Theoretical consideration on composite oxide scales and coatings

JOURNAL OF RARE EARTHS, Vol. 31, No. 5, May 2013, P. 435

Theoretical consideration on composite oxide scales and coatings HE Yedong (ԩϮϰ)1,*, GAO Wei (催 ଃ)2 (1. Beijing Key Laboratory for Corrosion-Erosion and Surface Technology, University of Science and Technology Beijing, Beijing 100083, China; 2. Engineering School, The University of Auckland, Auckland P.B. 92019, New Zealand) Received 19 April 2013; revised 3 May 2013

Abstract: The present paper discussed some fundamental aspects on composite oxide scales and coatings for protection of alloys from high temperature oxidation, the related thermodynamic conditions, special mechanical characteristics and a sealing mechanism. It was proposed that the oxide scales and coatings with a composite structure should possess superior mechanical properties than that with a single phase oxide. It also showed that the Al2O3 scales or coatings doped with Y2O3 and ZrO2 (or YSZ)-Al2O3 composite coatings possessed superior properties at high temperatures. In such composite oxide scales and coatings, the fracture resistance of the scales was increased by the toughening effect, the thermal stress was decreased owing to the increase of thermal-expansion coefficients, and Al2O3 phase could seal the alloy substrate well. In addition, the kinetic equation of thermal growth oxide on alloy covered with composite oxide coatings was derived. Keywords: rare earth oxides; composite oxide scales; composite oxide coatings; high-temperature oxidation of alloys

It is well known that the oxidation resistance of alloy coatings is determined by the formation of Al2O3, Cr2O3 or SiO2 scales and their failures caused by the stresses generated in oxide scales. The stresses mainly consist of growth and thermal stresses, due to which oxide scale cracking and spallation take place. With ceramic coatings, the oxidation resistance of alloys mainly depends on the integrality of ceramic coatings. Therefore, the processes of scale cracking and spalling are the key factor to influence the lifetime of alloy and ceramic coatings. A number of models on the processes of cracking and spalling of oxide scales have been proposed so far. These models normally assume that the oxide scale formed on alloys is a homogeneous single phase material[1,2]. Consequently, cracking and spalling of oxide scales and ceramic coatings with a single phase cannot be avoided completely. As ceramic composites can effectively enhance the strength and durability of ceramic components and improve their fracture toughness, it is reasonable to propose that the oxide scales and coatings with a composite structure should possess improved mechanical properties than that with a single phase oxide. In fact, the most of oxide scales formed on alloys are composites. For example, NiO/NiCr2O4 and NiO/NiAl2O4 muti-layered scales can form on Ni-Cr and Ni-Al alloys; Al2O3 and Cr2O3 scales doped with small amount of rare earth oxides can be formed on Ni-Cr-Al-RE and Ni-Cr-RE (RE denotes rear earth elements) alloys[3]. These composite oxide

scales often exhibit better resistance to cracking and spallation than the single phase oxide scale. In addition, recent researches have shown that the ceramic coatings with composite structures can effectively resist scale spallation and oxidation at high temperature[4,5]. In this paper, theoretical consideration on the composite oxide scales and coatings will be presented by taking Y2O3 doped Al2O3 scales and ZrO2-Al2O3 or YSZ (yttrium stabilized zirconia)-Al2O3 composite coatings as examples.

1 Fundamental aspects on composite oxide scales and coatings 1.1 Thermodynamic condition By the definition, composite oxide scales and coatings should have a multi-phase structure. Therefore, the ingredients of composite oxide scales and coatings should be located in a eutectic zone of phase diagrams, such as Al2O3-Y2O3[6], ZrO2-Al2O3[7] phases diagrams. By this way, composite oxide scales and coatings with various structure design for improving both mechanical properties and oxidation resistance must be thermodynamically stable. As shown in Fig. 1, the eutectic zones in the phase diagrams of binary metal oxides used for designing composite oxide scales and coatings can be divided into three types: (1) metala oxide –metalb oxide eutectic zone; (2) metala oxide–metala+metalb oxide compound (including solid solution) eutectic zone; and (3) metala+

