Wear tests on polypropylene for use in prosthetic heart valves

A medical grade of polypropylene, as used in the Abrams-Lucas prosthetic mitral valve, was tested in a crossed-cylinder wear machine under conditions which reproduce those in the heart. The results show an initial dormant period of running in, followed by a steady wear rate of 2.71 × 10⁻⁵mg/m. This figure is used to give an estimate of valve life which agrees well with clinical data and confirms that the method is suitable for future use as a valve design aid without the need to resort to protracted testing of completed valves     

Fretting wear of carburized titanium alloy against ZrO2 under serum lubrication

Fretting wear of carburized titanium alloys was investigated on the universal multifunctional tester (UMT) with the ball-on-flat fretting style under bovine serum lubrication. The tangential load and friction coefficient during the fretting process were analyzed, and the evolution of fretting log during the fretting process was investigated to understand the wear mechanism of the titanium alloy and carburized titanium alloy. Furthermore, the wear scar was examined using a SEM and three-dimension surface profiler. It was found that the friction coefficient of the titanium alloy increased faster than that of carburized titanium alloy in the first stage under serum lubrication, and then remained steady with a similar value in the second stage. The Ft-D curve indicated that there was wear mechanism transition from gross slip to mixed stick and slip. Finally, it was observed that there was a slight damage of the titanium alloy and carburized titanium alloy showed excellent performance during the fretting wear process under serum lubrication. All of the results suggested that carburized titanium alloy was a potential candidate for the stem material in artificial joints.     

Tribological properties of some water-based lubricants containing polyethylene glycol under boundary lubrication conditions

The metal-working industry is increasingly interested in synthetic metalworking fluids and coolants. The excellent lubricating properties of polyalkylene glycols have been recognised in areas such as gear lubricants, hydraulic fluids, and compressor lubricants. The main purpose of the present work was to investigate the friction and wear properties of several polyethylene glycols and their synergy with a common water-soluble EP additive, using four-ball and ball-on-plate tribometers.     

Next generation lubrication system for weapons

Current weapon system lubricants have performance limitations in extreme climatic conditions, specifically in sand and dust and cold environments. An alternative solid lubricant based lubrication system was investigated and validated using the ASTM subscale tests. The solid lubricants were selected and evaluated based on weapon system lubrication requirements. The solid lubrication systems were evaluated for coefficient of friction, wear, load carrying capacity, endurance (wear) life and corrosion resistance. The novel lubrication system showed superior subscale performance compared to current gun lubricants. It has the potential to provide much better performance than current gun lubricants, especially in sand, cold and humid environments.     

Mechanical wear behaviour of copper-base alloys

In an endeavour towards better understanding of the mechanical wear behaviour of copper and of the influence of the main contributing variables on it, wear test were carried out on some copper alloys with different percentages of alloying elements. The investigation concludes and confirms that mechanical wear increases with load and sliding speed both with and without lubricant. Results reveal that the resistance to wear of alloys may be improved by increasing their strength, toughness and hardness and by decreasing the Young's modulus of elasticity, which can be obtained by controlling the type and amount of alloying elements.     

In situ graphite lubrication of metallic sliding electrical contacts

Decoupled graphite lubrication of monolithic silver brushes on a copper rotor was studied in an ambient air environment under current varying from 0 A/cm² to 200 A/cm² using a custom designed electrical contact tribometer. Bifurcation of the positive and negative brush wear rates was observed at a current density of 200 A/cm². Energy dispersive spectroscopy showed transfer of copper from the rotor to the lower wear negative brush. Scanning electron microscopy of worn brush surface cross-sections created by focused ion beam milling revealed a fine-grained metallic layer below the graphite transfer layer on the negative brush surface; no such layer was found on the positive brush surface. At 40 A/cm², steady-state brush wear rates were very low (<10⁻¹¹ m/m). Friction coefficient at steady state was measured to be 0.15 ± 0.02 and was independent of current direction. Using a scanning white light interferometer, the thickness of the graphite transfer layer on the rotor surface was estimated to be 5 μm. Ultimately, the goal is to model lubricant buildup and removal as a competitive rates problem.     

