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Computers chem. Engng Vol. 20, Suppl., pp. S1185-SI 190, 1996 Copyright © 1996 Elsevier Science Lid

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C O M P U T E R - A I D E D M A N U F A C T U R I N G OF C E N T R I F U G A L SHS C O A T I N G S

Roberto Orru', Barbara Simoncini, Pier Fortunato Virdis and Giacomo Cao*

Dipartimento di Ingegneria Chimica e Materiali Universita' degli Studi di Cagliari,

Piazza d' Armi, 09123 Cagliari, Italy

ABSTRACT

A computer-aided process for coated-pipe manufacturing using a centrifugal self-propagating high-temperature synthesis (SHS) technique is proposed. The process is based on the occurrence of a thermite reaction involving Fe203 and aluminum in a field of centrifugal forces and it allows for the preparation of coatings constituted by a ceramic layer of alumina in the innermost region of the pipe and an iron layer between the substrate pipe and the alumina one. The effect of various operating conditions on coating stratification and adhesion is studied using this system. The centrifugal acceleration appears to be the most important processing variable and therefore a strategy to control the process by suitably adjusting the rotating speed of the pipe through the computer is suggested.

KEYWORDS

Computer-aided manufacturing, Coatings, SHS, Centrifugal forces.

INTRODUCTION

Ceramic coatings have acquired a role of considerable importance in a wide variety of technological applications such as electronic devices, optics, chemical processing and corrosion protection. The techniques for producing these coatings are correspondingly diverse, e.g. glazing, screen printing, plasma spraying, spray pyrolisis, sol-gel processing, physical and chemical vapour deposition (Atkinson, 1992). An alternative technique for the preparation of chemical and physical coatings is represented by SHS, as discussed in a recent review article (Grigor'ev and Merzhanov, 1992). It is worth mentioning that SHS is an attractive technique to synthesize a variety of advanced materials and is based on the concept that once initiated by means of a thermal source for relatively short times, highly exotermic reactions can become self- sustaining and the corresponding final products are progressively obtained without requiring additional heat. Some unique advantages of SHS over alternative methods of advanced materials production include lower energy consumption, high heating rates, short synthesis times and high product purity (Luss, 1990; Munir and Anselmi Tamburini, 1989; Varma and Lebrat, 1992).

By combining SHS with the presence of centrifugal forces, a novel technology, otherwise known as the centrifugal-thermite (CT) process, has been recently developed to produce long ceramic lined pipes (Odawara, 1990, 1992). These can be used for transportation of molten metals or highly corrosive chemical substances, due to their properties such as for example high resistance to corrosion, abrasion and heat. The process, as schematically shown in Fig. 1, is based on the occurrence of the thermite reaction Fe203 + 2Al --~ 2Fe + A120 3 + 836 kJ, which become self-sustained upon ignition due to the large value of the corresponding heat of reaction. The temperature values reached during this process (i.e. about 2500 K) allow for the complete

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melting of reaction products. These are separated because of their different densities under the action of a field of centrifugal forces. A coating constituted by a ceramic layer of alumina is formed in the innermost region of the pipe and an iron layer results between the substrate pipe and the alumina layer. Note, however, that the CT process has been developed at the industrial level only for certain applications (i.e. large diameter pipes), for which more attractive techniques (i.e. plasma spraying) are available. In addition, this process is influenced by several operating variables, such as the mass ratio of thermite to substrate pipe, the centrifugal acceleration, the thermite powder density, the presence of different additives, the distribution of reactant particle size, etc., which appear not to be studied in a systematic manner (Chandran et al., 1995; Odawara, 1990, 1992; Yin et al., 1993). Thus, the intrinsic complexity of this process clearly calls for a suitable monitoring and controlling strategy, in the light of the directions of the computer-aided manufacturing principles (Canfield and Nair, 1992; Chang et al., 1991), with the final goal of obtaining a coating material with desired microstructure and properties.

- Thermite mixture

Substrate pipe I Fe203 + 2AI ~ 2Fe + AI203+ 836 kJ

J i m - Alumina

- Iron

Fig. 1. Schematic representation of the principle of the centrifugal SHS process.

To this aim, we propose in this work a computer-aided system for the manufacturing of coated- pipe of small diameter. The synthesis process, developed at the bench-scale level, may be controlled by a computer which also acquires the signals from a pyrometer and a high-speed video camera. This set-up not only represents the first step towards the development of a fully automated process at the industrial level, but also allows us to systematically study the influence of some processing parameters, such as the mass ratio of thermite to substrate pipe, the centrifugal acceleration, the diluent type and its content in the starting mixture, on the characteristics of the final products. A strategy to control this process by appropriately adjusting the centrifugal acceleration is indicated, with the aim of synthesizing a coating with optimal stratification and adhesion properties. These, together with the microstructure of the coatings are examinated using optical and electronic microscopy, wavelength dispersive spectrometry (WDS) and X-ray diffraction (XRD).

