Fretting wear of TiN PVD coating under variable relative humidity conditions – development of a “composite” wear law

Fretting is defined as a small oscillatory displacement between two contacting bodies. The interface is damaged by debris generation and its ejection from the contact area. The application of hard coatings is an established solution to protect against fretting wear. For this study the TiN hard coating manufactured by a PVD method has been selected, and tested against a polycrystalline alumina smooth ball. A fretting test programme has been carried out at a frequency of 5 Hz, 100 N normal load, 100 μm displacement amplitude and at five values of relative humidity: 10, 30, 50, 70 and 90% at a temperature of 296 K. The intensity of the wear process is shown to be significantly dependent on the environmental conditions. A dissipated energy approach has been employed in this study to quantify wear rates of the hard coating. The approach predicts wear kinetics under constant medium relative humidity in a stable manner. It has been shown that an increase of relative humidity promotes the formation of hydrate structures at the interface and modifies the third body rheology. This phenomenon has been characterised by the evolution of wear kinetics associated with a significant variation of the corresponding energy wear coefficient. Hence, a ‘composite’ wear law, integrating the energy wear coefficient as a function of relative humidity, is introduced. It permits a prediction of wear under variable relative humidity conditions from 10 to 90% within a single fretting test. The stability of this approach is demonstrated by comparing various variable relative humidity sequences

Effect of test conditions on the temperature at which a protective debris bed is formed in fretting of a high strength steel

It is well known that mechanisms and rates of fretting wear of many metals are dependent upon the temperature of the environment; specifically, it is known that a transition temperature exists, above which the debris forms a protective bed in the contact which results in very low rates of wear being observed. This paper seeks to investigate the influence of contact geometry and slip amplitude on the transition temperature of a high strength alloy steel, and to understand these effects in terms of debris retention in (or expulsion from) the contact. Cylinder-on-flat fretting tests were performed at temperatures between 25 °C and 250 °C with two displacement amplitudes (25 μm and 100 μm) and two cylinder radii (6 mm and 160 mm). It was found that for the smaller cylinder radius, the transition temperature increased as the fretting displacement amplitude was increased. However, it was found that whilst the contacts with 6 mm radius cylinders and 160 mm radius cylinders exhibited very different mechanisms of wear at low temperature, the temperature at which the transition to forming of the protective debris bed was not strongly influenced by the contact geometry; moreover, at the higher temperature, the protective bed is formed irrespective of contact geometry. It is proposed that the reduction in wear rate at higher temperatures is associated with the retention of oxide debris within in the contact area for long enough that it sinters to form a protective ‘glaze’ layer. By increasing the displacement amplitude, the rate at which the oxide is ejected from the fretting contact increases and this reduces the ability to form a protective layer; as such, a higher temperature is required to form the protective glaze as the displacement amplitude is increased

The role of frictional power dissipation (as a function of frequency) and test temperature on contact temperature and the subsequent wear behaviour in a stainless steel contact in fretting

Temperature is known to affect the fretting wear behaviour of metals; generally, a critical temperature is observed, above which there are substantial reductions in wear rate, with these being associated with the development of protective oxide beds in the fretting contact. This work has examined the gross sliding fretting behaviour of a stainless steel as a function of bulk temperature and fretting frequency(with changes in the fretting frequency altering the frictional power dissipated in the contact amongst other things). An analytical model has been developed which has suggested that at 200 Hz, an increase in the contact temperature of more than 70°C can be expected, associated with the high frictional power dissipation at this frequency (compared to that dissipated at a fretting frequency of 20 Hz). With the bulk temperature at either room temperature or 275°C, the increase in contact temperature does not result in a transition across the critical temperature (and thus fretting behaviour at these temperatures is relatively insensitive to fretting frequency). However, with a bulk temperature of 150°C, the increase in temperature associated with the increased frictional power dissipation at the higher frequency results in the critical temperature being exceeded, and in significant differences in fretting behaviour

The role of geometry changes and debris formation associated with wear on the temperature field in fretting contacts

The temperature of a fretting contact is known to be a key factor in its development. However, as a test proceeds, the wear scar changes, both geometrically and through the formation of oxide-based debris-beds. Accordingly, the effects of these on the near-surface temperature field resulting from frictional heating in fretting has been analysed via numerical modelling. Under the test conditions examined, it was predicted that (i) the development of the wear scar geometry would result in a significant (up to ~ 25%) reduction in the mean-surface temperature rise, and (ii) the formation of a typical oxide debris bed would result in a significant (up to ~ 80%) increase in the mean-surface temperature rise

Usure par fretting d'un contact acier inoxydable / acier inoxydable sous hautes temperatures

