VILNIUS GEDIMINAS TECHNICAL UNIVERSITY

Jelena ŠKAMAT

EFFECT OF VIBRATORY TREATMENT ON THE STRUCTURE AND PROPERTIES OF NICKEL BASED THERMALLY SPRAYED COATINGS SUMMARY OF DOCTORAL DISSERTATION TECHNOLOGICAL SCIENCES, MATERIALS ENGINEERING (08T)

Vilnius 2013

Doctoral dissertation was prepared at Vilnius Gediminas Technical University in 2008–2013. Scientific Supervisor

Prof Dr Habil Algirdas Vaclovas VALIULIS (Vilnius Gediminas Technical University, Technological Sciences, Materials Engineering – 08T).

The dissertation is being defended at the Council of Scientific Field of Materials Engineering at Vilnius Gediminas Technical University: Chairman dr. Valentin ANTONOVIČ (Vilnius Gediminas Technical University, Technological Sciences, Materials Engineering – 08T).

Members: Prof Dr Habil Stasys BOČKUS (Kaunas University of Technology, Technological Sciences, Materials Engineering – 08T), Prof Dr Donatas Jonas SIDARAVIČIUS (Vilnius Gediminas Technical University, Technological Sciences, Materials Engineering – 08T), Prof Dr Raimundas ŠIAUČIŪNAS (Kaunas University of Technology, Technological Sciences, Chemical Engineering – 05T), Prof Dr Nerija ŽURAUSKIENĖ (State Research Institute Center for Physical Sciences and Technology, Physical Sciences, Physics – 02P).

Opponents: Dr Rimantas LEVINSKAS (Lithuanian Energy Institute, Technological Sciences, Materials Engineering – 08T), Prof Dr Rimas MASKELIŪNAS (Vilnius Gediminas Technical University, Technological Sciences, Mechanical Engineering – 09T).

The dissertation will be defended at the public meeting of the Council of Scientific Field of Materials Engineering in the Senate Hall of Vilnius Gediminas Technical University at 2 p. m. on 9 December 2013. Address: Saulėtekio al. 11, LT-10223 Vilnius, Lithuania. Tel.: +370 5 274 4952, +370 5 274 4956; fax +370 5 270 0112; e-mail: doktor@vgtu.lt The summary of the doctoral dissertation was distributed on 8 November 2013. A copy of the doctoral dissertation is available for review at the Library of Vilnius Gediminas Technical University (Saulėtekio al. 14, LT-10223 Vilnius, Lithuania).

© Jelena Škamat, 2013

VILNIAUS GEDIMINO TECHNIKOS UNIVERSITETAS

Jelena ŠKAMAT

VIBRACINIO APDOROJIMO POVEIKIS PURKŠTŲJŲ NIKELIO PAGRINDO DANGŲ STRUKTŪRAI IR SAVYBĖMS DAKTARO DISERTACIJOS SANTRAUKA TECHNOLOGIJOS MOKSLAI, MEDŽIAGŲ INŽINERIJA (08T)

Vilnius 2013

Disertacija rengta 2008–2013 metais Vilniaus Gedimino technikos universitete. Mokslinis vadovas prof. habil. dr. Algirdas Vaclovas VALIULIS (Vilniaus Gedimino technikos universitetas, technologijos mokslai, medžiagų inžinerija – 08T).

Disertacija ginama Vilniaus Gedimino technikos universiteto Medžiagų inžinerijos mokslo krypties taryboje: Pirmininkas

dr. Valentin ANTONOVIČ (Vilniaus Gedimino technikos universitetas, technologijos mokslai, medžiagų inžinerija – 08T).

Nariai: prof. habil. dr. Stasys BOČKUS (Kauno technologijos universitetas, technologijos mokslai, medžiagų inžinerija – 08T), prof. dr. Donatas Jonas SIDARAVIČIUS (Vilniaus Gedimino technikos universitetas, technologijos mokslai, medžiagų inžinerija – 08T), prof. dr. Raimundas ŠIAUČIŪNAS (Kauno technologijos universitetas, technologijos mokslai, chemijos inžinerija – 05T), prof. dr. Nerija ŽURAUSKIENĖ (Valstybinis mokslinių tyrimų institutas Fizinių ir technologijos mokslų centras, fiziniai mokslai, fizika – 02P).

Oponentai: dr. Rimantas LEVINSKAS (Lietuvos energetikos institutas, technologijos mokslai, medžiagų inžinerija – 08T), prof. dr. Rimas MASKELIŪNAS (Vilniaus Gedimino technikos universitetas, technologijos mokslai, mechanikos inžinerija – 09T).

Disertacija bus ginama viešame Medžiagų inžinerijos mokslo krypties tarybos posėdyje 2013 m. gruodžio 9 d. 14 val. Vilniaus Gedimino technikos universiteto senato posėdžių salėje. Adresas: Saulėtekio al. 11, LT-10223 Vilnius, Lietuva. Tel.: (8 5) 274 4952, (8 5) 274 4956; faksas (8 5) 270 0112; el. paštas doktor@vgtu.lt Disertacijos santrauka išsiuntinėta 2013 m. lapkričio 8 d. Disertaciją galima peržiūrėti Vilniaus Gedimino technikos universiteto bibliotekoje (Saulėtekio al. 14, LT-10223 Vilnius, Lietuva). VGTU leidyklos „Technika“ 2176-M mokslo literatūros knyga.

