Elsevier

Surface and Coatings Technology

Volume 318, 25 May 2017, Pages 309-314
Surface and Coatings Technology

Kinetics of composite coating formation process in cold spray: Modelling and experimental validation

https://doi.org/10.1016/j.surfcoat.2016.06.084Get rights and content

Highlights

  • Kinetics of multicomponent coating formation in cold spray is discussed

  • Model allowing to predict final coating composition is proposed

  • Experimental validation of the model is performed

Abstract

In this paper the kinetics of multimaterial (composite) coating formation in case of cold spraying of metal powder mixtures is considered. In particular, a semi-empirical probability simulation model was developed and applied for prediction of coating final composition in dependence on initial percentage of metal powders in the blend. Developed model permits to take into account the differences between deposition efficiencies and granulometry of each spraying component. Experimental validation using there-component 316L-Cu-Tribaloy mixtures with different component percentage shows that simulation results are in a good agreement with experimental ones. It was demonstrated that the dependence of coating composition on percentage of mixture component is non-linear. The further work will be aimed on the enlarging of the current model to the cases with mixtures with significant difference between particle sizes of the components.

Introduction

Cold spray is a coating deposition technology using a powder as a feedstock material [1], [2]. Coating is formed from the particles is solid state that differs cold spray from other thermal spray technologies [3], [4].

Possibility to deposit multicomponent (composite) coatings is an important advantage of cold spray [5], [6]. The simplest way of multicomponent coating deposition is spraying of previously prepared powder mixtures [6], [7]. Numerous papers devoted to cold spraying of metal and metal-ceramic composite coating using mechanical mixtures are available in the literature [7], [8], [9], [10], [11]. However, in spite of simplicity, this method has a significant drawback that is directly related to the physical principle of cold spraying. It is known that in cold spray the particles should reach some critical value of impact kinetic energy in order to adhere to the substrate. Absolute value of critical kinetic energy strongly depends on the mechanical and thermal properties of the spray material [1], [4]. In general, the harder metals demand higher values of impact energy [12]. At the same time, increasing of the particle or substrate temperature allows to decrease the value of critical velocity. The particles impact velocity and temperature are controlled by proper selection of stagnation pressure and temperature of working gas as well as nozzle geometry. In the case of spraying of powder mixtures, the gas flow parameters that may be appropriate for spraying one component of the mixture but ineffective for another. For example, in case of spraying of titanium-aluminium 15–45 μm powder mixture it is difficult to select working gas parameters optimal for both materials. In particular, typical parameters for deposition of Al powder with high deposition efficiency (~ 90%) are p0 = 2.5–3.0 MPa and T0 = 523–623 K for the nozzle with outlet Mach number M = 2.5–3 whereas titanium powder achieve similar deposition efficiency only at p0 = 3.5–4 MPa and T0 = 873–1073 K [1], [6]. From physical point of view, aluminium particles could be deposited at gas temperature appropriate for titanium, but experimental study demonstrated that overheating of aluminium leaded to immediate nozzle clogging that perturbs spraying process [5]. At the same time, if deposition of Ti-Al mixture is effectuated at the low gas temperature appropriate for aluminium, deposition efficiency of Ti particles drops down and deposition of titanium-rich coatings becomes impossible. Therefore, the main disadvantage of approach with spraying of powder mixture could be formulated as follows: the deposition efficiencies of mixture components are different that significantly changes the final coating composition in comparison with composition of the blend.

