Surface Degradation Mechanisms Research Articles

Abrasive erosive wear of powder steels and cermets

Composite materials produced by powder metallurgy provide solutions to many engineering applications that require materials with high abrasive wear resistance. The actual wear behaviour of a material is associated with many external factors (abrasive particle size, velocity and angularity) and intrinsic material properties of wear (hardness, toughness, Young modulus, etc.). Hardness and toughness properties of wear resistant materials are highly dependent on the content of the reinforcing phase, its size and on the mechanical properties of the constituent phase. This study is focused on the analysis of the (AEW) abrasive erosive wear (solid particle erosion) using different wear devices and abrasives. Powder materials (steels, cermets and hardmetals) were studied. Wear resistance of materials and wear mechanisms were studied and compared with those of commercial steels. Based on the results of wear studies, surface degradation mechanisms are proposed. The following parameters characterizing the materials were found necessary in materials creation and selection: hardness (preferably in scale comparable with impact), type of structure (preferably hardmetal type) and wear parameters characterizing material removal at plastic deformation.

The Influence of Thermally Sprayed Coatings Microstructure on their Mechanical and Tribological Characteristics

The tribological properties of parts surface, namely their wear resistance and friction properties, are in many cases determining for their proper function. To improve surface properties, it is possible to create hard, wear resistant coatings by thermal spray technologies. Using these versatile coatings it is possible to increase parts lifetime, reliability and safety. The thermally sprayed cermet composite coatings show, thanks to their specific properties, excellent resistance to abrasive and erosive wear, as well as corrosion resistance. To predict the behavior, lifetime and application area of thermally sprayed cermet coatings it is necessary to completely understand the relationships between technology, process parameters, microstructure and properties of the coatings. The finding of these relationships and use this understanding to develop deposits with improved wear resistance for coating of various applications is the main aim of the presented work. It was done by studying the coatings microstructure and mechanical properties. Four different tests of wear resistance were done to study the mechanism of surface degradation, to confirm the results of mechanical testing and to predict the lifetime of coated parts - the abrasive wear performance of the coatings was assessed using a dry/sand rubber wheel test according to ASTM G-65, wet slurry abrasion test according to ASTM G-75, pin-on-disc test according to ASTM G-99 and erosion wear resistance for three impact angles. On the basis of obtained data the new possibilities of coatings application was determined, tested and implemented.

Evolution of interfacial adhesion force in dynamic micromachines due to repetitive impact loading

A contact-mode surface micromachine was used to study the effect of repetitive impact loading on the evolution of the adhesion force at sidewall contact interfaces. Low and stable adhesion force was encountered during the initial stage of impact testing (run-in phase). A surface degradation phase occurred subsequently in which the adhesion force increased logarithmically with impact cycles. The experimental trend was used to derive a method for predicting micromachine failure due to excessive interfacial adhesion. High-magnification scanning electron microscopy did not reveal any modification of the surface topography even after 5.5×107 impact cycles, despite the significant enhancement of the adhesion force, attributed to the increase of the real contact area and the higher surface energy produced as a result of the removal of thin surface layers. The dominant surface degradation mechanisms are interpreted in the context of adhesion force measurements and microscopy images of the impacted surfaces.

An analysis of the in vivo deterioration of Co-Cr-Mo implants through wear and corrosion

The degradation of Co-Cr-Mo ASTM F75-92 hip implants after a harvesting period of 81 months in sheep was investigated. Hip prostheses and tissue samples were obtained from a medical study involving total hip arthroplasty of the cemented type in 12 sheep. Upon euthanasia, the explants were retrieved for analyses of the surfaces and evidence of degradation, while tissue samples from the interface regions were harvested for chemical analysis and evidence of Co, Cr, and Mo contents. Clear evidence of wear and corrosion was detected. Results also indicated that the modes of metal transport through the poly(methyl methacrylate) bone cement play an important role as the surface degradation mechanisms of the metal. The results are being discussed in terms of electrochemical and triboelectrochemical behaviour of the Co-Cr-Mo alloy.