© Jelena Škamat, 2013

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Introduction

The problem under investigation. Nowadays coatings technologies are one of the underlying areas of materials development. Technological developments in various industry fields give rise to constantly increasing demands on part surfaces. Wear, corrosion, high temperature oxidation are the main stresses that can be controlled by surface technology. The creation of protective layers allows effective protecting of working surface from the corrosion and tribological damage and gives associated economic effect related to the less consumption of high-priced metals alloys, hence fewer repair works, and increasing of products durability. Thermal spraying is a separate group of surface technologies. Nowadays thermal spraying is a high-tech branch of world economy with an annual turnover of about 5 billion USA dollars, which incorporates about 10 distinguish methods; where the majority of them have got a further wide spread developing in different industrial fields. The coating features are largely determined by the obtained microstructure, thus an introduction of some optimizations and modification of structure may lead to a significant improvement of the coating performance and quality. The chemical composition of spray materials, the deposition method and parameters, and, also, a lot of other factors determine the final structure. The part of the thermal spraying methods allows the porous lamellar anisotropic coating structures, and the monolithic layers with nearly cast-type structure can be obtained by application of other methods. In the last case the structure is obtained after solidification from the liquid or semi-liquid state and can be controlled by some structure affecting methods, known and applied in the metallurgy, such as magnetic field, ultrasound, adding of modifiers, low frequency vibrations, etc. However, the application of such type methods has not been studied adequately in the field of the thermal spraying. The aim of the present work is to investigate the influence of the low frequency vibratory treatment on the structure and properties of thermal sprayed and refused coatings.

The topicality of the problem. The use of the Ni-based protective coatings has received a widespread use in such fields, as chemical, petrol, glass mould industry and etc., where wear and corrosion resistance at low and moderate temperatures are required. A two-step process is usually used for producing of such type coating, where the powder is first deposed by the thermal spray and further the as-sprayed layer is remelted. Different remelting techniques may be applied for this purpose. A low-cost and versatile flame fusing is used most

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commonly. The advantages of this technique are especially related to coating of large size parts such as piston rods, earth-working machines, etc. Processes such as a fusion, deoxidation, degassing, nucleation and growth of grains are occurred during remelting. These factors determine the final coatings structure as well as their properties. Corrosion resistance of coatings is provided mainly by Cr-rich Ni-based metal matrix. The type and amount of hard precipitations, formed after solidification, largely determine the mechanical properties of the obtained coating. The structure optimization and the modification enable significant improving the coating quality and particularly their mechanical properties. It can be reached by two main approaches: first, the increasing of hard inclusions amount in deposited layer and/or refining of microstructure. In the last case the use of some specific structure affecting methods, such as a vibratory treatment, during the solidification may lead to a significant improvement of the coating performance and quality. The results of many researches testify vibratory treatment efficiency in controlling of solidification processes. The application of vibrations of appropriate parameters enables refining of a microstructure, increasing of layer homogeneity, reducing porosity and the amount of other defects, herewith the improving of mechanical and special features. The influence of vibratory treatment on thin Ni-based coatings has not been studied, and therefore the database has not been created adequately on what kind of an impact of vibrations obtainable when such a vibratory treatment is introduced during solidification of refusible layer. This research aims to establish possible effects caused by the introduction of the low-frequency vibratory treatment on the structure and properties of refusible Ni-based coatings. The object of research. The influence of a vibratory treatment on the structure and properties of Ni-based thermal sprayed coatings remelted with the introduction of mechanical vibrations. The aim of the work. To investigate the influence of vibratory treatment on the obtained structure and properties of the Ni-based coatings, in the case when the vibrations are introduced during the remelting. Tasks of the work. The aim of the dissertation is reached by solving two major tasks: 1. To investigate the microstructure and properties of the thermally sprayed coatings refused under the vibratory treatment of various parameters.

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2. To determine the dependences between the coatings characteristics (the microstructure, the hardness, the wear resistance, the corrosion resistance and the roughness) and the parameters of vibrations. Methods of the research. Analytical and experimental investigation methods combine with comparative analysis have been applied in the present research. The thermally sprayed coatings, refused with change of the direction, frequencies and amplitudes of the vibratory treatment have been investigated. The microstructure and properties of initial powders, vibrated coatings and unvibrated references have been examined with use of forward research methods such as optical microscopy, X-ray microanalysis, scanning electron microscopy (SEM), X-ray diffraction, differential thermal analysis (DTA). The measurements of potentiodynamic polarisation, hardness, microhardness and surface roughness as well as wear test have been accomplished in present work. Scientific novelty. The scientific novelty of the present study is proved by the following new results in the field of materials engineering: 1. The vibratory treatment has been applied in process of remelting of thermally sprayed thin Ni-based coatings. 2. The obtained coatings have been investigated using the complex of state of the art research methods and the influence of vibratory treatment on the coating structure and the properties has been determined as well as the importance of each of vibrations parameters: frequency, amplitude and direction. 3. The microstructure refining effect, accompanied by an increase of hardness and improving of wear resistance, has been established on the less alloyed Ni-based coatings with dominance of primary solidification of γ-Ni phase. 4. The range of significant grain refinement effect has been defined. It has been found also that various combinations of the frequency and amplitude allow reaching the same microstructure refining. 5. In more alloyed Ni-based coatings the application of the vibratory treatment leads to the formation of the layered structure with gradually increasing of hardness from the interface to the top of coating and with reduced roughness of the coating surface. In the vibrated coatings a harder microstructure of the main working layer is forming without a decrease in the corrosion resistance. 6. The amplitude of the applied vibrations more intensively influences the layered structure forming and surface roughness reducing in more alloyed Ni-based coatings than the frequency.

