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MECHANICAL PROPERTIES AND MICROSTRUCTURE OF VACCUM PLASMA SPRAYED C3C2 - 25(Ni20Cr) COATINGS
Mihailo R. Mrdak, Research and Development Center IMTEL Communications a. d., Belgrade
e-mail: miki@imtelkoiri.ac.rs
DOI: 10.5937/vojtehg63-4324
FIELD: Chemical Technology ARTICLE TYPE: Original Scientific Paper ARTICLE LANGUAGE: English
Summary:
This paper analyzes vacuum plasma spray VPS - Cr3C2 -25(Ni20Cr) coatings. Commercial powder marked Sulzer Metco Woka 7205 is used. The powder is deposited with a plasma gun F4 at a distance of 340 mm from the substrate. The main objective of the study was to eliminate, at the reduced pressure of inert gas Ar, the degradation of primary Cr3C2 carbide into Cr23C6 carbide which significantly reduces the microhardness and mechanical properties of the coating. The coating is deposited with a thickness of 100 -120 pm on a steel substrate. The microhardness of the coating was tested by HV03. The microhardness values were in the range of 1248 - 1342 HV03. The bond strength of the coating was tested by tension. It was found that the bond strength between the substrate and the coating has a value of 89 MPa. The microstructure of the coating was tested by the light microscopy technique. The structure of the coating consists of an NiCr alloy base with a dominant primary Cr3C2 carbide phase. In addition to the Cr3C2 phase, the Cr7C3 phase is also present. The coating etching was done with the reagent 1HNO3 : 4HCl : 4H2O that primarily dissolves nickel to enable the distribution of the carbide phase to be clearly seen in the coating. Etching the coating with this reagent revealed the presence of the largely undegraded primary Cr3C2 carbide phase which provides high hardness values to the coating.
Key words: vacuum, substrates, strength, property, phases, microstructures, microhardness, mechanical properties, coatings, carbides.
ACKNOWLEDGEMENT: The author is thankful for the financial support from the Ministry of Education and Science of the Republic of Serbia (national projects OI 174004, TR 34016).
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Introduction
Vacuum plasma spraying (VPS) is usually referred to as LPPS due to low pressure. At low pressure, a plasma jet becomes longer and smaller in diameter and with the use of convergent / divergent nozzles it has a higher rate of ions. Eliminating oxygen in the chamber and a possibility to preheat the substrate enable the creation of denser coatings with higher tensile bond strength and without the oxide content. For high performance applications, plasma spraying is carried out in a vacuum chamber at a reduced pressure of inert gas Ar. The vacuum plasma spray process (VPS) produces high-quality coatings, especially those sensitive to oxygen. One such coating is a cermet coating - Cr3C2 - 25(Ni20Cr) sensitive to oxygen due to the reaction of carbon from the carbide with the oxygen from the surrounding atmosphere. The VPS application process prevents the de-carburization of the primary Cr3C2carbide, so that coatings of high hardness are deposited. Traditionally, these coatings were deposited by APS and HVOF processes. In the last decade, a number of researchers have published results concerning the structure and properties of the deposited coatings by the HVOF process (Guilemany, et al., 2006, p.2998), (Guilemany, et al., 2002, p.207), (Ji, et al., 2006, p.6749), (Li, et al., 2005, p.229), (Picas, et al., 2006, p.477). In this process, as in the APS process, the main problem was the loss of carbon during deposition. The results clearly show that the major loss of carbon occurs during the process of depositing particles due to the surrounding atmosphere. It was also found that the initial size of carbide powder particles have a significant impact on the carbon loss during the deposition of Cr3C2 - 25(Ni20Cr) coatings in the HVOF process (Li, et al., 2002, p.137). In VPS coatings, the dominant phase is the Cr3C2 carbide phase with a hardness of 1600HV and a less significant phase is the Cr7C3 phase with a hardness of 1300HV (Marcano, et al., 2008, pp.4406-4410), (Tomita, et al., 2001, pp.699-704). In the coatings there is no the Cr23C6 carbide phase with a hardness of 1000HV, which, in APS and HVOF carbide coatings, reduces the coating hardness. Tomita, T. and other researchers have found that the 75Cr3C2 - 25(Ni20Cr) coating deposited by the VPS process has a higher hardness than the coatings deposited by APS and HVOF processes (Tomita, et al., 2001, pp.699-704). The hardness of the VPS C^C2 - 25(Ni20Cr) coating is HV1243 ± 80, which is much higher than the one of HVOF coatings with a hardness of HV958 ± 44 (Tomita, et al., 2001, pp.699-704). The tensile bond strength of the coating deposited by the VPS process is greater than 80 MPa with a porosity content of less than 5% (ASM Metals Handbook, 1987, p.367). Cr3C2 - NiCr plasma spray coatings have a high resistance to abrasive wear and a low friction coefficient, from room temperature to 850°C, due to their high thermal stability and resistance to oxidation
