Agata Dudek

Effect of sintering atmosphere on properties of porous stainless steel for biomedical applications

This study discusses manufacturing of metallic biomaterials by means of powder metallurgy with consideration for their unquestionable advantages, i.e. opportunities of obtaining materials with controllable porosity. The paper focuses on properties of 316 L stainless steel obtained using the method of powder metallurgy with respect to compacting pressure and sintering atmosphere. All the specimens were compacted at 700, 400 and 225 MPa, and sintered at 1250 °C. In order to analyze the sintering atmosphere, three different media were used: dissociated ammonia, hydrogen and vacuum. The study covered sintering density, porosity, microstructure analysis and corrosion resistance. The proposed method of powder metallurgy allowed for obtaining materials with predictable size and distribution of pores, depending on the parameters of sinter preparation (compaction force, sinter atmosphere). High corrosion resistance of the materials (sintering in the atmosphere of hydrogen and in vacuum) and high porosity in the sinters studied offer opportunities for using them for medical purposes.

Assessments of Shrinkage Degree in Bioceramic Sinters HA+ZrO2

In the aspect of growing popularity of hydroxyapatites and HA-based composites (with addition of ZrO2 phase) used in medicine, it seems to be crucial to determine accurate percentage content of ZrO2 phase addition in the mixture ensuring invariable or predictable changes in dimensions of implants or coatings after the process of sintering. In consideration of the aforementioned statement, combined mathematical and statistical analysis enabling prediction of such changes was carried out.

Structure and Properties of Bioceramics Layers Used for Implant Coatings

Ceramic materials included in the group of advanced materials are increasingly used in different fields of science and technology as well as in everyday life. Hydroxyapatite ceramics (HAp, HA), based on calcium phosphates, i.e. chemical compounds being a constituent of natural bone, is found to be one of the best implantation materials for bone surgeries and dentistry. Due to poor mechanical properties of hydroxyapatite ceramics, the attempts are made to modify HA materials in order to extend the scope of current applications. It is also necessary to maintain high biocompatibility and bioactivity. Reported investigations encompassed comparison of functional properties of HA and HA+ZrO2 coatings (modified with 8%wt. Y2O3) as well as coatings remelted with laser beam. Microstructural and phase analyses of the coatings before and after remelting were carried out. In order to determine biocompatibility of the coatings after plasma spraying and remelting, corrosion resistance testing was also performed in Ringer’s solution.

Structural Analysis of Sintered Materials Used for Low-Temperature Fuel Cell Plates

Bipolar plates (BPs) are key components of fuel cells. Functions of materials used for fuel cells include equal distribution of gas fuel and air, conduction of electricity between adjacent cells, heat transfer from the cell as well as prevention of gas leakage and cooldown. Moreover, the material must show particular corrosion resistance in cell’s working conditions. Meeting particular requirements or prevention of the abovementioned situations will enable efficient operation of cells. Due to multifunctional nature of fuel cell plates, choice of materials used for plates is immensely difficult. This paper presents opportunities of application of a new technology of powder sintering for creation of parts for electricity and heat generators. This work also presents analysis of structural and phase-related properties, porosity and strength tests.

Properties and Application of Sintered Stainless Steel as Interconnectors in Fuel Cell

This study presents opportunities of application of sintered materials for interconnectors in low-temperature fuel cells. The task of these cell components is to provide even distribution of both fuel and oxidant. Materials for interconnectors must possess good thermal conductivity, conduct electricity and have high strength. It is also required that the material is corrosion-resistant under cell operating conditions. Due to multifunctional nature of these elements, selection of materials for interconnectors is extremely difficult. This paper presents analysis of structural properties and effect of pH level in sulphate solutions (pH = 2, 4, 6) on corrosion resistance in austenitic and ferritic sintered materials.

