Thermal Shock Resistance Of PVD Nitride Coatings

Coating tools are subjected to great thermal shock during cutting process because the temperature of cutting area changes remarkably at the moment of cutting start and cutting over, or cooling of the coolant. The coatings are likely to crackle, wrinkle or peel off under the action of the thermal stress. So it’s very important to evaluate the thermal shock resistance of coatings. In this paper, the thermal shock resistance of four PVD nitrides coating (TiN, CrN, TiAIN, AlCrN) were studied systematically using laser irradiation method.Based on the elastic thermodynamics theory, the temperature distribution and the thermal stress distribution of the coating were analyzed. The failure reason of coating was deduced, and the critical stress formula was derived. The temperature distribution and the thermal stress distributions of the coatings under different types of loading which included the power density, coating thickness, transition layer and coating material were simulated using FEM software. The finite element simulation results show that, the thermal shock resistance of coatings is in proportion to coating thickness; transition layers whose physical properties fall in between coating and substrate can improve the thermal shock resistance of coatings; the thermal stress generated in the process of thermal shock become bigger with the increase of the power density, so the critical power density of laser beam is introduced as a measure to evaluate the thermal shock resistance of coatings. The critical power density of TiN, CrN, TiAIN and AlCrN coatings is32.88×108W/m2,45.53×108W/m2,32.62×108W/m2,46.81×108W/m2successively. If the critical power density is bigger, thermal shock resistance of the coating is better. So they can be arranged in the order of AlCrN>CrN>TiN>TiAIN.The four PVD nitrides coatings were studied using laser irradiation experimental method. The test results show that, the coating surfaces appeared netty crack after the laser irradiation, and the width of crack is in proportion to power density; When the power density lowers to a certain value, the cracks of coating surface disappeared. And the certain value was defined as critical power density; The critical power density of TiN, CrN, TiAlN and AlCrN coatings is19×108W/m2,51×1O8W/m2,41×108W/m2,39×108W/m2successively, and considering the influence of coating thickness, the thermal shock resistance of TiN, CrN, TiAIN and AlCrN coatings are arranged in the order of CrN>AlCrN>TiN>TiAIN. The simulation results get a good agreement with experimental results.