Running-in Stage Research Articles

Fracture and wear of the Sn11Sb5.5Cu Babbitt after equal-channel angular pressing

A computer simulation and natural experiments on the rod from babbit Sn11Sb5.5Cu deformation by the method of equal-channel angular pressing (ECAP) were carried out. Two structural states of the material were investigated. The first, obtained by usual crystallization, is characterized by large particles of intermetallic- and -phases. The second, obtained by rapid crystallization, has fine particles. ECAP was carried out in a matrix with a vertical and horizontal channels intersecting at a right angle. With the aim to ensure the least damage at the ECAP the computer simulation of the process in the software product DEFORM-2D was made, the deformation rate was selected. The wear was determined by the loss of the sample weight. Dependence of wear of babbitt, obtained by usual crystallization, had stages of running-in and stable wear. Wearing of the samples obtained by rapid crystallization showed that the running-in stage is absent. It was found that severe plastic deformation (SPD) by the ECAP method leads to the dispersion of intermetallic particles and the weakening of the adhesive bond of the particles with the matrix. In the case of coarse particles, their milling causes a decrease in wear. In the case of fine particles, the weakening of adhesion to the matrix causes an increase in wear. It can be said that for a material such as babbit Sn11Sb5.5Cu, an attempt to improve the tribological characteristics by means of the SPD method is untenable. There are less costly and simpler ways to obtain in babbits a structure with small intermetallic particles, which fully complies with the Charpy rule and has high tribological characteristics.

Research on Synchronous Coordination Development of Tourism-Economy-Environment System in Qinghai Section of Silk Road

Using coupling theory in physics, the paper analyzed the relationship of coordinated development of tourism-economy-environment system (abbr. TEES) in Qinghai. Results showed the comprehensive evaluation of Qinghai TEES has been improved greatly from 2000 to 2014. However, coupling degree has still been in the running-in stage and coordination degree only arrives at the primary coordination level, the development of economy and tourism has exceeded the threshold of ecological environment after 2008, and the ecological environment has become the bottleneck restricting the further improvement of coupling coordination. In the future, Qinghai must change its mode of development and focus on industrial upgrading and transformation so as to promote the harmonious and sustainable development of TEES in the Silk Road.

Fractal dimension and size distribution characteristics of ultrahigh- molecular-weight polyethylene wear particles in condition of artificial knee joint simulator

Background: As a typical friction pair, the artificial joints also generate a large number of wear particles due to the wear movement, which has become the direct cause of the debris disease called in clinical. The wear debris with different morphology and size contains a large amount of information relating to the wear status, and has the typical fractal characteristics. These fractal characteristics with a certain degree of regularity are favorable to study the wear mechanism under the different wear conditions. The aim of this study is to evaluate the fractal dimension and size distribution characteristics of UHMWPE wear particles in condition of artificial knee joint simulator by the automatic extraction and fractal identification system of wear debris based on the improved radar graph method. Methods: The forged CoCrMo alloy (ISO5832-12) and ultra-high-molecular-weight polyethylene (UHMWPE, ISO5834-2) were selected as the artificial joint prosthesis materials. The knee joint simulator produced by University of Leeds (Simulation Solutions Ltd.) was used to realize the knee joint wear motion. The wear debris extraction was based on the standard of ISO17853. Scanning electron microscope (Hitachi, S3000N) was used to observe the morphology characteristics of wear debris. The automatic extraction and fractal identification system of wear debris based on the improved radar graph method was used to investigate the fractal characteristics and size distribution of UHMWPE wear particles. Results: The fractal characteristics of the single UHMWPE wear debris are very obvious. When the wear particles changes from the larger sized strip to the smaller sized near-sphere, the radar fractal dimension decreases continuously. The fractal dimension D of ideal spherical shaped wear debris is close to zero. The fractal characteristics of UHMWPE wear debris group directly reflects the transformation trend of wear mechanism. In the initial stage of wear, the strip and flaky wear debris with the larger fractal dimension occupy the largest proportion in total wear debris. The adhesive and ploughing wear are the main wear mechanisms, corresponding to the running-in stage of the wear movement. With the extension of wear running-cycles, the proportion of strip wear debris reduces and the percentages of flaky and blocky wear debris increase obviously. The wear mechanisms gradually change to the abrasive and fatigue spalling wear and the wear movement enters into the composite wear stage. When the friction pair enters the stable wear stage, the proportion of all kinds of wear debris has little change. But the number of small-sized wear debris increases due to the action of cyclic stress, which causes the fractal dimension of wear debris group to decrease to some extent. Conclusions: The fractal dimension and size distribution of wear debris with different profiles can be obtained by the automatic extraction and fractal identification system of wear debris based on the improved radar graph method. It has enough numerical gradients to distinguish the wear debris morphology and avoid the effects of errors. It is not only used for the shape extraction, calculation of fractal dimension and statistics of wear particle parameters for the single and group of wear debris, but also provides a new digital analysis tool for the identification and diagnosis of the artificial joint wear debris.

