Microstructure and Wear Properties of Laser Clad NiCrBSi-MoS2 Coating

Friction and Wear Research Volume 2, 2014

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Microstructure and Wear Properties of Laser Clad NiCrBSi-MoS2 Coating Wei Niu1,a, Ronglu Sun*1,b, Yiwen Lei2,c School of Mechanical Engineering, Tianjin Polytechnic University, China; Advanced Mechatronics Equipment Technology, Tianjin Area Major Laboratory, China

1 2

newways_2005@126.com; *,brlsun@tjpu.edu.cn; cleiyiwen@163.com

a

Received 14 March 2013; Accepted 4 July 2013; Published 12 January 2014 © 2014 Science and Engineering Publishing Company

Abstract A self-lubricant Ni-based composite coating was fabricated on the H13 steel substrate by laser cladding a mixture of Ni-coated MoS2 particles and NiCrBSi particles using a CO2 laser. The microstructure, microhardness and wear properties of the clad coating were investigated. The results showed that the laser clad coating was composed of sphere-like CrxSy particle, net-like eutectic of the compound of sulfide of iron, and chromium, and dendritic γ-Ni solid solution. A metallurgical bonding between the laser clad coating and substrate was obtained. The micro-hardness of the coating is 350~420 HV0.2. The friction coefficient of the coating is 0.10~0.20 at room-temperature in air. The wear mass loss of the coating is reduced to 17.4% of that of the substrate in wear testing.

Keywords Laser Cladding; Composite; Microstructure; Micro-hardness; Wear Property

Introduction Many tribological systems are required to work in high-temperature, vacuum, strong radiant and chemical-pollution aggressive environments. Under these conditions, the presence of liquid lubricant is inevitably contaminated. Self-lubricating metal matrix composites combine both high wear resistance/ toughness of metal matrix and excellent lubricating characteristics of lubricants, exhibiting excellent tribological characteristics and good suitability to different environments. Metal sulfide (WS2, MoS2, etc.), graphite, hexagonal BN, CaF2 are chosen as lubricants because of their special structure [Wu et al. (1997), Xiong (2001), Renevier et al. (2000), Chen et al. (2008)].

At present metal matrix lubricant composite coating is generally applied instead of bulk composite. In addition, plasma spraying [Huang et al. (2009)] and laser cladding are familiar surface technologies.

However, the bonding between the coating (made by the plasma spraying) and substrate is mechanical. It is well known that laser cladding is an effective technique to improve the surface wear resistance of workpieces and has been widely investigated to produce metal, ceramet, ceramic coating containing ceramic particles (such as TiC, WC, TiB2, etc.)

[Majumdar & Li (2010), Yang et al. (2010), Candel et al.

(2010)]. And laser cladding has been recently developed to produce metal lubricant composite coating adopting solid lubricant WS2, MoS2, hexagonal BN [Xu et al. (2006), Wang et al. (2008), Zhang et al.

(2010), Avril & Courant (2006)].

Metal sulfide lubricant composite coating changes on the tribological contact of working surfaces and exhibits effective lubricating performance because sulfide has a lamellar structure with low shear strength.

In the present study, in situ Ni-based cladding coating was produced using Ni-coated MoS2 particulates and NiCrBSi particles on H13 steel by laser cladding. In situ sulfide lubricant with similar lamellar structure of MoS2, the invalidation was avoided due to the decomposition and reaction of MoS2 during laser cladding, and thus leading to surprising enhancement of self-lubricating properties. The microstructure of the composite coating was characterized by means of scanning electron microscopy, while its microhardness, friction and wear mass loss were evaluated.

Experimental Procedures H13 steel with a composition of 4Cr5MoSiV1 and size of 50 mm×20 mm×20 mm was adopted as the substrate for laser cladding. The surface of the steel was ground to a surface roughness of Ra=0.2 μm, and rinsed with ethanol and acetone successively. The powder mixtures used as the clad materials were prepared

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from NiCrBSi alloy (Ni60) and Ni-coated MoS2 powders (50wt. % MoS2) according to the weight ratio of 2:1. The size of the NiCrBSi alloy powder was 100~250 μm, and that of Ni-coated MoS2 powder was about 100 μm. The chemical composition (wt. %) of NiCrBSi alloy powder is: 16Cr, 3.3B, 4.5Si, 0.9C, less than or equal to 8.0Fe, and balance Ni. The mixtures were placed on the surfaced of H13 steel substrate using an organic binder. The thickness of the pre-placed powder was 1.0 mm.