An investigation on changes in chemical properties of pure ethylene-propylene-diene rubber in aqueous acidic environments

The influence of two aqueous acidic environments on two types of pure ethylene-propylene-diene (EPDM) rubber (i.e., elastomer) thin films is studied. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) results revealed the formation of several oxygenated species on the surface after exposure. Raman spectroscopy along with ATR-FTIR results confirmed the vulnerability of the olefinic double bonds (C C) of 5-ethylidene-2-norbornene (ENB) in acidic environments. In addition, the aggressive nature of 20% Cr(VI)/H 2SO 4 was also observed through an increase in oxygenated species formation on the surface of EPDM rubbers compared to 20% H 2SO 4 under identical conditions. Complex formation on the surface of EPDM samples exposed to 20% Cr(VI)/H 2SO 4 through reactions of carboxylic groups (generated due to EPDM degradation) with Cr(III) (formed due to reduction of Cr(VI)) was also evident in ATR-FTIR. Finally, all the oxygenated species combined to form crosslinking as observed by the increase in gel fraction with exposure time. Effect of molar mass on chemical changes was noticed as samples with higher molar masses were found to form relatively higher amount of gel upon exposure. However, no significant effect of long chain branching was noticed. On this basis, plausible surface degradation mechanisms are proposed.

Wear mechanism maps for thermal-spray steel coatings

Wear mechanisms in low-carbon-steel-based thermal-spray coatings deposited on aluminum alloys using a plasma-transfer wire-arc (PTWA) process and a high-velocity oxy-fuel (HVOF) process were studied. The coatings investigated consisted of PTWA 1020, HVOF 1020 deposited from a low-carbon steel wire stock, and HVOF 1020-2.5 pct Al produced using a wire stock with 2.5 wt pct Al added to the base AISI 1020 composition. Wear tests were performed using a pin-on-disc-type tribometer equipped with an environmental chamber within a load range of 5 to 75 N and a sliding-speed range of 0.2 to 2.5 m/s against tool steel pins in a dry air atmosphere (10 pct relative humidity). The wear rates of the three types of thermal-spray coatings and the micromechanisms that control the wear rates at different loads and sliding speeds were presented in the form of wear maps. Under dry sliding conditions, two basic wear and surface-degradation mechanisms were identified, consisting of (1) mechanical wear, which involved severe plastic deformation of the iron splats on the contact surfaces and their fracture and (2) oxidational wear that took place by the formation of various iron oxides, whose compositions and thicknesses depended on the loading conditions. The wear rates of PTWA 1020 and HVOF 1020-2.5 pct Al decreased with increasing sliding speed, as they showed a transition from mechanical wear to mild oxidation wear. The wear rates of HVOF 1020 also decreased with increasing speed, but they increased again, once they passed through a minimum where a transition from mild to severe oxidational wear occurred. An improvement in the wear resistance of HVOF 1020 was observed, in particular at high loading conditions, as a result of the addition of 2.5 pct Al to the wire feed stock. The wear maps demonstrated that the wear rates were sensitive to the compositions of the coatings and, hence, to the thermal-spray technique used in their production.

1Cr-0.5Mo 강의 재질열화가 프레팅 피로거동에 미치는 영향

Fretting is a kind of surface degradation mechanism observed in mechanical components and structures. The fretting damage decreases in 50-70% of the plain fatigue strength. This may be observed in the fossil power plant and the nuclear power plant used in special environments and various loading conditions. The thermal degradation of material is observed when the heat resisting steel is exposed for long period time at the high temperature. In the present study, the degraded 1Cr-0.5Mo steel used for long period time at high temperature (about 515) and artificially reheat-treated materials are prepared. These materials are used for evaluating an effect of thermal aging on the fretting fatigue behavior. Through the experiment, it is found that the fretting fatigue endurance limit of the reheat-treated 1Cr-0.5Mo steel decreased about 46% from the non-fretting fatigue endurance limit, while the fretting fatigue endurance limit of the degraded 1Cr-0.5Mo steel decreased about 53% from the non-fretting fatigue endurance limit. The maximum value of fatigue endurance limit difference is observed as 57%(244 MPa) between the fretting fatigue of degraded material and non-fretting fatigue of reheat-treated material. These results can be a basic data to a structural integrity evaluation of heat resisting steel considered to thermal degradation effect.

