Thermal crack formation in TiCN/α-Al2O3 bilayer coatings grown by thermal CVD on WC-Co substrates with varied Co content

In Situ Investigations on Stress and Microstructure Evolution in Polycrystalline Ti(C,N)/α-Al2O3 CVD Coatings under Thermal Cycling Loads

“…This indicates that the α-Al 2 O 3 layer presents two different stages: one consisting of stress relaxation of top-blasted-induced stresses in the first heating step, and a reversible tensile-compressive-tensile behavior of the same magnitude for the alternating subsequent cycles. The relaxation of residual stresses in post-treated α-Al 2 O 3 has also been observed previously in other studies [4,6,36] and the reversible behavior for one thermal cycle in [4,12]. The results of this study on several thermal cycles show that the top-blasting effect decreases continuously with the increased number of thermal cycles, namely, −900 to −450 MPa at 800 • C and +50 to +350 MPa at RT.…”

“…A cross-section image of the tip of the CVD coated cemented carbide investigated is shown in Figure 1a (Figure 1c) shows the typical topography of Al 2 O 3 grains, together with a regular microcrack network (indicated by white arrows), which formed during the cooling step of the CVD process due to CTE differences between the coating layers and the cemented carbide substrate as reported in [4,12,34,35]. The averaged surface roughness (Rz) was 0.6 ± 0.05 µm.…”

“…Additionally, residual stress of the bilayer coatings was determined using X-ray diffraction. A secondary stress relaxation mechanism (undefined as described by the authors, and probably associated with the binder phase transformation of the cemented carbide, which was not visible in dilatometry measurements [10]) in the Ti(C,N) film of the bilayer coatings was suggested to account for discrepancies between stresses calculated by FEM and those determined by XRD in the temperature range RT-1000 • C. A common observation of all previous works [8][9][10][11][12] is the reversible behavior of stresses for Ti(C,N) below the deposition temperature; however, if the heat treatment surpasses deposition temperature (890 • C), then the formation of additional thermal microcracks is suggested for a kink in stresses at around 450 • C [11,12]. However, the assumptions of relaxation due to microcracking and plasticity effects (of the carbonitride coating layers [9] or the Co binder phase of cemented carbides [11,12]) may not be conclusive to explain the observed stress variations in CVD coatings and their impact on the performance of industrial coated cutting tools.…”

“…A similar discrepancy is observed in α-Al2O3 for both measurements at RT (+1000MPa vs. +500MPa) and 800 °C (+250MPa vs. −400MPa). This difference between experimental and theoretical stresses is understood by the formation of cooling cracks during the cooling step of the CVD process (see Figure 1), and has been discussed previously considering the variation of CTE for the coating/substrate [4,[9][10][11], the density of cracks [4,[9][10][11], the widening of cracks [11,38] or even the formation of subcracks during the temperature steps [10][11][12]. For the case of the intermediate Ti (C,N) layer, a different stress behavior is observed.…”

“…Recently, the thermal crack formation in Ti(C,N)/α-Al 2 O 3 bilayer coatings grown by thermal CVD on WC-Co substrates with varied Co content has been investigated [11,12]. It was concluded that increased Co contents in the substrate reduced the CTE mismatch between the carbide and the coating layers, even suppressing the formation of CVD cooling crack networks for Co contents above 12 wt%.…”