Mikhail Petrzhik

Structure and properties of CaO- and ZrO2-doped TiCxNy coatings for biomedical applications

Abstract In the present work multicomponent thin films based on the systems Ti–Ca–C–O–N and Ti–Zr–C–O–N have been deposited and evaluated. TiC 0.5 +10% CaO and TiC 0.5 +10% ZrO 2 targets were manufactured by means of the self-propagating high-temperature synthesis (SHS) method. The synthesized targets were subjected to DC magnetron sputtering in an atmosphere of argon or in a gaseous mixture of argon and nitrogen. The films were characterized in terms of their structure, surface topography, mechanical properties and tribological behavior. The films deposited on Si substrates under optimal conditions showed high hardness in the range of 36–40 GPa, low Youngs modulus 260–300 GPa and high percentage of elastic recovery 70–75%. The CaO- and ZrO 2 -doped Ti–C–N films showed significantly lower friction coefficient and wear rate against WC+6% Co alloy in comparison to conventional magnetron–sputtered TiC and TiN films. The biocompatibility of the films was evaluated by both in vitro and in vivo experiments (in mice). In vitro studies involved the investigation of the proliferation of fibroblasts Rat-1 and epithelial cells IAR-2 at the tested films and morphometric analysis of the cells cultivated on the films. Fibroblasts and epithelial cells were seeded on the coverslips, coated with examined films and incubated at 37 °C for 24, 48 and 72 h. We did not detect statistically significant differences in the attachment, spreading and proliferation of cultured cells on the coated and the uncoated substrata. The adhesion and proliferation of cells was good at all investigated films. We also did not observe any inflammatory reactions on the implants, inserted under the mouse skin.

Structure formation during thermomechanical processing of Ti-Nb-(Zr, Ta) alloys and the manifestation of the shape-memory effect

The formation of structure during thermomechanical processing by the regime of cold plastic deformation by rolling and postdeformation annealing (PDA) and its influence on the mechanical properties of Ti-Nb-(Zr, Ta) shape-memory alloys (SMAs) have been investigated. A moderate strain (e ≈ 0.3) leads to the formation of a developed dislocation substructure in the β phase. With going to severe plastic deformation (e ≈ 2), a nanocrystalline structure can locally be formed without the amorphization of the structure. There are also present α″-, α-, ω phases in the deformed alloys. When the PDA (1 h) is performed below 450°C, the structure of the β phase changes only slightly. Above 450°C, a polygonized substructure is formed in the β phase, which is nanosubgrained at an annealing temperature of 500°C and transforms completely into a submicron one at 600°C. In the case of severe plastic deformation to e ≈ 2, in this range of annealing temperatures, high-angle misorientations of blocks are also observed. The recrystallization of the β phase in the Ti-Nb-(Zr, Ta) SMAs develops at temperatures above 600°C. The presence of the ω phase is detected at temperatures of up to 550°C. The lattice parameters of the strain-induced α″ martensite formed in the Ti-Nb-Ta alloy are independent of the PDA temperature in the range from 600 to 900°C, where the polygonized substructure transforms into the recrystallized structure of the β phase. The range of PDA temperatures that are most favorable for the manifestation of the effect of superelasticity in the Ti-Nb-(Zr, Ta) alloys is in the vicinity of 600°C.

Effect of Al, Si, and Cr on the thermal stability and high-temperature oxidation resistance of coatings based on titanium boronitride

The thermal stability and resistance to high-temperature oxidation of multicomponent nanostructured coatings in the Ti-X-B-N (X = Al, Si, Cr) system have been studied using X-ray diffraction analysis, X-ray photoelectron spectroscopy, secondary-ion mass spectroscopy, and transmission electron microscopy. The hardness, elastic modulus, elastic recovery, friction coefficient, wear rate, and adhesion strength of the coatings have been determined. It has been established that Ti-B-N and Ti-Cr-B-N coatings exhibit a stable nanostructure and high stable mechanical and tribological properties up to 1000°C. The coatings with an fcc structure can be also employed as barrier layers preventing diffusion of metal atoms from the substrate. It has been shown that the high resistance to high-temperature oxidation of Ti-Cr-B-N and Ti-Al-Si-B-N coatings is connected with the fact that protective oxide layers based on (Ti,Cr)BO3 and TixAlySiOz are formed on their surface.

Investigation of the structure stability and superelastic behavior of thermomechanically treated Ti-Nb-Zr and Ti-Nb-Ta shape-memory alloys

The superelastic parameters of Ti-Nb-Ta and Ti-Nb-Zr alloys, such as Young’s modulus, residual strain, and transformation yield stress after thermomechanical treatment (TMT), were determined during cyclic mechanical tests using the tension-unloading scheme (maximum strain 2% per cycle, ten cycles). The superelastic parameters and the alloy structure have been studied by electron microscopy and X-ray diffraction analysis before and after testing and after holding for 40 days, as well as after retesting. The Young’s modulus of the Ti-Nb-Ta alloy decreases from 30–40 to 20–25 GPa during mechanocycling after TMT by different modes; however, it returns to its original magnitude during subsequent holding for 40 days, and changes only a little during repeated mechanocycling. The Young’s modulus of the Ti-Nb-Zr alloy changes insignificantly during mechanocycling, recovers during holding, and behaves stably upon repeated mechanocycling. Surface tensile stresses arise during mechanocycling, which facilitate the development of martensitic transformation under load, orient it, and thereby promote a decrease in the transformation yield stress and the residual strain. The enhancement of the level of initial strengthening stabilizes the superelastic behavior during mechanocycling.

