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Study on the properties of FeCrNi/CBN composite coating with high velocity arc spraying
⁎Corresponding authors. lxjhenan@126.com (Xiao-juan Liu)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
FeCrNi/CBN cored wire is prepared by use of the pull-type flux cored wire machine (FCWM50 type), FeCrNi/CBN composite coating is made by high velocity arc spraying. The special properties of the composite coating can be improved by adding Cr, Ni and small amounts of rare earth elements to adjust the coating composition. The Tribological properties of FeCrNi/CBN composite coating with high velocity arc spraying is studied. Images and components and cross-section microstructure of coatings are analyzed by means of SEM, and EDS, etc. This research indicated that FeCrNi/CBN composite coatings have typical layered structure feature and high bond strength and hardness. Friction of coatings at near room temperature or higher has “Run-up” period. With the increase of temperature, friction coefficient of coatings becomes low and wearing capacity of coatings high. The adding of CBN powder highly improved the wearing capacity of coatings.
Keywords
FeCrNi/CBN
Tribological properties
Structure feature
Wearing capacity
1 Introduction
High-velocity arc spraying technique (HVAS) is a new kind of thermal spraying technique. It processes many advantages. Jet velocity of molten drop is high. Distribution of molten drops atomized is homogeneous. Bond strength of coating is high. Porosity of coating is low (Weipu et al., 2004a,b). In recent years, high- velocity arc spraying technique is being widely studied and applied. The quality of coating with this technique can be improved. The high economic benefit and social benefit have been achieved with use of this technique (Guo et al., 2015). Currently, flux-cored wire has been used in high-velocity arc spraying. It has the following advantages compared with solid-cored wire (Halin et al., 2017).
It has short cycle of production, cost low. So, It is suitable for high velocity arc spraying. At present, flux-cored wire composite material, nano material and new alloy material are used as spraying materials. It is difficult to made solid-cored wire with above materials. But it is easy to put above material in flux-cored wire (Samad et al., 2017).
Chemical composition of flux-cored wire is adjusted easily. Some kinds of special characteristics coating can be achieved (Roslan et al., 2017). And coating quality can be controlled conveniently. Compared with solid-cored wire, the problem that high hardness metal is hard to be wire drew can be solved by use of flux-cored wire. Some insulating materials possessing special chemical elements only are used in coating as flux-cored wire. The powder not being alloyed are mounted in fused base surface of work piece in coating as hard phase. The other metal droplet which has been alloyed fully becomes into solidified microstructure quickly. The metal droplet deposited can be re-melted with use of releasing heat to come from reaction of some unique metal elements. The performance of coating with flux-cored wire is excellent. The bond strength is enhanced greatly. The porosity is controlled efficiently. Oxide film is formed homogeneously. Stability of coating is improved (Shaoqing, 2009; Weipu and Binshi, 2004; Nordin et al., 2017).
The flux-cored wire of FeCrNi/CBN coating was made by authors in order to obtain coating of high hardness and wearing capacity. The FeCrNi/CBN composited coating was made successfully by high velocity arc spraying. Cross-section microstructure, thermal shock resistance, micro-hardness and wearing capacity were researched. The flux-cored wire in preparation of high wearing- capacity metal-ceramic composite coating has a greater advantage than solid wire. XU, etc. A group researcher made respectively different flux-cored wires with Cr7C3, Cr3C2, WC-Co powder and another low-carbon steel belt (Runsheng et al., 2006; Zhao-Feng and Wen-Ping, 2006; Jianhai and Banggu, 2001). The wearing capacity of the composite coating by coating spray is commensurate with once of plasma spray. He, etc. [8] obtained a high bonding strength, high hardness, wearing- capacity coating with the flux-cored wire of low carbon steel coated WC powder and with high velocity arc spraying (Dingyong et al., 2007). Li Zhuoxin etc. [9] studied with the increase of Tibz and Alzo in the volume fraction in the coating, the wear resistance of the coating is obviously increased, and the wear quality loss decreases linearly with the increase of the ceramic phase volume fraction (Zhuoxin et al., 2005; Yaacof et al., 2017). Adding alloying elements ni and Al can reduce porosity and increase wear resistance of coating. The coating has good wearing- capacity. JIANG Jian-min [10] studied that carbide ceramic powders can obviously increase the hardness and abrasive wear resistance of the coatings, the average values of microhardness are about 1 1 00∼1 200 HV0.1 (Jianmin et al., 2007). The researchers of Institute of Chinese Armored Force [11–13] studied preparation of flux-cored wire made by low-carbon steel coated with WC, Cr3C2, and high-speed arc spray coating which possesses higher hardness and wearing- capacity (Weipu et al., 2005; Zixin et al., 2004; Weipu et al., 2004a,b). And they considered that the coating obtained with Cr3C2 flux-cored wire possess higher coating corrosion resistance and wearing- capacity than WC flux-cored wire. At present, the coating in preparation of CBN hard phase has not been reported obtained with high velocity arc spraying. Authors prepared one kind of FeCrNi/CBN flux-cored wire which possesses high hardness and high wearing-capacity. FeCrNi/CBN composite coating was made by high velocity arc spraying. And also, the microstructures, thermal shock resistance, microhardness and wearing capacity were studied.
