Depth-sensing Indentation Tests Research Articles

Synergistic impact of corrosion pitting on the rotating bending fatigue of additively manufactured 316L stainless steel: Integrated experimental and modeling analyses

In this study, the ex-situ rotating bending corrosion fatigue behavior of laser powder bed fused stainless steel (SS) 316L has been studied incorporating experimental analyses and modeling. The findings revealed significant variations (almost 20 %) in fatigue lifetime in pre-corroded specimens relative to non-corroded (virgin) specimens because of corrosion-derived pitting phenomena on the surface. To this end, the fatigue strength of virgin (non-corroded) and pre-corroded SS 316L were recorded at 242 MPa and 203 MPa, respectively. Upon conducting the fatigue tests and extracting stress-life trends, advanced microstructural characterization and postmortem analyses were used to quantify fatigue crack behavior and fracture surface. An empirical model is also developed employing depth-sensing indentation testing, linear elastic fracture mechanics, and computer tomography scans to propose an optimum predictive trend. The analysis comparing existing models with our proposed model demonstrates reasonable agreement between experimental findings and the predicted life for the studied laser powder bed fused SS 316L material exposed to corrosion rotating bending fatigue test. The findings of this research carry important contributions to the structural integrity and longevity of marine engineering, ship construction, naval operations, and naval aviation where materials and components are exposed to cyclic loading and corrosion.

Erosion behaviour of cold sprayed stainless-steel coatings for civil infrastructures: An energetic approach

Along with corrosion, erosion is one of the main causes of damage to civil infrastructures. The continuous impact of fine particles carried by the air produces a deterioration in the elements of these structures. When this damage is significant, these elements are replaced by new ones. In this work, the possibility of extending the useful life of these components by spraying coatings on the damaged areas is proposed. In this sense, repair techniques such as cold spray deposition are possibilities for this purpose. Therefore, this study focuses on evaluating the erosion behaviour of stainless-steel coatings deposited on structural steel. In addition, the erosion results have been correlated with the model proposed by Hutchings, and an energetic interpretation of the erosion process through this model has been proposed. To carry out this study, stainless steel coatings have been deposited on carbon steel substrates to improve their resistance to corrosion. Different spraying temperatures, from 800 °C to 1100 °C, combined with different pressures of the carrier gas, from 40 to 70 bar, have been analysed. The elastic moduli of the coatings, and their stress-strain curve, have been determined by depth sensing indentation tests with Berkovich and spherical tips, respectively. Erosion tests with normal incidence have been programmed to obtain the corresponding erosion rates. From the results of this work, the optimal projection conditions of stainless steel on carbon steel substrates for the repair of civil infrastructures subjected to erosion processes could be identified.

Fatigue response of a cast Al-Cu-Mg-Ag-TiB2 (A205) alloy: Stress relieved and aged

The purpose of this study is to assess the influence of heat treatment on the fatigue behavior of cast A205 alloy (i.e., Al-Cu-Mg-Ag-TiB2 nanocomposite). To do so, A205 samples were heat-treated and divided into two categories (i) stress-relieved (SR) and (ii) T7 stabilizing over-aged. This research delves further into the microstructure, mechanical properties, and fatigue performance of A205 alloy in SR and T7 heat-treated (i.e., T7 aged) conditions. After production and post-heat treatments, the samples underwent force-controlled fatigue experiments, tensile and depth-sensing indentation testing, and advanced microstructural characterizations (using electron backscattered diffraction and transmission electron microscopy). Microstructural characterization revealed exceptionally equiaxed, texture-free microstructure with uniform grain distribution and the existence of Al2Cu precipitates and TiB2 particles. The T7 heat treatment did not change grain size significantly (grain size: 46.4 ± 13.5 µm for SR and 48.5 ± 13.7 µm for T7 material). The T7 heat treatment was found to be very effective in improving the fatigue resistance of A205. The higher fatigue resistance of cast T7 material is attributed to the accelerated precipitation of Al2Cu along with different habit planes. The results of this study offer a critical understanding for choosing the optimum heat treatment parameters to control the fatigue behavior of cast A205, which is crucial considering their applications in the aerospace industry.

Full Regional Creep Displacement Map of Articular Cartilage Based on Nanoindentation Array.