Theoretical Approach of Wear for Slide-Friction Pairs

A number of different mechanisms and devices in mechatronic systems may involve sliding-friction surfaces. The issues of service life and its prediction for the details of such surfaces have always been of particular importance. Having studied the wear process prediction theories that have been developed in the course of time, which can be classified by dividing them in definite groups based on similar theoretical approach one can state that each of them has different shortcomings, which might affect the result precision, when essential basic parameters have been disregarded, as well as create a need for useless additional practical experiments, as a result of which theoretical calculation becomes unnecessary. [4] The article determines the most suitable wear calculation model that allows considering the set of parameters necessary for calculating slide-friction pair. Wearing usually proceeds in three stages: the running-in stage, the normal wear stage, and the intensive wear stage. The proposed model is provided for normal wear stage calculations. The proposed model for wear calculation is based on the application of theories from several branches of science to the description of 3D surface micro-topography in accordance with random field theory, assessing the material’s physical and mechanical characteristic quantities, substantiating the regularities in creation of material particles separated during the wear process and taking into consideration definite service conditions of fittings. Since the wear process is variable and many-sided, it is influenced by numerous different parameters, for example, surface geometry (roughness, waviness, form deviation, etc.), physical and mechanical conditions of the upper layer, material components, wear regime, wear temperature, etc. Based on the regularities stated in the article one can propose the following wear calculation sequence [4]:1) Initial data should be stated which will be further necessary in calculations: constructive-kinematic characteristic quantities (rated area Aa of wearing component, load P, gliding movement speed v, movement time t), fatigue characteristic of friction component material (friction coefficient f (f≤0,1) and material fatigue destruction parameters (m, σ-1, N0), mechanical characteristic quantities of material (E, μ);2) Parameters should be stated after attachment: surface roughness parameters (Sa, Sm1, Sm2, Sm2a), initial wear Up and corresponding time Tp, tolerated wear Umax.

Properties of steels and chromium-nickel alloys under low-cycle combined deformation

The work is devoted to the experimental study of steels and alloys deformation properties with account to the mutual influence of the cyclic and static deformation components. Carbon steel and three different chrome-nickel austenitic steels were tested at different temperatures and durations. At strain-controlled cyclic loading these steels demonstrate cyclic hardening (or softening – depending on the previous history) at the running-in stage, then stabilization and subsequent softening at the pre-fracture stage. Experiments show that in case of comparable cyclic and static damage the mutual influence of the cyclic and static material hardening (softening) strongly affects the stress-strain diagrams; this effect should be taken into account in structural analysis. Models that use only the information about static or strain-controlled cyclic loading are incapable to describe this effect.

Friction mechanism in the running-in stage of copper: From plastic deformation to delamination and oxidation

The friction mechanisms during the running-in stage and its effect on the subsequent steady-state were investigated by examining the evolution of surface topography and subsurface microstructures in pure copper sliding against a WC-Co ball. Scanning electron microscopy observations indicated that plastic deformation induced grain refinement occurs after only several sliding passes. A transition in the dominant friction mechanism from plastic deformation to delamination and oxidation was found in the running-in stage, corresponding to a peak coefficient of friction and onset of detectable wear.

