WEAR NEWSVolume 9, Number 4 Fall 2011

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A New Fretting Test Method

Interlaboratory Tests

“ASTM Standard Test Method for Damage to Contacting Surfaces Under Fretting Conditions” was conditionally approved in 2010 pending addition of a statement on reproducibility obtained in interlaboratory tests. It is the purpose of this document to report on the status of an interlaboratory study (ILS) conducted on one test couple (52100 steel at 60 HRC vs. type A2 tool steel at 60 HRC) by four participating laboratories. Test specimens were made from the same lot of material and finished by the same laboratory. The test method covers the testing parameters but allows the use of different test rigs. All four labs that supplied data for this report had rigs of different design. We did not request design details on the test rigs used.

Procedure:

The following test conditions were communicated to the five laboratories that agreed to participate as of December 30, 2011. Four laboratories have submitted test data as of June 16, 2011.

Test configuration – 6.3 mm-diameter ball on a 3 x 8 x 30 mm flat

Test surface finish – less than 0.1 µm RaForce on rider – 10 NTest amplitude – 50 µm (total indicator movement)Test duration – one million cycles (rotations)Test frequency – 13 HzTest environment – room air, 50% RH, 20 C

The labs were asked to perform at least three replicate tests and to report ball scar diameter, ball wear volume, and counterface wear volume. We did not specify the procedure for measuring ball or counterface wear scars or wear volumes.

Results:

Ball scar diameters are compared in Figure 1. Two labs had ball scare diameters of between 740 and 812 µm and two labs had ball scar diameters in the range of 172 to 405µm. The within laboratory COV’s are listed below.

COV Average ballScar diameter

Lab A 0.04 780 µmLab B 0.008 745 µmLab C 0.03 177 µmLab D 0.09 367 µm

The average between-lab scar diameter was 517 µm. The between-lab COV was 0.56 (s=293). This suggests that the various test machines used in this study are not producing identical sliding and contact conditions. Some probable causes for these are:

1. There may be ball-on-flat compliance differences

2. The required normal force of 10N may not have been achieved

3. The relative slip amplitude may be different than the G 204 specifications

4. There may be test frequency differences between the labs

Of course, there could be scar measurement errors or biases, but the above are more likely. Compliance is almost certainly different since each machine has a different design for holding and moving the rider. Normal force difference could exist, but this is one of the easiest sources of specification departure to test for and correct. Relative slip difference between labs are quite possible unless each machine has a dynamic sensor to continuously record the displacement of counterface and rider throughout a test. For example, as wear progresses, the friction force can change and the forces on the rider are different

than they were at setup and rider deflection can be different than at setup

It is believed that frequency differences are not likely to be the source of the observed reproducibility differences because the first tests with this test method were conducted at 10.7 Hz and the wear volumes were not significantly different than those obtained at the current test frequency of 13 Hz. In addition, with machines using a motor for the source of oscillation, the frequency is simply measured by motor rpm and this is not a measurement usually subject to error.

Slip amplitude can be accurately measured by measuring the ellipse diameter of wear scars early in testing. This study suggested that measuring the difference between major and minor ellipse diameters after 100 to 300 cycles is the most accurate measurement of real slip amplitude. Later in testing, wear debris tends to obscure the differences between major and minor diameters. At one million cycles, the wear scars are almost circular under optical examination.

The interlaboratory tests did not include other test couples, but limited testing suggests that the ASTM G 204 test on the 52100/A2 tool steel couple used in these interlaboratory studies (ILS) will produce a significantly lower wear volume when tested in light-weight mineral oil (pharmaceutical grade). A lubricated 52100/A2 test couple could be a candidate for a second ILS. Under lubricated conditions, Laboratory A measured an average ball scar diameter of 215 µm compared with an average of 780 µm in the unlubricated tests. Each lab could retest the couple used in this study lubricated and determine if each lab obtained a similar decrease in wear volume (ball scar diameter). This would verify that ASTM G 204 can accurately rank materials even with a high interlaboratory COV.

In summary, this study determined that the ASTM G 204 test method is very repeatable with-lab, but the between-lab reproducibility is less than desired.

Compliance and slip differences are thought to be the sources for the observed high COV.

Discussion:

The test metric specified in ASTM G 204 is wear volume of both members of the fretting couple. However, the wear measurement that did not involve a measuring technique unique to each lab was ball scar diameter. This can be measured optically or with profilometry and wear volume can be calculated using the procedures in the ASTM G 99 pin-on-disk test. For this reason, we used ball scar diameter as the metric to calculate interlaboratory reproducibility.

ASTM G 204 is a reciprocating test, therefore, the ball and counterface scars are not really circular; they are not really diameters, but rather they are elliptical. When the labs reported major and minor ellipse diameters, they averaged to arrive at average ball scar “diameter”.

The test results show that the test couple and the ASTM G 204 procedure produce very repeatable results within a lab. The within-lab COV’s are all less than 10 percent which is a rule of thumb limit for reproducibility for abrasion tests. The between-lab COV were much higher than desired

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and it would it would be a monumental effort to compare machine designs and mechanics.

