Showing papers on "Indentation published in 1986"

A method for interpreting the data from depth-sensing indentation instruments

Abstract: Depth-sensing indentation instruments provide a means for studying the elastic and plastic properties of thin films. A method for obtaining hardness and Young’s modulus from the data obtained from these types of instruments is described. Elastic displacements are determined from the data obtained during unloading of the indentation. Young’s modulus can be calculated from these measurements. In addition, the elastic contribution to the total displacement can be removed in order to calculate hardness. Determination of the exact shape of the indenter at the tip is critical to the measurement of both hardness and elastic modulus for indentation depths less than a micron. Hardness is shown to depend on strain rate, especially when the hardness values are calculated from the data along the loading curves.

Plastic properties of thin films on substrates as measured by submicron indentation hardness and substrate curvature techniques

Abstract: Substrate curvature and submicron indentation measurements have been used recently to study plastic deformation in thin films on substrates. In the present work both of these techniques have been employed to study the strength of aluminum and tungsten thin films on silicon substrates. In the case of aluminum films on silicon substrates, the film strength is found to increase with decreasing thickness. Grain size variations with film thickness do not account for the variations in strength. Wafer curvature measurements give strengths higher than those predicted from hardness measurements suggesting the substrate plays a role in strengthening the film. The observed strengthening effect with decreased thickness may be due to image forces on dislocations in the film due to the elastically stiffer silicon substrate. For sputtered tungsten films, where the substrate is less stiff than the film, the film strength decreases with decreasing film thickness.

The effect of lateral crack growth on the strength of contact flaws in brittle materials

Abstract: The effect of lateral cracks on strength controlling contact flaws in brittle materials is examined. Inert strength studies using controlled indentation flaws on a range of ceramic, glass, and single crystal materials reveal significant increases in strength at large contact loads, above the predicted load dependence extrapolated from strength measurements at low indentation loads. The increases are explained by the growth of lateral cracks decohesing the plastic deformation zone associated with the contact from the elastically restraining matrix, thereby reducing the residual stress field driving the strength controlling radial cracks. A strength formulation is developed from indentation fracture mechanics which permits inert strengths to be described over the full range of contact loads. The formulation takes account of the decreased constraint of the plastic deformation zone by lateral crack growth as well as post-contact nonequilibrium growth of the radial cracks. Simple extensions permit the strengths of specimens controlled by impact flaws to be described, as well as those failing under nonequilibrium (fatigue) conditions. The implications for materials evaluation using indentation techniques are discussed and the dangers of unqualified use of strength measurements at large indentation loads pointed out. The work reinforces the conclusion that a full understanding of the residual stress field at dominant contact flaws is necessary to describe the strength of brittle materials.

Criteria for mechanical property modifications of ceramic surfaces by ion implantation

Abstract: Modifications to the mechanical properties of ceramic surfaces by ion implantation have been investigated as a means of producing increased wear resistance for tribological applications It has been established that ion implantation can result in a number of tribologically relevant parameters (eg hardness and indentation fracture toughness) being modified in various ways Examples of these changes will be given for ion implanted sapphire Further, these modifications have been related to critical structural changes occurring in or near the surface Radiation damage, amorphisation and surface stress generation are the most important of these and affect both hardness and indentation fracture behaviour This paper will describe how the observed mechanical properties are related to the structural changes occurring in the surface and presents a number of criteria designed to enable prediction of the mechanical property modifications that will ensure after implantation of a given substrate with a giv

Adhesion measurement of thin films by indentation

Abstract: An indentation technique is described for measuring the shear strength of the interface between a thin film and a rigid substrate. A simple elastic/plastic analysis is used to describe the experimental results and good agreement between the two is found. The method may also be used to estimate the interfacial toughness so that the important parameters characterizing both initiation and propagation of adhesive failure may be determined from one simple experiment. The analysis is extended to describe the effect of residual stress in the film. Also, the stresses around a pinhole defect in a stressed film are examined.

Implications of Transformation Plasticity in ZrO2‐Containing Ceramics: II, Elastic‐Plastic Indentation

Abstract: Indentation hardness of partially stabilized zirconia was found to be 8.2 times the uniaxial compressive yield stress. This ratio is anomalously high for ceramic materials and is attributed to pressure hardening in transformation plasticity. An analytic solution of elastic-plastic indentation using the spherical-hole model is given, based on a pressure-sensitive idealized yield criterion. The theory correctly predicts a higher hardness and a larger plastic zone, both in quantitative agreement with the experimental observations in Mg-PSZ. Data analysis further reveals a large disparity of yield stress and hardness values among cubic, tetragonal, and monoclinic zirconias, resulting from their different phase transformation and twinning capabilities. The broader implications of pressure sensitivity in elastic-plastic Indentation are rationalized.

