· ·118

Microstructure and wear-resistance performanceof WC/Cu-Ni-Mn composite coatings

(State Key Laboratory of Materials Processing and Die & Mould

Technology, Huazhong University of Science and Technology, Wuhan

430074, China)

Abstract: The Cu-Ni-Mn alloy matrix coatings reinforced by WC particles

(WC/Cu-Ni-Mn) are deposited on a steel substrate by a manual oxy-

acetylene weld. Microstructure and wear resistance performance of the

fabricated coatings are investigated. There are no cracks or other defects

observed in the hardfacing coating. Uniformly distributed WC particles

in the composite hardfacing coating are not dissolved, and its volume

fraction is up to about 63%. A sound interfacial junction form between

the WC particles and the Cu-Ni-Mn alloy metal matrix binder. Under three-

body abrasion testing with silica sands, the coatings shows about 4 times

better wear resistance performance than the high-Cr cast iron. The main

wear mechanisms are the plastic extrusion of the Cu-Ni-Mn matrix and the

fracturing of WC-reinforcement particles under three-body abrasive wear.

The wear behavior of the coatings also is investigated. At room temperature,

the sliding wear-resistances of the as-deposited WC/Cu-Ni-Mn coatings is

1.83 higher than the commercial Fe-Cr-C hardfacing coating which is mainly

ascribed to the high volume fraction and uniform distribution of WC in the

Cu-Ni-Mn metal matrix. At 350 °C in air, the relative wear-resistance values

is 1.90, owing to the good thermal stability of WC. The main sliding wear

mechanisms of the coatings are adhesion and abrasion. Besides，the wear

also is influenced by metal matrix softening at 350 °C.

Key words: Cu-Ni-Mn matrix; WC/Cu-Ni-Mn coatings; microstructure; wear

resistance

DOI: 10.7512/j.issn.1001-2303.2017.13.08

Prof. Chibin Gui Email: cbgui@sohu.com

Professor, works in college of material science and engineering, huazhong university of science and technology. He is engaged in welding physical and chemistry metallurgy research for a long time. He wrote many books about welding physical and chemistry metallurgy such as "Toughness and Toughening of Weld Joint in High Strength Steel Hull Structure".

0 IntroductionAs for engineering machines and components, there is an ever

increasing demand for wear resistant materials, which can reduce wear

and thus extend their service life. Hardfacing process is an effective way

to improve wear resistance performance by applying a wear resistant

coating on the surface of softer and tougher inner materials [1-3]. Such

composite coatings, commonly known as particle-reinforced metal-

matrix composites (PRMMCs), can significantly improve the tribological

properties of components by employing a metallic alloy as the matrix

and the ceramic particles as reinforcements. In addition, compared with

fiber reinforced metal-matrix composites (MMCs), PRMMCs can also be

Xuewei Meng, Weisheng Xia, Shuai Yang, Jun Liu, Chibin Gui

fabricated with low cost and obtain nearly isotropic properties. Hence, it

has been widely studied and applied [2, 4-7].

The wear-resistance performance of PRMMCs is mainly determined

by the based metal matrix and the reinforcement particles. Generally

speaking, in order to guarantee the good wear-resistance performance

of PRMMCs, the selection of reinforcement particles and the

metal matrix should follow the next three basic principles. Firstly,

reinforcement particles, mostly engineering ceramics, should have some

good properties, including high hardness, general chemical inertness,

excellent wear-resistance and the ability to work in severe thermal

conditions [8]. Secondly, the metal matrix binder should have the certain

· ·119

strength and good toughness, which can provide reliable supports for

reinforcement particles. Thirdly, reinforcement particles and the metal

matrix should have good compatibility (or wetting characteristics)

with each other, otherwise, some defects may form in the interfacial

junction, which will lead to the initiation and extension of cracks, even

the segregation of reinforcement particles and the metal matrix [9].

Tungsten carbide particles are often selected as the reinforcement

particles to fabricate PRMMCs which have been utilized extensively for

numerous wear-resistant applications [10-13]. Cu-Ni-Mn alloys have been

studied due to their excellent mechanical, conductivity and chemical

properties, as well as good corrosion resistance [14-17]. Furthermore, WC

particles and Cu-Ni-Mn ternary alloys have excellent wettability with

each other. Therefore, Cu-Ni-Mn alloy matrix composites reinforced

by WC particles are promising candidate to develop wear resistant

material.

