CHARACTERIZATION OF Cu 10 wt . % Al INTERMETALLIC COATINGS APPLIED BY THE ATMOSPHERIC PLASMA SPRAYING PROCESS

The atmospheric plasma spray process is one of the procedures used for the deposition of coatings resistant to wear due to friction, erosion, cavitation and corrosion. In this paper, the APS process produced a Cu10wt.%Al intermetallic coating which is a reliable candidate for use in tribological environments because of a combination of low price and exceptional resistance to abrasion under different work conditions. The aim of this study was to investigate the mechanical properties and the structure of the Cu10wt.%Al intermetallic coating and develop an efficient method for repairing and improving light alloy resistance to wear. Many components of copper alloys tend to be degraded due to corrosive environment, friction, erosion and cavitation. Such components can be saved by surface engineering with the use of appropriate coatings on surface areas exposed to degradation. A typical microstructure of a coating for the APS process is lamellar, with micro pores, unmelted particles, interlamellar oxides and precipitates present in it. The mechanical properties of Cu10wt.% Al coatings were investigated by measuring the microhardness of coating layers using the HV0.3 method while the


Summary:
The atmospheric plasma spray process is one of the procedures used for the deposition of coatings resistant to wear due to friction, erosion, cavitation and corrosion.In this paper, the APS process produced a Cu10wt.%Alintermetallic coating which is a reliable candidate for use in tribological environments because of a combination of low price and exceptional resistance to abrasion under different work conditions.The aim of this study was to investigate the mechanical properties and the structure of the Cu10wt.%Alintermetallic coating and develop an efficient method for repairing and improving light alloy resistance to wear.Many components of copper alloys tend to be degraded due to corrosive environment, friction, erosion and cavitation.Such components can be saved by surface engineering with the use of appropriate coatings on surface areas exposed to degradation.A typical microstructure of a coating for the APS process is lamellar, with micro pores, unmelted particles, interlamellar oxides and precipitates present in it.The mechanical properties of Cu10wt.%Al coatings were investigated by measuring the microhardness of coating layers using the HV 0.3 method while the Introduction APS -atmospheric plasma spraying is one of the technological processes used to manufacture coatings with a thickness between 0.1 and 0.5 mm.Plasma spray technology is one of possible solutions to produce copper-based coatings because of afordable production costs and possibilities to produce coatings on parts of complex shapes.The APS process includes the injection of powder particles into a plasma jet, its melting or semi-melting, and then accelerating and collision with the substrate where powder particles are deposited, forming a coating (Mrdak, 2015a, pp.137-159), (Mrdak, 2015b, pp.46-67).The microstructure of plasma spray coatings is characterized by a lamellar structure with limited inter-lamellar bonding due to the presence of micro pores.The coating microstructure and its mechanical properties are influenced by a large number of process parameters which directly affect the phase composition and porosity content for a specific powder and a range of powder particles (Mrdak, et al., 2015, pp.337-343), (Mrdak, 2016a, pp.1-25), (Mrdak, 2016b, pp.411-430), (Vencl, et al., 2011(Vencl, et al., , pp.1281(Vencl, et al., -1288)), (Vencl, et al., 2010, pp.591-604).Copper is a metal that is widely used in many applications because of its excellent thermal and electrical conductivity.The mechanical properties of copper can be improved by alloying.Some copper alloys such as Cu-Al, Cu-Si and Cu-Al-Fe are used for wider industrial applications, because they are quite resistant to wear and corrosion.Cu10wt.%Alalloys are widely used in the chemical industry thanks to their high corrosion resistance.In this alloy type, besides the α structure, there is the β structure as well.Wear resistant Cu10wt.