Deformation Twinning in Single-Crystal Monoclinic Zirconia: A First Report

The hardness of single crystals of flux-grown, untwinned monoclinic zirconia was determined using Knoop and Vickers microindenters. Significant hardness anisotropy existed on (100) and {110} faces, with a maximum Knoop hardness of ∼8 GPa, although large variance was observed between different batches of crystals. Extensive deformation twinning, primarily on {110}, but also on (001), accompanied microindentation at all temperatures between room temperature and 800°C.     

Deformation and Fracture of MnTe and MnSe-MnTe Solid Solutions

Single crystals of MnTe and solid solutions in the system MnSe-MnTe were prepared; selected surfaces were indented with either a Vickers or a Knoop microindenter. Information on mechanical behavior was obtained by observation of slip traces and by study of Knoop hardness anisotropy. Hexagonal NiAs-type MnTe and MnTe-rich Mn(Te,Se) solid solutions show plastic deformation attributable to {1012} twinning, basal slip, and pencil glide in 〈1120〉. Fracture occurs as primary {0001} and secondary {1010} cleavage. Vickers and Knoop hardnesses were greater on the basal plane than on prism planes. Cubic MnSe-rich Mn(Te,Se) solid solutions show both {111}〈110〉 and {110}〈110〉 slip with {100} and {110} fracture in crushed fragments and around surface indentations. Knoop hardness anisotropy is like that in MnSe. The rate of solid solution hardening is greater than for comparable substitutions of sulfide ions in the MnSe matrix.     

Comparison of conventional Knoop and Vickers hardness of ceramic materials

Ceramics generally have a lower Knoop than Vickers hardness. This difference is due to the elastic recovery occurring around a Knoop indentation and the difference in representative area considered to calculate the hardness value.Conventional hardness tests with Knoop and Vickers indenters were performed in order to show how Knoop hardness test can give the same hardness number obtained by Vickers hardness test. This is obtained when Knoop hardness number is calculated based on the residual plastically deformed area whether projected or true. Complementary hardness data obtained from the literature were used in this work in order to validate the method proposed in this work. A revision of the well-known relation of Marshall is proposed in order to determine the elastic modulus by means of one Knoop hardness test when the Vickers hardness is unknown.     

The indentation size effect and the hardness of single crystal diamond on the (001) 110

The hardness of single-crystal diamond is addressed through the analysis of independent Knoop and Vickers indentation microhardness data on the (001) 110. A proportional specimen resistance model is applied to separate the hardnesses into two components, one representative of the indentation size-load effect (ISE) and the other the load-independent hardness. The analysis yields a load-independent Knoop hardness of 4411 kg mm⁻² and a Vickers hardness of 9158 kg mm⁻². These values are compared with similar analyses for sapphire and silicon carbide and are related to the Plendl-Gielisse volumetric lattice energy concept of hardness. The extraordinarily high hardnesses which have been reported for diamond, values in excess of 10 000 kg mm⁻², are attributed to the elastic modulus and friction contributions to the ISE.     

Knoop Hardness Anisotropy and Plastic Deformation in Mn-Zn Ferrite Single Crystals

The Knoop hardness anisotropy and plastic deformation markings around the DPH indentation were investigated for an Mn-Zn ferrite single crystal. Knoop hardness values, which depended primarily on the crystallographic direction, were represented on a standard stereographic triangle. The hardness was maximum in the <001> direction and minimum in the <111> and <011> directions. The patterns of the plastic deformation markings show that the ferrite deforms plastically at room temperature by {110} <110> and {111} <110> slip.     

Thermally activated deformation under Knoop indentations in super-hard directions of high-quality synthetic type-IIa diamond crystals

The deformation resistance at room temperature against a Knoop indentation in (001)<110> (the<110> direction on the (001) plane) of high-quality synthetic type-IIa diamond is known to be extremely high. The behavior of deformation in the hard direction activated thermally by heating was investigated, using super-hard Knoop indenters prepared from high-quality diamond crystals by taking the tip orientation to (001)<110>. Indentation tests in (001)<110> with a load of 4.9 N revealed that the formation of normal Knoop impressions arises suddenly at a threshold temperature of 200–240 °C, whereas no impressions are observed up to 200 °C. The hardness values derived from the impressions in (001)<110> formed above the threshold temperatures are as low as 50–60% those in (001)<100> at the same temperatures. The anisotropy in the Knoop hardness at such high temperatures is consistent with the nature of anisotropy predicted by an effective resolved shear stress model for a {111}<110> slip deformation.     