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Practical value. The good workability is one of the major advantages of the vibratory treatment process, because it is applicable for various parts with nearly no limitation of shapes and sizes. The results obtained in the present research are related directly to the frequency and amplitude of the applied vibrations. Therefore, a commercially available industrial vibrator with the working parameters determined in the present work may be applied. Complex positive effect of the vibratory treatment has been obtained. The use of the vibrations with the parameters determined in the present work allows obtaining the coatings with a hardened microstructure, smother surface and an improved wear and corrosion resistance. Defended propositions 1. The low frequency vibratory treatment, when applied during the refuse stage, influences and improves the structure and properties of the obtained Ni-based coatings. The obtained effects differ when the vibrations are applied for the refused coatings of a different chemical composition. 2. The low frequency vibratory treatment has no influence on the phase constitution of the obtained coatings. The scope of the scientific work. The scientific work consists of the introduction, three chapters, conclusions, list of references, list of author’s publications and 3 annexes. The total scope of the dissertation is 94 pages of text (without annexes) containing 46 pictures, 18 tables and 3 numbered formulas. 1. The overview on the thermal spray technology and vibratory treatment in materials engineering The two step spray fuse process was developed firstly in order to improve the strength of the bond between the coatings and the substrate which is obtained with a normal thermal spraying process, and at the same time to increase the density of sprayed layer by eliminating pores and cavities, inevitably formed in the as-sprayed layer. Protective refusible Ni-based coatings have received widespread use due to their excellent wear and corrosion resistance at low and moderated temperatures. The change of a spraying powder chemical composition enables varying the final properties of Ni-based coatings in a wide range from the soft (~300 HV) antifriction coatings with excellent corrosion resistance to the hard (~780 HV) with good wear and erosion resistance. The analysis of recently research works has showed that the enhancing of wear resistance are obtainable by adding high-melting-point and high-hardness

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compounds such as carbides, nitrides, borides into the coatings. The rare earth elements have a more complex influence on the properties of the coatings and make a compromise between hardness and toughness besides improving the corrosion and oxidation resistance of the coatings. The application of different deposition and remelting methods normally leads to the partially change in the phase composition and morphology of layers. When the concentrated energy sources (such as a laser, electron or ion beam) are used, the extremely high cooling velocities lead to the forming of very fine nano-level crystal and amorphous microstructures with enhanced strength properties. Nanophase layers are also developed by an application of feedstock materials in a form of agglomerated nano-powders, nano-solutions and nano-suspensions. At the same time many researches in the other fields of metallurgy, such as casting and welding, show that the use of specific natures’ energies can be effective in control of the metals solidification processes. Thus, the complex positive impact of the low frequency vibrations on the microstructure and properties of ingots have been reported in a lot of works. The more homogeneous fine microstructure with the decreased amount of porosity and other defects and with the improved mechanical and special properties may be obtained when the vibrations of suitable parameters are introduced during the solidification. The impact of the vibratory treatment on the residual stresses is studied mostly in a field of welding technologies. Based on the obtained effects the vibratory stress relief equipment has been developed, which now is already commercially available one. In technologies of protective metal coatings the effects of vibratory energy have not been studied adequately. A few related publications on laser cladded and plasma sprayed Fe-base coatings have been appeared. The reported results testify the efficiency of the vibratory treatment in the refining of microstructure, improving of the typical features and the uniformity of obtained coatings. The question about the expedience of application of the vibratory treatment for the Ni-base coatings remains open, since there is no any related data.

2. The methodology of the vibrational refusing of the as-sprayed Ni-based coatings, used research methods and materials

The methodology of the experimental research has been performed considering the main aim of present research: to discover possible effects of the vibratory treatment on the microstructure and properties of refusible Ni-based coatings. Commercial self-fluxing Ni-based alloy powders of two compositions have been used as spraying material (Table 1). The use of A-type powder with lower content of alloying elements (about 82% Ni) allows obtaining soft monolithic layers with expressed grain-type microstructure and good resistance to

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corrosion and high-temperature oxidation. In very hard coatings, produced from more alloyed B-type powder (about 67% Ni), high amount of borides and carbides, incorporated in Ni-based matrix, determines good resistance to both wear and corrosion.

Table 1. Nominal chemical composition of the spraying powder (in wt. %) Powder Ni Cr B Si Fe C Mo Cu A ~82% 10.2 1.61 3.19 2.79 0.44 - - B ~67% 15.7 3.92 4.11 2.72 0.45 4.69 1.94 The microstructural and thermal studies of initial powders have been carried out. The morphology and microstructure of the powders are presented on Fig. 1. Nominally powders consist of spherical grains having grain diameter of 38–125 µm, in the reality it was found that part of powder particles has irregular shape or the shape of coalesced spheres. The particles with a size less than declared range have been also found, mostly in a form of small “satellites”, adhered to larger particles. The melting points and solidus-liquidus intervals have been established for both A and B powders.

Fig. 1. The morphology and microstructure of initial powders Initial powders were deposited on structural steel S235J0 (EN 10025-2) substrate plates (100x100x10 mm) by the flame spraying. The preparation of the substrate and the spraying were performed according to the recommendation of the powders and spraying/fusing tools producer (Castolin Eutectic). After spraying, the as-sprayed layers with thickness of 1.2–1.4 mm have been remelted by heating up to about 1100 ˚C with neutral oxy-acetylene

