Microstructures and Wear Resistance of Al-25 wt.%Si Coatings Prepared by High-Efficiency Supersonic Plasma Spraying

Hypereutectic Al-Si alloy is an ideal surface strengthening material for aluminum alloy cylinder body. However, the coarse eutectic silicon phase and primary silicon particles in Al-Si alloys prepared by traditional methods lead to poor mechanical properties of the alloys, and the high cost of preparation limits the practical application of hypereutectic Al-Si alloys. In this study, Al-25 wt.%Si coatings were prepared on substrate 7075 by high-efficiency supersonic plasma spraying (HESP). The micromorphology, Vickers hardness, bonding strength and wear resistance of the coatings were measured and characterized by experiments. The results show that the Al-25 wt.%Si coatings prepared by HESP have a denser structure, and the primary silicon phase in the coatings is refined. The hardness of the coatings is 235 HV_0.1, and the bonding strength between the coatings and the substrate is 44.1 MPa. The wear rate of the coatings is reduced by 20 times compared with that of the substrate, and the wear mechanism is mainly adhesion wear and oxidation wear.

Keywords: Al-25 wt.%Si coatings; mechanical properties; thermal spraying technology; wear resistance

Nowadays, with the development of automobiles, the use of aluminum engine is more and more widespread. The aluminum engine is to prepare a wear-resistant coating on the surface of aluminum alloy cylinder body to enhance its wear resistance. Hypereutectic Al-Si alloys have the advantages of lightweight, high stiffness, excellent wear resistance and low thermal expansion coefficient (Ref [1]-[3]). It is an ideal surface strengthening material for aluminum alloy cylinder body. Hypereutectic Al-Si alloy generally refers to Al-Si alloys with silicon content greater than 13%. Related studies have shown that (Ref [4]-[7]), silicon content has a significant effect on wear behavior of Al-Si alloys. With the increase in silicon content to eutectic compound, wear resistance is improved. At the same time, the shape, size and distribution of eutectic silicon phase and primary silicon also affect the properties of hypereutectic Al-Si alloys. At present, casting and spraying deposition are the main methods to prepare Al-Si alloys (Ref [8]-[10]). However, due to the relatively low solidification rate during the casting process, hypereutectic Al-Si alloys prepared by the conventional casting method result in the formation of a needlelike eutectic silicon phase and rectangular-shaped coarse primary silicon, which seriously deteriorates the mechanical properties of alloys and hinders their practical application. Spraying deposition technology is usually used to prepare hypereutectic Al-Si alloys by rapid cooling and solidification (Ref [11], [12]). It can improve the microstructure of Al-Si alloys and refine the eutectic silicon phase and primary silicon. However, the preparation conditions are severe, and some complicated technique and expensive equipment must be used, which limits its actuality application (Ref [13], [14]).

Compared with the traditional method of preparing high Al-Si alloy, other researchers use thermal spraying technology to prepare a layer of high Al-Si coatings on the surface of aluminum alloy, the surface hardness and wear resistance of the aluminum alloy can be improved, and its application in automobile engine can reduce the cost and improve the efficiency. Yi et al. (Ref [15]) studied on the Al-12 wt.%Si coatings were fabricated using cold spray technique and the results showed that the hardness of the coatings was 120HV0.1. XIE et al. (Ref [16]) studied on the Al-13 wt.%Si coatings were fabricated using droplet spraying process and the results showed that the hardness of the coatings was 170HV0.1. Laha et al. (Ref [17]) studied on the Al-23 wt.%Si coatings were fabricated using high-velocity oxy-fuel spray technology and the results showed that the hardness of the coatings was 193HV0.1 and the porosity of the coatings was 3.2%.

As an important technology in surface engineering and remanufacturing engineering, the HESP adopts the single anode structure of Laval nozzle profile, and plasma jet is formed by arc ionization of inert gas (argon) between the anode and cathode. As the length of nozzle channel is compressed and the mechanical compression of the initial arc is enhanced, the anode spots are forced to move forward and elongate the arc (the arc voltage can reach 200-400 V). The thermal efficiency of the spraying gun is improved. The spraying temperature is as high as 10,000 C, and the flying speed of particles is as high as 400-600 m/s. The spray nozzle adopts an inner powder feeding structure, and the spray material is sent to the plasma jet for heating and acceleration. When the molten particles collide with the substrate, they are cooled and solidified rapidly to form a coating. The inert gas (argon) is used in spraying gas, which can reduce particle oxidation and form fewer defects and high-quality coatings. Compared with the traditional thermal spraying technology, HESP technology can obtain high-quality Al-Si coatings. Thus, HESP technology has broad prospects in practical production and application.

