Wear measurement using radionuclide-technique (RNT)

Wear 254 (2003) 801–817

Wear measurement using radionuclide-technique (RNT) M. Scherge∗ , K. Pöhlmann, A. Gervé Head of Basic Research Department, IAVF Antriebstechnik AG, Leiter Grundlagen und Tribologieforschung, Im Schlehert 32, 76187 Karlsruhe, Germany

Abstract Wear measurement based on the radionuclide-technique (RNT) is a unique way to determine the tribological performance of a system continuously and in real-time. Due to its extremely high resolution and accuracy this method is especially suited to mechanical systems showing low wear rates. When simultaneously to wear, friction is also measured, dynamic processes like running-in can be assessed precisely. This leads to a deeper understanding of the fundamental mechanisms of the wear process. The method, therefore, can be used for applied as well as for detailed basic research. This paper deals with the fundamentals of RNT and its application. Although most of the examples come from combustion engine tribology, the findings can be generalized to other areas of mechanical engineering. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Wear mechanism; Continuous wear measurement

1. Introduction During the construction of the first cyclotron in 1930 by E.O. Lawrence (Nobel Prize winner 1939) in Berkeley, USA, the idea of wear measurement with radionuclides was developed. It was suggested that the location of suspected wear could be radioactively labeled directly by bombardment with heavy charged particles. Consequently, the radioactive wear of this area should be detectable by measuring the emitted radiation. The technique, assigning mechanical properties to emitted radiation was initially known as the isotope-technique. The correct designation today is radionuclide-technique (RNT). In the beginnings of this technique radioactive cores were generated almost exclusively in the neutron flux inside a nuclear reactor. The name isotope-technique existed, because the production of radioactive cores by reactor neutrons predominantly resulted in isotopes of the irradiated atoms. In many countries the isotope-technique was promoted also internationally, e.g. by the International Atomic Energy Agency (IAEA). The isotope-technique was developed in many different disciplines such as biology, medicine, geology, chemical process engineering and also in mechanical engineering. Most successfully, the isotope-technique was developed in medicine. Profes∗ Corresponding author. Tel.: +49-721-95505-30; fax: +49-721-95505-44. E-mail address: [email protected] (M. Scherge).

sionals in medicine soon used their own name: nuclear medicine instead of isotope-technique. In contrast to the isotope-technique in mechanical engineering and in process engineering the operators of nuclear medicine were physicists and chemists, who had become medical professionals. In mechanical engineering, however, the isotope-technique was operated world-wide with few exceptions by physicists or radiochemists, not being engineers and thereby not fulfilling adequately the needs of engineering. In 1958, the German Forschungsvereinigung Verbrennungskraftmaschinen e.V. (FVV, Research Association for Internal Combustion Engines) asked the Universität Karlsruhe to find out whether the isotope-technique could be used for internal combustion engines. Under Karl Kollmann, professor and director of the Institute for Machine Construction and Automobile Engineering, from 1958 to 1969, important contributions to the isotope-technique in mechanical engineering were made. These efforts were focused upon on-line wear measurement [1–4], the measurement of the dynamics of moving machine parts in the running engine [5] and the measurement of the oil film thickness in tribosystems [6]. During the above mentioned developments, two difficulties emerged. The electronics (at that time still equipped with tubes) was too unreliable and difficult to operate. However, this problem seemed solvable. More serious was the high radioactivity due to an activation exclusively performed with neutrons. The machine parts had to be installed with special remotely controlled tools behind lead shields. Wear investigation in cylinder bores

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could not be executed since the radioactivity was too high. Whole engine blocks (for engines without dismantable cylinder liners) could not be activated, because they did not fit into the irradiation positions of nuclear reactors. Besides which, the total activity of such large machine components would have been enormously high and therefore, the adherence to safety precautions would have made handling very complicated. The problem of excessive radioactivity of machine components in industrial applications was solved by the development of the thin-layer activation (TLA) method with heavy charged particles (protons, deuterons and ␣-particles) [7–15]. In contrast to neutron activation, where a machine part becomes thoroughly radioactive, thin-layer activation results in a layer of some 100 ␮m, because the heavy charged particles are decelerated in thin layers due to their electrical charge. Since during the activation with charged particles, existing elements are changed into new elements, the name isotope-technique was replaced by radionuclide-technique (RNT). This was also favorable, since historically the name “isotope-technique” in industry conveyed a bad reputation. The isotope-technique was too difficult to handle and too dangerous. The thin-layer activated machine parts could be handled without additional protective measures or adherence to strict radiation protection regulations. With the development of thin-layer activation the negative view changed. Under the direction of Gervé [16–18] the activation technique and the wear measurement were developed further in the Laboratory for Isotope Technique at the Nuclear Research Center in Karlsruhe. The solution of important problems of mechanical engineering, from 1970 to 1978, documented the applicability of this technique [19–23]. In addition to the “concentration measurement procedure”, where the concentration of the radioactive particles in the lubricating oil volume is related to the amount of wear, the “difference measurement procedure” was developed, relating the reduction of radioactivity by wear to the wear depth [24]. Besides the development of the activation and wear measuring technique, more fundamental work was undertaken on the application of the radionuclide-technique in tribology and engine research [25,26]. In particular, the application of RNT to determine the size of wear particles [27], and extensive work dealing with the continuous measurement of oil consumption with radioactively marked lubricating oil, must be mentioned [28–30]. Before using RNT wear measurement for basic research, it was established that the activation does not influence the tribological behavior of the activated machine parts, because the doses are too small (1010 –1011 nuclides/cm2 ). Tribologically effective doses are approximately four to six orders of magnitude larger [31]. In extensive research work, wear characteristics were examined for sliding bearings [32] and roller bearings [33]. A procedure for the optimization of the running-in process for internal combustion engines was developed [34]. The RNT

wear measurement was also applied to the analysis of wear protection of lubricating oils and the effect of additives. In the 1980s and 1990s RNT was predominantly applied to routinely monitor combustion engines to solve tribological problems and for basic research in tribology. In addition, the discovery of tribomutation, i.e. the modification of materials in thin layers of surface-near volumes by friction, was made possible only with the help of RNT [34–39]. The main advantages of this matured technique, which is commercially available, are high sensitivity (typically 0.1 nm/h), continuous wear measurement, allocation of wear to the area of its generation, and the assignment of wear to particular points of operation in the tribosystem. With the development of modern combustion engines and the analysis and development of new lubricating oils, RNT wear measurement is a necessary aid which is used without radiation hazard. Also for tribology research, the RNT technique has become an indispensable tool alongside the methods of surface science [40,41].

