Vickers Pyramid Research Articles

Wear Evaluation of Pack Boronized AISI 1060 Steel

Abstract Boronizing is used to obtain very high hardness and wear resistance on ferrous and non-ferrous materials and some super alloys. The process can be carried out in solid, liquid, or gaseous medium. In this work, samples of AISI 1060 steel were treated by the pack boriding method during 2 and 4 h at 900 and 1000°C. Optical microscopy, x-ray diffraction, and micro-hardness analyses were conducted and wear tests were performed using a micro-wear machine with a fixed-ball configuration. The pack boriding resulted in the formation of layers with high hardness and wear resistance. All the borided layers presented top hardness around 2000 HV (Vickers pyramid number). Raising time and temperature of treatment increased the layer thickness. X-ray diffraction patterns showed the presence of only Fe2B for samples treated at 900°C for 2 and 4 h. The samples borided at 1000°C for 2 and 4 h presented layers composed by a mixture of Fe2B and FeB. The wear resistance of the samples borided at 1000°C was higher than that borided at 900°C, probably because of the difference in chemical composition.

Evaluation of Mechanical Properties of Medium Carbon Steel Quenched in Water and Oil

Samples of medium carbon steel were examined after heating between 900℃ - 980℃ and soaked for 45 minutes in a muffle furnace before quenching in palm oil and water separately. The mechanical behavior of the samples was investigated using universal tensile testing machine for tensile test and Vickers pyramid method for hardness testing. The microstructure of the quenched samples was studied using optical microscope. The tensile strength and hardness values of the quenched samples were relatively higher than those of the ascast samples, suggesting improved mechanical properties. However, samples quenched in palm oil displayed better properties compared with that of water-quenched samples. This behavior was traced to the fact that the carbon particles in palm oil quenched samples were more uniform and evenly distributed, indicating the formation of more pearlite structure, than those quenched in water and the as-received samples.

On the possibility of detecting near-surface technological stresses in ceramics by photoacoustic microscopy

The influence of near-surface technological stresses on the photoacoustic images of Vickers pyramid indentations in silicon nitride ceramics is studied experimentally. It is found that grinding- and polishing-induced stresses in the ceramics influence the behavior of photoacoustic signals coming from the ends of surface cracks. Photoacoustic microscopy data obtained from Vickers-indented areas near the ends of cracks make it possible to estimate technological stresses.

Preparation of Fe and Co based amorphous wires by in-water quenching process

The investigation addresses the effect of in-water quenching process parameters on the magnetic properties of Fe and Co based amorphous wires. The in-water quenching process parameters such as water drum speed, crucible nozzle orifice diameter, crucible nozzle to water level separation and gas pressure for melt ejection were found to be critically important to obtain continuous and ductile wires. The thinner wires with a diameter of ∼124 μm revealed superior soft magnetic properties with coercivities as low as ∼5˙63 A m–1 (70˙8 mOe) which was attributed to their high amorphous nature and consequent isotropy. This was also manifested from their surface morphology as observed by scanning electron microscopy. The Fe77˙5Si7˙5B15 and Co72˙5Si12˙5B15 (at.-%) wires in their as prepared state revealed high fracture strength, above 3000 MPa, and high microhardness ∼1200 HV (Vickers pyramid number).

The effect of small flaws on the fatigue strength of HAZ at the weld toe

The effects of small artificial surface flaws on the fatigue strength of HAZ material are studied. The samples of coarse grain HAZ material were prepared using a welding thermal-cycle simulator. The artificial flaws were produced by indenting with a Vickers pyramid at different loads as either single indentations or as a series. The size of the flaws did not exceed the primary austenitic grain size of the material, which is the most relevant microstructural unit of carbon steels. The dependence of the experimentally determined bending fatigue strength of treated coarse grain HAZ materials on the properly evaluated size of the artificial flaws was compared with the law of long-crack propagation. The biggest flaws evaluated with the flaw-size parameter √area ≅ 100 m are still small for the studied coarse grain HAZ. In further experimental work, the effects of flaw size and the effects of residual stresses should be analysed separately.

IMPROVEMENT OF HARDNESS AND WEAR RESISTANCE OF T1- 6AL- 4V ALLOY BY THERMAL OXIDATION FOR BIOMEDICAL APPLICATION

Titanium and its alloys provide optimum metallurgical properties for implants. The formation of an oxide layer favours compatibility with the adjacent hard and soft tissues. This study is concerned with the improvement of hardness and wear resistance of Ti6Al-4V surface by thermal oxidation. In this study, comparative investigation of thermal oxidation treatment for Ti–6Al–4V was carried out to appropriate the oxidation conditions for further evaluation of wear performance. Hardness examination conducted under four different indentation loads of 200 gf to 1000 gf with a Vickers pyramid indenter revealed that surface hardness increased from 450 for untreated samples to 1300 Hv upon oxidation at 900oC for 45 h, which was accompanied by significant improvement in wear resistance. Characterization of modified surface layers was made by means of microscopic and X-ray diffraction analysis. This oxidation condition achieved 25 times higher wear resistance than the untreated alloy during reciprocating wear test conducted in a dry sliding.

