Margaret Stack

The role of triboparticulates in dry sliding wear

In this paper, wear processes and mechanisms for wear transitions with sliding time and temperature during sliding of a nickel-based alloy, N80A, in oxygen at temperatures to 250°C are discussed. Transitions in wear from high rates to low rates with sliding time were always observed at all the temperatures investigated. The transitions in wear were usually accompanied by transitions in contact resistance between the rubbing surfaces from nearly zero to positive high values. It was found that wear debris particles were heavily involved in the wear processes. The transitions in wear and contact resistance with sliding time mainly resulted from the development of wear-protective layers following the compaction of wear debris particles on the rubbing surfaces. The adhesion of triboparticulates to each other and to the rubbing surfaces played an important role in the rapid decrease in wear rate with sliding time and with increase in temperature. Processes involved in the development of the wear-protective particle layers and mechanisms for the wear transitions have been described on the basis of experimental observations. The importance of triboparticulates in wear and its implications for wear protection are discussed.

Modelling the tribo-corrosion interaction in aqueous sliding conditions

In corrosion wear, one of the most commonly observed phenomena is the synergism where both corrosion and wear are significantly increased by the tribo-corrosion interactions, leading to much greater material losses than produced by the sum of losses by either process alone. However, mechanisms for the synergistic effect are generally not well understood and there has been little effort in modelling synergistic processes. In this paper, mechanisms for the interactions between corrosion and sliding wear of metals are proposed and a mathematical model is developed which incorporates the various factors affecting corrosion wear, including experimental and environmental conditions and material properties. Many of the observed phenomena in corrosion wear can be explained. It also provides a good basis for wear map construction and would be especially useful in carrying out dimensional analyses for constructing such maps.

A mathematical model for sliding wear of metals at elevated temperatures

Abstract The transition in wear rate from a high value to a low value for metals after some time of sliding is a well known phenomenon. However, few models have been presented to account for such a transition. In this paper, a mathematical model, based on experimental observations that the transition is caused by the development of wear protective layers on the rubbing surfaces, is proposed. The protective layers are developed mainly from accumulated wear debris particles retained within the wear tracks; these can have various characteristics, depending on the experimental conditions and the properties of the metal, particularly the oxidation conditions and the contact between the rubbing surfaces. There is broad agreement between reported experimental observations and calculated predictions based on this model. For example, the development of protective layers occurs very quickly once the transition time/distance has been attained; whether or not ‘glaze’ layers develop on top of the compact particle layers depends on the sliding temperature, leading to the concept of a transition temperature. Wear debris particle size plays an important role in determining the wear transition; if the particles are too large and/or are difficult to fragment, such as those generated when the load or speed are high, they are more likely to be removed from the wear tracks and the severe to mild wear transition becomes difficult, or even impossible. The model is applicable to both room temperature and elevated temperature sliding wear.

The corrosion behaviour of macroparticle defects in arc bond-sputtered CrN/NbN superlattice coatings

The investigation concerned the corrosion behaviour of macroparticle and growth defects in PVD CrN/NbN superlattice coatings formed by are bond-sputtering (ABS) process on a mild steel BS6323. The electrochemical behaviour of the coatings was firstly studied by potentiodynamic polarising in de-aerated 0.5 M (Na2CO3-NaHCO3) buffer and 5% NaCl solutions, respectively. The coating and defects were then examined in planar view and cross-section by scanning electron microscopic analyses and the results were compared with those prior to the electrochemical measurement. It is found that the overall coating/substrate corrosion process is closely related to the deleterious effect of the macroparticles and growth defects in the PVD coatings. It is further demonstrated that for through thickness macroparticle inclusions, corrosion initiates by galvanic or crevice corrosion between the defect and the coating matrix, subsequently permitting solution access to those defects with eventual substrate pitting and corrosion at the coating/substrate interface. On the basis of the experimental findings and the macroparticle formation theory, the mechanisms of the growth defect-related coating/substrate corrosion are finally proposed

On erosion issues associated with the leading edge of wind turbine blades

The increasing developments in wind turbine technology, coupled with an unpredictable operating environment, present significant challenges regarding erosion issues on the leading edge of the blade tips. This review examines the potential degradation posed by the different environmental variables, with specific emphasis on both rain droplet and hailstone impact on the blade leading edge. Drawing on both the insights from experimental results and recent field data from the literature, the mechanisms of leading edge erosion are discussed. Meteorological tools that may enable rain and hailstone erosion prediction are addressed, as well as potential experimental and numerical approaches that may provide insight into the nature of impact and erosion on the blade surface.