2. RNT wear measurement methods Basically, two different continuous wear measurement methods are applied, both using the radionuclide-technique [9,42–50]: the concentration method and the difference method. Both techniques offer the advantage of continuous observation and measurement of the progress of wear on machine parts in service without having to stop or even to disassemble the machine. The time dependence of wear is determined from the radiation emitted by the activated material. In addition, both techniques exhibit extremely high sensitivity and are less time consuming and less costly compared to conventional methods. 2.1. Concentration method The so-called concentration method (radioactivity of wear particles per volume, e.g. oil volume) involves measuring wear by detecting the radiation emitted by wear particles carried away from an activated surface area by a medium. The transporting medium can be, for example, a lubricant or fuel that flushes the wear particles from the activated machine part, such as oil in an internal combustion engine. Both the medium and the wear particles are carried to a highly sensitive detector, which measures the radiation emitted by the wear particles. The wear measuring system determines the numerical activity values. From this, the actual worn mass (amount of volumetric wear in microgram) or the actual wear depth (amount of linear wear in nanometer) can be calculated by using the calibration data of the measuring system and by considering further relevant system data, such as the half-life of the radioactive nuclei of the wear particles. The instrument performs this calculation on-line and continuously plots the progress of wear for the machine part in service.

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Fig. 1. Concentration flow method with both flow-through and filter measuring system, mainly consisting of pump, filter, detector, lead shielding as well as measuring, evaluation and display unit.

2.1.1. Concentration flow method One possible configuration of the concentration method is the concentration flow method (Fig. 1). During operation of, for example, an internal combustion engine, wear particles are generated and then transported by the engine oil. Representative portions of both the oil and the wear particles are pumped through the flow-through measuring head, consisting mainly of the measuring vessel and a NaI scintillation detector. There the ␥-radiation emitted by wear particles originating from activated surface regions is detected by the NaI scintillator and corresponding signals are processed by the measuring system. In addition, the measuring vessel is shielded by lead to reduce underground signals originating from natural radioactivity as well as from the activated machine parts of the engine tested. The correlation between counting rate of signals during operation of the engine and the absolute amount of linear or volumetric wear is derived from the following calibration procedures. An initial procedure is performed once in the lifetime of the measuring head and a second calibration is conducted every time a newly activated machine part is tested. The initial calibration provides the correlation between the detector sensitivities for all the various radionuclides of interest as solid point sources in special geometric arrangement and as dissolved calibration liquid. The second procedure is conducted once before every new test run by using the same material with identical radioactive marking as the machine parts tested. The detector sensitivity of this solid calibration sample is then measured in the same special geometric arrangement as the initial procedure. From this a direct correlation between the well known activated mass of a given material and its count rate as dissolved liquid is derived and is used for calibration purposes. Knowledge of the material properties and

of the geometry of the activated areas can also provide the calibration factors for calculating linear and volumetric wear, in microgram and nanometer, respectively. When considering the mutual influences of the measured ␥-spectra originating from different activated machine components, wear can be calculated independently for every machine component. The concentration method, as described, only requires a relatively low radioactivity of typically 250 kBq for the machine parts tested. With the ␥-spectroscopy, several wear components can be measured simultaneously, as for example, shown in [7], whereas the standard setup measures two components at a time. The concentration method offers extremely high accuracy and sensitivity as compared to conventional techniques. For typical applications the accuracy amounts to about 0.1 ␮g/h or 1 nm/h (0.001 ␮m/h), respectively. To achieve this resolution of wear rates the duration of the test run needs to be only of the order of 1 h. The process thereby enables simultaneous and continuous measurement of two different machine parts or two different locations for the same machine part [51]. 2.1.2. Filter concentration method If filters are located in the flow circuit of the transport medium, for example, the oil filter in an internal combustion engine, the continuous-flow method must be supplemented by a filter probe. This is realized by using a second measuring unit, the so-called filter measuring head (Fig. 1), in addition to the flow-through measuring head. The ␥-quanta emitted by activated wear particles retained in the filter are measured by a NaI scintillator detector within the filter measuring head (for example, by using a special variant of an original passenger car oil filter). The setup and operation of the second unit introduced is basically the same as for the flow-through measuring head, as described above.

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Therefore the same requirements, sensitivities and resolution limits (1 ␮g/h or 1 nm/h, respectively) also apply for this measuring unit. Such a wear measurement system determines separately the wear particles in the filter, the wear particles not retained on the filter and the total amount of wear as the sum of both the components. The filter concentration method is the most commonly used method for wear studies. 2.1.3. Wear particle size analysis Using filters and the radionuclide-technique an analysis of the size distribution of wear particles can be performed easily. Moreover, by using RNT the size distribution functions of wear particles can be related to certain activated engine parts and/or certain running conditions [27,44]. The distribution functions can be measured with the setup shown in Fig. 2. An oil sample is taken from a combustion engine which has been running with one or more activated parts. This oil sample will contain wear particles, and among these particles there will be those originating from the radioactively marked engine parts. After dilution, the content of the sample is passed through a series of filters. A Nuclepore track etched membrane filter was chosen, as it offers the smallest scatter of pore diameters and finest gradation of available pore sizes, ranging from 8 to 0.05 ␮m (50 nm). An exemplary SEM micrograph of the surface of the track etched membrane filter with a nominal pore size of 1.0 ␮m, as well as a schematic of the cross-section of the filter, are shown in Fig. 2, illustrating its functionality. The diluted

