Better body armour

Published: 07 December 2005  03:23 PM
Source: The Engineer Online

Boron carbide is the US Defense Department's material of choice for body armour. It is the third hardest material on earth, yet it's extremely lightweight. But it has an Achilles heel that piqued the interest of Georgia Tech Professor of Materials Science and Engineering Robert Speyer five years ago.

He knew that the boron carbide powder used to form the armour had a reputation for poor performance during sintering -- a high-temperature process in which particles consolidate, without melting, to eliminate pores between them in the solid state. Poor sintering yields a more porous material that fractures more easily – not a good thing for a soldier depending on it to stop a bullet.

Determined to understand the sintering problem, Speyer built an instrument called a differential dilatometer to measure the expansion and contraction of materials during sintering heat treatments to temperatures as high as 4,300 degrees Fahrenheit.

"As a particle compact sinters, it shrinks 12 to 15%," Speyer explained. "There are nuances that occur in contraction, and if you monitor them accurately (with a dilatometer), it tells you what is happening at different stages in the sintering process. So we used that information in conjunction with additional materials characterisation techniques to figure out the reasons why boron carbide didn't sinter well, and then found ways around them."

From these findings, Speyer and his research team have created a new boron carbide formation process based on methodical control of thermal and atmospheric conditions during sintering. The method yields higher relative densities – and thus better ballistic performance – than currently available boron carbide armour.

The current commercial process, called hot pressing, squeezes boron carbide powders together between large dies, while heating to elevated temperatures. It yields armour materials with a 98.1% relative density.

Speyer's pressureless sintering method yields a 98.4% relative density and hardness greater than hot pressing. But it can be done faster and at a lower cost than hot pressing. For the most demanding applications, post-sintering hot isostatic pressing (HIP) is used. It increases the relative density of the part to 100 percent through the hydrostatic squeezing action of a high-temperature, high-pressure gas.

"Our material made using HIP is remarkably harder than the current ceramic armour used in the Iraq and Afghanistan theatres," Speyer said. "Plus, because we're not using uni-axial hot pressing, we can make complicated, curved shapes for use in form-fitting body armour and other applications. Hot pressing allows for some curvature so long as the parts can stack together, but there's no chance of making parts like a single-piece helmet."

To make such products, Speyer has formed a company called Verco Materials under the advisory support of Georgia Tech's VentureLab, which helps faculty members commercialise their research.

Ceramics expert Beth Judson is the company's general manager, and Jon Goldman is the VentureLab commercialisation catalyst helping Verco get started.

A Georgia Tech patent on Speyer's sintering process for boron carbide is pending, and when granted, Verco will have access to an exclusive license, Judson said.

The company has received two technology commercialisation grants - totalling $100,000 - from the Georgia Research Alliance to fabricate prototypes for potential military and industrial customers. The Georgia Tech Rapid Prototyping and Manufacturing Institute assisted with fabrication of model armour shapes. Also, VentureLab continues to analyse the company's potential markets.

Beyond body armour, potential military applications include aircraft/rotorcraft protective components. Civilian markets include industries "that can exploit the phenomenal abrasion resistance of theoretically dense boron carbide," Speyer said.

Military applications – body armour, in particular – would be Verco's first target market, and its potential is promising, Speyer noted. The US Army Soldier Systems Center in Natick, MA, has conducted ballistic testing on a small boron carbide disk provided by Verco. Detailed results are classified, but the Army says they are encouraging. With a $75,000 grant from the centre, Verco will produce 6- by 6-inch plates for more comprehensive military ballistic testing within the next few months.

Early next year, the Army Research Laboratory (ARL) at the Aberdeen Proving Ground in Maryland will be examining boron carbide materials (including complex shapes) they purchased from Verco. ARL is interested in Verco's potential ability to form complex shapes cost effectively.

Meanwhile, Verco expects to make thigh and shin plate prototypes in early 2006 for Concurrent Technologies Corporation (CTC). The plates will be evaluated for use in CTC's Ballistic Gauntlet, a new lower-body armour product for use in military and commercial vehicles in war zones to protect against the threat of improvised explosive devices.

The company's current design calls for the Ballistic Gauntlet's thigh and shin plates to be made from titanium, but its cost has risen recently, and it's hard to form and heavier than boron carbide, Judson and Goldman said.

In one other effort, Verco and the Georgia Tech Research Institute (GTRI) are collaborating. GTRI has developed a composite armour "blast bucket" for the ULTRA AP, a concept vehicle designed to illustrate potential technology options for improving survivability and mobility in future military combat vehicles. Verco and GTRI hope to modify the "blast bucket" by replacing heavier ceramic spheres with lightweight boron carbide spheres in the composite structure to make it attractive for use in new helicopters, as well as in retrofitting current rotorcraft, Judson said.

If Verco gets initial defense-related contracts from the customers it is courting, the company would need a tremendous productive capacity – enough to make thousands of parts a week, Judson and Goldman said. Plans call for a highly automated manufacturing facility in Georgia that would initially hire a significant number of engineering and manufacturing employees.

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