Ultra High Temperature Ceramics for Thermal Protection

Recent Developments in Oxidation Resistance and Fracture Toughness of Ultra High Temperature Ceramics for Thermal Protection Systems

(Dated: February 3, 2015)
For safer and faster space vehicles a reduction of the tipradius of the leading edges is inevitable. This leads to temperatures exceeding 2200 °C which the used material has to withstand. ZrB2/SiC and HfB2/SiC have suitable properties, but the oxidation resistance and fracture toughness at high temperatures have to be improved. This review describes the recent approaches to handle these problems that base mainly on the addition of a third material (La2O3, Gd2O3 or graphite). The addition of either La2O3 or Gd2O3 led to increased oxidation resistance, but the processing, the amount of additive and the testing methods have to be improved. Due to the addition of graphite flakes the fracture toughness of ZrB2/SiC (20 vol%) increased from
1. INTRODUCTION
The Thermal Protection System (TPS) of space ve­hicles is one of the most important parts of the whole vehicle [1–3]. This protects it from the heating during re-entry. During this process the temperatures are very high, especially at the nose cone and leading edges as shown in Fig. 1.
Figure 1: Temperature profile of a space vehicle during re­entry prepared by the NASA. The dark red colour represents the highest temperatures and the light blue regions are coolest parts (copied from [4]).
The tip of the leading edge has a radius of 10 cm, but a radius in the range of millimetres is wished so that sharp leading edges instead of blunt leading edges can be used [5]. This would have the advantage to “help reduce the vehicles drag, enhance maneuverability and performance, and also improve safety due to an increased cross-range-capability” [5]. The problem of a smaller tip radius is that this leads to higher surface temperatures, which can exceed 2000 °C [6, 7]. Jin et al. [8] investigated the maximum surface tempera­ture depending on the radius of the tip using an oxyacety­lene torch. Fig. 2 shows that the temperature increases from 1930 °C with a radius of 1.5 mm to 2100 °C with the radius of 0.15 mm.

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Due to the high requirements for these materials only a few can be considered. Very good potential for the usage show Ultra high temperature ceramics (UHTC). These are ceramic materials with high melting points (> 3000 °C), good thermal shock resistance and chem­ical and mechanical stability e.g. ZrB2, ZrC, HfB2 or HfC [6, 9, 10]. Although the carbides have a higher melt­ing point than the diborides, their thermal conductivity is lower, which is very important because the heat on the surface has to be transported as fast as possible away [5]. Therefore the main focus of research is on ZrB2 and HfB2. The problem with these diborides is their brittleness and their oxidation at temperatures exceeding 1200 °C [11]. To fabricate a more ductile material SiC was added to ZrB2 or HfB2 [12]. Although ZrB2/SiC and HfB2/SiC are promising materi­als for the usage in TPS, there are still two main problems which have to be solved. On the one hand side there is still the problem with the oxidation resistance at higher temperatures [11, 13]. On the other hand the fracture toughness decreases with the temperature to rather low values which also leads to mechanical problems [14]. This review gives an overview over solution attempts that have been made in the last years, with the main focus on ZrB2/SiC.
2. OXIDATION RESISTANCE
The oxidation resistance of MeB2/SiC (Me = Zr, Hf) depends on the ratio of MeB2/SiC [10, 15], the pres­sure [10, 16, 17], the temperature [18], the exposure time [19], on the processing [20, 21] and in case of the addition of additives also on their chemical structure and the amount of additive [22, 23]. Considering just ZrB2, the following happens during ox­idation [24]:
The liquid B2O3 forms a protection layer for the porous ZrO2 layer, where oxygen can diffuse very fast and easily through. But at 1100 °C this protective layer evaporates and cannot prevent the oxidation of the bulk material anymore. The addition of SiC leads to the following additional re­actions [10, 25, 26]:
During oxidation four different layers can form (Fig. 3). Above the bulk material the SiC depleted zone forms, where SiC oxidizes to SiO2 which forms a borosilicate (BS) glass on the surface. This layer is porous due to the formation and evaporation of CO (Eq. (2)). Above this layer there is the oxidized layer, which consists of porous ZrO2 and the upper layer is the Silica rich layer which consist of the BS glass, that also fills partly the pores of the ZrO2 layer. Due to the evaporation of B2O3, the protection layer is shrinking over time and shows an oxidation resistance up to 1600 °C [24, 26].
