Dark Etching Research Articles

Investigating the Process of White Etching Crack Initiation in Bearing Steel

White etching cracks (WECs) have been identified as a dominant mode of premature failure within wind turbine gearbox bearings. Though WECs have been reported in the field for over a decade, the conditions leading to WECs and the process by which this failure culminates are both highly debated. In previously published work, the generation of WECs on a benchtop scale was linked to sliding at the surface of the test sample, and it was also postulated that the generation of WECs was dependent on the cumulative energy that had been applied to the sample over the entirety of the test. In this paper, a three-ring-on-roller benchtop test rig is used to systematically alter the cumulative energy that a sample experiences through changes in normal load, sliding, and run-time, in an attempt to correlate cumulative energy with the formation of WECs. It was determined that, in the current test setup, the presence of WECs can be predicted by this energy criterion. The authors then used this information to study the process by which WECs initiate. It was found that, under the current testing conditions, the formation of a dark etching microstructure precedes the formation of a crack, and a crack precedes the formation of white etching microstructure.

Laser surface pretreatment of 100Cr6 bearing steel – Hardening effects and white etching zones

In order to achieve a surface pretreatment of the bearing steel 100Cr6 (1–1.5wt.% Cr) a laser-based process was used. The obtained modification may result in an optimization of the adhesive properties of the surface with respect to an anticorrosion polymer coating on the basis of PEEK (poly-ether-ether-ketone), which is applied on the steel surface by a laser melting technique. This work deals with the influence of the laser-based pretreatment regarding the surface microstructure and the micro-hardness of the steel, which has been examined by scanning electron microscopy (SEM), light microscopy and automated micro-hardness testing.The most suitable parameter set for the laser-based pretreatment leads to the formation of very hard white etching zones (WEZ) with a thickness of 23μm, whereas this pretreatment also induces topographical changes. The occurrence of the white etching zones is attributed to near-surface re-austenitization and rapid quenching. Moreover, dark etching zones (DEZ) with a thickness of 32μm are found at the laser path edges as well as underneath the white etching zones (WEZ). In these areas, the hardness is decreased due to the formation of oxides as a consequence of re-tempering.

Microstructures and Mechanical Properties of a Wear-Resistant Alloyed Ductile Iron Austempered at Various Temperatures

To further improve the mechanical performance of a new type of alloyed bainitic wear-resistant ductile iron, the effects of the various austempering temperatures have been investigated on microstructure and mechanical behaviors of alloyed ductile iron Fe-3.50C-1.95Si-3.58Ni-0.71Cu-0.92Mo-0.65Cr-0.36Mn (in weight percent). This alloyed ductile iron were firstly austenitized at 1123 K (850 °C) for 1 hour and then austempered in a salt bath at 548 K, 573 K, and 598 K (275 °C, 300 °C, and 325 °C) for 2 hours according to time–temperature–transformation diagram calculated by JMatPro software. The microstructures of austempered wear-resistant ductile irons consist of matrix of dark needle-like ferrite plus bright etching austenite and some amount of martensite and some dispersed graphite nodules. With increasing the austempering temperature, the amount of ferrite decreases in austempered ductile iron, while the amount of austenite and carbon content of austenite increases. There is a gradual decrease in hardness and increase in compressive strength with increasing austempering temperature. The increased austenite content and coarsened austenite and ferrite can lead to a hardness decrease as austempering temperature is increased. The increased compressive strength can be attributed to a decreased amount of martensitic transformation. The alloyed ductile iron behaves rather well wear resistance when the austempering is carried out at 598 K (325 °C) for 2 hours. Under the condition of wear test by dry sand/rubber wheel, the wear mechanisms of austempered ductile irons are both micro-cutting and plastic deformation.

