Rolling Contact Fatigue of HVOF-Sprayed WC-Based Hardmetal Coatings

Transcription

1 Manuscript Referee: Professor Herbert Danninger, TU Wien, Austria Rolling Contact Fatigue of HVOF-Sprayed WC-Based Hardmetal Coatings L.-M. Berger 1), K. Lipp 2), C. Jordan 1), U. May 2), T. Naumann 1) 1) Fraunhofer Institute for Materials and Beam Technology (Fh-IWS) Winterbergstr. 28, D Dresden, Germany 2) Fraunhofer Institute for Structural Durability and System Reliability (Fh-LBF), Bartningstr. 47, D Darmstadt, Germany ABSTRACT Mechanical properties of WC-based coatings deposited with modern HVOF (high velocity oxy-fuel) spray equipment have reached a level which allows coating solutions to be developed for contact fatigue applications. Following the promising results of a previous study presented at EURO PM 2006 [1] with WC-17%Co coatings on hardened substrates, in the current work WC-17%Co and WC-(W,Cr) 2 C-7%Ni coatings deposited on non-hardened substrates were studied. Both coating compositions were investigated with coating thicknesses of 100 µm and 600 µm and sprayed from commercially available powders with a liquid-fuelled HVOF system. The coating microstructures, phase compositions and mechanical properties are discussed. Rolling contact fatigue (RCF) results are presented by means of Woehler curves. The best coatings showed fatigue behaviour similar to that of the case hardened steel 16 MnCr 5. INTRODUCTION Application of thermally sprayed hardmetal coatings as structural components has been hindered in the past due to their insufficient mechanical properties under contact loading. With broad introduction of high velocity oxy-fuel spraying (HVOF) into industrial practice a process for preparation of high-quality hardmetal coatings became available, enabling the development of new applications including surface modification of structural components [2]. The potential for use of WC-17%Co coatings applied to hardened substrates has recently been demonstrated [1]. The choice of WC-17%Co for these experiments was based on the good results described in the literature for this composition [3]. This composition was also the best performer among several hardmetal coating compositions in a multi-stage testing and selection process for dry-running gears [4,5]. In particular, for the WC-17%Co coating in the rolling contact fatigue test (twin-disk tribometer) under self-mated, dry running conditions no failure up to a Hertz pressure of 1385 MPa and 30% slip ratio was observed. Interestingly enough, Nakajima et al. [6-8] investigated the rolling contact fatigue of HVOFsprayed WC-(W,Cr) 2 C-Ni coatings. This is one of the standard compositions for the preparation of thermally sprayed hardmetal coatings [2] and is based on an invention for which a patent application was filed in 1958 [9]. However, nowadays there are confusing commercial designations used for this one and the same composition, e.g. WC- CrC -Ni, WC- NiCr and WC-Cr 3 C 2 -Ni [10,11]. The coatings are mostly known and applied in industrial practice on account of their good corrosion resistance. The coatings also show an oxidation resistance that is much superior to that of other commercial WC-based coatings [12] as well as low wear rates and low coefficients of friction in dry sliding wear conditions when mated with alumina at 800 C. Other WC-based coatings cannot be applied at all at this temperature [13]. For these reasons WC-(W,Cr) 2 C-Ni coatings were included in the test program. In this paper the systematic studies on the rolling contact fatigue of WC-based hardmetal coatings based on the results of WC-17%Co coatings deposited on hardened substrates [1] were continued. In these new experimental series WC-17%Co and WC-(W,Cr) 2 C-7%Ni coatings deposited on non-hardened substrates were tested for the purposes of studying the rolling contact fatigue behaviour using soft substrates. Use of non-hardened substrates has two advantageous aspects: technological as the substrate preparation by grit blasting before 205

2 spraying is easier and economic because the cost-intensive hardening process is not necessary. EXPERIMENTAL Coating Materials Commercially available agglomerated and sintered spray powders WC-17%Co (Amperit ) and WC-(W,Cr) 2 C-7%Ni (Amperit , composition traded as WC-CrC-Ni 73/20/7), both produced by H.C.Starck GmbH, Goslar, Germany, were used. The WC- 17%Co powder was also used in the previous work [1]. The WC-(W,Cr) 2 C-Ni powder was selected from several recently investigated commercial materials [11]. Specimen Preparation The rolling contact fatigue test specimens shown in Figure 1 were manufactured from nonhardened 16 MnCr 5 steel with a hardness of about 190 HB. Samples with correspondingly decreased diameters were coated with coatings with two different thicknesses (greater than 100 and 600 µm). After finishing for all specimens a final diameter of 30 mm was obtained. These two coating thicknesses were selected after preliminary calculations with the expectation that the maximum stress would appear in the substrate in the case of the 100µm coating thickness and in the coating in the case of the 600µm coating thickness [1]. Fig. 1: Crowned sample for RCF tests Spraying was performed at Fh-IWS with an HVOF process using a paraffin-fuelled system (K2, GTV mbh, Luckenbach, Germany). The substrates were grit-blasted immediately before spraying. All coatings were deposited using optimised spray parameter sets. Coating Characterisation After testing the coating microstructures were studied by optical microscopy and scanning electron microscopy (SEM) using metallographically prepared cross sections of the RCF specimens and flat samples sprayed using the same parameter sets. In addition, the coating cross section microhardness was studied under a 300g load. The flat specimens were also used to study the phase composition by X-ray diffraction and to measure the Young s modulus using laser acoustic surface waves (LAWave, Fh-IWS) [14]. Rolling Contact Fatigue Testing As in the previous study [1] the rolling contact fatigue tests were performed at Fh-LBF on a ZF-RCF test bench using the loading principle shown in Figure 2. The advantage of this testing principle is its realistic simulation of load conditions on gear flanks [15,16]. The testing conditions were Hertz pressure with additional sliding of -24% as the most severe condition on gear flanks, gearbox oil SAE 80 (MIL-L 2105 = GL4) and an oil temperature at the surface of the specimens of 80 C. The test was terminated either when failure was detected or when the specimen endured cycles without failure. A vibration sensor detected failures such as pitting or spalling of the coating. 206