DESCRIPTION OF THE COMPUTER-AIDED MANUFACTURING PROCESS

The bench-scale set-up for coating preparation is shown in Fig. 2 and can be divided in four sections: the starting mixture preparation part, the synthesis section, the data acquisition part and the computer system. A centrifugal mill (Tecnotest, Italy) is used to prepare the starting mixture by blending reactants and additives in powder form together with acetone as dispersing agent. The synthesis section is constituted by an ignition source, a variable speed controller, a variable speed motor and a current source which provides the energy required for the rotation of the pipe mounted coaxially on a shaft of the variable speed motor. The data acquisition section consists of

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a video system and a temperature measurement device. A high-speed video camera (JVC, TK- 1280E model), a high speed video recorder (JVC, AG4700 model) allows us to observe and record the whole coating synthesis evolution. As shown in Fig. 2 an infrared pyrometer (Land Instruments, Cyclops 152A model) is located in front of the pipe for temperature monitoring. The computer system (PowerMacintosh 7200) is equipped with a data acquisition board (Model PCI-MIO-16XE-50, National Instruments) and makes use of a software package for data acquisition (LabVIEW, National Instruments).

Current source

Variable Varia speed controller speed n

Dispersing agent

i n

, , -

Printer Computer

Reactants

~ Diluent

Reacting mixture

preparation

meter Infrared

i " l n . - . = r -

ed video camera

"1 i ~ * I Video cassette High-speed [ recorder video recorder

Fig. 2. Schematic diagram of the computer-aided manufacturing process of centrifugal SHS coatings.

The manufacturing process consists first in distributing a uniform layer of the starting mixture into the inner wall of steel pipes (inner diameter of 38 mm, wall thickness of 2 mm and length of 80 mm) to be coated. Drying operations are then performed at room temperature, in order to avoid the formation of cracks, as acetone evaporates. During this step the pipe is rotating with a constant speed of 200 rpm. After drying is completed, the speed is increased to the desired level and ignition is accomplished by using a thermal ignition source, i.e. an oxyacetylene torch. As soon as the ignition is detected, the energy source is turned off. The reaction front then propagates rapidly through the reacting mixture, thus reaching the opposite end of the pipe within 2-3 s. During the propagation of the combustion front the pyrometer revealed maximum values of temperature of about 2500 K. After completion of the reaction, the pipe is kept rotating for cooling purposes.

RESULTS AND DISCUSSION

As noted in the introduction section, the centrifugal SHS process for coating preparation depends upon several processing parameters and, in addition, involves a variety of intricate physico- chemical phenomena (e.g. melting and diffusion of one reactant, formation of intermediate phases, etc.), and mechanisms which are related to several operating variables, such as the green density, the particle size distribution, the composition of the starting mixture, the combustion conditions, etc. (Luss, 1990; Munir and Anselmi Tamburini, 1989; Varma and Lebrat, 1992). The inherent complexity of the process makes the complete understanding of the influence of the various parameters very difficult. Moreover, the apparent lack of a systematic study of this process clearly prevents the direct development of a computer-aided manufacturing system. Thus, we first perform a systematic investigation of the effect, upon the stratification and

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adhesion of the obtained coating, of four important operating variables, i.e. the diluent type and its weight percentage content in the starting mixture (%D), the mass ratio of thermite mixture to outer pipe (M) and the centrifugal acceleration during the combustion evolution, which is proportional to the square of the rotating speed of the pipe (N). The corresponding operating conditions are summarized in Table 1. Note that the presence of diluents in the starting mixture is needed in order to reduce the violent character of the aluminothermic reduction, which may cause explosions and expulsion of reactants and products from the pipe, as observed clearly when totally eliminating them from the reacting mixture. The strategy adopted during the systematic study consists in keeping constant three of the operating variables mentioned above, while letting the fourth one to vary within a certain range.

Table 1. Summary of the operating conditions investigated for coating preparation.

Operating

variables

kept as

constant

diluent = SiO2

M = 0 . 3

N = 3500

diluent = SiO2

%D=7 .

N = 3500

diluent = SiO2

%D=7 .

M = 0 . 4

% D = 7 .