Macropitting in Microsegregated Induction-Hardened Gear Tooth

A sample from a large gear was submitted for investigation into the cause of apparent macropitting observed around the pitch line on the loaded face of one of the teeth. A banded (light and dark etching) martensitic structure was observed, which varied significantly in hardness. Cracking was observed to be isolated to the case-hardened surface. The macropitting observed appeared to be consistent with contact fatigue. The nonhomogeneous microstructure observed may have been particularly susceptible to fatigue initiation and propagation. The microstructure observed appeared to be due to a lack of homogenous diffusion of carbon during the austenization of the part from the annealed condition.

Residual Stresses in Roller Bearing Components

Rolling contact fatigue is a very complex process. The mechanism can only be described by speculative considerations. Because the loading conditions during the elastic- hydro- dynamical contact are not clearly described. The loading cycle runs within extremely short rates and structural alterations occur under high hydrostatic pressure. Widely unknown is therefore, how the materials conditions are influenced by these processes. But by means of simplified considerations an approach to the rolling contact fatigue process can be obtained. Following these conceptions simplifying quasi-static conditions are drawn. A lubricant film inhibits the metallic contact of the revolved bearing components. A HERTZian load stress will be accumulated over an elliptical contact area and within and beneath this contact area three dimensional stresses are acting. The materials strengthening can be described by the hypothesis of alteration of shape. During the fatigue period, the microstructure will be changed by micro- and macro- plastically deformation. By this residual stresses occur. These are superimposed to the operational -loading –stresses which change the distressing conditions of the material. The progressive plastically deformations accompanying the growing fatigue procedure cause perpetually alterations in the distress- conditions of the material. Structural alterations of the rolling contact fatigue process are shown by means of metallography as followed: by dark etching areas (DEA), and by white etching areas (WEA) showing bands, which are positioned beneath the contact area at an angle of 30° (30°WEB) and for instance at 80° (80°WEB), and furthermore by so called butterfly structures (butterflies with “white etched” flanks). All these white etching areas, regarding their morphological structure and the etching conditions, are commonly originated by two axial distressing. The three dimensional materials distressing within the roller-bearing component on the one hand and the two dimensionally originating of the WEA on the other seem to be an antagonism. But when the changes of residual stresses during the contact rolling fatigue process are to be analyzed, it is clear that this antagonism rises only virtually because there exists a real tri-axial stress condition, which tolerates a two axial distressing of the material. By the concept, that the growing plastically deformations cause residual stresses superposing the operational load stresses, the temporary cycle of the structural alterations and the local and angular positions of the 30° WEB can be explained.

Microstructural Alterations in Bearing Steels under Rolling Contact Fatigue Part 1—Historical Overview

Microstructural alterations in bearing steels during rolling contact cycling have been reported in the literature for more than half a century. These structural changes are primarily caused by the decay of parent martensite and have been designated as white and dark etching regions due to their preferential etching characteristics. One of the most striking features of the white etching bands is their repeatable directionality, which has puzzled investigators for decades. Despite numerous attempts, a satisfactory explanation for the orientation of these bands is still not available. In this article (Part 1), an overview of the phenomenon is presented with detailed discussion of various experimental observations from the literature. The article also examines the previous approaches adopted to explain white etching bands and address their limitations. In Part II of the article, a J2-based elastic–plastic finite element model coupled with a carbon diffusion model is developed that directionally predicts the occurrence and orientation of the white etching bands.

Development of Micro-Pores Including Nano-Pores on n-Si (100) Coated with Sparse Ag Under Dark Etching in 1.0 M NH4F Containing 5.0 M H2O2

Development of Micro-Pores Including Nano-Pores on n-Si (100) Coated with Sparse Ag Under Dark Etching in 1.0 M NH4F Containing 5.0 M H2O2

Rolling contact fatigue in bearings: multiscale overview

For over a century, rolling contact fatigue in bearings has been recognised as a key feature limiting bearing life. The phenomenon is manifested through dark etching regions, 30 and 80° white etching bands as well as white etching areas, the latter often forming intricate defects commonly referred to as butterflies. Their presence depends on testing conditions, such as contact pressure, temperature, number of revolutions and steel cleanliness. Microstructural inspection demonstrates that precipitate shearing, dissolution, cell and nanocrystal formation as well as matrix/inclusion debonding may take place throughout bearing life. Such microstructural features have a negative effect on bearing hardness, strength, ductility and toughness, usually preceding failure. The present review shows how such phenomena are interconnected, highlighting the need for integral characterisation and modelling across the scales. This will aid in the conception of new heat treatments, steel grades and microstructures for enhanced rolling contact fatigue, leading to increased bearing life.