Fretting wear is perceived as a plague in many industrial fields (aeronautics, nuclear engineering, etc.). It is defined as a small displacement amplitude oscillatory motion between two contacting bodies. In this study, Jethete M152 steel was tested against A-286 steel under low pressure gross slip fretting conditions throughout a wide range of constant and variable temperatures from 23 ◦C to 400◦C. Friction and wear quantifications using a dissipated energy approach were carried out. Tests performed under constant temperature conditions show tribological discontinuities around 220 ◦C. Above this threshold temperature (Tt) a decrease of the friction coefficient and wear rate is observed, which is related to a significant modification of the interface. At lower temperatures severe adhesive wear dominates whereas at higher ones a third body abrasive process, involving the formation of a compliant glaze layer structure takes place. Various structural analyses including Raman spectroscopy confirm the hypothesis that the glaze layer structure is activated through a tribo-sintering process of oxidized wear particles. Besides, it is shown that the energy wear approach allows the wear kinetics to be determined. However, two different energy wear coefficients were identified depending on the temperature range. This confirms the idea that each wear mechanism is characterized by a specific wear rate. Variable temperature test conditions consisting of repeated sequences at 200 ◦C and 400 ◦C demonstrate a rather high reactivity of the interface regarding the evolution of the wear kinetics. Finally, a ‘composite’ wear law integrating the energy wear coefficient as a function of the temperature is introduced and its stability evaluated.L'usure par frottement est perçue comme un fléau dans de nombreux domaines industriels (aéronautique, nucléaire, etc.). Il est défini comme un mouvement oscillatoire de faible amplitude de déplacement entre deux corps en contact. Dans cette étude, l'acier Jethete M152 a été testé par rapport à l'acier A-286 dans des conditions de fretting en glissement total sous faibles pressions de contact et sur une large plage de températures constantes et variables allant de 23 ◦C à 400 ◦C. La cinétique de l’usure est quantifiée à partir d’une approche énergétique (énergie de frottement cumulée). Des essais réalisés dans des conditions de température constante montrent des discontinuités tribologiques autour de 220 ◦C. Au-dessus de cette température seuil (Tt) on observe une diminution du coefficient de frottement et du taux d'usure, liée à une modification importante de l'interface et la mise en place d’une couche protectrice dite « glaze layer ». À des températures plus basses, une forte usure adhésive est observée, Diverses analyses structurelles, notamment la spectroscopie Raman, confirment l'hypothèse selon laquelle la structure de la couche de vernis est activée par un processus de tribo-frittage de particules d'usure oxydées. Par ailleurs, il est montré que l’approche énergétique de l’usure permet de déterminer la cinétique d’usure. Cependant, deux coefficients d’usure énergétique différents doivent être considérés en fonction de la plage de température. Ceci confirme l'idée selon laquelle chaque mécanisme d'usure est caractérisé par un taux d'usure spécifique. Les conditions de test à température variable consistant en des séquences répétées à 200 ◦C et 400 ◦C démontrent une réactivité assez élevée de l'interface concernant l'évolution de la cinétique d'usure. Enfin, une loi d’usure « composite » intégrant le coefficient d’usure énergétique en fonction de la température est proposée et discutée

Study of the properties of optimum spectral filtration of optical radiation with the use of the normed Kullback-Leibler divergence

Побудована математична модель процесу оптимальної динамічної спектральної фільтрації в оптикоелектронних системах, яка дозволяє дослідити властивості оптимальної обробки оптичних сигналів та перевірити працездатність інформаційного критерію для оцінювання її узгодженості. Приведені результати математичного моделювання оптимальної додетекторної обробки та визначено ступень її узгодженості для трьох випадків: відсутнє розузгодження між вхідними та опорними сигналами; змінилось середнє значення яскравості вхідних оптичних сигналів об’єкта та фону; відбулись зміни в спектральному складі вхідного оптичного сигналу фону.Построена математическая модель процесса оптимальной динамической спектральной фильтрации в оптикоэлектронных системах, которая позволяет исследовать свойства оптимальной спектральной фильтрации, а также проверить работоспособность информационного критерия для оценки ее согласованности. Приведены результаты математического моделирования оптимальной додетекторной обработки и определена степень ее согласованности для трех случаев: отсутствует рассогласование между входными и опорными сигналами; изменилось среднее значение яркости входных оптических сигналов объекта и фона; произошли изменения в спектральном составе входного оптического сигнала фона.The article is devoted to the study of the properties of the optimal dynamic spectral filtration in target detection electrooptical systems. The efficiency of optimal processing of optical signals depends on how correspondent the spectral characteristics of the input emissions of the target and the background to the information contained in the database are. Therefore, studies aimed at assessing the degree of signal matching in systems with dynamic spectral filtering are relevant. An information criterion the normalized Kulbak-Leibler divergence is used to assess the matching of optimal signal processing. The purpose of the article is to study the efficiency of the information criterion in order to estimate the matching of optimal processing of optical signals through mathematical modeling of the dynamic spectral filtration of radiation in electro-optical systems. Using the normalized Kulbak-Leibler divergence, the properties of optimal pre-detection processing of optical signals are studied. The examples show to what extent the differences in the statistical characteristics of input and reference signals affect the information matching indicator. In particular, the article poses and solves three problems that illustrate the process of optimal spectral filtration in the following situations: 1) the statistical properties of the input and reference signals coincide completely; 2) the mean brightness of the optical radiation of the target and the background at the input of the electro-optical system has changed; 3) the statistical characteristics of the background input signal differ from the reference values

Recovery of image elements lost by low spatial resolution of multispectral electro – optical systems

Описан метод частичного восстановления элементов изображений целей, утраченных в результате низкого пространственного разрешения оптоэлектронных систем, в случае, когда фон и объект представляют собой однородные, но различные по своему спектральному составу изображения. Проведено математическое моделирование метода восстановления элементов изображений. Показано улучшение сходства по геометрическим признакам обработанных и эталонных изображений.Викладено метод часткового відновлення елементів зображень цілей, втрачених в результаті низької просторової роздільної здатності оптоелектронних систем при умові, коли фон та об’єкт являють собою однорідні, але різні за своїм спектральним складом зображення. Проведено математичне моделювання методу відновлення елементів зображення. Показано поліпшення подібності за геометричними ознаками оброблених та еталонних зображень.The article describes the method of partial recovery of target image elements, lost as the result of low spatial resolution of electro-optical system, on condition, that background and target are homogeneous, but various by their spectral composition. It represents the mathematical modeling of the method how to restore the elements of images. It shows the improvement of similarity based on dimensional features of the processed and master images