Ball Counterface Research Articles

The role of test conditions on the microabrasive wear behaviour of soda-lime glass

In a recently developed microabrasive wear test, specimens are abraded as a fine abrasive slurry is drawn into a gap between the specimen and a rotating ball. In this work, the microabrasive wear behaviour of an ideally brittle material, soda-lime glass with 4 μm SiC particles has been examined as a function of applied load with polymeric, metallic and ceramic counterfaces. It has been shown that due to the nature of the processes occurring in the contact zone, wear does not always proceed as predicted by the Archard wear equation. Although abrasion by a particle in the contact zone becomes more severe as the load is increased, the ease of entrainment of the abrasive slurry into the contact zone is reduced. In certain circumstances, ribbons of material form within the wear scar which support the ball and reduce the wear of the material while in other cases, elastic deformation of the ball may lead to anomalously shaped wear scars. The intrinsic mechanisms of wear depend largely upon the nature of the counterface ball; stiff metallic and ceramic counterfaces resulted in chipping and fracture of the glass by three-body abrasion whereas the compliant polymeric counterface resulted in ductile cutting of the glass by a two-body mechanism. Care must be taken to ensure that when comparing test results the mechanisms of wear are known and the operation of other phenomena (such as ribbon formation) which will affect the results must be noted.

Wear behavior of Pb–Mo–S solid lubricating coatings

Amorphous Pb–Mo–S coatings 200 to 510 nm thick were deposited by dual ion-beam deposition (IBD) onto steel and Si substrates. Coating wear studies were performed using ball-on-flat reciprocating sliding with steel ball counterfaces in dry air. Tests were run between 1 and 100,000 sliding cycles, and wear depths measured by interference microscopy. Morphology and chemistry of the as-deposited coatings and worn surfaces were investigated with optical microscopy, micro-Raman spectroscopy and cross-section high resolution transmission electron microscopy (HRTEM). Pb–Mo–S coatings were found to be quite wear resistant; no more than 25% of the coating thickness was removed by 10,000 sliding cycles. Two wear mechanisms were identified. At the nanometer scale, wear proceeded in a two-part process: transformation of the coating surface to MoS 2, then layer-by-layer removal of MoS 2. At the micrometer scale, wear occurred by plowing. The long endurance of Pb–Mo–S coatings was attributed to slow wear of the coatings, with lubricant redistribution processes playing a minor role.

Effect of source gas and deposition method on friction and wear performance of diamondlike carbon films

We investigated the tribological performance of diamondlike carbon (DLC) films derived from methane, acetylene, and hydrogen + methane source gases in a magnetron sputtering system and an ion-beam-deposition system. Films have been deposited on AISI 440C bearing-steel substrates and were tested in a pin-on-disk machine under a wide range of conditions in open air and in dry nitrogen. We found that the films grown in a hydrogen + methane plasma resulted in significantly lower friction coefficients (i.e. 0.01) and ball wear rates during tests in dry nitrogen (N 2). The friction coefficients for the methane-derived films were also low (0.02 in dry N 2 but the friction coefficients of the acetylene-produced films were 2- to 10-times higher than those recorded for the methane- and hydrogen + methanegrown films, especially in dry N 2. The methane- and hydrogen + methane-grown films also resulted in very small wear losses on counterface balls, regardless of the test environments. Raman spectroscopy and electron microscopy were used to elucidate the structural chemistry of each film and correlate the findings with tribological performance.

Ion-beam deposited Cu-Mo coatings as high temperature solid lubricants

Thin coatings of Cu-Mo were deposited on alumina substrates via ion-beam deposition (IBD). Structure and composition of the coatings were examined using X-ray diffraction, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. Sliding tests of coated and uncoated substrates were performed in air under reciprocating sliding conditions against alumina ball counterfaces; mean Hertzian contact pressures were 1.0 and 1.6 GPa, and test temperatures were between 25 and 650 °C. The as-deposited coatings were metallic and amorphous, but after exposure to temperatures ±530 °C, they converted to oxides containing CuMoO 4 and MoO 3. With increasing temperature, the friction coefficients of the IBD Cu-Mo coatings decreased from ∼0.5 to ∼0.2. At high temperature, the coatings were capable of sustaining low friction sliding (μ0.2) for 3000 cycles. At high load and temperature, the ball wear coefficient against the IBD Cu-Mo-coated alumina was 20–100 times less than that of uncoated alumina. The low friction observed at high temperatures was attributed to transformation of the coatings to crystalline oxides, which are known to be lubricious at elevated temperatures.

