Running-in Stage Research Articles

State Evolution of Dry Gas Seal during Repeated Start–Stop Operation Using Acoustic Emission Method

Seal failure is a common phenomenon that occurs when using a dry gas seal. Many seal failures occur during the start-up process of the seal. In this study, the state evolution of a dry gas seal during repeated start–stop operations was investigated using an acoustic emission (AE) method and a modified empirical mode decomposition using masking signals (MS-EMD). It was evident that the liftoff speed and root mean square (RMS) of the full-speed period changed in stages. According to the change in these parameters, the state evolution of the dry gas seal during the experiment occurred in four stages: the running-in stage, normal working stage, transition stage, and abnormal stage. During the normal working stage, the RMS curve of the late liftoff period oscillated, but its wave trough was flattened due to misalignment of the seal face. After entering the transition stage, the period flattened at the wave trough disappeared and was replaced by a period of slow decline, which is attributed to the existence of abrasive particles between the seal faces. The unsteady accumulation of abrasive particles caused the liftoff speed to fluctuate significantly. By monitoring the liftoff speed of the seal and the RMS of the full-speed period, seal operators may assess the state of the seal and take intervention measures. This has great engineering value for the safe operation and life management of the seal.

The formation of a cobalt-based glaze layer at high temperature: A layered structure

The purpose of this study is to investigate the protective glaze layer structure formed during high temperature fretting wear process. The studied material is a cobalt-based alloy with a Cr-Ni-W solid solution (HS25) fretted against an alumina sample. It appears that a protective third body is spontaneously created at the interface for temperatures above 400 °C. The excellent tribological properties of the glaze layer leads to a stabilized friction coefficient of 0.3 and a quasi no-wear regime.Microstructure observations at the micro- and nanoscale were performed and revealed that the high temperature glaze layer is not homogeneous but rather composed of three layers with different grain sizes and chemical compositions. The investigations show that the effective glaze layer, the layer able to resist to fretting wear, is a thin layer made of nanocrystalline cobalt oxides. However, the chemical analysis reveals that before the formation of the effective glaze layer, the interface displays a high content of chromium oxides. Hence, a chemical segregation happens leading to the formation of a cobalt-rich effective glaze layer on the top of the former chromium-rich layer. In the light of these results, a chemical mechanism involving the consumption of chromium during the running-in wear stage is proposed to describe the glaze layer formation.

Improving the wear performance of feldspathic veneering porcelain by ion-exchange strengthening

ObjectivesThe present study examined the effects of Na+→K+ ion-exchange on the wear performance of feldspathic veneering porcelain. MethodsBar and disk specimens were prepared using IPS classic as the feldspathic veneering porcelain. After ion-exchange by immersion of the specimens in melted KNO3 at two temperatures for different time-periods, the bars were tested for flexural strength and Vickers surface hardness. The disks were paired with zirconia antagonists and tested with a pin-on-disk tribometer with 10 N for 70☓104 wear cycles in artificial saliva. Wear analysis of the porcelain and zirconia was performed using 3D profilometer and analysed with one-way analysis of variance and Tukey’s post-hoc pairwise comparison procedures. Worn surfaces were examined with scanning electron microscopy. ResultsThe feldspathic veneering porcelain exhibited strong time-dependent wear behaviour, with typical running-in and steady wear stages. Ion-exchange treatments at 380 °C and 440 °C both enhanced the mechanical properties, decreased the wear rates of running-in wear and steady wear. The wear performance of porcelain treated by ion-exchange at lower temperature (380 °C) was improved significantly, especially reducing the wear rate of the running-in stage. ConclusionA thicker ion-exchange layer with less stress relaxation may be obtained by ion-exchange at lower exchange temperature for a long processing time. Such a protocol improves the wear performance of the porcelain effectively. Clinical significanceRestorations with veneering porcelain may fail prematurely due to excessive wear. It important to improve the wear performance of the porcelain. Ion-exchange has the potential to strengthen dental veneering porcelain. Understanding the effect of ion-exchange on the wear performance of porcelain provides insight improving the wear performance of these restorations.

