balkantrib'14 proceedings

BALKANTRIB´14

21 YEARS OF BALKAN TRIBOLOGICAL ASSOCIATION

ROMANIAN TRIBOLOGY ASSOCIATION

PETROLEUM-GAS UNIVERSITY OF PLOIESTI, ROMANIA

BALKANTRIB’14

8th INTERNATIONAL CONFERENCE ON TRIBOLOGY 30th October - 1st November 2014 SINAIA, ROMANIA

BALKANTRIB´14

PROCEEDINGS

PETROLEUM-GAS UNIVERSITY OF PLOIESTI, ROMANIA

BALKANTRIB’14

8th INTERNATIONAL CONFERENCE ON TRIBOLOGY 30th October - 1st November 2014 SINAIA, ROMANIA

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ART 21 YEARS OF BALKAN TRIBOLOGICAL ASSOCIATION

ROMANIAN TRIBOLOGY ASSOCIATION

UNIVERSITY PETROLEUM-GAS OF PLOIESTI, ROMANIA

BALKANTRIB’14 8th INTERNATIONAL CONFERENCE ON TRIBOLOGY, 30thOct.-1stNov.2014, SINAIA, ROMANIA

Influence of the Ceramic Phase on the Wear of HVOF Composite Coatings Athanasios MOURLAS1), Pandora PSYLLAKI1)*, Michael VARDAVOULIAS2) 1)

Laboratory of Tribology, Department of Mechanical Engineering, Technological Education Institute of Piraeus (TEI), 250 Thivon Avenue, 122 44, Egaleo, Greece 2) PyroGenesis SA, Technological Park of Lavrion, 195 00 Lavrion, Greece * Corresponding author: [email protected] Abstract: In the present study, composite coatings with five different proportions of WC-Co/Cr and NiCrFeBSiC components were deposited on stainless steel by HVOF spraying. The microstructure of the obtained coatings was characterized by SEM observations, X-ray diffraction and microhardness measurements. In order to estimate their in-service performance and machinability two series of tests were performed on a pin-on-disc apparatus, according to the ASTM G99 specification: (a) Sliding friction tests using a Si3N4 ball counterbody, under a normal load of 10 N and (b) Micro-cutting tests using standard diamond-coated cutting inserts, under normal loads in the range 1-10N. In both cases, the volume loss was determined as a function of the cermet fraction, while the wear micro-mechanisms involved were identified by microscopic observations. Keywords: HVOF spraying, composite coating, sliding friction, machinability, wear mechanisms.

1. Introduction The term “cemented carbides” is used to describe a subgroup of cermets, where a ceramic phase of hard carbide particles, such as WC, TiC, Cr3C2, etc., in percentages of the order of 80-90%, is bound together by a metal binder phase, typically Co, Ni, Mo or a mixture of them. Their high hardness and wear resistance under non-lubricated conditions induced by the ceramic phase has established them as the materials of choice in applications under harsh conditions, like hard metal cutting, rock drilling, etc [1]. Their excellent properties as sintered monolithic anti-wear components have rendered cermets also attractive as coatings of metallic parts of tribosystems operating under sliding, abrasion and erosion conditions under high temperature and corrosive atmospheres [2]. Thermal spraying is the principal deposition technique to obtain such coatings, with thickness in the range of 200-400 µm. Carbide-based coatings are typically deposited onto metallic substrates by High Velocity Oxy-Fuel (HVOF) spraying, during which oxygen and fuel gas are mixed and burnt in a combustion chamber at high flow rates and pressures up to 12 bar, producing a high-speed jet. Feedstock particles are accelerated with high velocities and relatively low temperature compared to other thermal spray processes such as atmospheric or vacuum plasma spraying. The coatings obtained exhibit high density and superior bond strength to the metallic substrate [3,4]. However, the relevantly high roughness of thermal-sprayed cermet coatings often imposes their post-deposition conventional machining, which is difficult due to the inherent hardness of the carbide. Thus, the maximum performance of such coatings can be achieved only by the co-optimising of the two crucial, yet “contradictory” targeted properties, namely wear resistance and machinability. For this purpose, recent studies are focused on the tribological behavior of composite cermet/metal coatings, in which the metallic phase participates not as binder metal, but as a distinct matrix of the product in percentages higher than 20% [5,6]. The deformable metallic phase could have a beneficial effect of the coatings’ machinability, allowing them in parallel to retain their high anti-wear resistance during service. In the present study composite cermet/metal coatings with various cermet contents were deposited onto stainless steel substrates via HVOF technique. The influence of the cermet percentage on both the machinability and the in-service anti-wear behaviour of the coatings were estimated by performing sliding tests. 2. Experimental details Five series of coatings, each one containing different fractions of the cermet component, were deposited onto AISI 304 stainless steel substrates (60×70×5 coupons) via HVOF thermal spraying. The feedstock powders were prepared by premixing in a ball mill two commercially available powders: a self-fluxing NiCrFeBSiC metallic and a WC-Co/Cr cermet one. The cermet powder fraction in the five mixtures was 0, 25, 50, 75 and 100 %vol. to obtain coatings with the corresponding nominal compositions. The respective coatings are denoted herein as: 0% WC-Co/Cr (metallic), 25% WC-Co/Cr, 50% WC-Co/Cr and 75% WC-Co/Cr (composite), and 100% WC-Co/Cr (cermet). Deposition was realized in the facilities of the company Pyrogenesis, Greece. The values of the spraying parameters were in-house optimized to 506

