Bias voltage optimum adjustment considering coatings’ strength and adhesion requirements when cutting various steels

PVD coatings were deposited at various bias voltages on cemented carbide inserts. The coating’s mechanical properties, fatigue and adhesion were determined via FEM-supported evaluation of nanoindentation, perpendicular and inclined impact test results. The coated inserts were applied in milling hardened and normalized steel. For explaining the wear evolution based on the cutting loads and stress fields developed in the coating and its substrate, FEM calculations were performed considering among others the films’ strength and adhesion. According to the workpiece properties, certain coating’s parameters become prevailing for the tool life. These depend on the bias voltage and facilitate its optimum adjustment.     

Correlation between PVD coating strength properties and impact resistance at ambient and elevated temperatures

The performance of PVD films depends on the operational mechanical and thermal loads of coated tools or components. To withstand these loads, sufficient film strength properties, at ambient and elevated temperatures are required. The present paper deals with the mechanical properties and the impact resistance of a TiAlN coating at temperatures up to 400 °C. Nanoindentations were conducted by a nanoindentation device, enabling measurements at room and elevated temperatures in an inert gas atmosphere. The obtained results were evaluated using appropriate FEM algorithms for determining the film's stress-strain curves, at the temperatures of the conducted nanoindentations. Moreover, perpendicular impact tests on the coated specimens were carried out up to 400 °C, for investigating the film's impact behaviour. The developed impressions were recorded by scanning electron microscopy and white light confocal measurements. The attained results demonstrate a non-linear dependence of the film fatigue properties versus the temperature, whereas a significant impact resistance improvement at approximately 150 °C develops. Finally, a convergence between the yield and the fatigue endurance stress versus the temperature was revealed.     

Fatigue Life Properties and Availability of Proof Testing in Ceramics-Coated Glass

The long-term durability of high-performance ceramics-coated glass should be appropriately evaluated prior to their practical applications. Fatigue properties of such materials should be clarified to ensure the long-term durability. In this work, a borosilicate glass was coated with alumina or silicon carbide thin films by sputtering method. Fatigue tests of coated glass were conducted under three-point bending. It was clarified that the fatigue life was elongated by coating ceramic thin films on glass and the fatigue life distribution in glass coated with thicker films shifted toward longer life region. Proof testing was carried out for coated glass specimens to remove specimens having lower fatigue lives. It was suggested that proof testing for fatigue of ceramics-coated glass was effective as a screening procedure which can remove weaker specimens by static pre-loading before fatigue tests. In correlating average fatigue lives, fatigue resistance strength was introduced as the average bending strength divided by the applied maximum stress. It was revealed that the average fatigue lives of every coated glass, including average lives after proof testing, were well correlated by a power function of the fatigue resistance strength and its modified parameter, irrespective of film material and thickness and also applied stress level.     

Brittleness and fatigue effect of mono- and multi-layer PVD films on the cutting performance of coated cemented carbide inserts

The effect of brittleness and fatigue of mono- and multi-layer PVD films on coated tools cutting performance is introduced. Cemented carbide inserts were coated to the same overall film thickness with various numbers of layers. Nanoindentations were conducted to evaluate the hardness of the diverse coating structures. The film brittleness and fatigue were characterized by nano- and macro-impact tests respectively. The coated inserts’ wear behaviour was investigated in milling hardened steel. The attained results revealed the coatings’ brittleness and fatigue endurance enhancement by increasing the number of film's layers. This increase leads simultaneously to the coated tool life improvement.     

Cutting performance of coated tools with various adhesion strength quantified by inclined impact tests

Coated tools with PVD-films of almost the same mechanical properties may perform differently. This effect can be attributed to the film adhesion. For highlighting this phenomenon, the adhesion of coatings with practically identical mechanical properties deposited on cemented carbide and powder metallurgical high speed steel inserts was quantified by FEM-based evaluations of inclined impact test and nano-indentation results. Moreover, the films’ mechanical properties were assessed by repetitive nano-impacts. Based on these data and on FEM calculations of the cutting process, the coated tools cutting performance was explained in all investigated material and interrupted cutting process cases.     

