The mechanism of bonding in cold spraying is still a matter of some debate. In this work, copper has been cold sprayed onto aluminium alloy substrates, the surfaces of which had been prepared in a ...variety of ways. The coating-substrate bonding was assessed via a novel intermetallic growth method along with adhesive pull-off testing, and related to the substrate preparation method. The bond strength has been rationalized in terms of a modified composite strength model, with two operative bonding mechanisms, namely (i) metallurgical bonding and (ii) mechanical interlocking of substrate material into the coating. In most cases, mechanical interlocking is able to account for a large proportion of the total bond strength, with metallurgical bonding only contributing significantly when the substrate had been polished and annealed prior to spraying. In addition, grit-blasting has been shown to significantly reduce the bond strength compared to other substrate preparation methods.
Fretting generally results in either material removal or fatigue, or a combination of both. Although the term is rarely used now, in the early literature addressing this subject, fretting that ...resulted in material removal was sometimes termed “fretting corrosion” on account of the characteristic oxide debris that emanated from such contacts, with this description itself encapsulating the understanding that the material removal has both a mechanical and a chemical nature.
When the mechanical aspects of material removal in fretting dominate in the interpretation of the results, wear rates tend to be presented in terms of volume loss for a given exposure to wear (often measured by number of fretting cycles, total distance of sliding or energy dissipated). However, it is well understood that, in fretting, some aspects related to the formation of oxide-based debris are time-dependent (such as transport of species into and out of the contact and chemical reactions which take place at the contact surface) and this raises issues as to how to best present rate data associated with material removal. In this paper, recommendations are made as to how to be present volume loss data in fretting in a way that assists in the development of understanding of the rate-determining processes in material removal in fretting.
•Mechanical and chemical effects in fretting are addressed.•Graphical presentation of wear data is considered.•Time-based wear rates are used to identify competing processes.•Re-analysis of data points to the existence of rate determining processes.
The abrasive wear of three metallurgical structures with radically different hardnesses has been investigated for the same steel. The particular steel concerned is a recent innovation capable of ...generating extremely fine distributions of crystals. The austenite in the alloy nevertheless has the capability of uniformly transforming into extremely fine pearlite, nanostructured bainite, and plate martensite. It is found that although the abrasion rates and wear coefficients are not very different for the three states, the mechanisms of abrasion are quite different. We report detailed characterisation experiments together with comparisons with commercially available steels subjected to identical tests.
•Three-body abrasive wear of three different microstructures was studied.•Limited variation in wear loss observed in spite of large differences in hardness.•The mechanism of material removal from surface changes with microstructure.•Significant strain-hardening of bainite observed near the wear surface.•Pearlite and martensite showed significant softening near the abraded surface.
It has been long understood that fretting differs from sliding wear in that the relative displacement between the bodies is generally smaller than the size of the contact between them, with debris ...ejection from the contact thus playing an important role in the behaviour of the contact in fretting. Whilst these ideas were clearly articulated more than 30 years ago via Godet's third-body approach and Berthier's concept of the tribology circuit, calculation of wear rates in fretting have continued to employ Archard's wear equation (or approaches directly derived from it), despite this approach assuming that the rate of wear is controlled by the rate of generation of wear debris (as opposed to the rate of its ejection from the contact). It has been shown recently that when debris ejection is the rate-determining-process in fretting, the instantaneous rate of wear is inversely proportional to a characteristic dimension of the wear scar. When non-conforming specimen pair geometries (such as cylinder-on-flat) are employed in fretting testing, the wear scar size increases as wear proceeds, and thus the instantaneous rate of wear decreases. In this paper, wear equations have been derived for three commonly employed non-conforming pair specimen geometries, which all take the form Vw=KRn−1Edn (Vw is the wear scar volume, R is the radius of the non-plane specimen(s) in the pair and Ed is the frictional energy dissipated) where n varies between 0.67 and 0.8 depending upon the geometry and assumptions made regarding the governing equation. It is argued that the assumptions upon which the analysis is based are most valid for the cylinder-on-flat contact configuration with fretting perpendicular to the cylinder axis where the length of the line contact is large compared to the wear scar width. It is demonstrated that, despite the often apparently good fit of experimental data to an Archard-type equation, it is not appropriate to employ such Archard-type approaches to the analysis of fretting data in situations where debris ejection is the rate-determining-process. The equations derived in this paper relating wear scar size to some measure of the duration of the test should be used for such analysis instead of the linear relationships generally employed in previous work.
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•Fretting wear depends upon the rate of debris ejection from the contact.•Debris expulsion rate falls as the critical dimension of wear scar increases.•In non-conforming contact geometries, wear rate falls as a test proceeds.•Archard-type wear equations are not always appropriate for fretting wear.•New equations are presented for commonly employed fretting test configurations.
