We report on phase and strain changes in Ti1-xAlxN (0 <= x <= 0.61) coatings on cutting tools during turning recorded in operando by high-energy x-ray diffractometry. Orthogonal cutting of AISI 4140 steel was performed with cutting speeds of 360-370 m/min. Four positions along the tool rake face were investigated as a function of time in cut. Formation of gamma-Fe in the chip reveals that the temperature exceeds 727 degrees C between the tool edge and the middle of the contact area when the feed rate is 0.06 mm/rev. Spinodal decomposition and formation of wurtzite AlN occurs at the positions of the tool with the highest temperature for the x >= 0.48 coatings. The strain evolution in the chip reveals that the mechanical stress is largest closest to the tool edge and that it decreases with time in cut for all analyzed positions on the rake face. The strain evolution in the coating varies between coatings and position on the rake face of the tool and is affected by thermal stress as well as the applied mechanical stress. Amongst others, the strain evolution is influenced by defect annihilation and, for the coatings with highest Al-content (x >= 0.48), phase changes.

Titanium alloys are known for being difficult to machine. Within the groups of Ti alloys, the machinability is reduced when going from the alpha-alloys, to alpha + beta, and finally to near-beta or beta-alloys. Uncoated cemented carbide is traditionally used for machining these alloys and finding a suitable coating to improve the performance is a challenge due to the high strength of Ti alloys and the high chemical reactivity of Ti with tool and coating materials at the temperatures achieved during cutting. The PVD applied TixAl1-xN (x = 0.4-0.7) is generally recommended for machining Ti alloys and a top layer of NbN has shown promising performance in milling applications. This study systematically explores the wear mechanisms of either uncoated WC-12%Co or its Ti0.45Al0.55N-NbN coated version in the milling of Ti alloys ranging from near-alpha (Ti-6Al-2Sn-4Zr-2Mo), alpha + beta (Ti6Al-2Sn-4Zr-6Mo), and near-beta (Ti-5Al-5Mo-5 V-3Cr) alloys. The wear evolution is explored by studying as-worn tools and their cross-section using SEM-XEDS and EBSD after having reached 10%, 30%, 50%, and 100% of the full tool life at a flank wear criterion VB = 200 mu m. The coating is removed within seconds of engagement and fails by cracking within the bulk and PVD droplet defects can initiate such cracks. Diffusional dissolution of the coating may be active on a minor scale, but the rapid mechanical failure shows the need for a more robust coating formulation. Exposed cemented carbide is worn at varying intensities when machining the Ti alloys. The highest wear rate is achieved in milling alpha + beta Ti-6246, followed by a moderate wear rate in near-beta Ti-5553, and with a slower wear rate in milling near-alpha Ti-6242 which is explained varying intensities in oxidation wear, diffusional dissolution, and mechanical cracking. Diffusional loss of C gives rounder WC grains and remaining W at the interface is removed by the chip flow. Diffusional loss of Co gives reduced grain bonding and loss of the dampening effect that leads to fractures in WC grains. Specifically in milling alpha + beta Ti-6246 and in minor scale in near-alpha Ti-6242, there is formation of CoWO4 ceramic present several mu m into the tool and its formation is facilitated by the oxidation of binder with resolved W. Fractures within the CoWO4 explains the high wear rate. Cracks also propagate in binder regions initiated from weakened interface regions due to diffusional loss of C and Co.

