- What is the coefficient of friction of a material A ?
Friction and wear are system properties, which means that they may depend on the whole system of materials and mechanics that are in contact with each other, and also on the working conditions and the environment. So unfortunately, there isn’t a single value of coëfficiënt of friction for a material. This does not mean, that a coefficient of friction cannot be measured, in any system where the normal load and the resulting friction force are known, a coefficient between both can of course be be calculated. But it will only be a meaningful number when some conditions are met : did we measure the friction force under realistic conditions (i.e. correlating with your system or application ?), did we measure the friction force over a wide enough range to establish if it's constant or not, did we measure the friction force for a long enough duration to establish whether it's stable or not...
Besides, material properties vary for real engineering materials, so it is likely that there is variation in the coëfficiënt of friction even when comparing nominally identical material coupons.
- Isn’t the coëfficiënt of friction a fixed number ?
This is a widespread misconception, based on initial experimental work of Leonardo (Da Vinci, not Di Caprio) who found that the friction force of a big block of material was proportional to its weight (normal force) and that the coefficient didn't change when changing the size of his blocks. These observations were later confirmed and turned in to 'laws' by Coulomb.
But these laws are only valid within a limited range (sizes of the materials, forces, speeds, etc) and only for dry friction. Most industrial systems today do not operate under the same conditions as Leonardo's simple experiments, so coefficient of friction may vary widely for the same materials. Just rub your hands when they're dry (warming up) or wet (sticking) or when lubricated with soap : very different friction, isn't it ?
So it only makes sense to talk about 'a' coefficient of friction if it has been experimentally measured in a meaningful way (correlating to your application), repeatable and consistent.
From the above answers, it may be clear that it is not straightforward and obvious to extract a value for a coefficient of friction or to use tabulated values. If you are designing a part, and need to use a friction value, you may be interested to know that the coefficient of friction of steel-on-steel , even unlubricated, may vary from 0.1 to more than 1. So which one to choose ?
- What is the wear of material A vs material B ?
The 'wear' of a material has to be measured also in correlation with the intended application. For the same reason why there isn't a single coefficient of friction for a material, there isn't a single wear number of wear rate. In fact, the few standardized test methods that describe how to measure wear resistance of a material, always use a fixed procedure and even fixed test equipment, to be able to publish a single 'wear number'.
- Can we extrapolate a test with the reality ?
Yes and No. The No portion is the disappointing - or challenging- part of tribology testing : any lab test will be some simplification of the reality and as such does not replace reality. There may always be factors in real life, that can not be simulated in the lab, or that are maybe even unknown at the time of testing.
But there is also a YES portion : with the proper know-how it is possible to design meaningful lab tests that will approach reality. And although they will not be a 100% safe extrapolation, they do deliver a lot of data that are often difficult to obtain with a real life test. In lab tests, more data can be monitored and stored than in many field tests, so more knowledge about the materials and systems can be collected. And this knowledge, coupled with experience or modelling, can lead to fairly confident extrapolation to real life conditions. The message is : the lab test must be the right one !
- Can you calculate the result of test method A from a result of test method B ?
Usually not. Because friction and wear are system properties, changing the test method means changing the system and thus the values can and probably will change. If there are correlations between test methods, they are usually empirically established, or based on the fact that very similar wear mechanisms are being testing in both methods. But it is not the rule. It is even dangerous to use an inappropriate method (e.g. method A) to predict the behaviour of materials that should be tested by another more appropriate method (e.g. method B), because you may draw contradictory conclusions with desastrous consequences for your application.
- Aren’t there any material friction and wear data bases ?
Very few and not easily accessible, and besides very incomplete. The reason for this is simple : because there are too many applications, each with their own set of conditions and because there are too many materials, it is economically impossible to test them all and store the information in a database.
It is even an economic challenge to do enough repeats in wear tests (see FAQ about statistics !). Because material properties may vary, and because friction and wear mechanisms are highly dynamic, a minimum of repeats has to be used to construct databases.
- Can we perform a single test ?
You may want to just explore some different materials, and a single test result will do ? Yes, of course you can, if you are doing a standardized test that has already been used many times with your class of materials, and the typical variation in test results is known. Assume you do an extreme pressure test by the ASTM D3233B norm. Then you know from the standard, that a typical result variation is 250 lb. If you find that lubricants differ more than this 250 lb, you may conclude that they're significantly different. But keep in mind history and statistics ! The precision of this standard has been established many years ago, with the products of that era. Are they still relevant ? And the 'variation' in the standard refers to a 95% confidence interval. One result in 20 may still fall outside this bracket. Outliers are very common in friction and wear testing, due to the complexity of the testing, the mechanisms and the variation in materials properties. It is safer to plan for at least duplicate testing in any case, and certainly when exploring a new method and new materials. Because duplicate testing doubles the test time, the test price rises of course, which is often considered a loss/cost. But just as companies insure themselves against many external and internal risks and damages, duplicate or triplicate testing should be considered as an insurance against wrong decisions.
How expensive will it be ?
This is a very common and natural question of course. Whether it makes economic sense, to solve a problem by an expensive development project, is a question only you can answer. It also depends a lot on the amount of work that needs to be done and the complexity of testing, whether we’re talking about a 1000, 10.000 or multi 10’s of thousands. In the end, we have to plan some physical test work, and every test costs money : operator time, equipment cost, analysis and reporting.
Having said that, one of our branding themes is ‘Partner Solutions’ which means that we will adapt and find ways to delivery quality information that fits in your budget. A single QC test could be as low as 150 EURO/USD and may be sufficient for your question.
It is only possible to estimate a project cost after all technical expectations have been discussed and settled.
The best approach to keep budgets under control, is a step-by-step approach to a problem. We routinely offer a fixed price parameter study as an introduction to an unknown problem : we look at the application, suggest a limited set of test conditions and try them out in practice. We evaluate if the method makes sense, if it is repeatable and what can be learned from that. This fixed price study – at 1800 USD/EURO per day – is a good way to start a project that still has a lot of unknowns. ‘To measure is to know’. So take a chance and invest a little !
Why do I need to do a duplicate test ? That’s only doubling costs.
This is a matter of statistics. Whenever non-standard test methods are needed (i.e. most of the time), there aren’t any published repeatability data. We have to assume that repeatability of a test is a-priori unknown. So in many projects where a test procedure is adapted or developed, one of the first stages of the project is to evaluate the repeatability and the sensitivity of the test method (parameter study : see also 1.)
Aren’t there any more (cost) efficient ways to gain confidence ?
Yes, there are : the way is ‘multistation testing’ ….this is particularly useful for wear resistance testing. Wear data are by nature more variable than friction values, and harder to measure (after a test, weight losses, profilometry) so it usually takes longer. The only efficient way to do multple tests (for statistics) that last long (for measurement purposes) are parallel multistation tests. So we set these up whenever reliable wear rate data are needed.
A hardness test only costs 30 USD, why does a standard pin on disk test cost already more than 200 USD?
There are major differences between volume testing of material physical properties (hardness, elasticity, strength), which all take little time and are completely standardized in all ways, and a friction or wear test which is a system property. A friction test takes at least an hour of machine time, not including setup costs, specimen preparation time and reporting. A wear test (one that makes sense) may last for hours or days. Tribological testing also requires a good understanding of the application, and a lot of preliminary discussion to align the test method as much as possible with the real conditions. All this takes time as well. As a result, the cost of tribology testing is not limited to the physical execution of a test and should be viewed as a consulting cost. In that sense, a 200 USD cost to evaluate friction or wear resistance of a material in optimised conditions, is not a high price to pay for knowledge.
Without the knowledge of an underlying distribution, a measurement point or data point is meaningless. The term 'confidence' relates directly to statistical calculations, in fact everything in life is governed by statistics and the way we make decisions is based on chances, probabilities, averages, risk factors, and so on.
Although statistics mean duplicate or more repeats (and thus more costs), they have to be considered as part of the project and budgetted for.
Some economically interesting approaches to the dilemma between reliability and cost, will be shown in our application examples and will appear later in the download area.
1. What is the Falex Application Center for Tribology ?
Falex has been building and standardizing tribo test equipment for 90years now, since the 1990's there is also a test lab active in the USA. In 1999, Falex and the University of Leuven established the European Technical and Support Center, Falex Tribology, where we have been serving the European industry and research consortia with applied tribology testing and support on equipment.
The accumulated know-how and experience in testing solutions, is now being collected in the Falex Application Center for Tribology, an initiative within the group that focusses and emphasizes on special test protocols for special industrial needs. Although both labs are also providing standardized testing (see 'our tools-standardized tests'), our added value is the modification of test protocols and even test instruments to focus on practical industrial tribology questions. With our accumulated and accumulating experience in this field, we feel we are the best in our class.
Maybe call us 'tribo experts' ;)
2. How do you decide on a test proposal for my application ?
This is a mix of know-how and experience in tribology testing (of which we have more than 60 years accumulated), insights in materials (strong background of material engineering, coupled with lubricant and chemistry), and inspiration gained from a close discussion with you, the customer. Deciding on a test plan is a bit like deciding on a treatment for an illness, your doctor would also interview you first and then, based on this interview and his own knowledge, propose a treatment to be followed up.
You might call us 'tribo doctors' ;)
3. What do I get for this 'parameter study' ?
More than you think ! We estimate a fixed daily cost, including all overheads, this means we spend the estimated fulls day (and usually some more), on performing trials with your products. But these trials are based on our know-how and experience, on preliminary discussions and meetings, and a lot of thought, before we do the first lab test. In the end, the parameter study is intended to confirm or adjust our planning, and also give confidence to you, the data user, that what we measure is valuable and meaningful. Depending on the tasks necessary to reach this goal, we can do a study of one or several days. Repeatability of the method is one of the critical results we develop in this parameter study. If we plan a method for your application, we still need to know how precise, repeatable and sensitive the method is, in order to compare different materials or lubricants.
In fact, even a one day parameter study will cost us a few days, it's a kind of mini-project intended to clear the path towards an optimised and standardized routine test method taylored to your needs.
You might call us 'tribo taylors' ;)