oil sample with the wear particles is filtered using an ultrasound generator, compressed nitrogen and a pore filter with the largest nominal pore size. The filter retains the residue, which is separated, whilst the dropout is gathered in a test tube and is filtered again using a pore filter with the next smaller pore size, and so on. In between two filter stages, however, the test tube containing the dropout is inserted into a bore measuring head consisting mainly of a NaI scintillation detector to determine the mass of the wear particles, originating from the activated surface area after every single filter stage. The mass portions of the individual filter stages can then be standardized against the total mass of the activated wear particles and plotted as function of the pore size of the filter, resulting in the wear particle size distribution. The running-in procedure of a compression ring of a water-cooled four-cylinder otto engine [44], for example, resulted in the following particle size distribution for particles originating from the ring flank: about 80 mass% of the wear particles were smaller than 1 ␮m in diameter, 75% smaller than 0.6 ␮m, 55% smaller than 0.2 ␮m whilst about 50% were smaller than 0.1 ␮m. 2.2. Difference method (thin-layer difference method) The second continuous wear measurement method using the radionuclide-technique (besides the concentration method) is the so-called difference or thin-layer difference method [18,52]. In this technique, wear is studied by measuring the decrease in the residual activity of the irradiated

Fig. 2. Particle size analysis using track etched filters (right) for breaking down the grain size of wear particles (left, top) and bore hole measuring system (left, bottom for determining the mass portion of individual particle size fractions.

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Fig. 3. Thin-layer difference method for systems without capability of conveying particles, mainly consisting of detector as well as measuring, evaluation and display unit.

machine part. This particular method is used only when it is not possible to apply the concentration method, i.e. when there is no capability to have a fluid for conveying wear particles. With the engine running, wear is generated resulting in material loss from the surface of the activated surface region. Due to the loss of activated surface material, the total radioactivity of the machine part of interest is reduced. This reduction of activity is monitored by the difference measuring head through the housing of the tribological system (Fig. 3). The measuring head mainly consists of a NaI scintillation detector and its housing, which forms a lead shielding to reduce underground signals originating from natural radioactivity. The NaI scintillator crystal detects the ␥-quanta emitted by the activated machine part and the measured signals are processed by the measuring system. The decrease of radioactivity is thereby directly correlated to the amount of wear. As part of this process, the decay of the radioactivity of the machine component due to its limited half-life, typically ranging from 5 to 270 days, also has to be considered. Based on prior calibration, the wear depth is calculated automatically and on-line. The duration of the tests strongly depends on the problem and the required accuracy of the wear rate (typically 10 nm/h for measuring periods of 6–10 h). This process is applied in those instances only where there is no suitable transport medium, e.g. for analyzing the wear behavior of valves and valve seats, since its accuracy is about one to two orders of magnitude lower than to the concentration method. Although the principle of the thin-layer difference method is quite simple, many influencing factors—such as geometric limitations of the engine or the machine part itself, optimization of the distance between detector and machine part, especially if moving, or adsorption of the emitted radiation before it reaches the detector, e.g. by bubbles formed within the coolant—complicate the measuring situation greatly. For resolution purposes it is essential for this technique to activate only a very thin surface zone or surface layer. If the activated surface layer is too thick, then wear may only re-

sult in an irresolvable small change in total surface activity. On the other hand, if the activated surface layer is too thin, it may be worn through before the end of the test run. To produce this thin activated surface zone, thin-layer activation has to be applied using an accelerator such as a cyclotron and heavy charged particles such as protons, deuterons or ␣-particles. The thin-layer activation technique yields regions of constant activity of up to 20 or 500 ␮m depending on the activated radionuclides. Therefore, the maximum total linear wear should be less than the activated depth. The total activity of the complete machine part is about 250 kBq, which is quite low compared to activation by neutrons. For calibration reasons it is essential that the surface zone of interest exhibits an almost constant activity throughout the entire radioactively marked depth of the machine part. Consequently, the total activity of the marked machine part has to decrease linearly with increasing wear depth from its maximum value at the outermost surface, as shown schematically in Fig. 4. The relationship between total activity— strictly speaking, the counting rate Z of detected ␥-quanta, measured by the detector in the chosen location near the activated machine part—and the absolute amount of linear wear is derived from the following considerations. While the values of the nominal activation thickness d0 and the thickness dlin of the linear range (Fig. 4) are given by the activation parameters used, the actual wear depth d can be calculated from the actual counting rate Z and the counting rate Z0 of the unworn machine part by using the equation:
 1 − Z0 d = d0 (1) Z This relationship is valid as long as the total amount of wear d stays below the thickness dlin of the linear range. It should be mentioned, though that measuring the counting rate Z0 of the unworn machine part is quite challenging as the engine has to be in operation for this procedure but no wear should be produced at the same time, in order not to falsify the results. To determine Z0 with an accuracy of approximately 0.1%, it typically requires measuring periods of about 1 h.

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Fig. 4. Schematic diagraph of the counting rate Z of the detector as a function of wear depth d of a machine component. Parameters dlin (maximum linear range) and nominal activation thickness d0 are derived from activation parameters. The counting rate Z is proportional to the residual total activity A of the machine component.

2.3. Activation of materials and detection of radiation Both continuous wear measurement methods introduced above (concentration method and thin-layer difference method) employ the radionuclide-technique. To be able to take advantage of the continuous observation of wear progress on machine parts in service, without having to stop or even to disassemble the engine, the machine parts of interest have to be marked radioactively before the test run. Proper activation of machine components is thereby a very important issue for RNT wear studies. There are three different methods of activation: activation by thermal neutrons, activation by heavy charged particles and implantation of radioactive particles as, for example, by means of recoil activation [53]. The latter method has not been generally used to date and therefore will not be discussed further. However, the other two above mentioned methods will be explained below in more detail as well as their underlying nuclear reactions. 2.3.1. Nuclear reaction and activation Atomic nuclei of wear parts are marked radioactively by employing nuclear reactions. In such a nuclear reaction the atomic nuclei (N) of the target material are bombarded by various particles (a) such as protons (p), neutrons (n), ␣-particles (␣) or deuterons (d) with high kinetic energy (Ea ) in the range of 5–100 MeV. Upon impact by the bombarding particles the target nuclei are transformed into the intermediate state of highly activated compound cores. As a consequence of this high energy excitation an immediate emission of various other particles (b), e.g. neutrons, protons, deuterons, ␣-particles or ␥-quanta, from the compound cores takes place. As a result of the nuclear reaction the nu-

clei N+ can remain in unstable, radioactive states. The underlying nuclear reaction (activation) is symbolically written as: N (a,b) N+

