Characteristic Studies on Whisker-Reinforced Borosilicate Glasses

Glass composition of fixed values of the structural factor R(CdO/B2O3) = 0.5 and K(SiO2\B2O3) = 0.5 in borosilicate glass modified by CdO has been studied by different techniques. The amorphous structure of as prepared glass was confirmed by both XRD and EDP spectroscopy. The amorphous samples were treated thermally to be transformed to suitable type of crystalline glass ceramic. Based on SEM and TEM data, the bulk crystallized network was developed by treating the glasses at high treatment route (950 ̊C 1100 ̊C) for a specific time (8 hours). Crystalline Al4B2O9 and Al18B4O33 whisker phases are simply detected by X-ray diffraction spectroscopy. The scanning and transmission electron microscopes (SEM and TEM) have confirmed that aluminum borate crystalline phases in the form of whiskers (rods) species are the main formed type. Both size and concentration of the whisker species were increased with increasing temperature of thermal heat treatment. Formation of this type of crystals (whiskers) leads to interlocking microstructure and as a direct result, an increase in hardness number, glass transition temperature is simply evidenced. Glass-ceramics synthesized in this study can be applied as high temperature resistant materials because their microstructures can resist micro-cracking upon indentation. The crack length due to indentation process is reduced by increasing concentration of crystalline road or whisker like structure.

A. Behairy DOI: 10.4236/njgc.2019.92002 16 New Journal of Glass and Ceramics Their properties include high thermal stability, very low thermal expansion, low heat conductivity, high creep and corrosion resistance and high stiffness [1] [2] [3].Special attention has been paid toward glass ceramics containing some specific types of crystals.In this regard whiskers type is useful substance applied to resist deformation and crack propagation in materials [2] [3] [4] [5].Whisker is generally referred to a certain phase with high aspect ratio (i.e.length/diameter).
Presence of crystals of road like shapes in the glass ceramic matrix reinforces its network structure [5] [6] [7].The direct growth of dispersed whiskers in a glass matrix to form a glass-ceramic leads to improvements of material properties especially mechanical properties.This includes increased fracture toughness, hardness and improved thermal shock resistance [2] [4] [5].Aluminum borate whiskers are considered to have great potentials for fabricating hard and tough topographical microstructures stable for applications.
The crystallization behavior of glasses in the Al 2 O 3 -B 2 O 3 -SiO 2 system has recently been investigated [4]

Measurements
XRD diffraction patterns were carried out on aBrucker Axs-D8 spectrometer.
Emitting source of type (λCuKα) has been utilized.The numerical data was steeply accumulated with a small scanning step, 2θ rang of 5˚ -70˚ and a dwell time of 0.4 seconds have been applied.The obtained X-ray diffraction spectra were revised to reference samples related to standards which were gathered by the technique of powder diffraction and standards (JCDPS).
Transmission Electron Microscopy (TEM) is a common technique used to assess the shape, size, and morphology of the bulk of the material.TEM were performed on a JEOL-JEM-2100 (Mansoura University), with an electron acceleration voltage of 200 kV.During this technique, a high energy beam of electrons is transmitted through a very thin specimen, causing interactions between the electrons and the atoms and producing the TEM images.The surface structure of samples was characterized using JEOL JSM-6510 LV electron microscope operated at accelerating voltage 30 KV, with a magnification 10× up to 400.000×.
For the SEM study, the samples were coated with gold to prevent scattering of the electron beam.Hardness was obtained from a HV-1000B Vickers microhardness tester with a load of 300 g on a mirror finish surface.

Results
Figure 1 shows DSC curve for the as prepared sample.The crystalline temperature is evaluated to be around 940˚C.Exothermic peaks corresponding to crystallization process are dependent on the route of thermal heat treatment.Figure 2 represents XRD spectra of both as prepared and thermally treated glasses.The network structure of the as prepared glass was confirmed to be amorphous.This is can be evidenced by appearing of a broad X-ray diffraction pattern characterizing the amorphous state of the glass as is shown in Figure 2  Kg/mm 2 for glass treating at 1000˚C and 485 Kg/mm 2 for sample heat treated at 1100˚C, respectively.The as obtained glass has a lower number of hardness.In addition, the crack length and distortion due to indentation process at the corner of diamond pyramid was also observed to be lowered by increasing treating temperature (Table 1).This may lead that the microstructure of high hardness number and lower crack length can resist crack propagation in solid materials.

XRD Analysis
Several studies on oxide glasses and glass ceramics had indicated that the structure of glass network depends strongly on, morphology and crystallinity relationships [1] [14] [15] [16].To control formation of some specific types of crystalline species, employment of sensitive and specific technique has to be required.The applied method of thermal heat treatment (THT) is considered between the most suitable routes.The THT may be applied to obtain morphology with certain desired crystalline phases characterized glass ceramics.Figure 2 presents XRD patterns of crystallized glass samples treated at different tempera-  [11] [16] [17] could be identified by XRD patterns of glass treated at 1100˚C (Figure 2(d)).Presence of the same crystalline species is also documented in aluminum borate glasses treated at lower temperature [11].The formation of aluminum borates (Al 18 B 4 O 33 and Al 4 B 2 O 9 ) from alumina and boron oxide occurs between 600˚C and 800˚C in aluminum borate glasses [11].But in the present investigated aluminum borosilicate glasses these phases are obtained at more higher temperature 1000˚C and 1100˚C.Thermal treating technique is the most recommended to obtain morphologies with a better crystallinity as is evidenced from Figure 2(d) and Figure 3(b).These characteristics become essential to improve the structure of the used materials.

