Mechanical, Thermal, and Ablative Properties of Silica, Zirconia, and Titania Modified Carbon-Phenol Ablative Composites

Abstract

The effect of nanozirconia, nanotitania, and fumed silica on the mechanical, thermal, and ablation behaviour of carbon-phenolic (C-Ph) composites is investigated. The inorganic nanofillers at different loading percentage are used to prepare nano-C-Ph panels by the compression moulding technique. The dispersion of nanofillers is confirmed through SEM analysis. After manufacturing of C-Ph laminates, the mechanical properties such as tensile strength and hardness are evaluated and the effect of these fillers is investigated. Thermal conductivity, thermal erosion, and back wall temperatures were measured to understand the thermal and ablation behaviour of nano-C-Ph laminates. Additionally, the ablation mechanism is analysed by performing SEM analysis of partially and fully burnt composite laminates. The erosion resistance and burnout time of zirconia-C-Ph panels significantly improved with increase in filler loading percentage; however, the back wall temperature rises with filler loading. Titania-filled C-Ph panels show a better control over the back wall temperature but with a poor erosion control. Silica-filled composite panels have shown a balance between decreased back wall temperature with a reasonable erosion rate and burnout time.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Ablative materials represent the conventional approach towards thermal protection to aerodynamic structures, component rocket, and aircraft engines from aerodynamic heating [1]. Most of the ablative materials are fiber-reinforced composites. Thermoset resins such as epoxies, bismaleimides, polyimides, polyarylacetylene, cyanate ester, and phenolic are used as matrices for ablative materials [2–4]. These resins possess a high thermal stability and high char yield. However, phenolic resin, especially resol type, is still used mostly due to their superior mechanical strength, dimensional stability, high resistance against various solvents, good heat resistance, excellent ablative properties, and a high char yield (60%) [5, 6]. Moreover, low cost is the vital characteristic of phenolic resin, which enables them to be used so widely for these applications. The formation of carbonaceous “char” during ablation radiates the heat and acts as an insulating layer to the bulk material [7, 8]. Reinforcements used for these composites must provide “char” with mechanical stability. Carbon fibers have been extensively used as reinforcement in composites for thermal protection due to their high dimensional stability, nonflammability, low density, and excellent mechanical properties [9]. Composite materials based on phenolic resins and carbon fibers (C-Ph) were used by NASA as standard material for high-temperature applications such as MX-4926 [10–12]. However, there are certain restricting factors involved in conventional C/Ph composites. One such limitation comes from the fact that chars are structurally weak and suffer mechanical erosion which in turn reduces the lifetime of the ablative layer or the required insulation thickness [13, 14]. Again, these conventional C-Ph composites cannot tolerate severe application conditions, such as ultrahigh temperature and high heat flux during flight, for the leading edges and nose caps in hypersonic flight vehicles [2].

Many efforts have been made to evidently improve the performance of C-Ph composites in recent years. Phenolic resin modified by boron, synthesized from boric acid, phenol, and formaldehyde, is widely used as matrix of C–Ph ablation composites because of its good heat resistance, mechanical properties, electric properties, and absorbance of neutron radiation [15]. It has been investigated further that H₃PO₄-coated C-Ph composites can withstand more thermomechanical influence during ablation and give a lower erosion rate [8]. Recently, many researchers have used some nanostructured inorganic fillers such as nanosilica [16, 17], carbon black [13, 18], carbon nanotubes [19], nanoclay, and ZrB₂ [20–22] nanoparticles to improve the ablation and thermal resistance of phenolic matrix composites. Patton et al. developed NASA standard C-Ph nozzle materials containing 50 wt% of carbon fibers, 15 wt% of carbon black, and 35 wt% of phenolic resin [23]. Nanosilica filler when used in a controlled manner in C/Ph composite material could enhance the ablation resistance and interlaminar shear strength, thereby reducing thermal conductivity [16]. For high-temperature applications (above 1500°C), ZrB₂ can be used as a filler in an oxidizing environment [24–26] to evidently improve the ablation resistance of carbon composites [27].