3 a. Test principle Specimen Contact surface Loading roll Fig. 2: Test principle RESULTS AND DISCUSSION Coating Characterisation The phase compositions and some of the physical coating properties are compiled in Table 1. The typical structure of the WC-17%Co coating is described elsewhere [1,2]. Table 1: Phase composition, microhardness and Young s modulus for the coatings studied, * determined by tensile strength test WC-17%Co WC-(W,Cr) 2 C-Ni Phase composition WC, W 2 C, W, Co-based WC, (W,Cr) 2 C, Ni-based binder phase binder phase HV Young s modulus [GPa] 234* 228 The feedstock powder selected for the preparation of the WC-(W,Cr) 2 C-Ni coatings consists of WC, Cr 3 C 2, and metallic Ni [11]. The spray process results in disappearance of Cr 3 C 2 and in formation of (W,Cr) 2 C. Very little information on this phase and its properties is available in the literature. According to Stecher et al. [17] it presents a solid solution of Cr 2 C with the well-known W 2 C phase which is characterised by a linear dependence of the lattice parameters a and c on composition. Based on the lattice parameters the content of Cr 2 C in the (W,Cr) 2 C phase in the coating was estimated to be mole per cent [10,11]. For the purposes of illustration Figure 3 shows two SEM micrographs for a WC-(W,Cr) 2 C-Ni coating sprayed from this powder. Unlike WC-Co, WC-(W,Cr) 2 C-Ni is not a simple binary hard phase binder metal composite. Here a significant proportion of the fine WC grains are not embedded in a metallic binder matrix, but rather surrounded by (W,Cr) 2 C. Despite the latest progress [10,11] made in the investigation of the microstructures of WC-(W,Cr) 2 C-Ni coatings, additional work is required for the metallurgical processes occurring during feedstock powder preparation and as a result of the spray process to be fully understood. Coatings sprayed using liquid-fuelled HVOF processes at high chamber pressures have a strong tendency to produce compressive stresses [18, p.59]. Besides thermal and quenching stresses in these processes, peening stresses arise due to the kinetic energy of the impinging particles [19]. The compressive stresses of coatings sprayed from other commercial WC-(W,Cr) 2 C-Ni powders were found to lie at about 300 MPa, as determined using the hole-drilling method according to ASTM E

4 Fig. 3: SEM micrographs of the WC-(W,Cr) 2 C-Ni coating (left: overview, right: detail) Endurable Hertz Pressures Initial results of rolling contact fatigue tests are presented in Figure 4 for the samples with WC-(W,Cr) 2 C-Ni coatings and in Figure 5 for the samples with WC-17%Co coatings using a double logarithmic scale. In both figures Woehler curves of uncoated specimens made from 16 MnCr 5 in the normalised and case hardened states are also plotted for a probability of survival of 50% [1]. Applying a coating of thickness 100 µm to soft substrate materials did not result in an improvement in the RCF behaviour compared with that of uncoated soft specimens. Contrary to this, a coating thickness of 600 µm resulted in endurable Hertz pressures similar to those of case hardened specimens. Specimens which did not fail showed an excellent surface even after loading cycles for both coating materials (see Figs. 4 and 5). Fig. 4: Results of the RCF tests for WC-(W,Cr) 2 C-Ni-coated specimens 208