Study on fatigue and wear behaviors of a TiN coating using an inclined impact-sliding test

A new ball-on-plate inclined impact-sliding test method was developed to investigate wear phenomena of coated dies under simulated automotive stamping conditions where the stamping die material undergoes a combination of impact and sliding forces. The test coupon in this study was a TiN-coated M2 steel disc. A bearing steel (AISI 52100) ball with a diameter of 10 mm was used as the counterface material. Each cycle of the testing load consisted of an impact force (Fi) and a pressing force (Fp). Tests of different impacting and sliding cycles were conducted to observe progressive failure processes. The severity of the coating failures was investigated through electron microscope observations on the coating surfaces where the impact-induced crater and sliding-induced wear track existed. It was found that the coating failure behaviors included fatigue cracking, chipping, peeling and material transfer. The test results also suggested that the fatigue cracks initiated the chipping and peeling when the cracks formed networks.     

Nano-impact test on a TiAlN PVD coating and correlation between experimental and FEM results

Nano-impact test on PVD coatings is an efficient method for investigating film failure mechanisms. During this test, the coating is subjected to repetitive impacts by a diamond indenter, inducing high local deformations and stresses into the film material, which may lead to coating failure.In the paper, coated specimens with a TiAlN PVD film were investigated by nano-impact tests. The nano-impacts were conducted at several loads and for various test durations. For explaining the attained results, the nano-impact test was simulated by a developed three dimensional finite elements method (FEM) model, considering a piecewise linear plasticity material law. The employed software was the LS-DYNA package; its feature of constrained tied nodes failure was used for simulating crack formation and propagation, as the plastic strain develops and exceeds the coating failure strain. The film elasto-plastic properties, used in the FEM-calculations, were determined by nanoindentations and analytical evaluation of the related results. During the nano-impact indenter penetration, it was assumed that the coating material at the FEM model node regions can withstand the applied load up to a maximum value, which corresponds to the coating rupture stress. Over this load limit, the related nodes are disconnected from the neighboring finite elements. If all nodes of an element are disconnected, the element is released for simulating a crack formation and it becomes an inactive separate entity. In this way, the stress fields developed in the film material and its coating fracture progress in terms of imprint depth versus the repetitive indenter penetrations are analytically described. The attained results converge sufficiently with the experimental ones. The developed nano-impact FEM-simulation predicts the film failure initiation and evolution, which depend on the impact load.     

Investigation of coated tools’ cutting performance in milling Ti6Al4V and its correlation to the temperature dependent impact resistance of the film

Based on analytical and experimental test methods, an evaluation of coated tools efficiency in milling Ti6Al4V with coated cemented carbide inserts is introduced. The stress-strain curves and the fatigue critical loads of the investigated coating were determined at various temperatures by nanoindentations and impact tests respectively, employing FEM-supported algorithms for results evaluation. The stress and temperature fields in the cutting wedge region were obtained by force measurements and FEM calculations of the milling process at various cutting conditions. A sufficient correlation of the coating’s impact resistance at various temperatures with their cutting performance at corresponding cutting conditions is revealed.     

Quantification of PVD Film Adhesion with Critical Shear Stress by Using Dynamic Simulation of the Inclined Impact Test

The film adhesion can be evaluated by the ratio of the tangential to normal film-substrate interface stiffness, as already described in recent publications. In the present paper, a new method is introduced for assessing the film adhesion based on the critical shear failure stress (SFLS). To predict SFLS in the coating-substrate region, a new 3D-FEM model was developed for the dynamic simulation of the inclined impact test, using the LS-DYNA software. This model enables the explicit determination of SFLS and also the relevant maximum equivalent stress developed in the film during the inclined impact test. The occurring SFLS in the coating-substrate interface affects the stresses resulting in the film during the operation of a coated component. In this way, they may lead to potential film material overloading and its cohesive failure.     

Increasing tool life by adjusting the milling cutting conditions according to PVD films’ properties

The coated tools cutting performance in up and down milling depends significantly on the PVD film material properties. The related wear mechanisms at various cutting speeds can be sufficiently explained considering the developed tool loads and the non-linear coating impact resistance versus temperature. Various PVD coated cemented carbide inserts were tested at different cutting conditions. The corresponding cutting loads and temperatures were determined by FEM simulations and the films’ impact resistance by impact tests. A correlation between the impact resistance and the cutting performance at corresponding temperatures contributed to the optimum adjustment of the cutting parameters to the film properties.     

