The material of the distant future

The third hardest material known to man – boron carbide – lends itself to applications where high wear resistance is needed. DR ROGER GOWER of AWE describes what the material is and how best to use it

Boron carbide is a ceramic with the chemical formula B4C, a molecular weight of 55.29 and a rhombohedral crystal structure. It is the third hardest material known to mankind behind diamond and cubic boron nitride. The theoretical density is relatively low at 2.52g/cm-3. The melting point of boron carbide is around 2350 C and it is stated to boil at a temperature in excess of 3500 C.

Chemically inert

It is chemically inert towards water, acids (including hydrofluoric acid), alkalis and oxidising agents at room temperature. Molten sodium hydroxide will react appreciably with boron carbide at >550 C and oxidising agents will attack it at elevated temperatures. Boron carbide powder will burn at around 600 C but solid objects will only start oxidising at >800 C owing to the formation of a passivating layer of boron oxide. The non-metals: sulphur, phosphorus, bromine and iodine do not attack boron carbide at temperatures up to 1200 C but chlorine will form boron chloride above about 700 C. Some molten metals, and transition metal oxides, will corrode boron carbide by the formation of metallic borides, carbides and borocarbides.

The Young’s modulus, which has been found to be density dependent for the boron carbide test pieces, has been measured as typically 345 10GPa (at 94% theoretical) to 380 10GPa (at 96% theoretical). The Poissons ratio for boron carbide is 0.17 0.1 which is density independent. It has proved difficult to measure accurately the material’s hardness. The coefficient of linear expansion has been measured on sintered boron carbide as 4.66 0.06 3 10-6 C-1 and the room temperature thermal conductivity is -25 Wm-1 C.

Boron carbide currently has a number of industrial uses. Its high boron content (78.3%) makes it a technical source for boron and as a starting material for the production of other boron compounds. With its high neutron cross section for thermal neutron and low cost, boron carbide is well suited as a neutron absorber in controlling neutron flux in nuclear reactors and in the storage of spent fuel rods. High abrasion resistance makes it suitable for uses such as blasting nozzles, dressing sticks and as a grinding grit.

Powder pressing

The manufacture of many artefacts is through hot isostatic pressing of powder followed by machining to size and shape.

AWE uses boron carbide in the manufacture of complex piece-parts. Aldermaston scientists have devised and patented a process that allows rapid, cheap and accurate production.

In the process, boron carbide powder of known particle size distribution is mixed with an epoxy resin binder in a carrier solvent. The resulting slurry is dried and granulated. This material is used to form the parts by isostatic pressing. At this stage of the process, ie, the green state, the articles are strong enough to be machined if necessary. The parts are heat treated in a controlled manner to convert the epoxy resin binder into carbon prior to sintering at high temperature(>2280 C) which achieves densities of 93% to 96% of the theoretical value.

The new sintering process has several advantages. The conventional method involves complex, and expensive, graphite dies which do not allow intricate shapes nor large items to be produced. Hot pressing can only produce a limited number of artefacts from each run whereas, with the new method, the number of components that can be produced is limited only by the size of the furnace hot zone. The ability to machine in the green state gives the capacity to produce more complicated shapes and near true-form dimensions. At present the process will produce artefacts of up to 400mm diameter but there is no reason why objects of 600mm diameter and 1000mm long can not be made.