Determination of the life and current stress state of elements of power equipment from parameters of the operating load with the use of methods of mathematical statistics

A method of constructing a system for recording the current stress-strain state and life by means of methods of mathematical statistics and on-site investigation of stresses is shown. The results of strain measurements are analyzed for examples of various objects of thermal and nuclear power stations, illustrating the possibility of creating such a system.Translated from Problemy Prochnosti, No. 12, pp. 3–7, December, 1990.     

The Contribution of Thin PFPE Lubricants to Slider-Disk Spacing. 2. Effect of Film Thickness and Lubricant End Groups

We have investigated the effect of film thickness of a perfluoropolyether lubricant, Zdol 4000, on slider-disk spacing, or clearance. The major conclusion of this work is that the Zdol lubricant impacts the slider-disk spacing. A decrease in the film thickness of Zdol 4000 by 10Å results in an increase in the effective slider-disk spacing by 1 nm. The effect of end groups on slider-disk spacing is also investigated using the Zdol-TX and Z-Tetraol perfluoropolyether lubricants. At similar molecular weights and film thicknesses, the end groups do not affect the slider-disk spacing by causing any additional interference. The slider-disk interference is therefore attributed to the attractive van der Waals interaction between the slider and the disk surface that causes the Zdol main chain to interact with the slider surface, giving rise to a friction force.     

Autophobic Dewetting of Perfluoropolyether Films on Amorphous-Nitrogenated Carbon Surfaces

The dewetting of perfluoropolyether (PFPE) films on amorphous nitrogenated carbon, CNx, is investigated. An optical surface analyzer is used to image perfluoropolyether films on CNx-overcoated magnetic recording disks. An autophobic dewetting transition is observed to result when the PFPE film thickness applied to the disk surface exceeds a critical value. This critical dewetting thickness is linearly dependent on the PFPE molecular weight. Addition of the phosphazine, X-1P, to the PFPE film reduces the critical dewetting thickness compared to that of the neat lubricant. Dewetting in these molecularly-thin PFPE lubricant films is shown to occur at thicknesses where the total disjoining pressure is negative. The impact of this autophobic dewetting on the performance of a head--disk interface is inferred from take-off height measurements conducted as a function of PFPE film thickness. A steep reduction in the slider--disk clearance is observed when the PFPE film is present at thicknesses in excess of the critical dewetting thickness.     

The Tribological Properties of a New Cyclotriphosphazene-Terminated Perfluoropolyether Lubricant

We have investigated the tribological properties of a novel perfluoropolyether (PFPE) lubricant truncated on one end by a hydroxyl group and on the other end by a cyclotriphosphazene derivative. A measurement of the friction force as a function of molecular weight indicates that the dynamic clearance between the slider and the disk can be reduced by ～1.5 nm by decreasing the molecular weight from 5300 to 2400 g/mol. However, the thermodynamic film stability of the novel PFPE lubricants, as determined by surface energy measurements and ellipsometric imaging of lubricant dewetting, becomes increasingly unstable at lower film thicknesses with decreasing molecular weight. Measurements conducted on lubricant mobility indicate that the novel PFPE lubricants are relatively immobile compared to the Zdol perfluoropolyether lubricants and hence resist film thinning to a greater degree. These data provide the direction for the optimization of the molecular weight of these novel PFPE lubricants.     

Roughness effects on head–disk interface durability and reliability

This paper deals with the tribological reliability and durability issues in modern magnetic recording hard disk drives, where the slider flies at typically less than 10 nm. Specifically, we investigate the effect of disk surface roughness on the above performance characteristics. The durability of the interface is investigated using low-pressure on-track tests. The drive-level reliability data are presented to confirm general conclusions reached from the component-level testing. Component-level slider–disk clearance measurements are also used to better understand how the roughness affects both durability and reliability. Finally, we discuss possible explanations for the experimentally observed relationships.     

