Knoop and Vickers Indentation in Ceramics Analyzed as a Three-Dimensional Fracture

A three-dimensional fracture analysis is applied to the Knoop and Vickers indentation fracture of ceramics. A brief discussion of the accuracy of the analysis applied to model the step load on the crack face caused by the residual stresses is given. A study is made of the effect of the elongated plastic zone in Knoop indentation on the unloaded radial fracture. It is shown that for small indentation loads the published experimental data can be verified by varying the depth reached by the semielliptical plastic zone with given surface length. An analysis and interpretation of the interaction between the two halfpenny-shaped radial cracks induced by Vickers indentation is also given.     

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.     

高温陶瓷材料延迟断裂寿命预测：Knoop压痕小裂纹技术

本文对Knoop压痕小裂纹技术在陶瓷材料寿命预测研究中的应用进行了简要的评述,并举出具体的寿命预测实例说明了这一技术的实用性。     

Objective Evaluation of Short-Crack Toughness Curves Using Indentation Flaws: Case Study on Alumina-Based Ceramics

An objective methodology is developed for evaluating toughness curves ( T -curves) of ceramics using indentation flaws. Two experimental routes are considered: (i) conventional measurement of inert strength as a function of indentation load; (ii) in situ measurement of crack size as a function of applied stress. Central to the procedure is a proper calibration of the indentation coefficients that determine the K -field of indentation cracks in combined residual-contact and applied-stress loading, using data on an appropriate base material with single-valued toughness. Tests on a fine-grain alumina serve to demonstrate the approach. A key constraint in the coefficient evaluation is an observed satisfaction of the classical indentation strength–(load)^−1/3 relation for such materials, implying an essential geometrical similarity in the crack configurations at failure. T -curves for any alumina-based ceramic without single-valued toughness can then be generated objectively from inert-strength or in situ crack-size data. The methodology thereby circumvents the need for any preconceived model of toughening, or for any prescribed analytical representation of the T -curve function. Data on coarse-grained aluminas and alumina-matrix material with aluminum titanate second-phase particles are used in an illustrative case study.     

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.     

Quantifying Local Microcrack Density in Ceramics: A Comparison of Instrumented Indentation and Thermal Wave Techniques

Instrumented indentation and thermal wave techniques are used as local, quantitative probes of microcrack density in a silicon nitride that has been subjected to severe Hertzian contact stress. The techniques act as local probes, sampling volumes of material <10⁻³ mm³. Microcracked regions are created using a tungsten carbide spherical indenter, as a result of high shear stress in the compressive zone beneath the indenter. The microcracked zones, studied in cross section using a "bonded interface" technique, increased both in size and in degree of damage with increasing Hertzian load. Within the zones, Young's modulus is measured using instrumented indentation, and the thermal diffusivity using a thermal wave technique; both quantities are lower in the damaged regions than in the undamaged regions. The shifts in modulus and diffusivity are used independently to calculate microcrack densities using related models of the effect of microcracks on each property. The two techniques show the same functional relationship between Hertzian contact load and microcrack density, and yield densities that agree to within a factor of approximately 2.     

Spherical and Vickers Indentation Damage in Yttria-Stabilized Tetragonal Zirconia Polycrystals

Damage induced in an yttria-stabilized tetragonal zirconia polycrystal by spherical and Vickers indentations was investigated. Scanning acoustic microscopy revealed that, as indentation stress increased, the spherical indentation gradually developed subsurface damage, until it experienced a transition to a fully plastic state, characterized by a highly anisotropic variation in the leaky Rayleigh wave velocity, v _R, and very similar to that for Vickers indentation. The transition was a result of the formation of a microcracked core beneath the contact. Indenter geometry had an appreciable effect only within the core; the distribution of microcracks differed depending on the indenter used, as confirmed by direct observations using a scanning electron microscope. In contrast, the residual stresses in the elastic-plastic zone were insensitive to indenter geometry. The resulting plastic zone was not hemispherical but rather cylindrical, irrespective of indenter geometry.     

Mixed-Mode Fracture in Soda-Lime Glass Using Indentation Flaws

Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.     

Cracking and the Indentation Size Effect for Knoop Hardness of Glasses

The Knoop hardnesses of five glasses decreased with increasing load in accordance with the classic indentation size effect (ISE). At moderate loads, cracking dramatically altered the indentation sizes and the ISE trends in three of the five glasses. Cracked indentations were as much as 10 μm longer than uncracked indentations made under identical conditions. Diagonal length readings must be corrected for optical resolution limitations if low power lenses are used.     