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(Guilemany, et al., 2006, p.2998). These deposits are extensively used for S coating parts and components for energy conversion, such as steam and if gas turbine engines (Matthews, et al., 2003, p.4267). Recently, it was established that these coatings can improve the resistance to thermal fatigue and wear resistance under severe conditions of load and extend the life of components (Guilemany, et al., 2002, p.207). Thermally sprayed cermet o coatings Cr3C2 - 25(Ni20Cr) appeared as a good solution for a wide range of applications in machine parts. Because of the extended service life of o parts, coatings based on chromium carbides are widely used for many applications in gas turbines, steam turbines and aircraft engines to improve slip resistance, abrasion and erosion wear (Hillery, 1986, pp.2684-2688). i Thermally sprayed cermet coatings are a good alternative to hard chro- o mium, when high wear resistance is required (Erning, Nestler, 1999, o pp.462-466), (Sahraoui, et al., 2004, pp.654-660), (Ko, Robertson, 2002, pp.880-893), (Savarimuthu, et al., 2000, pp.1095-1104). When compared 2 to WC coatings, Cr3C2-NiCr coatings offer greater resistance to corrosion and oxidation, and also have a high melting point and maintain high hard- e ness, strength and wear resistance up to 900°C (Beczkowiak, et al., 1999), (Blatchford, 2001), (Doi, Yoshiaki Suda, 2000), (Liu, 1998), (Loubiere, et e al., 1995, pp.1535-1546), (Staia, et al., 2001, pp.553-562). Corrosion resistance is primarily provided by the NiCr alloy base, while wear resistance is mainly provided by the hard Cr3C2 carbide phase (He, Lavernia, 2000, ° pp.555-564). So, Cr3C2 carbide-based coatings are applied to a wide range of industrial components, including various accessories used in steam and gas turbines. Thermal spraying is an effective method to apply thin and thick coatings on mechanical components to change their surface properties (Erja Turunen, et al., 2006, pp.4987-4994), (Wang, et al., 2000. p.69.). APS plasma spray processes and VPS are used in a wide range of applications including automotive, aerospace industry, chemical processing equipment, pulp and paper, orthopedic and dental components, etc. (Erja Turunen, et al., 2006, pp.4987-4994), (Mrdak, et al., 2013, pp.559- 2 567), (Mrdak, 2013, pp.69-88), (Mrdak, 2012, pp.182-201), (Mrdak, et al., 2009, pp.27-32), (Vencl, et al., 2006, pp.151-157), (Vencl, et al., 2011, pp. 1281-1288). The plasma spray process has been used for more than four decades in manufacturing protective coatings based on metals, ceramics and even composite materials for various applications (Chuanxian Ding, et al. 2003, pp.455-458). Despite the long period of application of the plasma spray process, there is still a great interest among scientists in the development of new materials for coatings and in the study of their behavior under working conditions (Leblanc, 2003, pp.291-299).
This paper presents the results of the experimental investigation of the influence of the VPS - vacuum plasma spray process on the mechanical properties and the microstructure of the Cr3C2 - 25(Ni20Cr) cer-
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met coating. The main objective of the study was to avoid, at the reduced pressure of Ar inert gas during deposition, the degradation of the primary Cr3C2 carbide into a much softer Cr23C6 carbide and to deposit coating layers with the microstructure with the dominant Cr3C2 carbide phase which gives better performance in the coating operation. The tests have shown that the - Cr3C2 - 25(Ni20Cr) VPS coating has higher hardness and bonding strength than APS and HVOF coatings, which are in accordance with the coating microstructure dominated by the primary Cr3C2 carbide phase.
Materials and experimental details
The Sulzer Metco Woka 7205 powder was used for coating production ( Material Product Data Sheet, 2012, Woka 7205 Chromium Carbide - 25% Nickel Chromium Powders, DSMTS-0031.1, Sulzer Metco). The Woka7205 powder contains 75%Cr3C2 carbide and 25%(Ni20Cr) alloy. The Cr3C2 - 25(Ni20Cr) powder particles are spherical, produced by agglomeration and the sintering technique with a range of powder granules from 10 to 38 ^m. Fig. 1 shows a scanning electron micrograph (SEM) of the powder particle morphology. A spherical Cr3C2 - 25(Ni20Cr) powder particle can be seen, consisting of sintered Cr3C2carbide particles (dark blue) and 25(Ni20Cr) alloy particles (light blue).
Figure 1 - (SEM) Scanning electron micrograph of СГ3С2 - 25(Ni20Cr) powder particles Slika 1 - (SEM) Skening elektronska mikrografija cestica praha СгзС2 - 25(Ni20Cr) Рис. 1 - (SEM) Электронная микрография частиц порошка СгзС2 - 25(Ni20Cr)
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The substrates on which the coatings were deposited for micro S hardness testing and microstructural evaluation were made of steel C.4171 (X15Cr13 EN10027) in the thermally unprocessed state with the dimensions 70x20x1.5mm (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA). Also, the substrates for testing the bond strength are made of steel C.4171 o (X15Cr13EN10027) in the thermally unprocessed state with the dimension 025x50 mm (Turbojet Engine - Standard Practices Manual (PN o 582005), 2002, Pratt & Whitney, East Hartford, USA).