In this study, Al-25 wt.%Si coatings were prepared by HESP technology. The microstructure, mechanical properties and wear properties of the coatings were studied experimentally.

The spraying raw material was Al-25 wt.%Si powders (silicon content is 25 wt.%) produced by Changsha Tianjiu Metal Material Co., Ltd., China, using gas atomization, and the powder morphology is shown in Fig. 1. The substrate is 7075 with a size of 50 mm × 10 mm × 3 mm, and the main composition of substrate 7075 is shown in Table 1. Before the experiment, the Al-25 wt.%Si powders were dried for 3 h at 100 °C in the dry box, and the substrate 7075 was cleaned ultrasonically for 30 min in a bath of acetone to remove oil stain. Then, the surface of the substrate was grit blasted to Ra ≈ 6 µm by a manual blasting machine, using 500-µm angular alumina grits, to increase the bonding strength between coatings and the substrate. Finally, the Al-25 wt.%Si coatings were fabricated using HESP developed by National Key Laboratory for Remanufacturing, China. The spraying method is to stop spraying after one spraying, then cool the substrate to room temperature with cooling gas, then spray for the second time and then cycle in turn until the coating thickness is 0.4 mm to finish spraying. The internal structure of the HESP gun is shown in Fig. 2, and the process parameters of grit blasting and spraying are given in Table 2.

Graph: Fig. 1Micromorphology of the Al-25 wt.%Si powders: (a) low magnification and (b) high magnification

Chemical composition of substrate 7075 (mass fraction, %)

Graph: Fig. 2Internal structure of the HESP gun

Spraying and grit-blasting parameters

The surface morphology, cross section morphologies and composition of the coatings were analyzed by Nova NanoSEM50-type environmental scanning electron microscope (SEM, FEI, America) equipped with energy-dispersive spectroscopy (EDS, OXFORD, England). The phase identification of the samples was conducted by x-ray diffraction (XRD, Bruker, Germany) device with Cu Kα radiation. The tube voltage and current were 40 kV and 150 mA, respectively. The scanning speed and range were 3°/min and 20° ∼ 90°, respectively. Talos-F200X field emission transmission electron microscope (TEM, FEI, Talos-F200X, America) was used for the coating's crystal structure and high-resolution image.

The porosity of Al-25 wt.%Si coatings was measured by grayscale method. The pore size inside the coatings was expressed by their area distribution on the two-dimensional plane. The main steps were as follows: acquisition of 1000 × magnification SEM images of 10 coatings section, input ImageJ2x software, transformation images, processing images and recording porosity.

The microhardness of coatings was measured using a microhardness tester (MICROMET-6030, Buehler, America) with load 100 gf and hold time 15 s. The bonding strength of the coatings was tested according to ASTMC-633-01 standard. The coatings sample was bonded with the stretching fixture (35CrMo steel) by using E-7 glue (E-7, Shanghai Huayi, China). After E-7 solidification, it was measured on MTS809 universal tensile testing machine. The cross-head speed of the equipment was 1 mm/min. The bonding strength of each coating is the average of three experimental measurements. Tensile test specimen (diameter is 25.4 mm) is shown in Fig. 3.

Graph: Fig. 3Tensile test specimen for measuring bond strength of spray coatings

The wear resistance tests were carried out on a UMT-3 multi-functional friction and wear tester (CETR, America), as shown in Fig. 4. The samples of the coatings are 10 × 10 × 3 mm. The diameter of the GCr15 ball was 4 mm, and its Rockwell hardness was 61 HRC. In order to ensure the consistence of the experimental conditions, the samples were grinded with SiC sandpaper from #400 to #1000 before the experiment, until the surface roughness (Ra) of the sample was less than 0.8 μm and then polished. The experimental parameters were: load 2 N; frequency 2 Hz; test time 20 min; amplitude 4 mm. The morphologies and section profiles of the worn surfaces were characterized by means of SEM and LEXT (Olympus OLS4000, Japan) 3D laser measuring microscope.

