Philippe Kapsa

Quantification of fretting damage

Abstract The fretting behaviour of high speed steel SC 6-5-2 uncoated and coated with a TiN coating against an alumina ball was studied based on a fretting map approach. Cracking and material loss were observed depending on the contract loading. To quantify the damage, a methodology is proposed based on an elastic Hertzian-Mindlin contact description. The sliding regimes are clearly defined, applying some fretting sliding criteria. First appearing under partial slip conditions, the crack nucleation conditions were precisely identified for different number of cycles and normal forces. Comparison with various fatigue approaches indicates that, if the local friction coefficient as well as the local aspect of contact stressing are considered, the maximum principal and the equivalent Von Mises stresses permit a rather good quantification of cracking. Wear and debris formation mainly observed under gross slip conditions lead to a wear volume which is shown to present a linear evolution with the cumulated dissipated energy. This interfacial work approach is completed by a local description. Axial and lateral sliding axis linear energies are deduced allowing a rapid identification of the wear behaviour through different energy wear factors. Experimental comparison of the TiN coating with the steel substrate contact shows that, as long as the coating is present in the contact, the important compressive residual stresses prevent cracking whereas the energy wear analysis indicates a wear resistance of the coating up to 10 times superior to the studied steel.

Wear analysis in fretting of hard coatings through a dissipated energy concept

Abstract Fretting situations are becoming increasingly important for industrial applications. Numerous studies have been undertaken to understand the damage created by low-amplitude reciprocating motion, particularly in the case of a ball on flat contact in dry conditions. The loading condition of materials can be characterized by a “running condition fretting map” which describes, in a graph of normal load versus displacement amplitude, the various domains of sliding; partial or gross slip. By considering the time evolution of sliding conditions, various regimes can be defined: a partial slip regime when partial slip is observed throughout the test, a gross slip regime for gross slip,regime for gross slip, and a mixed regime when both partial and gross slip are observed. The response of the material is, of course, dependent on the sliding regime: crack initiation and propagation are related more to the mixed regime, while surface transformations and loss of matter are related more to the gross slip regime. This paper analyzes the fretting gross slip situation and describes the loss of matter in the case of hard coatings deposited on high-speed steel. Two aspects are considered: (i) the wear volume measurements are usually performed using 3D topography, whereas modeling of the wear scar morphology indicates that valuable estimates of the worn volume can be obtained by 2D profilometry, which is much less time-consuming than 3D topography; and (ii) the wear volume observed in experiments is related to the energy dissipated in the contact at both the global scale and the local scale where the wear depth is an important parameter. With this energy approach, the wear behaviour of a TiN-coated and uncoated high-speed steel can be compared quantitatively.

Effect of surface topography on formation of squeal under reciprocating sliding

Abstract Under reciprocating sliding there are at least four states available to distinguish whether squeal occurs or not and how squeal evolves if it is present: (1) no squeal is emitted in the whole reciprocating sliding as many as 3800 cycles or more; (2) squeal is not emitted when the friction coefficient is smaller in the beginning stage of reciprocating sliding; (3) squeal is emitted when the friction coefficient arrives at a larger (critical) value; (4) sometimes squeal disappears after more cycles. These four states are found to be to some extent in correlation with the topography characteristics of the wear scars. Acoustic and dynamic measuring techniques were applied to ascertain the area where squeal was present on a wear scar. By means of SEM observations of wear scars, a comparative investigation into the characteristics of scar surfaces was carried out under both silent and squealing conditions. The result shows that for flat–flat contact the area associated with squeal on a wear scar is characterized by uneven serious pit-like detachment of the specimen material. For flat–ball contact it is found that the area related to squeal is characterized by adhesively joined asperities. Moreover, the effect of a negative friction–velocity slope on squeal formation is examined. The result seems to demonstrate that not all negative friction–velocity slopes can cause squeal. A possible alternative mechanism of squeal formation is proposed and dependence of squeal generation on the friction coefficient are also discussed based on the detachment of the material or formation of adhesively joined asperities.

Development of a friction model for the tool-chip-workpiece interfaces during dry machining of AISI4142 steel with TiN coated carbide cutting tools

This work deals with the identification of a new friction model for high speed dry cutting of AISI4142 steel with TiN coated carbide tools. Both experimental and numerical studies were used to find the relationship between friction coefficient, sliding velocity, contact pressure and contact temperature. A tribometer, based on a plane-sphere contact configuration, has been developed to reach extreme contact conditions similar to the ones occurring during metal cutting. The apparent friction coefficient, provided by the ratio between the tangential and normal forces, is measured. It is assumed that this coefficient can be divided into two components: an adhesive coefficient and a plastic deformation coefficient. The proportion of each component is investigated by means of a numerical model simulating the friction of a sphere with similar contact conditions.

Impact of contact size and geometry on the lifetime of a solid lubricant

Abstract The selection of low friction coatings is of great interest to industrial applications. Nevertheless, regarding the lifetime of the coatings, the selection criteria often depend on the experimental apparatus and contact configuration and then cannot be applied to real cases. In this study, we use a model based on the local dissipated energy due to friction under gross slip conditions in fretting wear. Indeed, the maximum value of the local dissipated energy is a unique parameter that takes into account the two major variables in fretting wear experiments: the normal force and the sliding amplitude. Two kinds of contact geometry are tested to assess the stability of the proposed approach: a cylinder on plane contact and a plane on plane contact (which consists of a circular contact surface with rounded edges to remove the discontinuity). This approach is applied to a commercially available polymer bonded MoS2 solid lubricant film used in aeronautical applications to protect titanium surfaces in contact. The effects of normal force and displacement amplitude are investigated. The results show that, even if there is a great difference in contact size and pressure distribution between the two types of contact, the lifetime is the same if the maximum local dissipated energy is the same. Thus, the local variable used is representative of the wear kinetics and the “lifetime versus local dissipated energy” curve is independent of the contact configuration: the proposed approach can then be used to select coatings whatever the contact size.

