Depth-sensing Microindentation Tests Research Articles

Dry Sliding Wear Resistance of Cobalt Boride Coatings Formed on ASTM F1537 Alloy

New results about the wear resistance of CoB-Co2B coatings under dry sliding conditions were estimated in this work. The cobalt boride coatings were developed at the surface of the ASTM F1537 alloy by means of the powder-pack boriding process using two experimental conditions: 1223 K with 6 h of exposure and 1273 K with 10 h of exposure. Before the sliding wear tests, Vickers depth-sensing microindentation tests were conducted on the cross section of the cobalt boride coatings to estimate the distribution of hardness, Young’s modulus, and residual stresses. Otherwise, the sliding wear tests were performed on both boriding conditions and on the untreated material, using a ball-on-flat configuration comprised of an alumina ball as a counterpart with applied loads between 5 and 20 N. The wear rates of the borided ASTM F1537 alloy were ranged between 4.02 and 8.91 × 10−6 mm3 N−1 m−1 compared with the values of the untreated material (13.90 and 15.78 × 10−6 mm3 N−1 m−1) for the overall set of experimental conditions; nevertheless, the influence of boriding conditions (1273 K with 10 h of exposure) tended to increase the CoB coating thickness, developing a more brittle layer that decreased the sliding wear resistance at the surface of the borided ASTM F1537 alloy. Finally, the presence of failure mechanisms on the surface of the wear tracks was analyzed for both borided ASTM F1537 alloy and untreated material.

Determining the viscoelastic behavior of polyester fiberglass composite by continuous micro-indentation and friction properties

The purpose of this study is to characterize the influence of specific micro-indentation test methods and frictional behavior on the viscoelastic properties of polyester fiberglass composite. Previous work shows that viscoelastic parameters for neat thermoset polymers can be successfully determined using a recently developed phenomenological model. In this study, similar depth sensing micro-indentation tests were performed on polyester fiberglass composite surface to determine the effect of fiberglass reinforcement on viscoelastic properties of the composite. Data obtained from micro-indentation tests was used to calculate the viscoelastic properties of the composite using the previously developed model, and compared to those of neat polyester. Results show that fiberglass significantly increased the both viscoelastic indentation modulus parameters, E0* and E1*, of the composite. Scratch tests were also performed on this composite, showing a higher coefficient of friction for a diamond stylus in contact with a polyester fiberglass composite than when in contact with a neat polyester. Moreover, the coefficient of friction decreased as the diamond stylus׳s sliding speed increased for the composite. A model is then applied to illustrate that the von Mises stress distribution in the material during a scratch test is dependent on scratch velocity.

Estimation of local anisotropy of plexiform bone: Comparison between depth sensing micro-indentation and Reference Point Indentation

The recently developed Reference Point Indentation (RPI) allows the measurements of bone properties at the tissue level in vivo. The goal of this study was to compare the local anisotropic behaviour of bovine plexiform bone measured with depth sensing micro-indentation tests and with RPI. Fifteen plexiform bone specimens were extracted from a bovine femur and polished down to 0.05µm alumina paste for indentations along the axial, radial and circumferential directions (N=5 per group). Twenty-four micro-indentations (2.5µm in depth, 10% of them were excluded for testing problems) and four RPI-indentations (~50µm in depth) were performed on each sample. The local indentation modulus Eind was found to be highest for the axial direction (24.3±2.5GPa) compared to the one for the circumferential indentations (19% less stiff) and for the radial direction (30% less stiff). RPI measurements were also found to be dependent on indentation direction (p<0.001) with the exception of the Indentation Distance Increase (IDI) (p=0.173). In particular, the unloading slope US1 followed similar trends compared to the Eind: 0.47±0.03N/µm for axial, 11% lower for circumferential and 17% lower for radial. Significant correlations were found between US1 and Eind (p=0.001; R2=0.58), while no significant relationship was found between IDI and any of the micro-indentation measurements (p>0.157). In conclusion some of the RPI measurements can provide information about local anisotropy but IDI cannot. Moreover, there is a linear relationship between most local mechanical properties measured with RPI and with micro-indentations, but IDI does not correlate with any micro-indentation measurements.

