A composites of (Mo0.9Cr0.1)Si2 + 15vol% ZrO2 was prepared with powder metallurgy and Pressure- Less Sintering (PLS) method, aiming at applications of high temperature structural materials. Mechanical properties of the composites were assessed with hardness, indentation fracture toughness Kc and KIC tested using SEVNB, flexure strength at room temperature and 1200?C, and isothermal oxidation at 1400?C. The results showed that the native silica oxide and molybdenum-oxides on the silicide feedstock surface were significantly reduced in terms of Cr-alloying. (Mo0.9Cr0.1)Si2 and its composite also exhibited improved sinterability and grain growth, owing to the presence of (Cr, Mo)5Si3 at grain boundaries. Fracture toughness of the composite was increased by a factor of 1.6 to that in the monolithic silicide. Mechanical property of the composite at high temperature was not affected by Cr addition. However, the high temperature oxidation resistance was greatly improved in the (Mo0.9Cr0.1)Si2 + 15vol% ZrO2 compared with the non Cr-alloyed counterpart. The Cr-alloying effects on the microstructure, fracture behaviour, and high temperature oxidation resistance were discussed.
Molybdenum disilicide MoSi2 is a candidate for high-temperature structural materials due to high melting point, high specific strength (strength/density), high thermal conductivity, and excellent oxidation resistance at elevated temperature . However, the properties of fracture toughness and creep resistance have to be improved before engineering applications. Promising approaches include solid solution alloying the silicide (Al, V, Cr, Ta, Nb, and Re, etc.), and composite reinforced with refractory and ceramic particles and fibres (SiC, Si3N4, Al2O3 and ZrO2) . Significant increasing in fracture toughness and strengthening at high temperature has been reported in MoSi2-ZrO2 composites in terms of phase transformation toughening of ZrO2, but the degraded oxidation resistance was also observed, especially with a high ZrO2 content  .
In this investigation, Cr alloyed molybdenum disilicide (Mo0.9Cr0.1)Si2, and composite (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 were produced with Pressure-Less Sintering (PLS) that was the most economic and practical technique applied popularly in industrial production. The purpose of this study is to investigate the alloying effect of Cr addition on mechanical properties and high temperature oxidation resistance in comparison with the un-al- loyed MoSi2 + 15vol%ZrO2 composite.
2. Experimental Methods
(Mo0.9M0.1)Si2 (3.3 at%Cr) and (Mo0.9M0.1)Si2 + 15 vol%ZrO2 composites were prepared using a powder metallurgy process described in an earlier work . The average particle sizes were in a range of 2.3 - 2.6 and 0.87 μm for the Cr-alloyed silicide and un-stabilized ZrO2 feedstocks, respectively. The powder mixture was milled in gasoline for 4 hours, and pressed to 60% of theoretical density (T.D.) using Cold Isostatic Pressing (CIP) at 200 MPa. PLS process was performed at 1600˚C - 1620˚C in H2 atmosphere. The sintering density was measured using Archimedes method. Native oxides on the silicide feedstock powder surfaces were examined using X-ray Photoelectron Spectroscopy (XPS). Phases were determined using XRD with Cr-Kα radiation. The microstructure was characterized with optical microscope and scanning electron microscope (SEM) with Energy Dispersive Spectroscopy (EDS).
The fracture toughness was measured with two methods: Kc indentation fracture toughness (IF) calculated with the Anstis formula; KIC fracture toughness tested with a standard method of Single Edge V-Notch Beam (SEVNB). The 4-point bending tests were conducted with inner/outer spans of 20/40 mm, and with a cross head speed was 0.2 mm/s. As-sintered surfaces from PLS remained on the testing pieces. The flexure strength was tested at room temperature and at 1200˚C in ambient air. Discontinuous isothermal oxidation test was performed at 1400˚C for 1000 h in flowing air. The weight changes were carefully measured after each exposure with accuracy of 0.1 mg.