Failure of brittle layers on polymeric substrates from Vickers indentation

A study is made of median crack evolution in brittle coatings subjected to sharp contacts. A model bilayer system consisting of a glass plate bonded to a polycarbonate base, with a Vickers pyramid as indenter, is used to demonstrate the evolution in situ. The cracks undergo a phase of stable downward growth with increasing load. Beyond the coating mid-plane, flexural stresses (coupled with the local contact field) elongate and accelerate the median cracks through the plate to failure. Critical loads corresponding to this failure state diminish with decreasing layer thickness until, at small thicknesses, failure occurs spontaneously at initiation.

Performance of polymer blends on phosphated steel substrate

Polymer blends are used to protect mild steel structures from corrosive environment. Multi-coat protective coating systems are substituted by single coat polymer blend, which will protect the surface equally for long duration when compared to the multi-coat system. In our studies, we have prepared three polymer blends, viz. silicone–acrylic, silicone–titanate and epoxy–acrylic blends. These polymer blends are applied over sandblasted as well as phosphated mild steel panels by brush. The mechanical properties like hardness and adhesion of these blends are measured and found that nearly 2 Vickers Pyramid Number (VPN) has been increased for phosphate surface. The accelerated salt spray chamber exposure also confirms that the phosphate gives additional protection to the steel substrate. The electrochemical impedance spectroscopy measurements are carried out periodically for all the three blends for 7 days. The impedance observation of acrylic silicone and silicone titanate blends on phosphate surface gives resistance values above 10 6 Ω cm 2 after 7 days, whereas the same blends on sand blasted surface exerted resistance values below 10 6 Ω cm 2 after the same duration, the epoxy acrylic blends are found to give resistance values above 10 9 Ω cm 2 irrespective of the surface modification. Thus, this study concludes that the polymer blends of organic to organic binders protect the phosphate steel surface for longer duration than that of the blend composed of organic and inorganic system.

Microhardness and Plastic Deformation of Glasses upon Microindentation

The mechanics of plastic deformation of silicate and polymer glasses upon microindentation of a Vickers pyramid is analyzed. The microhardness characterizes the shear strength of a glass and is determined by the maximum shear stress. For silicate glasses, the microhardness virtually coincides with the yield point (plastic limit), above which the plastic deformation is observed.

Surface modification of a Ti–6Al–4V alloy by thermal oxidation

In this study, the effect of thermal oxidation on the dry sliding wear resistance of a Ti–6Al–4V alloy has been examined. Oxidation has introduced hard surface layers composed of TiO 2 and oxygen diffusion zone beneath it. Hardness survey conducted under a load of 10 g with a Vickers pyramid indenter revealed that surface hardness increased from 450 to 1300 HV 0,01 upon oxidation at 600 °C for 60 h, which was accompanied by significant improvement in wear resistance. Thus, the dry sliding wear rate of thermally oxidised Ti–6Al–4V alloy was almost negligible when compared to the as-received condition.

Viscosity of glass at high contact pressure during indentation experiments

The mechanism of plastic deformation of glasses by indentation near room temperature has not been understood completely up to now. For the analysis of indentation experiments with a Vickers pyramid and spherical indenter, respectively, viscoelastic deformation behaviour is assumed for the complete temperature range from room temperature to the transformation temperature. Therefore, the rheological analysis of all experiments was performed with the help of a simple Maxwell model. The results of indentation experiments were compared with cylinder compression experiments, which were analysed by the same rheological principles. Additionally, some indents were analysed by topographical investigation to get the first hints of viscoelastic deformation behaviour below the transformation temperature of glass.

Instrumented pyramidal and spherical indentation of polycrystalline graphite

The elastoplastic surface deformation of a polycrystalline graphite was studied by examining the indenter’s geometry dependence of load P versus penetration depth h relation (P–h relation) in instrumented pyramidal/spherical indentation tests. The tetrahedral pyramid indenters included inclined face angles β of 10.0°, 22.0° (Vickers pyramid), and 40.0°. The tip radius of spherical indenters used were 32 μm, 200 μm, 794 μm, 1.59 mm, and 6.35 mm. The true hardness H as a measure for plasticity was singled out of the elastoplastic loading parameter k1 in the quadratic expression of P = k1h2 and then quantitatively related to the yield stress Y that was determined from the mean contact pressure for spherical indentation at the onset of plastic yielding. The size effect of Y, decreasing with the increase in the tip radius of spherical indenter, is discussed using the model of geometrically necessary dislocations in terms of the material length scales for a plastic field with strain gradient.