In this study, we use in-situ synchrotron x-ray diffractometry to monitor the real-time evolution of stress, grain size, and crystallographic orientation during cathodic arc deposition of Titanium-Aluminium-Nitride (Ti1-xAlxN) films with varying Al content (x=0, 0.25, 0.5, and 0.67). We vary the substrate bias voltage and evaluate its effect on the film’s stress and grain size evolution. Our results show that films deposited at a floating potential develop a (111) fiber texture and converged tensile stress, while films formed at a finite negative substrate bias voltage have a compressive stress with an off-axis tilted (200) texture. The biaxial stress and grain size evolution are well described by a power law with an exponent and a pre-factor that have a systematic dependence on substrate bias and Al content. We fit a kinetic model to the stress evolution to extract kinetic parameters of the thin film growth. Under specific growth conditions, cohesive failure occurs in the films, which we have used to determine the fracture toughness to be 3.1 ± 0.1 and 4.8 ± 0.1 MPa·m1/2 for Ti0.75Al0.25N and Ti0.50Al0.50N, respectively.

Polycrystalline cubic boron nitrides (PcBN) have been increasingly used together with PVD coatings, mainly for hard turning operations. Within this context, effectiveness of coated PcBN as cutting tool is usually addressed by evaluation of its machining performance. Meanwhile, studies aiming to assess and understand the correlation between microstructural features and mechanical behaviour of the coating-substrate system are rather limited. Aiming to overcome such lack of information, in this study the influence of substrate bias voltage (-35 V as compared to -60 V) and microstructural assemblage (as a function of cBN content and binder chemical nature) on the mechanical integrity of TiAlN-coated PcBN systems is investigated. In doing so, contact damage response and coating adhesion strength of different coated-PcBNs are evaluated by means of indentation testing using distinct loading conditions (static and sliding) and tip geometries (spherical and conical). Such testing program is complemented by detailed FESEM inspection of the involved failure micromechanisms, as well as microstructural and micromechanical characterization of the deposited films. Results indicate that resistance against crack nucleation and propagation of coated PcBN, induced by either spherical or conical indentation, is enhanced by using harder (high content of cBN particles) and tougher (metallic binder) substrates (H-PcBN). Regarding bias voltage, systems with coatings deposited using a higher value (-60 V as compared to -35 V) show improved adhesive strength, this being particularly true for combinations involving low cBN content and ceramic binder substrate (L-PcBN). Similar beneficial effect was found, but exclusively in coated L-PcBN systems, regarding resistance to radial cracking emergence and to material removal through cohesive-failure chipping induced in Rockwell C tests. Although these findings are linked to the higher compressive residual stresses exhibited by coatings deposited under -60 V bias voltage, the latter does not translate in significant changes in microstructural and intrinsic mechanical properties of the TiAlN coating itself.

Polycrystalline cubic boron nitride (pcBN) is a very promising tool material for turning martensitic stainless steels at high cutting speeds (vc > 200 m/min). The competitive advantage of pcBN over cemented carbide increases as the cutting speed is increased. Changing the speed might lead to a shift in the wear balance and hence the knowledge about tool wear below vc = 200 m/min might not be applicable at vc = 600 m/min. The coatings designed for the lower speed range might also not be performing in the same way at higher speeds. This paper investigates the wear mechanism of uncoated and (Ti,Al)N coated pcBN tools when turning 17–4 PH in a hardened condition at speeds vc = 200–600 m/min. Both scanning and transmission electron microscopy are used to study the worn tools. The in-depth analysis reveals that adhesive wear is only active at low speeds. Increasing the speed does however lead to more wear by diffusion and oxidation. The cBN is preferentially worn out, leaving the TiC binder at the tool-chip interface. Oxidation results in the accelerated wear of the pcBN but also in the formation of metal oxides within the adhered build up layer. The (Ti,Al)N coating does not significantly extend the tool life within this speed range, but it suppresses the adhesive wear mechanism preventing premature tool failure.

