Mathias Galetz

Abrasion resistance of oxidized zirconium in comparison with CoCrMo and titanium nitride coatings for artificial knee joints

Most total knee replacement joints consist of a metal femoral component made from a cobalt-chromium- molybdenum (CoCrMo)-alloy and a tibial component with an ultrahigh molecular weight polyethylene (UHMWPE) bearing surface. Wear of the UHMWPE remains the primary disadvantage of these implants. The allergic potential ascribed to CoCrMo-alloys is a further concern. Other metallic alloys with and without ceramic coatings are clinically used to avoid these problems. This study compared the mechanical surface properties of an oxidized zirconium alloy with those of cast and wrought CoCrMo and TiAlV6-4. Additionally, the influence of a titanium nitride (TiN)-plasma coating on the surface properties was investigated. The composition of the oxidized zirconium layer was analyzed. Micro- and macrohardness tests as well as adhesion tests were used to reveal material differences in terms of their abrasive wear potential in artificial joints.

Potential for adhesive wear in friction couples of UHMWPE running against oxidized zirconium, titanium nitride coatings, and cobalt‐chromium alloys

The classical wear mechanisms abrasion, fatigue, and adhesion are the most frequent causes of surface changes of ultra high molecular weight polyethylene (UHMWPE) in artificial joints. The counterpart material has a strong influence on the wear and friction behavior of artificial joints due to its abrasive properties and adhesive interaction with UHMWPE. The formation of a transfer layer on the counterpart in UHMWPE bearing systems is often described as being a clear indication of strong adhesive forces. The influence of using a cobalt-chromium-molybdenum (CoCrMo) alloy, a titanium nitride plasma coating or an oxidized zirconium alloy on adhesive wear was studied. The surface free energy and the bonding forces of these counterpart materials to UHMWPE were investigated. Catalytic effects on the degradation behavior of polyethylene, the micro friction behavior, and the build-up and constitution of a transfer layer deposited under loads, and relative velocities that are relevant in knee joints were analyzed.

Determination of the temperature rise within UHMWPE tibial components during tribological loading

The wear of ultrahigh molecular weight polyethylene (UHMWPE) is considered as one of the major reasons for revision of artificial joints. While in vivo measurements have shown a significant temperature increase in knee implants, the amount of heat dissipated within the UHMWPE tibial component and its influence on the friction behavior when paired with a cobalt-chromium (CoCrMo) femoral component is unknown. Our goal was to address these questions by measuring the temperature rise over a wide range of tribological loading conditions that mimic certain spots on artificial knee joints. The temperature rise as a function of lubricant, sliding velocity, coefficient of friction and maximum load was determined and analyzed. Additionally, the heat gradient during constant loading was investigated that allows the calculation of heat flow. The test setup consists of a wheel-on-flat laboratory testing device. Tests were performed in ambient air and different lubricants. During the tests, the temperature rise in the polyethylene was recorded with embedded thermocouples. The temperature rise was high and shown to be directly linked to load, coefficient of friction and relative velocity. Because it is generally assumed that the applied energy is an indicator for the development of wear in particles, some considerations for the design of knee joints are proposed based on our observations. The amount of heat dissipated in the polyethylene under cyclic loading was measured and is discussed in comparison with the theoretical model of temperature in friction pairs.

Improved oxidation resistance of ferritic – martensitic steels in water vapour containing environments via diffusion coatings

Abstract Ferritic – martensitic steels are of high interest as superheater materials or as materials for interconnectors in solid oxide fuel cells. In comparison to austenitic steels and nickel base alloys they offer much better heat transfer behaviour and a lower coefficient of thermal expansion, as well as lower costs. Modern 9% Cr-steels have sufficient creep strength however their corrosion resistance particularly in H2O containing oxidising environments (e.g. up to 25% H2O in the combustion of biomass or the oxyfuel process) needs further improvement. A large number of studies have shown that the corrosion resistance of 9% Cr-steels in water vapour containing combustion environments is inferior to that in dry atmospheres, due to the formation of a volatile Cr-species of the type CrO2(OH)2. This investigation starts from the idea of a shift of partial pressure of CrO2(OH)2 to lower values if the solid oxide phase on the alloy surface is a manganese –chromium – spinel scale, so that at 630°C the formation of volatile chromium species can be suppressed. Therefore, the metal subsurface region was enriched with manganese and chromium via a diffusion coating (pack cementation) to promote the formation of Cr –Mn– spinel during oxidation. In the course of the project it was observed that the diffusion treatment can lead to the formation of a chromium carbide surface layer. The surface treated alloys show extreme stability under oxidising water vapour containing environments, as illustrated by oxidation exposures in a simulated combustion environment (N2 + 1%O2 + 10%H2O) at 650°C for 800 h.