In general the oxidation resistance is measured by the thickness of the reaction zone under the given parameters of the experiment. But also the diffusion coefficient of O2 is important but rarely measured.
2.1. Ratio ZrB2/SiC
The dependency of the oxidation resistance on the ra­tio of ZrB2/SiC was investigated by many researchers [10, 12, 15, 27–29]. The addition of SiC led in all cases to a re­duction in grain size, a homogeneous distribution of SiC, higher viscosity and higher density. Karlsdottir et al. [29] investigated not only the reaction
Figure 3: Schematic demonstration of the different layers that form during oxidation of ZrB2/SiC and their arrangement.
zone thickness but also the viscosity depending on the volume fraction of SiC. The results are shown in Table I and an increase in viscosity with the amount of SiC can be seen. This reduces the diffusion coefficient of O2 in the layer.
Seong et al. [10] compared ZrB2, ZrB2/SiC (20 vol%), ZrB2/SiC (30 vol%) and ZrB2/SiC (40 vol%) and mea­sured the resulting thickness of the reaction zone. All samples were densified by hot pressing and exhibit a ho­mogeneous distribution of SiC. The grain sizes were be­tween 1.0 µm and 3.0 µm. They investigated the oxi­dation under low and high pressure and the results are shown in Figure 4. The SiC depletion layer did not form and therefore the reaction zone consisted only of the Silica rich layer and the oxidized zone. In air (2∗104 Pa) the thickness of the reaction zone was in ZrB2 the thickest due to the missing protecting silica layer and with increasing amount of SiC the thickness de­creases. The problem with high amounts of SiC (30 vol%) is, that it does not form single grains anymore, but in­stead a network, which leads to higher porosity. Under low pressure the behaviour is vice versa. Because the space shuttle has to deal with low and normal pressure ZrB2/SiC with 20 vol% or 30 vol% SiC is the best choice. Apart from the improvements of oxidation resistance due to optimization of the ZrB2/SiC ratio, at temper­atures above 1800 °C active oxidation (Eq. (4)) of SiC takes place and this hinders the formation of the BS layer. 2.2. Additives
To increase the oxidation resistance at higher temper­atures transition metals were added to the ZrB2/SiC ma­trix [21, 30–32]. They are suitable due to their high melt­ing points and low reactivity with the environment [30]. The transition metal cations can be enclosed in the BS layer to form a higher viscous layer that decreases O2 diffusion [11]. Furthermore this may lead to a higher evaporation point of the protective layer so that the ma­terials are stable at T > 2000 °C. This is based on an assumption, the mechanism how the transition cations interact with ZrB2 and SiC is not understood yet but in general a positive trend to higher oxidation resistance can be seen [11]. Several attempts were made with many different transi­tion metal oxides [9, 30], borides [11, 33], carbides [21, 31] and silicates [32]. This review focuses on La2O3 and Gd2O3 because they are the most promising additives and introduce two different processing methods that ef­fect the properties [30].
2.2.1. Addition of La2O3
The addition of La2O3 to ZrB2/SiC was investigated by several researchers and led to different results, espe­cially various new phases were found [6, 9, 22, 23]. Ta­ble II gives an overview over the composition, the pro­cessing routes to densify the material and the new phases that were discovered.
Although Table II shows many different results, gen­eral trends due to the addition of La2O3 despite the usage of different processing routes (hot pressing, spark plasma sintering (SPS)) were observed [6, 22, 23]:
• Higher density

Higher Vickers Hardness
Reduction of grain sizes

Besides the use of La2O3 leads to a more homogenized distribution of SiC, because it is always close to it and therefore prevents agglomeration [9]. In the case of fracture toughness there are contradictory statements: Li et al. [22] measured an increased fracture toughness compared to the material without additive and Guo et al. [23] published a lower fracture toughness due to the addition of La2O3. After hot pressing at 1900 °C for 60 min Li et al. [22] dis­covered the formation of new phases: La2Zr2O7 (melting point: 2295 ± 10 °C [9]) and La2Si2O7 due to the follow­ing reactions:
2ZrO2(s)+ La2O3(s) → La2Zr2O7(s) (5)
2SiO2(s)+ La2O3(s) → La2Si2O7(s) (6)
La2Zr2O7 was also observed by Zapata et al. [6] and Jayaseelan et al. [9] but no other working group detected the formation of La2Si2O7. The addition of 10 wt% La2O3, densified by SPS and oxidized for 1 h in air at 1600 °C led to the formation of two different oxidized layers [9]. On the surface a La­BS-glass formed (Eq. (7)), below it two oxidized layers, one consisting of La2Zr2O7 (Eq. (5)) and the other one of ZrO2.
