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Chemical vapour deposited TiCN-Al 2 O 3 hard coatings applied on hard metals may remain in a residual tensile stress state after production and sometimes they show a surface crack network. Shot peening treatments are used to introduce compressive residual stresses in the coatings and hard metal. The current work investigates dry shot peening treatments of hard metal coated by TiCN as base layer and α-Al 2 O 3 as upper layer by means of finite element modelling. The simulations use experimentally parameterized material models for substrate and coatings. The influence of the coating roughness on plastification and stress development after shot peening is regarded and a number of 2D simulations with various combinations of speed, impact angles, and particle sizes are carried out. Additionally, the effect of compressive residual stresses and associated crack closure is investigated. The study reveals a number of recommendations for effective shot peening in terms of introduced compressive stresses and crack closure. Residual stress depth profiles from the calculations and experiments show good agreement for a chosen set of reasonable process parameters. • Verified shot peening model of a coated cutting tool insert. • A significant influence of the coating roughness on the residuals stress state is found. • A range of shot peening parameters is found producing compressive stresses in coatings and substrate. • It is shown that existing open cracks can be firmly closed by shot peening. [ABSTRACT FROM AUTHOR]

In metal cutting applications functionally graded near-surface zones in cemented carbide substrates are applied to optimize their properties, in particular toughness and hardness. Thus, the present work focuses on the influence of Co-enriched substrate surface zones and their thickness on the microstructure and mechanical properties of state-of-the-art TiN/TiC 0.6 N 0.4 /α-Al 2 O 3 coatings synthesized using chemical vapor deposition. Complementary cross-sectional energy dispersive X-ray spectroscopy and electron back-scatter diffraction maps provided insight into the grain size, preferred orientation and phase composition of coatings and substrates. While the hardness and Young's modulus of the coatings were hardly affected by the Co-enriched surface zone and its thickness, nanoindentation maps performed on the cross-sections of the substrates confirmed a lower hardness and Young's modulus in zones with higher Co content. Furthermore, it was found that the Co-enriched surface zone and its thickness have no effect on the thermal crack networks of the coatings. Rockwell-indentation tests demonstrated a reduction of the coating adhesion with increasing thickness of the Co-enriched surface zone. As determined by X-Ray diffraction, the tensile residual stress in both, the TiC 0.6 N 0.4 and α-Al 2 O 3 , decreased with increasing thickness of the Co-enriched surface zone. Complementary finite element method simulations have shown that plastic deformation due to creep in the substrate and the Co-enriched surface zone only has a minor influence on the residual stress in the coating. The results obtained within this work contribute to a better understanding of the influence of a Co-enriched surface zone and its thickness on the performance of TiN/TiC 0.6 N 0.4 /α-Al 2 O 3 coated cutting tools. • CVD TiC 0.6 N 0.4 /α-Al 2 O 3 on cemented carbides with Co-enriched surface zones. • Thicker Co-enriched surface zones reduce residual stress in the coating. • FEM simulations confirm residual stress trend observed in experiments. • Co-enriched surface zone has no effect on hardness and Young's modulus of coating. [ABSTRACT FROM AUTHOR]