In this paper, a Cu-Ni-Mn hardfacing coating reinforced with

WC particles is deposited on steel substrates by a manual oxy-

acetylene welding. Its microstructure and wear behaviors under

three-body abrasive wear condition as well as sliding wear condition

are investigated. The sliding wear condition test include both room

temperature and 350 °C, with which the high temperature wear

behaviors can investigated. The sliding wear mechanisms of WC/Cu-Ni-

Mn composite coatings under different conditions are also investigated.

1 Experimental details1.1 Materials and hardfacing welding

The morphology of the cast WC particles is shown in Fig. 1, and it

is the typical irregular appearance of angular shape. Its size is 100~150

mesh. Before pouring the powder into copper tubes, the initial mixture

powder is mixed for 8h by ball-milling, and then baked at 150 °C for 2h

to ensure the dry powders. Table 1 lists the chemical compositions of the

steel substrate and the comparative material.

Table 1 Chemical compositions of the steel substrate and

high-Cr cast iron (wt.%)

and eutectic (γ-Fe+(Cr, Fe)23C6). A large number of rod-like primary

M7C3 carbides uniformly distribute in the metal matrix with the volume

fraction of about 33%.

1.2 Microstructure characterization and hardness testMicrostructures are observed by standard optical microscopy

(OM, AxioCam ERc 5s, Carl Zeiss, Germany) and scanning electron

C Si Mn P S Cr Mo Ni Fe

steel substrate 0.203 0.260 1.39 0.012 0 0.00029 0.0454 <0.0020 0.0179 balance

high-Cr cast iron 2.96 0.779 0.858 0.0359 0.313 21.05 1.21 0.484 balance

Under 3-body abrasive wear condition, a conventional high-Cr cast

iron (KmTBCr20Mo, supplied by Zaoyang Qinhong New Materials

Co. Ltd., China) is taken as the comparative material. Fig. 2 shows the

optical micrograph of the comparative high-Cr cast iron, which is mainly

composed of blade like primary M7C3 carbides and eutectic (g-Fe þ

M23C6) structure. The volume fraction of primary M7C3 carbides is about

21%.

A commercial Fe-Cr-C hardfacing coating is taken as the

comparative sample in the sliding test. Its top-surface micrograph is

shown in Fig. 3. It is mainly composed of primary (Cr, Fe)7C3 carbides

Fig.1 SEM photograph of the WC particles

Fig.2 Optical micrograph of the comparative high-Cr cast iron

Fig.3 Microstructure of the commercial Fe-Cr-C hardfacing coating

on the top surface

· ·120

microscopy (SEM, Nova NanoSEM 450, FEI, Japan) with energy

dispersive spectrometer (EDS). All specimens are etched with

hydrochloric acid solution of ferric chloride. The volume fractions of

tungsten carbide particles on the top surface of hardfacing layers are

calculated based on Image Pro-Plus software. Microhardness is tested

by a MH-5 microhardness tester (the load of 200 g and the dwelling

time of 15 s). Ten readings are taken for each sample and an average

is calculated as the final hardness. It is hard to measure the accurate

bulk hardness of the WC-reinforced Cu-Ni-Mn metal matrix composite

because of the big hardness gap between WC particles and the Cu-Ni-

Mn metal matrix and the relative large dimension of WC particles used

in this study.

Table 2 Microhardness (HV0.2) results

Specimens Overall hardness Matrix Carbide

Cu-Ni-Mn 114 ---- ----WC/ Cu-Ni-Mn ---- 125 2436high-Cr cast iron 823 740 1434Fe-Cr-C coating 670 ---- ----

1.3 Wear tests by three-body abrasionHigh-stress abrasion wear tests are carried out by a MMH-5 type

ring-on-block three-body abrasion testing machine (Jinan Zhongyi

Instrument Co. Ltd., China). As shown in Fig. 4, the rotating arms rotate

clockwise around the rotation axes and drive two specimens to slide

on the table roller together (the outside diameter d3=380 mm and the

inside diameter d4=340 mm). Fig. 5 shows the shape and dimensions

of three-body abrasive wear test specimen. The bevel edge of the

specimen can make sure the silica sands entering into the interface of

the specimen and the table roller during the test. The wear tests are

conducted at room temperature and normal atmosphere conditions.

Prior to the wear test, each sample has been exposed to 800 mesh

sandpaper (the final average roughness is less than 0.1 mm) in order to

ensure that the wear surface of tested samples can touch the abrasive

surface completely. The nominal load is 8 kg and the corresponding

contact pressure is about 0.13 MPa. The rotation speed is kept constant

at 30 rpm, and each sample is tested for 6 h. The relative velocity of the

disc with the sample is 50.9mm/s, so the sliding distance is 1098m.

