Hot deformation behaviour of bimodal sized Al2O3/Al nanocomposites fabricated by spark plasma sintering

The deformation behaviour of bimodal sized Al₂O₃/Al nanocomposites were investigated by hot compression tests conducted in the temperature range 350–500°C and strain rates of 0.001, 0.01 and 0.1 s^–1. The dynamic recrystallisation behaviour of the nanocomposites strongly depended on the forming parameters. The bimodal sized Al₂O₃ particles played a crucial role in the recrystallised microstructure. The addition of bimodal sized Al₂O₃ particles led to a significant increase of activation energy of plastic deformation, corroborating the enhanced resistance of the nanocomposite to hot deformation. This was also reflected by the increased compressive yield strength in the nanocomposite due to both dislocation strengthening caused by n‐Al₂O₃ and preventing the grain growth due to the presence of μ‐Al₂O₃ at grain boundaries. It was found that with the decrease of Z values, local strain induced by deformation was released and the grain size of aluminium matrix gradually increased, indicating that the main softening mechanism of the bimodal sized Al₂O₃/Al nanocomposites was dynamic recrystallisation (DRX). The lower the Z value was, the easier the DRX occurred. The highly beneficial role of the bimodal sized Al₂O₃ reinforcement in improving the high‐temperature performance of aluminium matrix nanocomposite was discussed.     

Synthesis and characterization of permalloy-reinforced Al2O3 nanocomposite powders by mechanical alloying

Intermetallics of Fe and Ni, which are known as permalloy, are under attention due to their excellent magnetic performance. Besides, mechanical properties of the materials can be improved by decreasing crystallite size of FeNi intermetallics or by reinforcing them with hard secondary phases such as Al₂O₃. In this study, FeNi–Al₂O₃ nanocomposite powders with three different compositions were successfully synthesized through mechanical alloying of Fe₂O₃, Ni, and Al powders mixture. Characterization of the samples was accomplished by scanning electron microscopy, transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy and X-ray diffraction. Effects of various parameters such as chemical composition of received materials, milling time, and annealing on the phase evolution, morphology, and microhardness of samples were investigated. It was found that by the addition of Fe as diluent, the required milling time for formation of FeNi intermetallic increased. By increasing milling time, mean crystallite size of FeNi decreased and reach to about 28 nm for FeNi-30 wt% Al₂O₃ nanocomposite powder sample. TEM observations also showed that in situ-formed Al₂O₃ particles, with particle size of about 65 nm, were uniformly dispersed within FeNi matrix.     

Specimen preparation technique for high resolution transmission electron microscopy studies on model supported metal catalysts

A new technique is described which can be used for preparing transmission electron microscopy (TEM) specimens suitable for high resolution studies on supported metal catalysts. By conventional silicon processing techniques 200 × 200 μm² Si₃N₄ membranes on Si wafers are produced. These membranes are extremely flat and have a uniform thickness of 13 nm. They can be used as a support in various kinds of thin film deposition. A TiO₂ film, optimally structured with respect to the requirements for high resolution TEM work in TiO₂–metal cluster systems, is deposited on the Si₃N₄ layer. It consists of one monolayer of 10–25 nm TiO₂ crystallites. TiO₂ lattice images show that a line resolution down to 0.19 nm is possible. Examples of TiO₂–Pd and TiO₂–Rh are given using respectively photodeposition and impregnation reduction to produce l.5–4 nm metal clusters.     

