RAE 5

Last years many contact mechanical methods were developed for the investigation of me-chanical properties and tribological behaviour of material on the micro- and nano-scale. How-ever, some problems and disadvantages of these methods are kept up to now. The combined ap-plication of normal and lateral forces allows to extend the possibilities of conventional contact mechanical approaches for investigations of mechanical properties and tribological behaviour of bulk and coated materials. Due to the unique construction of the Lateral Force Unit the lateral force can be applied to the sample during normal indentation by the commercial nanoindenter UMIS 2000. Thus, the presented thesis reports the detailed study of the LFU capabilities for the determination of mechanical properties and tribological behavior on the micro-scale.

As the result of evaluation of the LFU the following conclusions about operation and analy-sis of experimental data were obtained.

•

It was found that the LFU plane is inclined by 3.3° with respect to the UMIS indenter shaft.

Thus, the most appropriate position for the combined application of normal and lateral forces is the LFU inclination by 3.3° with respect to the UMIS stage. As a result the inclination of LFU plane with the respect of the UMIS indenter shaft should be minimized. This position was defined as the standard position. It allows to minimize the influence of different factors on the measuring process.

•

It was shown that the shape of normal displacement-time curves is the most appropriate for the simplification of determination of such parameters as the maximal normal displacement at the lateral force application and the residual normal deformation after the removal of lat-eral force after the procedure of thermal drift correction.

It was found that the following problems of contact mechanical methods can be solved by the combined application of normal and lateral forces.

• The crack formation can be detected as the observation of sudden change of lateral and nor-mal displacements in lateral force-displacement and nornor-mal displacement-time curves re-spectively. These investigations were performed for single-crystal sapphire (0001). Failure was detected in the sample at the lateral force application. The observation of decrease of critical lateral force for failure by the repeated applications of normal and lateral forces to the same point on sapphire surface allowed to assume that just crack was the reason of failure.

Further analysis showed that plastic deformation could not be the reason of failure. As was established the tensile stress increases by more than one third at the application of lateral force. Therefore, just crack is the reason of failure. It should be noted that for the first time the crack in single-crystal sapphire was detected by the contact mechanical method in situ.

Using the detected critical forces the critical tensile stress of 9.68 ± 0.22 GPa for the crack formation in single-crystal sapphire (0001) was determined. This value corresponds to the critical tensile stress obtained by other methods.

• The onset of plastic deformation can be detected by the observation of shape change of lat-eral force-displacement curve together with the appearance of residual normal deformation in normal displacement-time curve. These investigations were done for bulk BK7 glass and sili-con dioxide film with thickness of 951 nm on silisili-con substrate. Moreover, for the comparison of results the investigation of onset of plastic deformation in silicon dioxide film was also performed by the standard loading-partial unloading method of nanoindentation. It was found that the values of yield strength of the investigated coating obtained by these two contact

me-chanical approaches are in a good agreement each other. It was established that the value of this yield strength is 6.83 ± 0.02 GPa.

• The static friction of materials couples can be evaluated by the analysis of lateral force-displacement curves with the error of 5-10 %. The static friction coefficients for fused silica, BK7 glass, single-crystal sapphire (0001) as well as SiO2, DLC and CrN0.08 coatings were determined against diamond, tungsten carbide and sapphire spherical indenter with different radii. The effect of normal load on static friction for fused silica and BK7 glass against the same diamond spherical indenter was also under consideration. It was shown that the influ-ence of normal load on static friction is individual for each materials couple. However, just onset of plastic deformation leads to a significant change of static friction.

In future the originality of measuring system, suitable models for theoretical simulation as well as the results presented in this thesis allow to attract the attention to the solution of the fol-lowing tasks.

• For the analysis of influence of contact area on the crack formation by the combined applica-tion of normal and lateral forces the investigaapplica-tions can be done by means of indenters with different indenter radii. Single-crystal sapphire (0001), for which the critical tensile stress was evaluated in this thesis, can be used for these investigations.

• As was shown the combined application of normal and lateral forces can allow to detect the onset of plastic deformation for very thin and hard coatings at the appropriate friction coeffi-cient between film surface and indenter. Consequently, the detection of critical normal and lateral forces for the onset of plastic deformation in those coatings can allow to determine correctly yield strength.

• By analogy with the investigation of the influence of normal load on the static friction the study of influence of indenter radius can be done. As a result the analysis of influence of con-tact area on static friction can be performed. Moreover, the influence of other external factors mentioned in this thesis on static friction can be investigated in detail.

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In document Elaborado por: Arnidis R. Bustamante y. Lucy Hernández (página 27-157)