Environmental Effects on Friction and Wear of Ceramics

It is well known that the characteristics of friction and wear of ceramics are extremely sensitive to the environment. It is possible to divide the mechanism of the environmental effects into three major steps : The formation of the adsorbed layers, the change of the surface properties and the formation of tribochemical reaction products. Under actual sliding conditions, the influence of tribochemical reaction plays an effective role. An attempt have been made to use this effect to activate an excellent lubrication. This work deals with the relationships between the environment and the tribological characteristics of ceramics.


Introduction
Friction and wear of solids are caused by phenomena which occur on material surfaces. Thus, friction and wear characteristics depend highly on the physical and chemical properties of the surfaces. One of the most important factors used for defining the properties of surfaces is the environmental effect which results in a thin surface layer. It is well known that the mechanical strength of ceramic materials is likely to be strongly affected by the surrounding atmosphere 1 l. In particular, their tribological properties are extremely sensitive to the environment. Many factors influence the usual phenomena of friction and wear simultaneously, so it is difficult to observe the environmental factor alone. Therefore, the first important step in the study of the tribology of ceramics is to single out those factors from among many factors that contribute to the environmental effect which exert a strong influence on the friction and wear characteristics and to elucidate the mechanisms of their action. If it were possible to start from this premise and construct lubricating systems which make use of the environmental effect and use this feedback of the tribology of materials as a guide for planning the development of ceramic materials, it would definitely be possible to design new l\amiki 1-2, Tsukuba-shi, lbaragi 305, JAPAl\ TEL. 0298-58-7196 t Received July 15th 1992 58 sliding systems possessing properties required for applications under different environmental conditions.

Mechanisms of environmental effect
The elucidation of environmental effect mechanisms is much easier if the effects are classified into three major steps depending on the environmental action, as shown in Table 1. Table 1 Mechanism of environmental effect on friction and wear of ceramics Formation of adsorption layers 2 Change of mechanical properties by adsorption 2--1 Chemo-mechanical effect 2-2 Stress corrosion cracking 3 Tribochemical reaction

Formation of adsorption layers
The adhesion theory 2 l concerning the mechanism of friction between solids proposed by Bowden-Tabor is based on experimental results obtained by studying friction phenomena, mainly between metal surfaces in friction. However, this adhesion theory can also be applied to NaCl crystals and to numerous types of hard and brittle materials as was proven by King and Tabor in 1954 3 ). At present 4 l, the adhesion theory is applicable to friction between ceramic materials 5 l, excluding the controversy 6 l concerning the anisotropy of friction observed in single crystalline materials. According to the adhesion theory, the shearing stress at the junction between two solids within the region of an actual contact decreases with an increase in the adsorbed molecular layer. The formation of the adsorbed layer results in a wear reduction through the reduction of the friction coefficient.

Change of mechanical properties of surfaces with adsorption
Molecules adsorbed from the environment cause a change in the surface properties of solids. At present the following two main factors are related to this process.

Chemo-mechanical effect
The mechanical strength of a solid surface is affected by the adsorption of extraneous molecules from the surface. The changes in the strength depend highly on the type and molecular weight of the molecules adsorbed. Rehbinder et. al.7l explained that these phenomena were due to the fact that the rupture stress of the solids is related to the surface energy y by the equation 8 • 91 : ( 1 ) where IJ': Rupture stress of ideal brittle material containing micro-cracks E: Young's modulus P: Surface plastic work C: Depth of micro-crack on the surface In brittle materials such as ceramics y > > P.
Since it is believed that other variables are not affected by the environment, is lowered with a reduction in r. The relationship between the surface energy and the strength was established for quartz glass 101 as shown in Fig. 1. Numerous data proving this relationship were also reported 111 .
On the other hand, Westwood et al. performed a series of studies on the mechanical strength of crystals and glass in various liquid surroundings. They proposed the WGL (Westwood, Goldheim, Lye) hypothesis 12 • 13 1, which states that the surface potential and distribution of mobile charge carriers in the near-surface region change with chemisorption of molecules. These changes, it is suggested, affects the ease with which kinks are generated on, and move along, near-surface dislocations in crystals, and hence affect line-defect mobility. For ionic solids, changes in the electron occupancy of near-surface point defects will also influence their interactions with dislocations, and this too will affect dislocation mobility and crystal hardness. It is possible to say that we are now at a stage where the existence of chemo-mechanical effects 171 is being confirmed, but a comprehensive insight into their mechanisms has not yet been reached. However, if the chemo-mechanical effect is accepted as an environmental effect on friction and wear behavior of ceramics, then such mechanisms as the reduction of the friction coefficient by a lowering of the yield stress in the subsurface layers and the restricting effect on the rupture by wear due to the avoidance of local stress concentrations are possible.

Stress corrosion cracking
This phenomenon appears following adsorption of polar molecules to solid surfaces. How-ever, it is different from the chemo-mechanical effect 18 >. In some types of ceramics, it is known that the crack growth speed at the surface of a > -" = " " c..