WC-Co hardmetals are popular tool materials, which are used in applications such as metal milling or turning. In these applications, elevated temperatures occur in the tools during the machining process, although they are also cooled. This results in a complex interaction of thermal and mechanical loads in the tools. Within this current work, a strain asymmetry of a WC-10 wt% Co hardmetal after tensile and compression uniaxial step-loading creep tests is described. Two types of tests were performed: Firstly, specimens were deformed to certain strain limits at 700 °C and 800 °C. Strain asymmetry was observed for tensile and compression stresses above 600 MPa at 700 °C and above 250 MPa at 800 °C. In the second type of test, the specimens were stepwise loaded up to a stress of 300 MPa under tensile and compressive load at 800 °C. The aim of test 2 was to identify the physical reason for the strain asymmetry from the first tests at 800 °C. The material's microstructure was analyzed for the specimens from test 2 by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The influence of the loading type was analyzed with regard to damage development and deformation behaviour of the WC- and Co-phase. SEM images showed that the faster increase in strain over time under tensile than under compressive loading was caused by the formation of cavities at WC-WC interfaces and at WC-Co phase boundaries. Due to the larger number of observed microdefects under tension than under compression, it was assumed that this was the physical reason for the strain asymmetry. In addition, EBSD data showed that during the compression and tension creep tests, the fcc Co-phase was partially transformed into the hcp Co-phase. • Uniaxial step-loading creep tests performed for WC-Co under tensile and compression load, with 50 MPa steps and holding time of 500 s for each stress level. • With increasing temperature, the strain asymmetry onset-stress decreases to lower stress levels. • At 800 °C and 550 MPa the tensile creep rate is 16 times higher than the compressive one. • Strain asymmetry is due to faster formation of microdefects under tensile than under compressive load. [ABSTRACT FROM AUTHOR]

In the metalworking industry, hard-coated WC-Co hardmetal tools are often used in machining applications. Generally, tool life is limited by pre-existing defects in the hardmetal substrate and defects that form during tool application. The complex, multi-axial loading situations and the high temperatures present in the cutting-edge area of machining tools are among the main reasons for the formation and growth of defects in operation. At present, there is a lack in experimental setups that can replicate these conditions. In the current work a novel material testing method is presented, which uses a spherical indenter and inclined specimen surfaces to apply multi-axial loads at 700 °C. The local stress state in the specimen was calculated using finite element simulation implementing an experimentally parameterized material model considering ratchetting and creep of the substrate material. Stresses ranging from mainly compressive to tensile-compressive were predicted. Initiation and accumulation kinetics of defects in the nm- to μm size regime were studied quantitatively. The comparison of stress calculations and damage development shows that positions with tensile-compressive stresses exhibited significantly higher defect formation rates than those with mainly compressive stresses. • Novel Ball-in-Cone test setup to induce significant multi-axial cyclic loads at high temperatures is introduced. • Experimentally parameterized FE model considering creep and cyclic creep used to determine test setup load situation. • Experimental results show defect initiation and accumulation kinetics in a hard-coated WC-Co substrate. • Numerical results demonstrate stress range and stress ratio dependency of defect initiation kinetics. • Load & damage situation as in milling cutting edges during milling are reproducible in laboratory experiment. [ABSTRACT FROM AUTHOR]

WC-Co hardmetals are utilized as tool materials in metal cutting applications in which they are exposed to high mechanical cyclic loads and elevated temperatures. A better understanding of the failure mechanisms of WC-Co hardmetals under these application conditions and the ability to diagnose the damage evolution state are key factors to understand the limits of endurable cyclic load at a certain temperature. The aim of the current work was the experimental determination of stress-strain-hysteresis loops for the investigation of damage indicators in uniaxial cyclic tests at a stress ratio of R = σ min /σ max = −1 for two WC-10 wt% Co hardmetals at 700 °C and 800 °C in vacuum. An increase in the stress-strain-hysteresis loop area and tension-compression-strain asymmetry was recorded with increasing number of load cycles at 800 °C, with earlier failure than at 700 °C. The relationship between the stress-strain-hysteresis loop parameters and the damage evolution state at the microstructure level, as well as the deformation behavior of WC- and Co-phases with increasing number of load cycles, were analyzed. To this end, the microstructure for one WC-Co hardmetal grade was analyzed by scanning electron microscopy and electron backscatter diffraction after cyclic testing up to defined numbers of load cycles at 800 °C. It was observed that the hysteresis loop area and strain asymmetry coincide with the formation of nanopores at WC/WC interfaces and WC/Co phase boundaries, which enlarge to form larger cavities with increasing number of load cycles. Additionally, electron backscatter diffraction data showed that the fcc Co-phase partially transformed into hcp Co under cyclic loading. All specimens, in which an increase in the stress-strain hysteresis loop area or strain asymmetry was observed, ultimately failed when a sufficiently high number of load cycles was applied. Thus, these results indicate that the investigated parameters are reliable indicators for bulk material damage. • Results of uniaxial cyclic tests for two WC-Co hardmetal grades at R = σ min / σ max = −1 at 700 °C & 800 °C. • The selected loads correspond to application-relevant stress amplitudes for cutting edges of milling insert plates. • Identification of material damage indicators from cyclic stress-strain-hysteresis loops. • Loop area & strain asymmetry increase with increasing load cycles are indicative of progressive formation of microdefects. [ABSTRACT FROM AUTHOR]

