Indentation Impression Research Articles

Real hardness of high-purity ultra-fine nano-polycrystalline diamond synthesized by direct conversion sintering under HPHT

Nano-polycrystalline diamond (NPD) with different grain sizes (10–1000 nm) was synthesized under ultrahigh-pressure and high-temperature conditions (16 GPa, 2200–2300 °C) starting from high-purity pyrolytic graphite having different crystallinity. The impurity content of these NPDs was 1 ppm or less, suggesting that the grain boundary of the structure has few impurities and the intergranular strength is high. These hardness characteristics were evaluated in detail by the indentation method using the super-hard (>140 GPa) Knoop indenter prepared from synthetic type IIa diamond. On ultra-fine NPD (UF-NPD) consisting of very small diamond grains of around 10 nm, it was found that the elastic recovery of the unloaded indentation impression is large and the length of the direction of the Knoop long diagonal also shrinks. Due to the shrinkage of indentation, the Knoop hardness of this UF-NPD erroneously yielded a high hardness value apparently exceeding 200 GPa. Detailed observations using high resolution FE-SEM around the unloaded Knoop indentations formed on the UF-NPD revealed line-like impressions (striations) along the outside length of the long diagonal as traces of contact with the Knoop indenter. Considering this striation length, the accurate hardness value of the UF-NPD was about 130 GPa. On the other hand the large elastic recovery of the indentation suggests that the UF-NPD has high critical tensile stress. Hertz fracture tests revealed that the crack initiation resistance (fracture strength) of NPD is improved by further grain refinement and high purification.

Fractal analysis of hillocks: A case of RF sputtered aluminum thin films

Abstract A monofractal and multifractal approach on quantification of hillocks on Al thin films deposited on glass substrates at a varying substrate temperature (Ts) has been reported in this work. The relationship between the fractal characteristics and mechanical properties of the hillocks are established. The lowest density and dimensions of hillocks were obtained at 95 °C while the highest at 65 °C. A power law relationship (with fractal dimension (D) as the power) is established between the perimeter and areas of the individual hillocks with a considerably large coefficient of regression (R2 ≈ 0.9) for all the temperatures. The fractal examinations of the segmented hillock structures revealed that these structures exhibit multifractal characteristics. The line profiles from optical profiling images of the hillock-dominant regions revealed non-uniform sinusoids which further confirm the fractal nature of these structures. The deformation mechanism of the hillocks under normal nanoindentation loading was described by evaluating the indentation impression against the nanoindentation curves.

Radial micro-cracks in pile-up region of single-crystal Cu in spherical indentation: experimental observation and crystal plastic simulation

Spherical indentations with deep penetration were conducted to single-crystal Cu along [001] direction. Zigzag slip bands induced by intersection of two groups of slip bands were found in the indentation pile-up region. On the material surface in the indentation pile-up region, micro-cracks were initiated along the slip bands radial to the indentation impression. The initiation of the micro-crack is affected by the indentation load and indenter radius. 3D crystal plasticity finite element simulation was applied to study the spherical indentation. Simulation results showed that the primary slip systems activated in indentation pile-up region were $$ (\bar{1}11)[0\bar{1}1] $$ , $$ (\bar{1}11)[101] $$ , $$ (1\bar{1}1)[10\bar{1}] $$ , and $$ (1\bar{1}1)[011] $$ , whose slip planes make intersection lines radial to indentation impression on the material surface. The secondary slip systems activated were $$ (111)[0\bar{1}1] $$ , $$ (111)[10\bar{1}] $$ , $$ (11\bar{1})[011] $$ , and $$ (11\bar{1})[101] $$ , whose slip planes make intersection lines tangent to indentation impression on the material surface. These two groups of slip bands, which both move outward the material surface, will intersect with each other and produce zigzag slip bands and jogs. The jogs would cause local stress concentration and thus provided the driving force to the initiation of micro-cracks. Meanwhile, simulation showed that the stress triaxiality in indentation pile-up region was around 0.6, which are closed to the stress state of plane strain tension and quite favorable to the initiation of cracks. Through simulations of spherical indentation along $$ [110] $$ and $$ [111] $$ , it illustrated that the initiation of cracks may be orientation dependent.

