microstructural study and tribological behavior of wc-co coatings on stainless steel produced by...

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 5, Issue 1, January (2014), © IAEME

132

MICROSTRUCTURAL STUDY AND TRIBOLOGICAL BEHAVIOR OF

WC-Co COATINGS ON STAINLESS STEEL PRODUCED BY HVOF SPRAY

TECHNIQUE

M. Mruthunjaya1 and K.I. Parashivamurthy

2

1Assistant Professor, Department of Mechanical Engineering, JSS Academy of Technical

Education, Bangalore-560060 2Professor, Department of Mechanical Engineering, Government Engineering College,

Chamarajanagar – 571313

ABSTRACT

Tungsten Carbide (WC) coatings exhibit high wear resistance at low and high temperatures,

WC - Cobalt coatings will demonstrate anti-resistive and wear characteristics better than those of conventional materials. Research in this area has shown that the service life of the WC-Co coatings depended on varying compositions of tungsten and cobalt. WC-Co coating is developed on the stainless steel AISI 304 by High Velocity Oxy-Fuel spray technique. The grain size of WC is varied in three ranges of 10-40µm, 15-63µm and 45-90µm. Microstructure, chemical composition, phases present in the coating on the steel substrate was studied by using Scanning Electron Microscope (SEM) and X-Ray Diffraction method. Microstructure shows uniform distribution of WC in the matrix. WC coatings exhibit increased in hardness and resistance to wear. The wear rate of tungsten carbide of sample C mesh size (45 to 90 µm) is less compare to two remaining two samples by considering different loads. Key words: Corrosion, High Velocity Oxy-Fuel, SEM, Sliding Wear, Tungsten Carbide.

1. INTRODUCTION

The components of thermal power plant coal handling systems (Mills, PF pipes and bends) are subjected to wear. For improved life expectancy of the such components, several attempts are being made by introducing advanced materials to combat wear situations. Carbide coatings have proven themselves an excellent choice for wear and corrosion applications. A large use of WC-Co coatings has been documented leading to good results in several industrial applications. The grain

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size of WC is varied in three ranges can be used effectively to combat abrasive, adhesive or erosive wear, including applications that combine wear and corrosion, either at ambient or elevated temperatures [1]. High Velocity Oxy-Fuel (HVOF) flame spraying has been recognized as the best choice for carbide coatings application [2]. An inherent advantage of the HVOF process is its ability to spray semi-melted particles to very high speeds (around 900m/s) resulting in a quite dense coating with high adhesion and low oxides content. The High Velocity Oxy-Fuel sprayed coats show more uniform and fine grained microstructure than plasma sprayed coats. The wear resistant strongly depends on the internal micro-structure of coatings. The nanometric features contributes to the increase of surface smoothness of coatings and increase the resistance against the wear[3]. The resistance to the high temperature and oxidation behavior of coatings were studied in detail previously [4-6]. Many investigators [7-14] investigate the adhesive and erosion behavior of WC coated specimens produced by HVOF technology. The purpose of this investigation is to investigate the wear behavior of WC-Co coated stainless steel specimen with varying mean grain sizes of WC (10-40µm, 15-63µm and 45-90µm) and cobalt 12% by HVOF spray technique for wear and corrosion applications. Micro hardness and specific wear tests evaluation are carried out as well as metallographic characterization.

2. EXPERIMENTAL PROCEDURE

WC-Co based coatings were produced by using HVOF model 80 kW HVOF spray system (Model: Sulzer-Metco Diamond Jet 80 kW, Japan). The coatings were applied on stainless steel substrates. Specimens are initially grit blasted at a pressure of 3 kg/cm2 using Al2O3 having grit size of 60µm for the average roughness of the surface was 6.8µm. The standoff distance in shot blasting was kept between 120-150 mm. The average roughness of the substrates was 6.8µm. The grit blasted specimens were cleaned with acetone in an ultrasonic cleaning unit. HVOF process carried out at operating power 80kW and current 550 A. The flow rate of fuel gas was 5.5 liter/minute, oxygen 2.90 liters/minute, spray distance 150 mm and standoff distance is 100 mm. Chemical analysis of base metal was determined by using vacuum emission spectrometer. Carbon of the base metal was analyzed by wet method. Chemical composition of WC-Co powder was analyzed using Energy Dispersive Spectroscopy (EDS). Chemical composition of base metal and WC-Co powder is tabulated in table 2.1 and table 2.2 respectively.