Nominalload

F Nominalload

F

Specimen SpecimenTest chamber

Table roller Specimen

Nominalload/F

Specimen

Sliding direction

Silica sands

Table roller

Fig.4 Schematics of MMH-5 type ring on block wear tester

30 m

m

15 mm

24 mm

Wear surface45°

Fig.5 Three-body abrasive wear test specimen: (a) dimensions of wear test specimen, (b) wear test specimens of the composite

(a) (b)

The abrasive particle employed is 100~160 mesh silica sands (150~

250 mm, Hs=1123 HV).

Three specimens are taken from each coating and tested separately.

The weight loss is obtained by weighting the specimen before and

after tests by an electronic scale with the accuracy of 0.01g. Two

rotating sand-scraping plates can push silica sands from the middle

place of the chamber to the area around the table roller. Replace the

silica sands every hour, in the meantime, the specimen is unloaded,

washed, cleaned, dried and weighed. The average of weight losses is

calculated and recorded to determine the volume loss. The densities

of tested samples are measured by the Archimedes principle of water

displacement. The worn surfaces are analyzed by SEM to determine the

wear mechanisms. The wear volume loss of each sample is calculated

based on the following formula:

(1)

Where m and ρ are the weight loss and density of the tested

specimen.

1.4 Sliding wear testThe tribological tests are carried out on a pin-on-disc sliding wear

V = mρ

· ·121

tester (MG-2000B, produced by Zhangjiakou integrity test equipment

manufacturing Co. Ltd., China). As schematically shown in Fig. 6, two

cylindrical pin-like specimens (6 mm in diameter and 25 mm in length)

slide on the top surface of a rotating disc-counter made of quenched

GCr15 steels with the hardness of HRC63.

Three specimens are taken from each coating and tested separately.

The sliding wear tests are conducted at 350 °C. Prior to the wear test,

the contacting surfaces of the disc and the pins are both ground and

polished by SiC grinding papers with the grit sizes up to 1200 grit (the

final average surface roughness is less than 800 nm) in order to ensure

that the wear surface of tested samples can touch the abrasive surface

completely. And then the pin-like specimens and the counterpart disc

are cleaned with acetone and dried in hot air. The main test parameters

include: the load of 270 N, the rotation speed of 300 rpm and the test

time of 5 min.

The average of weight loss is calculated and recorded to determine

the volume loss. The densities of tested samples are measured based

on the Archimedes principle of water displacement. The volume loss is

calculated based on the formula 1. The worn surfaces and sliding wear

debris are analyzed to determine the wear mechanisms.

1.5 Microstructure of coatingFig. 7 illustrates the optical micrographs of the Cu-Ni-Mn binder

and the composite hardfacing coating. The Cu-Ni-Mn matrix binder

in Fig. 7(a) shows a typical dendritic structure without cracks or other

defects. It can be observed in Fig. 7(b) that no concentration of WC

particles happens and they distribute uniformly throughout the

coating. Furthermore, WC particles retain their original typical irregular

appearance of angular shape, and its volume fraction is about 63%.

SEM images of the composite hardfacing coating are shown in

Fig. 8. The continuous and reliable bond between the substrate and

the composite hardfacing coating can be observed in Fig. 8(a), where

no separation or cracks are presented. Fig. 8(b) shows the interface

Ratation axesPin-like specimens

Electricfurnace

Specimenholder

Counterpart disc

Load

Fig.6 Schematic of the pin-on-disc sliding wear tester

Fig.7　Microstructure of (a) Cu-Ni-Mn binder material and (b) the

hardfacing composite coating

(b)

(a)

between the Cu-Ni-Mn alloy matrix and WC particles. The interface

between WC particles and Cu-Ni-Mn matrix is smooth, and there are no

evidences of interfacial delamination, debonding or other defects, which

are harmful to the mechanical properties of the composite coating. The

low stress concentration at the interface between WC particles and

the Cu-Ni-Mn matrix is the main reason to obtain the strong interfacial

bond between the reinforced particles and the metal matrix. This means

that WC particles and Cu-Ni-Mn matrix binder have good compatibility

and excellent wetting characteristics with each other.

The good wetting is essential for the generation of a sound bond

between the reinforcements and the liquid Cu-Ni-Mn metal matrix

during welding process. This also allows the transfer and distribution of

load from the matrix to the reinforcements without failure. Therefore,

cracks are less likely to initiate from the WC/Cu-Ni-Mn alloy interface

and then propagate through the composite coating during the sliding

wear.