3. Results and Discussion
3.1. Oxides on Stoking Powder Surfaces
Figure 1 shows XPS spectra of MoSi2 and (Mo0.9Cr0.1)Si2 silicide feedstock at the Si2p, Mo3d and Cr3d. The area ratios of Si2p peaks at SiO2 and MoSi2 are 2.9 and 1.0 for the MoSi2 and (Mo0.9Cr0.1)Si2 powder surfaces, respectively (Figure 1(a)). The area ratios of Mo3d peaks at Mo-oxides and MoSi2 are 1.7 and 1.0 in the MoSi2 and (Mo0.9Cr0.1)Si2 powder surfaces, respectively (Figure 1(b)). The Cr2p peak of Cr2O3 was found in the (Mo0.9Cr0.1)Si2 powder surface, and the area ratio of Cr2O3 to Cr-silicide is 1.4 (Figure 1(c)). It is apparent that the formation of silica and Mo-oxides on the (Mo0.9Cr0.1)Si2 powder surface are substantially impeded by the formation of Cr-oxides. The oxygen in the sintered bulk is believed to be introduced from the native oxide on the stocking powders. According to the chemical analysis, the oxygen contents were 2.16 and 1.13 wt% in the MoSi2 and (Mo0.9Cr0.1)Si2 sintered bulks. Meanwhile, substantially decreasing of SiO2 particles was observed in the sintered (Mo0.9Cr0.1)Si2 and composite bulks (see the following section). It was consistently testified that Cr-addition can efficiently reduce silica in molybdenum disilicide. It is widely agreed that low silica at grain boundaries is appreciable to the creep resistance of silicide composites . Thus, Cr-alloying is an alternative approach to reduce the SiO2 contents of silicide composites, which could contribute to creeping property at elevated temperature.
3.2. Sintering Density
Theoretical density of MoSi2 and (Mo0.9Cr0.1)Si2 silicides are 6.25 and 6.10 g/cm3, respectively, measured from XRD. Theoretical densities of MoSi2 + 15vol%ZrO2 and (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 are 6.16 and 6.04 g/cm3, calculated from a linear combination rule. The sintering density of different materials is shown in Table 1. High sintering density over 98% T.D. was obtained in both Cr-alloyed silicide and composite. Improved sinterability is directly related to the existence of (Cr, Mo)5Si3 type silicide as secondary phase presenting (see the following
Figure 1.XPS spectra at (a) Si2p; (b) Mo3d in the surface of the MoSi2 and (Mo0.9Cr0.1)Si2 feedstock powders, (c) Cr2p in the surface of the (Mo0.9Cr0.1)Si2 feedstock powder.
Table 1. Property of sintered silicides and composites.
section). The melting point of CrSi2 is 1551˚C, nearly 470˚C lower than that of MoSi2. According to a quasibinary section phase diagram of MoSi2-CrSi2, a peritectic reaction exists at 7 at%Cr at 1529˚C, taking place as L + Mo1?xCrxSi2 Û MoyCr1?ySi2 . A liquid phase might appear at the sintering temperature, which may greatly assist mass transport and densification process during sintering at 1600˚C - 1620˚C.
3.3. Material Characterization
The XRD analysis shows that the (Mo0.9Cr0.1)Si2 consists of a single-phase of tetragonal C11b structure. The Cr solid solubility in the (Mo0.9Cr0.1)Si2 is 2.5 - 2.6 at% by EDS, which is lower than the nominal composition of 3.3 at%. Typical microstructures of different materials are shown in Figure 2. The amount of SiO2 particles in (Mo0.9Cr0.1)Si2 was reduced to 1.4 vol% compared with 2.2 vol% in MoSi2, measured from selected dense areas with imaging process. A small amount of Mo5Si3 was observed in MoS2 (Figure 2(a)). In (Mo0.9Cr0.1)Si2, the Mo5Si3 phase was replaced by a Cr-rich silicide with a composition of Cr:Mo:Si = 45:15:40 (at%) close to (Cr0.75Mo0.25)5Si3. The Cr-rich silicide appears as discrete grains and continuous phase along grain boundaries (Figure 2(b)). Grain growth is substantial in (Mo0.9M0.1)Si2 by a factor of 3 of that in MoSi2 (Table 2), which could be ascribed to the liquid phase sintering effect of the existence of the (Cr, Mo)5Si3 silicide with lower melting point. ZrO2 particles were well-dispersed in both composites (Figure 3(c) and Figure 3(d)), and grain sizes in both composites were refined owing to the dispersion of ZrO2 particles (Table 2). The Mo5Si3 grains are still visible in MoSi2 + 15vol%ZrO2 (Figure 2(c)), but the (Cr ,Mo)5Si3 is hardly recognized in (Mo0.9M0.1)Si2 + 15vol%ZrO2 composite from the contrast in SEM images (Figure 2(d)).
