Surface Layer Activation
Monitoring techniques based on Surface Layer Activation (SLA) to produce gamma emitting surface "markers" were developed independently by the Atomic Energy Research Establishment(1) in the United Kingdom, and by Spire Corporation(2,3) in the United States for on-line measurement of material loss due to wear, erosion, or corrosion. SLA was an outgrowth of earlier radiotracer techniques(4,5), which required much larger bulk activations (initially by reactor neutrons) to achieve similar precision to that possible using a very slightly activated, shallow layer as a marker. With activated layers of a few micrometers or less in depth, precision in the nanometer range is possible with a minimal amount of radiation.
Both marker and tracer approaches rely on debris being carried away from the original activated surface by a working fluid such as oil or water. To measure buildup of tracers in this fluid, a scintillator detector is placed to register gamma rays from the fluid line or filter. Alternatively, wear can be measured directly by placing a detector to indicate activity remaining in the activated marker. When the marker is very shallow, this approach produces precision on the order of 1% of the activated depth.
Both marker and tracer approaches are still used today in engine research, primarily in Europe and Japan, but also at Southwest Research Institute in San Antonio (6), Fleetguard Filters in Cookesville, Tennessee, and other companies with engine related products. The marker approach has been successfully applied to a variety of wear problems including diesel engine parts (bearings, cams, piston rings, and valves), and centimeter-scale erosion problems (pipelines, drilling equipment, furnace walls, boiler tubes, and missile nose cones). Its high precision configurations have also been used to characterize head crash damage to magnetic recording media and erosion inside fuel injectors. Applications currently in development include precision machine tool bits and pump components. The tracer technique is often used to accurately indicate the onset of a wear or corrosion process, although it too can be used to track wear rates, particularly if a shallow activation is used and there is sufficient activity in the activated zone.
SLA markers are calibrated by incrementally polishing an activated specimen and measuring the activity remaining after each increment to determine the activity versus depth profile. Tracers are calibrated by introducing known quantities of activated debris into the system during operation and monitoring the counting rate at a down-stream filter or collection site.
The surface of a component or specimen is activated by bombardment with ions of a precisely known energy from a cyclotron or Van de Graaff. By choosing the incident species and energy, a suitable radionuclide and depth distribution in the part can be produced. The radionuclide must have a sufficiently deep profile and long half-life to allow completion of the monitoring and a sufficiently energetic gamma ray to penetrate any surrounding material and allow external detection.
If wear is of extremely small magnitude (<1µm deep), which is common for fretting, one of several ultra-shallow activation approaches may be used: bombardment at oblique angles, bombardment through a thin surface coating, or recoil implantation.(7) In recoil implantation, the part to be studied is not exposed directly to the particle beam. Instead the beam is directed through a separate metal foil. The reaction product atoms recoil with energies up to several hundred keV each; many escape from the foil to be implanted downstream in the target surface.
This method produces an extremely shallow activated layer with a depth of less than 0.2 µm. It has the advantage that the radionuclide species can be selected by simply changing the foil material; furthermore, the possibility of radiation damage from the direct beam in sensitive materials such as polymers, composites, and some ceramics is eliminated.
Below is a representative wear history for a mechanical part. Note that there is a trade-off between the total amount that can be monitored and the resolution of each individual wear increment. Often we can resolve about 1% of the total activated depth.
If you have an application for real-time wear monitoring or tribological study that this approach might help, we would like to hear from you. We are currently training students to perform laboratory calibrations of activations and would be interested in considering new configurations. Activation costs vary, starting around $500 for each component to be tested. Detectors usually cost between $2,000 and $3,000 each. Electronic packages to run these can now be installed on a single board inside a PC with analysis software readily available.
Please email me at firstname.lastname@example.org or call (620) 235-4398.
1. Conlon, T.W., "Thin Layer Activation by Accelerated Ions: Application to Measurement of Industrial Wear", Wear, Vol.29, No.1, pp.69-80, (1974).
2. Armini, A.J., "SPI-WEAR, A Real Time Wear Monitoring System", Spire Corporation, TR-75-07, (1975).
3. Armini, A.J., and Bunker, S.N., "A Re-Entry Vehicle Nosetip Shape Change Sensor", ISA Transactions, Vol.15, No.1, (1976), pp.22-29.
4. A fairly complete bibliography of early work in this area is in "Radionuclide Methods," C. Blatchley, in Metals Handbook: Friction, Lubrication and Wear Technology, Vol. 18, p.319-330, Peter Blau, Ed., ASM International, October, 1992.
5. Earliest reported application is Pinotti, P.L., Hull, D.E., and McLaughlin, E.J., California Research Corp., SAE Quarterly Transaction, (1949).
6. "Importance of Diesel Oil Filtration Demonstrated by Surface Layer Activation Wear Monitoring," J. Truhan, C. Covington, C. Blatchley, and C. Colerico, TR94-07, American Chemical Society Meeting in San Diego, CA, in March, 1994.
7. Conlon, T.W., "Indirect Recoil Implantation Following Nuclear Reactions: Theory and Potential Applications", AERE-Harwell, U.K., R9340, (1979).