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flame. In order to make the same spraying and fusing condition for each specimen, robotic equipment Motoman 100 have been used. The heat input has been controlled also by controlling O2 and C2H2 flow rates. The refusing experiments under vibratory treatment have been performed by three stages. The first and the second stages have covered the investigation of A-type coatings, refused with introducing of perpendicular (1st stage) and parallel (2nd stage) to substrate vibrations. B-type coatings have been investigated during the 3rd stage. Considering the results, obtained in the 1st and the 2nd stages, the vibrations’ parameters range have been narrowed down and mechanical vibrations of parallel direction have been applied. Mechanical vibrations were introduced during fusing and after fusing till temperature of coating reached 500 ˚C (the duration of vibratory treatment was 7 min. 30 s.). The microstructure of obtained coatings has been investigated on the etched transverse cross-sections with optical microscope Nicon MA 200 and SEM HITACHI SU-70 (SE/BSE) scanning electron microscope equipped with energy dispersive (EDS) and wavelength dispersive (WDS) spectrometers for X-ray microanalysis. The SEM operating conditions have been: 15 kV, 39 µA. The phase analyses were done on Philips X-ray powder diffractometer equipped with X-Pert goniometer. Graphite-monochromatized Cu Kα (λ = 0.1541837 nm) radiation was used in all examined cases. Experimental conditions were as follows: voltage – 40 kV; current – 40 mA; angle range 2θ – 20–120°; step ∆2θ – 0.032°; exposure time per step – 16 s. Measurements were performed on the polished tops of coatings. Microhardness measurements have been done by Knoop and Vickers methods using Zwick Roell ZHµ tester with different loads (exposure time 10 s). Measurements were performed on the polished and etched transverse cross-sections of remelted coatings. Electrochemical corrosion tests were accomplished through potentiodynamic measurements carried out using potentiostat / galvanostat AutoLab PGSTAT32N. A three-electrode cell arrangement was used, with a saturated calomel electrode as a reference electrode and a platinum wire as the auxiliary electrode. Two solutions, 3.4 % NaCl and 0.1 M H2SO4, naturally aerated at 21°C were used. Potentiodynamic polarization scans were carried out with a scan rate of 0.5 V/s in range of potential from –1500 mV to 180 mV using 3.4 % NaCl solution, and with a scan rate of 1 V/s in range of potential from –700 mV to 1500 mV in 0.1 M H2SO4 solution. Measurements were performed on the polished tops of coatings. The wear resistance of the coatings has been estimated by two-body dry sliding wear test and the coatings surface state after remelting, which has been evaluated by the measurements of roughness parameter Ra.

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3. The investigation of the vibrations influence on the structure and properties of refusible Ni-based coatings The third chapter of dissertation includes result obtained by microstructural and other studies of as-sprayed layers and remelted coatings, and their discussion. The as-sprayed coating exhibit discontinuous microstructure with a lot of unclosed voids and large gaps between the splats and is bonded to the substrate by poor mechanical adherence only. Monolithic layers of about 1 mm thickness with a strong metallurgical bond between the coatings and the substrate have been obtained after remelting of as-sprayed coatings. Typical microstructures of coatings are presented on Fig. 2.

Fig. 2. Scanning electron microscope micrographs of remelted A-type coating (a–c) and B-type coating (d, e): a – γ-Ni phase, b – Ni-B phase, c’ – carbides with lower C content; c’’ – carbides with higher C content, d – borides It was established that the microstructure of the coatings produced from powder A consists of soft metallic 2-phase γ-Ni solid solution and γ-Ni/Ni3B eutectic matrix. It hardened by insignificant amount of hard inclusions of two different compositions at least. One of them is Cr7C3 carbide, and the other is most probable carbide of type M23C6, where M = Cr, Ni. The remelted B-type coating consists of analogical 2-phases matrix. The hard inclusions of three types at the least were found in this type of coatings: M23C6 carbide, where M = Cr, Ni, Si, Mo; M7C3 carbide, where M = Cr, Ni; and M2B boride, where M = Cr, Mo, Ni. It was found, also, that both, the size and the shape of initial powder influence the final microstructure of the coatings. In coatings B the size difference of powder particles has led to the forming of zonal-type microstructure. In A-type coatings this effect was less intensive.

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The comparative analysis of A-type coatings has showed that grain size in layers, produced under vibratory treatment, differs from that in unvibrated coatings and vibrations promote the forming up to 2.4 times refined microstructure. The grain refining has been obtained in both vertically (Fig. 3) and parallel (Fig. 5) vibrated coatings.

Fig. 3. Microstructure of remelted A-type coatings (1st stage): 0_NV – unvibrated coating; 0_100, 0_200, 0_2000 – coatings, vibrated with 100 Hz/57 µm, 200 Hz/52 µm, and 2000 Hz/10 µm vibrations parameters

Fig. 4. Hardness of A-type coatings and thickness of layers, removed during two-body dry slide wear test after 16000 cycles (1st stage): NV – series of unvibrated samples

The 1st stage results of the hardness measurements and two-body dry sliding wear test of vertically vibrated A-type coatings are presented in Fig. 4.

NV 20 Hz 100 Hz 200 Hz 1000 Hz 2000 Hz 10 000 Hz 255 µm 57 µm 52 µm 31 µm 10 µm 1 µm

Parameters of vibratory treatment Hardness HV 10 Thickness of removed layer, µm Polynomial (Hardness) Polynomial (Thickness of removed layer)

310 300 290 280 270 260 250 240 230 220 210 200 190 180 170 160 150

Hardness

HV 10

Th

ickness

of rem

oved la

yer, µm

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As it was expected, coatings with more refined microstructure have showed the higher values of hardness (~10%) and better wear resistance (9–15%).

Fig. 5. Microstructure of remelted A-type coatings (2nd stage): A_NV – unvibrated coating; A_100_115, A_150_50, A_200_25 – coatings, vibrated with 100 Hz/115 µm, 150 Hz/50 µm, and 200 Hz/25 µm vibrations parameters

The analysis of the grains refining effect in A-type coatings showed that the grain size as well as the hardness of the coating varies with the varying of vibrations parameters (Fig. 6).

Fig. 6. A-type coatings: Knoope microhardness, measured parallel (HK║) and perpendicular (HK┴) to the substrate and average number of grains ANG (2nd stage): NV – series of unvibrated samples

NV 50 Hz 75 Hz 100 Hz 125 Hz 150 Hz 200 Hz 250 Hz 300 Hz 350 Hz 400 Hz 25 µm 115 µm 115 µm 25 µm 50 µm 25 µm 25 µm 25 µm 25 µm 25 µm

Parameters of vibratory treatment

HK║ HK┴ ANG Polynomial (HK║) Polynomial (ANG)

400

350

300

250

200

150

100

50 0

Micr

ohardn

ess H

K 1

Avera

ge nu

mber

of grain

s / 25

000 µ

m²

15

The quantitative evaluation of the grain refining effect has been carried out at the 2nd experiments stage through the calculation of grain numbers on particular cross-section area. It was found that the same effects can be achieved by choosing different combinations of amplitude and frequency and the significant effect is obtained in the 75–200 Hz range. A_200_25 coatings with the most refined microstructure has the maximum hardness (~272 HK), and this is ~19% more than in unvibrated coatings. The comparison of XRD patterns and X-ray microanalysis results for the vibrated and unvibrated coatings did not show any significant difference in the resulting phase composition. Therefore, improving of the mechanical properties in this case can be explained mainly due to the grain refinement effect. In B-type coatings parallel mechanical vibrations have formed near the interface relatively thick layer free from borides and including large-size carbides of both types (Fig. 7).

