The sharpest knives available are made of steel or ceramic, two synthetic materials that must be forged in furnaces at extreme temperatures. Now researchers have come up with a potentially more sustainable way to make sharp knives: using hardened wood. The method, presented on October 20e2021, in the magazine Questionmakes wood 23 times harder and a knife made from this material is almost three times sharper than a stainless steel table knife.
“The knife cuts through a moderately well-done steak with ease, with performance similar to that of a table knife,” says Teng Li, lead study author and materials scientist at the University of Maryland. Then the hardened wooden knife can be washed and reused, making it a promising alternative to disposable steel, ceramic and plastic knives.
Li and his team have also demonstrated that their material can be used to produce wooden nails that are as sharp as conventional steel nails. Unlike steel nails, the wooden nails developed by the team resist rust. The researchers showed that these wooden nails could be used to hammer three boards together without damaging the nail. In addition to knives and nails, Li hopes that in the future the material can also be used to make hardwood floors that are more scratch and wear resistant.
Although Li’s method of producing hardened wood is new, wood processing in general has been around for centuries. However, when wood is prepared for furniture or building materials, it is treated only by steam and compression, and the material rebounds somewhat after shaping. “When you look around at the hard materials that you use in your daily life, you see that many of them are man-made materials, because natural materials will not necessarily satisfy what we need”, says Li.
“Cellulose, the main component of wood, has a higher strength-to-density ratio than most engineered materials, such as ceramics, metals and polymers, but our current use of wood is barely reaching its full potential” , he said. Although it is often used in construction, the strength of wood is lower than that of cellulose. Indeed, wood is composed of only 40 to 50% cellulose, the rest being composed of hemicellulose and lignin, which acts as a binder.
Li and his team sought to treat the wood in such a way as to remove the weaker components without destroying the cellulosic skeleton. “It’s a two-step process,” Li explains. “First, we partially delignify the wood. Generally, the wood is very stiff, but after the lignin is removed, it becomes soft, flexible and somewhat spongy. In the second step, we perform a heat press by applying pressure and heat to the chemically treated wood to densify and remove water.
After the material is processed and sculpted into the desired shape, it is coated with mineral oil to extend its life. Cellulose tends to absorb water, so this coating preserves the sharpness of the knife during use and when washed in the sink or dishwasher.
Using high-resolution microscopy, Li and his team examined the microstructure of hardened wood to determine the origin of its strength. “The strength of a piece of material is very sensitive to the size and density of defects, such as voids, channels or pitting,” Li explains. “The two-step process we use to treat natural wood greatly reduces or removes the defects of natural wood, so that these channels for transporting water or other nutrients in the tree have almost disappeared.”
This process of hardening wood has the potential to be more energy efficient and have a lower environmental impact than the manufacture of other synthetic materials, although further analysis is required to say this with certainty. The first step requires boiling the wood at 100° Celsius in a bath of chemicals, which could potentially be reused from batch to batch. By way of comparison, the process used to manufacture ceramics requires heating the materials up to a few thousand degrees Celsius.
“In our kitchen, we have many wooden pieces that we have used for a very long time, such as a cutting board, chopsticks or a rolling pin,” Li explains. “These knives can also be used many times if you resurface them. , sharpen them and perform the same regular maintenance.”
Reference: “Hardened Wood as a Renewable Alternative to Steel and Plastic” by Bo Chen, Ulrich H. Leiste, William L. Fourney, Yu Liu, Qiongyu Chen and Teng Li, October 20, 2021, Question.
This work was supported by Advanced Research Projects Agency – Energy.