Indentation‐Induced Amorphization in Mullite Single Crystals

Hardness indentations with a Vickers pyramid under a load as low as 0.1 N applied at room temperature induce amorphization in single‐crystal mullite when applied to the (001) crystallographic face. Different zones of damage were identified via TEM. Directly under the indenter, in the region of high‐compressive stress, mullite becomes amorphous. Further out toward the matrix material, there exists a region of high plastic deformation in the form of dislocation networks, radial microcracks, and bend contours. The Vickers‐induced damage is comparable to that produced by dynamic shock or intense ball milling of mullite.

Residual stress estimation of ceramic thin films by X-ray diffraction and indentation techniques

The residual stresses in ceramic thin films obtained by the indentation method have been found to be three times higher than those of the X-ray diffraction method. This discrepancy can be eliminated by setting the geometrical factor for the Vickers pyramid indenter to 1 in the relevant equation of the indentation method.

Elastoplastic load–depth hysteresis in pyramidal indentation

Extensive indentation tests were conducted for nineteen different engineering materials ranging from brittle to ductile materials, and including hard ceramics, ductile metals, and a soft organic polymer. Three tetrahedral pyramid indenters with specific face angles β [shallow pyramid (β = 10°), Vickers (β = 22°), and sharp pyramid (β = 40°) indenters] were used. All the materials tested were subjected to the quadratic load P and penetration depth h relationship P = k1h2 on loading, and most of the tested materials to the quadratic unloading relationship of P = k2(h − hr)2 with the residual depth hr after a complete unload. To determine the contact area at peak indentation load, a specially designed depth-sensing instrument was constructed, on which the contact behavior during loading/unloading was examined by through thickness observation of transparent specimens. All the characteristic indentation parameters were investigated on the basis of simple elastoplastic model, and correlated well with the nondimensional strain E′ tan β/H, in which the elastic modulus E′ was a measure for elasticity, true hardness H was a measure for plasticity, and the inclined face angle β characterized the indenter. The ratio of the conventional Meyer hardness HM to the true hardness H of the materials tested ranged from 0.2 to 0.9 as a function of E′ tan β/H. The cavity model suggested that true hardness H is expressed by the yield stress Y through a constraint factor C as H = C · Y with C ≈ 5.

Subsurface deformations induced by a Vickers indenter in GaAs/AlGaAs superlattice

During the past century, indentation mechanics has been extensively explored. This effort led to the development of instrumented microand nanoindentation techniques that have proved to be of first importance in materials science research. As reviewed recently by Tabor [1], indentation mechanics straightforwardly used today was developed mainly in view of experimental results found on metals [2–4]. Therefore, the use of indentation theory for other materials (for instance ceramics) assumes that plastic-flow characteristics are not much affected. Furthermore, Chaudhri showed using pearlite grains as markers that the measured plastic strains in heavily work-hardened mild steel deformed by indenters of various geometries were much higher than previously thought [5]. In a more recent work, the same author used the hardness-strain relation to map the strain field in high-conductivity copper deformed by spherical indenters [6]. He showed then that the strain-contour geometry was more complex than what had been observed previously in other metals [2]. Determination of the strain field under a Vickers indenter in semiconductors was studied recently in a GaAs/AlAs superlattice where interfaces were used as markers [7]. However, severe delamination occurred at the interfaces. In this paper, we have deformed a GaAs/AlGaAs superlattice by a Vickers indenter and have used focused-ion beam (FIB) technique to prepare cross-sectional thin foils through the center of the indent site for transmission electron microscopy (TEM) observation. These techniques let us get information on the plastic and brittle deformations underneath the indent site and allow us to determine the strain-field map. Superlattices were grown on (001) surfaces of GaAs single crystals by metal-organic vapor-phase epitaxy at 650 ◦C. GaAs and AlGaAs layers (30 of each) were grown alternatively to form a structure about 7.6 μm thick (period 254 nm). The composition of the Alx Ga1−x As (x = 0.85) alloy was chosen so that the misfit with GaAs was only e = 0.13% and the structure was almost lattice matched. The samples were deformed by a Vickers diamond pyramid at room temperature under 500 mN for 30 s. They were set in the indenting machine in such a way that the diagonals of the indentations were along a 〈110〉 direction. The central zone of the indent site was protected from the gallium FIB by a tungsten thin film. Two trenches were milled with a 30 kV FIB in such a way that a thin wall was left containing the selected zone that was transparent to the electron beam as described elsewhere [8]. The periodical structure of the superlattice can be observed in Fig. 1. AlGaAs layers appear in clear contrast while GaAs layers appear in dark contrast. At the bottom of the superlattice, the structure was started by an AlGaAs layer and a GaAs layer about 510 and 235 nm thick respectively. It should be noted that during the milling two layers on top of the structure on the left-hand side of the indent were lost despite the tungsten protection. The indent site is situated on the top of the median crack (observed perpendicularly to the indented surface). Curvature of the surface at the indent site and of layers underneath is obvious and is the result of the plastic deformation generated by the indenter. The curvature of the layers was observed to decrease with depth. For instance, it is about 158◦ for the third GaAs layer and about 176◦ for the 41st layer below. Interestingly the value found in the vicinity of the indent site (158◦) differs from the Vickers pyramid angle (148◦) because of the elastic recovery of the sample while the indenter was unloaded [8]. Using a higher-magnification image just beneath the indent site (Fig. 2), converging slip bands could be observed. They are inclined to the indented surface by about 54◦ and correspond to the gliding of dislocations in the {111} slip planes. On a higher contrast image (Fig. 3) slip bands in {111} planes diverging from the indent site could also be observed. Such a plastic-flow geometry is now well established for bulk semiconductors and was shown to operate here [8–11]. The slip bands correspond to the gliding of dislocations with Burgers vectors inclined to the indented surface that generates deformation of the layers on their path [8–10]. This finally results in the curvature of the superlattice that was described above. At the intersection of the converging slip bands (underneath the indent site and along the load axis direction), we observed a median crack that crossed the layers. This crack was propagated straight into the GaAs substrate on 4 μm. A median-radial crack system is commonly observed in brittle material under sharp contact [12]. Here the crack was initiated at