The elucidation of the mechanisms underlying articular cartilage lesions poses a formidable challenge in the field of cartilage repair. Despite significant strides in cartilage mechanics research, the region-dependent creep properties of articular cartilage remain elusive. In this study, we employ depth-sensing indentation tests to experimentally determine the creep properties of four distinct regions of articular cartilage, thereby unveiling a region-dependent full map of creep parameters. The measured creep displacement-time response curves indicate that the creep properties of the articular cartilage exhibit a clear regional correlation. Accordingly, the full regional creep map of articular cartilage is experimentally constructed for the first time. The correlation between the microstructures and the creep properties of cartilage in different regions is revealed. A three-parameter model is established to describe the creep velocity-displacement response of cartilage. Raman spectra reveal that the proteoglycan content is positively correlated with creep properties. The Raman shift directly indicates diverse residual stresses in different microregions. The obtained data facilitate a deep understanding of the potential creep dependent damage mechanism of cartilage and the further development of artificial cartilage materials.

The effect of heat treatment on fatigue response of laser powder bed fused Al-Cu-Mg-Ag-TiB2 (A20X) alloy

The paper aims at assessing the effect of heat treatment on the fatigue behavior of a novel laser-powder bed fused (L-PBF) Al-Cu-Mg-Ag-TiB2 composite, otherwise known as A20X alloy. Heat treatments, (i) stress relieving, and (ii) T7 over-aging and stabilizing, were performed on L-PBF A20X materials, followed by advanced microstructural characterizations, mechanical property measurements (utilizing tensile and depth-sensing indentation testing), and force-controlled fatigue testing. Microstructural assessments revealed the ultra-fine and fine equiaxed grain structure of the L-PBF stress-relieved and aged A20X materials. Mechanical properties (yield stress, ultimate strength, and nano-hardness quantities) of the aged material were recorded as higher than the stress-relieved counter materials. By the same token, the aged material showed better low-cycle and high-cycle fatigue performance and higher fatigue strength relative to the stress-relieved conditions. Based on detailed fractography assessments, the life-limiting fatigue failures of the studied materials (stress relived and T7 aged) were primarily governed by sub-surface L-PBF-induced volumetric defects such as pores and/or lack of fusion. The findings of this study present useful information on the role/mechanism of inevitable process-induced interior defects on fatigue behavior of the Al-Cu-Mg-Ag-TiB2 composites and the effect of post-processing (i.e., aging in this research) on fatigue behavior, which is of great significance for the high-demanding and critical applications of lightweight high-strength L-PBF Al-Cu-Mg-Ag-TiB2 composites in the space and aviation sectors.

Accelerated versus Slow In Vitro Aging Methods and Their Impact on Universal Chromatic, Urethane-Based Composites.

Structural coloring of dental resin-based composites (RBC) is used to create universal chromatic materials designed to meet any aesthetic need, replacing the mixing and matching of multiple shades. The microstructural adjustments to create this desideratum involve nanoscale organic-inorganic core-shell structures with a particular arrangement. The generally higher polymer content associated with these structures compared to universal chromatic RBCs colored by pigments, which in their microstructure come close to regularly shaded RBCs, can influence the way the material ages. Accelerated and slow aging up to 1.2 years of immersion in artificial saliva at 37 °C were therefore compared in relation to their effects on the materials described above and in relation to the immersion conditions prescribed by standards. Quasi-static and viscoelastic parameters were assessed to quantify these effects by a depth-sensing indentation test equipped with a DMA module. The microstructure of the materials was characterized by scanning electron microscopy. The results convincingly show a differentiated influence of the aging protocol on the measured properties, which was more sensitively reflected in the viscoelastic behavior. Accelerated aging, previously associated with the clinical behavior of RBCs, shows a 2- to 10-fold greater effect compared to slow aging in artificial saliva of up to 1.2 years, highly dependent on the microstructure of the material.