Morphological Studies (SEM and TEM) Microscopy
The morphology and bulk structure of as prepared glass were characterized by

Conclusion
The formation of aluminum borates (Al Glass-ceramic treated at 1100˚C presented the highest crystallinity and hardness number and lowest crack length.SEM and TEM showed that a large number of whisker crystalline phases together with the glassy phases were evenly distributed in the network of glass ceramic treated at 1000˚C and 1100˚C.The results indicated that thermal heat treatment process creates more crystalline species of more chemical bonds which acted as a vital part in improving the holding power for the bonding of glass-ceramic bonds and increasing the glass hardness and reducing the crack concentrations. (a).The amorphous nature of as prepared sample is further confirmed by electron diffraction patterns (EDP), Figure4(c), since there is no any diffraction due to crystalline species could be observed.Treating the glass thermally leads to formation of some types of crystalline species as is evidenced from Figures2(b)-(d)and.In this regard, some sharp diffractions are simply appeared.This leads that the applied temperature plays the role of transformation of amorphous phase into more ordered crystalline ones.The present glass is treated thermally at higher temperature than that of crystalline one.Thermal heat treatment temperature is therefore ≥950˚C which may the proper temperatures valuable for transformation of the amorphous structure New Journal of Glass and Ceramics into crystalline one as is evidenced from DSC curve (Figure1) and both XRD and EDP (Figures2(b)-(d) and Figure4(d)).The will formed crystalline phases are indexed to some specific type of whisker phase[1]  [2] [3] [8][10] which are distributed in the residual glass matrix.The morphology of as prepared sample was studied by Scanning Electron Microscopy (SEM) and TEM (Figure3and Figure4).The photograph of base glass is shown by Figure3(a) which shows a homogenous morphology of the glass network.Noteworthy, heating the base glass at a specific temperature for fixed time of treating successfully led to the formation of crystalline nano-fibrous or whiskers bundles (Figure3(b) and Figure3(c)).Treatments at the higher temperature for the same time can simply form co-aligned and elongated nanodifused species (Figure3(d)).The formed nano rods may contain Al 4 B 2 O 9 and Al1 8 B 4 O 33 nano-crystals which are considered to be useful for using the materials in the field of tissue engineering and bio scaffolds applications.

Figure 1 .
Figure 1.DSC curve for as prepared sample.

Figure 4 .
Figure 4. TEM of as prepared glass-ceramic (a) and for heat treated glasses at treated at 1100˚C (b).EDP of as prepared glass (c) and of glass treated at 1100˚C (d).
18 B 4 O 33 and Al 4 B 2 O 9 ) crystalline phases in borosilicate glass ceramics occur between 1000˚C and 1100˚C.There is a development in microstructure and properties upon thermal heat treatment.The differential thermal analysis was carried out to evaluate the limit of the sintering process.Then the degree of crystal formation was confirmed by X-ray and electron diffraction.Finally, the developed microstructures were characterized by scanning and transmission electron microscopy.The obtained results have confirmed that Al 18 B 4 O 33 is the main crystalline phase.Needle grains like shape were dominant crystal type.The various heat treatment temperatures mainly influenced the content of (Al1 8 B 4 O 33 and Al 4 B 2 O 9 ) crystalline species in the glass-ceramic.

2. Experimental 2.1. Samples Preparation
[2].It was showing that whisker of aluminum borate phase was crystallized out above glass transition temperature of the as-prepared glass.Several studies have reported that the bulk crystallization of borosilicate glasses can be affected by different factors, for example change of the glass composition, thermal heat treatment and irradiation processes.All have a great effect on bulky crystallization of the glass or glass ceramics[2][3] [4] [8] [9] [10].Crystalline Al 4 B 2 O 9 and Al 18 B 4 O 33 phases are simply formed in aluminum borosilicate glasses by effect of heat treatment [2] [4] [11].The size of aluminum borate whiskers/rods increases with increasing time of thermal treating.Usually the whisker/rod-like crystals are uniformly oriented throughout the microstructure in the heat treated samples [2] [8] [9] [10] [11].This led to a structure characterized with several types of sub crystals which are joint together firmly and hence an increase in hardness and fracture toughness is observed.As a result, the microstructure of such materials can resist micro-cracking and scratching upon indentation.This work is aimed to determine the phase evolution, microstructures, micro hardness and crack propagation in the investigated glass and glass ceramics.The additional aim of this study is to shed more light on the effect of thermal heat

Table 1
presents the relation between the micro hardness number and tempering temperature of heat treatment.There is a significant change in the hardness number with the effect of increasing temperature of treatment.H v is increased gradually and the values were 350 Kg/mm 2 for as prepared glass, 396 in Figure 2(b) and Figure 2(c).Increasing crystallization temperature led to an enhanced growth of the crystals, since both Al 4 B 2 O 9 and Al 18 B 4 O 33 [17].However the intensity of peaks increased with increasing treating temperature from 950˚C to 1100˚C.Formation of whisker crystals at temperature 950˚C and 1000˚C can be correlated with Al 4 B 2 O 9 phase according to XRD data A. Behairy DOI: 10.4236/njgc.2019.9200219 New Journal of Glass and Ceramics as shown

Table 1
[23]rved to be lowered by increasing treating temperature.This may lead that the microstructure of high hardness number and lower crack length can resist crack propagation in solid materials[22][23].New Journal of Glass and Ceramics

Table 1 .
Change of hardness number and crack length with temperature.