Friction and wear performance of Titanium alloy against tungsten carbide lubricated with phosphate ester

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1 216 STLE Annual Meeting & Exhibition May 15-19, 216 Bally s Las Vegas Hotel and Casino Las Vegas, Nevada, USA Friction and wear performance of Titanium alloy against tungsten carbide lubricated with phosphate ester 1. Introduction Ye Yang Chenhui Zhang Yan Wang Yuanjing Dai Jianbin Luo State Key Laboratory of Tribology, Tsinghua University, Beijing 184, China Due to the relatively low density, outstanding strength-to-density ratio and excellent corrosion resistance, titanium alloys have been widely utilized in aeroengine and airframe manufacturing fields [1,2]. However, titanium alloys are recognized as difficult-to-cut materials due to their low thermal conductivity, high chemical activity and small elastic modulus [3,4]. Cutting fluid is considered as an accessory in a machining process in order to increase the productivity. It lubricates the contact areas between rake face and chips, flank face and machined surface as well as reduces adhesion and abrasion thus producing a good surface finish. As to the mechanism of this action, the chemical compositions of the cutting fluid are of vital importance and the interactions with the metal surface have been investigated [5]. Researches try to find effective additives which have good tribological properties with steel or aluminum alloys, for example, borate ester compounds, phosphorus-containing molecules and so on[6,7]. However, the chemical properties of metal surfaces differ significantly. As a consequence, the type and intensity of interaction of cutting fluid additives with titanium alloys are very different with steel and aluminum alloys because of the bad tribological properties of titanium alloys [8]. Up to now, few reaches have been done to investigate the effective cutting fluid additives with titanium alloys considering the lubricating and anti-adhesion properties. This paper aims to provide an effective water soluble additive for titanium alloys and investigate the mechanism of the additives acting with the metal surface. It can be further used for the development of titanium alloys cutting fluid. 2. Experimental details Nonylphenol polyoxyethylene ether phosphate ester (PPE) aqueous solution PPE was bought from Haian Company, China, which was synthesized by nonylphenol polyoxyethylene ether (NP-1) and phosphorus pentoxide in the presence of a catalyst. The triethanolamine borate used in this study was purchased from Haian Company. It was diluted to the concentration of 1 wt% by deionized water. As a comparison, one kind of commercial emulsion with concentration of 5% which is widely used for machining titanium alloys in aviation industry was also tested lubricating properties. The frictional tests were carried out utilizing a -on- apparatus SRV4 (Optimal) under different lubricating conditions at atmosphere during the tests. The s were TC4 (Ti6Al4V) alloy with hardness of HRC35. The tungsten carbide YG8 (WC-Co) with diameter of 1mm and hardness of 89HRA was chosen as the counter specimen. The samples were ultrasonically cleaned by acetone and ethanol and then ultrapure water successively each for 1 min before tests. A normal load of 1N was applied and the initial maximum Hertz contact pressure was calculated to be 1267MPa. The upper slid reciprocally against the stationary with amplitude of 1 mm and frequency of 2 Hz for 5 min. A Quanta2 scanning electron microscopy (SEM) combined with energy dispersion spectrometry (EDS) was used for the surface analysis of the investigated materials. The chemical compositions of the worn surfaces were characterized by a PHI Quantera SXM X-ray photoelectron spectrometer (XPS).

3. Results and Discussion 3.1 Tribological properties of various aqueous solutions Figure 1 showed the friction coefficients during the sliding process with various lubricants.

