Stress relaxation through thermal crack formation in CVD TiCN coatings grown on WC-Co with different Co contents

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

“…For the following thermal cycles (2 to 5), the stress behavior oscillates reversibly between almost the same values. Reversible stress behavior of similar Ti(C,N) wear resistant coatings for one temperature cycle has been observed in previous works of the authors [4,9,37] and more recently by Stylianou et al on cemented carbides with different cobalt binder contents [11]. Interestingly, by interpolating the measured values, it can be estimated that the condition for zero stresses for α-Al 2 O 3 is in a temperature range between 100 and 300 • C and for Ti(C,N) in a range of 500 ± 50 • C.…”

“…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.…”

“…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]. One remaining observation is the appearance of certain deviation from the expected linear behavior of the stress evolution at temperatures between 400 and 600 • C. This kink of stresses has been discussed with regard to the formation of additional cracks [8] or a phase transformation of the cobalt binder phase [11,12]. The latter can be a plausible explanation, since it is known that pure cobalt undergoes fcc-hcp transformation at 450 • C, and that this temperature can rise due to solid solution of tungsten in Co, which is the case for cemented carbides [2].…”

“…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%.…”