Indentation Fracture Test Research Articles

Composite vitreous enamel coatings with the addition of 316L stainless steel flakes: Novel insights on their behaviour under mechanical stresses

Vitreous enamel coatings are greatly appreciated for their ability to protect the covered metals from weathering and corrosion. On the other side, their applicability is somehow limited, as they show brittle behaviour when they are subjected to mechanical stresses. The addition of a second ductile phase inside a brittle matrix, thus the creation of a composite material, is known to be an effective way to improve the fracture toughness and cracking resistance of brittle materials and coatings. The main aim of this research work is to deeply characterize the mechanical properties of composite 316-L stainless steel flakes/enamel coatings under different mechanical solicitations, in particular performing 4-point bending tests, indentation fracture tests, and scratch tests. Three different sample typologies are taken under consideration for these tests: the reference sample without the addition of metallic flakes, the sample with 1 wt% of 316L steel flakes, and the sample with 5 wt% 316L steel flakes. The mechanical behaviour of the samples is compared, and the damaging mechanisms are always clarified by means of optical and scanning electron observations. The addition of 5 wt% stainless steel flakes resulted to be useful in improving the cracking resistance of the coatings and in limiting the extent of cohesive damage.

VERIFICATION OF BEHAVIOUR OF HUMAN ENAMEL FOR FRACTURE TOUGHNESS DETERMINATION

<p>Enamel is the hardest biological tissue in the human body because of its structure<br />and composition. The structure of interlocking rods enables this biomaterial to resist the stresses of mastication. Unfortunately, enamel is prone to fracture initiation and growth. Determining fracture toughness of enamel is a difficult task. The lack of thickness makes it impossible to prepare samples which could be analyzed by usual methods. Other authors use Vickers indentation fracture test (VIF) to determine fracture toughness of enamel ordinarily. VIF is, however, not generally acceptable.<br />The aim of this study is verification of fracture behaviour of enamel using nanoindentation. In the study the impact of changes of hardness (HIT) and reduced modulus (Er) caused by crack initiation and growth on fracture toughness determination is observed. The next goal is evaluation of loading rate impact.</p>

Improved force prediction model for grinding Zerodur based on the comprehensive material removal mechanism.

There have been few investigations dealing with the force model on grinding brittle materials. However, the dynamic material removal mechanisms have not yet been sufficiently explicated through the grain-workpiece interaction statuses while considering the brittle material characteristics. This paper proposes an improved grinding force model for Zerodur, which contains ductile removal force, brittle removal force, and frictional force, corresponding to the ductile and brittle material removal phases, as well as the friction process, respectively. The critical uncut chip thickness agc of brittle-ductile transition and the maximum uncut chip thickness agmax of a single abrasive grain are calculated to identify the specified material removal mode, while the comparative result between agmax and agc can be applied to determine the selection of effective grinding force components. Subsequently, indentation fracture tests are carried out to acquire accurate material mechanical properties of Zerodur in establishing the brittle removal force model. Then, the experiments were conducted to derive the coefficients in the grinding force prediction model. Simulated through this model, correlations between the grinding force and grinding parameters can be predicted. Finally, three groups of grinding experiments are carried out to validate the mathematical grinding force model. The experimental results indicate that the improved model is capable of predicting the realistic grinding force accurately with the relative mean errors of 6.04% to the normal grinding force and 7.22% to the tangential grinding force, respectively.

Critical evaluation of indentation fracture toughness measurements with Vickers indenter on yttria-stabilized zirconia dental ceramics

The purpose of this study was to investigate and analyze fracture toughness (KIc) of yttria stabilized tetragonal zirconia (Y-TZP) dental ceramics by the Vickers indentation fracture test. In order to determine fracture toughness, the Vickers indenter was used under the load of 294.20 N (HV30). The cracks, which occur from the corners of a Vickers indentation, were measured and used for fracture toughness determination, through five mathematical models according to (I) Anstis, (II) Evans and Charles, (III) Tanaka, (IV) Niihara, Morena and Hasselman and (V) Lankford. Morphology of indentation cracking was determined by scanning electron microscope. The most adequate model for determination of fracture toughness (KIc) of yttria stabilized tetragonal zirconia dental ceramics by the Vickers indentation fracture test is Lankford model.