3.4. Mechanical Property
Mechanical properties are shown in Table 1 and Table 2. The average hardness values of the (Mo0.9Cr0.1)Si2 matrix phase and (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 composite are 10% lower than their un-Cr alloyed counterparts (Table 1), which might result from the softening effect of the Cr-addition. The fracture toughness KIC of the two composites are comparable, which are higher than the monolithic silicides in a factor of 1.4 - 1.6 (Table 2). The fracture toughness values from indentation fracture (Kc) and SEVBN (KIC) methods are close in MoSi2 + 15vol%ZrO2. However, the Kc value is too high over KIC in the (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 composite, which
Figure 2. Microstructure of as-sintered silicides and composites (a) MoSi2, (b) (Mo0.9Cr0.1)Si2, (c) MoSi2 + 15vol%ZrO2, and (d) (Mo0.9Cr0.1)Si2 + 15vol%ZrO2.
Table 2. Mechanical property of silicides and composites.
is caused by the uncertainty in determination of the crack length. Figure 3 shows typical indentation cracks in the composites. The primary median cracks from the indent corners in MoSi2 + 15vol%ZrO2 are deflected (D), branched (Bra), bridged (Bri) by the dispersed ZrO2 additives (Figure 3(a)), which dissipates and absorbs the energy for crack propagation. In contrast, multi-cracks are developed around the indent edges in (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 (Figure 3(b)). In such a case, the measured crack length is not reliable and inaccurate, which results in the overestimated Kc result. It is implied that the indentation fracture method is not suitable to describe the fracture toughness property of the composite (Mo0.9Cr0.1)Si2 + 15vol%ZrO2, therefore, a verification with a standard method has to be considered. The room temperature flexural strength sf of the composites is lower than that of MoSi2 (Table 2). The sf of (Mo0.9Cr0.1)Si2 + 15vol%ZrO2is substantially reduced at 1200˚C compared with MoSi2 + 15vol%ZrO2. It is believed that interfacial fracture energy is changed between the alloyed silicide matrix and ZrO2 particles , and grain boundaries can be weakened by the present of the (Cr, Mo)5Si3 phase in (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 at elevated temperatures.
3.5. High Temperature Oxidation Test
Figure 4 shows time dependence of mass change after isothermal exposure at 1400˚C in air. All the samples present with parabolic kinetics in the steady oxidation stage, indicating the formation of protective oxide scales and diffusion controlled oxidation. Total weight gains in (Mo0.9Cr0.1)Si2 and MoSi2 are 1.7 and 3.6 mg/cm2 after 1000 h exposure, inferring that the thickness of the SiO2 layer on the Cr-alloyed silicide (Mo0.9Cr0.1)Si2 is thinner than that on MoSi2. The performance of (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 composite is similar to that of monolithic
Figure 3. Cracks at indents in the composites (a) MoSi2 + 15vol%ZrO2, and (b) (Mo0.9Cr0.1)Si2 + 15vol%ZrO2.
Figure 4. Exposure time dependence on weight change at 1400˚C in air.
MoSi2. In contrast, an erratic weight loss occurred in un-alloyed composite MoSi2 + 15vol%ZrO2in the initial oxidation stage within 50 h. A parabolic weight-gain in MoSi2 + 15vol%ZrO2 is established when a continuous silica layer is formed. The rate constants in the steady oxidation stage (after 50 h) are 0.0014 and 0.0006 (mg2∙cm−4∙h−1) in MoSi2 + 15vol%ZrO2 and (Mo0.9Cr0.1)Si2 + 15vol%ZrO2, respectively. Obviously, the oxidation rate is substantially decreased in the Cr-alloyed composite.
