Wear, 34 (1975) 251 - 260 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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FERROGRAPHY - AN ADVANCED DESIGN AID FOR THE 80’s *

D. SCOTT

National Engineering Laboratory, East Kilbride, Glasgow (Gt. Britain)

W. W. SEIFERT

Department of Electrical Engineering and Civil Engineering, Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.)

V. C. WESTCOTT

Foxboro/Trans-Sonics, Inc., Burlington, Mass. (U.S.A.)

(Received May 6, 1975)

Summary

With the sophisticated machinery of the 80’s it will be essential to effect design improvements during prototype testing to eliminate both maintenance and failure in service. Ferrography allows the in situ condition of inaccessible mechanisms to be monitored and thus enables design changes and their assessment to be completed prior to the use of the machinery in service; lubricant deterioration can also be diagnosed and necessary remedial measures effected.

The equipment used and the investigational techniques developed are described. Specific applications and the significant results obtained are illustrated from the work of the authors and their associates over a wide field.

Introduction

Many advances in engineering have been made by turning failure into success and the history of engineering is, in part, a tale of failure in service followed by the development of improved materials and designs based upon effective failure investigation [ 1, 21. Such investigations [ 3 - 91, besides indicating remedial measures, can lead to improved design incorporating greater reliability, reduced maintenance costs and increased productivity. However, from both safety and economic points of view, prevention of failure is better than any lessons which can be learned from it. Thus, increasing emphasis is now being given to on-line monitoring techniques as a means of detecting deterioration of machinery so that remedial action can

*Paper presented at the 3rd International Tribology Conference, “Tribology for the Eighties”, Paisley, 22 - 25 September, 1975.

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be taken before the breakdown point is reached [lo]. As machinery be- comes more sophisticated in the 80’s, any failure of the more complex machines will tend to have serious consequences. Also, the shutting down of vital machinery, such as on-line chemical plant and atomic reactors to effect remedial maintenance indicated by the monitoring system may have signif- icant economic implications. Thus, it will become essential to thoroughly screen prototype machinery during design proving tests and to effect design changes and necessary improvements at this stage to eliminate premature signs of distress in service.

The avoidance of design faults in comparatively, cheap, simple mass- produced machinery such as automobile engines and automatic transmis- sions will require their rigorous assessment during prototype testing. Owing to increasing production rates and the vast numbers of units produced, premature failure of a single component can cause considerable financial losses if recall of units for rectification becomes necessary.

In the past, many machines were simple and their longevity was ensur- ed by the fact that the engineer could see most of the working parts and, if any began to wear, he could take remedial action before the machine failed. Machines today are more compact and, as moving parts are totally enclosed, the maintenance and development engineer must periodically dismantle essential parts for routine examination. Such a procedure is not only time consuming and expensive but involves a danger of failure during run-in after reassembly.

In the future, machines are likely to become more complex and their vital components even more inaccessible. Also, the time between research on a new design and its industrial utilization is continuously decreasing so that prototype testing must be reduced to the absolute minimum. Time will not be available for routine dismantling to examine the faults, make changes and improve the design. Comprehensive monitoring of the condition of such machinery will become essential. While it is not possible to examine in situ the working parts of complex machines such as jet engines, the oil which circulates through the moving parts carries with it evidence of its experience in passage. Careful examination of the oil and any particles it contains allows interpretation of the conditions it experienced in passage through the machine and the condition of the moving parts encountered. Techniques such as S.O.A.P. are available to assess the volume of contaminant constit- uents, and equipment such as counters permit determination of the number of particles in lubricants. However, more specific information is required if the morphology of the particles in the lubricant is to be determined so that prognosis of the condition of the machine can be carried out. Ferrographic oil analysis is a convenient way of doing this. This paper outlines the application of Ferrography as a design aid for the production of the improv- ed, more reliable machinery which will be essential in the 80’s.

A long range objective of research in the machine design field is no less than the production of failure free machinery. This goal, which on first reflection appears to be unobtainable, has already been achieved in part. The

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Fig. 1. Ferrograph analyzer.

utmost reliability of the machinery which got man to the moon and back is today almost inherent in a limited range of current machines, particularly hermetically sealed household refrigeration units. Comparable reliability during an economically adequate machine life’can eventually be possible for a variety of simple components, such as bearings and gears, and of units, such as pumps and internal combustion engines.