Mechanical properties of thermally sprayed porous alumina coating by Vickers and Knoop indentation

Depending on the thermal spraying conditions, coatings obtained can present different defects, like pores, cracks and/or unmelted particles, and different surface roughnesses, that can affect the determination of the hardness and elastic modulus. The present work investigates the mechanical properties, determined by means of Knoop and Vickers indentations, of a plasma as-sprayed alumina coating, obtained with a nano-agglomerated powder sprayed using a PTF4 torch, in order to highlight how the surface defects interfere into the indentation process. As a main result, Knoop indentation compared to Vickers one gives less dispersive results (15% and 33%, respectively), that are, in addition, more representative of the coating properties. The mean values obtained are 110 ± 40 GPa for the elastic modulus and 1.75 ± 0.42 GPa for the hardness. In addition, and for the two indenter types used, multicyclic indentation has been performed because it allows a more appropriate characterization of such heterogeneous coatings due to the representation of the mechanical properties as a function of the indentation load and/or the penetration depth, leading to more reliable results according to the depth-variability of the coating microstructure.     

A Simple Method for Determining Elastic-Modulus–to-Hardness Ratios using Knoop Indentation Measurements

A simple indentation technique for measuring the hardness-to-modulus ratio of elastic/plastic materials was developed. The method, which is based on measurement of the elastic recovery of the in-surface dimensions of a Knoop indentation, allows ready evaluation of the hardness-to-modulus ratio to an accuracy better than 10%.     

Visualization of Knoop Indentation Damage of Silicon Nitride by Plasma Etching

A visualizing technique for indentation damage of ceramics was developed. Plasma etching was used to enhance the view of cracks and the subsurface microcracking crush zone following Knoop indentation of hot pressed Si₃N₄. The microcracking zone was readily identified from the surface view of the indented surface as a grain-falling-off region (GFOR), defined as a region in which grains were removed by preferential etching using CF₄ gas, followed by ultrasonic cleaning. A fissure-like opening corresponding to the indentation cracks was also observed. It is inferred that the formation of the GFOR region and the fissure-like opening were caused by the etching/cleaning treatment. Meanwhile, the etching on a section which included diagonals of the impression provided a section view of the microcracking zone.     

Cracking of Surface Mount Capacitors: Crack Susceptibility by Vickers Indentation Techniques

Crack susceptibility of surface mount capacitors fabricated using NPO, Z5U, and X7R materials have been evaluated using Vickers indentation techniques. Capacitors from two sources using X7R have been compared. A relative quality scale between the four surface mount capacitors has been made using the calculated critical stress factor necessary to initiate a crack. It is shown the Vickers indentation technique can be used to determine positional dependence of crack susceptibility. Z5U and NPO materials are used as examples. The Vickers indentation technique has potential of being used to distinguish between acceptable and rejectable lots. It appears from this study that the technique can be an aid to incoming inspection of devices that use ceramic material in their construction, such as surface mount capacitors, electronic packaging devices, and superconductor devices.     

Comparison of Uniaxial and Biaxial Strengths for Test Pieces with Controlled Flaws

Indentation cracks are used as controlled flaws in soda-lime glass specimens for failure tests in uniaxial and biaxial loading. The inert strength is independent of loading type within the scatter of data. This result is discussed in relation to the conclusions of other workers who have reported systematic differences in similar comparative tests on specimens containing natural flaws.     

Comparative Measurement of Indentation Fracture Toughness with Berkovich and Vickers Indenters

Measurements of the load dependence of the radial crack size with Vickers and Berkovich indenters are compared for a range of materials. It is found that the extent of radial cracks was slightly larger for the Berkovich than for the Vickers indenter. The observations reveal that cracks from a Berkovich indenter are best described by an expression developed by Laugier combined with a modification proposed by Ouchterlony to account for the number of radial cracks. It was also found that the Berkovich indenter, which offers advantages for ultramicroindentation, gave more consistent toughness values at lower loads than a Vickers indenter.     

Critical Load for Median Crack Initiation in Vickers Indentation of Glasses

Critical load characteristics for the indentation-induced median crack in ternary lead-zinc-borate and commercial silicate glasses were investigated. The critical load is directly proportional to the fracture toughness/Vickers microhardness term, supporting the Lawn-Evans model; however, the magnitude of the load is considerably greater than that estiated from the model. The relation of critical load to deformation/fracture properties is discussed in terms of the stress field around the indent area and fractography.     

Morphology of Vickers Indent Flaws in Soda–Lime–Silica Glass

The subsurface structure of Vickers indents in soda–lime–silica glass was investigated using confocal microscopy and conventional microscopy. It was determined that the lateral cracks that form beneath the indentation site propagate away from the surface. The median/radial (MR) crack system was found to be semielliptical in shape. The growth of the lateral and MR cracks was found to be codependent such that the depth of the lateral crack limited the depth of the MR crack, and the presence of the MR crack caused deflections in the direction and increased the extent of lateral crack growth.     

Controlled Surface Flaws in Hot-Pressed SiC

Controlled semi-elliptical surface flaws were produced in hot-pressed SiC by Knoop microhardness indentation. Flawed specimens were placed in 4-point bending in order to determine their critical stress intensity factor, K_IC, at both room and high temperatures. Room-temperature fracture and K_IC values after annealing were sensitive to the annealing environment; this behavior correlated with the active/passive nature of the oxidation process. Flaw healing was observed for annealing exposures in air. Room-temperature K_IC values increased with increasing annealing temperature. High-temperature K_ICvalues decreased with increasing temperature as a result of a decrease in the fracture surface energy.