Influence of Temperature on Single Twinning Events in a Mg Single Crystal Subjected to Nanoindentation

Two distinct observations of load-controlled nanoindentation elastic-plastic transition (pop-in) on twinning favourite orientations of Mg have been reported: one predicted that pop-in was attributed to homogeneous nucleation while the other suggested the {10‾12} tensile twin nucleation and immediate growth assisted the permanent displacement burst. Here, a series of rigorous sample preparation methods aiming to obtain a smooth and dislocation-free Mg (10‾10) surface to visualise the indentation impressions clearly were performed to resolve this dispute. With substantial results from AFM images of indentation impressions just after the pop-in events, not only slip lines but twin was precisely identified from room temperature to 250℃. A previous developed model was used to measure the activation volume and energy barrier for the onset of pop-in. The extracted activation parameters are in the similar range for a heterogeneous nucleation mechanism from surface. Therefore, the pop-in events are believed to be consistent with a thermally activated mechanism with heterogeneous nucleation of first dislocation from surface and then triggers {10‾12} twining. The immediate twin growth after nucleation would contribute to the permanent displacement burst under the constant applied stress. In addition, the nucleation with a nanoscale activation volume can lead to temperature sensitivity of pop-in stress, while the increasing in pop-in depth could be rationalized by the dislocation-arbitrated relaxation of misfit back stress near twin boundary at elevated temperature described by a developed governing formwork.

Damage controlled by brittle particles crush in AA7075-T6 beneath spherical indenter

The distribution of damage controlled by brittle particles crush in AA7075-T6 beneath spherical indenter was obtained experimentally and predicted analytically. Micro hardness tests were conducted on the vertical sections across the indentation impression and by comparing it with the estimated ideal micro hardness, the damage beneath spherical indenter can be quantified. In the present study, for a spherical indenter with 794 μm radius and a 117 μm penetration, maximum damage is 0.29, locating in the middle of the contact arc. The damage along path 1, which is right below the indentation impression, has generally smaller damage and the path 3, which is below the middle of the contact arc, has more extensive damage. Through a model which can describe the damage controlled by second phase particles crush, a modified expanding cavity model (ECM) coupling with the Ramberg-Osgood stress-strain relation was applied to predict the distribution of damage in spherical indentation deformation. By comparing with the experimental results, it proved that the method could give a prediction with reasonable accuracy.

Formation mechanism of abnormally large grains in a polycrystalline nickel-based superalloy during heat treatment processing

Controlling the final grain size in a uniform manner in powder metallurgy nickel-based superalloys is important since a number of mechanical properties are closely related to it. However, it has been widely documented that powder metallurgy superalloys are prone to suffer from growth of abnormally large grains (ALGs) during supersolvus heat treatment, which is harmful to in-service mechanical performance. The underlying mechanisms behind the formation of ALGs are not yet fully understood. In this research, ALGs were intentionally created using spherical indentation applied to a polycrystalline nickel-based superalloy at room temperature, establishing a deformation gradient underneath the indentation impression, which was quantitatively determined using finite element modeling and synchrotron diffraction. Subsequent supersolvus heat treatment leads to the formation of ALGs in a narrow strain range, which also coincides with the contour of residual plastic strain in a range of about 2%–10%. The formation mechanisms can be attributed to: (1) nucleation sites available for recrystallization are limited, (2) gradient distribution of stored energy across grain boundary. The proposed mechanisms were validated by the phase-field simulation. This research provides a deeper insight in understanding the formation of ALGs in polycrystalline nickel-based superalloy components during heat treatment, when subsurface plastic deformation caused by (mis)handling occurs or small residual strain has been retained from hot/cold working before supersolvus heat treatment.

