Zirconium Doped Hybrid Composite Systems for Ultra-High

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Review

Zirconium Doped Hybrid Composite Systems for UltraHigh Temperature Oxidation Applications: A Review Giridhar Gudivada, and Balasubramanian Kandasubramanian Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05586 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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Industrial & Engineering Chemistry Research

Zirconium Doped Hybrid Composite Systems for Ultra-High Temperature Oxidation Applications: A Review Giridhar Gudivada, Balasubramanian Kandasubramanian* Structural Composite Fabrication Laboratory, Department of Metallurgical & Materials Engineering, Defence Institute of Advanced Technology (DU), Ministry of Defence, Girinagar, Pune-411025, India. * Corresponding Author: Prof. (Dr.) Balasubramanian Kandasubramanian, Email: [email protected] Abstract The re-entry phase of a space vehicle demands thermal shielding against the heat elicited by an aerodynamic heating process. The quantity of heat spawned in such process is close to 11.35X105 KJ.m-2 and temperature reaching 8000oC at the stagnation point on the surface of the re-entry vehicle. Ablative materials are effectual for heat shielding re-entry vehicles and their performance is improved by various physio-chemical techniques. Utilization of ultra-high-temperature ceramics to enhance the performance of ablative materials has set a new milestone in heat shielding applications. The article briefly introduces the conditions that prevail during a re-entry phase, discusses thermophysical properties that are to be adorned by ablative materials other mere thermal insulation. This review article uniquely discusses the effect of the selection of ceramic filler system on the formation of an oxidation layer, the emissivity of the oxidation layer and thermophysical behavior of the oxidation layer.

Keywords: Re-entry; Hypersonic; Ablation; Oxidation; Matrix modification; Passivation; Mechanical denudation; Zirconium Functionalization.

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1. Introduction: Ablative materials are degenerative composite systems which are by design processed to degrade at projected rates when exposed to high aerodynamic heat rates (~105 BTU-ft2) at high temperatures (~8000oC). Ablative materials have diverse applications1–5 in the fields of aerospace as a protective layer for leading edges 6 of the control surface, in medicine for curing various diseases in form of ablating lasers and in space technology as thermal protecting systems at hyperthermal 7–9 environments. In the field of medicine, ablation 2 phenomenon is used cure tumors, irregularities in heart pulse rates; by focusing high dosages of energy over a small volume as in case of ablative radiography or catheter ablation for atrial fibrillation but in case of aerospace technology, heat energy is insolated upon a larger surface is to be considered. Ablation in the medical terminology implies complete material to be removed from the host system as in case of tumors and in the case of atrial fibrillation the paths of unnecessary impulses are cut down, whereas, for the field aerospace ACS Paragon Plus Environment

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technology, only a part of the system is necessarily required to ablate at known uniform rates at stable operating conditions. Technical understanding of the phenomenon, Ablation as early as 1983 states that, “Ablation is a complex energy dissipative process whereby a material undergoes combined thermal, chemical, and mechanical degradation accompanied with a physical change or removal of surface material”10. The degradation process has been a keen interest among the scientific community for many years and has evolved many techniques to converge upon a common idea, i.e., how an ablative material functions in severe aerodynamic condition. Recently, the multi-phase modified matrix technology unlike simple single phase matrix systems of two classes mentioned in next section has offered a platform for yielding knowledge investment from a multidisciplinary background of science and engineering for design the ablative materials. So, the performance of modern ablatives is tending towards euclidative application of ultra-high temperature ceramics, potentially with zirconium diboride due to its quick and timely response to the cataclysmic re-entry environments as witnessed by the thermo-kinetic approach and experimental procedures which are discussed in this article. 1.1. Re-entry Vehicle Structures: Re-entry 11–14 vehicle structures are marvels of modern engineering which have made humanspace travel conceivable by guaranteed safe landings surviving the extreme re-entry conditions which are discussed in the next section. There are two kinds of re-entry vehicle structure the first one being capsule type and the second being shuttle type 11,15. Both are meant for human travel to space but for one difference that the latter is more maneuverable than the former. The space capsule probably adapted from early spy photography satellites example Vostok the first manned capsule was adapted from during cold war, enters the planetary atmosphere at hypersonic velocities with high velocity gradient, whereas, the shuttle undeniably also has high Mach number, can alter its speed by a method called lifting re-entry where the speed of the shuttle is reduced considerably due to aero breaking attributed to the frictional resistance of air against the RVS. The lifting re-entry 11 is done based on the principle of conservation of frictional energy into heat energy due to repeated entry and controlled


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overshooting thereby, reducing the kinetic energy of the shuttle before re-entry, frictional heat generated on the surface 11 and increasing the window 11 for re-entry. Detailed studies16–19 have been done during the 20th century through the past decade for calibrating various design parameters for space capsules and space shuttles like blunt nose approach, lifting entry etc., which would enter at hypersonic speeds. Some of the major factors affecting the aerodynamic design of the RVS are air friction, the angle of attack, aerodynamic heating, re-entry velocity 11,20–22. There is a need for the introduction of thermal protection system for heat shielding management despite the fact that certain aerodynamic methods have been employed earlier to reduce the amount of aerodynamic heat that generated at the interface of surface and ambient air. Thermal Protection Systems (TPS) is broadly classified into ablative systems and non-ablative systems as shown in figure 17.

Figure 1: Classification of materials for Thermal Protection System Ablative material a sub-class in TPS is again classified into organic and inorganic ablative materials which sublimate certain percentage of material along with a fraction incident heat whereas the non-ablative materials are insulators and exhibit radiative cooling, yet both the materials are accompanied by inevitable limitations 6 such as fungus, flora, and fauna, environmental, chemical, thermal etc. ACS Paragon Plus Environment

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The irradiation (net heat incident) on the material surface has a significant impact on the bulk properties like residual stresses, the degree of curing of polymer ablatives like phenolic etc leading to desideratum to closely observe the energy content within the material system where materials of different thermophysical properties form an interface or interact with each other23–28. The irradiation of surface structures like moisture barrier, ablative coating and adhesives binding these elements at the ground base (before launching) is contributed by solar radiation, reradiation from nearby equipment, sky radiation, ground radiation and aerodynamic heating adds to net heat during re-entry. The net heat content or the enthalpy in the material system will significantly impact the performance of the materials either in a static situation or during dynamic re-entry, which is the cause of desideratum mentioned earlier 29. 1.2. Evolution of Ablative materials The first successful use of phenolic-based ablative material was for inter-continental ballistic (ICBM) in 1957 preceded by Bumper, a modified V-2 6 which had Teflon integrated into its design for thermal protection and in a decade the phenolic-ablative material has replaced Teflon for thermal shielding of RVS (Table 1)6 to space expeditions during the space race in the early 1960s where phenolic and epoxy based reinforced composite was coated on an aeroshell

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structure. Although

much detail of such a system is still partially classified yet is being used for specialty applications even today. Eventually, flexible ablative 31 materials based on the technologies of SiC-Fabric, foam and even cork 30 are being used as ablatives in various components of a space shuttle program since the 1980s. The concept of thermal curtains 29 has had also occupied a crucial role in heat shielding the components in the dorsal part of the shuttle during the lift-off modules. Many polymeric ablatives have been investigated in order to achieve better ablation properties and much recently researchers have developed new kind of polymeric materials system with nanofillers to achieve extreme values of the thermal gradient as high as ~3000oC implying a remarkable achievement in the domain of polymer ablative composites 32–42. The phenolic-based ablative material PICA-X developed by Space-X has shown remarkable results in its recent use in dragon reusable cargo capsule and has been claimed as the best ablative to ACS Paragon Plus Environment

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be used in re-entry in the history

43–47.