The mechanical and microstructural characterizations of the coatings were made according to Pratt & Whitney (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hart- § ford, USA). The evaluation of the mechanical properties of the layers was £ done by the hardness testing method HV03 and by bond strength tensile testing. The hardness testing was done in the direction along the lamellae, in the middle and at the ends of the sample. Five readings at three places were obtained and the paper presents the hardness range from the minimum to the maximum value.
The method of bond strength testing is a method of tensile testing. The testing was done at room temperature with a strain test rate of 1cm/60s. Three specimens were used, and the mean value is shown in the paper.
The morphology of powder particles was examined on the SEM -Scanning Electron Microscope. The analysis of the share of pores in the coating was done by processing 5 photos at 200X magnification. Through tracing paper, micro pores are labeled and shaded, and their total area was calculated related to the total area of the micrograph. This paper presents the mean value of the share of pores. The microstructural analysis of the coating was performed under a light microscope. In order to determine the distribution of the carbide phase in the coating, the coating etching was done with the reagent 1HNO3: 4HCl:4H2O. The microstructure of the coating after etching was examined by the light microscopy technique.
The Cr3C2 - 25(Ni20Cr) powder was deposited at a low pressure of Ar inert gas in the VPS system of the Plasma Technik AG company. An F4 plasma gun was used for the powder deposition. The process involves cleaning the substrate surface by the transferred arc and the Ja powder deposition at low pressure. A program for Cr3C2 - 25(Ni20Cr) powder deposition was designed in the microprocessor unit of the robot of the VPS Plasma Technik AG system. The program set and time-synchronized all process parameters such as: chamber vacuuming, plasma gas flow, cleaning the substrate by the transferred arc, powder flow, coating deposition, substrate cooling and ventilation of the vacuum
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chamber. The cleaning of the substrate surface and the powder deposition were performed with a mixture of Ar-He plasma gases. The VPS parameters of the deposition of Cr3C2 - 25(Ni20Cr) powder on the samples are shown in Table 1.
Table 1 - Plasma spray parameters Tabela 1 - Plazma sprej parametri Таблица 1 - Характеристики плазменного напыления
Parameters Values
Cleaning arc Spraying
Plasma current, I (A) 700 700
Plasma Voltage, U (V) 74 74
Primary plasma gas flow rate Ar (l/min) 50 50
Secondary plasma gas flow rate He (l/min) 20 140
Carrier gas flow rate Ar (l/min) — 5
Powder feed rate (g/min) -- 45
Stand-off distance (mm) 320 340
Chamber pressure (mbar) 30 70
Nozzle diameter (mm) 8 8
Speed of the gun (mm /s) 12 12
Results and discussion
In the Cr3C2 - 25(Ni20Cr) coating layers along the cross-section, the hardness values of 1248 to 1342 HV03 were measured. The obtained hardness values indicate that a greater proportion of the degradable primary Cr3C2 carbide phase is present in the microstructure, due to the inert atmosphere of Ar at low pressure (Marcano, et al., 2008, pp.4406-4410), (Tomita, et al., 2001, pp.699-704). The range of hardness of the deposited layers is caused by the presence of micro pores in the coating layers. The tensile bond strength between the substrate and the coating was 89 MPa which is typical for VPS coatings. The cleaning of the substrate surface with the transferred arc resulted in better adhesion of the deposited coating layers, which resulted in obtaining high values of bond strength. The values of the microhardness and tensile bond strength were correlated with their microstructures.
Figs. 2 and 3 show the microstructures of the VPS Cr3C2 -25(Ni20Cr) coating layers in the deposited condition.
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Slika 2 - Microstructure of the СГ3С2 - 25(Ni20Cr) coating in the deposited state Slika 2 - Mikrostruktura СгзС2 - 25(Ni20Cr) prevlake u deponovanom stanju Рис. 2 - Микроструктура покрытия C3C2- 25 (Ni20Cr) в нанесенном состоянии
Slika 3 - Microstructure of the Cr3C2 - 25 (Ni20Cr) coating in the deposited state Slika 3 - Mikrostruktura Cr3C2 - 25(Ni20Cr) prevlake u deponovanom stanju Рис. 3 - Микроструктура покрытия C3C2 - 25 (Ni20Cr) в нанесенном состоянии
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The qualitative analysis showed that at the interface between the substrate and the deposited coatings there are no defects such as discontinuities of the deposited layers on the substrates, microcracks, macrocracks and separation of the coating from the substrate. The boundaries on the interface between the substrate and the coating layers are very clean, which indicates a good cleaning of the substrates with the transferred arc. Through the coating layers, micropores of spherical and irregular shapes can be seen (marked with red arrows). There are no unmelted particles and precipitates in the coating layers. Microcracks are not present in the structure. Oxide lamellae cannot be observed through the layers of coatings. The VPS - vacuum plasma spray process allows depositing layers without oxide content in the coating, which is a big advantage over the APS and HVOF processes.