Investigation the temperature-dependent surface mechanical properties of PolyJet printed samples by cyclic indentation testing in a DMA system

Tools and products used in everyday life are regularly exposed to dynamic, cyclical stress. These stresses are particularly significant in the case of polymers, as their viscoelastic behavior means that creep can be considerable even at room temperature. Therefore, polymer components are often tested for maximum deformation rather than for maximum load-bearing capacity. Basic material testing methods are tensile, compression, and flexure tests, which are excellent for determining the material properties of the bulk material. However, in some cases, it is more appropriate to investigate surface mechanical properties. The ideal method for this is the depth-sensing indentation test. This method has been widely applied to polymers in the last ten years, but cyclic tests are rare, and creep is not considered in this type of indentation test. In the case of cyclical tests, temperature dependence is not fully examined, either. This paper presents the results of cyclic indentation tests on photopolymer specimens produced by additive manufacturing with the use of a dynamic mechanical analyzer with a unique indentation clamp. We performed the tests at three temperatures to investigate the influence of temperature on cyclic indentation tests. The test results show the variation of deformation between cycles and the significant effect of temperature.

Adhesive depth-sensing indentation tests: Slopes of the force–displacement curves

Depth-sensing indentation experiments are a very important tool for estimating mechanical properties of modern materials. A review of various aspects of contact problems that used as the theoretical basis for interpretation of depth-sensing nanoindentation experiments is presented. Usually, analytical treatment of the indentation tests is based on analysis of the slopes of the force–displacement curves according to the non-adhesive Hertz contact theory. However, molecular adhesion is crucially important for many physical processes at the micro/nano-scales. Here, depth-sensing indentation techniques are reviewed and analyzed using the recent results obtained for adhesive contact problems. Fundamental relations for adhesive nanoindentation tests are derived for both frictionless and no-slip boundary conditions within the contact region. It is argued that the adhesive effects may be very important for treatment of the nanoindentation tests because the slopes of the force–displacement curves may considerably differ from the slopes derived using the non-adhesive contact theory.

DIC/DSI based studies on the local mechanical behaviors of HR3C/T92 dissimilar welded joint during plastic deformation

In this study, both the digital image correlation (DIC) and depth-sensing indentation (DSI) measurements were performed to investigate the non-uniform local mechanical behaviors of HR3C/T92 dissimilar welded joint during plastic deformation. The macro-scale DIC experiments showed the uneven distribution and transfer phenomena of the plastic deformation in the welded samples. The grain-scale DIC measurements further revealed that the strain localization was closely related to the local microstructures. To understand the correlation of the non-uniform deformation behavior with the local microstructure in the welded samples, the electron backscatter diffraction (EBSD) tests in the as-received state and ex-situ optical microscope (OM) observations during the tensile deformation process were conducted. The slip morphologies were found to be different across the welded joint, and the grain-scale strain localizations were proved to be closely related with the behaviors of the slip bands. Moreover, ex-situ DSI tests were also carried out to determine the local mechanical properties of the HR3C/T92 dissimilar welded joint during the tensile deformation process. The basic elasto-plastic properties (elastic modulus, yield strength, and hardening parameters) in particular zones of the welded joint were estimated by using the multi-fidelity deep neural network method combined with indentation characteristic parameters. The non-uniform deformation behavior of the HR3C/T92 welded joint during tensile loading was further simulated by using the finite element method based on the estimated elasto-plastic properties of the welded joint, and a general agreement between the simulated result and experimental tensile curve has been obtained.

Universal Chromatic Resin-Based Composites: Aging Behavior Quantified by Quasi-Static and Viscoelastic Behavior Analysis.

Universal chromatic dental resin-based composites were recently developed in an attempt to speed up the restoration process with the aim of making it easier for the practitioner to decide on a suitable shade and to avoid time-consuming matching and mixing of materials. The way in which color is created in the analyzed universal chromatic materials is innovative, as it is not only induced by selective light absorption via pigments (Venus Diamond ONE, Venus Pearl ONE), as is usual in regular composites (Charisma Classic, Charisma Topaz, Venus, Venus Diamond), but also by selective light reflection via particularized microstructures (Omnichroma). Material properties were assessed at 24 h post-polymerization and after artificial aging. Flexural strength (n = 20) and modulus were measured in a 3-point-bending test and complemented with fractography and Weibull analysis. Quasi-static (Martens, Vickers, and indentation hardness; elastic and total indentation work; creep, indentation depth) and viscoelastic (storage, loss, and indentation moduli; loss factor) behavior (n = 6) was measured by a depth-sensing indentation test equipped with a DMA module. The nanoscale silica/zirconia polymer core-shell structure in the structural-colored material induces similar or poorer mechanical properties compared with pigment-colored materials, which is related to the higher polymer content. For all materials, aging shows a clear influence on the measured properties, with the degree of degradation depending on the measurement scale.