Hydrogen Accelerated Classical Rolling Contact Fatigue and Evaluation of the Residual Stress Response

The accelerating effect of absorbed hydrogen on subsurface rolling contact fatigue is studied by bearing rig testing under HF electric current passage. The additional chemical loading enhances dislocation mobility and microplasticity. Structural changes occur in a premature stage, as shown by comparative XRD residual stress analysis. Dark etching regions appear to be a precursor for phase transformation in regular white etching bands. Fatigue crack initiation is demonstrated.

Microstructural Development in Bearing Steel during Rolling Contact Fatigue

It is well known that microstructural changes occur in a steel bearing, when the bearing is operated under conditions involving high cyclic stresses. When combined with relatively high temperatures, such microstructural changes result in the flaking of the bearing raceway. In this paper, microstructural changes that occurred during rolling contact fatigue were investigated, and the relationship between these changes and fatigue life are discussed in association with the recrystallization behavior of martensite. Conventional bearing steel SUJ2 (SAE52100) was subjected to partial solution treatment at 1133K for 2.4ks followed by oil quenching. The quenched material with a martensitic structure was tempered at 443K for 7.2ks, and then subjected to rolling contact fatigue testing. The testing was performed at temperatures ranging from 373K to 443K and surface pressures of 4.6GPa or 5.5GPa. During testing at 373K, flaking occurred from the surface of the raceway due to non-metallic inclusion and without any marked microstructural changes. On the other hand, in the case of testing at 403K or more, flaking occurs after obvious microstructural changes. Firstly, dark etching constituent (DEC) formed around the area of maximum shear stress, which was followed by the formation of white etching constituent (WEC) within the DEC at 80 and 30 degrees to the rolling direction. TEM observations showed the change from martensite lath to dislocation cell structure within the DEC, and also the existence of fine ferrite grains of 20nm through 100nm within the WEC. Arrhenius plots for the fatigue life indicated that the activation energy of the fatigue process corresponded to that of carbon diffusion in bcc ferrite. These results suggest that rolling contact fatigue originated from the WEC is controlled by the diffusion of carbon in the ferrite matrix.

Formation of Ferritic Products during Continuous Cooling of a Cu-bearing HSLA Steel

A continuous cooling transformation (CCT) diagram was constructed by a combination of dilatometry and metallographic methods for a Cu-bearing HSLA steel, which is a low carbon low alloy variant of the ASTM A710 type structural steel. It was found that the decomposition of austenite was significantly depressed to lower transformation temperatures resulting in a prominent transformation region for bainitic structures, at temperatures intermediate between those of diffusional products and the displacive transformation to martensite. Polygonal and quasi-polygonal ferrite were observed to grow across and eliminate the prior austenite grain boundaries at relatively low cooling rates. At a cooling rate ranging from 0.35 to 20°C/s, the structure was characterised by a mixture of quasi-polygonal ferrite, Widmanstatten side-plate ferrite, and bainitic structures associated with minor dispersed islands of martensite and/or retained austenite which were dark etching on preparation for optical microscopy. This microstructure develops by the following processes. The Widmanstatten side-plate ferrite nucleates from the ferrite grain boundary allotriomorphs at the early stage of transformation, together with the bainitic ferrite plates which nucleate directly at the prior austenite grain boundaries. On further cooling, the neighbouring plates of Widmanstatten and bainitic ferrite each tended to coalesce and the volume of untransformed austenite decreased and the shapes of the enclosed y volumes evolved into residual islands between the ferrite plates. Provided the cooling rate was greater than 20°C/s the bainitic ferrite plates nucleated directly at the prior austenite grain boundaries, and the plate morphology was revealed by regions of elongated retained austenite or its decomposition products. At the fastest cooling rate obtained by dilatometry (∼375°C/s), the structure was largely characterised by a mixture of bainitic ferrite and martensitic packets surrounded by retained austenite films. Dilatometric and metallographic examination of the martensite and bainitic ferrite formed on rapid cooling failed to find a clear microstructural distinction between the two products. The packets of bainitic ferrite plates were generally nucleated directly from the prior austenite grain boundaries, whereas the martensite was characterised by thinner ferritic units with a higher dislocation density. There also appeared to be a larger number of variants of lath packets and apparent intragranular nucleation in the case of martensitic ferrite.