A significant body of work exists in the literature concerning the corrosion behaviour of zinc–magnesium coated steel (ZMG), describing its enhanced corrosion resistance when compared to conventional ...zinc-coated steel. This paper begins with a review of the literature and identifies key themes in the reported mechanisms for the attractive properties of this material. This is followed by an experimental programme where ZMG was subjected to an automotive laboratory corrosion test using acidified NaCl solution. A 3-fold increase in time to red rust compared to conventional zinc coatings was measured. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the corrosion products formed. The corrosion products detected on ZMG included simonkolleite (Zn
5Cl
2(OH)
8
·
H
2O), possibly modified by magnesium uptake, magnesium hydroxide (Mg(OH)
2) and a hydroxy carbonate species. It is proposed that the oxygen reduction activity at the (zinc) cathodes is reduced by precipitation of alkali-resistant Mg(OH)
2, which is gradually converted to more soluble hydroxy carbonates by uptake of atmospheric carbon dioxide. This lowers the surface pH sufficiently to allow thermodynamically for general precipitation of insoluble simonkolleite over the corroding surface thereby retarding the overall corrosion reactions, leaving only small traces of magnesium corrosion products behind. Such a mechanism is consistent with the experimental findings reported in the literature.
More than 25 years ago, Vingsbo and Söderberg published a seminal paper regarding the mapping of behaviour in fretting contacts (O. Vingsbo, S. Söderberg, On fretting maps, Wear, 126 (1988) 131–147). ...In this paper, it was proposed that in the gross-slip fretting regime, the wear coefficient increased by between one and two orders of magnitude as the fretting displacement amplitude increased from around 20µm to 300µm (defined as the limits of the gross-slip regime).
Since the publication of this paper, there have been many papers published in the literature regarding fretting in the gross-sliding regime where such a strong dependence of wear coefficient upon fretting displacement has not been observed, with instead, the wear coefficient being shown to be almost independent of fretting amplitude. Indeed, many researchers have demonstrated that there is a good correlation between wear volume and frictional energy dissipated in the contact for many material combinations, with the additional insight that a threshold in energy dissipated in the contact exists, below which no wear is observed (experimental data relating to fretting of a high-strength steel is presented in the current paper which supports this concept).
It is argued that in deriving a wear coefficient in fretting, there are two key considerations which have not always been addressed: (i) the far-field displacement amplitude is not an adequate substitute for the slip amplitude (the former is the sum of the latter together with any elastic deformation in the system between the contact and the point at which the displacement is measured); and (ii) there is a threshold in the fretting duration, below which no wear occurs and above which the rate of increase in wear volume with increasing duration is constant (this constant may be termed the wear coefficient, ktrue). Not addressing these two issues results in the derivation of a nominal wear coefficient (knominal) which is always less than ktrue. A simple analysis is presented which indicates thatknominalktrue=1−A−B
where A is associated with erroneously utilising the far-field displacement amplitude in place of the contact slip amplitude in the calculation of the wear coefficient and B is associated with the failure to recognise that there is a threshold in fretting duration below which no wear occurs.
A and B are shown to depend upon the tractional force required to initiate sliding (itself dependent upon the applied load and coefficient of friction), the system stiffness, the applied displacement amplitude, the threshold fretting duration below which no wear occurs and the number of fretting cycles in the test. Using typical values of these parameters, the ratio of knominal to ktrue has been shown to be strongly dependent upon the applied displacement amplitude over the range addressed by Vingsbo and Söderberg (with the ratio rapidly decreasing by an order of magnitude over this range). As such, it is argued that ktrue shows no strong dependence on slip amplitude in fretting, and that the strong dependence of knominal upon displacement amplitude presented by Vingsbo and Söderberg does not imply a change in ktrue as is often inferred.
The routine recording of force–displacement loops in fretting is a major experimental advancement which has taken place since the publication of the paper by Vingsbo and Söderberg. It is argued that this technique must be routinely used to allow the correct interpretation of wear data in terms of the actual slip amplitude (or energy dissipated); moreover, a range of conditions should be experimentally examined to allow the threshold fretting duration below which no wear has occurred to be evaluated and its significance assessed.
•Assumptions made in derivation of the fretting wear coefficient have been analysed.•The measured wear coefficient is generally smaller than the true wear coefficient.•To minimise such discrepancies, the recording of fretting loops is essential.•The influence of the threshold for initiation of wear is also recognised.•The strong dependence of wear on fretting amplitude in gross sliding is questioned.
As a candidate coating material for heat-exchanger surfaces in commercial power generation boiler, an amorphous/glass forming Fe-Cr-B alloy NanoSteel SHS 7170 was deposited by a 2 kW fibre laser onto ...a boiler grade steel substrate (15Mo3). A comprehensive trial with 28 single track optimisation runs was carried out to develop models of the influence of three processing parameters, laser power, laser traverse speed and powder feed rate, on powder deposition efficiency, dilution and porosity. It was found that deposition efficiency is dependent on laser power and powder feed rate, increasing with increasing power and decreasing powder feed rate when tested within the parameter window of laser power ranging from 0.4 to 2 kW; traverse speed varying from 150 to 1200 mm min‑1; and powder feed rate varying from 4 to 10 g min‑1. Similarly, it was found that dilution is also dependent on laser power and powder feed rate. Dilution increases with increasing power and decreases with increasing powder feed rate within the same parameter window discussed above. This means that through processing parameter selection, these properties can be adjusted to suit their application. Porosity was found to be independent of processing parameters and instead mostly dependent on the feedstock material. A model was produced for predicting porosity within a powder feedstock, found to be 8.5%. These models were used to successfully produce an optimised coating.
