Investigation of stand-off distance effect on structure, adhesion and hardness of copper coatings obtained by the APS technique
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The outbreak of the disease and infection in the hospital environment and medical equipment is one of the concerns of modern life. One of the effective ways for preventing and reducing the complications of infections is modification of the surface. Here, the handmade atmospheric plasma spray system is used for accumulating copper as an antibacterial agent on the 316L stainless steel substrate, which applies to hospital environment and medical equipment. As a durable coating with proper adhesion is needed on the substrate, the effect of stand-off distance (SOD) which is an important parameter of the spray on the microstructure, the hardness and adhesion of the copper coating on the 316L stainless steel were investigated. The structure and phase composition of copper depositions were investigated using scanning electron microscopy and X-ray diffraction. The adhesion and hardness of depositions are evidenced using the cross cut tester and Vickers hardness tester, respectively. The findings confirm that the voids in the coatings increase with increasing SOD, which leads to decreasing the hardness of coatings and also the adhesion strength between depositions and substrate. In addition, by increasing the SOD, the oxygen content and the size of grains in the lamellae (fine structure) of coatings also increase.
KeywordsAtmospheric plasma spray Copper Stand-off distance Fine structure Hardness and adhesion strength
The properties of depositions depend on spray methods [1, 2]. In recent years, the plasma spray technique is used for deposition of different coatings [3, 4]. The performance of a plasma spray system depends on the design of torch, type and rate of plasma gas, plasma power and suchlike. The structural modification of the plasma torch can increase the power of plasma, which leads to more hardness of the coatings . By adding helium or hydrogen gas to plasma gas, the power of plasma increases resulting in increase of coating hardness . The bond between the splats inside the depositions and layer structure of coatings is the factor which defines the features of coatings achieved by the plasma spray method . The temperature of particles plays an important role in the hardness of depositions . Roughness and temperature of substrates can improve solidification of copper droplets and modify the properties of depositions obtained by the plasma spray technique [9, 10]. The large diameter of the nozzle can improve cohesion between the lamellae in the coatings, because the dwelling time of particles in plasma torch increases . The depositions of nickel-based alloy formed by plasma spray under low power may have certain features comparable to depositions formed by conventional plasma spray .
The parameters of spray
Power of plasma, 18 kW
Plasma gas flow rate (argon), 25 l/min
Gas pressure, 0.3 MPa
Powder feed gas flow rate (argon), 15 l/min
Stand-off distances, 150, 175, 200 and 225 mm
Cooling water rate, 10 l/min
Substrate temperature, room temperature
The crystalline spectra of coatings are observed by the STADI MP X-ray diffraction (XRD) with a filament of tungsten at voltage 40 kV and current 30 mA in the presence of radiation copper anode (λ = 1.5402 Å). The XRD spectra of the coatings are recorded in the \(2\theta\) range of 20°–80° with steps 0.05. The structures of coatings are investigated with a Philips, XL-30 back-scattering electron microscopy (BSE) and scanning electron microscopy (SEM). The coatings are pushed by a diamond indentation under a load of 100 g for a total time 15 s. The average value of the three tests at different points for each of the polished coatings indicates the hardness evaluated by Vickers hardness tester (HV0.1). In adhesion test performed by cross cut tester (TQC), some scratches are formed in two normal directions on the coatings. Special glue is placed on the depositions for 5 min and then peeled off. The adhesion strength between depositions and substrate is determined with the ASTM standard.
Results and discussion
Increase in the proportion of weight percentages of copper oxide to copper from coating (a) to (c), in which SOD also increased, can be attributed to an increase of the interaction of copper particles with oxygen of the air due to the increased distance. But for coating at SOD 225 mm, (d), which has the greatest distance from the nozzle, not only the ratio of weight percentage of copper oxide to copper is higher than that of (a) and (b), but also we see the presence of CuO peaks in the XRD spectrum of this coating.
