THERMALLY SPRAYED COATINGS FOR RAILWAY APPLICATIONS

THERMALLY SPRAYED COATINGS FOR RAILWAY APPLICATIONS Michaela KAŠPAROVÁ a, František ZAHÁLKA b, Šárka HOUDKOVÁ c a Výzkumný a zkušební ústav Plzeň, Tylova 1581/46, 301 00 Plzeň, Česká republika, kasparova@vzuplzen.cz b Výzkumný a zkušební ústav Plzeň, Ltd., Tylova 1581/46, 301 00 Plzeň, Česká republika, zahalka@vzuplzen.cz c Výzkumný a zkušební ústav Plzeň, Ltd., Tylova 1581/46, 301 00 Plzeň, Česká republika, houdkova@vzuplzen.cz Abstract Thermally sprayed coatings are widely use in the area of energy industry, aircraft industry, metallurgical, glass and printing industry or for renovation and repairing, etc. Further possibility of the utilization of thermal spraying is in the area of railway traffic, in concrete terms in railway vehicles. The main aim of this work was aimed to find the suitable coatings for wheel-axle systems with ensuring good mechanical-physical properties during dynamic stress of the axles. Selected coatings sprayed using high pressure/high velocity and arc technologies were tested (abrasive wear, bond strength, macro and micro-hardness) and the coatings microstructure was evaluated (porosity, oxides, cracks, etc.). The results of experimental measurements provide beneficial information for a successive choice of suitable thermally sprayed material to given applications. Keywords: thermally sprayed coatings, mechanical properties, rail vehicle 1. INTRODUCTION A railway axle is a highly dynamically stressed part of railway vehicles. Very strict demands are impose on this component. One of parts of axles is a wheel-set where the wheels are cold pressing for ensuring the permanent joint. Requirements for axles materials and mechanical-physical properties of the axles and wheel-sets are standardized in ČSN EN 13261 and ČSN EN 13260 [1, 2]. In the standards there are mentioned fatigue limits during dynamical loading among others. These values changes in the relation to the axle material, geometry (hollow or full axles) and to the axle area (stepped or un-stepped part). The molybdenum plating which enables the fretting resistance, electrical conductivity and repeated withdrawing of the wheel from the axle is year-applied technology of the surface treatment of the wheel sets. The main aim of this work was to find a suitable thermally sprayed material that should be as an alternative to standardly used molybdenum (Mo is most of all sprayed by flame spray technology or by arc eventually). The alternative material has to show high adhesive/cohesive strength, high macro- and micro-hardness, good wear resistance, corrosive resistance, good sliding properties and a guarantee of electrical conductivity. The beginning of research, results of which are presented in this work, was aimed at comparative laboratory testing of several electrically conductive and corrosive resistant coatings. 2. EXPERIMENTS AND TESTED MATERIALS Following coatings tests and analyses were done: measurements of macro-hardness HR15N, microhardness HV 0.3, bond strength in tensile loading, abrasive wear resistance and evaluation of the coatings microstructure. The macro-hardness HR15N was measured using the PM13 Rockwell tester, microhardness HV 0.3 using the LECO DM 400A Hardness Tester. Five indentations for HR15N and seven indentations for

HV 0.3 performed to determine the average value. The bond strength was evaluated using the tensile test method in accordance with ČSN EN 582 [3]. The wear resistance was measured using the Dry Sand Rubber Wheel Test method according to ASTM G-65 Standard [5]. The parameters of the wear test were as follows: the applied load of 22N, abrasive line of 718 m, Al 2 O 3 abrasive sand in 212-250 µm of grain size. Coatings microstructure was determined using a light microscope and the coating thickness was measured using the Lucia programme. Two types of sprayed technology (HVOF-high velocity oxygen fuel system and arc technology) were used for samples preparing. Three types of arc coatings -13%Cr (austenitic stainless steel), 99.9%Mo, CuAl8 (aluminum bronze)- and two types of HVOF coatings -Triballoy 400 (CoMoCrSi) and Stellite Alloy 6 (CoCrWCSi)- were sprayed using the optimized deposition parameters (optimized amount of fuel and oxygen, carrier gas, spray distance, etc.). 