Fracture toughness of intermetallic Cu6Sn5 in lead-free solder microelectronics

Even though lead-free Sn-Ag-Cu based solder alloys are emerging as promising candidates to replace well-established but hazardous lead-containing solders, they suffer from limited knowledge about their mechanical performance. To ensure the high reliability demands in microelectronics, fracture properties need to be quantified with respect to crucial microstructural elements like Cu6Sn5 intermetallic compounds formed at common Cu metallization interfaces. While indentation fracture testing methods have shown varying fracture toughness values, micro-cantilever fracture testing is applied in this work on FIB-prepared single-crystalline Cu6Sn5 cantilevers to clarify on the microstructure-fracture relationship.

Mechanical Properties of Thermoelectric Ba8Al15Si31 Clathrate Prepared by Combining Arc Melting and Spark Plasma Sintering Techniques

The mechanical properties (the elastic moduli, hardness, and fracture toughness) were investigated for polycrystalline Ba8Al15Si31 clathrate as one of useful thermoelectric materials on a phonon glass and electron crystal concept. Samples were prepared by combining arc melting and spark plasma sintering techniques. The Young’s modulus E = 96.88 GPa, shear modulus G = 38.61 GPa, bulk modulus K = 65.77 GPa, and Poisson’s ratio ν = 0.2545 were calculated from longitudinal sound velocity vL = 6105 m/s and transverse sound velocity vT = 3503 m/s, measured by an ultrasonic test. The elastic constants c11 = 78.64 GPa, c12 = 40.03 GPa, and c44 = 38.61 GPa were calculated from E and G. The Vickers hardness HV and Young’s modulus E were simultaneously determined to be 634 and 109.7 GPa, respectively, by a nanoindentation test. The fracture toughness KIC was determined to be 1.1 MPa m1/2, which was comparable to typical values 1.1–1.2 MPa m1/2 for thermoelectric silicide Mg2Si, by a Vickers indentation fracture test.

Determination of fracture toughness of brittle materials by indentation

Fracture toughness is one of the crucial mechanical properties of brittle materials such as glasses and ceramics which demonstrate catastrophic failure modes. Conventional standardized testing methods adopted for fracture toughness determination require large specimens to satisfy the plane strain condition. As for small specimens, indentation is a popular, sometimes exclusive testing mode to determine fracture toughness for it can be performed on a small flat area of the specimen surface. This review focuses on the development of indentation fracture theories and the representative testing methods. Cracking pattern dependent on indenter geometry and material property plays an important role in modeling, and is the main reason for the diversity of indentation fracture theories and testing methods. Along with the simplicity of specimen requirement is the complexity of modeling and analysis which accounts for the semi-empirical features of indentation fracture tests. Some unresolved issues shaping the gap between indentation fracture tests and standardization are also discussed.

Prediction of cracking initiation in enamel coated product and its verification by four-point bending test

The cracking of enamel coating often occurs because of excessive tensile stress under the product usage environment. The cracks loosen the enamel coating, leading to spalling and peeling and subsequent corrosion of the metal substrate. The cracking is also affected by the residual stress, which complicates the prediction of cracking initiation. The objective of this study is to predict the cracking initiation of the enamel coating by the FE-analysis. The brittle fracture properties of the enamel coating in the FE-analysis were determined by an experimental Vickers indentation fracture (VIF) test and a uniaxial tensile test. The effect of the residual stress of the enamel coating generated by the firing and cooling process on the cracking was considered and analyzed with a four-point bending model by the FE-analysis. To verify the predicted displacement up to the cracking initiation of the enamel from the FE-analysis, a four-point bending test was also performed using the same conditions as the simulation and the results were in agreement with those of the simulation.

