Scaling study of cavitation pitting from cavitating jets and ultrasonic horns

Cavitation erosion prediction and characterization of cavitation field strength are of interest to industries suffering from cavitation erosion detrimental effects. One means to evaluate cavitation fields and materials is to examine pitting rates during the incubation period, where the test sample undergoes localized permanent deformations shaped as individual pits. In this study, samples from three metallic materials, an Aluminum alloy (Al 7075), a Nickel Aluminum Bronze (NAB) and a Duplex Stainless Steel (SS A2205) were subjected to a vast range of cavitation intensities generated by cavitating jets at different driving pressures and by an ultrasonic horn. The resulting pitted sample surfaces were examined and characterized with a non-contact 3D optical scanner and the resulting damage computer-analyzed. A statistical analysis of the pit population and its characteristics was then carried out. It was found that the various cavitation field strengths can be correlated to the measured pit distributions and that two characteristic quantities: a characteristic number of pits per unit surface area and unit time, and a characteristic pit diameter or a characteristic pit depth can be attributed to a given “cavitation intensity level”. This characterization concept can be used in the future to study the cavitation intensity of the full scale and to develop methods of full scale predictions based on model scale erosion data.     

Cavitation erosion of Fe–Mn–Si–Cr shape memory alloys

Cavitation erosion tests of three Fe–Mn–Si–Cr shape memory alloys were carried out at speed 34 and 45 m/s using a rotating disc rig, and their cavitation damage has been investigated by comparison with a referring 13Cr–5Ni–Mo stainless steel used for hydraulic turbine vanes. The research results proved that the cavitation erosion of the Fe–Mn–Si–Cr shape memory alloys is a failure of low cycle fatigue and fracture propagates along grain boundaries. After 48 h cavitation erosion the cumulative mass losses of the studied alloys at speed 45 m/s are more than theirs at speed 34 m/s; however, the effect of velocity on cavitation damage of the Fe–Mn–Si–Cr alloys is much lower than that of 13Cr–5Ni–Mo stainless steel. The cumulative mass loss of the 13Cr–5Ni–Mo stainless steel are 26.3 mg at speed 45 m/s and 3.2 mg at speed 34 m/s, and the mass losses of the Fe–Mn–Si–Cr alloys are within the range of 3.6–7.3 mg at speed 45 m/s and 2.0–4.1 mg at speed 34 m/s. The surface elasticity of the Fe–Mn–Si–Cr shape memory alloys is better than that of the 13Cr–5Ni–Mo stainless steel, and the effect of surface elasticity on cavitation damage increases with velocity. The excellent surface elasticity of the cavitation-induced hexagonal closed-packed (h.c.p.) martensite plays a key role in contribution of phase transformation to the cavitation erosion resistance of the Fe–Mn–Si–Cr shape memory alloys. The cavitation damage of the studied alloys at speed 45 m/s mainly depends on their surface elasticity, and the variation of 48 h cumulative mass loss (Δm) as a function of the elastic depth (h_e) can be expressed as Δm=2.695+[1371.94/(4(h_e−46.83)²+12.751)] with a correlation factor of 0.99345.     

Effect of carbide volume fraction on erosive wear behaviour of hardfacing cast irons

The present work reports the effect of carbide volume fraction on erosive wear behaviour of hardfacing cast irons. Five different grades of weld hardfacing cast irons were selected for the present investigation. The solid particle erosion experiments were carried out with blast furnace sinter, silica sand and alumina particles under mild (53–75 μm, 25 m s⁻¹), moderately severe (125–150 μm/100–150 μm, 50 m s⁻¹) and under severe erosion conditions (300–425 μm, 90 m s⁻¹) at impingement angles of 30 and 90°. The variation in erosion rate with carbide volume fraction was observed to be strong function of the erodent particle hardness, impingement angle and the impact velocity. Under mild erosion conditions, erosion rate decreased with increasing carbide volume fraction (CVF), whereas erosion rate increased with CVF under moderately severe erosion condition with alumina particles. With silica sand particles under moderately severe erosion conditions the beneficial effect of large volume fraction of carbides could only be observed at 30°, whereas at normal impact erosion rate increased with increasing CVF. The erosion rate showed power law relationship with ratio of hardness of erodent particle to that of the target material (H_e/H_t) and expressed as E=c(H_e/H_t)^p.With increasing severity of erosion conditions erosion rate showed stronger dependence on H_e/H_t as compared to those under mild and moderately severe erosion conditions. The mechanism of materials removal from the carbides involved Hertzian fracture with softer sinter particles, whereas harder alumina particles could plastically indent and cause gross fracture of the carbides.     

