View of the High Precision Microtribometer, used for extreme precision friction force measurements on real engineering surfaces.
Low wear rates: the simulation of wear under realistic conditions is necessary to estimate lifetime of products and components. The key to improving long-term reliable operation of parts is a uniform and low wear rate. This presents a dual challenge to the modern tribology lab: testing under realistic conditions so that low wear is generated, and measuring the low wear in a reasonable time frame. We apply two possible solutions to this problem: extremely precise on-line wear methods as Thin Layer Activation allow us to measure nanometer wear during testing. The method is extremely precise and can be used on lab scale, but requires a costly activation of test pieces. The cost of this activation can be offset against the efficiency gain, but is still holding back a lot of customers.
Otherwise, there is the multistation approach: testing simultaneously of 50 or 100 coupons for wear resistance. In this case, it is possible to operate the wear test machine for many weeks, because at the end of the run, 50 or 100 test results are available at the same time. This results in an efficiency gain, provided that enough different contacts need to be compared.
Both challenges are encountered by us very regularly, and met with depending on our client’s need by one or the other approach.
CONCLUSION: DO WE STILL NEED STANDARD TESTS ? There is discussion and a lot of criticism on standard tests. “The Four Ball test does not replicate any real industrial or automotive contact.” “Sliding point contacts aren’t found in industry.” The Pin&Vee Block and Block on Ring sliding line contacts are slightly more relevant but the operational parameters of the test methods aren’t often found in a real application either. Few contacts in industrial use are subjected to the severe boundary lubrication regimes and intolerable wear rates, found in these standard test methods.
A modern laboratory on tribology will perform customer-specific test procedures besides the many standard methods, because they give a better evaluation of products directly related to an application. This approach is not a simple one, it leads to a number of challenges in terms of measurement and lab scale-testing. First and foremost, real applications are characterized by relatively low wear rates, compared to typical lab tests. A typical component would wear at a rate of nanometers per hour, whereas lab wear tests are often conducted under accelerated conditions, leading to micrometers per hour. But by accelerating the wear process 1000 fold, we risk changing the wear mechanism and making the lab test irrelevant ! In our test lab, we address this challenge in one of two ways. The first way is to perform long duration tests at realistic wear rate, and with a state of the art 3D-confocal microscopy it is possible to measure significant wear volumes. Long duration means weeks, maybe months which is only economically possible with multistation machines, where up to 50 parallel wear tests can be performed simultaneously. Even when the test takes 60 days, at the end we obtain 50 wear test results, allowing us to perform statistics and compare different materials and conditions in one efficient test project.
An alternative approach for lubricated tests, is to make use of Thin Layer Activation or TLA (aka. RNT: Radio Nucleide Technique). In this approach, the surface of the test pieces is implanted with some low radioactive tracer elements. Oil is recirculated passed a sensor that picks up this low radiation, so when particles wear off, they can be counted on-line. Calibrating the radiation depth allows to recalculate on-line wear losses in the range of nanometers while running the test. The effect of changes in parameters can be immediately detected, as opposed to test methods where the wear volume is measured after a test. Both methods have their advantages and disadvantages but both approaches allow to measure realistic wear rates on the lab scale in an economic and efficient manner.
Coupled with high precision friction measurements with the newest sensor technology, this infrastructure allows to answer many of today’s new challenges in tribology (reduce friction, reduce wear, longer maintenance intervals, faster development, etc).
Having said all that, do we still need the standard tests? The answer is simply ‘yes’! Even though a standard procedure does not really reproduce
that many real world conditions and even though modern products are far beyond the intention of standards that were developed in the 1940s, the great benefits of standard procedures are still valid:
50 station test machine, allowing multiple wear results in parallel.
• they provide a convenient, agreed upon and – usually – easy way to compare products globally and can thus be used for specifications and quality control
• they represent an industry consensus and agreement on how to evaluate products, to protect the end users from catastrophic problems
• they provide, when well executed with quality equipment and experienced operators, a convenient and fast screening for product development
For more information please contact Dirk Drees, CEO, FALEX Tribology on Tel: +32 (0)16 407965 or email:
ddrees@falex.eu
LINK
www.falex.eu
16
LUBE MAGAZINE NO.123 OCTOBER 2014
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