J O U R N A L O F M AT E R I A L S S C I E N C E L E T T E R S 1 8 ( 1 9 9 9 ) 7 2 7 ± 7 2 9
Wear and thermal fatigue characteristics of plasma-sprayed alumina coatings P . P . B A N D Y O P A D H Y A Y , S . D A S { , S . M A D H U S U D A N { , A . B . C H A T T O P A D H Y A Y Departments of Mechanical Engineering and {Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India E-mail:
[email protected]
of these materials' combinations to cyclic thermal stress were also explored. The substrate specimens were cut from a plain carbon steel plate having a carbon content of 0.42 wt %. The dimensions were 13 mm 3 13 mm3 5 mm. The specimens were grit-blasted by 250 ìm alumina grits to an average roughness of about 6 ìm. The air pressure and stand-off distance were 60 kg cmÿ2 and 100 mm, respectively. Before spraying, the specimens were cleaned ultrasonically. Yttria powder was suspended in ether and painted onto the substrate just before spraying. In all cases, spraying was done immediately after shot blasting. The substrate=top coat=bond coat combinations are listed in Table I. The alumina powder used here was of Indian origin (Al2 O3 99.5 wt %, Na2 O 0.28 wt %, SiO2 0.045 wt %, Fe2 O3 0.02 wt %, TiO2 0.07 wt %, CaO 0.025 wt %). The Metco 450NS (Ni±5%Al) is a standard bond coat material for spraying alumina. It has been suggested by the vendor that Metco 449P (3.0 wt % Mo, 3.0 wt % C, 3.0 wt % Al, 0.1 wt % B and the rest Fe) can possibly act as a substitute for the expensive Metco 450NS. The plasma-spraying parameters are given in Table II. The bond and top
The advantage of coating technology, in general, is that it marries two dissimilar materials to improve in a synergistic way the performance of the aggregate. Usually, the substrate provides mechanical strength and toughness while the coating provides protection against environmental degradation including wear, corrosion and thermal attack. Thermal spraying has emerged as an increasingly sophisticated tool in surface engineering technology, research and development. Among the various thermal-spraying techniques, plasma spraying is gradually ®nding wide application mainly for its extremely high temperature, which is essential for dealing with coating materials like carbides and ceramics, whose melting temperatures are very high. The process, however, has certain limitations. The cost of equipment and the consumables are quite high and the process gives a porous coating with an irregular reticula of cracks running through it. This is especially true for brittle materials like alumina [1, 2]. The porosity adversely affects the wear properties [3], and the cracks allow the environment to attack the bond coat [4, 5], resulting in a premature failure in service. Being brittle, these coatings are also vulnerable to thermal fatigue [6]. This is a matter of concern because hard coatings are often used in wear and other applications [7, 8]. This letter deals with the characteristics of a plasma-sprayed alumina coating. The alumina powder is of Indian origin and is very inexpensive in comparison to commercially available plasma sprayable alumina powder. Here, an attempt has been made to use yttria as an oxidation and diffusion barrier. The coating substrate interface was investigated by scanning electron microscopy (SEM). The wear behavior of this alumina and the effect of using different bond coats was also studied. The responses
TA B L E I The base metal, bond coat and top coat combinations used in spraying Sl. no Base metal
Bond coat
Top coat
1
plain carbon steel
none
2
none
3
plain carbon steel with an yttria paste applied on plain carbon steel
alumina of Indian origin (INDAL make; HT grade) Do
4
plain carbon steel
Ni±5%Al (Metco 450NS) high-carbon iron (Metco 449p)
Do Do
TA B L E I I The parameters of plasma spraying Parameters Primary gas ¯ow Secondary gas ¯ow Stand-off distance Arc voltage Arc current Primary gas pressure (N2 ) Secondary gas pressure (H2 )
Units ÿ1
cft h cft hÿ1 mm volts amp psi psi
0261-8028 # 1999 Kluwer Academic Publishers
Ni±Al
Consumables high-carbon iron
alumina
75 10 75 72 500 50 50
75 10 75 70 500 50 50
75 10 75 74 500 50 50
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coat thicknesses were about 100 ìm and 400 ìm, respectively. The coated specimens were transverse-sectioned by wire electrodischarge machining (EDM). The exposed cross-section was polished with diamond paste and subsequently examined under SEM using the secondary electron imaging mode. For thermal fatigue, the specimens were soaked for about 45 min at 900 8C followed by air cooling for about 15 min. The same procedure was repeated until the coating peeled from the substrates. The dry sliding abrasive test was carried out on a laboratory polishing wheel with emery paper (SiC abrasive of sieve size 220) clamped onto it. The specimen was pressed against the emery paper with a constant normal pressure of 0.029 N mmÿ2 while the wheel rotated at a speed of 1.25 m sÿ1 . For this purpose, a special specimen holder was fabricated. Fig. 1 exhibits the top view of the alumina-coated surface. The splats of partially molten deformed alumina powders, along with a few unmelted particles, can be seen here. The ®gure con®rms the presence of a few branching cracks. Alumina is a very brittle material, and cracking occurs during the rapid cooling and accompanying shrinkage. Figs 2±5 include the scanning electron micrographs (secondary electron imaging mode) of several interfaces consisting of coating, bond coat and substrate. A ceramic coating does not adhere properly when
Figure 3 SEM micrograph of steel (MS)±yttria±alumina interfaces.
