Kirsten Bobzin

Tribological properties, phase generation and high temperature phase stability of tungsten- and vanadium-oxides deposited by reactive MSIP-PVD process for innovative lubrication applications

Abstract The tungsten and vanadium oxides are promising to be usable as solid lubricants at elevated temperatures because of their ability to form oxygen deficient Magneli-phases. As a matter of fact, metal-oxides are interesting for tribological insets at atmospheric conditions because of their expected oxidation stability and low adhesion. The study reports about the deposition of tungsten and vanadium oxides in a reactive d.c. mode by the MSIP- (Magnetron Sputtering Ion Plating) PVD process and especially about the influence of the oxygen content in the sputtering atmosphere as well as the deposition temperature on the phase generation. A simplified ‘sputtering phase diagram’ of the binary systems V–O and W–O as a function of the deposition temperature (378–650 K) and the oxygen content (0–50%) was determined. Furthermore, it was shown that the tested vanadium-oxides are phase stable up to 878 K and the tungsten-oxides up to 1100 K (measured in a high-temperature XRD facility). Additionally tribological properties of the deposited oxide coatings, like the friction coefficient vs. steel, will be presented. For polished and WOx coated samples a friction coefficient of μ≈0.2 against steel was measured at room temperature. The coatings were analyzed by various testing methods to characterize the tribological, mechanical and structural properties, like SEM, nanoindentation, (high-temperature)-XRD and pin-on-disk.

Wettability of PVD compound materials by lubricants

The wetting properties of coolants are important for tribological systems. In order to optimize the wettability on steel alloys coolants have been modified for decades. Using PVD-coatings for wear and corrosion resistance the thermodynamical interactions between coolant and surface have been changed. The characteristic factor, which affects interfacial interactions such as adsorption, wetting and adhesion, is the surface free energy. Test results indicate that all PVD hard film systems have much lower surface energies than uncoated low-alloyed steels. The surface free energy of solids and liquids can be examined by using a drop shape analysis where, due to spreading, the contact angle of coolants on metals is not measurable. In this work a theoretical solution is presented. The wetting behaviour can be calculated and is given as the spreading coefficient S. Results of these theoretical equations have been verified by experimental tests.

The effect of PVD layer constitution on surface free energy

Abstract The surface free energy of solids is a characteristic factor which affects the surface properties and interfacial interactions such as adsorption, wetting, adhesion etc. Therefore the surface free energy is of interest in the field or adhesive technologies, biomedical applications, cleaning procedures or for the wettability of tribological systems. One method of determining the polar and dispersive terms of the free surface energies of solids is based on measurements of the contact angles of pure liquids on solid surfaces. Within this paper the use of dynamic contact angle analysis for the evaluation of surface tension is described. PVD-coatings in the systems of Ti-Hf-N, Zr-C and Cr-Al-N were examined by using the dynamic contact angle analysis. The contact angles were measured by the sessile drop technique. The influences of chemical composition as well as macro- and microstructure on contact angles and surface tension were investigated. The test series presented here have shown that the surface free energy and above all its polar and disperse shares can be modified through the mechanical processing of hard metal samples, leading to different adhesions of the layer. The results show that a high polar share with a simultaneously high overall surface energy of the substrate lead to a better layer adhesion. Neither roughness nor phase structure, hardness or modulus of elasticity of the PVD layers investigated lead us to assume a correlation to the surface energy. It could be shown that adsorption and reaction layers lead to lower surface energies. The concomitant poorer wetting behaviour of the surfaces can significantly alter their useful and application features.

Mechanical properties of EB-PVD-thermal barrier coatings by nanoindentation

Abstract In the present work EB-PVD zirconia thermal barrier coatings were examined by nanoindentation. Because of the microstructure of EB-PVD-coatings, the mechanical properties in the vertical and the horizontal direction are completely different. Therefore the indents were performed on cross-sections and on the coatings’ surface itself. Because of the expected different behavior at the grain boundaries and the grains, the measurement locations were randomly chosen. The measured property was the Young’s modulus. For a better interpretation, the measurement results were described by the method of Weibull distribution. The measured values showed a significant dependency with the substrate temperature and a low dependency with oxygen partial pressure during the deposition process. Non-stoichiometric phases within the zirconia showed a complete different mechanical behavior.

Investigation of the residual stresses and mechanical properties of (Cr,Al)N arc PVD coatings used for semi-solid metal (SSM) forming dies

In many cases, high compressive stresses are an unwanted side effect of deposited PVD coatings, because they are known to reduce the adhesive strength of the coating on the substrate. However, in some applications a main focus of the PVD coatings consists of bringing the surface of a substrate into a compressive state. A surface being in a compressive state is more likely to withstand thermal and mechanical alternating stresses within the surface and has a higher resistance against forming cracks and increases the life span of semi-solid metal forming (SSM) dies. Arc ion plating is a PVD process, which is known to cause high compressive stresses in coatings due to its high ionisation rate and the applied bias voltage to the substrate. Therefore, the arc ion plating process is suitable for bringing a surface of a substrate into a compressive state. The investigated (Cr,Al)N coatings were deposited in such an arc ion plating PVD process and the thickness varies from 2.7 to 17 μm. The correlation of thickness vs. residual stresses of these coatings was investigated. In order to determine these residual stresses a stripe bending test is backed up and compared with a XRD stress analysis. Additionally, the coatings were exposed to impact tests to determine the influence of compressive stresses on the wear behaviour caused by alternating stresses.