Assessing the Residual Stresses in Dissimilar Welded Joint: Depth-Sensing Indentation Testing and Numerical Simulation

Assessing the Residual Stresses in Dissimilar Welded Joint: Depth-Sensing Indentation Testing and Numerical Simulation

Practical method to determine the effective zero-point of indentation depth for continuous stiffness measurement nanoindentation test with Berkovich tip

The zero-point of indentation depth in nanoindentation or depth-sensing instrumented indentation tests should be precisely set to evaluate the indentation hardness and indentation elastic modulus of materials to be tested, especially at shallow depths. A critical contact stiffness value has been widely used to determine the zero-point in nanoindentation tests with a Berkovich tip using the continuous stiffness measurement (CSM) method. However, this criterion occasionally gives an inadequate zero-point owing to the surface roughness of materials, the vibration of the testing system, and the flaws of the CSM method at shallow depth. This study proposes a practical method to determine the effective zero-point of indentation depth, which was obtained linearly at the zero-point of contact stiffness and extrapolated from the depth-dependent contact stiffness values, except for those at initially unstable contact depths. The proposed method enables nanoindentation tests to obtain a constant indentation elastic modulus and low deviation of nanoindentation hardness of homogenously fused silica and metallic materials, which provides an efficient way to obtain more accurate test data.

Effect of interface roughness on the tribo-corrosion behavior of diamond like carbon coatings on titanium alloy

Substrate surface morphology can significantly affect the functional performance and durability of the subsequently deposited coatings. In the present work, diamondlike carbon films were prepared by radio frequency plasma enhanced chemical vapor deposition on Ti-6Al-4V alloy substrates with different premediated surface roughness parameters (average roughness Ra, Skewness Rsk and Kurtosis Rku), and their mechanical, electrochemical, and tribo-corrosion properties were studied. The surface parameters, the microstructure, and the chemical composition were assessed by optical profilometry, scanning electron microscopy, Raman spectroscopy, and energy dispersive spectroscopy. The mechanical properties were evaluated using depth-sensing indentation and scratch testing, and the films' tribo-corrosion behavior was determined using a reciprocating tribometer in a ball-on-flat configuration with the tribological contact (Al2O3 counterpart) immersed in a 3.5% NaCl sea waterlike solution. The evolution of the corrosion potential as a function of time before, during, and after the wear tests indicated that the tribo-corrosion behavior is strongly affected by the surface roughness parameters. The potential of samples with Ra = 20 nm was unaffected by the rubbing process under the chosen tribological conditions compared to samples with higher Ra values. A similar trend was observed for samples with negative Rsk as opposed to those with Rku values greater than 3. The poor tribo-corrosion behavior of samples with Ra > 20 nm and high Rku (greater than 3) is mainly due to the significant height of asperities that constitute initiation sites for stress and strain failure on the surface. The predominant degradation mechanism was abrasive wear for samples with high surface roughness, tested under dry wear conditions. In the tribo-corrosion process, a synergy between the abrasive wear and corrosion was found to contribute to the overall material loss.

Comparison of modern light-curing hybrid resin-based composites to the tooth structure: Static and dynamic mechanical parameters.

The study aims to compare the way modern resin-based composites (RBCs) respond to mechanical stress related to the tooth structure they are designed to replace. Eight representative light-cured RBCs, including ormocers, giomers, RBCs with nano and agglomerated nanoparticles, prepolymerized, or compact fillers, were selected. Flexural strength, FS and modulus/E, were measured in a three-point bending test. A fractographic analysis determined the origin of fracture. The quasi-static (indentation hardness/HIT , indentation modulus/EIT ) and viscoelastic (storage modulus/E', loss modulus/E″, loss factor/tan δ) behavior was assessed by a depth-sensing indentation test equipped with a dynamic-mechanical analysis module. One and multiple-way analysis of variance (ANOVA), Tukey honestly significant difference (HSD) post-hoc tests (α=0.05), and Weibull statistics were applied. Parameter material exhibited the highest effect on E (p < .001, ηP2 =.857), followed by FS (ηP2 =.729), and the strain (ηP2 =.553). Highest material reliability was identified in the RBCs with nano and agglomerated nanoparticles. The most frequent type of failure originated from volume (81.3%), followed by edge (10.6%), and corner (8.1%) flaws. Enamel evidenced three times higher HIT , EIT , and E' values as RBCs and dentin, and the smallest deviation from ideal elasticity. Ormocers exhibited the highest damping capacity, followed by the RBCs with prepolymerized fillers. Damping capacity and static mechanical properties are mutually exclusive. Analyzed RBCs and the tooth structure are better adapted to the relevant frequency for chewing than for higher frequencies. RBCs are comparable to dentin in terms of their mechanical performance, but apart from the damping behavior, they are far inferior to enamel. Damping ability of analyzed material could be exploited for correlation with the clinical behavior.