Addition of Ta and Y in a hard Ti-Al-N PVD coating: Individual and conjugated effect on the oxidation and wear properties

A comparative study of as-deposited structural properties, oxidation and wear properties of industrially arc-deposited Ti-Al-N, Ti-Al-Y-N, Ti-Al-Ta-N and Ti-Al-Ta-Y-N is presented. In the as-deposited coatings, Ta leads to more compressive stress while Y reduces the compressive stress but increases the nanohardness. The nanohardness of Ti-Al-Ta-Y-N is shown to be dominated by Y while its oxidation rate is dominated by Ta. Interestingly, Ta specifically favours the growth of oxide at the interface between Al-rich and Ti-rich oxide layers.In 850°C ball-on-disk wear tests, there is a pronounced effect of the running-in stage, which in turn depends strongly on the oxidation properties of the coating.The transferability of these findings to industrial machining at higher temperatures are discussed.

Experimental characterisation and computational modelling of cyclic viscoplastic behaviour of turbine steel

Fully reversed strain controlled low cycle fatigue and creep-fatigue interaction tests have been performed at ±0.7% strain amplitude and at three different temperatures (400 °C, 500 °C and 600 °C) to investigate the cyclic behaviour of a FV566 martensitic turbine steel. From a material point of view, the hysteresis mechanical responses have demonstrated cyclic hardening at the running-in stage and subsequent, hysteresis cyclic softening during the rest of the material life. The relaxation and energy behaviours have shown a rapid decrease at the very beginning of loading followed by quasi-stabilisation throughout the test. A unified, temperature- and rate-dependent viscoplastic model was then developed and implemented into the Abaqus finite element (FE) code through a user defined subroutine (UMAT). The material parameters in the model were determined via an optimisation procedure based on a genetic solver. The multi-axial form of the constitutive model developed was demonstrated by analysing the thermo-mechanical responses of an industrial gas turbine rotor subjected to in-service conditions. A sub-modelling technique was used to optimise the FEA. A 2D global model of the rotor with a 3D sub-model of the second stage of the low pressure turbine were then analysed in turn. The complex transient stress and accumulated plastic strain fields were investigated under realistic thermo-mechanical fatigue loading (start-up and shut-down power plant loads). The sub-model was then used for local analysis leading to identification of potential crack initiation sites for the presented types of blade roots.

Investigation of wear behavior of graphite coating on aluminum piston skirt of automobile engine

Abstract The graphite coatings were prepared by the screen-printing technique. The surface and interface morphology were observed, respectively. The engine test under the standard condition was employed to estimate the performance of graphite coating. Results shows that the deposited graphite coatings own the uniform thickness and desirable bond strength. The deposition of graphite coating plays an important role on decreasing friction and resisting the scuffing of cylinder bore. The solid lubricant characteristic of graphite coating can give desirable protection to the coupling elements under harsh conditions, such as cold start or running-in stage. The main wear of graphite coating occurs in the expansion stage of the engine for the high load and thermal deformation.

Complex Behavior of Nano-Scale Tribo-Ceramic Films in Adaptive PVD Coatings under Extreme Tribological Conditions.

Experimental investigations of nano-scale spatio-temporal effects that occur on the friction surface under extreme tribological stimuli, in combination with thermodynamic modeling of the self-organization process, are presented in this paper. The study was performed on adaptive PVD (physical vapor deposited) coatings represented by the TiAlCrSiYN/TiAlCrN nano-multilayer PVD coating. A detailed analysis of the worn surface was conducted using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) methods. It was demonstrated that the coating studied exhibits a very fast adaptive response to the extreme external stimuli through the formation of an increased amount of protective surface tribo-films at the very beginning of the running-in stage of wear. Analysis performed on the friction surface indicates that all of the tribo-film formation processes occur in the nanoscopic scale. The tribo-films form as thermal barrier tribo-ceramics with a complex composition and very low thermal conductivity under high operating temperatures, thus demonstrating reduced friction which results in low cutting forces and wear values. This process presents an opportunity for the surface layer to attain a strong non-equilibrium state. This leads to the stabilization of the exchanging interactions between the tool and environment at a low wear level. This effect is the consequence of the synergistic behavior of complex matter represented by the dynamically formed nano-scale tribo-film layer.

