INVESTIGATION OF INFLUENCE OF SURFACE ROUGHNESS ON THE STRUCTURE AND PROPERTIES OF GAS THERMAL COATINGS

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1 INVESTIGATION OF INFLUENCE OF SURFACE ROUGHNESS ON THE STRUCTURE AND PROPERTIES OF GAS THERMAL COATINGS Prof. Vasily KLIMENOV, Ph.D. Zhanna KOVALEVSKAYA, Konstantin ZAITZEV, Vyacheslav BOROZNA Yurga Technological Institute of Tomsk Polytechnical University Leningradskaya str.,26, Yurga, Russia Abstract The features of structures and properties and the bonding of coatings sprayed by the high-speed method over the steel base with different surface morphology are analyzed. It is shown that the use of before spraying modificating ultrasonic processing of surface instead of processing it with broken electrocorundum blasting allows obtaining coatings of sufficient adhesion. Thermal spray coatings offer practical and economical solutions to a variety of industrial problems. They are most commonly applied to resist wear, oxidation, heat, and corrosion; provide electrical conductivity or resistance; and restore worn or undersized dimensions. Although the coating techniques have been around for long time, ongoing improvements are leading to lower application costs and a better understanding of how these coatings are in use (FRANK 1998). Thermal spray is somewhat related to the welding process. In welding, the added material is actually fused to the base metal, forming a metallurgical bond; whereas a thermally sprayed coating generally adheres to the substrate through a mechanical bond. That is why in gas flame spray technologies it is conditioned to prepare surface to be sprayed by roughening. This can be done by means of processing surface with electrocorundum powder or by making ragged thread on it. Besides, in V.Kudinov's works (KUDINOV 1981) much attention was paid to the possibility of bonding process of coating particles with the base. At the same time centers of welding generally appear at the ridge of the roughness. This also points to the necessity of abrasive blast processing of the base before spraying. Nonetheless, some thermal spray processes are capable of achieving mechanical bond strengths that exceed 70 MPa. At the same time one of the most effective ways of achieving high value of bonding strength is the acceleration of particles to high speed (the detonation gun process, the cold gas-dynamics spray process and the high velocity oxygen fuel thermal spray process). The high velocity oxygen fuel (HVOF) thermal spray process is closely related to the flame spray process, except that combustion take place in a small chamber rather than in ambient air. The HVOF combustion process generates a large volume of gas caused by the formation and thermal expansion of such exhaust gases as carbon dioxide and water vapor. These gases must exit the chamber through a narrow barrel several inches long. Because of the extremely high pressure created in the combustion chamber, the gases exit the barrel at supersonic velocities, thereby accelerating the molten particles. Although the particles do not reach the speed at which the gases are traveling, they do reach very high velocities - about 800 m/s). This allows obtaining high bond strengthening of coatings with the base. This method was first created as a method of spraying materials subjected to decomposition at high temperature such as carbides. Then this method started to compete with detonation and high speed plasma spraying. In the later case as a result of expanding of the application of new 1

