Size Effects and Mechanisms in the Friction and Wear Processes of Ceramics Based on Zirconium Dioxide

This paper presents a study of the tribological properties of ceramics based on ZrO2 stabilized with CaO in a ceramic-ceramic friction pair. The influence of the scale factor on the coefficient of friction in the micro- and nanoscale is established. A qualitative correlation of the theoretical and experimental curves is shown for friction coefficient dependence on the body-counterbody contact force. Friction and wear micromechanisms at a micro- and nanoscale are proposed for the investigated TZP ceramics.


Introduction
Today rather strict requirements are imposed on modern engineering materials, especially on the ones used in high-load applications. Advanced nanostructured ceramics is one of the novel materials that satisfy these requirements. One of them is the engineering ceramic based on stabilized zirconium dioxide [1] that form an important class of materials with a unique set of properties, where strength and tribological characteristics come to the fore. Although the result of friction and wear is usually seen at the macrolevel, initially the interaction of interacting surfaces takes place at submicroscale and nanoscale through multiple local short-term contacts. As a rule, these contacts arise between the promontories of the surface due to the roughness; typical size of the contact regions ranges from a few nm to a few um [2][3][4][5][6][7][8][9]. It is a promising approach to simulate physically the processes occurring during dry friction at the level of single micro-/ nanocontacts. This approach may allow us to get more detailed understanding and description of this process. It lets us to characterize the deformation processes occurring in locally loaded submicrovolumes, under fully controlled conditions.

Methods
The technique, used in this study, makes it possible to study the behavior of the material at local lateral interaction during sliding into the technique of nanoindentation [10][11][12][13][14], consists in measuring of local deformation with nanometer resolution. The local deformation occurs under the action of normal and lateral forces applied to the test material surface according to the test protocol [8][9][10][11][12][13][14][15]. The technique allows us to control of all essential parameters during the experiment (normal and lateral components of the forces, contact area and geometry of interacting areas of interacting bodies, local deformation and the lost material volume, and fracture toughness). It leads to creating completely defined conditions in dynamic micro-/ nanocontact, vary them as desired, and investigate the mechanical behavior of materials on a nanoscale. The tribomechanical studies were carried out on a TI 950 Hysitron Triboindenter (USA) and DNT Nanoindentometers developed in the Research Institute  The study shows tribological behavior of ZrO2 stabilized with calcium oxide and ceramics with Nb2O5 additives were also used. A spherical tip made of ceramics based on ZrO2 with a radius of R = 250 μm was used as a counterbody and the Berkovich indenter was used for small parts of tests.

Processes at micro-and nanoscale
Conducted research at the micro-and nanoscale has shown that the friction coefficient depends on the value of the normal force FN under the tip (Figure 2). At the first stage, the friction coefficient was instantiated by quite a sharp decrease, and then a slight increase, and had a minimum at achieving a certain critical value of the normal force FNcr. Such behavior was represented typically for each set of tests and interacting bodies. However the FN value varied and depended on the materials of body and counterbody. The plot until critical load FNcr corresponds to elastic interaction, which was also confirmed by the dependences of the load on the counterbody penetration depth P = f(h) by reorganizing the obtained data during friction curves into P-h curves (Figure 3) where the load branch has the same slope like the unload curve. It is also confirmed by the absence of the tracks on the surface verified by atomic force microscopy.   [15,16], and the coefficient of friction is determined by the next equation f = fmol + fdef [15,17,18].
According to the equation and the classical approach to tribology, the friction coefficient contains both the molecular and deformation components. The ratio of the contribution to the friction coefficient depends on many factors, such as surface topology, the nature of the contacting bodies, etc. [22][23][24].
The adhesive part of the friction coefficient fmol at nanoscale (Figure 4) is dominant and contribute the main part into the obtained friction coefficient that was true for all friction pairs that were studied. The deformation part of friction coefficient fdef plays an unsignificant role when the normal loads are not high, and opposite, fdef gives the main contribute at relatively high loads FN> FNcr.
The contribution of the deformation part is arising from 0 to 80% at FN = 1000 mN, but at the same time, the contribution of the adhesive component is falling from ≈100% to 20% (at FN = 0.2 mN and FN = 1000 mN respectively) for the ZrO2 ceramic with 6.5 mol%. CaO. for the ZrO2ceramics with 6.5% CaO additives 1 -friction coefficient obtained in the tests; 2 -fmol adhesive part of the friction coefficient; 3 -fdef deformation part of the friction coefficient.

Processes at macroscale
The carrying out of friction and wear at a macroscale were simulated by lateral sliding of a sharp tip (Berkovich indenter) under the impact of relatively high normal loads FN (up to 1 N). This approach made it possible to form a sufficiently large deformation zone, where the processes of crack formation and destruction begin to be observed in addition to the typically observed track because of the counterbody sliding.
The results of experiments showed that separated cracks are detected under loads up to 0.2 N, further increasing of the load (FN ≥ 0.5 N) leads to the appearance of multiple cracking and spalling of ceramics separate grains. Figure 5. shows a typical example of the onset of cracking at dry friction of zirconium ceramics with 6.5 mol%. CaO.
A more detailed study of the identified cracks shows that they have a noticeable tendency towards a predominant direction of propagation, and it is to be about 60 degrees with the direction of the Berkovich's triangular indenter motion and corresponding to the highest tangential mechanical stresses existing along the ribs of the sliding tip.
The load increasing up to FN = 500 mN leads to the fact that except the typical cracks, areas of greater destruction of the material were revealed inside the track left by the counterbody, in which spalling of individual grains was observed. Such spalling is characterized by a change in spatial orientation and practical exfoliation of individual grains. The further increasing of the load When FN ≥ 1 N leads to the increasing in crack formation, and microerrosion of the surface is observed with chipping of fine particles like individual grains or their intergrowths and the formation of the debris. The formation of cracks and further spalling of the material can be caused by the generation, displacement and further coalescence of point defects (for example, vacancies), destruction of grain boundaries, as well as phase transitions in the contact zone of the counterbody, which were revealed by micro-Raman spectroscopy.

Conclusions
It can be concluded that, purely elastic deformation mechanisms are initially implemented under the experiment conditions of dry friction, which are replaced by elastoplastic mechanisms with an increase of the normal load. When the load FN keeps growing, crack formation and spalling of individual grains are added to elastic and elastoplastic mechanisms of deformation.
Based on this, we can say that the wear of the studied ceramics is based by a decrease in roughness at low loads (by cutting off singular protrusions of nano size), then mechanisms of plastic deformation begin to turn on with increasing in the load, and at the end, cracking and micro-erosion of the material are added at high loads, due to cutting off of the singular grains and restructuring of intergrowths borders. science, 3 327 358-70