Foundation item: Project supported by National Natural Science Foundation of China (51071030) * Corresponding author: HE Yedong (E-mail: [email protected]; Tel.: +86-10-62332715) DOI: 10.1016/S1002-0721(12)60300-7

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Fig. 1 Three types of eutectic zones in phase diagrams of binary metal oxides

metalb oxide compound (D)–metala+metalb oxide compound (E) eutectic zone. 1.2 Special mechanical properties The mechanical properties of ceramics including strength, fracture toughness and plasticity can be effectively improved by the formation of multi-phase composite. According to the types of reinforcements, the toughening effects of composite ceramics can be divided into nano-particle toughening, micro-particle toughening, whisker toughening, fiber toughening and multilayer toughening. The main toughening mechanisms in multiphase composite ceramics include phase transformation toughening, microcrack formation, crack deflection, crack bridging, etc., as shown in Fig. 2. It is reasonable to propose that above mechanisms can be used to explain the behavior of composite oxide scales and coatings. However, because the real materials studied for high temperature oxidation involve composite oxide scales, coatings and alloys substrate, the stresses generated in composite oxide scales or coatings are restricted by the alloy substrates as the oxidation progresses. The mechanical behavior in composite oxide scales or coatings is more complicated than that in bulk composite ceramics. Moreover, owing to the limitation in thickness, composite oxide scales or coatings are often designed and fabricated to possess finer structures than that of the bulck composite ceramics. Therefore, it is necessary to study the special mechanical characteristics in composite oxide scales or coatings. Cracking and spalling of oxide scales and coatings are often caused by thermal stresses, which arise from dif-

ferential thermal expansion between the alloy substrate and scale. Timoshenko[8], derived the thermal stress, ox, in oxide scale during cooling as:  Eox (ox  m ) 'T (1)  ox 1  Where Eox is the elastic modulus, ox and m are the linear thermal-expansion coefficients for the oxide and metal, and 'T is the change in temperature. As analyzed by Evans et al.[9,10], spallation of a single phase oxide scale will occur when the elastic strain energy stored in the scale exceeds the fracture resistance, Gc, of the interface. The criterion for failure is given as the following equation: (1   ) ox h / Eox ! Gc 2

(2)

Where  is the Poisson’s ratio of the oxide scale, h is the scale thickness, and ox is the equal biaxial residual stress in the scale. Comparing with the single phase oxide scales and coatings, the fracture resistance, Gc, of composite oxide scales or coatings can be enhanced by toughening actions. According to Eq. (2), there are two conditions to avoid scale spallation: increasing Gc that permits the oxide scale to withstand bigger stresses, or decreasing the stress, ox, in oxide scales by increasing the thermal-expansion coefficients, ox, in Eq. (1). 1.3 Sealing mechanism

In order to improve the oxidation resistance of an alloy, we propose a “sealing mechanism” for designing the composite oxide scales or coatings. Such oxide scales or coatings should have a special structure, in which at least

Fig. 2 Toughening mechanisms in multi-phase composite ceramics (a) Phase transformation toughening; (b) Microcrack formation; (c) Crack deflection; (d) Drack bridging

HE Yedong et al., Theoretical consideration on composite oxide scales and coatings

one phase with the lowest oxygen diffusion coefficient could seal the alloy substrate. Below we present several examples to show this principle.