Phase transformation during cavitation erosion of a Co stainless steel

Cavitation erosion is a surface degradation mechanism that affects hydraulic turbine blades. Materials have been developed in order to minimise the detrimental effect. This work uses a Co stainless steel specially developed to exhibit improved cavitation resistance. The role of the γ austenite to ε-martensite phase transformation during cavitation erosion is assessed and the influence of surface finish on the cavitation erosion behaviour is evaluated as welded surfaces were ground and remelted before ultrasonic cavitation testing. Results reveal that structure refinement due to remelting contributes to a longer incubation period and that phase transformation is responsible for up to 25% of this erosion stage.

Poly(ethylene terephthalate) copolymers containing nitroterephthalic units. III. Methanolytic degradation

AbstractThe methanolytic degradation of poly(ethylene terephthalate) (PET) copolymers containing nitroterephthalic units was investigated. Random poly(ethylene terephthalate‐co‐nitroterephthalate) copolyesters (PETNT) containing 15 and 30 mol % nitrated units were prepared from ethylene glycol and a mixture of dimethyl terephthalate and dimethyl nitroterephthalate. A detailed study of the influence of the nitro group on the methanolytic degradation rate of the nitrated bis(2‐hydroxyethyl) nitroterephthalate (BHENT) model compound in comparison with the nonnitrated bis(2‐hydroxyethyl) terephthalate (BHET) model compound was carried out. The kinetics of the methanolysis of BHENT and BHET were evaluated with high‐performance liquid chromatography and 1H NMR spectroscopy. BHENT appeared to be much more reactive than BHET. The methanolytic degradation of PET and PETNT copolyesters at 80 °C was followed by changes in the weight and viscosity, gel permeation chromatography, differential scanning calorimetry, scanning electron microscopy, and 1H and 13C NMR spectroscopy. The copolyesters degraded faster than PET, and the degradation increased with the content of nitrated units and occurred preferentially by cleavage of the ester groups placed at the meta position of the nitro group in the nitrated units. For both PET and PETNT copolyesters, an increase in crystallinity accompanied methanolysis. A surface degradation mechanism entailing solubilization of the fragmented polymer and consequent loss of mass was found to operate in the methanolysis of the copolyesters. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2276–2285, 2002

Water-associated surface degradation of CsLiB6O10 crystal during harmonic generation in the ultraviolet region

A study of the mechanisms of CsLiB6O10 (CLBO) crystal performance degradation during harmonic generation in the ultraviolet (UV) region was performed. The degradation rate of transmissivity during irradiation with the fourth harmonic light of an Nd3+ doped YLiF4 laser (4.7 eV) was linearly proportional to the concentration of hydroxyl (OH) groups in the CLBO crystal. In this study, the formation of OH groups was recognized by infrared absorption spectroscopy, and a method for determination of OH groups was proposed. The concentration of OH groups in the CLBO was linearly proportional to water vapor pressure and the CLBO containing OH groups absorbed UV light. We propose that one of the main mechanisms of surface degradation is the generation and growth of the mutants and that these mutants are generated by a UV photon-assisted reaction via OH group formation associated with absorbing water molecules into CLBO crystal.

High-speed tribological behaviour of a carbon/silicon-carbide composite

Increasing attention is being paid to carbon-fibre/SiC-matrix composites (C/SiC) for their friction and wear performance because the carbon-fibre reinforcement enables a significant reduction in the coefficient of friction through self-lubricating properties. This study was performed by using a C/SiC composite at high-speed (15 m s −1). Two distinct types of tribological behaviour, as a function of the rubbing pin features (100Cr6 or alumina), were found. Analysis of both the surface degradation and velocity adaptation mechanisms has made it possible to establish the major role of debris trapped in the contact zone. The wear debris from the 100Cr6-C/SiC pair is a heterogeneous layer adhering strongly to the friction track. Consequently, the velocity adaptation is a direct effect of the shearing of the transfer layers and fracturing of bulk materials, resulting in severe wear. In contrast, with an alumina-C/SiC pair, wear particles are finely ground owing to the fact that they remain for a long time in the contact zone. The velocity adaptation here mainly occurs through debris rolling and shearing within the powder bed, thus resulting in reduced wear.