Athanasios MOURLAS, Pandora PSYLLAKI, Michael VARDAVOULIAS

ensure minimal WC decarburization. Prior to deposition, the substrate surface was sand-blasted in order to remove surface impurities and achieve surface roughness promoting the mechanical anchorage of the coatings onto the metallic substrate. The average roughness (Rα) of the as-sprayed coatings was measured 8.0±0.5 µm. All coatings were tested under sliding friction conditions in dry air (25 %RH, 20 °C) using a standard tribometer apparatus (Centre Suiss d' Electronique et de Microtechnique, CSEM, Figure 1a) that allows the real-time recording of the friction coefficient. For all tests, the sliding speed was kept at 200 m.s-1 and the total sliding distance was 10000 m. In order to simulate surface machining and in-service performance, two types of tests were carried out, respectively: (a) Pin-on-disc tests of the as-sprayed coatings, using as counterbody a cutting insert with a diamond-coated tip (Figure 1b) and applying normal loads in the range of 1-10 N. (b) Ball-on-disc tests of specimens polished to a roughness level of 1.0±0.2 µm, using as counterbody a Si3N4 ball of 6 mm in diameter and applying a normal load of 10 N. The total wear volume was calculated by measuring with a stylus profilometer (Taylor–Hobson) the track cross-sectional area at ten different locations along the wear track and by multiplying the average track area by the circumference of the slide cycle.

Figure 1 (a) Experimental apparatus used for all tests, (b) Cutting insert used for the estimation of the machinability. Microstructure characterization was carried out by means of stereo-, optical- and scanning electron-microscopy, X-ray diffraction and microhardness measurements applying a load of 0.3 kg. The wear micro-mechanisms were also determined by microscopic observations of the worn surfaces. 3. Results and discussion 3.1. Microstructure The characteristics of the five coating’s series after deposition are summarized in Table 1. The total average thickness of the coatings obtained, as measured by optical observations on the respective cross sections, was varying from 205 µm, for the metal-only, to 355 µm, for the cermet-only coatings. In the case of the composite coatings, the average thickness was found to be around 280 µm for all the three cermet contents (Figure 2). Table 1 CerMet content 0% WC-Cr/Co 25% WC-Cr/Co 50% WC-Cr/Co 75% WC-Cr/Co 100% WC-Cr/Co

Total thickness [µm] 205 ± 18 280 ± 14 274 ± 20 270 ± 15 354 ± 25

Characteristics of the coatings examined Mean thickness/ pass [µm] 15.8 17.5 15.2 13.5 13.5

Hardness [HV0.3] 709 ± 63 814 ± 73 856 ± 170 960 ± 180 1208 ± 223

Adhesion strength [MPa] > 75 > 75 > 75 > 75 > 75

The average micro-hardness of the coatings was increasing rather linearly with the cermet fraction, whilst values’ scattering was found to exhibit the same tendency. Such a behavior can be attributed to the “mean free path” approach developed by Lee and Gurland [7] for the case of bulk cermets, according to which the hardness values can be correlated to the inter-carbide spacing. In the present study, higher cermet fraction results in smaller “mean free path” length between splats of completely different mechanical behavior in the vicinity, due to the constrained plastic deformation of the metallic part of the coating [5]. Adhesion measurements, according to ASTM C633-13 specification: “Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings”, demonstrated adhesion strength values higher than 75 MPa, which is the threshold value beyond which adhesion for ceramic thermal sprayed coatings onto steel substrates is considered to be excellent [8]. X-ray diffraction (Figure 2) indicated that metallic-only coatings (0% WC-Co/Cr) consist of f.c.c. nickel solid solution (γ-Ni), whilst the cermet-only ones (100% WC-Co/Cr) of WC, W2C and W. The presence of the last two phases is indicative of the partial dissociation of the feedstock powder during spraying. Similar observations by other researchers have been attributed to the thermal-only decomposition of WC, its reaction with oxygen or its dissolution within the metallic binder [9-12]. The XRD-spectra of the composite coatings exhibited all the four phases described 507

above, indicating that reaction between the distinct particles had not taken place during spraying.