Figs. 4 and 5 show the microstructures of the VPS Cr3C2 -25(Ni20Cr) coating in the etched condition.
Slika 4 - Microstructure of the Cr3C2 - 25 (Ni20Cr) coating in the etched state. Slika 4 - Mikrostruktura СгзС2 - 25(Ni20Cr) prevlake u nagrizenom stanju. Рис.4 - Микроструктура покрытия СгзС2 - 25 (Ni20Cr) в протравленном состоянии
The microstructure of the coating clearly shows the two phases. The dark blue phase represents the lamellae of Ni20Cr alloy and the light blue one shows the primary Cr3C2 undegraded carbides and the secondary Cr7C3 carbides which give high values of microhardness to the coating (Marcano, et al., 2008, pp.4406-4410), (Tomita, et al., 2001, pp.699704). The Cr3C2 and Cr7C3 carbide phases are evenly distributed in the coating structure. The structure of the coating is quite uniform in the cross-section, with no history of micro and makrocracks.
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Slika 5 - Microstructure of the Cr3C2 - 25 (Ni20Cr) coating in the etched state Slika 5 - Mikrostruktura СгзС2 - 25(Ni20Cr) prevlake u nagrizenom stanju Рис.5 - Микроструктура покрытия C3C2 - 25 (Ni20Cr) в протравленном состоянии
This indicates that the coating layers are deposited evenly. Micropores are present in the coating structureand seen as dark fields in Figs.2 and 3. The porosity of the coating was determined by the image analysis technique, where 5 fields were analyzed at 200X magnification in the coating cross section. The average value of the porosity was 4%. Primary Cr3C2 carbide particles and secondary Cr7C3 carbides phases are located in the interlamellar regions of the Ni20Cr alloy (Marcano, et al., 2008, pp.4406-4410), (Tomita, et al., 2001, pp.699-704). Due to coating etching, Ni is dissolved from the solid solution of the Ni20Cr alloy while Cr3C2 and Cr7C3 carbides are raised in the light blue relief. Since the incident light falls obliquely onto the sample surface, and casts a shadow over the raised carbide phases, the Ni20Cr alloy phase is dark blue.
Conclusion
In this paper, the vacuum plasma spray - VPS procedure is used to deposit Cr3C2 - 25(Ni20Cr) cermet coatings with cleaning the substrate surface with the transferred arc at a distance of 320mm of the F4 plasma gun from the substrate and deposit powder particles at a distance of 340mm of the plasma gun from the substrate. The paper investigated the
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mechanical properties and the microstructure of the coatings in the deposited and etched state in the reagent 1HNO3: 4HCl: 4H2O. The investigation came to the following conclusions.
The VPS Cr3C2 - 25(Ni20Cr) cermet coating had high hardness values from 1248 to 1342 HV0 3 along the cross-section. The measured hardness values indicate the presence of a large share of the nonde-graded primary Cr3C2 carbide phase in the coating microstructure. The range of the hardness of the deposited layers is a consequence of the presence of micro-porosity in the coating layers. The tensile bond strength of the Cr3C2 - 25 (Ni20Cr) coating had a high value of 89 MPa. The cleaning of the substrate surface with the transferred arc resulted in better adhesion of the deposited coating layers, which resulted in obtaining high values of bond strength. The values of the microhardness and tensile bond strength correlated with their microstructures.
The microstructure of VPS 75Cr3C2 - 25(Ni20Cr) cermet coatings is lamellar. Micro pores with a share of 4% are present in the deposited layers. Through the deposited layers,unmelted powder particles and precipitates cannot be observed. The microstructure of the coating in the etched condition clearly shows the dark layers of the Ni(Cr) alloy in which light fields of evenly distributed primary Cr3C2 carbide phases can be seen as well as the secondary Cr7C3 carbide phases. in the coating layers deposited at low pressure in an inert atmosphere of Ar, there are no Ni and Cr oxide phases.
The tests have shown that the VPS - C^C2 - 25(Ni20Cr) cermet coatings have higher hardness and bond strength values than the APS and HVOF coatings, which are in accordance with the coating microstructure. The deposition of powder in a protective atmosphere at low pressure enabled the deposition of the coating layers with the prevailing primary Cr3C2 carbide phase which provides better performances to the coatings in operation.
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МЕХАНИЧЕСКИЕ СВОЙСТВА И МИКРОСТРУКТУРА ПОКРЫТИЯ СГ3С2 - 25(1\П20Сг) НАНЕСЕННОГО МЕТОДОМ ВАКУУМНОГО ПЛАЗМЕННОГО НАПЫЛЕНИЯ.