Conclusions:

1. ASTM G 204 produces very repeatable within-lab test results.

2. ASTM G 204 can have a high between-lab COV when different test rigs are used.

What’s Next

Bud Labs has conducted many additional tests to identify another material couple to test that would produce significantly different results than the 52100 steel/A2 tool steel couple tested in the “round one” tests. The concept of testing a significantly different couple is that if each lab found it, for example, to be one half of the wear volume of the 52100/A2 couple, it would suggest that labs can rank couples the same even though their test wear volume different by as much as 50%.

In an attempt to find a significantly different couple for a second round of interlaboratory tests, we tested a hard rider on a soft counterface, two hard/hard steel couples, a ceramic/hard steel couple, a stainless couple, and a hard/hard couple lubricated with mineral oil.

The steel couples produced similar rider wear results regardless of alloy type and hardness differences. The 316 stainless steel/soft-steel (1020) couples and the aluminum alloy/A2 steel couple produces counterface divots with negligible ball wear (only adhesion). The 52100/A2 couple in air produced ball scars that were only 25% of those produced in the same couple in air.

Thus, a second round of tests could be attempted with the same material couples as the first test, but fretting under a drop of mineral oil. This is what is recommended for a second ILS.

ASTM G2 Wear and Erosion Activities

Abrasion Subcommittee – Brian Merkle (Nanosteel) was appointed subcommittee chair. He chaired the Fall 2011 meeting in New Orleans. The standards needing review for reapproval are:

1. G 81 – Jaw crusher gouging abrasion test2. G105 – Wet sand rubber wheel abrasion

test3. G132 – Pin-on-sandpaper drum abrasive

test

The G81 Standard will be reballoted without change. Troy LeValley (Falex) will review the G105 test before reballoting as will the G132.

There was a paper presented in New Orleans on work conducted to determine if neoprene rubber could be used to replace the chlorobutyl rubber currently specified in the G65 dry-sand rubber wheel test. After many tests and machine checks, it was concluded that it can be a suitable substitute to solve the rubber problems that have plagued users for years. It is difficult to make, difficult to dress, has extremely limited availability and it costs more than $1,000 for wheel that could be used in one test. An interlaboratory study will be conducted to determine the suitability of the neoprene.

Ken Budinski reported that the ASTM B611 standard will be reballoted for the third time after resolution of concerns expressed in prior ballots.

Friction Subcommittee – Scott Hummel (Lafayette College) chaired the meeting. Scott discussed the laws of friction appendix proposed for addition to the G115 friction measurement guide. It was recommended that Ken Budinski (Bud Labs) ballot G115 with the proposed appendix to determine its usefulness.

Data Acquisition in Tribosystems – Chair, Greg Dalton (Tribsys) reported that he will review ASTM G190 for reapproval. This guide is used to

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select an appropriate wear test for an application. Scott Hummel recommended balloting G117 on calculating measures procession tribotest for withdrawal because it used outdated computer methods. Greg and Bill Ruff (NIST – Retired) will discuss this and G118 for consideration for reapproval or withdrawal.

Non-abrasive Wear Subcommittee – Chair, Nick Randall (CSM), requested Ken Budinski to issue a report on the status of the G204 fretting test round on tests that were conducted over the past two years.

Terminology Subcommittee – Chair Peter Blau (ORNL), led discussions of some recently balloted terms including:

Incubation periodWear resistanceAbrasion resistanceAbrasion

Negatives were received on a previous ballot. After word-smithing by attendees, the terms were to be reballoted.

Erosion Activities – Swami Swaminathan presented an update on their high temperature erosion test. They are using a solid particle erosion rig with a larger nozzle (1.5 mm dia vs. 1.0 mm) and longer standoff distance (14 vs 10 mm) than G76 to reduce the severity of the test.

Miscellany

Future Symposium – The next G2 meeting will precede the 3rd International Symposium on Tribocorrosion to be held April 19-20 at Georgia Tech University in Atlanta, GA, USA. The symposium is jointly sponsored by ASTM and other organizations.

Future Tribology Events

The appended tabulation of upcoming tribology events was prepared by Dr. Peter Blau of Oak Ridge National Labs. Thank you, Peter.

Future ASTM G2 Meetings

Spring 2012April 18 – Georgia Tech, Atlanta, GA

Fall 2012September 20-21, CSM, Boston, MA

Spring 2013May, ASTM Headquarters, PA

Editor’s Note

Normally, we report on papers presented at major conferences in the fall. This year, I could not attend any because of my wife’s health problems. We reported in our internal research efforts in fretting as a substitute.

NOTE: Wear News is the informal account of selected tribology events and the activities of the ASTM G2 Committee on Wear and Erosion.

Contributed tribology articles are welcome. Send them and other inquiries to:

Ken BudinskiBud Labs3145 Dewey AvenueRochester, NY 14616 (USA)

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