Electrochemical Corrosion Resistance of Ni and Co Bonded Near-Nano and Nanostructured Cemented Carbides

Abstract

The advantages of nanostructured cemented carbides are a uniform, homogenous microstructure and superior, high uniform mechanical properties, which makes them the best choice for wear-resistant applications. Wear-resistant applications in the chemical and petroleum industry, besides mechanical properties, require corrosion resistance of the parts. Co as a binder is not an optimal solution due to selective dissolution in an acidic environment. Thus, the development of cemented carbides with alternative binders to increase the corrosion resistance but still retaining mechanical properties is of common interest. Starting mixtures with WC powder, grain growth inhibitors GGIs; VC and Cr3C2, and an identical binder amount of 11-wt.% were prepared. GGIs were added to retain the size of the starting WC powder in the sintered samples. The parameters of the powder metallurgy process were adapted, and samples have been successfully consolidated. A very fine homogeneous microstructure with relatively uniform grain-size distribution and without microstructural defects in the form of carbide agglomerates and abnormal grain growth was achieved for both Ni-bonded and Co-bonded samples. Achieved mechanical properties, Vickers hardness, and Palmqvist toughness, of Ni-bonded near-nanostructured cemented carbides are slightly lower but still comparable to Co-bonded nanostructured cemented carbides. Two samples of each grade were researched by different electrochemical direct current corrosion techniques. The open circuit potential Ecorr, the linear polarisation resistance (LPR), the Tafel extrapolation method, and the electrochemical impedance spectroscopy (EIS) at room temperature in the solution of 3.5% NaCl. From the carried research, it was found that chemical composition of the binder significantly influenced the electrochemical corrosion resistance. Better corrosion resistance was observed for Ni-bonded samples compared to Co-bonded samples. The corrosion rate of Ni-bonded cemented carbides is approximately four times lower compared to Co-bonded cemented carbides.

A Review on Fatigue Mechanisms and Approaches for Hard metals

“…The creation of specimen thickness on effects of static fatigue is greatly influenced by larger WC grain size and the content of cobalt in the metal.The influence of carbide grain size is a pronounced under cyclic loading rather than under monotonic ones [8,9]. Fatigue sensitivity evaluated for NiMo-based grades is within the lower limit of values that allows to speculate on improved fatigue response accompanied with binder chemical nature or coarser microstructures [27]. About 85% of cemented carbide component failure happens through a fatigue mechanism approach.…”

“…Therefore, hardness value is considered as an important mechanical property of metals and significantly this increases the performance and wear resistance of hard metals [25]. The micro-cantilever and Tensile testing were used in investigations of strength and deformability of the Tungsten carbide (WC) grains [26] and tungsten carbide boundaries were exposed according to strength of fracture which influences crack tip [27]. The Cobalt phase in the region of slip goes into a fatigue phase change process and this influences effects of Cobalt (Co) content and Cobalt phase conversion are closely related to fatigue crack growth characteristic of WC-Co cemented carbides [28].…”

“…The major achievement to cemented carbides occurs in their outstanding wear resistance, strength and durability is thus explained. Thus, boron powder was used as an additive to reduce the formation of intermetallic brittle compounds during sintering and dissolution of liquid phase into molten binder phase [26], [27].The research carried was to understand influence of boron addition on nature of phases, microstructure, and mechanical properties of cemented carbide [44]. The coatings of microstructures are generally characterized on mechanical properties from micro indentation test [45].…”

“…Vickers hardness (HV30) were equally used to test polished specimens hardness value after standard metallographic preparation, and test observation shows an average indentations done on each sample at different regions [48].The indentation response technique isused to measure accurate measurement of plane using local elastic and short diagonal of coating surface were equally observed [57]. The direct joining method between fragments due to limiting factorontotal fractureof surface area and mechanical strength was observed [27], [28]. The fractography was used as confirmatory test on surface defects and validation of biaxial testing results that was demonstrated at room and elevated temperatures, that is having special interest for small samples in evaluating the mechanical strength [28], [29], three different ratios at high temperature were used on fatigue bending tests performed on commercial cemented carbide [58] as seen in the Fig.…”

“…For example, crack growth due to applied compressive load have been coated with cemented carbide that produces a strong surface with a thin film.Surface of cemented carbide exposed to tensile stressposes greater risks under the action of compressiveload stress. The samples of mechanical behaviour was examined with small cracks and without artificial notcheswhere behaviour of materials with applied load on cemented carbide were tested [27].The first-order residual stress may rise due to external forces that acts on surfaces of cemented carbide and this cause changes in product geometry of cemented carbide in terms ofdistortion. Thus, the difference between external loads results into crack growthand individual microstructure constituents with relative movement may result into high residual stress between binder of carbide grains.…”