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Since the first use of Phenolic Impregnated Carbon Ablator

(PICA), the phenolic-based composites hybridized/functionalized with ultra-high temperature ceramics have shown increment in the ablation performance by 65 to 72% in contrast to basic phenolic / carbon systems 48. 1.3. Ultra-High Temperature Ceramics (UHTC) for ablative applications The UHTCs are the ceramics with melting points greater than 2700oC 1. These materials possess properties like good oxidation resistance, ablation resistance, thermal expansion and damage tolerance among other characteristic features which are discussed in further sections. The best contender among UHTCs for ablative is ZrB2, nevertheless, there are other ceramics like Tantalum Carbide (TaC), Hafnium-Diboride (HfB2) and Hafnium-Carbide (HfC) with melting point temperatures higher than that of ZrB2 but there are other aspects like cost, ease of processing, availability and temperature range of chemical activity (1500oC-1800oC). It is necessary for the ceramic to be used as a matrix modifier while ablation during the re-entry phase that they have to respond to the changes in the environment in the vicinity of the boundary layer 49–53. 1.4. The re-entry Conditions The re-entry is a dynamic phenomenon where a re-entry vehicle interacts with the planetary atmosphere with significantly high momentum (approximately 1500Kg x 11m/s) holding a total energy at least twice a score of Giga-joules6, an arduous situation for designers to come up with a material system with good mechanical characteristics, aerodynamic performance and least possible weight addition to the payload carrier. Additionally, the re-entry velocities range from 8 Km-s-1 to 10 Km/s, heating rates are near to 105 BTU-ft2 (1.135GJ/m2) 10, stagnation pressure, the pressure at a point where the movement of free stream air would abruptly cease is 8 GPa and stagnation temperature at that pressure is around 8000oC 11.


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Table 1 Retrospective Data on various ablative materials used in past seven decades for space missions S. No

Re-entry vehicle

Year

Ablative material

Re-entry velocity, m/s (Mach)

1949

Teflon

1600 (9)

-

14.0

12700

6,54

-

-

-

1678.3

6,45

M= 16

-

18288.0

-

6,17

8000 (24)

41

120000

Energy (GJ)

Height (m)

Weight (Kg)

Ref

Bumper 1 V2 Missile Mark 2 Metal based (name 2

(I heat sink

1958 not specified)

RV) Ph-Ny1 and 3

Jupiter 1C

1957 HPFSIN2

Mercury 4

1958

BH3 and FG-Ph4

6,12

re-entry 140000 Evaluation for 5

Big Joe

1959

6700 (20)

-

to

1159

6

5533.83

6,23,55

Mercury 2300000 11000 6

Apollo

1962

ENQF and PMB 5

340

6

120000

(33.33)

This review article discusses the ablation behavior of phenolic based ablative materials, and the comparison is done with ablation performance of zirconium functionalized composites which include both ceramic as well as polymer matrix ablatives. This review also discusses the rationality in modifying a composite with any of the zirconium ceramics, their synthesis roots, and ablation mechanisms. The formation of zirconia is the primary requirement for the mitigation of oxidation

Phenolic Nylon Ablator Fused silica hot pressed into Inconel 3 Beryllium Heat Sink 4 Fiber Glass reinforced Phenolic 5 Epoxy Novoloac / quartz fibre 6 Phenolic Micro Ballon 1 2


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which is best done when zirconia forms binary or tertiary phases with other oxides of ablation. It has been discussed how modifying ZrB2 improves the structure of the scale formation and its functionality by improving its response to the high-temperature environment. Additionally, volatility diagrams have been used to understand the response and behavior of zirconium diboride with varying temperatures at known oxygen partial pressures to compare the oxidation behavior of ZrB2 with other UHTCs. 2. The physical properties of ablative materials: The foremost properties of ablative material are the ablation resistance and rate of ablation. The motive of designing thermal protection system has always been tossing between insulation and ablation. The insulation, as the name suggests, is to just reduce thermal gradient across the material whereas the ablative material has two functional requirements firstly, to reduce back surface temperature56 by insulation and secondly, to expel the heat along with the sublimated mass into the free stream57. The performance of the ablative system is undeniably dependent on aerodynamic factors58 such as ballistic coefficient 11, angle of attack and drag coefficients 11 friction reduction methods like aerobraking but, our focus is limited to thermophysical parameters like heat generation due to aerodynamic heating, phase changes in the material constituents and oxidation behavior at various temperatures in the presence of secondary phases and structural integrity. Optimization of ablative performance demands certain modifications in matrix systems like the addition of fillers which are in general accompanied by variations in physical properties. Srikanth et al., have reported that the content of zirconium in Zirconium-Carbon fabric and phenolic matrix system have significantly affected the physical properties of the composite. A particular composition consisting of 6.5 wt% of Zirconium has offered the least back surface temperature of 104oC when subjected to the plasma arc jet test for 30 seconds at an energy rate of 4.0 MW/m2. The reduction in back surface temperature is reported to accompany with a reduction in flexural strength to a value of 146 MPa from 225 MPa for blank Carbon-Phenolic sample, which approximates to 35% reduction. Similarly, a 30 % reduction in the value interlaminar shear strength was reported i.e., from 20 MPa to 14MPa and thermal ACS Paragon Plus Environment

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conductivity by 44 % from 0.59 W/mK to 0.33W/mK56. John et al., have processed syntactic foam of phenol formaldehyde reinforced with silica fibers in order to achieve higher thermal stability. Increasing content of silica fiber up to 15% by volume increases tensile strength and a further increase in silica content deteriorates the tensile performance. Similarly, compressive strength, the coefficient of thermal expansion and char yield have been reported to enhance with increasing silica content but for compressive strength, under lower solid loading of silica have shown lesser value in contrast to blank carbon- phenolic syntactic foam and then slowly augmented59. Carbon-phenolic systems, when modified with silica-alumina based cenosphere ceramic filler, are found to improve the ablative performance along with thermal stability enhanced upon addition of cenosphere loading ranging from 5 to 20 wt% and fiber loading of 65wt%. The residual mass after ablation test increased along with cenosphere content and the temperature of 10% weight loss enhanced from 638oC to 677 oC while the back face temperature during oxyacetylene flame ablation test was ~400 oC for the sample with 20wt% cenosphere content at the time when from face temperature was 1500 oC. It was also reported qualitatively that the addition of cenosphere in compliment to improving ablation performance of cenosphere also has improved resistance to denudation during ablation process60. The main issues in the processing of polymer-based ablative material are to improve the thermal stability of the material by retaining the mechanical performance close to that of the virgin matrix material. Further, the absence of functional interactions at the molecular level between UHTC and matrix material is one of the major reason for the deterioration of mechanical performance in ablative materials. Silicate modified polymeric systems render improved mechanical properties, but, they do not offer elevated heat shielding performance which is primarily required in ablative materials, however, they can be specifically utilized for fire retardant applications. It can be concluded that in order to maintain the balance between ablative and mechanical performance simultaneously, can be achieved by selecting a proper filler system suitable for specific temperature ranges.


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2.1. Applications of Ablation: The literature on ablative materials is expanded to various fields like medicine, engineering (naval and aerospace applications). Typical examples of ablation application in the medical field are percutaneous ablation to cure renal tumors, Catheter ablation to cure atrial fibrillation, etc. 2,3 Ablatives in the naval application had been developed into a diverse variety of ablatives namely melting type, sublimating and charring type ablators. Another kind of ablative which had been developed for fire resistance coating is intumescent type ablator 10. The versatility of the ablative family is a platform for intense material selection and engineering, owing to the re-entry condition exhibits a cataclysmic variation in its characteristics in terms of free stream velocity, temperature and pressure

11,20,21

which necessitates considering

ergonomics of design hence driving towards a design developed out of concurrent engineering for TPS. In case of a general space shuttle vehicle, where the TPS is composed of Carbon-Carbon at the nose part, Carbon-phenolic on the fuselage, high-temperature reusable ablative materials on the leeward face during re-entry (figure 2)

Figure 2 Ablative Material selection for thermal protection system


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There has been an exponential increase in the interest of the scientific community in ablative materials like zirconium diboride modified phenolic class which is coetaneous with the increasing space expeditions by private stakeholders 61. 2.2. Polymer-based matrix systems for ablative materials:

Figure 3: Representation of the ongoing ablation process The phenolic resins obtained by condensation polymerization of aldehydes and phenols have proven to be better char materials. The key property of the matrix system to be chosen for the ablative composite is that it has to maintain its structural integrity, while not allowing the heat to pass through its thickness. It is being achieved by allowing the material to pyrolyze and form a char layer above the virgin material. Additionally, when the polymer matrix is modified by UHTCs it also forms a glassy film over char layer acting as an oxygen barrier and also protects base material from mechanical denudation (figure 3). The selection of a polymer system for a matrix for ablative composite depends on following factors flash point, melting point, glass transition temperature, fire retardancy, thermal gradient between the front face and back face, finally the ablation characteristics 62–65. The back face temperature of the material should be well within the permissible limits such that it doesn’t damage the substrate either thermally or chemically. In general, the structure of the substrate ACS Paragon Plus Environment

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is made of aluminum alloys with honeycomb structure as in case of the space capsule of Apollo mission. However, the recent developments have enabled the engineers to opt for polymeric materials for structural purposes like in the case of Airbus 380 where 20 % of the structure is made by polymerbased composites. The structural requirements of the ablative grade polymers are high thermal gradient before and after pyrolysis, low density, good mechanical strength against chemical erosion and denudation of char layer (turbostratic carbon) and shall also have resistance to undesirable circumstances like the interaction with flora and fauna at ground base

6,66–70.