Lubrication of Si3N4 and Al2O3 in water with and without addition of silane coupling agents in the range of 0.05–0.10 mol/l

To improve water lubrication of ceramics at a lower sliding velocity, the effect of the addition of silane coupling agents was investigated. Si₃N₄ and Al₂O₃ were slid against themselves in water with and without the addition of silane coupling agents in amounts ranging from 0.05 to 0.10 mol/l. Silane coupling agents containing one or more amino groups were effective in reducing the friction of Si₃N₄ and Al₂O₃ in water. Si₃N₄ also showed significant wear reduction but not Al₂O₃. However, the addition of a silane coupling agent containing an epoxy group increased both friction and wear of Si₃N₄. Improved lubricative characteristics of Si₃N₄ in water and in silane coupling agent solutions were obtained when Si₃N₄ contained smaller amounts of sintering additives. The adsorption behaviour of a silane coupling agent on ceramics was examined using both Fourier transform infrared spectroscopy and thin layer chromatography to clarify the interaction between the silane coupling agent and the ceramics. The role of polysiloxane film formation on ceramics is discussed to demonstrate the lubrication properties of ceramics.     

Microtribological Properties of Two-Phase Al2O3 Ceramic Studied by AFM and FFM in Air of Different Relative Humidity

Commercially available, monolithic alumina ceramic was modified using CO₂-laser irradiation by surface remelting and adding HfO₂ powder. Scanning electron microscopy revealed that the microstructure of the modified ceramic consisted of a fine lamellar eutectic Al₂O₃–HfO₂ phase embedded in the Al₂O₃ matrix. Differences in the microtribological properties of the matrix and the eutectic phase could be measured by friction force microscopy (FFM) during unlubricated sliding contact with a silicon tip at room temperature as a function of relative humidity of the surrounding air and normal load. The dependence of the friction coefficient and the pull-off force on humidity was explained by the formation of lubricating tribochemical surface layers and described by theoretical models.     

Sliding wear characteristics of plasma-sprayed Al2O3 and Cr2O3 coatings against copper alloy under severe conditions

Al₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying and their tribological properties dry sliding against copper alloy were evaluated using a block-on-ring configuration at room temperature. It was found that the wear resistance of Al₂O₃ coating was superior to that of the Cr₂O₃ coating under the conditions used in the present study. This mainly attributed to its better thermal conductivity of Al₂O₃ coating, which was considered to effectively facilitate the dissipation of tribological heat and alleviate the reduction of hardness due to the accumulated tribological heat. As for the Al₂O₃ coating, the wear mechanism was plastic deformation along with some micro-abrasion and fatigue-induced brittle fracture, while the failure of Cr₂O₃ coating was predominantly the crack propagation-induced detachment of transferred films and splats spallation.     

Frictional deformation and fracture in polycrystalline SiC and Si3N4

Electron optical microscopy was employed to study the friction and wear of commercial polycrystalline varieties of SiC and Si₃N₄ in air at ambient temperature. Friction and wear tests were conducted in a reciprocating configuration with conical riders (both diamond and ceramic) sliding on a flat ceramic substrate. Worn surfaces were examined by both scanning electron microscopy and transmission electron microscopy. In general, friction and wear in the diamond-ceramic couples were severe. Friction with ceramic-ceramic couples was low, with friction coefficients between 0.1 and 0.4, wear being absent in single-pass tests.With ceramic-ceramic couple multipass systems, wear of Si₃N₄ occurs by plastic deformation which increases in severity with sliding distance accompanied by a corresponding increase in friction coefficient. With SiC, wear occurs by a mixture of intergranular fracture due to grain boundary weakness and plastic deformation.     

Triboemission, tribochemical reaction, and friction and wear in ceramics under various n-butane gas pressures

The triboemission behaviour of negatively and positively charged particles and photons generated by scratching a Si₃N₄ disc with a conical diamond stylus was studied under various gas pressures of n-butane as a model compound of typical paraffinic lubricating oil. The triboemission behaviour of photon emission in a frictional system of an Al₂O₃ ball sliding on Al₂O₃ and Si₃N₄ discs was also measured under various n-butane gas pressures. Further, the friction and wear in the frictional system of an Al₂O₃ ball sliding on an Si₃N₄ disc were studied under various n-butane gas pressures. The triboemission intensities of three kinds of energetic particles, negatively and positively charged particles and photons, become a maximum at a particular n-butane gas pressure. When the n-butane gas pressure is such that triboemission is a maximum, the formation of friction polymer-like high molecular weight products is at a maximum, and friction and wear of the ball decreases to a minimum. It has been concluded that the friction polymer-like high molecular weight products are formed through tribochemical reactions triggered by triboemission phenomena and help reduce the friction and wear of the solids.     