A novel analytical procedure for estimating the solids’ energy dissipation capacity from Depth-Sensing indentation tests

In emergent nanotechnology areas like Micro Electro-Mechanical Systems (MEMS) the components capacity to dissipate strain energy is a key factor to evaluate their potential lifespan. In this context, we present the direct derivation of an analytical expression to estimate the elastic–plastic solids energy dissipation capacity from Depth Sensing Indentation (DSI) data. An expression to the fast calculation of the elastic (Ue)-to-total (Ut) strain energy ratios (Ue/Ut) from loading and unloading curves was particularly proposed. Assuming reasonable power law behaviors for these curves this expression allows calculating the strain energy ratios only considering the maximum displacement (hm) and plastic hysteresis (hp), which are experimentally easy to estimate. The simplifications and assumptions taken in this work ensure, in the worst of the cases, Ue/Ut ratios with uncertainties about 8 %. Despite this, the results could represent an interesting improvement in the time spending-to-accuracy relation for estimating these ratios.

Scratch test in boride layers: Influence of indenter tip radius on failure mechanisms

In this work, the influence of indenter tip radius (ITR) on scratch tests in boride layers was studied. The thermochemical treatment of boriding was performed on an AISI H13 steel at 1000 °C for 1 h. Depth sensing indentation test was developed to estimate both, hardness, and elastic modulus of the boride-layer/substrate system. Indenter radius of 200, 100, 50 and 20 μm were used for the scratch tests on borided steel. Failure mechanisms were analyzed by scanning electron microscopy (SEM). Predominant failure mechanisms were lateral cracking, chipping, and gross chipping. ITR of 20 μm developed more severe damage on the borided AISI H13 surface and the more severe substrate plastic deformation was obtained by ITR of 200 μm.

Microstructural-micromechanical correlation in an Al–Cu–Mg–Ag–TiB2 (A205) alloy: additively manufactured and cast

In this paper, the processing-structure-property correlation of a high-strength titanium-modified aluminum-copper alloy, i.e., Al–Cu–Mg–Ag–Ti–B (A205), in the laser powder-bed fusion (LPBF) and casting conditions are studied. The as-fabricated and heat-treated materials were considered for this study. As A205 is a heat-treatable aluminum alloy, the effects of different post-fabrication heat-treatment processes, (i) solution heat-treatment (SHT: 520°C for 2 h) and (ii) over-aging and stabilizing treatment (T7), were also assessed. To test the stated relationship in LPBF and cast conditions depth-sensing indentation testing and extensive structural analyses were performed. The morphology and distribution of precipitates and nanoparticles in the microstructure were analyzed using optical, scanning electron, and transmission electron microscopy. The average nanoindentation hardness at as-fabricated, SHT, and T7 conditions are 1.48 ± 0.04, 1.50 ± 0.03, and 1.56 ± 0.06 GPa for the LPBF samples and 1.34 ± 0.04, 1.52 ± 0.10, and 1.72 ± 0.09 GPa for the cast samples. Detailed microstructural analyses confirmed the presence of precipitates and reinforced particles and revealed their morphology. The fine equiaxed grains with the average grain size of 1.1 ± 0.4 μm in the as-built condition highly enhanced the mechanical properties compared to the cast counter material, where the average grain size was measured as 46.4 ± 10.5 μm. Heat treatment played a more effective role on the cast samples and the strength of the cast material exceeded the LPBF material considerably after T7 treatment. This study offers a new compositional window for the additive manufactured lightweight high-strength material categories for various applications including automotive and aerospace industries. Besides, our findings demonstrate the significance of controlling the precipitates through the post-fabrication heat treatment processes.