3. RESULTS 3.1 Microstructure The micrographs of coatings microstructure are shown in Fig.1. Evident difference between individual coatings microstructures is given by the used spray technology. HVOF coatings are dense with a low oxide content and any appearance of un-melted particles, whereas the arc sprayed coatings are characterised with high porosity and high oxides content. The coatings roughness between the used spray technologies is also very different. From the coatings micrographs non-uniform surface of arc comparing to HVOF coatings is clearly evident. The results of coatings thickness measurements are mentioned in Table 1. High differences between maximal and minimal value for arc coating corresponds with their ultrahigh surface roughness. Table 3. Coatings thickness measured on the coatings cross sections using light microscope and Lucia programme Coating Average value [µm] St.dev. Min. [µm] Max. [µm] Stellite Alloy 6 488.9 ±19.2 445.7 528.7 Tribaloy 400 540.4 ±23.7 493.4 578.4 13%Cr 419.1 ±66.8 333.8 545.2 CuAl8 370.9 ±75.0 257.1 501.7 99.9%Mo 250.0 ±89.3 109.9 429.1 High coating roughness is a negative factor especially in term of operating cost due to high stock for machining. Nevertheless, the coatings properties can be controlled by the used spray parameters. Wilde et. al [12] stated that for spraying alloys it is important to reduce the burn-up of the elements, or that low particle temperature process is often required to maintain the alloy composition and that using low voltage along with high atomizing gas pressure and high current, the particle temperature and surface roughness can be reduced. On the contrary, the HVOF coatings are very uniform regarding the surface roughness. The roughness values of HVOF coatings are as follows: for Stellite Ra 5 µm and for Triballoy 400 Ra 9 µm. The surface roughness of arc sprayed coating was out of range and therefore impossible to measure.

The microstructure of Mo-coating is shown in Fig.1e. The Mo coating contains, besides fine 99.9% molybdenum, oxides of Mo, see thin strips surrounding the splats. The molybdenum oxides created in Mocoatings acts as an anti-corrosive protection. The majority oxides phase MoO 2 increases coating hardness but thereby also a brittleness, which influences negatively the cohesive strength of a coating [6]. Microstructure similar to Mo-coating can be seen at both other arc sprayed coatings. Fig. 1d shows the microstructure of the CuAl8 coating. The phases that surround individual splats of the CuAl8 coatings are mainly the aluminium oxides [7]. These oxides also occur on the surface of coating. They act as the passive layer decreasing the corrosive attack there. In Fig 1.c the micrograph of 13%Cr coating cross section is shown. High oxides content is also evident in the coating microstructure. Cifuentes et al. [11] showed that spinel oxides M 3 O 4 form majority part in coating. However, the amount of oxides depends on the spray parameters, most of all on the spray distance and type of the air cap. In all arc sprayed coatings there are evident large globular pores between individual splats increasing uncompactness of coatings. However, these pores are probably the results of a manual metallographic preparation, during which whole un-melted particles detach from coating due to lower cohesion of these structural particles. The microstructure of HVOF sprayed coatings is shown in Figs.1a,b. The microstructure of the Stellite Alloy 6 coating is documented in Fig.1.b. The coating is homogenous, dense and structured with visible interface between individual splats. Melted and semi-melted particles have a featureless appearance while the unmelted particles have a dendritic structure. The oxides (spinel oxides CoNiO 2 [8]) appeared in the microstructure in the form of intersplat lamellae or globules are oriented parallel to the substrate surface. Sidhu et al. [8] performed the X-ray mapping of the different elements present across the coating region to clarify the elements distribution. They found that the coating area is found to be rich with main elements of the feedstock powder as Co and Cr, which were uniformly distributed throughout the coating region. Cr provides oxidation and corrosion resistance as well as strength by the formation of M 7 C 3 and M 23 C 6 carbides. Metals such as Mo and W contribute to the strength via precipitation hardening by forming MC and M 6 C carbides and intermetallic phases such as Co 3 (Mo,W). These elements were also homogenously distributed in the coating cross section. Further additive Ni, increasing the strength, hardness and chemical resistance of the coating, showed a tendency to present mainly at the splat boundaries in the form of thin stringers, whereas Fe showed a tendency to make thin streaks at the coating substrate interface. Alloying addition Ni, C and Fe promotes the stability of an fcc structure of Co-rich matrix, which is stable at high temperatures up to the melting point (1495 C), while Cr, Mo and W tend to stabilize, hexagonal closepacked (hcp) crystal structure, which is stable at temperatures bellow 417 C [8, 13]. Si (element that