2 Test materials and methods
2.1 Flux-cored wire preparation
2.1.1 Cored wire composition determine
The cored materials of the FeCrNi/CBN flux-cored wire is made up of CBN powder, Cr, Ni, Al, B, Si and small amount of rare earth elements (composition shown in Table 1). Nickel has good strength and ductility. Its density is 8.9 g/cm3 at 20 °C. Its melting point is 1455 °C. Its thermal conductivity is 88.5 w/(mk) [14] between 0 °C and 100 °C (Wenyu, 2007). Nickel is a moderately active metal. The anti-corrosion of fluorine, alkali, salt and many organic substances is better. Nickel-base has a physical capacity of a large number of alloying elements, and forms a stable phase (Rozainy et al., 2017).
CBN
Cr
Ni
Mo
V
Mn
Fe
40–55
15–25
20–35
<0.05
<0.05
<0.15
Margin
The coefficients of thermal expansion of spray coating and of the substrate are different, their shrinkages of different temperature change are different too. There by it will inevitably lead to the generation of internal stress, the greater the differences of their thermal expansion coefficients, the greater the stress. The bond strength between the coating and substrate is also lower (Sukor et al., 2017; Hassan et al., 2017). If Large differences in coefficient of thermal expansion exist between them, residual stress will be generated in molten drop during crystallization. Nickel joined will lead to promote bonding integration among the components, to reduce the surface free energy, to enhance combining power, and not easy to separate the two binding surfaces. Ni, Fe, Cr and other elements form solid solution and compounds. A fresh surface is maintained. So, it is difficult to be oxidized. That promotes the reaction between the molten drop and the substrate surface, and results in the micro-diffusion layer interface, Thereby the bonding strength of the coating is enhanced. Cr is silver and refractory metals, its density is 7.19 g/cm3 at 20 °C, melting point 1875 °C, boiling point 2660 °C. Cr possesses good infiltration with super-hard material (Shamsudin and Majid, 2017).
2.1.2 Flux-cored wire rolling
The flux-cored wire is prepared with flux cored wire drawing machine (FCWM50) in Luohe Hengguang Ltd. The steel belt is with a selection of 10 mm × 0.4 mm cold rolling stainless steel belt (composition shown in Table 2). The steel belt rolled after a few rolls gradually formed the groove cross-section drawing. The powder is put into the steel tank, then after a few rolls, the groove cross-section is gradually closed in circular shape cross section. The diameter of wire being drew through the die after a 5–6 road progressively reduced up to 2.5 mm on the wire drawing machine. In order to improve the performance of the coating, there are other elements, filling powder in addition to CBN, whose particle size is 60–80 screen mesh. The rolling process of forming flux-cored wire is shown in Fig. 1.
Belt
C
Cr
Ni
Mn
Si
S
P
304 stainless steel
0.028
19.8
10.1
0.86
0.4
<0.03
<0.045
Forming flux-cored wire rolling process.