5 Fig. 5: Results of the RCF tests for WC-17%Co-coated specimens Typical failures of the investigated specimens are shown in Figure 6 for both coating materials and thicknesses. Analysis of the photographs presented in Fig. 6 could suggest that failure initiates at the substrate for both coating thicknesses. WC-(W,Cr) 2 C-Ni: Test no. 2, specimen no. 06: p 0 = 1250 MPa, N = cycles Coating thickness 100 µm 600 µm WC-(W,Cr) 2 C-Ni: Test no. 7, specimen no. 13: p 0 = 2700 MPa, N = cycles WC-17%Co: Test no. 5, specimen no. 05: p 0 = 1400 MPa, N = cycles WC-17%Co: Test no. 7, specimen no. 07: p 0 = 2300 MPa, N = cycles Fig. 6: Failed specimens The results of metallographic investigations of the specimens show, at least for the WC- 17%Co coating with a thickness of 600 µm, that failure occurred at the interface between coating and substrate as well as in the coating itself at the depth at which the maximum von Mises stress occurred, as shown in Figure 7. In addition, a further crack running from the interface into the substrate material was found. Such cracks propagating into the material at an angle of about 30 to the interface typically indicate the presence of shear stresses [16], as was proven by numerical calculations [1]. 209

6 WC-(W,Cr) 2 C-Ni: Test No. 6, specimen No. 08: p 0 = 2300 MPa, N = cycles without failure WC-17%Co: Contact area Test No. 2, specimen No. 02: p 0 = 2000 MPa, N = cycles cracks in coating Fig. 7: Metallographic investigations of specimens with coatings of thickness 600 µm For the investigated specimen with the WC-(W,Cr) 2 C-Ni coating, which was considered as a run-out without visible damage after testing, only a small radial crack starting at the interface could be found (see Fig. 7). Due to the high Hertz pressure of 2300 MPa with high subsurface stresses a structural change could be observed in the most highly stressed area of the substrate material. Figure 8 shows the subsurface stress distribution below the centre of contact for a constant Young s modulus of E = 206 GPa (assuming the same Young s modulus for the substrate and the coating) for Hertz pressures of p 0 = 2000 MPa and p 0 = 1250 MPa. Fig. 8: Von Mises stress distribution below the centre of contact for different Hertz pressures It can be seen that the maximum von Mises stress for a Hertz pressure of 1250 MPa occurs at a depth of 150 µm. This implies that for the specimens with coatings of thickness 100 µm the highest local stress occurs below the coating in the substrate material. Increasing the Hertz pressure increases the maximum stress as well as the depth where the maximum local stress appears. For a Hertz pressure of 2000 MPa the maximum von Mises stress was calculated to be σ a = 1195 MPa and at the interface between coating and substrate it is σ a = 905 MPa, already significantly higher than the subsurface stresses for the endurable Hertz pressure of the uncoated material. Based on these stress distributions the probability of failure starting in the substrate material is high for the soft substrate. In order for the failure mechanism to be studied in detail, further investigations including metallographic and 210

7 fracture analyses are necessary and should be correlated with numerical simulations such as those presented by Lipp et al. [1]. The results of the rolling contact fatigue tests demonstrated that with a soft substrate material and a coating thickness of 600 µm endurable Hertz pressures similar to those of case hardened steels (16 MnCr 5) could be achieved. Thus, the rolling contact fatigue behaviour is better for a soft substrate material than for a hard substrate material, as shown in Figure 9 [1]. Fig. 9: Comparison of Woehler plots for WC-17%Co coatings deposited on hard and soft substrates The limiting factor for the Hertz pressure of coated specimens using hardened substrate material was the coating bond strength [1]. Employing a soft substrate material significantly improves the bonding between the coating and the substrate. The limiting factor for the results presented here might be the strength of the soft substrate material. CONCLUSIONS The durability of HVOF-sprayed WC-17%Co coatings under rolling contact fatigue loading was significantly improved by use of non-hardened substrates rather than the case hardened substrates used in the previous experimental series [1]. Coatings of the composition WC- (W,Cr) 2 C-7%Ni deposited on non-hardened substrates were favourable in comparison with the WC-17%Co coatings and reached endurable Hertz pressures similar to those of case hardened steels (16 MnCr 5) at a coating thickness of 600 µm. The limiting factor for the endurable Hertz pressure for the investigated specimens shown here was the fatigue strength of the coating in the case of WC-17%Co and the substrate material in the case of WC-(W,Cr) 2 C-Ni. Further improvement in the rolling contact fatigue behaviour could be achieved by optimisation of coating thickness in combination with substrate strength. ACKNOWLEDGEMENTS This work was supported by the FhG Internal Programs under Grant No. WISA The authors would like to thank Dr Bojan Podgornik (Centre for Tribology and Technical Diagnostics, University of Ljubljana, Slovenia) for the measurement of the internal stresses. Ms B. Wolf (Fh-IWS) and Ms A. Till (Fh-LBF) are gratefully acknowledged for metallographic sample preparations. Ms A. Richter (Fh-IWS) is thanked for the SEM studies. REFERENCES [1] K. Lipp, L.-M. Berger, U. May, M. Wiener, Proc. EURO PM, October 23-25, 2006 (Ghent, Belgium), 1, Shrewsbury/U.K., EPMA, 2006, [2] L.-M. Berger, Powder Met., 2007, 50 (3),

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