Plastic deformation of 25CrMo4 steel during wear: Effect of the temperature,the normal force,the sliding velocity and the structural state

This article follows a previous study on friction and wear of 25CrMo4 steel [N. Khanafi-Benghalem, K. Loucif, E. Felder, F. Delamare, Influence de la température sur les mécanismes de frottement et d’usure des aciers X12NiCrMoSi25-20 et 25CrMo4 glissant sur du carbure de tungstène, Matériaux et techniques 93 (2005) 347–362]. The aim of our work is to study in more details the process of plastic deformation and the wear rate of this steel in lubricated sliding against cemented tungsten carbide, process observed in the previous work. The considered parameters are the temperature T (from 20 to 200 °C), the normal force P (from 500 to 1500 N), the steel structure (normalised HV 220 and quenched/tempered HV 480 states) and the sliding velocity v (from 0.05 to 0.3 m/s). We measured the friction coefficient and the sample total volume loss. A displacement sensor follows the volume loss evolution during the test; this follow-up is approximate because of the sample plastic flow which leads to the formation of peripheral burrs. All the tests conditions generate a significant plastic deformation of the sample steel, even in the quenched/tempered state: it produces a marked increase of the surface hardness, the work hardened layer being much finer for the quenched/tempered state (15 μm) than for the normalised state (40 μm at 20 °C). For temperatures T ≤ 100 °C in normalised state, the wear follows the Archard's law with an increasing rate with temperature. For T ≥ 120 °C, the wear rate decreases during the test, the global volume of wear being a decreasing function of T. For the quenched/tempered state, the wear rate decreases with the increase of the normal force, this decrease is less than 30% of the normalised state value. The material heating during the wear tests is well correlated with the friction dissipated power, but remains small, except in extreme cases (v maximum, great friction at high temperatures). These results suggest the existence of two wear mechanisms: abrasion by sample debris and burrs emission by plastic flow. The abrasion is probably the dominating mechanism for the tests carried out at the lowest temperatures. The plastic flow becomes a significant component at the highest temperatures. Using a contact model, we discuss to what extent the influence of the temperature and the strain rate on the steel hardness and ductility could explain the temperature and the sliding velocity effect on wear. Other phenomena are probably present: the influence of the steel microstructure and the lubricant on the size and/or the number of particles responsible for abrasion.     

Erosive smoothing of abrasive slurry-jet micro-machined channels in glass,PMMA, and sintered ceramics: Experiments and roughness model

Abrasive slurry jet micro-machining (ASJM) was used to machine channels in glass, PMMA, zirconium tin titanate, and aluminum nitride. The channel roughness was measured as a function of the ASJM process parameters particle size, dose, impact velocity, and impact angle. The steady-state roughness of the channels was reached relatively quickly for typical ASJM abrasive flow rates. The roughness of channels having depth-to-width aspect ratios up to about 0.25 could be reduced by approximately 35% compared to the roughest channel by decreasing particle impact velocity and angle. However, machining at such conditions reduced the specific erosion rate by 64% on average. It was therefore quicker to post-blast reference channels (225 nm average root mean square (R_rms) roughness) using process parameters selected for peak removal. It was also found that the roughness of reference channels could be reduced by about 78% by post-blasting using 3 μm diameter silicon carbide particles at 15° jet incidence. The smoothest post-blasted channels had an R_rms roughness of about 23 nm in glass, PMMA, and zirconium tin titanate, and 170 nm in aluminum nitride. Computational fluid dynamics was used to predict the particle impact conditions that were used in a model to predict the steady-state roughness due to ductile erosion with an average error of 12%.     

