Studies on engineering ceramics and composites in the temperature range 800-1500C aim to: (i) to obtain basic data on the ceramics to maximize utilisation; (ii) attain an understanding of the mechanisms controlling friction and wear - i.e., adhesion, CRSS, creep, hardness, fracture toughness, thermal fatigue and chemical stability - and the formation of lubricant surface film, (iii) examine lubricating behaviour of coatings, deliberately introduced constituents in the bulk, and reactions between surfaces and atmosphere to produce self-lubricating films; (iv) identify the best materials systems for the aforementioned high temperature range; (v) deduce guidelines to be used as the basis for specifying materials and lubricants for future applications.
The frictional and lubricant properties of a number of commercially available ceramics and composites were investigated in the temperature range 800 C to 1800 C. The lubricating behaviour of coatings was studied, as was the reactions between the surfaces and atmosphere to produce self-lubricating films. Variations were made to the ceramics by introduction of other constituents in bulk and these were similarly assessed.
The materials under investigation were alumina based and silicon carbide based with either particulate or whisker additions of silicon carboboride, carbon, titanium carbide, titanium nitride/carbide or titanium boride. The physical studies determined that whisker reinforcement is more effective than particulate additions, both in terms of hardness and toughness. The silicon carbide based ceramics displayed the higher hardness of the temperature range and this included the aluminium-based composite with silicon carbide whisker reinforcement, especially around 900 C. However, all the ceramics are susceptible to thermal degradation; this must be considered at high temperatures above 1300 C.
The self-sliding properties of the ceramics was not smooth. A stick slip motion was experienced, which was only overcome with the silicon carbide whisker reinforced alumina composite while cooling from higher temperatures. It was presumed that on cooling, a graphitic lubricating film was present having been formed by thermal degradation.
Incomplete work on the influence of dissimilar sliding material combinations indicated that it may be possible to improve or influence the frictional status of components by choosing surface components at the lower end of the temperature range. Operating pressures will be less than 10% of the indentation hardness of the materials. This research indicates that by choosing suitable matrices, it will be possible to have components whose performance improves wile working.
MUCH OF THE INITIAL WORK CONCERNED WITH SINGLE CRYSTALS EG SIC, AL2O3, TIC, BN, B4C AND READILY AVAILABLE WELL CHARACTERISED POLYCRYSTALLINE MATERIALS. SHORT FIBRE REINFORCED CERAMICS EG SIC WHISKERS IN ALUMINA, CARBON FIBRE REINFORCED SILICON NITRIDE AND UNIAXIAL, CERAMIC FIBRE REINFORCED METALS WILL BE PREPARED FOR INCLUSION IN THE WORK. IN ADDITION, LONG CERAMIC FIBRE REINFORCED CERAMICS, INCLUDING CARBON FIBRE REINFORCED CARBON AND SILICON CARBIDE FIBRE REINFORCED SILICON CARBIDE, WILL BE INCLUDED.
MODERN TECHNIQUES FOR SURFACE (AND NEAR SURFACE) ANALYSIS WILL BE EXPLOITED TO STUDY WORN SURFACES OF THE TEST MATERIALS.
THESE TECHNIQUES WILL BE EMPLOYED TO IDENTIFY:
- THE WEAR DEBRIS AND SURFACE FILMS FORMED DURING FRICTION EXPERIMENTS
- CHEMICAL CHANGES IN LUBRICANTS/SURFACE LAYERS
- THE DISTRIBUTION OF LUBRICANTS/SURFACE LAYERS
- IN-DEPTH CHEMICAL PROFILE MEASUREMENTS OF SURFACE LAYERS
- GRAIN BOUNDARY SEGREGANTS
Funding SchemeCSC - Cost-sharing contracts