(2)

As radioactive nuclei N+ are unstable, their excitation decays on average after a mean lifetime ranging from fractions of a second to million of years, depending on the excited core, accompanied by the emission of radiation, e.g. ␣-, ␤-, ␥- or induced X-radiation. The emitted radiation then is detected and evaluated by the RNT measuring system. Basically, two different types of activation will be discussed due to their industrial relevance: activation by neutrons in a nuclear reactor and activation by heavy charged particles using an accelerator, e.g. a cyclotron. This latter procedure is often referred to as the thin-layer activation method. 2.3.2. Activation by neutrons Low activities and sensitivities, as well as reliable measuring equipment, provide the foundation of an advantageous use of wear measurement in mechanical engineering using the radionuclide-technique. The necessary activation of machine parts may be carried out in a nuclear reactor using thermal neutrons [54]. The machine components to be activated are kept in the reactor and are bombarded by neutrons for a specific duration. This mainly results in the formation of radioactive nuclides, mostly via (n,␥)-reactions, e.g. 58 Fe (n,␥) 58 Fe, which emit ␥-radiation. The main difference between activation with thermal neutrons, as compared to charged particles, is that the targets, in nearly all cases, become radioactive throughout the entire sample and not only in the wear zone close to the surface. This means the

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activity is higher than needed. Moreover, in some cases it is not possible to get suitable radioisotopes by bombarding a part of an engine with neutrons. For example, most of the sliding bearings manufactured from lead bronze material cannot be activated by neutrons. As the total activity depends on the mass of the component, the application of this method of activation is limited to small engine components. As a consequence of high total activities produced by using large machine parts (typically 25–500 MBq, as compared to the thin-layer activation method with values of 25–250 kBq), protective measures and special safety facilities have to be established when using machine parts activated by neutrons. However, activation by neutrons is comparatively inexpensive. Thermal neutrons can furthermore be used very effectively when activating chrome, e.g. chromium-plated piston rings. Since the 50 Cr (n,␥) 50 Cr activation cross-section of thermal neutrons and Cr is orders of magnitude higher than that of neutrons and Fe (58 Fe (n,␥) 58 Fe), the chromium layer is entirely activated, while activation of the iron ring beneath is almost negligible. 2.3.3. Thin-layer activation by heavy charged particles In addition to activation by thermal neutrons the target can be activated by the bombardment with heavy charged particles originating from an accelerator, such as a cyclotron [8–10,13,15,16]. The particles may be protons, deuterons, or ␣-particles. When using this technique the target becomes radioactive only in a thin zone near its surface since the charged particles quickly lose their kinetic energy as a consequence of intense interactions with the target atoms due to the electrical charge of the target nuclei and the solid itself. This is in total contrast to thermal neutrons, which behave electrically neutral. Therefore, this activation method is called thin-layer activation. Activated machine parts emit characteristic ␥-rays that can be detected by the RNT measuring system. The great advantage of this method is the small total activity of tested engine parts and the capability of activating small surface areas even of machine components of large extent, such as wheel flanges of railroad wheels [55]. In most cases the total activity of engine parts using thin-layer activation is more than a thousand times lower than that of parts activated, as normally done, by thermal neutrons (250 kBq as compared to 500 MBq). Thus, wear measurements using RNT can be carried out even in industry without special protective measures such as special tools or lead shields, therefore eliminating handling problems. This method allows the activation of any visible part of a machine element, large or small. With an extracted cyclotron beam, it is even possible to activate narrowly demarcated areas of a machine part. Finally, by choosing bombarding particles of different types and energy levels, different patterns of radioisotopes may be obtained in targets of identical material. Thus, for example, in a combustion engine, wear of different machine parts or dif-

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ferent locations of the same machine part may be measured simultaneously [7]. The maximum penetration depth of charged particles in a target depends on the energy of the particles and various properties of the target material. The probability of certain nuclear reactions between the charged particles and the nuclei of the target producing radionuclides also strongly depends on the energy of the particles. For this reason initial particle energies of about 5–100 MeV are typically used. As an example, the upper part of Fig. 5 shows the energy of deuterons as a function of the thickness of the bombarded steel target. At a depth of about 2.8 mm (maximum penetration depth) the deuterons have completely lost their initial energy of 50 MeV. The lower part of Fig. 5 shows the depth profile of the specific activity (activity per unit depth) of the radioisotopes 56 Co, 57 Co and 52 Mn produced by the nuclear reactions 56 Fe (d, 2n) 56 Co, 56 Fe (d, 2n) 57 Co and 52 Cr (d, 2n) 52 Mn, respectively, of the deuterons with the steel target. In order to get a linear relationship between the thickness of worn surface layers and the loss of total activity of the target, for accuracy reasons, the activity per unit depth (Fig. 5) has to be sufficiently constant. On the other hand, the time of irradiation (activation) has to be kept as short as possible for monetary reasons. Therefore, the specific activity (activity per unit depth) has to be as high as possible. For the hashed areas of the depth profiles the desired linear relationship does exist and the specific activity is sufficiently high. Therefore, the hashed areas are most suitable for the activation. It has to be considered, however, that the activation is an exothermal reaction resulting in elevated temperatures of the machine part. As a consequence, the maximum permissible temperature of the machine component is another limiting factor. The thin-layer activation technique yields depth ranges typically of 20–500 ␮m. If for example, the iron target is shielded during activation by an iron masking plate with a thickness of d1 = 2.155 or d2 = 2.575 mm (see Fig. 5), regions of constant specific activity for the radionuclides 56 Co and 57 Co, respectively, can be produced at the very surface of the machine part. Alternatively, particle energies E1 or E2 can be selected to activate the outermost surface instead of using a masking plate. By such means it is possible to activate a surface layer with different radionuclides at different depths. The sensitivity (total activity per depth) of the activated surface region can be influenced very strongly by varying the angle between the surface of the target and the particle beam of the accelerator. At grazing incidence the sensitivity is distinctively larger as compared to normal incidence when using the same activation parameters. This is also accompanied by a thickness decrease of the region of constant activity as the total activity of the target is constant. By this means very large machine parts can also be activated. The above discussed method of activation has made RNT very versatile and as a consequence, the wear of almost every engine component can be measured.