Mechanical and electrochemical characteristics of thermomechanically treated superelastic Ti – Nb – (Ta, Zr) alloys

Such parameters of superelasticity as elastic modulus, residual strain, phase yield point and mechanical hysteresis are determined by cyclic mechanical tests in a tension-unloading mode (2% maximum deformation, 10 cycles) for alloys of the Ti – Nb – Ta and Ti – Nb – Zr systems to be used as materials for medical implants. The testing is preceded by a thermomechanical treatment including multiple cold deformation and post-deformation annealing at various temperatures with water cooling. The stability of the parameters of superelasticity in subsequent aging at room temperature and repeated tests is studied. The annealing modes yielding an oxide film with maximum cohesion strength are determined.

Mechanocyclic and Time Stability of the Loading-Unloading Diagram Parameters of Nanostructured Ti-Nb-Ta and Ti-Nb-Zr SMA

The Ti-21.8Nb-6Zr and Ti-19.7Nb-5.8Ta (at.%) shape memory alloys are thermomechanically treated by cold drawing and post-deformation annealing at 550-600°C forming a nanosubgrained structure in the β-phase. Cyclic mechanical testing using a “loading-unloading” mode with 2% tensile strain in each half-cycle reveals the non-perfect superelastic behavior of both alloys during the very first cycles of testing, which becomes perfect during further mechanocycling. The Young’s modulus of thermomechanically-treated alloys is low (about 45 GPa), and it decreases during mechanocycling (n=10 cycles) down to 25-35 GPa, approaching the Young’s modulus of cortical bone tissues. The Young’s modulus obtained in the 10th cycle is stable or changes only slightly during a further 40-day pause at room temperature and then during repeated mechanocycling. The residual strain per cycle, the transformation yield stress and the mechanical hysteresis decrease during mechanocycling. Subsequent to a 40-day pause at room temperature, they restore their initial values. Repeated mechanocycling is accompanied by a repeated decrease of these parameters.

A comparative study of structure formation in thermomechanically treated Ti-Ni and Ti-Nb-(Zr, Ta) SMA

The processes of structure formation in Ti-Ni and in Ti-Nb-Zr, Ti-Nb-Ta shape memory alloys (SMA) under thermomechanical treatment (TMT) were studied. The TMT comprised cold rolling with true strains from e=0.25 to 2 and post-deformation annealing. Differences in these processes between two groups of alloys are considered. The main conclusions are as follows: nanostructures created by TMT are useful for radical improvement of the SMA functional properties, and an optimum nanostructure (nanocrystalline structure, nanosubgrained structure or theirmixture) should be selected by taking into account other structural and technological factors.

Estimation of the crystallographic strain limit during the reversible β ⇄ α″ martensitic transformation in titanium shape memory alloys

Three methods are described to calculate the crystallographic strain limit that is determined by the maximum deformation of the crystal lattice in the reversible βbcc ⇄ α″orth martensitic transformation and ensures pseudoelastic deformation accumulation and shape recovery in Ti-Nb-Ta alloys.

Estimation of the internal stresses in amorphous glass-covered microwires

A technique is developed to estimate the internal stresses in amorphous glass-covered Co69Fe4Cr4Si12B11 alloy microwires with micron-scale cross sections that are produced by melt quenching by the Taylor-Ulitovskii method. In the developed model, a microwire is considered as a microcomposite to take into account the internal stresses that are induced by the difference between the thermal expansion coefficients of glass and metal during rapid cooling from the solidification temperature to room temperature. The radial, tangential, and axial stresses are calculated in both a glass shell and a metallic core using experimental data on the elastic moduli and the geometric sizes of these structural components of a microwire. The mechanical properties are determined by selective nanoindentation of transverse polished sections of microwires.

A comparative study of microstructure, oxidation resistance, mechanical, and tribological properties of coatings in Mo–B–(N), Cr–B–(N) and Ti–B–(N) systems

M–B–(N) (M = Mo, Cr, Ti) coatings were obtained by the magnetron sputtering of MoB, CrB2, TiB, and TiB2 targets in argon and in gaseous mixtures of argon with nitrogen. The structure and composition of the coatings have been investigated using scanning electron microscopy, glow-discharge optical emission spectroscopy, and X-ray diffraction. The mechanical and tribological properties of the coatings have been determined by nanoindentation, scratch-testing, and ball-on-disk tribological tests. The experiments on estimating the oxidation resistance of coatings were carried out in a temperature range of 600–1000°С. A distinctive feature of TiB2 coatings was their high hardness (61 GPa). The Cr–B–(N) coatings had high maximum oxidation resistance (900°С (CrB2) and 1000°С (Cr–B–N)) and possessed high resistance to the diffusion of elements from the metallic substrate up to a temperature of 1000°С. The Mo–B–N coatings were significantly inferior to the Ti–B–(N) and Cr–B–(N) coatings in their mechanical properties and oxidation resistance, as well as had а tendency to oxidize in air atmosphere after long exposure at room temperature. All of the coatings with nitrogen possessed a low coefficient of friction (in a range of 0.3–0.5) and low relative wear ((0.8–1.2) × 10–6 mm3 N–1 m–1.