Molybdenum and Cobalt Silicide Field Emitter Arrays

Journal of Mechanical Science and Technology 00 (2012) 0000~0000 www.springerlink.com/content/1738-494x

A study on erosive wear behaviour of HVOF sprayed nanostructured WC-CoCr coatings Lalit Thakur1 and Navneet Arora1* 1

Mechanical and Industrial Engineering Department, Indian Institute of Technology Roorkee, Roorkee-247667, India (Manuscript Received 000 0, 2012; Revised 000 0, 20121; Accepted 000 0, 2012)

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Abstract WC-CoCr cermet coatings were deposited on stainless steel substrate using high-velocity oxy-fuel (HVOF) thermal spray process. The coatings were developed with two different thermal spray powders, one having WC grains of conventional micron size and other one is composed of nanosized (near-nanostructuted) grains. HVOF spraying was assisted with in-flight particle temperature and velocity measurement system to control the process parameters that have resulted in quality coatings. Cavitation erosion testing was performed using a vibratory test apparatus based on ASTM standard G32-98. Surface morphology of powders and coatings was examined using the FESEM images and phase identification was performed by XRD analysis. The erosion behaviour of coatings and mechanism of material removal was discussed by examining the microstructual images of worn-out surfaces. It was observed that WC-CoCr cermet coating deposited with nanosized WC grains exhibits higher cavitation erosion resistance as compared to conventional coating. Keywords: At least four keywords; Coatings; Electron microscopy; HVOF; Nanostructured materials; Wear ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction Cavitation erosion is a very serious problem in most of the hydraulic machines, especially pumps and turbines. It is mainly caused by the pressure variations in fluid system of hydraulic machine that results in formation of bubbles. These bubbles collapse at the surface and cause shock loading which leads to the progressive removal of material. This problem can be minimized either by modifying the design of hydraulic component or by developing an advance erosion resistant material. Other different approach is the use of thermal spray coatings and high-velocity oxy-fuel (HVOF) sprayed cermet coatings are mainly used for erosion resistance application [1-3]. WC-Co based coatings have been successfully used for combating the cavitation erosion problem [4, 5]. It has been observed that, erosion resistance of WC-Co cermet coatings increases significantly by reducing the size of carbide grains to nanometer scale in ductile cobalt matrix [6, 7]. But there are some problems associated with these nanostructured coatings such as higher decomposition of WC phase in powder feedstock material that results in poor performance [8]. Several researchers have reported the cavitation resistance of conventional WC-Co based coatings but there is scarcity of *

Corresponding author. Tel.: +91-9412307242, +91-9359054297 E-mail address: [email protected], [email protected] † Recommended by Editor 000 000© KSME & Springer 2012

work related to the cavitation erosion behaviour of nanostructured coatings. Therefore, in the present study WC-Co-Cr coatings have been deposited by HVOF process on alloy steel substrate. These coatings were developed by using the powder feedstock material in which the size of WC grain was conventional micron and near nanostructured range. The cavitation erosion behaviour of conventional and nanostructured coatings has been investigated by using a vibratory apparatus producing the bubbles over the coated surface. The performance of both coatings under cavitation erosion was compared with each other. The mechanism of material removal was also studied and discussed to length.

2. Experimental Procedure 2.1 Material and coating process In this study, HVOF spray process was selected to deposit the WC-CoCr based coatings on AISI 304 stainless steel substrate. The coatings were developed by using two different commercially available thermal spray agglomerated powders. A detailed description of feedstock powders is presented in Table 1. Prior to the coating deposition, alloy steel substrate was degreased with acetone and grit blasted using 300-500 µm size alumina abrasive powder. This operation was performed to impart an average surface roughness of Ra≈ 5µm which has resulted in increased bonding.

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L.Thakur and N. Arora. / Journal of Mechanical Science and Technology 00 (2012) 0000~0000

Table 1. Material used for coating deposition. Coating powder Conventional WC-10Co-4Cr Nanostructured WC-10Co-4Cr

Morphology Spherical (spray dried & sintered) Spherical (spray dried & sintered))

WC grain size 2-4 μm

Agglomerated size +15/-45 μm

200-500 nm

+15/-45 μm

Table 2. HVOF process parameters used for CWC and NWC coatings. Parameters

WC-CoCr coatings

Oxygen flow rate (slph)

Fuel (Kerosene) flow rate (slph)

Carrier gas flow rate (scfh)

Spray distance (mm)

Powder feed rate (gm/mi n)

1800

22

22

280

78

Table 3. In-flight particle temperature and velocity measurement result.

Parameters CWC powder NWC powder

Fuel (splh) 18-24 17-22

O2 (scfh) 16002000 16501900

N2 (scfh) 22 22

Particle temp. °C 17002400 15002000

Particle velocity (m/s)

Fig. 1. Cavitation erosion test apparatus.