%Alcoatings are used on working parts to reduce damage due to friction, erosion, corrosion and cavitation (Bartuli, et al., 2007, pp.175-185).Cu-Al intermetallic coatings are good candidates for use in tribological environments because of a combination of low prices and exceptional resistance to abrasion under different work conditions.Optimum protection against wear of light metal substrates can be provided with effective application of thermal spray processes for coating and powder depending on the working environment and working conditions (Sartale, Yoshitake, 2010, pp.353-360), (Wang, Seitz, 2001, pp.755-761).Intermetallic coatings or metal-ceramic composite coatings can be obtained by thermal spray powder spraying.Cu-Al intermetallic systems have been actively researched for use in aircraft, automotive, marine, construction, etc.The Cu-Al system have long been used for wheel bearings for planes and screws for ships because of its resistance to wear and corrosion (Sartale, Yoshitake, 2010, pp.353-360).Cu10wt.%Alpowder marked Metco 445 is mechanically coated aluminum bronze which shows self-bonding for substrates during the thermal spray process as a result of the chemical reaction of the coated components which build intermetallic phases.The powder contains aluminum from 7.0 to 12.0 wt%Al (Material Product Data Sheet Aluminum Bronze Thermal Spray Powders Thermal Spray Powder Products Metco 445, 2012, DSMTS-0103.0,Sulzer Metco).Coatings have good resistance to friction and scuffing at low and moderate temperatures and excellent resistance to abrasion and cavitation.Testing of Cu10wt.%Alcoatings to abrasive wear and sliding wear using the ringon-disk method and a load of 150 g over the sliding speed of 4.5 cm/s showed that the abrasive coating wear is 0.52mgm -1 , and that sliding wear is 2.8x10 -5 mm 3 m -1 .The main coating wear mechanism is plastic deformation (Limpichaipanit, et al., 2011, pp.123-126).Coatings machine easily and excellently.Typical components covered by Cu10wt.%Alcoatings are: supports of bearings, sleeves of hydraulic presses, piston guides, air compressor seals, water pumps, turbine nozzles, etc.The presence of aluminum in bronze increases the resistance of the coating to corrosion because of the formation of a thin cohesive surface oxide which acts as a protective layer on the alloy rich in copper (Material Product Data Sheet Aluminum Bronze Thermal Spray Powders Thermal Spray Powder Products Metco 445, 2012, DSMTS-0103.0, Sulzer Metco).To understand better the reaction processes which occur between Cu and Al during the process of powder melting and the formation of intermetallic phases, it is necessary to know the Cu-Al equilibrium diagram.In the equilibrium phase diagram of Cu-Al, there are five stable intermetallic phases, i.e.: Cu 9 Al 4 , Cu 3 Al 2 , Cu 4 Al 3 , CuAl and CuAl 2 with two solid solutions Cu(Al) which are often described as α-Cu and Al(Cu) (ASM Handbook, 1992, Volume 3, Alloy Phase Diagrams, ASM International, Metals Park).Studies have shown that in the process of powder melting in plasma, due to the reaction of Al and Cu, various intermetallic phases are formed, such as: CuAl 2 , Cu 9 Al 4 , Cu 3 Al 2 , Al 4 Cu 9 (Altuncu, et al., 2012, pp.181-183).The main intermetallic phases which affect the wear resistance of the coating are Cu 9 Al 4 and Cu 3 Al 2 .Plasma spray deposited Cu10wt.%Alcoatings have a lamellar structure, with present unmelted particles, precipitates, and inter-lamellar pores.According to the authors (Hang, et al., 2008, pp.416-424), (Li, Sun, 2004, pp.92-101) in the microstructure there are present intermetallic phases of CuAl 2 and Cu 9 Al 4 .The alloy base is a solid solution of α-Cu (90.6-100wt%Cu) and β-Cu (85.0-91.5wt.%Cu).In the microstructure of the coating, there are not black oxide lamellae of Al 2 O 3 oxide (Li, Sun, 2004, pp.92-101).Cu10wt.%Alcoatings consist of a bright phase of copper and a dark phase of copper oxide Cu 2 O. Copper oxide is primarily formed when temperatures ≥ 1000 °C and in the presence of oxygen, which occurs when using the plasma spray process (Hang, et al., 2008, pp.416-424), (Li, Sun, 2004, pp.92-101).