Tribological behavior of WS2/oil-impregnated porous polyimide solid/liquid composite system

PurposeThis paper aims to study the tribological behavior of the WS2/oil-impregnated porous polyimide (PPI) solid/liquid composite system, in which both PFPE (perfluoropolyether) and SiCH (silahydrocarbons) oils with different hydrocarbon chains were used, respectively. Lubricating mechanism of the composite system was also explored.Design/methodology/approachThe tribological behaviors of the WS2 films against the PPI cylindrical pins before and after immersing oil were evaluated under different loads by a reciprocating-type ball-on-disc tribometer.FindingsThe composite system exhibited the low and stable friction coefficient after the running-in stage, and the lubricant oil played a positive effect. It was found that the WS2/PFPE composite system exhibited more excellent lubricating property, although sole SiCH far exceeds PFPE in lubrication. The abnormal phenomenon mainly resulted from the influence of the oil amount. XRD results on the wear track surfaces indicated that PFPE and SiCH oils with different hydrocarbon chains were likely to preferentially adsorb to the edge plane and basal plane of the WS2 crystals, respectively.Originality/valueIn previous studies, liquid lubricants were directly dripped or spin-coated on the solid lubricant surface. Based on its potential advantage in application, the tribological behavior and mechanism of the solid lubricating film/oil-impregnated PPI composite system were investigated in this study.

An investigation of the effect of a new tool treatment technique on the machinability of Inconel 718 during the turning process

Though Inconel 718 alloy possesses excellent material properties and has various key applications in different industries, it still suffers from severe machinability issues and is considered one of the most difficult-to-cut materials. In this paper, a new and simple method is proposed to improve the machinability of Inconel 718 by forming different ductile and lubricious layers at the tool tip prior to its application for machining. Previous research showed that this method is very successful at improving the tool performance. In the current study, an enhancement of the proposed method is presented. Prior to the actual machining of Inconel, very short cuts of around two seconds were performed on an Al-Si and/or cast iron workpiece to form very thin layers of these materials on the tool rake face. During the subsequent machining of Inconel, the built-up material on the tool face has melted and the excess material was pushed out of the contact zone, with just a thin film remaining on the tool. This thin film protected the tool from chipping and considerably improved the tool life and the integrity of the machined surface. Results indicate that the tool treated with both cast iron and aluminum possessed a maximum tool life increase of 204%, a 45% lesser cutting force, and a 59% reduction in machining-induced work-hardening compared to the uncoated tool. All of the treatments displayed significantly reduced chipping. The following mechanisms contributed to these improvements: filling of the tool microcracks and prevention of their propagation, friction reduction enabling greater control over adhesion, seizure and built-up edge formation, improvement of the running-in stage of tool wear by preconditioning the tool surface prior to its main application, formation of various lubricious and thermal barrier tribofilms on the tool tip, control of different tool wear mechanisms such as adhesion, abrasion, and oxidation. All these mechanisms are discussed in details using various characterization techniques.

Macroscale Superlubricity Enabled by Hydrated Alkali Metal Ions.

Superlubricity based on hydration lubrication provides a near-frictionless lubrication state for the extreme reduction of friction in aqueous conditions. Nevertheless, how to obtain the hydration superlubricity under macroscale conditions with higher load-carrying capacity still remains a challenge and the mechanisms governing macroscale superlubricity with hydrated ions are still not well comprehended. Here, we demonstrate that macroscale superlubricity based on hydrated alkali metal ions (Li+, Na+, K+) can be realized under high contact pressure between the Si3N4 ball and sapphire disk. The ultralow friction coefficients of 0.005 are obtained under average contact pressure up to 0.25 GPa by a universal micro-tribometer after a running-in period with acid solutions. The results reveal that running-in stage with acid solutions can not only make the worn region smoother but also generate a silica layer that is easy to shear, which provides excellent boundary lubrication. The hydration superlubricity occurs because hydration shells surrounding the alkali metal ions could generate the hydration repulsive force to sustain a large normal load and have a fluid response to shear simultaneously. These findings pave the way to the scale-up of hydration superlubricity and thus to the wide application of new water-based lubricants.