Using shifts in the electronic emission curve to evaluate polymer surface degradation

The polymer surface degradation and/or modification evolution of Teflon FEP and Mylar C films caused by a low energy electron beam were analyzed using a new method that consists in measuring the second crossover energy shift in the electronic emission curve. Upon prolonged irradiation, the second crossover energy shifts irreversibly to lower values in Teflon FEP but to higher values in Mylar C, indicating distinct mechanisms of surface degradation for the two polymers. The method represents a relatively inexpensive way to monitor early stages of surface degradation since the secondary electron emission comes from a maximum depth below the geometric surface of 100 nm in insulators.

Metal matrix composite layer formation with 3 μm SiC p powder on IMI318 titanium alloy surfaces through laser treatment

Metal surface modification is frequently carried out to increase the life and performance of engineering components that undergo severe service conditions such as wear, erosion and corrosion, without the cost of treating the entire component. The development of a suitable high performance surface layer on substrate materials can combat many surface degradation mechanisms. The formation of an in-situ surface layer MMC by injecting ceramic particulates on laser melted surfaces can, in particular, be used in applications requiring a localised treatment without changing the properties of the bulk material. Using a high volume fraction (25–60vol%) of large (50–150μm) ceramic particles (TiC, SiC) on titanium surfaces, the surface hardness was found to increase between 1.1 to 4 times the base hardness (150 Hv). In the present work, a 5kW continuous wave carbon dioxide laser was used under both helium and nitrogen environments to introduce a smaller volume fraction of 3 μm SiC p particles using the powder preplacement technique on a laser melted IMI318 titanium alloy (Ti-6Al-4V) surface. By changing the laser processing parameters, graded microstructures with hardness gradients were obtained over a melt depth of about 1 mm. The tracks produced surface cracks, but of low intensities (0.1 to 0.3 No./mm). However, in some glazing conditions cracks were absent. Under a helium environment, most of the ceramic particles dissolved in the melt pool, and the microstructure consisted mainly of thread-like particles; some dendrites were also present near the surface and at the edges of the melt pool. Under a nitrogen environment, the tracks produced mostly dendritic structure with no SiC p particles in the melt zone. The hardness developed from a base hardness of 350 Hv to the range 600 to 1200 Hv and 600 to 1450 Hv for tracks processed in helium and nitrogen atmospheres respectively.

Surface degradation features and microstructural properties of ultra-high molecular weight polyethylene (UHMWPe).

Surface degradation of UHMWPe is recognized as a leading clinical concern, limiting the long-term performance in total knee replacements. Eight retrieved tibial plateaux and six wear screening test samples were evaluated for surface degradation features and microstructural features. The surface degradation features were assessed using stereomicroscopy and scanning electron microscopy. Microstructural features were evaluated using optical microscopy of thin cross-sections and a permanganate etching technique. The pitting mechanism of wear was observed on all eight retrieved TKR and covered an average of 12.6% of the surface area. The size of the pits were similar to the size of grains observed in the microstructural evaluation - approximately 100 to 200 microm. The presented observations of pitting in retrieved knee implants have shown that the post-processing microstructure may influence this mechanism of surface degradation and hence the wear products.

Hydrolysis kinetics of okara and characterization of its water-soluble polysaccharides.

As a part of a study of reuse of waste okara, the kinetic mechanism of okara hydrolysis and the properties of the water-soluble polysaccharides (WSP) extracted were investigated. Okara was hydrolyzed by being autoclaved at pH 4.5 in two volume of water with or without a chelator such as hexametaphosphate. Okara hydrolysis proceeded by surface degradation mechanism without a chelator. Characterization of WSP suggested that the "egg-box regions" in okara were susceptible to degradation by a chelator and that the "non-egg-box regions" contained WSP linked with hydrophobic proteins. WSP with a molecular weight of more than 10(5) and the amount of protein bound to the polysaccharides seemed to govern their emulsifying characteristics.