Bubble raft model for indentation with adhesion

Abstract: Indentation hardness tests are now widely used to measure the mechanical properties of solid surfaces1–3. Recent developments of this technique4,5 permit the analysis of the outermost 10 nm of materials. Experimental and theoretical questions arise regarding the physical and mechanical processes involved in such small indentations. We describe here an indentation experiment on a microscopic scale, using soap bubbles blown onto a water surface. Bubble rafts provide a simple two-dimensional model for indentation behaviour; as for other materials, their behaviour is governed by two principal attraction–repulsion forces6, and by geometrical constraints. A crystalline two-dimensional lattice is obtained by using bubbles of uniform size7–9, whereas bubbles of two sizes give an amorphous structure10,11. Indentation can be represented by the contact between a triangular crystalline raft and a rectangular crystalline raft bordered by an amorphous layer. The flow of the materials, which is dependent on both adhesion and the force between the two rafts, can be analysed during the experiment.

Effect of test method and crack size on the fracture toughness of a chain-silicate glass-ceramic

Abstract: The fracture toughness of a canasite glass-ceramic with a highly acicular, interlocked grain structure was measured by a number of different methods. The values at room temperature obtained by the chevron-notch, short-bar and notched-beam methods ranged from 4 to 5 M Pa m−1/2, well above literature values for other glass-ceramics. Similar values of toughness were obtained by the fracture of bars with indentation cracks introduced with loads ranging from 1.96 to 400 N, but only for crack sizes >200 μm, with lower values for cracks of smaller size. The toughness values obtained by the direct measurement of the size of the indentation cracks were appreciably lower than the values obtained by all other methods over the total range of indentation loads and corresponding crack size. SEM fractography showed that the surface within the indentation cracks was appreciably smoother than the surrounding fracture surface. The high values of fracture toughness were attributed to the combined mechanisms of crack-deflection and microcrack-toughening due to the stress-enhanced creation of microcracks caused by the residual stresses which arise from the thermal expansion anisotropy of the canasite monoclonic crystal structure. The strong negative temperature dependence of the fracture toughness suggests that at room temperature microcrack toughening represents the primary contributing mechanism to the fracture toughness. The combined effects of crack-deflection and microcrack-toughening can lead to the development of glass-ceramics with greatly improved resistance to crack propagation.

Pressure release vent

Abstract: A pressure release vent for a container which comprises an indentation in a wall of the container which is interrupted by a scored hollow rib having an apex which connects the opposing sidewalls of the indentation at a distance above the bottom of the indentation which is equal to about 5% to about 75% of the depth of the indentation.

Influence of Surface Stress on indentation Cracking

Abstract: A simple, two-dimensional fracture mechanics analysis was used to determine the influence of nonuniform residual surface stresses on the formation of radial indentation cracks. The indentation behavior depends on the depth of the compressive stresses, such that the apparent fracture toughness passes through a maximum with increasing indentation load. The analysis was used to estimate the surface stress from indentation data for a zirconia-toughened ceramic and was compared to previous X-ray diffraction measurements of this stress. The comparison gives only fair agreement; the sources of possible error are discussed. Such surface stresses also influence the accuracy of KIC measurements when an indentation crack length technique is used; surface preparation is a critical factor in the measurement. Finally, the KIC values obtained from indentation crack sizes were compared with those obtained by the double-cantilever-beam technique.

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

Abstract: 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.

Indentation phenomena and wear of surfaces and edges

Abstract: Indentation tests are well established methods of measuring the hardness and toughness of surfaces, and the results can often be related to wear behaviour. Response to indentation in simple tests has been used in models for sliding wear, abrasion, and impact erosion; more sophisticated procedures can provide additional information about wear by stress corrosion. Despite the increased refinement of indentation fracture models, the scope for assessing the accuracy of theoretical predictions is limited considerably by the approximations used in the theories and the poor reproducibility of wear test data. An aspect of wear usually neglected is the brittle failure of edges and corners of components and tools. Recently, many of the experimental problems associated with using indentation to measure the susceptibility to edge flaking have been overcome, and it may soon be possible to recommend a test procedure for measuring this property.MST/452

A Comparison of Vickers and Knoop Indentations in Soda-Lime Glass

Abstract: Flaw and strength characteristics of Vickers and Knoop indentations in soda-lime glass were studied over a wide range of indenter loads. Vickers indentation produced ideal, well-developed deformation/crack patterns over the entire load spectrum; whereas, Knoop indentation patterns contained significant amounts of extraneous surface damage and radial cracks were consistently well-developed only at high indenter loads. Despite these complications, both Knoop and Vickers results showed very good fit to the indentation fracture mechanics model. However, unlike results reported for silicon nitride and glass-ceramic materials, Vickers and Knoop radial cracks on soda-lime glass were of comparable length for the same indentation load and equivalent crack lengths yielded equivalent fracture strengths independent of indenter geometry. Strength behavior of annealed flaws suggests that the flaw shape parameter for Knoop indentations is less than for Vickers indentations (0.21 vs. 0.28). Aging test results indicate greater relief of the indentation-generated residual stress in Knoop flaws than in Vickers flaws.

Compressive Thin Films For Increased Fracture Toughness

Abstract: Improved fracture toughness by as much as a factor of two in infrared window materials has been observed after the deposition of thin compressive surface films by reactive ion beam techniques. The relationship between film stress, film thickness, substrate properties, and observed fracture toughness are being investigated, using Vickers indentation to determine mechanical properties and a bending plate capacitance method to independently determine film stress. Comparisons are made to a theoretical model developed by Lawn and Fuller.