Studying and designing improved coatings for labyrinth seals of gas­turbine engine turbines

An analysis of improving efficiency of aircraft engine turbines by means of improvement of composition of sealing coatings used in labyrinth seals was made. It was established that a number of contradictory requirements to properties of such coatings are imposed at the initial stage of engine running-in and during further operation. Main types of damage of above coatings used in the design of labyrinth seals during operation of gas turbine engines were shown. In connection with the necessity of raising temperature of gases in turbines of aircraft engines, it was proposed to additionally dope the serial nickel-based coatings with yttrium-containing master alloys. The results of study of influence of doping of the wearing-in sealing coatings on operating properties in conditions of action of a high-temperature gas flow were presented. It was found that doping of the KNA-82 serial coating with a multicomponent Co-Ni-Cr-Al-Y master alloy is the most rational solution.It was established that the use of the developed coating in a temperature range of 1,100...1,200 °C makes it possible to reduce specific consumption of fuel by aircraft engines by improving turbine efficiency and prevent wear of the end faces of ridges of the rotor labyrinth seal. Based on simulation of flow in the labyrinth seal clearance by numerical method, it was shown that the use of the developed coating materials in seals of the compressor turbine and the free turbine makes it possible to reduce amount of cooling air leakage into the turbine air-gas channel by reducing wear of tops of the labyrinth rotor seal ridge

Nanoscale Reciprocating Sliding Contacts of Textured Surfaces: Influence of Structure Parameters and Indentation Depth

Textured surfaces are widely used in engineering components as they can improve tribological properties of sliding contacts, while the detailed behaviors of nanoscale reciprocating sliding contacts of textured surfaces are still lack of study. By using multiscale method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. The influence of indentation depth, texture shape, texture spacing, and tip radius on the average friction forces and the running-in stages is studied. The results show that the lowest indentation depth can make all the four textured surfaces reach steady state. Surfaces with right-angled trapezoid textures on the right side are better for reducing the running-in stage, and surfaces with right-angled trapezoid textures on the left side are better to reduce wear. Compared with other textured surfaces, the total average friction forces can be reduced by 82.94%–91.49% for the case of the contact between the tip with radius R = 60r0 and the isosceles trapezoid textured surface. Besides, the total average friction forces increase with the tip radii due to that bigger tip will induce higher contact areas. This research proposes a detailed study on nanoscale reciprocating sliding contacts of textured surfaces, to contribute to design textured surfaces, reduce friction and wear.

A novel treatment for cutting tools for reducing the chipping and improving tool life during machining of Inconel 718

Chipping and high tool wear are considered the most challenging issues associated with machining of superalloys, including Inconel 718. Despite numerous attempts in this area, it is still considered a critical drawback of this class of materials for application in different industries. A new tool treatment technique is proposed for improving the tool wear and reducing the chipping during machining of Inconel 718. The treatment involves less than two seconds of machining on an aluminum-silicon (Al-Si) workpiece prior to the actual machining of Inconel. In this way, the “pre-machining process”, deposited a very thin layer of Al-Si on the tool face. During the subsequent machining of the Inconel bar, the Al-Si was melted due to the high temperatures of Inconel machining. The molten material channeled itself through the microcracks on the tool surface and seized their propagation. The sliding of the tool on the low friction Al-Si layer resulted in much lower forces, less sticking, seizure and built-up edge formation and, thereby, in less tool wear and chipping. Several beneficial lubricious and thermal barrier tribo-films were also formed on the tool face, which further protected the tool from chipping and severe tool wear. Reducing the contact pressure, friction and temperature during machining resulted in considerable reduction in the machining induced work-hardening of the workpiece material, which is considered as one of the most important factors affecting chipping during machining of this class of materials. Moreover, the ductility of Al helped to dampen the vibrations during machining and helped in tool chipping prevention. Finally, the preconditioning of the tool itself through this process resulted in tool performance improvement by lowering the tool wear at the running-in stage. The new method resulted in a significant reduction in tool chipping and an overall improvement in tool life. Considerable improvements in the surface integrity of the machined part were also obtained. The contributing mechanisms are discussed using detailed material characterization techniques.