Long-term mechanical stability and light transmission characteristics of one shade resin-based composites

ObjectiveThe study aims to investigate the long-term mechanical characteristics, reliability, and light transmission of novel, one shade resin-based composites (RBC). Methods120 Specimens (n = 20) of three RBCs (Venus Diamond/VD, Venus Pearl/VP, Omnichroma/OC) were used in a three-point bending test, to determine flexural modulus (E) and flexural strength (σ). Testing ensued after 24 h or thermocycling (TC/10,000 cycles, 5/55 °C). Each fracture mechanism and filler system were documented, using light and scanning electron microscopy. A depth-sensing indentation test (n = 6) quantified the indentation modulus (Y), Martens/Vickers hardness (HM, HV) and creep (Cr). Incident (I0) and transmitted irradiance (IT) plus radiant exposure (RET) per RBC were measured employing a USB4000 spectrometer (n = 3) to calculate spectral absorbance. Data was tested for significant differences (α = 0.05) utilizing Student's t-tests, one- and multiple-way analysis of variance (ANOVA) and Tukey's post-hoc tests along with Pearson's correlational and Weibull analysis. ResultsInitially, σ, E and Weibull modulus m were the highest for VP. After TC σ was comparable for VP and VD, E was higher for VD and m higher for VP. Maximum Y, HM, HV and Cr were always measured for VD. Inferior parameters, except Cr, were always recorded for OC. IT, RET and absorbance differed marginally, with OC on top. ConclusionsThe variation in long-term mechanical stability and light transmission is significantly dependent on RBC formulation, most notably due to filler system and resin matrix. Clinical SignificanceOne shade RBCs vary substantially in their material composition and characteristics, facilitating the diversity present in RBC direct restoratives.

Strain rate dependent micromechanical properties of NiTi shape memory alloys: Laser powder bed fusion versus casting

As the strain rate influences the loading sensitivity of the metallic materials over time, it is very important to assess the associated plastic behavior exposed to a diverse strain rate. Strain rate sensitivity is a fundamental property used to assess the controlling mechanisms of plastic deformation and the susceptibility of a material to creep. In materials with high strain rate sensitivity, plastic deformation can occur even under the application of a small stress value with relatively low rates of strain. Because of the diverse use of NiTi shape memory alloys in small size applications, it would be crucial to understand how strain rates, locally and on small scale, influence plastic deformation, especially in superelastic conditions. Thus, proper and optimal implementation of their functional properties can be achieved. In this paper, the local strain rate sensitivity of heat-treated NiTi samples in the additive manufactured and cast conditions have been investigated employing depth-sensing indentation testing technique at ambient temperature subjected to various indentation loading rates. To this end, using a self-similar pyramidal (Berkovich) indenter, the materials are loaded with different indentation loading rates of 1, 5, 10, and 50 mN/s to a peak load of 200 mN and then unloaded. Upon conducting the indentation tests, the extracted results including indentation load-depth curves, indentation stress-depth curves, and indentation strain rate sensitivity values have been analyzed and discussed for the cast and additive manufactured materials. Experimental results demonstrate that the hardness values increase linearly with the increase in indentation loading rate while the elastic modulus stays relatively constant. These findings are important in understanding the local and small-scale deformation behavior of additively manufactured NiTi in which properties are location-dependent, as compared with the cast counterparts.

Static Hardness Testing of Cement Mortars Containing Different Types of Recycled Construction Waste Powders

The present paper deals with the hardness of cement mortars prepared with recycled materials that are potential supplementary cementitious materials (SCM). Two potential SCMs (aerated concrete powder (ACP) and concrete powder) were investigated and compared with a reference (neat cement) sample and a sample containing metakaolin (MK). The long-term performance of the mortars was studied up to the age of one year. Based on the compressive strength tests at different ages, neither concrete powder nor ACP significantly decreases the compressive strength at a 10% substitution ratio. The samples were studied with two types of static hardness tests: the Brinell hardness test and the depth sensing indentation test at two different load levels. The hardness test results indicated that the standard deviation of the results is lower at a higher load level. In the case of metakaolin and concrete powder, the change in the compressive strength was observable in the hardness test results. However, in case of the ACP, the compressive strength decreased, while the hardness increased, which can be traced back to the filler effect of aerated concrete powder. Finally, using the DSI test, the hardness results were analyzed on an energy basis. The analysis highlighted that the change in the hardness is connected to the elastic indentation energy, while it is independent from the dissipated (plastic) indentation energy.