Pure B4C Research Articles

Mechanical and Electrical Properties of Strong TiB2-B4C Ceramic System by Hot-Pressing.

TiB2-B4C ceramic composites containing no additives were hot-pressed from a mixture of TiB2 and B4C powders containing 0 up to 100mol% B4C (20mol%). Hot-pressing was conducted at 1950°C for 3.6ks under a pressure of 31.2MPa in an Ar gas atmosphere. The mechanical and electrical properties were examined as a function of compositional change from TiB2 to B4C. TiB2-B4C composites containing 20-60mol% B4C had a relative density of 97.4-98.9%, Vickers hardness of 23.8-27.2GPa, flexural strength of 589-705MPa, Young's modulus of 490-568GPa, fracture toughness (KIC) of 6.46-8.05MPa·M1/2. The mean surface roughness (Ra) was 25.6-30.5nm and the electrical conductivity 0.30-3.96MS·m-1. Thus, these composites possessed a high electrical conductivity and a smooth surface. Their mechanical properties were 1.5 times higher than those of pure TiB2 or B4C ceramics, and comparable to those of SiAlON or hot-pressed Si3N4 ceramics. The present composites are suitable as ceramic mold materials, machinable by electric discharge, corrosion-resistant electrode components, tool materials and high-temperature structural materials.

Evaluation of B4C as an Ablator Material for NIF Capsules

Boron carbide (B4C) is examined as a potential fuel container and ablator for implosion capsules on the National Ignition Facility (NIF). A capsule of pure B4C encasing a layer of solid DT implodes stably and ignites with anticipated NIF x-ray drives, producing 18 MJ of energy. Thin films of B4C were found to be resistant to oxidation and modestly transmitting in the infrared (IR), possibly enabling IR fuel characterization and enhancement for thin permeation barriers but not for full-thickness capsules. Polystyrene mandrels 0.5 mm in diameter were successfully coated with 0.15–2.0 µm of B4C. Thicknesses estimated from optical density agreed well with those measured by scanning electron microscopy (SEM). The B4C microstructure was columnar but finer than for Be made at the same conditions. B4C is a very strong material, with a fiber tensile strength capable of holding NIF fill pressures at room temperature, but it is also very brittle, and microscopic flaws or grain structure may limit the noncryogenic fill pressure. Argon (Ar) permeation rates were measured for a few capsules that had been further coated with 5 µm of plasma polymer. The B4C coatings tended to crack under tensile load. Some shells filled more slowly than they leaked, suggesting that the cracks open and close under opposite pressure loading. As observed earlier for Ti coatings, 0.15-µm layers of B4C had better gas retention properties than 2-µm layers, possibly because of fewer cracks. Permeation and fill strength issues for capsules with a full ablator thickness of B4C are unresolved.

Thermomechanical characterization of B4C vacuum plasma sprayed coatings on stainless steel tubular substrates

Boron carbide coatings have attracted increasing interest because of their promising properties. Their utilization for thermonuclear plasma-facing components and, in particular, as armour for the separate tubular first wall [1] could contribute to limiting the radiating power losses due to impurity particles and simultaneously protect the structural material from excessive thermal fatigue. In view of such applications, a characterization of the coatings with respect to the properties affecting their therm0mechanical behaviour in the presence of thermal shocks must be performed [1]. Feasibility tests carried out recently on B4C [2] have shown that vacuum plasma spraying (VPS) is a promising technique for producing thick coatings (up to 2 mm). Another technological aspect to be assessed is whether B4C can be deposited on an AISI 316L stainless steel tubular substrate of significant length (350 mm) and diameter (33.7 mm) without damage or detachment. The coatings were deposited on a 400 ~m thick cermet bond coat of molybdenum and B4C with a composition ranging from pure Mo on the stainless steel to pure B4C. The pure B4C coatings on the bond coat reached layer thicknesses of 500, 1000 and 1500 ktm. The absence of microcracks at the stainless steel/ bond coat interface and inside the B4C layer was first assessed by optical metallographic examinations, but later scanning electron microscope (SEM) examinations showed a significant amount of porosity (Fig. 1). Pure B4C ring specimens (Fig. 2) obtained by cutting away the stainless steel and the bond coat and grinding all of the external surfaces were used to

Mechanical behaviour of hot-pressed boron carbide in various atmospheres

Hot-pressed boron carbide, due to its high strength and low density, is a promising material for industrial applications. In argon the strength and fracture toughness of such a hot-pressed material are practically unchanged up to 1230°C [1], whereas in air its strength decreases at temperatures above 600°C, though critical stress intensity factors remain constant [2]. However, these data can hardly be compared, since they were obtained for different compositions. Therefore, in the present work we studied the hightemperature behaviour of silicon- and aluminiumdoped hot-pressed boron carbide with a density of 2.52gcm -3 [3] in air and argon up to 1400°C, using the procedures of [2]. The mechanical properties of a similar material at room temperature were described in [4]. As can be seen from Fig. 1, the variation in strength as a function of temperature in air and argon is quite different. Whereas above 1000 ° C the strength of the samples heated in argon is increasing, the strength of the samples tested in air is steadily decreasing. The fractographic studies revealed brittle fracture of the samples in the temperature range of interest. As the investigations of fracture and lateral surfaces of the samples demonstrated, the material appeared to be phase non-uniform with regions of pure B4C up to 500#m in size (dark zones in Fig. 2b) located in a silicon- and aluminium-doped matrix (bright zones in Fig. 2b). In inert atmosphere up to 1400 ° C and in air at room temperature fractures, in all cases, originated from pores (Figs 2c and d), surface and subsurface cracks (Figs 2e and f) as well as from other flaws usually formed in the regions of pure B4C. In these regions the fracture was of transcrystallite character, whereas in the dope-containing regions the traces of intercrystallite fracture were observed. The samples tested in air at 1000 and 1400°C were investigated after the removal of an oxide layer from their surfaces. To remove the oxide layer, the samples were boiled in distilled water for 10 min. Despite the etching of fracture surfaces associated with oxidation, we found that craters formed in the surface layer were major causes of failure. In the centre of several craters knobs of some kind were present (Fig. 3a). Proceeding from the results of the investigation of lateral surfaces, one can