The porosity of the char layer also plays an

important role as it leads to enhanced oxidation, and the fillers added to tackle this challenge are dealt in sections ahead. The response of the polymer to the incumbent high-temperature environment during re-entry is related to the thermodynamics of the material as reported in an early analysis article by Norman et al.,57 the effective or gross free energy when maximum leads to the total instability in the material. Such forms of thermal analysis of ablative are being done since the time of peacemaker. The motive behind such kind of analysis is to understand the temperature distribution through the material, linear ablation rate, and mass ablation rate as evident from the common interest among different research groups. The radiation characteristics of the ablatives also have been studied in several circumstances as the reradiation capacity of the surface help us in reducing the temperature by radiative cooling, the mechanism is extrapolated in (figure 3). Different kinds of polymeric systems are being developed for the ablative purpose to enabled to improve the functioning of an ablative system. These materials are usually Nano organic material like Polyhedral Oligomeric Silsesquioxane (POSS)

36,71

modified phenolic systems based on

resorcinol-formaldehyde (RF). These materials have gathered a significant interest by offering an excellent thermal gradient of 3000oC without being modified by UHTCs like conventional resol or novolac type phenolic materials36, additionally the RF is a long carbon chain with aromatic functions33, this could probably be expected to give better physical properties to carbonaceous char like thermal insulation and hardness as reported in an article by wen-Shyang Kuo 72 on Raman study ACS Paragon Plus Environment

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of pyrolysis process for phenolic resin infers that, at pyrolytic temperature for carbon phenolic composites carbonization and graphitization occur with low ordering but with some ordering at basal planes. Such kind of disorder could be because of the shorter carbon backbone chain of the phenolic matrix in contrast to the massive carbon chain of RF. 2.3. Ultra-High Temperature Ceramics (UHTC): The application of ablative materials has been done by two ways for TPS applications the first of which is by ceramics tiles used in high-temperature applications like nozzle, nose tip, and the second method is by reinforced polymers with suitable blends and fillers 7,32,48,54,56,73–75. In this section, we will discuss the developments of ceramic matrix composites modified with zirconium ceramics with other conventional ceramics like silicon carbide and tungsten carbide 38. The fundamental reason behind introducing zirconium ceramics alongside classical high-temperature ceramics like C/C composites is their high melting and eloquent thermo-physical performance. Zirconium-based ceramics belong to a different class of material called as ultra-high temperature ceramics(UHTCs) which have a melting point temperature more than 3000oC 76–78. In contradiction to their high melting temperature and impact strength79, Feng et al., had reported that monolithic ultra-high temperature ceramics are brittle materials with low values of fracture toughness and most of all have got poor thermal shock resistance80. Thermal Shock Resistance (TSR) is an important parameter to be tested for high-temperature ceramics’ performance81–86. A lot of research groups have established that the TSR of a ceramics is sensitive to temperature and other external parameters while D. J. Li et al.,87 had reported that the TSR of UHTC also depends on mechanical and thermal properties and they fail at surfaces when the surface stress due to thermal shock exceed fracture strength. Also mathematical evaluation of mechanical properties have been undertaken for fracture at high time rate of thermal loads based on certain assumptions which state that the model (figure 4) is a two well-bonded plate, which doesn’t consider the interface damage, there is no heat exchange between the UHTC plate and base plate, both the layers geometrically confirm with each other which make calculation easier,


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finally plate is continuous, isotropic, elastic and is restricted in the domain of small deformation hypothesis.

Figure 4: Different loads on a typical UHTC coated thermal protection system Then the equations for the effective linear expansion in the ceramic layer is given by equation (1)

[

𝜎

]

[∆𝑥 = ∆𝑥1 + ∆𝑥𝜎] = ∝ (𝑇 ― 𝑇1) ― 𝑌𝑐(1 ― 𝜇𝑐) ∗ 𝑥

(1)

Where ∆𝑥1 is the elongation in cthe eramic plate without external restrictions and ∆𝑥𝜎 is the value of restricted elongation due to complimentary compressive stress at the interface, from the above equation it is evident that the net elongation of the system when assumed the changes in Young’s modulus (𝑌𝑐) and Poisson’s ratio (𝜇𝑐) for ceramic material are devoid of temperature changes, could generate an internal stress 𝜎 in the ceramic plate plausibly at the interface, which was derive by D. J. Li et al.,87 as shown in equation (2) 𝑌𝐵𝑌𝐶𝛼(𝑇 ― 𝑇1)

𝜎 = (1 ― 𝜇𝐶)𝐸𝐵 + (1 ― 𝜇𝐵)𝐸𝐶 .…

…………(2)

Considering the above equation with the effects of temperature would be modified to equation (3).


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As shown where the pressure stress or internal compressive stress has been taken into account by considering Young's modulus (Y) and the coefficient of thermal expansion (𝛼) 𝑌𝐵𝑌𝐶(𝑇)𝛼(𝑇)(𝑇 ― 𝑇1)

𝜎 = (1 ― 𝜇𝐶)𝑌𝐵 + (1 ― 𝜇𝐵)𝑌𝐶(𝑇)…………………………(3) The most effective way to mitigate failure by thermal shock is to increase the Critical Temperature Difference of Rupture (CTDR). According to R. Z. Wang et al., the CTDR increases as the temperature of surroundings increase up to a certain extent and then decreases and the governing equation for CTDR by R. Z. Wang et al.,82 is as shown below in equation (4) 𝑅′

Δ𝑇𝐶 = 𝐶ℎ𝑡𝑆…………….……………… …(4) Where ‘h’ is the heat transfer coefficient, tS is the thickness of the ceramic plate and R’ is a constant parameter called as second thermal shock resistance parameter and is given by equation (5)

[

𝑌𝐵𝑌𝐶(𝑇)𝛼(𝑇)(𝑇 ― 𝑇1)

]

𝐾(𝑇 + Δ𝑇𝐶)(1 ― 𝜐𝐶)

𝑅′ = 𝜎𝑓(𝑇 + Δ𝑇𝐶) ― 𝑌𝐵(1 ― 𝜐𝐶) + 𝑌𝐶(𝑇)(1 ― 𝜈𝐵) ∗ 𝑌𝐶(𝑇 + Δ𝑇𝐶)𝛼(𝑇 + Δ𝑇𝐶)……….(5) It is to be noted that the physical properties of mechanical interest like Young's modulus and fracture stress vary along with temperature as described by equation (6).

𝑌 = 𝑌0 ― 𝐵0𝑇𝑒

―

𝑇𝑚 𝑇

𝑇𝑚

― + 𝐵1(𝑇 ― 𝐵2𝑇𝑚 + |𝑇 ― 𝐵3𝑇𝑚|)𝑒 𝑇 ………..

…….(6)

Where B1, B2, B3, and Bo are material constants and Yo is Young's modulus at ambient conditions. The fracture stress as a function of temperature is given according to D. J. Li et al.,87as in equation (7). In order to understand the dependence of the fracture stress with temperature, it has to be inferred from the function of Young's modulus that it is dependent on temperature in a transcendental fashion and so does the fracture strength.

𝜎𝑓 =

2

[

(𝜎0𝑡ℎ)2 𝑌0

[

𝑌(𝑇) 1 ―

𝑇

∫00𝐶𝑃𝑑𝑇 𝑇

]]

∫0𝑚𝐶𝑃𝑑𝑇

…………

………….……..(7)


15


[

𝑇

It is mentioned that the term 𝜑 = 1 ―

∫00𝐶𝑃𝑑𝑇 𝑇

]

∫0𝑚𝐶𝑃𝑑𝑇

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belongs to temperature dependent fracture surface

energy term 82 and indicates that as the operating temperature approaches to melting point the ratio 𝜑 tends to unity as a result the temperature dependent fracture stress tends to theoretical stress leading to failure. The thermal shock resistance can be increased by incorporating micro-flaws in to the ceramics like crack, pores, grains, residual stress due to thermal expansion anisotropy and as such eliminating the initial rupture temperature reaching the danger zone of temperature for thermal shock resistance as reported by H. B. Kou et al., 88 and R. Z. Wang et al., for materials like hafnium diboride and Zirconium diboride respectively along with mathematical reasoning 82. These micro-flaws also cause deterioration in mechanical performance as said by R. Z. Wang et al., which is testified by the following equations of fracture mechanics. As per T. B. Cheng et al.,83 there are two theories of studying TSR namely thermal shock fracture theory and thermal shock damage theory. The evaluation of TSR is done with data of some qualified experiments like quenching strength test, arc discharge technique, moving electron beam heating, hydrogen-oxygen torch and electric resistance method 83. A detailed explanation of methods will be given in later sections. All the parameters regarding TSR discussed above have been studied on ceramic matrix composites of UHTCs, whereas the thermal shock in polymer-based ablative will take a different course as it has better TSR. TSR of UHTC filler modified polymer matrix composites is now main interest among the research community. 3. Ablation Mechanism: The mechanism of ablation is a dynamic phenomenon that occurs at hyperthermal and turbulent environments, as an aggregation of chemical reaction, phase transformations in char layer as well as filler material, mass transfer along with latent heat of sublimation as characteristic of sublimating ablative materials 57, finally the aerodynamic shear forces causing the mechanical denudation of char off layer as described in figure 5. ACS Paragon Plus Environment

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Figure 5 Schematic of ablation cycle of a material Ablation mechanism has to be dealt differently with polymer matrix ablative and ceramic matrix ablatives as the former involves charring of polymeric resin followed by erosion of the char layer while the ablation in ceramics involves only oxidation followed by secondary phases as a barrier to further oxidation and as heat shields. However, the polymeric matrix ablative when modified with ultra-high temperature ceramics as reported by research groups, show better ablative resistance. Additionally, the polymer ablative materials provide excellent flexibility over ceramic matrix ablatives with tunable thermo-physical properties and better acoustic performance. 3.1. Ablation in ceramics: The ablation of ceramic-based ablatives is generally by chemical corrosion followed by mechanical denudation, H. Junli et al. has reported that as the ablation phenomenon changes its regime from chemical erosion to mechanical denudation as the heat rates increase from 2400 to 4200Kw.m-2 89. Coincidentally, in case of entry into a planetary atmosphere, there is variation in the heat rate at the surface due to the gradual increase in the density of the atmosphere, also due to increasing Mach number reaching as high as 20 6,90. The heat rate later drops down due to deceleration of the re-entry vehicle to safe landing speeds reducing the heat rate too. As we are discussing the ACS Paragon Plus Environment

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ablation mechanism of ceramics it is important to understand that unlike polymeric materials ceramic do not char off rather they would oxidize or melt off. Many research groups have been working on ultra-high temperature ceramics to achieve best suitable ablative system like carbon fiber reinforced carbon composites modified with UHTCs like Zirconium diboride, Zirconium carbide etc. or in combinations, which have reported to give satisfactory results with the reduction in the mass ablation about 90%. X. Yao et al., have reported that the carbon-carbon composite coated with ZrB2 and SiC system when undergone ablation test for 60 seconds have shown 92% reduction in linear ablation while mass ablation reduced from 1.43 to -0.02 mg.s-191, the negative sign indicates the deposition of mass due to formation of solid combustion products 192. In this case, it was reported that a glassy film of zirconia on the ablated surface functions as an oxidative barrier preventing further oxidation and denudation. However according to Y. Chen et al., and Rouhallah et al., ZrB2 has no role over temperature distribution and change in thermal stability, which depends on other nanofillers like Graphene, polyhedral oligomeric silsesquioxane (POSS) etc.,

48,71,

interestingly graphene when

wedged about Nano zirconium diboride under certain charged conditions at low temperatures repel oxygen and condenses it with nearby nascent hydrogen atoms to form water (Figure 6) 93.

Figure 6 Combined effect of graphene and ZrB2 under the influence of ionized platinum on oxidation properties at low temperatures ACS Paragon Plus Environment

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Similar kind of results have been reported by K. Z. Li et al.,92 for carbon-carbon (C/C) composite where the porous C/C composite has been modified by precursor infiltration and pyrolysis with ZrC and SiC, resulting in enhancement of ablation and oxidation resistance that attributed to the formation of filmy layer of zirconia but interesting aspect about silicon carbide according to M. Y. Zhang et al., was that SiC improves the adhesion of ablation products 94 and matrix in addition SiO2 which is formed out of oxidation of SiC acts as self-healing agent 7, probably are couple of the desirable qualities that an ablative material could offer in order to sustain the operational features of the material at such extreme erosive environment which arises due to high free stream velocities (Mach number ~30). Another interesting study was the impact of two solid condensates over ablation surface as it was reported by A. Vinci et al.,95 that in a system of hot pressed carbon fiber reinforced ZrB2-SiC composite during ablation at 750oC zirconia and Boria (B2O3) are formed, Boria although volatile at that temperature crystallizes at the surface along with zirconia forming a dense layer filling up that surface flaws as reported by X. Shen et al.,96. Zirconia film aids in the reduction of shear forces in the surface impressed by boundary layer in addition to the mitigation of oxidation and ablation rates. M. Naderi et al.,

97

reported that during the cooling phase of ablation test zirconia

formed in ZrB2-SiC system undergoes phase transformation and hence induces volume changes. This can be potentially used as heat sinking

98

before the onset of mechanical denudation and after

chemical interaction with the free stream leading to the coincidental formation of oxide layers. This phase change has been reported by another research group K. Edalati et al.,99 that Zirconia, when subjected to plastic deformation, undergo phase transformations from monoclinic at room temperature to tetragonal at 1373K, then to cubic phase at 2673K and finally liquidates at 3963K, but when a pressure of 10 GPa is exerted it may transform to orthorhombic phase. It is clear by now that the while re-entry, the pressure is as high as 8 GPa at stagnation point and so it can be inferred that the phase transformations could be counted during compositional characterization of zirconiumbased ablatives. It is quite fascinating to acknowledge that the binary phase scale formations are giving a better edge over single phase scale formation as it could be understood from the close observation from the binary phase diagrams of UHTCs. One example to testify is that, modifying a ACS Paragon Plus Environment

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ceramic composite like C/C with ZrB2 would decompose into zirconia film and Boria gas

100

that

escapes into free stream through boundary layer but using multiple fillers like SiC and ZrB2 would form fragile and thin binary scale like borosilicate by combination of Boria and Silica

97

(figure 8

and figure 9), otherwise Boria would escape owing to its volatile nature at high temperatures. So, the significance of the entire design of ablation materials is to sufficiently mitigate the jeopardy of the high-temperature oxidative environment. A research outcome on microstructure and ablation properties of C/C composites doped with ZrC has been reported by X. Shen et al.,96 which has discussed that the ZrC during ablation leads to the formation of honeycomb structure (Figure 7) on the surface and also provides a pinning effect to the zirconia film which in turn is formed by partial oxidation of ZrC. However, ZrC is used as a toughing agent in multiphase ZrC-SiC-ZrB2

101

or in a

ternary phase with HfC-SiC 102 which is achieved by pinning effect reportedly due to the formation of Zirconium Silicate (ZrSiO4) thereby increases porosity with subsequent reduction in char during ablation process56. Albeit, the ceramic-based ablative systems offer such an outstanding performance and some systems even offer reusability. There are certain limitations in ceramics, which could be mitigated with polymer-based ablatives owing to their tunable density, resistance to thermal shock (a is a key parameter for the structural integrity ablative material at first place), where ceramic matrix ablatives usually fail attributed to their brittleness. The next section deals with the mechanism of ablation in the polymer matrix material and their edge over ceramic matrix materials.


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Figure 7 Honeycomb structure of post-ablative specimen doped with 3.24 wt% of ZrC. Reprinted with permission from ref 96. Copyright 2010 Elsevier B. V. 3.2. Modification of Zirconium diboride The performance of ZrB2 based ablatives can further be enhanced by modifying ZrB2 with other elements and compounds, which have assisted in yielding significant and constructive changes in ablation performance of the material. ZrB2 modified with different compounds or elements either by mechanical mixing method followed by compaction or by chemical deposition over ZrB2 have displayed formation of multiphase oxidation barrier at ablating temperatures aiding in the improvement of performance of the material. This unique quality of the Zirconium diboride to form multiphase protective layer takes an upper edge (table 2) if it is mechanically alloyed with transition or rare earth elements103. Many scientists have investigated aforesaid studies and concluded that mechanical alloying with rare earth elements form multi-layer protective glass coat yet each layer may still be multiphase. W. Tan et al.,104 modified ZrB2 with samarium and thulium through two processes, first, chemical doping by CVI technique and second, by dry mixing in ball followed by compaction in a press. Further, they reported that chemically doped ZrB2 best performs by enhanced surface emissivity, an ingenuine technique to deal with ablating environment as radiation can transfer 90% of heat. It is required to recollect that addition of one atom to other effects cation field strength ACS Paragon Plus Environment

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and the addition of transitional metals to ZrB2 due to optimal cation field strength (equation 8), there would be immiscibility which increases viscosity as explained by Einstein-Stokes equation (equation 9) of the melt at oxidation temperatures. As a result, oxygen transport into material reduces in proportion to the increasing viscosity of the melt. Additionally, mechanical mixing has not given many admirable results when compared to chemically modified ZrB2 as reported by Monteverde. F et al.,105. ∁ =

Z

…………………

r2

KT

D = 6πηp………………

……………….(8)

………………. (9)

Where C denotes cation field strength, Z denotes valency, r denotes ionic radius, D denotes diffusion rate, K denotes Boltzmann constant, η denotes viscosity, p denotes particle dimension, T denotes absolute temperature. Jayaseelan et al. concluded that fusion of ZrB2 with 10% La2O3 through spark plasma sintering and SiC which as sintering aid gives the best performance for oxidation resistance over lanthanum boride and Gadolinium oxide 106. Another innovative idea to form multilayer was reported by S. C. Zhang et al.,107 by doping Zirconium Diboride with tungsten carbide (WC) which lead to the formation of dual glass layer (Figure 11), the top layer was porous and depleted of tungsten oxide and appeared light in complexion while the bottom layer was rich in WC which appeared dark and was dense. However, the addition of WC was limited to 6 mol% beyond which, the material cracks and as much as 230 vol% changes were recorded. The desirable achievement in this work was that WC reduced the thickness significantly, from 3 mm for nominal ZrB2 to 0.75 mm for ZrB2 doped with WC after 6 hours of ablation test at 1600oC projecting the future prospects for long duration supersonic flights. An illustration of the effect of different filler systems has been described in (Figure 10).


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Table 2 Effect Transition metal (Samarium) on Ablation characteristics Samarium Maximum Mass Concentration Surface Density S.No Porosity change in ZrB2-SiC temperature (g/cm3) (Δm/m) (Wt%) achieved 1 0 4.5 18.2

Surface roughness (microns)

3.47

1727 2

3

3

5

0.19

4.1

26.1

8.58

0.21

3.7

32.0

16.07

0.26

3.5

35.7

21.10

1631

1661 4

8

Figure 8 Illustration of ZrB2 – SiC response in typical re-entry environment


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Figure 9 Detailed illustration of highlighted area for the typical response of ZrB2 – SiC to re-entry environment

Figure 10 Effects of various fillers on the behavior of the oxidation barrier


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Figure 11 Effect of modifying ZrB2 with WC on the formation of barrier coat 4. Thermodynamic Analysis of Ablation Mechanism in polymeric systems: The thermodynamic analysis of ablation behavior in resin-based plastics with inorganic reinforcements was reported by Norman et al., that it was feasible to comprehend the ablation mechanism in glass fiber reinforced plastics through thermodynamic and kinetic principles. In this model, three processes had been considered first, the thermal decomposition of resin-based plastic, second the reaction of the gaseous products of this decomposition and third the reaction between carbon and fiberglass. Additionally, it has been mentioned that unlike the first two events the last event of carbon and fiberglass interaction is endothermic in nature and acts as heat sink to about 4 Kcal.g-1 of chemical heat. However, in this model, there is a correction as reported by Norman et al., of 750 BTU/lb (1.74 MJ.Kg-1) due to insufficiency in heat accounting during the reaction involving pyrolysis of the resin matrix and inorganic reaction. 4 Kcal accounts to over 16,700 joules whereas the heat insolation during re-entry would vary from 12 GJ to 340 GJ. Considering Apollo space capsule which had a mass of 12200 lb 6 which is approximately 5500 Kg and the total energy was reported to be 340 GJ (both potential and kinetic energies of 5500 Kg), which is 62 KJ.g-1 and in contrast to the chemical heat of 16.7 KJ per gram we previously mentioned, is around 27 percent of ACS Paragon Plus Environment

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former energy value (62 KJ.g-1), absorbing 27% of aerodynamic heat and conserving it in product form of a chemical reaction is quite significant phenomenon, however, the much modern ablative systems which include ultra-high temperature ceramics like zirconium ceramics have reported that the upon the onset of oxidation of zirconium ceramics which certainly leads to the formation of zirconia, much of the heat was consumed by zirconia for phase transformation and dilation 99,108 with the magnitude of heat sunk into the material albeit not been recorded. But few of modern research groups viz., Badhe et al., 33,38 have doped resorcinol formaldehyde with carbon soot obtained from controlled combustion of camphor which could be a suitable matrix modifier for the requirements of moderate char yield of 59% after pyrolysis at 800oC with an interesting ablation rates of 0.019 mm/s and 0.053 g/s, Badhe et al.,35 had come up with a novel idea by modifying resorcinol formaldehyde a super-organic chain with recycled waste rubber, have reported that the degradation temperature was enhanced by 13% and mass ablation rates by 11%. Another method for modification of resorcinol with 3 wt% boron nitride content, which has 44% of the reduction in mass ablation under laser generated heating and increasing the ablation heat by 50% to that of pure resorcinol. Additionally, resorcinol has exhibited proven results for heatsinking by phase changes 32,38. Understanding the ablation process by mathematical analysis helps in improving the rationale of ablation behavior and other concerned entities of material performance like strength, thermal shock etc. The utility of these mathematical models to design the real scale material has to done by correlating experimental results revealing more details of material performance from time to time. Norman et al., 57 had given early mathematical models to understand the kinetic behavior of plastic decomposition as shown below 𝑑𝜆 𝑑𝑡

= ― 𝑘′′𝜆 reaction rate expression is taken as 𝑑2𝑇 𝑑𝑥

2

+

(𝜐𝜌𝑐𝑝)𝑑𝑇 𝑑𝑥

―

𝑘′′𝜌ΔΗ𝑓 𝑘

=0

(10)

(11)

(energy balance at constant physical properties)


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The above equation (11) is valid for boundary conditions with T=T∞ when x=∞, λ=1 when x=∞ and T=Ts (surface temperature) when x=0, K = thermal conductivities, ΔΗ = heat of reaction, 𝑓= fraction of reactable species in the initial material, 𝑐𝑝= specific heat, 𝜌= density It has also been mentioned that there is a continuous thermal gradient which exists through the char region, reaction/ pyrolysis zone and unaffected virgin material (figure 12). An Arrhenius type temperature dependent reaction rate equation(12) has been mentioned as (12)

𝑘′′ = 𝑘′exp [ ― Δ𝐸/𝑅(𝑇 ― 𝑇∞)]

A significant work put forth by Norman et al. was temperature distribution as a function of char depth and energy balance (equation 13). ∂2𝑇

∂2𝑇

∂𝑇

∂𝑇

[

𝑘𝑠(1 ― 𝜖)∂𝑥2 ― 𝑘𝑔𝜖∂𝑥2 + 𝜐𝑠(1 ― 𝜖)𝜌𝑠𝑐𝑝𝑠∂𝑥 + 𝑚𝑔𝑐𝑝𝑔∂𝑥 + 𝑘𝑟𝜌𝑟𝜖

( )ΔΗexp ( ― )] = 0 (13) 1

Δ𝐸

𝑀𝑊

𝑅𝑇

The first term apparently is the rate of heat flow through non-porous part of the material calculated from Fourier’s principles for 1-D heat flow, the second term excludes the conductive heat flow into the trapped gases inside the voids which could otherwise flow away to the surface with the velocity 𝜐𝑠, the fourth term is probably the heat rate exchanged between the hot entrapped gases and elemental material of depth dx, while the gases are expelled out of reaction zone of the material with no effectiveness of heat exchange taken into consideration, finally the last term has been mention by Norman et al., 57 as a result of heat of decomposition. With the above analysis


