Investigation of sliding wear surfaces in alumina using transmission electron microscopy

In this study, the subsurface microstructure of alumina wear surfaces and the microstructure of agglomerated debris generated from unlubricated sliding wear at room temperature have been investigated through transmission electron microscopy (TEM). Specimens were thinned through the use of a focused ion beam miller (FIB). TEM studies, including analysis of electron diffraction patterns from the agglomerated region of the specimen, revealed the presence of an aggregate of nano crystalline particles embedded in an amorphous phase, together with some larger alumina particles. These larger alumina particles appear at the base of pits in the alumina surface, whereas the finer material appears at the contact surface. The agglomerated debris was readily distinguished from the alumina substrate, which contained localised dislocation damage and microcracking. It is proposed that the wear process involves the removal of ‘large’ alumina particles from the surface by a combination of trans- and intergranular microcracking. These particles are then ground into very fine, nanometer-sized particles that react on the surface with moisture in the air to form an amorphous hydroxide film. These are then compacted to form a nanocrystalline structure within an amorphous matrix that may also be viewed as a grain boundary phase.     

Thermal stability of Al2O3-5 vol% SiC nanocomposite

An alumina nanocomposite containing 5.0 vol% SiC, which exhibits a fracture toughness of more than three times that of conventional alumina ceramics, was fabricated by hot pressing a powder mixture consisting of submicrometre alumina and silicon carbide powder particles at temperatures in the range 1600–1700 °C. SiC particles, 20–30 nm in size, were observed to occur primarily at the intragranular positions in the alumina matrix. The nanocomposite was then thermally aged in air for various periods at 1400 °C. The thermal treatment led to the formation of a reacted surface scale, the thickness of which increases with increasing ageing time at the ageing temperature. The microstructure of the reacted surface scale was studied using XRD, SEM and TEM equipped with an EDX analysis facility. At the thermal ageing temperature, SiC particles within the surface scale are oxidized to form silica, which subsequently reacts with the alumina matrix. The oxidation of the SiC particles and the subsequent reaction with the alumina matrix, together with the grain growth of various phases, resulted in the formation of a porous microstructure, which consists of alumina grains, mullites of differing composition, and amorphous silica and aluminosilicate pockets. Two types of mullite phase, which contain a high and a low level of silica, respectively, were identified in the surface scales. The high silica-containing mullite phases, the composition of which is close to that of stoichiometric mullite, occur as large, irregularly shaped matrix grains. The low silica-containing mullite phases (15–20 wt% SiO₂), which exhibit a rounded morphology, are observed to occur as second-phase particles entrapped within the high-silica containing mullite grains. The nanocomposite structure in the bulk region remains almost intact when compared with that of the unaged nanocomposite. The only noticeable difference is that the alumina matrix in the thermally aged nanocomposite exhibits a slightly larger grain size than that of the unaged nanocomposite.     

An electron microscopy study of wear in polysilicon microelectromechanical systems in ambient air

Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS). While the reliability of polysilicon MEMS has received extensive attention, the mechanisms responsible for this failure mode at the microscale have yet to be conclusively determined. We have used on-chip polycrystalline silicon side-wall friction MEMS specimens to study active mechanisms during sliding wear in ambient air. Worn parts were examined by analytical scanning and transmission electron microscopy, while local temperature changes were monitored using advanced infrared microscopy. Observations show that small amorphous debris particles (∼50-100 nm) are removed by fracture through the silicon grains (∼500 nm) and are oxidized during this process. Agglomeration of such debris particles into larger clusters also occurs. Some of these debris particles/clusters create plowing tracks on the beam surface. A nano-crystalline surface layer (∼20-200 nm), with higher oxygen content, forms during wear at and below regions of the worn surface; its formation is likely aided by high local stresses. No evidence of dislocation plasticity or of extreme local temperature increases was found, ruling out the possibility of high temperature-assisted wear mechanisms.     