Figure 4 SEM micrograph of steel (MS)±high-carbon iron (BC)± alumina interfaces.
Figure 1 Top view of the alumina coating using SEM.
Figure 5 SEM micrograph of steel (MS)±Ni±5%Al (BC)±alumina interfaces.
Figure 2 SEM micrograph of steel (MS)±alumina interface.
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applied directly onto steels. Although Fig. 2 does not show any discontinuity at the steel±alumina interface, the coating appears to be porous. A thin layer of yttria is visible between the substrate and top coat, as shown in Fig. 3. Applications of both bond coats, high-carbon iron and Ni±Al result in apparently sound coating interfaces, as depicted in Fig. 4. and Fig. 5, respectively. The results obtained from a thermal fatigue test are shown in Table III, which reveals the number of cycles the coatings survived before completely
TA B L E I I I Thermal fatigue test results showing the number of cycles the coatings survived before peeling Bond coat
No of cycles
1 2 3 4
none high carbon iron only yttria paste is applied Ni±Al
2 21 51 55
peeling. The specimens without a bond coat peeled in no time owing to rapid oxidation of the mild steel substrate just below the coating. The same observation is true for the high-carbon iron bond coat (3 wt % carbon), as well, although in this case the coating lasted a little longer. It should be noted that the porous ceramic top coat cannot prevent oxygen diffusion through it to the substrate. A thin layer of yttria, when applied to the substrate, prevents the ingress of oxygen through the alumina layer to the substrate, which is prone to oxidation, especially at a high temperature. Therefore, the yttria-painted substrate has a much better life. In the case of the Ni± Al bond-coated specimens, such improvements can be attributed to a different reason altogether: During coating, nickel and aluminum react to form an intermetallic compound at high temperature, and such compounds are capable of absorbing oxygen in their interstitial sites. Thus, oxidation of either bond coat or substrate can only occur after the bond coat becomes saturated with oxygen. The plots of wear versus time are depicted in Fig. 6. Coatings without a bond coat suffer from a premature failure, whereas the ones having a bond coat perform satisfactorily. This is most likely due to the fact that bond coats by themselves are directly contributing to the wear resistance after the top alumina coats are worn off. Incomplete melting of ceramic powders has presumably contributed to the rapid failure of the coatings when subjected to the abrasive wear test. It seems that further adjustment of plasma parameters is necessary to develop a stronger and denser coating from the alumina of Indian origin. In summary, alumina of Indian origin can be used to develop wear-resistant plasma-sprayed coatings, provided the porosity of the alumina coatings is minimized to a great extent. The spraying parameters are to be selected more carefully in the future to improve the general coating qualities. Yttria
0.0004
0.0003 Wear in g/mm2/N
Sl. no.
0.0002
BOND COAT: Ni-Al NO BOND COAT BOND COAT: HIGH CARBON IRON NO BOND COAT, YTTRIA PASTE APPLIED
0.0001
0.0000 0
200
400 600 Time in seconds
800
1000
Figure 6 Plot showing the cumulative wear of plasma-sprayed alumina coating. Substrate: plain carbon steel; load: 4.905 N; speed: 1.25 m sÿ1 .
seems to act as a barrier for oxidation and interdiffusion. Performance of Ni±Al is better than that of high-carbon iron as a bond coat material for coating alumina on steel.
References
1. T. J. K AT TA M I S , M . C H E N , R . H U I E , J. K E L LY, C . F O U N T Z O U L O U S and M . L E V Y, J. Adhesion Sci. Technol. 9 (1995) 907. 2. A . O H O M O and K . K A M A D A , J. Mater Sci. Lett. 11 (1992) 108. 3. D. C H UA N X I A N , H . B I N G TA N G and L . H U I L I N G , Thin Solid Films 118 (1984) 485. 4. R . S I VA K U M A R and M . P. S H R I VA S TAVA , Oxidation of Metals 20 (1983) 69. 5. S . L . S H I N D E , D. A . O L S O N , L . C . D E J O N G H E and R . A . M I L L E R , Ceram. Engng. Sci. Proc. 7 (1986) 33. 6. T. K U R U S H I M A and K . I S H I Z A K I , Mater. Manuf. Processes 8 (1993) 465. 7. K . H O L M B E R G and A . M AT H E W S , Thin Solid Films 253 (1994) 173. 8. H . O N O , T. T E R M O T O and T. S H I N O D A , Mater. Manuf. Processes 8 (1993) 541.
Received 4 September and accepted 10 November 1998
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