Design and validation of a grinding wheel optical scanner system to repeatedly measure and characterize wheel surface topography

This paper describes the design and validation of an upgraded grinding wheel scanner system that controls the position of a Nanovea CHR-150 Axial Chromatism sensor along the x- and y-directions of the wheel surface to measure and characterize wheel surface topography. The scanner features a novel homing system that enables the wheel to be removed from the scanner, used on a grinding machine and then re-mounted and re-homed so that the same location on the wheel surface can be repeatedly measured and monitored. The average standard deviation for homing was 27.6 μm and 19.3 μm in the x- and y-directions, respectively, which is more than adequate for typical area scans of 25 mm². After homing, the scanner was able to repeatedly measure features that were similar in size to an abrasive grain (∼200 μm diameter) with an average error of 9.3 μm and 5.9 μm in the x- and y-directions, respectively. The resulting topography measurements were compared with Scanning Electron Microscope images to demonstrate the accuracy of the scanner. A custom particle filter was developed to process the resulting data and a novel analysis technique involving the rate of change of measured area was proposed as a method for establishing the reference wheel surface from which desired wheel topography results can be reported such as the number of cutting edges, cutting edge width and cutting edge area as a function of radial depth.     

Revision of cavitation erosion database and analysis of stainless steel data

Cavitation erosion data have been accumulated in our laboratory for about 32 years since 1970. The database was constructed as electronic data in MS Excel files. The data files are able to offer quick search in terms of the test material, test method and test conditions from among 859 data. In this study, 131 data since 2003 were newly added to the database constructed in our previous study. The stainless steel data were analyzed, including various stainless steels such as ferritic, austenitic, duplex and martensitic stainless steels. Vibratory cavitation test results for different stainless steels, obtained with varying test conditions of frequency, amplitude and attachment of specimen, were converted analytically to obtain average erosion rates under assumed standardized conditions of a stationary specimen test with 1 mm standoff distance, and with frequency and amplitude as specified by ASTM G32. The average of erosion rate under the standardized condition (ASTM G32, stationary specimen method, standoff distance 1 mm) was determined for different stainless steels. The erosion resistance was defined as a reciprocal of erosion rate, and the correlation between erosion resistance and hardness of the specimen after erosion test was better than with the other mechanical properties. The erosion resistance is equal to 2.6E?07 × (HV × F_mat)^2.4 (HV; Vickers hardness, F_mat; material factor), and the correlation coefficient is 0.98. It was concluded that the erosion resistance of different stainless steels could be estimated with high reliability from the material hardness and the material factor.     

Optimisation of parameters affecting surface roughness of Co28Cr6Mo medical material during CNC lathe machining by using the Taguchi and RSM methods

This study involves modelling of experimental data of surface roughness of Co28Cr6Mo medical alloy machined on a CNC lathe based on cutting parameters (spindle rotational speed, feed rate, depth of cut and tool tip radius). In order to determine critical states of the cutting parameters variance analysis (ANOVA) was applied while optimisation of the parameters affecting the surface roughness was achieved with the Response Surface Methodology (RSM) that is based on the Taguchi orthogonal test design. The validity of the developed models necessary for estimation of the surface roughness values (Ra, Rz), was approximately 92%. It was found that for Ra 38% of the most effective parameters is on the tool tip radius, followed by 33% on the feed rate whereas for Rz tool tip radius occupied 43% with the feed being at 33% rate. To achieve the minimum surface roughness, the optimum values obtained for spindle rpm, feed rate, depth of cut and tool tip radius were respectively, 318 rpm, 0.1 mm/rev, 0.7 mm and 0.8 mm.     

Nanowire cutting by an ultrasonically vibrating micro tool

Cutting of a nanoscale workpiece is useful in nano testing and fabrication, and novel cutting methods with little gasification of cut nano samples and simple device structures are needed for practical applications. In this paper, an ultrasonic nanowire cutting strategy is demonstrated, in which the linear and elliptical vibration of the tip of a micro cutting tool and the adhesion force between a substrate and nanowire are employed to cut and fix the nanowire, respectively. With this strategy, cutting of individual silver nanowires with a diameter from 50 nm to 400 nm is implemented, in which the vibration velocity amplitude of the micro cutting tool’s root is from 18 to 220 mm/s, and the working frequency is about 96.9 kHz and 45.2 kHz, respectively. The dependency of the minimum cutting velocity and optimum cutting velocity range’s lower limit on the AgNW diameter is experimentally clarified. Also, the cutting principle is analyzed, which can well explain the incision morphology and cutting characteristics.     

