1. Novosibirsk State Technical University, Novosibirsk, Russia. 2. Khristianovich Institute of Theoretical and Applied Mechanics SB RAS,. Novosibirsk, Russia.
Thermophysics and Aeromechanics, 2006, Vol. 13, No. 1
STRUCTURE AND PROPERTIES OF ALUMINUM COATINGS OBTAINED BY THE COLD GAS-DYNAMIC SPRAYING METHOD 1
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L.I. TUSHINSKY , A.P. ALKHIMOV , V.F. KOSAREV , A.V. PLOKHOV , 1 and N.S. MOCHALINA 1
Novosibirsk State Technical University, Novosibirsk, Russia
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Khristianovich Institute of Theoretical and Applied Mechanics SB RAS, Novosibirsk, Russia (Received October 18, 2005) Structure and properties of aluminum coatings deposited onto steel substrates by the cold gas-dynamic spraying (CGS) method were examined. Aluminum CGS coatings fundamentally differ from their thermal counterparts as they enable the formation of heavy-duty layers of metal particles on substrates at temperatures below 500 K. A dense, low-porosity coating is found to form, tightly bound to the base metal. The adhesion strength is shown to weakly depend on the thickness of the sprayed coating due to the compressive stress present in the surface layer. A qualitative model for the coating formation process is proposed. INTRODUCTION
Understanding of coating formation regularities and coating properties as dependent on the characteristics of the gas-powder flow impinging on the work surface makes it possible to obtain controllable coating-base compositions. Previously (see [1−3]), gasdynamic spraying regularities were examined versus the main physical parameters of the supersonic two-phase flow, including the velocity, size, material density and concentration of particles, the velocity and temperature of the gas flow, the angle of incidence of the gas-powder jet flow onto the surface, etc. In the present paper, data are reported which demonstrate the main properties of aluminum coatings formed on steel substrates as dependent on determining process parameters such as the spraying distance and the working-gas pressure and temperature in the nozzle chamber. EXPERIMENTAL SETUP AND PROCEDURE
ASD-1 powder was sprayed (at an angle of 90°) onto substrates to deposit on them aluminum coatings under various spraying conditions on a setup described in [1, 2]. The substrates were 2-mm thick St.3 steel plates. Prior to spraying, the plates were given a cleaning treatment to be subsequently pre-processed on a needle-milling machine. The powder particles had almost spherical shape. The coating-to-base metal bonding (adhesion) strength and the cohesion strength were estimated from the indentation depth hin _____________________________________________________________________________________________________
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This work was supported by the Russian Foundation for Basic Research (Grants Nos. 03-02-16329 and 05-07-90172).
L.I. Tushinsky, A.P. Alkhimov, V.F. Kosarev, A.V. Plokhov, and N.S. Mochalina, 2006
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registered on a tester with a spherical 10-mm diameter indenter. The indentation tests were performed on an Ericssen device (a tool generally accepted for such tests in Japan). The cohesion and adhesion were judged respectively from the onset of the crack formation process on the free surface of the coating and from the separation of the coating from the substrate [3, 4]. The coating-to-base metal bonding strength was also determined using specimens with annular pins [5]. Compared to specimens with round pins, such specimens normally show a higher probability of pure pin detachment; besides, with such specimens, a smaller spread of measured data could be achieved due to data averaging over a greater contact area. The coating porosity was determined by the point Glagolev method [3] on nonetched metallographic sections considering the proportion between the points contained in the observed voids and the points felt onto the coating material. The residual coating stress was measured by the mechanical method using coatings with thicknesses δc = 100−400 µm deposited onto 1.5-mm thick plates prepared from 60С2А laminated spring steel. The radii of curvature of the plates with sprayed coatings were measured to subsequently calculate the average residual coating stress [4]. The coating morphology was examined on metallographic sections obtained by etching coating cuts in a 0.5 ml 40 %-НF + 100 ml Н2О solution. RESULTS