Preparation and mechanical characterization of dense and porous zirconia produced by gel casting with gelatin as a gelling agent

A modified gel casting procedure based on a natural gelatin for food industry and commercial polyethylene spheres as pore formers was successfully exploited to produce dense and porous ceramic bodies made of yttria stabilized tetragonal zirconia polycrystal (Y-TZP). Vickers and Knoop microhardness, elastic modulus and fracture toughness measurements on dense samples obtained by experimental investigation closely matched results found in the literature for similar materials. However, after a careful analysis of obtained results, no indentation size effect and a lower scattering of experimental data from low load indentations were observed, in comparison with literature.     

Fabrication of New Cemented Carbide Containing Diamond Coated with Nanometer-Sized SiC Particles

A simple process for depositing a coating of silicon carbide (SiC) crystallites ∼10 nm in size onto diamond particles has been developed. SiO powders react with diamond in a vacuum at 1350°C to form a uniform β-SiC polycrystalline layer ∼60 nm thick. The SiC coating improves the oxidation resistance of the diamond. A cemented carbide material containing 20-vol%-SiC-coated diamond particles was sintered to a relative density of 99.5% by pulsed-electric-current sintering. A Vickers hardness and indentation fracture toughness of 15 GPa and 16.3 MPa·m^1/2, respectively, were obtained. This toughness is two times higher than that of cemented carbide containing no particles. The higher toughness is attributed to deflection and blockage of crack propagation by the diamond particles.     

Real indentation hardness of nano-polycrystalline cBN synthesized by direct conversion sintering under HPHT

The hardness characteristic of nano-polycrystalline cBN synthesized by direct conversion sintering was thoroughly investigated using Vickers and Knoop indenters. It was found that nano-polycrystals consisting of smaller cBN grains increase the elastic recovery of indentations during unloading of the indenters and the diagonal of Vickers indentations and the minor diagonal of Knoop indentations significantly decrease in length. Thus, if a Vickers indenter is used, the apparent hardness value increases, making it impossible to perform an accurate evaluation, e.g. incorrect Vickers hardness values in excess of 80 GPa were obtained from nano-polycrystalline cBN with a grain size of 50 nm or less. On the other hand, it was verified that a Knoop indenter ensures an accurate hardness evaluation even if the constituent grains are fine because its major diagonal length which is used for measurement is less susceptible to elastic recovery. In an accurate evaluation of the hardness of different types of nano-polycrystalline cBN using a Knoop indenter, the hardness of each type of cBN was around 45 GPa, and there was no clear Hall–Petch relationship between hardness and grain size without a slight bell-like correlation. These results suggest that reported hardness values higher than 80 GPa of similar nano-polycrystalline cBN evaluated using a Vickers indenter are incorrect values caused by elastic recovery occurring at the indentation.     

Comparison of instrumented Knoop and Vickers hardness measurements on various soft materials and hard ceramics

Vickers and Knoop hardness measurements performed on various ceramics (hard metals) and light alloy materials (soft metals) are compared. The results show that the Knoop hardness number is, in general, lower than the Vickers hardness number for the highest values of hardness, and this behaviour is reversed when the hardness values are low. This change in values, which occur at 8 GPa, has no real physical meaning and, therefore, it is difficult to interpret such behaviour in terms of the elasto-plastic deformation around the indent such as sinking-in, piling-up, and bulging of the indent faces, phenomena which take place during indentation or after the withdrawal of the indenter.Prior to interpreting the hardness difference, it is very important to consider the same area in the hardness calculations. That is why we have compared the available hardness data obtained from the literature and recalculated them by considering the projected and true areas of the contact. If the objective is to compare the two hardness numbers, it seems more suitable to consider the true area of contact, procedure which will provide a Vickers hardness number higher than the Knoop hardness number all over the range of the hardness values.     