Using image analysis, the thicknesses of depositions with the measurement software are measured 110, 106, 100 and 92 μm, respectively. It can be said that the thickness of coatings decreases slightly by increasing SOD. Based on empirical and computational studies, the temperature of the plasma jet decreases along the torch axis with increase in SOD . The powder particles have different sizes as seen in “Methods”. In addition, to the energy of the plasma jet is reduced on heating the powder and its carrier gas. Therefore, the particles injected in the plasma jet appear in three states: fully molten, partially molten and solid, during flight and at the moment of impact. By increasing the SOD, because of greater drop in the particle surface temperature, the number of solid particles increases. The partial molten and solid particles may be fragmented and weakly connected to the coating and or are bounced when they encounter the surface of the substrate or accumulate as splats without forming the coating.
To detect the fine structure of the coatings, they are etched by a solution of ammonia 125 ml, hydrogen peroxide 250 ml and distilled water 125 ml. The images suggest a layer structure for all coatings. Comparison of Fig. 3a–d confirms that by increasing SOD, the number of spherical particles (not melted) increases and the compactness of the structures decreases. As seen in Fig. 3a, b, at the distances 150 and 175 mm, the lamellae are compressed. However at SOD 175 mm, the lamellae are more regular and also wider than those formed at SOD 150 mm. Because the molten particles after collision can flow well on the substrate and flatten sufficiently. At SOD 200 mm, the lamellae are even but some voids appear in coating as shown in Fig. 3c. The voids are marked with black color. The air or the plasma gas dissolved in the liquid particles is trapped during impacting on the substrate and the splats are accumulated; therefore, the voids are formed. The voids located on the interface of the substrate coating and also between lamellae reduce the adhesion of the coating as verified by TQC. Figure 3d indicates the heterogeneous structure at SOD 225 mm, which has a lot of formless voids. In this structure, the lamellae are uneven and dissimilar. Because the lamellae created are exposed to insufficient heat of the plasma jet, they are less integrated with each other.
The voids reduce the area of loading tolerance. They create a state of multilateral stress which concentrates the local pressure in their vicinity. As a result, more voids lead to less hardness. As seen in the fine structure of coatings, since larger grains do not match well with each other and more voids are formed in the coating, the cohesion between grains, particles, splats and lamellae decreases by increasing SOD. The decrease of coating hardness by increasing SOD can be associated with increase in the size of grains inside the lamellae and also voids. The percent of voids in coatings for different stand-off distances is evaluated by image analyzer software and is 2, 3.09, 9.98 and 22.3% respectively.
The specific dissection of the etched lamellae with more magnification as seen in Fig. 4 (with no Alphabet letters) authenticates the grain structure inside each of the lamellae. The grains grow perpendicular to the direction of lamellae progress. As seen in Fig. 4, the size of the grains is sub-micrometer. At more distances, grains are less ordered and larger. Therefore, more and larger voids emerged in the coatings. At distances of 150 and 175 mm, the grains are cohesive and nearly coaxial. Figure 4 shows that by increasing the distance, the binding between grains decreases and the space between grains increases. This space depends on the cooling rate of splats which is inversely proportional to splat thickness or its square . The bigger grains are seen at SOD 225 mm (Fig. 4d). The mean sizes of grains for different stand-off distances are evaluated by image analyzer software which is 80, 85, 110 and 230 nm, respectively. This means when stand-off distance increases, the grains size also increases.
Copper coatings are prepared using the handmade atmospheric plasma spraying (APS) system at different stand-off distances onto the stainless steel 316L substrate. Here, the system works under the fixed spray conditions and the only changing parameter is the stand-off distance (SOD). It is proven that the fine structure of the coating depends on SOD. By increasing SOD, the size of grains and the space between grains in the lamellae increase and the content of oxygen in the coating also increases. More and larger voids emerge in the deposition. Therefore, the cohesion between lamellae and the coating hardness decrease. Moreover, adhesion strength also decreases. At a critical distance, we achieved the highest degree of adhesion in ASTM pattern classification. The SOD is one of the important parameters for creating a coating with excellent mechanical properties.
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