2 3. Results and Discussion 3.1 Tribological properties of various aqueous solutions Figure 1 showed the friction coefficients during the sliding process with various lubricants. As for deionize water and borate solution, the friction coefficient increased from 3 to 45 with increasing sliding time. The fluctuation of the friction coefficient was fairly large during the tests and cacophony was heard caused by rubbing. It meant that deionized water failed to lubricate titanium alloy and tungsten carbide. Compared to them, emulsion showed better lubricating properties. After a short period of running-in, the friction coefficient kept stable with a value of 18. During the tests, it always happened that the friction coefficient jumped to a high value (35 in fig.1 for example) and then went back to the original value and kept constant till next jump. This might be caused by material transfer and stick-slip friction. At the beginning of the test of PPE solution, the friction coefficient wasn t stable for running-in but it dropped to 14 quickly and kept stable to the end of the test. From this figure we can conclude that for tungsten carbide and titanium alloy tribopair, PPE can provide effective lubrication, even better than the emulsion used in our tests while triethanolamine borate has no effect on this system under the experimental conditions. Material:Ti6Al4V Ball:φ1mm WC-Co Fn:1N f=2hz L=2mm Fig.1 The friction coefficient of water, borate solution, emulsion and PPE solutions The wear tracks on the surfaces of TC4 and the surface of s were observed by SEM as shown in Figure 2. The SEM pictures showed that there were many delaminated scars and irregular abrasive particles on the tracks lubricated by water and triethanolamine borate solutions. Furthermore micro patches were detected on surface of the s (Fig. 2a, b). To ascertain the ingredients of the material on the s, EDS analysis were made. The results indicated that titanium alloys existed on the surfaces of the s which were transferred from the s and adhered on the surface (Fig. 2e). It was deduced that seriously adhesive wear occurred for the chemical activity of titanium and the poor lubricating condition. The rubbing surfaces lubricated by emulsion and PPE solutions were much smoother, only some micro-grooves exhibited (Fig. 2c, d). What s more, micro particles but not patches existed on the surface of s. The elements were so alike that it was just listed once in Fig. 2e. This meant that adhesive wear was strongly decreased and a well finish surface was obtained. It was easy to understand that oilcontained emulsion can provide effective lubrication. To our surprise, the PPE solution exhibited even better lubrication and anti-wear ability to titanium alloy.

(a) (b) 5 μm 2 μm 5 μm 2 μm (c) (d) 5 μm 2 μm 5 μm 2 μm (e) 1. 4 KCnt 1. 1 9 6 3 O Ti C A l Ti V 1. 2. 3. 4. Energy 5. 6. 7. 8. 9. 1 (kev) Fig.

Figure3 (a) showed typical XPS survey scans inside and outside the wear track.

It demonstrated that phosphoric compounds remained after frictional test when lubricated with PPE solution.

To further investigate the mechanism, detailed high resolution XPS scans of oxygen were recorded.

According to the possible existent compounds in this work, the spectrum was resolved by Gaussian fitting.

Because of the high chemical activity of titanium, it adsorbed oxygen and water molecules easily thus an oxidation layer formed on the surface of

9eV (assigned to Ti-OH) can be found in Fig.3 (b).the O1s peaks were quite different. As shown in Fig.3 (c), a peak at 531.