1.6 Wear test results and analysisThe volume loss of high-Cr cast iron and the composite hardfacing

coating as a function of sliding distance is given in Fig. 9. The volume

loss increases with the increasing of the sliding distance, which shows an

approximately linear tendency. The WC particles reinforced composite

coating possesses markedly better wear resistance performance, about

· ·122

materials. An abrasive particle can be called “hard”, if its hardness

is about 1.2 times greater than that of the target material, then the

abrasive particles can indent into the target material [20]. According to

Table 3, the two reinforced particles, WC particles and primary M7C3

carbides, can offer protection to their own matrices. So the WC particles

shows much better wear resistance performance than the primary

M7C3 carbides against the indentation or cutting of silica.

(b) the interface between the Cu-Ni-Mn matrix and WC particles

(a) the interface between the composite coating and the substrate

Fig.8 SEM images of the hardfacing composite coating

Fig.9 Effect of sliding distance on the wear rate of materials

Table 3 Microhardness (HV0.2) and hardness ratios (Hs/Hm, Hs/Hc)

Material Matrix (Hm) Carbides (HC) Hs/Hm Hs/HC

High-Cr cast iron 740 1434 1.52 0.78

WC/Cu-Ni-Mn 124 2436 8.98 0.46

Besides the hardness, the mean free path, which is the inter-

space between the reinforcements, also plays an important role in the

abrasion wear resistance of PRMMCs. Quantity and uniform distribution

of reinforcements have a beneficial effect on reducing wear [18, 21-23]. If

the reinforcements distribute uniformly in the PRMMC, the mean free

path between the reinforcements depends on both the reinforcement

particle size d and the volume fraction f :

(2)

Where d is the size of reinforcement particles, and f is the volume

fraction of reinforcement particles.

The length of the primary M7C3 carbides (in Fig. 2) in the high-Cr

cast iron is much bigger than that of the WC particles in the composite

coatings. The blade-like primary M7C3 carbides in the high-Cr cast iron

distribute non-uniformly, while the composite coatings have a higher

volume fraction of WC particles. Therefore, it can be concluded that the

composite hardfacing coating has smaller mean free path between the

reinforcement particles than that of the high-Cr cast iron.

Fig. 10 indicates the worn surface of high-Cr cast iron and the WC-

reinforced composite coating after the 6 h test. The continuous grooves

in high-Cr cast iron are apparent in Fig. 10(a), which are running parallel

to the sliding direction. On the contrast, no continuous grooves are

observed in the worn surface of the WC-reinforced composite coating

in Fig. 10(b), which indicates that there is no significant penetration of

the silica particles into the matrix. Fig. 11 is higher-magnification SEM

images of the worn surfaces. Obvious traces of ductile flow of the

matrix can be observed in the worn surface of two kinds of materials

caused by silica sands. It is worth noting that the ductile flow of the

matrix in high-Cr cast iron is more notable than that of WC-reinforced

composite coatings.

In addition, the overall wear behaviors of both composites

appear to be further associated with the delamination/detachment

of reinforcement particles, which is often reported as the major

mechanism in PRMMCs. The wear mechanism is the presence of

cracking and fracturing of reinforcement particles which can be

observed in both two kinds of materials (Fig. 11).

The reason for this wear behavior can be summarized: once a

portion of the metal matrix binder is extruded from the composite,

λ ∝ df√

4 times more resistant against wear than that of the high-Cr cast iron

after the 6 h test under three-body abrasive wear condition.

It is well known that the hardness of materials has a significant

influence on their wear resistance performance. In general, high

hardness is helpful to improve the abrasive wear of materials [18-19].

Generally speaking, the magnitude of abrasive wear is mainly depends

on the relative hardness between the abrasives particle and the target

· ·123

the silica sands are able to attack the sides of the carbide grains

continuously, and the carbides will be exposed to shear loads and then

result in their fracture/detachment.

1.7 Sliding wear test results and analysisTable 4 summarizes the sliding wear test results. WC/Cu-Ni-

Mn coating shows better sliding wear-resistance than that of the

commercial Fe-Cr-C hardfacing coating. These results indicate that the

addition of WC particles results in significant improvement in the sliding

wear-resistance for Cu-Ni-Mn matrix alloy. The volume loss of Cu-Ni-

Mn coating is unmeasurable, because the plastically deformed metal

is pushed and adhered to the edge of the specimen instead of being

separated from the specimen because of its low hardness (in Fig. 12).