Finding a wear resistant coating for cemented carbide cutting tools in the machining of difficult to cut Ti alloys is a challenge due to their high strength and chemical reactivity. Tool manufacturers recommend physical vapor deposited (PVD) Ti x Al 1-x N (x = 0.4-0.7), and an extra NbN overlayer has shown promising potential. This study explores wear mechanisms of PVD Ti 0.45 Al 0.55 N with and without NbN overlayer and its WC -Co substrate in machining Ti alloys. To achieve an accurate understanding of tool-chip-workpiece interaction and related wear mechanisms, several approaches were employed. Tests with controlled variation of cutting speeds were complemented by process freezing experiments using the quick stop method and imitational experiments of diffusion couples. Advanced microscopy techniques were employed for accurate detection of wear products and phenomena across length scale. Findings reveal that any new design of coatings for Ti machining must combine both high mechanical integrity and resistance to diffusional dissolution and oxidation. Observed diffusional loss of Al and N from the coating results in a TiN layer which is mechanically weaker than the original coating, while the NbN overlayer reduces the Al diffusion rate, but NbN is subjected to diffusional dissolution itself. On dissolution, Nb stabilizes -Ti beta and thus facilitating loss of Al, but the observed formation of intermetallic Nb 3 Al at the NbN-Ti interface works as a diffusion barrier. However, brittle Nb 3 Al can be more easily removed during machining. It was found that the coating retains longest on the edge line and protects the tool edge from failure because substrate cemented carbide wears at a faster rate than the coating with outward diffusion of C from WC grains and Co binder.

This study is focused on how the application of pulsed substrate bias during cathodic arc deposition affects the microstructure, texture, grain size and phase composition of (Ti,Al)N coatings. A series of Tix-1AlxN, 0.25 <= x <= 0.55 coatings were deposited on WC-Co cemented carbide substrates with -30 V, -60 V and -300 V pulsing bias (duty cycle 10 % and a frequence of 1 kHz) under controlled chamber conditions at 4.5 Pa N-2-gas and a substrate temperature about 400 degrees C. The pulsing parameters for the bias (voltage, duty cycle and frequency) were deliberately selected to influence structure, microstructure and composition of the deposited coatings. All Tix-1AlxN coatings had a consistent columnar cubic B1 structure regardless of their chemical composition. Coatings grown at -30 V and -60 V pulsed bias exhibited a pronounced <111> texture attributed to a kinetically driven mechanism influenced by the relative flux of ion species, affecting the surface migration of adatoms during growth. In contrast, the coatings grown with a pulsed bias of -300 V exhibited a reduced <111> texture and the onset of grains with <100> preferred orientation. The transition to the <100> orientation with increased ion energy agrees with the fact that the <111> directions expose the densest array of atoms to the ion beam during growth while the <100> are the most open channeling directions in a B1 structure. The correlation to the preferred with respect to pulsing conditions during growth, correlated to microstructure, grain size and phase composition be further discussed. Surface roughness was highest (S-a approximate to 0.17-0.22 mu m) for coating deposited at pulsed bias -30 V.

Structural changes in Ti1-xAlxN coated tool inserts used for turning in 316L stainless steel were investigated by XANES, EXAFS, EDS, and STEM. For coarse-grained fcc-structured Ti1-xAlxN coatings, with 0 <= x <= 0.62, the XANES spectrum changes with Al-content. XANES Ti 1s line-scans across the rake face of the worn samples reveals that TiN-enriched domains have formed during turning in Ti0.47Al0.53N and Ti0.38Al0.62N samples as a result of spinodal decomposition. The XANES spectra reveal the locations on the tool in which the most TiN-rich domains have formed, indicating which part of the tool-chip contact area that experienced the highest tem-perature during turning. Changes in the pre-edge features in the XANES spectra reveal that structural changes occur also in the w-TiAlN phase in fine-grained Ti0.38Al0.62N during turning. EDS shows that Cr and Fe from the steel adhere to the tool rake face during machining. Cr 1s and Fe 1s XANES show that Cr is oxidized in the end of the contact length while the adhered Fe retains in the same fcc-structure as that of the 316L stainless steel.