2 Typical composite oxide scales and coatings systems 2.1 Al2O3-Y2O3 composite scale

Al2O3 and Cr2O3 scales doping with rare earth oxide are well known examples of composite oxide scales, which can form on MCrAlY and MCrY alloys or alloy coatings respectively[11]. The resistances of high temperature oxidation and spallation of these oxide scales doping with rare earth oxide are increased obviously. There are many researches on this topic; most of them related to the effects of rear earth element on the selective oxidation of Al and Cr elements in the alloys, the transport properties in these oxide scales and the bonding behavior at the alloy/scale interface[3]. But from the composite materials point of view, the beneficial effects on the mechanical properties of oxide scales doped with rare earth oxide can be attributed to the multiphase composite structures. Taking Al2O3-Y2O3 composite for example, recent studies show that the Al2O3-YAG (Y3Al5O12) eutectic is a potential candidate material used at ultra- high temperature with excellent mechanical properties from room temperature to 1973 K[12]. Gil et al.[13] studied Y-rich oxide distribution in the alumina scale on plasma sprayed MCrAlY coatings by SEM with a cathodoluminescence detector and Raman spectroscopy. The presented results indicate that the type of the Y-rich oxide phases, such as YAG (Y3Al5O12) and YAP (YAlO3), after heat-treatment can be correlated with the content and/or reservoir of metallic Y in the coating. The distribution of the Y-rich oxide precipitates formed during manufacturing has been shown to affect the growth rate and mechanical stability of the alumina scales during service. From thermodynamics, Al2O3 scales doped with a small amount of yttrium oxide should have an Al2O3YAG eutectic structure finally, as shown in Fig. 3(a), according to the Al2O3-Y2O3 phase diagram[6]. Because

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the amount of YAG phase is very low in the composite scale, YAG particles appear to be segregated at the Al2O3 grain boundaries. The mechanical properties of oxide scales with such structure have been improved by the following factors. Firstly, YAG particles can inhibit the grain growth of Al2O3 grain and increase the strength of oxide scale following the Hall-Petch relation (Vy=V0+ kd–1/2). Secondly, YAG particles can change the propagation direction of cracks in oxide scale, consuming more energy. Consequently, the fracture resistance, Gc, of Al2O3-YAG composite scale in Eq. (2), is increased, so Al2O3-YAG composite scale can withstand higher stresses without spallation. Research has also shown that the mass transport mechanism in Al2O3 and Cr2O3 scales is changed by doping with rare earth oxides[3]. It was suggested that the diffusion of oxygen along grain boundaries is accelerated by the reactive element oxide particles segregated at the oxide grain boundary areas. However, the diffusion of oxygen in such oxide scales is very slow[14]. Therefore, Al2O3 and Cr2O3 scales doped with rare earth oxides formed on alloys during high temperature oxidation can effectively seal the substrate alloys from further oxidation. 2.2 Al2O3-Y2O3 composite coating

Al2O3 coatings doped with a small amount of Y2O3 have an Al2O3-YAG eutectic structure as well, as shown in Fig. 3(b). In our research work, Al2O3-YAG composite coatings were successfully fabricated on NiCoCrAlY alloy substrates by electrophoretic deposition in Y3+ doped composite sol-gel suspensions and pressure filtration microwave sintering[15]. The composite coating was dense, crack-free and had good adherence with substrate even after cycling oxidation at 1000 ºC for 200 h. Filed emission SEM showed that nano-size YAG particles were embedded in D-Al2O3 matrix. The high-temperature cyclic oxidation tests revealed that the oxidation resistance and spallation resistance were greatly improved. The oxidation resistance of alloy is mainly determined by D-Al2O3 sealing effect and rare earth elements effects on the thermally grown oxide scale. Moreover, the enhanced

Fig. 3 Schematic diagrams of Al2O3-Y2O3 composite scale (a) and coating (b)

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fracture toughness and strength of this composite coating can be attributed to the nano/submicro composite structure and YAG particles toughening effects that such coatings exhibit excellent resistance to high-temperature oxidation, while pure Al2O3 coating demonstrates poor resistance to oxidation due to serious oxide scale spallation. Therefore, Al2O3-Y2O3 composite scales or coatings have a similar mechanism on improving mechanical properties. These examples show that the improvement of mechanical properties caused by the Al2O3-YAG eutectic structure in scales and coatings is responsible to the so-called “reactive element effects”. 2.3 Two types of ZrO2 (or YSZ)-Al2O3 composite coatings

