Lessons learned from the test-to-test variability of different types of wear data.

Contrary to the established principles of the scientific method, a surprising number of experimentally-based papers submitted to tribology journals and conferences report only one test result for each material pair or set of applied conditions. However, like hardness, yield strength, fatigue life, and other material properties, wear data exhibit varying degrees of repeatability and reproducibility (R/R). Repeatability concerns the replication of experiments within the same laboratory using the same equipment and materials. Reproducibility concerns testing on different equipment, usually at a different location, but using the same lot of specimens and procedures. An important question is: How many replicate measurements are needed to validate trends in wear behavior or to relatively rank materials, surface treatments, or lubricants? Without repeatability information, it is virtually impossible to establish whether reported material rankings or the effects of variables are real or fall within normal data scatter. The purpose of this paper is to characterize and analyze the R/R of wear data that result from a variety of sources, including material homogeneity, choice of units of measure, and choice of experimental variables. Case studies compare R/R for different forms of wear and their test methods, including ASTM standards. Lessons learned are presented for five forms of wear: (1) cavitation erosion, (2) three-body abrasion, (3) solid particle erosion, (4) dry sliding wear, and (5) fuel lubricity using the ball-on-cylinder (BOCLE) test. Wear transitions can also affect R/R. These examples provide insights for validating wear models, deciding how many repeated tests to make, and when ranking wear-resistance.

Tribo-mechanical properties of thin boron coatings deposited on polished cobalt alloy surfaces for orthopedic applications.

This paper presents experimental evidence that thin (< approximately 200 nm) boron coatings, deposited with a (vacuum) cathodic arc technique on pre-polished Co-Cr-Mo surfaces, could potentially extend the life of metal-on-polymer orthopedic devices using cast Co-Cr-Mo alloy for the metal component. The primary tribological test used a linear, reciprocating pin-on-disc arrangement, with pins made of ultra-high molecular weight polyethylene. The disks were cast Co-Cr-Mo samples that were metallographically polished and then coated with boron at a substrate bias of 500 V and at about 100 degrees C. The wear tests were carried out in a saline solution to simulate the biological environment. The improvements were manifested by the absence of a detectable wear track scar on the coated metal component, while significant polymer transfer film was detected on the uncoated (control) samples tested under the same conditions. The polymer transfer track was characterized with both profilometry and Rutherford Backscattering Spectroscopy. Mechanical characterization of the thin films included nano-indentation, as well as additional pin-on-disk tests with a steel ball to demonstrate adhesion, using ultra-high frequency acoustic microscopy to probe for any void occurrence at the coating-substrate interface.