Determination of the slow crack growth susceptibility coefficient of dental ceramics using different methods

This study compared three methods for the determination of the slow crack growth susceptibility coefficient (n) of two veneering ceramics (VM7 and d.Sign), two glass-ceramics (Empress and Empress 2) and a glass-infiltrated alumina composite (In-Ceram Alumina). Discs (n = 10) were prepared according to manufacturers' recommendations and polished. The constant stress-rate test was performed at five constant stress rates to calculate n(d) . For the indentation fracture test to determine n(IF) , Vickers indentations were performed and the crack lengths were measured under an optical microscope. For the constant stress test (performed only for d.Sign for the determination of n(s) ) four constant stresses were applied and held constant until the specimens' fracture and the time to failure was recorded. All tests were performed in artificial saliva at 37°C. The n(d) values were 17.2 for Empress 2, followed by d.Sign (20.5), VM7 (26.5), Empress (30.2), and In-Ceram Alumina (31.1). In-Ceram Alumina and Empress 2 showed the highest n(IF) values, 66.0 and 40.2, respectively. The n(IF) values determined for Empress (25.2), d.Sign (25.6), and VM7 (20.1) were similar. The n(s) value determined for d.Sign was 31.4. It can be concluded that the n values determined for the dental ceramics evaluated were significantly influenced by the test method used.

Mechanical properties of polycrystalline silicon solar cell feed stock grown via fluidized bed reactors

Polysilicon granular beads grown via a fluidized bed reactor, a feedstock for silicon solar cell production, were annealed, sectioned, and indented using a combination of nanoindentation and microhardness testing to determine the mechanical response of this commercially available raw material. The granular material, with macroscopic dimensions on the order of millimeters and an internal grain size on the order of 20 nm, has an indentation modulus of approximately 160 GPa, and a hardness prior to fracture of 9.6 GPa; these values are relatively insensitive to annealing at temperatures between 600 and 1100 °C. Indentation fracture testing suggests the toughness of this material is on the order of 0.6 MPa m1/2. The fracture sequence has been verified using acoustic emission testing during indentation. Annealing in air at 600 °C for 3 days increases the toughness by approximately 50% with little change in grain size. The as grown material contains solute hydrogen, identified by infrared spectroscopy, from the growth process; annealing in air tends to remove solute hydrogen from the material at temperatures above 1050 °C. The removal of solute hydrogen appears to cause slight increases in toughness, while grain growth at elevated annealing temperatures or the formation of hydrogen complexes in the silicon appears to decrease toughness. The results suggest thermal treatments of silicon grown with this method can moderately alter the friability of the final product.

Improvement of Wear Resistance in Alumina Matrix Composites Reinforced with Carbon Nanotubes

Alumina matrix composites reinforced with carbon nanotubes (CNTs) fabricated by CNT purification, mixing, compaction, and sintering processes, and the effects of the CNT addition on wear resistance were investigated in relation to the relative density, hardness, and fracture toughness. Wear resistance and fracture toughness were measured by the dry sliding wear test method and the indentation fracture test method, respectively. Zero to ~3 vol pct of CNTs were homogeneously distributed in the composites, although some pores existed. The wear resistance and fracture toughness increased with an increasing CNT fraction, but the composite specimen containing 3.0 vol pct of CNTs hardly showed an increase over the specimen containing 2.25 vol pct of CNTs. Observations of worn surfaces revealed that the wear mechanism involved both the abrasive and delamination wear modes in the specimens containing 0 to ~0.75 vol pct of CNTs, whereas the surface was worn largely in an abrasive wear mode in the specimens containing 1.5 to ~3.0 vol pct of CNTs. This was because CNTs helped to change the delamination wear mode to the abrasive wear mode by preventing crack initiation and propagation at alumina grains. The fracture toughness increase provided beneficial effects in the resistance to crack initiation and propagation, the reduction in delamination wear on the worn surface, and the consequent improvement in wear resistance. Because the effect of the porosity increase due to the CNT addition unfavorably affected the improvement of wear resistance and fracture toughness in the specimen containing 3.0 vol pct of CNTs, the appropriate level of CNT fraction was 1.5 to ~2.25 vol pct.

