Morphology, structure and formation mechanism of silicide coating by pack cementation process

XIAO Lai-rong(肖来荣), CAI Zhi-gang(蔡志刚), YI Dan-qing(易丹青), YING Lei(殷 磊),
LIU Hui-qun(刘会群), HUANG Dao-yuan(黄道远)

School of Materials Science and Engineering, Central South University, Changsha 410083, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The MoSi₂ coating on C103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic rule during pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. The main layer is composed of MoSi₂, the two phases’ transitional layer consists of NbSi₂ and a few Nb₅Si₃ and the diffuse layer is composed of Nb₅Si₃. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance can effectively prevent the diffusion of oxygen into coating. 

Key words: niobium based alloy; pack cementation; silicide coating; formative mechanism

1 Introduction

Nb-based alloys have been used as important high temperature structural materials due to their high melting point and good high temperature strength[1, 2]. However, the poor high temperature oxidation resistance properties become an inevitable obstacle in application[3, 4]. As the requirement of new two-component orbit attitude controlled engine, Nb-based components should be endured at high temperature circumstance longer. Therefore, the studies of corresponding high temperature oxidation resistance coating get more and more attractive. With high melting point, moderate density and excellent high temperature oxidation resistance properties, MoSi₂ becomes one of important candidate materials which can be applied as protective coating on refractory alloys and C/C composites [5, 6]. 

RYOSUKE et al [6-8] used melting salt, laser fused and CVD method respectively to produce MoSi₂ coating on molybdenum and alloyed steel substrates. In the present work, MoSi₂ coating was produced on C103 Nb alloy by preparing a Mo layer using slurry firing technology with subsequently packed silicide cementation treatment. The formation mechanism of coating during silicification, characterizations of surface and crossed-section, transformation of structures after oxidation were investigated to provide benefic references for improving the properties of coating.

C103 Nb alloy with composition of 89Nb-10Hf-1Ti was cut into d 3 mm×40 mm sheets as specimens. After grinding, the specimens were treated by alkali solution, ultrasonic cleaning in ethanol, acid solution and distilled water respectively. Then the treated specimens were left in the drying chamber until the surface was dry.

Slurry was produced by ball milling Mo powder of 3.3 μm in diameter with ethanol in stainless steel milling medium. C103 Nb alloy specimens were dipped into the slurry and sintered in vacuum atmosphere to form a Mo layer on surface. Then the final MoSi₂ coating samples were prepared by pack cementation process.

Static oxidation testing were carried at 1 600 ℃ in air. Analysis balance was used to measure the liveweight of coating samples at different time during pack cementation process. XD98 X-ray diffraction analysis was used to identify the phases on surface of coating. Morphologies of surface and cross-section of coating were observed by scanning electronic microscopy (SEM). Energy analysis was carried out to investigate the distribution of compositions in surface and cross-section of coating. The transformations of compositions and structures were compared between original and oxidized MoSi₂ coating samples.

3.1 Morphology on surface

SEM images and X-ray diffraction patterns of original and oxidized MoSi₂ coating samples are shown in Fig.1 and Fig.2 , EDS data are listed in Table 1, respectively. As we can see from Fig.1(a),  the surface of Mo coating is relatively smooth after vacuum sintering except some tiny holes. The diameter of bigger hole can reach 3 μm. There are a few spherical particles composed of Fe, Ni and Cr on surface of Mo coating. These contaminations may be introduced by stainless

Fig.1 SEM images of Mo layer (a), Silicized layer (b) and Oxidated layer (c) at 1 600 ℃ for 10 h

Fig.2 XRD patterns of surface: (a) Mo layer; (b) Silicided coating; (c) Oxidated coating

Table 1 EDS data of different zones on coating surface(mole fraction, %)

steel milling medium during milling process. The eutectic phases with relatively low melting point introduced by contaminations form sphere on the surface of Mo coating after solidification at a high temperature. Fig.2(b) shows that the surface of silicified Mo coating consists of MoSi₂ and Al₂O₃ phases. EDS data show that the half spherical particles are composed of Al and O elements caused by Al₂O₃ particles in cemented powder during high temperature processing. A small quantity of Si and Na are also found in Al₂O₃ as contaminations. Fig.2(c) shows the oxidized coating surface consists of MoSi₂, Mo₅Si₃ and amorphous phases composed of Si, O and Al elements. A dense oxidized film with smooth surface is formed on coating after oxidation (Fig.1(c)). The low melting point eutectic phase is created by the reaction between Al₂O₃ and SiO₂ from the oxidation of MoSi₂[9]. In Fig.2(c), an amorphous bump can be found in a low angle diffraction area. The oxidized samples were rapidly cooled to room temperature, causing the melting oxides rapid solidification into amorphous glass. Due to the great gap of thermal expansion coefficient between amorphous phases(a(SiO₂) =2.4×10^-6/℃) and MoSi₂(a(MoSi₂)=8.1×10^-6/℃), some micro cracks can be found on the coating’s surface after rapid cooling form high temperature (Fig.1(c)).

