The influence of thermally sprayed coatings microstructure on their mechanical and tribological characteristics
Šárka Houdková1, a, František Zahálka1,b and Michaela Kašparová1,c 1
Škoda Research Ltd., Tylova 57, Plzeň, 300 00, Czech Republic
a
[email protected] , b
[email protected], c
[email protected]
Keywords: coatings, thermal spray, HVOF, tribology, wear resistance.
Abstract. The tribological properties of parts surface, namely their wear resistance and friction properties, are in many cases determining for their proper function. To improve surface properties, it is possible to create hard, wear resistant coatings by thermal spray technologies. Using these versatile coatings it is possible to increase parts lifetime, reliability and safety. The thermally sprayed cermet composite coatings show, thanks to their specific properties, excellent resistance to abrasive and erosive wear, as well as corrosion resistance. To predict the behavior, lifetime and application area of thermally sprayed cermet coatings it is necessary to completely understand the relationships between technology, process parameters, microstructure and properties of the coatings. The finding of these relationships and use this understanding to develop deposits with improved wear resistance for coating of various applications is the main aim of the presented work. It was done by studying the coatings microstructure and mechanical properties. Four different tests of wear resistance were done to study the mechanism of surface degradation, to confirm the results of mechanical testing and to predict the lifetime of coated parts - the abrasive wear performance of the coatings was assessed using a dry/sand rubber wheel test according to ASTM G-65, wet slurry abrasion test according to ASTM G-75, pin-on-disc test according to ASTM G-99 and erosion wear resistance for three impact angles. On the basis of obtained data the new possibilities of coatings application was determined, tested and implemented. Introduction Thermal spraying is an expanding area within the technology of surface engineering. It is a process that involves the deposition of molten or semi-molten droplets of powder onto a substrate to form a coating. In high velocity oxy-fuel (HVOF) thermal spraying, oxygen and fuel gas flow at high pressures and flow rates with internal combustion to produce very high particle velocities with relatively low temperatures compared with other thermal spray process such as air or vacuum plasma spraying [1]. As a result, HVOF has a capability for producing dense coating with low degrees of decomposition which are well bonded to the substrate. A wide range of material can be thermally sprayed for a variety of applications ranging from gas turbine technology to the electronics industry. Applications include protection from wear, high temperatures and chemical attack. Carbide-based cermets including tungsten carbide (WC) and chromium carbide (Cr3C2) are commonly used for wear resistant coatings. Coating performance is strongly dependent on the coating microstructure, which in turn is dependent on the characteristic of the starting powder from which the coating is formed and the spray process parameters employed. Experimental Seven commercially used materials were examined in this study to find out its ability to resist to abrasive wear in different wear conditions. Table 1 shows their chemical composition together with measured basic coatings properties. All coatings were deposited on grit blasted mild steel substrates using HP/HVOF JP-5000® (TAFA) spray equipment in SKODA Research Ltd. Details of the
spraying parameters and procedures employed in the spraying of the coatings can be found elsewhere [2]. Table 1. Coatings properties Coating
Powder Trade Name
Microhardness HV0,1
Hardness HR15N
IFT [MPa.m
WC-Co WC-CoCr WC-Hasteloy Cr3C2-NiCr (Ti)(Mo)(C,N)-NiCo NiCrSiB AISI 316L
FST K-674.23 FST K-647.23 AMPERIT 529.074 1375VM Experimental powder FST M-771.33 FST M-684.33
1240 ±116 1369 ± 114 1167± 109 786 ±123 799 ± 131 735 ± 59 321 ± 31
92.7 ± 1.2 92 ± 1.0 92.8 ± 0.7 91 ± 0.8 91.5 ± 1.5 89 ± 1.6 54 ± 5.4
2.41 ± 0.46 2.37 ± 0.26 0.68 ± 0.17 1.45 ± 0.47 0.5 ± 0.05 0.92 ± 0.18 0.33 ± 0.04
-1/2
]
The microhardness of coatings HV0,1 were measured on coatings cross section, while superficial hardness HR15N on the top of the coatings, using Rockwell cone indenter loaded at 15 kg. The value of indentation fracture toughness (IFT) were determined according to Lawn and Fuller formula [3] from the length of cracks, rising from the corners of Vickers indent made in coatings cross section at a load of 200N. The technique of indentation fracture toughness evaluation is described in detail in [4]. The parameters of wear tests are summarized in Table 2. Table 2. Tests parameters Parameter
Dry abrasion
Wet abrasion
Pin on disc
Erosion
Test standard Abrasive media
ASTM G-65 Al2O3, flow rate 440 g/min
ASTM G-75
-
Test loads Test distance (cycles) Mass loss measurement Number of specimens Specimen dimensions
22 N 1436 m 283 m intervals 3 (76 x 25 x 5) mm
ASTM G-99 Al2O3 ball, Steel ball 6 mm diameter 10 N 50 000 cycles at the end of the test 3 25 mm diam x 5 mm
50wt.%Al2O3with 50wt.%water 20 N 9216 m 2304 intevals 4 (25 x 15 x 10) mm
Casting sand, 1000HV, grain size 0,8 mm 50 m/s impact velocity 5 kg of abrasive media at the end of the test 3 (20 x 15 x 4 ) mm
Results The microstructure studies showed that all cermet coatings have a homogenous structure with regularly dislocated hard particles in the matrix and very low degree of porosity. The particles seem to have a good binding with the surrounding matrix. No evidence of cracks or another discontinuity can be seen. The microstructure of metal NiCrSiB coating has low size of precipitates that also exist in the powder feedstock. Porosity value is under 1%. In some cases, imperfectly unmelted particles are built-in coating and surrounding melted particle can be observed. That has a significant influence on interlaminar bonding strength and mechanical behaviour at wear tests. The CrB, Cr7C3 and Ni3B are ordinary most frequently phases occurring in coating after spraying [5]. However, its volume fraction in coating is in comparison with volume fraction of carbides in cermets substantially lower. The AISI 316L coating has a different shape of individual splat after impact during spraying process. The deflection from the standard shape is caused by the deliberately low temperature during spraying process, that is favorable to reduce the oxidation of sprayed particles. Nevertheless, this shape has a negligible influence on the mechanical properties of the coating. For this type of coating, no evidence of hard phases in microstructure can be seen. The results of wear tests are summarized in the graphs in Fig. 1 - 3. From them it can be seen, that the highest wear resistance under all types of loading succeed WC-based cermets coatings, followed by the other two cermet coatings. The metal coatings reached distinctively lower results, which is in agreement with their lower hardness.