Park et al. [28] investigated the effect of carbon nanotube on ablation properties and thermal conductivity of carbon fiber/phenolic composites and reported that both mechanical and thermal properties of phenolic-polymer matrix composites were significantly improved by the addition of carbon nanotubes. They have used the carbon fiber and carbon nanotube reinforcement at a percentage of 30 vol% and 0.5 wt%, respectively. Mirzapour et al. [7] have used micron-size zirconium oxide (ZrO₂) to improve the thermal stability and ablation properties of asbestos fiber/phenolic composites and to reduce their final cost. They have prepared the composite in an autoclave, and the results indicated that after addition of 14 wt% of ZrO₂, the linear and mass ablation rates of the composites were decreased by 58% and 92%. Chen et al. [21] studied the effect of ZrB₂ particles on ablation performance of C-Ph composites. They reported a significant decrease in linear ablation rate with increasing ZrB₂ content, and the ZrB₂–C–Ph composites have shown lowest linear ablation rate when the ZrB₂ to phenolic resin weight ratio is 0.09 : 1. Recently, Stawarz et al. have reported the thermal and ablative property enhancement of epoxy nanocomposites using titanium nanofillers [29]. They have observed that epoxy resin with TiO₂ showed better thermoprotective properties compared to virgin unmodified epoxy resins. However, very few efforts have been made to explore the effects of titanium filler on the ablation performance of C/Ph composites.

The main objective of this work is to enhance the mechanical, thermal, and erosion resistance of phenolic carbon composites with the use of inorganic nanofillers. Three inorganic nanofillers, zirconia and titania fillers at different loadings from 0–10% and fume-silica, were used to prepare nanofilled-phenolic carbon fiber composite (C-Ph) laminates. The filler loading for fume-silica could not go beyond 3% as proper mixing over this loading percentage was not possible during experimentation. The dispersion of nanofillers in the resin matrix was confirmed through SEM analysis. Nanofilled C-Ph prepregs were prepared, and composite laminates were manufactured using these prepregs by following the compression moulding technique. The mechanical and thermal stability and ablation performance of nanofilled C-Ph composite laminates were studied, and the effect of nanozirconia, nanotitania, and fumed silica was investigated. Several mechanical tests such as tensile and Rockwell hardness tests were used for evaluating the effect of nanofillers on the mechanical performance of C-Ph laminates. Thermal conductivity was measured for all the nano-C-Ph laminates for evaluating the effect of nanofillers on the thermal property. Ablation property of these nanocomposites was further evaluated by conducting the oxyacetylene test. The present work targets an increased laminate thermal erosion resistance and decreased back wall temperatures on varying the filler loadings. The workflow of the proposed research is shown in Figures 1(a) and 1(b).

[figure(s) omitted; refer to PDF]

2. Experimental Details

2.1. Materials

Resol-type phenolic resin (PR100WS) supplied by ABR Organics Ltd., Hyderabad, was used as resin matrix and rayon-based carbon fabrics were used as reinforcement fiber in the present study. The nanoforms of zirconia, titania, and fumed silica supplied by Zirox Technologies, Ahmedabad, were used as the inorganic fillers for enhancing mechanical, thermal resistivity, and ablative properties of the C-Ph composite panels. The details of resin matrix and reinforcement fiber are given in Tables 1 and 2, respectively.

Table 1

Resol phenolic resin specifications.

Sl. no.	Properties	Specifications	Test values
1	Appearance	Honey red in color	Honey red in color
2	State	Liquid	Liquid
3	Specific gravity at 30°C	1.12–1.16	1.153
4	Viscosity at 30°C	150–350 cps	190.72
5	Point of trouble at sg: 0.860	6–12 ml	10.1
6	Volatile content (%) (160°C for 20 min)	32–38	36.20
7	Solid content (%) (170°C for 1 hr)	60–65	62.08

Table 2

Specification of rayon-based carbon fabric.

