Multilayered and functionally graded coatings are extensively used for protection against wear of the working surfaces of mechanisms and machines subjected to sliding contact. The paper considers the problem of wear of a strip made of a functionally graded material, taking into account the heating of the sliding contact from friction. Wear is modeled by a moving strip along the surface of a hard abrasive in the form of a half-plane. With the help of the integral Laplace transform with respect to time, the solutions are constructed as convolutions from the law of the introduction of an abrasive into the strip and the original in the form of a contour integral of the inverse Laplace transform. The study of the integrands of contour quadratures in the complex plane allowed determination of the regions of stable solutions to the problem. Unstable solutions of the problem lead to the concept of thermoelastic instability of the contact with friction and formed regions of unstable solutions. The solutions obtained made it possible to determine a formula for the coefficient of functionally graded inhomogeneity of the coating material and to study its effect on the occurrence of thermoelastic instability of the contact taking friction into account, as well as on its main characteristics: temperature, displacement, stress and wear of the functionally graded material of the coating. The effects of the abrasive speed, contact stresses and temperature on wear of the coating with the functionally graded inhomogeneity of the material by the depth were investigated.

Crack resistance of silicon wafers plays a vital role in development of MEMS technologies containing beam elements. In the present research, this characteristic was determined using the Vickers tip indentation method. The critical stress intensity factor K-IG and fracture energy G(IG) of silicon wafers of (1 0 0), (1 1 0), and (1 1 1) crystallographic orientations were evaluated. The measurements were supplemented by imaging of indents using atomic force microscopy (AFM). The correlation of these parameters with the specific surface energy, Young's modulus E and microhardness H was conducted. The values of E and H were evaluated by nanoindentation. The dependences of K-IG and G(IG) on the load of silicon wafers of (1 0 0), (1 1 0), and (1 1 1) orientations were obtained.

In the present paper X-ray microtomographic research of a molar tooth was conducted. The study revealed regions with a reduced mineral density in the vicinity of the fissure tip. The basic assumption investigated is that corrosion induced enamel mineral density decrease is enhanced by high tensile stresses generated by mechanical load on the occlusal surface of the tooth during crushing of food. Magnitude and location of tensile stress concentration occurs at the fissure tip and may be determined by solving the problem of the stress-strain state of the tooth crown enamel with a wedge-shaped notch. The study of stresses in the vicinity of fissure tip make it possible to construct the boundaries of enhanced enamel virtual fracture. Comparison of the sizes and locations of areas with a reduced enamel mineral density with the sizes and locations of areas of virtual enamel fracture made it possible to establish their approximate congruence. This circumstance made it possible to recreate by mathematical means the nature and magnitude of the force load on the lateral surface of the fissure. Degree of influence of the main parameters of the fissure on the geometrical characteristics of the virtual fracture, such as its area and diameters, were determined.

The application of two-layer coatings is one of the most widely used ways to increase the performance capacity of heavily loaded frictional joints. Design of two-layer coatings for dry sliding friction surfaces and prediction of their efficiency involve the need to develop mathematical models of sliding contact allowing for wear and frictional heating. To achieve this goal, we consider a nonsteady quasi-static contact problem of uncoupled thermoelasticity of a rigid punch in the form of half-plane sliding with a constant velocity over the surface of a two-layer elastic coating bonded on its bottom face to a flat rigid substrate. During the sliding, the punch also penetrates into the two-layer coating in a direction normal to its surface. The punch friction against the coating surface is mathematically described by the Coulomb friction model. The heat flow generated by friction is directed into the coating depth. The frictional sliding contact gives rise to wear of the two-layer coating. The stated problem is solved using the Laplace integral transform. The basic characteristics of each layer, namely, displacements, temperature, and stresses, are presented in the form of the contour integrals of meromorphic functions. The analysis of the properties of the integrand poles in the complex plane of integration and determination of thermoelastic instability domains in the space of problem parameters are followed by evaluation of the contour integrals. The influence of the problem parameters on the sliding contact main characteristics, i.e., temperature, wear, and stresses, is studied. As an example, the wear of a two-layer coating with its upper layer made of titanium nitride and lower layer made of titanium (TiN/Ti) is considered. The numerical results show that by properly selecting the coating layer thicknesses, better wear resistance and lower contact temperature of the coating can be achieved.

