Christina Berger

Betriebsfestigkeit in Germany — an overview ☆

Abstract The German word “Betriebsfestigkeit” can be translated into English with several terms. The best expression seems to be structural durability or structural integrity. This article gives a brief description of the efforts taken especially by the German automotive industry and its cooperating research institutes to improve the accuracy of life time prediction and to optimise the proof-out by usage of experimental and numerical procedures. The advantages of standardised load–time histories (SLHs) for the user particularly in the pre design stage are shown and an overview of the existing SLHs is given. At the end of the article some aspects of practical applications of “Betriebsfestigkeit” in the German automotive industry are presented.

Development of chromium nitride coatings substituting titanium nitride

Abstract The deposition of chromium nitrides by reactive magnetron sputtering on machine parts seems to be very promising, because the mechanical properties are similar to those of TiN hard coatings. Especially the corrosion resistance of CrN is an interesting aspect related to the known problems of TiN films. The presented research deals with the deposition of CrN films by using r.f.-magnetron sputtering units. Starting with common deposition parameters for titanium nitride and a variation of the deposition parameters, partial pressure and bias voltage, chromium nitride coatings are deposited on steel substrates. The results show that the reactive sputtering process of CrN is more sensitive to minor changes of the adjustable deposition parameters and the related plasma conditions than that of TiN. The deposition of CrN allows the production of coatings with a wide field of properties but only within small windows of deposition parameters, especially when sputtering Cr2N films. The characterization of the coatings by their basic properties, such as thickness, hardness, adhesion, phase analysis and residual stress, indicates possibilities and limits of CrN coating systems.

Development of a plasma surface treatment for magnesium alloys to ensure sufficient wear and corrosion resistance

Extensive investigations of the authors showed the possibility of producing reliable plasma vapour deposition (PVD) hard coatings on magnesium alloys with good wear properties comparable to PVD coated steel or titanium based alloys. In presence of corrosive mediums crucial contact and pitting corrosion, induced by process related coating pinholes are the main failure mechanisms of coated magnesium alloys, even without any tribological load applied. The authors developed the new method of plasma anodisation to ensure acceptable corrosion resistance. In contrast to conventional anodisation processes the aqueous electrolyte is substituted by an oxygen plasma. The main advantages are that the process uses no toxic electrolytes, there is no waste and anodising and PVD-coating can be done in one process. The article gives a brief view of our extensive work starting from the first approach in PVD deposition of magnesium alloys up to the present work in establishing the plasma anodisation.

Plasma anodisation as an environmental harmless method for the corrosion protection of magnesium alloys

Sufficient corrosion resistance of technical magnesium alloys is generally ensured by anodising the components in aqueous electrolytes. In the majority of cases, these processes bear environmental risks due to the highly toxically electrolytes and are subjected to substantial legal regulations. Considering these facts, the plasma anodisation seems to be a prospective method to obtain comparable corrosion resistance without using toxic substances. The main goal of the method of the plasma anodisation is the substitution of the aqueous electrolyte by an oxygen plasma. The plasma anodised surfaces were well characterized and the corrosion resistance was proved in the salt spray test according to DIN 50021 SS. For the tests the magnesium die cast alloy AZ91hp was used. The tests included plasma anodised, plasma anodised and PVD coated and only PVD coated specimens.

Tribological properties of sputtered CrN coatings under dry sliding oscillation motion at elevated temperatures

In the future dry running machine elements may play a more important role, due to environmental legislation. The economical demand for cost and timesaving causes higher mechanical and thermal stresses for tools in high-speed production processes. Is it possible to improve the wear resistance by the use of thin hard coatings and will they find an area of application especially at elevated temperatures where lubricants fail? Therefore, oscillating sliding motion dry wear experiments at specific temperatures ranging from 30 to 700°C (900°C is possible) were performed on PVD-coated steels and light metal alloys. The characterisation of the coatings includes fundamental properties such as thickness, hardness, structure and residual stresses. The tribological tests were done with an SRV3 apparatus from Optimol, the coatings were tested against 100Cr6 balls using a ball-on-disk arrangement. The friction and wear behaviour was documented. The wear traces were characterised using a Hommel Tester T 8000. Topographical microanalysis was done with SEM and EDX.

Tribological Studies of CrN coated Magnesium AZ91 at Temperatures up to 250°C

The tribological behaviour of PVD Chromium-Nitride coated magnesium alloy AZ91hp was investigated in wear tests by variation of temperature and load. A combination of different investigation methods revealed information about the failure modes and wear mechanisms. The authors show that failure of the CrN-AZ91 systems is due to the unsatisfactory load-carrying capacity of the magnesium.

Measurement of the hardness of hard coatings using a force indentation function

Abstract Today the so-called dynamic micro hardness with analysis of the force indentation function is replacing in many cases the conventional Vickers hardness measurement for evaluating the hardness of thin films such as coatings for wear protection with thicknesses in the range of several micrometers. This test procedure requires equipment with a numerically controlled loading unit and a high resolution measurement of the indentation depth. This contribution shows, using various different examples, that the load indentation measurement can solve problems such as the influence of the operator, but the test conditions should be well selected to give comparable and meaningful results. Therefore, this paper analyzes the most important reasons for deviations in practice, and makes suggestions for a standardized test procedure and test documentation.