2.1.3 Add powder coefficient
Add powder coefficient of flux-cored wire is an important parameter to ensure the alloy contents of flux-cored wire. The add powder coefficient is the weight of internal alloy powder ratio of the flux-cored wire. According to the formula: Calculate the flux-cored wire add powder coefficientwhere P - add powder coefficient
m0 - the total weight of flux-cored wire (g)
m1 - skin weight of flux-cored wire (g).
Measurements of add powder coefficient of flux-cored wire are shown in Table 3.
Samples
Total weight/m0 (g)
Skin weight/m1 (g)
Add powder coefficient/%
The average coefficient/%
1#
16.3
11.8
27.5
28.0
2#
16.5
11.8
28.4
3#
17.5
12.6
28.2
4#
16.7
12
27.8
The diameter of flux-cored wire is drawn into Φ2.5 mm at last. The component of powder in flux-cored wire includes: CNB about 40–55%, Cr about 15–25%, Ni about 20–35%, and a small amount of rare-earth metal. The average add powder coefficient filling rate of flux-cored wire with powder is 28%.
2.2 High-speed arc spraying process
Coatings were produced by ZPG-400A type electric supply and HVAS gun (QDIII-250 V type). The parameters of spraying were the same for all coatings, i.e. voltage 32 V, current 150-210A. The distance between work piece and nozzle of gun is 180–250 mm. Atomizing pressure is 0.45–0.5 MPa. The thickness of coating is typically 300–400 μm.
2.3 Determination of bond strength of coating and thermal shock resistance
The bond strength of coating was determined by the method of dual sample test according to the ASTMC633-79 standard on material stretch test machine. The coating thickness is between 0.3 mm and 0.4 mm. Each thickness of coating was measured with five times. The average bond strength value is as the one of the coating (Shamsudin et al., 2017).
The box furnace used for determination of thermal shock resistance of the coating was made by Shanghai Experimental Furnace Ltd. The sample size is 50 mm × 50 mm × 5mm. Its material is No. 45 steel. The sample was scrubbed with gasoline, and was blasted with corundum. Then the surface coating thickness of 0.4–0.5 mm was made by arc spray. During the testing, the sample was placed in the box furnace heated for 15 min at the different heating temperatures which were 650 °C, 700 °C, 750 °C. After that, the sample was removed from the furnace, quickly placed in water of the room temperature cooling, so forth to complete the heating - cooling - heating cycle. This was reacted 15 times, then the coating surface was observed whether the coating from the skin surface, the coating off, oxidation discoloration, etc., to check the coating thermal fatigue performance (Basheer et al., 2017).
The stresses suffered by the sum of all which exit in the coating and the substrate interface lead to cracks, the coating peeling off during the testing period of thermal shock resistance. These stresses mainly come from two aspects: one is the residual stress which is caused by solidification shrinkage of the melten drop during the preparation of the coating with the arc spraying process. Another is the additional stress which is caused by the difference (Δα) of thermal expansion coefficient between the coating and the substrate resulting in mismatch the elastic strain between them in the process of thermal shock resistance testing. When the total stress achieved on the interface of the coating and the substrate is at critical cracking stress point of them, the interface will crack.
2.4 The coating porosity
The spray coating process determines that there is a certain level of porosity. Numbers of pores, shape of pore, size of pore, distribution of pores directly affect the coating performance. Therefore, the porosity is an important indicator of the quality of the coating. In the test, the linear intercept method was used to determine the porosity of the coating which belong to manual determination method using quantitative metallography according to national standard GB/T15749-1995 (Binshi, 1996; Ismail and Hanafiah, 2017).
Test principle: The pores in the coating are regarded as a phase. The coating entity is regarded as another phase. The pore content of three-dimension was calculated according to the measurement of two-dimensional parameters of the grinding metallography
Operation: ① take the coating cross section, take picture after mounting, grinding, metallographic polishing and cleaning picture. ② take ten equally spaced parallel lines in microstructure picture. ③ measure the segment length of the pores cutting and parallel line to the segment length.
calculated as follows:
- the percentage of pore cut-off line, %,
- pore cutting the segment length, mm,
- parallel to the segment length, mm
2.5 Coating micohardness measurement
Coating micohardness measured by HX-500 durometer, under 150 g load and 30 s residence time. The hardness was measured in the perpendicular direction of coating substrate according to 40 μm internal in order to observe the hardness distribution rules.