Ferrography

Ferrography [ 11 - 131 is a technique developed to separate wear debris from the lubricant and arrange it according to size on a transparent substrate for examination in an optical or scanning electron microscope. The Ferrograph analyser, Fig. 1, consists of a pump to deliver an oil sample at a low rate (approximately 0.2 ml/min), a magnet to provide a high- gradient magnetic field near its poles and a treated transparent substrate on which the particles are deposited. The oil sample, diluted with a special solvent to promote the precipitation of the wear particles, is pumped across a transparent substrate which is mounted at a slight incline, the magnetic particles adhere to the substrate, distributed approximately according to size. After about 2 ml of oil has been pumped across the slide a washing and fixing cycle removes the residual oil and causes the wear particles to adhere permanently to the slide, Fig. 2. The quantity of wear particles and their size distribution can be determined by optical density measurement. When successive oil samples yield Ferrograms with essentially constant density

readings it may be concluded that the machine is operating normally and producing wear particles at a steady rate.

A rapid increase in the quantity of particles and, in particular, the ratio of large to small particles is indicative of severe wear. If additional informa- tion on the morphology of the deposited particles is desired, they may be examined with the aid of a Bichromatic microscope which uses simultaneous- ly reflected red light and transmitted green light. Metal particles even as small as 1 pm reflect the red light and block the green light and thus appear red. Particles consisting of compounds allow much of the green light to pass and appear green or, if they are relatively thick, yellow or pink.

Fig. 2. Ferrogram.

Particles generated by different wear mechanisms have characteristics which can be identified with the specific mechanisms [ 10, 131. Rubbing or adhesive wear particles found in the lubricant of most machines have the form of platelets and are indicative of normal permissible wear. Cutting or abrasive wear particles take the form of miniature spirals, loops and bent wires and a concentration of such particles is indicative of a severe, abrasive wear process; a sudden increase in the number of such particles in successive oil samples indicates imminent machine failure. Particles consist- ing of compounds can result from an oxidizing, or corrosive, environment. Steel spherical particles are a characteristic feature associated with fatigue crack propagation in rolling contacts [ 14, 151. The concentration of spheres is indicative of the extent of crack propagation [ 161. Examination of Ferrograms with the aid of a scanning electron microscope can reveal specific details of wear particles [lo, 13, 14, 171. Figure 3 shows typical cutting or abrasive wear particles. Figure 4 shows strings of platelet-type rubbing wear particles and Fig. 5 shows typical spherical particles. X-ray energy analysis in the scanning electron microscope and electron probe X-ray micro-analysis provide means of determining the composition of individual particles and thereby establishing from which specific component in the machine the particles were detached. The particle morphology establishes the mechanism of wear by which the particle was formed.

Progressive heating of Ferrograms can reveal changes in their appearance as revealed in the Bichromatic microscope and allow deduction of additional information on particle composition.

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(a) (b)

Fig. 3. Cutting wear particles. (a) Optical micrograph; (b) scanning electron micrograph.

(a) (b)

Fig. 4. Strings of platelet type rubbing wear particles. (a) Optical micrograph; (b) scanning electron micrograph.

Use of ferrography

Wear particles display characteristics unique to the wear mechanisms by which they are produced. The particles can be examined optically and in greater detail with an electron microscope. The composition of individual particles may be determined by X-ray energy analysis. These techniques provide a powerful diagnostic method for the detection of distress in vital industrial machine elements and the prevention of catastrophic failure by allowing timely maintenance. A prognostic approach is thus available by which failure can be prevented. Monitoring of the condition of expensive machines by this technique permits a safe change from periodic, expensive, planned maintenance with its inherently lower reliability at the time of

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(a) (b)

Fig. 5. Spherical metallic particles. (a) Optical micrograph; (b) scanning electron micrograph.

machine startup to the generally more commercially acceptable failure prevention maintenance.

In test-bed engine condition monitoring Ferrography can indicate premature distress of a specific component such as a bearing, gear or cam and thus lead to its replacement by one of improved design based on the experience gained and knowledge of the mechanism of impending failure. The redesigned component can be assessed in situ during continued test- bed running so that the design is proved before the test engine is finally dismantled and before service operation is required. The almost instantan- eous feed back possible with Ferrographic analysis permits the determination of the factors which could cause premature failure. Thus, changes in the nature, concentration or composition of the particles produced as condi- tions of load, speed, temperature and environment are changed can indicate how, when, and which components are predominantly affected. Where numerous bearings or similar self contained units are vital parts of a complete machine, rinsing these with particle-free lubricant and subsequent Ferrographic analysis of the lubricant will reveal the condition of each unit [ 181 without the need to dismantle it.

