Near-interface cracking in a TiN coated high speed steel due to combined shear and compression under cyclic impact loading

Hard coated tool steels are commonly used for metal working applications, where they are exposed to high levels of contact loads comprising normal and lateral force components. For such complex loading conditions, information on early stages of damage of hard coated steels is lacking. As these early stages provide information about damage mechanisms and possible weaknesses of the coated material, the current work investigates the damage behavior of the interface between a titanium nitride hard coating and a high speed steel. Specimens were subjected to cyclic impacts that combine shear and compression loading under unlubricated or lubricated conditions using an inclined impact tester. Investigation of crack formation below the substrate-coating interface by scanning electron microscopy on cross sections prepared by focused ion beam milling suggests that the predominating damage mechanism for unlubricated specimens is gradual coating wear. For lubricated specimens, the damage mechanisms are dominated by carbide fracture and fatigue crack growth between carbide and matrix occurring just below the substrate-coating interface.