In this paper, plasma spraying at atmospheric pressure was used to deposit an aluminum bronze coating which contains 7.0 to 12.0 wt.%Al.The coating microstructure was analyzed with a light microscope and the coating surface was analysed with the SEM -scanning electron microscope.The aim of this study was to investigate the mechanical properties and the microstructure of the Cu10wt.%Alintermetallic coating and to develop an economically efficient method of depositing intermetallic coatings for improving resistance of worn aircraft parts made of Cu alloys exposed to a combination of corrosion and wear.

Materials and experimental details
The material on which layers of Cu10wt.%Al(7.0 to 12.0 wt%Al) intermetallic coatings were deposited was made of stainless steel X15Cr13 (EN 1.4024) in the thermally unprocessed state.Powder of the Sulzer Metco company labeled Metco 445 was used to produce the Cu10wt.%Alcoating.The powder was manufactured using mechanical coating and spheroidization to a specific granulation density of 3.1-4.3g/cm 3 .The powder melting point is 1040 °C.Powder with granules in a range of 45 -106 μm was used for the experiment (Material Product Data Sheet Aluminum Bronze Thermal Spray Powders Thermal Spray Powder Products Metco 445, 2012, DSMTS-0103.0,Sulzer Metco).Figure 1 shows the (SEM) scanning electron photomicrographs of the Cu10wt.%Al powder particles morphology.The powder particles are approximately spherical in shape.The testing of the mechanical properties of the Cu10wt.%Al coating was done in accordance with the Pratt & Whitney standard (Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).The bases on which were deposited layers of coatings for microhardness testing and evaluation of the microstructure in the deposited state are made of steel Č.4171 (X15Cr13 EN10027) in the thermally unprocessed state with the dimensions of 70x20x1.5 mm.The substrates for testing the bond strength are also made of steel Č.4171 (X15Cr13EN10027) in the thermally unprocessed state with the dimensions of Ø25x50 mm.The microhardness of layers was tested using HV 0.3 and bond strength tensile testing.The microhardness measurements were performed along the lamellae.Five readings of microhardness values of the layers were performed in the middle and at the ends of the samples while two extreme values were rejected.Out of three remaining values, the average value is shown.The bond strength was tested at room temperature with a tensile speed of 1cm/60s.Five specimens were tested, out of which two extreme values were rejected.Out of the three remaining values, the mean value of the bond strength is shown.The morphology of the powder particles and the surface of the deposited coating was examined using scanning electron microscopy (SEM).The microstructure of the deposited layers was examined on an optical microscope (OM).The share of micro pores in the coating was analysed by analysing 5 photos at 200x magnification.In this paper, the mean value of the share of micro pores in the coating is presented.
Cu10wt.%Al powder is deposited with the atmospheric plasma spray system of the Plasmadyne company and the plasma gun SG-100, with controlled plasma spray parameters.The plasma gun SG-100 consists of a cathode type K1083-129, anode type A 2083-175 and the gas injector type GI 1083-130.Ar in combination with He was used as an arc gas, and the power supply was 40kW.The plasma spray deposition parameters of Cu10wt.%Alpowder are shown in Table 1.Before the deposition process, the substrate surfaces were roughened with white aluminum oxide particles of the size 0.7-1.5 mm.Coatings were deposited on the test samples with a thickness of 0.45-0.5 mm.