Graph: Fig. 4Schematic diagram of the friction pair for linear reciprocating sliding contact

The typical surface topography of the Al-25 wt.%Si coatings is shown in Fig. 5(a). The particles melt well, spread evenly, and the coating surface is denser with a few pores. The cross-sectional morphology of Al-25 wt.%Si coatings is shown in Fig. 5(b). The coatings have compact structure, and there are no large pores and impurities, only a small number of micropores, and no cracks at the interface between the coatings and the substrate, which is a typical mechanical bonding mode (Ref [18]). The measured porosity of the coatings is 2.02%. This is attributed to HESP spraying technology, which has high flame temperature, good melting state and high-flying speed of particles. When molten or semi-molten particles reach the substrate, they have high kinetic energy and thermal enthalpy, and strong impact force, making them fully flattened and evenly spread.

Graph: Fig. 5Microstructure and morphology of Al-25 wt.%Si coatings: (a) surface morphology and (b) cross-sectional morphology

The TEM image of Al-25 wt.%Si coatings is shown in Fig. 6(a). Some spherical and bulk particles appear in the coatings. The EDS scanning of the coatings (in Fig. 6b) indicates that only Al and Si elements were found in the coatings, and the distribution of both elements was very uniform. Moreover, the EDS analysis of these spherical and bulk particles shows that the silicon content is very high, so we can judge that these spherical and bulk particles are primary silicon phase. SAED patterns (in Fig. 6a) insinuate the presence of aluminum and silicon in Al-25 wt.%Si coatings. The less intense, but well-defined ring pattern indicates the presence of ultrafine crystalline structure. The average grain size was estimated from the x-ray diffraction spectra using Scherer's formula and the size range of primary silicon phase in Al-25 wt.%Si coatings was 50-120 nm. Compared with the coarse bulk or rod-shaped primary silicon with the size range of 40-200 µm of high Al-Si alloys prepared by conventional casting method, the size range of block primary silicon phase of high Al-Si alloys prepared by spraying deposition technology is 10-30 µm (Ref [1], [19]), indicating that the primary silicon phase of Al-25 wt.%Si coatings prepared by HESP is refined. This is attributed to the fact that the spraying heating temperature of HESP ranges from 5000 to 11,000 °C. When Al-25 wt.%Si powders are heated at high temperature and flying at high speed, they impact on the substrate and are cooled by cooling gas in a very short time (106-108 K/s) (Ref [17]) and then rapidly solidify to form coatings. Under this extremely hot and cold spraying condition, the growth of primary silicon phase is limited. However, there is no eutectic silicon phase in the Al-25 wt.%Si coatings. This is because the cooling time of droplets is too short to form eutectic silicon phase.

Graph: Fig. 6TEM morphology and EDS of Al-25 wt.%Si coatings: (a) TEM morphology and (b) EDS surface scanning

The XRD patterns of Al-25 wt.%Si powders and Al-25 wt.%Si coatings are shown in Fig. 7. It can be seen from the patterns that the phases of the powders and coatings are identical, all of which are aluminum, silicon and Al3.21Si0.47.

Graph: Fig. 7XRD patterns of Al-25 wt% Si powders and coatings

Five points were selected to measure the hardness of coatings and substrates. The microhardness values of Al-25 wt.%Si coatings and substrate 7075 are shown in Table 3. The average microhardness of coatings is 235 HV0.1, which is 2.2 times higher than that of substrate 107.4 HV0.1. This is because the presence of nanosized grains in the Al-25 wt.%Si coatings will result in higher mechanical strength of coatings. These nanosized primary silicon phase can act as potential barrier for dislocation movement and thus can improve the mechanical strength and hardness of the Al-25 wt.%Si coatings, which can improve the wear resistance of the coating (Ref [20], [21]).