Figure 13 shows the worn surfaces of Cu-Ni-Mn coatings at room

temperature and 350 °C respectively. Plenty of adhesion scars are

observed on worn surfaces, and deep grooves are parallel to the sliding

direction due to abrasion and ploughing. This kind of severe abrasion

Fig.10 Low-magnification SEM photographs of worn surfaces

Fig.11 High-magnification SEM photographs of worn surfaces

(a) high-Cr cast iron (b) tungsten carbide reinforced composite

(a) high-Cr cast iron (b) tungsten carbide reinforced composite

Table 4　Wear volume loss and the relative wear-resistance

after the test of 5 min

Temperature Cu-Ni-Mn WC/Cu-Ni-Mn Fe-Cr-C

Wear volumeloss/×106 μm3

room temperature ---- 87.10 159.40350 °C ---- 92.85 176.74

Relative wear-resistance/1

room temperature ---- 1.83 1350 °C ---- 1.90 1

Fig.12　Worn surface of the Cu-Ni-Mn after the test of 5 min at

room temperature

· ·124

Fig.13　SEM morphologies of the worn surface of the Cu-Ni-Mn alloy coatings

(a) room temperature (b) 350 °C

and ploughing is caused by the protruding hard asperities of the

counter-disc and the accumulation of the wear debris mainly formed

during the running-in period [24]. It is obvious that the high contact stress

during sliding test leads to the heavily plastic deformation of metal

matrix, which results in the delamination and fracture of Cu-Ni-Mn

coatings.

Figure 14 shows the worn surfaces of WC/Cu-Ni-Mn coatings

at room temperature and 350 °C respectively. The WC/Cu-Ni-Mn

coatings show similar worn surface feature.Additionally, ploughing

grooves and a large amount of flaking pits can be observed in both of

the worn surfaces with different temperature. It is the direct metal-to-

metal contact between the coatings and the counter-disc that cause the

formation of adhesion. Therefore, the dominant wear mechanisms of

WC/Cu-Ni-Mn coatings are also severe abrasion and adhesion.

To sum up, the sliding wear performances at room temperature

are similar to 350 °C. The sliding wear mechanisms of all the specimens

include abrasive wear and adhesion wear. Different with the specimens

tested at room temperature, all of the specimens suffered oxidation

during the sliding wear test at 350 °C. The wear resistance ordered

from highest priority to lowest is WC/Cu-Ni-Mn, Fe-Cr-C. In general, the

Fig.14 SEM morphologies of the worn surface of WC/Cu-Ni-Mn

(a) room temperature (b) 350 °C

excellent wear-resistance performance results from the high volume

fraction of carbides and the good toughness of the matrix. Therefore,

the hard WC particles can significantly improve the sliding wear-

resistance of the Cu-Ni-Mn alloy.

The WC/Cu-Ni-Mn shows better sliding wear resistance than Fe-

Cr-C harfacing coating for possessing more and harder reinforcements.

It is worth pointing that the volume of all specimens loss are increase

from room temperature to 350°C but WC/Cu-Ni-Mn increase less than

Fe-Cr-C coating. The reason is WC possess high thermal stability which

can effective against the thermo-softening.

2 Discussion To sum up, the reasons why the composite coating can obtain good

wear resistance performance in this study are as follows. Firstly, WC

particles are worn preferentially and protect the soft metal matrix from

being extruded. Secondly, the good bond between WC particles and

the matrix binder is essential because the particles are hard to be pulled

out from the matrix when the binder around them are extruding away

gradually. Thirdly, high carbide volume fraction of WC particles in the

composite coating makes it hard for abrasive silica sands to indent into

· ·125

the matrix and extrude the binder away due to the small mean free

path. Finally, WC particles provide good protection for the metal matrix,

and the metal matrix with good toughness can accordingly keep WC

particles from being dug out and fractured by abrasive particles.

Therefore, it is the comprehensive results of “protect function” of

the WC particles and “support function” of the Cu-Ni-Mn metal matrix

that makes the WC-reinforced hardfacing coating obtain good wear

resistance performance.

3 Conclusions(1) WC particles are distributed uniformly in the Cu-Ni-Mn matrix

alloy. There is no delamination or other defects observed at the interface

between the WC particles and the Cu-Ni-Mn matrix. A sound bond

is formed between the composite hardfacing coating and the steel

substrate. WC particles are not dissolved in the composite hardfacing

coating, which is beneficial to improve the final wear resistance

performance.

(2) The major wear mechanisms of 3-body wear are the plastic

deformation and ductile flow of the Cu-Ni-Mn matrix, and the

fracturing of WC particles.

(3) The coating is characterized by adhesive and abrasive wear as it

sliding against the GCr 15 steel counter disc. Besides, it also experienced

the negative softening.

(4) The WC/Cu-Ni-Mn shows good wear resistance and higher

relative wear resistance at high temperature. The reason is WC possess

high thermal stability which can effective against the thermo-softening.

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