Development of a laboratory test device for electrorheological fluids in hydrostatic lubrication

Electrorheological (ER) fluids exhibit changes in their rheological behaviour in the presence of an external electric field. Research and development activities related to ER fluids and their applications have increased dramatically during the last few decades. If commercialised, ER fluids could have a significant impact on hydraulic equipment, and be utilised in the automotive, marine, aerospace, robotics, and machinery industries. The objective of the present study is to demonstrate the feasibility of using ER fluids to produce ‘smart’ lubricants that can control friction and wear of sliding components. The ER lubricant effect is demonstrated using a unique test machine that utilises hydrostatic lubrication. It is found that the friction torque increases by as much as 30% when a voltage of 2000 V is applied to an ER fluid in the newly developed test machine.     

On the hydrodynamic liquid lubrication analysis of slider/disk interface

A numerical solution for ultrathin hydrodynamic liquid lubrication of slider/disk interface is introduced. Both surface roughness effects and non-Newtonian behavior of the liquid lubricant are incorporated into the hydrodynamic lubrication analysis. A non-Newtonian liquid is used as the lubricant, and its behavior is described by a power-law rheological model. The contact pressure is calculated for a Gaussian surface roughness. The hydrodynamic load capacity is calculated by using an averaged form for the Reynolds equation. The finite difference scheme, with Gauss–Seidel iterative-relaxation method, is applied to solve the average Reynolds equation. The effects of surface roughness parameter, surface pattern parameter, and the power-law exponent on hydrodynamic pressure distribution, hydrodynamic load capacity are studied and discussed.     

Elastohydrodynamic lubrication of circular contacts at pure squeeze motion with non-Newtonian lubricants

In this study a numerical method for general applications with non-Newtonian fluids is developed to investigate the pure squeeze motion in an isothermal elastohydrodynamic lubricated spherical conjunction under constant load conditions. The coupled transient modified Reynolds, the elasticity deformation, and the load equilibrium equations are solved simultaneously. Computer simulation is carried out to investigate the effects of flow rheology and operations on the relationship between the pressure and film thickness distributions. The simulation results reveal that the larger the flow index (n), the larger the film thickness and the smaller the maximum central pressure. This results in larger time needed to obtain maximum central pressure. In addition, the elastic deformation is more significant for the lower flow index. Therefore, the smaller the flow index becomes, the greater the difference between the hydrodynamic lubrication (HL) solution and elastohydrodynamic lubrication (EHL) solution becomes.     

Time of exposure to lubricating liquid for predicting wear of UHMWPE in joint prostheses

It is generally agreed that contact pressure and sliding speed are the predominant factors for the prediction of wear of ultrahigh‐molecular‐weight polyethylene (UHMWPE) in joint prostheses. A new parameter for predicting the wear of UHMWPE has been introduced with a wear test in vitro. The parameter is the time of exposure to a lubricating liquid on a bearing surface. A pin‐on‐disc machine was designed such that the exposure time of a Co Cr Mo alloy disc to a lubricating liquid could be varied. The specific wear of UHMWPE was increased by a decrease in the exposure time, even if the contact pressure and the sliding speed were held constant. The parameter is able to account for the contact pressure set in the experiment (2.0–20.0 mPa), and clarifies the conditions under which the specific wear of UHMWPE is found to be high.     

