Meeting report

Citation

(1998), "Meeting report", Industrial Lubrication and Tribology, Vol. 50 No. 1. https://doi.org/10.1108/ilt.1998.01850aab.018

Meeting report

Meeting report

UK forum on friction and wear

Progress of the DTI sponsored guide to wear testing, results of a survey of current wear testing methods, testing of commercial oil-free compressor piston ring materials, the wear and testing of dental materials, and details of the findings from the NEL hard metal abrasion tests were programmed for the day.

Industrial wear survey CAM 8

The first two items arose from the DTI (Department of Trade and Industry) sponsored CAM 8 project to improve the effectiveness of wear testing for the benefit of British industry. CAM stands for characterisation of advanced materials. Mail shot questionnaires to 1,800 organisations and follow-up visits paid to 22 companies by Michael Neale Associates had revealed that wear in the UK cost around 0.25 per cent of a company's turnover on average. Two thirds of wear problems were with production equipment and one third with wear of the product. Nationwide, taking this percentage of the 40 per cent of the gross national product (GNP) which involves wear, the annual cost is £650 million.

For comparison, the 1996 Lubrication (Tribology) Education and Research report (Jost report) had suggested that a figure of 1.5 per cent of GNP, or £515 million per annum, could be saved through the application of improved tribology, of which wear was just one part. The 1966 estimate included £230 million savings in reduced maintenance and replacement costs, £115 million savings from fewer breakdowns and £100 million from increased machinery life. These figures would be ten times greater at today's costs.

The main wear categories and percentages from the 1997 survey were as shown in Table III.

It was concluded that the six types of wear test needed, based on the results of the survey were: high speed jet abrasion, rubber pad abrasion, pin-on-disc, blade-on-ring, a small movement reciprocating test and a thrust ring test.

Companies which co-operated in the survey are being sent a preliminary report, A Guide to Solving Wear Problems.

Current test methods

Dr Mark Gee, NPL, reported on his survey of current test methods. From the 50 replies he received from 600 firms surveyed he has listed more than 400 standard tests. He has produced a database drawn from the literature, published standards and conference papers, including the recent international conference on wear test methods. The pin-on-disc test was by far the most used configuration reported, followed by reciprocating, scratch and block-on-ring tests, in that order. In abrasive tests Dr Gee differentiated between 3-body (where the abrasive particles can move) as in an abrasive bed and 2-body (where the particles cannot move) as in the G65 and G105 rubber wheel abrasive tests. Tests identified for development included the G65 (dry sand/rubber wheel), the scratch test, the ball cratering test, the sliding wear test and the G133 ball-on-flat reciprocating test.

Dry piston ring testing

Dr Roger Bayliss, Morgan Materials Technology, described friction and wear testing by the Graflon Division of Morganite SpecialCarbon Limited at Stourport-on-Severn. The company has over 30 years' experience of PTFE bearing materials, with the polymer both as the matrix and as a filter. Morganite had a modified Plint T77 reciprocating test rig built to permit testing of compressor piston ring materials as 18mm diameter hemispherical samples under realistic conditions: up to 200°C, 4m/s, 50mm stroke, 40Hz, 35 kPa load with controlled atmospheres having moisture contents between 1 and 1,000 parts per million by volume.

Counterface materials were En 57 rust resistant steel or grade 20 grey cast iron with a surface finish of 0.6 micrometres Ra. The materials tested in a nitrogen atmosphere at 150°C were HY22 (general purpose carbon/graphite filled PTFE grade), HY54 and the HY50 premium grade material which is used with very dry gases. Differences in the performance of the three grades agreed well with practical experience.

Repeatability in the tests was within 10 per cent for both friction and wear. HY22 was sensitive to moisture control with a large step change in wear and friction occurring at around 150ppm. Samples took 12 to 24 hours to run in. Transfer films were detected on both samples and counterfaces. The HY50 material produced a thicker, more complete transfer film than HY22. Two poster papers describing these tests and the Plint rig were presented at the recent World Tribology Congress.

Future tests will include stop/start running, other ring material grades, hydrogen and methane atmospheres, and materials with PEEK (polyetheretherketone) and polyimide matrices.

Tooth filling wear

After lunch Professor John McCabe, University of Newcastle upon Tyne, described the testing of material for repairing human teeth. Glass powder filled modified methacrylate resin is preferred to the traditional mercury amalgam because of worries about the toxicity of heavy metals and because its appearance is closer to that of tooth enamel. The volume fraction of filler to resin is important in determining the modulus and strength. Small particles with a large surface area to volume ratio reduce the amount of glass filler that can be mixed in. To get an acceptable appearance the filler has to have roughly the same refractive index as the resin. The laboratory wear test uses a steatite (aluminium silicate) abrader (in place of tooth enamel) and a 2mm stroke. Surface contact fatigue of the back teeth is simulated with a rolling 2mm diameter ruby ball. Four other wear mechanisms were present. Corrosive wear from pH values from 4 to 9, erosion, 2- and 3-body wear. Front teeth, where there is no direct tooth to tooth contact, experience 3-body wear while the tooth to tooth contact arising from the chewing action of the back teeth is combined with 3-body wear. Over zealous tooth brushing with toothpaste can cause abrasive wear. Plaque formation can lead to the presence of lactic acid, lowering the pH to 4 and corrosive attack.

Abrasive wear tests of tungsten carbide

Dr Gee showed high quality electron microscope photographs of the wear of 6 per cent cobalt bonded 2 to 3 micrometre size WC grains abraded by alumina in high stress abrasion tests. The work was part of the DTI sponsored CAM 9 ceramics and hard metals project. The WC grains were shown to break up and re-embed in the surface. A thin layer of cobalt was formed on the surface of the wear grooves. Parallel tests with silica abrasive are planned.

A scratch test has been developed in which a 2mm radius pointed indentor with a 50 N load traverses at 0.1mm/s in a single pass. The wear mechanism is considered similar to that of the B611 abrasion test above. Friction coefficients were around 0.22, decreasing with harder surfaces. Tests with sharper indentors and multiple passes are planned.

Ball cratering is a newish test in which a ball is loaded against a surface and rotated about its axis in the presence of abrasive. Wear is measured by the diameter of the wear scar produced. An explanation of why there was no trend for the wear crater size to decrease with increase in hardness is sought. It may have something to do with the abrasive size being similar to the size of the hard phase of the cemented carbide. Wear took the form of removal of the cobalt phase with little damage to the WC. Tungsten carbonitride is to be tested.

VAMASS TW1 progress

Dr Gee is the UK representative on the VAMASS technical working group on wear. After a period with little activity three projects are planned:

1. 1.

   The wear debris from human joint prostheses, a cause of failure of joints, is to be characterised.

2. 2.

   There are to be round robin ball cratering tests of titanium nitride coatings.

3. 3.

   The testing standards database is to be validated.

NPL will be involved in the last two of these. Professor John Fisher, Leeds University, is involved in the first project.

The attendance of around 30 workers in the field ensured lively questioning and there was ample opportunity for information discussion of mutual interests. There is no charge for attending these twice yearly meetings, only a request for £10 for the lunch. No papers or abstracts are distributed but presenters are asked to make copies of their overhead transparencies available to delegates. The next meeting is planned for next spring.