Stability Analysis of Tribosystem Based on the Energy Feature of Friction Vibration

The tribosystem stability in different wear stages was analyzed by the friction vibration energy feature, which was extracted from the phase-space-matrix of friction vibration attractor with singular value decomposition (SVD). An energy feature parameter K of friction vibration was defined as the norm of the singular value feature vector, and the variation of K in different wear stages was also investigated in this paper. Results show that K becomes larger in the running-in wear stage, friction vibration energy becomes higher and tribosystem stability gets worse; K fluctuates smoothly and steadily in the stable wear stage, friction vibration energy is stable and the tribosystem is dynamically stable; and K increases sharply in the severe wear stage, the friction vibration energy increases dramatically and the tribosystem stability decreases greatly. Therefore, the friction vibration energy can reflect the tribosystem stability in different wear stages with the energy feature parameter K.

Effect of contact stress on the cycle-dependent wear behavior of ceramic restoration

AimCeramic restoration experiences the non-linear wear process during the chewing simulation, which contains running-in, steady and severe wear stages. However since various levels of contact stress may be applied on the occlusal surface during chewing, the cycle-dependent wear behaviors of ceramic crowns may differ. The aim of this study was to investigate the effect of contact stress on the development of wear behavior, as tested in a chewing simulator. Materials and methodsThirty-six anatomical metal-ceramic crowns using Ceramco III as the veneering porcelain were randomly assigned to two groups based on the contact stress applied in the wear testing. Stainless steel balls served as antagonists. The specimens were dynamically loaded in a chewing simulator up to 2.4×106 loading cycles, with additional thermal cycling between 5 and 55℃. For each group, several checkpoints were employed to measure the substance loss of the crowns’ occlusal surfaces and to evaluate the microstructure of the worn areas. ResultsAfter 2.4×106 cycles, the ceramic restorations with lower contact stress demonstrated a long steady wear stage following the running-in, but without the severe wear stage. And a slowly microstructural degradation was observed that the subsurface defect could not be seen until final. With higher contact stress, however, the ceramic restorations experienced a faster transition from running-in to severe wear stage that the steady wear stage nearly disappeared. And an early formation of subsurface defects and the deterioration of microstructure were observed. ConclusionsContact stress is a key factor affecting the wear development of ceramic restoration. The higher contact stress promotes the veneering porcelain to evolve into severe wear stage. In contrast, lower contact stress is prone to keep the veneering porcelain operating in steady wear stage, which delays the arrival of severe wear region.

In-situ study of structural evolution of tribological materials using synchrotron radiation

In this paper, we propose an approach for estimation of structural changes at the surface of metallic materials during running-in stage of friction. The application of synchrotron microbeam for in-situ observation of structural evolution in thin layers subjected to friction load is substantiated. A design of experimental apparatus intended for in-situ studies of friction processes using synchrotron radiation is described. This apparatus maintains constant position of the sample surface with respect to the incident beam. The ring shaped experimental sample with a thickness of 1 mm is pressed with constant pressure against a pin shaped indenter and rotated with constant speed. The pressure on the sample is determined by the mass of the load placed in the loading basket. The surface of the sample that comes out of indenter is probed by synchrotron beam. The proposed experimental scheme allows receiving X-ray diffraction patterns even under condition of severe plastic deformation of the surface. The preliminary results on the synchrotron assisted study of the dry friction of a Cu sample are described.

Finite element analysis of fretting wear under variable coefficient of friction and different contact regimes

Fretting wear is a material damage in contact surfaces due to micro relative displacement between two bodies. It causes some unexpected results, such as loosening of fasteners or sticking in components supposed to move relative to each other. Since this micro motion of fretting wear is difficult to measure in experiments, finite element method (FEM) is widely used for investigating the evolution of contact variables and wear scars during fretting wear process. In most FEM simulations of fretting wear, coefficient of friction (CoF) is assumed to be constant in order to simplify the models. As measured in experiments, however, the evolution of CoF has a relation with the wear number of cycles, especially during the running-in stage. In this research, the effects of variable CoF are considered in both gross sliding and partial slip conditions of fretting wear. The wear scar and wear volume predicted by FEM models for constant and variable CoF cases are calculated. Results indicate that, in gross sliding condition, whether or not using a variable CoF has little effect on wear volume at the end of the steady state stage of fretting wear cycles. However, when considering partial slip or running-in stage of gross sliding conditions, FE models with variable CoF achieve predictions that are closer to experimental results.

