Investigation of the Corrosion and Tribological Properties of WC-Co Tools Hardened with PVD Coatings in Solid Oak Wood Processing

Abstract

Corrosion and friction coefficient tests were performed on solid oak wood machined with hard-metal woodworking tools coated with PVD coatings (AlCrN, AlTiN, TiAlN, TiCN and CrN). The tannic acid attacks the carbide more intensively than the PVD coatings. During cutting, corrosion spreads on the cutting edge of the cutter due to mechanical action, which dissolves the cobalt binder of the hard-metal and causes the carbide grains to flake off. After 80 min of contact with the wood, the cobalt content decreases from 3.53 to 1.74%. Depending on the PVD coating material, cracks of 4 to 40 µm in width appear after 120 min (9000 m cutting path). After 120 min of machining, wear, corrosion effects and the influence of corrosion on the coefficients of friction were evaluated for tools with and without PVD coatings. TiCN is the most sensitive to corrosion, while AlCrN and CrN coatings are the least sensitive, with the AlTiN coating being the most affected under real cutting conditions (with mechanical + thermal + corrosion effects) and the tools with CrN and AlCrN coatings being the least affected. Corrosion affects the hard-metal and PVD coatings and reduces the coefficient of friction. The angle between the directions of sliding and sharpening of the cutting edge sharpening significantly influences this parameter. The coefficient of friction of hard-metal WC-Co and PVD coatings is higher in the parallel machining direction than in the perpendicular machining direction and ranges from 16.03% (WC-Co) to 44.8% (AlTiN). The coefficient of friction of hard-metal WC-Co decreases by 5.13% before and after exposure to tannic acid, while the corrosion of PVD coatings reduces it by 4.13% (CrN) to 26.7% (TiAlN).