2.6 Determining the wearing capacity of coating
The wearing capacity of coating is completed on MM-200 abrasion tester which was made by Hebei Xuanhua Material Testing Machine Factory. Rolling - sliding test diagram was shown in Fig. 2. The sample as bottom wheel was prepared by coating the surface of the test loop, and then the surface will be grinded to the desired size (0.3–0.4 mm thickness of the coating) after grinding. Upper wheel is acted as the comparison pair of bottom one, which made up of No. 45 steel quenched from 800 °C. The bottom wheel (sample) velocity is 10% lower than the upper wheel velocity when testing, So, that the testing condition is constituted with 90% of rolling friction and 10% of the sliding friction. The test can be on lubrication conditions. And also, can be on dry friction conditions. Lubrication conditions in the test were selected by author. The weight difference of wheels before and after wear was measured by abrasion tester worked on 10,000 revolutions of the sample.
Schematic diagram of abrasion tester.
2.7 Coating microstructure analysis
The sample was prepared with mounting, grinding sample, metallographic polishing and cleaning. The thickness of coating is about 0.6 mm, which was made up of No. 45 steel and whose finish size is 25 mm × 16 mm×5mm. One piece of the sample in cross-section was prepared for observing. Cross- sectional microstructure was observed by Model PME Olympus optical microscope. Coating surface morphology was studied by Quanta200 scanning electron microscope (SEM). The chemical composition of coating section was analyzed by Genesis 60 s energy spectrometer.
3 Results and analysis
3.1 Bond strength and thermal shock resistance
The average bond strength of the coating is 30.5 MPa from the Table 4. The composite coatings of FeCrNi/CBN have high bond strength. It has the following these reasons are that the coefficient of thermal expansion of coating is low, so the internal stress of coating is small, and that the room plasticity is improved by the method of adding element Cr and that the bond strength of coating is improved with use of the exothermic reaction of Ni (Yangshan et al., 1991). Ni can form solid solution and compounded with Fe and Cr. The effects can protect fresh surface against oxidization and to promote the reaction between droplet and substrate. Micro-area diffusion layer appears in the interface.
Sample
1
2
3
4
5
Average data
Data
30.9
30.8
30.6
30.2
30
30.5
AS the results of thermal shock resistance are shown in Table 5, it is easy to find that macro-defects between coatings and substrate haven’t appear obviously in the conditions of fifteen times according to the order 650 °C, 700 °C and 750 °C. It also shows that the sample has an excellently thermal shock resistance.
Test sample
650 °C 15times
700 °C 15 times
750 °C 15 times
Thermal shock
Thermal shock
Thermal shock
1
Nothing
Nothing
Color yellow and microcrack
2
Nothing
Nothing
Color a little yellow
3
Nothing
Nothing
Color blue
3.2 The coating porosity
As can be seen from Table 6, the coating porosity is of 7.0% or less full. It indicates that the coating organization possessed the higher densities. The ideal value of the arc spray was achieved basically. This is because the surface of the rare earth elements is active. The trace of the rare earth elements joined could reduce the surface tension of molten drop. It could improve the mobility of molten drop. It could enhance infiltration of the molten drop on the substrate. It could reduce thermal expansion coefficient, and reduce internal stress of the coating. Thereby, it could lead to enhance the bond strength of the coating and to reduce porosity of the coating.
Sample
1
2
3
4
5
Porosity average
Porosity
6.3
6.1
6.2
6.1
6.4
6.2
3.3 The microhardness of coatings
The transverse distribution of the microhardness on cross section of coatings in the perpendicular direction of substrate is shown in Fig. 3. It is easy to find that the average microhardness of coating is 2–3 times of substrates from Fig. 3. There are some areas whose microhardness is very high because they have hard phase element of CBN. The microhardness is also relatively high in certain areas. It is in conformity with dispersive distribution of oxide. The microhardness is low in certain area of coating, which shows that deficiency such as pore space exists in the coating. In addition, it is shown that microhardness changes gently from substrate to coating from Fig. 3. It contributes to improve bearing capacity of coatings and to decrease residual internal stress between coating and substrate.
