Hosung Kong

Effect of environment on the tribological behavior of Si-incorporated diamond-like carbon films

An experimental study was performed to discover the effect of environment on the tribological behavior of Si-incorporated diamond-like carbon (Si-DLC) film slid against a steel ball. The films were deposited on Si (1 0 0) wafers by a radio frequency glow discharge of mixtures of benzene and dilute silane gases. Experiments using a ball-on-disk test-rig were performed in vacuum, dry air and ambient air. It was observed that coefficient of friction decreased as the environment changed from vacuum to dry air. Results also showed that low and stable friction related closely to the smoothening of track surfaces and the formation of silicon-rich oxide debris.

Tribological behavior of sliding diamond-like carbon films under various environments

Abstract Tribological behaviors of amorphous diamond-like carbon (DLC) films were experimentally evaluated under various environments using a steel ball-on-disk wear-rig at dry sliding surfaces. The DLC films were prepared on Si wafer by r.f. PACVD method using benzene (C 6 H 6 ). Every test was performed under the normal load 4.9 N and sliding velocity of 0.05 m/s, and the coefficients of friction of DLC films were measured with the contact cycle. When the test was performed under a vacuum, the coefficient of friction of DLC films showed a significant fluctuation with the contact cycles, remarkably accompanied with roll-shaped polymeric wear debris. Another test using thermally-baked DLC films showed that the friction was lower and more stable. Under a humidity controlled air, friction of DLC films was also stable. Tests with dry O 2 and N 2 gas were extended to find any other environmental factors which could affect the tribological behavior of DLC films. It was found that tribological behavior of DLC films was dependent on the formation of friction layers which were mainly affected by the environment. In the final discussion of this work, effects of environment on the tribological behavior of DLC films were combined and discussed in terms of tribo-chemistry of DLC films.

Classification of wear debris using a neural network

Abstract In this work, an evaluation was made to prove the possibility of employing a neural net method for the classification of debris and monitoring of a lubricated contact pair. We trained a neural net to classify the severity of wear into two types from the morphological features of the wear debris. The following procedures were used. First, the shape of wear particles was characterized by Fourier descriptors. The Fourier descriptors were considered as coordinates of a point in multidimensional feature space. A set of points from a cluster, and the location and structure of the cluster depend on the morphology of the wear particles and the current conditions of the contact system. A distance distribution between the debris in the feature space was used to represent the location of the cluster. Second, we trained a back-propagation neural net. To train the neural net, we used the distance distribution corresponding to the different stages of the wear process as an input vector and encoded the wear rate as a desired response. The network was then further trained until the desired error goal was achieved. Finally, we tested the trained neural net. The ability of the neural net method to monitor wear is shown.

The use of color in wear debris analysis

A method and results of classification of metallic wear debris using their color features are presented. The color images of wear debris were used as initial data. The color features of the debris are specified by an HSI color model. Each particle is characterized by a set of statistical features derived from a distribution of HSI color model components. The initial feature set was optimized by principal component analysis. A multidimensional scaling procedure was used for definition of the classification plane. The selected features allow us to distinguish copper-based alloys, red and dark iron oxides and steel particles. A method was proposed of probabilistic decision of class assignment based on analysis of debris coordinates distribution in the classification plane. The obtained results have demonstrated the feasibility of using color features for automated wear particle analysis.

Self-formation of protective oxide films at dry sliding mild steel surfaces under a medium vacuum

The friction and wear at dry sliding steel surfaces under different vacuum conditions have been investigated to understand the mechanisms of wear of mild steel in vacuum. For the test, a ball-on-disk type of wear-rig has been built and implemented, allowing control of sliding speed, load and vacuum. A theoretical analysis of frictional heating at sliding surfaces and an experimental analysis of the influence of oxidation wear under different vacuum conditions are described. Results show that, at a low sliding velocity, a low load and under a medium vacuum, effective, protective oxide films can be formed on the sliding surface in a friction process with a self-regulating way, which results in both very low coefficient of friction (about 0.3) and mild wear. It is found that significant reduction in friction and wear is caused by formation of thin layer of Fe3O4 steel oxides on the surface which are promoted by a high surface temperature and a high plastic deformation at the local surface area, and they are formed of the compaction of wear debris while supporting the bearing load and preventing the direct intermetallic contact on the surface. Oxide films under a high vacuum and at a high sliding velocity are found that they are ineffective to protect the surface against severe wear. Results suggest that there is a critical regime with contact conditions for producing effective, protective oxide films of mild steel.