Environmental degradation of glass‐ionomer cements: A depth‐sensing microindentation study

This study investigated the effects of various environmental conditions on the hardness and elastic modulus of restorative glass-ionomer cements (GICs). Two resin-modified GICs (RMGICs) (Fuji II LC [FL]; Photac-Fil Quick [PQ]) and three highly viscous GICs (HVGICs) (Fuji IX Fast [FN]; KetacMolar [KM]; KetacMolar Quick [KQ]) were evaluated in this study. Specimens were fabricated according to the manufacturers' instructions and stored under a variety of conditions (n = 7): 100% humidity, distilled water, pH 5 demineralization solution, and pH 7 remineralization solution. The hardness and elastic modulus were measured using a depth-sensing microindentation test after 4 weeks. The results were analyzed using the independent samples T-test and ANOVA/Scheffe's post hoc test (p < 0.05). HVGICs showed significantly higher hardness and elastic modulus than RMGICs under all storage conditions. Storage in distilled water significantly increased the hardness and elastic modulus of FN, but decreased that of PQ. All HVGICs and RMGICs stored in remineralization solution had hardness values and elastic moduli comparable to those stored in water. Compared to remineralization solution, demineralization solution had no significant effects on the modified GICs with the exception of KQ. The results suggest that the mechanical properties of glass-ionomer restoratives are material-type and storage condition dependent. Therefore, the clinical selection of a glass-ionomer material should be based on the oral environment to which it will be subjected.

Influence of environmental calcium/phosphate and pH on glass ionomers

This study investigated the effects of environmental calcium/phosphate and pH on the hardness and elastic modulus of two glass-ionomer cements (GICs) [Fuji IX Fast (FN) and KetacMolar (KM)]. Specimens were randomly subjected to storage media of pH 3, 5, and 7. The calcium and phosphate levels of the storage solutions ranged from 0 to 2.4 mM. After 4 wk of conditioning, hardness and elastic modulus were determined using a depth-sensing microindentation test. Sectioned surfaces were observed with scanning electron microscopy. For both FN and KM, no significant change in hardness, elastic modulus, or surface structure were observed at pH 7 and 5, regardless of the concentration of calcium and phosphate. FN and KM specimens conditioned at pH 3 had lower hardness and modulus in comparison to those conditioned at pH 7. An increased level of environmental phosphate led to higher hardness and elastic modulus of FN and KM at pH 3. In general, a microscopic surface reaction layer was observed in specimens conditioned at pH 3. The thickness and structure of the reaction layer varied depending on environmental phosphate levels. The results suggest that the influence of environmental calcium and phosphate on GICs was pH dependent.

Identification of Plasticity Parameters of Metallic Coatings by Means of Depth-Sensing Micro-Indentation

A method of plasticity parameter identification for metallic coatings using depth-sensing micro-indentation test is presented. In this method, a set of plasticity properties are identified by an inverse approach, where finite element (FE) simulation of micro-indentation is iteratively carried out, by correcting material parameters in a constitutive model in each simulation iteration, until the calculated results of indentation load vs. penetration depth (P-h) response fits well to the corresponding experimental data. For coatings, the guideline for appropriate indentation penetration depth, where the deformation of substrates does not influence P-h response, was proposed based on FE simulation experiments. In order to validate this method, plasticity parameters of Zn and Zn-Al coatings on steel sheets were identified.

Effects of surface finish on indentation modulus and hardness of dental composite restoratives

The depth-sensing micro-indentation testing was recently introduced for the characterization of dental composites. One of the critical issues raised was the possible influence of surface finish on material properties. The aim of this study was to investigate the effects of surface finish on the indentation modulus and micro-hardness of resin-based dental composite materials. The materials used included minifill (Z100, 3M ESPE), microfill (A110, 3M ESPE) and poly-acid modified (F2000, 3M ESPE) composites. The specimens were polished successively using SiC grinding papers of different grit size and diamond suspensions to achieve varying surface roughness. The arithmetic mean of the roughness (R(a)) was measured using profilometry. In the depth-sensing micro-indentation test, specimens (n=7) were indented to 10N with Vickers indenter and the load-displacement (P-h) data was obtained using a universal testing system. The indentation modulus (E(in)) and hardness (H) were then computed using the developed analytical solutions. Data was analyzed using ANOVA/post-hoc Scheffe's test at significance level 0.05. The polished specimens had surface roughness ranging from 0.02 to 0.81 microm. The roughness of F2000 was significantly higher than A110 and Z100. The E(in) and H for Z100 ranged from 14.02 to 14.83GPa and 1.18 to 1.27 GPa, respectively. E(in) for F2000 and A110 ranged from 12.25 to 13.82 GPa and 5.26 to 5.52 GPa and hardness ranged from 0.89 to 0.98 GPa and 0.52 to 0.55 GPa, respectively. The indentation modulus and hardness of dental composite restoratives were independent of the surface finish provided indenter penetration is sufficiently deep (h(max)/R(a)>30).