M = 0 . 4

N = 3500

%D M N diluent

7. 0.3 1300 SiO2

8. 0.4 2500 SiO2 : A1203 = 1:1

10. 0.5 3500 A1203

Although the pictures of the cross sections of the various coatings obtained under the conditions above are not shown here for brevity reasons (the interested reader should refer to Orru' et al., 1995a, 1995b, 1995c), the corresponding results are summarized as follows. By increasing the diluent (SIO2) content, smaller amounts of the heat generated by the reaction are available for maintaining products under melting conditions, where centrifugal forces can display their separation effect. In fact, the formation of stratified reaction products may be obtained only if the time needed for separation is much less than the corresponding one for the solidification process. When this condition does not occur, phase separation is hindered and the produced iron results to be highly dispersed into the oxide products. In the next set of experiments, indicated in the second column of Table 1, we explore the effect of the mass ratio of thermite to substrate pipe by maintaining fixed the silica content and N. We find that as M increases phase separation is promoted due to the fact that more heat is liberated during the reaction. However, M should be kept less than the value of 0.5 grams of thermite mixture / grams of substrate pipe, at least under the operating conditions investigated in the present work. In fact, a value of M equal to 0.5 is responsible of an excessive stratification of the reaction products, which results in a total separation of the oxide layer from the coating. The third set of experiments reported in Table 1 refers to the investigation of the effect of centrifugal acceleration upon phase separation and adhesion. A very strong binding between the produced iron and substrate pipe is obtained for low values of the rotating speed (i.e. N = 1300 - 2500 rpm). However, phase separation may be improved by increasing the centrifugal acceleration (i.e. N = 3500 rpm). Moreover, as the centrifugal acceleration is increased, the porosity of the obtained coating decreases due to volumetric compression of partial gasifying of substances which occurs owing to high temperatures involved. By maintaining constant the same weight percentage of diluent, as well as the above values of M and N, we next investigate the influence of the type of diluent on the final coating characteristics. As it may be seen in the last column of Table 1, we considered also alumina as diluent, since in SHS technology reaction products are typically used for this purpose (Varma and Lebrat, 1992). By substituting the 50 % of the silica content with alumina, although phase separation between the produced iron and oxide products is partially achieved, the bonding of the coating with the substrate pipe becomes worse. This finding is even more evident when

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totally substituting silica with alumina. The considerations above lead to the conclusion that, for the set of variables here considered, the operating conditions corresponding to N = 3500 rpm, M = 0.4 grams of thermite mixture / grams of substrate pipe and %D(SiO2) = 7, appears to be the combination which is responsible for the coating having relatively good properties in terms of phase separation and adhesion. In particular, a SEM view of a cross section of the coated pipe obtained under these conditions is shown in Fig. 3, where it may be seen that the thickness of the iron and the oxide layers is found to be approximately 0.5 mm and 1 mm, respectively. The oxide layer is then characterized in detail by performing XRD analysis, SEM and WDS investigations and it reveals corundum (A1203)and hercynite (FeA1204), as structural components and iron-aluminosilicates in amorphous form.

Fig.3. SEM view of the cross section of the coated pipe: (a) substrate pipe, (b) produced iron and (c) oxide layer.

As observed from Fig. 3, although a satisfactory level of stratification is achieved, the adhesion properties of this coating may be improved. For this, we suggest to select a suitable program of variation of the centrifugal acceleration during the synthesis and the cooling steps of the process. In fact adhesion, which is related to the binding between the metal layer and either the ceramic one or the substrate pipe, depends primarily on pressure and temperature evolution in time during the coating preparation (Howe, 1993). These variables may be in turn controlled by properly varying the centrifugal acceleration (Merzhanov and Yukhvid, 1990), i.e. the rotating speed of the pipe. Thus, by continuously monitoring the temperature during the synthesis process using the infrared pyrometer depicted in Fig. 2, we plan to establish through the computer a desired program of variation, either continuous or discrete in time, of the rotating speed.

CONCLUDING REMARKS

In the present work we propose a computer-aided system for coated-pipe manufacturing using a centrifugal SHS technique. The computer integration appears to be the only direction to be followed in order to control such an extremely fast process. At this stage, this may be done by properly manipulating the rotating speed of the pipe, and therefore the centrifugal acceleration, according to the temperature measurements recorded during the entire coating formation process. Currently, we are also devoting our attention towards the automation of the process with respect to both the ignition source and the distribution procedure of the starting mixture. This work, together with the detailed study of the structure and phase formation which occur during the evolution of the reactions involved in this process (the interested reader should refer to Orru' et al., 1995b), may represents a contribution of the reaction engineering methodology for obtaining coated-pipes with tailored microstructure and properties.

ACKNOWLEDGEMENTS

The authors are grateful to the Regione Autonoma della Sardegna (Italy) for financial support.

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