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Figure 12: Temperature profile of the ablative material Where for equation (13) 𝜖 is fractional void in the solid (for a unit length volume fraction and area fraction do not vary significantly), 𝜐𝑠 is a relative velocity between the material surface and incoming mass of air, 𝜌𝑠 the density of the material, 𝑐𝑝𝑠 stands for the specific heat of material, 𝑐𝑝𝑔 specific heat of gases, 𝑚𝑔 is the mass rate of gases, ks is conductivity of solid, 𝛥𝐸 is activation energy of phenolic matrix (11 Kcal.mole-1), MW is a constant with the value of 10 and finally kg the Conductivity of gas. The above equation gives the energy balance whereas the temperature distribution through the thickness of materials is comprehended by the equation (14) 𝑇 ― 𝑇𝑂 𝑇𝑋 ― 𝑇𝑂

1 ― exp ( ― 𝑥𝑟)

= 1 ― exp ( ― 𝑋𝑟)

(14) To is surface temperature while x and X are the arbitrary distance from the surface and distance enters the material prior to pyrolysis. The value ‘r’ is quantity with a dimension of distance and is formulated as 𝑚𝑠𝑐𝑝𝑠 + 𝑚𝑔𝑐𝑝𝑔

𝑟 = 𝜖(𝑘𝑔 ― 𝑘𝑠) + 𝑘𝑠

(15)


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Alongside energy balance, material balance too is important and is expressed as

[

𝑧 = 𝑧𝑋 ―

𝑃(𝑀𝑊)𝑎𝑣𝑔𝑘𝑔 𝑋1 ∫𝑥 𝑇exp 𝑚𝑔𝑅

𝐸

]

( ― 𝑅𝑇)𝑑𝑥

(16)

Norman et al. have gone further by formulating pre-exponential rate constant as for a heterogeneous reaction occurring at the interface of carbon and silica. 𝑑𝑥𝑐 𝑑𝑡

Δ𝐸

= 𝑘𝑉𝑒𝑥𝑝( ― 𝑅𝑇)

(17)

XC = grams of carbon and the value of k is 2*1014g.cm-3.min-1 and V is the volume of reacting mass. 5. Processing Techniques: Processing of ablatives composites usually involves three steps firstly processing of fillers like fibers or fabrics which altogether have developed into a different field of engineering in recent times109, Nanosized particulates secondly selection of proper polymer blend for matrix and finally rational desegregation of all these individual composite building units. The methods of desegregating materials into a composite system, in general, are tape-winding method in case of long carbon fiber reinforced phenolic resin but in case of preparation of ceramic fillers or ceramic matrix composites a variety of innovative processing techniques like Slurry Infiltration (SI), Chemical Vapor Infiltration, Polymer Infiltration and Pyrolysis (PIP), Chemical Vapor Deposition (CVD), Hot Pressing (HP), Liquid Infiltration (LSI), Vapor Silicon Infiltration (VSI) and Pack Cementation (PC) are being studied ever since the use of ceramics has been ventured into engineering applications as functionalizing substances. yang et al., had employed sol-gel to process SiBCN-Zr powder followed by spark plasma sintering to sinter the as-prepared SiBCN-Zr system powder at 2000oC and a heating rate of 1000C.(min)-1 110. Sol-gel is a process usually adopted to process Nanopowders to be used as fillers for matrix modifications or as a surface coating for high- temperature applications. yang et al. opted Zirconium n-propoxide as precursor supplied by Sigma Aldrich and resol type phenolic resin by Resitan Co. The sol-gel ( figure 13) process usually involves three stages, the first is to prepare a sol by dispersing the required precursor111,112 into suitable dispersant (usually ethanol) and ACS Paragon Plus Environment

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surfactants (to improve surface interaction between constituents of sol-gel process), the second is to drench the preform in this sol, and finally, allow the solvent to evaporate. Another interesting processing technique called pack cementation was applied by Feng et al., to coat ZrB2-SiC-Si over C/C composites firstly, by preparing a mechanical mixture of required compounds, (in this case ZrB2, SiC, and pyrolytic carbon) and heat treating the mixture at a temperature range of 2000 k to 2200 k for 2 to 3 hours in an inert argon atmosphere. The use of pyrolytic carbon had two interconnected functionalities, one was to densify the fiber matrix interface and second was to protect the fibers from thermo-mechanical vulnerabilities113. K. Li et al 94., had processed C/C composites modified by an Ultra-high temperature ceramic ZrB2 and a high-temperature ceramic SiC through precursor infiltration and pyrolysis (PIP) (figure 14), this processing technique was yet times used alongside reactive melt infiltration (RMI) which was effective in forming an excellent filler-matrix interface since the infiltration of the precursor into the surface of filler material like particulates significantly increases the adhesion strength between preform and matrix. PIP in the work presented by K. Li et al., involved densification of 2D needle punched carbon felt, a type of carbon fiber based preform material and the others being carbon fiber and fabric, was densified from 0.45 g.cm-3 to 0.7g.cm-3 by isothermal chemical vapor infiltration process (ICVI) (figure 15) by depositing pyrolytic carbon onto the fibers for protection from oxidizing species during further infiltration of SiC into this densified C/C preform to obtain C/C-SiC. Further modification as reported by Kezhi li el al., was done by liquid infiltration of liquid ZrC and C/C -ZrC-SiC composite was obtained which was further densified by thermal gradient chemical vapor infiltration (TG-CVI) for 130 hours to obtain the required product.


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Figure 13: Sol-Gel Process

Figure 14: Precursor infiltration and pyrolysis


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Figure 15 Schematic of typical CVI process The above processes discussed are mostly employed for wide range of composites and are done mostly by hot press method, in order to incorporate compaction to the material, wherein the fiber and matrix are mixed in suitable mass ratio and hot pressed to temperature as high as 150oC and at the pressure of 9 MPa, one such procedure has been adopted by Aghdam et al., where the sol-gel processed ZrB2 particles were dispersed into the resole type phenolic resin by ultrasonication for 1 hour along with graphene oxide nanoparticles. This mixture was hot pressed to 150oC for 2 hours at 9 MPA along with carbon fiber preform after drying at 70oC. Chen et al., 114 had processed zirconium diboride modified carbon phenolic composited by impregnating method where phenolic resin, zirconium diboride particles were dispersed in ethanol in the ratio of 1:1:0.2 (ethanol: phenolic: ZrB2), then carbon fiber fabric was dipped for 20 minutes in this mixture later subjected to curing after drying for 24 hours to remove ethanol solvent. The curing cycle which is a crucial step in processing the composites has been adjusted to 80oC for 30 minutes, 110oC for 90 minutes and 150oC for 90 minutes at a heating rate of 1oC. min-1 and finally cooled at room temperature. The process techniques although might have evolved due to the requirement of precise technical niche yet the rudimentary obligation is same for all the processing techniques i.e., better interfacial tenacity between filler-matrix conjunction 115, well distributed optimal density finally with the properties upon which the performance relies have to optimum and homogeneous throughout. ACS Paragon Plus Environment

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Table 3 Applications and hindrances in various processing techniques Processing Technique

Purpose

Sol-gel

Glass and Ceramic Preparation (particles, fibers, and films)

Pack Cementation

For the purpose of coating ceramics over Carbon-carbon substrates

Precursor Infiltration and Pyrolysis Reactive Melt Infiltration

CVI

Insitupolymerizati on

Suitable for fiber reinforced ceramic matrix composites Suitable for coating CarbonCarbon composites Suitable most reinforced ceramic matrix composites Suitable for polymer-based ablatives

Advantages

Disadvantages

Ref.

1. Difficult to determine Gelation point 2. Time taking and Skilled labor-intensive process 3. Green body vulnerable to failure while sintering due to microcracks if any 1. Requires skilled labor 2. Time-consuming process 3. Areas not to be coated are difficult to be masked due to very high temperatures

116,11

1. Precursors are usually volatile hence the process has to be done as quick as possible

121,12

1. Short processing method 2. Low cost 3. Thick coatings can be quickly applied

1. High temperatures of melts may damage the fibers 2. Less energy efficient

123

1. Superior deposition and mass transport rates in contrast to precursor deposition 2. Compositionally controlled depositions are feasible

1. Time-consuming process 2. Heavy Investments may incur 3. If proper retrospect on the kinetics of process is not available, results are harder to predict 1. Difficult to incorporate ceramic particle and solid filler due to higher densities of ceramics over polymer systems 2. Material stability is sensitive to process parameters like temperature and pressure

124

1. 2. 3. 4.