Table 2.1 Chemical composition (in weight %) of base metal

C Mn Si P S Cr Ni

0.08 2.00 1.00 0.04 0.03 20.0 10.5

Table 2.2 Chemical composition (in weight %) of WC-Co

Coatings Mesh size range in

µm C W Co Ni O

A 10-40 10.58 61.24 14.35 3.72 8.54

B 15-63 11.08 63.16 15.16 2.66 7.54

C 45-90 13.85 64.75 14.64 2.53 8.23



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Microstructures of the surface coated specimens are taken using Scanning Electron Microscope(SEM) connected to Energy Dispersive X-ray analysis equipments (EDX). SEM images were used for study the distribution of WC particles on the substrate. XRD pattern was carried out using X-Ray diffract meter with CuK radiation using wavelength of 1,790 Å at operating voltage 40.0kV and current 30 mA. The 2 range scanned was 10 to 70 degree at the step size 0.02 degree and count time 0.60 seconds. The micro-hardness measurements were made on carbide particles in the surface coating using ZWICK 3212 micro-hardness tester at a test load of 0.03 kgf and average of five different readings is computed. The wear test was carried out using a pin on disc tribometer (Model: TR-20 LE, DUCOM Bangalore make) as per ASTM G 99. A pin on disc tribometer consists of a stationary "pin" under an applied load in contact with a rotating disc. The used Pin-on-disc methodology was described in detail in [15] The wear performance of the coating was evaluated at room temperature (250C).Samples to be tested were cleaned ultrasonically with isopropanol, dried and weighed before testing. In the present experimental work, speed and time varied while the load was varied from 20 N to 40 N & speed varied from 300 rpm to 500 rpm, time remained constant throughout all the coatings are given in the table 2.3.

Table 2.3 Parameters used in Wear test

The sliding wear test have been repetitively conducted for all the coatings and the average of

all the reading were recorded. The wear rate of each sample was calculated from the weight loss, the amount of wear is determined by weighing the specimen before and after the test using precession electronic weighing machine. Since the mass loss is measured it is converted to volume loss using the density of the specimen. In order to study tribological behavior of coatings in severe conditions, the tests were carried out without any lubrication. The worn-out surfaces of the pins were subsequently examined under a SEM to identify the possible wear mechanisms. The wear debris was collected during wear tests and was subjected to morphological characterization under SEM.

Parameters Operating Range

Wheel speed (RPM) 300, 400 and 500

Load w (N) 20, 30 and 40

Time t ( minutes) 5

Wheel Diameter Ǿ(mm) 145

Micro hardness of wheel substrate (Brass) HV0.1 ( kg/mm2) 3000



135

Fig. 2.1 SEM Micrographics of HVOF sprayed WC-Co fine Powder size range 10

to 40µm)

3. RESULTS AND DISCUSSION

The chemical composition of the base metal and WC-12Co carbides was determined using vacuum emission spectrometer. Carbon contents in the composites were analyzed by wet method and provided in table 2.2. The coatings are designated as A, B and C. Fig. 2.1 (a) to (c) illustrate the surface SEM micrographs of samples prepared using HVOF method with varying mean grain size of WC varying from 30µm to 65µm As observed from the SEM structure the grain sizes are varied from sample A to sample C. SEM micrographs confirm the WC-12%Co uniformly coated on the surface of the steel substrate. All the coating revealed uniform distribution of WC-12Co particles and good bonding existing between the metal matrix and reinforcements with varying grain sizes from sample A to sample C. From the microstructure it was found that WC uniformly coated with thickness about 15 to 30µm on the surface of stainless steel. Fig. 2.1(c) indicates SEM structure at higher magnification and clearly indicate porous of the samples. The SEM results indicate that the WC can be effectively coat on the steel surface at required thickness with varying grain sizes of WC with using HVOF method.