SiO2 + La2O3 → La − BS − glass (7)
The large expansion coefficient of La2O3 causes filling of the pores that appear after the evaporation of B2O3 and therefore still protects the bulk material. The same composition and the same processing was used by Guo et al. [23] but they could not detect the new phases. Instead they found out that La2O3 reacts
Table II: Overview over the composition (always ZrB2/SiC (20 vol%) + the given amount La2O3), the processing route and the new phases that formed.
amount La2O3 processing new phase
5 vol% hot pressing La2Zr2O7 (1900 °C, 60 min) La2Si2O7
3 vol% hot pressing La2O3-SiO2 (1900 °C, 60 min)
10 wt% SPS La2Zr2O7 oxidized (1600 °C, 1 h, air) La-BS-glass
2 wt% SPS La2Zr2O7 oxidized (1400 °C, 16 h, air) La-BS-glass with SiO2 to form La2O3-SiO2 as a protective layer.
Further studies were made by Zapata et al. [6] who used less La2O3 (2 wt%). Due to the proximity of the La2O3 particles to the SiC particles they are also included in the BS melt whereby this results in a higher viscosity, a higher thermal stability and in general a better protec­tion against O2 diffusion. The oxidation tests show that at 1400 °C the sample with La2O3 has a better oxidation resistance but at 1500 °C and 1600 °C it is worse. The reason is that because of the addition of La2O3 the BS layer has a higher viscos­ity and therefore the ZrO2 particles cannot precipitate directly to the top of the surface layer. This leads to a more homogeneous mixing with the BS melt. The oxy­gen diffusion through ZrO2 is much easier than through B2O3 and therefore a homogeneous distribution of ZrO2 makes it easier for O2 to diffuse through this layer, al­though the La-BS-glass has a higher viscosity due to the addition of La2O3. Moreover at 1600 °C ZrOxCy and SiOxCy form with dif­ferent O/C ratios which were found in the BS melt and in the oxide layer [6]. This can be seen as another pro­tection layer because when O2 diffuses into the oxidation layer it will react with ZrOxCy or SiOxCy, so it can be seen as a puffer zone and it takes longer until the oxygen reaches the bulk material. HfB2 differs from ZrB2 because the formation of HfO2 is more difficult [6]. Therefore a smaller amount is formed which leads to a lower amount of B2O3 and therefore a thinner protection layer compared to ZrB2. But it has the advantage that the diffusion coefficient for O2 through HfO2 is smaller. Another problem in the case of HfB2 is that SiC is not as homogeneously distributed as in ZrB2 and it forms large agglomerates. When these agglomerates become oxidized they leave behind a highly porous material where O2 can easily diffuse through. This shows that further improvements are inevitable in a more homogeneous distribution, further analysis of the new formed phases must be made and the C/O ratio can
Figure 5: Topview (a) and sideview (b) of the surface with the dimensions of the cavities for Gd2O3.
oxidation layer was 15 ± 3 µm and formed in accordance with the following reaction equation [9]:
2Gd2O3(s)+2ZrO2(s) → Gd2Zr2O7(s) (8)
Gd2O3(s)+ SiO2(s)+ B2O3(l) → Gd − BS − glass (9)
The thickness of the layer below it was 160 µm and consisted mainly of porous ZrO2 due to the oxidation and evaporation of the glassy phase. The advantage of Gd-BS-glass compared to BS glass is the higher viscosity and therefore the reduced diffusion of O2 through this coating. At higher Gd2O3 fractions Gd stops ZrO2 particles at the glassy phase and they cannot diffuse further. This leads to O2 vacancies which is the driving force for inward O2 diffusion. Using a distance of 20 µm between the cavi­ties, the Gd2O3 fraction is high enough to get a higher viscosity in the BS melt but ZrO2 can still precipitate so that no O2 inward diffusion occurs.
3. FRACTURE TOUGHNESS
In 2009 the first tests were made to include graphite in the ZrB2/SiC matrix [34, 35]. Hu et al. [34] investigated the fraction of additive graphite to ZrB2/SiC (20 vol%). They found out that the addition of graphite led to a
1high dense material with an increasing fracture toughness be optimized. But the addition of La2O3 is already a from 4.5 MPam
(ZrB2 + SiC (20 vol%)) to 6.1 MPam1
22
very promising approach for a better oxidation resistance
although further research is necessary.