Experimental investigation into squeal under reciprocating sliding

Abstract Experiments on squeal under reciprocating sliding were performed by means of a ball against a block. Vibration accelerations, sound pressure level of squeal and tangential force were measured simultaneously. Under certain test conditions, the reciprocating sliding can create a whole process from squeal generation to disappearance. Based on power spectral density (PSD) and short-time Fourier transform (STFT) analyses on the vibration accelerations in that process, it was found that the dominant frequencies of the friction-induced vibrations associated with squeal is not varied. Examination of the friction–velocity slope shows that there is no invariable correlation between the negative friction–velocity slope and occurrence of squeal. Squeal can occur in regions with both negative and positive friction–velocity slopes.

Sliding damage on hot-pressed and sintered silicon nitride caused by a diamond tip under controlled humidity

Abstract The friction and wear behaviour of sintered silicon nitride (SSN) and hot-pressed silicon nitride (HPSN), when sliding against a diamond tip under various conditions, was investigated. Friction coefficients were found to be dependent on the roughness of the tip during the initial sliding cycles, subsequent values obtained were about 0.1 at 20% relative humidity. Ceramic wear was greater for a rough tip. Using the cathodoluminescence mode of a scanning electron microscope and scanning acoustic microscopy were very helpful in studying surface damage. Wear was a combination of plastic deformation, the formation of a hydrated oxide layer and fracture. The occurrence of fracture and plastic deformation, leading to the formation of “craters”, was observed for SSN at low humidity. This phenomenon is associated with the presence of subsurface flaws in the ceramic; they represent weak points from which fractures originate. The effect of humidity is beneficial for SSN, as fracture is eliminated and corrosive wear takes place. The wear rate is decreased by the effect of humidity. In contrast, the effect of humidity on HPSN, where no fractures appeared, was detrimental as corrosive wear took place. However, the effect on the friction coefficient is detrimental for both ceramics: the friction coefficient is 0.1 at 60% relative humidity and 0.04 at 10% relative humidity.

Fretting behaviour of hard coatings under high normal load

Abstract Surface damage of worn material and cracks due to fretting can be offset by the use of hard coatings. The present study shows the results which have been obtained using titanium nitride and diamond-like carbon coatings 5 μm thick deposited on high speed steel. The fretting experiments were conducted with a 10 mm diameter steel ball (AISI 52100) sliding on a coated flat surface with a normal load ranging from 100 to 500 N. The sliding amplitude (peak-to-peak displacement) of the ball is constant during the test, that is between 5 and 50 μm. The experiments were performed under room atmosphere at ambient temperature. The results obtained demonstrate the changes related to the presence of the coating: the fretting maps are modified because of a different surface nature and mechanical properties. An energy ratio (dissipated energy divided by total energy during a fretting cycle) has been found to be very useful to describe the contact phenomena.

On the mechanisms of abrasive wear of polyamide fibres

The abrasion resistance of fibres is a basic property in the textile industry since the fibres can be damaged during the manufacturing process. To understand the fundamental mechanisms of abrasion of 40 μm diameter polyamide fibres, a tribometer using a fibre-cylinder contact configuration immersed in water has been developed. This instrumented device controls tribological parameters such as the sliding speed, the environment and the tension applied to the fibre. A reference experimental procedure has been defined in order to run comparative experiments where the former parameters were varied. The number of rod revolutions leading to the fibre rupture has been selected as a macroscopic wear criterion. It is found to be independent of the sliding speed, in the investigated range of velocities. A simple thermal analysis of the sliding contact confirms that the contact heating is limited by the cooling effect of water. Microscope observations of worn fibres reveal abrasive scars and defibrillations on the fibre surface and are used to characterize the wear kinetic. From these observations, it is shown that the abrasive process is responsible for a continuous diminution of the fibre cross-section until the creep failure stress is achieved locally.

Influence of Material Nature and Surface Texturing on Wear of Heavy-Duty Diesel Engine Cylinder Liners

The aim of this work is to study the influence of the material (chemical composition and mechanical properties) and the surface texturing on wear of heavy-duty diesel engine cylinder liners. Three kinds of lamellar grey cast iron have been studied: a classical grey cast iron, a surface-quenched grey cast iron, and a micro-alloyed grey cast iron. Honed samples have been tested with different surface roughness parameters. Reciprocating friction tests with the configuration “sphere against a piece of cylinder liner” in lubricated contact have been carried out on a Cameron-Plint test rig. The friction coefficient and the electrical contact resistance were measured during the test. The tribochemical film formation on the wear scars has been studied after different test durations (during and after the running-in stage). The wear volume of the cylinder and the sphere has been determined. Surface heat treatment and phosphorus eutectics can improve the tribochemical film formation and the wear resistance of cylinder liners. Moreover, the surface topography has an important influence; a flat surface with deep valleys shows a better wear resistance than a surface with lots of asperities.