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Fig. 3.1 XRD patterns of WC/Co fine Powder size range (10 to 40µm)

Fig. 3.1 illustrate the XRD pattern of the WC-12%Co coating on the steel surface. It is

observed that the WC coating using HVOF consists of WC and Co and WC is dominating phase. During HVOF spraying the different phase transformations are taking place, notable quantity of binder phase is present in the coating.

Fig. 3.2 Comparison of Wear rate vs. coating with different loads 20, 30 & 40N

Fig. 3.3 Comparison of Wear rate vs. coatings with Speeds 300,400 & 500 rpm

0

1

2

3

4

5

6

7

A B C

Wea

r ra

te (

mm

3 /m

)

Coatings

Load 20N

Load 30N

Load 40N

0

1

2

3

4

5

6

7

8

A B C

Wea

r ra

te (

mm

3/m

)

Coatings

300 rpm

400 rpm

500 rpm



137

The wear test is conducted under three different loads of 20. N, 30 N and 40 N and it is obtained from the test such that the weight loss during the wear test is highest for the load of 40 N and is lowest for the load of 20 N. so it is concluded from the test, as the load increases the wear rate is also increases. The wear rate of tungsten carbide of sample C mesh size (45 to 90 µm) is less compare to two remaining two samples by considering different loads. The wear test is conducted under three different speeds of 300, 400 and 500 rpm and it is obtained from the test such that the weight loss during the wear test is highest for the speed of 500 rpm and is lowest for the load of 300 rpm. So it is concluded from the test, as the speed increases the wear rate is also increases. The wear resistance of the sample C is good compared remaining two samples A & B. Pin-on disc wear test shows, the sliding wear resistance increases with increasing mesh size. This is because of large sized WC particles can have higher wear resistance. During sliding wear test it shows low wear compared to small mesh size carbide.

Fig. 3.4 SEM micrographs of WC-Co fine Powder size range (10 to 40µm)surfaces generated

under wear tests speed of 500rpm. (a)100x (b)200x (c)500x (d)1000x

The microstructure of the WC-Co fine Powder size range (10 to 40µm) coating is shown in

fig. 3.4 (a), (b) and (c), the SEM micrograph of fig3.4 (a) shows clear smoothened wear track. Grooves and lip mechanism were observed on the wear surface zone of coating. The pulled out of the carbide grains can also be seen in some regions. The coating uniformly distributed in the soft substrate can increase hardness of the coating and has a good binding force on the interface between particles and substrate. It is less easy to form a large plastically deformed region around the smaller craters, so that deformation-induced wear is decreased. The fig. 3.4 (c) indicates more oxide content



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and roughened wear surfaces in the coating, the micrograph also showed external and inter granular cracks, the atoms are not strongly bonded which disperse with loose phases and more tiny granular pores. A higher concentration of high-aspect-ratio porosity was found in the coating surface direction. This indicates that the entrapped interlamellar pores were formed due to the incomplete filling of the pores by successive molten layers along the axial direction and the intersplat boundary region. This is attributed to the partial melting and decarburization of the tungsten carbide particles.

4. CONCLUSIONS

The result of the present study WC-12Co HVOF coatings can be summarized as follows.

• Among the WC-12Co HVOF coatings, the fine mesh size coating having fine structure and the homogenous distribution of hard face particles can be used for components of thermal power plant coal handling systems.

• All the three coatings exhibits better wear resistance, the wear rate of tungsten carbide of sample C mesh size (45 to 90 µm) is less compare to two remaining two samples by considering different loads and speeds.

• The wear resistance of the sample C is good compared remaining two samples A & B. Pin-on disc wear test shows, the sliding wear resistance increases with increasing mesh size. This is because of large sized WC particles can have higher wear resistance. During sliding wear test it shows low wear compared to small mesh size carbide.

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