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advances in nanomaterials Feasibility of nanoceramics as binder in cemented carbide tools Researchers from Shandong University in China explored the feasi- bility of using nanoceramics as a binder in cemented carbide tools. Cemented carbides are metal matrix composites used extensively as cutting tool materials because of their high hardness, wear resistance, and fracture strength. Tungsten carbide–cobalt (WC– Co) cemented carbides, in which cobalt is the cementing phase between the tungsten carbide grains, are one of the main types of cemented carbides used. The use of cobalt in cemented car- bides has some drawbacks, however. For example, the solubility of many work- piece materials in cobalt causes WC–Co to have high sensitivity to crater wear, especially when machining steels. Plus, high-temperature operation of the cobalt- bonded cemented carbide can result in tool failure due to plastic deformation caused by softening of the binder phase. These drawbacks, in addition to poor corrosion resistance, high cost, and high toxicity of cobalt, have inspired research- ers to explore ways to reduce or elimi- nate the cobalt binder. Early investigations on alternative binders mainly involved iron, nickel, and their alloys. Intermetallic materials such as titanium aluminide and alumi- num nitride were considered as second- generation alternative binders. Recently, ceramic phases have started attracting significant attention as bind- ers. Studies on several ceramic-bonded tungsten carbides have found they exhibit superior hardness, corrosion/ oxidation resistance, and high-temper- ature performance in comparison with cemented carbides with metal binder or intermetallic binder. Cobalt is a main material used as the binder in cemented tungsten carbide. A recent study dives further into the feasibility of using nanoceramics as a binder instead. Rapid development of nanopow- der technology has led scientists to start researching nanoceramic-bonded cemented carbides as well, following the hypothesis that nanoparticles may improve the densification and properties of cemented carbide. The authors of the recent study select- ed nano aluminum oxide (Al2O3), yttria- ceramic stabilized zirconium dioxide (ZrO2), and magnesium oxide (MgO) as the binders to investigate. Following microstructural and mechanical analyses of the nano- composite cemented carbides, which were fabricated through hot-pressing sintering, they determined that all three ceramic binders led to cemented carbides with near-full densification. In addition, the ceramic-bonded cemented carbides achieved excellent comprehen- sive mechanical properties, specifically • WC-6Al2O3: hardness of 23.5 GPa, flexural strength of 1,173.6 MPa, fracture toughness of 8.13 MPa∙m1/2 • WC-6ZrO2: hardness of 22.6 GPa, flexural strength of 1,229.7 MPa, fracture toughness of 9.35 MPa∙m1/2 • WC-6MgO: hardness of 21.1GPa, flexural strength of 906.3 MPa, fracture toughness of 8.62 MPa∙m1/2 Looking closer, the researchers determined that the ceramic-bonded cemented carbides used interlacing dis- tribution of crack deflection, crack bridg- ing, and crack branching as toughening mechanisms. In the case of the zirconia- bonded tungsten carbide, stress-induced transformation toughening significantly enhanced the toughness as well. Based on these results, “These ceram- ic bonded WC materials may be promis- ing candidates for high-speed machining tools,” they conclude. The paper, published in Journal of Alloys and Compounds, is “Nano-ceramic replacing cobalt in cemented carbide as binder phase: Is it feasible?” (DOI: 10.1016/j.jallcom.2021.162968). n Tech chat www.ceramics.org/ceramic-tech-chat 22 www.ceramics.org | American Ceramic Society Bulletin, Vol. 101, No. 2 Credit: Sandvik Coromant, YouTubePDF Image | hydrogen as an alternative fuel
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