2.3 Cavitation erosion testing

400-800 600-700

The conventional and fine grain WC-10Co-4Cr coatings were deposited by using Praxair-Tafa JP5000 HVOF torch with process parameters listed in Table 2. The range of inflight particle temperature and velocity is given in Table 3. Both coatings were deposited with a thickness range of 200400 µm; same is used for commercial purpose with minimum residual stresses. Effect of coating thickness on erosion behaviour has been neglected in this study. 2.2 Characterization Microstructural features of powders and coatings were examined by using Quanta 200F (FEI, Netherland) field emission scanning electron microscope (FE-SEM). Identification of phases was performed by XRD analysis with the help a diffractometer (Bruker AXS-D8). The coating micro-hardness was measured by using a Vickers indenter (Leitz:MM6, Germany) at a load of 300 gm. Surface roughness (Ra) value of as-sprayed coatings was measured by an optical profiler (Vecco Wyko 1100NT). Porosity analysis was performed by using an optical microscope and image analysis software (Dewinter Material Plus 4.1). Fracture toughness of the coatings was determined by Vickers indentation method and employing the correlation used in other study at 10 kg load with 15 s dwell time [9]. The indentations were taken on polished cross-section of coated samples to determine the micro-hardness and fracture toughness. An average of ten readings taken at different locations is reported.

Cavitation erosion test was carried out in an ultrasonic vibratory test setup according to ASTM G32 standard as shown in Fig. 1. For each new test specimen, the vessel was cleaned, filled with fresh liquid and vibrated for 30 min to stabilize the gas content. Coated samples of dimension 20mm X 15mm X 5mm were used for this testing. Prior to erosion testing, coated specimens were ultrasonically cleaned in acetone, dried up until all the moisture was removed and then weighted to ±0.1 mg precision. Test specimen was held stationary below the vibrating horn, at a distance of 1 mm. The erosion test parameters are listed in Table 4 and distilled water was taken as test liquid. The cavitation erosion was conducted for a total duration of 18 hrs and weight loss was measured at required intervals of time. The test specimen was removed after required intervals of time and was cleaned in acetone, dried, weighed using an electronic balance to determine the weight loss. To evaluate the performance of both the CWC and NWC coating during the cavitation erosion, cumulative weight loss was calculated against the test time in hours. 3. Results and discussion Fig. 2(a) and (b) are scanning electron (SEM) micrographs of conventional WC-CoCr (CWC) powder, exhibiting spherical morphology formed by agglomeration of particles. Fig. 2(c) and (d) are SEM micrograph of second powder having fine WC grains and it also possess spherical morphology by agglomeration and sintering process. Fig. 3(a) and (b) are SEM images of as-sprayed CWC coating whereas Fig. 3(c) and (d) are SEM images of as-sprayed second coating showing the distribution of WC grains in binder material. Initially the second powder with primary carbide size ex-


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ceeding 100 nm was sprayed with optimized process paraTable 4. Cavitation erosion test parameters. Testing parameters Frequency 20 kHz Amplitude 50 µm Fluid Distilled water Temperature 28 °C Total testing time 18 hrs

(d)Higher magnification image of NWC particle showing nano-WC. Fig. 2. SEM images of spherical agglomerated particles of CWC and NWC powder.

(a) Lower magnification image of CWC agglomerated particles.

(b)Higher magnification image of CWC particle showing WC.

(c) Lower magnification image of NWC agglomerated particles.

-meters; it has resulted in the development of near nanostructured WC-CoCr (NWC) coating. Fig. 4(a) and (b) are crosssectional SEM image of CWC and NWC coating showing the predefects like pores and some of WC pullout regions that were formed during the sectioning and polishing procedure. Fig. 5 represents the XRD analysis of the deposited coating showing the major WC phase and without the formation of W2C phase. Both the coatings show only dissolved W and WC1-x phase indicating a mild decomposition of WC phase in NWC coating. Results of micro-hardness, porosity, assprayed coating roughness, and fracture toughness testing are presented in Table 5. The results have shown that as-sprayed NWC coating possess lower surface roughness (Ra) and porosity value as compared to CWC coating. This lower value of surface roughness may be attributed to smaller mean size of WC grains of NWC coating. The micro-hardness value of NWC coating is significantly very high as compared to CWC coating. The high micro-hardness of NWC coating is due to decreased mean size of WC grains resulting in increased surface bonding with binder material, offering greater resistance to indentation. The increased micro-hardness value may also be attributed to lower value of defects like porosity at nano-scale leading to dense microstructure. The indentation fracture toughness test had shown that the cracks are thick and large in CWC coating as compared to NWC coating as shown in Fig. 6(a) and (b). It shows that NWC coating possess higher fracture toughness as compared to CWC coating. The dislocation through nano-scale grain is suppressed due to misorientation and higher volume of grain boundaries in NWC coatings that jammed the slip, ultimately resisted the crack propagation i.e. increase fracture toughness [10, 11]. Fig.7 presents the cumulative weight loss of CWC and NWC coating with respect to time as a result of cavitation erosion. Curves of cumulative weight loss for both the CWC and NWC coatings were drawn by considering the average