Fracture Modes in MoSi2

The room-temperature fracture behavior of polycrystalline MoSi₂ was characterized using Vickers indentation fracture. Fracture analysis was aided by the optically active grain structure of MoSi₂ revealed under polarized light. Radial crack propagation from indentations was found to be predominantly transgranular. The approximate indentation fracture toughness of MoSi₂ was 3 MPa.m^1/2, while the measured hardness was 8.7 GPa. Fracture behavior is believed to be controlled by anisotropy and cleavage energy of the tetragonal MoSi₂ crystal structure.     

A microindentation study of polyethylene composites produced by hot compaction

Microindentation measurements are reported on a range of single polymer polyethylene (PE) composites, which are produced by hot compaction of high modulus PE fibers. It is possible to measure two hardness values, parallel and perpendicular to the fiber direction respectively, from which the microindentation anisotropy is defined. The hardness values relate to the instantaneous elastic recovery of the fibers, and the results show that the microindentation measurement is deforming a material volume below the surface of the sheets comparable to the dimensions of the fibers. It appears that the microindentation anisotropy approaches a limiting value with increasing fiber orientation, i.e., as the Young's modulus of the fibers increases. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1659–1663, 2006     

Mechanical properties of synthetic type IIa diamond crystal

The mechanical behavior of synthetic type IIa diamond has been investigated by the Knoop hardness measurement and observation of the cleavage surfaces. It was clarified that the Knoop hardness in (100)100 of synthetic diamonds increases with decreasing of the nitrogen impurities concentration, and that the synthetic type IIa diamond, having few nitrogen impurities, has the highest hardness of synthetic diamonds. In addition, it was found that the Knoop hardness in (100)110 of synthetic type IIa diamond is extremely high, and the anisotropy in the hardness of the diamond is different from those of natural diamond and synthetic type Ib diamond. The cleavage surfaces of the synthetic type IIa diamonds were very smooth and showed remarkably regular cleavage patterns. These results indicate that there are very few impurities and crystal defects in the synthetic type IIa diamond, and also suggest that the diamond has high resistance to plastic flow.     

Recommendations for Determining the Hardness of Armor Ceramics

It is empirically known that an armor ceramic should be as hard or harder than the projectile it intends to defeat. Quasi-static indentation testing is one of the most widely utilized techniques for determining the hardness of armor ceramics. Hardness measurements can also be used to generate other property values that may be relevant to ballistic performance (fracture toughness, elastic properties, and even the yield strength). While the indentation methodologies are simple and straight forward, the resultant hardness values for ceramic materials can be influenced by the indenter geometry, indentation load, loading rate, specimen surface finish, and microstructure. This presentation will summarize the results of a study to determine the hardness of a variety of armor-grade ceramics with different indenter geometries (Vickers and Knoop) over a range of indentation loads (0.98-98 N) and discuss the implications for armor ceramics. The resulting data strongly indicate that the best means of determining the hardness of armor ceramics is the use of 19.6-N Knoop indentations.     

Ferroelastic Domain Switching in Polydomain Tetragonal Zirconia Single Crystals

As-received, yttria-doped (4.2 wt% Y₂O₃) single crystals of zirconia were heated to ≥2100°C in air. Cube-shaped samples with faces perpendicular to 〈100〉 axes on the basis of the pseudocubic symmetry were cut from the crystals. X-ray and electron diffraction indicated that the crystals are polydomain with [001] axes, on the basis of the tetragonal symmetry, in three mutually orthogonal directions (perpendicular to the cube faces). The cube-shaped crystals were tested in compression at temperatures as high as 1400°C. X-ray diffraction indicated that ferroelastic domains underwent reorientation (switching) in compression. Subsequently, notched samples with the long direction of the beams along 〈100〉 on the basis of the pseudocubic symmetry were fractured in three-point bending at temperatures as high as 1000°C. X-ray diffraction from fracture surfaces showed that domain reorientation had occurred and that no monoclinic phase was observed on fracture or ground surfaces. The fracture toughness at room temperature and at 1000°C was ∼12 and ∼8 MPa · m^1/2, respectively. Preliminary experiments on polycrystalline tetragonal zirconia samples containing 5.4 wt% Y₂O₃ and sintered at ≥2100°C also showed no evidence of the monoclinic phase on fracture or ground surfaces. The toughness of the polycrystalline samples was typically 7.7 MPa · m^1/2. These results indicate that ferroelastic domain switching can occur during fracture and may contribute to toughness.     