3.2 Morphology of cross-section

As shown in Fig.3 (a), Mo layer, whose thickness is about 35 μm,  is dense except some cavities inside. The transitional layer with deep color between Mo layer and substrate is composed of Nb, Fe and small quantities of Ni elements as listed in Table 2. The contaminations, such as Fe and Ni, were come from the milling of Mo powder. Compared to Mo, the self-diffusion coefficients of Fe and Ni are higher but the activated diffuse energy is lower [10]. Consequently, the contaminated elements in Mo layer diffus to substrate and out-layer and form two segregated areas. The Fe element diffuses to out-layer and forms the spherical shape low melting point eutectic phases on surface after cooling as shown in Fig.1(a).

The morphology of coating’s cross-section after silicification is shown in Fig.3 (b). The coating’s thickness increases to 125 μm after pack cementation, due to the silicates increased by reactive diffusion. Some cavities caused by incompletely sealing of relatively big cavities in Mo layer can be found in out-layer of coating. From outside to inside, the coating can be divided into main layer, micro cavities zone and diffused layer. The main layer composing of MoSi₂ with about 75 μm is dense.

The micro cavities zone with about 15 μm in thickness is formed by the aggregation of vacancy during the process of reactive diffusion. During the process of Si atom diffuses through MoSi₂ layer and Nb substrate, vacancy source is generated and formed the dislocation creep deformations which are impelled by stress, where is created in new phase layer following its volume changed [11]. In the early stage of reactive diffusion, non-equilibrium vacancy distributes on the whole new thin layer.

With the proceeding of reactive diffusing, the concentration of vacancy increases gradually in interface. After diffusion, a low energy area appeares at the interface between MoSi₂ and NbSi₂ layers to create many vacancy traps. The consequential vacancy zone is formed for the aggregations of vacancies. TORTORICI et al [12] also found the similar phenomenon between Mo₅Si₃ and Nb₅Si₃ in diffusion couple of MoSi₂/Nb. The formation of cavity zone can effectively release the thermal stress inside the coating, and then decreases the possible creat-

Fig.3 Cross-section morphology: (a) Mo layer; (b) Silicide coating; (c) Micropore zone; (d) Interlayer of silicide coating

ion of inner micro crack. On the other hand, the cavity zone is one of weaknesses for its relative lower strength. The mechanical properties of coating may be degraded under the flushing of high velocity and temperature flow due to the shear stress acting on cavity zone.

As shown in Fig.3(d), the bright layer closing to substrate with a straight interface and 2 μm in thickness is Nb₅Si₃. The dark area is NbSi₂ phases with a few Nb₅Si₃ phases. After cooling to room temperature, Nb₅Si₃ separates out and deposites from NbSi₂ to form the two phases’ transitional layer, due to its solid solution decreasing. As we known from Nb-Si binary phase diagram, NbSi₂ and Nb₅Si₃ coexist in the range from 37% to 67% in mole ratio. The similar phenomenon is also found by CHIARA et al[13] in Nb/Si diffuse couple. The growing direction of needle shape Nb₅Si₃ phases in two phases’ transitional layer is perpendicular to the interface of coating, corresponding to the diffuse direction of Si in Nb substrate.

Table 2 EDS data of different zones in cross section(mole fraction, /%)

Dense Nb₅Si₃ phases with high strength and stiffness can prevent the further extension of cracks as illustrated in Fig.3(d). The precipitation of Nb₅Si₃ improves the strength of NbSi₂ layer which also can resist the oxidation after the inactivation of main layer [14]. Nb₅Si₃ phases, which distributes near the interface between transitional layer and diffuse layer, improves the thermal shock resistance properties of coating whereby strengthens the combination of interface.

The whole thickness of coating increases to 180 μm after oxidation, and the dense oxidative film which is formed on the surface of coating reaches to 20 μm (Fig.4(a)). From EDS data and XRD patterns, oxidative film is composed of SiO₂ and a small quantity of Al₂O₃. After the formation of oxidative film, the process of oxidation is controlled by the diffusion of oxygen through the oxidation. The diffuse rate of oxygen in SiO₂ and Al₂O₃ is so slow that the oxidation resistance is improved by this dense oxidative film[14]. The cavity layer still exists between the main layer and transitional zone, but the micro cavities have come into bigger holes. The thickness of Nb₅Si₃ increases from 2 to 46 μm after oxidation, due to the Si in Si rich layer diffusing to substrate driven by chemical potential gradient. At the same time, Si diffuses to outer layer to remedy the loss in main layer. The reverse flow of vacancy achieves, the diffusion of Si and also can be aggregated by vacancy traps.

Because of the stronger appetency of oxygen with Hf in substrate, the dispersed HfO in white color are formed preferentially when the oxygen diffuses into coating or substrate (Fig.4(b)).

