is 0 which represents black. The a*
and b*
have
no numerical scale limit, a positive a*
indicates red and the
negative one represents green. The positive b*
is yellow
and the negative one is widely known as blue[9]. The
morphology of fracture surfaces of the composites was
observed by a scanning electron microscope (SEM, JOEL,
JSM-5400). The specimens were fractured cryogenically
after immersion in liquid nitrogen. All fractured surfaces
were coated with a thin layer of gold of about 300 angstrom
before being subjected to the microscopic examination.