Study of the fracture features of layered ceramics in its microvolumes by indentation methods

Technology and modes of ZrB2―SiC layered ceramic composites manufacturing have been developed. The structures, elastic characteristics and strength properties of the materials under investigation have been studied. Effect of internal stress fields on fracture processes in the indentation area and mechanical properties of the ceramics in its microvolumes has been investigated both in layers and at their interfaces. Using values of contact tensile strength along different directions in layers of the composites, effective residual thermal stresses have been calculated (≈180 MPa). The obtained data on contact tensile strength and effective crack resistance, taking into consideration the contributions of residual stresses to their values, have been used for estimations of contact strength and crack resistance of the layer materials themselves. The fracture toughness measured by the three-point bending method is 3,3—3,7 MPa · m1/2. Analysis of the data obtained indicates that the spark notch provides a greater sharpness of the crack tip and better conditions for measuring K1c, while processing with a blade picks up a sharp thermal crack in the notch tip. The elastic properties of the multilayer system (SiC—15% ZrB2) + (SiC—30% ZrB2) were studied using ultrasonic research methods. The values of the velocities of sound and elastic characteristics are sufficiently large and close to those expected from the models of the composite, which does not contain noticeable porosity and microcracks in the layers themselves and in the region of their boundaries. For directions along and across the plane of the layers, the values of Young's moduli differ by about 6%. For the directions of propagation of an ultrasonic wave along and across the layers, anisotropy of ultrasonic velocities of ~5% and elastic moduli of ~10—12% is observed, which may be due to the texture that develops in the structure of the layers during hot pressing. Keywords: layered ceramics, indentation, strength properties, thermal stresses.

Regularities and properties of instrumented indentation diagrams obtained by ball-shaped indenter

The history of appearance and the current state of instrumented indentation are briefly described. It is noted that the materials instrumented indentation methods using a pyramid and ball indenters are actively developing and are currently regulated by several Russian and international standards. These standards provide formulas for calculating the Young’s modulus and hardness at maximum indentation load. Instrumented indentation diagrams «load F – displacement α» of a ball indenter for metallic materials were investigated. The special points on the instrumented indentation diagrams «F – α» loading curves in the area of elastic into elastoplastic deformation transition, and in the area of stable elastoplastic deformation are revealed. A loading curve area with the load above which the dF/dα begins to decrease is analyzed. A technique is proposed for converting «F – α» diagrams to «unrestored Brinell hardness HBt – relative unrestored indent depth t/R» diagrams. The elastic and elastoplastic areas of «HBt – t/R» diagrams are described by equations obtained analytically and experimentally. The materials strain hardening parameters during ball indentation in the area of elastoplastic and plastic deformation are proposed. The similarity of «HBt – t/R» indentation diagram with the «stress σ – strain δ» tensile diagrams containing common zones and points is shown. Methods have been developed for determining hardness at the elastic limit, hardness at the yield strength, and hardness at the ultimate strength by instrumented indentation with the equations for their calculation. Experiments on structural materials with different mechanical properties were carried out by instrumented indentation. The values of hardness at the elastic limit, hardness at the yield strength and hardness at the ultimate strength are determined. It is concluded that the correlations between the elastic limit and hardness at the elastic limit, yield strength and hardness at the yield strength, ultimate tensile strength and hardness at the ultimate strength is more justified, since the listed mechanical characteristics are determined by the common special points of indentation diagrams and tensile tests diagrams.

INFLUENCE OF STRUCTURAL-PHASE CONDITION ON THE MECHANICAL-TRI BOLOGICAL PROPERTIES OF Ti3SiC2 COATINGS OBTAINED BY THE DETONATION METHOD

The article provides the results of the research of the structure and properties of powder coatings based on titanium carbosilicide Ti3SiC2 obtained by detonation spraying on the surface of tool steel U9/У9 (equivalent to N9). Micro-indentation methods and abrasive wear tests for mechanical and tribological properties of Ti3SiC2 based coatings were conducted. The microstructure of the coating has a layered structure. The border between the coating and the base has a characteristic crinkled appearance. It was determined that the phase composition of the coatings changes during detonation spraying is a result of the decomposition of Ti3SiC2 powder into titanium carbide and titanium carbosilicide (secondary phases). Selected consolidation conditions ensure the formation of a Ti3SiC2/TiC composite material. The influence of the second phase content (TiC, TiO2) on the properties of coatings was studied. Studies of the microhardness of samples with coatings showed that, in the entire range of annealing temperatures, the microhardness of the Ti3SiC2/TiC composite material increases compared to the coating before annealing. It was found that the maximum microhardness of the Ti3SiC2/TiC composite material after annealing at a temperature of 800 ○C is explained by an increase in the content of the Ti3SiC2 phase. It was established that, during detonation spraying of Ti3SiC2 powders, a coating with a higher microhardness and wear resistance is formed.