Friction-formed liquid droplets

The formation of nanoscale liquid droplets by friction of a solid is observed in real-time. This is achieved using a newly developed in situ transmission electron microscope (TEM) triboprobe capable of applying multiple reciprocating wear cycles to a nanoscale surface. Dynamical imaging of the nanoscale cyclic rubbing of a focused-ion-beam (FIB) processed Al alloy by diamond shows that the generation of nanoscale wear particles is followed by a phase separation to form liquid Ga nanodroplets and liquid bridges. The transformation of a two-body system to a four-body solid-liquid system within the reciprocating wear track significantly alters the local dynamical friction and wear processes. Moving liquid bridges are observed in situ to play a key role at the sliding nanocontact, interacting strongly with the highly mobile nanoparticle debris. In situ imaging demonstrates that both static and moving liquid droplets exhibit asymmetric menisci due to nanoscale surface roughness. Nanodroplet kinetics are furthermore dependent on local frictional temperature, with solid-like surface nanofilaments forming on cooling. TEM nanotribology opens up new avenues for the real-time quantification of cyclic friction, wear and dynamic solid-liquid nanomechanics, which will have widespread applications in many areas of nanoscience and nanotechnology.     

Ni-Al包覆增强Fe基非晶涂层的结构与耐磨性

为了降低Fe基非晶涂层的孔隙率,并提高其耐摩擦性能,采用机械球磨法在铁基非晶颗粒表面包覆Ni-Al颗粒,得到具有壳-核结构的涂层复合喷涂粉末,并采用等离子喷涂法将其在45钢表面制备铁基非晶涂层;分析了包覆型复合粉末和Fe基复合涂层的相组成和微观结构,测试了复合涂层的孔隙率、非晶相比例和干摩擦性能,研究了Ni-Al粒子包覆前后涂层的微观结构与摩擦机理变化。结果表明:Ni-Al粒子成功包覆在Fe基非晶表面形成了结构致密的壳-核结构复合粉末,粉末粒径小于50μm,包覆层厚度1～3μm。涂层孔隙率下降5%左右,由于Ni-Al稀释作用,非晶含量由72.5%降低为64.8%,复合涂层的结构显示为Ni3Al相和Fe基非晶形成的层片状。相对于传统涂层结构,复合涂层更能够强化非晶涂层的层状堆积的优点。相比Fe基非晶涂层,复合涂层的摩擦系数由0.412 3降低到0.256 1,磨损量为原涂层的1/2左右。Ni-Al粒子通过改进Fe基非晶粒子的熔解形态和延展能力,有效提高了非晶涂层的致密度,涂层非晶含量降低不明显,耐磨性显著增强。     

Sliding wear behaviour of Al-Si-Cu composites reinforced with SiC particles

Abstract

The sliding wear behaviours of an unreinforced monolithic Al-Si-Cu alloy and SiC particles reinforced composites containing 5, 13, 38 and 50 vol.-% with diameters of 5.5, 11.5 and 57μm were investigated. The results showed that the wear resistance of the composites is much higher than the monolithic alloy, and the larger and the more SiC particles, the higher the enhancement of the wear resistance. Metallographic examinations revealed that the subsurface of worn composites was composed of both fragmented particles and deformed matrix alloy. The depth of the particle fracture zone in the subsurface varied in the range of 20-35 μm at a sliding distance of 1.8 km, while the plastic deformation zone of the worn subsurface on monolithic alloy was more than 100 μm. Scanning electron microanalyses of the worn surface, subsurface microstructure and debris suggested that the depth of the particle fracture zone became smaller as the diameter of SiC particles increased. Increasing the hardness and decreasing the applied wear stress changed the debris morphology from flake to very small lumps.     