Sl. no.	Parameter	Nominal range	Test method
1	Carbon content	≥94	M/S Aerospace Materials Company Ltd.
2	Ash content	≤10	M/S Aerospace Materials Company Ltd.
3	pH	7–11	IS-1390 : 1983
4	Specific gravity	1.8 ± 0.1	Specific gravity bottle method
5	Sodium content	≤800 ppm	M/S Aerospace Materials Company Ltd.
6	Breaking strength (kg/25 mm)	≥warp:9.1 ≥ fill:6.8	ASTM D1682E-64(R1975)
7	Areal density (g/m2)	300 ± 25	ASTM D3776-1985
8	Fabric content	≥warp:48–56 ≥ fill:45–55	ASTM D3775-1985
9	Thickness (mm)	0.4–0.5	ASTMD1777-1964(R1975)
10	Width (mm)	1000 ± 50	ASTM D3774-1989

2.2. Methods

2.2.1. Dispersion of Nanofillers to Resin Matrix

The incorporation of inorganic fillers into the resin matrix and the attainment of better dispersion were obtained with the use of rheometer, mechanical stirrer, and ultrasonicator. The mixing trials were performed using either a single mixing device or the combinations of mixing devices, which are all operated either under room temperatures or at elevated temperatures. Haake Rheomix 254 with counterrotating sigma blade and an ultrasonicator with probe distribution model of Sonics Vibra-Cell were used to mix the filler and resin contents. The rheometer was used for the dispersion of zirconia and titania nanofillers, whereas combinations of mechanical stirring and ultrasonication at elevated temperatures were adapted for the dispersion of fumed silica fillers to the phenolic resin. The fumed silica and phenolic resin were mixed with ultrasonication of 35 Hz frequency for an hour under 40°C, as shown in Figure 2. An optimization of mixing time with the degree of resin-filler mixing for the Haake Rheomix was studied by analysing nanofilled resin casts of specified intervals through electron microscope scanning. The casts of 4% nanozirconia-filled phenolic resin were prepared from the samples drawn from the Haake rheomixer at every 1-minute interval and cured at 80°C for 3 hours.

[figure(s) omitted; refer to PDF]

2.2.2. Prepreg Preparation

The prepregs were prepared using the hand lay-up (HLU) method by applying nanofilled resin over each layer of the carbon fabric. It was ensured an adequate resin impregnation and wetting during the lay-up process. The prepared prepregs were sealed with the polythene film to contain the effect of environmental factors and were stored under refrigeration conditions for week duration before the composite moulding. The details of the filler ratio, fabric dimensions, and resin and fabric weights used in the prepreg preparation are given in Table 3.

Table 3

Details of filler ratio, fabric dimensions, and resin and fabric weights used in the prepreg preparation.

Sl. no.	Wt% of nanozirconia/nanotitania/fumed silica	Fiber dimensions (cms)	Fiber (wt) (gms)	Resin (wt) (gms)	Nanozirconia (wt) (gms)
1	0	40 × 40	61	81	0.00
			61	81	0.00
2	4	40 × 40	62	82	3.28
			61	81	3.24
3	5	40 × 40	61	81	4.05
			62	82	4.10
4	6	40 × 40	62	82	4.92
			62	82	4.92
5	7	40 × 40	61	81	5.67
			61	81	5.67
6	8	40 × 40	63	83	6.64
			61	81	6.48
7	9	40 × 40	62	82	7.38
			60	80	7.20
8	10	40 × 40	60	80	8.00
			62	82	8.20
Total			982	1302	79.75

2.2.3. Composite Laminate Moulding

The tack-free prepregs from the cold storage were cut to the dimensions of 10 cm × 10 cm. 16 layers each of 0.4 mm thick prepregs were stacked to constitute the elements of composite laminate. The stacked prepreg layers were moulded under specified pressure and cure cycle using 12-ton Carver Press, USA. The details of the cure and pressure cycle used to manufacture the composite laminates are given in Table 4.

Table 4

Details of the cure and pressure cycle followed.

Time, minutes	Temperature, °C	Pressure, tons
1	100	Atmospheric pressure
1	125	10
2.15	165	12

3. Mechanical, Thermal, and Ablation Resistance Property Evaluation

3.1. Nanofiller Particle Size Analysis

The particle size analysis for the fillers was performed using a scanning electron microscope (JEOL, JSM-6390LV, Japan), and the particle agglomeration was scanned.

3.2. Tensile Testing

Tensile strength and other mechanical properties of composite laminates prepared were tested by using a Universal Testing Machine (UTM) (Instron, 8801.UK) equipped with a 100 KN load cell using a cross head velocity of 5 mm/min. The dimensions of test sample are 100 mm × 10 mm × 6 mm (L × B × W) and were tested for each formulation. The tests were carried out in accordance with ASTM D 3039.

