Holger Hoche

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.

Structural evolution of SnO2TiO2 nanocrystalline films for gas sensors

Thin films (50‐200 nm) of SnO2TiO2 were deposited on SiO2:(001)Si substrates by RF-sputtering and by molecular beam before they were annealed in vacuum at 200‐900°C. In-situ TEM, XRD, SEM, Raman and IR-spectroscopy were used to analyze the structure transformations in the SnO2TiO2 films. In the as-deposited state, the films are amorphous. They crystallize at higher temperatures (starting at about 500°C) forming nanosized grains. The problem of the spinodal decomposition in the SnO2TiO2 system observed earlier at high temperatures is discussed also for low-temperature processing. The stoichiometry of the films of both groups (reactive ion sputtered and high-vacuum e-gun sputtered) is being compared.

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.

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.

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.

Optimisation of process parameters for lattice structures

Purpose – This paper aims to develop a set of process parameters tailored for lattice structures and test them against standard process (SP) parameters. Selective laser melting (SLM) is a commonly known and established additive manufacturing technique and is a key technology in generating intricately shaped lattice structures. However, SP parameters used in this technology have building time and accuracy disadvantages for structures with a low area-to-perimeter ratio, such as thin struts. Design/methodology/approach – In this research work, body-centred cubic structure specimens are manufactured using adapted process parameters. Central to the adapted process parameters is the positioning of the laser beam, the scan strategy and the linear energy density. The specimens are analysed with X-ray micro-computed tomography for dimensional accuracy. The final assessment is a comparison between specimens manufactured using adapted process parameters and those using SP parameters. Findings – Standard parameters f...

Liquid Metal Assisted Crack Formation in Zinc Melts, Representation of the Damage Pattern Using 3D Computed Tomography

Abstract In the past, liquid metal induced galvanizing cracks were either analyzed on the metallographic section in the respective partially ground plane of view, or the exposed main fracture surface was examined in the scanning electron microscope (SEM). Thus, possibly existing secondary cracks remained undetected. Three-dimensional computed tomography (CT) allows for a representation of all cracks including their branchings and ramifications and makes possible an analysis of an overall picture of the network of cracks. Differences in the effect of different zinc melts can be further identified by means of 3D analysis and the load situations that promote the crack initiation during hot-dip galvanizing can be differentiated more clearly using laboratory samples. Key results are the representation of cracks parallel to the direction of the main load (B-cracks) which are oriented with the pearlite bands and apparently penetrate significantly deeper into the base material than the macroscopically visible crack which is orientated perpendicular to the direction of the main load (A-crack). A relationship between the occurrence of B-cracks and the used zinc alloys could only conditionally be shown. Examinations on flat samples, which had been dipped into various metal melts under load-free condition, yielded more or less comparable results, in particular for molten pure tin and molten pure lead. In this case, the pearlite phase is preferentially damaged even without introducing an external load into the samples.

Fractures in High-Strength Bolts due to Hydrogen Induced Stress Corrosion – Causes and Corrective Actions

Abstract Delayed brittle fractures of high-strength bolts of the strength class 10.9 are presented, taking the example of three damage cases. The respective damage mechanisms could be attributed to hydrogen induced stress corrosion which was caused, in turn, by hydrogen absorption during operation. The examples were chosen with a particular focus on the material condition‘s susceptibility which explains the cause for the occurrence of the damage mechanism. However, in only one of the three cases the susceptibility was evident and could be explained by violations of normative specifications and an unfavorable material choice. Whereas in the two other examples, only slight or no deviations from the standards and/or regulations could be found. The influencing parameters that caused the damage, those that further promoted the damage, as well as possible corrective actions are discussed taking into account the three exemplary damage cases.