HfB2-doped ZrB2-30 vol.% SiC composites: oxidation resistance behavior

To investigate the HfB2 on microstructure and oxidation resistance of ZrB2-30 vol% SiC, ZrB2-30 vol% SiC composites with different amounts of HfB2 (4, 8, and 12 vol%) were consolidated by Spark Plasma Sintering method (SPS). Microstructural evaluations were done by scanning electron microscopic (SEM). To investigate the oxidation resistance, the samples were placed in a box furnace at the temperature of 1400 C for different times. The samples were weighed before and after the oxidation and the Δw was applied as a criterion of oxidation. The thickness of SiO2 layer and Si depleted layer were also used as oxidation criterion. The results showed that HfB2 addition caused to decrease Δw and better oxidation resistance.


Introduction
Due to the growing tendency for high-temperature oxidation resistance materials for several structural and aerospace applications, ultra-high temperature ceramics (UHTCs) have been at the center of attention during the last recent years. UHTCs which are known as materials with a melting point above 3000 K, seem to be potential candidates for military and aerospace applications, e.g. nozzle inputs for rockets, air injection combustion systems, missile heads, and supersonic vehicles [1][2][3][4][5]. Historically, to reduce the aerodynamic friction-derived heat in primary aerospace vehicles, high radios and thickness components were designed. Also, such a design may minimize the generated heat, but challenges the achievable velocity of the vehicles. Therefore, sharp leading edges have been used in the new generation of aerospace components which can improve the lift/ drag ratio and consequently, the overall efficiency of the vehicle. Anyway, such designs can result in higher friction-derived heat input and promote high-temperature reactions at the sharp leading edges. Hence, the successful design of leading edges depends on the available high-temperature materials, e.g. UHTCs [3].
Due to its lower density, ZrB 2 is the highest studied TMB and several brilliant research works have been carried out on densification, properties, and applications of this material [13][14][15][16][17][18][19][20][21][22][23][24]. The surface of ZrB 2 particles is naturally covered by a mixture of low-melting-point boron oxide (B 2 O 3 ) and ZrO 2 which challenge its densification and sintering, as well as high-temperature oxidation behavior. Many attempts were conducted by researchers to solve both problems. For example, the impact of AlN on the microstructural features and consolidation behavior of the ZrB 2 -SiC system was studied. The addition of AlN had a remarkable effect on the sintering behavior of ZrB 2 -SiC, attaining fully dense composite samples [16]. Also, the various densification mechanisms in the ZrB 2 -SiC system at different sintering temperatures were investigated. It was released, sintering temperature has an important role in densification. Besides the difficulty in densification, low oxidation resistance is another problem. When ZrB 2 ceramic encounters a high-temperature oxidative environment, B 2 O 3 surface impurity evaporates and leaves a non-protective porous ZrO 2 layer remains on ZrB 2 Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
surface, which cannot effectively control the oxidation of ZrB 2 particles. It has been repeatedly reported that the addition of silicon carbide (SiC) to ZrB 2 can promote the formation of a borosilicate glassy layer and consequently, improve the oxidation behavior of the material [1][2][3][25][26][27][28][29]. Therefore, numerous research projects have been dedicated to ZrB 2 -SiC composites through the last decades. Typically, Gao et al [30] reported that when the ZrB 2 -SiC ceramic was oxidized in low-pressure O 2 /N 2 mixture gas, the ZrSiO 4 was promoted to form at below 1600°C. Also, it was [31] reported that during the oxidation of ZrB 2 -SiC composite in lowpressure O 2 /N 2 mixture gas at 1500°C, oxidation kinetics was change from parabolic law to linear law when the oxygen partial pressure reduces. Also, Jin et al [32] reported this a change at ZrB 2 -SiC-Graphite composite when it was oxidized at 1800°C in low-pressure O 2 /Ar mixture gases. All in all, there are still several challenges in densification and increased oxidation behavior of ZrB 2 -SiC composites. Researches indicated the positive influence of additives and sintering aids on the oxidation of the mentioned composites, as well as its densification and sintering behavior.
Among several sintering additives, it has been revealed that HfB 2 (melting point: 3380°C) as another member of HUTCs family, not only improves the mechanical properties and thermal shock resistance of ZrB 2 -SiC composites, but also can influence the oxidation behavior of the composite as it forms a protective oxide layer, HfO 2 [2,7,10,11]. Anyway, to the best of our understanding, there is no comprehensive report on the effects of HfB 2 on the oxidation resistance of ZrB 2 -SiC composites. Therefore, the present study is dedicated to the role of HfB 2 in the oxidation behavior of ZrB 2 -SiC-HfB 2 ternary composites.

