effect of co content on the properties of hvof sprayed coatings based on tungsten...

23. - 25. 5. 2012, Brno, Czech Republic, EU

EFFECT OF CO CONTENT ON THE PROPERTIES OF HVOF SPRAYED COATINGS BASED

ON TUNGSTEN CARBIDE.

Jan SCHUBERT a, Šárka HOUDKOVÁ b, Michaela KAŠPAROVÁc, Zdeněk ČESÁNEKd

a VZÚ Plzeň s.r.o., Tylova 46, 301 00 Plzeň, Czech republic, [email protected]

b NTC ZČU, Univerzitní 22, 306 14 Plzeň, Czech Republic, [email protected]

c VZÚ Plzeň s.r.o., Tylova 46, 301 00 Plzeň, Czech Republic, [email protected]

d ZČU Plzeň, Univerzitní 8, 306 14 Plzeň, Czech Republic, [email protected]

Abstract

Thermal sprayed coatings based on tungsten carbide are the most durable materials in terms of wear

resistance. Although they are not suitable for high temperature applications, they can be applied in many

areas of industry due to the combination of very hard carbides and tough matrix. The good wettability of

carbides WC in Co matrix contributes to the high cohesive strength of WC-Co cermets. The hardness and

toughness rate of WC-Co coatings is, in the case of thermal sprayed coatings, determined by mutual

proportion of carbide phase and matrix, and by spraying parameters. Depending on the application, either

hardness of coating (abrasive wear) or higher level of toughness (erosive wear) can be preferred for different

types of wear.

The presented study was conducted to determine the effect of cobalt matrix content on the resulting coating

mechanical properties. Samples were prepared using the HVOF (high velocity oxy-fuel) spraying equipment

HV-50. Three powder types with different cobalt matrix content, namely WC-12%Co, WC-17%Co and WC-

25%Co, were used. The basic mechanical properties were evaluated for each of Co content. Firstly, special

attention was dedicated to the abrasive wear resistance evaluated by Dry Sand Rubber Wheel test

performed according to ASTM G-65 [1]. Secondly, the paper focuses on the sliding friction coating properties

using the pin-on-disc test according to ASTM G-99 [2] and, finally, the evaluation of coatings potential

application with higher matrix proportion was investigated.

Key words: (coating, wear resistance, HVOF, mechanical properties, cobalt matrix content)

1. INTRODUCTION

WC-Co cermet surface coatings are used to enhance the wear resistance of many types of engineering

components. The common practice how to deposit the WC-Co surface coatings is thermal spraying; typically

high velocity oxy-fuel (HVOF) spraying. HVOF is one of the leading thermal spray techniques [3, 4]. It allows

the fabrication of variety of coatings characterized by low or intermediate melting point (mainly metals and

polymers). The main advantage of HVOF, compared to other thermal spray techniques, is the ability to

accelerate the melted powder particles of the feedstock material at relatively high velocity. High velocity

obtained using HVOF provides the designed thick-formed components a fairly dense microstructure. In

addition, the lower temperature regimes in HVOF, compared to plasma spraying technique, ensure less

decomposition of WC [5]. This statement does not mean that HVOF is the best processing solution. Indeed,

the disadvantage of HVOF coatings, compared to sintered WC–Co, is decarburization and decomposition as

shown by several authors. These phenomena support the formation of undesirable phases such as W2C, W

and Co–W–C [6]. Based on Pin-On-Disc experimental results, cermet-based coatings form the smooth and

compact tribofilm by local plastic deformation, which gives these materials similar performance as Cr2O3 and


superior properties than Al2O3–TiO2 coatings. They key parameter for the best performance of cermet-based

coatings is related to the stability of the tribofilm, which is usually brittle. If the coating process conditions are

not well optimized, critical contact pressure would enhance tribofilm detachment, which in turn provokes

severe wear. [6].

This article presents research of three WC-Co cermets with different binder phase contents: 12, 17 and 25

wt.% Co. HVOF deposited coatings of these materials were examined for the effect of binder phase content

on mechanical and wear properties using Dry sand rubber wheel test and Pin-on-disc test.