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Fig. 5. Residual energy E of heavy charged particles (deuterons) used for activation as well as activity per depth A/ d (specific activity) of the activated target material as a function of sample depth. Particle energies E1 or E2 can be used instead of using masking plates of thickness d1 or d2 to activate the outermost surface by producing 56 Co or 57 Co, respectively.

2.3.4. Decay of activation and detection of γ-radiation Activated machine parts consist, to small extents of 0.1–10 ppb, of unstable radioactive nuclei. As those nuclei are unstable, the excitation will decay on average after a half-life, ranging from fractions of a second to million of years, depending on the excited nuclei. The time dependent decay of the radioactive nucleus is written in terms of its half-life T1/2 :
 −ln 2t n(t) = n0 exp (3) T1/2 with the number of radioactive nuclei n0 at the time t = t0 and the number of remaining instable nuclei n(t) after the time t. Typical radionuclides used for wear measurements on machine parts show half-life values of 5–270 days, permitting test runs over desired durations of several hours to several weeks.

The decay process of the radioactive material is accompanied by the emission of radiation, which is characteristic for the corresponding nuclei. Possible types of radiation are, for example, ␣-, ␤-, ␥- or induced X-rays. This radiation, strictly speaking the ␥-radiation, is monitored by using a NaI scintillation detector with electronic output coupling for high counting rates. Individual count pulses are amplified, recorded and classified. The determined energy distribution of the measured counts is then correlated to the corresponding radionuclides, whilst their count rates are converted to the amount of linear wear (wear depth) in nanometer or volumetric wear (worn mass) in microgram, respectively, of the different machine parts. Thus, with an automated procedure the progress of wear can be plotted on-line very easily. By considering underground signals from natural radioactivity as well as Compton counts, accuracy can be optimized and as already has been done in commercially available systems.

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Therefore, continuous wear measuring instruments are easy to operate, even without any special knowledge of nuclear physics and radiometry. However, the measured data must be converted to tribological data, which mainly requires adequate knowledge of the tribological system and of tribology in general. From the point of view of RNT wear measurements, solving a tribological problem then only entails converting tribological data such as material composition, sensitivity required, test duration, etc. into activation data (type of bombarding particle, irradiation angle and time, etc.).

3. Wear analysis 3.1. Realization of measurements and optimization of technical tribosystems Using the radionuclide-technique, wear analysis [43,56] can be performed as described above. As an equation, wear analysis can be written in terms of the wear rate w ˙ of different components as a function of various parameters and conditions b: A = w(b) ˙ = w(r, ˙ p, e, τ)

(4)

with r is the running conditions such as speed, load, temperatures, etc.; p the parameters of the tribosystem such as materials, micro and macro geometry, lubricants and lubrication, etc.; e the environmental conditions such as humidity, dust, temperature, etc. and τ the lifetime of the tribosystem.The wear analysis of the tribosystem therefore represents a multi-dimensional wear characteristic with the dimensions r, p, e and τ. In this multi-dimensional wear characteristic conditions causing high lifetime relevant wear rates can be identified. Moreover, transient operating conditions jeopardizing the complete tribosystem, as well as those yielding time dependent wear rates, can be revealed. If in addition the actual distribution f(b) of operating conditions and parameters b is known (for example, the actual distribution of operating conditions and parameters for the average driver of a specific passenger car), the total amount of wear w or the wear rate w ˙ can be evaluated by integral calculation over conditions b and over runtime t:  bmax w ˙ = w(b)f(b) ˙ db (5) 0



τ

wmax =

w ˙ dt

(6)

0

The wear-related lifetime τ then can be approximated form the maximum permissible amount of wear wmax of the system. For the approximation, however, only the practicerelevant conditions and parameters have to be considered, e.g. those yielding both a non-negligible wear rate and a non-negligible probability within the distribution. Those conditions can be identified by measuring three-dimensional

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wear maps (for example, wear rate as function of speed and torque). Of course, special peculiarities, such as transient operation conditions, have to be taken into consideration also, as for example, wear rates are frequently higher immediately after start–stop action as compared to wear rates after long periods of running at the same constant conditions. In addition, wear analysis strongly depends on the lifetime of the tribological system with the successive stages of running-in, normal life and end of lifetime. The influence of operating conditions and parameters, e.g. torque or speed, on the wear behavior can be optimized by sensitivity analysis, without risking severe damage of the analyzed machine parts. Also, the influence of macroscopic factors on the wear behavior, such as the clearance of bearings, can be investigated very easily by the so-called tolerance analysis resulting in optimization of production tolerances and costs. During the test, the wear data, for example, the amount of volumetric or linear wear in microgram or nanometer, respectively, are determined continuously (see Fig. 6). Wear rates can be calculated on-line using the slope of the wear curves over time at any chosen time frame. Any distinct changes in the wear behavior of the engine will be indicated at once, e.g. due to adding lubricant additives to a base oil, or by increasing the engine speed, as shown in Fig. 6. Therefore, it is possible to determine quickly and exactly the economically and technically optimum dosage of wear-reducing additives or the load limits of the analyzed engine parts. Since expensive test engines must not be risked, the test run can be stopped automatically once a predetermined amount of wear has been exceeded or in response to a sudden increase in wear rate caused by a change in operating conditions. 3.2. Interpretation of results 3.2.1. Analysis of running-in performance The running-in or breaking-in performance of a tribological system supplies important indications as to whether both machine parts and lubricants (breaking-in oils) will suffice for the loads required, and whether during breaking-in the wear surfaces will be conditioned sufficiently, by the formation of a tribomutated zone in surface-near volume showing typical thicknesses of 0.1 ␮m [39,57–59]. As shown in Fig. 6, three characteristic values are evaluated and used to evaluate the running-in behavior: • the wear rate immediately after a variation of the operating conditions towards higher loads; • the period of time until a constant value of wear rate is reached; • the wear rate upon reaching a stationary wear behavior. During the running-in process, it often emerges (by extrapolation of the wear behavior) that machine parts will not stand the required loads. In such cases the tests will be stopped. By analyzing the running-in behavior both machine components and breaking-in procedures can be optimized.

