Jacob Marx, Marc Portanova and Afsaneh Rabiei have published their findings regarding a composite armor structure consisting of a boron carbide faceplate, a "foamed" energy-absorbing interlayer and an aluminum backing plate, with most of their attention on the novel energy-absorbing layer.
Phys.org wrote up an article about their findings, though the headline is a little misleading. The interlayer is not quite "foamed" like most people think of it, but rather a composite of hollow hardened steel microspheres sintered in a metallic matrix.
The mass efficiency of their system vs 12.7x99mm (Ball and AP) varies between tests, but averages out to about 2.1. It's hard to tell from their figures, but my first attempt to calculate its thickness efficiency puts it at about 1.3. I'd like to revisit that to get a higher confidence figure.
The article is here:
Their findings were published in Composite Structures (2019). I have a copy archived here: