# Surface Engineering of Woodworking Tools, a Review
## 5 Coatings Used for Woodworking Tools
### Multilayer Coatings  
Multilayer coatings generally have better wear resistance compared to single-layer coatings. This takes into account many factors such as: total thickness of the coating and individual layers, type of layer, functions performed in the coating (adhesive, abrasive, sliding, etc.). Greater wear resistance is connected with reduction in crack propagation at grain boundaries and layers. The coated carbide tools provide better wear resistance, surface roughness, and lower noise level compared to uncoated tungsten carbide tools in the cutting of asbestos, WPC, LVL, and OSB. Among the coated carbide tools, the multilayer TiAlN/TiBON coated carbide tool is the highest in wear resistance and is proposed for cutting wood composites. The abrasives contained in the wood composite are important in the wearing of the tungsten carbide tools. The structures of the wood composites are important in determining the roughness of the machined surfaces. The noise level and roughness increase due to an increase in wear and should be a good indication for determining the wear of the coated and uncoated tungsten carbide tools [81]. This is probably related to the high hardness of the coating, low coefficient of friction, and the reported lubrication effect at high temperatures in cutting. Ti-W-N/Ti-W and Cr-W-N/Cr-W multilayer coatings with different periods, as well as comparison of single-layer coatings Ti-W-N and Cr-W-N with a thickness from 0.5 to 2.0 \(\upmu\)m, deposited on WC-Co substrates, were the subject of research conducted by [PERSON] et al. [41]. The results showed that both 1 \(\upmu\)m thick single-layer coatings, tested when cutting OSB, increased wear resistance approximately 2.5 times compared to uncoated cutting tools. Using the same cutting parameters (as those used in industry), they showed that the best result was achieved by applying the Cr-W-N/Cr-W multilayer coatings (three layers). This coating increases the cutting ability of wood-based products by 500% compared to the uncoated cemented carbide tool. [PERSON] et al. [54] conducted a comparative study of the wear resistance of HSS and cemented carbide knives with a single-layer CrN and CrN/CrCN multi-layer coatings during rounding of wood. They showed that the service life of CrN/CrCN-coated tools increased by 170% (HSS) and 110% (cemented carbide) compared to uncoated tools and by approximately 33% and 7%, respectively, compared to CrN-coated tools. Two important conclusions can be drawn from this work: a harder substrate contributes to an increase in durability more than a lower hardness substrate when covered with the same coating, and a multi-layer coating gives a greater increase in durability compared to a single-layer coating. A similar result was obtained for the same multilayer coating on HSS knives [71]. Figure 2 shows the parts of two planer knives, without a coating (top) and with a CrN/CrN coating (bottom) [82]. The left side shows the knife wear after a distance of 77 km of machining pinewood at feed speed--90 m/min, cutting speed--36 m/s and cutting  
depth 2 mm. The right side shows a fragment of the knife after sharpening, without wood, machining. The tests were performed using Weinig Hydromat 22A. Plastic deformation caused by high temperatures during cutting is visible on the edge of the uncoated tool. The blade was heavily worn after the machining test. On the edge of the CrN/CrCN-coated tool, slight wear is observed compared to the uncoated tool. In the middle part of Figure 2 is an enlargement of the edges of both knives. In the case of a tool with a CrN/CrCN coating, wear and delamination of the coating on the edge are hardly visible. The test conditions favored an increase in temperature on the cutting edge and on the rake face. This leads to accelerated wear of the tools, resulting in, e.g., an increase in power consumption by the milling machine, and on the other hand, a deterioration in the quality of the processed material. Dependent on the type of processed wood, the wear rate and its symptoms may vary.