2 erosion and abrasive wearing protective materials and making thermal barrier coatings this method attracts more and more attention of researchers and practicing engineers (BALDAEV 2003). The efficiency of this method becomes the highest when the coatings of the main high loaded details of machines, mechanisms and equipment are sprayed. At the same time practice shows that there are a lot of details and constructions which cannot be processed with the help of abrasive grinding or abrasive blasting which are able to cause the decrease of the fatigue resistance of production (axle, bar, etc.). That is why it is necessary to find new methods of preparing a surface to spraying. Taking into consideration the fact that the high speed of particles being sprayed by means of the HVOF method allows to decrease roughness requires to the surface and that the abrasive blast processing is faulty technology we can speak about advanced application of the alternative method of processing. In the work under consideration the research of the ability of ultrasonic modification of steels and alloys surfaces was carried out (KLIMENOV 2004). This method, in spite of the facts that it decreases surface roughness, forms specific surface morphology, refines grain structure and increases the strength of grain boundary, activates and hardens the surface layer and thereby decreases the leap of properties at the boundary of the coating and the base. So it can positively influence the workability of especially wear proof hard surfaces. Performance attributes of the plant are: the speed of gas jet outflow at the burner nozzle section is 800 m\s; the consumption of gas fuel (propane) is 250 l\min; the productivity at metal and alloys is up to 18 kg\hr; the productivity at carbide spray is up to 22 kg\hr; the thickness of sprayed layer is mm. As a spraying material the powder on the base of PRCH28N10M5S1 iron and the powder on the base of N65CH25S3R3 nickel were used. Chemistry of these powders is shown in Table 1. Table1 Fraction, Sort Fe C Cr Ni Mo Si B mkm PRCH28N10M5S1 the rest the N65CH25S3R rest Several methods were used for the surface pretreatment. After turning at a lathe pins were subjected abrasive blast machining, grinding and ultrasonic smothering. Abrasive blast machining was accomplished in a chamber by a short-blast machine, which directs electrocorundum particles measuring mm at the processing surface in a compressed air jet. Grinding with abrasive material and ultrasound smoothering with a special device of pins were carried out up to the same level of roughness as Ra equals to mkm. The optical profilometric complex MICRO MEASURE 3D station was applied for the study of morphology and roughness of the base and the analysis of the joint character of the coating with the base after the tearing the coating off. With the help of the graphic program they estimated areas of bonding the base and the coating; according to this size they forecast bond strength of the coating and the base. The coating structure was investigated on cross section using optical microscope MIM-10, coating roughness being estimated with the help of a computer program using its photo. Measuring of coating and base micro hardness was carried out by means of CSEM Nano Hardness Tester. Surface geometry after turning has a certain periodicity of ridges and cavities which depends on the turning modes (Fig. 1a). The profilometric analysis results showed that roughness is characterized values of Rzmax about 9mkm and Ra = 1.15 mkm (Fig. 1b). 2

3 Fig.1. Surface of the sample after turning: a) morphology; b) surface profilogram. The surface of the sample after the abrasive blast machining has numerous peaks and pits that emerge after striking the surface by electrocorundum particles (Fig. 2a). In the process of repeated action of abrasive particles on the surface of the sample its roughness increases up to Ra = 4.38 mkm (Fug. 2 b). The resulting geometry has dentate geometry with ridge height up to 15 mkm. Fig.2. Surface of sample after abrasive blast machining: a) morphology; b) surface profilogram. The surface has completely different geometry after the ultrasonic smoothing by a hard metal indenter vibrating with ultrasonic frequency (Fig. 3a). Ultrasonic finish treatment creates the surface micro relief produced by relative motion of the smoothing tool and the pin and that is characterized by roughness Ra = 0.7mkm (Fig. 3b). The micro asperities crosssection in the direction of advance of tool has wavy structure with roughness width of 0.2 mm and height Rz = 4mkm (Fig. 3b). Repeated discontinuous pulse action of the tool forms submicro geometry lengthwise the tool motion. Geometry periodicity comes to about 5mkm. It is typical that the morphology of the surface grinded up to the same cleanliness level by means of an ordinary abrasive grinding tool (Rmax about 6 mkm and Ra= 0,85 mkm) differs greatly first of all in the presence of microincision on the surface being processed (Fig.4). 3

4 Fig.3. Surface of sample after ultrasonic final processing: a) morphology; b) surface profilogram. Fig.4. Surface of sample after grinding: a) morphology; b) surface profilograme. Powder coatings on pins made of steel 20 and prepared by means of the method under consideration were sprayed by means of HVOF method. Spraying was accomplished on best performance providing maximum strength with the variation of spray distance and oxygen outlay (FROLOV 2003). In all cases the obtained coatings were formed without any peeling. Roughness of coating thickness about 300 mkm. did not practically depend on base roughness and was defined first of all by the heterogeneity of particles warming up in jet so by the deformation degree of definite particles. Coating structure heterogeneity in cross section also depends on correlation of particles in jet heated up to the melting point and underheated accordingly. Fig.5. Microstructure of coating and base: a) after finish ultrasound processing; b) after abrasive blast processing. 4