Hexagonal germanium formation at room temperature using controlled penetration depth nano-indentation

Thin Ge films directly grown on Si substrate using two-step low temperature growth technique are subjected to low load nano-indentation at room temperature. The nano-indentation is carried out using a Berkovich diamond tip (R ~ 20 nm). The residual impressions are studied using ex-situ Raman Micro-Spectroscopy, Atomic Force Microscopy combined system, and Transmission Electron Microscopy. The analysis of residual indentation impressions and displacement-load curves show evidence of deformation by phase transformation at room temperature under a critical pressure ranging from 4.9GPa–8.1GPa. Furthermore, the formation of additional Ge phases such as r8-Ge, hd-Ge, and amorphous Ge as a function of indentation depth have been realized. The inelastic deformation mechanism is found to depend critically on the indentation penetration depth. The non-uniform spatial distribution of the shear stress depends on the indentation depth and plays a crucial role in determining which phase is formed. Similarly, nano-indentation fracture response depends on indentation penetration depth. This opens the potential of tuning the contact response of Ge and other semiconductors thin films by varying indentation depth and indenter geometry. Furthermore, this observed effect can be reliably used to induce phase transformation in Ge-on-Si with technological interest as a narrow band gap material for mid-wavelength infrared detection.

The effect of modulated matrix microstructure on the deformation behavior in SiCf/Ti17 composites

Typical lamellar and equiaxed microstructure of matrix in SiC fiber reinforced Ti17 alloy composites (SiCf/Ti17 composites) can be obtained through matrix composition tuning. The discrepancy of microstructure characteristic results in lower hardness but superior deformation resistance (lower strain-rate sensitivity exponent) in lamellar structure than equiaxed structure. The further microstructure analysis on indent impressions indicates the micro-scale deformation mechanism of lamellar microstructure is principally dominated by dislocation movement in each layer along phase interfaces, rather than in the normal direction due to existence of the large amount of phase interfaces. In contrast, higher hardness comes from dislocation pinning in uniform-distribution grain boundaries in equiaxed microstructure, while quasi-isotropy character makes it more strain-rate sensitivity than lamellar structure.

Failure of DLC Films Caused by Cyclic Sphere Indentation and Effects of Film Structures on Failure Mode

As a new strength evaluation method for hard coatings such as diamond-like carbon (DLC), the cyclic indentation test was proposed. Then, a test rig was fabricated and used for the evaluation of DLC. DLC and a Cr/C inclined interlayer for the improvement of the adhesion strength of DLC were prepared by the unbalanced magnetron (UBM) sputtering method on heat-treated and polished JIS SCM415 substrates. Si3N4 spheres having diameters of 10 mm were used for the cyclic indentation test. 10000 cycle indentations brought about DLC fracture and detachment in the indentation impression. Also ring cracks were observed around the impression. I(D)/I(G), which is the ratio of the D to G peak intensities, decreases towards the center of the impression. It was considered to indicate decreasing sp2 cluster size in DLC. On the other hand, compressive stress increased near the surface of DLC in the contact area, as indicated by the change in its G peak position. It seemed to be the cause of the fracture and detachment of DLC in the impression.

Effects of Cr doping on the mechanical properties of AlN films grown by the co-sputtering technique

We report the mechanical properties of Cr-doped AlN thin films synthesized by reactive magnetron co-sputtering technique with different Cr concentrations (0, 2, 4, 6 and 11 at%). Surface chemical analysis and crystal structure studies of Cr doped AlN films are carried out using X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXRD) techniques. All these films are crystallized with wurtzite structure and grown predominantly along a-axis orientation in the absence of secondary phases belonging to Cr. The surface morphology and RMS roughness are measured by atomic force microscope (AFM). The residual stress of these films exhibit a tensile behavior and it decreases with Cr concentration as calculated by sin2ψ technique. Indentation hardness of these films is measured by nanoindentation technique, which is ranged from 17.5 to 23.0 GPa. However, indentation modulus is enhanced with Cr concentration due to strong p-d hybridization between Cr and N atoms. Indentation impression of these films exhibit a sink-in behavior and gives an evidence of high elastic recovery without radial cracks.

In-situ Indentation and Correlated Precession Electron Diffraction Analysis of a Polycrystalline Cu Thin Film

In-situ TEM nanoindentation of a polycrystalline Cu film was cross-correlated with precession electron diffraction (PED) to quantify the microstructural evolution. The use of PED is shown to clearly reveal features, such as grain size, that are easily masked by diffraction contrast created by the deformation. Using PED, the accompanying grain refinement and change in texture as well as the preservation of specific grain boundary structures, including a ∑3 boundary, under the indent impression were quantified. The nucleation of dislocations, evident in low-angle grain boundary formations, was also observed under the indent. PED quantification of texture gradients created by the indentation process linked well to bend contours observed in the bright-field images. Finally, PED enabled generating a local orientation spread map that gave an approximate estimation of the spatial distribution of strain created by the indentation impression.