Fig. 7. Layered structure of B-type coating, remelted under vibrations of 100 Hz and 75 µm: 1 – planar solidification layer, 2 – borides-free layer

The thickness of “borides-free” layer varies with changing of the vibration frequency as well as with changing of the amplitudes. Large-size carbides predominance in the borides-free layer can testify the change in the solidification process caused by solidification process modifying to conditions, when the amount of spontaneous crystallization nucleus is limited. The wave extending during vibrating may also promote the growing of the inclusions by destroying of the part of crystallization centres. However, neither of mechanisms explains the absence of borides in the near interface layer for sure. Hardness measurements results showed that average hardness of vibrated coatings was increased and the thicker borides-free layer was obtained near interface, the higher hardness value was determined in main layer (Fig. 8). At the same time the hardness of the borides-free layer has the value intermediate between the hardness of the substrate and the hardness of the main coatings’ layer. Though, the application of vibratory treatment lead to the forming of coatings with harder outer layers, whereas the formation of a softer layer near

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interface may promote more uniform distribution of residual stresses. On the other hand, if the increasing of coatings hardness is achieved due to the increasing of precipitations amount in upper layers, this effect can be accompanied by the decrease in corrosion resistance, since this property is provided mainly by Cr-rich Ni-based metal matrix. In order to evaluate the caused impact of the obtained microstructure changes on the coatings behaviour in corrosive environments, potentiodynamic polarization tests were carried out. Then, it was established that creating of the hardened outer layers are achieved without loss of corrosion resistance.

Fig. 8. B-type coatings: thickness of borides-free layers and microhardness of the main coating layer (NV – series of unvibrated samples)

Fig. 9. Roughness Ra of B-type coatings (NV – series of unvibrated samples)

NV 100 Hz 100 Hz 100 Hz 100 Hz 150 Hz 200 Hz 25 µm 50 µm 75 µm 115 µm 25 µm 25 µm

Parameters of vibratory treatment

HK 0.5 Thickness of borides-free layer 778 818 796 755 690 703 717

1000 900 800 700 600 500 400 300 200 100 0

Micr

ohardn

ess H

K 0.5

Th

ickness

of bo

rides-

free lay

er, µm

199 89 98 76 82 48

0

NV 25 50 75 115 NV 100 150 200 Amplitude of vibration, µm Frequency of vibration, Hz Measurement 1 Measurement 2 Measurement 3 Average

8 6 4 2 0

8 6 4 2 0 Ro

ughn

ess Ra,

µm

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Evaluating of the coatings surface roughness was carried out through the measurement of Ra parameter. In A-type coatings the positive impact of the vibrations on the surface state is less intensive: vibrated A-type coatings exhibit Ra values similar to that in the unvibrated references or slightly improved. In B-type coatings the vibrations have led to the significant reduction of the roughness with the increase of the frequency as well as with the increase of the amplitude (Fig. 9). When maximum amplitude of 115 µm was applied, the roughness minimum of 2.269 µm was achieved, and this is nearly 3 times less than the roughness values obtained on the unvibrated coatings. General conclusions

1. The low frequency vibratory treatment, when applied during the refuse stage, influences and improves the qualitative characteristics (microstructure, hardness, roughness, wear resistance) of the thermally sprayed and refused Ni-based coatings. 2. The effect of the low frequency vibratory treatment differs when the vibrations are applied for the coatings of different chemical composition: • in the less alloyed Ni-based coatings (~82% Ni), with dominated solidification of solid solution phase, parallel and perpendicular vibrations lead to the refining of grains (up to 2.4 times) with the increasing of the coatings hardness (up to 19%) and improving of wear resistance (up to 9–15%); the most intensive effect is obtainable when the frequency of 100–200 Hz is applied; • in a more alloyed boride-containing Ni-based coatings (~67% Ni) vibrations of 100–200 Hz frequency form the layered structure with gradually increasing of the hardness from the interface to the surface and forming up to 19% harder main working layers without a decrease in the corrosion resistance and with the reduce of coating surface roughness up to 3 times. 3. The efficiency of the vibratory treatment is mainly determined by the amplitude of the applied vibrations. The amplitude of the applied vibrations more intensively influences layered structure forming and the surface roughness reduce in a more alloyed Ni-based coatings, and also the microstructure refining in a less alloyed Ni-based coatings. 4. No influence of vibratory treatment on the phase constitution of the obtained coatings has been observed.

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List of published works on the topic of the dissertation In the reviewed scientific periodical publications Škamat, J., Valiulis, A. V., Černašėjus, O. 2010. The influence of mechanical vibrations on properties of Ni-based coatings, Journal of Vibroengineering 12 (4): 604–610. ISSN 1392-8716 (Thomson ISI Web of Science). Škamat, J., Valiulis, A. V., Kurzydlowsky, K. J., Černašėjus, O., Lukauskaitė, R., Zwolinska, M. 2014. NiCrSiB thermal sprayed coatings refused under vibratory treatment, Materials Science (Medžiagotyra) 20. ISSN 1392-1320. (Thomson ISI Web of Science, accepted). Škamat, J., Valiulis, A. V. 2010. Apie galimybes ultragarsą naudoti terminio purškimo technologijose, Mokslas – Lietuvos ateitis = Science – future of Lithuania: mechanika, medžiagų inžinerija, pramonės inžinerija ir vadyba 2 (4): 39–41. ISSN 2029-2341 (IndexCopernicus). Valiulis, A. V., Škamat, J. 2010. Advanced materials research and technologies development: Lithuanian experience, Solid State Phenomena 165: 210–215. ISSN 1012-0394. (Scopus). In the other editions Škamat, J., Černašėjus, O., Valiulis, A.V. 2012. Исследование влияния механических колебаний на структуру и свойства термических покрытий на основе никеля, Proceedings of the International Scientific-Practical Conference "Science and education as a leading factor in strategy Kazakhstan – 2030" (Saginov's readings No. 4), June 28–29, 2012. Part 1: 77–79. ISBN 9786012960532. About the author