Research on improvement of machining accuracy of micro-rods with electrostatic induction feeding ECM

Micro-rods were machined by electrochemical machining using the electrostatic induction feeding method, with which ultra-short current pulse duration of several tens of ns can easily be obtained. A tungsten plate and stainless steel (SUS304) rod were used as the tool electrode and workpiece, respectively. To improve the machining accuracy, the machining characteristics when the workpiece is fed in the axial and radial directions were investigated using NaCl aqueous solution as the electrolyte. When fed in the axial direction, the machinable length of the micro-rods was found to peak at the optimum feed speed because of the influence of pitting corrosion and collision between electrodes. When the workpiece was fed in the radial direction, the influence of pitting corrosion decreased, however, the micro-rod was shortened with increasing feed distance in the radial direction because of the stray current flowing through the end of the micro-rod. The simulation results of the material removal process agreed qualitatively with experimental results. Next, machining characteristics were compared between the electrolytes, NaCl and NaNO₃ aqueous solutions, by feeding the workpiece in the axial direction. It was found that the influence of pitting corrosion was eliminated with the NaNO₃ aqueous solution, and there was no machinable length limitation with suitable feed speeds. In addition, the taper angle and gap width were smaller with the NaNO₃ aqueous solution, compared with those of the NaCl aqueous solution. Stainless steel micro-rods of 100 μm in diameter with a high aspect ratio of 20 were fabricated with the NaNO₃ aqueous solution. According to the preliminary research results, the machinable minimum diameter of the micro-rods was investigated and micro-rods with an average diameter of 9 μm and length of 78 μm were machined successfully.     

Improved critical flow factors and representative equations for four calibration gases

The critical flow nozzle is widely used to calibrate flowmeters in gas flow measurement. Its use requires the critical flow factor, C*, a parameter dependent upon the thermophysical properties of the gas at the nozzle throat, and the upstream temperature and pressure. This paper presents C* values for four calibration gases (air, argon, nitrogen and methane), calculated from the most recent reference quality equations of state, over a wider range of temperature and pressure than previously available, 200–600 K and up to 20 MPa. In addition, a new empirical equation has been developed to represent the calculated values accurately, thus eliminating the need for complex calculations or interpolations from tables.     

Upstream installation and misalignment effects on the performance of a modified electromagnetic flowmeter

The performance of a modified Danfoss 50 mm diameter electromagnetic flowmeter has been investigated when installed downstream from three different pipe diameters—50 mm, 55 mm and 45 mm. The effects of a 3 mm misalignment of the flowmeter, in both the vertical and horizontal planes, with respect to each of the three upstream pipe diameters has also been identified. The largest percentage errors are reported for the 45 mm upstream diameter pipe, with the flowmeter misaligned by 3 mm in the horizontal plane. The vertical and horizontal mean velocity and root-mean-square velocity profiles, measured within the flowmeter using laser Doppler anemometry, show significant variations in comparison with the ideal, fully developed profiles.     

Cavitation erosion behaviour of niobium

Cavitation erosion behaviour of niobium was investigated by means of a 20 kHz ultrasonic vibrator at peak-to-peak amplitude of 50 μm, aiming to determine the niobium potential as a material for the manufacturing of hydraulic machine components. The study was emphasized for the three first cavitation stages of the cumulative erosion–time curve. The modification of the niobium surface morphology as a function of the testing time in the incubation, acceleration, and maximum erosion rate stages was verified by SEM analysis. Samples were prepared from 98.9% purity and 90% reduction cold-rolled niobium bar. The study was performed for niobium samples in both the cold-worked and annealed conditions. Samples of CA-6NM martensitic stainless steel, a typical material utilized for hydraulic turbines manufacturing, were also analysed for comparison purpose. Annealing treatment of niobium decreases its hardness and increases its ductility, leading to an increase of the incubation period when compared with the cold-worked niobium. Cavitation erosion failure mechanism in niobium occurs in a sequence of events comprising the work-hardening effect and the fracture of debris allied to the effect of fatigue and microcracks formation. Finally, annealed niobium presents similar incubation period but worse behaviour in the maximum erosion rate stage than CA-6NM steel.     

