Development of a fretting-fatigue mapping concept: The effect of material properties and surface treatments

Fretting-fatigue induced by combined localized cyclic contact motion and external bulk fatigue loadings may result in premature and dramatic failure of the contacting components. Depending on fretting and fatigue loading conditions, crack nucleation and possibly crack propagation can be activated. This paper proposes a procedure for estimating these two damage thresholds. The crack nucleation boundary is formalized by applying the Crossland high cycle fatigue criterion, taking into account the stress gradient and the ensuing #size##effect#. The prediction of the crack propagation condition is formalized using a short crack arrest description. Applied to an AISI 1034 steel, this methodology allows the development of an original material response fretting-fatigue map (FFM). The impact of material properties and surface treatments is investigated

On the greatest prime factor of ab+1

We improve some results on the size of the greatest prime factor of integers of the form ab+1, where a and b belong to finite sets of integers with rather large density.Comment: 38 pages. Theorem 3 of this version is improved. One bibliographical item is adde

Development of a Wöhler-like approach to quantify the Ti(CxNy) coatings durability under oscillating sliding conditions

The selection of a proper material for the particular engineering application is a complex problem, as different materials offer unique properties and it is not possible to gather all useful characteristics in a single one. Hence, employment of different surface treatment processes is a widely used alternative solution. In many industrial applications, coating failure may be conducive to catastrophic consequences. Thus, to prevent the component damage it is essential to establish the coating endurance and indicate the safe running time of coated system. To this study PVD TiC, TiN and TiCN hard coatings have been selected and tested against polycrystalline alumina smooth ball. The series of fretting tests with reciprocating sliding at the frequency 5Hz have been carried out under 50-150N normal loads and under wide rage of constant as well as variable displacement amplitudes from 50µm to 200µm at a constant value of relative humidity of 50% at 296K temperature. To quantify the loss of material a dissipated energy approach has been applied where the wear depth evolution is referred to the cumulative density of friction work dissipated during the test. Different dominant damage mechanisms have been indicated for the investigated hard coatings, which is debris formation and ejection in case of TiC coating and progressive wear accelerated by cracking phenomena in case of TiN and TiCN coatings. Energy-Wöhler wear chart has been introduced, in which the critical 1 dissipated energy density corresponds to the moment when the substrate is reached after a given number of fretting cycles. Two different methods to determine the critical dissipated energy density are introduced and compared. The Energy-Wöhler approach has been employed not only to compare the global endurance of the investigated systems but also to compare the intrinsic wear properties of the coatings. It has been shown that the fretting wear process is accelerated by the stress-controlled spalling phenomenon below a critical residual thickness and a severe decohesion mechanism is activated. Finally the applicability of the investigated method to other coated systems subjected to wear under sliding conditions is discussed and analyzed. The perspectives of this new approach are elucidated

An energy description of wear mechanisms and its applications to oscillating sliding contacts

To quantify wear rates, the Archard approach is classically applied. It relates the wear volume to the product of the sliding distance and the normal load. A wear coefficient is then extrapolated and is supposed to establish the wear resistance of the studied material. This synthesis shows that this approach does not work when the friction coefficient is not constant. It appears to be much more relevant to consider the interfacial shear work as a significant wear parameter. This approach is applied to study the wear response of different steels and then extended to different hard TiN, TiC coatings under reciprocating sliding conditions. By identifying wear energy coefficients the wear quantification can be rationalized and the wear resistance of the studied tribosystems can be classified. This also appears to be a convenient approach to interpret the different wear mechanisms. Metallic materials involving plastic strain are analyzed from FEM computations. The energy balance confirms that a minor part of the dissipated energy is consumed by plasticity, whereas the major part participates in the heat and debris flow through the interface. When a load energy approach is introduced an accumulated density of the dissipated energy variable is considered to quantify the TTS (Tribologically Transformed Structure) formation. A wear ”scenario” of metallic structures is then discussed. This energy wear approach is applied to analyze hard coating wear mechanisms focusing on abrasion and oxidation phenomena. The local wear energy analysis is transposed, thus allowing the lifetime of hard coatings to be quantified

Behaviour of shot peening combined with WC-Co HVOF coating under complex fretting wear and fretting fatigue loading conditions

This study investigated the fretting and fretting fatigue performance of tungsten carbide–cobalt (WC–Co) HVOF spray coating systems. Fretting wear and fretting fatigue tests of specimens with shot peening and WC–Co coatings on 30NiCrMo substrates were conducted. The WC-Co coating presents very good wear resistance and decreases by more than 9 times the energy wear coefficient (α) under fretting conditions. The tested coating reduces crack nucleation under both fretting and fretting fatigue studied situations. Finally the crack arrest conditions are evaluated by the combined fretting and fretting fatigue investigation. It is shown and explained how and why this combined surface treatment (shot peening and WC–Co) presents a very good compromise against wear and cracking fretting damage

Development of a friction energy capacity approach to predict the surface coating endurance under complex oscillating sliding conditions

In the case of surface coatings application it is crucial to establish when the substrate is reached to prevent catastrophic consequences. In this study, a model based on local dissipated energy is developed and related to the friction process. Indeed, the friction dissipated energy is a unique parameter that takes into account the major loading variables which are the pressure, sliding distance and the friction coefficient. To illustrate the approach a sphere/plane (Alumina/TiC) contact is studied under gross slip fretting regime. Considering the contact area extension, the wear depth evolution can be predicted from the cumulated dissipated energy density. Nevertheless, some difference is observed between the predicted and detected surface coating endurance. This has been explained by a coating spalling phenomenon observed below a critical residual coating thickness. Introducing an effective wear coating parameter, the coating endurance is better quantified and finally an effective energy density threshold, associated to a friction energy capacity approach, is introduced to rationalize the coating endurance prediction. The surface treatment lifetime is then simply deduced from an energy ratio between this specific energy capacity and a mean energy density dissipated per fretting cycle. The stability of this approach has been validated under constant and variable sliding conditions and illustrated through an Energy Density–Coating Endurance char