Surface micromechanics of ultrahigh molecular weight polyethylene: Microindentation testing, crosslinking, and material behavior.

The wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) is critical to the success of total joint replacements. Recent attempts to modify the wear behavior of UHMWPE by processing, in particular, crosslinking UHMWPE have shown promise to increase wear resistance, but concerns persist regarding other mechanical properties. It is also unclear what specific surface mechanical properties govern the wear resistance seen in these materials. The goal of this study was to demonstrate a custom-built surface mechanical test system and method that measures the micromechanical response of microtomed UHMWPE surfaces to depth-sensing microindentation tests. The surface structure of these UHMWPE materials was also examined using scanning electron microscopy and atomic force microscopy. A custom designed microindentation test system assessed the microindentation behavior of three UHMWPE resins: 1. Hylamertrade mark, 2. GUR-1020 CMS, and 3. Marathontrade mark-a lightly crosslinked material. The effects of material and indentation depth were studied. Microindentation tests were performed with indentation depths ranging from 2 to 45 microm. Four different measurements of surface micromechanical behavior were obtained including the surface modulus, microhardness, hysteresis energy (irreversible work done to the sample per unit cycle) and its associated energy dissipation factor, and loading slope. Statistically significant differences in each of these parameters were found for each material. Generally, Hylamer had the largest values for these parameters, followed by the GUR resin and then the Marathon. Surface modulus was independent of depth of testing and found to be 651 MPa for Marathon, 738 MPa for GUR, and 1015 MPa for Hylamer (Modulus for bulk UHMWPE is 540 MPa for Hylamer, 620 for GUR, and 1380 for Hylamer). The microhardness varied between 67 and 162 MPa depending on material and depth of testing. Surface structural characterization shows that the microtoming process for surface preparation generated distinct surface features that varied between materials. Intermittent drawn ribbons of polymer with oriented crystals were observed in both scanning electron microscopy and atomic force microscopy. The surface density and size of these features were characteristic of the materials with the Hylamer having the fewest, but largest ribbons, followed by GUR and then Marathon.

Design of contact loaded coated components using data from depth sensing microindentation tests

Microindentation tests provide information about mechanical properties of sUliace layers that can be useful for design purposes. This paper summarises the quantities obtainable with depth sensing devices, such as hardness, elastic modulus, yield stress, work of indentation, critical contact pressure, and analytical expressions of load–penetration curves. In the second part of the paper, the principles of design of contact loaded components are explained, and it is shown how the measured data can be used.

Errors associated with depth-sensing microindentation tests

In the determination of mechanical properties by ultra-microindentation, various errors can appear. This paper analyzes various sources of errors in estimation of elastic modulus and hardness. These errors arise from uncertainties of the indenter geometry and properties, as well as measuring instrument limitations and errors, such as the minimum detectable load, compliance, and noise of the system. Other sources of errors are thermal drift, shape of the impression, and scatter of properties of the tested material. Characteristic features and the magnitude of individual kinds of errors are discussed, together with formulas and recommended methods for their reduction.

A new method for analyzing data from continuous depth-sensing microindentation tests

A new parameter, hardness/modulus2 (H/E2), has been derived from the equations used to calculate the hardness and elastic modulus from data taken during continuous depth-sensing microindentation tests. This paper discusses the use of this parameter to treat the data obtained from a sample whose surface roughness was of the same scale as the size of the indents. The resulting data were widely scattered. This scatter was reduced when the data were plotted in terms of H/E2 versus stiffness. The effect of surface roughness on the hardness and elastic modulus results is removed via stiffness measurements, provided single contacts are made between the indenter and the specimen. The function relating the cross-sectional area of the indenter versus the distance from its point is not required for calculation of H/E2, but the hardness and modulus cannot be determined separately. The parameter H/E2 indicates resistance to plastic penetration in this case.