ASSESSMENT OF DURABILITY OF COATINGS IN DIFFICULT STRESS CONDITIONS

The mechanical properties of protective powder coatings based on epoxy-polyester applied to steel substrates were studied. The mechanical characteristics of coatings, as well as their influence on the behavior of samples under mechanical loading, were determined. Tests of samples with coatings, created by Nano indentation methods, with subsequent processing of experimental data using standard analytical simplified models and using numerical finite element modeling were performed. In tensile tests it was demonstrated that presence of coatings leads to changes in the nature of fracture of the samples. In stability tests, the results of experimental data were compared with the results of analytical and numerical calculations, which demonstrated the effect of coatings on the stability of thin-walled samples. The mechanical properties of samples with coatings on one or both sides of the sample were studied, which also confirmed the elastic modulus of the studied coatings at a level of up to 10 GPa (with the average value for bulk materials up to 3.5 GPa). In the process of testing at elevated temperature, the importance of ensuring tight clamping of the samples was established, in the absence of which significant increase in the measurement error takes place. Due to temperature deformations, stiffness of the samples is reduced and, accordingly, deviate from conditions of hard seal assumed in calculations. The results obtained are aimed at developing an understanding about mechanical properties, as well as about methods for changing and calculating mechanical properties, thin protective coatings used in various industries and, in particular, a wide class of coatings used to protect aircraft structural components from corrosion.

3D Imaging of Indentation Damage in Bone

Bone is a complex material comprising high stiffness, but brittle, crystalline bio-apatite combined with compliant, but tough, collagen fibres. It can accommodate significant deformation, and the bone microstructure inhibits crack propagation such that micro-cracks can be quickly repaired. Catastrophic failure (bone fracture) is a major cause of morbidity, particularly in aging populations, either through a succession of small fractures or because a traumatic event is sufficiently large to overcome the individual crack blunting/shielding mechanisms. Indentation methods provide a convenient way of characterising the mechanical properties of bone. It is important to be able to visualise the interactions between the bone microstructure and the damage events in three dimensions (3D) to better understand the nature of the damage processes that occur in bone and the relevance of indentation tests in evaluating bone resilience and strength. For the first time, time-lapse laboratory X-ray computed tomography (CT) has been used to establish a time-evolving picture of bone deformation/plasticity and cracking. The sites of both crack initiation and termination as well as the interconnectivity of cracks and pores have been visualised and identified in 2D and 3D.

Evaluation of Soft Material Fracture Behavior via Indentation Testing With a Needle-Like Indenter

Because the fracture behaviors of soft materials are complex, high-precision technology is needed to perform the detailed analysis necessary to produce more effective materials. Indentation methods exist to characterize the fracture behavior of soft materials, with fractures varying in accordance with the shape of the indenter. Thus, it is important to clarify the relationship between indenter shape and the fracture behaviors of soft materials, and necessary to consider the complicated deformation patterns induced by the mechanical nonlinearity of the materials. To this end, various needle-like indenter shapes are modeled via a finite element method (FEM). In this study, we employ a cone-shaped reference needle. Then, the shape is changed to have the tip form a curvilinear indentation, resembling a steep mountain ridgeline. In addition, shear strain is set as the fracture criterion for soft material in the FE analysis since the shear force resulting from penetration primarily damages the soft materials. Regarding the force applied to the needle, there is a tendency for the force to become smaller as it more resembles a cone shape. There are three fracture types: holing, opening, and slitting. Holing is generated by the cone needle, and the fracture of soft material appears along the shape of the needle. Opening fractures are generated by the needle with a curvilinearly spreading tip, forming a small slit on the surface of the soft material. Lastly, slitting fractures are also generated by the needle with a curvilinearly spreading tip, but the resulting slit is deepened without damaging the tissue surrounding the needling. These fracture types can also be classified according to the fracture area of the soft material surface, with slitting resulting in the smallest fracture area.

Error estimation of the physical and mechanical characteristics measurements by indentation

The active application in the practice of testing the indentation methods, in particular to measure the physical and mechanical properties of metals, polymers, biological technologies demands to development techniques for the measurement error estimation. At the same time existing traditional measurement error evaluation system, based on the using of the reference blocks, is not always suitable for use in testing and research laboratories. The aim of this work was development the technique for estimating the indirect measurements error of materials physical and mechanical characteristics that can be applied in practice and based on the existing standards. Checking of the proposed approach using the experimental values of the hardness and elastic modulus obtained during static indentation for various metals.It is shown that since the initial information about the material is an indentation curve representing the dependence of the load versus penetration depth of the indenter into the material tested, then it is better to confirm the metrological characteristics of the indentation measuring devices using the applied force and achieved displacement, but to estimate the accuracy of determining the properties through the error of indirect measurements. The equations for calculating the hardness and modulus of elasticity are derived. It allows to determine the component value most influencing the error magnitude. The calculation of error on the base of the value of boundary of a random and non-exclusive systematic error was carrying out.The advantage of the developed technique is the fact that the measurement of the physical and mechanical characteristics is based on the experimental data and does not require the creation of the additional metrological assurance. The proposed approach seems appropriate to extend for the determination of the measurement error of other characteristics: the yield point, the strain hardening exponent, creep, relaxation, determined by the indentation methods.