3.3. Hardness Test for Nanocomposite Panels

Rockwell Hardness B type was used for the panels filled with nanofillers. The laminates were tested for hardness at five different positions on the laminate, and the average value was taken.

3.4. Thermal Conductivity

The thermal conductivity of the nanocomposite laminates was measured using Lee’s disc apparatus. Lee’s disc is generally used for measuring the thermal conductivity of insulating materials, and the results proved that the composite panels were also within that range. The Lee disc apparatus consists of two metallic discs and a deep hollow cylinder (steam chamber). The disc has two tubes for steam inlet and outlet, respectively. Additionally, it has provision for radial holes to insert thermometers. The nanocomposite sample is placed within the discs; the upper disc is connected to the hot chamber with the steam inlet. Steam is allowed to pass through the cylindrical vessel till a steady state is reached. At the steady state, heat conducted through the sample is equal to heat radiated from Lee’s disc. The chamber was set at 70°C for 2 h. [30].

3.5. Thermal Erosion Test Using Oxyacetylene Flame

The apparatus consists of an oxyacetylene burner, a specimen holder, and means for measuring the time to burn-through and for recording the back-face temperature history of the specimen. A specimen test hood was fabricated to measure the time of flame reach through the laminate thickness and back wall temperature rise during the flame test. In the test hood, a light sensed-time triggering electronic circuit was placed opposite to the sample holder which enables to measure the time of through thickness flame reach. The electronic circuit triggers off the timer when the light is sensed out of laminate thickness during the flame test. The oxyacetylene flame was adjusted to a tip temperature of 2000°C, and the flame was fixed 50 mm away from the center of the sample. The time of light sensed was recorded as burnout time, and temperature at the back side of flame exposed laminate face during the burnout time was measured. The back wall temperature was measured using the noncontact IR thermometer ST-8869H RoHS. The assembly oxyacetylene flame apparatus and timer electronic circuit are shown in Figures 3(a) and 3(b). The erosion rate is calculated from the ratio of laminate thickness to the burn-through time.

[figure(s) omitted; refer to PDF]

4. Results and Discussion

4.1. Nanofiller Particle Size Analysis

The particle size of the nanofillers is analysed by performing scanning electron microscopic analysis by using a scanning electron microscope (JEOL, JSM-6390LV, Japan). The images shown in Figures 4(a) and 4(b) show an agglomeration of different particle sizes of nanozirconia and nanotitania, respectively. The average agglomeration size of each inorganic nanofiller is tabulated in Table 5. Hence, all these fillers were dried and ball milled an hour before the dispersion to the resin medium.

[figure(s) omitted; refer to PDF]

Table 5

Average agglomeration size.

Inorganic nanofiller	Average agglomeration size, nm
Zirconia	326.4
Titania	410
Fumed silica	213.6

4.2. Dispersion of Nanofillers to the Resin Medium

The 4% zirconia-filled resin cast with different intervals of Haake Rheomix was analysed through a scanning electron microscope (SEM), and the degree of filler dispersion as a function of mixing time is shown in Figure 5. From the results, it can be seen that more agglomerated particles are present at the lower mixing times. Also, it shows increased degree of dispersion as the mixing time is increased. The results showed an effective mixing time of 7 minutes in which the degree of mixing has not shown an appreciable change in the degree of dispersion. Hence, 7 minutes of Haake Rheomix was considered to be the optimized mixing time and the same was implemented for fumed silica and titania filler loadings to the C-Ph resin.

[figure(s) omitted; refer to PDF]

4.3. Tensile Strength of Nanozirconia, Nanosilica, and Nanotitania Filled C-Ph Laminate

The tensile strength of C-Ph laminates filled with nanozirconia showed an increased trend as filler concentration increases, as shown in Figure 6(a). This is due to reinforcing effect of filler which reinforces itself due to improvement in interfacial surface area and decreased interparticle distance [31]. As interparticle distance decreases, the bonding effectiveness increases. Hence, it gives an additional reinforcing effect because it compromises for the bonding loss between matrix and reinforcement. This additional reinforcing effect increases the tensile strength and other properties [31].

