investigation of influence of surface...

METAL 2007 22. – 24. 5. 2007 Hradec nad Moravicí

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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

E-mail: [email protected]

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


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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 0.03…10mm.

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

Sort Fe C Cr Ni Mo Si B Fraction,

mkm

PRCH28N10M5S1 the rest 1.7 28 10 5 1 - 5-53

N65CH25S3R3 ≤5.0 1.5 26 the

rest - 2.3 3 30-50

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 1.5 - 2 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 0.7 – 0.85 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).


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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.

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 cross-

section 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 sub-

micro 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).

Fig.1. Surface of the sample after turning: a) morphology; b) surface profilogram.

Fig.2. Surface of sample after abrasive blast machining: a) morphology; b) surface

profilogram.


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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.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.

Fig.5. Microstructure of coating and base: a) after finish ultrasound processing; b)

after abrasive blast processing.


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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).

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

Fig.6. Distribution of microhardness (2,4) and nanohardness (1,3) in depth in base

and in coating (PRCH28N10M5S1).


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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.

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

Fig.7. Surface of samples after coating segregations: a, d) abrasive; blast; b, e)

ultrasound finish processing; c, f) grinding.


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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 06 – 08 – 01220 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, 43 -46.

FRANK M.J., VAN DEN BERGE, 1998, Thermal spray processes, Advanced Materials and

Processes, 12, 31-34.

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, 26 – 30.

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, 183-187.

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, 66 – 71.

investigation of influence of surface...

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