A microstructural and mechanical study on the effects of carbon ion implantation on zirconia-toughened-alumina

Biomedical grade (>99.97% purity) alumina, zirconia and zirconia-toughened-alumina (ZTA) have been implanted with carbon ions at a dose of 5 × 10¹⁷ C ions/cm² using an ion energy of 75 keV. The near-surface hardness of these bioceramics was examined using a load partial-unload indentation technique, both before and after implantation. The surfaces of the bioceramics have also been examined in cross-section using transmission electron microscopy (TEM) both before and after implantation and the implantation data correlated with a computer based simulation, TRIM (Transport and Range of Ions in Matter). The grinding and polishing treatment used prior to the implantation treatment has been found to have a strong influence on the surface microstructures for all three ceramics, although more significant modifications are brought about by carbon ion implantation. A comparison was made between the near-surface hardness of the unimplanted and carbon ion implanted surfaces of these bioceramics with relation to the modified microstructure. TEM examination of the implanted surfaces has demonstrated the formation of a sub-surface amorphous layer in all three materials as well as other microstructural modifications, such as microcracking and an increase in the near-surface dislocation density, that are characteristic of ion damage. The hardness data reveals that carbon ion implantation tends to decrease the surface hardness of alumina and zirconia with increasing ion dose, with a significant decrease occurring at the immediate near surface for both materials.     

Factors controlling dry sliding wear behaviour of a leaded tin bronze

The sliding wear behaviour of a leaded tin bearing bronze was investigated over a range of applied pressures and sliding speeds with respect to the influence of microconstituents such as lead on the wear response. Significantly high wear rates were found at the minimum sliding speed due to extensive microcracking. This was evinced by the formation of coarse debris and considerable subsurface/wear surface cracking. The (micro) cracking tendency of the alloy prohibited the occurrence of subsurface deformation. The absence of a lead film was primarily due to the lead particles being engulfed in the coarse debris. Higher sliding speeds led to increased frictional heating making the alloy matrix viscoplastic. This in turn greatly suppressed the tendency of the alloy to exhibit microcracking, thereby facilitating interaction between the materials of the mating surfaces through wear induced plastic deformation. As a result, a stable transfer layer formed on the specimen surface. Interestingly, the formation of a lead film on the wear surface was also observed under these conditions. The above factors were mainly responsible for the improved wear behaviour of the alloy at higher speeds. Finer debris formation, less surface and subsurface damage, and the presence of both a deformed and stable transfer layer and a lead film strongly supported these observations. Material removal mechanisms involved delamination of the undeformed subsurface region causing chipping off at the minimum sliding speed. Higher speeds, however, caused delamination of the transfer layer. In addition to adhesion, three body abrasion was found to contribute considerably towards material removal. The formation and stability of a transfer layer and the presence of a lead film are at least two major factors which control the wear behaviour of leaded tin bronzes. It is found that the above phenomena which provide improved wear characteristics occur only under specific sliding conditions.

MST/3218     

Fractographic study of the alumina and zirconia particles embedded in mullite prepared by reaction sintering

Four different mullite-alumina-zirconia composites have been prepared by reaction sintering between alumina and zircon powders using magnesia or spinel (MgAl₂O₄) to increase the sintering and reaction rates. The microstructure of these materials can be described as composed of two parts: the first one is the mullite matrix containing various kinds of zirconia and alumina particles, whereas the second part is an amorphous phase in which alumina submatrices, zirconia and spinel particles are embedded. Examination of fracture surfaces allows one to identify the crack paths and shows that the main differences are related to zirconia inclusions. Analysis of mechanical properties and fracture features leads to the conclusion that crack deflection and microcracking are operative toughening mechanisms for the various materials. Moreover, a crack bowing mechanism is proposed to explain the higher modulus of rupture found for the series of materials prepared with magnesia as a reaction sintering aid. On leave from Instituto de Ceramica Y Vidrio, CSIC, Arganda del Rey, Madrid, Spain.     