The chemistry,mechanism and function of tricresyl phosphate (TCP) as an anti‐wear lubricant additive

The detailed understanding of the physical and reaction chemistry of engineering lubricants is key to new developments in the future. Here, we draw together the main chemical/engineering literature in the first systematic review of the standard anti‐wear additive used as an aviation lubricant, tricresyl phosphate (TCP), focusing on understanding the links between the surface chemistry, tribology and decomposition of TCP. While there is still debate concerning the mechanism of TCP, it is clear that it is activated by moisture, oxygen or oxidised metal surfaces. Its anti‐wear properties arise from the resulting formation of iron phosphate or polyphosphate on the contact surfaces. Evidence stresses the importance of chemistry at the boundary layer and a mechanism involving pre‐coordination of TCP at the surface, resulting in activation at the P‐centre and subsequent attack by residual H₂O or surface O²⁻. This perspective provides a potential baseline for the development of future phosphorus‐based high‐performance additives. Copyright © 2015 John Wiley & Sons, Ltd.     

Etched-pocket, dry-bearing materials

Novel, dry-bearing materials have been fabricated by photo-etching an array of very small, blind holes in metal surfaces and filling them with solid lubricant formulations. Friction and wear were assessed against a tool-steel counterface in reciprocating line-contact conditions. Preliminary experiments demonstrated feasibility with a range of substrates (bronze, steel, cobalt alloy) and lubricants (PTFE + Pb, MoS₂ + polyimide). Detailed studies were made with different copper alloys to examine the effects of substrate hardness and the distribution, size, spacing and depth of the etched pockets. Be---Cu containing PTFE + Pb exhibits wear lives over a range of stresses (Hertzian) to 375 MPa which are appreciably longer than those of a porous bronze composite containing the same lubricant. Wear rates of etched Be----Cu are also much lower than those of a typical, woven PTFE fibre-type of dry-bearing composite. Wear life is not, in general, limited by the depth of the etched pockets and several failure mechanisms have been identified.     

Ferrofluid lubrication of finite journal bearings using Jenkins model

The present study investigates hydrodynamic lubrication by ferrofluids of finite journal bearings using the Jenkins model. A magnetic field created by displaced finite wire is used. A numerical solution for the modified Reynolds equation using the finite difference method is obtained. Static characteristics of finite journal bearings are analyzed using 2 control parameters: magnetic force coefficient and Jenkins viscosity. The obtained results are compared to those from Neurenger‐Rosensweig model. It is shown that pressure, load capacity, attitude angle, and side leakage increase and friction factor decreases when increasing the value of each control parameter at low and medium eccentricity ratios. However, the Jenkins viscosity parameter decreases the load capacity and increases the friction factor at high eccentricity ratios.     

Tribological performances of heterocyclic‐containing ether and/or thioether as additives in the synthetic diester

The load‐carrying capacity, wear and friction properties of 2‐octoxyl methylthio‐benzothiazole (DEOY) and 2‐dodecylthio methylthio‐benzothiazole (DEMB) added to a synthetic lubricant (diester) were evaluated using a four‐ball test machine. The results indicate that the two compounds added to the diester possess good load‐carrying capacities and excellent anti‐wear and friction reduction properties. The thermal stability of the two compounds under nitrogen atmosphere was investigated by thermogravimetric analysis (TGA). It was found that the compounds DEOY and DEMB possess excellent thermal stability. The surface topography of the rubbed surface was investigated with scanning electron microscopy, the elemental chemical nature of the anti‐wear films generated on steel counterface were investigated with X‐ray photoelectron spectroscopy. Copyright © 2009 John Wiley & Sons, Ltd.     

Hard turning using HiPIMS-coated carbide tools: Wear behavior under dry and minimum quantity lubrication (MQL)

In the present work, experimental investigations carried out to assess the applicability of HiPIMS (High Power Impulse Magnetron Sputtering)-coated carbide tools to hard turning (55 HRC) and to address the widely debated topic about the use of coolants in hard turning are presented. Tool wear progressions and hence, tool life, different tool wear forms and wear mechanisms observed for tools coated with HiPIMS coating technique, namely, nanocomposite AlTiN, nanocomposite multi-layer TiAlN/TiSiN and nanocrystalline AlTiCrN are presented along with the images captured by digital and electron microscope. Characterization results of all the coated tools in terms of their average coating thickness (measured using Calotest and Fractographs), adhesion strength of the coating(s) (determined using Scratch test), composition and microhardness (using EDAX and Vickers microhardness test, respectively) are presented. Experimental observations indicate higher tool life with nanocrystalline AlTiCrN coated carbide tools which shows encouraging potential of these tools to hard turning. Improvement in tool life of almost 20–25% has been observed under minimum quantity lubrication (MQL) due to better cooling and lubricating effects. However, this effect was more prominent at higher cutting speed of 150 m/min.     