High chemical homogeneity and purity Control over morphology Low processing temperature Enable containerless processing for film coatings

1. Coating of high thermal shock resistance can be processed 2. Enable coating over the substrate with mismatched thermal expansion coefficients 3. Physical properties depend on the temperature of sintering 4. Constructively affects interface strength of coating than other processes 1. Superior surface wetting of fiber which ensures better compaction between fiber and matrix after pyrolysis 2. Skilled labor is not a mandate

1. 2. 3. 4. 5.

Suitable for any kind of reinforced composite Highly energy efficient Facile process Skilled labor not required Maximum flexibility to modify the chemical composition to achieve better results

7

118– 120

2

39,40,1 25,126


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The selection of processing technique(s) entirely depends on the desired properties for the material being designed for a particular task. It is difficult to decide which process is the best process for manufacturing composite material. However, in Table 3 applications and hindrances of various processing techniques are discussed. 6. Testing standards: From the literature survey, it is clearly understood that the research on ablative composites has been confined re-entry space explorations, yet many testing methods have been developed to evaluate the ablative materials in terms of ablation rates. The main purpose of the testing ablative material is to procure its ablation rates in terms of mass and reduction in thickness. It is interesting to understand that some ablative composites give positive ablation rate while some give negative, the physical significance of the former being that the amount of mass sublimated into the free stream while that of a negative ablation rate infers mass deposition which has been reported in few of many cases 127. Other testing methods like for evaluating mechanical properties128 like interlaminar shear stress, indentation, density, porosity have been discussed in this section. 6.1. Ablation testing: Ablation rates are measured by usually oxyacetylene torch test for a brief period of 60 seconds to 120 seconds (figure 16) depending upon the intuit of the researchers of the concerned field, albeit the genuine ablation test standard is ASTM E285-08 (2015) 129. The standard gives the flexibility to change the source of the energy to conduct the tests hence few research groups have altered the energy sources prudently with Nd: YAG laser (Neodymium-doped Yttrium Aluminum Garnet) and plasma arc jet test

32,42,56,130.

It is worthy to note that few research groups have mentioned the

aerodynamic test conditions 56,131 i.e., the Mach number of the test conditions, since the velocity and temperature of the plume of oxy-acetylene flame hardly mimics the real-time scenario, ergo recording of the aerodynamic data is of equal importance to reciprocate the performance of the materials in practical situations. In Table 4 ablation performance of various materials in been enlisted featuring effect of material composition, processing route and ablation rates. ACS Paragon Plus Environment

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Figure 16 Schematic of Ablation test


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Table 4: Outcome of Ablation Testing Acetylene Flow Pressure rate (MPa) (L/s) 0.095 0.31

Sr. No.

Matrix

Preform

Process

1

ZrC-C

PI7+TCVI8

2

ZrB2-SiC

C-fiber N.A. Coated structure

-

-

3

ZrC-ZrB2-SiC

3-D Carbon

PIP9+CVI10

4 5 6

ZrC-SiC ZrB2-ZrC-SiC ZrB2-ZrC-SiC

2D needled felt 2D needled felt 3D carbon fiber

7

HfC-ZrC-C

8

ZrB2-Phenolic

Oxygen

Ablation rate

Pressure (MPa)

Flow rate (L/s)

Mass (g/s)

Linear (mm/s)

Ref.

0.4

0.42

5.1*10E-4

96

0.088

-

0.09444

2.45*10E-3

132

-

-

-

-

10E-4

133

PIP HPIP11 PIP

0.095 -

0.31 -

0.4 -

0.42 -

1.0210E-3 1.8*10E-3 0.002

92 134

C-fiber

CVD12

0.95

0.32

0.4

0.42

6.2*10E-4 0.0095*10E-3 (for Sq.cm) 2.3*10E-6 (for Sq.cm) -4.01*10E-4 1.7*10E-3 0.01 (0.36 to 1.07)*10E-3 (SCM)

-

136

C-fiber

I13

-

0.111

-

0.138

5.2*10E-3 (0.09:1)14

114

-

135

Precursor Infiltration Thermal gradient CVI 9 Precursor Infiltration and Pyrolysis 10 Chemical Vapor Infiltration 11 Hybrid PIP 12 Chemical Vapor Deposition 13 Impregnation method 14 Mass ratio of Filler to Matrix. 7 8


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Sr. No.


Matrix

Preform

Process

Acetylene Flow Pressure rate (MPa) (L/s)

Oxygen Pressure (MPa)

Ablation rate

Flow rate (L/s)

Mass (g/s)

Linear (mm/s)

Ref.

6.1*10E-3 (0.11:1) 9

SiC-ZrC coat

Carbon-Carbon composite

PC15+SI

0.95

0.18

0.4

0.24

0.038*10E-3 (SCM)

2.42*10E-3

137

10

ZrSi2-phenolic

Carbon fiber

I+CM16

-

-

-

-

0.0244

1.7*10E-2

138

15 16

Pack Cementation Compression Molding ACS Paragon Plus Environment

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6.2. Interlaminar shear strength: Interlaminar shear strength(ILSS) is a mechanical property of a layered composite that keeps the adjacent laminar from sliding over each other due to shear action. The ILSS measured according to ASTM D 2344 139. It is necessary to know the ILSS of the ablative composites since the volume fraction of ablative composites unlike structural composites does not go beyond 60% and additionally the presence of particulate fillers like zirconia as reported by Srikanth et al., reduces the ILSS of the composite as in case of carbon -phenolic system coated carbon fabric with 9.5 wt% of zirconia has been witnessed to give poor ILSS properties due to formation of zirconia interface between fiber and matrix reducing the direct interaction between fiber and matrix making it trying for simple machining operations too. 6.3. Bending Strength: This property, tested according to ASTM D 970 140 is equally important as aerospace devices like aircrafts, rockets and missiles are usually enormous structures with their dimensions dominating in one of the three axes and hence maintaining the structural integrity 141 is an engineering challenge whenever there is a call for maneuver which certainly induces complex loads in the structures. Bending is one important parameter to be considered to evaluate the ablative composite which has low fiber volume fractions as mentioned earlier. 6.4. Thermal Conductivity: The key role of the ablative grade thermal protection systems is to maintain the thermal gradient within the safety zone and to expel the heat. The former is however dealt with sections but the thermal conductivity of the material decides what magnitude of the cumulative aerodynamic heat can pass through the material thickness and make sure that the material offers less depth of penetration to the temperature reducing the back face temperature which is generally measured by K-type thermocouples whereas the front face temperature is measured by pyrometers. Needless to say that there is a standard procedure to measure as per ASTM E1225-99 now superseded by ASTM E122513

142

recognized as hot guard method which works on the basic principles of Fourier’s 1-D heat

conduction. ACS Paragon Plus Environment

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7. Best possible combination for ablative composites

Figure 17 Design parameters of an Ablative material The prime focus of this review has been on zirconium-based ceramics as matrix modifiers for both polymer and ceramic systems, but still to justify the selection of zirconium-based ceramics in polymer systems it is necessary to understand that there are UHTCs which have higher melting point than Zirconium diboride, like Hafnium diboride, tantalum carbide etc., do have melting point higher than ZrB2. But the goal of the material design should be that the ablative composite designed shall have responsive properties that would come into play in the intended environment, like the oxidation, melting, phase transformation or even the formation glassy film that act as thermal or oxidation barrier (figure 17) all must occur in right instant of time within a brief period of re-entry phase i.e., ~ 120 seconds. However, the formation of passive oxidation layer which is attributed for enhancing ablation resistance depends on the thermodynamics of the material, which is generally explained by ACS Paragon Plus Environment