Results and discussion
The microhardness and the tensile bond strength of the coating had values characteristic for this type of coatings.The Cu10wt.%AlIntermetallic coating had an average microhardness of 176 HV 0.3 .The measured average value of microhardness was higher than the value specified by the powder manufacturer -158 HV 0.3 (Material Product Data Sheet Aluminum Bronze Thermal Spray Powders Thermal Spray Powder Products Metco 445, 2012, DSMTS-0103.0,Sulzer Metco), which indicates that a large share of micro pores was not present in the coating.This was confirmed by metallographic examinations of the coating layers.The mean value of the tensile bond strength measured on the samples with coated Cu10wt.%Al was 35 MPa.The coating was destroyed at the substrate / coating interface because of good bonding with the substrate.The measured values of the microhardness and the tensile bond strength of Cu10wt.%Alcoatings were correlated with the microstructure of the deposited layers.The photomicrographs clearly show the interfaces of coating layers and the substrate.The substrate/coating interface is very clean indicating a very good bonding of the coating with the substrate, which indicates a good substrate surface preparation.Because of good surface preparation, the interface does not show the presence of corundum particles left from roughening, which resulted in good adhesion of the coating with the substrate.At the interface between the substrate and the deposited coating layers, there are no defects such as discontinuities of deposited layers, microcracks, macrocracks, coating peeling and separation from the substrate.Generally, the layers are uniformly deposited on the substrate.The coatings have a lamellar structure, inter-lamellar oxides and interlamellar pores.The microstructure of the coating showed that powder particles are uniformly and homogeneously distributed.Through the coating layers, coarse micro pores, micro cracks and macro cracks cannot be seen.The average share of micro pores in the coating layers was 8%.At a higher magnification, in Figure 3, we can see the light gray lamellae of oxide Cu 2 O with a uniform distribution in the coating (Hang, et al., 2008, pp.416-424), (Li, Sun, 2004, pp.92-101) as well as micro pores in black.In the coating, unmelted powder particles were not detected, which indicates that powder particles had been deposited with the optimal deposition parameters.The Cu10wt.%Al coating base consists of solid solutions α-Cu and β-Cu rich in copper containing CuAl 2 and Cu 9 Al 4 intermetallic phases (Hang, et al. 2008, pp.416-424), (Lee, Sun, 2004, pp.92-101).
Figure 4 shows a SEM photomicrograph of the surface of the Cu10wt.%Alintermetallic coating.The analysis of the surface morphology of the Cu10wt.%Alintermetallic coating shows complete melting and regular melting of powder particles on the previously deposited layer.The red lines in SEM micrographs mark boundaries between the melted particles.The molten powder particles formed thin discs -splates in a collision with the substrate.Thus formed shapes of deposited particles have good cohesive bonding with the previously deposited particles, indicating that the powder particles were deposited with the optimum deposition parameters.Coarse micro pores cannot be seen on the coating surface.SEM micrographs clearly show black micro pores surrounded by yellow color of a size up to 10µm.The coating surface shows precipitates formed as a result of the collision of molten droplets with the substrate.At the moment of the collision of molten droplets with the substrate, the ends of molten particles chip and solidify as precipitates in the deposited coating layers.

Conclusion
In this paper, the APS -atmospheric plasma spray process produced a Cu10wt.%Alintermetallic coating based on copper with a content of aluminum from 7.0 to 12.0wt.%Al.The coating deposited on the test samples had a thickness of 0.45-0.5 mm.We analyzed the mechanical properties and the microstructure of the coatings in the deposited state, which led to the following conclusions.
The Cu10wt.%Al intermetallic coating had good mechanical properties, with a microhardness value of 176 HV 0.3 which was above the value of 158 HV 0.3 prescribed by the powder manufacturer.The bond strength was 35 MPa.The microstructure is lamellar, consisting of lamellas of oxide Cu 2 O formed by Cu oxidation in the process of deposition of the powder and inter lamellar pores with an average proportion of 8%.
The base of the Cu10wt.%Alcoating consisted of solid solutions α-Cu and β-Cu rich in copper with CuAl 2 and Cu 9 Al 4 intermetallic phases as a result of the thermal reaction between Cu and Al in plasma during melting and deposition.

Figures 2
Figures 2 and 3 show the micrographs of a typical microstructure of the plasma spray coating.