Understanding the link between titanium-aluminium nitride’s (Ti1-xAlxN) physicochemical properties and structural dynamics during operation is essential to designing and fabricating advanced ceramic coatings. Herein we directly visualise the oxidation of Ti1-xAlxN (for x = 0, 0.18, 0.44, & 0.67) coatings from 100 to 1000 °C using environmental transmission electron microscopy and energy dispersive X-ray analysis. The high-frame-rate ETEM movies show that oxidation in TiN proceeds at the grain boundaries and cracks; in contrast, Ti1-xAlxN coatings transform from large as-deposited grains into oxide nanoparticles. Moreover, high-resolution ETEM imaging showed the presence of anatase TiO2 at the early stages of oxidation across all compositions. Above ~850 °C, the oxide nanoparticles grow through crystal merging, diffusion and recrystallisation to form rutile TiO2. The EDX elemental maps coupled with secondary electron imaging reveal a uniform TiO2 sublayer decorated with increasing coverage of Al2O3 particles for x = 0.18 to 0.44. In contrast, coatings with x = 0.67 reveal a complete in-plane phase separation of Al- and Ti-oxides, which can rationalise their decreasing long-term oxidation resistance. Finally, the study provides unique insight into the real-time structural dynamics underpinning the oxidation resistance of Ti1-xAlxN coatings.

The residual stress plays a vital role in determination of the device performance that uses thin films coating and thus the accurate determination of stress and its optimization with process parameters is an ongoing research work for many decades. In line with this, the microscopic origin of the stress at the atomic scale and its development during the thin film deposition is a matter of major scientific interests. The development of stress is a complex phenomenon and has a complex dependence to process parameters, film microstructure and its morphology. In this work, by utilizing a custom-designed cathodic arc deposition system and synchrotron radiation based 2D x-ray diffraction (XRD) technique, we determine the real-time evolution of stress, crystallite sizes and their preferential orientations of Aluminum-Titanium-Nitride (AlxTi1-xN) films with varied Al-content (x=0.0, 0.25, 0.50, and 0.67) on Si-100 substrate. The energies of incoming ions and hence stress in the films is tuned by applying different direct current substrate bias (Vs = floating potential, -20, -40, -60, -80, and -100 V). The instantaneous stress is evaluated by the well-known d vs. sin2ψ technique, while crystallite sizes are determined by analyzing line profiles of x-ray diffractograms. The evolution of stress and crystallite sizes are modelled with multiple numerical models from which kinetic parameters associated with the thin film depositions are extracted. The ex-situ microstructure characterizations of AlxTi1-xN coatings are carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The formation of ex-situ microstructure of the films is discussed considering the results obtained from in-situ XRD data. Finally, we demonstrate that the method utilized here is a powerful approach towards estimation of the fracture toughness of thin film coatings.

Non-metallic inclusions offer one of the most effective routes for improving the machinability of steels. However, the wear-reducing mechanisms activated by such inclusions are not fully understood. The interactions are notoriously difficult to predict due to the wide variety of steel grades, cutting conditions, and tool materials employed in industry. The interaction between PVD (Ti,Al)N coated cemented carbide tools, non-metallic inclusions, atmospheric oxygen, and the stainless steel 316L in a turning operation is therefore investigated here as a case study. The study includes turning experiments, nanometer resolution microscopy, and thermodynamic calculations. The paper explains how not only too high a contact pressures hinder the formation of protective deposits at the tool edge, but also how too low a contact pressure leads to excessive wear. A range of conditions specified in this paper must therefore be met for the two observed protective non-metallic inclusions Mg1Al2O4 and Al2Ca2Si1O7 to be preferentially deposited on a tool. Hence the coating wear is experimentally investigated, explained, and a thermodynamic calculation method for predicting the protective or degenerative potential of a deposit on the coating is presented.