ОБЛАСТЬ: химические технологии
ВИД СТАТЬИ: оригинальная научная статья
ЯЗЫК СТАТЬИ: английский
Резюме:
В данной работе анализируется метод вакуумного плазменного напыления покрытия Сг3С2 - 25(М120Сг) с использованием промышленного порошкового состава Биквг МвСо №ока 7205 при применении плазматрона Р4 на расстоянии 340мм от основания.
Основной целью работы является проверка утверждения, что при пониженном давлении инертного газа исключается распад первичного карбида Сг3С2 до карбида Сг23С6, который значительно снижает микротвердость и механические свойства.
Покрытие толщиной 100 - 120 ут наносилось на стальное основание. Испытания покрытия на микротведость проводились по методу НУ03. Значения показателей микротвердости на-
ходятся в промежутке 1248 - 1342 НУ03. Испытание адгезии покрытия к основанию проводилось методом натяжения, полученное в результате испытания значение составляет 89 МРа.
Изучение микроструктуры покрытия, методом световой микроскопии показало, что покрытие состоит из основного сплава №Сг с преобладанием первичной карбидной фазы 0г302 и присутствием фазы Сг7С3.
Травление покрытия проводилось с использованием реагента 1НМ03: 4НС1: 4Н20, растворяющего в первую кочередь никель, что позволяет увидеть распределение карбидной фазы в покрытии. Травление покрытия показало, что в слое преобладает карбидная фаза Сг3С2, обеспечивающая высокое значение микротвердости покрытия.
Ключевые слова: вакуум, основание, прочность, свойства, микроструктура, механические свойства, покрытие, карбиды.
MEHANICKA SVOJSTVA I MIKROSTRUKTURA VAKUUM PLAZMA NAPRSKANE C3C2 - 25(Ni20Cr) PREVLAKE
OBLAST: hemijske tehnologije VRSTA CLANKA: originalni naucni clanak JEZIK CLANKA: engleski
Sazetak:
U radu je analizirana vakuum plazma sprej prevlaka VPS -Cr3C2 - 25(Ni20Cr). Upotrebljen je komercijalni prah oznake Sulzer Metco Woka 7205. Prah je deponovan sa plazma pistoljem F4 na od-stojanju substrata od 340 mm. Glavni cilj rada bio je da se na sma-njenom pritisku inertnog gasa Ar eliminise razgradnja primarnog kar-bida Cr3C2 u karbid Cr23C6 koji bitno umanjuje mikrotvrdocu i meha-nicke karakteristike prevlake. Prevlaka je deponovana debljine od 100 do 120 pm na celicnom substratu. Mikrotvrdoca prevlake ispitana je metodom HV03. Vrednosti mikrotvrdoce bile su u rasponu od 1248 do 1342 HV03. Cvrstoca spoja prevlake ispitana je metodom na zate-zanje. Utvrdeno je da Cvrstoca spoja izmedu substrata i prevlake ima vrednost 89 MPa. Mikrostruktura prevlake ispitana je tehnikom sve-tlosne mikroskopije. Struktura prevlake sastoji se od osnove NiCr le-gure sa dominantnom primarnom karbidnom fazom Cr3C2. Pored Cr3C2 faze prisutna je i faza Cr7C3. Nagrizanje prevlake uradeno je reagensom 1HNO3: 4HCl: 4H2O koji prvenstveno rastvara Ni da bi se videla raspodela karbidne faze u prevlaci. Nagrizanjem prevlake reagensom utvrdeno je da je u slojevima prevlake u velikom udelu prisutna primarna nerazgradena karbidna faza Cr3C2 koja prevlaci daje visoke vrednosti mikrotvrdoce.
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Vakuum plazma prskanje (VPS), zbog niskog pritiska, obicno se naziva i LPPS. Na niskim pritiscima mlaz plazme postaje vece duzine i manjeg precnika sa upotrebom konvergentnih/divergentnih mlaznica i o ima vecu brzinu jona. Eliminisanje kiseonika u komori i mogucnost pri-
mene predgrevanja substrata omogucuje izradu guscih prevlaka, vise zatezne cvrstoce spoja bez sadrzaja oksida u prevlaci. Primenom VPS cn procesa sprecava se dekarburizacija primarnog karbida Cr3C2, tako da
of se deponuju prevlake visoke tvrdoce. U poslednjoj deceniji veliki broj
istrazivaca publikovao je rezultate koji se odnose na strukturu i svoj-5 stva prevlaka deponovanih HVOF procesom (Guilemany, et al., 2006,
8 p.2998), (Guilemany, et al., 2002, p.207), (Ji, et al., 2006, p.6749), (Li,
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et al., 2005, p.229), (Picas, et al., 2006, p.477). Kod ovog procesa, kao o i kod APS procesa, glavni problem bio je gubitak ugljenika tokom talo-
zenja prevlake. U VPS prevlakama dominantna je karbidna faza Cr3C2 o tvrdoce 1600HV sa manjim udelom faze Cr7C3 tvrdoce 1300HV (Mar-
m cano, et al., 2008, pp.4406-4410), (Tomita, et al., 2001, pp.699-704).