Many research works have proved that ZrO2 (or YSZ)Al2O3 composite with eutectic structure possesses very good mechanical properties. For example, the strength of the Al2O3/(t-ZrO2+m-ZrO2) composites after surface grinding can reach the value as high as 940 MPa, which is roughly three times of that of Al2O3. The strengthening effect is attributed to the microstructural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding t-phase and m-phase zirconia, which can reach values as high as two times of Al2O3. The toughening effect is attributed mainly to the zirconia t-m phase transformation[16]. There are two kinds of ZrO2-Al2O3 composite materials: (a) ZrO2 reinforced with alumina particles, and (b) alumina reinforced with zirconia particles. In both cases, the fracture toughness of the ceramic matrix material is increased[17]. Attention has also been paid to develop ZrO2 (or YSZ)-Al2O3 composite coatings recently. For example, a novel ZrO2-Al2O3 thermal barrier composite coating was produced using a gas-tunnel-type plasma spraying torch[18]. The unique microstructure features of this composite coating include a relatively even distribution of embedded thin ZrO2 splats in a Al2O3 matrix, parallel alignment of the ZrO2 splats relative to the substrate surface, absence of porosity between the ZrO2 splats and the

Al2O3 matrix, etc. This anisotropic composite coating combined with the large difference in thermal conductivity between ZrO2 and Al2O3 will alter the thermal behavior of the coating. However, because the Al2O3 phase in this coating can not seal the alloy substrate completely, this coating can not protect the alloy substrate from oxidation efficiently. In order to enhance the oxidation resistance of coatings, two kinds of ZrO2 (or YSZ)-Al2O3 composite coatings with special structures have been developed as shown in Fig. 4. The first kind of coating with a structure: micro-sized ZrO2 (or YSZ) particles packed in nano-sized Al2O3 films, as shown in Fig. 4(a). The second kind of coating with a multi-layered structure, as shown in Fig. 4(b). In both cases, the Al2O3 phase can seal the alloy substrate and inhibit the oxygen diffusion by a step to step way. Therefore, these coatings can protect the alloy substrate from oxidation effectively. In our research work[19], a novel sol-gel process has been utilized to fabricate Al2O3/YSZ (6 wt.% yttria partially stabilized zirconia) composite coatings on Ni-based superalloy. The green coatings were obtained by electrophoretic deposition in a suspension containing aluminium oxide sol, nano-Al2O3 and micro-YSZ particles, and then treated by so-called pressure filtration microwave sintering process. The as-sintered composite coatings were dense, uniform and crack-free and the phases mainly present -Al2O3, m-ZrO2 and t-ZrO2 with aluminium oxide sol content decreasing. The cyclic oxidation tests at 1000 ºC for 200 h demonstrate that both oxidation rate and oxide spallation decreased with increasing ratio of [YSZ]:[Al2O3] in coatings. The beneficial effects can be attributed to the special structure of microYSZ coated with nano/submicron Al2O3 particles, which can seal the alloy substrate and inhibit the diffusion of oxygen ions, and the improvement of the mechanical properties of the composite coating as the thermal stress was decreased with increasing ratio of [YSZ]:[Al2O3]. YSZ/Al2O3 multi-layered coatings have been prepared by electrolytic deposition and microwave sintering in our laboratory[20,21]. Laminated structures of alternated Y2O3