The aim of the current work was the experimental determination of limit stresses in the uniaxial cyclic compression test that lead to advancing ratcheting in different WC-Co hardmetal grades at elevated temperature. At stresses below the limit stress the plastic strain per cycle reduces and plastic strain accumulation stops after a characteristic number of load cycles. Special attention was paid to the microstructural influence on the onset of advancing ratcheting and the associated damage development at the microstructure level. WC-Co hardmetals are used in various areas such as forming and forging tools, where they are exposed to high temperatures and pressure loads. Their good high-temperature properties allow them to be used under these conditions, but these properties are influenced by the microstructure. Investigations of the mechanical properties of hardmetals had been carried out under monotonously increasing loads and cyclic tests at room temperature and elevated temperatures. In these tests, the effect of different stress ratios R = σ min / σ max on the fatigue behaviour of hardmetals were studied, However, no studies are known for WC-Co hardmetals and their limit stresses in relation to strain ratcheting under cyclic compressive loading at elevated temperature. Hence, in the current work the influence of increasing stresses as a function of microstructure and their effect on the evolution of the strain of six different WC-Co hardmetal grades are discussed. For this purpose, the materials were investigated by uniaxial cyclic compression tests at a stress ratio of R = −∞ at 700 °C and 800 °C in vacuum. The investigated hardmetal grades differ on the one hand in their WC grain size, which varies between 0.4 μm and 2.0 μm, and on the other hand in their Co-content, which varies between 6 wt% and 12 wt%. The residual strain value ε res at zero applied stress was observed to stabilize with increasing number of load cycles at low applied stress ranges. Strain ratcheting occurred above a critical stress range, referred to as the limit stress for strain ratcheting. Strain ratcheting is the accumulation of plastic strain with increasing number of load cycles in which no strain stabilization occurs. Further, for all investigated hardmetal grades, the limit stresses were observed to decrease with increasing temperature. In the following, the microstructure of one hardmetal grade was analysed after loading below and above the limit stress by scanning electron microscopy and electron backscatter diffraction (EBSD). The influence of strain stabilization and ratcheting was analysed with regard to damage development and deformation behaviour of the WC and Co-phases. Strain ratcheting was observed to result in the formation of cavities and nanopores at phase boundary triple points and WC/Co interfaces. Additionally, the EBSD data showed that the fcc Co phase was transformed into hcp Co. Therefore, it is assumed that on the one hand, a certain strain value needs to be exceeded for strain ratcheting to occur and, on the other hand, that besides dislocation movement, microdefect formation and phase transformation significantly contribute to the increase in strain. • Determination of limit stresses for the onset of advancing strain ratcheting for six WC-Co hardmetal grades. • Investigations under uniaxial cyclic compression loading with a stress ratio of R = −∞ at 700 °C & 800 °C. • The onset of advancing strain ratchetting was found at stress ranges Δσ just below the compressive yield strength R p0.2. • Loading above limit stress for onset of strain ratchetting leads to microdefect formation that may trigger premature failure. • Of the six studied WC-Co hardmetal grades the grades with the lowest Co-content exhibit the highest limit stresses at 700 °C. [ABSTRACT FROM AUTHOR]

