Ultra High Temperature Ceramics for Thermal Protection

Ultra High Temperature Ceramics for Thermal suretyRecent Developments in Oxidation Resistance and Fracture Toughness of Ultra High Temperature Ceramics for Thermal Protection SystemsKatrin Abrahams(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/ place and HfB2/ go down have suitable properties, but the oxidisation confrontation and violate hardness at high temperatures have to be cleansed. This review describes the late approaches to handle these problems that base mainly on the impr everyplace of a third material (La2O3, Gd2O3 or plumbago). The humanitarian of either La2O3 or Gd2O3 light-emitting diode to change magnitude oxidisation resistance, but the processing, the amount of additive and the testing methods have to be improved. Due to the addition of graphite akes the whirl toughness of ZrB2/ lay out (20 vol%) increased from1. INTRODUCTIONThe Thermal Protection System (TPS) of space vehicles is champion of the most serious parts of the whole vehicle 13. 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. variety 1 Temperature prole of a space vehicle during reentry prepared by the NASA. The dark red colour represents the highest temperatures and the light down(p) regions are coolest parts (copied from 4).The tip of the leading edge has a radius of 10 cm, but a radius in the chuck of millimetres is wished so that piercing 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 as well as improve safety referable to an increased cross-range-capability 5. The problem of a smaller tip radius is that this leads to higher push through temperatures, wh ich can exceed 2000 C 6, 7. Jin et al. 8 investigated the maximum cake 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.Due to the high requirements for these materials exclusively a few can be considered. Very good potential for the usage show Ultra high temperature ceramics (UHTC). These are ceramic materials with high break up 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 mel lower-rankinging point than the diborides, their thermal conductivity is lower, which is very important because the heat on the coat has to be transported as fast as possible away 5. Therefore the main focus of look for 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 much ductile material fructify was added to ZrB2 or HfB2 12. Although ZrB2/ congeal and HfB2/ set on 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 RESISTANCEThe 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 cartridge clip 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 24The liquid B2O3 forms a protection degree for the porous ZrO2 layer, where atomic number 8 can diuse very fast and easily through. just now at 1100 C this protecting(prenominal) layer evaporates and cannot prevent the oxidation of the bulk material any more than(prenominal). The addition of SiC leads to the following additional reactions 10, 25, 26During oxidation four dierent 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 oxidate layer, which consists of porous ZrO2 and the velocity layer is the Silica rich layer which consist of the BS glass, that also lls 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 t he weightiness of the response zone under the disposed parameters of the experiment. But also the diusion coecient of O2 is important but rarely measured.2.1. Ratio ZrB2/SiCThe dependency of the oxidation resistance on the ratio of ZrB2/SiC was investigated by numerous researchers 10, 12, 15, 2729. The addition of SiC led in all cases to a reduction in grain size, a homogeneous diffusion of SiC, higher viscousness and higher minginess. Karlsdottir et al. 29 investigated not only the reactionFigure 3 Schematic demonstration of the dierent layers that form during oxidation of ZrB2/SiC and their arrangement.zone thickness but also the viscousness 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 diusion coecient of O2 in the layer.Seong et al. 10 equationd ZrB2, ZrB2/SiC (20 vol%), ZrB2/SiC (30 vol%) and ZrB2/SiC (40 vol%) and measured the resulting thickness of the reacti on zone. All samples were densied by hot pressing and exhibit a homogeneous diffusion of SiC. The grain sizes were between 1.0 m and 3.0 m. 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 then the reaction zone consisted only of the Silica rich layer and the oxidized zone. In business line (2104 Pa) the thickness of the reaction zone was in ZrB2 the thickest due to the absent 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 outdo choice. Apart from the improvements of oxidation resistance due to optimization of the ZrB2/SiC ratio, at temperatures abo ve 1800 C active oxidation (Eq. (4)) of SiC takes place and this hinders the formation of the BS layer. 2.2. AdditivesTo increase the oxidation resistance at higher temperatures transition metals were added to the ZrB2/SiC matrix 21, 3032. 