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Fig. 6. Progress of wear during running-in process of a technical tribosystem with three characteristic values for every increase in speed or load: wear rate w ˙ A immediately after an increase in speed or load as well as the period of time τ A to reach the stationary wear rate w ˙ B.

Therefore it is possible to operate engines safely throughout the full range of speed/load conditions after only a few minutes of running-in. 3.2.2. Analysis of three-dimensional wear maps The wear rates of a machine part measured as a function of the operating conditions of engine speed and torque (speed-load-plane, for example, as shown in Fig. 7) yield important wear characteristics. Therefore the magnitude of wear rates and the shape of wear characteristics is of special interest. When combined with the characteristics of oil consumption, blow-by or constituents of the exhaust gas as a function of the speed-load-plane, important hints regarding the operational safety of lubricants and machine components of the technical tribosystem can be obtained. In the case when the distribution of practice-relevant speed and load conditions is known, it is also possible to evaluate (extrapolate) the wear-related lifetime. Therefore, special peculiarities of transient running conditions also have to be considered.

In addition, wear maps measured after different periods of running can be used as a very distinct indication of the aging resistance of both machine parts and lubricants, which is important for the optimization of oil drain intervals. Besides the optimization of technical tribosystems, the analysis of wear maps also facilitates the optimization of endurance tests. For example, it is not well known that in diesel engines of modern passenger cars or trucks, maximum wear of the radial surface of piston rings occurs at low speed and full load [60]. Unfortunately, verification of piston ring and piston designs frequently consists of tests at high speed and full load. 3.2.3. Sensitivity analysis Sensitivity analysis [43] shows the influence of operating conditions and parameters (e.g. torque, speed, temperatures, pressures) on the wear behavior of the engine. As an example, the wear rates of two different piston rings as a function of the mean effective pressure are displayed in Fig. 8. Whilst the wear behavior of piston ring A is superior to that of ring

Fig. 7. Wear map over speed and load plane showing wear rates as a function of engine speed and torque as well as key operation conditions at full load/low speed and full load/full speed.

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Fig. 8. Sensitivity analysis showing the wear rate of two different piston rings as a function of the mean effective pressure and indicating different sensitivities close to rated power.

B below a pressure of 1.5 MPa, it shows a distinct sensitivity to pressure close to the rated power. Sensitivity analysis allows the evaluation of various influencing factors without any risk of severe damage to the analyzed machine parts, as for example, the load limits of piston ring packages in the case of a power enhancement, the effects of a diminution of the oil pressure on the wear of the bearings, or the need for oil coolers, charged air coolers, and oil spray jets for pistons. All such information is indispensable for the development of modern engines. For the development of lubricants temperature sensitivity is of particular importance. In many cases additives become effective only above a certain minimum temperature. However, if a certain maximum temperature is exceeded the additives may deteriorate. Special attention should be paid also to the fluctuation of the running conditions of engines in everyday practice, since some machine parts might become impaired beyond any remedy after exceeding the admissible working stress for only very short periods of time. As a consequence of these fluctuations, high wear rates might result even at low-load levels. The wear behavior of the valve train (cams, tappets, cam followers) in some cases strongly depends on speed, which might be due to vibration-related disturbances or adverse tappet rotation. A regular tappet rotation is generally desirable, although not always necessary, since increased wear is always accompanied by longer periods without rotation of the tappets. Excessive tappet rotation, however, is an indication of excessive friction between the cam and tappet and therefore should be prevented also.

face roughness or roundness and misalignment of journals) is investigated by the so-called tolerance analysis. The aim is to establish low-cost production tolerances, which guarantee sufficient wear resistance. This point is gaining in significance as, during the design stage of a combustion engine, questions are now more frequently asked about low-cost production tolerances.

3.2.4. Tolerance analysis Comparable to sensitivity analysis, the influence of macroscopic factors on wear behavior (e.g. clearance of bearings, clearance between piston and cylinder liner, sur-

4.1. Wear behavior of different engine components

3.2.5. Analysis of transient operation When analyzing and optimizing machine components and lubricants the wear-related lifetime can be evaluated from wear characteristics under constant running conditions and from knowledge of the distribution of speed and load conditions during practical application. However, the evaluation of transient operating conditions, e.g. start–stop operations, has to be considered also, as it is indispensable. For example, poor design along with a low-quality lubricant may result in complete wear of the lead layer of a bearing due to starting-procedures of the engine after short-term stops. Other examples of transient conditions are: the effects of a long-time operation of diesel engines at low load with exhaust gas recirculation on the wear of piston rings during the following full load operation, or the effects of high oil temperatures at low engine speed and full load immediately after rated power or full load and full speed operation on the wear of bearings (simulation of highway driving with traffic congestions).

4. Case studies

Significant work has been done by a number of authors on wear studies of different engine components using the

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radionuclide-techniques [42,43,57,61–71]. Based on the results obtained in various case studies, the wear behavior of different engine parts will be discussed briefly.

The biggest advantage of using RNT for the development of modern engines lies in the combination with other techniques, as shown above.

