Effect of temperature on tribological performance of a silicon nitride ball material with a linear perfluoropolyalkylether

The effect of temperature in rolling contact performance of a hot isostatically pressed (HIP) silicon nitride ball material with a linear perfluoropolyalkylether (PFPAE) was studied using a ball-on-rod type rolling contact fatigue tester. The test temperature ranged from ambient to 343°C for a period of 24 h at a stress of 5.5 GPa using thin dense chrome (TDC)-coated T-15 bearing races. The lubricant and its decomposition products, specifically acid fluoride and acids, attacked Si₃N₄ balls at all test temperatures resulting in corrosion pitting. The presence of metal fluoride on all the Si₃N₄, transferred from the races, was detected by X-ray photon spectroscopy (XPS). The thickness of the oxide layer formed on the balls, as determined by Auger electron spectroscopy (AES) increased with temperature. The changes in physical properties of post-test lubricant showed that the lubricant was stable at temperatures up to 288°C. The change in viscosity was constant up to 288°C and with a significant change above 288°C. The FTIR analysis of 316 and 343°C post-test lubricant showed the presence of carboxylic acid. The total acid number (TAN) increased linearly up to 288°C and accelerated at 316 and 343°C. The study indicates that the use of Si₃N₄ balls with a linear PFPAE results in an incompetent tribo system.     

Effect of Molecularly Thin Lubricant on Roughness and Adhesion of Magnetic Disks Intended for Extremely High-Density Recording

For extremely high-density recording using conventional technologies, the fly-height needs to decrease to less than ten nanometers. To allow such operation, disk and slider surfaces must become extremely smooth, down to root-mean-square (RMS) roughness values of a few angstroms. For super-smooth disks, molecularly thin lubricants are applied to improve tribological performance of head/disk interfaces. The focus of this study is to quantify the effect of lubricant thickness in terms of detailed roughness parameters and to evaluate the effect of roughness and molecularly thin lubricant on adhesion of magnetic disks intended for extremely high-density recording. Three identical ultra-low-flying disks have been fabricated from the same batch for this particular experiment. To investigate the effect of molecularly thin lubricants on disk roughness, super-smooth magnetic disks with increasing lubricant thickness have been measured and studied, using a primary roughness parameter set. It describes amplitude, spatial, hybrid, and functional aspects of surface roughness and is used to quantify the extremely smooth disk roughness as a function of lubricant thickness. It is found that in addition to simple amplitude parameters, hybrid and functional parameters also capture small features on the disk roughness and show distinct trends with increasing lubricant thickness. Subsequently, a continuum-based adhesion model that uses three parameters from the primary roughness parameter set, is used to predict how the varying thickness of molecularly thin lubricant and the resulting disk roughness affect intermolecular forces at ultra-low-flying head-disk interfaces. It is found that a thicker lubricant layer of 2nm causes higher adhesion forces for ultra-low-flying-heights in the range of 1–3 nm     

Ultra-low-flying-height design from the viewpoint of contact vibration

The design of a head-disk interface for ultra-low flying height has been studied from the viewpoint of contact vibration. It is known that a super-smooth disk is necessary for a slider to fly at an ultra-low flying height; however, such a disk increases the friction force, which potentially increases the vibration of the slider. To solve this problem, the head-disk interface must be optimized to reduce this increased vibration. It has been shown that a large pitch angle and center-pad-mounted read/write elements have advantages in terms of slider/disk contact. It has also been found that a micro-texture on the air bearing surface can prevent contact vibration. Moreover, a frequency-shift-damping slider was found to damp the vibration effectively. To further investigate these findings, numerical simulation and modeling of slider dynamics during contact have been performed. Their results revealed two zones of contact vibration: a stable zone and an unstable zone.     

Pulse Train Microprocessor Programmer for NMR Relaxometers-Diffusiometers

A pulse train programmer for a spin–echo NMR relaxometer–diffusiometer is based on an AT89C51-24PC microcontroller. The programmer ensures the arbitrary generation of pulse trains with a minimum train step of 30 s and recording of up to 30000 10-bit amplitudes of spin–echo NMR signals with a minimum quantization step of 3 s. The all-in-one one-board programmer is connected to a computer serial asynchronous data port, which is compatible with NMR relaxometer–diffusiometers of almost any design.     