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a set of thermodynamic tools like free energy analysis, volatility analysis etc. A volatility diagram, technically a plot of the partial pressure of the vapors of the compounds against the partial pressure of the oxygen(usually) at mentioned temperature is instrumental in predicting the sequence of oxidation species formation of different filler systems employed for modifying matrix systems. Since it is not much common that the material with a high melting point at mean sea level deteriorates first in harsh and uncertain conditions that prevail in re-entry situation, as adopted by Y. Zhang et al.,131 the partial pressure of oxygen while the simulating re-entry was chosen between 10 to 30KPa. Eventually, the volatility diagrams were drawn at a partial pressure of 20KPa of oxygen enabling the prediction of oxidation of ceramic species, from the works of Ji Zou143 it is inferred that ZrC and SiC form a primordial temporary protective layer of oxides over ZrB2 at a temperature of 1600oC at oxidation and ablation at 1930oC. An experimental result put forth by K. Li et al.,130 where SiC and Nano carbon tubes had been used to form a protective cost over ZrB2 during oxidation delaying oxidative decomposition of ZrB2 into zirconia and Boria. It is inferred from the values of vapor pressures that the important event, the evaporation of Boria occurs at a temperature 1000oC higher for HfB2 when compared to ZrB2 144,145. Additionally, Zou et al., 143 have made a detailed study on the oxidative behavior of a ternary system of ZrB2-SiC-ZrC, where at an oxidative temperature of 1930oC the domination of gaseous species of boron oxides vary with the partial pressure of oxygen i.e., partial pressure, P of oxygen (O2) of 10-8Pa. Oxidation of the system inversely varies with P(O2) up to 10-6Pa. ZrC was the first to oxidize followed by SiC enveloping ZrB2 and ZrO2. Upon further increasing the P(O2), exceeding 10-6Pa SiC was passively oxidized which is its characteristic to SiC but finally the SiO2(l) formed from oxidation of SiC was driven off due to its vulnerability to fluid dynamics at high temperature leading to the onset of ZrB2 depletion. Hence, it is important to understand that the mere possession of higher physical properties by the any of the element of ablative composite does not improve the performance, their activity is needed to be optimum at the appropriate time, i.e., when the temperature reaches to the peak value of ~ 3000oC. Further for the best utilization of the ablatives materials the knowledge required is not just limited to the technical know-how, further the results obtained from experiments and mathematical stipulation have to be ACS Paragon Plus Environment

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optimized. One method that has developed a reliable method to optimize the composition of the composite system is Taguchi technique and had been implemented in recent past for the optimization of passive TPS material as reported by Daniel et al.,32,108. Further, the phenolic matrix ablative in comparison offer less density than that of ZrC and SiC doped Carbon-carbon composites56,146 and it could be prudent enough to infer that for better results an ablative must have high mechanical strength, high thermal shock resistance, high ablation, and oxidation resistance. It is important to acknowledge that for phenolics resins as reported by Swann et al., the densities of 400 to 480 Kg/m3 have offered best results of ablation performance, however, the ablation performance has been reported to be intensive property147. Such abilities could be achieved by incorporating UHTCs into the phenolic resins with high molecular weight and dense aromatic structure since Trick et al.,148 had reported that the phenolic resin upon pyrolysis are stripped of hydroxyl group and the residual aromatic rings coalesce (figure 18) in absence of moisture in three steps, which might add to the order and alignment of atoms in the char layer as previously the alignment of the atoms have been discussed accordingly with Raman spectroscopy analysis of phenol pyrolysis72. Finally, the phenolicbased matrix which has enormous thermal shock resistance, unlike their ceramic counterparts, were reported to have better interfacial strength with epoxy-aramid sized carbon fiber due to the affinity between phenol and epoxy groups as reported by Sturiale et al.,149.

Figure 18 Mechanism of coalescence of phenol rings during pyrolysis ACS Paragon Plus Environment

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8. Latest Developments in Ablative Materials: Challenges in achieving an ablative composition with enhanced mechanical performance have motivated the scientific community to explore processing methods effective to improve ablative and mechanical performances simultaneously. Pulci et al., have reported that surface of nano zirconia modified with methacrylic acid surfactant before incorporating into low-density phenolic matrix system reinforced with carbon felts in order to prevent agglomeration of nanoparticles, improves the mechanical performance of the composite. It was reported that 5wt% content of nano zirconia when exposed to 4 W/m2 of heat flux given by oxyacetylene flame for 60 seconds with a front face temperature of 2000 oC, the back face temperature was ~180 oC, while the virgin matrix back-face temperature was ~215 oC. It was also reported that 5 wt% loading of nano zirconia improved mechanical strength66. The performance of a composite as an ablative significantly depends on surface emissivity among other factors popularly recognized as radiative dissipation of heat. Cai et al., have reported that silica fiber reinforced phenolic matrix composite when subjected to high temperatures form carbonized layer over the surface. It was reported that the emittance of the carbonized layer was higher than 0.8 when measured for emittance test in a wavelength range of 2.5to 25 microns and maintained at temperatures varying from 200oC to 500 oC150. Yun et al., have reported that char yield improvement of 76% has been achieved at a temperature as high as 800 oC when phenolic resin system containing boron and silicon were subject to oxy-acetylene ablation test. The result was attributed to the utilization of methyltriethoxysilane as silicon source replacing tradition silane. It was observed that the linear ablation rate was 0.0544mm/s and mass ablation rate was 0.0517g/s for the composite system of high silica glass fiber reinforced phenolic resin modified with boric acid and methyltriethoxysilane. It was reported qualitatively that the system also has improved flexural strength and interlaminar shear strength151. 9. Conclusion and Future Prospects There has been an increasing trend in demand for ablative materials owing to number of re-entry missions that have increased exponentially by different research groups like NASA, Space X, European space agency etc and has huge potential of growth in global market with the fact that the ACS Paragon Plus Environment

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developing nations have gone ahead in space explorations which have testified to be a proven market reserve for nations with surplus production in the past decade. It is now clear that the majority of the ablative market is shared between phenolic based polymer ablative and ultra-high temperature ceramic based reusable ablative with both class of materials like in DARPA, NASA space shuttle, Ariane 5 etc, which are host to a unique set of pros and cons. Yet, the limited quantum of understanding has been presented in the literature regarding the functioning of the ablative mechanism. Since a few research articles have actually considered the aerodynamic impact on the resistance mechanism of the material and hardly few have discussed the contribution of the char layer in increasing the thermal resistance of material, impact of selection of resin material on thermal conductivity of char off layer and factors that promote the existence of in-process glassy barrier coat against erosive forces of the boundary layer. Interestingly, zirconium-based ceramics and zirconium diboride, in particular, has met both technical and economic requirements of the market with its optimal response to the ablative environment over other UHTCs with high melting temperature owing to its active Boria, high volatility (at even temperatures as high as 2500oC). Additionally, its compatibility with transitional as well as rare earth elements which are proven methods to engineer an oxidation barrier. However, there is a requirement of development in testing standards of ablative performance as standards do not specify heating methods thereby many techniques have been rationally developed like a flame test, spark ignition, infrared ablation etc. But this is in contradiction to the real-time scenario where the ablative surface is heated beyond the flash point of the material by the conservation of frictional drag into aerodynamic heating and so there is need to reconsider the method of ablation testing which closely mimics the conditions experienced by RVS modules. Latest developments in understanding ablative materials have included considering the importance of enhancing mechanical performance which was previously overseen as ablative materials design was only focused on heat shielding. Additionally, other parameters like emittance of ablative composites of traditional materials processed novel yet facile processing methods have developed in recent times. Finally, polymer-based ablative materials modified with zirconium diboride based UHTC may have a good application in domestic envisioned space travel. The future of polymer-based ablatives tend ACS Paragon Plus Environment

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towards Heat shield for Extreme Entry Environment Technology (HEEET) are expected to have an important role in far future, however, the polymeric based ablatives modified with UHTC and ZrB2, in particular, have the potential to play the role of best available alternative under current circumstances for kick start commercial space travel. In the current paper, we have discussed the reentry environment and the properties to be possessed by a composite material to function as an Ablative material. The mechanism of performance of various material systems have been explained taking into account the processing techniques adopted by different research groups and a unique effort has been made to comprehend the role of modifying ZrB2 in improving the performance of ablative systems, considering formation of oxidation layer as one of the key requirements of ablation performance to insulate heat influx to the material. Finally, different test methods to evaluate the mechanical and ablative performance have been discussed. Acknowledgment The authors would like to thank Dr. CP Ramanarayanan, Vice Chancellor, Defence Institute of Advanced Technology (DU), Pune for the support. Authors also acknowledge Mr. Prakash Gore, Mr. Swaroop Gharde and Ms. Prasansha Rastogi for their continuous technical support during the preparation of the manuscript. The authors are thankful to Dr. Robin McIntyre, Director, Iconiq Innovations Pvt. Ltd, The United Kingdom thoroughly checking the manuscript for grammatical errors. References: (1)

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