Effect of Counterface on the Tribological Behavior of Ti3AlC2 at Ambient

In this paper, we investigated the effect of counterface of AISI 52100 steel and ceramics of SiC, Al₂O₃, and Si₃N₄ on the tribological behavior of Ti₃AlC₂ at ambient. The results showed that the tribological properties of Ti₃AlC₂ are strongly dependent on the counterfaces. Under present test conditions, the tribocouple of Ti₃AlC₂/SiC exhibits lower coefficient of friction (CoF) 0.40, Ti₃AlC₂/AISI 52100 steel shows a rising CoF from 0.1 to 0.63, both Ti₃AlC₂/Si₃N₄ and Ti₃AlC₂/Al₂O₃ tribopartners exhibit higher CoF of 1.22 in average. The Ti₃AlC₂ exhibits the lowest wear rate sliding against AISI 52100 steel, the wear rate of Ti₃AlC₂ sliding against Al₂O₃, Si₃N₄, and SiC are higher and comparable. The morphologies of the worn surfaces of the Ti₃AlC₂ are observed by scanning electron microscopy, and the element states are analyzed by X-ray photoelectron spectroscopy. The wear mechanisms are discussed.     

Microstructural and wear properties of sputtered carbides and silicides

Sputtered Cr₃C₂, Cr₃Si₂ and MoSi₂ wear-resistant films (0.05–3.5 μm thick) were deposited on metal and glass surfaces. Electron transmission, electron diffraction and scanning electron microscopy were used to determine the microstructural appearance. Strong adherence was obtained with these sputtered films. Internal stresses and defect crystallographic growth structures of various configurations within the film have progressively more undesirable effects for film thicknesses greater than 1.5 μm. Sliding contact and rolling element bearing tests were performed with these sputtered films. Bearings sputtered with a duplex coating (a 0.1 μm thick undercoating of Cr₃Si₂ and subsequently a 0.6 μm coating of MoS₂) produced marked improvement (more than 10.5 × 10⁷ cycles) over straight MoS₂ films.     

The Tribological Behaviors of Different Mass Ratio Al2O3-TiO2 Coatings in Water Lubrication Sliding against Si3N4

The tribopairs of water hydraulic plunger pumps are usually operated under severe conditions, due to the poor lubrication of water and silt suspended in natural water. It is essential to identify the desired engineering materials and material combinations for designing water pumps. As the candidate materials of tribopairs, the tribological characteristics of different Al₂O₃-TiO₂ coatings combined with Si₃N₄ ceramics under silt-laden water and tap water lubrication were investigated. The Al₂O₃-TiO₂ coatings with different weight percentages of TiO₂ in a wide range from 3 to 100% were tested. The tribological characteristics of the various couple pairs were researched using a ring-on-ring test rig. The microstructures of the contact surfaces were analyzed with a scanning electron microscope before and after the test to study the wear mechanism. The experimental results indicate that the friction coefficient of the Al₂O₃-TiO₂/Si₃N₄ tribopairs increases with an increase in the percentage of TiO₂ content in the Al₂O₃-TiO₂ coating. However, the TiO₂ content does not present a clear relationship with the wear rate. Considering the friction and wear properties, Al₂O₃-13%TiO₂ is the preferred coating to use in water hydraulic pumps when sliding against Si₃N₄.     

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Friction and wear behaviors of diamond-like carbon (DLC) film in humid N₂ (RH-100%) sliding against different counterpart ball (Si₃N₄ ball, Al₂O₃ ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si₃N₄ ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si₃N₄ ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al₂O₃ ball caused the highest wear rate of DLC film when sliding against Al₂O₃ ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.     