Adaption of graded Cr/CrN-interlayer thickness to cemented carbide substrates' roughness for improving the adhesion of HPPMS PVD films and the cutting performance

High Power Pulsed Magnetron Sputtering (HPPMS) techniques jointly with the deposition of a graded Cr/CrN-nanointerlayer on cutting inserts can increase the film adhesion and consequently the tool life. These improvements depend on the roughness of the employed cemented carbide substrates. The investigations described in the present paper intend to explain the effect of Cr/CrN-interlayer thickness and substrate roughness on the coating adhesion and cutting performance. To attain various roughnesses, the applied cemented carbide inserts were superficially treated. These treatments were grinding at a medium roughness level, or grinding with subsequent polishing for enhancing the surface integrity and finally, in all cases, micro-blasting by fine Al₂O₃ grains. After Ar-ion etching, graded Cr/CrN adhesive layers with different thicknesses were deposited by HPPMS technology on the variously pretreated substrates. Subsequently, an approximately 3 μm thick (Ti,Al)N film was deposited by HPPMS PVD on all used inserts. Rockwell C indentations and inclined impact tests were performed to assess qualitatively and quantitatively the films' adhesion. The cutting performance of the coated tools was investigated in milling of 42CrMo4 QT. FEM supported calculations of the developed stresses during the material removal process contributed in explaining the obtained tool wear results. In these calculations, the adhesion, dependent on the substrate roughness characteristics and on the adhesive interlayer thickness, was taken into account. The results revealed that the effectiveness of HPPMS adhesive graded Cr/CrN-nanointerlayer strongly depends on the substrate surface integrity and on the interlayer thickness. Thus, the film adhesion and consequently the cutting performance can be significantly improved if the interlayer thickness is adapted to the substrate roughness.     

Effect of the crystallinity of diamond coatings on cemented carbide inserts on their cutting performance in milling

Micro-crystalline diamond (MCD) coatings were deposited on cemented carbide inserts at different temperatures using hot filament chemical vapor deposition technique. For investigating the effect of the developed diamond crystallinity on the fatigue strength and wear behaviour of the prepared MCD coated inserts, inclined impact tests and milling investigations were conducted correspondingly. Raman spectra were recorded for capturing the crystalline phases after the film deposition and their potential changes after the impact and milling experiments induced by the mechanical and thermal loads. Thus, the explanation of the cutting performance of the employed diamond coated inserts with various crystalline phases was enabled.     

Micro-blasting of PVD Films, an Effective Way to Increase the Cutting Performance of Coated Cemented Carbide Tools

This paper investigates the feasibility of increasing the wear resistance of cemented carbide tools through micro-blasting of their PVD-coatings. The enhanced and graded film strength properties before and after micro-blasting are determined by means of a FEM-based evaluation of nanoindentation results. The coating topomorphy, induced by micro-blasting, was monitored and correlated to the substrate roughness and film adhesion. The cutting performance of inserts, coated with micro-blasted films, was investigated in milling and explained with the aid of a cutting process FEM simulation. The obtained results reveal a tool life growth through micro-blasting of coatings, deposited on substrates with appropriate roughness characteristics.     

Using the plasma surfacing method for depositing corrosion-resisting layers on gas and petrochemical equipment operating in hydrogen-containing media

Summary

In the previous report, to establish an evaluation method for rolling contact fatigue strength of a coating film, the endurance limit of a coated roll under contact loading was investigated and crack initiation was found to dominate the determination of the endurance strength. The authors proposed that the mechanism of cracking was a fatigue process caused by a compressive tangential stress under contact loading.

So, in this study, static and fatigue properties of Cr‐plating films under bending, tension and compression were obtained. And these properties were compared with rolling contact fatigue strengths of Cr‐plating film on a fatigue endurance diagram. As a result, we found that the rolling contact fatigue strength agrees well with the. value estimated from the static and fatigue properties, and the rolling contact fatigue strength of Cr‐plating film could be estimated from the static and fatigue properties. Also, it was found that residual stress in the Cr‐plating had a large effect on the rolling contact fatigue strength.     

Blistering and spalling of thin hard coatings submitted to repeated impacts

The endurance life of thin DLC coatings, obtained by plasma assisted chemical vapour deposition (PACVD), has been evaluated under repeated impact conditions. The observations of the damaged surfaces have permitted to identify several failure mechanisms. Impact tests, interrupted after various test durations, enable us to clarify the damage origin and to follow the failure evolution up to the complete coating failure. In particular, a blistering phenomenon has clearly been revealed. Some hypotheses for this unusual behaviour are proposed in reference to the film mechanical properties and residual stresses level.     

Impact Resistance of PVD Films and Milling Performance of Coated Tools at Various Temperature Levels

Impact tests were conducted on coated cutting inserts, revealing that a temperature increase at first enhances the coating impact resistance, and then leads to its non-linear deterioration. On the other hand, milling investigations with coated cemented carbide inserts demonstrated that the cutting speed increase, up to a certain level, causes initially a significant improvement of the cutting performance. FEM calculations of the developed temperature and stress fields during cutting provided additional insights. The joint study and the subsequent correlation of the aforementioned results, renders the impact test as a convenient and efficient methodological tool for the characterization of coated tools' cutting performance.     