Laser cladding was carried out using a 5 kW CO2 laser.

The parameters of laser cladding process were selected as: 2 kW power, 3 mm/s scanning speed and 3 mm laser beam diameter. Fig.1 shows schematic diagram of the laser cladding processing.

FIG. 1 SCHEMATIC DIAGRAM OF THE LASER CLADDING PROCESSING

Microstructure and chemical composition of the laser clad coating were characterized using QUANTA200 scanning electron microscope (SEM) equipped with a dispersive X-ray spectroscopy analyzer (EDS). The phases were identified by X-ray diffraction (XRD) method using the Rigaku D/max 2500 PC X-ray diffractometer with Cu target Kα radiation. The specimens for SEM observations were cut transversely to clad coating, mechanically polished and etched using FeCl3: HCl: H2O=5 g: 5 ml: 100 ml etchant reagent. The microhardness of the cladding coating was measured using a HXD-1000T digital microhardness tester at an applied load of 200 g with a dwell time of 15 s. Wear properties of laser clad coating in air were evaluated using a block-on-ring friction tester (M100). The samples of 7 mm×7 mm×25 mm were used as the blocks, and the GCr15 steel (HRC65 with an initial surface roughness of Ra=0.2 μm) with size of ф43.5 mm×10 mm as the ring. The wear tests were as follows: rotating speed of ring was 200 rpm, the load was 49 N, room-temperature running time was 30 min. The friction coefficient μ was

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Friction and Wear Research Volume 2, 2014

calculated using the expression μ=M/(R·F), where M is the friction moment, R is the radius of the ring, and F is the applied normal load. The samples were cleaned by ethanol after test and weighed by an analytical balance with accuracy of 10-5 g.

Results and Discussions Fig. 2 shows the X-ray diffraction pattern of NiCrBSi-MoS2 composite coating. The result indicated that the phase of composite consisted of γ-Ni solid solution, binary element sulfide Cr3S4, Cr5S6, Fe9S10. The sulfide of Cr3S4, Cr5S6 can form the eutectic compound which was marked as CrxSy (in which y=x+1). It can be seen that the MoS2 peaks disappeared after laser cladding, and new phase such as CrxSy and Fe9S10 appeared. It is well known that MoS2 has some special physicochemical properties, such as low melting point (1180℃) and decomposition temperature (1370℃). The MoS2 was decomposed, melted and even vaporized sequently with a gradual increase in the temperature of the laser-irradiated zone during laser cladding process. The decomposition of MoS2 resulted in the formation of S and Mo, of which S reacted with Cr and Fe in the laser-generated pool. The reaction as follows happens: Cr+MoS2 → CrxSy+ Mo(x=3,5; y=x+1) Fe+MoS2 → Fe9S10+ Mo And Mo melted to Ni matrix form γ-Ni solid solution.

Although molybdenum disulfide reacted with matrix and reduced its lubrication function to some degree, but new phases such as CrxSy formed through reaction were also excellent solid lubricants.

FIG. 2 X-RAY DIFFRACTION SPECTRUM OF LASER NiCrBSi-MoS2 COMPOSITE COATING

Fig. 3 shows the scanning electron micrographs of the laser cladding Ni-based composite coating. The microstructure is characterized by like-spherical particles, net-like eutectic structure and dendritic matrix, shown in Fig. 3(a). The results of EDS analysis (shown in Fig. 4) indicated that the like-spherical

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particle is rich of Cr and S element, which is non-stoichiometric compound of sulfide of chromium and defined CrxSy containing 7.84Ni, 53.4Cr, 5.61Fe and 33.15S element (wt. %), and the net-like eutectic (marked B in Fig.3) contains more Fe (57.62Fe, 18.91Cr and 23.47S, wt.%) which are compound of sulfide of chromium and iron.