Improved wear resistance of Si3N4 tool inserts by addition of Al2O3 platelets

In the present work, Si₃N₄ matrix composites reinforced with different amounts of Al₂O₃ platelets (0, 30 and 50vol%) were produced with the aim of increasing the tribochemical resistance in the machining of steels. Tool wear was related to the linear increase of the main cutting force (F_c) with time (dF_c/dt); a real-time parameter that can be used to assess the cutting edge damage and to stop machining before the tool fails. For all machined steels, tool wear resistance increased with increasing Al₂O₃ platelet content.     

High-temperature low-friction properties of vanadium-alloyed AlCrN coatings

V-alloyed AlCrN hard coatings were deposited by reactive arc-evaporation in a commercial Balzers RCS coating system. The Al/(Al + Cr + V) atomic ratio in the target was kept constant at 0.7, while the Cr was stepwise substituted by V. Alloying V into the AlCrN coatings led to the partial transformation to a hexagonal phase of the otherwise metastable cubic lattice, whereas the hexagonal phase fraction was observed to increase with the V concentration in the coating. This results in a reduction of hardness (from 28 to 19 GPa) and biaxial coating stress (from − 1700 to − 900 MPa). The performed tribological tests at high temperature revealed a significantly lower coefficient of friction down to a value of 0.2 for Al_0.67Cr_0.05V_0.28N at 700 °C. This behaviour is due to the formation and melting of the Magnéli phase oxide V₂O₅ forming a low-friction layer on the surface as indicated by Raman spectroscopy and scanning electron microscopy.     

Wear Behavior of Composites Based on ZA-27 Alloy Reinforced by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particles Under Dry Sliding Condition

In present study, the effect of Al₂O₃ particle reinforcement on the sliding behavior of ZA-27 alloy composites was investigated. The composites with 3, 5, and 10 wt% of Al₂O₃ particles were produced by the compocasting procedure. Tribological properties of unreinforced alloy and composite were studied, using block-on-disk tribometer under unlubricated sliding conditions at different specific loads and sliding speeds. The worn surfaces of samples were examined by the scanning electron microscopy (SEM). The test results revealed that those composite specimens exhibited significantly lower wear rate than the ZA-27 matrix alloy specimens in all combinations of applied loads and sliding speeds. The difference in the wear resistance of composite with respect to the matrix alloy, increased with the increase of the applied load/sliding speed and Al₂O₃ particle content. The highest degree of improvement of the ZA-27 alloy tribological behavior corresponded with change of the Al₂O₃ particles content from 3 to 5 wt%. At low sliding speed, moderate lower wear rate of the composites over that of the matrix alloy was noticed. This has been attributed to micro cracking tendency of the composites. Significantly reduced wear rate, experienced by the composite over that of the matrix alloy at the higher sliding speeds and loads, could be explained due to enhanced compatibility of matrix alloy with dispersoid phase and greater thermal stability of the composite in view of the presence of the dispersoid. Level of wear rate of tested ZA-27/Al₂O₃ samples pointed to the process of mild wear, which was primarily controlled by the formation and destruction of mechanical mixed layers (MMLs).     

Chemical reactions and morphological stability at the Cu/Al2O3 interface

The microstructures of diffusion‐bonded Cu/(0001)Al₂O₃ bicrystals annealed at 1000 °C at oxygen partial pressures of 0.02 or 32 Pa have been studied with various microscopy techniques ranging from optical microscopy to high‐resolution transmission electron microscopy. The studies revealed that for both oxygen partial pressures a 20–35 nm thick interfacial CuAlO₂ layer formed, which crystallises in the rhombohedral structure. However, the CuAlO₂ layer is not continuous, but interrupted by many pores. In the samples annealed in the higher oxygen partial pressure an additional reaction phase with a needle‐like structure was observed. The needles are several millimetres long, ～10 µm wide and ～1 µm thick. They consist of CuAlO₂ with alternating rhombohedral and hexagonal structures. Solid‐state contact angle measurements were performed to derive values for the work of adhesion. The results show that the adhesion is twice as good for the annealed specimen compared to the as‐bonded sample.     