Experimental
Commercially pure ZrB 2 , SiC, and HfB 2 powders were used as the starting materials. The characteristics of the used materials and their weighting ratio as well as sintering conditions are presented in tables 1 and 2.
The powders were then weighed and consequently ball-milled using WC balls through ethanol as milling media in a tungsten carbide cup. The milling process was carried out at 250 rpm for 3 h. Consequently, the milled powder mixture was dried via a heater/stirrer at 160°C-200°C and loaded into a graphite die covered with 0.1mm thick graphite foil. The spark plasma sintering process was then carried out via SPS furnace (SPS: 20T-10, manufacturer company, country of origin) at 1800°C for 9 min under the applied pressure of 30MPa. Detailed SPS parameters are presented in table 2. To remove the remained graphite foil, the obtained diskshaped samples were ground and then, wire cut to achieve 2×4×10 mm beam-shaped samples for oxidation test. The oxidation test was performed in a box furnace at 1400°C for 1, 2, 3, 6, 7, 15, 17 1st 20 h. The weight change was then measured as the oxidation progress criterion, based on the following equation: In which, W C-O shows the percentage of weight change due to oxidation, and W i and W a show initial and after oxidation weights of the samples, respectively. Microstructural investigations were carried out using a scanning electron microscope (SEM: Vega Tescan, Czech Republic) equipped with an energy dispersive spectrometer (EDS) on a polished surface.    Figure 3 shows the variation of weight change due to oxidation (W C-O %) of samples versus the volume fraction of HfB 2 in all test durations. As it can be clearly seen, increased HfB 2 content leads to reduced W C-O percentage or in other words, higher oxidation resistance. Better understanding the oxidation behavior, cross-sectional SEM micrographs of the sample oxidized for 20 h in 1400°C are presented in figure 4. EDS line scan and x-ray map analyses of the mentioned samples are also presented in figures 5 to 9. As can be clearly seen in figure 4, the polished cross-section of the samples reveals four distinct layers including a very thin surface layer (white), a dark and dense layer, a dark and porous layer, and the uninfluenced substrate (original microstructure of the composite). Based on the x-ray maps and EDS line scans (figures 5-9), a

( )
Actually, the formation of SiO 2 layer can result in the migration of silicon atoms toward the surface and consequently, a silicon-depleted sublayer. Anyway, SiO 2 may form a highly adhesive glassy layer on the surface (2nd layer) and increase the oxidation resistance of the composite, mainly due to interrupted atomic diffusion paths. On the other hand, although higher amounts of HfB 2 can lead to thinner SiO 2 layer, as the thickness of Sidepleted layers simultaneously decreases; improved oxidation resistance of the composite is expected. It has been reported that SiC can increase the oxidation resistance of ZrB 2 and HfB 2 -based ceramics. Such a positive role can be attributed to the formation of a glassy borosilicate layer (B 2 O 3 -SiO 2 ) instead of B 2 O 3 , which has a higher viscosity and thermal stability, as well as lower vapor pressure. Therefore, borosilicate layers not only decrease the diffusion rate of oxygen atoms (as the main oxidation controlling parameter) due to higher viscosity but also tolerate higher temperatures (thermal stability) which prolong their protection role. Besides its viscosity, the molten borosilicate layer provides a favorable wettability with both ZrO 2 and HfO 2 compounds which promotes the filling of surface porosities and consequently, prevents oxygen atoms from diffusion in porosities. Such a mechanism is known as the main oxidation controlling parameter of HfB 2 -SiC composites up to 1400°C. Although a similar mechanism can be proposed for ZrB 2 -SiC composites, the oxidation resistance of ZrB 2 is lower than HfB 2 , due to volume changes accompanied by the phase transformation of ZrO 2 , as well as a higher diffusion coefficient of oxygen atoms in ZrO 2 compared with HfO 2 [2]. Such a phenomenon can be clearly seen in x-ray maps of figures 5, 7, and 8.
The oxidation behaviors of hot-pressed ZrB 2 -20 vol.% SiC and HfB 2 -20 vol.% SiC composites were investigated by Mallik et al [2], through non-isothermal TGA up to 1300°C, 1, 24 and 100 h isothermal TGA at 1200 and 1300°C and full-cycle (24 cycles, each one for 1 h) oxidation test (including heating up to 1300°C and cooling down in the air), and similar results were reported. It also worth noting that thickening the SiO 2 layer leads to a more scattered distribution of ZrO 2 grains (lower fraction of ZrO 2 grains), which is clearly emphasized from x-ray maps and indicates the consumption of Zr atoms, and consequently results in weaker oxidation resistance. Figure 10 shows the weight change of sample 17 versus time after oxidation at 1400°C, which indicates a higher oxidation rate (higher weight change) when holding time is increased. It can be attributed to increased diffusion of oxygen atoms into the surface of the samples and thickening the oxide layers and oxidation-affected zones. The following function can be fitted to the curves presented in figure 10:

Oxidation mechanism
Where n=2.24 here. It has been confirmed that when n > 2.00, the oxidation process is parabolically controlled by diffusion, or in other words, the diffusion rate of O atoms plays the key role in the oxidation behavior of the composite [18].

Conclusions
Results indicated that the presence of HfB 2 can promote the formation of HfO 2 , which reduces the diffusion rate of oxygen atoms and consequently, results in improved oxidation resistance of the composite.
It was also revealed that the oxidation products are formed as three distinct layers, including a very thin ZrO 2 surface layer, a ZrO 2 +SiO 2 middle layer, and the inner Si-depleted layer. It was also indicated that the middle