Phase transformations associated with micropitting in rolling/sliding contacts

Metallographic investigation of the discs used in tests to simulate gear tooth contact has revealed the occurrence of several microstructural features that are present only in fatigued specimens: e.g. dark etching regions (DER) and white etching bands (WEB). In addition, a new type of microstructural constituent has been observed close to the surface, below asperities. This feature is referred to as the plastic deformation region (PDR). For the first time the hardness of these fatigue-induced phases was accurately determined by the nanoindentation technique. It has been found that DER initiates at the prior austenite grain boundaries. Fatigue cracks initiate and propagate preferentially at the boundaries of PDR and ultimately lead to the formation of micropitting.

Martensite decay in micropitted gears

Metallographic examination performed on a number of micropitted gears has revealed microstructural features similar to those reported in the literature in fatigued bearings, namely, dark etching regions and white etching bands. In addition, a new type of fatigue-induced microstructural constituent was observed close to the surface below asperities referred to as the plastic deformation region. Finally, it is shown that the initiation and propagation of cracks leading to the formation of micropits are related to the phase transformations mentioned previously.

Thermal Oxidation of Porous Silicon: Study on Reaction Kinetics

Porous silicon (PS) samples obtained by dark etching of p+-type silicon wafers are oxidized in dry air, at various temperatures from 200 °C up to 800 °C for 1−20 h durations, to determine the kinetics of the reaction. The extent of oxidation calculated from mass gains is plotted as a function of oxidation time and temperature. By fitting the general reaction kinetic solutions of different-order reactions, one finds that the function valid for first-order kinetics gives the best matches. From the obtained reaction rates, the pre-exponential factors and activation energies of the oxidation process are determined using Arrhenius plots. It is found that, depending on the temperature range of the oxidation process, either one or two chemical reactions take place. At lower temperatures (200−400 °C), a reaction with an activation energy Ea,1 ≈ 7 kJ·mol-1 and a pre-exponential factor A1 ≈ 0.2 h-1 dominates. When the temperature is increased (400−800 °C), a second reaction starts, showing Ea,2 ≈ 50 kJ·mol-1 and A2 ≈ 100 h-1.

Analysis of micropitting of prototype surface fatigue test gears

AbstractThe early stages of micropitting were examined on experimental gears designed for surface fatigue studies by the Helical Gear Rating Committee of the American Gear Manufacturers Association. Optical profilometry, scanning electron microscopy, and metallographic examinations were conducted on the first prototype gearset tested. The gearset was tested in an FZG test rig. To evaluate tooth bending fatigue strength, the gears were loaded to load stage 12. A tooth broke from bending fatigue in 0.88 h (the expected time). This short runtime allowed for the examination of micropitting at an early stage of development. The distribution and morphology of micropitting over the tooth face and associated relationships with surface micro‐ and macrogeometry features and traction have been examined. Micropitting was primarily associated with finish geometry. A few pits were associated with sulphide inclusions and one pit was possibly initiated by a small non‐metallic inclusion. Metallographic examinations revealed asperity‐related lines of dark etching alterations caused by high asperity contact and traction stresses.