[figure(s) omitted; refer to PDF]

Further deviation is showed because of improper mixing and mould pressure. Above an optimum level of filler percentage, further addition does not have any significant effect because of formation of agglomeration. These may decrease the interfacial surface area. Similar observations can be made for nanotitania-filled C-Ph and fumed silica-filled C-Ph composite laminates from Figures 6(b) and 6(c). Comparable observations are also reported by other researchers [32].

4.4. Hardness of Nanozirconia, Nanosilica, and Nanotitania Filled C-Ph Laminate

The Rockwell hardness of nano-C-Ph laminates was tested at five different positions, and the average was considered. The variation of hardness of the nanocomposites with nanofiller loading is shown in Figure 7.

[figure(s) omitted; refer to PDF]

The hardness was found to be maximum for nanofiller percentages of 6% and 7% and minimum for filler percentages of 9% and 10% both for nanozirconia and nanotitania C-Ph laminates. Addition of fillers increases the hardness up to an optimum weight fraction, and then it gets decreased. For fumed silica-C-Ph laminates, the hardness values recorded were maximum for filler concentrations of 2% and 3%. The addition of nanofillers did not show any significant increase in hardness due to very low weight concentrations of fillers used, but generally shows increasing trend [31].

4.5. Thermal Conductivity

The thermal conductivity variation with nanofiller loading for all the three nanofillers is plotted and shown in Figure 8. It can be observed that thermal conductivity decreases on incorporation of all the nanofillers. Addition of nanofillers arrests the lattice vibrations and increases the microboundaries and porosity within the laminate. Because of decrease in lattice vibrations, the heat cannot be conducted as thermal agitation and motion of atoms are arrested and cannot strike against each other which will decrease the heat conducting effect as presented by Srikanth et al. [16]. The thermal conductivity decreases with increase in filler concentration, the lowest being recorded for a filler concentration of 10%.

[figure(s) omitted; refer to PDF]

Thermal conductivity of nanotitania-filled C-Ph composites is higher compared to nanozirconia- and fumed silica-filled ones. Fumed silica filler has lowest conductivity values compared to zirconia and titania. Hence, the thermal conductivity values are low compared to other nanofilled composites. Srikanth et al. [19] have reported a decrease in thermal conductivity of C-Ph composite with increase in ZrO₂ content and observed the lowest value at 6.5 wt% loading which is similar to the results shown in Figure 8. Thermal conductivity values recorded were maximum for a fumed silica filler concentration of 1% and lowest for a nanosilica filler concentration of 3%.

4.6. Thermal Erosion of Nanozirconia, Nanotitania, and Nanosilica Filled C-Ph Composite Laminates

The results of thermal erosion of nanozirconia composite panels are tabulated in Table 6; similar results were obtained for nanotitania and nanosilica C-Ph laminates. The effect of erosion rate, burnout time, and back wall temperature as a function of nanofiller loading is shown in Figures 9–11.

Table 6

Thermal erosion of nanozirconia C-Ph composite laminate.

Zirconia weight %	Laminate thickness, mm	Burnout time, sec		Burnout time, sec	Back wall temperature, °C	Erosion rate, x10−4 m/s
		Sample 1	Sample 2
0	6.1	33.5	34	33.8	420	1.82
4	6.0	39.6	40	39.8	460	1.50
5	6.0	41.2	41	41.1	450	1.45
6	6.0	41.4	42	41.7	520	1.43
7	6.0	41.0	44	42.5	550	1.41
8	6.0	44.2	44	44.1	550	1.36
9	6.0	45.3	45.1	45.2	560	1.32
10	6.0	45.6	45.2	45.4	640	1.32

[figure(s) omitted; refer to PDF]

The rate of erosion for zirconia-C-Ph, titania-C-Ph, and fumed silica-C-Ph with nanofiller loading percentage is presented in Figures 9(a)–9(c), respectively. From the results, it can be seen that the erosion rate of nanocomposite panel decreases with increase in nanofiller loadings for all the laminates. This indicates the formation of char during the oxyacetylene test. It can be noted from Figure 9(b) that, for titania-C-Ph laminate, the erosion rate decreases initially up to 7% of filler loading and then it increases with the increase in the filler loading percentage. It can be observed from Figure 9(c) that, at the lower concentrations of the fumed silica fillers, significant changes in the composite panel erosion resistance were obtained. This may be attributed to the higher surface area of fumed silica even at the lower filler loadings [17]. As the temperature goes up during the oxyacetylene test, silica melts and forms a viscous layer which plays a role of high temperature binder and holds the underlying phenolic matrix, which in turn reduces the erosion rate [16].