5 Considerable difference is present at the boundary of coating and base joint. As it was expected the boundary on the samples after smoothing with ultrasound (Fig. 5a) is flat in comparison with samples processed with electrocorundum (Fig. 5b). It does not include drastic lugs, discontinuity flaws or oxide inclusions. At etched sections it is clearly seen that when blast processed in surface grains the tracks of deformation appear. In samples processed with ultrasound the grain size decomposition and forming happens. Essential difference in distribution of micro hardness value in the coating base composition is also seen (Fig.6). Fig.6. Distribution of microhardness (2,4) and nanohardness (1,3) in depth in base and in coating (PRCH28N10M5S1). It is seen that the distribution of micro - and nanohardness of coatings demonstrates both base strengthening and its distribution in depth. The increase of micro hardness value in layers processed with ultrasound is the evidence of grain size decomposition, deficiency of grain structure and of compression in surface layers. It is necessary to pay attention to the evening out of the leap of micro hardness value, which takes place when we spray hard coatings. The efficiency of coating and base joint was investigated by means of studying the structure of sample surface layers after coatings segregation (Table 2). Table 2 Surface pretreatment method Base starting roughness Ra, mkm Roughness after coating tearing off Ra, mkm Joint area of grip center of spraying particles and the base, % Abrasive blast 4,38 6,99 53 Ultrasound finish processing 0,7 1,97 38 Grinding 0,85 1,05 24 On the base of profilometric analyses it was determined that the effective maximum contact area of surface is formed being blast processed. In all cases when we tear coating off the base the grip center of spraying particles with the base alternate with the area where the adhesive segregation of coating happened. Spraying particles are characterized by cohesive 5

6 fracture of coating in single areas. Correlation of these areas defines the efficiency of coating and base grip. The size and the character of grip areas distribution along the stylus method line are seen well at the surface profilograms obtained on the samples after coating segregation shown at Fig. 7. It is significant that the under review increase of additive value of roughness due to lugs, which were formed by sprayed particles left after coating segregation and by fragments of single particles proves high strength of coating joint sprayed on the base after jet processing. Coating joint on the grinded surface is conditioned by microcenters of grip. For joint coatings made on the surface pressed with ultrasound mixed character of joints is typical. Mixed character of joint means that along with areas of microgrip there are areas of single particles joints. But the portion of these areas is much smaller than the portion we have when we use traditional method of surface preparation for spraying. This defines lower strength of bond. Fig.7. Surface of samples after coating segregations: a, d) abrasive; blast; b, e) ultrasound finish processing; c, f) grinding. At the same time taking into consideration the character of microhardness distribution near the boundary and the favorable influence of the process of structure decomposition and the formation of compressive stress when ultrasound processed on the base strengthening 6

7 allow to speak about the availability of the application of this kind of processing when obtaining wear proof coatings first of all on the figure of revolution. The work was executed in the context of RFFI grant. Authors thank the staff of The center of physical and exploitation properties of new materials and coatings measurement of INP of TPU for their help in execution phase. LITERATURE REFERENCES BALDAYEV L.H., SHESTERKIN N.G., LAPANOV V.A., SHATOV A.P.,2003, Features of highspeed gas and flame sputtering processes. Svarochnoe proizvodstvo, 5, FRANK M.J., VAN DEN BERGE, 1998, Thermal spray processes, Advanced Materials and Processes, 12, FROLOV V.A., POKLAD V.A., RYABENKO B.V., VICTORENKOV D.V., SHIMBIREV P.A.,2003, Technical features of coating the elements of gas turbine engines by means of HVOF method.// Svarochnoe proisodstvo. 11, KLIMENOV V.A., KOVALEVSKAYA ZH.G. et all, 2004, Ultrasonic modification of surface and its influence on covering properties, Proceedings of 20-th International Conference on Heat Treatment, Jihlava, KUDINOV V.V., IVANOV V.M., 1981, Spraying refractory coatings with plasma. М.:Machinostroenie,, 192 p. SHMAKOV A.M., YERMAKOV S.S., 1986, Percussive interaction of a particle with a base under gas and thermal sputtering. FiHОМ. - 3,