LOAD-CARRYING CAPACITY AND IMPACT WEAR RESISTANCE OF COATED TOOL STEEL - INFLUENCE OF FRACTURE TOUGHNESS

摘要

As the market focus toward the use of new high-strength materials, which are more and more difficult to form, coated forming tools are becoming necessity. However, limited load-carrying capacity and impact loading resistance greatly restrict the use of hard coatings in forming applications. Load-carrying capacity can be simply improved by increasing substrate hardness, however, under cyclic impact loading resistance to crack initiation and propagation is equally important. The aim of this research work was to investigate the effect of substrate fracture toughness and hardness on the load carrying capacity and impact wear resistance of coated tool steel. Investigation was performed on monolayer (TiAlN), multilayer (AlTiN/TiN) and nano-composite ((Ti,Si)N) PVD coatings deposited on P/M cold work tool steel. By using different combinations and parameters of vacuum heat treatment and deep cryogenic treatment potential for improving load-carrying properties and impact wear resistance has been evaluated and the effect of hardness vs. fracture toughness determined. Effect of the substrate fracture toughness and hardness on the load-carrying properties was determined under progressively loading dry sliding conditions, while ball-on-plate impact fatigue test was employed to investigate impact wear resistance. Results clearly show that substrate hardness is the most important property influencing load-carrying capacity and impact wear resistance of the coated surface. However, with increased hardness and brittleness of the coating increase in fracture toughness on the expense of the reduced hardness becomes beneficial.