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Characterization and tribological properties of boride coatings of steels in a fluidized bed reactor K. David Department of Mechanical Engineering, Technical University of Serres, Serres, Greece

K.G. Anthymidis Applied Research Center of Serres, Serres, Greece

P. Agrianidis Department of Mechanical Engineering, Technical University of Serres, Serres, Greece, and

G. Petropoulos Department of Mechanical and Industrial Engineering, University of Thessaly, Volos, Greece Abstract Purpose – The aim of the current research is to characterize boride coatings on steels and steel alloys produced in a CVD fluidized bed reactor. Design/methodology/approach – Heat treatments of alloys in fluidized bed reactors have been carried out for more than 25 years. Recently, this technology has been used for surface engineering applications in the deposition of hard and/or corrosion-resistant layers. The present paper used fluidized bed technology (FBT) to deposit boride coatings on to ferrous materials. The coatings were examined by means of optical microscopy, Vickers microhardness measurements and X-ray diffraction in terms of coating thickness and morphology, phase formation and hardness determination. The coating’s tribological properties were evaluated under dry wear. Impact tests were also carried out to determine the fatigue resistance of the examined coatings under dynamic impact loading. Findings – Boriding in a fluidized bed reactor is a simple, environmentally friendly and fast-coating process. The produced iron-boride coatings are characterized by excellent quality and uniform tooth-shaped morphology. Fe2B was the predominant boride phase formed, exhibiting superior tribological properties under dry wear conditions. Impact testing investigations revealed high-fatigue strength of boride coatings in combination with limited deformable substrates. Research limitations/implications – The investigated coatings were deposited only on some structural and tool steel substrates. Practical implications – Boride coatings deposited using FBT are satisfactory abrasive wear- and fatigue-resistant coatings in comparison with those produced using common boride coating methods. Originality/value – The outcome of the research is of great importance for the industry using abrasive wear coatings. Keywords Tests and testing, Coatings technology, Steels Paper type Technical paper fluidization is described in great detail elsewhere (Gupta and Sathiyamoothy, 1999; Howard, 1989). Briefly, fluidization is a process in which a bed of particles, e.g. Al2O3, behaves like a liquid, when a carrier gas is fed through the bed. There are several types of fluidized beds and their main advantages are the high rates for mass and heat transfer. This leads to uniformity of temperature throughout the volume of the reactor and flash mix of all compounds in it, which results in quality improvement of the coatings. Some other advantages of this process are the capability of immediate adjustment of the furnace atmosphere to specific requirements, the relatively low-investment costs for equipment and the low-operation costs. Some of the parameters that affect the quality of fluidization in a fluidized bed reactor are the properties of the solids and fluids used, the bed geometry, the gas flow rate, the type of gas distributor and finally the reactor design. This paper is concerned with the study of boride coatings deposited on various types of steel, produced by the fluidized-

Introduction Fluidized bed technology (FBT) has been successfully used for the formation of different types of coatings, e.g. aluminizing, chromizing (Kingel et al., 1995; Pe´rez et al., 1999; Tsipas and Flitris, 2000), nitriding (Japka, 1983; Reynoldson, 1993), carburizing and carbonitriding (Reynoldson, 1993). On the other hand very limited information exists on boride coatings obtained using FBT, although this method is simple, efficient, environment friendly and the boride coatings have been reported to have an excellent combination of properties (Anil Kumar Sinha, 1997; Arai et al., 1986; Tsipas et al., 1999). The theory of The current issue and full text archive of this journal is available at www.emeraldinsight.com/0036-8792.htm

Industrial Lubrication and Tribology 60/1 (2008) 31– 36 q Emerald Group Publishing Limited [ISSN 0036-8792] [DOI 10.1108/00368790810839918]

The authors would like to thank the Research Committee of Technical University of Serres for the financial support of this research project.

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Characterization and tribological properties of boride coatings

Industrial Lubrication and Tribology

K. David, K.G. Anthymidis, P. Agrianidis and G. Petropoulos

Volume 60 · Number 1 · 2008 · 31 –36

Figure 2 Impact testing principle

bed process. The boride coatings, which have been prepared using FBT, were characterized by optical microscopy, X-ray diffraction measurements and Vickers microhardness testing. Finally, the coating fatigue strength ascertained by means of impact testing, and the tribological properties of the investigated coatings under dry wear conditions were evaluated.

The Impact Test carbide ball Fimp

Methodology

coating

Coatings were deposited using a typical fluidized bed reactor system, which is schematically shown in Figure 1. The system consists of five main parts: 1 the fluidized bed reactor unit; 2 the gas preheating and reactants providing system; 3 the furnace for reactor heating; 4 the control panels and measuring instruments; and 5 the trapping of hazardous substances unit.

