Thermal cracking behavior of multi-layer LAFAD coatings on nitrided die steels in liquid aluminum processing

Thermal cracking is the primary mode of failure of large steel dies in aluminum die casting of automotive engine blocks or transmission housing. This cracking is due to the large thermal shock experienced by the die steel when it is quenched by cold water after a die casting cycle. The propagation of cracks depends on the severity of the thermal fatigue cycle (peak and range), that is caused by die surface being in contact alternatively with the liquid melt at 700°C and cold water at room temperature. Previous work at The Ohio State University has shown that single layer hard PVD and CVD coatings do not protect the die steel surface from cracking. On the other hand, they may enhance cracking. This paper presents an interesting multi-layer duplex coating approach with a nitriding H-13 steel substrate and a multi-layer, hard outer film applied by LAFAD technique that prevents reaction with the liquid melt, and alters the thermal fatigue behavior. Results of thermal cycling tests indicate that the multi-layer duplex coating system helps reduce the density of the thermal cracks.