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 diusion 11. onlymore 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 mum yet but in general a positive trend to higher oxidation resistance can be seen 11. Several attempts were made with numerous dierent 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 dierent processing methods that effect the properties 30.2.2.1. Addition of La2O3The addition of La2O3 to ZrB2/SiC was investigated by several researchers and led to dierent 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 dierent results, general trends due to the addition of La2O3 scorn the usage of dierent processing routes (hot pressing, spark plasma sintering (SPS)) were observed 6, 22, 23 Higher densityHigher Vickers HardnessReduction of grain sizesBesides the use of La2O3 leads to a more homogenized distribution of SiC, because it is always close to it and accordingly 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 reactions2ZrO2(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 pigeonholing detected the formation of La2Si2O7. The addition of 10 wt% La2O3, densied by SPS and oxidized for 1 h in air at 1600 C led to the formation of two dierent 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 coecient of La2O3 causes lling 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 dete ct the new phases. Instead they found out that La2O3 reactsTable 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 phase5 vol% hot pressing La2Zr2O7 (1900 C, 60 min) La2Si2O73 vol% hot pressing La2O3-SiO2 (1900 C, 60 min)10 wt% SPS La2Zr2O7 oxidized (1600 C, 1 h, air) La-BS-glass2 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 diusion. 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 fall down directly to the top of the surface layer. This leads to a more homogeneous mixing with the BS melt. The oxygen diusion through ZrO2 is much easier than through B2O3 and therefore a homogeneous distribution of ZrO2 makes it easier for O2 to diuse 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 diuses into the oxidation layer it go out react with ZrOxCy or SiOxCy, so it can be seen as a puer zone and it takes longer until the oxygen reaches the bulk material. HfB2 diers from ZrB2 because the formation of HfO2 is more dicult 6. Therefore a smaller amount is formed which leads to a lower amount of B2O3 and therefore a repressner protection layer compared to ZrB2. But it has the advantage that the diusion coecient 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 diuse 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 canFigure 5 Topview (a) and sideview (b) of the surface with the dimensions of the cavities for Gd2O3.oxidation layer was 15 3 m and formed in accordance with the following reaction equation 92Gd2O3(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 m 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 reduc ed diusion of O2 through this coating. At higher Gd2O3 fractions Gd stops ZrO2 particles at the glassy phase and they cannot diuse further. This leads to O2 vacancies which is the driving force for inward O2 diusion. apply a distance of 20 m 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 diusion occurs.3. FRACTURE TOUGHNESSIn 2009 the rst 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 a1high 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 MPam122very promising approach for a better oxidation resistancealthough further research is necessary.2.2.2. Addition of Gd2O3For an improved surface and at the same time unchanged bulk material a new processing method was invented 11 At rst the sample was prepared and densied using the bulk material ZrB2/SiC. Afterwards they used a laser to make equal coat cavities on the surface that were lled 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 dierences between 10 vol% and 15 vol% graphite were negligible small. Moreover there were investigations about the inuence of the diameter size of the graphite akes 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-akes. 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 m to 3.2 m. The reason is the homogeneous distribution of graphite which stop grain growth. Because of the reactions of graphite with the surface impurities the addition of graphite results in higher dense samplesZrO2(s)+ B2O3(l)+5C(s) ZrB2(s)+5CO(g) (10)The particles that form due to this reactions can ll 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-akes pull-out, crack bridging, branching and deection.1Figure 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%) +1graphite (15 vol%) on the temperature in vacuum and in air (Fig. 7). Over the whole temperature range the fracture toughness in air was hig her than that in vacuum. In vacuum the fracture toughness decreases fromThis oxidation layer densies 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 deection which absorbs the energy leads to higher fracture toughnesses at higher temperature. These mechanisms all result in a sulky decrease in fracture toughness in air than in vacuum.Figure 7 Fracture toughness depending on the environment and on the temperature 37.4. CONCLUSIONThe 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 resis tance 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 espec ially the analytical approaches. Due to the varying experimental parameters and insucient analytical tests it is dicult 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 ofEq. (1), (2) and the following reactiongraphite from 4.5 MPamto 7.1 MPam creases, but also this is slowed down due to the graphite tives, because extensive testing of the dierent samples akes. 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 dicult to announce the best addi