The wear behavior of TiAlN coatings during turning of stainless steel 316L at low cutting speeds (60-120 m/min) was investigated using scanning electron microscopy. In this speed range, the coatings fail by fracture due to an adhesive wear mechanism. The fracture of the coating is described in detail, including the strong influence of Alcontent and cutting speed on the rate of wear. Low Al-content (x <= 0.23) coatings showed worse wear resistance than high Al-content (x >= 0.53) samples. Less substrate is exposed when the cutting speed is increased, because of reduced adhesive wear. The TiN and Ti0.77Al0.23N coatings are severely worn for all cutting speeds while Ti0.47Al0.53N and Ti0.38Al0.62N remain essentially unaffected at the highest speed. The difference in wear behavior is interpreted as a difference in the fracture toughness of the coatings.

This study investigates the wear of W- and Mo-alloyed Ti1-x-yAlxMeyN coatings (Me = W, Mo) with x asymptotic to 0.55 and y asymptotic to 0.10 during high-speed turning of stainless steel 316L. A difference in the crater wear rate was observed between TiAlN and Ti1-x-yAlxMeyN coatings. The wear behavior in the sliding area is characterized in detail for two different regions by scanning and transmission electron microscopy and energy dispersive X-ray spectroscopy. A thin adhered layer constituted of elements from the workpiece material is observed on the top of all coatings, followed by diffusion of species from the stainless steel 316L into the coatings. Co from the cemented carbide substrate also diffuses through column boundaries of the coating. The temperature varies in the sliding area. The presence of Mo or W retards the spinodal decomposition and the formation of h-AlN as compared to TiAlN coatings, leading to lower crater wear rate in alloyed coatings.

Through the use of tool coatings, a significant increase in tool life is possible. Particularly in the case of coatings obtained by physical vapor deposition, compressive residual stresses show a clearly positive influence on the wear behavior of coated carbide tools. In addition to being influenced by the coating process itself, the residual stress state of the coating can also be influenced by post-treatment processes like heat treatment or wet blasting. Compressive residual stresses can be measured using known X-ray diffraction techniques. However, these methods are very limited on strongly curved surfaces such as cutting edges. Raman spectroscopy has great potential for measurements in such areas. In order to use the Raman method to measure residual stresses on cutting tools, the TiAlN coatings were subjected to wet blasting and heat treatment. The residual stress state was determined by X-ray diffraction using the sin2ψ method and the depth-resolved scattering vector method. Then, measurements of Raman scattering of light were carried out and the peak shift of the transversal/longitudinal optical mode in the Raman spectrum was measured. Finally, the peak shift in the cutting edge area was determined. The investigations show that residual stresses mechanically induced by wet blasting can be reliably detected by Raman spectroscopy. Heat treatment effects are also clearly detectable, but here the correlation between peak shift and residual stresses determined by X-ray diffraction methods differs from the previous results due to the change in the crystal lattice dimensions. The residual stresses in the cutting edge area, converted from Raman peak shifts, show values of a plausible order of magnitude.

The residual stress state in tool coatings can positively influence tool life. Measurement in strongly curved surfaces e.g. in the cutting edge area is only possible to a limited extent by means of X-ray diffraction (XRD). Raman spectroscopy offers great potential for determining the residual stress state in this area. Therefore, the aim is to determine the fundamental limits of residual stress measurement by XRD on coated carbide tools and to determine and evaluate the suitability of Raman spectroscopy. On typical cutting tools only a small area on the rake face can provide reliable measurement results using conventional XRD methods. Using the XRD results as reference, Raman spectroscopy shows plausible results for residual stresses induced into the coating by mechanical or thermal post-treatment. Coating-induced residual stresses cannot be reliably detected because other coating properties are also changed by modified coating processes that induce higher compressive residual stresses.

Reciprocating sliding wear tests were performed on TiAlN coating against 316L stainless steel and carbide-free bainitic steel at temperatures of 40, 400 and 800 degrees C. The results indicate that material transfer is more pronounced for the softer stainless steel at lower temperatures but at 800 degrees C, carbide-free bainite exhibits relatively more material transfer. Friction coefficient of stainless steel increases when temperature increases. However, for carbide-free bainite, there is a reduction in friction coefficient at elevated temperatures. This can be attributed to formation of an easily sheared iron oxide layer at elevated temperatures. In case of stainless steel, generation of a thin tribofilm containing aluminium oxide and oxidised transferred material can protect the TiAlN coating against wear at 800 degrees C.

In this research, a fundamental study was conducted on damage behavior of cathodic arc evaporated TiN and Ti0.44Al0.56N coatings, in terms of oxidation and cracking/spallation, when they were exposed to single-pulse laser treatment in a temperature range of 1200-2100 degrees C. Moreover, a multiple-pulse laser treatment was designed to apply thermo-mechanical loads on the coatings in order to evaluate their thermal degradation during rapid heating/cooling cycles between 200 and 1200 degrees C. Single-pulse treatment of TiN up to 1500 degrees C led to the intercolumnar cracking and formation of ultrafine TiO grains. An increase in temperature up to 2100 degrees C resulted in a notable bulging of the surface, and formation of TiO2 of various morphologies such as grainy structure, dense molten and re-solidified structure, droplets from melt expulsion and, more interestingly, nanofibers. Multiplepulse treatment of TiN was accompanied by a severe cracking and spallation, which divided the surface into two layers: a heavily cracked top layer composed of dense TiO2 grains, and a bottom layer having porous TiO2 grains indicating incomplete oxidation. Conversely, Ti0.44Al0.56N did not show any visible cracking and oxidation after single-pulse treatment. Multiple-pulse treatment did not also yield cracking and spallation for Ti0.44Al0.56N, and its ablated region consisted of TiO2 grains combined with thin Al2O3 platelets. An excellent combination of properties including higher oxidation resistance and greater fracture toughness at high temperatures led to a higher thermal damage resistance for Ti0.44Al0.56N coating compared to TiN when undergoing single- and multiple-pulse laser treatments.

The influence of the aluminium content (x) on crater wear mechanisms of Ti1-xAlxN coated WC-Co inserts in highspeed turning of 316L stainless steel was investigated. Electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the wear behaviour. Ti1-xAlxN coatings with x <= 0.53 showed, after 1/3 of the tool life, a thick adhered layer composed of oxides and metallic species from the steel, and no diffusion of workpiece material into the coating. These coatings presented the best wear resistance and least abrasive wear. The high aluminium content Ti0.38Al0.62N coating showed the worst crater wear resistance. This is assigned to interdiffusion of workpiece elements and oxygen into the coating as a consequence of spinodal decomposition of the cubic TiAlN-phase, resulting in more severe abrasive wear.

Detailed knowledge of correlations between direct current (DC) cathodic arc deposition process parameters, plasma properties, and the microstructure of deposited coatings are essential for a comprehensive understanding of the DC cathodic arc deposition process. In this study we have probed the plasma, generated by DC arc on a Ti-50 at.% Al cathode in a N 2 ambience, at the growth front of the TiAlN coating. Several consequences of an increasing N 2 pressure are observed, including a decreased electron temperature, an increased electron density, and a loss of energetic ions. As a result, the preferred growth texture switches from 220 to 111. It is also observed that neutrals in the plasma can significantly contribute to the growth of TiAlN coatings.

Residual stress measurements directly in the coated cutting edge are not possible with X-ray diffraction (XRD) due to the diameter of the X-ray beam. On the other hand, Raman microscopy enables measurements on the micrometer scale. Parameter variations in the PVD process were used to provide different residual stress states in (Al,Ti)N coatings on carbide cutting tools. They were examined by XRD in regions that can be reliably measured. The same area was then examined by Raman microscopy to determine the relationship of Raman peaks to the residual stress. Local high-resolution Raman measurements were then taken at the cutting edge and further influences on the Raman peak position besides residual stresses were excluded. In order to analyze the relationship between Raman peak shift and residual stress state, measurements were performed during a bending load. Finally, an outlook on further investigations is given.