Effects of sequential He+ and Ar+ implantation on surface properties of polymers

Three important polymers: polystyrene (PS), poly ether ether ketone (PEEK), and polyimide Kapton, were irradiated separately with 1 MeV He+, 1 MeV Ar+, and 1 MeV He+ followed by 1 MeV Ar+ sequentially, to a fluence of 3 × 1019 ions/m2 for each ion. The specimens were characterized for changes in surface hardness using a nanoindentation technique, and wear resistance using a reciprocating sliding wear apparatus with a steel ball counterface. Results indicated that while all polymers showed higher hardness values after ion irradiation, the dual irradiation resulted in the largest hardness increase, greater than for the single ion-irradiated specimens. Wear test results also indicated that the dual He+ + Ar+ irradiation resulted in the best improvement in wear resistance of the polymers. These improvements in properties are a consequence of cross-linking of the polymer material caused by the ion irradiation. Linear energy transfer considerations showed that the dual He+ + Ar+ implantation was better because it combined a deeper implant, in the form of He, along with Ar irradiation which resulted in a shallower but more highly cross-linked layer at the near surface. Thus a deeper and graded cross-linked surface region was formed. The study shows that there is greater flexibility for tailoring surface properties of polymers by using a judicious combination of ion species, ion energies, and fluences.

Improved wear properties of high energy ion-implanted polycarbonate

Polycarbonate (LexanTM) (PC) was implanted with 2 MeV B+ and O+ ions separately to fluences of 5 × 1017, 1 × 1018, and 5 × 1018 ions/m2, and characterized for changes in surface hardness and tribological properties. Results of tests showed that hardness values of all implanted specimens increased over those of the unirradiated material, and the O+ implantation was more effective in improving hardness for a given fluence than the B+ implantation. Reciprocating sliding wear tests using a nylon ball counterface yielded significant improvements for all implanted specimens except for the 5 × 1017 ions/m2 B+-implanted PC. Wear tests conducted with a 52100 steel ball yielded significant improvements for the highest fluence of 5 × 1018 ions/m2 for both ions, but not for the two lower fluences. The improvements in properties were related to Linear Energy Transfer (LET) mechanisms, where it was shown that the O+ implantation caused greater ionization, thereby greater cross-linking at the surface corresponding to much better improvements in properties. The results were also compared with a previous study on PC using 200 keV B+ ions. The present study indicates that high energy ion irradiation produces thicker, more cross-linked, harder, and more wear-resistant surfaces on polymers and thereby improves properties to a greater extent and more efficiently than lower energy ion implantation.

Wear properties of argon implanted poly(ether ether ketone)

The sliding wear properties of Ar +-implanted poly(ether ether ketone) (PEEK) using nylon 66 and SAE 52100 high carbon chrome steel ball counterfaces have been investigated in this study. PEEK was implanted with 1 MeV Ar + to fluences of 5 × 10 18, 1 × 10 19 and 5 × 10 19 ions m −2. Reciprocating sliding wear tests were conducted using a 1 N normal load, 3 mm sliding distance and 100 cpm frequency for 10 000 sliding cycles. The implantation significantly improved the wear properties of PEEK for the nylon ball tests to the extent that no wear tracks were evident on the PEEK surface. This was attributed to significant strengthening of the PEEK surface by ion beam induced cross-linking which, in turn, caused corresponding wear of the softer nylon counterface. For the steel counterface tests, the intermediate-fluence implanted specimen showed no wear damage on the surface. There was wear damage for the unimplanted, lower and higher fluence implanted specimens. The results indicate that the best wear properties appeared at some optimum level of ion-beam induced cross-linking of PEEK for the steel counterface. Friction coefficient data were correlated with the observed wear behavior. Mechanisms of energy transfer from the energetic ions to the polymer molecules were analyzed to explain the wear improvements. This study shows that ion implantation is a promising technique for significant improvements in the tribological properties of PEEK.

Chemomechanical effects in ion-implanted MgO

Experiments have been undertaken to explore the effects of ion implantation on the low-load hardness (Knoop), friction (with a steel ball counterface) and absorbed water content (as measured by infrared spectroscopy) of single-crystal MgO surfaces. While the hardness behaviour shows the usual hardening and softening associated with implantation-induced structural changes leading to radiation hardening and eventual surface amorphisation, a large increase in friction at low implantation doses of Ti+ has been found to be associated with the removal of both chemisorbed and physisorbed water. Further, water readsorption has been found not to recur even several years after implantation leading to the postulation that implantation-induced neutral charge defect clusters are responsible for the change. This effect seems most marked for Ti+ ions which are known to both substitute for magnesium ions in MgO and to segregate to the surface of MgO samples. The experimental observations are reported and the implications for both the tribology of ion-implanted ceramics and the authors' understanding of water absorption on MgO are discussed.