With the development of minimally invasive treatment technology, coronary stents made of corrosion-resistant steel are in demand for restoring the patency of blood vessels. The effectiveness of coronary stenting depends on various factors, but the quality of the surface of the stents is a major factor. The higher the quality of the surface of the stent is, the less negative the effect on the circulatory system, arterial walls, and the higher the biocompatibility of the stent is. The complex shape, small cross-section, size, and low rigidity of coronary stents are the main reasons for the inability to ensure high surface quality using mechanical finishing methods. Therefore, electrochemical methods are used to polish stents. For electrochemical polishing (ECP) of stents, an electric mode based on direct current is traditionally used. The disadvantages of direct current ECP are excessive metal removal and the need to use electrolytes of complex compositions, often containing toxic components. As an alternative to the traditional ECP with the use of direct current, we have proposed a method of pulsed ECP using pulses of microsecond duration for polishing stents. The use of pulsed current allows one to achieve a significant increase in the efficiency of the SEC process, when, due to the localization of the anodic dissolution, the smoothing speed of the microroughness of the treated surface, referred to the total metal removal, increases significantly. The paper presents a comparative analysis of ECP modes using direct and pulse current to change the surface roughness, removal, radius of curvature of the edges, and corrosion resistance on the example of stents made of the 316LVM stainless steel. Based on the results of the studies, technological regimes of pulsed ECP were established that provide the highest quality polishing of the stent surface with a small metal removal with a slight rounding of the edges.

Today, world enterprises specializing in the production of titanium and niobium alloy products and having modern high-tech equipment are still forced to use outdated, costly and environmentally unsafe methods of polishing and surface cleaning, based on electrochemical treatment in toxic solutions. The complex geometric shape of the processed products does not allow the use of mechanical polishing methods, due to high labor costs and low efficiency. These materials are difficult to process, and the processes of their polishing require the use of toxic electrolytes. An alternative to existing methods of electrochemical polishing is plasma electrolyte polishing in aqueous solutions of salts with a concentration of up to 5%. We have developed fundamentally new modes of plasma electrolyte polishing of products from titanium and niobium alloys using simple electrolytes based on an aqueous solution of fluoride salt, the use of which can significantly increase the surface quality class. The advantage of the plasma electrolyte polishing over classical electrochemical polishing is its safety due to the use of an electrolyte based on an aqueous solution of salt. The paper presents the results of an investigation of the characteristics and morphology of the surface after processing using the developed method. Modes that provide the maximum change in roughness with minimal metal removal were obtained.

In the manufacture of implants that are subject to increased cyclic loads, cobalt-chromium alloys with high hardness- and wear resistance have recently been widely used. Roughness of working surfaces is one of the most important characteristics of such products. The traditional processes of finishing the surface of cobalt-chromium alloy implants are based on mechanical and electrochemical methods. The disadvantages of mechanical methods are low productivity, susceptibility to the introduction of foreign particles, difficulties in processing of complex geometric shapes. For electrochemical technologies the treated materials are considered intractable, harmful electrolytes, consisting of solutions of acids, are used in the process of polishing. As an alternative to existing methods, it was proposed to use an environmentally safe method of electrolytic-plasma polishing, the main advantage of which is the use of aqueous solutions of salts with a concentration of 3–5 % as electrolytes. According to the results of the technological process, it has been established that at most electrolyte-plasma polishing modes of cobalt-chromium alloys for medical purposes, a relief in the form of a grid of protrusions occurs on the surface, the origin of which can be explained by the heterogeneity of the material structure that occurs at the stage of casting. Moreover, the height of the formed relief protrusions has a direct impact on the amount of surface roughness. As a result of studies, electrolyte-plasma polishing process modes were established, ensuring the formation of a smooth surface without the presence of embossed protrusions, smoothing the microrelief with the removal of scratches resulting from pre-grinding, achieving a low roughness value (Ra 0.057 micron) and a high reflection coefficient (0.7), which fully meets the requirements for the surface of the implants.

Nitinol is widely used in the production of medical devices, especially the ones that are designed for minimally invasive treatment, such as stents to restore vascular patency, stent grafts to eliminate aneurysms, and cava filters to trap blood clots. One of the most important characteristics that determines the reliability of the functioning of such products in the human body is the state of the surface layer. The higher the surface quality, the less negative impact is on the circulatory system, the walls of blood vessels and the higher the biological compatibility of the product. Electrochemical polishing methods are mainly used to improve the surface quality of nitinol products. The disadvantage of the applied electrochemical methods is the need to use aggressive electrolytes that contain toxic components, such as hydrofluoric acid, sulfuric acid, perchloric acid, nitric acid, methanol. As an alternative to the existing methods of electrochemical polishing, we have developed electrolytic-plasma polishing (EPP), a new highly efficient process for improving the surface quality of nitinol products. The most important advantage of the method over traditional electrochemical polishing is the use of aqueous salt solutions with a concentration of 4 % as electrolytes. Based on the results of the studies performed, the most rational EPP mode was established, the use of which during polishing of nitinol provides surface cleaning from scale, polishing with a decrease in the roughness parameter Ra by 0.344 µm and an increase in pitting potential by 33 %.

Реферат. В последнее время в промышленности появилась тенденция использования мето-дов электрохимической обработки, основанных на применении милли-и микросекундных импульсов различной полярности и амплитуды вместо постоянного тока. Применение им-пульсного тока позволяет во многих случаях получить необходимый эффект более деше-выми средствами и обеспечить дополнительную управляемость электрохимическим про-цессом за счет регулировки временных параметров импульсов тока, снизить энергетические затраты на процесс полирования и очистки поверхностей по сравнению c обработкой при постоянном токе, повысить эффективность обработки, при которой скорость сглаживания микронеровностей обрабатываемой поверхности, отнесенная к общему съему металла, зна-чительно возрастает. Например, применение биполярных импульсов при полировании мно-гих металлических материалов позволяет отказаться от использования дорогостоящих и вредных хромсодержащих электролитов. Применение импульсного режима при электро-литно-плазменном полировании помогает добиться снижения энергопотребления и повы-шения эффективности процесса при сохранении высокой интенсивности, качества обработ-ки и экологической безопасности. Для исследования влияния временных параметров им-пульсов тока, а также длительности пауз между ними на характеристики поверхности деталей из различных металлических материалов в процессе электрохимической обработки и при переходе процессов в область электролитно-плазменной обработки при повышении напряжения смоделирован, разработан и изготовлен специальный источник питания, обес-печивающий возможность регулирования частоты, длительности положительного и отрица-тельного импульсов, а также пауз между ними в широком диапазоне. Разработанный источ-ник питания позволяет формировать импульсы тока до 50 А при напряжении от 0 до 400 В положительной и отрицательной полярностей и способен регулировать их длительность в диапазоне от 10,0 мкс до 8,1 с при возможности изменения соотношения длительности импульсов и пауз от 1:1 до 1:9.Ключевые слова: источник питания, биполярное импульсное электрохимическое полиро-вание, электролитно-плазменное полирование, частота, амплитуда