Figure 2 Microstructure details and XRD spectra of three typical coatings examined.

3.2. Machinability The coatings’ machinability was simulated by performing tests in a pin-on-disc setup using a diamond-coated tip insert. In Figure 3, the experimental findings are presented as a function of the normal load applied. The relevant results for the case of metallic-only coatings (0%WC-Co/Cr) are not included, since the tip was rapidly failing, rendering impossible to perform the tests: (a) For coatings with the same cermet fraction, the steady-state friction coefficient was found to decrease with increasing normal load, whilst with increasing cermet fraction within a coating the F.C.-Load curves were displaced towards lower values. For example, in the case of 25%WC-Co/Cr composite coating the friction coefficient decreased from ~0.70 to ~0.55 as the normal load increased from 1 up to 10 N, respectively. In the case of cermet-only coatings (100%WC-Co/Cr), the friction coefficient decreased from ~0.55 to ~0.48 as the normal load was increased from 1 up to 10 N, respectively (Figure 3a). (b) The apparent wear volume per sliding lap presented an almost linear evolution with the normal load applied (Figure 3b), exhibiting higher values in the case of “more-metallic” coatings (25 and 50% WC-Co/Cr). For example, in the case of 10 N normal load, the wear volume per lap was increasing from around 10-5 mm3 up to almost the double value (2×10-5 mm3), as the cermet fraction in the coating decreased from 100-75% to 50-75%. This tribological behavior of the coatings examined against the diamond tip can be directly correlated to the presence of the metallic phase within the coating: • In the case of the metallic-only coatings (0%WC-Co/Cr), their plastic deformation without material removal in the front of the sliding tip, results in the diamond coating detachment and, thus, the premature failure of the cutting edge even at the early stages of testing. • In the case of the cermet-only coatings (100%WC-Co/Cr), the material is removed via micro-cutting and micro-cracking mechanisms due to its’ brittle nature; the wear volume, as measured by the stylus profilometer, corresponds indeed to the volume removed. • In the case of composite coatings (75, 50 and 25% WC-Co/Cr), the deformable metallic phase intervenes in both the friction coefficient and the wear volume determined experimentally. On one hand, the plastic deformation of the metallic phase in the front of the tip results in higher friction forces and consequently higher values of friction 508


coefficient; whilst, at the same time, the brittle micro-cracking of ceramic phase leads to ceramic fragments on the worn area that act as micro-slippers, reducing the friction forces. The higher the cermet fraction in the coating, the higher is its percentage within the debris mixture at the contact interface, resulting in lower values of the friction coefficient. On the other hand, the plastic deformation of the metallic phase that is not only limited at the worn surface but it can be extended in-depth below the surface, introduces a “volume loss” that does not correspond to material removal due to machining. Moreover, the higher the normal load and the metallic fraction, the higher the “apparent wear” volume is. In Figure 4, stereo-micrographs of characteristic worn surfaces are presented; however, detailed observations on the respective cross-sections are required to justify the mechanism described above.

Figure 3 Pin-on disc tribosystem: (a) Friction coefficient and (b) Wear volume per sliding lap, as a function of the normal load applied. 3.3. In-service performance The in-service performance of the coatings examined, as the anti-wear protective ones, was simulated by performing tests in a ball-on-disc setup using a Si3N4 ball as counterbody. In Table 2, the experimental findings are presented as a function of the cermet faction in the coating: (a) The friction coefficient is increasing with the cermet content to a maximum value of 0.64, for the 50% WC-Cr/Co coating series, and afterwards is decreasing to a minimum value of 0.44, for the 100% WC-Cr/Co coating series. (b) The wear coefficient exhibits a linear decrease with increasing cermet fraction. Table 2

Ball-on-disc testing results for all the coating series examined.

