Microabrasion-corrosion of cast CoCrMo alloy in simulated body fluids

Wear and corrosion of metal-on-metal hip replacements results in wear debris and metal-ion release in vivo, which may subsequently cause pain and hypersensitivity for patients. Retrieved metal-on-metal hip replacements have revealed that two-body sliding wear and three-body abrasive wear are the predominant wear mechanisms. However, there is a lack of understanding of the combined effects of wear/corrosion, especially the effect of abrasion-corrosion.

This study investigates the sliding-corrosion and abrasion-corrosion performance of a cast CoCrMo alloy in simulated hip joint environments using a microabrasion rig integrated with an electrochemical cell. Tests have been conducted in 0.9% NaCl, phosphate buffered saline solution, 25% and 50% bovine serum solutions with 0 or 1 g cm(-3) SiC at 37 degrees C. Experimental results reveal that under abrasion-corrosion test conditions, the presence of proteins increased the total specific wear rate. Conversely, electrochemical noise measurements indicated that the average anodic current levels were appreciably lower for the proteinaceous solutions when compared with the inorganic solutions. A severely deformed nanocrystalline layer was identified immediately below the worn surface for both proteinaceous and inorganic solutions. The layer is formed by a recrystallisation process and/or a strain-induced phase transformation that occurs during microabrasion-corrosion. (C) 2008 Elsevier Ltd. All rights reserved.

Abrasive size and concentration effects on the tribo-corrosion of cast CoCrMo alloy in simulated body fluids

Some retrieved CoCrMo hip implants have shown that abrasive wear is one of the possible wear mechanisms invoked within such joints. To date, little work has focused on the third body abrasion of CoCrMo and therefore there is a general lack of understanding of the effect of abrasive size and volume concentration on the tribo-corrosion performance of the CoCrMo alloys. The present work assessed the tribo-corrosion behaviour of cast CoCrMo (F-75) under various abrasion-corrosion conditions by using a modified microabrasion tester incorporating a three-electrode electrochemical cell. The effects of reduced abrasive size/hardness and volume concentration, as well as the role of proteins on the tribo-corrosion performance of the cast CoCrMo alloy were addressed. The correlation between electrochemical and mechanical processes for different abrasion-corrosion test conditions has been discussed in detail. Results show that the reduction in abrasive size and volume concentration can significantly affect the abrasion-corrosion wear mechanisms and the wear-induced corrosion response of the material. The finding of this study implies that the smaller/softer third body particles generated in vivo could also result in significant wear-induced corrosion and therefore potential metal ion release, which could be potentially detrimental to both the patient health and the life span of the implants. © 2009 Elsevier Ltd. All rights reserved.

Evaluation and mechanistic investigation of a AuPd alloy catalyst for the hydrocarbon selective catalytic reduction (HC-SCR) of NOx

The ability of a gold palladium bimetallic catalyst to selectively oxidise toluene has been used to enhance the hydrocarbon selective catalytic reduction of NOx, a reaction in which the interaction of partial oxidation intermediates is considered important. The combination of gold with palladium has a synergistic effect, producing a catalyst that is more active for NOx conversion than the arithmetic sum of the corresponding mono-metallic materials. Three regimes in the conversion profile of the AuPd catalyst are proposed relating to production and consumption of toluene derived species, such as benzaldehyde and benzonitrile. The possible role of these reaction intermediates in the toluene HC-SCR reaction is examined. Using 15NO, the formation of N2 and N2O is observed via the direct interaction between the nitrogen atom of benzonitrile and 15NO. The higher activity of the bimetallic catalyst for the NOx reduction reaction by toluene is discussed in the context of these partial oxidation intermediates.

Effect of fretting on fatigue performance of Ti-1023 titanium alloy

The plain fatigue and fretting fatigue tests of Ti-1023 titanium alloy were performed using a high-frequency push-pull fatigue testing machine. Both σmax versus number of cycles to failure curves were obtained for comparative analysis of the fretting effect on fatigue performance of the titanium alloy. Meanwhile, by analyzing the fracture of plain fatigue and fretting fatigue, the fretting scar and the fretting debris observed by scanning electron microscopy (SEM), the mechanism of fretting fatigue failure of Ti-1023 titanium alloy is discussed. The fretting fatigue strength of Ti-1023 titanium alloy is 175 MPa under 10 MPa contact pressure, which is 21% of plain fatigue strength (836 MPa). Under fretting condition, the Ti-1023 titanium alloy fatigue fracture failure occurs in a shorter fatigue life. When it comes to σmax versus number of cycles to failure curves, data points in the range of 106–107 cycles under plain fatigue condition moved to the range of 105–106 under fretting fatigue condition. The integrity of the fatigue specimen surface was seriously damaged under the effect of fretting. With the alternating stress loaded on specimen, the stress concentrated on the surface of fretting area, which brought earlier the initiation and propagation of crack.

Tool life and hole surface integrity studies for hole-making of Ti6Al4V Alloy

With a significant growth in the use of titanium alloys in the aviation manufacturing industry, the key challenge of making high-quality holes in the aircraft assembly process needs to be addressed. In this work, case studies deploying traditional drilling and helical milling technologies are carried out to investigate the tool life and hole surface integrity for hole-making of titanium alloy. Results show that the helical milling process leads to much longer tool life, generally lower hole surface roughness, and higher hole subsurface microhardness. In addition, no plastically deformed layer or white layer has been observed in holes produced by helical milling. In contrast, a slightly softened region was always present on the drilled surface. The residual stress distributions within the hole surface, including compressive and tensile residual stress, have also been investigated in detail.