Hardness profile of FeCrNi/CBN coating along T direction.
3.4 The wearing capacity of coatings
The test results of wearing capacity of FeCrNi/CBN composite coatings are shown in Table 7. It is easy to find that wearing capacity of coating is good because there are hard phase elements of CBN which is diffusion. The hardness elements of coating in contact with wear material during the friction and wearing protect efficiently soft substrate and control abatement of surface of workpiece, and reduce abrasion loss of coating. Thereby, so the coatings possess good wearing capacity.
Coatings
Original weight (g)
Weight worn (g)
Weight loss (mg)
Wear resistance
Average
sample1
66.3907
66.0875
303.2
6.3532
6.4240
wheel1
64.7623
62.8360
1926.3
1
sample2
64.6201
64.3125
307.6
6.2893
wheel2
75.1439
73.2093
1934.6
1
sample3
69.2715
68.9779
293.6
6.6297
wheel3
72.8216
70.8751
1946.5
1
3.5 Coating microstructure
Fig. 4 shows the micrograph of the cross section of FeCrNi/CBN composite coating. The typical lamella structure of coatings was shown by Fig. 4. Organization of the coating is dersification and it has not bulky pore space. Some irregular flat particles are found for example A point in the coatings. The observing results of SEM and EDS are shown in Fig. 5 and Table 8, It indicates that particles are CBN. Element distribution map in the region is shown in Fig. 6 through observing the region around the CBN particles. Table 8 shows the chemical compositions of FeCrNi/CBN composite coating. Through analysis, it indicates that metals of around CBN surface are melted in spraying process. CBN as a core surrounded by melted metal powder hits against the surface of substrate. Dense coating is formed in process of coating expansion.
Structure of FeCrNi/CBN coating.
EDS spectra of FeCrNi/CBN coating Full-scale:313cts Pointer 0.119 keV(37cts).
Elements
B
N
C
Ni
Cr
Fe
w(x)
17.42
22.56
3.28
21.36
16.32
Margin
The face composite of FeCrNi/CBN coatings distribution element distribution map of the region.
4 Conclusion
The FeCrNi/CBN composite coating by HVAS has characteristics of high bond strength and compactness. The bond strength of FeCrNi/CBN composite coating by HVAS is 30.5 MPa. It has excellently thermal shock resistance. The results of the microhardness of the coating indicate that the average hardness is high. Microhardness changes gently from substrate to coating. It contributes to improve bearing capacity of coating and to decrease residual internal stress between coating and substrate. The results of dynamics performance experiments of coating indicate that synthetic dynamics performance of FeCrNi/CBN composite coating is good. The typical lamella structure of coating was shown. The FeCrNi/CBN composite coating has excellent wearing capacity. It can be used as wearing surface of workpiece in order to reduce loss by wearing.
Acknowledgements
We are grateful to the anonymous referees for their many valuable and helpful comments. This work was supported by Project of science and technology tackling key project in 2017, Henan Provincial Science and Technology Department, (No. 172102310355); Key scientific research projects of colleges and universities in 2016, Henan Province, (No. 16A460035); Science and Technology Research Program of Henan Province, (No. 162102210271, No. 172102310737, No. 182102110010, No. 182102110296); Basic and Frontier Technology Research Program of Henan Province, (No. 142300410294); Key scientific research projects of colleges and universities in Henan Province, (No. 17B416001); Science and technology project of Zhengzhou Airport Economic Zone: Key technologies to improve ecological benefits of roof greening trees, (Economic Development Bureau of Zhengzhou Airport Economic Zone No. [2016] 152); Key scientific research project of Colleges and universities in Henan Province: Virtual simulation of root growth and functional characteristics under root restriction based on GPR technique, (No. 17A220002) and Technology research and development project of Zhengzhou: Study on the construction of sponge city in Zhengzhou (No. 153PKJGG145).
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