A Study On Tribological Properties Of Ceramic Class Composite Materials In Water

The technology of water lubrication has been widely studied and applied in the area of mechanical transmission since 70's, because water lubricated friction pair has many advantages over the conventional oil lubricated one. The key issue of water lubrication technology is the correct choice of friction materials under water lubrication. Ceramic friction pair is a focus of the study in water lubrication technology, because ceramic materals have excellent tribological performance in water. However, the brittleness of ceramic materials are difficult to overcome, so their application has been limited in many ways. Ceramics class composites are made of ceramic and metal composite, contain the some of the features both of ceramic and metal materials, has a high potential in water lubricaton. In this thesis, the tribological properties of two ceramics classes composites (the WC-Co cemented carbide and the Ti(C,N) Cermets) in water-lubrication were researched and discussed. The experiments were carried out by a pin-on-disk tribo-meter, Si₃N₄ ceramic balls as upper specimen, the lower specimens were disks made of Ti(C,N), WC, Si₃N₄ and 316 stainless steel materials respectively.Deionized water lubrication was mian situation in this research, and both the 3% NaCl solution and anhydrous ethanol liquid were also selected as lubricant, to make a comparative analysis with deionized water. 3% NaCl solution used to simulate the lubrication of sea water. The purpose of ethanol use was to isolate water and friction pairs. Analysis the role of water through the performance of the friction pairs in the process of friction experiments in ethanol liquid without the hydration reaction.The the friction and wear conditions of friction pairs in de-ionized water, 3% NaCl solution, ethanol liquid were observed. The results of different loads and defferent sliding speeds were compared with. The changes in the different stages of worn surface were analyzed. Observed the three-dimensional micro-morphology of wear scars by optical microscope and non-contact profilers. The results show: the friction coefficient of Si₃N₄ ball/ Ti(C,N) disk and Si₃N₄ ball/ WC disk friction pairs can be reduced to less than 0.01 in de-ionized water. That means they can realize hydrodynamic lubrication. The Na⁺ ions in 3% NaCl solution may have effects on the friction process, which make the reduction in the friction coefficient smaller, and unable to realize hydrodynamic lubrication in some experimental conditions. The friction coefficient curve of all four kind of firicton pairs maintain as a horizontal line basically in the ethanol liquid, which demonstration that water lubrication is the main factor of friction reduction. In all conditions, the wear loss of both Ti (C, N) disk and WC disk is smaller than that of Si₃N₄ disk. It can make a conclusion that these two materials have excellent tribological performance in water.Under water lubrication the friction pair of Si₃N₄ ball and Ti(C,N) disk can realize hydrodynamic lubrication in both high speed low load and high speed high load conditions, while its friction coefficient only has a little decrease in low speed condition. This indicates that as the sliding speed increases the effects of water lubrication and load carrying capacity of the friction pair are improved. The high hardness and good corrosion resistance of Ti(C,N) material result in the least wear loss of the the friction pair in most conditions. And disk's wear loss at high load is less than that at low load. It is deduced that the running-in process is as follows: metal and its oxide layers of disk were removed gradually at first, then the inner ceramic layer was polished, finally the lubrication film produced by tribo-chemical reaction was accumulated on the surface and hence reduced the friction coefficient. The Ti(C,N) cermet material contains seven kinds of chemical compositions, which may have negative effects on the formation of lubrication film, so its amount of lubrication film is less than that of Si₃N₄/WC pair.Under water lubrication the friction pair of Si₃N₄ ball and WC disk can realize hydrodynamic lubrication in all speed-load conditions. The wear can be mainly attributed to tribo-chemical wear. The friction products adhere to wear scar of the disk, which results in less wear loss of the disk. And the disk's wear loss decreases as the load increases, showing good wear resistance. It can be deduced that the running-in process of Si₃N₄-WC pair is similar to that of Si₃N₄-Si₃N₄ pair: the polishing effects make surface more smooth; the lubrication film produced by tribo-chemical reaction between Si₃N₄ and water reduces the friction; and more lubrication film produced in high load condition result in better antifriction effect。The fact that friction pair of Si₃N₄ ball and WC disk can realize hydrodynamic lubrication at low speed shows they possess good adapatability.