Tribochemical reaction
At the sliding surface, the peculiar chemical reaction which is caused by friction is generically called a tribochemical reaction22l. When subjected to the mechanical action of friction, surfaces are activated by the exposure of fresh surfaces which are free from any adsorbed molecules or by a change in the crystalline structure (lattice defects, dislocation, amorphous state) 23 >. During friction, sites of specific reactions are initiated by high temperature, high pressure and high shear stress. The occurrence of secondary or tertiary physical and chemical phenomena, such as exoelectron (Kramer electron) radiation 24 >, triboelectrification 25 >, triboluminescence 26 > and tribo-plasma generation 27 > promote peculiar chemical reactions. Reactions between adsorbed molecules, reactions between adsorbed molecules and surface molecules, or reactions between atoms of solids through adsorbed molecules result in the formation of a layer of a third material which differs from the initial materials on the sliding surfaces. This tribochemical reaction is believed to promote lubrication trough the formation of reaction prod-ucts at the sliding surface which reduce friction and wear in ceramics. However, removal of these reaction products leads to a slight increase in wear.

Friction and wear characteristics and tribochemistry
We will now attempt to establish the relationships between the actual friction and wear characteristics taking into account tribochemistry. Friction and wear in ceramics from the view point of tribochemistry were analyzed by Fischer and Tomizawa 28 l who reported their studies on the friction characteristics of silicon nitride and silicon carbide in water. Figure 4 shows a typical example of friction and wear behavior of ceramics being lubricated with pure water 29 l. The friction coefficients of silicon nitride and silicon carbide have very small values even under low sliding velocity conditions, and are comparable with that of hydrodynamic lubrication. In addition, the specific wear rate of these materials decreases with an increase in sliding velocity. These phenomena of friction reduction in water are explained by the formation and the dissolution of silicon hydrates as a result of tribochemical reactions and by the simultaneous synergistic effect of the smoothing of the sliding surfaces. The form of wear in which the wear proceeds with a gradual dissolution due to tribochemical reactions is called tribochemical wear. This KONA No. 10 (1992) wear mechanism was established by Tomlinson 30 l 60 years ago; it was called "atomic wear" 31 l. Figure 5 shows the effect of humidity on the friction and wear properties 29 l. Under such dry conditions, especially in low humidities, the distractive wear dominates the wear properties. The friction coefficient decreases with an increase in relative humidity for all ceramics, but the wear rates shows no such definite tendency. For example, while the wear rates of alumina and silicon carbide decrease as the relative humidity increases, the highest wear rate of partially stabilized zirconia appears at 20% relative humidity. In the case of silicon nitride, the lowest wear rate is obtained at 50% relative humidity. The increase in humidity restrains the distractive wear, but it accelerates the tribochemical reaction of silicon nitride with water, resulting in the formation of reaction products, silicon hydrates.  Figure 6 shows the influence of an organic compound atmosphere on the friction and wear behavior of alumina. Both friction and wear in these organic compounds are very low as compared to those in nitrogen. In hexane and benzene, the formation of reaction products similar to friction polymers can be observed on the sliding surfaces as shown in Fig. 7. These reaction products are highly viscous greaselike substances, and have lubricating effects 32 l. Such lubricating effect of the reaction products can also be observed for alumina being lubricated with a water solution of fatty acid sodiums 33 l as shown in Fig. 8. The friction and wear of oil

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-lubricated ceramics are likely to be strongly affected by the formation of reaction products which result from tribochemical reactions that contribute to the lubrication effect of a particular introduced additive 341 • Studies on the formation of friction polymers by Hermance and Egan 351 were first published more than 30 years ago 361 , but mechanisms of their formation still remain unclear in many aspects. When the surfaces of different materials are mated, a reaction through the surrounding atmosphere between them occurs. For example, when sliding alumina and iron surfaces, the formation of a spinel-type solid solution occurs between the iron oxide and alumina. This is attributed to the process of wear37l. The characteristics of friction and wear for silicon nitride and alumina in water are shown in Fig. 9  the other hand, when the wear process proceeds with the dissolution resulting from the reaction between silicon nitride and alumina, the distractive wear of silicon nitride is restrained. In this case the friction characteristics are significantly improved. The improvement of the lubrication characteristics using the reactions between mated surfaces of different materials is one of the important methods for future tribosystems. Figure 10 shows the wear characteristics of alumina particles dispersed zirconia and partially stabilized zirconia (PSZ) in water 39 l. This composite material was developed to improve the mechanical properties of PSZ; it raised the stress level required for the martensitic trans- formation of tetragonal phase to a monoclinic phase and restrained the stress corrosion cracking. The wear rates of both PSZ (ZYM, ZYH) increase with the sliding velocity. On the other hand, the specific wear rate of the composite materials (ZYS) decreases slightly with sliding velocities of over 0. 2 m/s, and is found to be less than 1/10 that of ZYM and ZYH. A larger superiority in the wear characteristics of ZYS is obtained in organic liquids as shown in Fig. 11. Such an excellent tribological property is believed to be strongly related to the chemical activity of alumina, in addition to an improvement in the mechanical strength.

Conclusion
Efficient applications of tribochemical reactions in lubricating processes require a quantitative estimation of the reactions at specific stages and the possibility of controlling them. One of the methods to achieve this objective is to clarify the tribochemical reactions that are necessary, giving a deeper insight into the tribochemical activation and development of materials while taking their tribology into account. Therefore, fundamental approaches to the tribochemical reactions will be more and more important with the sophistication of analytical equipments.