2.2.2. Addition of Gd2O3
For an improved surface and at the same time un­changed bulk material a new processing method was in­vented [11]: At first the sample was prepared and densi­ï¬ed using the bulk material ZrB2/SiC. Afterwards they used a laser to make equal sized cavities on the surface that were filled with Gd2O3 nanopowder. The dimen­sions of the best sample can be seen in Fig. 5.
Due to this new processing it was possible to create only a thin protection layer that consisted of BS mixed with Gd2O3 ( Eq. (9)). After 1 h in air under 1600 °C the thickness of the outer (ZrB2 + SiC (20 vol% + graphite)). The differences be­tween 10 vol% and 15 vol% graphite were negligible small. Moreover there were investigations about the influence of the diameter size of the graphite flakes [36]. They found out that in the range of micrometres the diameter size does not change the fracture toughness. Asl et al. [14] used soft graphite nano-flakes. They found out that ZrB2 + SiC (20 vol%) + graphite (10 vol%) showed a higher density than the samples without graphite. Furthermore the addition of graphite led to a decrease in grain size from 6.9 µm to 3.2 µm. The reason is the homogeneous distribution of graphite which stopped grain growth. Because of the reactions of graphite with the surface impurities the addition of graphite results in higher dense samples:
ZrO2(s)+ B2O3(l)+5C(s) → ZrB2(s)+5CO(g) (10)

The particles that form due to this reactions can fill the pores in the ZrB2/SiC matrix and therefore lead to a higher density. The resulting fracture toughness can be seen in Fig. 6. An increase in fracture toughness due to the addition of graphite is obvious. The following mechanisms led in this case to a higher fracture toughness: nano-flakes pull-out, crack bridging, branching and deflection.
1
Figure 6: Fracture toughness depending on the composition of the sample at RT [14].
Wang et al. [37] investigated the dependency of the fracture toughness of ZrB2 + SiC (20 vol%) +
1
graphite (15 vol%) on the temperature in vacuum and in air (Fig. 7). Over the whole temperature range the fracture toughness in air was higher than that in vac­uum. In vacuum the fracture toughness decreases from
This oxidation layer densifies with higher tempera­ture and yields in higher fracture toughnesses than with­out this layer. That is the reason why there is nearly no decrease in fracture toughness between 1200 °C and 1300 °C. Moreover crack deflection which absorbs the energy leads to higher fracture toughnesses at higher temperature. These mechanisms all result in a slower decrease in frac­ture toughness in air than in vacuum.
Figure 7: Fracture toughness depending on the environment and on the temperature [37].
4. CONCLUSION
The recent developments to improve the oxidation re­sistance and the fracture toughness based mainly on the addition of a third component (La2O3, Gd2O3 or graphite). Concerning the oxidation resistance, the best matrix composition is ZrB2/SiC (20 vol%) because it shows the best oxidation protection over the whole range of O2 partial pressure. Above 1800 °C active oxidation of SiC begins and oxidation resistance is not given anymore.
at 1300 °C because The approaches for a better oxidation resistance at higher ual thermal stresses between the ZrB2/SiC matrix and temperatures due to the addition of La2O3 or Gd2O3 arethe graphite inclusions are released. The residual stresses very promising, but more research to understand the real acted at low temperature as toughening mechanism and function of the additives and the interaction with the ma­with the release of these stresses the fracture toughness trix is necessary. Furthermore there are many parameecreases.
At 1300 °C the group observed a brittle to ductile trans­formation which leads to a slight increase of fracture toughness. But afterwards the fracture toughness de­creases further due to the distorted graphite and the larger ZrB2 grain sizes. In air at higher temperature the material starts to ox­idize and a oxidation layer forms on the surface due to ters that have to be optimized, e.g the amount of ad­ditive, the processing route and especially the analytical approaches. Due to the varying experimental parameters and insufficient analytical tests it is difficult to compare results. To solve this problems standard tests have to be introduced and a wider temperature range for oxidation has to be investigated.
The fracture toughness increased due to the addition of
Eq. (1), (2) and the following reaction:
graphite from 4.5 MPam
to 7.1 MPam creases, but also this is slowed down due to the graphite tives, because extensive testing of the different samples flakes. is missing. Especially tests under real atmospheric and Taken into account the oxidation resistance and the frac-re-entry conditions are important but not done yet. ture toughness it is difficult to announce the best addi­