Jelena Škamat was born in Vilnius, on 12 of July 1977. First degree in Industrial Engineering, Faculty of Mechanics, Vilnius Gediminas Technical University, 2006. Master of Science in Industrial Engineering, Faculty of Mechanics, Vilnius Gediminas Technical University, 2008. The qualification of International Welding technologist, 2008. Since 2006 – constructor at UAB “Pavlidi ir Co”. In 2009–2013 – assistant at Vilnius Gediminas Technical University. In 2008–2013 – PhD student of Vilnius Gediminas Technical University. In 2011 was on internship at Warsaw University of Technology. At present – lecturer in Materials and Welding Department of Vilnius Gediminas Technical University.

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VIBRACINIO APDOROJIMO POVEIKIS PURKŠTŲJŲ NIKELIO PAGRINDO DANGŲ STRUKTŪRAI IR SAVYBĖMS

Problemos formulavimas. Dangų technologijos yra viena iš svarbių sričių medžiagų inžinerijoje. Sparčiai besivystanti pramonė kelia vis didesnius reikalavimus konstrukcinėms medžiagoms, dažnai dirbančioms korozijos, aukštatemperatūrės oksidacijos, įvairių trinties ir dilimo rūšių sąlygomis. Apsauginių sluoksnių formavimas yra vienas iš veiksmingiausių metodų apsaugoti detalių paviršius nuo kenksmingų aplinkos veiksnių, sutaupyti brangiai kainuojančius metalus, sumažinti remonto išlaidas, gerokai padidinti gaminių darbinį resursą. Yra žinoma ir taikoma daug įvairių paviršių dengimo būdų. Atskirą grupę sudaro terminio purškimo technologijos. Šiuo metu terminis purškimas yra labai išsivysčiusi pasaulio ekonomikos šaka su metine apyvarta apie 5 mlrd. JAV dolerių. Terminį purškimą sudaro apie 10 savarankiškų metodų, daugelis iš kurių plačiai taikomas įvairiausiose pramonės srityse. Gaunamų dangų eksploatacinės savybės didele dalimi priklauso nuo susidarančios struktūros. Todėl struktūros optimizavimas yra viena iš terminio purškimo technologijų vystymo krypčių. Struktūrai turi įtakos purškiamos medžiagos cheminė sudėtis, pasirinktas purškimo būdas ir jo parametrai ir kiti veiksniai. Taikant vienus metodus gaunamos porėtos sluoksninės anizotropinės struktūros, taikant kitus – gaunami monolitiniai sluoksniai su struktūra, artima lieto metalo struktūrai. Pastaruoju atveju besikeičiantys proceso parametrai ypač turi įtakos tiek fazinei, tiek struktūrinei gaunamų dangų sudėčiai. Toks kristalizacijos proceso kinetikos jautris rodo, kad dangų struktūra gali būti kontroliuojama, kryptingai veikiant jos susidarymą. Specialūs kristalizacijos procesą kontroliuojantys metodai yra gerai žinomi ir taikomi metalurgijoje. Veikiant besikristalizuojančių metalų lydalus magnetinio lauko impulsais, ultragarsu, žemo dažnio vibracijomis, įvedant inokuliatorius ir modifikatorius bei taikant kitus specialius metodus gaunami geresnės struktūros ir savybių lydiniai, metalų ruošiniai ir liejiniai. Vibracinis apdorojimas taip pat sėkmingai naudojamas kaip efektyvi priemonė struktūrai ir savybėms gerinti ir vidiniams įtempiams mažinti. Duomenų apie vibracinio apdorojimo taikymą terminio purškimo technologijose labai mažai ir tai neleidžia spręsti apie jų taikymo naudingumą.

Darbo aktualumas. Darbe tiriamos nikelio pagrindo purkštosios perlydomosios dangos, plačiausiai taikomos chemijos, naftos perdirbimo, stiklo ir kitose pramonės šakose siekiant padidinti paviršių atsparumą dilimui esant korozinei

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terpei ir aukštatemperatūrei oksidacijai. Šioms dangoms pagaminti taikomas dviejų pakopų procesas: miltelių užpurškimas ant paruošto substrato ir užpurkštos dangos perlydymas. Perlydyti dangas galima įvairiais metodais. Labiausiai paplitęs ekonomiškas ir universalus liepsninis perlydymas. Dėl didelio proceso našumo jis ypač tinka didelių matmenų dengiamiems paviršiams: naftos gręžinių gręžimo įrenginiuose, žemės ūkio technikoje ir pan. Perlydant užpurkštą dangą medžiaga išlydoma, o aušimo metu susidaro kristalizacijos užuomazgos, vyksta lydinio dezoksidacija, degazacija ir sparti kristalizacija. Šie veiksniai lemia susidarančios dangos struktūrą ir savybes. Antikorozines dangos savybes užtikrina gausiai legiruota chromu nikelio pagrindo matrica. Mechaninės savybės labiau priklauso nuo kristalizacijos metu susidarančių kietųjų intarpų rūšies ir kiekio. Nikelio dangų kokybė gerinama optimizuojant ir modifikuojant struktūrą. Tai pasiekiama dviem pagrindiniais būdais: didinant kietųjų intarpų kiekį dangos sluoksnyje ir formuojant smulkesnę mikrostruktūrą. Daugeliu tyrimų įrodyta, kad vibracijų taikymas lydalų kristalizacijos metu leidžia homogenizuoti ir susmulkinti liejamų gaminių struktūrą, sumažinti porų ir kitų defektų kiekį, tuo pačiu pagerinti mechanines charakteristikas ir specialiąsias savybes, todėl šį būdą tikslinga pritaikyti formuojant perlydomąsias apsaugines dangas. Šiuo metu trūksta duomenų apie vibracijų poveikį perlydomajai dangai jos kristalizacijos metu, todėl šiame darbe siekiama ištirti žemo dažnio vibracinio apdorojimo poveikį perlydomųjų Ni pagrindo dangų struktūrai ir savybėms.