Effect of self-assembled monolayers modified slider on head-disk tribology under volatile organic contamination

Self-assembled monolayers (SAMs) with different endgroups were established on slider surface. The effect of the SAMs coated slider on head-disk tribology under volatile organic contamination (VOC) of octamethylcyclotetrasiloxane (D₄) was investigated using a contact start/stop (CSS) tester. The slider surfaces before and after the CSS tests were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). The contact angle measurement and TOF-SIMS analysis proved that the SAMs were successfully formed on the slider surface. All the SAMs reduced the friction under the pollutant vapor. The transfer of lubricating oil onto the slider surface was detected after the CSS tests. It was found that the slider with a low surface free energy associated with small amount of lubricating oil transfer. The little the lubricating oil transfer was, the low the frictions were. These results indicate that a slider with low surface free energy can reduce the loss of lubricating oil from the disk surface, and hence improve the tribological properties of hard-disk interface (HDI) under VOC.     

Temperature Modeling in a Total Knee Joint Replacement Using Patient-Specific Kinematics

This paper reports the implementation of a computer modeling approach that uses fluoroscopically measured motions of total knee replacements as inputs and predicts patient-specific implant temperature rises using computationally efficient dynamic contact and thermal analyses. The multibody dynamic simulations of two activities (gait and stair) were generated from the fluoroscopic data to predict contact pressure and slip velocity time histories for individual elements on the tibial insert surface. These time histories were used in a computational thermal analysis to predict average steady-state temperature rise due to frictional heating on each element. For the standard condition, which assumes an ultra-high molecular weight polyethylene (UHMWPE) tibial component and cobalt-chrome femoral component, 1Hz activity frequency, friction coefficient of = 0.06, and convective heat transfer coefficient of h = 30 (W/(m²·K)), the predicted maximum temperature rise on the medial compartment was 9.1 and 14°C for continuous activities of gait and stair respectively. The sensitivity of the temperature rise to activity rate, heat partitioning to the femoral component, and convective heat transfer coefficient was explored. The model is extremely sensitive to the thermal properties of the femoral component and predicts order of magnitude changes in contact temperature with order of magnitude changes in thermal conductivity. A survey of thermal conductivity for current and proposed scratch resistant femoral component implant materials shows variations greater than an order of magnitude.     

Deterministic Friction Model of a Rough Surface Sliding Against a Flat Layered Surface

In this paper, a layered surface is modeled like a solid that has effective mechanical properties (E _eff(), _eff() and H _eff()) as a function of indentation depth () and the rough surface is modeled as a population of spherically shaped asperities with different radii and heights (not necessarily Gaussian distributed). The contact behavior and the resistant to motion experienced by each asperity is analyzed locally and summarized as the total friction force based on the adhesion and ploughing mechanisms. The present model extends the capability of Halling's model to predict friction of layered surfaces. With this model, one is able to predict the friction of soft layer on a hard substrate and hard layer on a soft substrate in contact with a rough counter surface.     

Tribological study on the surface modification of metal-on-polymer bioimplants

The tribological performance of artificial joints is regarded as the main factor of the lifespan of implanted prostheses. The relationship between surface roughness and coefficient of friction (COF) under dry and lubricated conditions is studied. Results show that under dry test, friction coefficient is not reduced all the time with a decrease in surface roughness. On the contrary, a threshold of roughness value is observed, and frictional force increases again below this value. This critical value lies between 40 and 100 nm in S_a (roughness). This phenomenon is due to the transfer of friction mechanisms from abrasion to adhesion. Under wet test, COF always decreases with reduction in surface roughness. This result is mainly attributed to the existence of a thin layer of lubricant film that prevents the intimate contact of two articulating surfaces, thus greatly alleviating adhesion friction. Furthermore, surface texturing technology is successful in improving the corresponding tribological performance by decreasing friction force and mitigating surface deterioration. The even-distribution mode of texturing patterns is most suitable for artificial joints. By obtaining the optimal surface roughness and applying texturing technology, the tribological performance of polymer-based bioimplants can be greatly enhanced.     

Tribological characteristics of silicon nitride at elevated temperatures

A sliding friction-and-wear test for silicon nitride (Si₃N₄) was conducted using a ball-on-disk specimen configuration. The material used in this study was HIPed silicon nitride. The tests were carried out from room temperature to 1000°C using self-mated silicon nitride couples in laboratory air. The worn surfaces were observed by SEM and the debris particles from the worn surfaces were analyzed for oxidation by XPS. The normal load was found to have a more significant influence on the friction coefficient of the silicon nitride than an elevated temperature. The specific wear rate was found to decrease along with the sliding distance. The specific wear rate at 29.4 N and 1000°C was 292 times larger than that at room temperature. The main wear mechanism from room temperature to 750°C was caused by brittle fracture, whereas from 750 to 1000°C the wear mechanism was mainly influenced by the oxidation of silicon nitride due to the increased temperature. The oxidation of silicon nitride at a high temperature was a significant factor in the wear increase.     