A quantitative connection between shear band mediated plasticity and fracture initiation toughness of metallic glasses

While it is well recognized, albeit qualitatively, that shear band mediated plasticity ahead of crack or notch tips is the raison d'être for the high fracture toughness of 'ductile' bulk metallic glasses (BMGs), quantitative connection between those two material properties is yet to be established. In an attempt to study this, we examine if mode I fracture initiation toughness, KIc, of a number of BMGs can be related to the shear band number, Ni, which is a discretized measure of plasticity in MGs, around spherical indentation impressions that are made to a fracture mechanism based predetermined indentation strain. Results show that KIc scales with (Ni)3/2. Then, the relation between the shear band density in the notch tip plastic zone, Nn, and KIc is examined, which shows that a power law: KIc∝(Nn)1/2, captures the data reported in literature for a number of BMGs. This result confirms that it is indeed the notch tip plasticity that determines KIc of BMGs. The power law exponent of 0.5 is rationalized by recourse to elasto-plastic fracture mechanics. Possible connections between Ni and Nn, ways of enhancing the latter so as to increase KIc, and the central role played by the relative density of MGs in determining both elastic, plastic, and fracture responses are discussed.

Anisotropy of the Mechanical Properties of TbF3 Crystals

TbF3 (sp. gr. Pnma) crystals up to 40 mm in diameter have been grown from melt by a Bridgman technique. The anisotropy of their mechanical properties is studied for the first time. the technical elasticity constants are calculated, and room-temperature values of Vickers microhardness for the (010) and (100) planes are measured. The shape of indentation impressions is found to correlate with Young’s modulus anisotropy for TbF3 crystals.

Preparation of Mullite–TiC–TiN Materials by a Plasma Spark Method and Their Properties

The phase composition of synthesized TiC and TiN, microstructure, relative density, open porosity, linear shrinkage, elasticity modulus, Vickers hardness, ultimate strength in compression, and linear dependence of elasticity modulus and ultimate strength in compression of mullite–TiC–TiN-specimens with a different ratio of TiC and TiN sintered by a plasma-arc method in the range 1200 – 1600°C with a compaction load of 30 MPa, are studied. Synthesized powders have intense crystallization of TiC and TiN. Sintered specimens with a different ratio of TiC and TiN demonstrate intense mullitization in the range 1200 – 1600°C. Specimens with a TiC/TiN ratio of 50/50, 70/30, and 90/10 mol.% demonstrate a gradual increase in TiC from 1200 – 1400°C compared with a more intense increase in TiC from 1400 to 1600°C. An increase in TiC concentration and a reduction in TiN content in powder mixtures sintered at 1500°C facilitates formation of a uniform density sintered microstructure with presence of small pores. Specimens in the ratios TiC/TiN of 50/50 and 70/30 mol.% point to a more intense increase in relative density and linear shrinkage; a reduction in open porosity and an increase in elasticity modulus, Vickers hardness, and ultimate strength in compression. This leads to an increase in specimen crack formation resistance in the presence of microcracks with a relatively rectilinear propagation trajectory (composition M50TiC50TiN), and absence of microcracks (composition M70TiC30TiN) around an indenter impression, with the greatest linear dependence of elasticity modulus and ultimate strength in compression in the range 1200 – 1600°C.

Quantitative estimates of the content of the metastable Si-XII, Si-III, and α-Si phases of silicon in the areas of indenter impressions

A quantitative estimate is presented of the specific and total volumes of the metastable Si-XII, Si-III, and α-Si phases of silicon in a locally strained (Berkovich pyramid) area. Calculations are performed using experimental data obtained via Raman spectroscopy and in situ registration of the Si-I → Si-II phase transitions of silicon under the indenter.

High temperature indentation tests of YSZ coatings in air up to 1200 °C

In this paper, high temperature indentation tests in air up to 1200°C were firstly conducted to measure hardness and elastic modulus of nanostructured 8wt% yttria partially stabilized zirconia (YSZ) coatings. An in-house constructed elevated temperature indentation apparatus was developed. The load-displacement curves of YSZ coatings testing at 25, 1000 and 1200°C were obtained. Hardness and elastic modulus were evaluated from the load-displacement curves. It is found that the value of hardness and elastic modulus both decrease with increasing temperature. This may be attributed to the enhanced plasticity of YSZ coatings at high temperatures. The microphotographs of residual indentation impressions also show that plastic deformation tends to occur with increasing temperature.