Some frictional features associated with the sliding wear of the nickel-base alloy N80A at temperatures to 250 °C

Abstract The time-dependent variations of friction coefficient and the contact resistance of a nickel-base high-temperature alloy, N80A, during like-on-like sliding in pure oxygen at temperatures of 20–250 °C were simultaneously recorded and the tribological behaviour correlated with the nature of the sliding contact. A transition to a positive contact resistance always occurred after some time of sliding in the temperature range investigated. However, the time-dependent variations of coefficient of friction showed quite different features at the various temperatures. Corresponding to the times of the transitions in contact resistance, at 20 °C, the friction coefficient increased to a higher value from the initial value while, at 250 °C, it decreased to a lower value; however, at 150 °C, the coefficient of friction remained unchanged after the transition. Scanning electron microscopy observations have shown that the predominant factor for such different tribological responses at the various temperatures is the nature of the contact between the surfaces. At 20 °C, the real contact areas mainly consisted of loosely compacted particles while, at 250 °C, the load-bearing areas were smooth wear-protective oxide layers. At the intermediate temperature, 150 °C, the load-carrying areas comprised both types of contact. A mathematical model has been proposed and is used to relate the frictional behaviour to morphological features of the wear surfaces developed at the various temperatures.

Characterization of Synergistic Effects Between Erosion and Corrosion in an Aqueous Environment Using Electrochemical Techniques

Abstract Synergistic effects between erosion and corrosion processes on mild steel (MS) in an aqueous carbonate-bicarbonate solution were studied using electrochemical measurements from a rotating ...

Bridging the gap between tribology and corrosion: from wear maps to Pourbaix diagrams

Abstract Wastage as a result of the combined effects of wear and corrosion occurs in many environments, ranging from offshore to the healthcare industries. In such cases, the degradation is dependent on a wide range of parameters relating to the materials in contact and the nature of the corrosive environments. Defining conditions in which the wastage is minimised is critically important for engineers charged with monitoring such processes. The nature of the tribological contact plays a critical role in determining the effect of corrosion on the wastage rate. In some cases, as in sliding wear, frictional heating may arise at high velocities and applied loads, leading to oxide film formation, even at room temperatures. In other cases, as in solid particle erosion, frictional heating may play a significant role only at very high fluxes of particle impact. The action of a corrosive medium, either in gaseous or in liquid form, thus may have very different effects on such diverse tribological processes. The interactions between tribological processes and corrosion environments are examined in the light of recent work in the area. The role of tribo-corrosion maps will be discussed for coated and uncoated materials. In addition, areas that should be addressed in future work are discussed in terms of predictive modelling and experimental and analytical work for new and emerging materials.

The influence of low concentrations of chromium and yttrium on the oxidation behaviour, residual stress and corrosion performance of TiAlN hard coatings on steel substrates

Ti0.43Al0.52Cr0.03Y0.02N films, which have been shown to exhibit a fine grain near equiaxed microstructure were found to exhibit a compressive residual stress of - 6.5 GPa in contrast to conventional columnar Ti0.44Al0.53Cr0.03N coatings which demonstrated - 3.8 GPa compressive stress. Novel coatings with this modified microstructure were also found to possess improved resistance to both dry oxidation and wet aqueous corrosion. Glancing angle parallel beam geometry X-ray diffraction (GAXRD) studies showed that in conventional Ti0.44Al0.53Cr0.03N films, severe oxidation initiated above 850 degrees C whilst oxidation of Ti0.43Al0.52Cr0.03Y0.02N started close to 950 degrees C. In an alkaline aqueous medium, Ti0.43Al0.52Cr0.03Y0.02N coatings deposited on steel showed an extended passive potential range and a significantly lower passive current compared with Ti0.44Al0.53Cr0.03N films of similar thickness. A similar improvement was evident in sulphuric acid where yttrium containing coatings passivated at high potential (Ti0.44Al0.53Cr0.03N films did not passivate). These effects may be ascribed to reduced porosity in the fine-grained Ti0.43Al0.52Cr0.03Y0.02N as well as the well-known effects of low concentrations of yttrium on high-temperature oxidation performance

A methodology for the construction of the erosion-corrosion map in aqueous environments

For the erosion of alloys under aqueous conditions, there have been some recent attempts to identify the transitions between the erosion-corrosion regimes using laboratory results. Such transitions can be superimposed on an erosion-corrosion map as a function of the main corrosion and erosion variables. Defining the magnitude of the wastage rate on such maps enables the relevant process parameters involved in the damage process to be optimized. The object of this work was to model the erosion-corrosion behaviour of mild steel exposed to an aqueous solution containing alumina particles. The results were used to identify the transitions between the erosion-corrosion regimes on erosion-corrosion mechanism maps. Process control maps, which show the magnitude of the wastage rate, were also constructed using such an approach. This paper describes the basis of the mathematical models used to construct the boundaries on the maps. The effects of particle size, flux, and solution pH on such boundaries are demonstrated. In addition, a qualitative comparison is made between the results of the model, and a series of slurry erosion-corrosion tests of mild steel in aqueous media.