U prevlaci nije prisutna karbidna faza Cr23C6, tvrdoce 1000HV, koja u < APS i HVOF karbidnim prevlakama umanjuje tvrdocu prevlaka. Tvrdo-
ca VPS Cr3C2 - 25(Ni20Cr) prevlake je HV1243 ± 80, sto je mnogo vece od HVOF prevlake sa tvrdocom HV958 ± 44 (Tomita, et al., 2001, pp.699-704). Zatezna cvrstoca spoja prevlake deponovane VPS procesom veca je od 80 MPa sa sadrzajem poroznosti manjom od 5% Q (ASM Metals Handbook, 1987, p.367). Plazma sprej prevlake Cr3C2 -
¡3 NiCr imaju visoku otpornost protiv abrazionog habanja i nizak koefici-
2 jent trenja, od sobne temperature do 850°C, zbog visoke termicke sta-
y bilnosti i otpornosti na oksidaciju (Guilemany, et al., 2006, p.2998). Ne-
davno je utvrdeno da ove prevlake mogu poboljsati otpornost na toplot-w ni zamor i otpornost na habanje u teskim uslovima opterecenja i produ-
o ziti radni vek komponentama (Guilemany, et al., 2002, p.207). Termicki
^ naprskane kermet prevlake dobra su alternativa tvrdom hromu, kada
> se zahteva visoka otpornost na habanje (Erning, Nestler, 1999,
pp.462-466), (Sahraoui, et al., 2004, pp.654-660), (Ko, Robertson,
2002, pp.880-893), (Savarimuthu, et al., 2000, pp.1095-1104). Plazma sprej procesi APS i VPS koriste se u sirokom spektru aplikacija, ukljucujuci automobilsku industriju, avionsku industriju, hemijsku proce-snu opremu, industriju celuloze i papira, ortopedskih i stomatoloskih komponenti i dr. (Erja Turunen, et al., 2006, pp.4987-4994), (Mrdak, et al., 2013, pp.559-567), (Mrdak, 2013, pp.69-88), (Mrdak, 2013, pp.182201), (Mrdak, et al., 2009, pp.27-32), (Vencl, et al., 2006, pp.151-157), (Vencl, et al., 2011, pp.1281-1288). Plazma sprej proces koristi se vise od cetiri decenije za izradu zastitnih prevlaka na bazi metala, keramike i cak kompozitnih materijala za razlicite aplikacije (Chuanxian Ding,
2003, pp.455-458). Uprkos dugom periodu primene plazma sprej procesa, medu naucnicima je jos uvek prisutno veliko interesovanje za razvoj novih materijala za izradu prevlaka i istrazivanje njihovog pona-sanja u radnim uslovima (Leblanc, 2003, pp.291-299).
U radu su predstavljeni rezultati eksperimentalnih ispitivanja uticaja VPS - vakuum plazma sprej procesa na mehanicke karakteristika i mikro-strukturu kermet prevlake Cr3C2 - 25(Ni20Cr). Glavni cilj rada bio je da se na smanjenom pritisku inertnog gasa Ar u procesu depozicije izbegne raz-gradnja primarnog karbida Cr3C2 u mnogo meksi karbid Cr23C6 i deponuju slojevi prevlake sa mikrostrukturom u kojoj ce biti dominantna karbidna fa-za Cr3C2 koja daje bolje performanse prevlaci u eksploataciji. Ispitivanja su pokazala da VPS - Cr3C2 - 25(Ni20Cr) prevlaka ima vece vrednosti mikrotvrdoce i cvrstocu spoja od APS i HVOF prevlaka, koje su u saglasnosti sa mikrostrukturom prevlake u kojoj dominira primarna karbidna faza Cr3C2.
Materijali i eksperimentalni detalji
Za izradu prevlaka koristio se prah firme Sulzer Metco s oznakom Woka 7205 ( Material Product Data Sheet, 2012, Woka 7205 Chromium Carbide - 25% Nickel Chromium Powders, DSMTS-0031.1, Sulzer Metco). Prah Woka7205 sadrzi 75%Cr3C2 karbida i 25%(Ni20Cr) legure. Ce-stice praha Cr3C2 - 25(Ni20Cr) sfernog su oblika, proizvedene tehnikom sinterovanja i aglomeracije sa rasponom granulata praha od 10 do 38 pm.
Osnove na koje su deponovane prevlake za ispitivanje mikrotvrdoce i za procenu mikrostrukture napravljene su od celika C.4171 (X15Cr13 EN10027) u termicki neobradenom stanju, dimenzija 70x20x1,5mm (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA). Takode, osnove za ispitivanje cvrstoce spoja napravljene su od celika C.4171(X15Cr13EN10027) u termicki neobradenom stanju dimenzija 025x50 mm (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).