2. EXPERIMENTAL

The aim of this experiment was to determine the effect of binder content changing (cobalt) on the resulting

coating properties. Three variants of WC-Co were selected with the binder content of 12%, 17% and 25%.

They were sprayed by HP/HVOF JP-5000® (TAFA) spraying technology in the VZÚ Plzeň s.r.o., using the

standard preparation procedure on the grit blasted substrate of carbon steel (ČSN 11 523) and by the

previously optimized spraying parameters for WC-12%Co, WC-17%Co and WC-25%Co. Grain size of

powders ranged from 15 to 45 micrometers. Such prepared samples were examined for their mechanical

properties and ware resistance. Microstructure of the coatings is shown in Fig. 1. There is recognizable the

low porosity and good bonding between coating and substrate.

a) b) c)

a) b) c)

Fig. 1 Microstructure of the coatings: a) WC-12%Co, b) WC-17%Co, c) WC-25%Co

Samples were measured in two laboratories; VZÚ Plzen s.r.o and New Technology Research Centre of the

University of West Bohemia in Pilsen (ZČU NTC). Parameter like deposition efficiency, mechanical

properties like roughness, micro-hardness HV0,3 (results are from 7 measurements) and hardness HR15N


(results are from 5 measurements) were measured in VZÚ Plzeň. Finally, the abrasion resistance was

measured using test Dry sand rubber wheel, which proceeded in accordance with ASTM G-65 [1]. All profiles

after abrasion tests and also tests of ware resistance using Pin-on-disc testing which took place according to

ASTM G-99 [2] were performed in laboratories of NTC ZČU.

3. RESULTS AND DISSCUSION

The deposition efficiency was one of the first observed values of the WC-Co samples and was determined

by measurements made on the cross-section of samples. Specific values of these measurements are shown

in Table 1.

3.1. Mechanical properties

Basic mechanical properties of WC-Co layers were measured on all samples and their average values are

given in Tab. 1. The most important values were HV0, 3 and HR15N, which should have the greatest

influence on the abrasion resistance further investigated.

Table 1 Mechanical properties of WC-Co coatings

Material Deposition rate (Thickness per

pass [μm])

Roughness [Ra]

Thickness [μm]

HV0,3 HR15N

WC-12%Co 56,5 3,79 ± 0,473 509 ± 9,67 1052 ± 148,42 86,2 ± 3,14

WC-17%Co 44,125 2,88 ± 0,144 353 ± 17,43 1148 ± 61,69 92,2 ± 2,38

WC-25%Co 52,1 3,08 ± 0,33 521 ± 11,88 1009 ± 74,25 92 ± 1,61

The measurements clearly imply that the content of cobalt binder does not have the significant influence on

the resulting macro-hardness of the prepared coating and deviations are within the limits of tolerance.

Hardness increases with the decrease of porosity level, which may be caused by the higher content of binder

material [6]. Surprisingly, slightly higher micro-hardness was proved in the sample with 17% cobalt binder,

which has already been observed in a study [7], where the increase of binder to 17% caused also the slight

increase in micro-hardness values. However, this increase does not exceed values of tolerance and stays

generally within the range of values commonly reported by other authors [6].

3.2. ASTM G-65 Dry Sand Rubber Wheel test

All samples were tested using the Dry Sand Rubber Wheel test according to the ASTM G-65 [1]. These tests

were performed in the VZÚ Plzeň s.r.o. Test parameters are listed further. Sand Al2O3 as an abrasive

material. Contact force was 22N, speed of wheel was 180 rev/min and the total distance of measuring was

718m. The test was performed on unpolished samples with surface roughness listed in Table 2.

The Fig. 3 shows the standard profile of the sample surface after the test G-65. Measurements were

performed on the device HIROX in the New Technology Research Centre of the University of West Bohemia

in Pilsen (NTC ZČU). The measurement results are shown in Figure 4, which are plotted on a graph.

The resulting values clearly show that the increasing binder content progressively reduces abrasive wear

resistance of this type. The difference between 12% and 17% is negligible but, in the difference between

17% and 25%, it is possible to see the trend of decreasing abrasion resistance depending on the content of

cobalt binder, which corresponds to other papers [8, 9]. This phenomenon can be explained as the result of

increased proportion of hard WC particles within the coating [5, 8, 10].