The oxide scales on both composites consist of tetragonal ZrSiO4 and α-SiO2 by XRD. Typical morphologies of the composite surfaces after 1000 h exposure are shown in Figure 5. The scale on MoSi2 + 15vol%ZrO2 surface contains large amounts of smaller zircon particles with a size of 1 - 2 mm (Figure 5(a)). In contrast, the surface of (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 is comprised zircon particles with a size of 3 - 5 mm (Figure 5(c)). Scale thickness is between 50 - 60 and 20 - 30 µm in MoSi2 + 15vol%ZrO2 and (Mo0.9Cr0.1)Si2 + 15vol%ZrO2, respectively. SEM cross sectional images show that Mo5Si3 grains are located at the interface between the base silicide and oxide scale of MoSi2 + 15vol%ZrO2 (Figure 5(b)). The fine zircon particle network in the scale of MoSi2 + 15vol%ZrO2 can provide fast diffusion paths for oxygen to the interface of scale-base silicide, which results in higher oxidation rate. In comparison, the oxide scale of (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 consists of nearly pure silica. Some large zircon particles mainly appear on the outermost surface of this composite. A few small discrete Cr2O3 grains were found at the interface (Figure 5(d)), which resulted from the outer diffusion and oxidation of Cr from the alloyed base silicide after the formation of the protective silica scale.
It is known that the formation of protective silica on MoSi2 at high temperature surface is implemented by selective oxidation of Si: MoSi2 + O2 ® MoO3(g) + SiO2 at high pO2; and MoSi2 + O2(g) ® Mo5Si3 + SiO2, at low pO2. In the latter case, and silica can form at surface and Mo5Si3 silicide usually form at the interface between scale and base silicide as oxygen diffuses through the surface oxide layer. In presence of ZrO2 additives, SiO2 will react with ZrO2 to form zircon: SiO2 +ZrO2 = ZrSiO4, thus, the resultant zircon particles are retained in
Figure 5. Plane view and cross section of the composites after exposure at 1400˚C for 1000 h, (a) and (b) MoSi2 + 15vol%ZrO2, and (c) and (d) (Mo0.9Cr0.1)Si2 + 15vol%ZO2.
the oxide scale. The improved oxidation behaviour of the Cr- alloyed composite can be attributed to the chemistry change at the as received surfaces sintered in reduced atmosphere. It was reported that as-sintered surface of PLSed MoSi2 + 15vol%ZrO2 is comprised with a silicon depleted silicide (Mo-Zr-Si) . The substantial weight loss presenting in initial oxidation occurred in this composite was ascribed to the rapid oxidation of the high Zr-content silicide in the outermost surface. In comparison, the PLS sintered (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 surface consists of Cr-rich silicide with a low Zr content. It is believed that this silicide in the as-sintered surface could assists the formation of SiO2 scale with a favourable microstructure (showed in Figure 5(d)) at relative lower temperature. The outer diffusion of Cr can also promote the mobility and diffusion of Si at high temperature in the Cr-alloyed composite. The investigation details will be published in a later paper.
(Mo0.9Cr0.1)Si2 + 15vol% ZrO2 composite with high sintering density 99% T.D. was prepared with pressure-less sintering method. The native silica and Mo-oxides in the Cr-alloyed silicide feed stocking surface was substantially reduced by means of forming Cr-oxides. As a result, the amount of silica particles in the sintered silicide bulk was reduced by 50%. The sinterablity of the composite was improved considerably due to the existence of small amounts of (Cr0.75Mo0.25)Si2 silicide at grain boundaries. Toughening effect of the composite was not influenced by Cr-addition, and flexure strength was 200 MPa at 1200˚C. However, the as-sintered (Mo0.9Cr0.1)Si2 + 15vol%ZrO2 composite exhibited an excellent oxidation behaviour at 1400˚C, and the weight loss occurred at the starting oxidation in the as-sintered MoSi2 + 15vol%ZrO2 was avoided.
The research was financially supported by Svenska Elföretagens Forsknings-och Utvecklings Elforsk-AB (KME), Sandvik Heating Technology AB, and Siemens Industrial Turbomachinery AB.