Lubricants, lubricant additives, lubricant contaminants and their influence on machine condition can also be assessed by Ferrography. For example, two synthetic lubricants manufactured to the same specific aero- engine specification but formulated by different suppliers exhibited differ- ent wear characteristics when run in a test rig [19]. Ferrograms of the wear debris revealed that the friction polymer formed by rubbing action differed with the different lubricants. Figure 6 shows an amorphous polymer termed “polymud” which was formed between rubbing steel surfaces lubricated with a polyester oil to specification Mil-L-23699. This oil was formulated with a normal, light additive package. Identical rubbing wear tests were conducted employing a similar Mil-L-23699 oil but with a “high load”

(a) (b)

Fig. 6. Amorphous polymer (Polymud) intermixed with bright metallic particles. (a) Optical micrograph; (b) scanning electron micrograph.

(a) (b)

Fig. 7. Rolling pin type polymer. (a) Optical micrograph; (b) scanning electron micrograph.

additive package. The Ferrogram prepared from this oil after tests contained the “rolling pins” shown in Fig. 7. It appears’that the polymer forms a tough film on the rubbing surface and the rolling pin like particles are form- ed by part of the friction polymer being rolled up between the rubbing sur- faces. The lubricant which provided the surface friction polymer rolling pins gave a significantly lower wear rate than the lubricant with the agglomera- tion of polymer nucleated in the lubricant. The Ryder gear tester load rating also correlated with the type of polymer, the lubricant with the rolling pins having the higher rating. Other particles, Fig. 8, “spherical baskets”, similar in composition to the “rolling pins” have been observed on Ferrograms prepared from Mil-L-23699 used in a jet engine under test. Controversy

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Fig. 8. Scanning electron micrograph of “basket” type polymer particles.

exists as to their mechanism of formation which is still being investigated [20]. The ability to gather such information on additive packages during machine testing allows lubricant formulation to meet subsequent service requirements.

Wear in systems where fluid is circulated is a costly aspect of their operation as it is not economically feasible to manufacture components completely insensitive to fluid contamination or to remove all the contam- inants which enter the fluid system. Thus, filtration is a compromise to limit contamination to an acceptable level. Ferrography can assess filter performance on test rigs as well as in critical industrial applications. Thus, suitable compromises can be achieved before service difficulties arise or production filter units are found to be inefficient or overdesigned. Ferro- graphy has shown that disintegration or degradation of the materials from which filter units are constructed can aggravate rather than control contamination. Assessment of specific types of filters on test rigs can aid design changes, and their evaluation, prior to replacement of ineffective service units.

Discussion and conclusions

The era of making advances in engineering by improved design result- ing from successful investigation of failures in service appears to be past. Monitoring of expensive machinery to detect failure initiation and allow a prognostic approach to failure prevention by timely remedial maintenance is now becoming increasingly important. The presence in a lubricant of unique wear particles easily and quickly revealed by Ferrography is a powerful diagnostic technique for the detection of distress in critical mechanisms. Monitoring by this technique can, in many instances, currently allow the safe change from expensive planned periodic dismantling of machines for maintenance to the more economic failure prevention maintenance. More-

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over, maintenance can be time absorbing and expensive as the time required for it lowers productivity. This, together with the danger of start-up implicit in the dismantling-for-maintenance procedure, makes desirable the elimina- tion of maintenance by the design of machinery to ensure a maintenance-free life of sufficient duration to adequately cover capital outlay.

With the sophisticated machinery of the 80’s it will be essential to effect design improvements quickly during prototype testing. Ferrographic oil analysis monitors the in situ condition of inaccessible mechanisms to provide almost instantaneous feed back of information to enable required design changes and their assessment to be completed prior to putting machinery into service or commencing its mass production. The information obtained by Ferrographic oil analysis thus allows the reliable prediction of long term performance in service. Such information also prevents expensive overdesign incorporating a large undesirable safety factor or more appropriate ignorance factor in design. The influence of different lubricant additives on machine performance and the efficiency of filters can also be investigated during testing and the results used to improve system designs for subsequent industrial applications.

It may be concluded that Ferrographic oil analysis, in addition to enabling a prognostic approach to failure prevention and the safe change from regular scheduled to failure-prevention maintenance, can be an aid to the design of advanced machinery for immediate maintenance and failure free service without costly development work.

Acknowledgements

The authors express their appreciation for the many discussions with Dr. Richard Miller of the Office of Naval Research and for the support of that office under contract No. NOOO-14-74-C-0135 for the analysis and study of the meaning and characteristics of the various particles of wear. They are also indebted to Dr. E. C. Van Rueth of the Advanced Research Project Agency, U. S. Dept. of Defense, for technical and contract support in investigating many aspects of wear and how particles are generated. They thank the Director of the National Engineering Laboratory of D.T.I. for permission to publish the paper and Mr. G. H. Mills for the provision of electron.micrographs.

References

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