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weight loss of three samples of each coating. It is also evident from the graph that NWC coating possesses better cavitation resistance than CWC coating. The curves for both the coatings show three stages of erosion i.e. incubation stage, acceleration stage and deceleration stage. During the initial stage of erosion, weight loss is not yet measurable or negligible compared to later stages which is called incubation period. The incubation period is usually thought to represent the accumulation of stresses by shock loading at the predefects like pores, oxides, inhomogeneous carbide distribution regions, and thermal cracks that are formed during the initial coating deposition [3, 5]. There is no exact measure of the duration of incubation period. It is evident from the graph that incubation takes place in both the coatings and it is more in NWC coating. Acceleration stage is characterized by an almost increasing slope of cumulative weight loss curve with time. It is evident from graph that weight loss is more in acceleration stage when compared to other stages. The third stage is a deceleration stage; the curve shows a decreasing slope and then achieving the steady state. The decreasing trend of slope may be attributed due to continuous increasing gap between the vibrating horn tip and coated surface during 18 hrs of testing, which ultimately results in reduced effect of impacting bubbles. Fig. 8(a) and (b) represents the BSE-SEM images of eroded surfaces of CWC and NWC coated samples exposed to cavitation erosion. It can be inferred from the figures that CWC coating suffers more erosion than NWC coating. The material has been removed from the surface leaving pits or cell like features. These cells are formed probably as a result of shock loading caused by bubbles impact, leading to the eruption of binder matrix as a result of growth of cracks. These cracks originate from predefects like porosity, brittle phases, and thermal cracks which propagate along interlamellar boundaries and individual splats, followed by the detachment of WC grains.

(b) Higher magnification image of as-sprayed CWC coating.

(c) Lower magnification image of as-sprayed NWC coating.

(d) Higher magnification image of as-sprayed NWC coating. Fig. 3. SEM images as-sprayed CWC and NWC coating.

(a) Lower magnification image of as-sprayed CWC coating.

The other mechanism of material removal is observed as the brittle cracking of WC grains due to elastic shock loading and then these cracks have propagated to the binder.


(a) Image showing the distribution of pores and thickness of CWC coating.

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Fig. 5. Result of XRD analysis of as-sprayed coating showing different phases.

(b) Vickers indentation at CWC coating cross-section. (b) Image showing the distribution of pores and thickness of NWC coating. Fig. 4. SEM-BSE cross-sectional images of CWC and NWC coating showing coating thickness and distribution of pores. Table 5. Result of micro-hardness, porosity, as-sprayed roughness, and fracture toughness testing.

Coating

CWC NWC

Microhardness (HV, 300g)

Porosity%

1150±45 1650±36

0.75±0.5 0.60±0.6

Assprayed roughness (Ra µm) 4.9±0.5 3.6±0.6

Indentation fracture toughness (Kc) at 10 kg load (MPa m1/2) [9] 4.05±0.12 7.21±0.20

Kc = 0.089(HvP/2l)1/2 (Shetty et al.), Hv is the Vickers hardness of the coating; E (300 GPa, assumed) is the elastic modulus, “a” is the indentation half diagonal, “c” is the crack length from the center of indent, P is the load, l is the crack length from indenter corner.

It can be seen from Fig. 8(c) and (d), that cracks are developed on WC grains by the continuous impacts of bubbles in both the coatings but some crack arrest is seen in case of NWC coating.

(b) Vickers indentation at NWC coating cross-section. Fig. 6. SEM image of Vickers indentation (10 kg load) taken at (a) CWC (b) NWC coating.

The propagating cracks are arrested in case of NWC coating due to its high fracture toughness. The material has been removed by same mechanism in both coatings but NWC coat-

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then it has propagated to the binder. The material has been removed by same mechanism in both coatings but NWC coating has suffered less erosion due to its high micro-hardness and fracture toughness.