Fissure caries in a form of brown spot lesion is a serious problem for patients. Having passed the dentine-enamel junction, cariogenic bacteria cause demineralization of dentine in its vicinity. Over time, the area of demineralization spreads deeper towards the pulp of the tooth, creating a gradient of strength characteristics in dentine. The aim of this work was to determine the mechanical properties and mineral density in the direction from the dentine-enamel junction to the dental pulp using nanoindentation and X-ray computed microtomography. The results can be used as control ones for the further research of remineralization techniques used in clinical practice.

BACKGROUND:Current assessment methods of penile cavernous fibrosis in animal models have limitations due to the inability to provide complex and volume analysis of fibrotic alterations.OBJECTIVE:The aim was to evaluate micro-computed tomography for assessment of cavernous fibrosis and compare it with histological, histochemical, immunohistochemical, and RT-PCR analysis.MATERIALS AND METHODS:A controlled trial was performed involving 25 New Zealand male rabbits with induced testosterone deficiency by orchidectomy. Penile samples were obtained before and after 7, 14, 21, and 84 days from orchidectomy. We consistently performed (a) gray value analysis of corpora cavernosa 3D models reconstructed after micro-computed tomography, (b) morphometry of smooth muscles/connective tissue ratio, collagen type I/III ratio, and area of TGF-beta-1 expression in corpora cavernosa, and (c) RT-PCR of TGF-beta-1 expression.RESULTS:Micro-computed tomography allowed visualization of penile structures at a resolution comparable to light microscopy. Gray values of corpora cavernosa decreased from 1673 (1512-1773) on the initial day to 1184 (1089-1232) on the 21st day (p < 0.005). However, on the 84th day, it increased to 1610 (1551-1768). On 21st and 84th days, there was observed a significant decrease in smooth muscle/connective tissue ratio and a significant increase in collagen type I/III ratio (p < 0.05). TGF-beta1 expression increased on the 84th day according to immunohistochemistry (p < 0.005). RT-PCR was impossible to conduct due to the absence of RNA in obtained samples after micro-CT.DISCUSSION AND CONCLUSIONS:Micro-computed tomography provided 3D visualization of entire corpora cavernosa and assessment of radiodensity alterations by gray value analysis in fibrosis progression. We speculate that gray value changes at early and late fibrosis stages could be related to tissue reorganization. RT-PCR is impossible to conduct on tissue samples studied by micro-CT due to RNA destruction. We also suggest that micro-computed tomography could negatively affect the immunohistochemical outcome, as a significant increase of TGF-beta-1 expression occurs later than histological fibrotic signs.

Polylactic acid (PLA) porous composite scaffolds with three different infill patterns were subjected to compression testing. The composites were fabricated by 3D printing based on Fused Filament Fabrication (FFF) technology. Stress-strain curves were obtained which allowed to identify Young’s modulus, yield point and residual strain of samples. 3D tomography images were taken before and after deformation. Comparison between images corresponding to a particular sample were used to identify deformation and failure mechanisms.

The protection of the working surfaces of mechanisms in sliding contact conditions is often carried out by applying protective multilayer and functionally graded coatings, which prevent wear of the working surfaces and reduce the temperature heating of the contact. The problem of grinding the surface of oxidized and other materials with a functionally graded change in properties along the depth of the product leads to the need to control the wear rate and contact heating from friction. The effectiveness of studying the processes of wear, grinding, polishing and early diagnostics of thermoelastic instability of sliding contact is facilitated by mathematical modeling of the process of wear of products made of functionally graded materials. The thermoelastic contact problem of the wear of a functionally graded coating with an arbitrarily varying shear modulus with a hard abrasive, taking into account the heating of the contact from friction, is considered. The solutions of the problem are constructed in the form of Laplace convolutions. Analysis of the obtained solutions in the complex plane makes it possible to determine the regions of thermoelastic stability and instability of the solutions in the space of dimensionless parameters of the problem. Unstable solutions give rise to the concept of thermoelastic instability of a sliding contact. The constructed analytical solutions made it possible to study the effect of the functionally graded inhomogeneity coefficient of the coating material on the thermoelastic instability regions of the sliding contact, temperature, displacements, stresses and wear of the functionally graded coating material.

