Enhanced corrosioon resistance by sol‐gel‐based ZrO2‐CeO2 coatings on magnesium alloys

The physical, chemical and mechanical properties of magnesium alloys make them attractive materials for automotive and aerospace applications. However, these materials are susceptible to corrosion and wear. This work discusses the potential of using sol‐gel based coatings consisting of ZrO₂ and 15 wt.% of CeO₂. The CeO₂ component provides enhanced corrosion protection, while ZrO₂ impart corrosion as well as wear resistance. Coating deposition was performed by the dip coating technique on two magnesium alloy substrates with different surface finishes: AZ91D (as‐casted, sand‐blasted, and machined) and AZ31 (rolled and machined). All as‐deposited coatings (xerogel coatings) were then subjected to 10 h annealilng: a temperature of 180°C was applied to the AZ91D alloy and 140°C to the AZ31 alloy. Morphological and structural properties of the annealed coatings were investigated by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Coating composition was examined using energy dispersive X‐ray analysis. Adhesion of the annealed ZrO₂‐CeO₂ coatings on the substrates, assessed by scratch tests, showed critical loads indicative of coating perforation of up to 32 N. Hardness and elasticity, measured using depth‐sensing nanoindentation tests, gave a hardness and elastic modulus of 4.5 GPa and 98 GPa, respectively. Salt spray corrosion tests performed on these coatings showed superior corrosion resistance for AZ91D (as‐casted and machined) and AZ31 (machined), while severe corrosion was observed for the AZ31 (rolled) and AZ91D (sand‐blasted) magnesium alloy substrates.     

High‐precision positioning and measurement systems for microtribotesting

A key problem in microtribology is measurement equipment in the micronewton force regime. This work reports on a new generation of test devices for materials research that closes the gap existing between the so‐called ‘macrotribometers’ and ‘nanotribometers’. The systems incorporate novel mechatronic systems for force measurements based on a combination of photostructured glass and high‐resolution fiber‐optic sensors. In addition, precision drives operating at high speed enable rapid sample positioning and sample motion in different modes. The flexibility of the concept allows tribological examinations of a wide variety of materials and material pairings to be performed with various geometrical configurations. An important feature of the system is modularity, which gives the researcher tremendous flexibility in designing experiments. Furthermore, such systems can easily be added in multi‐analysis environments. Modularity is realized at the hardware level by the possibility of rapidly exchanging drive modules and/or force transducer(s) depending on the user‐defined measurement requirements. Also, the test equipment presented can operate in a wide normal and tangential force regime (exchange of measurement head, force transducer) as well as in different motion configurations. It thus bridges the gap between ‘macro’ and ‘nano’ measurement systems. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of nanoscale topography and chemical composition of surfaces on their microfrictional behaviour

The influence of nanoscale topography and chemical composition on microfriction has been studied at different humidities. Structured surfaces exhibit lower friction than smooth ones. Among the structured surfaces, the crater-like morphologies show lower friction than pyramid-like morphologies. No significant differences in friction were observed when varying the roughness of the crater-like structures. On pyramid-like morphologies, friction increases with decreasing roughness. Additional hydrophobization of surface nanostructures results in only small reductions in friction.     

Microtribology of silicon,oxide, and carbide surfaces

This paper presents an overview of the microtribological properties of silicon oxide, sapphire, and titanium carbide surfaces as well as a self‐assembled monolayer with respect to their application in microsystems. Testing was performed with a reciprocating microtribometer with normal loads in the micronewton to millinewton range. Silicon and titanium carbide balls were used as counterbodies. For silicon oxide, sapphire and a perfluorodecyltrichlorosilane self‐assembled monolayer (FDTS), the microfriction corresponds to the water contact angle when the smoother titanium carbide ball or the relatively rougher silicon ball was used as a counterbody. Microfriction measurements performed on tribopairs of the same material, but having different roughnesses, showed that the friction of the rougher tribopairs is lower than that of the smoother ones. Interestingly, in the microforce regime, reduction in friction was significant and almost as much as when hydrophobic self‐assembled monolayers are applied on smooth surfaces. This investigation showed that comparative microtribological investigations between different material systems can be very challenging due to the fact that comparable roughness values on samples and countersamples are difficult to realize. Copyright © 2006 John Wiley & Sons, Ltd.     