Effects of friction on topography and vice versa

Experimental results of friction and topography measurements are presented which demonstrate the mutual modification of friction and contact topography. The effect of topographical ‘landmarks’ on friction was tested by Al₂O₃-balls sliding over Ti-ridges on Ti-surface and by Si₃N₄-balls sliding over grooves in SiO₂-surfaces. However, experiments of 100Cr6-balls sliding against 100Cr6-substrates in ultrahigh vacuum, Al₂O₃-balls sliding on DLC coated 100Cr6 and Si₃N₄-balls sliding on SiO₂-surfaces reveal that the formation of triboreaction layers and moreover the creation of wear particles can screen the effect of the topographical ‘landmarks’ completely. Wear particles and their exact behaviour in the contact area can affect friction in a stochastical and hence unpredictable way. Most modern friction theories have difficulties in coping with this problem.     

TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy

The structure of Al₃Zr precipitates in Al‐1.0Mg‐0.6Si‐0.5Zr (in wt.%) alloy was investigated using conventional transmission electron microscopy (TEM) and high‐resolution TEM (HREM). After annealing of the alloy in the temperature range 450–540 °C, spherical precipitates of metastable L1₂‐Al₃Zr phase appeared nearly homogeneously within the matrix, and elongated particles were found at grain boundaries. L1₂‐structured Al₃Zr were about 20–30 nm in diameter and coherent with the matrix. Inside some of them, planar faults parallel to {100} planes were revealed by use of HREM. Most probably, these faults are an indication of the transition stage of transformation to the stable D0₂₃‐type Al₃Zr phase. The elongated precipitates (about 100 nm) were identified as D0₂₂‐type Al₃Zr. Energy‐dispersive X‐ray analysis showed that they contain, apart from Al, mainly Zr with small amounts of Si. The substitution of Al by Si increased the stability of the D0₂₂‐Al₃Zr as compared with D0₂₃‐Al₃Zr.     

A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition

The friction and wear properties of the polyetheretherketone (PEEK) based composites filled with 5 mass% nanometer or micron Al₂O₃ with or without 10 mass% polytetrafluroethylene (PTFE) against the medium carbon steel (AISI 1045 steel) ring under the dry sliding condition at Amsler wear tester were examined. A constant sliding velocity of 0.42 m s⁻¹ and a load of 196 N were used in all experiments. The average diameter 250 μm PEEK powders, the 15 or 90 nm Al₂O₃ nano-particles or 500 nm Al₂O₃ particles and/or the PTFE fine powders of diameter 50 μm were mechanically mixed in alcohol, and then the block composite specimens were prepared by the heat compression moulding. The homogeneously dispersion of the Al₂O₃ nano-particles in PEEK matrix of the prepared composites was analyzed by the atomic force microscopy (AFM). The wear testing results showed that nanometer and micron Al₂O₃ reduced the wear coefficient of PEEK composites without PTFE effectively, but not reduced the friction coefficient. The filling of 10 mass% PTFE into pure PEEK resulted in a decrease of the friction coefficient and the wear coefficient of the filled composite simultaneously. However, when 10 mass% PTFE was filled into Al₂O₃/ PEEK composites, the friction coefficient was decreased and the wear coefficient increased. The worn scars on the tested composite specimen surfaces and steel ring surfaces were observed by scanning electron microscopy (SEM). A thin, uniform, and tenacious transferred film on the surface of the steel rings against the PEEK composites filled with 5 mass% 15 nm Al₂O₃ particles but without PTFE was formed. The components of the transferred films were detected by energy dispersive spectrometry (EDS). The results indicated that the nanometer Al₂O₃ as the filler, together with PEEK matrix, transferred to the counterpart ring surface during the sliding friction and wear. Therefore, the ability of Al₂O₃ to improve the wear resistant behaviors is closely related to the ability to improve the characteristics of the transfer film.