increases the hardness of materials) significantly diffused from the coating to the substrate and was also distributed uniformly in the coating area. The Triballoy 400 coating, see Fig.1a, can be also characterized as a dense and homogenous coating but a higher porosity as at the Stelite coating is observed. Triballoy 400 can be generally characterized as a composite composed of a brittle metallic phase embedded into a solid solution of a soft cobalt matrix. Przybylowitz et al. [9] described the coatings microstructure, chemical and phase composition. They mentioned that the coating contains 50% of Laves phases which ensure good adhesive (cohesive) strength of coating to the substrate. The molybdenum, that is contained in the powder in the amount around 29wt.%, increases the strength of Co matrix. The elements of Ni+Fe are added to the coating for the stabilization of a facecentered crystal lattice of matrix. Bolelli et al. [10] studied mainly the phase composition of the coating by means of X-ray diffractometry, they found following phases: Co 7 Mo 6, Co 3 Mo 2 Si, CoSi 2 and CoMoCr solid solution with fcc lattice. 3.2 Hardness, Wear resistance The results of coatings hardness and microhardness measurements are shown in Table 2 and graphically depicted in Fig.2. Both measurements show the highest hardness for HVOF sprayed coatings, whereas the hardest coating is Tribaloy 400. Very low hardness values were measured for CuAl8 coating. The wear resistance of coatings is shown in Fig.3, other characterised corresponding values are mentioned in Table 3. Values of coating density had to be determined for possible comparison of wear characteristics between different materials. The coatings densities were calculated from the equation: ρ = m c /V c. Where m c is the coating weight [g] and V c is the coating volume [cm 3 ]. For using this formula the precise shape of sample must be known. The results of the coatings wear rate show a strong influence of the hardness on the wear resistance. As it is shown in the Fig. 3a, the coatings resistance against abrasive wear behaves similarly as the hardness values. The highest wear resistance was recorded for HVOF coatings, whereas the wear rate of Stellite Alloy 6 and Triballoy 400 coating was nearly identical. Slightly higher wear rate, volume losses respectively, of Triballoy 400 coating can be given by higher coatings porosity. The pores weaken the splats boundaries, which leads to decrease in cohesion strength of the coating. Due to invasion of cohesive strength whole splats can unfasten from free surface, thus the volume loss rises. However, the differences of wear rates between these coatings are minimal and the porosity effect would be probably more distinctive in high-stress abrasive conditions. The worst wear resistance was recorded for the CuAl8 coating. Nevertheless, poor hardness values were measured for this coating. The total volume loss for the CuAl8 coating was not possible to

measure because in the third cycle the substrate was uncovered and the abrasive test had to be stopped. The wear resistance of the Mo and the 13%Cr coatings in comparison to other tested materials was intermediate. Table 2. Coatings hardness Coating Stelite Alloy 6 Triballoy 400 99.9%Mo 13%Cr CuAl8 Microhardness HV 0.3 615.6±57.9 701.8±71.3 285.9±12.6 480.3±90.1 128.3±15.1 Hardness HR15N 86.4±0.6 88.4±0.5 59.0±1.5 73.5±2.5 43.2±1.2 Hardness HRC (converted) 51.9 56.1 -* 27.3 -* *very low value of HR15N, impossible to converted into HRC Tab.3. Wear resistance of coatings Materials TVL* [mm 3 ] AVL* [mm 3 ] v* [mm 3 /m] k* [mm 3 /N.m] CuAl8-28.6 0.194 0.0088 99.9%Mo 69.7 14.1 0.098 0.0045 13%Cr 55.8 11.3 0.081 0.0037 Triballoy 400 38.8 7.9 0.052 0.0024 Stelite Alloy 6 35.0 7.1 0.048 0.0022 *TVL=total volume loss, AVL=average volume loss, v=wear rate, k=coefficient of wear 3.3 Bond strength The results of coatings bond strength are listed in Table 4. In case of both HVOF sprayed coatings the failure occurred in the glue at ~76 MPa (Triballoy 400) and at ~88 MPa (Stellite Alloy 