Improvement of flexure strength and fracture toughness in alumina matrix composites reinforced with carbon nanotubes

In this study, alumina matrix composites reinforced with carbon nanotubes (CNTs) were fabricated by CNT purification, mixing, compaction, and sintering processes, and their relative density, electrical resistance, hardness, flexure strength, and fracture toughness were evaluated. 0–3 vol.% of CNTs was relatively homogeneously distributed in the composites, although some pores existed. The three-point bending test results indicated that the flexure strength increased with increasing volume fraction of CNTs, and reached the maximum when the CNT fraction was 1.5 vol.%. The fracture toughness increased as the CNT fraction increased, and the fracture toughness of the composite containing 3 vol.% of CNTs was higher by 40% than that of the monolithic alumina. According to the observation of the crack propagation path after the indentation fracture test, a new toughening mechanism of grain interface bridging-induced CNT bridging was suggested to explain the improvement of fracture toughness.

Indentation techniques for evaluating the fracture toughness of biomaterials and hard tissues

Indentation techniques for assessing fracture toughness are attractive due to the simplicity and expediency of experiments, and because they potentially allow the characterization of both local and bulk fracture properties. Unfortunately, rarely have such techniques been proven to give accurate fracture toughness values. This is a concern, as such techniques are seeing increasing usage in the study of biomaterials and biological hard tissues. Four available indentation techniques are considered in the present article: the Vickers indentation fracture (VIF) test, the cube corner indentation fracture (CCIF) test, the Vickers crack opening displacement (VCOD) test and the interface indentation fracture (IIF) test. Each technique is discussed in terms of its suitability for assessing the absolute and relative toughness of materials or material interfaces based on the published literature on the topic. In general, the VIF and CCIF techniques are found to be poor for quantitatively evaluating toughness of any brittle material, and the large errors involved (∼±50%) make their applicability as comparative techniques limited. Indeed, indentation toughness values must differ by at least by a factor of three to conclude a significant difference in actual toughness. Additionally, new experimental results are presented on using the CCIF test to evaluate the fracture resistance of human cortical bone. Those new results indicate that inducing cracking is difficult, and that the cracks that do form are embedded in the plastic zone of the indent, invalidating the use of linear elastic fracture mechanics based techniques for evaluating the toughness associated with those cracks. The VCOD test appears to be a good quantitative method for some glasses, but initial results suggest there may be problems associated with applying this technique to other brittle materials. Finally, the IIF technique should only be considered a comparative or semi-quantitative technique for comparing material interfaces and/or the neighboring materials.

Residual stresses in YAG phase of melt growth Al 2O 3/YAG eutectic composite estimated by indentation fracture test and finite element analysis

The residual stresses in YAG phase of the melt growth Al 2O 3/YAG eutectic composite were studied at room temperature by means of the indentation fracture test and finite element stress analysis. The YAG phase taken out from the composite was used as a reference sample without thermally induced residual stresses. The apparent fracture toughness of YAG phase with residual stress in the composite was higher than that of the reference sample of bare YAG. From the difference in fracture toughness value between the YAG in the composite and the bare YAG, the residual stress of YAG in the composite was estimated to be −170 ± 100 and −220 ± 130 MPa in the cross-sections perpendicular and parallel to the solidification direction, respectively. The experimentally measured spatial distribution of the residual stress in YAG phase in the composite was accounted for by the result of finite element analysis, which showed that the residual stress is different from position to position due to the variation in local morphology of YAG and Al 2O 3 within the composite.

Influence of mechanical surface treatments on the indentation fracture toughness of glass infiltrated zirconia toughened alumina "GI-ZTA" disks

The main objective of this study was to evaluate the influence of mechanical surface treatments for excess glass removal on the fracture toughness of a glass infiltrated zirconia toughened alumina "GI-ZTA" disks (In-Ceram® Zirconia). The GI-ZTA disks were submitted to three different mechanical surface treatments after glass infiltration (grinding, sandblasting and grinding/sandblasting/annealing). Fracture toughness was evaluated through indentation fracture (IF) test. Reliability of tests results was accessed through Weibull statistics. Results: Indentation fracture tests (IF) of GI-ZTA disks have shown that grinding was the surface treatment that presented the lowest K Ic and reliability. An annealing treatment after grinding and sandblasting promoted an increase in K Ic, mainly due to monoclinic-tetragonal reverse transformation recovering the tetragonal zirconia lost during the different mechanical surface treatments. The highest K Ic values were observed after sandblasting and grinding/sandblasting/annealing. Significance: The proposed mechanical surface treatments played an important role on the metastability of tetragonal zirconia and strongly influenced the mechanical performance of GI-ZTA. Kruskhall-Wallis test indicated that K Ic values of the three mechanical surface treatments were statistically distinct.