We examined how the three most important process parameters, namely, the spraying distance, the working-gas pressure, and the working-gas temperature in the nozzle chamber of the setup influenced the coating-to-base metal bonding strength, the coating structure, and the morphology of the coating/base metal interface. The spraying distance is the most easily adjustable process parameter. At the same time, in the course of the process this parameter undergoes variations resulting from base-metal roughness, and process imperfections and nonstationarity. That is why it is necessary to determine the optimum spraying distance and the admissible range of this parameter for the coating properties to still satisfy necessary requirements. At a constant nozzle-chamber pressure of 1.5 MPa and at fixed gas-flow temperature, 100 °С, we examined the variations in the structure and properties of the sprayed coatings as dependent on the spraying distance, 5, 15, and 40 mm. Figure 1 shows the adhesion (1) and cohesion (2) strength for various coating thicknesses δc versus the spraying distance ls. In spite of the considerable differences in the coating thickness, the emergence of a first crack (cohesion) and the coating separation from the base metal (adhesion) were found to uniquely depend on the spraying distance. A decrease in the spraying distance from 40 to 5 mm results in increased adhesion and cohesion strengths. By way of example, Fig. 2 shows the structure of a coating sprayed at a spraying distance of 5 mm. The coatings sprayed from a shorter distance showed a lower
Fig. 1. Effect of spraying distance on the adhesion and cohesion strength at various coating thicknesses. hin indentation depth for the 10-mm diameter spherical indentor at which a first crack appears (cohesion) or the coating separates from the substrate (adhesion). 1 and 2 – adhesion and cohesion domains.
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Fig. 2. Cross-sectional image of an asdeposited (non-etched) aluminum coating obtained by the CGS method at a spraying distance of 5 mm, 150× magnification.
porosity, the structure of the coating-base interfacial region in such samples being more favorable from the standpoint of conformity and composition integrity. In some areal parts, the interface in samples with coatings sprayed from the distance 40 mm was of worse quality, the coating loosely mating the base metal. Alongside, even in the case of coatings sprayed from the largest distance the interface was, for the most part, of satisfactory quality, the bonding strength was rather high, and the porosity was lower than 10−12 %. To find the optimum value of the stagnation pressure, we varied this pressure in the range from 0.5 to 1.5 MPa. In these tests, the dependence of the adhesion-cohesion strength and coating structure on the pressure was examined at a gas-flow temperature of 100 °С and at a spraying distance of 15 mm. Figure 3 shows that an increase in the stagnation pressure in the examined range of pressures resulted in improved adhesion strength. At the highest pressure of 1.5 MPa the adhesion and cohesion were maximal, and the coating structure possessed the best quality. The interfacial region displayed almost no discontinuity flaws, the mean porosity was 4−6 %, and the voids were fine and elongated. In this case, the porosity distribution was nonuniform over the height of the coating, with the coating being denser near the contact interface. The latter regularity seems to be probably related to the cold dynamic compaction effect [6, 7]. In our experiments, no direct influence of gas-flow temperature (in the examined range of temperatures) on the bonding strength was found (see Fig. 4). DISCUSSION
A joint analysis of Figs. 1 and 3 shows that the coating thickness has almost no influence on the coating-to-base metal strength. This conclusion is not typical of the majority of coatings in use deposited by various methods [3, 6]. In the case of detonation,
Fig. 3. Effect of pipeline air pressure on the adhesion and cohesion strength at various coating thicknesses. (Notation is the same as in Fig. 1).
Fig. 4. Effect of particle pre-spray temperature on the adhesion and cohesion strength at various coating thicknesses. Spraying distance 15 mm, pressure 1.5 MPa. (Notation is the same as in Fig. 1).
