# Surface Engineering of Woodworking Tools, a Review
## 5 Coatings Used for Woodworking Tools
### Multilayer Coatings
Tool wear is observed in the rounding of the edge of the blade and the abrasion of the rake face of the knife. Figure 3 shows a 3D view of the knife blade without a coating ready to work (Figure 3a), as well as the knife blade without coating (Figure 3b) and the knife blade with a CrN/CrCN multilayer coating (Figure 3c) after planing dry beech wood. The cutting path in this case was 6000 m. After sharpening, the uncoated knives had a blade radius of about 7 \(\upmu\)m, while the coated knives had a slightly larger blade radius of about 10 \(\upmu\)m. After cutting beech wood along a path of 6000 m, the radius of the knife blade without the coating increased more than three times and amounted to 24 \(\upmu\)m. Under the same cutting conditions, the tip radius of the coated knife was approximately two times greater, at 20 \(\upmu\)m.  
Figure 2: Fragments of uncoated HS6-5-2 steel knives (**top**) and CrN/CrCN (**bottom**). The left part after processing pine wood; the right part, after sharpening, did not process the wood [82]. The rake face (Figure 3a) shows a different surface morphology in the blade working zone, caused by the temperature increased by machining, higher than the tempering temperature of the HS6-5-2 steel, i.e., above 550 \({}^{\circ}\)C (see Figure 2). This is represented by an image of the rake face profile perpendicular to the edge of the blade. There is visible wear of the knife rake face without the coating as a result of abrasion of the knife material. The coated knife does not show this type of wear (Figure 4). This indicates the good anti-wear properties of the coatings. The coatings also restrict the heat flow to the tool [50], which can reduce or eliminate the tempering effect of the tool material. High temperature causes plastic deformation of the material in the area of the blade edge, which promotes edge rounding and accelerates tool wear, and also deteriorates the quality of the machined wood surface. When sharpening the tool, restoring its original cutting properties, the material with mechanical properties changed by temperature should be removed. In the case of the tested knives made of HS6-5-2 steel, it is even 0.3 mm. This requires both an increase in the tool regeneration time and a reduction in the number of such processes, resulting in an increase in machining costs. In the case of knives with coatings, no negative influence of temperature and no reduction in the hardness of the tool material was observed. The greater efficiency of tools with a CrN/CrCN multilayer coating is manifested in the lower surface area of the blade wear in relation to the uncoated knife. For such selected tool-machined material systems and cutting process parameters, the surface area of the blade wear for the coated knife is approximately 25% lower, which translates into approximately three times longer cutting distance. Figure 4: The cutting edge profile of planer knife with CrN/CrCN multilayer coating after 6000 m of dry beech wood cutting [83]. Figure 3: 3D image of cutting edge of planer knife without the coating after sharpening (**a**), the cutting edge of planer knife without the coating (**b**), and with CrCN/CrN multilayer coating (**c**) after 6000 m of dry beech wood cutting [83]. The analysis of the data presented in Tables A1-A3 shows that they are very diverse. The substrate, coatings, material treatments, and the material itself are different, as well as the presentation and evaluation of the results. The records in the above Tables are arranged according to the sequential year of publication. It is not possible to unambiguously observe an increase in durability due to the variety of coatings, types of material being processed, and probably different machining parameters. One can also notice the important role of the deposition method and the tool material used as a substrate for the coating. The above data confirm that there is no universal coating that will be equally effective in the machining of various materials.