Vickers hardness of Al-25 wt.%Si coatings and substrate 7075

Through tensile test, all three samples were disconnected at the interface between coatings and substrate (in Fig. 8). The bonding strength between coatings and substrate is shown in Table 4. The three bonding strength values between coatings and substrate are 45.2 MPa, 43.6 MPa and 43.5 MPa, and the average value is 44.1 MPa, which is higher than the 30 MPa bonding strength between the Al-Si coatings and aluminum alloy substrate prepared by traditional thermal spraying technology (high-velocity oxy-fuel spray technology and common plasma spraying technology, etc.) (Ref [22]). Porosity, oxide and impurities in the coatings have a great influence on the bonding strength of the coatings (Ref [23]). As Al-25 wt.%Si coatings prepared by HESP is uniform and compact, the bonding strength between coatings and substrate is better.

Graph: Fig. 8Tensile fracture morphology of Al-25 wt% Si coatings samples

Bonding strength between Al-25 wt.%Si coatings and substrate 7075

The friction coefficient curves of Al-25 wt.%Si coatings and substrate 7075 are shown in Fig. 9. Three friction tests were carried out on each specimen, and the average values of friction coefficients were obtained, and the trend of friction coefficients was the same. The friction coefficient of the substrate increases sharply at the initial stage of friction and tends to be stable gradually. The reason for this trend is that in the initial stage of friction, the friction coefficient rises sharply because the wear surface is rough and the contact between GCr15 ball and substrate 7075 surface is not uniform. With the development of friction, the contact between GCR15 ball and substrate 7075 surface becomes tight and the friction coefficient tends to be stable. The friction coefficient of Al-25 wt.%Si coatings is generally stable and is significantly lower than that of substrate 7075. This indicates that the Al-25 wt.%Si coatings have a good anti-friction performance compared with substrate 7075.

Graph: Fig. 9Friction coefficient with time for Al-25 wt.%Si coatings and substrate 7075

Figure 10 shows the three-dimensional morphologies of wear surfaces of Al-25 wt.%Si coatings and substrate 7075. The width and depth of wear tracks of Al-25 wt.%Si coatings and substrate 7075 are shown in Table 5. The wear rate K (mm3/N m) is calculated by the following formula, and the result is shown in Fig. 9:

$$K=\frac{S·l}{L·F}$$

Graph

where S is the area of cross section in mm2, l is the length of wear track in mm, L is the total sliding distance in m and F is the applied load in N.

Graph: Fig. 103D morphologies of wear track: (a) Al-25 wt%Si coatings and (b) substrate 7075

Width and depth of wear track

It can be seen from Table 5 that the width and depth of the wear track of Al-25 wt.%Si coatings are 453.5 µm and 16.5 µm, respectively, and those of substrate 7075 are 1207.1 µm and 108.8 µm. It can be seen from Fig. 11 that the wear rate of substrate 7075 is 187.5 × 10−4 mm3/N m, which is 20 times higher than that of Al-25 wt.%Si coatings of 9.37 × 10−4 mm3/N m. This is because the hardness of substrate 7075 is lower than that of Al-25 wt.%Si coatings and is softer than that of coatings. For the softer metals, the wear rate is greatly affected by the wear contact area (Ref [24]). As can be seen from Fig. 10 and Table 4, the width and depth of the wear track of the substrate are larger, indicating that the wear contact area is larger and the wear is serious. The hardness of Al-25 wt.%Si coatings is higher than that of substrate 7075. In Al-25 wt.%Si coatings, aluminum-rich phase is a soft substrate, which plays the role of fixing primary silicon phase. In the process of wear, primary silicon phase as hard phase bears load pressure, and reduces grinding depth and friction contact area, which results in lower wear rate.

Graph: Fig. 11Wear rate of Al-25 wt.%Si coatings and substrate 7075

The SEM diagrams of the wear surfaces of substrate 7075 and Al-25 wt.%Si coatings are shown in Fig. 12(a) and (c). It can be seen from Fig. 12(a) that the wear surface of substrate 7075 is very rough, and there are a lot of furrows, debris and delamination along the direction of the wear track, which indicates that serious abrasive wear and adhesion wear have occurred. This is due to the low hardness and rough surface of substrate 7075. Adhesive wear occurs at the beginning of wear, and shear failure mainly occurs on the surface layer of the soft substrate. As wear progresses, the area of adhesive points between GCr15 ball and substrate surface increases, and many abrasive debris attach to the wear track surface. Serious plastic deformation occurs on the contact wear zone. At the same time, serious abrasive wear occurs and serious scratches occur on the wear surface.