Experimental investigation and mechanism of material removal in nano finishing of MMCs using abrasive flow finishing (AFF) process

Aluminum alloy and its composites appear to have a good future as a candidate material for engineering and structural components. Finishing of these materials is a big challenge as they are heterogeneous in nature having abrasive particles randomly distributed and oriented in the matrix material. Metal matrix composite (MMC-aluminum alloy and its reinforcement with SiC) workpieces were initially ground to a surface roughness value in the range of 0.6 ± 0.1 μm, and later were finished to the R_a value of 0.25 ± 0.05 μm by using Abrasive Flow Finishing (AFF) process. The effects of different process parameters, such as extrusion pressure, number of cycles and viscosity of the medium were studied on a change in average surface roughness (ΔR_a) and material removal. The relationship between extrusion pressure and ΔR_a shows an optimum at about 6 MPa. In the same way, the relationship between weight percentage of processing oil (plasticizer) and ΔR_a also shows an optimum at 10 wt%. Further, an increase in workpiece hardness requires more number of cycles to achieve the same level of improvement in ΔR_a. Material removal also increases with an increase in extrusion pressure and number of cycles while it decreases with an increase in processing oil content in the medium. It is also concluded that the mechanism of finishing and material removal in case of alloys is different from that in case of MMC.     

A ring area formed around the erosion pit on 1Cr18Ni9Ti stainless steel surface in incipient cavitation erosion

After a 1 min cavitation experiment performed in an ultrasonic vibration system, needle-like erosion pits appeared on a polished stainless steel surface, and a special ring area was formed around each pit. The shape of the pit and the plastic deformation of the ring area indicate that the mechanical impaction on the surface is the main reason for the cavitation erosion. On the other hand, the iridescent color, the decreased surface hardness and the appearance of the precipitated carbon ring prove that the ring area has experienced a tempering process with the temperature higher than 300 °C. Also, the lack of oxygen in the ring area proves that it is not a chemical oxygen result.     

Effects of materials and solution temperatures on cavitation erosion of pure titanium and titanium alloy in seawater

Cavitation erosion was studied for various pure titanium and titanium alloy samples using a rotating disk method in seawater at 303, 318, and 333 K. Their respective erosion resistances were evaluated in terms of Vickers hardness (HV). The resistance increased in order with increasing hardness: pure titanium samples of first, second, and third types, and titanium alloy (Ti–6Al–4V). The relative temperature was defined as 273 K for freezing temperature and 373 K for boiling temperature under pressurized water. The volume loss rate of test specimens increased with rising seawater temperature of 289–316 K of the relative temperature, as well as in cases using cavitating liquid jet and vibratory apparatuses.     

Study on the evaluation of cylinder’s global sizes

In ISO 14405-1, the global sizes, such as least-squares diameter, minimum circumscribed diameter and maximum inscribed diameter are defined. The diameters above can be measured by using cylindrical coordinate measuring method like the circular section measuring method of cylindricity error. The determination method of the least-squares diameter was firstly given based on the cylindrical measuring system, and the optimization models of the minimum circumscribed diameter and the maximum inscribed diameter were built, respectively. The corresponding objective functions were unified as “minimax” expressions. For the four axis parameters of the cylinder with the minimum circumscribed diameter or the maximum inscribed diameter, the searching ranges of cylinder’s axis parameters for their optimal solutions were defined numerically. Thereafter, the genetic, steepest decent and BFGS-0.618 algorithms were introduced, and the optimization evaluation algorithms of two kinds of diameters mentioned above were given. Based on many cylinders’ profiles obtained by the circular section measuring method on a measuring instrument of cylinder’s global sizes which was developed by Zhongyuan University of Technology, Zhengzhou, China. The accuracy, efficiency and suitability of three optimization algorithms were investigated through the evaluation of a lot of the minimum circumscribed diameters and the maximum inscribed diameters. The measurement uncertainty of the global sizes for the cylindrical specimen was analyzed, and the measurement uncertainties of the sizes in the radial and z directions are ±0.95 μm and ±0.5 μm, respectively. The total measurement uncertainties of the global sizes of the cylindrical specimens with the specifications of ϕ10 × 120 mm and ϕ100 × 300 mm are ±3.8 μm and ±5.7 μm, respectively. The investigation results showed that for the evaluation of the globe sizes, any one of three algorithms above is not absolutely prior to the other two algorithms while considering both evaluation accuracy and efficiency, and the difference of their evaluation results do not exceed 0.5 μm. On the other hand, many points between the maximum value and the least value do not affect the evaluation results in optimization process. For improving the evaluation efficiency, by de-selecting those points while considering the characteristic parameter was also studied based on the statistic method and experiment. Coefficient t should be less than 0.3 to ensure the evaluation accuracy. This research may be useful for developing the next generation measurement instrument for the global sizes and the way forward for the digital manufacturing.     