Tribological and mechanical properties of carbon nitride thin coating prepared by ion-beam-assisted deposition

Carbon nitride thin films may become good competitors for diamond-like carbon, due to their high hardness, high wear resistance, and low friction coefficient. At present, there are only a few studies of the effect of CN _x coating hardness and internal stress on its tribological properties, such as coating life and frictional behaviour. This work deals with tribological and mechanical properties of a carbon nitride coating prepared by ion-beam-assisted deposition (IBAD). Friction coefficients in the range of 0.10–0.12 were observed for the best CN _x coatings sliding against silicon nitride under ambient conditions. A nonlinear correlation between coating life and its internal stress and hardness was found.     

Strength, Durability, and Safety of Engineering Systems

The evolution of approaches to the solution of machine dynamics and strength problems on the basis of determining static and dynamic nominal and local stresses of the operating load is discussed. It is shown how the material strength and plasticity characteristics, the cyclic strength characteristics in the high-cycle and low-cycle fatigue regions, the high-temperature long-term strength and creep characteristics, and the linear and nonlinear characteristics of fracture mechanics were sequentially used as the basic criterial parameters of the deformability and strength of structural materials. Particular emphasis was given to the results of studies on machine safety and catastrophe mechanics. Integrated approaches to the solution of strength and safety problems of potentially dangerous facilities (nuclear power stations, spacecraft complexes, aircraft, chemical plants, etc.) by analysis of all stages of their life cycle, including design, manufacture, testing, and operation, are discussed.     

Nano-wear of the diamond AFM probing tip under scratching of silicon, studied by AFM

In order to improve such a widely used microtribological testing procedure as surface scratching by an AFM diamond tip, an experimental study has been carried out using single-crystalline silicon as the tested material. Wear of the AFM diamond tip under scratching was observed by a decrease in the scratch depth with increasing wear cycles and by the direct imaging of the diamond tip shape using a Si₃N₄ AFM tip. It was shown that the current widely used experimental method, which assumes the diamond tip to be non-wearable, introduces uncontrollable error into the obtained values for the tested material's wear rate. The harder the tested material, the larger may be the tip wear, and, therefore, the bigger may be its effect on the obtained wear rate values. The specific wear rates for the diamond tip and a silicon wafer were estimated to be 1.4 × 10^-9 and 4.5 × 10^-4 mm³/(N m), respectively.     

The Effect of Carbon Overcoat Thickness on the Zdol Boundary Lubricant Film

Formation of a tribologically reliable interface between the read-write head and the computer disk in hard-disk drives is accomplished by the use of a thin, wear-resistant carbon overcoat in conjunction with a molecularly-thin perfluoropolyether (PFPE) lubricant film. The intermolecular interactions that develop between the PFPE lubricant and the carbon overcoat govern the adhesion, coverage, and physical properties of the lubricant, e.g. the lubricant structure and mobility. Consequently, the molecular interactions at the lubricant-carbon interface will contribute to the overall tribological performance of the disk-drive. Due to the ever-increasing demands for storage capacity, pressure exists to reduce the separation distance between the read-write head and disk surface. One means of reducing this separation distance is to use thinner protective overcoats on both the head and disk surfaces. In this study the interactions between Fomblin Zdol and both amorphous hydrogenated (CHx) and nitrogenated (CNx) carbon overcoats were investigated as a function of overcoat thickness from 0 to 100Å. The Zdol film structure was probed by titrating the magnetic alloy, the CHx and CNx surfaces with Zdol. The molecular weight dependence of the maximum bonded Zdol thickness on these surfaces is used to deduce structural information on the adsorbed Zdol film. In progressing from CHx to CNx to the magnetic alloy, we find the Zdol boundary layer film to be characterized by an increase in average distance between the PFPE backbone and the surface, or equivalently an increase in the average Zdol monolayer thickness. On the CHx overcoat, Zdol preferentially lies more parallel to the surface, whereas on the magnetic layer, Zdol is oriented more perpendicular to the surface. When these experiments were conducted as a function of carbon overcoat thickness, we found that interaction of Zdol with the field of the underlying magnetic film becomes important at carbon film thicknesses 30Å. The dependence of the Zdol adhesion on carbon overcoat thickness was quantified by determining the Zdol film thickness dependence of both the dispersive and polar components of the Helmholtz free energy. The Zdol bonding kinetics were also studied as a function of carbon thickness.