E. Lugscheider

(Cr:Al)N coatings deposited by the cathodic vacuum are evaporation

(Cr:Al)N coatings were deposited using two cathodic vacuum arc evaporators fitted with chromium and aluminium cathodes. Both monolayer and multilayer coatings were deposited with different aluminium content. Selected mechanical and tribological properties of the coating were investigated as well as the oxidation behaviour. It is shown that the oxidation rate decreases with increasing aluminium content. The hardness of the (Cr:Al)N coating is higher than that of CrN coatings.

Sliding wear behavior of thermally sprayed 75/25 Cr3C2/NiCr wear resistant coatings

Abstract One of the foremost coating methods for combating wear is thermal spraying, however, despite its widespread industrial use, little is known about the basic friction behavior and the mechanisms by which such coatings wear. The manner of processing in thermal spraying inevitably leads to inhomogeneities, such as unmelted particles, oxide inclusions and porosity, in the sprayed deposits resulting in a structure markedly different from that of cast, wrought or even powder metallurgy materials. It is expected that thermal spray coating behavior would be even more complicated than that of homogeneous, uncoated bulk materials. Results of an investigation to determine the effects of some wear test variables on high velocity oxy-fuel (HVOF) sprayed Cr 3 C 2 NiCr coatings using a pin-on-disk tribometer are presented. Room temperature sliding friction and wear behavior of coatings are discussed with respect to load, relative velocity and counterbody material. The present investigation shows that the tribological behavior of HVOF sprayed Cr 3 C 2 NiCr coatings is significantly affected by its microstructural constituents such as splats, porosity and form and dispersion of second phases. It is also shown that changes in imposed sliding wear test conditions varied the friction and wear behavior of thermally sprayed coatings considerably. The break-in sliding coefficient of friction is found to be more significantly affected by load than other test parameters. Results also indicate that friction decreased with increasing velocity but wear decreased then increased with increasing velocity. By proper control of test conditions and by selected changes of those conditions, the physical wear mechanisms involved in thermally sprayed coatings could be understood. This study showed that the pin-on-disk is a well controlled test and can be used to understand certain basic relationships between the sliding friction and wear behavior of thermally sprayed coatings.

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.

Comparison of the structure of PVD-thin films deposited with different deposition energies

Abstract Under the various available PVD processes, thin films can be deposited with higher or lower deposition energy, e.g., with or without ion bombardment. Related to this deposition energy the structure and consequently the properties of the deposited films can be directly influenced. The wide range of possible deposition parameters for the PVD-processes enables the use of materials in form of thin films in a large scope of applications, as a result of the different properties which can be achieved. Particularly high adhesion of thin films is always desired, which depends among other things on the microstructure of coatings. The aim of this paper is to compare three different PVD deposition processes: cathodic arc evaporation, magnetron sputtering and electron beam evaporation. These PVD processes are related to their film structure produced under three different conditions and studied interms of their deposition energies. Structure and morphology of the coatings are compared at identical temperatures resulting in a model, which is suggested to explain how excitation of deposited films takes place. Besides condensation effects, the momentum transfer of ions caused by biasing the substrates is obviously important and leads to film densification without increasing the substrate temperature.

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.

Effect of thermal aging on the erosion resistance of air plasma sprayed zirconia thermal barrier coating

To increase the performance, efficiency and reliability of thermal barrier coatings, it is important to understand the influence of the microstructure on the erosion resistance. Therefore, the erosion behavior of air-plasma-sprayed 7 wt% yttria stabilized zirconia (7YSZ) thermal barrier coatings (TBC) at elevated temperatures was investigated. The paper focuses on both the porosity of the coating as well as the thermal pre-treatment of the ceramic. To simulate operating conditions on the ceramic samples, high-temperature erosion experiments at 1093°C were carried out on as-sprayed samples as well as on samples, thermally aged at different temperatures up to 1482°C in air prior to erosion testing. A significant dependence of erosion rate on porosity and thermal pre-treatment was found. Finally, a technique was developed to predict the erosion rates of air-plasma-sprayed 7YSZ TBCs independent of aging conditions or porosity levels. Erosion rates seem to be highly correlated to the micro-hardness of the zirconia TBC. A power-law model was empirically derived to estimate erosion rates of plasma-sprayed 7YSZ ceramic thermal barrier coatings.

Investigations on hard coated reamers in different lubricant free cutting operations

Abstract The reaming finishing process sets special high demands on quality and wear resistance of used reamers. Only the best adapted cutting geometries and parametric cuts allow to obtain high surface qualities and minimum deviations from circular form. Cutting operations carried out free of cooling lubricant will cause an extreme increase in thermic and abrasive load of tools. Details given in this paper will show how, using PVD coatings, it nevertheless is possible to obtain relevant file-proven tool life results, and certainty of operation in lubricant-free reaming and lubricant-weak reaming of gray cast iron (GG25) and aluminium alloy (AlSi12). Various arc-coatings and coatings deposited by sputtering are analyzed as to their mechanical properties, and wear behaviour is compared in application. In dry reaming of gray cast iron, good tool life could be obtained, even for a high cutting speed of 100m min−1, with modified coating materials (TiAlN, TiAlCN, for instance). These are compared with other coating materials (for instance TiZrN). Cutting of AlSil2 was done successfully with TiAlN-and TiB2-coated reamers.

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.

Structure and properties of PVD-coatings by means of impact tester

Abstract Machine parts like rolling bearings or gears are stressed during operation in a changing mechanical strain. This causes wear by impacts and wear by rolling which is marked by the so-called surface ruin. The appearance of surface fatigue is based upon structural transformation, cracking and cracking-growth processes and ends with the separation of debris particles caused by the above-mentioned permanent changing strain [K.-H. Habig, Verschleis und Harte von Werkstoffen, Carl Hanser Verlag Munchen Wien, S. 203ff, 1980]. The final stage, which is equivalent to the component failure, is the so-called ‘pitting’ on the technical surface, which is characteristically named ‘surface fatigue’. The impact tester is used for detailed research on failure mechanisms of thin films. Statements about the adherence of hard material coatings under dynamic compressive stress can be made using this test method, due to the possibility of simulating some effects of rolling strain. Therefore, a hard metal ball strikes with a frequency of up to 50 Hz on to the surface. The altitude stress can be varied to obtain a detailed evaluation of fatigue strength under reversal strain. Selected hard material coatings were analyzed after testing with the described method applying an impact force of 300 N, 500 N and 700 N. In the framework of this presentation, MSIP (Magnetron Sputter Ion Plating) coatings on titanium- and chromium basis were used. The fatigue defects and the results of this study will be discussed relative to structure and morphology of thin films.

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.