all of the problems mentioned above [10]. However, WC-Ni tools have lower toughness when compared to WC-Co [9], as a result of the lower stacking fault energy of Ni compared to Co [9], the lower wettability of WC by Ni than by Co [11] and/or the lower fracture strain of Ni than Co [12]. Efforts have been made to improve the wettability of WC by the Ni binder, including the addition of Mo 2 C to the system [12]. Another approach involves the replacement of Ni by its alloys [13]. According to phase diagrams, the addition of Si, Al, Mn, Cr, Nb and/or Fe to the Ni matrix can strengthen it by a solid solution mechanism [14]. Correa et al.[15] used a Ni-4.1Si alloy binder and reported a relative density of 98.3 -98.8% and an enhancement in both strength and toughness. Worauaychai et al. [16] evaluated the effect of adding Cu, Sn and P to the Ni matrix on the sintering behavior of WC-Ni composites and reported a significant increase in hardness and a remarkable decrease in sintering temperature on alloying the Ni binder with less than 0.05% P (but not Cu or Sn). Both references mention the importance of fluidity for the densification and toughening of such composites. Another approach to improve both the hardness and the toughness of WC-Ni composites is refine the grain size to the nanoscale [9]. WC-based tools are typically brazed easily using Cu or Ag-based brazing alloys, not Ni-based ones. Nevertheless, Ni-based hardfacing allo...

Sintering of nanocrystalline materials: Sintering parameters

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Parametric optimisation of high-velocity oxy-fuel nickel-chromium-silicon-boron and aluminium-oxide coating to improve erosion wear resistance

“…Although scarce, there are some studies where NiCrSiB coating has been studied. However, the studies have been in combination with Tungsten carbide cobalt (WC-Co) [39,40]. Furthermore, the only study where NiCrSiB was combined with Al2O3 was by Praveen et al [40] where a Plasma spraying technique was used.…”