An Infrared Pyrometer for Monitoring the Temperature of Materials in Vacuum Systems

A pyrometer of IR radiation for monitoring the melting and strengthening temperatures of metals in vacuum systems is described. A diaphragmed optical system is used, which ensures the required spatial resolution and protects the pyrometer from the vaporizing metal. The measured temperature range is 20–1200°C, and the measurement accuracy is 2% at a time constant of 1 s.     

Improving the Anti-seizure Ability of SiC Seal in Water with RIE Texturing

The demand for higher pressure and higher speed of sealing systems is currently increasing due to stricter standards for permissible emissions. Silicon carbide (SiC) is thought to be a promising material for sealing systems operating in water, since the combination of SiC and water is both environmentally friendly and energy saving. The purpose of this study is to improve the anti-seizure ability of SiC seals working in water by means of surface texturing. The texture pattern of micro-pits evenly distributed in a square array is formed on one of the contact surfaces by reactive ion etching (RIE). Experiments which simulate the working condition of mechanical seals were carried out to evaluate the effect of micro-pits on the critical seizure load. It is found that micro-pit texturing is an effective way to stabilize friction, to reduce the friction coefficient, and to expand the low-friction range ( < 0.05) of SiC seals working in water.     

Study of the Interaction of ZDDP and Dispersants Using X-ray Absorption Near Edge Structure Spectroscopy—Part 1: Thermal Chemical Reactions

The interactions of ZDDP and different dispersants have been investigated both in oil solutions and on steel substrates at 150–185°C. X-ray absorption near edge structure (XANES) spectroscopy at P and S L-edge and K-edge has been used to identify the chemical species both in solution and on the surface of the steel. It was found that noticeable ZDDP decomposition in solution starts at 175°C when no dispersant is present. In contrast, thermal oxidative films begin to form on the steel at 150 °C in the same solution. The products of decomposition in solution and in the film are phosphates and sulfides. N K-edge XANES spectroscopy has also been utilized to identify the reaction of the dispersant with the steel surface and ZDDP. The results show that dispersants enhance the decomposition of ZDDP in oil solutions as well on the steel surface. Dispersants, on their own, react/adsorb with the steel surface at 150–175 °C and also interact with ZDDP to form new products. Depending on the composition of the dispersants, the product is different.     

A Two-Section Gas-Filled Ionization Chamber for Neutron Flux Measurements

A promising design of a -type two-section gas-filled ionization chamber is described for the first time. A relationship between the design parameters, gas pressure, and characteristics of the material is determined, under which full compensation of the background currents from the -radiation and the measurement of thermal neutron fluxes in a range of 400 to 7 ×10⁹cm^–2s^–1at a load characteristic with a 5% nonlinearity are provided. Test results of the chamber are presented.     

A novel representation of tandem systems with inter-stage storage applied to on-line system monitoring

One of the goals of operating a tandem manufacturing system with finite inter-stage storage and asynchronous operations is to meet the demand without over-producing, under-producing or carrying large quantities of material in storage. We believe that analysing the operation of such a system on a real-time basis helps achieve this goal. The first step in this real-time analysis would be to quantitatively associate the causes and effects of over-production or under-production as they occur. This requires determining the cumulative effect that the performance that any stage has on the system, based on its history, the current system state and the interrelationships between the stages. This paper proposes a method which first represents uniquely and completely each stage and surrounding storage as an element. While this system, which consists only of this simple type of element functions in exactly the same way as the original one, each element is put into an ideal world for decoupled measurement. Though an element behaves in exactly the same way whether it is in the ideal world or in the real world, the elapsed times in the two worlds since the beginning of production can be different, since the responses (occurrences and durations of the blockings and starvations) of the two worlds can be different. A phase parameter is introduced for each element to represent this difference. Once the formation of the phase parameter of the output element is formulated, quantitative relationships between causes and effects of over-producing or under-producing can be explained as they occur.     