Structural Analysis of Hydroxyapatite Sinters with Addition of ZrO2 Phase

A range of benefits of implants containing hydroxyapatites results, among other things, from their phase composition and degree of porosity. Poor mechanical properties of hydroxyapatite (HA) ceramics considerably limit its wider use. One of the methods for improvement of poor HA properties is addition of solid solution of Y2O3 in ZrO2. [1-8]. The investigations focused on compositions of ceramic powders based on hydroxyapatite with addition of zirconium dioxide (ZrO2 + 8%wt. Y2O3 and ZrO2 + 20%wt. Y2O3). The powders were axially compacted and then sintered at the temperature of 13000C for two hours. After the process of sintering the samples were subjected to analysis of microstructure, phase composition and geometrical measurements in order to determine volume density in each sample.

ANALYSIS OF THE STRUCTURE AND TRIBOLOGICALPROPERTIES OF SINTERED STAINLESS STEELS

Increasing development of civilization encourages the search for modern engineering materials. At present, very interesting and promising materials are sintered stainless steels that are considered to be the most rapidly developing group of materials, which is related to the demand for materials with good mechanical properties and high resistance to corrosive agents. Taking into account sintered austenitic–ferritic steels, it is possible to obtain a structure with different proportions of the two basic structural components. The main problem limiting the applicability of sintered stainless steels is porosity, which deteriorates not only the strength properties but also the usability as compared to their cast and materials after plastic working. Extremely valuable factors influencing tribological properties are the chemical composition, proportion of individual powders and sintering conditions. In this study, the microstructure and tribological properties of sintered stainless steels obtained by sintering in different proportions of 316L austenitic steel and 409L ferritic steel were compared. The abrasion wear resistance test for individual sintered steels was performed by pin-on-disc method. Słowa kluczowe: zużycie tribologiczne, odporność na ścieranie, pin-on-disc test, spiekane stale nierdzewne, metalurgia proszków (PM), porowatość, stale austenityczno-ferrytyczne. Streszczenie: Zwiększający się rozwój cywilizacyjny zachęca do poszukiwania nowoczesnych materiałów inżynierskich. Obecnie bardzo interesującymi i obiecującymi materiałami są stale spiekane zaliczane do najprężniej rozwijającej się grupy materiałów, co jest związane z zapotrzebowaniem na materiały o dobrych własnościach mechanicznych i wysokiej odporności na oddziaływanie czynników korozyjnych. Biorąc pod uwagę spiekane stale austenityczno-ferrytyczne, możliwe jest uzyskanie struktury o różnych udziałach dwóch podstawowych składników strukturalnych. Głównym problemem ograniczającym możliwości aplikacyjne spiekanych stali nierdzewnych jest porowatość, która pogarsza nie tylko własności wytrzymałościowe, ale również użytkowe w porównaniu z ich odlewanymi i przerabianymi plastycznie odpowiednikami. Niezwykle cennymi czynnikami wpływającymi na własności tribologiczne są skład chemiczny, udział poszczególnych proszków oraz warunki spiekania. W niniejszym opracowaniu dokonano porównania mikrostruktury i własności tribologicznych spiekanych stali nierdzewnych, uzyskanych poprzez spiekanie w różnych proporcjach stali austenitycznej 316L oraz stali ferrytycznej 409L. Badanie odporności na zużycie ścierne dla poszczególnych stali spiekanych zostało przeprowadzone metodą pin-on-disc. * Czestochowa University of Technology, Institute for Material Engineering, Faculty of Production Engineering and Materials Technology, al. Armii Krajowej 19, 42-200 Czestochowa, Poland, e-mail: dudek.agata@wip.pcz.pl, e-mail: lisiecka.barbara@ wip.pcz.pl, e-mail: strzelczak.katarzyna@wip.pcz.pl. INTRODuCTION One of the most modern modifications of stainless steel widely used in many industries is sintered duplex stainless steel – SDSS. The characteristic feature of this group of materials is a two-phase structure consisting of ferrite and austenite [L. 1–2]. The sintered duplex stainless steels are very attractive materials for numerous applications due to their improved high mechanical strength, corrosion resistance, and superior weldability in different atmospheres [L. 3–7]. The SDSSs can be manufactured in very different ways, for example, by 100 ISSN 0208-7774 T R I B O L O G I A 2/2017 mixing ferritic powder with austenite stabilizer element powder [L. 8–9], but, in this paper, the authors selected powder metallurgy for sample preparation. Powder metallurgy has become an excellent alternative to conventional steel production methods, allowing for large-scale production of complex shape components, with great precision in dimensions and surface quality even in the case of small components, which significantly affects the competitiveness of sintered steel against their solid counterparts [L. 10–11]. Powder metallurgy allows modifying chemical composition in a very wide range. This gives the opportunity to produce duplex steel with a very varied microstructure, i.e. the different proportions of the individual phases, giving them similar properties to austenitic or ferritic steels. Therefore, sintered duplex steels can be excellent substitutes for the conventional austenitic, ferritic, or martensitic steels