Analysis of Elastic-Plastic Ball Indentation to Measure Strength of High Strength Steels

Abstract: Ball-indentation experiment ware performed with A723 steel of 100- to 1200-MPa ultimate strength. Results are compared with conventional tension tests and with an elastic-plastic finite-element model of the ball indentation. Finite-element analysis shows the ball-indentation process to be insensitive to friction effects. Comparison of indentation and conventional tests shows that slip-line field analysis closely predicts the ball contact stress. The indentation tests gave an accurate measure of ultimate tensile strength because of the following test procedures: using a large ball size and a fixed ratio of indentation depth to ball size; accounting for directional material properties; accounting for extraneous system deflections.

Word processor permitting modification or addition of data in response to particular indentation of data

Abstract: A text processing apparatus or word processing system has an input unit for entering character data, an indentation input unit for entering indentation data, a memory for storing the character data and the indentation data, which is connected to or coupled with both the input unit and the indentation input unit, an indentation modifying unit for modifying the position of the indentation data which is stored in the memory, and a control unit for modifying the data stored in the memory according to the position of the indentation data as modified by the indentation modifying unit. This system provides for the modification of document data in response to a change in the position of indentation, the modification or editing of character data which precedes the position of indentation, and the automatic deletion of spaces, which precede an indent start position, when the indentation is released.

Electrical contact resistance and its relationship to hardness

Abstract: Studies have been made of the electrical resistance at the contact between a silicon carbide indenter and various metals. It has been found that reproducible results can be obtained if the contact under load is subjected to a low-energy condenser discharge prior to the measurement of the resistance. After this pre-test operation, the electrical resistance is inversely proportional to the length of the diagonal of the indentation, as observed under a microscope. This allows one to define a hardness value, from the contact resistance measurement, that correlates well with the Vickers hardness. It is shown that the technique can be adapted to the continuous study of hardness under load for viscoelastic non-conducting materials.

Effect of hardness on erosion of WC-Co composites

Abstract: Erosion behaviour of a range of WC-Co composites is investigated using 200 to 500μm Al2O3 grit normally incident at 140±40 m sec−1. A simple relation is obtained linking erosion rateW(g g−1) and hardnessH(M Pa), W = 1.44 × 1011H−3.5. Current erosion models based on indentation fracture mechanics are not found to apply; an explanation is suggested in terms of an indentation size effect.

Deformation and fracture of superhard materials in concentrated loading

Abstract: 1. The results show that the elastic stress-strain state of the material has a controlling effect on the processes of deformation and fracture of diamond in the penetration of the diamond pyramid. Analysis of the dimensions and form of the experimentally measured residual indentation and of the calculated elastic contact area shows that the resistance of the diamond to the penetration of the indentor is mainly elastic. The processes of plastic deformation leading to the formation of the residual indentation visible on the surface are of secondary importance. 2. The formation of the cracks around the pyramidal indentor is in qualitative (and satisfactory quantitative) agreement with the processes of formation of Hertzian cracks during penetration of the sphere. The existence of the critical load of formation of circular cracks shows the strong scale dependence of the fracture stresses of diamond which is identical with that described by Averbach's law in penetration of the sphere.

Microstructure Around Indentation of Chemical Vapor Deposition (CVD)-SiC Observed by Transmission Electron Microscopy

Abstract: The high temperature mechanical properties of chemical vapor deposition CVD-SiC are superior to those of hot pressed- and normally sintered-SiC. In order to have an insight into the origin of the superiority of CVD β type SiC, transmission electron microscopy (TEM) observation has been done around the areas indented with a Vickers diamond pyramid at room temperature and 1000°C. At room temperature, two types of microstructure were distinguished in the region around indentation. Around a hole of indentation, there was a plastically deformed region (a) containing a number of stacking faults formed by motion of Shockley partial dislocations on the close packed planes and micro-cracks orientated randomly from the hole. Surrounding this region, there exists a circular region (b) with extensive elastic lattice strain. In region (a) of indentation at 1000°C, stacking faults formed by dislocation movement were seen more clearly. But no micro-cracks were observed and macro-cracks occurred from two of the fonr corners of indentation. It is concluded that the formation and the movement of partial dislocations play an important role for strain relaxation in CVD-SiC indented at room temperature as well as at 1000°C.

Some features of indentation creep of LiF single crystals

Abstract: The dependence of the size of the indentation and dislocation rosettes on loading time was investigated on the (001) plane of LiF single crystals. The measurements were performed in temperature range from room temperature to 170° C. The indentation time was varied from 0.2 to 103 sec. It was revealed that the change of the indentation size during creep was more significant than the change in dislocation ensemble tracks in the field of the concentrated load. It was shown that the dependence of the length of the dislocation rosette edge arms on loading time, when plotting in double logarithmic scale, was linear. This fact allowed the determination of parameterm, characterizing the dependence of the dislocation velocity on stress, using creep experiments. The values ofm proved to be in good agreement with the results obtained by different methods.