Topography-related effects on the lubrication of nanostructured hard surfaces

This study reports the effect of nanoscaled surface structure of some hard coatings on the (micro-) frictional behaviour of systems under minimum lubrication conditions with modest contact pressures and low sliding speeds (below 1 mm/s). For this purpose, Cr-N coatings with a randomly crater-like topography and with varying dimensions of surface features as well as a smooth Cr-N surface were tested with a microtribometer. The friction on the samples was measured as a function of the viscosity of the applied mineral base oil and the sliding velocity. For all tests, the structured surfaces exhibited lower friction than the smooth surface. Furthermore, it was possible to detect variations in the lubrication-promoting effect of the structures depending on the oil viscosity and the sliding speed. Indications for the existence of an optimum topographic scale for this type of surface structure were found.     

In-process structuring of CrN coatings, and its influence on friction in dry and lubricated sliding

The growing trend to improve component lifetimes coupled with the need to conserve resources is driving new technologies in fields such as tool and forming industries. The use of in-process structuring applied to hard coatings on surfaces is one way of creating lubricating surfaces on the microscale, with superior tribological properties and improved lifetimes.In this study, CrN coatings deposited by plasma-activated physical vapor deposition (PAPVD) on hard metal substrates were structured by variation of the deposition parameters. The parameter combinations favorable for surface structuring were identified.Of the various deposition parameters that were varied in this study, the bias voltage was determined to have a dominating influence on the surface structure of the coatings. A wide variety of structures were fabricated, ranging from flat to highly creviced, with grain sizes ranging from 5 to 500 nm, as determined using scanning electron microscopy (SEM). Profilometer measurements show that the surface roughness, R_a, could be varied from 0.04 to 0.12 μm. The highly creviced surfaces however exhibit a somewhat reduced hardness as well as lower adhesion to the substrates, relative to flat CrN surfaces. Even so, ball-on-disk (BoD) experiments, performed under conditions of minimum lubrication at high loads exhibited a longer wear life on the highly structured coatings compared to the relatively flat, unstructured surfaces. This is attributed to lubricant accumulation in microfissures present in the structured coatings. These microreservoirs not only provide critical lubrication at the contacting surfaces but also act as traps for wear-generated debris. Furthermore, the advantages of surface structuring are even more evident under low load conditions; this effect is the result of the reduced contact area and directed lubrication provided by the surface structuring.     

MoS2‐based alloys and nanocomposites for solid lubrication

The development of MoS₂ coatings has involved the modification of substrate surfaces, the addition of metals or compounds to the MoS₂, and variation in the deposition process parameters affecting the properties of deposited films. More recently, multilayer and periodic nanolayer coating structures have also been investigated. At present, work is concentrated on alloys of MoS₂, mainly with various metals, and targeted at terrestrial (ambient air) applications. The addition of metals or compounds to physical‐vapour‐deposited MoS₂ has led to improvements in coating performance, for example, greater stability of friction coefficient, greater film endurance, and increased temperature/oxidation resistance. The metal or compound can be either in the form of nanoscale multilayers or mixed with the MoS₂, sometimes leading to nanoclusters within a MoS₂ matrix. Microstructural analysis seems to show that the primary function of these additives is to suppress the formation of low‐density, columnar structures. At certain concentrations an added metal can also enhance the formation of the tribologically favourable (002) orientation of the MoS₂ crystallites. Other changes in the properties of MoS₂—metal composites may be due to their oxidation resistance, as indicated by the stability of these films against storage in air and their increased endurance when in sliding contacts at elevated temperatures.