Key Engineering Materials Vol. 799

Abstract: In the present study, static coefficients of friction of pure and friction modified (FM) polyamide 6 (PA6) polymers against primer-coated steel surfaces were investigated under a series of nominal contact pressures and by considering the influences of water absorption by the polymer, temperature, counter-body surface roughness and lubrication conditions. Under the majority of the test conditions investigated, FM PA6 exhibited lower static friction than pure PA6. Under unlubricated conditions, this was due to the low adhesion of the FM PA6 provided by its friction modifying inclusions; while under lubricated conditions, a combination of softening due to water absorption and decreased adhesion provided by its friction modifiers enabled lower static friction, especially at medium and high contact pressures.

Abstract: The travelling waves ultrasonic motors (TWUM) with nonmagnetic, large torque, high precision and simple structure was driven by frictional force, which had been applied aerospace, intelligent and precise instruments. In order to reduce the total weight of TWUM, the phosphor bronze stator with the biggest density among the all parts of TWUM (8.89 g/cm³) were substituted with light weight polymer or polymer composites. This study designed and prepared one types of low density polyimide (PI) composite (1.41g/cm³) reinforced with carbon fibers (CF) which can reduce the weight of stator over 85%. Importantly, this PI composite can meet the main requirements of TWUM, such as high elastic modulus, wear resistance, and suitable friction coefficient. The output stalling torque of TWUM with CF/PI stator still have 0.22 N·m (18.3% compared with TWUM with phosphor bronze stator). Moreover, this study systematically investigated the mechanical and tribological properties of CF/PI composite. For comparison, the pure PI, polyetheretherketone (PEEK) and polyphenylene sulfide (PPS) were also analyzed to reveal the wear mechanisms. The experimental results indicated that CF/PI had better wear resistance and lower friction coefficient than other polymer sliding against Si₃N₄. This light weight polymer composites would be an ideal candidate to reduce the weight of TWUM, which can broaden the lightweight application in the field of aerospace.

Abstract: The paper is aimed to the methodology for estimation of service life of mechanical engineering components in the case of elastic-plastic contact of surfaces. Well-known calculation methods depending on physics, theory of probability, the analysis of friction pair’ shape and fit include a number of parameters that are difficult or even impossible to be technologically controlled in the manufacturing of mechanical engineering components. The new approach for wear rate estimation using surface texture parameters as well as physical-mechanical properties and geometric parameters of components is proposed. The theoretical part of the calculations is based on the 3D surface texture principles, the basics of material fatigue theory, the theory of elasticity and the contact mechanics of surfaces. It is possible to calculate the service time of the machine, but the process of running-in of the components is relatively short (less than 5%), therefore, the service time is mainly determined by a normal operating period, which also was used to evaluate this period. The calculated input parameters are technologically and metrologically available and new method for calculating the service time can be used in the design process of the equipment. The results of approbation of the method for estimation service time of mechanical engineering, which prove the applicability of mentioned method, are offered as well.

Abstract: Compaction of granular raw materials is a crucial process in modern heavy industries. Extreme operating conditions such as high temperatures, high loads and high press rotational velocities are applied in applications such as hot briquetting processes. For fast and smooth removal from the mold, the adhesion forces are a crucial parameter. To set up a model test for investigations of the adhesive behavior of several release agents under application-oriented harsh operating conditions an available forming tribometer was adapted. Special release agents to reduce adhesive wear and improve the release properties are considered. Surfaces and wear tracks of test samples were characterized by means of optical microscopy, 3D-topographical evaluation as well as SEM analysis. The benefit of the release agents on the subjacent microstructure was evaluated on cross-sections of tribo-stressed areas. Resulting adhesion forces could be well distinguished for all release agents and significant improvements to uncoated operating conditions were achieved. It was proven that the developed test instrumentation is suited to characterize the performance of release agents. The obtained results strongly indicate a significant decrease of severe surface deteriorations when suitable release agents are applied.

Abstract: Abrasive wear limits the lifetime of key components and wear parts used in various applications. Damage is caused by indentation of harder particles into the wearing materials and subsequent relative motion resulting in ploughing, cutting, and fracture phenomena. The wear mechanisms depend mainly on the applied materials, loading conditions, and abrasives present in the tribosystem, hence material choice is often a difficult task and requires careful evaluation. For this, a variety of laboratory abrasion tests are available of which the scratch test is discussed in this work as the most fundamental abrasive interaction. For further insight into the acting wear mechanisms and microstructural effects, large-scale molecular dynamics simulations were carried out as well as meso-/macroscopic scratch simulations with the mesh-free Material Point Method. The prediction of abrasive wear is of high relevance for industrial applications. Up to now, no general one-to-one match between field application and lab system is known. Here, a simulation-based transfer of experimentally determined wear rates via a lab-2-field approach enables the prediction of wear rates in real applications.