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plasma, and flame coatings an increase in thickness worsens the strength of bonding with the base. In the final analysis, all other conditions being identical, the adhesion strength is determined by the level, sign, and distribution of the residual stress in the coating [8]. It is found that, with increase in the coating thickness, an increase in the internal compressive stress from 48 MPa (200-µm thick coating) to 63 MPa (400-µm thick coating) is observed. Most probably, in coatings sprayed under the examined process conditions favorable compressive stress prevails. Indeed, the crystallization of liquid particles on a cold substrate typical of thermally sprayed coatings is accompanied by coating shrinkage, gives rise to a tensile stress and, finally, worsens the coating-to-base bonding strength. A different picture is observed with coatings sprayed by the proposed technology. First, the particles, as they undergo acceleration, are never melted and remain solid until they come in contact with the substrate. No shrinkage, typical of the majority of coatings, is observed. Second, during the pre-spray treatment of the substrate on the needle-milling machine the base temperature rises to a value comparable with the particle temperature. Since the spraying is performed right after the pre-spray treatment, there is no substantial temperature gradient in the coating-base metal system. Third, a metallographic study shows that the aluminum particles undergo heavy deformation, which is accompanied by cold hardening and induces an additional compressive stress. In other words, in the cold gasdynamic method effects analogous to surface plastic deformation are observed. The generated internal compressive stress defines rather high adhesion strength almost independent of the coating thickness. For instance, in a 200-µm thick coating the bonding strength measured by the pin method with an annular pin is 48 MPa, whereas in a 400-µm thick coating this strength amounts to 52 MPa. SUMMARY
Undoubtedly, the high bonding strength of comparatively thick (up to 400 µm) sprayed coatings can be considered as a major advantage offered by the gas-dynamic spraying method. The performed analysis of the coating-base metal interface suggests that the bonding is a result of physico-chemical and mechanical interactions. Moreover, in the contact zone the formation of diffusion layers, intermetallides, and iron-aluminum spinel is possible. Yet, it is important to create, in addition, a developed rough surface on the base providing, due to the enlarged coating-substrate contact area, for effective bonding of coating particles and improved adhesion. The structure of gas-dynamic coatings sprayed onto steel substrates under all the examined regimes differs from the structure of analogous metallization coatings considered, for instance, in [4]. First, no lamination is observed, largely typical of metallization coatings. In gas-dynamic coatings, the particles are tightly driven in between their neighbors to completely fill the voids. No indications for particle twisting are observed (see Fig. 2) typical of metallization. Second, the structure involves almost no brittle oxide films that form when melted aluminum interacts with atmospheric oxygen in a plasma jet or in an electric arc discharge. In the method under consideration, the particles undergo no melting and, hence, no additional oxides are being formed. Third, the porosity of gasdynamic coatings deposited under optimum process parameters is lower than the porosity of their metallization counterparts. And (which is most important for the protective (anticorrosion) properties to be improved), the gasdynamic coatings display no open through-voids typical of thermal spray coatings and related to the gas evolution proceeding during crystallization in thermal spraying methods. 128
REFERENCES 1. A.P. Alkhimov, V.F. Kosarev, and A.N. Papyrin, Cold gas-dynamic spraying methods, Dokl. Akad. Nauk SSSR, 1990, Vol. 315, No. 5, P. 1062−1065. 2. A.N. Papyrin, A.P. Alkhimov, V.F. Kosarev, A.V. Plokhov et al., A process and setup for depositing anticorrosive coatings onto pipes by the gas-dynamic spraying method, in: Novel zinc and zinc-alloy based coating for protecting metal rolls and pipes, Sci. and Production Union, All-Union Sci. Res. Thechnol. Inst., Dnepropetrovsk, 1990, P. 14−21. 3. L.I. Tushinsky, A.V. Plokhov, A.O. Tokarev, and V.I. Sindeev, Methods of Material Research: Structure, Properties, and Deposition of Inorganic Materials, Mir, Moscow, 2004. 4. A. Khasui, Spraying Engineering, Mashinostroenie, Moscow, 1975. 5. V.M. Rogozhin, D.V. Smirnov, and V.Ya. Vetrov, Determination of adhesion strength of thermal spray coatings, Poroshkovaya Metallurgiya, 1982, No. 7, P. 87−91. 6. L.I. Tushinsky, V.I. Sindeev, and A.V. Plokhov, Structure and Mechanical Properties of Modified Engineering Surfaces, Novosibirsk State Tech. Univ., Novosibirsk, 1996. 7. A.P. Alkhimov, V.F. Kosarev, A.V. Plokhov, L.I. Tushinsky et al. Novel Materials and Technologies, in: Strengthening under Extreme Conditions: Theory and Practice, M.F. Zhukov and V.E. Panin (Eds.), Nauka, Novosibirsk, 1992. 8. L. Tushinsky, I. Kovensky, A. Plokhov, V. Sindeev, and P. Reshedko, Coated Metal. Structure and Properties of Metal-Coating Compositions, Springer-Verlag, Berlin, New York, 2002.
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