NiCrBSi
3
Cumulative volume loss [mm ]
25,0000
20,0000 316 L
15,0000
10,0000
5,0000
3
Cr C2-NiCr
S 49 WC-Co WC-Hasteloy WC-CoCr
0,0000 0
200
400
600
800
1000
1200
1400
Abrasive distance [m]
Fig.1. The results of dry sand rubber wheel abrasion test
50,0000 NiCrSiB
AISI 316L 40,0000
30,0000
20,0000 Cr3C2-NiCr 10,0000
S 49 WC-Co WC-CoCr
0,0000 0
1000
2000
3000
4000
5000
6000
7000
WC-Hasteloy 8000 9000
10000
Abrasive distance [m]
Fig. 2. The results of slurry abrasion test 3,5 3 3
impact angle 15° impact angle 45° impact angle 90°
2,5 2 1,5 1 0,5
Fig. 3. The results of erosion test
AISI 316L
NiCrSiB
(Ti,Mo)(C,N)NiCo
Cr3C2-NiCr
WC-Hasteloy
WC-CoCr
0
WC-Co
Total volume loss [mm ]
Cumulative volume loss [mm3]
60,0000
The results of pin on disc test were not confirmative. The volume loss for WC based cermet coatings were to small too be measured by available equipment (profilometer HomelTester 1000). The other cermet coatings wear resistance was also higher comparing to metal coatings, especially AISI 316L coating showed a very poor resistance against two body abrasion. The values of coefficient of friction measured by pin on disc test are summarized in Table 3. Table 3. Coefficient of friction Coating
CoF for Al2O3 ball
CoF for steel ball
WC-Co
0.369 ± 0.54 0.398 ± 0.009 0.382 ± 0.011 0.549 ± 0.008 0.653 ± 0.016 0.645 ± 0.026 0.644 ± 0.021
0.801 ± 0.002 0.820 ± 0.023 0.862 ± 0.006 0.852 ± 0.049 0.765 ± 0.052 0.759 ± 0.108 0.808 ± 0.127
WC-CoCr WC-Hasteloy Cr3C2-NiCr (Ti)(Mo)(C,N)-NiCo NiCrSiB AISI 316L
In the case of alumina ball the coefficient of friction is lower for WC-based coatings. In the case of steel ball, the values of the friction coefficient are similar for all measured coatings and are for all coatings higher compared to alumina ball. It could be caused by a transfer of steel ball material on the coatings surface. The mechanism of wear was studied for all provided tests by SEM evaluation. Generally the gradual loss of metal matrix followed by decrease of carbide-matrix cohesion and pull-off the carbides was observed for cermet coatings. The main mechanism of metal coatings wear was established a repetitious plastic deformation that led to fatigue cracks initiation, spreading and resulting delamination of coatings. Summary The contribution includes the results of wear tests and mechanical properties measurements of seven selected HVOF coatings, designed for wear protection of coated parts. It was proved, that the WC-based cermet coatings have superior properties and are the most resistant coatings for all measured types of wear. The other evaluated cermet coatings shown also very good wear resistance and together with metal NiCrSiB coating is suitable for application in wear condition. On the other hand, AISI 316L coating does not have high wear resistivity and his application can be recommended only for less severe conditions. Acknowledgement: The paper was prepared thanks to financial support of project COST OC 532.002. References [1] E. B Smith, T.J. Power, T. J. Barber: United Technology Research Centre Report, East Hartford, CT, (1991), p.91 [2]
F. Zahalka, R. Enzl, S. Houdkova, O. Blahova: 12th International Symposium on Metallography, 28th - 30th April, Stará lesná, (2004), ISSN 1335-1532
[3]
B. R. Lawn, E. R. Fuller: J. Mater. Sci., 10, (1975), p. 2016
[4]
C. B. Ponton, R. D.Rawlings, Mat. Science and Technol.,Vol. 5., (1989), p.113
[5]
J.M. Miguel, J.M. Guilemany, S.Vizcaino: Tribology International 36 (2003), p. 181
