Examination of tribological properties of oxide-polymer and carbide-polymer coatings formed by flame, plasma and HVOF spray processes

Transcription

1 Examination of tribological properties of oxide-polymer and carbide-polymer coatings formed by flame, plasma and HVOF spray processes R. Samur 1, H. Demirer 2 1 Department of Metallurgy, Faculty of Technical Education, Marmara University, 34722, Goztepe/Istanbul, Turkey, rsamur@marmara.edu.tr 2 Department of Metallurgy, Faculty of Technical Education, Marmara University, 34722, Goztepe/Istanbul, Turkey, hdemirer@marmara.edu.tr Abstract In this paper, the tribological performance of composite (oxide and carbide) coatings applied on a 2014-T6 aluminum alloy substrate is presented. Before coating process oxides were mixed with 10% polytetrafluoroethylene(ptfe) and carbides were mixed with 6% ethylene trifluoroethylene (ETFE). Plasma sprayed Al 2 O 3 13TiO 2 +10%PTFE coating exhibited higher hardness and wear resistance than flame sprayed Al 2 O 3 13TiO 2 +10% PTFE coating. For carbide coatings, WC12Co+6% ETFE coating formed by HVOF process exhibited superior wear resistance than WC12Co+6% ETFE coating formed plasma process. Keywords: Oxide, Carbide, Coating, Polymer, Wear. Introduction Aluminum and its alloy are attractive engineering materials for many applications in chemical, automobile and aerospace industries, owing to their high strength-to-weight ratio, high electrical and thermal conductivities, and excellent corrosive resistance [1]. However, low hardness and wear resistance greatly limit their use in friction and wear related applications. In order to improve wear resistance, many surface technologies, such as a hard anodizing, electroplating and physical vapor deposition have been applied to aluminum alloys. But these surface modification layers are too thin to sustain the applied high load [2,3]. Hence, it is necessary to form a thick layer on the surface, thermal spraying is the most favorable surface modification technique for aluminum alloys [4]. The wear resistant layer

2 can be successively achieved by various thermal spraying techniques with introduction of hard oxide and carbide particles into the wear resisting surface. The work of this paper describes the effect of thermal spraying techniques (specifically flame, plasma spraying and high velocity oxy-fuel) on the properties of oxide and carbide coatings (Al 2 O 3 TiO 2, WC-Co) containing fluoropolymer materials applied on an age hardenable aluminum alloy. Microstructure and wear resistance behaviour of the different coatings obtained with optimized parameters were studied and compared. Experimental The substrate material used in this study was a 2014-T6 aluminum alloy having hardness of 130 HV 0,5. Disc shaped samples having 25 mm diameter and 10 mm thickness were degreased and sandblasted prior to processing and then coated with Ni-Cr bond layer. The mixtures of 10% PTFE with alumina-titania powders (87 wt.% small Al 2 O 3 13 wt.% TiO 2 ) and 6% ETFE with tungsten carbide cobalt powders (88 wt.% WC 12 wt.% Co) were sprayed over the bond layer by flame, plasma and HVOF spray processes. The grain sizes of alumina-titania and tungsten carbide cobalt powders were µm and µm, respectively. Table 1 lists the coating properties of examined samples. The spraying conditions adopted for the three processes are given in Table 2. Table 1. Coating properties of examined samples. Samples Sample 1 Sample 2 Sample 3 Sample 4 Al 2 O 3 13TiO wt.% PTFE Composition Al 2 O 3 13TiO wt.% PTFE WC12Co + 6 wt.% ETFE WC12Co + 6 wt.% ETFE Spraying conditions Flame process Plasma process Plasma process HVOF process

3 Table 2. Spraying conditions adopted for flame, plasma and HVOF processes. Coating process Coating Sample 1 Sample 2 Sample 3 Sample 4 1. Flame process O 2 pressure (MPa) O 2 flow rate (m 3 /h) Acetylene pressure(mpa) Acetylene flow rate (m 3 /h) Feed Rate (kg/h) Spraying distance (m) 1. Plasma process Current (A) Voltage (V) Feed Rate (kg/h) Spraying distance (m) 1.HVOF process O 2 flow rate (m 3 /h) Ratio of O 2 /Kerosene Carrier gas Spraying distance (m) Argon 0.2 The characterization of the coatings was made by microscopic examinations, thickness and hardness measurements. Microscopic examinations were conducted on the surfaces of the coated samples by a scanning electron microscope (SEM), after grinding and polishing in standard manner. The thickness and the hardness of the coatings were measured on the polished cross sections. Hardness measurements were carried out with a Vickers indenter under the indentation load of 500 g. The tribological performance of the coatings was examined by a reciprocating wear tester at normal atmospheric condition (room temperature and 50 % relative humidity). The surfaces of the coatings were ground by 1200 grit SiC abrasive paper before testing. Wear tests were performed by rubbing 10 mm diameter Al 2 O 3 balls to the coatings under test load of 2 N. The stroke of the ball on the surface of the samples was 12 mm with sliding speed of 0.02 m/s. During testing, friction force was continuously recorded by a computer. After the wear test, the wear tracks developed on the surfaces of the coatings were examined by a SEM and a light optical microscope (LOM).