The burnout time of all the nanocomposite panels was plotted with different nanofiller loading as shown in Figure 10. It can be noted from Figure 10 that the burnout time increases as there is an increase in the zirconia filler loading. This may be attributed to the increased thermal stability of composite panel with increase in the uniform concentration of thermally stable zirconia fillers. Likewise, from Figure 10, it can be observed that the burnout time increases initially until 7% titania filler loading and then the erosion rate increases with the increase in the filler loading. The biphasic trend may be attributed to the poor surface properties due to the agglomeration of filler particles loaded more than 7% by weight.

Also, from Figure 11, it can be noted that the back wall temperature increases with the increasing zirconia filler loading. The reason could be the increased heat capacity of the composite panel as a cause of uniform zirconia filler loadings. From Figure 11, it can be observed that the back wall temperature decreases with the increasing titania filler loading. This may be due to the charring nature of titania filler at the flame temperature, thereby resisting through thickness heat transfer, and hence, the back wall temperature gets decreased with the increase in titania filler loadings. More thermal insulation capability of nanotitania-C-Ph laminate was confirmed.

For fumed silica-C-Ph laminates, shown in Figure 11(c), the back wall temperature distribution clearly shows the better insulating property of silica nanofillers. The uniformly distributed nanofillers on composite laminate act as a thermal protection layer and prevent the heat flow from one surface to the other.

4.7. Analysis of Ablation Mechanism by Microscopy

The ablation test was continuously performed until the specimen was burnt through its entire thickness. When the burnt samples are observed under SEM (JEOL, JSM-6390LV, Japan) and optical microscope (Leica, DFC-320, Germany), the matrix region could not be seen (Figures 12 and 13, respectively). As the carbon-phenolic composite is exposed to high thermal environment, the matrix region in the outer layers of the composite is carbonized, fractured, and separated from the carbon fibers because of the thermomechanical influences. Figures 12 and 13 show the images of partially and complete burnt laminates. It was observed from these images that, at the point of burn-through, the unstable polymer matrix in the inner layers decomposed leaving behind the fibers.

[figure(s) omitted; refer to PDF]

Two important mechanisms of degradation process are observed. The first one is the chemical degradation of the composite which involves pyrolysis of phenolic resin as per the following equation:$$\begin{array}{}\text{(1)}& \mathrm{phenolic}\mathrm{matrix}⟶C\text{Char}+C{H}_4+CO+H_2-\mathrm{heat}\mathrm{.}\end{array}$$

Also, the second mechanism is mechanical erosion due to high shear force of the plasma jet which removes char and C-fibers from the ablating surface [19]. The optical images of cured C-Ph composite are also presented in Figure 14 for better interpretation of the thermal test.

[figure(s) omitted; refer to PDF]

5. Conclusions

(i) From the nanofillers’ dispersion trials, it was found that the combination of mechanical stirring and ultrasonication at slightly elevated temperatures gives the effective mixing with the uniform distribution of fillers incorporated to the resin medium. Also, mechanical stirring followed by ultrasonication at the elevated temperatures ensures the release of bubbles tracked during mechanical stirring.

(ii) The dispersion of zirconia and titania fillers to the resin medium was easy compared to fumed silica filler due to their lower surface areas.

(iii) Zirconia-filled composite panels showed a better erosion resistance and burnout time with the increased filler loadings. However, the panel starts conducting heat with increased filler loading which is evident by back wall temperature rise.

(iv) Titania-filled composite panels showed a good control of back wall temperatures with increased filler loading. However, these panels burn out quickly with a poor erosion rate as the filler loading increases.

(v) Silica-filled composite panels have shown a balance between decreased back wall temperature with a reasonable erosion rate and burnout time.

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