Reverse Indentation Size Effect Research Articles

Study of indentation size effect and microhardness of SrLaAlO4 and SrLaGaO4 single crystals

The load dependence of the Vickers micro-hardness of SrLaAlO4 and SrLaGaO4 single crystals, using a PMT-3 hardness tester, has been investigated and analysed from the standpoint of various theoretical models. On the (100) and (001) planes of these crystals, reverse indentation size effect was observed. Analysis of the experimental data revealed: (1) the indentation size effect is best described by Meyer's law and the proportional specimen resistance model of Li and Bradt, (2) indentation-induced cracking model for reverse indentation size effect and Meyer's law cannot be used to determine the hardness of the crystals, (3) as shown by the negative values of the load-dependent quantities in Hays-Kendall's approach and Li-Bradt model, the origin of indentation size effect is associated with the processes of relaxation of indentation stresses, and (4) the load-dependent and load-independent quantities of different models are interrelated and are intimately connected with the orientation and chemical composition of the crystals. It was also found that the plots of the ratio of indentation load to indentation diagonal against indentation diagonal for a sample exhibit two different slopes with a transition in the slopes occurring at an indentation diagonal, whose value depends on the indenter orientation, indented plane and chemical composition of the crystals. The physical significance of the appearance of these transitions and the nature of load-independent indentation microhardness are discussed.

Study of indentation size effect and microhardness of SrLaAlO4 and SrLaGaO4 single crystals

The load dependence of the Vickers microhardness of SrLaAlO4 and SrLaGaO4 single crystals, using a PMT-3 hardness tester, has been investigated and analysed from the standpoint of various theoretical models. On the (100) and (001) planes of these crystals, reverse indentation size effect was observed. Analysis of the experimental data revealed: (1) the indentation size effect is best described by Meyer's law and the proportional specimen resistance model of Li and Bradt, (2) indentation-induced cracking model for reverse indentation size effect and Meyer's law cannot be used to determine the hardness of the crystals, (3) as shown by the negative values of the load-dependent quantities in Hays-Kendall's approach and Li-Bradt model, the origin of indentation size effect is associated with the processes of relaxation of indentation stresses, and (4) the load-dependent and load-independent quantities of different models are interrelated and are intimately connected with the orientation and chemical composition of the crystals. It was also found that the plots of the ratio of indentation load to indentation diagonal against indentation diagonal for a sample exhibit two different slopes with a transition in the slopes occurring at an indentation diagonal, whose value depends on the indenter orientation, indented plane and chemical composition of the crystals. The physical significance of the appearance of these transitions and the nature of load-independent indentation microhardness are discussed.

Indentation responses of plasma sprayed ceramic coatings

This investigation aims at finding the Vickers and Knoop indentation responses of a few plasma sprayed ceramic coatings, namely Indian alumina, imported alumina, zircon, plasma dissociated zircon (PDZ) and zircon—20 wt% CaO. Some of the sprayed coatings under Vickers indentation show typical Indentation Size Effect (ISE) while others exhibit Reverse Indentation Size Effect (RISE). Such coating behaviour may be attributed to the heterogeneous phase composition and microstructure of the coatings. “True Hardness” and “Elastic Modulus” of the ceramic coatings have been calculated. Also the ISE phenomenon observed for all the coatings undergoing Knoop indentation are explained using models such as Meyer's law, Normalised Meyer's law, Hays' Kendall Approach and Proportional Resistance models. These models have so far been used for explaining the ISE behaviour of only the sintered crystalline ceramics. In this paper, it has been shown that these models hold good to an appreciable extent for plasma sprayed ceramic coatings as well.

Load-dependence of the measured hardness of Ti(C,N)-based cermets

The Vickers hardnesses of a series of Ti(C,N)-based cermets were measured in the indentation load range from 1.47 to 40.67 N. It was found that the examined materials exhibit a reversed indentation size effect, i.e. the measured hardness increases with increasing indentation load. Both Meyer's law and the energy balance model can not provide a proper description for the observed experimental phenomena, while the polynomial series representation can describe the experimental data very well. A possible explanation for the cause of the observed reverse indentation size effect was also proposed.

On the reverse indentation size effect and microhardness measurement of solids

The reverse type of indentation size effect (ISE), where the apparent microhardness increases with increasing applied test load, was critically examined for the experimentally reported data for a number of single crystals differing in crystal structure and chemical bond, using the theoretical models reported in the literature. The analysis revealed that the indentation-induced cracking model, initially proposed to explain the reverse ISE, is not satisfactory. Examination of the data from the standpoint of the models of the normal ISE showed that, in the case of the reverse ISE, a specimen does not offer resistance or undergo elastic recovery, as postulated in some of the models, but undergoes relaxation involving a release of the indentation stress along the surface away from the indentation site. It was found that, for the comparison of microhardness of a face along different directions, of different planes or that of the same face of the crystals of the same substance subjected to different thermal treatment, the Meyer law and the Hays–Kendall approach are not reliable but that the proportional specimen resistance and elastic/plastic deformation response models give similar results. It was also found that the reverse ISE phenomenon occurs only in materials in which plastic deformation is predominant. The possible origins of the occurrence of reverse ISE are suggested.

The effect of indentation-induced cracking on the apparent microhardness

The phenomenon of apparent microhardness increase with increasing applied indentation test load, the reverse indentation size effect (RISE), was addressed from the viewpoint of indentation-induced cracking. The apparent microhardness when the cracking occurs was found to be related to the applied indentation test load as P5/3. Previously published results on single crystals of silicon, GaAs, GaP and InP, which differ by a factor of four, all fall on the same line when analysed through this concept. It is concluded that the RISE is a result of the specimen cracking during the indentation.