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knowing this, how can the relevant parameters be controlled to improve materials behavior? To answer the above questions involves a concerted effort in both theoretical and experimental tribology. Innovative research concepts and laboratory techniques are needed to obtain the necessary information and are being developed in this task. Wear test methodology for lubricated ceramics is being developed to provide a meaningful set of guidelines for future data collections.

In the early stages of the F&W research at ORNL, friction and wear values were determined for candidate heat-engine materials, e.g., silicon carbide, silicon nitride, partially stabilized zirconia and alumina, and the principal wear mechanisms were identified. It was also recognized that a transition from mild to severe wear can occur suddenly in ceramics and that most machine applications of ceramics will require wear of the ceramic component to remain in the mild-wear regime. Current activities are focused on determining the friction and wear characteristics, as well as the wear mechanisms of an alumina/silicon-carbide-whisker composite. This material is of interest because of significant increases in strength and fracture toughness resulting from the incorporation of whiskers. Friction and wear studies of a 20 volume percent whisker composition are in progress. Pin-on-disc testing, in which friction-and-wear coefficient values are being determined for both the composite and the whisker-free matrix material in laboratory air and in dry nitrogen, are currently underway.

To facilitate intercomparisons with data obtained for other test geometries, the test data are being derived as a function of constant apparent stress on the contact interface by periodic interruption of the test for load adjustment. Repetitive tests of the composite at room temperature in laboratory air have shown that the mild-regime wear coefficient is 1 x 1O^mrrrVN-m over an applied stress range of 30 to 75 MPa at a sliding velocity of 0.1 m/s. A slightly greater value for the mild wear coefficient is observed at a sliding velocity of 0.5 m/s, 1.7 x lO ^mmtyN-m. Also, at an apparent stress of 50 MPa and a velocity of 0.5 m/s, the transition from mild to severe wear occurs at a sliding distance of 6 to 7 kilometers, whereas at a velocity of 0.1 m/s and the same stress, the transition had not occurred after 12 kilometers of sliding. At a sliding velocity of 1.0 m/s, severe wear begins almost immediately over the entire range of stresses. As compared to the response in air, the composite sliding in a dry nitrogen atmosphere undergoes the transition from mild to severe wear in relatively short sliding distances. Detailed examination of the wear surfaces shows that the transition is initiated by the formation of microfractures. These data define the limits for unlubricated sliding of the composite for the mild-wear regime. Among the critical questions remaining are the effects of. a lubricant layer on the onset of the transition and the effects of further toughening of the composite microstructure.

Sliding-wear experiments on titanium diboride using diamond as the counterface slider have shown that an adhered debris film forms on the sliding path as the diamond tip wears. The surface of the TiB2, which is polished at the start of the experiment, remains unworn, although some cracks or displacements at grain boundaries are observed after extended sliding. The sliding friction coefficient for this couple is remarkably low (0.02 to 0.04). The composition of the adhered layer has not been fully determined, but results indicate the presence of a non-diamond carbon and an oxide of titanium.

A model for the unlubricated sliding wear of ceramics has been developed at Georgia Institute of Technology based on the observation of hot-spot formation in such interfaces. The contact zone during sliding was directly observed by using transparent sapphire discs in a pin-on-disc test. The size, number, duration and temperature of the hot spots were recorded by an infrared camera and a television image recording system. The model considers the hot spots to be the primary sites of material removal from the surface and incorporates the effects of thermal and stress gradients as well as the variation of materials properties with temperature.

The development of analytical techniques for the use of x-ray diffraction as a means of measuring subsurface strain in wear specimens was accomplished at Virginia Polytechnic Institute/State University. A principal difficulty which was overcome by this work is the treatment of diffraction data from irregular surfaces. X-ray diffraction analyses typically assume a planar surface whereas real surfaces, especially wear surfaces, are generally rough. The analysis evaluates the diffracted intensities taking the surface irregularities into account, and provides data from which accurate subsurface strains are derived. Measurement of near-surface strains on ground, fully stabilized zirconia indicated surface strains as great as 4.4% in a zone within 200A of the free surface.

Development of wear test methodologies for lubricated ceramics being conducted at NBS involves several essential features including: 1) assessment and selection of wear test apparatus and contact geometries, 2) determination of standardized sample preparation procedures, 3) performance of systematic wear tests over a wide range of operating conditions, 4) data analysis and 5) specification of the optimum wear test methodology. After careful examination of available test machines and possible contact geometries the four-ball-test apparatus was selected for standard wear tests. This selection was based on availability of apparatus, ranges of test variables such as speed, load and temperature, and ease of specimen fabrication. Furthermore, a simpler contact geometry, i.e., ball-on-three-flats, was designed to accommodate ceramic materials that are not easily fabricated into precision balls. A cleaning procedure for oxide ceramics has been developed which consists of washing the specimens with a detergent, rinsing with deionized water, and heating for twelve hours at 560'C. This procedure eliminates the effect of surface contamination on the test results. Friction and wear tests were performed on Al203, Si3N4 and SiC

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