Untersuchung des Sputter‐Ätzens auf die Eigenschaften von PVD‐CrN Hartstoffschichten auf Magnesium AZ91hp

Eine gangige Methode, zu beschichtende Oberflachen von Verunreinigungen zu befreien und die Adhasionseigenschaften von PVD-Schichten zu verbessern, ist das sogenannte Sputter-Atzen vor dem eigentlichen PVD-Beschichtungsprozess. Beim Sputter-Atzen wird der Beschichtungsprozess umgekehrt und die Oberflache des Substrates selbst wird gesputtert. Im Rahmen dieser Veroffentlichung wurden vor der Abscheidung von PVD-CrN Schichten die Substrate der Magnesium Druckgusslegierung AZ91 unterschiedlichen Atzzeiten unterzogen und deren jeweiliger Einfluss auf die Grenzflache Schicht-Grundwerkstoff, auf die Schichtoberflache und auf die mechanischen Eigenschaften des beschichteten Systems hin untersucht. Die Grenzflachenuntersuchungen wurden mit tiefenaufgeloster XPS und mit SIMS durchgefuhrt. Die Oberflachen wurden mit hochauflosender Rasterelektronenmikroskopie und AFM untersucht. Die Charakterisierung der mechanischen Eigenschaften beinhaltet Schichtdicke, Schichtharte, Hartetiefenprofile, Schichthaftung, Schichteigenspannungen und die Schichtstruktur. Es wurden zum Teil sehr starke Eigenschaftsanderungen mit zunehmender Atzzeit festgestellt. Dies betrifft vor allem einige mechanische Eigenschaften sowie die Oberflachenrauheit der beschichteten Systeme. Beim Schichtsystem AZ91-Cr/CrN fuhren langere Atzzeiten zu einer Verbesserung der Schichtqualitaten. Investigating the Influence of the Sputter Etching Process on the Properties of PVD-CrN Coatings on Magnesium Die Cast Alloy AZ91hp A common method prior to the PVD deposition is the sputter etching process of the substrate itself to clean the surface from adhesion products and to improve the coating adhesion. This report deals with the sputter etching of magnesium die cast alloy AZ91hp to investigate the influences on the coating-substrate interface, the surface properties and the mechanical properties of PVD-CrN hard coatings. The coating-substrate interface of the Cr-AZ91 coating systems was investigated with XPS and SIMS. Surface studies were carried out by high resolution electron microscopy and AFM. The characterization of the mechanical properties of the CrN-AZ91 compound systems includes thickness, coating hardness and hardness depth profiles, coating adhesion, structure and residual stresses. Some properties show a strong dependency of the etching time, especially the mechanical properties and the coating roughness. Increasing etching times lead to an improvement of the coating quality.

Corrosion of metals for automotive applications in ethanol blended biofuels

Abstract With the objective of reduction carbon dioxide emissions, gasoline fuels are increasingly being blended with ethanol of biogenous origin. These blends cause changes in the corrosivity of the fuels. As a result, different forms of corrosion, e.g. general, wet and dry corrosion, of metallic materials must be considered to ensure the corrosion resistance of constructions in contact with fuels. The corrosion behaviour of a stainless steel, a brass alloy and wrought and cast aluminium alloys has been investigated in ethanol containing gasoline fuel blends of practical interest, studying the possible forms of electrochemical and chemical corrosion under long term exposure in ambient conditions and under high temperatures and pressures.

PVD‐CrN Beschichtungen auf Magnesium AZ91hp und Stahl 100Cr6 – Untersuchung des Substrateinflusses auf die Schichteigenschaften

An PVD-Chromnitrid beschichteten Proben der Magnesiumlegierung AZ91hp und dem Walzlagerstahl 100Cr6 wurden vergleichende Untersuchungen hinsichtlich Struktur, mechanischen Eigenschaften, Haftfestigkeit und Eigenspannungen durchgefuhrt. Fur die Beschichtungen wurden die Parameter Schichtdicke und Substrat-BIAS variiert. Beide Substratwerkstoffe wurden in einer Charge zusammen beschichtet. Bei der rontgenographischen Analyse der Eigenspannungen in den Schichten zeigten sich signifikante Unterschiede zwischen dem beschichteten Magnesium und dem beschichteten Stahl. Beim Stahl war eine Abhangigkeit der Eigenspannungen vom Substrat-BIAS erkennbar, beim Magnesium war dies nicht der Fall. Die Schichtstruktur wurde im Rasterelektronenmikroskop untersucht. An den beschichteten Substraten wurden mit SIMS Element-Tiefenprofile aufgenommen. PVD-CrN coated magnesium alloy AZ91hp and steel 100Cr6 – Investigation on the influence of the substrate material on coating properties PVD-chromium-nitride coated samples of substrates of the magnesium alloy AZ91hp and the roller and ball bearing steel 100Cr6 were investigated regarding structure, mechanical characteristics, adhesion and internal stresses. For the coatings the parameters layer thickness and substrate BIAS voltage were varied. Both substrate materials were coated in one lad. Results of the x-ray analysis of the internal stresses show significant differences between the coated magnesium and the coated steel substrates. In the case of the variation of the substrate BIAS voltage, for the coated steel a dependency of the internal stresses to coating parameters could be obtained. For the coated magnesium no dependency was recognizable. The coating structure was examined with scanning electron microscopy. Element depth profiles of the coated samples were performed with SIMS.