Spherical indentation of elastic–plastic solids

The finite–element method is used to perform an accurate numerical study of the normal indentation of an elastic–plastic half–space by a rigid sphere. The effects of elasticity and strain–hardening rate of the half–space are explored, and the role of friction is assessed by analysing the limiting cases of frictionless contact and sticking friction. Indentation maps are constructed with axes of contact radius a (normalized by the indenter radius R and the yield strain of the half–space. Competing regimes of deformation mode are determined and are plotted on the indentation map: (i) elastic Hertzian contact; (ii) elastic–plastic deformation; (iii) plastic similarity regime; (iv) finite–deformation elastic contact; and (v) finite–deformation plastic contact. The locations of the boundaries between deformation regimes change only slightly with the degree of strain–hardening rate and of interfacial friction. It is found that the domain of validity of the rigid–strain–hardening similarity solution is rather restricted: it is relevant only for solids with a yield strain of less than 2 x 10 −4 and a / R

Shape Evaluation of HV-Indents by AFM

Vickers-type hardness patterns were investigated by atomic force microscopy. The resulting bitmap images were analyzed to evaluate the elastic relaxation of the impression. A procedure, which is suitable for the comparison of the shape of the residual hardness impression and that of the Vickers pyramid, is presented. With the help of difference images can be verified that the strongest relaxation takes place at the tip and at the horizontal edges of the Vickers pyramid.

Mechanical Properties of Gallium Nitride and Related Materials

ABSTRACTBasic mechanical properties, such as hardness and fracture toughness are examined for GaN and related materials. The mechanical properties are explored by indentation with a diamond (Vickers) pyramid under less than 20N load. Testing on bulk single crystal GaN indicated hardness and fracture toughness similar to GaP, but with much greater values than GaAs. Data for compound semiconductors are compared with values of a number of substrate materials. The following materials are examined in bulk form: AlN, Al2O3 (sapphire), Al2O3-TiC (ALTIC), GaAs, GaN, GaP, Ge, Si, SiC, ZnS, and ZnSe.

Elastic properties determination from indentation tests

The knowledge of mechanical properties, and elastic modulus in particular, is necessary for a good understanding of the behaviour of materials subjected to static or dynamic loading. Accurate determination of elastic modulus is possible in the case of massive and homogeneous materials. For coatings the problem is more complicated but this determination is rendered possible using load and displacement sensing indentation tests or ultrasonic wave tests, for example. The purpose of this study is to determine elastic modulus of materials and specially that of coatings by analysing the morphology of indents obtained by the Vickers hardness test. During the withdrawal of the indenter, distortions of the faces of the indent are directly linked to the elastic properties of the material. Comparison between the true volume of the deformed indent and the ideal volume of the Vickers pyramid allows one, starting with Bulychev's relation, to achieve good estimations of the elastic modulus of massive materials as well as that of coatings. The model is first tested on massive material of known elastic properties (middle carbon steel, ferritic stainless steel and aluminium alloy), then it is applied to a chromium carbide coating obtained by hypersonic thermal spraying. Calculated elastic moduli are in accordance with literature data, except for the aluminium alloy, for which plastic flow around the indent is different from the other materials.