CerMet content

Friction coefficient

0% WC-Cr/Co 25% WC-Cr/Co 50% WC-Cr/Co 75% WC-Cr/Co 100% WC-Cr/Co

0,48 0,61 0,64 0,58 0,44 509

Wear coefficient [mm3/N.m] 2,96E-05 2,65E-05 1,63E-05 9,65E-06 3,62E-06

Figure 4 Wear tracks of two representative coatings after testing applying 1, 2, 5 and 10 N (Stereo-micrographs). 3.4. Wear micro-mechanisms In Figures 5-7, SEM micrographs of characteristic worn surfaces after ball-on-disc tests are presented, revealing the wear micro-mechanisms that take place during sliding: • In the case of metallic-only coatings (0% WC-Cr/Co), a combination of micro-ploughing together with micro-cracking at surface splat level can be recognized (Figure 5). • In the case of cermet-only coatings (100% WC-Cr/Co), surface micro-cracking and splat fragmentation that lead to carbide particle pull-out can be observed (Figure 6). • In the case of composite coatings (25, 50 and 75% WC-Cr/Co), on the worn surface both the two previously mentioned micro-mechanisms can be detected (Figure 7). However, the nature of the splats (brittle/ cermet or ductile/ metallic) underneath the contact area alternates the wear mechanisms of the surface layer [5]. 510


Figure 5 Worn surface of the metallic-only (0% WC-Co/Cr) coating (SEM micrographs).

Figure 6 Worn surface of the cermet-only (100% WC-Co/Cr) coating (SEM micrographs).

Figure 7 Worn surface of composite (50% WC-Co/Cr) coatings (SEM micrographs). 511

4. Conclusions The present study is focused on the effect of the CerMet fraction on the tribological behavior of CerMet-Metal composite coatings deposited via HVOF spraying onto steel substrates. More precisely, the machinability and the in-service performance of the composite coatings were evaluated as a function of the WC-Co/Cr fraction in a NiCrFeBSiC metallic matrix, using two simulation setups: pin-on-disc and ball-on-disc, respectively. In both cases, the cermet fraction was found to affect significantly both the friction and wear coefficient, as well as the wear micro-mechanisms taking place at and underneath the contact surface. 5. Conclusions [1] [2]

Basu B., Kalin M.: Tribology of ceramics and composites. Hoboken, New Jersey: John Wiley & Sons, (2011). Heydarzadeh Sohi M., Ghadami F.: Comparative tribological study of air plasma sprayed WC-12%Co coating versus conventional hard chromium electrodeposit. Tribology International, 43, (2010), 882-886. [3] Wood R.J.K.: Tribology of thermal sprayed WC-Co coatings. International Journal of Refractory Metals and Hard Materials, 28, (2010), 82-94. [4] Wirojanupatump S., Shipway P.H., McCartney D.G..: The influence of HVOF powder feedstock characteristics on the abrasive wear behaviour of CrxCy-NiCr coatings. Wear, 249, (2001), 829-837. [5] Kekes D., Psyllaki P., Vardavoulias M.: Wear micro-mechanisms of composite WC-Co/Cr - NiCrFeBSiC HVOF coatings. Part I: Dry sliding. Tribology International (2014 submitted). [6] Kekes D., Psyllaki P., Vardavoulias M., Vekinis G.: Wear micro-mechanisms of composite WC-Co/Cr NiCrFeBSiC HVOF coatings. Part II: Cavitation erosion. Tribology International (2014 submitted). [7] Lee H.C., Gurland J.: Hardness and deformation of cemented tungsten carbide. Materials Science and Engineering, 33, (1978), 125-133. [8] Rosa G., Psyllaki P., Oltra R., Montesin T., Coddet C., Costil S.: Laser ultrasonic testing for estimation of adhesion of Al2O3 plasma sprayed coatings. Surface Engineering, 17, (2001), 332-338. [9] Venter A.M., Oladijo O.P., Luzin V., Cornish L.A., Sacks N.: Performance characterization of metallic substrates coated by HVOF WC-Co. Thin Solid Films, 549, (2013), 330-339. [10] Verdon C., Karimi A., Martin J.-L.: A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures. Materials Science and Engineering A, 246, (1998), 11-24. [11] Morks M.F., Gao Y., Fahim N.F., Yingqing F.U., Shoeib M.A.: Influence of binder materials on the properties of low power plasma sprayed cermet coatings. Surface and Coatings Technology, 199, (2005), 66-71. [12] Morks M.F., Gao Y., Fahim N.F., Yingqing F.U.: Microstructure and hardness properties of cermet coating sprayed by low power plasma. Materials Letters, 60, (2006), 1049-1053.

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