Reciprocating Sliding Wear Behavior of 60NiTi As Compared to 440C Steel under Lubricated and Unlubricated Conditions

ABSTRACT

60NiTi is a hard (～60 HRC) and highly corrosion-resistant intermetallic with a relatively low elastic modulus (～100 GPa). In addition, this alloy exhibits a high compressive strength (～2,500 MPa) and a high elastic compressive strain of over 5%. These attributes make this alloy an attractive candidate to be employed in structural and mechanical component applications. However, sliding wear behavior of this intermetallic has not yet been studied in a systematic way. In this study, lubricated and unlubricated reciprocating sliding wear behavior of 60NiTi is compared to 440 C steel as a conventional bearing and wear-resistant alloy. Results of experiments carried out under different loads show that 60NiTi, despite having a higher hardness, exhibits a significantly inferior wear behavior under dry conditions in comparison to 440 C steel. These unexpected results indicate that 60NiTi does not follow conventional wear theories where the wear of materials has an inverse relationship to their hardness. On the other hand, under lubricated conditions with castor oil and a synthetic gear oil, 60NiTi exhibits low specific wear rates. These results exhibit the importance of proper lubrication in sliding mode applications where 60NiTi is exploited as a wear-resistant alloy.     

液体空间润滑剂的现状与发展

空间润滑剂对于宇航飞行器的安全性,可靠性至关重要。综述了液体空间润滑剂的现状及发展,介绍了几种新型液体空间润滑剂的性能、合成方法。     

Nanoscale boundary lubrication of diamond‐like carbon coatings with fluorinated compounds

Progress in the technology of magnetic media has brought about a remarkable increase in recording density. The most important factor determining the utility of magnetic disks is durability against head wear, and this durability is controlled by several factors. The present paper discusses the tribology of these media, particularly from the viewpoint of boundary lubrication. In that context there are two characteristic features of this lubrication regime: specific standard lubricants (fluoropolyethers such as Z‐DOL and perfluo‐ropolyethers such as Z‐15) and the newer application of these lubricants in the form of films only a few nanometers thick Advanced phosphazene‐type fluorinated compounds are of most interest at present, so these compounds are discussed in more detail. The emphasis is on X‐1P lubricant used either alone or as an additive for fluoro‐ and perfluoropolyethers deposited on protective diamond‐like carbon coatings.     

Influence of solid lubricants and fibre reinforcement on wear behaviour of polyethersulphone

Polyethersulphone (PES), is an amorphous, brittle and high temperature engineering thermoplastic. Two composites of PES containing short glass fibres (GF) and solid lubricants viz. PTFE and MoS₂; and two composites containing short carbon fibre (CF) [30% and 40%] were selected for the present studies. Compositional analysis of selected materials was done with various techniques such as gravimetry, solvent extraction and thermal analysis viz. thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). These materials were studied for adhesive and abrasive wear performance by sliding against a mild steel disc and silicon carbide abrasive paper respectively, under different loads. It was observed that GF reinforcement along with incorporation of solid lubricants (PTFE and MoS₂) enhanced the wear performance of PES by an order of two. In the case of solid lubricants, PTFE proved to be more beneficial than MoS₂. CF reinforcement, however, proved to be the most effective in enhancing wear performance of PES. PES reinforced with 40% CF exhibited a specific wear rate in the order of 10⁻¹⁶m³/Nm which is considered to be very good for the thermoplastic composite. In the case of abrasive wear behaviour, however, incorporation of fibres or solid lubricants deteriorated the performance of the neat matrix. SEM was employed to investigate the wear mechanisms.