Miha Čekada

Comparison of structural and chemical properties of Cr-based hard coatings

Abstract A series of chromium nitride, carbide and carbonitride coatings were prepared by ion plating using nitrogen and acetylene as reactive gases. The depositions were made at different partial pressures of the reactive gases while maintaining other parameters constant. The crystal structure and microstructure were studied by TEM. The CrN and Cr 2 N phases were detected in the chromium nitride coatings. In the carbide coatings, orthorhombic Cr 7 C 3 and the metastable cubic CrC phase were confirmed. For oxidation studies, the samples were annealed at 750 °C. Afterwards, the depth profile was measured by glow discharge optical emission spectrometry (GDOES). A complex diffusion was observed involving outdiffusion of nitrogen and segregation of carbon at the substrate-coating interface.

A new experimental method for studying the cracking behaviour of PVD multilayer coatings

Abstract One way to improve the cracking resistance of PVD hard coatings is to prepare them in the form of a multilayer structure, where micro-cracks tend to branch and deflect at the interface between two alternating layers of different materials. However, it is still not clear how a layer structure influences the cracking resistance. One of the main reasons for the shortage of data in this field is the lack of appropriate experimental methods to prepare the samples and to observe the micro-cracks that appear through the multilayer structure. Due to the extremely small layer thicknesses, a SEM analysis on a perpendicular sample cross-section is too coarse, while a cross-sectional TEM analysis is time consuming and, therefore, expensive. In this study a new experimental method for observating the propagation of micro-cracks in different multilayer structures is presented. It is based on microscopic observations of the micro-cracks on the cross-section of a ball crater over a scratch track. The first step of this test is to prepare a scratch on the sample using a diamond stylus. Next, a ball crater must be ground over the scratch track at the position where the first cracks are observed in the scratch track. Along the edges of the scratch track the low-angle cross-section of the multilayer structure can be observed. On such a cross-section we could clearly see how the micro-cracks propagate from the coating surface through all the layers that compose the multilayer structure. For the inspection of the micro-cracks we used optical as well as SEM microscopy. The applicability of this new technique was tested on a 2.6 μm-thick TiN/CrN multilayer structure with 6, 8, 16 and 32 layers and on a 2.4 μm-thick CrN/TiAlN multilayer structure with 8 layers. The coatings were prepared by reactive sputtering in a Sputron (Balzers) plasma–beam sputtering apparatus. The coating adhesion was characterized with a conventional scratch tester (CSEM Revetest). For the ball–crater preparation we used a steel ball with a diameter of 20 mm and a diamond paste with a grain size 0.25 μm. The ball craters were prepared on the scratch track at the positions that corresponded to the different scratching loads. We discovered that the individual layer structure has a strong influence on the crack propagation as well as on the plastic deformation of multilayer coatings.

PVD CrN coating for protection of extrusion dies

In this paper, we present a comparison of tribological behaviour between conventional gas nitrided and PVD CrN coated extrusion dies used in industrial production. We found that PVD CrN coated dies proved to be superior to nitrided ones.

Properties of Cr(C,N) hard coatings deposited in ArC2H2N2 plasma

Abstract Several chromium carbonitride (Cr(C,N)) coatings were prepared with different C:N ratios by varying the N 2 and C 2 H 2 flow. Chromium nitride (CrN) and chromium carbide (CrC) coatings were also prepared for comparison. The coatings were deposited in two different ion-plating systems: by reactive evaporation in BAI730M (Balzers) apparatus at high temperature (450 °C) and by reactive sputtering in plasma-beam Sputron (Balzers) apparatus at low temperature (200 °C). Among mechanical properties microhardness, adhesion (measured by scratch test) and surface roughness were evaluated. Oxidation of the coatings was carried out by heating the samples at temperatures of 750–900 °C in an oxygen atmosphere. Crystal structure and microstructure were studied by XRD, TEM and SEM. Chemical State of the elements was observed by XPS. The concentration and depth profiles of the samples oxidized at various temperatures were measured by AES, EDX and GDOES.

Interface characterization of combination hard/solid lubricant coatings by specific methods