Scanning Electron Microscopy And Electron Probe Microanalysis Of Extraterrestrial Materials

One of the first samples analyzed by Castaing in his electron probe microanalyzer (EPMA) some 50 years ago was an iron meteorite. The Widmanstatten pattern microstructure of iron meteorites can be observed at very low magnifications ( Fig. 1). These meteorites are ideal samples for microanalysis because of the Ni gradient which extends over 10 to 1000 microns in the parent taenite phase of these Fe-Ni samples (Fig. 3). The Ni gradient is the result of very slow cooling of the iron meteorite, in terms of millions of years, within a parent'asteroid.The scanning electron microscope (SEM) has been used to characterize the microstructure of meteorites, as well as samples from the moon and mars. For example, the microstructure of the dark etching taenite areas (T in Fig. 1) of the Carleton iron meteorite is shown in Fig 2. In this example, precipitates are observed along original martensite laths which form during the cooling of the iron meteorite at low temperatures.

Rolling-contact fatigue lives and steel AISI 52100 balls with eight mineral and synthetic lubricants : Wang, Y., Fernandez, J.E. and Cuervo, D.G. Wear (1996) 196, 110–119

Substantial microstructural changes have been found to occur in bearing steels when subjected to high stress Rolling Contact Fatigue (RCF) and have been mainly reported in literature between the 1940s and 1990s. However, owing to limitations in the characterisation techniques available at the time, inconsistent interpretation and use of discrepant terminology have caused considerable difficulties in defining the microstructural changes accurately and unambiguously. In the present work, we have investigated the typical microstructural alterations, including Dark Etching Region (DER), Low Angle Bands (LABs) and High Angle Bands (HABs), and their formation mechanisms in RCF failed AISI 52100 (100Cr6) bearing steels using a combination of advanced microstructure characterisation techniques, including Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) coupled with Energy Dispersive X-ray Spectroscopy (EDX), Transmission Electron Microscopy (TEM), and nanohardness measurements. Based on this combined approach, we are now able to give detailed insight in the plasticity-induced transformation and degradation mechanisms during high-stress RCF. The results show that new globular and elongated grains with distinct textures form during all stages of RCF, however a redistribution of chemical elements was only observed during the later stages of RCF. This has provided a significant insight in the formation mechanisms of DER, LABs and HABs. A model of the sequence of microstructure alterations during RCF is thus been proposed based on the findings.

Production of very hard surface layers by repetitive use of high energy plasma pulses

At the Fachgebiet Raumfahrttechnik of the Technical University at Munich, electrothermal and electromagnetic launchers are used to accelerate projectiles to velocities of 5–15 km s −1 to simulate the impact of micrometeorites on structure of spacecraft. The discharge of stored electrical energy (in the order of some kJ) across selected materials generates a plasma armature, which accelerates the projectile. Without a projectile a high pressure (in the order of 10–50 kbar) plasma pulse (duration 10–50 μs, temperature approximately 1–10 4 K, 10 6–10 7 W cm −2) is accelerated to velocities of up to 20 km s −2. This pulse, to which selected materials (e.g., Cr, C) can be deliberately added, is directed towards the surface of a target for treatment. Experiments with steel targets show that an extremely hard surface layeer of average 30 μm thickness is created by a single pulse [1]. The surface layer thickness could be increased to approximately 400 μm by repetitive impacting. After 10 process steps, where the target is placed with a small offset after each “shot”, an adhesive surface layer with a fairly high surface roughness is produced. The surface layer has then to be ground to a thickness of 200 μm in a subsequent step to achieve a smooth surface. The surface layers are investigated by means of metallography, scanning electron microscopy (SEM) and X-ray diffraction. The layer consists of two different zones with a measured microhardness up to 1300 HV 0.01. There is a very hard white etching zone immediately below the surface, and a dark etching zone between the white zone and the core workpiece which has a needle-shaped martensitic microstructure. Preliminary results indicate that the white zone, which is created after every new impact, transforms into the dark zone in dependence on temperature during subsequent treatments. The production of very hard adhesive surface layers with thicknesses of minimum 200 μm is a further step towards industrial application of this method.