4.2. Wear analysis of sliding bearings such as the big end bearing of a connecting rod

4.3. Wear analysis of sliding bearings as a function of surface hardness

The connecting rod big end bearing of an engine is a dynamically loaded journal bearing. The lubricating oil film thickness in the journal depends in part upon the combustion pressures and inertia forces of rotating and reciprocating parts. For wear studies the bearing shells are irradiated using a cyclotron. The wear behavior of the upper shell is generally more sensitive to the clearance, which strongly depends upon the type of assembly as well as operating conditions and its surface properties. The wear behavior of bearing shells is very strongly correlated to the pressures acting upon the journal bearing during the combustion cycle. Therefore helpful information can be obtained from force diagrams and displacement curves of the bearing. From those diagrams it can be clearly seen that the maximum forces and minimum lubricant film thickness occur more frequently on the upper shell, for example, at low engine speed. This has been confirmed by using RNT wear measurement as wear of the upper shell was higher than that of the lower one, which exhibits almost negligible amount of wear. RNT wear analysis provides more detailed information about the scuffing tendency, corrosion susceptibility and failure probability of the big end bearing of a connecting rod. The normal wear rate of a big end bearing in a diesel engine typically ranges from 0 to 6 nm/h and running-in periods last for 10–50 min, when using an optimized procedure.

The wear characteristics of a sliding bearing was examined under static and sinusoidal loading conditions using different materials. Based on using the concentration method with the flow-through measuring head, shafts with different values of surface hardness but the same surface finish were evaluated in contact with the same bushing. The applied load was set to drive the system into the boundary friction regime. Fig. 9 shows the result and it can be seen that the amount of total wear decreases with increasing Vickers hardness ratio of shaft to bushing. 4.4. Wear analysis of sliding bearings using different surface roughness Another example of the analysis of the sliding bearing/shaft system deals with the influence of surface texture of the shaft and bearing on the minimum admissible lubricant film thickness at the transition from fully hydrodynamic lubrication (almost no wear) to boundary lubrication. This is an important extension of the theory for the design of sliding bearings that departs from ideally smooth and geometrically perfect surfaces which are frequently not available after manufacturing and surface finishing. RNT was applied to characterize the transition point from mild to severe wear for different rough surfaces. Fig. 10 illustrates the experimental results. Fig. 10 shows wear rates

Fig. 9. Total wear of the bushing of a journal bearing as a function of the Vickers hardness ratio of shaft and bushing. Runtime and hardness of the shaft were constant while hardness of the bushing was varied.

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Fig. 10. Determination of the transition load pT from mild to severe wear and determination of the corresponding minimum admissible lubricant film thickness for different rough surfaces by analyzing the measured wear rate of a journal bearing and the calculated lubricant film thickness as a function of the applied mean pressure.

measured as function of the applied load for three differently configured shaft roughnesses (A, B, C), which were produced by different running-in procedures. The transition load clearly shifts to higher values for smoother shafts: shaft A (axial peak to valley height Rt,ax = 0.4 ␮m) and B (Rt,ax = 1 ␮m) in comparison to the rough shaft C (Rt,ax = 3.5 ␮m). Such an accurate determination of the transition values, as they were determined here using RNT and by measuring the transition wear rate, cannot be achieved by means of any other measuring technique. An accompanying calculation, assuming perfectly smooth surfaces and isothermal elastohydrodynamic lubrication provided thickness values of the lubricant film thickness as function of the applied load as expected from smooth surfaces. By exceeding the transition wear rate w ˙ T the specific transition load pT and the computed transition lubricant film thickness were determined. Consequently, the admissible minimum lubricant film thickness can be approximated for the measured roughness values of the surfaces using Fig. 10. Thus, the analysis can be used to support the computational design of sliding bearing systems with the help of hydrodynamic theory. It should be mentioned, however, that after running-in the roughness of both contacting surfaces cannot only be smoother, but also rougher, as compared to the original finished surface, depending on surface properties, surface finish and running-in conditions [38,39]. An example of the effect of increasing the roughness of both contacting surfaces in a pin-on-disc test [39] is shown in Fig. 11. The smoother surface does not necessarily have to be the one with superior wear behavior, as can be seen in the wear analysis in the cam/tappet system in Fig. 12, where the optimum

wear behavior, i.e. the smallest wear rate, was found for a medium-sized roughness value. As a measure for roughness the profile depth at 80% micro-profile bearing ratio was used. In the inset of Fig. 12 the micro-profile bearing ratio curves (profile depth as function of bearing ratio) are displayed for various rough surfaces. 4.4.1. Lubricant analyses in tribological systems Practically all tribological systems lubrication is applied to reduce friction and wear. Apart from the type of lubrication the selection of a suitable lubricant is of great importance. For such lubricant tests RNT is particularly useful. On the one hand, RNT is suitable to determine the effect of formulated lubricants on critical components of engineering systems within a short time. On the other hand, RNT permits the optimization of lubricants for certain applications by the analysis of the wear characteristics in response to direct variation of active substances in the assigned lubricant. This will be demonstrated by the following case study. In experiments using a water-cooled four-cylinder diesel engine, different oils were tested with a range of cylinder wall temperatures under hot test operation (maximum speed at full load). The wear of the cylinder liner around the top dead center position for the top piston ring (critical wear zone of diesel engines) was measured using RNT. The results are presented in Fig. 13. Without dealing with all details, it is noticeable that below 150 ◦ C the wear rates of different formulated oils and different viscosity indices are below 500 ␮g/h (Fig. 13). This is also the normal temperature range of the engine. In the case of increased wall temperatures achieved by increasing coolant temperatures, the oils E–G showed strongly

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Fig. 11. Wear rates w ˙ and roughness values Ra (parallel and normal to the sliding direction) of both pin (AlSn20) and disk (100Cr6) as a function of runtime.

Fig. 12. Mean wear rate of cams as a function of surface finish roughness (using the parameter profile depth at 80% micro-profile bearing ratio), measured over a wide range of engine speed. The inset displays the micro-profile bearing ratio curves showing profile depth as function of bearing ratio.