Tyrimų objektas – vibracinio apdorojimo poveikis termiškai užpukštų ir taikant vibracinį apdorojimą perlydytų Ni pagrindo dangų struktūrai ir savybėms.

Darbo tikslas – išaiškinti vibracinio apdorojimo poveikį purkštųjų perlydomųjų Ni pagrindo dangų struktūrai ir savybėms, kai vibracinis apdorojimas taikomas perlydymo metu. Darbo uždaviniai. Darbo tikslui pasiekti buvo iškelti du uždaviniai: 1. Ištirti purkštųjų perlydomųjų Ni pagrindo dangų, gautų taikant perlydymo metu įvairių parametrų vibracinį apdorojimą, mikrostruktūrą ir savybes. 2. Nustatyti perlydytų dangų kokybinių charakteristikų (mikrostruktūros, kietumo, atsparumo dilimui, atsparumo korozijai, šiurkščio) priklausomybes nuo vibracijų parametrų. Tyrimų metodika. Disertacijoje taikomi analitiniai ir eksperimentiniai tyrimo metodai bei lyginamoji analizė.

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Darbe buvo tiriamos purkštosios perlydomosios Ni pagrindo dangos, vibruotos perlydymo metu keičiant vibracinio apdorojimo parametrus – kryptį, dažnį ir amplitudę. Eksperimentams naudoti 2 rūšių nikelio pagrindo milteliai. Mažiau legiruoto lydinio milteliuose buvo ~82 % Ni, daugiau legiruoto – ~67 % Ni. Užpurkštos dangos buvo perlydytos be vibracijų ir taikant vibracinį apdorojimą. Dangų struktūra ir savybės buvo tiriamos taikant šiuolaikinius tyrimo metodus: optinę mikroskopiją, nuskaitančiąją elektroninę mikroskopiją, rentgeno mikroanalizę, rentgeno spindulių difrakciją bei diferencinę terminę analizę. Dangų savybės tirtos taip pat atliekant potenciodinaminės poliarizacijos, kietumo, mikrokietumo, paviršiaus šiurkštumo matavimus ir dilimo bandymus. Mokslinis naujumas. Rengiant disertaciją buvo gauti šie medžiagų inžinerijos mokslui nauji rezultatai: 1. Vibracinis apdorojimas panaudotas Ni pagrindo plonų purkštųjų dangų perlydymo metu. 2. Kompleksiškai ištirtos Ni pagrindo dangos, kristalizacijos metu paveiktos mechaninių virpesių. Nustatyta, kad vibracijos turi įtakos dangos mikrostruktūros susidarymui ir savybėms (kietumui, atsparumui dilimui, šiurkštumui). Nustatyta, kad susidarantiems mikrostruktūros ir savybių pokyčiams turi įtakos taikomų vibracijų dažnis, amplitudė ir kryptis. 3. Taikant vibracinį apdorojimą mažiau legiruotoms (~82 % Ni) dangoms su vyraujančia γ-Ni kietojo tirpalo kristalizacija vibracijos smulkina struktūros darinius, tuo pačiu didina dangos kietumą ir gerina atsparumą dilimui. 4. Tiriant struktūros susmulkėjimą dangose nustatyta, kad ženklūs struktūros susmulkėjimo efektai gaunami tam tikrame vibracijų parametrų intervale taikant skirtingus dažnio ir amplitudės derinius. 5. Gausiai legiruotose (~67 % Ni), boridus sudarančiose, perlydomosiose Ni pagrindo dangose vibracinis apdorojimas dangoje sukuria sluoksninę struktūrą su tolydžiu kietumo didėjimu nuo sulydymo linijos link paviršiaus, tuo pačiu didina dangos pagrindinio darbinio sluoksnio kietumą nebloginant antikorozinių dangos savybių ir mažina gaunamų dangų paviršiaus šiurkštį po perlydymo. 6. Gausiai legiruotų Ni pagrindo dangų sluoksninės struktūros susidarymui ir paviršiaus šiurkštumo sumažėjimui taikomų vibracijų amplitudės didinimas turi stipresnį poveikį negu dažnio kitimas.