Non-Contact Roughness Investigation of Ball-Cratered Molybdenum Thermal Spray Coatings

The effects of micro-abrasion wear on the surface roughness of molybdenum coatings deposited by electric arc thermal spray on steel support were investigated. The 2D surface roughness was measured and correlated with the experimental results of the micro-abrasion tests. Different worn surfaces which correspond to running-in and steady stage of wear were investigated in terms of the microstructure, 3D image and 2D surface roughness. The micro-abrasion tests were made in the presence of a SiC abrasive slurry for test durations between 200 – 1600 s and the worn surfaces were evaluated by scanning electron microscopy and stereomicroscopy.

On-line detection of fretting fatigue crack initiation by lock-in thermography

Lack of accurate methodologies to detect micro-cracks in fretting fatigue experiments is a barrier to better understand the physics of the crack initiation phenomenon. In this study, the feasibility of thermography for on-line detection of micro-cracks in a closed contact during fretting fatigue experiments is investigated. Temperature variations in four regions of interest at the specimen surface are recorded and processed on-line based on a lock-in methodology to extract the first harmonic temperature amplitude. Following running-in and stabilization stages, the initiation of cracks leads to a rise of the stabilized first harmonic temperature amplitude for the specific region of interest where cracks are located. After that, tests are stopped and samples are inspected for cracks by optical microscopy.

The tribological behavior evolution of TiB2/Al composites from running-in stage to steady stage

The purpose of this study was to investigate the characteristics of a friction and wear transition from the running-in stage to the steady-state stage for an Al-based composite containing TiB2 particles and fabricated by a pressure infiltration technique. The dry sliding friction and wear behavior was investigated using a 3.98mm diameter AISI 52100 steel ball-on-disk geometry under applied loads of 1N and 3N. Microstructural characterization and element distribution were carried out using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). 3D surface topographies and roughness of the wear surfaces were measured using confocal laser scanning microscopy (CLSM). The results indicated that the formation of a transfer film and boric acid (H3BO3) was the main reason for the low friction coefficient of about 0.16 at a load of 1N. An increase of friction coefficient was caused by the abrasion of pulled-out TiB2 particles. In the running-in stage at an applied load of 3N, abrasive wear dominated and resulted in the formation of grooves and pulled-out TiB2 particles. In addition, at a steady-stage friction coefficient of 0.6, severe three-body abrasive wear and oxidative wear were the dominant wear mechanisms. An excellent correlation was found between friction coefficient and surface roughness, as measured by 3D topography.

On the Prediction of Transient Wear

During the running-in process, a friction pair experiences drastic evolution in many of its tribological parameters, such as surface roughness, wear rate, and coefficient of friction until steady-state is attained. In this paper, we present a model for predicting the behavior of the running-in process. Specifically, we determine a general relationship between the wear loss and surface roughness during the running-in stage and test the validity of its prediction of wear rate by comparing to available experimental results. We show, by using a dimensional analysis and applying the Buckingham Pi theorem, that there exists a linear relationship between the transient dimensionless wear, the dimensionless initial surface roughness, and dimensionless running-in time.

The role of load on wear mechanisms in high temperature sliding contacts

The role of contact pressures in high temperature, high speed sliding wear has been studied in the context of alignment plates for the hot rolling of steel. To this end, a lab-scale simulation of the process was performed under varying normal loads, using several iron-based materials to be implemented in the real field application. The resulting wear rates and friction coefficients were compared, and it turned out that applied loads exerted no discernible effect on wear rates at room temperature, while leading in some cases to significant wear decrease at temperatures equal or higher than 500°C. This was attributed to a tribolayer formation influenced not only by sample temperature but also by applied load. Running-in stage duration was also observed to be related to decreased wear rates for several of the materials.