Fig. 4 Schematic diagrams of two types of ZrO2 (or YSZ)-Al2O3 composite coatings

HE Yedong et al., Theoretical consideration on composite oxide scales and coatings

stabilized t-ZrO2 (YSZ) and Al2O3 layers formed in the coating with the phases of t-ZrO2, -Al2O3 and -Al2O3. The experiment results indicate that such coatings exhibit not only excellent oxidation resistance but also good spallation resistance under thermal cycling. It is demonstrated that there is a size effect for the (Al2O3-Y2O3)/ YSZ micro-laminated coating on high-temperature oxidation resistance[22]. Experimetal results of high temperature oxidation indicate that both oxidation resistance and spallation resistance of the coating have been improved significantly, by increasing the thickness ratio of YSZ layer to Al2O3-Y2O3 layer and layer number in a specific range. As these ZrO2 (or YSZ)-Al2O3 composite coatings showed superior mechanical properties and oxidation resistance, we believe that the beneficial effects can be attributed to the composite structures. The strength and fracture toughness of the oxide are improved due to the crack deflection or bifurcation at the ZrO2/Al2O3 interfaces and ZrO2 t-m phase transformation. The fracture resistance, Gc, of the ZrO2-Al2O3 composites is increased. Furthermore, the level of thermal stress, ox, in these coatings should be decreased comparing with the single phase Al2O3 coating due to the lower Young’s modulus, Eox, and higher thermal- expansion coefficient, ox, in Eq. (1). At the same time, these composite oxide coatings are effective barriers for oxygen diffusion, because that the Al2O3 films can seal the alloy substrate completely. In addition, these two kinds of YSZ -Al2O3 composite coatings can be used as thermal barrier coatings when the coatings are thick enough and the ratio of [YSZ]:[Al2O3] is high[23]. Furthermore, our research has proved that these coatings can be used as the bottom coatings to replace the alloy bonding layer in the traditional yttrium stabilized zirconia (YSZ) thermal barrier coatings[24].

3 Kinetics of TGO on alloy covered with composite oxide coatings If the growth kinetics of thermal growth oxide scale (TGO) on alloy follows the parabolic law, it may be assumed that the oxygen barrier efficiency of the supplied composite oxide coating on the surface of alloy is equivalent to a TGO with the thickness, x0, grew for time, t0: (3) x02=kt0 So, the thickness, x, of TGO on alloy covered with composite oxide coating is the function of time, t, and can be written as: (4) (x0+x)2=k(t0+t) Substitute equation (3) in Eq. (4): (5) 2x0x+x2=kt As x0>>x, Eq. (5) can be simplified as kt 2x0 x | kt or x | (6) 2 x0

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Therefore, the growth kinetics of TGO on alloy covered with composite oxide coating follows a linear relation with time, as shown in Fig. 5. When the thickness, x0, of composite coating is thick enough, the oxidation rate of an alloy covered with such composite coating should tend to zero.

Fig. 5 Growth kinetics of TGO of alloy covered with composite oxide coating

4 Summary (1) Oxide scales and coatings with a composite structure possess superior mechanical properties than that with a single oxide phase. (2) The composition of composite oxide scales and coatings should be located in the eutectic zone of phase diagrams. The composite oxide scales and coatings with various structural designs and preparations must be stabile thermodynamically. (3) The stresses generated in the composite oxide scales or coatings are affected by the restriction of the alloy substrates as the oxidation progresses. The mechanical behavior in the composite oxide scales or coatings is more complex than that in the bulk composite ceramics. (4) A sealing mechanism for designing the composite oxide scales and coatings is proposed. In these composite scales or coatings at least one phase with the lowest oxygen diffusion coefficient should seal the alloy substrate. (5) It is demonstrated that the property improvement of Al2O3 scales or coatings doped with rare earth oxides and ZrO2 (or YSZ)-Al2O3 composite coatings can be attributed to these factors: in composite oxide scales and coatings, the fracture resistance is increased by second phase toughening, the thermal stress is decreased owing to the increase of thermal-expansion coefficients, and Al2O3 phase can seal the alloy substrate completely. (6) The kinetics of TGO on alloy covered with composite oxide coatings follows a linear law. When the thickness of composite coating is thick enough, the oxidation rate of an alloy covered with such composite coating should tend to zero.

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