Fig.4 Cross-section morphologies of coating oxidated at 1 600 ℃ for 10 h (a) and interface between NbSi₂/Nb₅Si₃ (b)

3.3 Formation process of coating

Pack cementation method to prepare the MoSi₂ coating on Nb substrate includes two independent processes. The first procedure is to prepare Mo layer on Nb substrate by immersing or spraying the Mo slurry, and then followed with vacuum sintering. The second process is pack cementation to silicify. Halide activated pack cementation method is the combination of high temperature chemical vapor deposition and reactive diffusion. The reactive procedures as follows[15, 16]. 1) NaF reacts with silicon powder to form volatile SiF₂; 2) The gaseous halide diffuses to the surface of Mo layer through porous pack driven by chemical potential gradients; 3) MoSi₂ layer is formed by reactive diffusion of Si and Mo; 4) The chemical potential gradient of gaseous SiF₂ is kept by the consume of deposition elements due to the growth of MoSi₂ layer, subsequently the process of vapor diffusion is carried on; 5) The thickness of MoSi₂ layer increases with the processing of Si deposition and reaction on Mo layer which is driven by chemical potential. The following is the procedures of vapor deposition [16, 17]:

2NaF+SiSiF₂(g)+2Na                       (1)

2SiF₂(g)SiF₄(g)+Si                         (2)

5Mo+3SiMo₅Si₃                                      (3)

Mo₅Si₃+7Si5MoSi₂                                  (4)

As known from the analysis above, the growth of coating is controlled by two processes: one is the gas flux of halide reaching on surface of Mo layer; the other is the growth rate of MoSi₂ during the reactive diffuse between Si and Mo.

The gas flux of halide reaching on the surface of Mo layer can be denoted as

                               (5)

where D_i is the coefficient of gaseous diffusion;ΔP_i/ΔX is the partial pressure gradient on substrate’s surface. With fixed D_i and sintering processing parameter, and the Si which is used to form gaseous SiF₂ in pack powder is always enough, then the gas flux of halide reaching on surface of Mo layer is a fixed value under the invariable partial pressure gradient [17].

During the practicable pack cementation process, the reactions (1) and (2) are controlled by the equilibrium partial pressure of SiF₂ and SiF₄   which relate to the consumption rate of Si or the growth rate of coating. Fig.5 shows the relative curve between gained mass on coating surface and time of silicification. As the time of pack cementation increases, the mass gained in unit area of coating increases, but the rate of mass gained decrease obeys the parabolic rule, which shows that the formation process of coating is controlled by reactive diffusion.

Fig.5 Relationship between gained mass and pack time

The formation of new phase is decided by its chemical stability and diffuse dynamics during the process of reactive diffusion. Because the formative enthalpy of Mo₅Si₃ is lower than that of MoSi₂, during the instantaneous time of beginning of reaction, Si and Mo react to form a thin Mo₅Si₃ layer which is defined as reaction 3. As the proceeding of Si diffusion, MoSi₂ layer is formed. The following reactive diffusion is controlled by the diffuse rate of Si through the MoSi₂ layer. The front of Mo₅Si₃ layer is moving to Mo layer as the interface of reactive diffusion. After the whole Mo layer transforms into MoSi₂ phases, the Si begins to diffuse into the substrate. The diffusion of Si is achieved by the reverse flow of vacancy, and subsequent micro cavity layer is formed by the aggregation of vacancy between the MoSi₂ layer and substrate. Because of the formative enthalpy of Nb₅Si₃(-516.8 kJ/mol) is lower than that of NbSi₂’s(-161 kJ/mol)[18], the Nb₅Si₃ layer forms firstly by reaction of Nb and Si which diffuses through MoSi₂. As the proceeding of reactive diffusion, Si reacts with Nb₅Si₃ to form NbSi₂, and the front of Nb₅Si₃ phase layer moves to substrate as reactive interface. The diffuse rate of Si in substrate is lower than before because of it is controlled by diffuse rate of Si in both MoSi₂ and NbSi₂. Furthermore, the diffuse path prolongs with the increase of coating’s thickness, as well as the time for diffusion of Si reaches the reactive interface. Effected by the decrease of reactive diffuse rate, the growth of coating gets slower.

1) The silicide coating prepared by pack cementati-

nmethod is formed by reactive diffusion. The process of silicification obeys the parabolic rule.

2) The coating has a typical composite structure as follows. The main layer composes of MoSi₂. The two phases transitional layer composes of NbSi₂ and small quantity of Nb₅Si₃ and the diffuse layer composes of Nb₅Si₃.

3) The dense amorphous glass layer formed under high temperature oxidative circumstance can effectively prevent the further diffusion of oxygen into coating.

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(Edited by YANG Hua)

Corresponding author: XIAO Lai-rong; Tel: +86-731-8830263; E-mail: caicsu@126.com; xiaolr368@sina.com