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Praktinė vertė. Vibracinio apdorojimo procesas yra paprastas bei nebrangus ir gali būti taikomas įvairios formos ir masės detalėms. Darbe gauti rezultatai tiesiogiai susiję su pačių vibracijų dažniu ir amplitude, todėl galima pritaikyti bet kokį pramoninį vibratorių, leidžiantį gauti darbe apibrėžtus vibracijų parametrus. Tyrimų metu nustatytas vibracinio apdorojimo kompleksinis poveikis. Pritaikius darbe apibrėžtų efektyviausių parametrų vibravimą perlydomosioms dangoms galima gauti glotnesnio paviršiaus, sustiprintos struktūros, atsparesnes dilimui ir korozijai dangas. Ginamieji teiginiai 1. Žemo dažnio vibracinis apdorojimas, taikomas perlydymo metu, turi įtakos purkštųjų perlydomųjų Ni pagrindo dangų struktūrai ir savybėms, skirtingai veikia skirtingos cheminės sudėties perlydomąsias dangas ir gerina jų savybes. 2. Taikant žemo dažnio vibracinį apdorojimą purkštųjų Ni pagrindo dangų perlydymo metu naujos fazės ar struktūros dariniai nesusidaro. Darbo apimtis. Disertaciją sudaro įvadas, trys skyriai, rezultatų apibendrinimas, literatūros šaltinių ir autoriaus publikacijų sąrašai ir trys priedai. Įvadiniame skyriuje aptariama tiriamoji problema, darbo aktualumas, aprašomas tyrimų objektas, formuluojamas darbo tikslas bei uždaviniai, aprašoma tyrimų metodika, darbo mokslinis naujumas, darbo rezultatų praktinė reikšmė, ginamieji teiginiai. Įvado pabaigoje pristatomos disertacijos struktūra bei autoriaus paskelbtos publikacijos ir pranešimai konferencijose. Pirmajame skyriuje apžvelgiamos terminio purškimo ir vibracinio apdorojimo technologijos, analizuojama įvairių veiksnių įtaka Ni pagrindo dangų sandarai ir galimybė pritaikyti vibracinį apdorojimą termiškai purkštų dangų perlydymo metu. Skyriaus pabaigoje formuluojamos išvados ir tikslinami disertacijos uždaviniai. Antrajame skyriuje pateikiama tyrimo metodika. Trečiajame skyriuje pateikiami ir analizuojami tyrimų rezultatai. Darbo apimtis yra 94 puslapiai, neskaitant priedų, tekste panaudotos 3 numeruotos formulės, 46 paveikslai ir 18 lentelių. Rašant disertaciją buvo panaudota 100 literatūros šaltinių. Bendrosios išvados Atlikus termiškai užpurkštų Ni pagrindo dangų vibracinio perlydymo eksperimentus ir ištyrus gautų dangų mikrostruktūrą bei savybes palyginamosios analizės rezultatų pagrindu suformuluotos šios išvados:

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1. Žemo dažnio vibracinis apdorojimas, taikomas perlydymo metu, turi įtakos purkštųjų perlydomųjų Ni pagrindo dangų kokybinėms charakteristikoms (mikrostruktūrai, kietumui, šiurkščiui, atsparumui dilimui) ir leidžia jas pagerinti. 2. Žemo dažnio vibracinis apdorojimas skirtingai veikia skirtingos cheminės sudėties Ni pagrindo dangas: • mažiau legiruotose (~82 % Ni) perlydomosiose Ni pagrindo dangose su vyraujančia γ-Ni kietojo tirpalo kristalizacija vibracijos lygiagrečiai ir statmenai substratui sukelia struktūros grūdo smulkėjimo (iki 2,4 karto) efektą, tuo pačiu didina dangos kietumą (iki 19 %) ir jos atsparumą dilimui (iki 9–15 %); stipriausias poveikis nustatytas 100–200 Hz dažnių intervale; • gausiai legiruotose ir boridų turinčiose Ni pagrindo (~67 % Ni) perlydomosiose dangose 100–200 Hz dažnio vibracinis apdorojimas lygiagrečiai substratui sukuria sluoksninę struktūrą, kurioje kietumas, nuo sulydymo linijos link paviršiaus, laipsniškai didėja. Tuo pačiu dangoje susidaro iki 19 % kietesnis pagrindinis darbinis sluoksnis, kurio atsparumas korozijai padidėja arba išlieka nepakitęs, o dangų paviršiaus šiurkštis sumažėja iki 3 kartų. 3. Vibracinio apdorojimo efektyvumas daugiausia priklauso nuo mechaninių virpesių amplitudės. Amplitudės pokytis turi stipresnį poveikį mažiau legiruotų Ni dangų struktūros darinių smulkėjimui, o gausiai legiruotų Ni dangų – sluoksninės struktūros susidarymui ir dangų paviršiaus šiurkščio sumažėjimui. 4. Tiriant vibracinio apdorojimo poveikį gaunamų dangų struktūros fazinei sudėčiai esminių pokyčių nenustatyta.

Trumpos žinios apie autorių Jelena Škamat gimė 1977 m. liepos 12 d. Vilniuje. 2006 m. įgijo pramonės inžinerijos bakalauro laipsnį Vilniaus Gedimino technikos universiteto Mechanikos fakultete. 2008 m. įgijo pramonės inžinerijos mokslo magistro laipsnį Vilniaus Gedimino technikos universiteto Mechanikos fakultete. 2008 m. įgijo tarptautinio suvirinimo technologo kvalifikaciją. Nuo 2006 m. dirba konstruktore UAB „Pavlidi ir Ko“. 2009–2013 m. dirbo asistente Vilniaus Gedimino technikos universiteto Medžiagotyros ir suvirinimo katedroje. 2008–2013 m. – Vilniaus Gedimino technikos universiteto doktorantė. 2011 m. stažavosi Varšuvos technologijų universitete. Šiuo metu dirba lektore Vilniaus Gedimino technikos universiteto Medžiagotyros ir suvirinimo katedroje.

Jelena ŠKAMAT EFFECT OF VIBRATORY TREATMENT ON THE STRUCTURE AND PROPERTIES OF NICKEL BASED THERMALLY SPRAYED COATINGS Summary of Doctoral Dissertation Technological Sciences, Materials Engineering (08T) Jelena ŠKAMAT VIBRACINIO APDOROJIMO POVEIKIS PURKŠTŲJŲ NIKELIO PAGRINDO DANGŲ STRUKTŪRAI IR SAVYBĖMS Daktaro disertacijos santrauka Technologijos mokslai, medžiagos inžinerija (08T) 2013 11 08. 1,5 sp. l. Tiražas 70 egz. Vilniaus Gedimino technikos universiteto leidykla „Technika“, Saulėtekio al. 11, 10223 Vilnius, http://leidykla.vgtu.lt Spausdino UAB „Ciklonas“ J. Jasinskio g. 15, 01111 Vilnius