6). It means that these coatings have a higher bond strength then the recorded tensile strength. All of arc sprayed coatings were fractured cohesively inside the coatings. The failure of the 13%Cr and the CuAl8 was recorded at the same loading force of ~33 kn (65 MPa). The maximal tensile force recorded for the Mo-coating was slightly lower about 27 kn (55 MPa). Table 4. Coatings bond strength (tensile) Material Triballoy 400 Stelite Alloy 6 13%Cr CuAl8 99.9 % Mo Bond strength [MPa] 76.6±4.6 88.4±5.5 65.9±1.4 65.6±2.7 55.5±1.0 Failure glue glue cohesive cohesive cohesive 4. CONCLUSION In the presented study two types of the HVOF sprayed coatings (Triballoy 400 and Stellite 6) and three types of the arc sprayed coatings (13%Cr, CuAl8 and 99.9%Mo) were investigated to evaluate their suitability for surface protection of two-wheeled systems of rail vehicles. All selected materials fulfilled the conditions of electrical conductivity and corrosive resistance in compliance with the requirements of the rail vehicles. Electrical properties are generally measured on the real piece of the whole wheel-axle system. The standards for railway materials determine the maximal value of electrical resistance between wheels placed on the axle. This value must not exceed 0.1 µω. The main aim of this study was evaluation of basic properties of selected materials and narrowing of the choice of given materials for consequent experiments, as the fatigue measurements and shear strength measurements are. The results of this study showed that the least suitable coatings are arc sprayed CuAl8 and 99.9%Mo for low hardness of the CuAl8 and low adhesive-cohesive strength of the 99.9%Mo. Others three coatings show good wear resistance, high hardness and bond strength. However, the 13%Cr coating contains higher amount of oxides in the microstructures, which can decline the electrical conductivity of this material. Nevertheless, both the HVOF sprayed coatings can show high potential in the railway applications due to

their good mechanical properties and unique microstructure and they deserve deeper attention and exploration. ACKNOWLEDGEMENT This contribution was written thanks to the project No. 1M0519 supported by Ministry of Education, Youth and Sports, CZ. REFERENCES [1] ČSN EN 13260, Railway application Wheelsets and bodies Wheelsets Product requirements, Czech office for standard, metrology and testing, Praha, 2009. [2] ČSN EN 13261, Railway application Wheelsets and bodies Axles Products requirements, Czech office for standard, metrology and testing, Praha, 2009 [3] ČSN EN 582, Thermal spraying Determination of tensile adhesive strength, Czech Office for Standards, Praha, 1995. [4] ČSN EN 15 340, Thermal spraying Evaluation of the shear strength of thermally sprayed coatings, Czech Office for Standards, Praha, 2007. [5] ASTM G-65 Standard, Evaluation of coatings wear resistence using DrySand Rubber Wheel Test Apparatus, ASTM International, West Conshohocken, PA, 2010, DOI: 10.1520/G0065-04R10, www.astm.org. [6] MODI, S.C., CALLA, E.: A study of high-velocity combustion wire molybdenum coatings, Journal of Thermal spray technology, 2001, Vol 10 (3), p. 480-486. [7] AKDOGAN, et al.: Surface fatigue of molybdenum and Al-bronze coatings in unlubricated rolling/sliding contact, Wear, 2002, Vol. 253, p. 319-330. [8] SIDHU, T.S., et a.: Studies of the metallurgical and mechanical properties of high velocity oxy-fuel sprayed satellite-6 coatins on Ni- and Fe-based superalloys, Surface & Coatings Technology, 2006, Vol. 201, p. 273-281. [9] PRZYBYLOWICZ, J., KUSINSKI, J.: Laser cladding and erosive wear for Co-Mo-Cr-Si coatings, Surface & Coatings Technology, 2000, Vol. 125, p. 13-18. [10] BOLELLI, G., et al.: Microstructural and tribological comparison of HVOF-sprayed and post-treated M-Mo-Cr-Si (M=Co, Ni) alloy coatings, Wear, 2007, Vol. 263, p. 1397-1416. [11] CIFUENTES, L., et al.: Composition and microstructure of ARC-sprayed 13% Cr steel coatings, J. of Thin Solid Films, 1984, Vol. 118, p. 515-526. [12] WILDEN, J., et al.: Investigation about the chrome steel wire arc spray process and the resulting coating properties, Proceedings of International Thermals Spray Conference: Global Coating Solution, (Ed.) B.R. Marple, M.M. et al., Published by ASM International, Materials Park, Ohio, USA, 2007, p. 319 323. [13] CROOK, P., Properties and Selection: Non-Ferrous Alloys and Special Purpose Materials, 10 edn., Metals Handbook, Vol. 2, ASM International, 1993, p. 446.