Hyperthermal Atomic Oxygen Interaction with MoS<Subscript>2</Subscript> Lubricants Relevance to Space Environmental Effects in Low Earth Orbit—Atomic Oxygen-Induced Oxidation

Effect of 5 eV atomic oxygen beam exposure on the surface properties of sputter-deposited and single-crystal molybdenum disulfide (MoS₂) were evaluated in the light of space environmental effects in low earth orbit. X-ray photoelectron spectra indicated that the loss of sulfur from the atomic oxygen exposed MoS₂ surface was significant, especially for the sputter-deposited samples. This is due to the formation and gasification of the volatile product, SO. It was also identified that Mo atoms at the surface were oxidized to form MoO₃. The amount of oxygen increased within a depth of 22nm from the surface, whereas loss of sulfur was only observed within 3nm. It was thus concluded that the chemical change of MoS₂ due to atomic oxygen attack is limited to the surface layer of the MoS₂-sputtered lubricant.     

A Solar Spectromagnetograph

A new solar spectromagnetograph for measuring the full magnetic-field vector and line-of-sight velocities is described. A new version of a polarization analyzer ensuring parallel measurements of six polarization components of spectral lines is considered. The spectromagnetograph allows the use of any algorithms for obtaining the magnetic fields vector, in particular, the Babcock algorithm and the Fourier transform technique. The sensitivity of the instrument for the longitudinal and transverse magnetic field is 3–5 and 20–30 G, respectively, and 10 m/s for the line-of-sight velocities.     

Tribochemistry of ZDDP and MoDDP chemisorbed films

By means of a UHV analytical AES/XPS pin-on-flat tribotester, we have investigated tribochemical change as well as transfer mechanisms in static reaction films on steel from ZDDP (anti-wear) and MoDDP (friction modifier) lubricant additives. The results show that solid ZDDP reaction films are able to strongly reduce friction to a value of 0.1–0.3 (in the absence of a film, steel-on-steel gives a friction coefficient value of up to 2). MoDDP reaction films produce ultra-low friction (below 0.05) after an induction period. In situ AES analysis and energy-filtered XPM gave definitive evidence of frictioninduced chemical changes and selective material transfer from the flat to the pin. In the case of MoDDP, the thiophosphate film is decomposed and a MoS₂ residual film is found in the wear scar on the pin.A lecture based on this paper was presented at the Satellite Forum on Tribochemistry, International Tribology Conference, Yokohama 1995, organized by the Tribochemistry Research Committee, Japanese Society of Tribologists.     

Sliding Friction and Wear Characteristics of Novel Graphitic Foam Materials

While the use of ORNL developed high-thermal-conductivity graphitic foam materials has been focused on thermal management applications, recent experiments have revealed their potential as bearing surfaces as well. The three primary tribological advantages are: (1) they can efficiently remove frictional heat, (2) their natural porosity can trap wear debris, and (3) their porosity can serve as a lubricant reservoir for the contact surface. A series of pin-on-disk experiments were conducted at both room temperature and 400 °C to compare the sliding friction and wear characteristics of the densified graphitic foam (mated against M-50 tool steel or against alumina) to those of conventional bearing materials like graphite, bearing bronze, poly- tetrafluoroethylene, bearing steel, and a Co-based superalloy. At room temperature and under low contact pressure, the tribological behavior of the densified graphitic foam material was comparable to that of graphite and better than that of other bearing materials. At 400 °C, traditional graphite exhibited a dusting wear regime accompanied by a high friction coefficient. In contrast, the graphitic foam demonstrated an ability to maintain low friction and wear at that temperature.     

Time-of-Flight Trigger Based on a Time-to-Amplitude Converter

A time-of-flight trigger based on a time-to-amplitude converter and differential discriminator is described. The trigger has a short decision time (60 ns) and high (100%) efficiency of useful event selection.     

An Energy Source Based on Helical Explosion–Magnetic Generators for High-Impedance Loads

A transportable standard module developed at the Institute of Experimental Physics as a component of an energy source for modeling current pulses of positive lightning is described. The source is designed as a combination of several unified helical explosion–magnetic generators with output transformers and an inner diameter of the stator of 200 mm. According to our calculations, the source generates current pulses with an amplitude of up to 100 kA in loads with an inductance of 100 H and a resistance of several tens of ohms. The results from testing a typical module operating with a circuit having an inductance of 100 H and a resistance of 4 are presented. The experimental data coincide well with the results of a mathematical simulation of the module's operation.