Fig. 3 Standard profile of specimens after G-65 testing. (WC-17%Co)

3.3. ASTM G-99 PIN-ON-DISC TEST

Pin on disc test was performed in the laboratory NTC ZČU and conducted according to ASTM G-99 [2]. All

samples were treated under the same conditions and the resulting values are the average of several

measurements. Measurements were processed under the following conditions. Linear sliding speed was

10cms-1

, normal load was 10N, ball diameter was 6mm and samples were always purified using ethanol. The

sliding distance was kept constant at 50 000

cycles for all the tests. The environmental

conditions of relative humidity and temperature

Hr= 15-20% and 20°C, respectively, were held

constant during the test.

Fig. 5 shows photos of ware track and its profile

taken by optical microscope HIROX (in this

case are shown ware tracks on a sample of

WC-17% Co). The results of the depth

measuring of the ware track are shown in Table

3. Complete results of friction tests of pin on

disc are recorded in the chart and displayed in

Figure 6.

Fig. 4 Results of ASTM G-65 testing shown in graph

The values (listed in Table 3) using Al2O3 balls exhibit the gradual reduction of wear of the coating

(expressed as depth of wear track) with the increasing content of cobalt binder. The values measured after

using steel balls are not relevant; because the method is not able to determine the true depth of the wear

track. The use of steel ball has following disadvantage: adhesive bonding of the steel ball to the coating

creates on the sides of ware track deposit that changes the relief of the wear track and thus affects the real

depth of the track [5].

The graph in Figure 6 clearly shows that the cobalt binder content does not significantly affect the resulting

value of friction while using Al2O3 ball or steel ball. All measurements exhibit the same or very similar

progress of the test procedure with only minor variations. Occasional deviations when using Al2O3 ball can

be caused by the collapse of tribofilm, which is formed in the contact layer [5, 6].

Table 3 Wear track depth after Pin on disc testing

Coating Al2O3 ball [μm] Steel ball [μm]

WC-12%Co 2531 6004

WC-17%Co 2411 8113

WC-25%Co 2267 2499


a) b)

Fig. 5 The standard display of ware track after the pin on disc test and its profile: a) WC-17% Co - Al2O3

ball, WC-17% Co - steel ball

Fig. 6 Summary chart of pin on disc test results.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 10000 20000 30000 40000 50000

Fric

tio

n

Laps

WC-12%Co - Al2O3

WC-17%Co - Al2O3

WC-25%Co - Al2O3

WC-25%Co - steel

WC-17%Co - steel

WC-12%Co - steel


4. CONCLUSIONS

1) The measured increase in abrasive wear (Sand rubber wheel test) with increasing content of cobalt binder

is likely to be connected with the differences between macro and micro hardness of WC-Co coatings. While

the macro hardness remains almost identical, the micro-hardness is gradually reduced. This affects the

resulting abrasion resistance against Al2O3 grains that can easily tear the individual carbides out of binder

material. Abrasion resistance increases with decreasing binder content [8, 9]. This phenomenon can be

explained as the result of an increased proportion of hard WC particles within the coating [5, 8, 10].

2) Pin on disc test for determining the wear resistance showed that the WC-Co coatings are less dependent

on the cobalt binder content. It was proved that an increase in the binder material can decrease the depth

of wear tracks, which was the result of using AL2O3 ball. The fact that friction is not affected by the content

of binder material is an interesting result, which is probably caused mainly by two factors. The micro-

hardness, which progressively decreases with increasing binder content, does not play a major role in this

test. The macro-hardness, which is for all coatings almost identical, plays the main role in the wear

resistance of coating. This also corresponds exactly to the graph in Figure 6. The second factor is the

formation of tribofilm on the surface between coating and ball, which stabilizes the friction [6, 9, 11].

ACKOWLEDGEMENT

This paper was prepared thanks to the project of No. TA02010486.

REFERENCES

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[2] ASTM Standard G99 - 05, 2010, " Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus," ASTM

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effect of co content on the properties of hvof sprayed coatings based on tungsten...

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