Mehanicke i mikrostrukturne karakterizacije prevlaka uradene su prema standardu Pratt & Whitney (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA). Procena mehanickih osobina slojeva uradena je ispitivanjem mikrotvrdoce metodom HV03 i cvrstoce spoja ispitivanjem na zatezanje. Ispitivanje mikrotvrdoce uradeno je u pravcu duz lamela, u sredini i na kra-jevima uzorka. Uradeno je pet ocitavanja na tri mesta, a u radu je pri-kazan raspon mikrotvrdoce od minimalne do maksimalne vrednosti.
Metoda ispitivanja cvrstoce spoja je metoda ispitivanja na zatezanje. Ispitivanje je uradeno na sobnoj temperaturi sa brzinom zatezanja 1cm/60s. Za ispitivanje su upotrebljene tri epruvete, a u radu je prika-zana srednja vrednost.
Morfologija cestica praha uradena je na SEM - skening elektron-skom mikroskopu. Analiza udela mikropora u prevlaci uradena je obra-dom 5 fotografija na uvelicanju 200X. Preko paus papira mikropore su oznacene i osencene, a njihova ukupna povrsina racunala se na ukupnu povrsinu mikrofotografije. U radu je prikazana srednja vrednost udela mikropora. Mikrostrukturna analiza prevlaka uradena je na svetlosnom mikroskopu. Radi utvrdivanja raspodele karbidne faze u prevlaci radeno je nagrizanje prevlake u reagensu 1HNO3:4HCl:4H2O. Mikrostruktura pre-vlake posle nagrizanja ispitana je tehnikom svetlosne mikroskopije.
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Depozicija praha Cr3C2 - 25(Ni20Cr) izvrsena je na niskom pritisku inertnog gasa Ar u VPS sistemu firme Plasma Technik AG. Za depozici-ju praha koriscen je plazma pistolj F4. Proces obuhvata ciscenje povrsi-ne substrata transferovanim lukom i deponovanje praha na niskom pritisku. Na mikroprocesorskoj jedinici robota VPS sistema Plasma Technik AG uraden je program deponovanja praha Cr3C2 - 25(Ni20Cr). U progra-mu su zadati i vremenski sinhronizovani svi parametri procesa kao sto je: vakuumiranje komore, protok plazma gasova, ciscenje substrata transferovanim lukom, protok praha, depozicija prevlake, hladenje substrata i ventilacija vakuum komore. Ciscenje povrsine substrata i depozicija praha uradena je sa mesavinom plazma gasova Ar-He.
Rezultati i diskusija
U slojevima prevlake Cr3C2 - 25(Ni20Cr) duz poprecnog preseka su iz-merene vrednosti mikrotvrdoce od 1248 do 1342 HV03. Dobijene vrednosti mikrotvrdoce ukazuju da je u mikrostrukturi u vecem udelu prisutna neraz-gradena primarna karbidna faza Cr3C2, sto je omogucila inertna atmosfera Ar na niskom pritisku (Marcano, et al., 2008, pp.4406-4410), (Tomita, et al., 2001, pp.699-704). Raspon mikrotvrdoce deponovanih slojeva je posledica prisustva mikroporoznosti u slojevima prevlake. Zatezna cvrstoca spoja iz-medu substrata i prevlake bila je 89 MPa, sto je karakteristicno za VPS pre-vlake. Ciscenje povrsine substrata transferovanim lukom uticalo je na bolje prijanjanje deponovanih slojeva prevlaka, sto se odrazilo na dobijanje viso-ke vrednosti cvrstoce spoja. Vrednosti mikrotvrdoce i zatezne cvrstoce spo-ja bile su u korelaciji sa njihovim mikrostrukturama.
Kvalitativna analiza je pokazala da na interfejsu izmedu substrata i deponovanih prevlaka nisu prisutni defekti kao sto je diskontinuitet deponovanih slojeva na substratima, mikropukotine, makropukotine i odvajanje prevlaka od osnove. Granice na interfejsu izmedu substrata i slojeva prevlake izuzetno su ciste, sto ukazuje na dobro ciscenje povrsine substrata transferovanim lukom. Kroz slojeve prevlake uocavaju se mikropore sfernog i ne-pravilnog oblika obelezene crvenim strelicama. U slojevima prevlake nisu prisutne nestopljene cestice i precipitati. U strukturi nisu prisutne mikropukotine. Kroz slojeve prevlaka ne uocavaju se oksidne lamele. VPS - vakuum plazma sprej proces omogucuje deponovanje slojeva bez sadrzaja oksida u prevlaci, sto je velika prednost u odnosu na procese APS i HVOF.