used to date [L. 12–13]. The main recipient of sintered materials is the automotive industry. The use of sintered materials in the various industries is shown in Figure 1 [L. 11]. Fig. 1. Relation between surface tension of aqueous solutions of sulfosuccinate derivatives as a function of their concentration Rys. 1. Zależność napięcia powierzchniowego wodnych roztworów pochodnych sulfobursztynianów w funkcji ich stężenia The crucial role in determining the final microstructure of sintered austenitic-ferritic steels, their mechanical and utility properties, and the transformation occurring during sintering have the chemical composition, the proportion of individual powders, and the sintering conditions. During the sintering of austenitic-ferritic steels, diffusion of Fe, Cr, and Ni occurs. The Fe selffusion coefficient in ferrite is, on average, 100 times higher than in austenite (1.2 μm2/s and 8.9∙10-3 μm2/s, respectively), which accelerates the particle bonding process and thus affects the sinter density; the greater the proportion of ferritic steel powder, the higher the density. The effect of the diffusion of the alloy elements by the contact surface of the powder particles of ferritic and austenitic steel is the the formation of inter-diffusion zones [L. 14–15]. Limitations of the application of sintered steels result from the reduced wear resistance [L. 16]. So far, the wear of sintered duplex steels has received relatively little attention, and most of the available studies concern ferritic steels [L. 17–19] or general abrasive wear concepts [L. 20]. Abrasive wear of sintered steel is a complex process, depending on many factors, i.e. porosity, microstructure, hardness, and working conditions. For sintered steel, the following wear mechanisms may be present: – Mechanical: plastic deformation, abrasion process associated with the presence of loose particles causing scratching, fissuring, machining; – Physical: mainly the phenomenon of adhesion, consisting in the alternating tearing of particles of one material, tacking with the other material and rupturing of these joints; and, – Chemical: associated with the chemical reactions between the friction materials and the medium, e.g., oxidation leading to the chipping of the material particles [L. 21]. Generally, it can be assumed that a higher degree of microstructural inhomogeneity of the sintered austenitic-ferritic steels as well as high hardness are reflected in a high coefficient of friction and a smaller wear of the material [L. 22]. The role of porosity in the process of tribological wear is ambiguous [L. 21]. Under friction conditions involving lubricants, pores accumulate lubricant, which limits the rate of wear of the material [L. 20]. Under 101 ISSN 0208-7774 T R I B O L O G I A 2/2017 dry friction conditions, it is generally assumed that steels with lower porosity are characterized by better wear resistance. However, the presence of pores may reduce the contact load and inhibit the deformation of the material in areas around the pores. Pores may also play the role of “traps” that accumulate loose particles, resulting in a greater contact surface and a slower and more uniform wear [L. 21]. In addition, by filling open pores with the particles, the material undergoes surface “arming,” which limits the plastic deformation and further chipping of the particles from the pores around the pores. For sintered steels with a small fraction of ferritic steel powder whose ductility is close to austenitic steel, open pores can be gradually closed with gliding due to plastic deformation [L. 22]. MATERIAL AND METHODS The specimens for the examinations were made of water–atomized commercial powders of 316L and 409L steels manufactured by Höganäs (Sweden). Table 1 presents the chemical compositions of the powders. Both powders selected had a nominal particle size of 150 μm. Table 1. Chemical composition of steel powders (%wt.) Tabela 1. Skład chemiczny proszków stalowych (% mas.) The analysis of the microstructure was carried out using an Axiovert 25 optical microscope and scanning microscope Jeol JSM-6610LV. Vickers methodology (with load of 980.7 mN) was used to measure the hardness of the sintered duplex stainless steel. Identification of phase composition was carried out using the X-ray diffractometer (Seifert 3003 T–T) with a cobalt lamp with characteristic radiation wavelength of lcoka = 0.17902 nm. Other parameters are presented in Table 3. Table 2. Percentage contribution of individual powders Tabela 2. Procentowy udział poszczególnych proszków Table 3. x-ray diffractometer parameters Tabela 3. Parametry dyfraktometru The powders were mixed at different proportions of ferritic and austenitic steel powders to give the two different series of the samples (see Table 2). The powders were compacted with the addition of the Acrawax C lubricant at 720 MPa. The molded pieces were heated at the temperature of 1250°C for 30 minutes and then cooled at the rate of 0.5°C/s. In order to significantly limit the oxidation of the batch and protected from reduction of the chromium content, the whole process was carried out in the reducing atmosphere using hydrogen. Tribological properties of obtained samples were compared with 100% 316L sintered steel. The analysis of tribological properties was performed using a pin-on-disc type Tester T–0.1M under unlubricated sliding contact against the steel ring (HRC 58–63). The steel specimens had