2α

tcoat substrate

A detailed description of the experimental set up has been given in a previous publication (Anthymidis et al., 2002). In the present experiments boriding was carried out on constructional (St37, Ck60, 42CrMo4), cold worked tool (X210Cr12) and hot worked tool (X40CrMoV51) steels, at temperatures of 9508C. Argon was used as fluidizing gas, while the fluidizing media were composed of Al2O3, B4C and halogen containing compounds. The tribological properties of the coatings were evaluated using a pin-on-disc testing machine under dry wear conditions. The coating impact test was used as the most convenient experimental method to study the fatigue strength of the boride hard coatings under alternative impact loads. The theory of impact testing is described in great detail elsewhere (Knotek et al., 1992). Briefly, during the impact testing a coated specimen is cyclically loaded by a hard ball that repetitively impacts on the specimen surface (Figure 2). The superficially developed Hertzian pressure induces a complex stress field within the coating, as well as, in the interfacial zone. Both stress states are responsible for distinct failure modes, such as a cohesive or adhesive one.

The exposure of the layered compounds against impulsive stresses generates the real conditions for the appearance of coating fatigue phenomena, based upon structural transformation, cracking generation and cracking growth, which are responsible for the gradual micro-chipping and the degradation of the coating. In all impact craters resulted from the experiments three different zones inside the impact cavity were identified (Figure 3). A central zone in the middle of the impact cavity, where the coating is strained with compressive stresses and a gradual cohesive degradation takes place. The intermediate zone inside the piled up rim formed around the impact cavity, where tensile and shear stresses build up and both cohesive and adhesive delamination arises. Finally, the peripheral zone of the impact cavity, where macrocracks might propagate and coating failure occurs. The coating failure mode and its extent were assessed by SEM observations and EDX analysis. The contact load leading to coating fatigue fracture was recorded in diagrams (endurance strength curves) versus the number of impacts (Figure 4). The impact load for which the

Figure 1 The fluidized bed reactor system Activator's access

Cyclone

Temperature Measurement Field Point

Fluidized bed

Plenum Flowmeter

Manometer Argon

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Figure 3 Impact crater with the developed coating failure Imprint Carbide ball

Impact direction Coating

te

~500µm

Substra

Central Zone Middle Zone Outer Zone

Figure 4 Typical coating endurance strength curve obtained by impact testing Endurance strength curve

Impact force (N)

1000 800 600 400 200 0 0

5,00,000

10,00,000

15,00,000

Number of impacts

coating does not fail after 106 impacts is called limit of continuous endurance of the coating.

Figure 5 Typical tooth-shape morphology of boride coating deposited on 0.5%wt C steel (St37) in a fluidized bed reactor

Results and discussion

100µm

In Figure 5, a typical morphology of boride coating deposited on 0.5%wt C steel (St37) after 1 h and 30 min. of treatment is shown. This coating had an average thickness of 80 mm, Vickers microhardness values of 1,800-2,000 HV (Figure 6) and it is characterized by very good adherence due to its tooth-shape morphology. With the aid of the X-rays patterns it was concluded that the as-prepared coatings consist of a uniform compound, which was found to belong to a Fe2B phase. Small traces indicated of a-Fe phase are present in the thinner than in the average coatings. It was also observed that in the case of the coating with less than 1 h treatment crystallization proceeds by formation of Fe2B crystallites with a preferred orientation. Apparently, longer treatment induces secondary recrystallization resulting in improvement of the coating properties. In Figure 7, the typical morphologies of boride coatings obtained on Ck60, 42CrMo4, X210Cr12 and X40CrMoV51 after 3 h of treatment at 9508C are shown. From the X-rays patterns it was found that mainly one phase ˚ belonging to Fe2B (space group I4/mcm, a ¼ 5.110 A ˚ ) was formed during the treatment. In the case c ¼ 4.249 A of the cold worked tool steel X210Cr12 FeB phase is also present. The thickness of the boride layers was 35 mm for

Ck60, 35 mm for 42CrMo4, 30 mm for X210Cr12 and 25 mm for X40CrMoV51. Via the tribological tests under dry wear conditions (pressure 15 kp, SiC paper 220 grit, testing time 2 min, velocity 30 rev/min.) the Fe2B layer showed an up to 50 percent increased resistance to abrasive wear compared to the 33




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Figure 6 Measurement of the microhardness of the boride coating deposited on 0.5%wt C steel (St37) in a fluidized bed reactor

imposed by high deformation of the X40CrMoV51 substrate. The main failure of the most examined coatingsubstrate compounds there occurred in the central zone of the impact cavity with coating delamination. Figure 9(c) shows coating delamination in the central zone of the impact cavity (substrate 42CrMo4) and Figure 9(d) extended coating fatigue failure both appearing in the central zone, in the form of layer degradation and in the peripheral zone of the impact cavity as adhesive wear (substrate Ck60). It is evident that when the tensile stresses at the peripheral zone of the impact crater were too high, spalling of the coating layer due to poor adhesion may occur. For the examined hard boride coating in combination with a relatively plastically deformable substrate (Figure 10(a)), the high-tensile stresses in the immediate vicinity of the impact caused the development of a large number of macrocracks in the peripheral zone of the impact cavity. In general, macrocracks arise inside the coating layer and perpendicularly to its surface when the coating is not tough or ductile enough to accommodate the stress induced by the ball indenter and to follow the flexure and deformation of the substrate. On the contrary, in the case of slight plastically deformable substrates resulting in small cavity volume, the coating layer sustains the repetitive impacts without fatigue failure. Only superficial abrasive wear could be observed though (Figure 10(b)). Figure 11 shows an overview of the endurance performance of the boride coating deposited on St37 steel, produced by the fluidized-bed process by means of the experimentally