Studies on Dynamic Tribology Properties of Friction Materials by Using an Approximate In Situ Observation for Worn Surfaces

This paper primarily focused on the dynamic tribology properties of one certain nonasbestos organic (NAO) friction material by using an approximate in situ method. This study was performed through a pad-on-disk type friction tester under different temperature conditions. Results showed that temperature has a significant effect on the dynamic tribology performance. At 100 °C, friction coefficient and wear rate after the running-in stage varied little with time. At 250 °C, friction coefficient after the running-in stage increased gradually and then tended to be stable, while wear rate decreased gradually. From 100 to 350 °C, friction coefficient increased first as a function of temperature, but decreased sharply when the temperature was over 250 °C. Simultaneously, wear rate also increased sharply over 250 °C. Additionally, three dynamic evolution models of worn surfaces corresponding to different cases were established.

On the integrated degradation coefficient for adhesive wear: A thermodynamic approach

Normal adhesive wear process in tribological components is characterized by a non-linear behavior during the transient running-in stage followed by a constant rate of steady-state wear. Even though the running-in stage of wear has an imperative significance in defining the component's long-term performance, its characteristics have been largely overlooked in the literature. In general, many of the available predictive models do not consider the contact temperature and the variation in the friction coefficient with time. The present work is an endeavor to propose a complete integrated wear model using the principles of thermodynamics. In this regards, an integrated degradation coefficient is proposed which correlates the time-dependent wear rates (at both running-in and steady-state stage), friction coefficient, and the contact temperature. The efficacy of the proposed degradation coefficient compared to established wear coefficient is demonstrated by considering test results using a vertical pin-on-disk apparatus. Results reveal that the degradation coefficient provides a realistic measure of wear during running period.

A new approach to the description of height distribution of plateau honed cylinder liner surface texture during the initial stage of wear

This article presents a new approach to the description of the height distribution of plateau honed cylinder liner surface topography after the initial stage of wear. In the literature, the two-Gaussian model is often used to describe the height distribution of cylinder liner surfaces. However, in the case of plateau honed surfaces, during the running-in period different changes of surface take place, mostly involving truncation or plastic deformation processes of the highest summits located in the plateau area. Therefore, to describe the height distribution of plateau honed surfaces during the running-in or initial stage of wear, an analytical model called three-Gaussian was used. This model was applied to determine the surface height distribution of cylinder liner after the initial stage of wear. Many plateau honed surfaces with traces of wear were analysed using this model, and some of them are shown as typical examples in this paper. The results of use of the three-Gaussian model to describe the height distribution of surface texture after the initial stage of wear are compared with the existing two-Gaussian approach. A comparison of the fit of these two models to the height distribution of plateau honed surface texture after the initial stage of wear shows that the three-Gaussian model fits the data better than the two-Gaussian model. The observed, measured and calculated results are elucidated and commented on.

Tribology assessment of surface-contact sliding pairs with streak defect under dynamic harmonic excitation loading

PurposeStreak defect and dynamic harmonic excitation (DHE) loading play important roles in machine operating conditions. The purpose of this paper was to assess the effects of streak defect and DHE loading on the tribological properties of surface-contact friction pairs, for example the differential gear end-face on the washer, via experimental investigation.Design/methodology/approachStreak defect was artificially introduced into the washer surface, which was loaded with DHE loads produced by a spring-connecting weight system. The wear scar of the washers and the monitored friction force signals were respectively scanned using scanning electron microscope (SEM) and analyzed using wavelet simulation.FindingsThe friction force curves, SEM images and discrete wavelet transform results indicate that DHE loading tends to increase friction force, to accelerate plowing damages and result in side-flow of material and plastic deformation on the surfaces of the washer. Whereas, streak oil-channel textures on washer specimen can be machined to modify the lubrication condition in the running-in stage so as to improve the tribological properties of the sliding pairs which were even subjected to DHE loading.Originality/valueOn the basis of this thesis research, the effect of streak defect and DHE loading on tribological performance of surface-contact sliding pairs is discussed. The results of wear form and friction state with the effect of streak defect and DHE loading facilitate to optimize the operating condition of mechanical parts.