Erosion and Erosion-Corrosion of Metals

Abstract The loss of material from the surface of a metal being impacted by small, solid particles carried in a flowing gas occurs by a sequence of mechanical actions. The present work, which was presented as the plenary lecture at CORROSION/95, describes the mechanisms that occur on ductile and brittle materials and relates these mechanisms to surface loss rates. The ability of these mechanisms to be represented by mathematical models is assessed. The effects of material properties on erosion behavior is discussed. Combined erosion-corrosion surface degradation mechanisms are explained.

Relationship Between Feedstock Characteristics and Erosivity of FBC Bed Materials

The erosion-corrosion (E-C) metal wastage mechanisms and rates that occur in 1018 plain carbon steel used in tubular heat exchangers of fluid bed combustors (FBC) are discussed. The characteristics of FBC bed material erodent particles such as composition, shape, size, and strength were found to have a major effect on the surface degradation mechanisms and rates that occurred. A total of 16 different bed material particles from ten different FBCs were tested. It was determined that when the particles were strong enough not to shatter when they impacted the steel’s surface, their shape and composition were the most important factors in determining their erosivity. The relative amounts of SiO2, CaO, CaCO3, CaSO4, and alkali compounds in the bed materials were related to the metal wastage by using laboratory mixtures of the compounds as erodents. The ratio of SiO2 to CaO and CaCO3 was especially important in determining the erosivity of the bed materials. It was found that increasing this ratio increased the metal wastage. A sharp transition ratio occurred on either side of which the metal wastages were linear. The slope of the linear curve was low for the lower sand content mixtures and higher for higher sand content mixtures. The transition ratio required a higher CaO content for higher velocity particles.

Surface degradation mechanisms in brittle material structural systems : Int J Fracture V28, N4, 15 April 1991, P231–244

A mechanics based theory for surface degradation in brittle material systems is introduced. Surface degradation is directly related to damage progression. For this reason the mechanics of damage evolution is presented first. Subsequently, relations governing surface degradation mechanisms are derived and discussed in detail. It is shown that surface degradation can capture important properties of brittle materials such as scale (size) and shape effects, surface damage growth and subsequent bursting instabilities. Finally, the problem of transferring information from laboratory experiments to large scale problems is discussed; the need for further experimental and theoretical research is pointed out.

Erosivity of particles from operating fluidized bed combustors

The erosion-corrosion (E-C) metal wastage mechanisms and rates that occur in 1018 plain carbon steel used in tubular heat exchangers of fluid bed combustors (FBCs) are discussed. The characteristics of FBC bed material erodent particles such as composition, shape, size and strength were found to have a major effect on the surface degradation mechanisms and rates which occurred. A total of 16 different bed material particles from ten different FBCs were tested. It was determined that when the particles were strong enough not to shatter upon impact, their shape and composition were the most important factors in determining their erosivity. The basic E-C mechanism was the formation of an outer layer of bed material deposit or mixed bed material and iron oxide that was eroded away in a brittle manner by cracking and chipping of a segmented scale. Beneath the outer layer a mixture of varying amounts of iron oxide and bed material occurred, with the iron oxide content increasing as the base metal was approached. The morphology and behavior of the outer layer are closely related to the metal wastage of the tubing steel.

Surface degradation mechanisms in brittle material structural systems

A mechanics based theory for surface degradation in brittle material systems is introduced. Surface degradation is directly related to damage progression. For this reason the mechanics of damage evolution is presented first. Subsequently, relations governing surface degradation mechanisms are derived and discussed in detail. It is shown that surface degradation can capture important properties of brittle materials such as scale (size) and shape effects, surface damage growth and subsequent bursting instabilities. Finally, the problem of transferring information from laboratory experiments to large scale problems is discussed; the need for further experimental and theoretical research is pointed out.