Wear of advanced ceramics
Introduction
Advanced engineering ceramics such as Al2O3, Si3N4, SiC and ZrO2 made by modern fine manufacturing technology have higher hardness (HV>15 GPa) and acceptable toughness (KIC>4 MPa m1/2) which are useful for practical tribo-elements.
Although, the toughness of these ceramics are not yet high enough to give reliability to the machine structure in the case of tensile stress usage, it is high enough to give reliability to tribo-elements which work mainly under compressive contact stress. The excellent performance of low friction and/or low wear of ceramic tribo-elements are shown in operation at high speed, at high temperature or in corrosive and erosive media.
The successful applications of Si3N4 to ball bearings [1], [2], [3], [4] and of SiC to mechanical water seals or water lubricated sliding bearings [5], [6], [7] are good examples to show the usefulness of advanced ceramics as tribo-materials. Al2O3 and other ceramics have been used as cutting tool materials [8], [9], [10], [11], that are better than traditional high speed steel. ZrO2 has been widely used as material for guides and dies [12] in manufacturing process. All these examples show the high potential of ceramics for more application in tribo-elements in future machines and devices.
These performances result mainly from the unique tribochemical reactions and the high hardness of ceramics together with the high elastic modules.
These chemical and physical properties work effectively to reduce wear and to form smooth wear surface, which is necessary for good lubrication and low friction.
This paper describes the wear mechanisms of ceramics from the viewpoints of more applications and the reliable designs of tribo-systems.
Section snippets
Wear in unlubricated rolling contact in air
Fig. 1 [13] shows the decrease in average roughness Ra on the wear surface with the number of contact cycles observed during the repeated pure rolling contacts between Si3N4 rollers in air. This takes place when the contact pressure is small enough to avoid crack propagation in the contact region. The wear debris looks transparent in the optical microscope. They are formed tribochemically at room temperature and are detached from the contact surfaces as thin films and leave very smooth wear
Wear in unlubricated rolling–sliding contact in air
If sliding is introduced to the rolling contact with a controllable gear system, the transition from tribochemical wear to mechanical wear takes place easily. In Fig. 4 observed with Si3N4 against Si3N4 [15], tribochemical wear dominates in the region A and mechanical wear dominates in the regions of C and D. The region B shows the transition from the tribochemical to mechanical wear. The wear surface in region A is very smooth as in the case of Fig. 1; it is very rough in the regions of C and
Wear in unlubricated sliding contact in air
Fig. 6 [17] shows the observed specific wear amount ws and friction coefficient μ in sliding of Si3N4, SiC, ZrO2 and Al2O3 against themselves, where μ varies in the range of 0.1–1.0 and ws in the range of 10−9 to 10−2 mm3/(N m) depending on the frictional conditions. It is noticed that specific wear amount can change by six orders of magnitude for even same friction coefficient.
If the wear mode is distinguished by the roughness of wear surface on these data, two distinct wear modes can be
Wear map described with parameters of mechanical and thermal severities of contact for sliding in air
Wear modes of ceramics in unlubricated sliding in air are very briefly distinguished as mild or severe wear with the definition explained earlier. Either one of these two wear modes appears depending on the contact pressure and sliding velocity as shown in Fig. 7 [18] for the frictional pairs of Al2O3, ZrO2 and SiC sliding against themselves.
If severe wear is assumed to take place by the propagation of vertical surface cracks by tensile stress at crack tips in the sliding contact region, the
Wear maps for sliding in water or purified paraffin oil
The wear mode transition between mild and severe takes place in lubricated sliding with water or oil, too. Fig. 9(a)–(d) shows the shifts of transition boundaries between mild and severe wear regions observed with Al2O3, Y-TZP, Si3N4 and SiC sliding against themselves in dry air, water and purified paraffin oil [23].
It is obvious from Fig. 9 that the severe wear region shrinks and the mild wear region expands as the frictional condition changes from unlubricated to water lubricated and then to
Tribochemical wear
As Fig. 1, Fig. 2 show, Si3N4 oxidizes in pure rolling contact at room temperature in air. The oxide grows well at high contact pressure and asperity peaks are worn as a result. This is the reason why an initial rough surface becomes smoother in progressive tribochemical wear.
Fig. 10(a) shows the decrease in roughness on Si3N4 surface caused by mild mechanical wear followed by tribochemical wear. Fig. 10(b) shows the decrease in friction coefficient corresponding to the decrease in surface
Discussions
In the cases of metals, a rubbed surface easily deforms plastically and workhardens. These metallic responses decide the wear properties of metals together with their frictional properties. Ceramics do not deform plastically and do not workharden. The top surface only may plastically deform and flow when frictional heating raises the temperature at the contact region to enough to soften it.
Therefore, the contact asperity has high contact pressure at the contact region, which generates the
Conclusions
Fundamental wear properties of ceramics are observed with Al2O3, Si3N4, ZrO2 and SiC, in various contact situations, and basic wear mechanisms are confirmed as follows:
- (1)
The state of ceramic wear is briefly recognized by mild wear or severe wear from the view points of wear surface roughness and wear rate.
- (2)
In mild wear, the wear surface roughness is smaller than the grain size by about one-tenth, and the wear rate is in the range of 10−9 to 10−6 mm3/(N m).
- (3)
In severe wear, the wear surface roughness
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