Development of a superlattice (Ti,Hf,Cr)N coating for cold metal forming applications

Abstract Tribosystems in mechanical engineering put high demands towards tools, surface technology and lubrication. Cold forming processes in particular represent a significant challenge for wear reduction. Mineral-based oils with high contents of ecologically harmful additives are state of the art due to high abrasive and adhesive wear. In order to reduce the ecological impact and to increase tool life, biodegradable lubricants and hard coatings have been developed, which are deposited by modern vacuum technologies. In the collaborative research center ‘Environmentally friendly tribological systems’ (SFB 442), which has been established by the German Science Foundation (DFG), a new arc-PVD Ti–Hf–Cr–N coating has been developed. This nanolayer system with single layers of different hardness is supposed to fulfill the requirements of high hardness and abrasive wear resistance while possessing low Youngs modulus values at the same time. In this paper, the results of coating characterization based on various standardized tests and on microscopic analysis are shown. The influence of the structure and the coating process on the performance is evaluated using nanoindentation and pin-on-disc tests.

Determination of mechanical properties of electron beam-physical vapor deposition-thermal barrier coatings (EB-PVD-TBCs) by means of nanoindentation and impact testing

Abstract EB-PVD-zirconia coatings are well known as thermal barrier coating materials for gas turbine applications. Using these materials, the gas turbine can work at higher temperatures, and thus the turbine efficiency increases. Due to the fact that not only the turbine efficiency, but also its reliability is a very important issue, prediction of the lifetime of thermal barrier coatings by assessment through simulation and modeling becomes very important. Developing a model of an anisotropic EB-PVD coating requires knowledge of the mechanical behavior of the coating material. In the present paper, the mechanical properties of the coatings investigated were determined by means of advanced experimental analytical procedures. Stress–strain curves for the coatings examined were determined by a continuous, finite element method (FEM)-supported simulation of indenter penetration into the coating surface. Moreover, the superficial EB-PVD ZrO 2 coating was examined by means of an impact tester to determine its time-dependant plasticity (apparent creep) under dynamic load. The critical stresses for apparent creep of the coating are defined by means of an FEM-supported algorithm presented in previous publications.

Investigations of mechanical and tribological properties of CrAlN + C thin coatings deposited on cutting tools

Abstract Physical vapor deposition (PVD) of hard coatings such as titanium nitride have been used in industry reality since the beginning of the 1980s. Two processes, Arc-Ion plating (AIP) and Magnetron-sputtering-ion plating (MSIP), were responsible for the early commercial success of hard coatings on high speed steel tooling. There are many similarities and differences between these two PVD hard coating processes, but not all of the commonly used PVD hard coatings can be well deposited with the two systems. In this paper, CrAlN+C thin coatings were deposited on cutting tools materials using a new Physical Vapour Deposition (PVD) hybrid process. The hybrid process is a combination of random Arc-Ion-Plating (AIP) and balanced Magnetron-Sputter-Ion-Plating (MSIP) process, where both processes work simultaneously. The cutting tools should possess the following mechanical and tribological properties: hardness, adhesion, low coefficient of friction and good bond strength between coating and substrate. Coatings that were deposited with the PVD hybrid process showed the required properties for cutting tools. From the machining test results (made from the IPT Institute in Dresden) we can conclude that the CrAlN+C/C coatings can be applied in machining industry. The present work was an advanced AIF-project.

Industrial tribology : tribosystems, friction, wear and surface engineering, lubrication

Definition of Tribology Economic Importance of Tribology FRICTION AND TRIBOSYSTEM ANALYSIS AND TESTING Fundamentals of Friction Machine Elements: Tribology of Sliding Bearings, Roller Bearings, Gear Teeth, Seals, Shock Absorbers, Machine Guide Ways, Wet Clutches Machines: Tribology of Internal Combustion Gasoline and Diesel Engines, Industrial and Automotive Gears, Other Transmissions, Compressors, Turbines, Hydraulic Systems with Different Pump Types, Machine Tools for Cutting and Forming Processes WEAR AND WEAR PROTECTION, MATERIAL SELECTION AND SURFACE ENGINEERING Fundamentals of Wear Materials Selection Wear Protection and Surface Engineering Developing, Selection, Testing and Application of Special Coatings e.g. PVD or CVD Coatings for Gears, Bearings, Hydraulic Cylinders Cutting and Forming Tools LUBRICANTS AND LUBRICATION Base Oils Rheology Additives Lubricant Families as Automotive Engine Oils and Gear Oils, Industrial Gear Oils, Compressor Oils Including Refrigerating Oils, Hydraulic Fluids, Turbine Oils, Lubricating Greases, Metalworking Fluids and Compounds (Cutting and Forming) Lubricants in the Environment Lubricant Testing Lubricant Specifications

Oxidation characteristics and surface energy of chromium-based hardcoatings for use in semisolid forming tools

Abstract Dies for SSM-forming suffer from multiple combinations of wear mechanisms. The modes of wear are similar to those occurring in tools for pressure die casting. One of the most important problems for semisolid processes with aluminum is the sticking of the produced components in the die. Sliding contact of solid aluminum particles with the metallic die surface is one origin for sticking. Chromium-based PVD coatings are found to give satisfying solutions for pressure die casting tools. Their coefficients of thermal expansion are matched to those of tool steels as well as they form stable oxide scales protecting against surface interaction with the molten metal. The paper presents the effect of oxidation as a concrete means for reducing the sticking of aluminum on tools surfaces. Therefore, hardcoatings of CrC, CrN and CrAlN were characterized for their oxidation behavior. The surface energies of differently oxidized specimens were measured and set into relation to the oxidation temperature. Clear correlations between the oxidation state and the surface energies were found. The course of oxidation was investigated by in situ XRD measurements. Finally, the technological performance of the coatings was tested by pin-on-disk investigations. The oxidation was found to have a strong influence on the technical performance.