Interrupted cutting tests of cemented carbide tools coated by physical vapor deposition and chemical vapor deposition techniques

Abstract

Refractory compound coatings prolong the life of cemented carbide inserts. The structure of these coatings is vastly different when the same coating is produced by chemical vapor deposition (CVD) and physical vapor deposition (PVD) methods. TiC and HfN coatings were applied to cemented carbide tools by both CVD and PVD processes. The coated inserts were tested under interrupted cutting conditions using slotted bar tests. The CVD-coated inserts failed after a few (less than 100) cycles whereas the PVD-coated inserts lasted well past 2000 cycles without failure as did the uncoated inserts. PVD coatings have a much greater fracture toughness than CVD coatings due to their very fine-grained microstructure with a distribution of fine cavities which act as crack stoppers. In contrast, CVD coatings have a fully dense microstructure with a large grain size which does not have much fracture toughness. Another reason for the difference in behavior is the much lower deposition temperature (about 500°C) used in the PVD process as compared with the much higher deposition temperature (about 1000°C) used in the CVD process. Chemical attack of the cemented carbide substrate occurs at high deposition temperatures, thus weakening the area near the coating-substrate interface.