Surface Treatment Proposals for the Automotive Industry by the Example of 316L Steel

Abstract Nowadays, stainless steels are very interesting and promising materials with unique properties. They are characterized high mechanical strengths, high toughness and good corrosion resistance, so that can be used in many industrial sectors. An interesting alternative to steels obtained using the conventional methods is sintered stainless steel manufactured using the powder metallurgy technology. AISI 316L stainless steel is one of the best-known and widely used austenitic stainless steel. Modification of surface properties of stainless steels, in particular by applying the Cr3C2 coating is becoming more and more popular. The technique of atmospheric plasma spraying (APS) was used to deposit Cr3C2 - NiAl powder on stainless steel surface. In this study presents arc surface remelting of two types of stainless steel was used by GTAW method in order to improve function and usability these materials. The results of optical microscope metallographic, hardness and scratch test are presented. The main assumption for this study was to analyze the microstructure and hardness after remelting and alloying the surface of 316L steel (using GTAW method) with current intensity 50 A.

Martensitic transformation in TiNi alloy after surface modification done by hydroxyapatite layer deposition

ABSTRACT A hydroxyapatite (HAp) layer was deposited on the surface of an NiTi alloy using a spark plasma technique. The effects of spraying conditions on the martensitic transformation were investigated in three heat-affected zones: the zone under the handle, the transition zone and in a representative zone with HAp coverage. In the representative area, a reduction in the amount of non-transformable phases was observed. A part of the sublayer showed a nanocrystalline or completely amorphous structure. This sublayer was approximately 30 µm thick. Structural changes contributed to the reduction of the temperature of the reverse martensitic transition down to 25°C. Moreover, modification of the surface revealed a positive influence on the transformation course by narrowing its temperature range and increasing its enthalpy. This paper is part of a thematic issue on Titanium.

Analysis of Structure of Elements for Automotive Industry

The paper presents the results of the structure and chemical composition of materials used to manufacture of gear wheel for the automotive industry. Analyzed gear wheel that is a part of one of the mechanical systems of an automotive vehicle was made of sinter Sint-D 32 in the technology of powder metallurgy and alloy structural steel for quenching and tempering 42CrMo4. The cause of the analysis was to research for an alternative material for sinter Sint-D 32 after identified low static strength according to the requirements applicable in the automotive industry. For the analysis were used standard test methods applicable in materials science. Based on microstructure and mechanical properties analysis performed according to requirements applicable in the automotive industry, the research found that steel 42CrMo4 is relevant material to be used in serial production for this particular gear wheel.