In Situ High Temperature X‐Ray Diffraction Reveals Residual Stress Depth‐Profiles in Blasted TiN Hard Coatings

Abstract

High temperature X‐ray diffraction (XRD) is used to characterize residual stress depth‐profiles and volume‐averaged residual stresses in blasted 9 µm thick TiN coatings on WC/Co substrates in the temperature range of 25–1100 °C using a dedicated high temperature attachment from Anton Paar GmbH. Room temperature measurements reveal that the blasting process changes the almost constant tensile residual stress of 560 MPa across the coating to compressive stress which decreases exponentially within the coating surface region of 1–3 µm. The magnitude of the compressive stresses is dependent on the particle pressure used during the blasting process. Heating up results in rapid compressive stress relaxation already at relatively low temperatures and is accompanied by relaxation of stresses of II and III order. During cooling down from 1100 °C, tensile residual stresses are formed. After reaching the original stress value of about 560 MPa, the tensile stresses do not increase upon cooling anymore but are relaxed by cracks. The analysis of diffraction peaks shows that blasting results in anisotropic nano‐structural changes in the coating surface. In summary, the approach documents that HT‐XRD can provide important insights into the high temperature behavior of engineering components.

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

“…This corroborates that the top-blasting process in -Al2O3/Ti(C,N) multilayer CVD coatings is mainly confined to the α-Al2O3 top-layer. Though, in previous works on top-blasted TiN [8] and Ti(C,N) [9] CVD layers (without top -Al2O3 layers on it), a strong change in defect density TEM investigations of Ti(C,N) thin films show no difference among the as-coated, top-blasted and heat treated conditions. Mainly stacking faults and twin boundaries defects for all conditions were observed, as displayed in Figures 9 and 10.…”

“…This corroborates that the top-blasting process in -Al2O3/Ti(C,N) multilayer CVD coatings is mainly confined to the α-Al2O3 top-layer. Though, in previous works on top-blasted TiN [8] and Ti(C,N) [9] CVD layers (without top -Al2O3 layers on it), a strong change in defect density and residual stress at RT were observed. The stresses induced by top-blasting were annealed upon heating, indicating that Ti(C,N) may undergo a similar mechanism-as observed for the -Al2O3 in this work if defect introduction by mechanical impact is possible.…”

“…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.…”

“…However, a complete description of the correlation between stress behavior of multilayer coatings under thermal cycling and the microstructural changes associated with the development of residual stresses needs further understanding. Stress depth profiles, average stresses and microstructure development on single titanium nitride (TiN) layers deposited on cemented carbide in a temperature range of 25-1100 • C were reported by Bartosik et al [8]. It was observed that top-blasting influenced the surface microstructure of the TiN film and that the residual stresses are relaxed already at low temperatures in the heating up part of the cycling step.…”

Sample tilt-free characterization of residual stress gradients in thin coatings using an in-plane arm-equipped laboratory X-ray diffractometer

“…a 0 ðÞ and ij ðÞ are the stress-free lattice constant and the stress value corresponding to the penetration depth . The stress state is supposed to be equibiaxial ( 11 ¼ 22 ¼ , i3 ¼ 0) according to investigations done with similar samples (Stefenelli et al, 2013;Bartosik et al, 2011). a 0 is a lattice parameter corresponding to the stress-free bulk state, which can be used to approximate the actual a 0 ðÞ value; this approximation can be utilized since the first term of the equation is insensitive to a minor variation of a 0 ðÞ.…”