100 µm

170 Hv

254 Hv

1925 Hv

untreated steel. When the layer was removed (after approximately 2 min of testing) the wear performance was similar to that of the uncoated steel (Figure 8). Impact tests were also carried out to determine the fatigue resistance of the examined coatings under dynamic impact loading. In Figure 9(a), spalling of the boride coating is evident due to poor adhesion with the X210Cr12 substrate in the peripheral zone of the crater. Figure 9(b) shows the development of perpendicular macrocracks inside the coating layer due to high tensile stresses

Figure 7 Typical morphologies of boride coating obtained on various steels and steel alloys in a fluidized bed reactor: (a) X210Cr12; (b) 42CrMo4; (c) Ck60; (d) X40CrMoV51 100 µm

200 µm

(a)

(b)

200 µm 100 µm

(c)

(d)

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Figure 8 Tribological properties of uncoated and borided steel (St37) UNCOATED

COATED

Weight lost (gr)

0.02 0.015 0.01 0.005 0 0

0.5

1

1.5

2

Treatment Time (min) Notes: Test conditions: pressure 15 kPa, SiC paper 220 grit, testing time 6 min, velocity 30 rev/min

Figure 9 Coating failure modes

(a)

(b)

(c)

(d)

Figure 10 (a) Development of perpendicular macrocracks inside the coating layer (substrate X40CrMoV51). (b) Coating abrasive wear without coating fatigue failure (substrate 42CrMo4)

(a)

(b)

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Figure 11 Experimental determined fatigue curve of Fe2B coating deposited on steel (St37)

reactor and their properties”, Science and Technology of Advanced Materials, Vol. 3 No. 4. Arai, T., Endo, J. and Takeda, H. (1986), “Chromizing and boriding by use of a fluidized bed”, paper presented at the 5th International Conference on Heat Treatment of Materials, 3, pp. 1335-41. Gupta, C.K. and Sathiyamoothy, D. (1999), Fluid Bed Technology in Materials Processing, CRC Press, Boca Raton, FL. Howard, J.R. (1989), Fluidized Bed Technology Principles and Applications, Adam Higler, Bristol. Japka, J.E. (1983), “Using the fluidized bed for nitriding-type processes”, Metal Progress, February, pp. 27-33. Kingel, S., Angelopoulos, G.N., Papamantellos, D. and Dahl, W. (1995), “Feasibility of fluidized bed CVD for the formation of protective coatings”, Steel Research, Vol. 66 No. 7, pp. 318-24. Knotek, O., Bosserhoff, B., Schrey, A., Leyendecker, T., Lemmer, O. and Esser, S. (1992), “A new technique for testing the impact load of thin films: the coating impact test”, Surf. Coat. Technol., Vol. 54/55, pp. 102-7. Pe´rez, F.J., Hierro, M.P., Pedraza, F., Go´mez, C. and Carpienero, M.C. (1999), “Aluminizing and chromizing bed treatment by CVD in a fluidized bed reactor on austenitic stainless steels”, Surface & Coating Technology, Vol. 120/121, pp. 151-7. Reynoldson, R.W. (1993), Heat Treatment in Fluidized Bed Furnaces, ASM International, Materials Park, OH. Tsipas, D.N. and Flitris, Y. (2000), “Surface treatment in fluidized bed reactors”, Journal of Material Science, Vol. 35 No. 21, pp. 5493-6. Tsipas, D.N., Triantafyllidis, G.K., Kipkemoi, J. and Flitris, Y. (1999), “Thermochemical treatments for protection of steel in chemically aggressive atmosphere at high temperature”, Materials and Manufacturing Processes, Vol. 14 No. 5, pp. 697-712.

600

no failure coating failure coating fatigue curve

Impact force (N)

500 400 300 200 100 0 1.0E+04

1.0E+05

1.0E+06

1.0E+07

Number of impacts

determined fatigue curves. The Fe2B coating revealed high-fatigue strength against cycle impact loading.

Conclusions A simple, environment friendly, fast-boriding process was carried out in a fluidized bed reactor. Samples of a typical morphology of iron-boride coatings and of excellent quality were produced. Fe2B was the predominant boride phase formed and it showed improved tribological properties under dry wear conditions. Impact testing investigations revealed the high-fatigue strength of boride coatings in combination with limited deformable substrates.

References Anil Kumar Sinha, S. (1997), “Boriding (boronizing)”, ASM Handbook, Vol. 4, pp. 437-47. Anthymidis, K.G., Stergioudis, G. and Tsipas, D.N. (2002), “Boride coatings on non-ferrous materials in a fluidized bed

Corresponding author K. David can be contacted at: [email protected]

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