Estimation of fracture toughness of nitride compound layers on tool steel by application of the Vickers indentation method

Nitriding and nitrocarburizing treatments are established methods of improving the wear performance of tool and die steels. However, our understanding of the relationship between nitriding process parameters, and microstructure and fracture behaviour of the surface layers is far from complete. This paper describes an investigation of the application of Vickers indentation fracture testing to nitrided and nitrocarburized AISI H11 hot work tool steel. Optical microscopy and microhardness testing were used to characterize structure and mechanical properties of the compound and diffusion layers. The results show that, where a sufficiently thick compound layer has formed, this method has the potential to be applied as a pseudo non-destructive method of monitoring the fracture properties of treated surfaces on actual tool parts. However, the validity of the method appears to be a function of the presence and thickness of the compound layer, and possibly the mechanical properties of the diffusion layer, and further work is proposed to establish the sensitivity of the method for fracture toughness calculations.

Fracture behaviors of piezoelectric materials

Theoretical analyses and experimental observations of the failure and fracture behaviors of piezoelectric materials are presented. The theoretical analyses are based on the Stroh formalism. A strip dielectric breakdown model is proposed to estimate the effect of electrical non-linearity on the piezoelectric fracture of electrically insulated cracks. The reviewed experiments include the indentation fracture test, the bending test on smooth samples, the fracture test on pre-notched or pre-cracked samples, the environment-assisted fracture test, etc. For electrically insulated cracks, the experimental results show a complicated fracture behavior under combined electrical and mechanical loading. Fracture data are greatly scattered when a static electric field is applied. For electrically conducting cracks, the experimental results demonstrate that static electric fields can fracture poled and depoled lead zirconate titanate (PZT) ceramics. A charge-free zone model is introduced to understand the failure behavior of conducting cracks in the depoled lead zirconate titanate ceramics under electrical and/or mechanical loading. These theoretical and experimental results indicate that fracture mechanics concepts are useful in the study of the failure behaviors of piezoelectric materials.

119 リン酸カルシウム系セラミックスの成形プロセス及び力学的挙動の評価(OS 新材料の加工技術(1))

Two types of calcium phosphate ceramics, hydroxyapatite and tricalcium phosphate, were sintered with Spark plasma sintering (SPS) technique. Sintering condition were investigated and sintered body of these calcium phosphate ceramics were obtained with SPS technique. Four points bending tests and indentation fracture tests were conducted to charactrize the mechanical properties. Bending strength was lower than the previous studies. This is attributed to the initiation of microcracks during cooling process.

Variables that Affect the Indentation Fracture Testing (IF) of a Dental Porcelain

The aim of this study was to investigate the influence of the indentation load, the period waste between indentation and measurement, and the gold coating on the fracture toughness (K Ic ) of a feldspatic porcelain (d.Sign, Ivoclar) measured by the Indentation Fracture Technique (IF). Porcelain beams (∼3x4x20mm 3 ) were fabricated by sintering green bodies at 870 C under vacuum with rapid heating and cooling rates. The samples presented microstructure composed of a glassy matrix, second phase particles and residual pores. Vickers hardness tester was used to produce indentation patterns on mirror polished surfaces. The diagonal of indentation (2a) and the length of crack (2c) were measured in an optical microscopy with better resolution than the hardness tester. The increase of indentation load from 0,2 to 5kg caused: i) increase of c/a ratio from 1,8 to 3,3; ii) almost no variation of Vickers hardness number; and iii) constancy of K Ic value from 0,3kg load. It was observed no influence of a gold coating on K Ic measurement. K Ic suffered significant reduction with time, mainly in the first hour.

Measurement of Residual Stress in ZnO Thin Films Deposited on Silicon Wafers by the Indentation Fracture Test

ABSTRCT:The indentation fracture test was used to evaluate residual stress in ZnO thin films deposited on silicon wafers. Using the calibrated constants involved in a simple semi-empirical formula, which was developed based on fracture analysis and experimental results, we estimated the residual stress in the ZnO thin films by measuring the indentation-induced crack length as a function of the indentation load.