ТЕОРЕТИЧЕСКИЕ ОСНОВЫ ХИМИЧЕСКОЙ ТЕХНОЛОГИИ, 2012, том 46, № 5, с. 594-600
УДК 66.088
SURFACE COATING BY MEANS OF VELOCITY SHEAR INSTABILITY IN PLASMA © 2012 R. K. Tyagi", R. S. Pandey4, A. Kumar"
aDepartment of Mechanical Engineering, Birla Institute of Technology Mesra Ranchi, Jharkhand, India
tyagi_rk1@rediffmail.com
bDepartment of Applied Physics, Amity School of Engineering and Technology, Amity University Uatter Pradesh, Noida, India
rspandey@amity.edu Received 31.10.2011
In this paper the effect of different parameters like magnetic field, homogenous direct-current electric field, shear scale length, temperature anisotropy, inhomogeneity in direct-current electric field and density gradient on ions velocity is discussed. A mathematical model for ions/micron size particles velocity is discussed and its values are calculated by taking experimental parameters and by applying computer technique. A model of plasma spray machine is also suggested, which contains plasma production with velocity shear instability in laboratory, powder injection and mass and momentum transfers between particles. The coating process by means of velocity shear instability in plasma has possibility to spray hard and arduous material (alloy) with minimum defects and maximum technical and economic efficiency.
INTRODUCTION
Plasma spraying is one way to apply protective coatings. The hot, high-speed flame of a plasma gun can melt a powder of almost any ceramic or metal and spray it to form a coating, for protection against corrosion, wear or high temperature. The technique carries much less risk of degrading the coating and substrate than many other high-temperature processes because the gas in the plasma flame is chemically inert and the target can be kept fairly cool. And yet a plasma gun can be only a little more cumbersome than a paint sprayer. Investigators are applying this technique to new materials [1].
Plasma spray deposition is one of the most important technologies available for producing the high— performance surfaces required by modern industry. Over the past 25 years, there have been significant advances in the understanding of plasma physics and in the development of spraying equipment and techniques. This has enabled a range of materials including metals, alloys, ceramics, and cermets to be plasma sprayed on to a great variety of substrate types and geometries. During this period, the uniquely aggressive environment within the gas turbine engine has provided not only some of the greatest challenges to plasma spraying technology, but also some of its most successful applications [2]. Plasma engineering is usually reliable, reproducible, non-line-of-sight, relatively inexpensive, and applicable to different sample geometries as well as different materials such as metals, polymers, ceramics, and composites. Plasma processes can be monitored quite accurately using in situ plasma diag-
nostic devices. Plasma treatment can result in changes of a variety of surface characteristics, for example, chemical, terminological, electrical, optical, biological, and mechanical. Proper applications yield dense and pinhole free coatings with excellent interfacial bonds due to the graded nature of the interface. Plasma processing can provide sterile surfaces and can be scaled up to industrial production relatively easily. On the contrary, the flexibility of non-plasma techniques for different substrate materials is smaller [3]
In atmospheric pressure plasma spraying, the powders of the sprayed materials are introduced into the plasma area of the plasma torch. Because of the high-temperature and flux velocity of the plasma, the melted or partially melted powders are accelerated towards the substrate at a high speed forming a coating with a lamellae structure. Owing to the high-temperature of the plasma core, plasma spraying offers the ability to deposit almost any metals and various combinations of materials. The high-temperature enables the use of coating materials with a high melting point such as ceramics, cermets, and refractory materials. Materials can be processed as long as there is a temperature difference of at least 300 K between the melting temperature and decomposition or evaporation temperature [4].
Plasma spraying has undergone a rapid expansion in the last 30 years due to a couple of important factors. Firstly, it can conveniently treat specimens with a complex geometry. Secondly, the wide spectrum of materials that can be handled by this technique has spurred applications in the area of corrosion resistant, high-temperature, and ablation resistant coatings as
well as biocompatible films. One of the disadvantages of plasma spaying is the poor adhesion between the substrate and coating, but several measures can be used to improve it. For instance, the thermal gradient at the substrate/coating interface caused by the rapid quenching of the molten particle splats that leads to deposition of an amorphous coating can be reduced. In addition, one can prevent a steep gradient in the coefficients of thermal expansion between the substrate and coating to avoid the formation of strong tensile forces that give rise to crack generation, chipping, and/or delaminations [5].