Graph: Fig. 12SEM diagrams of the worn surfaces and EDS spectrums: (a) the worn surfaces of substrate 7075; (b) the EDS spectrum of substrate 7075; (c) the worn surfaces of Al-25 wt.%Si coatings; (d) the EDS spectrum of Al-25 wt.%Si coatings

It can be seen from Fig. 12(c) that the wear surface of the Al-25 wt.%Si coatings is smooth. There are obvious cracks in the vertical direction of the wear track, and spalling coatings on both sides of the wear track, indicating that adhesive wear has taken place. Because the hardness of Al-25 wt.%Si coatings is higher than that of substrate 7075, the separation force is produced in the direction of the wear track, and cracks are produced along the vertical direction of the wear track. Previous studies have shown that the morphology and size of eutectic silicon phase play an important role in the wear resistance of Al-Si alloys (Ref [25]). The primary silicon phase in Al-25 wt.%Si coatings is bulky (Fig. 12c), showing some spheroidizing effects. This effect can effectively reduce the localized stress concentration at the interface of spheroidized silicon/alpha aluminum, making the surface primary silicon and alpha aluminum combine perfectly, and effectively inhibit the crack propagation at the interface of spheroidized silicon/alpha aluminum. At the same time, the spheroidizing effects of primary silicon phase can increase the fracture toughness of the coatings, making the spheroidized silicon/alpha aluminum interface more difficult to fracture. This can effectively reduce the formation of crack nucleation sites and improve the wear resistance of Al-25 wt.%Si coatings.

Through EDS analysis of the wear surface, it can be seen from Fig. 12(b) and (d) that the oxygen element appears on the worn surface of Al-25 wt.%Si coatings and substrate 7075, and the oxygen content on the worn surface of Al-25 wt.%Si coatings is much higher than that of substrate 7075. The reason is that the temperature at sliding surface interface gradually increases with the sliding continues. Relevant work indicated that the increase in temperature within the limit range can increase the ability of soft aluminum substrate to accommodate hard and brittle polyhedral shaped primary silicon crystals (Ref [26], [27]), resulting in a higher hardness of the Al-25%Si coating. Moreover, the formed oxide film reduces the direct contact between GCr15 ball and Al-25 wt.%Si coatings, thus reducing the interaction and enhancing the wear resistance of the coatings. Therefore, oxidation wear also occurs in the Al-25 wt.%Si coatings.

The following main conclusions can be drawn:

• High-efficiency supersonic plasma spraying technology has the characteristics of inert heat source, high temperature, high speed and short spraying distance. The Al-25 wt.%Si coatings prepared by this technology is uniform and compact and the coatings mechanically bonded with the substrate. The porosity of Al-25 wt.%Si coatings is 2.02%, the hardness of Al-25 wt.%Si coatings is 235 HV0.1, and the bonding strength between coatings and substrate is 44.1 MPa.
• The heat source temperature range of high-efficiency supersonic plasma spraying technology is 5000-11,000 °C, and the substrate temperature is usually controlled at room temperature by cooling gas. Al-25 wt.%Si powders can undergo extremely hot and cold spraying conditions in a very short time (106-108 K/s), so that the primary silicon phase in Al-25 wt.%Si coatings can be refined, and the size range is 50-120 nm.
• Al-25 wt.%Si coatings prepared by high-efficiency supersonic plasma spraying technology have stable and lower friction coefficient, and it has a good anti-friction performance compared with substrate 7075. At the same time, the wear rate of Al-25 wt.%Si coatings is 20 times lower than that of the substrate, indicating it also has a good wear resistance. The main wear mechanisms of the coatings are adhesion wear and oxidation wear.

This work was supported by the National Natural Science Foundation of China (Grant Numbers 51535011, 51605451, 51675531), the Pre-Research Program in National 13th Five-Year Plan (Grant Number 61409230603), Joint Fund of Ministry of Education for Pre-research of Equipment for Young Personnel Project (Grant Number 6141A02033120) and the Fundamental Research Funds for Central Universities (Grant Number 2652018095).

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