The paper presents the results of a study of the surface fracture tested for bending at negative temperatures of a ferrite-martensite composite (FMC) obtained on the basis of K 02704 structural hypoeutectoid steel by atomic force microscopy. Metallographic studies of the sample were carried out on longitudinal sections. Investigations of microstructures were performed on optical microscopes Metam RV-22 and Neophot-21. Impact bending tests were carried out in accordance with ISO 148-1:2016. The quantitative assessment of martensite was carried out using the digital imaging program and amounted to 30%. A three-dimensional image of the fracture of the sample surface was obtained by atomic force microscopy (Nanoeducator II) in the semi-contact mode. An AFM study of the fracture surface showed that the data of optical and atomic force microscopy are in agreement. The three-dimensional AFM image of the surface fracture and its profiles, extracted in different areas, as well as the roughness parameters, show the presence of a dual line structure in steel.

Micro-CT visualization allows reconstruction of eye structures with the resolution of light microscopy and estimation of tissue densities. Moreover, this method excludes damaging procedures and allows further histological staining due to the similar steps in the beginning. We have shown the feasibility of the lab-based micro-CT machine usage for visualization of clinically important compartments of human eye such as trabecular outflow pathway, retina, iris and ciliary body after pre-treatment with iodine in ethanol. We also identified the challenges of applying this contrasting technique to lens, cornea, and retina and proposed alternative staining methods for these tissues. Thereby this work provides a starting point for other studies for imaging of human eyes in normal and pathological conditions using lab-based micro-CT systems.

Current imaging methods in pre-clinical and animal model studies of penile disease are limited because of the small penis of a rat – standard laboratory animal used in this type of research. Routine visualization methods of surface and inner structures of the penis used in clinical medicine are not suitable in experimental animal researches. The only method available for these purposes is histopathological analysis, which does not provide complex view on penile structures due to two-dimensional imaging per slide. In the pre study we evaluated and compared capability of ultrasound, histopathological and micro-computed tomography imaging methods to visualize anatomical structures of laboratory rabbit penis. Ultrasound provides limited imaging in comparison with other methods. However, it could be useful in assessment rough structural alterations of tunica albuginea and corpora cavernosa as a secondary and supportive method. Micro computed tomography provides nearly the same image quality as histopathological analysis. Moreover, it gives a complex view on the whole penile sample due to three-dimensional reconstruction. This fact gives reason to use this method as a basic one in the pre-clinical trials and animal model studies.

Microposit S1813 is very popular photoresist due to high resolution, simplicity of deposition methods, and high adhesion to semiconductors. The least controlled parameter of photoresist thin films deposition is the tanning process, where everything depends on the operator and therefore it is quite difficult to ensure perfect reproducibility of the process. In this work, we studied thin films of Microposit S1813, deposited by spin coating on silicon substrates, and then tanned in 30, 45, 60, 75, and 90 s at 110 $$^\circ $$ C. The thickness of the films was studied using optical microscopy. The waviness and surface roughness were investigated by atomic force microscopy. To evaluate the mechanical properties of Microposit S1813 thin films, the nanoindentation method in the quasistatic mode was used. The results obtained will help to determine the region of the indenter-sample contact area as well as the measurement error, which is necessary for modern analytical mathematical models used to study the mechanical characteristics of multilayer coatings.