Effects of surface texturing on the tribological behavior of piston rings under lubricated conditions

Purpose – The piston ring-cylinder liner pair is one of the most important tribological systems of an internal combustion engine. The friction loss of the piston ring-cylinder liner pair accounts for the largest portion of total efficiency losses. Therefore, improving the tribological system design of the piston ring-cylinder liner pair can reduce friction losses and bring tremendous economic benefits to society. This paper aims use surface texturing, which is proving to be an effective method, for improving the tribological performance of sliding surfaces. Design/methodology/approach – In this paper, an experimental study using a pin-on-disk tribometer was carried out to evaluate the effects of surface texturing on friction reduction of piston rings under various loads and sliding velocities. Rectangular- and circular-shaped textures with different depths and area densities were produced by a Femtosecond laser. Comparison experiments were conducted with un-textured rings. Findings – The results indicate that the friction performance of the ring surface was significantly improved by surface texturing, and the running-in stage was also shortened. More specifically, it was found that the rectangular-shaped texture had a better effect on friction reduction than the circular-shaped texture. Results also indicate that an optimum texture density existed for the rectangular-shaped texture. Additionally, it was observed that the average friction coefficient reduction of the textured ring decreased with increasing load and increased with increasing sliding velocity. Originality/value – Consequently, these findings provide a more in-depth understanding of the relationship between micro-textures and tribological properties of piston rings in lubricating sliding.

Tribo-films control in adaptive TiAlCrSiYN/TiAlCrN multilayer PVD coating by accelerating the initial machining conditions

Abstract A nano- multilayer TiAlCrSiYN/TiAlCrN was deposited by Physical Vapor Deposition (PVD) on cemented carbide turning inserts. Assessment of the performance of the coated inserts in machining of DA718 Inconel was made at various cutting speeds during the initial, running-in stage of wear. Three types of machining conditions were used: i. Regular cutting speed (40 m/min) used in industrial practice; ii. Higher cutting speed of 60 m/min, and iii. Accelerated (higher speed followed by regular cutting speed). Comprehensive characterization of the tribofilms formed on the surface of the worn cutting tools was made using Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy analyses. SEM/EDS elemental mapping was used for evaluation of the wear patterns. It was shown that an initial short-term increase in the cutting speed during the running-in stage (condition iii) noticeably improves tool life. This is because during initial cutting at high speed, enhanced formation of protective/lubricious tribo-ceramic films on the friction surface takes place, with the subsequent slowdown in speed preventing total wearing out of the beneficial tribofilms. In this way, the tribofilm formation process can be enhanced at the start of the process and the benefits of these films can be realized over the life of the tool.

Fractal properties of worn surface of Fe-based alloy coatings during rolling contact process

The rolling contact fatigue (RCF) failure procedure of Fe-based alloy coating, fabricated by high efficient plasma spray (PS) technology, was investigated by a double-roller test machine with oil lubrication under pure rolling contact condition. The fractal dimension (FD) was utilized to quantitatively characterize the profile of the worn surface at different experiment stage and the failure mechanism of the coating was discussed. The results indicated that the nonlinear morphologies of the worn surface of Fe–Cr alloy coating possessed excellent fractal properties. The failure procedure could be divided into four stages according to the value and change rule of FD, i.e. (1) running-in stage, (2) stable abrade stage, (3) accelerated damage stage, (4) unstable removal stage.

Dry friction and wear performance of co-continuous Al-23Si/SiC composites

The dry friction and wear behaviour of co-continuous Al-23Si/SiC composites were evaluated using a MMD-1 wear testing machine under the testing conditions of various sliding distance, load and rotation rate. The worn surfaces as well as the wear debris were photographed using optical microscope and scanning electron microscope. The experimental results show that the wear behaviour of the composites contains running-in stage and steady state, the wear mass loss increases in accordance with the growth of sliding distance, load and rotation rate. The composites whose SiC framework is with small pore have better wear resistance than that of the one with big pore. The composites exhibit superior friction and wear performance over the matrix material. The wear mechanisms of co-continuous Al-23Si/SiC composites are adhesive wear, abrasive wear, surface fatigue wear and oxidation wear. The wear mechanisms of matrix material are severe adhesive wear and oxidation wear.

Monofractal and multifractal behavior of worn surface in brass–steel tribosystem under mixed lubricated condition

The dynamic evolution law of brass surface in brass–steel tribopair during wear process is studied by using monofractal and multifractal methods. Wear tests are performed on a ring-on-disc tribometer under mixed lubrication. The profile of brass surface is measured by the reposition-measurement apparatus. Both the maximum peak-to-valley height Rt and the width of multifractal spectrum Δα decrease in running-in wear stage, maintain at a small value in steady wear stage, and increase rapidly in severe wear stage. While the evolution of fractal dimension is opposite to that of Rt and Δα. The results are instructive to wear state identification.