U mikrostrukturi prevlake jasno se vide dve faze. Tamnoplava faza su lamele legure Ni20Cr, a svetloplava faza su primarni nerazgradeni karbidi Cr3C2 i sekundarni karbidi Cr7C3 koji daju prevlaci visoke vrednosti mikrotvrdoce (Marcano, et al., 2008, pp.4406-4410), (Tomita, et al., 2001, pp.699-704). Karbidne faze Cr3C2 i Cr7C3 su ravnomerno raspore-dene u strukturi prevlake koja je dosta ujednacena po preseku, bez pri-sutnih mikro i makropukotina. To ukazuje da su slojevi prevlake ravnomerno deponovani. U strukturi prevlake prisutne su mikropore koje se vide kao tamna polja. Poroznost prevlake odredena je pomocu tehnike analize slike, gde je 5 polja na uvecanju od 200X analizirano na poprec-nom preseku prevlake. Prosecna vrednost poroznosti iznosila je 4%. Primarne cestice karbida Cr3C2 i sekundarne karbidne faze Cr7C3 nalaze se
u interlamelarnim regionima legure Ni20Cr (Marcano, et al., 2008, pp-p.4406-4410), (Tomita, et al., 2001, pp.699-704). Nagrizanjem prevlake Ni se rastvara iz cvrstog rastvora legure Ni20Cr, dok karbidi Cr3C2 i Cr7C3 stoje izdignuti u reljefu svetloplave boje. Posto upadna svetlost ko-so pada na povrsinu uzorka i baca senku iznad izdignutih faza karbida, faza Ni20Cr legure je tamno- plave boje.
Zakljucak
U ovom radu su vakuum plazma sprej - VPS postupkom deponovane kermet prevlake Cr3C2 - 25(Ni20Cr) sa ciscenjem povrsine substrata tran-sferovanim lukom na odstojanju 320 mm plazma pistolja F4 od substrata i depozicija cestica praha na odstojanju 340 mm plazma pistolja od substrata. Ispitane su mehanicke karakteristike i mikrostrukture prevlaka u depono-vanom i nagrizenom stanju u reagensu 1HNO3:4HCl:4H2O. Na osnovu izvr-senih ispitivanja doslo se do odredenih zakljucaka.
VPS kermet prevlaka Cr3C2 - 25(Ni20Cr) duz poprecnog preseka ima-la je visoke vrednosti mikrotvrdoce od 1248 do 1342 HV03 Izmerene vred-nosti mikrotvrdoce ukazuju na prisustvo veceg udela nerazgradene primarne karbidne faze Cr3C2 u mikrostrukturi prevlake. Raspon mikrotvrdoce de-ponovanih slojeva posledica je prisustva mikroporoznosti u slojevima prevlake. Zatezna cvrstoca spoja Cr3C2 - 25(Ni20Cr) prevlake imala je visoku vrednost od 89 MPa. Ciscenje povrsine substrata transferovanim lukom uti-calo je na bolje prijanjanje deponovanih slojeva prevlaka, sto se odrazilo na dobijanje visoke vrednosti cvrstoce spoja. Vrednosti mikrotvrdoce i zatezne cvrstoce spoja bile su u korelaciji sa njihovim mikrostrukturama.
Mikrostruktura VPS kermet prevlake 75Cr3C2 - 25(Ni20Cr) je la-melarna. U deponovanim slojevima prisutne su mikropore sa udelom od 4%. Kroz deponovane slojeve ne uocavaju se neistopljene cestice praha i precipitati. U mikrostrukturi prevlake u nagrizenom stanju jasno se vide tamni slojevi legure Ni(Cr) u kojoj se nalaze svetla polja ravno-merno rasporedene primarne faze karbida Cr3C2 i sekundarne faze karbida Cr7C3. U slojevima prevlake koje su deponovane na niskom pritisku u inertnoj atmosferi Ar nisu prisutne oksidne faze Ni i Cr.
Ispitivanja su pokazala da VPS - Cr3C2 - 25(Ni20Cr) kermet prevlake imaju vece vrednosti mikrotvrdoce i cvrstoce spoja od APS i HVOF prevlaka, koje su u saglasnosti sa mikrostrukturom prevlake. Deponovanje praha u za-stitnoj atmosferi na niskom pritisku omogucilo je da se u prevlaci deponuju slojevi sa dominantnom primarnom fazom Cr3C2 u kojoj dominira primarna karbidna faza Cr3C2 koja u eksploataciji daje bolje performanse prevlaci.
Kljucne reci: vakuum, substrat, cvrstoca, svojstva, faze, mikrostrukture, mikrotvrdoca, mehanicka svojstva, prevlaka, karbidi.
Datum prijema clanka / Paper received on / Дата получения работы: 15. 08. 2013. Datum dostavljanja ispravki rukopisa / Manuscript corrections submitted on / Дата получения исправленной версии работы: 29. 08. 2014.
Datum konacnog prihvatanja clanka za objavljivanje / Paper accepted for publishing on / Дата окончательного согласования работы: 31. 08. 2014.
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