The microstructure and wear resistance characteristics of electroformed nickel and partially stabilized zirconia composite coatings

Ni-PSZ composite coatings with various PSZ particle content were prepared by the electroforming technique. The microstructure and surface components of the coatings have been examined by optical microscopy, electron microscopy and X-ray photoelectron spectroscopy analysis and the wear properties of the coatings tested on a reciprocating wear test machine. The results show that the PSZ particles are uniformly dispersed in the coatings and thus increase the wear resistance of the coatings by inhibiting plastic deformation of the nickel matrix. The co-deposition of the PSZ particles in the electrolyte is mainly in the form of agglomeration and is accompanied by the incorporation of Ni(OH)₂. When the PSZ content in a coating is higher than a critical value, the wear resistance of the coating could deteriorate because of the decrease in the integrity of the nickel matrix. After heat-treatment at high temperature, Ni(OH)₂ in the coating is turned into Ni₂O₃ and NiO which can wet the PSZ particles and increase the bonding strength between the PSZ and nickel. In addition, the agglomerated PSZ particles are sintered when heat-treated. These are all beneficial to increasing the wear resistance of the coating.     

First evidence of tyre debris characterization at the nanoscale by focused ion beam

In this paper, we present a novel technique for the nanoscale characterization of the outer and inner structure of tyre debris.

Tyre debris is produced by the normal wear of tyres. In previous studies, the microcharacterization and identification were performed by analytical electron microscopy. This study is a development of the characterization of surface and microstructure of tyre debris. For the first time, tyre debris was analysed by focused ion beam (FIB), a technique with 2- to 5-nm resolution that does not require any sample preparation. We studied tyre debris produced in the laboratory. We made electron and ionic imaging of the surface of the material, and after a ionic cut, we studied the internal microstructure of the same sample. The tyre debris was analysed by FIB without any sample preparations unlike the case of scanning and transmission electron microscopy (SEM and TEM). Useful information was derived to improve detection and monitoring techniques of pollution by tyre degradation processes.     

Mechanical-contact-induced transformation from the amorphous to the partially crystalline state in metallic glass

Sliding friction and wear experiments and electron microscopy and diffraction studies were conducted to examine the metallurgical microstructure of a metallic glass surface strained in sliding contact. Friction and wear experiments were conducted with aluminium oxide spheres 3.2 and 6.4 mm in diameter sliding, in reciprocating motion, on a metallic foil with a composition of Fe₆₇Co₁₈B₁₄Si₁ at a sliding velocity of 1.5 mm s^-1 (frictional heating is negligible) with a load of 2.5 N at room temperature and in a laboratory air atmosphere.The results of the investigation indicate that the amorphous alloy (metallic glass) can be crystallized during mechanical contact. Crystallites with a size range of 10–150 nm are produced on the wear surface of the amorphous alloy. A diffused honeycomb-shaped structure formed by dark gray bands is also produced during sliding. Considerable plastic flow occurs on an amorphous alloy surface with sliding and the flow film of the alloy transfers to the aluminium oxide pin surface. Multiple slip bands due to shear deformation are observed on the side of the wear track. Two distinct types of wear debris were observed as a result of sliding: an alloy wear debris and/or powdery and whiskery oxide debris. The wear rate of Fe₆₇Co₁₈B₁₄Si₁ was 5 × 10^-9 mm³ N^-1.     

Evidence of graphitization of diamond-like carbon films during sliding wear

Diamond-like carbon (DLC) exhibits excellent wear and friction characteristics. Transmission electron microscopy (TEM) has been used to investigate the substructures of as-deposited DLC and DLC debris after wear testing. The as-deposited DLC was found to consist of a dense, three-dimensional network structure with a medium range order (<3 nm) present. Diffraction pattern analysis showed that DLC was mainly amorphous. Two diffuse diffraction rings with d111=0.21 nm and d220=0.12 nm were observed, suggesting the presence of a short-range cubic diamond structure (sp3). Morphologically, the wear debris was found to be a discontinuous segregation of carbon particles ranging from nano- to micro-size. Diffraction pattern analysis showed that the debris consisted of graphite (sp2) and distorted DLC (sp3). A wear mechanism has been proposed based on the transformation of DLC to graphite. The transformation is related to the frictional energy and includes two stages: hydrogen release from the structure causing lattice relaxation and shear deformation of the DLC structure producing graphite.     

