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

Abstract

The stress behavior and the associated microstructure evolution of industrial Ti(C,N)/α-Al2O3 coatings subjected to thermal cycling are investigated by in situ energy dispersive synchrotron X-ray diffraction and transmission electron microscopy. Temperature-dependent stresses and changes in microstructural parameters (domain size and microstrain) are analyzed by in situ measurements at different temperatures between 25 and 800 °C, both in the heating up and cooling down step, including several thermal cycles. Transmission electron microscopy is used to evaluate defects before and after the thermal treatment. The introduction of high compressive stresses in α-Al2O3 by top-blasting is connected to a high defect density at the basal planes of the alumina layer. The stress relaxation of the alumina layer at high temperatures is associated with a successive annihilation of defects until a reversible temperature-dependent stress condition is set. Top-blasting does not change the initial microstructure and residual stress of the Ti(C,N) layer. Ti(C,N) shows a cyclic stress behavior associated with the heat treatment and an elastic deformation behavior in the temperature range investigated.

Design of Comb Crack Resistant Milling Inserts: A Comparison of Stresses, Crack Propagation, and Deformation Behavior between Ti(C,N)/α-Al2O3 and Zr(C,N)/α-Al2O3 CVD Coatings

“…Stress analysis on the investigated layers was performed for the diffraction lines summarized in Table 2 and the average of the resulting stress values was taken to monitor the stress evolution during thermal cycling. Further details about instrumentation and method have been extensively reported in a previous work by the authors [13]. [20,21] using the respective single crystal elastic constants.…”

“…Transmission Electron Microscopy (TEM) observations were done using a doublecorrected Thermo-Fisher-Scientific Titan G2 microscope operated at 300 KV. Dark field scanning TEM (STEM) images were taken at a camera distance of 73 mm from crystal grains tilted into low-index-zone axes to observe crystalline defects on the α-Al 2 O 3 top layer, as described in [13].…”

“…The replacement of the Ti(C,N) by the Zr(C,N) intermediate layer (average CTE: 9.0 × 10 −6 /K vs. 7.65 × 10 −6 /K) should impact the stress behavior both after coating deposition and during milling with cycling thermal loads. Furthermore, the cycling stress distribution at the cutting-edge during machining connected to the CVD pre-crack network impact the development of comb cracks [13]. Residual and thermal stress measurements are conducted in both systems in the as-deposited and top-blasted condition.…”

“…Crystals 2021, 11, x FOR PEER REVIEW 9 of 16 layer remain in compressive stress in the first heat cycle, releasing the induced top-blasted stresses after the second thermal cycle. It can be seen from STEM images (Figure 6) that a high density of defects (dislocations and stacking faults [13]) are introduced after blasting in the α-Al2O3 for both Ti(C,N) and Zr(C,N) grades, and that after annealing the density of these defects decreases. The physical understanding of this phenomenon connecting the residual stress behavior and the microstructural changes during thermal cycling has been recently reported [13], concluding that the high stresses introduced by blasting are connected to a high density of defects in the α-Al2O3 and the subsequent relaxation by thermal treatment is driven by annihilation of defects in a diffusion driven phenomena.…”

“…The study and understanding of residual and thermal stresses are therefore a key feature to control the crack formation and hence improve the tool life of CVD coated milling inserts [10][11][12]. In a recent work, the authors have investigated the thermal cycling behavior of Ti(C,N)/α-Al 2 O 3 coating systems in both as-coated and top-blasted conditions [13]. Main observations were the reversible correlation of stresses with the temperature cycles, the introduction of compressive stresses in the α-Al 2 O 3 layer by top-blasting associated with a high density of defects; and the relaxation of such induced compressive stresses by annihilation of defects in a thermally activated process.…”