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•An optimised Fe-based alloy coating was deposited with laser cladding.•Model for deposition efficiency showed it to be power and powder feed rate dependent.•Model for dilution showed it to be dependent on power and powder feed rate.•Model of true pore volume in powder was developed and used to predict coating porosity.
Temperature is known to affect the fretting wear behaviour of metals; generally, a critical temperature is observed, above which there are substantial reductions in wear rate, with these being ...associated with the development of protective oxide beds in the fretting contact. This work has examined the gross-sliding fretting behaviour of a stainless steel as a function of bulk temperature and fretting frequency (with changes in the fretting frequency altering the frictional power dissipated in the contact amongst other things). An analytical model has been developed which has suggested that at 200Hz, an increase in the contact temperature of more than 70°C can be expected, associated with the high frictional power dissipation at this frequency (compared to that dissipated at a fretting frequency of 20Hz). With the bulk temperature at either room temperature or 275°C, the increase in contact temperature does not result in a transition across the critical temperature (and thus fretting behaviour at these temperatures is relatively insensitive to fretting frequency). However, with a bulk temperature of 150°C, the increase in temperature associated with the increased frictional power dissipation at the higher frequency results in the critical temperature being exceeded, and in significant differences in fretting behaviour.
•Roles of temperature and frequency in fretting of stainless steel are characterised.•Fretting wear damage falls rapidly above a critical environmental temperature.•The thermal field due to frictional power dissipation in fretting is modelled.•The critical temperature is dependent upon the fretting frequency.•Observed wear behaviour is understood in terms of the modelled contact temperatures.
The paper ‘The third-body approach: a mechanical view of wear’ by Maurice Godet (Wear, 100 (1984), pp 437–452) was perhaps the first to articulate clearly the key role of the rate of debris expulsion ...from a fretting contact in controlling the overall rate of wear. Whilst subsequent research over the past four decades has acknowledged this, the issue is generally addressed qualitatively rather than quantitatively. There are many parameters which will affect the rate of debris expulsion from a fretting contact, and amongst them is the physical size of the fretting contact. In this paper, for the first time, a physically-based relationship is proposed between the debris-expulsion-limited wear rate and the contact size. This relationship is able to account for differences in wear rates observed in tests conducted with different (and evolving) contact geometries (non-conforming contacts) over a range of durations, thus clearly demonstrating the validity of the approach.
•Wear volume in cylinder-on-flat contacts is geometrically consistent.•W-shaped wear scars transform to U-shaped scars as the number of cycles is increased.•Debris egress from the contact normally controls the wear rate of the system.•Instantaneous wear rate is to proportional to the inverse of the scar width.•The contact-geometry dependence of wear rate is fully reconciled via this methodology.
It is well known that mechanisms and rates of fretting wear of many metals are dependent upon the temperature of the environment; specifically, it is known that a transition temperature exists, above ...which the debris forms a protective bed in the contact which results in very low rates of wear being observed. This paper seeks to investigate the influence of contact geometry and slip amplitude on the transition temperature of a high strength alloy steel, and to understand these effects in terms of debris retention in (or expulsion from) the contact. Cylinder-on-flat fretting tests were performed at temperatures between 25°C and 250°C with two displacement amplitudes (25μm and 100μm) and two cylinder radii (6mm and 160mm). It was found that for the smaller cylinder radius, the transition temperature increased as the fretting displacement amplitude was increased. However, it was found that whilst the contacts with 6mm radius cylinders and 160mm radius cylinders exhibited very different mechanisms of wear at low temperature, the temperature at which the transition to forming of the protective debris bed was not strongly influenced by the contact geometry; moreover, at the higher temperatures, the protective bed is formed irrespective of contact geometry. It is proposed that the reduction in wear rate at higher temperatures is associated with the retention of oxide debris within in the contact area for long enough that it sinters to form a protective ‘glaze’ layer. By increasing the displacement amplitude, the rate at which the oxide is ejected from the fretting contact increases and this reduces the ability to form a protective layer; as such, a higher temperature is required to form the protective glaze as the displacement amplitude is increased.
•Low wear in fretting of steel associated with glaze formation.•Glaze forms when debris residence time is long enough for sintering.•Transition temperature for glaze formation exists.•Transition temperature increases with increasing fretting amplitude.•Transition temperature broadly independent of fretting pain contact geometry.
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