Physicochemical Properties of Indomethacin and Related Compounds Co-Spray Dried with Polyvinylpyrrolidone

Abstract

The spray drying of indomethacin produced a viscous liquid phase which then solidified to an amorphous glassy solid mass. This amorphous phase was physically unstable and converted on storage to crystalline indomethacin forms II and I. Co -spray drying indomethacin with up to 20% PVP also gave a fused amorphous solid. Apparent solubility and dissolution studies illustrated the higher energy of indomethacin in these systems. The presence of PVP in the solid retarded conversion of indomethacin to a crystalline phase, the effect increasing with increasing PVP content. Scanning electron microscopy revealed the growth, with time, of needle-like whiskers on the surface of the amorphous products. Co-spray drying indomethacin with more than 20% PVP resulted firstly in products composed of a fused network of spherical particles, then partially coalesced spheres and ultimately, above 25% PVP, in individual agglomerated microspherical particles. The spray drying of either naproxen, ketoprofen or ibuprofen did not result in the formation of an amorphous glassy solid, Co-spray drying with PVP led to reduced crystallinity; the size of the melting endotherm decreased with increasing PVP content and became absent at 50% PVP. As the PVP percentage increased a microspherical product also developed, but at PVP levels greater than that observed for indomethacin.     

Surface analysis of argon-implanted zircalloy-2 and influence of bubble formation on the corrosion behavior

In order to simulate the irradiation damage, argon ions were implanted into zircalloy-2 alloy with a fluence ranging from 1×10¹⁶ to 1×10¹⁷ ions/cm², using an implanter at an extraction voltage of 190 kV, at liquid nitrogen temperature. Then the effect of argon ion implantation on the aqueous corrosion behavior of zircalloy-2 alloy was studied. The valence states of elements in the surface layer of the samples were analyzed by X-ray photoelectron spectroscopy (XPS). Transmission electron microscopy (TEM) was used to examine the microstructure of the argon-implanted samples. Glancing angle X-ray diffraction (GAXRD) was employed to examine the phase transformation due to the argon ion implantation. The potentiodynamic polarization technique was employed to evaluate the aqueous corrosion resistance of implanted zircalloy-2 alloy in a 1 M H₂SO₄ solution. It was found that the bubbles were formed on the surface of implanted samples; the bubbles grew larger with increasing argon fluence. The microstructure of argon-implanted samples changed from amorphous to partial amorphous, then to polycrystalline and finally to amorphous. The bubble forming and changing and microstructure changes affected the corrosion properties of implanted samples. Finally, the mechanism of the corrosion behavior of argon-implanted zircalloy-2 alloy is discussed.     

Microstructure and wear properties of Fe–TiC surface composite coating by laser cladding

AISI 1045 steel surface was alloyed with pre-placed ferrotitanium and graphite powders by using a 5-kW CO₂ laser. In situ TiC particles reinforced Fe-based surface composite coating was fabricated. The microstructure and wear properties were investigated by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, as well as dry sliding wear test. The results showed that TiC carbides with cubic or flower-like dendritic form were synthesized via in situ reaction between ferrotitanium and graphite in the molten pool during laser cladding process. The TiC carbides were distributed uniformly in the composite coating. The TiC/matrix interface was found to be free from cracks and deleterious phase. The coatings reinforced by TiC particles revealed higher wear resistance than that of the substrate.     