Influence of substrate microstructure on the contact fatigue strength of coated cold-work tool steels

The contact fatigue behavior of three microstructurally distinct tool steels coated with a physical vapor deposited TiN film is studied. Substrate microstructural differences come from variations in either chemical composition or processing route. Experimental procedure is based on determining critical applied loads and pressures, under both monotonic and cyclic spherical indentation loading conditions, for emergence and evolution of distinct damage modes at the coating surface: circumferential cracking, cohesive spallation and interfacial decohesion. Experimental results indicate that all coating/substrate systems evaluated are susceptible to mechanical degradation associated with repetitive contact loading. This is clearly discerned from the fact that some damage mechanisms, such as cohesive spallation at the coating and adhesion failure at the interface, are exclusively observed under cyclic loading. Substrate microstructure effects are evidenced by consideration of coating detachment as the critical damage mechanism. In this regard, crack nucleation resistance of primary carbides is pointed out as the main reason for the distinct response against decohesion observed under cyclic contact loads. Hence, finer and tougher, as well as less irregular and more homogeneously distributed primary carbides are pointed out as key microstructural features for enhancing contact fatigue strength of coated cold-work tool steels.     

Diamond like carbon film as a protective coating for high strength steel and titanium alloy

Hard chromium plating is currently used to protect the surfaces of wear and fatigue sensitive aircraft parts such as the landing gear, flap tracks etc. At Boeing, this process has been targeted for elimination due to environmental and workplace safety considerations. Our objective is to develop a non-hazardous, non-line of sight (NLOS) dry process replacement for chromium plating that does not adversely affect the engineering properties such as fatigue and wear. In this work, we report the results for Diamond Like Carbon (DLC) coating as a replacement for chrome plating. Excellent adhesion of DLC film to the steel surface is achieved using a variety of surface modification techniques such as in-situ inert ion sputter cleaning and ion implantation or deposition of an intermediate bonding layer between the DLC film and the substrate. Our tests show that DLC coated 4340 M steel exhibit better wear and fatigue characteristics when compared to those of chromium-plated 4340 M. DLC coating improved the wear resistance of Ti-6Al-4V drastically and did not cause any coating induced fatigue degradation in this alloy.     

Determination of reduced Young＇s modulus of thin films using indentation test

The flat cylindrical indentation tests with different sizes of punch radius were investigated using finite element method (FEM) aimed to reveal the effect of punch size on the indentation behavior of the film/substrate system. Based on the FEM results analysis, two methods was proposed to separate film's reduced Young's modulus from a film/substrate system. The first method was based on a new weight function that quantifies film's and substrate's contributions to the overall mechanical properties of the film/substrate system in the flat cylindrical indentation test. The second method, a numerical approach, including fitting and extrapolation procedures was put forward. Both of the results from the two methods showed a reasonable agreement with the one input FE model. At last, the effect of maximum indentation depth and the surface micro-roughness of the thin film on the reduced Young's modulus of the film/substrate system were discussed. The methods proposed in the present study provide some new conceptions on evaluating other properties of thin films, e.g. creep, for which a flat-ended punch is also employed.     

Corrosion and Nano-mechanical Behaviors of Magnetron Sputtered Al-Mo Gradient Coated Steel

A gradient three-layer Al-Mo coating was deposited on steel using magnetron sputtering method. The corrosion and nano-mechanical properties of the coating were examined by electrochemical impedance spectroscopy and nano-indentation tests and compared with the conventional electroplated cadmium and IVD aluminum coatings. Electrochemical impedance spectroscopy was performed by immersing the coated specimens in 3.5% NaCl solution, and the impedance behavior was recorded as a function of immersion time. The mechanical properties (hardness and elastic modulus) were obtained from each indentation as a function of the penetration depth across the coating cross section. The adhesion resistance of the coatings was evaluated by scratch tests on the coated surface using nano-indentation method. The results show that the gradient Al-Mo coating exhibits better corrosion resistance than the other coatings in view of the better microstructure. The impedance results were modeled using appropriate electrical equivalent circuits for all the coated systems. The uniform, smooth and dense Al-Mo coating obtained by magnetron sputtering exhibits good adhesion with the steel substrate as per scratch test method. The poor corrosion resistance of the later coatings was shown to be due to the defects/cracks as well as the lesser adhesion of the coatings with steel. The hardness and elastic modulus of the Al-Mo coating are found to be high when compared to the other coatings.