4 Result and Discussion Figure 1 illustrates the SEM surface micrographs of the examined coatings. In coatings dark spots corresponded to the polymer. For the oxide coatings (Fig 1 a, b) plasma process resulted in lower porosity fine microstructure than that of flame spray. In addition, in the microstructure of flame sprayed coating (Fig 1 a) a certain amount of unmolten particles which adhere weakly to the coating are present. When compared to flame spraying, plasma process deposited an extraordinarily dense alumina/titania ceramic + polymer coating because of its higher energetic jet. On the other hand for carbide coatings, the HVOF yielded denser microstructure when compared to the plasma process (Fig 1 c, d). HVOF is usually employed for depositing these coatings to avoid significant amount of reduction of carbides to brittle carbides and oxy-carbides due to the much lower temperature of the powder particles in the exhaust gas stream and less in-flight time as compared to that in plasma [5,6]. Also, the higher particle velocities in the high velocity process lead to better coating properties like higher bond strength, density and lower oxide content. Fig. 1. SEM surface micrographs of (a) Sample 1, (b) Sample 2, (c) Sample 3 and (d) Sample 4 coatings.

5 The characteristics of the coatings examined in this study are listed in Table 3, as thickness, and hardness data. The thickness of the coatings varied in between 100 and 420 µm. Hardness measurements conducted on the cross sections of the coatings revealed that, Sample 4 is the hardest and Sample 1 is the softest coating. The hardness of oxide coating applied by plasma method is higher than that of the flame sprayed method. For carbide coatings, HVOF coating resulted in a slightly lower porosity than that of the plasma coating. For both oxide and carbide coatings, high porosity content caused substantial reduction in hardness value. Table 3. The coating properties. Coating Thickness (µm) Hardness (HV 0,5, kg/mm 2 ) Sample Sample Sample Sample Fig.2 shows the variation of friction coefficient of coatings against Al 2 O 3 ball during reciprocating wear testing. Steady state friction coefficient value of the flame sprayed oxide coating was about 0.6. While the friction coefficient value of plasma sprayed oxide coating was high at the beginning of the test, then decreased gradually to about Heavy fluctuations of friction coefficient were evident for oxide coatings through out the testing period, which can be attributed to the production of third body particles during wear testing [4]. Upon application of carbide coatings, fluctuations were eliminated with severe reduction in the value of friction coefficient. The steady friction coefficient values of both carbide coatings varied in between 0.19 and The friction coefficient of WC-Co-ETFE coating is lower than that of the conventional coating (0.4). (a) (b) Figure 2. The variation of friction coefficient during reciprocating wear testing of (a) oxide and (b)carbide coatings.

6 Fig. 3 depicts 3D - profile images of the wear tracks produced on the coatings. For oxide coatings, wider and deeper wear track was developed on the coating produced by flame spray method than that of coating formed by plasma process. Among the carbide coatings, the wear track developed on the coating produced by HVOF method Sample 4 was difficult to be identified by the profilometer, since wear progressed mainly by smoothening of the asperities. Figure 3. Optical views and 3-D profile images of the wear tracks developed on the oxide and carbide coatings. The area of the wear tracks, calculated from the 3D - profile images are presented in Table 3. The area of the wear track is smaller for carbide coating than oxide coatings in accordance with their hardness. For oxide coatings, the area of the wear track produced by plasma spraying yielded higher wear resistance than flame spraying. Among the carbide coatings, WC Co-ETFE coating deposited by HVOF method has a lower wear area compared to that of WC Co-ETFE coating produced by plasma method. It has been reported that carbide containing coatings deposited by high velocity processes have good wear resistance compared to plasma-sprayed coatings due to the better coating properties achievable in case of high velocity processes [7]. Table 3. Wear track areas of oxide and carbide coatings. Coatings Wear Track Area (mm 2 ) Sample 1 12 x 10-3 Oxide Sample x 10-3 Sample x 10-3 Carbide Sample x 10-3

7 Fig. 4 depicts the SEM micrographs of the wear tracks. Oxide coatings exhibited relatively rough wear contact surface than carbide coatings. In some regions of the wear tracks, Ni-Cr bond layer, which were probably detached from the coating during testing, was present. These layers were more frequently observed in the wear track of the flame sprayed oxide coating, when compared to plasma sprayed coating. Therefore, heavy fluctuations in friction coefficient of oxide coatings (Fig.2) can be correlated with third body abrasive effect of these particles detached from coatings. Microcracks were observed within the wear tracks of plasma sprayed carbide coating. Owing to high porosity of the plasma sprayed carbide coating, the new microholes resulted from the brittle carbide phase such as W 2 C removal during wear test and there are more holes and microcracks exist on the worn surface. Figure 4. SEM views of the wear tracks developed on the (a) Sample 1, (b) Sample 2, (c)sample 3, and (d) Sample 4 coatings.

8 Conclusion In the present study, the wear behaviour of oxide+polymer and carbide+polymer coatings deposited on 2014-T6 aluminum alloy by flame, plasma and HVOF spraying processes has been examined. The results of examinations can be summarized as follows: 1. Plasma sprayed coating as 10 % PTFE with alumina-titania resulted in improvement in wear resistance when compared to flame sprayed coating. 2. HVOF yielded higher wear resistance than plasma spraying for 6% ETFE with tungsten carbide cobalt coating. 3. Carbide coatings exhibited higher wear resistance and lower friction coefficient than oxide coatings. References 1. S. Tomida, K. Nakata, S. Saji, T.Kubo, Surf Coat Tech, , , PH. Chong, H.C. Man, T.M. Yue, Surf Coat Tech, 145:51, T. Nagae, S. Tomida, A. Okada, N. Inada, Surf Coat Tech, ,174 7, U. Akın, H. Mindivan, E.S. Kayali, H.Cimenoglu, Key Eng Mater; : G. Barbezat, A.R. Nicoll, A. Sickinger, Wear (1993) A. Karimi, Ch. Verdon, Surf. Coat. Technol. 57 (1993) J.K.N. Murthy, B. Venkataraman, Surface & Coatings Technology 2004.