On the fracture characteristics of leaded aluminium alloys

Aluminium alloys with proper distribution of lead have high potential as materials for plain bearings. In fact, lead has been found to be either equivalent to or even more effective than tin as a soft phase alloying addition [1] in aluminium and its alloys. However, some unfavourable factors, such as the wide miscibility gap, large range of solidification and high disparity in the densities of aluminium and lead, cause practical problems in preparation of aluminium-lead alloys with a uniform distribution of lead. Pathak et al. [2] successfully demonstrated, using an improved design of impeller mixer and a modified technique of chill casting, that a fairly uniform distribution of lead up to 50 wt % can be obtained in aluminium and its alloys. The microstructures and mechanical properties of the aluminium-lead alloys, with varying lead content from 5 to 50 wt % are reported elsewhere [2, 3]. Ichikawa and Ishizuka [4] prepared leaded aluminium alloys with varying lead content from 5 to 50 wt % by rheocasting. They selected the same base alloys that were studied by the previous investigators [2, 3] and reported an improvement in tensile strength but a reduction in ductility of leaded aluminium alloys at room temperature, and Mohan et al. [5] prepared aluminium-lead alloys by stir casting and found a uniform distribution of lead both in longitudinal as well as transverse sections of the ingot castings. This letter is concerned with the effect of lead content on the fracture characteristics of aluminium-lead ingot castings tested in tension at room temperature. Commercially pure aluminium (99.5% purity) and lead (99.9% purity) were obtained from the Indian Standards Company, Bombay. Aluminium-lead alloys with lead content of 5-50 wt % were prepared, using an especially designed impeller for mixing and the technique of bottom discharge chill casting [2, 6]. The specimens for optical microscopy were mechanically polished and etched by Keller's reagent. Cylindrical Hounsfield tensile specimens with gauge diameter of 4.5 mm and gauge length of 16 mm were machined from the cast ingots. Tensile tests were conducted at room temperature using a servomechanical Instron machine at a crosshead speed of 2 mmmin -t. The fracture surfaces were examined by a Philips scanning electron microscope PS EM500. The tensile properties of the commercial purity aluminium, lead and the lead alloys are recorded in Table I. It may be seen that whereas the strength parameters decrease, the ductility parameters increase with the lead content. These variations in the properties of the aluminiumlead alloys may be attributed to the soft and ductile nature of lead. It may be noted that whereas the effect of lead on the ductility is high up to 25 wt % Pb, its effectiveness decreases at higher lead content. Hence, the fracture behaviour of only two alloys, one with 15 wt% Pb and the other with 45 wt % Pb is discussed. The optical micrographs in Fig. la and b show the size, shape and distribution of the lead-rich phase (dark etching) in the aluminium-rich matrix. It may be seen that average thickness of the lead-rich pockets is considerably higher in the A1-45 wt % Pb than that in the A1-15 wt% Pb. The scanning electron micrographs in Fig. 2 show the fracture characteristics of the A1-15 wt% Pb. The dark regions with irregular shape in the scanning electron micrographs (Fig. 2a) are depressions arising from the lead-rich regions. The lead-rich aggregates appear to have undergone extensive plastic deformation (Fig. 2b). In addition, there are two types of dimples, one with dark and the other with light contrast. The dimples are mostly equiaxed. The dark dimples are due to the lead-rich constituent and the

Selective Etching of Al2O3 on GaAs using Excimer Lasers

Selective etching of Al2O3 thin films on GaAs immersed in a H3PO4 aqueous solution by irradiation with excimer lasers is demonstrated. The dark etching of Al2O3 films (0.4 nm/min) is negligibly small compared with that of the laser-induced etching. It is found that the process cannot etch the GaAs substrates. Therefore, the etching stops at the Al2O3/GaAs interface, and the Al2O3 films are selectively etched only in laser irradiation regions. The etching characteristics of different wavelengths of ArF and KrF excimer lasers are also discussed.