Sliding Behaviour of Friction Material Against Cermet Coatings: Pin-on-Disc Study of the Running-in Stage

Running-in is the initial stage during which a tribological system reaches a steady-state condition. In this study, the running-in behaviour of a commercial brake friction material, pin-on-disc tested under dry sliding conditions, has been investigated to understand the role of different surface finishing of hard coatings. These coatings were deposited onto cast iron counterface discs and then mechanically polished to achieve set values of average roughness, Ra, and roughness skewness, Rsk. The tribological data were modelled using an exponential relation for the wear rate, according to a literature approach. The model parameters were related to the above-mentioned disc surface roughness parameters. The results provided indications on the wear mechanisms acting during the sliding action. Furthermore, these mechanisms were correlated with the formation of the friction layers on the pins and on the disc wear tracks. Of particular relevance is the finding that a negatively skewed surface roughness is fundamental to achieve the best running-in performances. The beneficial effects, coming from this surface treatment, are derived from the reduction in abrasion and from the improved formation dynamics of the friction layer, with particular regard to those parts (secondary plateaus) made of compacted wear debris.

Effects of crystal refining on wear behaviors and mechanical properties of lithium disilicate glass-ceramics.

The purpose of this study is to improve wear resistance and mechanical properties of lithium disilicate glass-ceramics by refining their crystal sizes. After lithium disilicate glass-ceramics (LD) were melted to form precursory glass blocks, bar (N = 40, n = 10) and plate (N = 32, n = 8) specimens were prepared. According to the differential scanning calorimetry (DSC) of precursory glass, specimens G1-G4 were designed to form lithium disilicate glass-ceramics with different crystal sizes using a two-step thermal treatment. In the meantime, heat-pressed lithium disilicate glass-ceramics (GC-P) and original ingots (GC-O) were used as control groups. Glass-ceramics were characterized using X-ray diffraction (XRD) and were tested using flexural strength test, nanoindentation test and toughness measurements. The plate specimens were dynamically loaded in a chewing simulator with 350 N up to 2.4 × 106 loading cycles. The wear analysis of glass-ceramics was performed using a 3D profilometer after every 300,000 wear cycles. Wear morphologies and microstructures were analyzed by scanning electron microscopy (SEM). One-way analysis of variance (ANOVA) was used to analyze the data. Multiple pairwise comparisons of means were performed by Tukey's post-hoc test. Materials with different crystal sizes (p < 0.05) exhibited different properties. Specifically, G3 with medium-sized crystals presented the highest flexural strength, hardness, elastic modulus and fracture toughness. G1 and G2 with small-sized crystals showed lower flexural strength, whereas G4, GC-P, and GC-O with large-sized crystals exhibited lower hardness and elastic modulus. The wear behaviors of all six groups showed running-in wear stage and steady wear stage. G3 showed the best wear resistance while GC-P and GC-O exhibited the highest wear volume loss. After crystal refining, lithium disilicate glass-ceramic with medium-sized crystals showed the highest wear resistance and mechanical properties.

Friction and wear behaviors of a gradient nano-grained AISI 316L stainless steel under dry and oil-lubricated conditions

A gradient nano-grained (GNG) surface layer was fabricated on an AISI 316L stainless steel (SS) by using the surface mechanical rolling treatment (SMRT). Reciprocating dry and oil-lubricated sliding tests of the GNG 316L SS in air at room temperature were conducted in comparison with the coarse-grained (CG) counterpart. Worn surface morphologies and subsurface microstructures were investigated for both 316L SS samples. 316L SS with a GNG surface layer shows a significantly improved wear resistance, especially under oil-lubricated condition. The notably wear resistance enhancement of the GNG 316L SS is attributed to the GNG surface layer with high strain accommodation ability and high hardness, which can reduce the wear volume in the running-in stage effectively.