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Fig. 13. Wear rate of a cylinder liner at the upper dead center of the top piston ring as a function of the average cylinder wall temperature for different lubricants showing intersecting curves. Therefore depending on the distribution of actual cylinder wall temperatures, different oils will result in superior wear behavior.

increased wear. Whereas in contrast, the oils A–D exhibited only small changes. The reasons for this behavior are differences in the composition and structure of the oils used and their viscosity. This example demonstrates how a tribological analysis can be examined using RNT with variation of sensitive parameters. However, as also shown above, valid results can only be obtained if the whole system is considered. Oils are not only used to reduce wear but also to keep the engine clean and to protect the system from corrosion. This must be considered in each case. It is therefore not advisable to reject the oils E or G (Fig. 13), since the data were obtained for the hot state only. To achieve a comprehensive picture of oil performance, the test regime has to be extended. It is possible that even oil G may show the most favorable characteristics when operated at lower temperatures. This is exactly the essential point that can be drawn from Fig. 13: as the curves are intersecting, the performance of certain oils above the crossover point is worse, though below the crossover point it is superior. Therefore, conclusions for certain oils should only be drawn when the entire practice-relevant range of parameters (the distribution function of parameters) has been considered.

5. Conclusions The use of radionuclides for the purpose of wear measurements has proven to be an easy to handle, powerful tool for analyzing and also optimizing technical tribosystems. The radionuclide-technique (RNT) offers a series of advantages, such as:

1. Continuous measurement and display of wear as a function of time on machine parts in operation. 2. Simultaneous measurement of wear for different selected machine parts, or different locations of the same machine part. Selected areas may even be narrowly demarcated. 3. Wear behavior and performance can be correlated to different operating conditions and parameters, enabling the analysis, e.g. for breaking-in, of three-dimensional wear maps, of the sensitivity or stability of the tribosystem, and of the influence of engineering tolerances. 4. High sensitivity, capable of resolving wear rates as low as 0.1 nm/h or 1 ␮g/h. 5. Correctly applied, RNT cannot only be a valuable tool for supporting the development of, for example, modern engines, but also makes this process faster, more rigorous and cheaper. Continuous wear measurement using the radionuclidetechnique comprises two methods: the concentration method and the thin-layer difference method, which is used only when it is not possible to apply the concentration method, i.e. when there is no capability to have a fluid for conveying wear particles. Each method enables continuous observation of the progress of wear without having to stop or even to disassemble the machine. The time dependence of wear is determined from the ␥-radiation emitted by the activated material. The activation of the machine parts can be performed by neutrons in a nuclear reactor or by heavy charged particles, e.g. protons, deuterons or ␣-particles, using a cyclotron. When using heavy charged particles, the machine component is activated only in a thin zone near its surface, since the charged particles quickly lose their kinetic energy

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as a consequence of intense interactions with the atoms of the machine part. The great advantage of this method is the low total activity of the machine components. Thus, wear measurements using RNT can be carried out even in industry without special protective measures, such as special tools or lead shields, thereby eliminating handling problems. In addition, this method of activation has made RNT very versatile and the wear behavior of almost every engine component can be measured. Continuous wear measurement using the radionuclidetechnique provides very accurate and detailed information on the wear behavior of different engine parts whilst an engine is running. In particular, it enables the analysis of breaking-in performance, the analysis of three-dimensional wear maps, and the analysis of the sensitivity and the stability of the tribological system, as shown in many case studies. References [1] W. Kaspar-Sickermann, Kolbenringverschleißversuche mit radioaktiven Isotopen, Kerntechnik 3 (7) (1961) 301. [2] K. Kollmann, D. Stegemann, Anwendung radioaktiver Isotope für Forschungsaufgaben des Maschinenbaus, Kerntechnik 4 (2) (1962) 41. [3] K. Kollmann, W. Kaspar-Sickermann, D. Stegemann, Verschleißmessung mittels radioaktiver Isotope an Kolbenringen und Gleitlagern von Verbrennungsmotoren, MTZ 24 (2) (1963) 33. [4] K. Kollmann, H. Sitzler, Zweikomponenten-Verschleißmessung mit Hilfe radioaktiver Isotope, MTZ 27 (5) (1966) 197. [5] K. Kollmann, D. Stegemann, Messung der Drehbewegung von Kolbenringen im Motorbetrieb und deren Einfluß auf die Abdichtung, MTZ 24 (3) (1963) 73. [6] K. Kollmann, H. Sitzler, Beitrag zur Messung von Schichtdicke und Dichte mit Hilfe der Beta-Strahlung, MTZ 27 (6) (1966) 237. [7] A. Gervé, A special measuring method of examining several components of wear by means of radioactive isotopes, in: Proceedings of the IAEA-Symposium on Radioactive Tracers in Industry and Geophysics, Prague, Czechoslovakia, November 1966, IAEA, Vienna, Austria, 1967. [8] A. Gervé, H. Kamm, G. Katzenmeier, Zur Deuteronenaktivierung von Maschinenteilen aus Stahl und Gußeisen, Forschungsbericht 2-213/10, Heft 77, FVV Forschungsvereinigung Verbrennungskraftmaschinen e.V., Frankfurt, Germany, 1968. [9] A. Gervé, Radioisotopes in mechanical engineering, in: Proceedings of the Fourth United Nations International Conference on the Peaceful Uses of Atomic Energy, Geneva, Switzerland, AED-Conference 71-100-55, 1971. [10] B. Herkert, Die Aktivierung von metallischen Maschinenteilen mit geladenen Teilchen zur Durchführung von Verschleißmessungen, KfK-Bericht 2096, Kernforschungszentrum Karlsruhe, Gesellschaft für Kernforschung mbH, Karlsruhe, Germany, 1975. [11] A. Gervé, G. Schatz, Applications of cyclotrons in technical and analytical studies, in: Proceedings of the Seventh International Conference on Cyclotrons and their Applications, Zürich, Switzerland, 1975. [12] G. Essig, Zur Aktivierung von Maschinenbauteilen für Abtragsmessungen, Kerntechnik 18 (1976) 470–474. [13] T.W. Conlon, Thin layer activation by accelerated ions—application to measurement of industrial wear, Wear 29 (1) (1974) 69–80. [14] G. Winkhaus, Progress in the technical application of isotopes, Atomwirtschaft-Atomtechnik 19 (11) (1974) 546–550. [15] T.W. Conlon, B.H. Armitage, The application of energetic ion beams in the study of wear and porosity, Wear 34 (3) (1975) 409–418.

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