Experiments on Deposition of Nano-Structured Alumina-Titania Coatings by Normal Detonation Waves

Deposition of functional coatings for wear, corrosion and thermal resistance is a commonly employed process in many industries. Coatings are typically applied by spraying powders through a high temperature and high velocity gas jet. Powder particles are heated up and accelerated in the jet and they subsequently impact onto the surface to be coated. The two common means of depositing such coatings are high velocity oxy-fuel (HVOF) torches and plasma jets [1,2]. Typically, materials with lower melting points such as metallics are sprayed by HVOF systems whereas high melting point ceramic materials require higher temperature jets and they are applied using plasma torch systems. The HVOF systems produce combustion jets that can be supersonic at the jet exit (typically greater than kilometer per second velocities) and can reach gas temperatures up to 3000 K using pure oxygen as the oxidant. On the other hand, plasma jets can produce velocities and temperatures of the order of 102 m/s and 104 K respectively. Hence, these processes cover different velocity-temperature regimes for deposition of different types of coatings. While these processes are most commonly used in industry, another technique involves the use of detonation waves (i.e. supersonic premixed flames) for deposition of high density coatings [3–7]. The advantage of using detonation waves to deposit coatings stems from the fact that very high (supersonic) speeds can be sustained and thus this process provides the means to accelerate particles to very high velocities and produce dense coatings. However, the main disadvantage of this process is its discontinuous, repetitive nature. The detonation deposition process had originally been developed as a proprietary technology. Recently, a number of publications have appeared in this field such as references [6,7]. While the detonation process may have its practical limitations, it has the distinct advantage of enabling detailed, systematic study of particle deposition process since it subjects the particles to a well-characterized one dimensional flow field. Plasma and HVOF systems provide a turbulent flow field in which the injected powder particles are accelerated and heated up before impact. Description of particle properties and the control of the particle states (velocity and temperature) in a turbulent flow field are difficult at best. Experimentally, the measurements of particle temperatures and velocities are presented in a statistical form as probability density distributions of particle properties [10]. Computationally, particle properties are computed in a turbulent flow simulation by Lagrangian tracking of particles and developing statistics of particle properties [9] much like those determined experimentally. Because of the dispersion of particle states in a turbulent flow, a systematic study of coating properties as a function of particle processing conditions becomes difficult. In contrast, one dimensional uniform flow behind a detonation wave front subjects the particles to the same flow conditions, thus allowing examination of the deposition process as a function of particle parameters. Furthermore, flow and particle parameters can be computed in a relatively straightforward manner. Earlier work on detonation deposition of coatings have concentrated primarily on comparisons of this process with plasma processing in terms of the coating properties (hardness and wear resistance) and microstructure [3–6]. For example, Kharlamov has studied the relationship between adhesion strength of coatings and the parameters of detonation spraying and compared them with those for plasma spraying [4]. Saravanan et al [7] had performed a full Taguchi design of experiments to investigate the effects of detonation process parameters for alumina coatings. A similar study was conducted by Sundararajan et al [6] for tungsten carbide-cobalt, alumina and nickelchromium coatings. Their study concentrated on the tribological behavior of these coatings under different wear modes and compared both plasma and detonation generated coatings. Semenov and cetegen have recently discussed the deposition of nano-structured coatings by the detonation process [7]. One of the main objectives of this presentation is to use the detonation process to determine if highly nano-structured coatings can be generated with the detonation process that provides higher velocitylower temperature processing conditions compared to the plasma process. Secondly, a more basic understanding of the deposition process is sought using the detonation deposition without the turbulent, dispersive nature of the plasma and HVOF processes. In the following, we first describe the experimental systems used in this study and the features of both nano-structured and conventional alumina-titania coatings created by the detonation process. The effects of high particle kinetic energies, partition of the particle energies between thermal and kinetic and their influence on the deposition process also evaluated.