AN ADSORBATE-STABILIZED VACANCY STRUCTURE FOR CU ON W(100) - A SURFACE ALLOY

Deposition of 0.5 ML of Cu on W(100) leads to the formation of a sharp c(2 x 2) structure when the surface is annealed at 800 K. A LEED intensity analysis reveals that the Cu atoms are adsorbed displacively into W sites, forming an ordered 2D surface alloy. Due to the lattice mismatch between copper and tungsten, a substantial buckling of the first layer of the alloy is also observed. The clean, bulk terminated W(100) surface is only just stable relative to the c(2 x 2) vacancy covered W(100) surface. This relative stability of the vacancy structure explains the driving force behind the formation of this alloy.

Insight of the interface of electroless Ni-P/SiC composite coating on aluminium alloy, LM24

Electroless nickel composite coatings with silicon carbide, SiC, as reinforcing particles deposited with Ni–P onto aluminium alloy, LM24, having zincating as under layer were subjected to heat treatment using air furnace. The changes at the interface were investigated using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to probe the chemistry changes upon heat treatment. Microhardness tester with various loads using both Knoop and Vickers indenters was used to study the load effect clubbed with the influence of second phase particles on the coating at the vicinity of the interface. It was observed that zinc was absent at the interface after elevated temperature heat treatment at 400–500 °C. Precipitation of copper and nickel with a distinct demarcation (copper rich belt) along the coating interface was seen with irregular thickness of the order of 1 μm. Migration of copper from the bulk aluminium alloy could have been the factor. Brittleness of the coating was confirmed on heat treatment when indented with Vickers. However, in composite coating the propagation of the microcrack was stopped by the embedded particles but the microcracks continue in the matrix when not interrupted by second phase particles (SiC).

Phase composition, microstructure and microhardness of electroless nickel composite coating co-deposited with SiC on cast aluminium LM24 alloy substrate

Electroless Ni–P (EN) and composite Ni–P–SiC (ENC) coatings were developed on cast aluminium alloy substrate, LM24. The coating phase composition, microstructure and microhardness were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and microhardness tester, respectively, on as-plated and heat-treated specimens. The original microstructure of the Ni–P matrix is not affected by the inclusion of the hard particles SiC. No formation of Ni–Si phase was observed up to 500 °C of heat treatment. The microhardness is increased on incorporation of SiC in Ni–P matrix. The hardening mechanism is the formation of intermetallic phase Ni3P on annealing at elevated temperature.

Microstructure and high cycle fatigue fracture surface of a Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy

Because of the requirements for the damage tolerance and fatigue life of commercial aircraft components, the high cycle fatigue (HCF) properties of Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy forgings are important. The effects of microstructure types of the α+β titanium alloy on fatigue properties need to be understood. In this paper, by analysing the fracture surfaces of the titanium alloy having four types of microstructure, the effects of microstructure are investigated. The differences of initiation areas and crack propagation among different microstructures were studied. It was found that the area of the initiation region decreases in the order of coarse basketweave, fine basketweave, Widmanstätten, and bimodal microstructure.

Electroless nickel, alloy, composite and nano coatings - A critical review

The development of metal deposition processes based on electroless nickel, alloy and composite coatings on various surfaces has witnessed a surge in interest among researchers, with many recent applications made possible from many excellent properties. In recent years, these coatings have shown promising corrosion and wear resistance properties and large number of newer developments became most important from macro to nano level applications. After a brief review of the fundamental aspects underlying the coating processes, this paper discusses in detail about different electroless nickel alloy, composite, nano plating, bath techniques, preparation, characterization, new depositing mechanism and their recent applications, including brief notes on difficult substrate and waste treatment for green environment. Emphasis will be onto their recent progress, which will be discussed in detail and critically reviewed.

Stress and Strain Distributions During Compressive Deformation of Titanium Alloy Affected by Microstructure

In the research of the microstructural influence on dynamic compression, an assumption that the α and the β phases in titanium alloys were linearly strengthened was proposed, and a two-dimensional model using ANSYS (ANSYS, Inc., Canonsburg, PA) focusing on the role of microgeometrical structure was developed. By comparing the stress and strain distributions of different microstructures, the roles of cracks and phase boundaries in titanium compression were studied.

Characterisation of phase composition, microstructure and microhardness of electroless nickel composite coating co-deposited with SiC on casting aluminium LM24 alloy substrate

Electroless Ni-P (EN) and composite Ni-P-SiC (ENC) coatings were developed on cast aluminium alloy, LM24. The coating phase composition, microstructure and microhardness were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and microhardness tester, respectively, on as-plated and heat-treated specimens. The original microstructure of the Ni-P matrix is not affected by the inclusion of the hard particles SiC. No formation of Ni-Si phase was observed upto 500°C of heat treatment. The microhardness is increased on incorporation of SiC in Ni-P matrix. The hardening mechanism is the formation of intermetallic phase Ni3P on annealing at elevated temperature. Overall, the composite coating (ENC) was found to be superior as compared to particles free (EN) coating in both as-deposited and heat-treated conditions.

Micro-scale abrasion behaviour of electroless Ni-P-SiC coating on aluminium alloy

Electroless nickel (EN) and electroless nickel composite (ENC) coatings were deposited on aluminium alloy substrate, LM24. The micro abrasion test was conducted to study the wear behaviour of the coatings with the effect of SiC concentration. Microhardness of the coatings was tested also. The wear scars were analysed using optical microscope and scanning electron microscope (SEM). The wear resistance was found to be improved in composite coating that has higher microhardness as compared to particles free and the bare aluminium substrate. In as-deposited condition for the composite coating, the wear volume increases on increase in SiC percentage in the coating but is found to be minimum for lower SiC percentage. The increase in hardness on heat treatment at 400°C is due to the hardening or grain coarsening with the formation Ni3P.