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.
Highlights
Metal part components in the automotive and aerospace industry are manufactured to their final shape with cemented carbide coated tools [1]
A fracture image showing the polycrystalline layers with a larger magnification is shown in Figure 1b consisting of a thin titanium nitride (TiN) starting layer of 0.4 μm followed by a Ti(C,N) layer of 4.5 μm, a Ti(C,N,O) bonding layer (BL) and a α-Al2O3 top-layer of 4.5 μm
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, as shown in the TEM image analysis
Summary
Metal part components in the automotive and aerospace industry are manufactured to their final shape with cemented carbide coated tools [1]. WC-Co-based cemented carbides coated with Ti(C,N)/α-Al2O3 multilayers of few micrometers by chemical vapor deposition (CVD) represent more than 50% of the total world market. The reason for this is the outstanding balance of hardness and toughness of cemented carbides [2] combined with the excelled chemical/heat resistance (provided by the aluminum oxide layer) and wear resistance (provided by the carbonitride layer) of the coatings [3]. During interrupted machining, thermomechanical wear-induced comb cracks form on existing CVD cooling microcracks, which leads to limited performance and failure of the tools [4]. Compressive stresses retard the opening of microcracks, which leads to the formation of thermomechanical cracks [4,5,6,7]
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