Abstract Double or combination coatings and multilayered coatings were recently developed as a new generation of protective PVD coating for cutting tools. Some of these have already been applied in selected cases in modern machining techniques such as dry high-speed milling, turning, tapping and drilling, or when using a minimum quantity of lubricant (MQL). In this work we studied details of the combination of two coatings, one a solid lubricant (WC/C) coating sputtered onto a hard, wear- and temperature-resistant (TiAlN) coating, deposited onto high-speed steel and cemented carbide substrates. Using specific analytical methods, we focused our investigation on interface (interlayer) characterization between individual layers and the substrate. A detailed knowledge of the interlayer properties is the basis for understanding and interpretation of the macroscopic properties of WC/C–TiAlN combination coatings in a given tribological system. Depth profiles of Vickers microhardness, as determined on low-angle cross-sections of test samples, showed that the TiAlN layer had approximately 3000 HV 0.05, the W-rich interlayer approximately 2500 HV 0.05 and the WC/C solid lubricant approximately 1000 HV 0.05. High adhesion strength was proved by the REVETEST scratch test and by additional SEM+EDS analyses and depth profiles of the scratch tracks. Total delamination L c was observed at critical loads from 80 to 115 N. A low scratching coefficient μ* (0.05–0.20) was typical of thin (3 μm) and also of thick (10.8 μm) coatings. The width of the WC/C–TiAlN interlayer (0.15–0.20 μm) was measured from the SEM image of the kalotest ball crater and from fracture cross-sections. This narrow transient area was also detected and analysed by AES depth profile analysis and by EDS linescan and SEM microanalysis at low-angle cross-sections of the coated samples. All the results were correlated successfully with the declared basic macroscopic properties of the WC/C–TiAlN combination layer.

Characterization of (Cr,Ta)N hard coatings reactively sputtered at low temperature

Abstract Two (Cr,Ta)N coatings with different composition were deposited in a Sputron plasma beam sputtering apparatus at a temperature below 200°C on polished tool steel disks, polished alumina substrates and silicon wafers. Microhardness, adhesion and internal stress were measured by Vickers microindentation, the scratch test and sample deflection, respectively. The mechanical and electrical properties were compared to the values of CrN and TaN binary nitride coatings prepared in the same device under similar conditions. Like CrN and TaN, (Cr,Ta)N crystallizes in a cubic (NaCl) structure. Oxidation resistance was tested in the temperature range 550–850°C. We observed that a chromium-rich protective oxide layer developed on the Cr 0.58 Ta 0.42 N coating, while on the Cr 0.17 Ta 0.83 N coating the oxidation was more rapid, with cracks appearing at temperatures as low as 600°C.

Carbon-containing Ti–C:H and Cr–C:H PVD hard coatings

Abstract Ti–C:H and Cr–C:H coatings were prepared by reactive sputtering in a Balzers Sputron deposition system at a substrate temperature of approximately 150°C. Acetylene was used as reactive gas. Coatings were deposited onto polished tool steel and silicon substrates. Transmission electron microscopy, Auger electron spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scratch test and microindentation were used to characterize the properties of the coatings.

Comparison of mechanical properties of various PVD hard coatings for forming tools

Abstract In the present paper the basic mechanical properties of PVD hard coatings, which are currently used in Končar Alati company for wear protection of forming tools manufactured on request of the Electrolux Group, with emphasis on the Electrolux Home Products company, Denmark A/S, have been compared. This company has high demands on tool performance for machining and forming of several different workpiece materials, such as sheet steel, stainless steel, aluminium and Teflon coated sheet. Two different deposition methods were used for preparation of hard coatings: sputtering using unballanced magnetron sources – CC800 (CemeCon) and cathode arc evaporation [BAI 1200 (Balzers)]. The coatings for test were deposited on four different tool steels. It was found that the hard coatings investigated in the present work showed a wide range of mechanical properties: microhardness in the range of 900–3200 HV, critical load for total delamination in the range of 75–135 N, indentation modulus in the range of 120–390 GPa and the scratching coefficient in the range of 0·02–0·09.

Improvement of tool life of die-casting tools with duplex treatment

A comparative tool life test of protected tools was made in aluminium die-casting of driving wheels for automobiles with two types of cores in one casting plate. One set of cores was improved with plasma nitriding process, while another one was improved with duplex treatment (plasma nitriding + PVD CrN coating). After 2700 and 4150 shots, surfaces on both types of tools were analysed using optical and scanning electron microscopes to identify damages of tool working surfaces. The performance tests, made under real manufacturing conditions, showed that wear of duplex treated tool is smaller compare to only plasma nitrided tool.

Energy-resolved mass spectroscopy of Ar–C2H2–N2 plasma, performed during ion plating of TiC and TiN hard coatings

Abstract Energy-resolved mass spectroscopy studies during discharge in Ar+N 2 and Ar+C 2 H 2 gas mixtures in a triode ion-plating apparatus (Balzers BAI 730) revealed a high degree of acetylene (C 2 H 2 ) decomposition under typical conditions suitable for titanium evaporation. In the case of the Ar+C 2 H 2 gas mixture, the most abundant neutral species besides argon is hydrogen, and not acetylene as would be expected. On the other hand, the 12 C + ion is the most abundant carbon-containing species. The ratio between C + , CH 3 + and C 2 H 2 + ions and titanium ions (Ti + , T 2+ ) decreases with increasing arc current. By careful adjustment of the discharge parameters, either a plasma rich in C x H y radicals with a low metal evaporation rate or plasma with a high evaporation rate but with almost completely decomposed acetylene, i.e. with C + and H n =1–3 + only, can be obtained.