Integrated Computational Materials, Process and Product Engineering – IC-MPPE 2022

Due to its high hardness and wear resistance as well as its good chemical stability, chemical vapor deposited TiB2 serves as protective coating in machining of Ti and Al alloys. To prevent B diffusion into the substrate, typically a TiN diffusion barrier layer is deposited beneath the TiB2 layer. However, the sharp transition from the face-centered cubic (fcc) TiN with its tensile residual stress to the hexagonal (h) TiB2, which typically exhibits compressive residual stress, results in poor adhesion. In order to strengthen the interface, a graded Ti(B,N) interlayer was implemented. In a first step, an interlayer providing a smooth transition from fcc-TiN, via TiBN containing 5, 15, 30 and 45 at.% B to h-TiB2 was synthesized. The graded coating and corresponding single layers were investigated in detail using cross-sectional and advanced high resolution methods. With increasing B content, a decreasing grain size and a rising h-TiB2-based phase fraction was observed, which strongly affects the residual stress, provoking a change from ~1 GPa tensile to ~1 GPa compressive across the coating thickness. The hardness almost linearly increases with rising B content from ~18 GPa for fcc-TiN to ~41 GPa for h-TiB2 and also the fracture stress and toughness increase from ~7 to ~13 GPa and ~4.6 to ~5.5 MPam1/2, respectively, with the only exception of the TiBN coating containing 45 at.% B, which exhibits a significant drop in hardness and fracture properties. The deteriorated mechanical properties can be related to a transition from a fcc-Ti(B,N) dominated structure at lower B contents to a highly defective h-Ti(B,N)2 dominated structure at 45 at.% B. Based on these findings, in a next step a further improved graded interlayer with B contents adjusted to below 15 at.% was synthesized, which exhibits excellent adhesion and superior performance in cutting application compared to the original TiN/TiB2.