Experiments with deposition of powder mixtures permitted to reveal another important phenomenon. In case of cold spray deposition of metal mixture of type “soft” metal – “hard” metal (or ceramics) at low-temperature spray parameters close to the optimal ones for deposition of soft metal, composite coatings containing some amount of hard metal embedded in soft matrix could be formed [6], [13]. At the same time spraying of pure powder of the same “hard” metal (or ceramics) at these spraying parameters does not permit to form a coating due to very low deposition efficiency. This phenomenon could be explained by following suggestion. It is known that the bonding mechanism in cold spray is a combination of metallurgical bonding and mechanical interlocking [14]. The contribution of each mechanism strongly depends on material properties of powder and substrate as well as on particle impact velocity. In general, the highest values of cold spray coating adhesion were obtained for the cases where the mechanical interlocking was considered as a predominant mechanism [15]. At the same time interlocking is mainly observed in the cases where the deposition of harder materials on significantly softer substrate was performed. In this case the metal jetting due to the impact of incoming particles causes lips of soft substrate material to form which partially envelop the impacting particles as was shown in [14]. It is important to note that if the impacted surface is significantly softer than the particle material the particle deposition occurs at lower critical velocity than in the case of spraying of the same powder on surface with similar or higher hardness. For example Bray et al. [16] clearly demonstrated that local laser thermal softening of impacted surface changes the adhesion mechanism towards mechanical interlocking of impacting particles by deformed substrate. In this case bonding particle-substrate occurs at significantly lower particle impact velocity in comparison with non-softened substrate surface [17].

One can suggest that the similar effect takes place during cold spray of powder mixture “soft” metal – “hard” metal at compromising spraying parameters (Fig. 1). The impact energy of the particles of the soft component is high enough to adhere on a substrate surface as well as on previously deposited soft or hard particles. Whereas the impact energy of hard material is enough only to adhere on the surface previously covered by soft material due to mechanical embedding (Scenario 1). If the hard particle impact the surface covered by hard particle its impact energy is not high enough for bonding and particle rebounds (Scenario 2).

In this case the deposition efficiencies of mixture components are significantly different. The deposition efficiency of soft component is fixed and does not depend on the type of impacting surface. At the same time the deposition efficiency of the hard component depends on the type of the surface that hard particle could meet at the impact that by turn depends on the percentage of soft component in the mixture. The Scenario 1 should be predominant in cases with spraying of mixtures with low concentration of hard particles. Increasing of hard powder percentage will shift the probability towards to scenario 2.

Therefore, one can conclude that:

  • The composition of multimaterial coating containing “soft” and “hard” component will differ from the initial blend composition if spraying parameters are optimal only for the “soft” component.

  • Variation of percentage of “hard” particles in the blend will lead to variation of deposition efficiency of “hard” component as a function of concentration. This fact makes impossible simple prediction of coating composition by interpolation of results obtained only for one type of the blend with one percentage to another.

However, from practical point of view it is important to develop a model permitting to predict the final coating composition in case of spraying of powder mixtures in order to limit the number of experimental tests. In current paper a simple mathematical model permitting to evaluate final coating composition is discussed and its experimental validation is provided.

Section snippets

Model description

In order to estimate the composition of two-component coating the following model could be applied. Firstly it is considered that deposition is performed using two-component powder blend of type “hard metal” + “soft metal” and the particles are well mixed in the gas flow. It can be assumed in this case that at some time during a short time interval a total number dn of particles falls onto unit area. Among this number, the number of hard particles is (1  c)dn and the fraction of abrasive particles

Experimental validation

Proposed model was experimentally validated using three-component blend. Experiments were performed by spraying of preliminary prepared powder blends using commercially available powders of 316L stainless steel (Sandvik Osprey, UK), copper (TLS, Germany) and Tribaloy T700 (Sandvik Osprey, UK). The purpose of the validation was to predict the percentage of T700 particles in obtained coating using a model and to compare with experimental results. Properties of the coatings obtained using these

Conclusion

Kinetics of coating formation in case of cold spraying of powder mixture is a complicated process. Previous experiments demonstrated that deposition efficiency of blended powders depends not only on the spraying parameters but also on share of the different powder phases in the mixture. Model based on the simple assumption that deposition efficiency of mixture components depends on the type of impacting surface allowed developing a simple approach for prediction of coating composition in

Cited by (0)

View full text