In this article, a method is proposed for determining parameters of the exponentialy varying shear modulus of a functionally graded half-space. The method is based on the analytical solution of the problem of pure shear of an elastic functionally graded half-space by a strip punch. The half-space has the depth-wise exponential variation of its shear modulus, whose parameters are to be determined. The problem is reduced to an integral equation that is then solved by asymptotic methods. The analytical relations for contact stress under the punch, displacement of the free surface outside the contact area and other characteristics of the problem are studied with respect to the shear modulus parameters. The parameters of the functionally graded half-space shear modulus are determined (a) from the coincidence of theoretical and experimental values of contact stresses under the punch and from the coincidence of forces acting on the punch, or (b) from the coincidence of theoretical and experimental values of displacement of the free surface of the half-space outside the contact and coincidence of forces acting on the punch, or (c) from other conditions. The transcendental equations for determination of the shear modulus parameters in cases (a) and (b) are given. By adjusting the parameters of the shear modulus variation, the regions of "approximate-homogeneous" state in the functionally graded half-space are developed.

Using atomic force microscopy, wear is performed and the results of determining wear resistances of 100-nm-thick Cu and SiO2 coatings and 3-μm-thick ZrN are presented. The dependences of the specific volumetric wear on the normal mechanical stress at the contact of an AFM probe with the surface during wear are found.

The study focuses on in vitro tracing of some fundamental changes that emerge in teeth at the initial stage of caries development using multiple approaches. The research was conducted on a mostly sound maxillary molar tooth but with a clearly visible natural proximal white spot lesion (WSL). Values of mineral density, reduced Young’s modulus, indentation hardness and creep as well as the molecular composition and surface microstructure of the WSL and bordering dentine area were studied. The results obtained were compared to those of sound enamel and dentine on the same tooth. A decrease of mechanical properties and mineral density both for the WSL and bordering dentine was detected in comparison to the sound counterparts, as well as increase of creep for the enamel WSL. Differences in molecular composition and surface microstructure (including the indenter impressions) were found and described. WSL induces a serious change in the state of not only the visually affected enamel but also surrounding visually intact enamel and dentine in its vicinity. The results provide the basis for future studies of efficacy of minimal invasive treatments of caries.

In the present paper, the mechanical characteristics (reduced Young’s modulus and indentation hardness) were measured for three sound human enamel and dentine samples using nanoindentation. Each sample was subjected to 60 s exposure in citric acid with varying concentration (0.5 wt.%, 3 wt.%, 5 wt.%). Each sample is a longitudinal section obtained from a single human maxillary molar. For each sample, a series of characteristic curves for the dependence of indentation depth on indentation load (load-displacement curves) for both enamel and dentine were obtained.

Alteration of the phase composition of a coating and/or its surface topography can be achieved by changing the deposition technology and/or introducing additional elements into the coating. Investigation of the effect of the composition of CrN-based coatings (including AlCrN and CrON) on the microparticle height and volume, as well as the construction of correlations between the friction coefficient at the microscale and the geometry of microparticles, are the goals of this study. We use atomic force microscopy (AFM), which is the most effective method of investigation with nanometer resolution. By revealing the morphology, AFM allows one to determine the diameter of the particles, their heights and volumes and to identify different phases in the studied area by contrasted properties. The evaluation of the distribution of mechanical properties (modulus of elasticity E and microhardness H) on the surfaces of multiphase coatings with microparticles is carried out by using the nanoindentation method. It is found that the roughness decreases with an increase in the Al concentration in AlCrN. For the CrON coatings, the opposite effect is observed. Similar conclusions are valid for the size of the microparticles and their height for both types of coating.

The results are presented for studies concerning the microstructure, phase composition and mechanical properties of an Al–Cr–N coating obtained by means of the cathode-arc evaporation of Al70Cr30 alloy. The values of the elastic modulus (E) and microhardness (H) of the coating are determined using the nanoindentation method. The contrasts of the adhesion forces in the Al–Cr–N coating, the friction coefficients and the specific volumetric wear are obtained using atomic-force microscopy.