Fig. 7. Plot showing cumulative weight loss of CWC and NWC coated samples during cavitation erosion testing performed for 18 hrs.

ing has suffered less erosion due to its high micro-hardness and fracture toughness. Different researchers have already reported that high micro-hardness and fracture toughness affects the cavitation erosion resistance of WC-Co based coatings [12, 13]. It has been observed from the present study that hardness and toughness of the coatings can be improved by decreasing the WC grain size to near nanometer range that highly affects the cavitation erosion resistance of WC-Co based coatings. In this study coatings were deposited using HVOF process with the assistance of in-flight particle velocity and temperature monitoring system to minimize the decarburization of WC in W2C phase by lowering the surface temperature of powder particle travelling in the flame. The NWC coating was achieved with improved properties and very less degree of decomposition. The decomposition was observed due to high surface to volume ratio of WC particle in NWC powder and thin hollow spherical morphology of agglomerated particle leading to rapid heating [13, 14].

(a) Image showing material removed from CWC coating surface during cavitation erosion testing.

4. Conclusion HVOF sprayed CWC and NWC coatings were deposited on AISI 304 stainless steel substrate. The spraying parameters were adjusted to decrease the in-flight particle surface temperature and dwell time to an optimum level in order to lower the degree of decarburization. The cavitation erosion behavior of both the coatings has been studied using vibratory test apparatus. The cavitation erosion of both the coatings is a three stage process consisting of incubation, acceleration, and deceleration phenomenon. The material has been removed by two different mechanisms leaving the pit or cell like feature. Firstly the shock loading caused by the bubble impacts has resulted in the growth of cracks originating from predefects present inside the coating. These cracks propagate along the interlamellar boundaries and individual splats and when they meet at a point caused the eruption of binder followed by the WC grain detachment. Secondly the material has been removed by the brittle cracking of WC grains due to elastic shock loading and

(b) Image showing material removed from NWC coating surface during cavitation erosion testing.

(c) Magnified image of cell feature and wear mechanism in CWC coating.


(d) Magnified image of cell feature and wear mechanism in NWC coating. Fig. 8. SEM images showing the eroded surfaces of CWC and NWC coating after 18 hrs of cavitation erosion testing.

Acknowledgement The authors gratefully acknowledge the facilities provided by M/s Industrial Processors and Metallizers Pvt. Ltd. (IPM), New Delhi, India for the successful completion of experimental work. The authors would like to thank Mr. Rahul Sood (Technical Director, IPM Pvt. Ltd.) for his wonderful suggestions about thermal spraying and in-flight diagnostics study.

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[7] L. Thakur, N. Arora, R. Jayaganthan, and R. Sood, An investigation on erosion behavior of HVOF sprayed WC–CoCr coatings, Applied Surface Science, 258 (2011) 1225-1234. [8] D.A. Stewart, P.H. Shipway, and D.G. McCartney, Abrasive wear behaviour of conventional and nanocomposite HVOFsprayed WC-Co coatings, Wear, 225-229 (1999) 789-798. [9] P.S. Babu, B. Basu, and G. Sundararajan, Processing– structure–property correlation and decarburization phenomenon in detonation sprayed WC–12Co coatings, Acta Materialia, 56 (2008) 5012-5026. [10]L. Thakur, N. Arora, Sliding and abrasive wear behaviour of HVOF sprayed WC-CoCr coatings with different carbide sizes, Journal of Materials Engineering and Performance, ISSN 1059-9495, 10.1007/s11665-012-0265-5. [11]M. Li, Y. Yang, and H. Chan, Effect of WC grain size on the abrasive wear resistance of HVOF spraying WC-Co coatings, Advanced Materials Research, 97-101 (2010) 1344-1347. [12]D. Zhang-xiong, C. Wei, and W. Qun, Resistance of cavitation erosion of multimodal WC-12Co coatings sprayed by HVOF, Transactions of Nonferrous Metals Society of China, 21 (2011) 2231-2236. [13]D.A. Stewart, P.H. Shipway, and D.G. McCartney, Microstructural evolution in thermally sprayed WC–Co coatings: comparison between nanocomposite and conventional starting powder, Acta Materialia, 48 (2000) 1596-1604. [14]Y. Qiao, Y.R. Liu, and T.E. Fischer, Sliding and abrasive wear resistance of thermal-sprayed WC–Co coatings, Journal of Thermal Spray Technology, 10 (2001) 118-125.

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Lalit Thakur received M.Tech. degree in Mechanical Engineering from IIT Roorkee, India, in 2010. Mr. Thakur is currently a research scholar at Mechanical & Industrial Engineering department of IIT Roorkee. His research areas are Thermal Spray Wear Resistant Coatings, Advance Material Development, Welding Processes, and Optimization techniques. Navneet Arora is in the faculty of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, India. He received his Ph.D. degree from Kurukshetra University, Kurukshetra in 1997. His research work focuses on the Welding Process, Process modeling, Dissimilar Weld Joining, System Reliability, Stochastic Modeling and Maintenance Management of Systems in Process Industries.

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