Knoop Microhardness Anisotropy of Single-Crystal Cassiterite (SnO2)

The Knoop microhardness anisotropy of single-crystal cassiterite (SnO₂) was measured on the (100), (110), (001), and (111). That anisotropy is depicted as the microhardness profiles for those planes. The results are addressed first in terms of the elastic anisotropy of SnO₂ and then on the basis of the effective resolved shear stress (ERSS), the latter an approach initially advanced by Brookes and co-workers. The load dependence of the Knoop microhardness is also evaluated in terms of the classical Meyer's law for which it is demonstrated that the Meyer's law coefficient and Meyer's law exponent are related.     

Effect of Indenter Geometry on Controlled-Surface-Flaw Fracture Toughness

Fracture toughness values obtained using both Knoop and Vickers-indentation-produced controlled surface flaws were compared as a function of indentation load for a well-characterized glass-ceramic material. At the same indentation load, Knoop cracks were larger than Vickers. As-indented K_c values calculated from fracture mechanics expressions for surface flaws were higher for Knoop flaws than Vickers, but both types gave low K_c values due to indentation residual stress effects. Analysis suggested that theoretical formalisms for indentation residual stress effects based on fracture mechanics solutions for a center-loaded penny crack in an infinite medium should apply to both indentation types. K_c values calculated using the residual stress approach were identical for Knoop and Vickers controlled surface flaws when a "calibration" value for a constant term in the expression for K_c was used for both indentation types.     

Effect of Sintering Temperature on Phase Transition and Mechanical Properties of Lead Zirconate Ceramics

Phase transition and mechanical properties of PbZrO₃ (PZ) at different sintering temperatures were studied. The Curie temperature depends on the sintering temperature. Hardness and fracture toughness of the PZ were measured using Vickers and Knoop microhardness testers. The lower density at the higher sintering temperature resulting from the loss of lead oxide (PbO) causes the lower value of the Curie temperature, hardness and fracture toughness. The results were well corresponding to the microstructure of the PZ ceramics.     

Temperature Dependence and Fracture Toughness and Elastic Moduli of a Waste Glass

The temperature dependence of elastic moduli, of the crack lengths formed by Vickers indentations, and of the hardness of the nuclear waste borosilicate glass GP 98/12 has been measured up to 300°C. The temperature dependence of the fracture toughness K_IC exceeds that predicted by the variations of Young's modulus E and hardness H with temperature.     

Micromechanical properties of Dy3+ ion-doped (LuxY1-x)3Al5O12 (x = 0, 1/3, 1/2) single crystals by indentation and scratch tests

Three kinds of Dy³⁺ ion-doped (Lu_xY_1-x)₃Al₅O₁₂ (x = 0, 1/3, 1/2) single crystals fabricated by the Czochralski method with 4 at.% Dy³⁺ ion doping were investigated by indentation and scratch techniques under Vickers, Knoop, Berkovich, and spherical indenters to understand the influence of Lu ion on micromechanical properties and fracture behavior of Y₃Al₅O₁₂ (i.e. YAG for x = 0) single crystals. The largest (or smallest) values of hardness, elastic modulus, and fracture toughness were found for x = 1/3 (or 1/2). The indentation size effect was explained by four different models with the Hays-Kendall approach being the most suitable one to determine the true hardness. Fracture toughness values of YAG crystals obtained by the Vickers hardness method agreed with those obtained by scratching with a spherical indenter based on linear elastic fracture mechanics.