Cold nanoindentation of germanium

Diamond cubic Ge is subjected to high pressures via nanoindentation at temperatures between −45 °C and 20 °C. The residual impressions are studied using ex-situ Raman microspectroscopy and cross-sectional transmission electron microscopy. The deformation mechanism at 20 °C is predominately via the generation of crystalline defects. However, when the temperature is lowered, the analysis of residual indentation impressions provides evidence for deformation by phase transformation and formation of additional phases such as r8-Ge, hd-Ge, and amorphous Ge. Furthermore, these results show that at 0 °C and below, dc-Ge will reliably phase transform via nanoindentation.

Indentation and erosion behavior of electroless Ni-P coating on pipeline steel

Electroless Ni-P coating has a potential use in oil and gas pipeline industry due to its good corrosion and wear resistance. However, the erosion and indentation behavior of the coating have not been fully explored yet. The objective of this study is to investigate the suitability of utilizing electroless Ni-P coating in the oil and gas industry by characterizing its mechanical damage under erosive and denting conditions. In this study, electroless Ni-P coating with different thicknesses were deposited on API X100 pipeline steel and characterized using XRD, SEM and EDS. Structural characterization indicates that as-deposited Ni-P coating has an amorphous structure with 9-11wt% phosphorus content. Hertzian-type indentation and solid particle erosion tests were performed in order to investigate the extent of the mechanical damage in the coatings. Optical microscopy and SEM observations of the indentation impressions show the presence of Hertzian, radial and bend cracks. It is found that the indentation behaviour of electroless Ni-P coating is significantly influenced by coating thickness. Eroded surfaces were examined and operative erosion mechanisms were identified. It was found that the erosion rate of electroless Ni-P coating is higher than the substrate.The suitability of utilizing electroless Ni-P coatings in the oil industry was assessed.

Fracture behaviour of nc-TiAlN/a-Si3N4 nanocomposite coating during nanoimpact test

Cyclic nanoimpact tests were carried on nc-TiAlN/a-Si3N4 nanocomposite, TiN and multilayered TiN/nanocomposite (NC) coatings to evaluate their resistance to fracture under cyclic impact loads. Fracture behaviour of the coatings was ascertained from fracture probability obtained from time-depth curves and focus ion beam milling images of resulting indentation impressions. TiN coating mainly showed intercolumnar cracks while the other coatings showed other modes of cracking, that is, lateral, inclined, bending, edge cracks, during testing. The performance ranking of the coatings, TiN > TiN/NC > nc-TiAlN/a-Si3N4, is linked to their β0 value, representing relative indentation depth of the coating-substrate composite hardness system at which the fractional hardness improvement equal to 50% of the maximum is retained and also their corresponding microstructure. Apart from enabling prediction of fracture resistance of the coatings, these studies provide useful insights into design and selection of coating materials for targeted machining applications.

Preparation of Mullite–TiC–ZrC-Ceramic Materials by a Plasma-ARC Method and Their Properties

The phase composition for synthesized titanium carbide TiC and zirconium carbide ZrC powders is provided, and development of the crystal phases, microstructure, relative density, open porosity, true shrinkage, hardness, strength in compression of mullite–TiC–ZrC-specimens with a different ratio of TiC and ZrC sintered by a plasma-arc method in the range 1200 – 1500°C is demonstrated. Synthesized TiC and ZrC powders have intense crystallization of TiC and ZrC phases, and sintered specimens with a different ratio of TiC and ZrC have different mullitization in the range 1200 – 1500°C and degree of TiC and ZrC phase development at up to 1500°C. Above 1500°C there is development of TiC or ZrC phases in relation to the ratio of TiC and ZrC within sintered materials. Specimens with a higher TiC concentration have a denser sintered microstructure compared with specimens containing a greater amount of ZrC. This facilitates an increase in relative density, linear shrinkage and a reduction an open porosity, formation of harder specimens with uniform indentation impression without damage formation around it, and with the best strength indices in compression.