From bottom region to upper region of laser cladding Ni-based composite coating, the coverage of sulfide phase increased correspondingly, the amount and size of CrxSy particles gradually increased. Because the density of CrxSy is about 3.9~4.8 g·cm-3 lighter than Ni-based alloy, the CrxSy particles have a tendency to concentrate at the top of the coating. It can be seen from Fig. 3(b) that there is a planar crystalline band of 2~5 μm thickness between the bonding zone and heat-affected zone, from which directional dendrites are formed. The thin layer exhibiting epitaxial solidification with a planar front growth is the liquid-solid interface between the melted and non-melted regions. This shows good metallurgical bonding between clad coating and substrate.

(a)

(a)

(b)

FIG. 4 EDS SPECTRUMS OF DIFFERENT PHASES IN THE NiCrBSi -MoS2 COMPOSITE COATING SHOWN IN FIG.3 (a) SPHERICAL BLOCKS (MARKED A); (b) EUTECTIC (MARKED B)

Microhardness (HV0.2)

700

B A

10μm (b)

600 500 400 300 200 0.0

0.5

Substrate

HAZ

CL

1.0

1.5

2.0

2.5

Distance from surface (mm) FIG. 5 MICROHARDNESS DISTRIBUTION PROFILE ALONG THE DEPTH OF CROSS-SECTION OF LASER CLAD COATINGS 1.2

Substrate Laser clad coating

Substrate

20μm

FIG. 3 SEM MICROGRAPH OF LASER CLAD COMPOSITE COATING (a) THE UPPER SECTION AND (b) THE BOTTOM SECTION

Friction cofficient

1.0 0.8 0.6 0.4 0.2 0.0

0

5

10

15

20

25

30

Time (min)

FIG. 6 FRICTION COEFFICIENT OF LASER CLAD NiCrBSi-MoS2 COMPOSITE COATING AND SUBSTRATE

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Fig. 5 shows microhardness distribution profile along the depth at the middle of cross-section of laser clad Ni-based composite coatings. The profile can be divided into three regions corresponding to the cladding zone (CL), heat-affected zone (HZA) and substrate, respectively. The addition of Ni-coated MoS2 to Ni60 alloy decreased the hardness of the clad coating. The microhardness of laser clad Ni-based composite coating ranged from 350 to 420 HV0.2. In the heat-affected zone, the microhardness of the laser cladding composite coating is 620-400 HV0.2 higher than that of 260 HV0.2 in the substrate, since martensitic transformation occurs.

Wear test results, as shown in Fig. 6 and Fig. 7, indicate that the laser clad Ni-based composite coating has excellent self-lubricating properties under roomtemperature dry sliding wear test conditions. The friction coefficient of the laser cladding Ni-based composite coating and substrate is 0.10~0.20 and 0.55~0.75, respectively, as shown in Fig.6. The friction coefficient of laser clad coating is considerably lower than that of substrate. The wear mass loss of the laser cladding Ni-based composite coating and substrate is 3.1 mg, and 17.8 mg. The wear mass loss of laser clad Ni-based composite coating is markedly lower than substrate, and which is 17.4% of that of the substrate. It is clearly illustrated that the “soft” sulfide solid lubricating phase in Ni-based coating leads to surprising enhancement of self-lubricating properties under room-temperature sliding wear test condition.

the compound of sulfide of chromium and iron, and dendritic γ-Ni solid solution. The microhardness of the laser clad Ni-based composite coating ranged from 350 to 420 HV0.2, higher than that of 260 HV0.2 in the substrate. The friction coefficient of the laser clad Ni-based composite coating was 0.10~0.20, considerably lower than substrate. The wear mass loss of the composite coating was 17.4% of the substrate. In situ self-lubricant sulfide of chromium and iron in the composite coating reduced the friction coefficient led to surprising enhancement of self-lubricating properties.

ACKNOWLEDGMENT

The authors would like to acknowledge the financial support provided by the National Nature Science Foundation of China (Grant No. 51371125), Tianjin Research Program of Application Foundation and Advanced Technology (No.11JCZDJC21400).

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