Evaluation of frictional characteristics for a pin-on-disk apparatus with different dynamic parameters

Frictional characteristics in response to changes of the dynamic parameters of a pin-on-disk apparatus were experimentally and theoretically investigated for dry sliding. The apparatus has been designed and built to allow control of sliding speed, load, stiffness and loading mechanism. Dynamic friction and normal force were measured for various conditions of the system parameters, e.g. different stiffness in the normal direction, loading mechanisms and test materials. For a normal load, mechanisms using a dead weight, a compression-type spring and a pneumatic cylinder were selectively applied. Stiffness in the loading direction was also varied using a spring with different stiffness. Steel, rosin and polytetrafluoroethylene were tested because they have different types of frictional behavior. Test results showed that frictional characteristics at various loading mechanisms were different even though the operating parameters were the same. They are also strongly dependent on both the sliding velocity and the run-out of the disk surface. Effects of the surface run-out on the friction behavior were further evaluated using a tapered steel disk. When the surface run-out was high, the coefficient of friction was computed differently with the data processing. A theoretical analysis for a simple model of a pin-on-disk apparatus showed that surface irregularities, such as surface roughness, waviness and run-out, result in dynamic normal forces which depend on both the dynamic parameters of the system and the sliding velocity.

Wear monitoring based on the analysis of lubricant contamination by optical ferroanalyzer

Abstract Lubricant contamination analysis is one of the common and prospective methods of machine condition monitoring. Wear debris formed in rubbing is a source of valuable information on the wear mechanism and severity, while total oil contamination gives information on oil lubricity. Development of new methods and means of reliable condition monitoring of friction units remains a challenging task for rating the condition of equipment, reducing losses for idle time or failures, and saving lubricants. Despite the fact that direct reading (DR) ferrograph was proposed more than 20 years ago its main principle—accumulation and estimation of deposited particles content—underlies the basis of creation of new effective built-in devices and criteria for condition monitoring. In the presented paper, the principle of operation of the optical ferroanalyzer (OF) jointly developed by V.A. Belyi Metal-Polymer Research Institute of Belarus National Academy of Sciences and Korea Institute of Science and Technology is presented. Optical ferroanalyzer in addition to ferrograph allows us to estimate total contamination of oil, increasing reliability of tribosystem condition monitoring. The results of bench tests of the analyzer are described and example of its application for condition monitoring of engine is shown.

The Effect of Topography on Water Wetting and Micro/Nano Tribological Characteristics of Polymeric Surfaces

The effect of surface topography on the water-wetting nature and micro/nano tribological characteristics of polymer surfaces was experimentally studied. A plasma-treated thin polymer film and ion-beam-treated PTFE (polytetrafluoroethylene) were used as flat specimens. Thin polymer films were deposited on Si-wafer (100) by the parallel-plate plasma technique. The ion-beam-roughening treatment was performed to change the PTFE surface topography using a hollow cathode ion gun under different argon-ion dose conditions in a vacuum chamber. Micro and nano tribological characteristics, water-wetting angles and roughness were measured with a micro-tribotester, SPM (scanning probe microscope), contact anglemeter and profilometer, respectively. The plasma-treated thin polymer film and ion-beam-treated PTFE surfaces were rendered increasingly hydrophobic as a function of the treatment time. The wetting characteristics were strongly dependent on the micro-roughness and the sharpness of the asperities. The tribological characteristics on the nano-scale showed different results from those on the micro-scale. It was noted that the contact-size ratio, especially the ratio of the tip or ball to the average surface roughness at a standard cut-off length, should be considered for evaluating the micro- and nano-scale tribological test results.

Effect of polymeric debris on the tribological behavior of diamond-like carbon films

Abstract Tribological behaviors between diamond-like carbon (DLC) films and steel ball (AISI 52100) were investigated under a vacuum (4 × 10 −3 Pa) and ambient air with controlled relative humidity. The DLC films were prepared on Si wafer by r.f. plasma assisted chemical vapor deposited using benzene (C 6 H 6 ) at a pressure of 1.33 Pa. In the present work, we focused on the friction behavior with increasing number of passes. In high vacuum, the coefficient of friction under the normal load of 4.9 N, decreases from 0.15 to a minimum value of 0.04 at about 200 passes. For higher number of passes, however, the coefficient fluctuated in the range from 0.1 to 0.3. This instability of friction was not observed under higher loads in high vacuum. The present work showed that the oxidation of the DLC surface occurs during the tribo-test even in high vacuum, which results in the formation of polymeric roll-shaped debris. The instability in friction was related to the formation of the polymeric debris. In humid air, the coefficient of friction was measured between 0.15 and 0.2. Fluctuated behavior of friction, however, was not observed in humid air.

A study on the precision wear measurement for a high friction and high pressurized gun barrel by using a diamond indenter

Abstract A precision wear measurement system is developed to measure the wear rate of a high friction and high pressurized gun barrel. The system consists of an indentation apparatus and an optical measuring system. The indentation apparatus has a pair of diamond indenters so that it makes indents on the land of rifling by using a pneumatic system. The speed of indentation is regulated by a control valve to get the same effect of dead weight as that of a hardness tester. The X – Y – θ stage of the measuring system can trace the positions of the indents. The optical measuring system sends the surface image of the diamond indent to the monitor and the video printer. The variation of the diagonal of each indent after the test is converted to the wear depth. The system makes it possible to measure the wear of bore precisely in a short period experiment. It is effectively applied to the gun barrel for predicting its life expectancy.