Microstructure and crystal structure development in porous titania coatings prepared from anhydrous titanium ethoxide solutions

Porous titania coatings were prepared by spin coating anhydrous titanium ethoxide–ethanol solutions in a controlled humidity atmosphere. Ti ethoxide reacted with atmospheric moisture during deposition, to form amorphous particles (approximately 200 nm), a dense layer or a combination of the two, depending on the processing conditions. Relatively humid atmospheres, low concentrations of Ti ethoxide in the coating solution and slow spinning rates favoured particle formation. These particulate coatings were typically composed of agglomerated particle clusters. Agglomeration could be prevented by adding hydroxypropyl cellulose to alkoxide solution to act as a steric stabilizer for newly formed particles. During thermal treatment, the coatings crystallized into the anatase phase and then transformed into the rutile structure at higher temperatures. The anatase–rutile transformation in porous coatings occurred over a range of 850–1150°C and strongly depended on microstructural features. More porous coatings with larger particle clusters transformed to rutile at lower temperatures. Tensile stress in the coating caused by constrained shrinkage inhibited the phase transformation. The substrate constraint slowed the transformation rate in coatings relative to free powder. Stress relief through rupture of particle cluster connections allowed transformation to occur at lower temperatures. © 1998 Kluwer Academic Publishers     

断裂方式对氧化铝基复合陶瓷耐磨性的影响

通过磨粒磨损试验测定了氧化铝 陶瓷、氧化铝/碳化硅复合陶瓷和氧化铝/莫来石复合陶瓷的耐磨性. 利用透射电子显微镜(TEM)观察了样品的微观结构, 采用扫描电子显微镜(SEM)分析了样品的断口形貌和磨损表面的剥落情况. 研究了断裂方式对磨损表面剥落和耐磨性的影响. 结果表明: 氧化铝陶瓷的磨损主要由断裂磨损机制控制, 氧化铝/碳化硅复合陶瓷的磨损主要由塑性磨损机制控制, 氧化铝/莫来石复合陶瓷受这两种磨损机制共同作用. 相对于氧化铝陶瓷, 氧化铝/碳化硅复合陶瓷和氧化铝/莫来石复合陶瓷的耐磨性提高2～4倍, 这主 要是由于其断裂方式转变为以穿晶断裂为主, 减少了磨损面的脆性剥落.     

Microstructural and magnetic characterization of alumina-iron nanocomposites

Iron-alumina nanocomposite powders containing 10 wt % iron were prepared by selective reduction of alumina-haematite solid solutions. Microstructural study showed three types of metal dispersion in the alumina matrix according to the elaboration process: iron grains that were >70 nm, most of the iron particles were <10 nm and directly epitaxied in the alumina matrix, and iron particles that were surrounded by an interfacial phase. In agreement with transmission electron miscroscopy (TEM) observations, magnetic study confirmed a distribution of the iron particles size, showing the superposition of a ferromagnetic behaviour (larger particles) and a superantiferromagnetic behaviour (smaller particles). Furthermore, analysis of thermoremanent behaviour, coercive field and dissymmetry of hysteresis loops allowed the interfacial phase surrounding some iron particles to be identified as an antiferromagnetic phase, Fe_1+x Al_2–x O₄. Nevertheless, at the interface of metallic iron epitaxied on the alumina matrix some atomic planes always existed where iron was ionic (even if no other phase was detected). As a consequence the mean magnetic moment of iron in these nanocrystals is larger than that in bulk metallic iron.     

干摩擦应变诱导板条马氏体固态非晶化的能量分析及其模型

使用定点离子束切割制样(FIB)并根据透射电镜(TEM)表征，分析了板条马氏体钢干摩擦层内部板条马氏体协调塑性变形、演变为纳米层片结构并发生非晶化的全过程。结果表明，高密度位错缠结和缺陷集中是纳米层片结构的典型特征，这种结构产生的界面在高应变驱动下发生非晶化。这些非晶产物，为进一步细化磨屑和形成表面自润滑层提供结构条件。基于上述实验结果并结合摩擦学和材料学理论建立了干摩擦过程中的非晶化形核模型，计算了发生非晶化的热力条件和能量壁垒。结果表明，根据经典形核理论和晶体向非晶转变的吉布斯自由能壁垒计算公式所建立的干摩擦非晶化形核能量模型，可用于计算发生非晶化必需的临界位错密度值。根据对应的计算结果，可控制摩擦条件用干摩擦应变诱导板条马氏体的固态非晶化。