At elevated temperatures, the material behaviour of WC-Co hardmetals shows differences under similar loading conditions depending on WC grain size and Co-content. Variations in the chemical composition and microstructure in hardmetals cause different material properties such as strength or creep resistance. In the current work, the influence of WC grain size on creep mechanism and creep resistance was investigated for WC-12 wt% Co hardmetals with 0.4 μm, 0.7 μm and 2.0 μm average WC grain size. Specimens were tested in uniaxial tensile and compression step-loading creep tests at 700 °C and 800 °C under vacuum conditions. Time-dependent creep behaviour with steady-state secondary creep was observed for all hardmetal grades investigated, with specimens creeping faster under tensile than under compressive loading. At 700 °C, the medium-grained hardmetal grade exhibited the highest minimal creep rates ε ̇ min compared to the submicron and ultrafine-grained grades. In contrast, the ultrafine-grained hardmetal grade showed higher ε ̇ min at low stresses and 800 °C, because of the high amount of grain boundary area per unit volume, which is advantageous for vacancy diffusion at grain boundaries. Therefore, the ε ̇ min of the medium-grained hardmetal grade was less affected by temperature than that of the finer-grained grade. Also two stress exponent n -ranges were observed at 700 °C and 800 °C: At low stress levels, n was in the range of about 1. Above a critical stress level, n reached values between about 4 and 6. Beside the influence of the WC grain size on the creep mechanism and creep resistance, damage evolution with increasing stress levels was analysed for the ultrafine-grained grade at 800 °C. The microstructures of three compression step-loading creep tested specimens were examined after maximum stress levels of −350 MPa, −950 MPa and −1350 MPa. Microstructural investigations performed via scanning electron microscopy showed that more and larger cavities had formed at WC/WC interfaces and WC/Co phase boundaries in the specimen tested up to −1350 MPa compared to the ones tested up to −350 MPa and −950 MPa. • Compression & tensile step-loading creep behaviour for WC-12wt.% Co hardmetals (WC grain 0.4 to 2.0 µm) at 700 °C & 800 °C. • Flexible test setup, as specimens can be loaded at low or high temperatures up to high operating loads. • Medium & submicron grades' creep rate is less affected by increasing temperature & vacancy diffusion than ultrafine grade. • Two stress exponents n at 700 °C & 800 °C: low stress levels n ≈ 1, & above a critical stress level, n is between ≈ 4 & 6. • Microstructure study of the ultrafine grade shows an increase in the number of microdefects & their size at high stress levels. [ABSTRACT FROM AUTHOR]

TiCN coatings were grown by chemical vapor deposition (CVD) on WC-Co substrates with different Co contents, in order to control thermal stress. The driving force for the development of thermal stress is attributed to the difference between room and deposition temperature (ΔT ≈ −780 °C), and the mismatch of the coefficient of thermal expansion (CTE) between substrate and coating. Co contents of 6, 7.5, 10, 12.5, and 15 wt% were utilized to adjust the CTE of the substrate, and therefore tune the stress in TiCN coatings. Dilatometry of the substrates and high temperature X-ray diffraction of a powdered TiCN coating indicate a decreasing CTE-mismatch for increasing substrate Co contents. In consequence, residual stress in TiCN determined by X-ray diffraction increases up to 662 ± 8 MPa with decreasing Co contents down to 10 wt%. For Co contents below 10 wt%, the residual stress decreases. The formation of thermal crack networks in TiCN, analyzed by scanning electron microscopy, coincides with 10 wt% Co. Stress relaxation in TiCN coatings through the formation of thermal cracks becomes evident. A finite element simulation utilized for the calculation of residual stress distributions reveals shielding effects, which occur with the introduction of thermal cracks. Discrepancies between experimental and simulated thermo-elastic stresses imply the presence of secondary relaxation sources. High temperature residual stresses in TiCN, determined up to 1000 °C (i.e. above deposition temperature), suggest additional thermal crack formation for substrate Co contents of 6 wt%. • Coefficients of thermal expansion (CTEs) of CVD TiCN and WC-Co substrates • CTE mismatch tuning of TiCN and WC-Co through adjustment of Co content • Elimination of thermal crack networks in TiCN for 12.5 and 15 wt% Co • Illustration of crack shielding effects by finite element simulations • High temperature residual stress of TiCN coatings grown on WC-Co substrates [ABSTRACT FROM AUTHOR]