Abstract: This work presents an experimental case study of 200 MW low-pressure steam turbine rotor blades damage after 310 thousand hours of operation and 404 start-ups on the power plant. Damaged blade material was analysed by experimental testing of mechanical properties as well as crack initiation and microstructure in the damaged area by using optical and scanning electron microscopes, and investigation of chemical composition by the SEM-EDS method. Results of the investigation show that the material was die forged martensitic steel 20Cr13. The mechanical properties of the blade steel were in accordance with standard technical requirements for new material. The cross-sections of damaged and undamaged blades were investigated and compared. It was found that thickness of damaged blades was significantly less than undamaged ones. SEM-EDS analysis has shown the presence of brass fragment on the surface of one damaged blade. The microstructural analysis has shown that one of the cracks was initiated due to impact of brass fragment. Based on the given case study results it is concluded that the reason of the blade damage was a combination of at least two factors: accelerated high cycle fatigue due to increased stresses caused by excessive vibration of damaged blades with lower thickness and impact of brass fragment against blades.

Abstract: The main aim of the study is recycling of niobium slag, separating valuable metallic Nb and mineral ballast – calcium aluminate. Niobium slag is a waste product from Nb production at NPM Silmet OÜ. For the retreatment of slag, an efficient disintegrator milling technology was used. Laboratory grindability tests of slags were carried out. We propose a potential principal scheme of slag treatment, consisting of preliminary size reduction by impact milling at direct mode and final at separative mode using a disintegrator milling system DSL-115. As a result of slag pretreatment, pure Nb from a coarse fraction was separated. To elaborate an industrial technology, semi-industrial tests for specifying recycling modes and finding optimal milling parameters were performed. Granulometry studies of milled products and chemical composition as of Nb as well as that of remained products were conducted. Yield of Nb with purity 90–95% was in the range of 0.4–1.0%. Preliminary cement tests for using the main product as a binder or a substitute of cement confirmed its potential for use in building materials production.

Abstract: In the present work, we use a modified Equal Channel Angular Pressing technique for structure and properties change of Tantalum and Niobium at room temperature. The main advantage of this modified technique is the possibility to produce relatively large samples with ultrafine-grained microstructure in all volume of the workpiece by reduced deformation load up to 25% via friction decrease, and also to prevent the punch fracture under high compression stress during pressing. The various microstructures and properties were produced in metals by using different von Mises strain levels up to Ɛ_vM = 13.8. The changes in microstructure were studied by using SEM and TEM techniques. The change of mechanical properties was measured by using various tension and hardness testing setups. We can conclude that during processing the ultrafine-grained microstructure in as-cast Nb and Ta was formed. The uniaxial tensile strength, Vickers hardness, and plasticity of Nb and Ta significantly increased as compared to coarse-grained counterparts. We believe that the relatively large workpieces of pure bulk Ta and Nb metals with improved microstructure and exploitation properties are suitable materials for the modern industry.

Abstract: In the past decade, research into High Entropy alloys (HEAs) have gained significant attention due to their outstanding properties and approach to design alloys for high temperature applications. Strengthening of face centered cubic (FCC) based HEAs, by addition of intermetallic phase or precipitate forming elements is a very captivating direction of alloy designing for high temperature structural applications. However, the knowledge regarding the influence of intermetallic phases on the properties of FCC HEAs is rare. The current study focuses on annealing effects on the microstructure of Cr₂₀Co₂₀Fe₂₅Ni₂₅V₅Mo₅ (at. %) alloy, this alloy was synthesized using induction melting, and was homogenized at 1200 °C for 12h. X-ray diffraction analysis indicated that the principle phase was (FCC) identified. Scanning electron microscopy (SEM) together with Energy Dispersion X-ray Spectroscopy (EDS) showed that there is an additional phases that is Mo-rich. In order to understand the effect of the high temperature annealing on phase stability, the homogenized samples were annealed at 700 °C, 800 °C, 900 °C, 1000 °C each for 6h and quenched. The annealing treatments had considerable effect on the crystal structure and the elemental distribution. The Mo-rich phase is precipitated at the grain boundaries at all temperatures. Additionally, at 1000 °C annealing temperature Mo-rich phase had precipitated inside the grains. The lower annealing temperatures inhibited diffusion of Mo, which restricted the Mo-rich phase formation. Additionally, the hardness is increased to 195 HV at 1000 °C due precipitation hardening. At other annealing temperatures the hardness is reduced to 145 – 158 HV.

Abstract: The main objective of this paper was to investigate machinability and technological assessment of powder metallurgy (PM) parts, especially bushings made by PM process. The bushings are made of CuSn10-C powder. Machining of the PM bushings was performed under various technological parameters, such as a cutting speed, a depth of cut and a feed. The main objective was to achieve or improve surface roughness (Sa – 0.3 to 0.6 μm) of machined bushing. Surface roughness is one of the main parameters, in result of which satisfactory performance of the bushing is achieved. The values of technological parameters were changed starting from the suggested technological parameters by a tool manufacturer and were further modified on step-by-step basis by evaluation of surface roughness of each machined PM bushing. Moreover, for evaluation and characterization of surface roughness not only 2D profile roughness measurements were used, but also 3D surface roughness measurement methodologies were used additionally allowing to investigate correlation between these two types measurement techniques. The results showed that technological assessment might be achieved in several ways not only by one best result.