3 (a) (b) 5 μm 2 μm 5 μm 2 μm (c) (d) 5 μm 2 μm 5 μm 2 μm (e) 1. 4 KCnt O Ti C A l Ti V Energy (kev) Fig.2 SEM micrographs of worn surface on titanium alloy and carbide lubricated by (a) water (b) triethanolamine borate (c) emulsion (d) PPE and (e) EDS analysis of the s 3.2 Chemical characterization XPS is a practical method to clarity the chemical states of elements within adsorption film on surface of tribopair. Figure3 (a) showed typical XPS survey scans inside and outside the wear track. Phosphoric peak at 133eV appeared at the position, while no phosphorus was detected outside the track. It demonstrated that phosphoric compounds remained after frictional test when lubricated with PPE solution. The peak of Ti2p was apparently lower than the substrate which further demonstrated that some film existed on the wear track. To further investigate the mechanism, detailed high resolution XPS scans of oxygen were recorded. The high resolution O1s spectrum outside the wear track was shown in Figure3 (b). According to the possible existent compounds in this work, the spectrum was resolved by Gaussian fitting. Figure3 (c) showed the O1s spectrum which was fitted with three peaks. Because of the high chemical activity of titanium, it adsorbed oxygen and water molecules easily thus an oxidation layer formed on the surface of titanium alloy though the titanium alloy used in our work were freshly polished. So a high peak at 533eV (assigned to TiO 2 ) and 531.9eV (assigned to Ti-OH) can be found in Fig.3 (b).the O1s peaks were quite different. As shown in Fig.3 (c), a peak at (assigned to P-O-Ti) and (assigned to P-OH) were detected. It can be deduced that PPE molecules were adhered onto Ti6Al4V surface through covalently bond P-O-Ti. The test results showed that the PPE can adsorb on the surfaces of titanium alloy. The viscosity of 1wt% PPE solution was about 96mPa.s which was almost the same as the viscosity of water at 22. Even the concentration increased to 1 wt%, the viscosity of solution was 1.3 m Pa.s. It was not high enough to form hydrodynamic film under this situation. So the adsorption film played the most important role to reduce the friction coefficient and significantly improve the anti-wear property of water by separating the contact surfaces to some extent.

4 7 x 14 C/S outside the wear track O1s O1s Ti2p C1s P2p C/S P2p Binding energy (ev) 2 (a) outside the wear track Binding energy (ev) P-O-Ti TiO2 Ti-OH P-OH TiO2 (b) outside the wear track (c) Fig.3 (a) XPS survey scans of the titanium alloy surface after tribological test; O1s components (b) ouside and (c) 4. Conclusions The friction and wear performance of titanium alloys against tungsten carbide s under four kinds of aqueous solutions were investigated. The results showed that triethanolamine borate had no effect on lubricating titanium alloy. The friction coefficient was high and the worn surface was as rough as that lubricated with water. While PPE solutions reduced the friction coefficient and wear volume thus a well finish surface being obtained. It exhibited even better lubricating and anti-wear abilities than the commercial emulsion used for cutting titanium alloys. The adhesive wear was weaken at the same time. The XPS analysis demonstrated that adsorption film formed by P-O-Ti covalent bond on titanium alloy surface avoided the directly contacting with the counter surface. This paper found a new aqueous system to lubricate titanium alloys/tungsten carbide effectively which gave a direction on choosing water-based additives for machining titanium alloys cutting fluid. References: [1] Leyens C, Peters M, Titanium and Titanium Alloys. Wiley Online Library: 23. [2] Ezugwu EO, Wang ZM. Titanium Alloys and their Machinability - A Review. Journal of Materials Processing Technology 1997; 68 (3): [3] Ezugwu EO, Bonney J, Yamane Y. An Overview of the Machinability of Aeroengine Alloys. J Mater Process Tech 23; 134 (2): [4] Budinski KG. Tribological Properties of Titanium Alloys. Wear 1991; 151 (2): [5] Brinksmeier E, Meyer D, Huesmann-Cordes AG, Herrmann C. Metalworking Fluids Mechanisms and Performance. CIRP Annals - Manufacturing Technology 215; 64 (2):

5 [6] Li J, Li Z, Ren T, Zeng X, van der Heide E. Hydrolytic Stability and Tribological Properties of N- Containing Heterocyclic Borate Esters as Lubricant Additives in Rapeseed Oil. Tribol Int 214; 73: [7] Liu S, Guo D, Li G, Lei H. Lubricating Properties of Organic Phosphate Ester Aqueous Solutions. Tribol Lett 21; 37 (3): [8] Niu QL, Zheng XH, Ming WW, Chen M. Friction and Wear Performance of Titanium Alloys Against Tungsten Carbide Under Dry Sliding and Water Lubrication. Tribol T 213; 56 (1): Key words Nonylphenol polyoxyethylene ether phosphate ester(ppe), titanium alloy, water-based lubrication, anti-wear properties

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