Copper alloys disintegration using pulsating water jet

This paper deals with on the investigation of surface topography, morphology and anisotropy of copper alloys – brass and bronze, created by pulsating water jet with frequency 20.38 kHz. The material was disintegrated using more passes of a pulsating water jet using flat nozzle, at pressure 40 MPa and stand off distance z = 55 mm. The average values of Ra, Rq, Rz roughness were analyzed at changing traverse speed and number of transitions. The effect of tensile strength and material hardness as mechanical properties of material affecting the average value of the roughness has been evaluated. It is assumed that this new way of metal eroding can be used in the automotive and engineering industries in the future e.g. for surface treatment.     

Evolution of wear,roughness, and friction of Alloy 600 superalloy surfaces in water-submersed sliding

Self-mated wear and friction of Alloy 600 superalloy was studied in a water-submersed ring-on-rod configuration, loading the side of a 6.35 mm diameter rod across the flat surface of a rotating annular ring of 100 mm outer diameter and 70 mm inner diameter producing two sliding contacts along the ring. Tests were conducted at sliding speeds of 0.178 and 0.330 m/s for sliding distances of 100 m. Normal loads of 51 and 204 N were applied, and initial R_a surface roughnesses of the rings along the sliding direction were either smooth (～0.2 μm) or rough (～7.5 μm). Increased initial ring roughness caused a ～20-fold increase in rod wear at the lighter load, whereas at the heavier load increased initial roughness only caused a ～4-fold increase in wear. At lower initial ring roughness the 4-fold decrease in normal load caused a large (one order-of-magnitude) decrease in rod wear, whereas for rings of higher initial roughness the 4-fold decrease in normal load caused only minor (2-fold or less) decreases in rod wear. Wear during this 100 m sliding distance only experienced a minor effect from the 1.8-fold change in sliding speed, as did friction. In all cases friction coefficient rapidly settled into the range 0.6–0.7, except in the cases of lower load on rings of lower initial roughness where friction coefficient remained above 1 for most of this sliding duration. At this lower load the initial ～0.2 μm rod roughnesses increased to nearly 0.8 μm by the 100 m sliding distance, whereas at the higher load this same sliding distance resulted in roughnesses returning near to the initial 0.2 μm. It was hypothesized more highly loaded cases also went through initial roughening prior to smoothening back to 0.2 μm roughness within the 100 m sliding distance, and given additional sliding the more lightly loaded cases would also experience subsequent smoothening. Increasing sliding distance to 400 m, roughnesses indicated a smoothening back to 0.2 μm level during those lightly loaded tests, with friction coefficient correspondingly dropping from 1 into the 0.6–0.7 range observed in all other cases. Extended sliding to 400 m at light loading against rings of lower initial roughness also allowed a rod wear rate which increased with increased sliding distance to be observed, approaching the same rate observed against initially rough rings within the 100 m sliding distance.     

The nanostructure of soot-in-oil particles and agglomerates from an automotive diesel engine

The characteristics of soot particles and agglomerates of particles extracted from samples of lubricating oil drawn from the sump of a diesel engine have been investigated. The engine was a high pressure common rail, direct injection diesel designed for light duty automotive applications. Soot from the samples was prepared for imaging by sample dilution with heptane, followed by washing in diethyl ether and in some cases, sample centrifugation. The size and shape of agglomerates were defined from measurements of projected length and width allowing for chain contortion. When used, centrifugation is shown to alter the size and shape of agglomerates, increasing the proportion of chain agglomerates and reducing clusters. Without centrifugation, roughly half of the soot in oil exists in long-chain agglomerates with average length of 130 nm and 50 nm in width. Clusters with modest branching account for the remaining 46%. The average aspect ratio (L/W) was of 2.9. The diameter of spherical primary particles that form the agglomerates ranges between 10 and 35 nm grouped in a Gaussian distribution with a mean value of 20.2 nm. All primary particles exhibit an inner core and an outer shell. The inner core is composed by several nuclei of around 4 nm diameter. Inner core is of around 8–15 nm in diameter and outer shell 4–12 nm thick.