Hezhuo Miao

On the description of indentation size effect in hardness testing for ceramics: Analysis of the nanoindentation data

Abstract The nanoindentation hardnesses of a commercially available soda-lime glass, a tetragonal ZrO 2 polycrystalline and a hot-pressed Si 3 N 4 were measured in the peak load range from 7.5 to 500 mN. The experimental results revealed that, for each material, the measured hardness exhibits a peak-load- dependence, i.e., indentation size effect (ISE). Such a peak-load-dependence was then analyzed using the Meyer’ law, the Hay–Kendall approach, the proportional specimen resistance (PSR) model, the elastic recovery model and the modified PSR model. The analyses revealed that: (1) Meyers law provides a satisfactory description for the experimental data for each material but cannot provide any knowledge of the origin of the observed ISE; (2) the Hays–Kendall approach, the elastic recovery model and the PSR model yield meaningless values of the parameters included in the corresponding equations, invalidating the applicability of these models in analyzing the ISE in the nanoindentation region; (3) the modified PSR model is sufficient for describing the observed ISE but the physical meaning of this model seems to be more complex than those proposed originally. For each material, the true hardness was also determined based on the PSR model, the elastic recovery model and the modified PSR model, respectively. It was found that the true hardness values deduced based on different models are similar with each other and this similarity was attributed to the similarity between the empirical equations adopted in these models.

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 Meyers 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.

Load-dependence of Knoop hardness of Al2O3-TiC composites

Abstract Nine samples of Al2O3–30 wt.% TiC composites were prepared by hot-pressing the Al2O3 powder mixed with TiC particles. The average sizes of the TiC particles used for preparing the nine samples were different with each other. Knoop hardness measurements were conducted on these nine samples, respectively, in the indentation load range from 1.47 to 35.77 N. For each sample, the measured Knoop hardness decreases with the increasing indentation load. The classical Meyers power law and an empirical equation proposed originally by Buckle were verified to be sufficiently suitable for describing the observed load-dependence of the measured hardness. Analysis based on Meyers law can not provide any useful information about the cause of the observed ISE while true hardness values, which are load-independent, can be deduced from the Buckles equation. It was found that the deduced true hardness increases with the average size of TiC particles existing in the sample.

Hard and wear-resistant titanium nitride coatings for cemented carbide cutting tools by pulsed high energy density plasma

Abstract Hard and wear-resistant titanium nitride coatings were deposited by pulsed high energy density plasma technique on cemented carbide cutting tools at ambient temperature. The coating thickness was measured by an optical profiler and surface Auger microprobe. The elemental and phase compositions and distribution of the coatings were determined by Auger microprobe, x-photon electron spectroscope, and X-ray diffractometer. The microstructures of the coatings were observed by scanning electron microscope and the roughness of the sample surface was measured by an optical profiler. The mechanical properties of the coatings were determined by nanoindentation and nanoscratch tests. The tribological properties were evaluated by the cutting performances of the coated tools applied in turning hardened CrWMn steel under industrial conditions. The structural and mechanical properties of the coatings were found to depend strongly on deposition conditions. Under optimized deposition conditions, the adhesive strength of TiN film to the substrate was satisfactory with the highest critical load up to more than 90 mN. The TiN films possess very high values of nanohardness and Young’s modulus, which are near to 27 GPa and 450 GPa, respectively. The wear resistance and edge life of the cemented carbide tools were improved dramatically because of the deposition of titanium nitride coatings.

Gelcasting of alumina ceramics in the mixed acrylamide and polyacrylamide systems

Ceramic gelcasting without inhibition of gelation due to the presence of oxygen must be performed in a nitrogen atmosphere to avoid surface spallation of the green body, which is a complex and expensive process. This study shows that the surface spallation of ceramic green bodies gelcast in air could be suppressed by introducing a proper amount of water-soluble polymer, polyacrylamide (PAM), to the acrylamide monomer solution. The dispersion of Al2O3 powder in a water solution containing PAM was investigated. The rheological behavior and the gelling behavior of the alumina suspension were analyzed. Flexural strength and microstructure of the gelcast green and sintered bodies were studied.

Hard and wear-resistant titanium nitride films for ceramic cutting tools by pulsed high energy density plasma

High-quality and wear-resistant titanium nitride films were deposited onto silicon nitride ceramic cutting tools by pulsed high energy density plasma technique at ambient temperature. The adhesive strength of TiN film to the ceramic substrate has been satisfactory with very high critical load up to more than 80 mN measured by nanoscratch tester. The TiN films possess very high values of nanohardness and Youngs modulus, which are near to 28 and 350 GPa, respectively. The wear resistance and edge life of the ceramic tools were increased greatly because of the deposition of titanium nitride coatings. The cutting performance of the silicon nitride ceramic tools with such TiN films could be optimized by varying the deposition conditions, including the (shot) number of pulsed plasma and the discharge voltage between the inner and outer electrodes, etc.

Effect of peak load on the determination of hardness and Young's modulus of hot-pressed Si3N4 by nanoindentation

Abstract The indentation load–displacement curves of a hot-pressed silicon nitride were measured under different peak load levels. The unloading segments of these curves were analyzed using the widely adopted Oliver–Pharr method. It was found that both the hardness, H , and the Youngs modulus, E , exhibit significant peak-load-dependence. Empirical approaches were then proposed to determine the load-independent hardness and modulus. The hardness and modulus deduced from these empirical approaches were proven to be comparable with the literature reported data measured with conventional methods.

Effect of metallic binder content on the microhardness of TiCN-based cermets

Microhardness testing was conducted on a series of TiCN-based cermets with different contents of metallic binders in the applied load range from 0.49 to 9.8 N. Significant indentation size effect (ISE) in the measured microhardness was observed for each test sample. The traditional Meyers law and the modified proportional specimen resistance (MPSR) model were employed to analyze the load-dependence of the measured hardness. It was found that the Meyers exponent n increases with the metallic binder content, implying that increasing the metallic binder content would restrain the ISE. The load-independent hardness was found to decrease with the increasing metallic binder content. This can be attributed to the fact that the hardness of metallic binder is lower than that of the TiCN matrix.

The influence of TiC-particle-size on the fracture toughness of Al2O3-30 wt.%TiC composites

Abstract Seven grades of Al 2 O 3 –30 wt.% TiC composites were prepared by hot-pressing the Al 2 O 3 powder mixed with TiC particles. The average sizes of the TiC particles used for preparing these composites were different with each other. The fracture toughness K IC was measured in three-point bending for each composite. It was shown that the variation in the size of the TiC particles results in large changes in fracture toughness and there is a particular particle size for which the fracture toughness of the resultant composite reaches its maximum. A possible toughening mechanism was also proposed to explain these experimental findings.

Effect of TiC particle size on the toughness characteristics of Al2O3-TiC composites

Abstract The effect of TiC particle size on the toughness of Al 2 O 3 –30 wt.% TiC composites was examined by quantitatively analyzing the R -curves measured with direct indentation method for four samples having different TiC particle sizes. It was shown that the toughening effect in the composite is strongly dependent on the size of the TiC particles.