A vacuum plasma coating apparatus is comprised of a plasma torch, arranged in a low pressure chamber and displaceable along a plurality of axes relative to the part to be coated and a device for moving the part to be coated with multiple degrees of freedom simultaneously with the plasma torch. In order to be able to coat a variety of parts simultaneously with great effectiveness, this device is constructed in such manner that it can contain a number of parts simultaneously, the parts being movable sequentially through the plasma jet. In each case the part nearest the plasma torch is arranged in the plasma jet in such manner that this extends at least occasionally, laterally beyond an outer dimension of this part and all parts to be coated move within a predetermined spray distance range when they are located in the plasma jet [6].
Ceramic materials with high hardness and high resistance to thermal and corrosive conditions and relatively low densities offer many advantages over metallic materials. The use of ceramic coating is an effective way to protect some metal components in power and refractory industries against chemical corrosion, abrasive wear, and high temperature oxidation.
Plasma spraying is one of the powerful techniques for preparing coatings with variety of properties required for industrial applications. Ceramic coatings produced by thermal spray techniques are increasingly and widely used for a range of industrial applications to provide wear and erosion resistance, corrosion protection and thermal insulation. Due to high flame temperature typically higher than 5000°C, a satisfactory melting state can be achieved, which is beneficial for formation of a dense coating structure [7].
The effect of processing parameters for coating was assessed in terms of adhesion, density (micro-hardness), roughness, composition, thickness and continuity in Table, which presents the range of these characteristics and those of the best coating [8].
The aspire of this exertion is to suggest a model of spray machine and effect of different parameters on velocity of metallic micron sized particles to perform plasma spraying. The plasma spray process is a spray
Effect of different parameters on spray properties
Parameters Zn Coating I Zn Coating II Zn-Al Coating
Thickness, p.m 12 15 18
Deposition rate, ^m/s 0.37 0.47 0.56
Roughness, m 3.5 2.5 2.3
Element content Zn Zn 3.0 % Al, Zn
Adhesion strength, MPa >70 >70 >70
Hardness, GPa 0.97 1.06 1.30
technique that is used to create a coating through the high velocity impact of particles on to the substrate. It is the only spray technique that solely relies on kinetic energy and heat of plasma to create a coating. The basic design of spray machine is obtained from literature. The basic intend of the design is to accelerate and transfer heat to metallic micron sized particles so that melted and solid/semi solid particles would reach at the critical velocity necessary for the particles to bond with the substrate. Above suggested machine is free from nozzle which eliminates the problem ofwear and tear and would be suitable for wide range of materials. By the above suggested machine, the combined properties of cold spray process and spray process in which molten material is used can be achieved.
MODEL DESCRIPTION
The collision-less plasma with velocity shear instability is created due to ionization of K+ and from SF6-. The magnitude of velocity shear instability depends upon the value of electric and magnetic field [9]. In plasma parallel shear is controlled by biasing each segment of ions source [10—13].
The model of plasma spray machine (Figure 1) mainly consists of plasma generator, which contains velocity shear instability, particle injector, carrier gas, etc. Spot size ofspray machine is controlled by degree of focusing which is achieved by electromagnetic lenses. The dimensions of potassium plasma column are 4 cm in diameter and 2.5 m in length [14]. The description of generation of plasma with velocity shear instability and its dimensions is described in detail by Tyagi et al. [15].
Particle injection is carried out by using a carrier gas and an injector (a simple tube whose diameter varies from 1.2—2 mm) for a given injector, at the exit, particles exhibit velocity that are not parallel to the injector axis. It depends on their collisions with the injector wall and between itself and it is also linked to their size distribution and carrier gas flow rate. This di-
Particle injector
Mixing between ions and micron size metallic particles
Object for spray process
Fig. 1. Model of plasma spray machine nozzle.
vergence increases when mean particle size decreases, especially when it is below 20 ^m in
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