An activated energy approach for accelerated testing of the deformation of UHMWPE in artificial joints

The deformation behavior of ultrahigh molecular polyethylene (UHMWPE) is studied in the temperature range of 23-80 degrees C. Samples are examined in quasi-static compression, tensile and creep tests to determine the accelerated deformation of UHMWPE at elevated temperatures. The deformation mechanisms under compression load can be described by one strain rate and temperature dependent Eyring process. The activation energy and volume of that process do not change between 23 degrees C and 50 degrees C. This suggests that the deformation mechanism under compression remains stable within this temperature range. Tribological tests are conducted to transfer this activated energy approach to the deformation behavior under loading typical for artificial knee joints. While this approach does not cover the wear mechanisms close to the surface, testing at higher temperatures is shown to have a significant potential to reduce the testing time for lifetime predictions in terms of the macroscopic creep and deformation behavior of artificial joints.

Initial Aluminizing Steps of Pure Nickel from Al Micro-Particles

Novel, unconventional type of high temperature coating systems can be elaborated by depositing Al micro-particles on nickel base substrates, using an appropriate binder, and converting them into a thermal barrier type coating by a two-step heat treatment under argon. Final result is a coating structure consisting of a quasi-foam top coat, constituted by spherical hollow alumina particles, surmounting a β-NiAl diffusion layer able to form during high-temperature oxidation a protective alumina scale. In this work, pure nickel was employed as a model material to evaluate the effects of moderate temperatures (550-700°C), dwelling times and Al particle size on the final characteristics of the coatings. Almost no diffusion occurred below 600°C. In contrast, a Ni2Al3 layer very quickly formed at 650 or 700°C. The rapidity of coating formation was attributed to the appearance of a liquid phase at the coating/substrate interface. The increase of dwelling time did not provide any significant thickness increase as the Al particles got practically emptied after 2h. In addition, the use of different micro-sized particles resulted in similar Al diffusion coatings under the investigated conditions.

Resistance of Coatings for Boiler Components of Waste-to-Energy Plants to Salt Melts Containing Copper Compounds

The accelerating effect of heavy metal compounds on the corrosive attack of boiler components like superheaters poses a severe problem in modern waste-to-energy plants (WTPs). Coatings are a possible solution to protect cheap, low alloyed steel substrates from heavy metal chloride and sulfate salts, which have a relatively low melting point. These salts dissolve many alloys, and therefore often are the limiting factor as far as the lifetime of superheater tubes is concerned. In this work the corrosion performance under artificial salt deposits of different coatings, manufactured by overlay welding, thermal spraying of self-fluxing as well as conventional systems was investigated. The results of our studies clearly demonstrate the importance of alloying elements such as molybdenum or silicon. Additionally, the coatings have to be dense and of a certain thickness in order to resist the corrosive attack under these severe conditions.

Fireside Corrosion of Chromium- and Aluminum-Coated Ferritic–Martensitic Steels

In modern fossil power plants, biomass is used more and more as secondary fuel in addition to coal. This leads to a significant decrease of the carbon footprint of such power plants. However, the demands on the corrosion resistance of the materials in the boilers increase because of chlorine in the atmosphere and salt-containing sulfides and chlorides. Heat-resistant ferritic–martensitic steels such as P91 are of great interest as superheater material. However, their corrosion resistance has to be improved for an application in modern fossil power plants with biomass combustion. For this purpose, chromium and aluminum diffusion coatings were developed and applied on P91 steel. The uncoated and coated material was investigated in a simulated biomass–brown coal ash with CaSO4, Na2SO4, K2SO4, KCl, and Al2O3 deposits and an atmosphere containing nitrogen with H2O, CO2, O2, SO2, and HCl. The improvement of the corrosion resistance is illustrated using metallographic methods such as electron probe micro-analysis.

Protective Aluminide Coatings for Refractory Metals

Refractory metals are promising materials for high-temperature applications. However, these materials exhibit low oxidation resistance at elevated temperatures. To overcome this problem, aluminum diffusion layers were applied to molybdenum, niobium, tantalum, and tungsten using a pack cementation process. The coated samples were characterized using EPMA, optical microscope, and XRD. Homogeneous diffusion layers of different intermetallic phases were identified. The observed phases were in agreement with phase predictions made using thermodynamic calculations. Oxidation tests at 1300